ArticlePDF AvailableLiterature Review

RNA transport


Abstract and Figures

RNA molecules synthesized in the nucleus are transported to their sites of function throughout the eukaryotic cell by specific transport pathways. This review focuses on transport of messenger RNA, small nuclear RNA, ribosomal RNA, and transfer RNA between the nucleus and the cytoplasm. The general molecular mechanisms involved in nucleocytoplasmic transport of RNA are only beginning to be understood. However, during the past few years, substantial progress has been made. A major theme that emerges from recent studies of RNA transport is that specific signals mediate the transport of each class of RNA, and these signals are provided largely by the specific proteins with which each RNA is associated.
Content may be subject to copyright.
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
Annu. Rev. Neurosci. 1997. 20:269–301
1997 by Annual Reviews Inc. All rights reserved
Sara Nakielny, Utz Fischer, W. Matthew Michael,
and Gideon Dreyfuss
Howard Hughes Medical Institute, and Department of Biochemistry and Biophysics,
University of Pennsylvania School of Medicine, Philadelphia,
Pennsylvania 19104-6148
KEY WORDS: nucleocytoplasmic trafficking, mRNA, U snRNA, rRNA, tRNA
RNA molecules synthesized in the nucleus are transported to their sites of func-
tion throughout the eukaryotic cell by specific transport pathways. This review
focuses on transport of messenger RNA, small nuclear RNA, ribosomal RNA,
and transfer RNA between the nucleus and the cytoplasm. The general molecular
mechanisms involved in nucleocytoplasmic transport of RNA are only beginning
to be understood. However, during the past few years, substantial progress has
been made. A major theme that emerges from recent studies of RNA transport is
that specific signals mediatethe transport of each class of RNA, and these signals
are provided largely by the specific proteins with which each RNA is associated.
Most eukaryotic RNAs are produced in the nucleus by RNA polymerase I, II,
or III. The RNA molecules undergo a variety of posttranscriptional processing
events, after which they are transported to their sites of function throughout
the cell. Clearly, the subcellular locations of each type of RNA—namely mes-
senger RNA (mRNA), small nuclear RNA (snRNA), ribosomal RNA (rRNA),
and transfer RNA (tRNA)—are critical to the normal functions of the cell.
A few RNAs are retained in the nucleus, and may be targeted to subnuclear
domains, but the majority of RNAs are transported to the cytoplasm. Some
RNAs need to be reimported to the nucleus for their function, for example U
snRNA and 5S rRNA.
The exchangeof macromolecules between thenucleus and the cytoplasm oc-
curs mostly, if not exclusively, through nuclear pore complexes (NPCs). Each
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
NPC is a large structure comprised of upwards of 100 different proteins that
assemble to form a pore across the nuclear envelope. The NPC allows the free
diffusion of ions, metabolites, and small proteins, but large molecules, in con-
trast, traverse the NPC by an active, energy-dependent mechanism (Rout &
Wente 1994, Davis 1995). The best understood active transport process is pro-
tein import to the nucleus, which can be reproduced in vitro (Adam et al 1990).
Protein import is mediated by specific sequences in the protein, termed nuclear
localization sequences (NLSs) (Boulikas 1993). The NLS is recognized in the
cytoplasm by a specific receptor, termed importin, karyopherin, or NLS re-
ceptor. Translocation into the nucleus requires several other factors, including
a GTPase cycle (Sweet & Gerace 1995, G¨orlich & Mattaj 1996). While the
molecular mechanism of import of proteins to the nucleus has been character-
ized in some detail, our understanding of RNA transport across the NPC has
lagged behind considerably, because of the difficulty of analyzing RNA export
from the nucleus. Despite this limitation, some general principles of RNA ex-
port have been established. All cellular RNA species exit the nucleus through
NPCs by an active, mediated mechanism (Stevens & Swift 1966, Zasloff 1983,
Dworetzky & Feldherr 1988, Featherstone et al 1988, Khanna-Gupta & Ware
1989, Bataill´e et al1990, Guddat et al1990, Mehlin etal1992, Mehlin &Dane-
holt 1993, Jarmolowski et al 1994). Recently, it has become clear that RNA
transport through the NPC is mediated by proteins associated with the RNA
molecules, and that these proteins, like proteins that are actively imported to
the nucleus, utilize signal-dependent, receptor-mediated pathways to transport
RNA molecules. The export pathway for each class is unique: Class-specific
factors are used (Jarmolowski et al 1994, Pokrywka & Goldfarb 1995). How-
ever, the pathwayslikelymergeat some point, sotheymayalso utilize common
This review focuses on transport of RNA molecules between the nucleus and
thecytoplasm. Wediscusswhatisunderstood aboutthe transportmechanismof
each class of RNA, and then consider some common themes and perspectives.
Experimental Approaches
Unlike transcription, RNA splicing, and nuclear import, RNA export from the
nucleus to the cytoplasm has not been reproduced in an in vitro system. Most
information on the molecular mechanisms of RNA trafficking between the nu-
oocytes. The large size of the oocyte (1 µl, or up to 200,000 times the size
of a HeLa cell) facilitates the microinjection of a large amount of radiolabeled
RNA into either the nucleus or the cytoplasm. The subcellular distribution of
the injected RNA can be directly visualized by manually dissecting the nucleus
from the cytoplasm and analyzing theRNA by gel electrophoresis (e.g. Zasloff
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
et al 1982a). Although the oocyte system offers several attractive features for
the analysis of RNA transport, the oocyte is a highly specialized cell type.
Therefore, results obtained from X. laevis oocyte studies may not always be
relevant to RNA transport in other cell types.
Some information on the molecular mechanisms of RNA trafficking, in par-
ticular mRNA trafficking, has been provided by the analysis of Saccharomyces
cerevisiae mutants. In this approach, yeast cells are usually randomly muta-
genized, and mutants displaying defects in mRNA localization are selected.
RNA (i.e. mRNA) is detected by in situ hybridization to a fluores-
cently tagged oligo (dT) probe. In wild-type cells, themajority of the poly(A)
RNA is in the cytoplasm. When mRNA export is defective, poly(A)
accumulates in the nucleus. Genes required for RNA export are identified
by screening temperature-sensitive mutant strains for those that accumulate
RNA in the nucleus at the restrictive temperature (Kadowaki et al
1992, 1994a; Amberg et al 1992).
In the following sections, we integrate the information obtained from both
of these approaches to the study of RNA transport.
Nuclear Export of mRNA
VISUALIZATION OF NUCLEAR EXPORT OF mRNA Balbiani ring transcripts in
Chironomus tentans salivary glands have been particularly useful for study-
ing mRNA export at the ultrastructural level. This RNA is very large, 40 kb,
and abundant in gland cells, which allows it to be easily identified by electron
microscopy. Moreover, Balbiani ring pre-mRNA undergoes minimal splicing,
allowing the transcripts to be followed from their site of synthesis, through
the nucleoplasm to the NPC (reviewed in Mehlin & Daneholt 1993). The
protein-coated transcript released from chromatin appears as a ribbon that is
bent into an asymmetric ring-like structure 50 nm in diameter. Of the four
domains of this structure that can be distinguished by electron microscopy,
one can be identified that contains the 5
end of the mRNA, and another that
contains the 3
end (Skoglund et al 1986). During translocation through the
NPC, the mRNA-protein complex undergoes a dramatic reorganization, which
appears to occur in ordered steps (Stevens & Swift 1966; Mehlin et al 1991,
1992). During the first step of RNA-protein particle translocation, the complex
associates with fibrous material extending from the rim of the NPC into the
nucleoplasm. It then orients itself in front of the NPC; the domain containing
the 5
end facesthe opening of the pore, and the domain containing the 3
end is
closely aposed to the rim of the pore. Finally, thering structure of the transcript
is gradually relaxed and translocated through the pore, with the 5
end in the
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
lead. Thus, several temporally and spatially coordinated events, all of which
appear to involve contacts between the RNA-protein complex and the NPC,
lead to the translocation of Balbiani ring mRNA through the NPC. Although
in the absence of other examples to draw on, it cannot yet be established that
these ordered structural rearrangements leading to translocation are typical for
all transcripts, Balbiani ring RNA currently serves as a useful model for the
analysis of mRNA export at the ultrastructural level.
transcribed as pre-mRNAs, which generally undergothree co- or posttranscrip-
tionalprocessingevents: 5
capping,i.e. additionofa 7-monomethylguanosine
G) cap structure to the 5
end of the transcript; removal of introns by splic-
ing; and polyadenylation at a defined site within the 3
untranslated region (for
reviews, see Banerjee 1980, Moore et al 1993, Keller 1995, Shuman 1995,
Adams et al 1996). The relationship between processing and export has been
addressed in many studies. As discussed below, these studies have shown that
splicing signals and introns are, in general, cis-acting nuclear retention ele-
ments, whereas the 5
cap and 3
end appear to enhance the rate of mRNA
Splicing signals and introns Pre-mRNA molecules bearing splice sites are
largely retained in the nucleus. The block in cytoplasmic accumulation of
pre-mRNA is overcome when splicing complex formation is disturbed by mu-
tating either the splicing signals or the trans-acting factors required early in
the formation of splicing complexes (e.g. U1 snRNA). These observations led
to the spliceosome retention hypothesis, which predicts that certain splicing
factors can function as negative trans-acting factors in mRNA export (Legrain
& Rosbash 1989, Chang & Sharp 1989, Hamm & Mattaj 1990). It is unclear
how retention of pre-mRNA is accomplished. Certain splicing factors may
interact with nuclear structures and thereby hold the RNA in the nucleus, or the
spliceosome may prevent interaction of the RNA with export factors.
Removal of introns by splicing does not, inmost cases, appearto be essential
for export of RNA; for example, overexpression of an intron-containing gene
in yeast results in the appearance of pre-mRNA in the cytoplasm. This sug-
gests that an interaction of the pre-mRNA with retention factors is saturable,
allowing the pre-mRNA to leave the nucleus (Legrain & Rosbash 1989). Fur-
thermore, some mechanism must exist to allow export of alternatively spliced
(i.e. intron-containing) transcripts. It is currently unclear whether alternatively
spliced mRNAs are exported by a pathway similar to that which exports fully
spliced transcripts, or whether a specialized mechanism ensures that alterna-
tively spliced mRNAs can leave the nucleus (see section on Viral Regulation
of mRNA Export, Rev and Rex Proteins).
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
Although splicing is believed to influence export indirectly by a retention
mechanism, the export of some mRNAs appears to be directly dependent on
splicing. For example, β-globin mRNA is not exported to the cytoplasm when
expressed from intronless transfected DNA (Collis et al 1990). The process
of splicing appears to convert the mRNA from an export-incompetent to an
export-competent form. It is unknown why splicingis apparently necessary for
efficient export of these transcripts but not for those encoded by other intron-
containing genes, such as cellular thymidine kinase, or for those encoded by
natural intronless genes, such as α-interferon and c-jun (Nagata et al 1980,
Gross et al 1987, Hattori et al 1988).
Recently, two naturally occurring intronless genes, herpes simplex virus
thymidine kinase andhepatitis B virusS transcripts, were each foundto contain
an element that when fused to the intron-dependent β-globin gene, allowed
efficient cytoplasmic accumulation of β-globin RNA in the absence of splicing
(Liu & Mertz 1995, Huang & Yen 1995). These viral RNA sequences may
function as cis-acting RNA export elements.
Cap structure and 3
end Mattaj and coworkers have demonstrated a corre-
lation between the export rate of a transcript and its 5
cap structure. Tran-
scripts synthesized in vitro with a trimethylguanosine m
G) cap or
an adenosine (A) cap (uncapped transcripts cannot be analyzed because they
are degraded in cells) were found to be exported from X. laevis oocyte nuclei
more slowly than an mRNA synthesized with an m
G cap (Hamm & Mattaj
1990, Jarmolowski et al 1994). The m
G cap structure can therefore enhance
the rate of mRNA export, but it does not appear to be essential. Several obser-
vations indicate that the m
G cap structure is neither necessary nor sufficient
for mRNA export. For example, Caenorhabditis elegans mRNAs processed by
trans-splicing acquire a trimethylated cap and are still translated in, and there-
fore transported to, the cytoplasm (Liou & Blumenthal 1990, Van Doren &
Hirsh 1990), and transfected histone genes lacking their normal 3
end produce
an m
G-capped mRNA that does not leave the nucleus (Sun et al 1992).
The observation that the m
G cap influences the rate of export of some
mRNAssuggeststhatcap-bindingproteinsmaybeinvolvedinexport. Anuclear
Gcap-bindingcomplex(CBC)comprisingtwocap-bindingproteins, CBP80
and CBP20, has been characterized (Ohno et al 1990, Kataoka et al 1994,
Izaurralde et al 1994; see also the section on U snRNA Export). Although it
is not certain whether CBC mediates the effect of the cap on mRNA export,
Visa et al (1996b) haveshownby immunolocalization that CBP20 accompanies
Balbiani ring mRNA out of the nucleus.
Several studies suggest that the 3
poly(A) tail and the histone 3
end stem-
loop structure can stimulate mRNA export, but that they are not an absolute
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
requirement for transport (Eckner et al 1991, Sun et al 1992, Jarmalowski et al
1994). Neither is a 3
poly(A) tail alone sufficient to export an mRNA, since
and omega-n RNA;Brockdorffet al 1992, Brownet al 1992, Hogan et al 1994).
In summary, although the m
G cap and 3
end may influence the rate of
transport, the role these structures play in mRNA export is unclear. Since
neither structure is essential, other RNA sequence elements yet to be identified
may be important for mRNA export.
TRANS-ACTING mRNA EXPORT FACTORS This section describes evidence for
the involvement of trans-acting protein factors in mRNA export, which is sum-
marized in Figure 1. These potential trans-acting factors include hnRNP pro-
teins, NPC proteins, components of a GTPase cycle, and various yeast proteins
identified in poly(A)
RNA export mutants.
hnRNPproteins From the moment pre-mRNAemergesfrom the transcription
complex and throughout its lifetime in the nucleus, pre-mRNA/mRNA proba-
bly never exists as free RNA, but rather is associated with proteins (reviewed
in Dreyfuss et al 1993; see the section on Visualization of Nuclear Export of
erogeneous nuclear RNA-bindingproteins (hnRNP proteins)and otherproteins
involved in transcription and pre-mRNA processing (Pi˜nol-Roma et al 1988;
reviewed in Dreyfuss et al 1993).
The hnRNP proteins, which have been most extensively characterized in
human cells, comprise a group of about 20 major proteins that associate with
nascent pre-mRNA, and participate in the processing reactions that generate
mature mRNA (Munroe & Dong 1992, Mayeda & Krainer 1992, Dreyfuss
et al 1993, Portman & Dreyfuss 1994, Yang et al 1994, C´aceres et al 1994).
All transcripts appear to be associated with most of the hnRNP proteins, but
the stoichiometry varies, so the protein constellation that assembles on each
transcript is probably unique (Pi˜nol-Roma et al 1989, Matunis et al 1993).
Several hnRNP proteins, for example, hnRNP A1 and hnRNP K, although
predominantly nuclear, shuttle continuously between the nucleus and the cy-
toplasm. While in the cytoplasm, hnRNP A1 is associated with mRNA, sug-
gesting that shuttling hnRNP proteins accompany mRNA from the point of
its emergence from the transcription machinery, through the nucleoplasm to
the NPC, and during translocation of the RNP complex through the NPC
(Pi˜nol-Roma & Dreyfuss 1992, 1993). This suggestion is now supported by
the recent observation that an insect A1-like hnRNP protein in Chironomus
tentans, Ct-hrp36, translocates through the NPC associated with RNA (Visa
et al 1996a). The question of whether mRNA is carrying, or is being carried
by, the shuttling hnRNP proteins has been addressed with the identification of a
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
Figure 1 Potential trans-acting factors for mRNA nuclear export. Messenger RNA is transcribed
in the nucleus by RNA polymerase II as a precursor with an m
G cap, and, in the majority of
highereukaryotic transcripts, introns and a3
poly(A)tail. Evidenceindicates that shuttling hnRNP
proteinsmediate exportofmRNAfrom thenucleus tothe cytoplasm. Interactionwith spliceosomes
and nonshuttling hnRNP proteins prevents mRNA export. The roles of the cap-binding complex
(CBC), the Ran GTPase cycle, and individual nuclear pore complex (NPC) proteins are less clear.
See text for details.
proteinnuclear exportsignal (NES) withinhnRNPA1 (Siomi &Dreyfuss1995,
Michael et al 1995). The A1 NES (a 38–amino acid segment of the protein
termed M9) is not involved in RNA binding and, when fused to a protein that is
normallyrestrictedtothenucleus, is capable of promoting its export to the cyto-
plasm in a temperature-sensitive manner (Michael et al 1995). Together, these
observations support the model that hnRNP A1, and probably other shuttling
hnRNP proteins, mediate, via their NESs, nuclear export of cellular mRNA.
While the shuttlinghnRNP proteins appearto remain associatedwith mRNA
until it reaches the cytoplasm,nonshuttling hnRNPproteins, such ashnRNP C1
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
and hnRNP U, do not leave the nucleus. Recently, a nuclear retention sequence
(NRS) has been identified in the hnRNP C proteins (Nakielny & Dreyfuss
1996). The NRS is capable of overriding hnRNP protein NESs, suggestingthat
pre-mRNA/mRNA that is associated with both NES-bearing and NRS-bearing
hnRNP proteins cannot leave the nucleus. It is therefore an attractive possibil-
ity that nonshuttling NRS-containing hnRNP proteins prevent transcripts that
have not been fully processed from exiting the nucleus to the cytoplasm. Non-
shuttling hnRNP proteins are removed from mRNA prior to, or during, mRNA
export. This removal is likely to be critical to allow mRNA to be exported, and
may be an important regulatory step in mRNA transport.
After translocation through the NPC as components of the RNP complex,
shuttling hnRNP proteins must dissociate from the mRNA and be reimported
to the nucleus. Import of hnRNP A1 is mediated by the same region of the
protein (M9) that mediates A1 export (Siomi & Dreyfuss 1995). It is not
yet clear whether the import and export signals in A1 are one and the same,
although so far, mutations within M9 that abolish import also prevent export
(Michael et al 1995). Interestingly, nuclear import of A1, and of the majority of
other shuttling hnRNP proteins, is dependent upon ongoing RNA polymerase
II transcription, which suggests that import of these candidate trans-acting
mRNA export factors is coupled to the nuclear content of exportable mRNA
(Pi˜nol-Roma & Dreyfuss 1991, 1993; Michael et al 1996). Import of proteins
bearing classical basic NLSs does not depend on transcription, indicating that
the import mechanism of shuttling hnRNP proteins is fundamentally different
from that of basic NLS-bearing proteins.
Nuclear pore complex proteins At least nine yeast NPC proteins have been
identified that, when they bear a temperature-sensitive mutation or when they
are deleted, cause poly(A)
RNA to accumulate in the nucleus. These in-
clude Rat2p/Nup120p, Rat3p/Nup133p, Nup100p, Nup49p, Nup116p, Nup1p,
Rat10p/Nup145p, Nup82p, and Rat7p/Nup159p (reviewed in Schneiter et al
1995, Maquat 1996). Fragments of two of these proteins, Nup145p and
Nup116p, are capable of binding RNA homopolymers in vitro, leading to
the suggestion that these NPC proteins interact with RNA molecules being
exported, or with a putative RNA component of the NPC (Fabre et al 1994).
For most of these genes, disruption results in phenotypes additional to that
of nuclear poly(A)
RNA accumulation, for example, defective protein im-
port and abnormalities in NPC morphology (reviewed in Schneiter et al 1995,
Maquat1996). Also, ina numberofcases, only asmall proportionofthemutant
cells show poly(A)
RNA accumulation. Although it may well be that some
of these NPC proteins are not only involved in mRNA export, but also function
in other processes (such as protein import and NPC assembly), a direct role for
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
these yeast NPC proteins in mRNA export cannot yet be assigned. One possi-
ble exception is Rat7p/Nup159p. Identified in a screen for genes required for
mRNA export, a temperature-sensitive mutation causes a rapid accumulation
of poly(A)
RNA in the nucleus at thenonpermissive temperature (Gorsch et al
1995). No protein import defect couldbedetected, and although NPC morphol-
ogyis abnormal, thisphenotypeis apparentat thepermissive temperature, when
mRNA export appears normal. Further characterization of Rat7p/Nup159p
should establish whether it plays a direct role in mRNA transport.
GTPasecycle Importofproteinstothenucleushasbeenfoundtobedependent
upon a GTPase cycle comprising a GTPase (termed Ran in higher eukaryotes,
spi 1 in fission yeast, and GSP1 and 2/CNR1 and 2 in budding yeast), aguanine
nucleotideexchangefactor (termed RCC1 in higher eukaryotes, pim1in fission
yeast, and PRP20/MTR1/SRM1 in budding yeast), and a GTPase-activating
protein (termed RanGAP1 in higher eukaryotes and rna1 in fission and budding
yeasts). This GTPase cycle may also play a role in mRNA export (Dasso 1993,
Melchior et al 1993, Moore & Blobel 1993, Tachibana et al 1994, Schlenstedt
et al 1995, Sweet & Gerace 1995, Tartakoff & Schneiter 1995, Sazer 1996).
Mutation of the genes encoding the GTPase cycle components in yeast or
RNAtoaccumulateinthenucleus. However,
mutation of these genes results in a variety of defects in addition to a block in
mRNA export, for example, abnormalities in nuclear morphology (Aebi et al
rRNA and tRNA maturation (Kadowaki et al 1993), and pre-mRNA splicing
(Vijayraghavan et al 1989). A direct role for the GTPase cycle components in
mRNA export remains to be demonstrated.
Other potential trans-acting factors In addition to NPC proteins and Ran
GTPase cycle proteins, yeast genetic screens have identified a number of other
genes, defects in which result in inhibition of mRNA export (Schneiter et al
1995, Maquat 1996). The proteins encoded include Rat1p, an exonuclease
(Amberg et al 1992), Mtr2p, a novel nuclear protein (Kadowaki et al 1994b);
Mas3p, a heat shock transcription factor (Kadowaki et al 1994a); Rpa190p, a
subunit of RNA polymerase I (Schneiter et al 1995); Rae1p, a novel protein
with β-transducin repeats (Brown et al 1995); Mtr3p, a novel nucleolar protein
(Kadowakiet al 1995); andMtr4p, anovelprotein with sequence motifs charac-
teristic of DEAD-box proteins (Liang et al 1996). For all the mutants that have
been analyzed, defects additional to that in mRNA export have been described.
These frequently include abnormal nucleolar morphology and deficiences in
rRNA processing. It is currently unclear whether the mRNA export defect is
primary or secondary to the other effects of the mutations, and/or whether the
gene products are multifunctional.
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
processing and transport have uncovered the first clear examples of regulated
RNA export (reviewed in Izaurralde & Mattaj 1992, 1995; Elliott et al 1994;
Krug 1993). A number of viral proteins, or protein complexes, are currently
known to influence the nuclear export of viral and cellular RNA molecules.
The best understood of these are human immunodeficiency virus 1 (HIV-1)
Rev protein, human T cell leukemia virus 1 (HTLV-1) Rex protein, adenovirus
early region 1B 55-kDa protein/early region 4 34-kDa protein complex (E1B
55-kDa /E4 34-kDa), and M1 matrix protein and nonstructural protein 1 (NS1)
of the influenza virus.
Rev and Rex proteins Rev and Rex proteins induce the nuclear export of par-
tially spliced or unspliced transcripts encoding structural proteins of HIV-1 and
HTLV-1, respectively (reviewed in Cullen & Malim 1991; Cullen 1992, 1995).
In the absence of Rev or Rex, these transcripts are retained in the nucleus un-
til they are either spliced or degraded (Malim et al 1989b, Felber et al 1989,
Emerman et al 1989). Since Revand Rex are functional equivalents (Hope et al
1991, Weichselbraun et al 1992), the following discussion focuses on Rev, the
more extensively characterized of the two proteins.
It has been suggested that Rev promotes the export indirectly by dissociating
splicing factors, directly by activating transport, or by a combination of these
two mechanisms (Cullen & Malim 1991, Cullen 1992). As outlined below,
several lines of evidence have shown that Rev can directly promote export,
although it is unclear whether it also functions in part by dissociating splicing
factors (Chang & Sharp 1989, Lu et al 1990, Kjems et al 1991, Luo et al 1994,
Stutz & Rosbash 1994).
Rev, a small protein of 116 amino acids, binds via an arginine-rich re-
gion to a complex 234-nucleotide RNA stem-loop structure in the partially
spliced/unspliced viral transcripts, termed the Rev response element (RRE).
This Rev-RRE interaction is essential for Rev function (Zapp & Green 1989,
Daly et al1989). Revshuttles between thenucleus and thecytoplasm, an activ-
ity that requires another domain critical for Rev function, termed the activation
domain(Malimet al1989a, 1991; Mermeretal1990; Venkatesh& Chinnadurai
1990; Malim & Cullen 1991; Daly et al 1993; Meyer & Malim 1994; Kalland
et al 1994; Wolff et al 1995). The leucine-rich activation domain (sequence:
LPPLERLTL) is capable of promoting nuclear export of heterologous proteins
in a saturable and temperature-sensitive manner, and it can function in the
absence of the RRE (Wen et al 1995, Fischer et al 1995, Meyer et al 1996).
Evidence that Rev directly promotes RNA export came from microinjec-
tion studies in X. laevis oocytes. Rev protein coinjected with RRE-containing
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
pre-mRNA into oocyte nuclei induces export of the RNA independently of
splicing (Fischer et al 1994b). Rev is therefore capable of carrying an RRE-
containing RNA molecule from the nucleus to the cytoplasm.
These findings together showed that the leucine-rich Rev activation domain
is an NES and that an RNA can be transported to the cytoplasm by associating
with an NES-containing protein.
The effect of Rev on RNA export is evident in the absence of any other
viral protein, suggesting that Rev promotes export by interacting with cellular
factors. Several cellular proteins that interact with Rev and may mediate Rev
functionhaverecentlybeendescribed, includingeukaryoticinitiationfactor5A,
human Rev interacting protein or Rev/Rex activation domain binding protein
(hRIP/Rab) and yeast Rev interacting protein (Rip1p) (Fritz et al 1995, Bogerd
et al 1995, Stutz et al 1995, Bevec et al 1996). Rip1p and hRIP/Rab are
distantly related, and both proteins contain phenylalanine-glycine (FG) repeats
and other sequence repeats, all of which are characteristic of NPC proteins. It
is still unclear if either hRIP/Rab or Rip1p are bona fide components of NPCs.
The Rev interaction domain of hRIP/Rab and Rip1p lies within the FG repeat-
containing region of the proteins (Fritz et al 1995, Stutz et al 1995). Rev can
one of which is Rat7p/Nup159p, that has been identified as a potential cellular
mRNA export factor (Stutz et al 1995, Gorsch et al 1995; see the section on
Trans-Acting mRNA Export Factors, NPC proteins). A model that arises from
this information is that translocation of Rev from the nucleus to the cytoplasm
is mediated by sequential interactions between Rev and FG repeat proteins in
the nucleoplasm and the NPC.
Saturation of the Rev export pathway, by nuclear injection of bovine serum
albumin (BSA) coupled to peptides comprising the Rev activation domain, has
no effect on the export of coinjected mRNA, and saturation of mRNA transport
does not interfere with Rev-mediated RNA export (Fischer et al 1995). Thus,
Revaccesses an exportpathway that ismechanistically differentfrom the cellu-
lar mRNAexport pathway, eventhough Rev exports RNAmolecules belonging
to the mRNA class. The reason for this redirection of RRE-containing mRNAs
by Rev is still unclear, but the Rev NES, in contrast to NESs that may medi-
ate cellular mRNA export, may direct the RNA to an export pathway that can
override nuclear retention of intron-containing pre-mRNA molecules.
The export of tRNA and rRNA (in the form of ribosomes) is also unaffected
underconditionsthatsaturate Rev-mediatedexport,indicatingthatthese RNAs,
like cellular mRNA, use export pathways that are different from that used by
Rev. Under the same conditions, however, export of U snRNA and 5S rRNA
is inhibited. These observations indicate that at least one limiting factor for
Rev-mediatedtransport is shared by the cellular5S rRNA and U snRNAexport
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
pathways (Fischer et al 1995). It has therefore been proposed that export of
5S rRNA and U snRNA is mediated by proteins that harbor an NES that is
functionally equivalent to the NES of Rev (see the sections on U snRNA and
5S rRNA Transport, below).
Adenovirus early region 1B 55-kDa protein/early region 4 34-kDa protein
complex A compex of two adenovirus proteins, E1B 55-kDa and E4 34-kDa,
facilitates the cytoplasmic accumulation of late viral mRNAs and blocks the
cytoplasmicaccumulation of hostcell mRNAs. In mutant viruses lacking func-
tional E1B or E4 protein, reduced levels of late viral mRNAs accumulate in
the cytoplasm, and cytoplasmic accumulation of host cell mRNAs is normal
(Babiss & Ginsberg 1984, Babiss et al 1985, Halbert et al 1985, Pilder et al
1986). Also, in the absence of the E1B protein, the nuclear distribution of
viral transcripts is altered (Pilder et al 1986, Leppard & Shenk 1989). The
molecular mechanisms that underlie the effects of the E1B/E4 protein complex
are not understood, but it has been suggested that the complex recruits a factor
that is essential for cellular mRNA export to viral replication/transcription cen-
ters, thereby enhancing viral mRNA transport and at the same time inhibiting
cellular mRNA export (Ornelles & Shenk 1991).
Influenza virus NS1 and M1 proteins Influenza virus encodes two proteins
that appear to regulate export of RNA molecules from the nucleus at differ-
ent stages during the production of viral particles (reviewed in Krug 1993,
Whittaker et al 1996). The NS1 protein affects cytoplasmic accumulation of
viral protein-coding transcripts, whereas M1 protein regulates transport of ma-
ture viral ribonucleoproteins (vRNPs).
Influenza virus proteins are synthesized in two phases, early and late, and the
switch between these two phases appears to be regulated at the level of mRNA
export from the nucleus (Shapiro et al 1987). The NS1 protein mediates this
regulation by inhibiting the export of late protein transcripts until the appropri-
ate time. The molecular mechanisms involved are unknown, although NS1 has
been shown to bind the poly(A) tails of all RNA molecules tested (Qiu & Krug
1994,Qianetal1994). NS1proteinfunction mustbe inactivatedtoallowexport
of late protein transcripts, and this appears to be mediated by post-translational
modifications, since protein kinase and methyltransferase inhibitors block ex-
port of late protein transcripts (Kurokawa et al 1990, Martin & Helenius 1991,
Vogel et al 1994). NS1 also indirectly inhibits transport of intron-containing
viral and cellular transcripts by inhibiting pre-mRNA splicing (Lu et al 1994,
Fortes et al 1994).
A prerequisite for the production of virus particles in the cytoplasm is ex-
port of vRNPs, which assemble in the nucleus. Export of vRNPs is dependent
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
on nuclear-localized M1 protein (Fraser 1967, Martin & Helenius 1991). The
mechanism by which M1 functions has not yet been characterized, although
studies of the transport properties of vRNPs of a virus bearing a temperature-
sensitive transport mutation in M1 have been informative (Whittaker etal 1995,
Yeetal1995). Atthenonpermissivetemperature, mostofthemutantM1protein
appearstoberestrictedtothenucleus, andthevRNPsareexportedwithoutbind-
ing detectable M1 protein. This suggests that M1 facilitates vRNP export not
by associating with them and accompanying them to the cytoplasm, but rather
by allowing their release from a nuclear retention mechanism. Viral RNP com-
ponents that mediate export of the released vRNPs have not yet been identified.
regulated nuclear export of cellular mRNA have yet to be described at the
mechanistic level. However, several reports suggest that like viruses, cells reg-
ulate nucleocytoplasmic mRNA transport. For example, heat-shocked yeasts
accumulate mRNA in their nuclei, but can selectively export mRNA-encoding
heat shock proteins (Saavedra et al 1996), and overexpression of eukaryotic
initiation factor 4E appears to alter the nucleocytoplasmic distribution of cy-
clin D mRNA (Rousseau et al 1996). In addition, it is of interest that the
export of unspliced RNA of two simple retroviruses, Mason-Pfizer monkey
virus (MPMV) and Rous sarcoma virus (RSV), is mediated by cis-acting RNA
elements, which can function in the absence of any viral protein (Bray et al
1994, Ogert et al 1996). Furthermore, these RNA elements, termed constitu-
tive transport elements (CTEs), can substitute for RRE RNA/Rev protein to
induce nuclear export of unspliced HIV transcripts, and the MPMV CTE can
promote export of a cellular intron-containing RNA that is normally retained
in the nucleus (Bray et al 1994, Ogert et al 1996). These observations suggest
that a cellular protein(s) can mediate the effects of these viral CTEs. It will be
of considerable interest to identify this factor, since it may play some role in
regulating export of alternatively spliced cellular transcripts.
Nuclear Import of Viral RNA
Many RNA viruses, including HIV-1 and influenza virus, replicate their ge-
nomes in the nucleus of the infected cell. In nondividing cells, import of
these nucleic acids has been shown to require ATP and to be protein mediated.
HIV nucleic acid import is mediated by viral matrix protein (MA) and viral
protein R (Vpr). MA contains a classicalbasic NLS and facilitates viral nucleic
acid import by utilizing the cellular classical NLS protein import pathway
(Bukrinskyetal1993,vonSchwedleretal1994). Thesequenceelementswithin
Vpr required for nuclear localization have not been delineated (Heinzinger et al
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
1994). Influenza virus RNA import appears to be mediated by viral NP, which
like HIV MA, utilizes the cellular classical NLS import machinery (O’Neill
et al 1995, Whittaker et al 1996). In an in vitro import assay, viral RNA is
imported only when NP is present, indicating that this RNA is transported as a
specific RNP (O’Neill et al 1995).
Overview of U snRNA Transport
The snRNPs U1, U2, U4/6, and U5 are RNA-protein complexes and are part
of the spliceosome in which pre-mRNA processing takes place. Each snRNP
complex consists of one (U1, U2, and U5) or two (U4/6) snRNAs, a common
set of proteins (the Sm proteins B, B
, D1, D2, D3, E, F, and G), and proteins
that are specific to each U snRNP (L¨uhrmann et al 1990).
All spliceosomal U snRNAs, with the exception of U6, undergo bidirec-
tional transport across the nuclear envelope as part of their maturation pathway
from precursor transcripts to functional nuclear U snRNP complexes (Figure 2)
(DeRobertis1983, Mattaj1988,Izaurralde&Mattaj1992).ThesnRNAsU1-U5
are synthesized in the nucleus by RNA polymerase II and acquire a 5
G cap cotranscriptionally (Reddy & Bush 1988). In contrast, the U snRNP
proteins are synthesized and stored in the cytoplasmand do not migrateon their
own into the nucleus (DeRobertis 1983, Mattaj & DeRobertis 1985). Instead,
the newly transcribed snRNAs are exported from the nucleus to the cytoplasm,
where they assemble with the Sm proteins to form the Sm core of U snRNP—a
common structure of these particles. Thereafter, the m
G cap is hypermethy-
lated to form the m
G cap structure, and the U snRNP is then imported into the
nucleus (Figure 2) (Mattaj 1986, 1988). When and where the specific proteins
are incorporated into the snRNP particles is unknown.
Figure 2 Schematic drawing of the U snRNP biogenesis cycle in oocytes as exemplified by U1
snRNP. The snRNA is transcribed in the nucleus by RNA polymerase II as a precursor with an
G cap and in some cases a 3
-terminal extension. The RNA is then transported to the cytoplasm
by virtue of binding to the nuclear CBC, and possibly by other factors that are so far unknown. In
the cytoplasm the snRNA associates with the Sm proteins B, B
, D1, D2, D3, E, F, and G, which
form, together with the RNA, the Sm core domain. The m
G cap is then hypermethylated to form
the m
G cap, and this is dependent on the prior formation of the Sm core domain. Eventually, the
U snRNP particle is transported to the nucleus, which requires interaction of the m
G cap and the
Sm core domain with a U snRNP-specific import receptor. The time and place of association of U
snRNP-specific proteins are in most cases unclear and are therefore not included in the figure.
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
Nuclear Export of U snRNA
ceosomal snRNAs appear to exit the nucleus via a common export pathway,
because nuclear injection of large amounts of any one particular snRNA inter-
feres with the export of all the others. This suggests that common structures in
all spliceosomal snRNAs contribute to their export (Jarmolowski et al 1994).
In search for such structures that may mediate snRNA nuclear export, Mattaj
and coworkers identified the m
G cap structure as one important signal element
(Hamm & Mattaj 1990). This observation led to the purification of a nuclear
Gcap-bindingcomplex(CBC) (Ohno et al 1990; Izaurraldeet al 1992, 1994,
1995; Kataoka et al 1994, 1995). CBC consists of two proteins, CBP80 and
CBP20, whose properties suggest a role in export. Antibodies against CBP20
interfere with CBC binding to the cap, and specifically inhibit snRNA export,
thereby demonstrating a direct involvement of CBC in the nuclear export of
spliceosomal snRNA (Izaurralde et al 1995). The mechanism by which CBC
inducesUsnRNAnuclear exportisstillunclear. OnepossibilityisthattheCBC
contains an NES that allows access to a specific export pathway. Alternatively,
CBC may interact with cellular components that contain NESs.
The m
G cap alone, however, is insufficient for mediating the export of
snRNAs. Other sequences that influencethe export of U1 snRNAhave been lo-
calizedto the 5
-terminal124 nucleotides as well as tothe 3
of the RNA (Terns et al 1993).
TRANS-ACTING U snRNA EXPORT FACTORS GTPase cycle Evidence suggests an
involvement of the Ran GTPase cycle in nuclear export of spliceosomal U
snRNAs(seealsothemRNAtrans-actingexportfactorssection). IntheChinese
hamster cell line tsBN2, whose mutant RCC1 is unstable at 40
C, nuclear
export of spliceosomal snRNAs is inhibited (Cheng et al 1995). Under the
same conditions, U3 snoRNA targeting to the nucleoli is also inhibited. Since,
unlike spliceosomal U snRNAs, U3 snoRNA is not exported to the cytoplasm,
the Ran GTPase cycle may be generally required to enable the intranuclear
movement of certain classes of RNPs.
Nuclear Import of U snRNA
UsnRNAsexportedfrom the nucleus assemblein the cytoplasminto U snRNPs
before returning to the nucleus. Because neither the snRNA nor the snRNP
proteins alone are imported independently of each other, the functionally active
NLS is generated during assembly of the U snRNP particle.
NUCLEAR IMPORT OF U snRNA IN X. LAEVIS OOCYTES Microinjection studies in
X. laevis oocytes showed that the assembly of the common Sm proteins with
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
the snRNA is an essential step in the formation of the snRNP NLS. Mutant U
snRNAs lacking the Sm protein binding site (Sm site) fail to accumulate in the
oocyte nucleus, whereas binding sites for the specific proteins are dispensible
for import (Mattaj & DeRobertis 1985, Mattaj 1986).
Because cap hypermethylation is also dependent on the assembly of the
common proteins on the Sm site of the U snRNA, the possibility of whether
the m
G cap may also contribute to the formation of a functional NLS was
tested. U1 snRNA carrying either no cap or the artificial adenosine (A) cap
assembles in the cytoplasm with the common proteins but fails to localize to
the nucleus (Fischer & L¨uhrmann 1990, Hamm et al 1990). In addition, the
GpppG cap dinucleotide specifically inhibits nuclear import of U snRNAs
(Fischer & L¨uhrmann 1990, Fischer et al 1991, Michaud & Goldfarb 1991).
Thus, the m
G cap has an essential signaling role in nuclear import of these
RNAs. However, the m
G cap alone is not sufficient for import of U snRNPs.
Another signal on the Sm core has been defined that, together with the m
cap, is necessary and sufficient for nuclear import (Fischer et al 1993). Not
all U snRNAs, however, have the same m
G cap requirement for their nuclear
import. For example, U4 and U5 snRNAs, or an artificial RNA that contains
only the Sm site flanked by two stem loops, do not absolutely require the m
cap for import, since they can enter the nucleus with an A cap. However, in
these cases import is considerably slower than that of m
G-capped snRNAs
(Fischer et al 1991, 1993; Jarmolowski & Mattaj 1993).
The mechanism by which the m
G cap and the signal on the Sm core domain
cooperateto create the fully activecomposite NLS isnot yet clear. In particular,
it is unclear whether one import receptor recognizes both signals or whether
two different receptors are required that recognize the cap and the Sm core
separately. Another possibility is that the m
G cap induces a conformational
change in the U snRNP particle and thereby exposes a preexisting signal on the
Sm core domain.
Competitionexperimentshavebeencarriedout inordertodeterminewhether
sharecommon features. These experimentsindicated that protein and U snRNP
import are mediated by different rate-limiting factors (Michaud & Goldfarb
1991, 1992; Fischer et al 1991, 1993, 1994a; van Zee at al 1993). However, it
is unknown whether proteins and U snRNAs use entirely different import path-
ways or whether their pathways converge at some point and therefore share
some common factors.
NUCLEAR IMPORT OF U snRNAs IN SOMATIC CELLS In somatic cells, import of
microinjected U1 snRNPs is not strictly dependent on the m
G cap. However,
transport is acceleratedby the presence of an m
G cap. The Sm core domain, in
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
contrast,isnecessaryandsufficientfornuclearimport. Thesamesignalrequire-
mentisobservedindifferentcelltypes, includingX.laevissomaticcells(Fischer
et al 1994a). The development of an in vitro transport system that faithfully
mimics in vivo snRNA import has provided further insights into mechanistic
aspects of U snRNA import. As in vivo, the Sm core domain is required for nu-
clear import of U1 and U2 snRNPs in vitro. However, the m
G cap dependence
varies, depending on the source of cytosol. When the assay is provided with X.
laevisoocyte cytosol, U1 andU2 snRNPnuclear importis strictly dependent on
the m
G cap. In contrast, somatic cell cytosol supports the import of ApppG-
capped U snRNPs. Therefore, cell type-specific rather than species-specific
differences in the transport machinery apparently account for the differential
G cap requirement for nuclear import of U1 snRNPs in X. laevis oocytes and
somatic cells (Fischer et al 1994a, Marshallsay & L¨uhrmann 1994).
5S rRNA is a component of large ribosomal subunits. Synthesis of 5S rRNA
differsfromthat oftheother rRNAs(18S,28S,and5.8S)bytwogeneralcriteria.
First, it is transcribed by RNA polymerase III, as opposed to RNA polymerase
I.Second, the 5S rRNAgenes are not included in the rDNArepeats that produce
thenucleolar organizerregions; consequently 5S rRNAissynthesized at sites in
the nucleoplasm distinct from nucleoli (Warner 1990, Scheer & Weisenberger
Much of the analysis of 5S rRNA transport has been accomplished by using
X.laevisoocytesas a model system, butasexplainedbelow, the oocytepathway
is highly specialized and more complicated than the somatic cell pathway.
Therefore, we discuss the oocyte experiments first, followed by what is known
about the pathway in somatic cells, and attempt to highlight general features
that relate to both cell types.
Overview of 5S rRNA Transport in X. laevis Oocytes
X. laevis contains two types of 5S rRNA genes, the oocyte and the somatic,
which are differentially expressed during development and differ by only six
nucleotide substitutions (Wolffe & Brown 1988). During oogenesis, oocyte-
type 5S rRNA is expressed before the other ribosomal components (Mairy &
Denis 1971) and migrates to the cytoplasm, where it is found in one of two
storage particles, the 7S and 42S RNPs. The 42S particle is a mixture of 5S
rRNA, tRNAs, and several proteins, while the 7S particle is composed of a 1:1
ratio of 5S rRNA and the transcription factor TFIIIA (reviewed in Tafuri &
Wolffe 1993). This composition remains the case until the onset of vitelloge-
nesis, when synthesis of the other ribosomal components commences. At this
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
stage the concentration of the storage particlesdecreases (Dixon & Ford 1982),
and 5S rRNA rapidly localizes to the nucleoli of the oocyte nucleus, where it
becomes incorporated into large ribosomal subunits(Allison et al 1991). These
subunits are then exported to their site of function in the cytoplasm. Thus, the
oocyte 5S rRNA pathway is unique in that two distinct nuclear export events
occur and can be summarized as follows: transcription in the nucleus, export
to the cytoplasm as a 7S or 42S storage particle, nuclear import, nucleolar
localization, and re-export in the context of the large ribosomal subunit. Per-
haps because of this complex, bidirectional transport pathway, 5S rRNA is the
only RNA examined for which nuclear export cannot be completely saturated
(Jarmolowski et al 1994).
proteinsare knowntospecificallybindto 5SrRNA:theLa protein, transcription
factor IIIA (TFIIIA), and ribosomal protein L5. La protein, which transiently
interacts with all pol III products, functions in pol III transcription termination
(Rinke & Steitz 1982, Gottlieb & Steitz 1989). TFIIIA binds both to the 5S
rRNA gene, where it facilitates transcription, and to 5S rRNA itself (Honda &
Roeder 1980). The other known 5S rRNA-binding protein is ribosomal protein
L5 (Chan et al 1987).
and protein factors by immunoprecipitationof defined5S RNP complexesfrom
the nucleus and cytoplasm of fractionated oocytes (Guddat et al 1990). These
experiments revealed that the La-5S RNP is confined to the nucleus at all times,
thus excluding a role for La in nuclear export of 5S rRNA, and that RNP
complexes containing either TFIIIA or L5 form initially in the nucleus and then
migrate to the cytoplasm. Additionally, using 5S rRNA mutants, they found
that as long as a given 5S mutant can interact with either TFIIIA or L5, then
export to the cytoplasm occurs. However, mutants that no longer bind either
protein are retained in the nucleus. These studies demonstrate that both TFIIIA
and L5 have the capacity to export 5S rRNA to the cytoplasm.
is similar to import of karyophilic proteins in that it is temperature- and ATP-
dependent and occurs through NPCs (Allison et al 1993). Analysis of 5S rRNA
nuclear import inoocytes reveals that L5 plays a significantrole in this process.
Allison and colleagues haveshownthat during vitellogenesis, when production
of ribosomal proteins begins, 5S rRNA is exchanged from the cytoplasmic 7S
storage particle onto L5 and then enters the nucleus (Allison et al 1991, 1993).
Additionally, it has been shown that 5S rRNA mutants are only imported into
the nucleus if they retain L5-binding activity (Rudt & Pieler 1996).
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
strategy for stockpiling ribosomes within the egg for later use during embryo-
genesis. The picture that emerges en toto is that, in previtellogenic oocytes,
oocyte-type 5S rRNA is transcribed, transiently associates with La protein, and
is then bound by TFIIIA to form the 7S RNP. This RNP is then exported to
the cytoplasm, where it exists as a storage particle until synthesis of other ri-
bosomal components begins. Consistent with a role for TFIIIA in 5S rRNA
export is the fact that the protein contains an NES similar to the Rev/PKI-type
of NES and that the Rev NES conjugated to BSA efficiently competes for ex-
port of 5S rRNA (Fischer et al 1995, Fridell et al 1996). The storage particles
remain in the cytoplasm until L5 becomes available and displaces TFIIIA to
form the 5S RNP. This RNP is then rapidly imported into the nucleus, be-
comes concentrated within the nucleoli, and is assembled into large ribosomal
subunits that are then exported to the cytoplasm. This model, summarized in
Figure 3, can account for the fact that both TFIIIA and L5 mediate export of 5S
5S rRNA Transport in Somatic Cells
The 5S rRNA transport pathway is simpler in somatic cells than in X. laevis
oocytes. Using HeLa cells, Steitz and coworkers showed that after a transient,
posttranscriptional association with La protein, 5S rRNA immediately binds
L5 to form a particle that is a precursor to assembly of 5S rRNA into nascent
ribosomal subunits within the nucleoli (Steitz et al 1988). Therefore nascent
Figure 3 Intracellular trafficking of 5S rRNA. 5S rRNA is transcribed in the nucleoplasm by
RNA polymerase III and then briefly associates with La antigen, which functions in transcription
termination. At this point the pathway as it is known in X. laevis oocytes appears to differ from
the pathway that is operational in somatic cells. The area shaded in gray depicts the X. laevis
oocyte-specific portion of the pathway: 5S rRNA is bound by TFIIIA, which also functions in
its transcription, to form a 7S RNP that is then rapidly exported to the cytoplasm, where it is
sequestered as the 7S storage particle. As the level of L5 accumulates in the oocyte, an exchange
reaction occurs in which 5S rRNA is transferred to L5 to form the 5S RNP. Free TFIIIA, which is
now competent for nuclear import, then presumably returns to the nucleus. The 5S RNP is rapidly
imported into the nucleus and is subsequently concentrated within nucleoli, where it is assembled
into nascent large ribosomal subunits. After complete assembly, the large ribosomal subunits
are exported to the cytoplasm, where they function in translation. In somatic cells, which do not
stockpile ribosomes for use later indevelopment,the pathway is less complex. After disassociation
from La, 5S rRNA immediately interacts with the L5to form the 5S rRNPin the nucleoplasm. The
RNP then localizes directly to the nucleolus, where it is incorporated into nascent large ribosomal
subunits and then exported to the cytoplasm.
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
5S rRNA does not shuttle out of and back into the nucleus in somatic cells but
instead accumulates directly in the nucleoli as a complex with L5. Consistent
with these findings is the fact that, despite considerable effort, the 5S rRNA-
containing storage particles have not been found in the cytoplasm of somatic
cells (e.g. see Honda & Roeder 1980).
What is common between the two pathways is the fact that transcription of
the 5S genes occurs in the nucleoplasm and that the L5-5S RNP forms prior to
ribosomalsubunitassemblyin thenucleolus, a pointwhich promptedSteitz etal
(1988) to suggest that L5 functions in part to deliver 5S rRNA to the nucleolus.
cells the pathway is confined to within the nucleus (summarized in Figure 3).
Recent work on the domain structureof L5 supports this model, as it was found
that the 5S rRNA-binding and nucleolar localization domains are separable
within the protein. Furthermore, L5 mutants that maintain 5S rRNA-binding
domainis interrupted, suggestingthatanintactnucleolar localization domain of
L5is required for efficientaccumulationof the L5-5S RNP within thenucleolus
(Michael & Dreyfuss 1996). Once targeted to the nucleolus, the L5-5S RNP is
probably assembled into nascent ribosomal subunits that are then exported to
the cytoplasm. Therefore, the problem of 5S rRNA export in somatic cells can
be reduced to the problem of ribosomal subunit export.
All rRNAs except 5S rRNA—namely 18S, 28S, and 5.8S rRNAs—are tran-
scribed as a commonprecursor thatis completely processed and assembledinto
ribosomal subunits, all within the nucleolus (Warner 1990, Scheer & Weisen-
berger 1994, Shaw & Jordan 1995). Therefore, nuclear export of these rRNAs
occurs in the context of ribosomal subunits. Beyond the fact that ribosomal
subunit export is a unidirectional, saturable process that requires energy and
occurs through NPCs (Khanna-Gupta & Ware 1989, Bataill´e et al 1990), very
little is known about export of ribosomes.
Nuclear Export
transcribed in the nucleus by RNA polymerase III, and like RNA polymerase
II–transcribedmRNA,it is produced as a precursortRNA.Pre-tRNAmolecules
are processed in the nucleus before transport to their functional location in the
cytoplasm, although they are further modified in the cytoplasm. Nuclear tRNA
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
processing reactions comprise excision of 5
and 3
terminal sequences, addi-
tion of CCA to the 3
end, base modifications, and for some tRNA molecules,
removal of introns and base editing (reviewed in Deutscher 1995; Altman et al
1993, 1995; Westaway & Abelson 1995; Bj¨ork 1995).
OnlytRNAmoleculeswithmatureterminiandwithout intronsaredetectedin
the cytoplasm of X. laevis oocytes, suggesting that 5
and 3
terminal extensions
and intron sequences prevent pre-tRNA export (Melton et al 1980, Zasloff et al
1982a; reviewed in Westaway & Abelson 1995). It has been suggested that the
process of splicing is coupled to transport through the NPC, because in yeast,
components of the splicing process (an endonuclease and a ligase) appear to be
localized in close apposition to NPCs, and mutations in certain NPC proteins
result in accumulation of unspliced tRNA (Peebles et al 1983, Clark & Abelson
1987,Sharmaetal1996;reviewedinWestaway&Abelson1995). However,the
in the nuclear import of a tRNA splicing factor. In addition, microinjection of
large amounts of pre-tRNA into the nucleus of X. laevis oocytes results in the
appearance of unspliced pre-tRNA in the cytoplasm, suggesting that tRNA
splicing is not coupled to the export process (Boelens et al 1995).
defined the critical regions of human tRNAmet for nuclear export in X. laevis
oocytes. Although point mutations in many regions of the molecule perturb
transport, the most drastic effects are caused by mutations in the most highly
conserved regions of tRNA, the D and T stem loops (Santos & Zasloff 1981,
Zasloffet al 1982a,b, Tobian et al 1985). The most critical regions for transport
alsoappear tobethe mostimportantregionsfor tRNAtranscription, processing,
and ribosome binding (see discussion of Tobian et al 1985). All transport
mutants described so far are also defective in processing, suggesting that a
processing factor also serves as an export factor or, more likely, that the same
structural features ofthe tRNA molecule are recognized by both processing and
transport machineries (Zasloff et al 1982a, Tobian et al 1985).
Different tRNA species competitively inhibit the export of each other when
microinjected into X. laevis oocyte nuclei, demonstrating that different tRNAs
share a common rate-limiting factor(s), which likely recognizes the highly
conserved stem loops D and T (Zasloff 1983). The glycolytic enzyme glycer-
aldehyde 3 phosphate dehydrogenase has been identified as a factor that binds
to wild-type tRNAs but not to transport-defective mutants, although a function
for this enzyme in tRNA export has not been described (Singh & Green 1993).
It has recently been suggestedthat sometransport-defective mutanttRNAsmay
be defective, not because they fail to interact with proteins of the transport ma-
chinery, but rather because they are retained in the nucleus by interaction with
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
nuclear components (Boelens et al 1995). This is unlikely to be the case for
all transport-defective tRNA mutants, so some of them should be of use in the
identification of tRNA export factors.
A common theme that emerges from the mechanistic information on nucleocy-
toplasmic trafficking of RNA is that transport of possibly all RNA classes, in
both directions across the NPC, is mediated largely by signals on proteins that
are associated with the RNA (Figure 4). Thus, mRNA export is most likely
mediated, at least in part, by NESs of the shuttling hnRNP proteins such as the
M9 NES of hnRNP A1; U snRNA export is mediated, at least in part, by CBC
(whose NES remains to be identified); and 5S rRNA export is mediated by L5
(whose NES remains to be identified) and TFIIIA (which bears a leucine-rich
NES similar to that of HIV-1 Rev protein) (Figure 4). The factors that mediate
rRNA and tRNA export are still obscure.
Nuclear import of U snRNAs, 5S rRNA (in oocytes), and viral RNAs is also
largelyproteinmediated. ThenuclearimportsignalofUsnRNAsisacomposite
cap. This signal accesses a different import pathway than that used by classical
NLS-containing proteins. In contrast, import of 5S rRNA in oocytes appears
Figure 4 RNA export from the nucleus into the cytoplasm is mediated by RNA-binding proteins.
Proteins that specifically interact with each class of RNA carry the signals for RNA transport to
the cytoplasm. Messenger RNA export is most likely mediated by shuttling hnRNP proteins; U
snRNA export is mediated by the cap-binding complex (CBC); and 5S rRNA export is mediated
by TFIIIA and L5. Factors involved in tRNA and rRNA transport remain to be identified.
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
to be mediated exclusively by the L5 protein via the classical NLS pathway.
Viral RNA import is also, in some cases, mediated by proteins that utilize the
cellular classical NLS import pathway (HIV-1 MA and influenza virus NP) or
by proteins whose import pathway has not yet been characterized (HIV-1 Vpr).
In most cases, RNA-binding proteins dictate the transport pathway taken by
a particular RNA molecule. However, the sequence of the RNA molecule, in
turn, determines the complement of proteins with which it will interact, so the
pathway taken is ultimately specified by RNA sequence.
This review has focused on transport of RNAmolecules between the nucleus
and the cytoplasm. However, once exported to the cytoplasm, some mRNAs
(and probably other classes of RNA that function in the cytoplasm) are trans-
ported and anchored to very precise subcytoplasmic locations. This targeting
is essential in processes as diverse as development of the embryo and neuronal
cell function, and the molecular mechanisms appear to follow a principle sim-
ilar to that of nucleocytoplasmic transport of RNA molecules. Transport and
anchoring of mRNAs within the cytoplasm is also mediated by proteins that
interact with the RNA (reviewed in Ding & Lipshitz 1993, Wilhelm & Vale
1993, Singer 1993, Okita et al 1994, Stebbings et al 1995, St Johnston 1995a,b,
Micklem 1995, Steward 1995). This basic principle of RNA transport is likely
to be of considerable generality.
Our understanding of cellular RNA transport pathways is at an early and
exciting stage. For most RNA classes, known mediators/effectors of transport
are few, but for mRNA export, at least, potential mediators are plentiful, and
a number of these may become bona fide export factors with more extensive
investigation. Immediate questions, such as the number and diversity of pro-
tein NESs and protein NLSs that mediate RNA export and import, seem quite
tractable. Transportofeach typeofRNAmayverylikelyinvolveboth pathway-
specific components and components that are shared with other pathways. The
identification of points of pathway convergence should also be possible. In
addition, regulatory mechanisms of mRNA export, as exemplified by viruses,
have yet to be characterized for cellular transcripts. However, with progress
in our understanding of viral regulation of mRNA export, cellular regulatory
mechanisms (for example, those required for export of alternatively spliced
transcripts) are expected to be uncovered.
Because the detailed molecular mechanisms by which RNA molecules move
from their site of synthesis to the NPCs, and by which they are translocated
through the NPCs, remain mysterious, we are currently left to speculation. We
doknowthat there areenergy-consumingsteps, which may includeintranuclear
transport of RNA and, most likely, translocation through the NPC. Maybe
translocation is motor protein driven, which would certainly account for part of
the energy requirement. If the Ran GTPase cycle is directly involved in RNA
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
export, this will also account for some of the energy consumption. It will be
fascinating to learn how directionality is imposed on RNA-protein complexes
as they move through the NPC, and how shuttling components of the transport
machineries that move with them through the NPC are recycled.
A major tenet of RNA transport, that it is mediated by proteins associated
with the RNA, has been firmly established. With this understanding, a more
detailed knowledge of these fundamental transport pathways is expected in the
near future.
The research in this laboratory is supported by a grant from the National
Institutes of Health and by the Howard Hughes Medical Institute. UF received
a grant from the Deutsche Forschungsgemeinschaft and an AIDS fellowship
from the Deutsches Krebsforschungszentrum.
Visit the Annual Reviews home page at
Literature Cited
Adam SA, Marr RS, Gerace L. 1990. Nuclear
protein import in permeabilized mammalian
cells requires soluble cytoplasmic factors. J.
Cell Biol. 111:807–16
Adams MD, Rudner DZ, Rio DC. 1996. Bio-
chemistryandregulationof pre-mRNAsplic-
ing. Curr. Opin. Cell Biol. 8:331–39
Aebi M, Clark MW, Vijayraghavan U, Abelson
J. 1990. A yeast mutant, PRP20, altered in
mRNA metabolism and maintenance of the
nuclear structure, is defective in a gene ho-
mologous to the human gene RCC1 which is
involved in the control of chromosome con-
densation. Mol. Gen. Genet. 224:72–80
Allison LA, North MT, Murdoch KJ, Romaiuk
PJ, Deschamps S, Le Marie M. 1993. Struc-
tural requirements of 5S rRNA for nuclear
transport, 7S ribonucleoprotein particle as-
sembly, and 60S ribosomal subunit assem-
bly in Xenopus oocytes. Mol. Cell. Biol.
Allison LA, Romaniuk PJ, Bakken, AH. 1991.
RNA-protein interactions of stored 5S RNA
with TFIIIA andribosomal protein L5 during
Xenopus oogenesis. Dev. Biol. 144:129–44
Altman S, Kirsebom L, Talbot S. 1993. Recent
studies of ribonuclease P. FASEB J. 7:7–14
Altman S, Kirsebom L, Talbot S. 1995. Recent
studies of RNase P. In tRNA Structure,
Biosynthesis, and Function, ed.DS¨oll,
UL RajBhandary, 6:67–78. Washington, DC:
Amberg DC, Goldstein AL, Cole CN. 1992.
Isolation and characterization of RAT1:an
essential gene of Saccharomyces cerevisiae
required for the efficient nucleocytoplasmic
trafficking of mRNA. Genes Dev. 6:1173–89
5 early region 1b gene productis required for
efficient shutoff of host protein synthesis. J.
Virol. 50:202–12
Babiss LE, Ginsberg HS, Darnell JE Jr. 1985.
Adenovirus E1B proteins are required forac-
on cellular mRNA translation and transport.
Mol. Cell. Biol. 5:2552–58
Banerjee AK. 1980. 5
-terminal cap structure
in eukaryotic messenger ribonucleic acids.
Microbiol. Rev. 44:175–205
Bataill´eN, HelserT, FriedHM. 1990.Cytoplas-
mic transport of ribosomal subunits microin-
ageneralized, facilitatedprocess.J.Cell Biol.
Bevec D, Jaksche H, Oft M, W¨ohl T, Him-
melspach M, et al. 1996. Inhibition of HIV-1
replication in lymphocytes by mutants of the
Rev cofactor e1F-5A. Science 271:1858–60
Bj¨ork GR. 1995. Biosynthesis and function
of modified nucleosides. In tRNA Structure,
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
Biosynthesis, and Function, ed.DS¨oll, UL
RajBhandary, 11:165–205.Washington, DC:
Boelens WC, Palacios I, Mattaj IW. 1995. Nu-
clear retention of RNA as a mechanism for
localization. RNA 1:273–83
Bogerd HP, Fridell RA, Madore S, Cullen BR.
1995. Identification ofa novel cellular cofac-
tor for the Rev/Rex class of retroviral regula-
tory proteins. Cell 82:485–94
Boulikas T. 1993. Nuclear localization signals
(NLS).Crit.Rev.Eukaryot. GeneExp. 3:193–
Bray M, Prasad S, Dubay JW, Hunter E, Jeang
K-T, et al. 1994. A small element from the
Mason-Pfizer monkey virus genome makes
human immunodeficiency virus type 1 ex-
pression and replication Rev-independent.
Proc. Natl. Acad. Sci. USA 91:1256–60
Brockdorff N, Ashworth A, Kay GF, McCabe
mouseXist geneis a 15kb inactiveX-specific
transcript containing no conserved ORF and
located in the nucleus. Cell 71:515–26
Brown CJ, Hendrich BD, Rupert JL, Lafreni`ere
RG, Xing Y, et al. 1992. The human XIST
gene: analysis of a 17 kb inactive X-specific
RNA that contains conserved repeats and
is highly localized within the nucleus. Cell
Brown JA, Bharathi A, Ghosh A, Whalen W,
Fitzgerald E, Dhar R. 1995. A mutation in
the Schizosaccharomyces pombe rae1 gene
causes defects in poly(A)
RNA export and
inthe cytoskeleton.J.Biol. Chem.270:7411–
Bukrinsky MI, Haggerty S, Dempsey MP,
Sharova N, Adzhubel A, et al. 1993. A nu-
clear localization signal within HIV1 matrix
cells. Nature 365:666–69
aceres JF, Stamm S, Helfman DM, Krainer
AR. 1994. Regulation of alternative splic-
ing in vivo by overexpression of antagonistic
splicing factors. Science 265:1706–9
primarystructure ofratribosomal proteinL5:
a comparison of the sequence of amino acids
in the proteins that interact with 5S rRNA.
J. Biol. Chem. 262:12879–86
Chang DD, Sharp PA.1989. Regulation by HIV
Revdepends upon recognitionof splice sites.
Cell 59:789–95
Cheng Y, Dahlberg JE, Lund E. 1995. Di-
verse effects of the guanine nucleotide ex-
change factor RCC1 on RNA transport. Sci-
ence 267:1807–10
Clark MW, Abelson J. 1987. The subnuclear
localization of tRNA ligase in yeast. J. Cell
Biol. 105:1515–26
Collis P, Antoniou M, Grosveld F. 1990. Defini-
tion of the minimal requirements within the
human β-globin gene and the dominant con-
trol region for high level expression. EMBO
J. 9:233–40
Cullen BR. 1992. Mechanism of action of reg-
ulatory proteins encoded by complex retro-
viruses. Microbiol. Rev. 56:375–94
Cullen BR. 1995. Regulation of HIV gene ex-
pression. AIDS 9(Suppl. A):S19–S32
Cullen BR, Malim MH. 1991. The HIV1 Rev
protein: prototype ofa novel class of eukary-
otic post-transcriptional regulators. Trends
Biochem. Sci. 16:346–50
Daly TJ, Cook KS, Gray GS, Maione TE,
Rusche JR. 1989. Specific binding of
HIV1 recombinant Rev protein to the Rev-
responsive element in vitro.Nature342:816–
Daly TJ, Rennert P, Lynch P, Barry JK, Dundas
M, et al. 1993. Perturbation of the carboxy
terminus of HIV1 Rev affects multimeriza-
istry 32:8945–54
DassoM.1993.RCC1inthecellcycle: Thereg-
ulator of chromosome condensation takes on
new roles. Trends Biochem. Sci. 18:96–101
Davis LI. 1995. The nuclear pore complex.
Annu. Rev. Biochem. 64:865–96
DeRobertis EM. 1983. Nucleoplasmic segrega-
tion of proteins and RNAs. Cell 32:1021–25
Deutscher MP. 1995. tRNA processing nucle-
ases. In tRNA Structure, Biosynthesis, and
Function, ed.DS¨oll, ULRajBhandary, 5:51–
65. Washington, DC: ASM Press
Ding D, Lipshitz HD. 1993. Localized RNAs
and their functions. BioEssays 15:651–58
Dixon LK, Ford PJ. 1982. Regulationof protein
synthesisand accumulation during oogenesis
in Xenopus laevis. Dev. Biol. 93:478–97
Dreyfuss G, Matunis MJ, Pi˜nol-Roma S, Burd
CG. 1993. hnRNP proteins and the biogene-
sis of mRNA. Annu. Rev. Biochem. 62:289–
Dworetzky SI, Feldherr CM. 1988. Transloca-
tion of RNA-coated gold particles through
the nuclear pores of oocytes. J. Cell Biol.
Eckner R, Ellmeier W, Birnstiel ML. 1991.Ma-
endformation stimulates RNA
export from the nucleus. EMBO J. 10:3513–
mRNA nuclear export. Curr. Opin. Genet.
Dev. 4:305–9
Emerman M, Vazeux R, Peden K. 1989. The
rev gene product of the human immunode-
ficiency virus affects envelope-specific RNA
localization. Cell 57:1155–65
Fabre E, Boelens WC, Wimmer C, Mattaj IW,
Hurt ED. 1994. Nup145p is required for nu-
clear export of mRNA and binds homopoly-
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
meric RNA in vitro via a novel conserved
motif. Cell 78:275–89
Featherstone C, Darby MK, Gerace L. 1988. A
monoclonalantibodyagainstthenuclear pore
complex inhibits nucleocytoplasmic trans-
port of protein and RNA in vivo. J. Cell Biol.
FelberBK, Hadzopoulou-Cladaras M, Cladaras
C, Copeland T, Pavlakis GN. 1989. Rev pro-
teinof human immunodeficiencyvirus type 1
affects the stability and transport of the viral
mRNA. Proc. Natl. Acad. Sci. USA 86:1495–
Fischer U, Darzynkiewicz E, Tahara SM,
Dathan NA, L¨uhrmann R, et al. 1991. Di-
versity in the signals required for nuclear ac-
cumulation of U snRNPs and variety in the
pathways of nuclear transport. J. Cell Biol.
Fischer U, Heinrich J, van Zee K, Fanning E,
uhrmann R.1994a. Nuclear transport of U1
snRNP in somaticcells: differences in signal
requirement compared with Xenopus laevis
oocytes. J. Cell Biol. 125:971–80
Fischer U, Huber J, Boelens WC, Mattaj IW,
uhrmann R. 1995. The HIV1 Rev activa-
tion domain is a nuclear export signal that
accesses an export pathway used by specific
cellular RNAs. Cell 82:475–83
Fischer U, L¨uhrmann R. 1990. Anessential sig-
U1 snRNP to the nucleus. Science 249:786–
Fischer U, Meyer S, Teufel M, Heckel C,
uhrmann R, Rautmann G. 1994b. Evidence
that HIV1 Rev directly promotes the nuclear
exportofunspliced RNA.EMBO J.13:4105–
Fischer U, Sumpter V, Sekine M, Satoh
T, L¨uhrmann R. 1993. Nucleo-cytoplasmic
transport of U snRNPs: definition of a nu-
clear location signal in the Sm core domain
that binds a transport receptor independently
of the m
G cap. EMBO J. 12:573–83
Forrester W, Stutz F, Rosbash M, Wickens M.
1992. Defects in mRNA 3
-end formation,
transcription initiation, and mRNA transport
associated with the yeast mutation prp20:
possible coupling of mRNA processing and
chromatin structure. Genes Dev. 6:1914–26
NS1 protein inhibits pre-mRNA splicing and
blocks mRNA nucleocytoplasmic transport.
EMBO J. 13:704–12
Fraser KB. 1967. Immunofluorescence of
abortiveandcomplete infectionsbyinfluenza
A virus in hamster BHK21 cells and mouse
L cells. J. Gen. Virol. 1:1–12
Fridell RA, Fischer U, L¨uhrmann R, Meyer BE,
Meinkoth JL, et al. 1996. Amphibian tran-
scription factor IIIA proteins contain a se-
nuclear export signal of human immunodefi-
ciency virus type I Rev. Proc. Natl. Acad. Sci
USA 93:2936–40
Fritz CC, Zapp ML, Green MR.1995. A human
nucleoporin-like protein that specifically in-
teracts with HIV Rev. Nature 376:530–33
transport. Science 271:1513–18
Gorsch LC, Dockendorff TC, Cole CN. 1995.
A conditional allele of the novel repeat-
containing yeast nucleoporin RAT7/NUP159
causes both rapid cessation of mRNA export
plexes. J. Cell Biol. 129:939–55
Gottlieb E, Steitz JA. 1989. Function of the
mammalian La protein: evidence for its ac-
tion in transcription termination by RNA
polymerase III. EMBO J. 8:851–61
GrossMK,Kainz MS,MerrillGF.1987. Introns
are inconsequential to efficient formation of
cellular thymidine kinase mRNA in mouse L
cells. Mol. Cell. Biol. 7:4576–81
Guddat U, Bakken AH, Pieler T. 1990. Protein-
mediated nuclear export of RNA: 5S rRNA
containing small RNPs in Xenopus oocytes.
Cell 60:619–28
Halbert DN, Cutt JR, Shenk T. 1985. Aden-
ovirus early region 4 encodes functions re-
quired for efficient DNA replication, late
gene expression, and host cell shutoff. J. Vi-
rol. 56:250–57
Hamm J, Darzynkiewicz E, Tahara SM, Mattaj
IW. 1990. The trimethylguanosine cap struc-
ture of U1 snRNA is a component of a bipar-
tite nuclear targeting signal. Cell 62:569–77
structures facilitate RNA export from the nu-
cleus. Cell 63:109–18
Hattori K, Angel P, Le Beau MM, Karin M.
1988. Structure and chromosomal localiza-
tion of the functional intronless human JUN
protooncogene. Proc. Natl. Acad. Sci. USA
Heinzinger NK, Bukrinsky MI, Haggerty SA,
Ragland AM, Kewalramani V, et al. 1994.
TheVpr proteinof humanimmunodeficiency
virustype 1 influencesnuclear localization of
viral nucleic acids in nondividing host cells.
Proc. Natl. Acad. Sci. USA 91:7311–15
Hogan NC, Traverse KL, Sullivan DE, Pardue
M-L. 1994. The nucleus-limited Hsr-omega-
n transcript is a polyadenylated RNA with a
regulated intranuclear turnover. J. Cell Biol.
Honda BM, Roeder RG. 1980. Association of
a 5S gene transcription factor with 5S RNA
andaltered levelsof the factorduring cell dif-
ferentiation. Cell 22:119–26
Hope TJ, Bond BL, McDonald D, Klein NP,
Parslow TG. 1991. Effector domains of
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
human immunodeficiency virus type 1 Rev
and human T-cell leukemia virus type I Rex
arefunctionally interchangeable and sharean
essential peptide motif. J. Virol. 65:6001–7
HuangZ-M, YenTSB. 1995.Roleof the hepati-
tis B virus posttranscriptional regulatory ele-
ment in export of intronless transcripts. Mol.
Cell. Biol. 15:3864–69
Izaurralde E, Lewis J, Gamberi C, Jarmolowski
A, McGulgan C, et al. 1995. A cap-binding
protein complex mediating U snRNA export.
Nature 376:709–12
Izaurralde E, Lewis J, McGuigan C, Jankowska
M, Darzynkiewicz E, Mattaj IW. 1994. Anu-
clear cap binding protein complex involved
in pre-mRNA splicing. Cell 78:657–68
Izaurralde E, Mattaj IW. 1992. Transport of
Cell Biol. 3:279–88
Izaurralde E, Stepinski J, Darzynkiewicz E,
Mattaj IW. 1992. A cap binding protein that
may mediate nuclear export of RNA poly-
merase II-transcribed RNAs. J. Cell Biol.
Jarmolowski A, Boelens WC, Izaurralde E,
Mattaj IW. 1994. Nuclear export of differ-
ent classes of RNA is mediated by specific
factors. J. Cell Biol. 124:627–35
Jarmolowski A, Mattaj IW. 1993. The deter-
minants for Sm protein binding to Xenopus
U1 and U5 snRNAs are complex and non-
identical. EMBO J. 12:223–32
Kadowaki T, Chen S, Hitomi M, Jacobs E,
Kumagai C, et al. 1994a. Isolation and char-
acterization of Saccharomyces cerevisiae
mRNA transport-defective (mtr) mutants. J.
Cell Biol. 126:649–59
Kadowaki T, Goldfarb D, Spitz LM, Tartakoff
AM, Ohno M. 1993. Regulation of RNApro-
cessing and transport by a nuclear guanine
nucleotide release protein and members of
the Ras superfamily. EMBO J. 12:2929–37
Kadowaki T, Hitomi M, Chen S, Tartakoff AM.
1994b. Nuclear mRNA accumulation causes
nucleolar fragmentation in yeast mtr2 mu-
tant. Mol. Biol. Cell 5:1253–63
Kadowaki T, Schneiter R, Hitomi M, Tartakoff
AM. 1995. Mutations in nucleolar proteins
lead to nucleolar accumulation of polyA
Cell 6:1103–10
Kadowaki T, Zhao Y, Tartakoff AM. 1992. A
conditional yeast mutant deficient in mRNA
transport from nucleus to cytoplasm. Proc.
Natl. Acad. Sci. USA 89:2312–16
Kalland K-H, Szilvay AM, Brokstad KA,
Sætrevik W, Haukenes G. 1994. The human
immunodeficiency virus type 1 Rev protein
shuttles between the cytoplasm and nuclear
compartments. Mol. Cell. Biol. 14:7436–
Kataoka N, Ohno M, Kangawa K, Tokoro Y,
Shimura Y. 1994. Cloning of a complemen-
tary DNA encoding an 80 kilodalton nu-
clear cap binding protein. Nucleic Acids Res.
Kataoka N, Ohno M, Moda I, Shimura Y. 1995.
Identification of the factors that interact with
NCBP, an 80 kDa nuclear cap binding pro-
tein. Nucleic Acids Res. 23:3638–41
Keller W. 1995. No end yet to messenger RNA
processing! Cell 81:829–32
Khanna-Gupta A, Ware VC. 1989. Nucleocy-
toplasmic transport of ribosomes in a eu-
karyotic system: Is there a facilitated trans-
port process? Proc. Natl. Acad. Sci. USA
Kjems J, Frankel AD, Sharp PA. 1991. Specific
regulation of mRNA splicing in vitro by a
peptide from HIV1 Rev. Cell 67:169–78
Krug RM. 1993. The regulation of export of
mRNA from nucleus to cytoplasm. Curr.
Opin. Cell Biol. 5:944– 49
Kurokawa M, Ochiai H, Nakajima K, Niwa-
yama S. 1990. Inhibitory effect of protein
kinase C inhibitor on the replication of in-
fluenza type A virus. J. Gen. Virol. 71:2149–
Legrain P, Rosbash M. 1989. Some cis- and
trans-acting mutants for splicing target pre-
mRNA to the cytoplasm. Cell 57:573–83
Leppard KN, Shenk T. 1989. The adenovirus
E1B 55 kd protein influences mRNA trans-
port via an intranuclear effect on RNA
metabolism. EMBO J. 8:2329–36
Liang S, Hitomi M, Hu Y-H, Liu Y, Tartakoff
AM. 1996. A DEAD-box family protein is
required for nucleocytoplasmic transport of
yeast mRNA. Mol. Biol. Cell 16:5139–46
Liou R-F, Blumenthal T. 1990. trans-spliced
Caenorhabditis elegans mRNAs retain
trimethylguanosine caps. Mol. Cell. Biol.
Liu X, Mertz JE. 1995. HnRNP L binds a cis-
acting RNA sequence element that enables
intron-independent gene expression. Genes
Dev. 9:1766–80
Lu X, Heimer J, Rekosh D, Hammarskj¨old
M-L. 1990. U1 small nuclear RNA plays a
direct role inthe formation of a rev-regulated
human immunodeficiency virus env mRNA
that remains unspliced. Proc.Natl. Acad. Sci.
USA 87:7598–602
Lu Y, Qian X-Y, Krug RM. 1994. The influenza
virus NS1 protein: a novel inhibitor of pre-
mRNA splicing. Genes Dev. 8:1817–28
uhrmann R, Kastner B, Bach M. 1990. Struc-
ture of splicesomal snRNPs and their role in
pre-mRNA splicing. Biochim. Biophys. Acta
Gene Struct. Express. 1087:265–92
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
Luo Y, Yu H, Peterlin BM. 1994. Cellular pro-
ciency virus type 1 Rev. J. Virol. 68:3850–56
Mairy M, Denis H. 1971. Recherches biochim-
iques sur l’oogenese. 1. Synthese et accumu-
lation du RNA pendent l’oogenese du cra-
paud sud-africain Xenopus laevis. Dev. Biol.
Malim MH, B¨ohnlein S, Hauber J, Cullen BR.
1989a. Functional dissection of the HIV1
Rev trans-activator: derivation of a trans-
dominant repressor of Rev function. Cell
Malim MH, Cullen BR. 1991. HIV1 structural
gene expression requires the binding of mul-
tiple Rev monomers to the viral RRE: impli-
cations for HIV1 latency. Cell 65:241–48
BR.1989b.The HIV1revtrans-activatoracts
through a structured target sequence to acti-
vatenuclearexport ofunspliced viral mRNA.
Nature 338:254–57
Malim MH, McCarn DF, Tiley LS, Cullen BR.
1991. Mutational definition of the human im-
munodeficiency virus type 1 Rev activation
domain. J. Virol. 65:4248–54
Maquat LE. 1996. RNA transport from the
nucleus. In Modern Cell Biology, Post-
ford, DK Morris, New York: Wiley. In press
Marshallsay C, L¨uhrmann R. 1994. In vitro nu-
clear import of snRNPs: cytosolic factors
mediate m
G-cap dependence of U1 and U2
snRNP transport. EMBO J. 13:222–31
Martin K, Helenius A. 1991. Nuclear transport
of influenza virus ribonucleoproteins: the vi-
ral matrix protein (M1) promotes export and
inhibits import. Cell 67:117–30
Mattaj IW. 1986. Cap trimethylation of U
snRNA is cytoplasmic and dependent on U
snRNP protein binding. Cell 46:905–11
Mattaj IW. 1988.U snRNP assembly and trans-
port. In Structure and Function of Major
and Minor Small Nuclear Ribonucleopro-
tein Particles, ed. ML Birnstiel, pp. 100–
14. Berlin/Heidelberg/New York: Springer-
Mattaj IW, DeRobertis EM. 1985. Nuclear seg-
regation of U2 snRNA requires binding of
specific snRNP proteins. Cell 40:111–18
Matunis EL, Matunis MJ, Dreyfuss G. 1993.
Association of individual hnRNP proteins
and snRNPs with nascent transcripts. J. Cell
Biol. 121:219–28
Mayeda A, Krainer AR. 1992 Regulation of al-
ternative pre-mRNA splicing by hnRNP A1
and splicing factor SF2. Cell 68:365–75
Mehlin H, Daneholt B. 1993. The Balbiani ring
particle: a model for the assembly and export
of RNPs from the nucleus? Trends Cell Biol.
Mehlin H, Daneholt B, Skoglund U. 1992.
Translocation of a specific premessenger ri-
bonucleoprotein particle through the nuclear
pore studied with electron microscope to-
mography. Cell 69:605–13
Mehlin H, Skoglund U, Daneholt B. 1991.
Transport of Balbiani ring granules through
nuclear pores in Chironomus tentans. Exp.
Cell Res. 193:72–77
MelchiorF, PaschalB, EvansJ, Gerace L. 1993.
Inhibition of nuclear protein import by non-
hydrolyzable analogues of GTP and iden-
tification of the small GTPase Ran/TC4 as
an essential transport factor. J. Cell Biol.
Melton DA, De Robertis EM, Cortese R. 1980.
Order and intracellular location of the events
involved in thematuration of a spliced tRNA.
Nature 284:143–48
Mermer B, Felber BK, Campbell M, Pavlakis
GN. 1990. Identification of trans-dominant
HIV1 rev protein mutants by direct transfer
of bacterially produced proteins into human
cells. Nucleic Acids Res. 18:2037–44
Meyer BE, Malim MH. 1994. The HIV1 Rev
trans-activator shuttles between the nucleus
and the cytoplasm. Genes Dev. 8:1538–
MeyerBE,Meinkoth JL,Malim MH.1996. Nu-
clear transport of human immunodeficiency
virus type 1, visna virus, and equine infec-
tious anemia virus Rev proteins: identifica-
tion of a familyof transferable nuclear export
signals. J. Virol. 70:2350–59
Michael WM, Choi M, Dreyfuss G. 1995.
A nuclear export signal in hnRNP A1: a
signal-mediated, temperature-dependent nu-
clear protein exportpathway.Cell 83:415–22
Michael WM, Dreyfuss G. 1996. Distinct do-
mains in ribosomal protein L5 mediate 5S
rRNA binding and nucleolar localization. J.
Biol. Chem. 271:11571–74
Michael WM, Siomi H, Choi M, Pi˜nol-Roma
S, Nakielny S, et al. 1996. Signal sequences
that target nuclear import and nuclear export
of pre-mRNA-binding proteins. Cold Spring
Harbor Symp. Quant. Biol. LX:663– 68
Michaud N, Goldfard DS. 1991. Multiple path-
ways in nuclear transport: The import of U2
snRNP occurs by a novel kinetic pathway. J.
Cell Biol. 112:215–23
Michaud N, Goldfarb D. 1992. Microinjected
U snRNAs are imported to oocyte nuclei via
the nuclear pore complex by three distin-
guishable targeting pathways. J. Cell Biol.
Micklem DR. 1995. mRNA localisation during
development. Dev. Biol. 172:377–95
Moore MJ, Query CC, Sharp PA. 1993. Splic-
ing of precursors to mRNAs by the spliceo-
some. In The RNA World, ed. RF Gesteland,
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
JF Atkins, 13:303–57. Plainview, NY: Cold
Spring Harbor Lab.
Moore MS, Blobel G. 1993. The GTP-binding
protein Ran/TC4 is required for protein im-
port into the nucleus. Nature 365:661–63
Munroe SH, Dong X. 1992. Heterogeneous nu-
clear ribonucleoprotein A1 catalyzes RNA-
RNA annealing. Proc. Natl. Acad. Sci. USA
Nagata S, Mantei N, Weissmann C. 1980. The
structure of one of the eight or more distinct
chromosomal genes for human interferon-
alpha. Nature 287:401–8
Nakielny S, Dreyfuss G. 1996. The hnRNP C
proteins contain a nuclearretention sequence
that can override nuclear export signals. J.
Cell Biol. 134:1365–73
Ogert RA, Lee LH, Beemon KL. 1996. Avian
retroviral RNA element promotes unspliced
RNA accumulation inthe cytoplasm. J.Virol.
Ohno M, Kataoka N, Shimura Y. 1990. A nu-
clear cap binding protein from HeLa cells.
Nucleic Acids Res. 18:6989–95
Okita TW, Li X, Roberts MW. 1994. Targeting
of mRNAs to domains of the endoplasmic
reticulum. Trends Cell Biol. 4:91–96
O’Neill RE, Jaskunas R, Blobel G, Palese P,
Moroianu J. 1995. Nuclear import of in-
fluenza virus RNA can be mediated by viral
nucleoprotein and transport factors required
Ornelles DA, Shenk T. 1991. Localization of
the adenovirus early region 1B 55-kilodalton
protein during lytic infection: Association
with nuclear viral inclusions requires the
early region 4 34-kilodalton protein. J. Virol.
Peebles CL, Gegenheimer P, Abelson J. 1983.
Precise excision of intervening sequences
from precursor tRNAs by a membrane-
associated yeast endonuclease. Cell 32:525–
PilderS,MooreM,Logan J,Shenk T.1986. The
adenovirus E1B55K transforming polypep-
tide modulates transport or cytoplasmic sta-
bilization of viral and host cell mRNAs. Mol.
Cell. Biol. 6:470–76
Pi˜nol-Roma S, Choi YD, Matunis MJ, Drey-
fuss G. 1988. Immunopurification of hetero-
geneous nuclear ribonucleoprotein particles
reveals an assortment of RNA-binding pro-
teins. Genes Dev. 2:215–27
Pi˜nol-Roma S, Dreyfuss G. 1991. Trans-
cription-dependent and transcription-inde-
pendentnuclear transportof hnRNP proteins.
Science 253:312–14
Pi˜nol-Roma S, Dreyfuss G. 1992. Shuttling of
pre-mRNAbindingproteins betweennucleus
and cytoplasm. Nature 355:730–32
Pi˜nol-Roma S, Dreyfuss G. 1993. hnRNP pro-
teins: localization and transport between the
nucleus and the cytoplasm. Trends Cell Biol.
Pi˜nol-Roma S, Swanson MS, Gall JG, Dreyfuss
G.1989.A novelheterogeneousnuclear RNP
protein with a unique distribution on nascent
transcripts. J. Cell Biol. 109:2575–87
Pokrywka NJ, Goldfarb DS. 1995. Nuclear ex-
port pathways of tRNA and 40S ribosomes
include both common and specific interme-
diates. J. Biol. Chem. 270:3619–24
PortmanDS, Dreyfuss G.1994. RNAannealing
activities in HeLa nuclei. EMBO J. 13:213–
Qian X-Y, Alonso-Caplen F, Krug RM. 1994.
NS1 protein are required for regulation of
nuclear export of mRNA. J. Virol. 68:2433–
QiuY, KrugRM.1994. Theinfluenza virusNS1
protein is a poly(A)-binding protein that in-
hibits nuclear export of mRNAs containing
poly(A). J. Virol. 68:2425–32
Reddy R, Bush H. 1988. In Structure and
Function of Major and Minor Small Nuclear
Ribonucleoprotein Particles, ed. ML Birn-
stiel, pp. 1–37. Berlin/Heidelberg/New York:
RinkeJ, SteitzJA.1982. Precursormolecules of
both human 5S ribosomal RNA and transfer
RNAs are bound by cellular protein reactive
with anti-La lupus antibodies. Cell 29:711–
Rousseau D, Kaspar R, Rosenwald I, Gehrke
L, Sonenberg N. 1996. Translation initiation
of ornithine decarboxylase and nucleocyto-
plasmic transport of cyclin D1 mRNA are
increased in cells overexpressing eukaryotic
initiation factor 4E. Proc. Natl. Acad. Sci.
USA 93:1065–70
Rout MP, Wente SR. 1994. Pores for thought:
nuclear pore complex proteins. Trends Cell
Biol. 4:357–65
Rudt R, Pieler T. 1996. Cytoplasmic retention
taining RNPs. EMBO J. 15:1383–91
Saavedra C, Tung K-S, Amberg DC, Hopper
AK, Cole CN. 1996. Regulation of mRNA
exportinresponse tostress inSaccharomyces
cerevisiae. Genes Dev. 10:1608–20
St. Johnston D. 1995a. The intracellular local-
ization of messenger RNAs. Cell 81:161–
St. Johnston D. 1995b. New role for tropo-
myosin. Nature 377:483
Santos T, Zasloff M. 1981. Comparative anal-
ysis of human chromosomal segments bear-
ing nonallelic dispersed tRNA
genes. Cell
Sazer S. 1996. The search for the primary func-
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
tion of the Ran GTPase continues. Trends
Cell Biol. 6:81– 85
Scheer U, Weisenberger D. 1994. The nucleo-
lus. Curr. Opin. Cell Biol. 6:354–59
Schlenstedt G, Saavedra C, Loeb JDJ, Cole CN,
Silver PA. 1995. The GTP-bound form of
protein import and appearance of poly(A)
RNA in the cytoplasm. Proc. Natl. Acad.Sci.
USA 92:225–29
Schneiter R, Kadowaki T, Tartakoff AM. 1995.
mRNA transport in yeast: time to reinvesti-
gatethe functions of the nucleolus.Mol. Biol.
Cell 6:357–70
Shapiro GI, Gurney T Jr, Krug RM. 1987. In-
fluenza virus gene expression: control mech-
anisms at early and late times of infection
and nuclear-cytoplasmic transport of virus-
specific RNAs. J. Virol. 61:764–73
Sharma K, Fabre E, Tekotte H, Hurt ED, Toller-
vey D. 1996. Yeast nucleoporin mutants are
defective in pre-tRNA splicing. Mol. Cell.
Biol. 16:294–301
Shaw PJ, Jordan EG. 1995. The nucleolus.
Annu. Rev. Cell Dev. Biol. 11:93–121
ShumanS.1995.Cappingenzyme ineukaryotic
mRNA synthesis. Prog. Nucleic Acid Res.
Mol. Biol. 50:101–29
Singer RH. 1993. RNA zipcodes for cytoplas-
mic addresses. Curr. Biol. 3:719–21
Singh R, Green MR. 1993. Sequence-specific
bindingoftransfer RNAbyglyceraldehyde3-
phosphate dehydrogenase. Science 259:365–
Siomi H, Dreyfuss G. 1995. A nuclear localiza-
tion domain inthe hnRNP A1 protein. J. Cell
Biol. 129:551–60
SkoglundU,Andersson K,StrandbergB, Dane-
holt B. 1986. Three-dimensional structure of
a specific pre-messenger RNP particle estab-
lished by electron microscope tomography.
Nature 319:560–64
Stebbings H, Lane JD, Talbot NJ. 1995. mRNA
translocation and microtubules: insect ovary
models. Trends Cell Biol. 5:361–65
SteitzJA,BergC,Hendrick JP, Branche-Chabot
H, Metspalu A, et al. 1988. A 5S rRNA/L5
complex is a precursor to ribosome assembly
in mammalian cells. J. Cell Biol. 106:545–
Stevens BJ, SwiftH. 1966. RNA transport from
nucleus to cytoplasm in Chironomus salivary
glands. J. Cell Biol. 31:55–77
Steward O. 1995. Targeting of mRNAs to sub-
synaptic microdomains in dendrites. Curr.
Opin. Neurobiol. 5:55–61
tion of a novel nuclear pore-associated pro-
tein as a functional target of the HIV1 Rev
protein in yeast. Cell 82:495–506
Stutz F, Rosbash M. 1994. A functional inter-
action between Rev and yeast pre-mRNA is
J. 13:4096–104
Sun J, Pilch DR, Marzluff WF. 1992. The his-
of histone mRNA to polyribosomes. Nucleic
Acids Res. 20:6057–66
Sweet DJ, Gerace L. 1995. Taking from the cy-
toplasmand givingto the pore: soluble trans-
port factors in nuclear protein import. Trends
Cell Biol. 5:444–47
Tachibana T, Imamoto N, Seino H, Nishimoto
T, Yoneda Y. 1994. Loss of RCC1 leads to
suppression of nuclear protein import in liv-
ing cells. J. Biol. Chem. 269:24542–45
Tafuri SR, WolffeAP. 1993. Dual roles for tran-
scription and translation factors in the RNA
storage particles of Xenopus oocytes. Trends
Cell Biol. 3:94–98
Tartakoff AM, Schneiter R. 1995. The nuclear
GTPase cycle: promoting peripheralization?
Trends Cell Biol. 5:5–8
Terns MP, Dahlberg JE, Lund E. 1993. Muli-
ple cis-acting signals for export of pre-
U1 snRNA from the nucleus. Genes Dev.
Tobian JA, Drinkard L, Zasloff M. 1985. tRNA
nuclear transport: defining the critical re-
gions of human tRNA
by point mutage-
nesis. Cell 43:415–22
Van Doren K, Hirsh D. 1990. mRNAs that ma-
ture through trans-splicing in Caenorhabdi-
tis elegans have a trimethylguanosine cap at
their 5
termini. Mol. Cell. Biol. 10:1769–72
vanZee K, DickmannsA, Fischer U, L¨uhrmann
R, Fanning E. 1993. A cytoplasmically an-
chored nuclear protein interferes specifically
with the import of nuclear proteins but not
U1 snRNA. J. Cell Biol. 121:229–40
Venkatesh LK, Chinnadurai G. 1990. Mutants
nusof HIV1 Revidentify functionally impor-
tant residues and exhibit a dominant negative
phenotype. Virology 178:327–30
Vijayraghavan U, Company M, Abelson J.
1989. Isolation and characterization of pre-
mRNA splicing mutants of Saccharomyces
cerevisiae. Genes Dev. 3:1206–16
Visa N, Alzhanova-Ericsson AT, Sun X, Kise-
leva E, Bj¨orkroth B, et al. 1996a. A
pre-mRNA-bindingprotein accompaniesthe
RNAfrom thegene through thenuclear pores
and into polysomes. Cell 84:253–64
Visa N, Izaurralde E, Ferreira J, Daneholt
B, Mattaj IW. 1996b. A nuclear cap bind-
ing complex binds Balbiani ring pre-mRNA
co-transcriptionally and accompanies the ri-
bonucleoprotein particle during nuclear ex-
port. J. Cell Biol. 133:5–14
Vogel U, Kunerl M, Scholtissek C. 1994. In-
fluenza A virus late mRNAs are specifically
November 11, 1997 22:21 Annual Reviews AR024-11 AR24-11
retained in the nucleus in the presence of
a methyltransferase or a protein kinase in-
hibitor. Virology 198:227–33
von Schwedler U, Kornbluth RS, Trono D.
1994. The nuclear localization signal of the
matrix protein of human immunodeficiency
virustype1allowstheestablishmentof infec-
tioninmacrophagesandquiescent Tlympho-
cytes.Proc.Natl. Acad.Sci. USA91:6992–96
Warner JR. 1990. The nucleolus and ribosome
formation. Curr. Opin. Cell Biol. 2:521–27
Weichselbraun I, Farrington GK, Rusche JR,
ohnlein E, Hauber J. 1992. Definition of the
human immunodeficiency virus type 1 Rev
and human T-cell leukemia virus type I Rex
protein activation domain by functional ex-
change. J. Virol. 66:2583–87
Wen W, Meinkoth JL, Tsien RY, Taylor SS.
1995. Identification of a signal for rapid
export of proteins from the nucleus. Cell
Westaway SK, Abelson J. 1995. Splicing of
tRNA precursors. In tRNA Structure, Biosyn-
thesis, and Function, ed.DS¨oll, UL Raj-
Bhandary, 7:79–92. Washington, DC: ASM
Whittaker G, Bui M, Helenius A. 1996. The
role of nuclear import and export in influenza
virus infection. Trends Cell Biol. 6:67–71
Whittaker G, Kemler I, Helenius A. 1995. Hy-
perphosphorylationof mutant influenza virus
matrix protein, M1, causes its retention in the
nucleus. J. Virol. 69:439–45
Wilhelm JE, Vale RD. 1993. RNA on the move:
the mRNA localization pathway. J. Cell Biol.
Wolff B, Cohen G, Hauber J, Meshcheryakova
D, Rabeck C. 1995. Nucleocytoplasmic
transport of the Rev protein of human im-
munodeficiency virus type 1 is dependent on
Res. 217:31–41
Wolffe AP, Brown DD. 1988. Developmental
regulation of two 5S ribosomal RNA genes.
Science 241:1626–32
Yang X, Bani M-R, Lu S-J, Rowan S, Ben-
David Y, Chabot B. 1994. The A1 and A1
proteins of heterogeneous nuclear ribonucle-
oparticles modulate 5
splice site selection in
vivo. Proc. Natl. Acad. Sci. USA 91:6924–
Ye Z, Robinson D, Wagner RR. 1995. Nucleus-
targeting domain of the matrix protein (M
of influenza virus. J. Virol. 69:1964–70
Zapp ML, Green MR. 1989. Sequence-specific
RNA binding by the HIV1 Rev protein. Na-
ture 342:714–16
Zasloff M. 1983. tRNA transport from the nu-
cleus in a eukaryotic cell: carrier-mediated
translocation process. Proc. Natl. Acad. Sci.
USA 80:6436–40
Zasloff M, Rosenberg M, Santos T. 1982a. Im-
paired nuclear transport of a human variant
. Nature 300:81–84
Zasloff M, Santos T, Hamer DH. 1982b. tRNA
precursor transcribed from a mutant human
gene inserted into a SV40 vector is processed
incorrectly. Nature 295:533–35
... We observed that "RNA transport" KEGG pathway genes were significantly upregulated in L. marina with dietary stearic acid supplementation ( Figure 3A). RNA transport is a process in which RNA molecules are actively transported from one position within the cell to another, including the export of RNA from the nucleus to the cytoplasm through the nuclear pores, and the microtubule-assisted movement of specific RNAs along the axon [37][38][39]. Among eight upregulated DEGs enriched in the "RNA transport" pathway, five of which were translation initiation or elongation factors, including EVM0011266/eif-1.A (eukaryotic translation initiation factor 1A), EVM0015069/eif-2Bbeta, EVM0003319/eif-2Bdelta, EVM0016427/eif-5, and EVM0014509/eef-1A.1 (eukaryotic translation elongation factor 1α) ( Figure 5B). ...
Full-text available
Stearic acid represents one of the most abundant fatty acids in the Western diet and profoundly regulates health and diseases of animals and human beings. We previously showed that stearic acid supplementation promoted development of the terrestrial model nematode Caenorhabditis elegans in chemically defined CeMM food environment. However, whether stearic acid regulates development of other nematodes remains unknown. Here, we found that dietary supplementation with stearic acid could promote the development of the marine nematode Litoditis marina, belonging to the same family as C. elegans, indicating the conserved roles of stearic acid in developmental regulation. We further employed transcriptome analysis to analyze genome-wide transcriptional signatures of L. marina with dietary stearic acid supplementation. We found that stearic acid might promote development of L. marina via upregulation of the expression of genes involved in aminoacyl-tRNA biosynthesis, translation initiation and elongation, ribosome biogenesis, and transmembrane transport. In addition, we observed that the expression of neuronal signaling-related genes was decreased. This study provided important insights into how a single fatty acid stearic acid regulates development of marine nematode, and further studies with CRISPR genome editing will facilitate demonstrating the molecular mechanisms underlying how a single metabolite regulates animal development and health.
... However, the viral RNA expressed by the cellular RNA polymerase II in the nucleus should be transported from the nucleus to the cytoplasm. To address this problem, an intron sequence downstream of the CMV promoter was inserted to ensure splicing and capping, thus enabling efficient export to the cytoplasm and a stabilization of the RNA [29][30][31][32][33]. ...
Full-text available
Reverse genetics is a technology that allows the production of a virus from its complementary DNA (cDNA). It is a powerful tool for analyzing viral genes, the development of novel vaccines, and gene delivery vectors. The standard reverse genetics protocols are laborious, time-consuming, and inefficient for negative-strand RNA viruses. A new reverse genetics platform was established, which increases the recovery efficiency of the measles virus (MV) in human 293-3-46 cells. The novel features compared with the standard system involving 293-3-46 cells comprise (a) dual promoters containing the RNA polymerase II promoter (CMV) and the bacteriophage T7 promoter placed in uni-direction on the same plasmid to enhance RNA transcription; (b) three G nucleotides added just after the T7 promoter to increase the T7 RNA polymerase activity; and (c) two ribozymes, the hairpin hammerhead ribozyme (HHRz), and the hepatitis delta virus ribozyme (HDVrz), were used to cleavage the exact termini of the antigenome RNA. Full-length antigenome cDNA of MV of the wild type IC323 strain or the vaccine AIK-C strain was inserted into the plasmid backbone. Both virus strains were easily rescued from their respective cloned cDNA. The rescue efficiency increased up to 80% compared with the use of the standard T7 rescue system. We assume that this system might be helpful in the rescue of other human mononegavirales.
... The ZGA process is important during early embryonic development; particularly, RNA synthesis is involved in differentiation into ICM/TE and formation of the blastocyst. hnRNPA2/B1 is an RNA-binding protein and a member of the hnRNP family, and contains several RNA recognition motif (RRM) sites that can bind to mRNAs and regulate various mRNA processes such as mRNA transport, alternative splicing, and maintenance of mRNA stability 22,23,40 . In embryonic stem cells, hnRNPA2/B1 is crucial for stabilizing pluripotency by regulating key pluripotency-related proteins; however, the functional roles of hnRNPA2/B1 during mammalian early ...
Full-text available
Heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNPA2/B1) plays an important role in RNA processing via in m6A modification of pre-mRNA or pre-miRNA. However, the functional role of and relationship between m6A and hnRNPA2/B1 in early embryonic development are unclear. Here, we found that hnRNPA2/B1 is crucial for early embryonic development by virtue of regulating specific gene transcripts. HnRNPA2/B1 was localized to the nucleus and cytoplasm during subsequent embryonic development, starting at fertilization. Knockdown of hnRNPA2/B1 delayed embryonic development after the 4-cell stage and blocked further development. RNA-Seq analysis revealed changes in the global expression patterns of genes involved in transcription, translation, cell cycle, embryonic stem cell differentiation, and RNA methylation in hnRNPA2/B1 KD blastocysts. The levels of the inner cell mass markers OCT4 and SOX2 were decreased in hnRNPA2/B1 KD blastocysts, whereas that of the differentiation marker GATA4 was decreased. N6-Adenosine methyltransferase METTL3 knock-down caused embryonic developmental defects similar to those in hnRNPA2/B1 KD embryos. Moreover, METTL3 KD blastocysts showed increased mis-localization of hnRNPA2/B1 and decreased m6A RNA methylation. Taken together, our results suggest that hnRNPA2/B1 is essential for early embryogenesis through the regulation of transcription-related factors and determination of cell fate transition. Moreover, hnRNPA2/B1 is regulated by METTL3-dependent m6A RNA methylation.
... To allow for the latter in the postsynaptic compartment, selected mRNAs become localized into dendrites, near synapses (Holt & Bullock, 2009). This is achieved by a set of RNAbinding proteins (RBPs) that recognize and bind to cis-elements preferentially within the 3′-untranslated region (3′-UTR) of target mRNAs (Nakielny, Fischer, Michael, & Dreyfuss, 1997) allowing the assembly of ribonucleoprotein particles (RNPs). With the help of molecular motor proteins, these RNPs are then transported along the neuronal cytoskeleton towards synapses. ...
Staufen2 (Stau2) is a double-stranded RNA-binding protein (RBP) involved in posttranscriptional gene expression control in neurons. In flies, staufen contributes to learning and long-term memory formation. To study the impact of mammalian Stau2 on behavior, we generated a novel gene-trap mouse model that yields significant constitutive downregulation of Stau2 (Stau2GT). In order to investigate the effect of Stau2 downregulation on hippocampus-dependent behavior, we performed a battery of behavioral assays, i.e. open field, novel object recognition/location (NOR/L) and Barnes maze. Stau2GTmice displayed reduced locomotor activity in the open field and altered novelty preference in the NOR and NOL paradigms. Adult Stau2GTmale mice failed to discriminate between familiar and newly introduced objects but showed enhanced spatial novelty detection. Additionally, we observed deficits in discriminating different spatial contexts in a Barnes maze assay. Together, our data suggest that Stau2 contributes to novelty preference and explorative behavior that is a driver for proper spatial learning in mice.
Full-text available
Hypoxia is a common stressor in shrimp, but the molecular mechanisms underlying the adaptation of the ridgetail white prawn, Exopalaemon carinicauda, to hypoxia and reoxygenation remain poorly understood. In the present study, ridgetail white prawns were exposed to gradual changes in hypoxia for 6 h via oxygen consumption in a static respiration chamber and then rapidly aerated to allow reoxygenation for 8 h. The dissolved oxygen concentration after 6 h of hypoxia was 1.04 mg/L, and the survival rate of the shrimp was 47%. A transcriptomic analysis of hepatopancreas tissues after 0 (control), 3 (hypoxic starting point), and 6 (hypoxic lethal half point) h of exposure to hypoxia and after 1 and 8 h of reoxygenation was conducted using the Illumina HiSeqTM 4000 high-throughput sequencing platform. A total of 93,227 genes were obtained, and 4315 of these genes were identified as differentially expressed genes (DEGs) that respond to hypoxia and reoxygenation. A KEGG pathway enrichment analysis suggested that all identified DEGs were mostly enriched in ribosome biogenesis in eukaryotes, apoptosis, the longevity regulating pathway, the MAPK signaling pathway, and protein processing in the endoplasmic reticulum. In addition, 20,203 genes with RPKM values ≥1 were parsed into 20 modules through a weighted gene coexpression network (WGCNA), and three of these modules were related to hypoxia and reoxygenation. GO and KEGG enrichment analyses of these three modules were then performed, and three hub genes were identified based on their connectivity: RREB1 (Ras-responsive element-binding protein 1-like), UBE1 (ubiquitin-activating enzyme E1), and an unknown gene. This study might help further elucidate the hypoxia tolerance mechanism of E. carinicauda.
RNA helicase A (RHA) as a member of DExH‐box subgroup of helicase superfamily II, participates in diverse biological processes involved in RNA metabolism in organisms, and these RNA‐mediated biological processes rely on RNA structure conversion. However, how RHA regulate the RNA structure conversion was still unknown. In order to unveil the mechanism of RNA structure conversion mediated by RHA, single molecule fluorescence resonance energy transfer (smFRET) was adopted to in our assay, and substrates RNA were from internal ribosome entry site (IRES) of foot‐and‐mouth disease virus (FMDV) genome. We first found that the RNA structure conversion by RHA against thermodynamic equilibrium in vitro, and the process of dsRNA YZ converted to dsRNA XY through a tripartite intermediate state. In addition, the rate of the RNA structure conversion and the distribution of dsRNA YZ and XY were affected by ATP concentrations. Our study provides real‐time insight into ATP‐dependent RHA‐assisted RNA structure conversion at the single molecule level, the mechanism displayed by RHA may help in understand how RHA contributes to many biological functions, and the basic mechanistic features illustrated in our work also underlay more complex protein‐assisted RNA structure conversions. This article is protected by copyright. All rights reserved.
This chapter presents the basic knowledge about the function of cell divisionCell division and the cell cycle process. In fine detail, the architecture and function of the largest compartment of the eukaryotic cell, the nucleus and its components, such as the nuclear envelopeNuclear envelope, nuclear pore complexNuclear pore complex (NPC), chromosomesChromosomes and nucleoskeleton, are emphasized. Centrioles play an important role in cell divisionCell division and are being discussed, including their replication. It is discussed how forces are transmitted from the cellular microenvironmentMicroenvironment, which are sensed by cell–matrix adhesionCell-matrix adhesion receptors, coupled via focal adhesion proteins to the cell’s cytoskeletal network, and subsequently they have an impact on the mechanical phenotype of the innermost organelle of the cell, the cell nucleusCell nucleus. The biophysical aspects of these main functions of cells under normal physiological conditions and under pathological conditions such as cancer and acute or chronic inflammationChronic inflammation are at the center of attention. Finally, the effect of mechanical force on gene expressionGene expression is addressed, how the cells deal with it.
The localization of long noncoding RNAs (lncRNAs) within the cell is the primary determinant of their molecular functions. LncRNAs are often thought of as chromatin-restricted regulators of gene transcription and chromatin structure. However, a rich population of cytoplasmic lncRNAs has come to light, with diverse roles including translational regulation, signaling, and respiration. RNA maps of increasing resolution and scope are revealing a subcellular world of highly specific localization patterns and hint at sequence-based address codes specifying lncRNA fates. We propose a new framework for analyzing sequencing-based data, which suggests that numbers of cytoplasmic lncRNA molecules rival those in the nucleus. New techniques promise to create high-resolution, transcriptome-wide maps associated with all organelles of the mammalian cell. Given its intimate link to molecular roles, subcellular localization provides a means of unlocking the mystery of lncRNA functions.
Full-text available
The functions of long non-coding RNAs (lncRNAs) in myocardial infarction (MI) remain largely unknown. Thus, we used the subp athway-LINCE method to characterize the potential roles of lncRNAs in MI. Candidate lncRNA-mRNA interactions were obtained from miRNA-mRNA interactions and lncRNA-miRNA interactions. Then the lncRNA and mRNA co-expression relationship pairs (LncGenePairs) were screened from the lncRNAs and mRNA intersections, which were extracted through candidate lncRNA-mRNA interactions and sample gene expression profiles. The lncRNAs in LncGenePairs were embedded into pathway graphs as nodes through linking to their regulated mRNAs, which resulted in obtaining condition-specific lncRNA competitively regulated signal pathways (csLncRPs). Finally, the csLncRPs were calculated using lenient distance similarity to obtain the lncRNA competitively regulated subpathways. Based on the statistical significance of signal subpathways, lncRNA-mRNA networks were constructed, in which hub lncRNAs were selected. A total of 65 lncRNAs competitively regulated subpathways and 13 hub lncRNAs were obtained, which associated with a risk of MI. Identifying lncRNAs competitively regulated subpathways not only provides potential lncRNA biomarkers for MI, but also helps the understanding of pathogenesis of MI.
Epidermal cells of leaf petioles, pedicles, and sepals in Caragana arborescens L. are characterized with a unique biogenesis of intracellular bodies, the presence of which continues during 10–12 days in spring, from budding till flowering and fruit inception. Initially, a nuclear body is formed as a derivative of the nucleolus at the beginning of elongation of the protodermal cells, whereas a cytoplasmic body is formed in the proximity of the nuclear envelope later. Nuclear bodies and cytoplasmic bodies do not contain DNA, lipids, and starch, and they consist of RNA tightly packaged with proteins mainly in the form of short thin fibrils with thickness of 6 nm. By the end of cell elongation and the beginning of differentiation, nuclear bodies disappear, while cytoplasmic bodies become surrounded by a homogenous zone (halo). Later, the bundles of parallel-oriented fibrils derived from the body radially pass through the homogenous zone and gradually disperse in the cytoplasm. In the differentiated epidermal cells, no traces of cytoplasmic bodies are observed; there is only one nucleolus in the nucleus. It is hypothesized that cytoplasmic bodies may function as an RNA depot, which is utilized later in cell metabolism during the formation of fruits and seeds.
Full-text available
We have screened nucleoporin mutants for the inhibition of tRNA splicing, which has previously been proposed to be coupled to transport. Strains mutant for Nup49p or Nup116p, or genetically depleted of Nup145p, strongly accumulated unspliced pre-tRNAs. Splicing was inhibited for all 10 families of intron-containing pre-tRNA, but no effects on 5' or 3' end processing were detected. Strains mutant for Nup133p or Nsp1p accumulated lower levels of several unspliced pre-tRNAs. In contrast, no accumulation of any pre-tRNA was observed in strains mutant for Nup1p, Nup85p, or Nup100p. Other RNA processing reactions tested, pre-rRNA processing, pre-mRNA splicing, and small nucleolar and small nuclear RNA synthesis, were not clearly affected for any nucleoporin mutant. These data provide evidence for a coupling between pre-tRNA splicing and nuclear-cytoplasmic transport. Mutation of NUP49, NUP116, or NUP145 has previously been shown to lead to nuclear poly(A)+ RNA accumulation, indicating that these nucleoporins play roles in the transport of more than one class of RNA.
The purpose of this summary is to present extensively the conclusions of this chapter, and to point out some areas where further research is required. References to the cited work are to be found in the text of the review. Assembly of UsnRNPs and Transport into the Nucleus. The study of UsnRNP assembly began with the definition of protein binding domains within the UsnRNAs. Since the majority of UsnRNP proteins are found in all the major UsnRNPs immunoprecipitable with anti-Sm antisera, the first attempts to define binding sites simply looked for structural domains conserved among the major UsnRNAs. By a combination of nuclease protection experiments and the analysis of the protein binding properties of mutant UsnRNAs one of these domains, a single-stranded region of consensus sequence PuA(U)nGPu, has been shown to be the site through which the common UsnRNP proteins bind. Since these proteins include those recognized by anti-Sm antisera, this region is now generally called the Sm binding site. In the case of U2 snRNA, this domain has been shown to be necessary for nuclear accumulation and for the modification of the co- transcriptionally added monomethyl guanosine cap structure to the UsnRNA- characteristic trimethyl gaunosine cap. This suggests that the Sm binding site, where it is present, has an important role in allowing an RNA to acquire the characteristics of a UsnRNA. This naturally begs the question of how RNAs, such as U3 snRNA, which do not detectably associate with Sm antigens, acquire their trimethyl cap structure and segregate into the nucleus. The importance of the Sm binding site is also supported by the observation that it has been extremely conserved in evolution, snRNAs from humans to fungi are capable of binding to Xenopus laevis Sm antigens. Indeed it has further been demonstrated that fungal RNAs with Sm binding sites are associated in fungi with proteins recognized by human anti-Sm antisera. Thus, at least this epitope has also been conserved on some snRNP proteins throughout eukaryotic evolution. Much less is known about the RNA binding sites of proteins unique to particular snRNPs. In the case of U2 hairpins towards the 3’ end of the molecule are required, in addition to the Sm binding site, for the interaction of the U2 specific A’ and B” proteins with the RNA. The order of assembly which this suggests, first interaction between the RNA and common UsnRNP proteins followed by addition of the unique proteins, is consistent with the in vivo analysis of UsnRNP assembly. These in vivo experiments further suggest that the core proteins D, E, F and G assemble, in the initial step of UsnRNP biogenesis, into a complex, in the absence of RNA. It is worth noting that one of the gaps in our current knowledge concerns the homogeneity of UsnRNPs. The molar ratios of different UsnRNP components is not known, and it is not clear whether all U2 snRNPs, for example, contain one molecule of all the common proteins plus A’ and B″ or whether there is a mixed population, some of which contain only A’ and others only B”. In order that we understand the role of the proteins in the function of snRNPs it is important that an answer is found to this question. A current model of the U2 snRNP is given in Fig. 1. From this model it is evident that we are ignorant of which snRNP proteins interact directly with UsnRNAs and where exactly they bind. A more direct analysis of UsnRNP structure would be extremely useful in this respect. The location of UsnRNP assembly is thought to be the cytoplasm. Evidence for this has come from many different experimental approaches. The fact that UsnRNAs injected into enucleated Xenopus oocytes assemble into anti-Sm immunoprecipitable snRNPs is the most direct demonstration that cytoplasmic assembly can occur. Oocytes are, however, unusual in that they store large amounts of UsnRNP proteins for use in the early stages of embryonic development and therefore the results which suggest that the location of assembly is the same in mammalian culture cells provide important confirmatory evidence. These results are mainly based on the observation that UsnRNAs, at least Ul, U2, and U4, are synthesized as precursors in HeLa cells. These precursors differ from the mature RNAs by having short 3’ extensions. They are short-lived, and are processed to the mature length within an hour of synthesis. The precursors are found mainly in the cytoplasm, as determined by many different cell fractionation methods. In this cytoplasmic phase immunoprecipitation studies have shown that the precursors are already associated with both common and at least some unique snRNP proteins. The short 3’ extensions are apparently removed from the UsnRNA precursors either prior to or during re-entry into the nucleus. This RNA processing event is not the only to occur in the cytoplasm. The conversion of the monomethyl guanosine cap structure to the unique trimethyl cap structure has also been shown to occur in the cytoplasm of Xenopus oocytes. It is currently not known where the other abundant internal modifications which occur on UsnRNAs take place. A final piece of evidence supporting the fact that UsnRNP assembly occurs in the cytoplasm is the observation, again made with Xenopus oocytes, that the common UsnRNP proteins in the free state are cytoplasmic. Migration of these proteins to the nucleus requires their interaction with RNA. This interaction is specific, and depends on the presence on the RNA of an Sm binding site in addition to some other, as yet undefined, structural characteristics. There are two gaps in our knowledge of UsnRNP assembly. The first concerns the details of the order of assembly of the B/B’ proteins and the unique snRNP proteins. The second is that we do not know where in the cell the unique proteins are found in the free state, nor where they assemble onto UsnRNPs, although there is evidence that at least some of these proteins join the snRNPs in the cytoplasm. As an example of assembly the order of events involved in the biogenesis of the U2 snRNP are diagramed in Fig. 2. Intranuclear Location of UsnRNPs. Making use of both light and electron microscopy, immunohistological studies have led to the elucidation of UsnRNP location in the nucleus. UsnRNPs are distributed throught the nucleoplasm, i. e., are not associated with chromatin or the nuclear lamina, in a pattern which reveals local concentrations. These aggregates of staining material are seen both at the light and the electron microscopic level, although the correlation between the two types of analysis is not yet perfect. UsnRNPs are also not associated with chromosomes during mitosis. They simply disperse throughout the cell and re-enter the daughter nuclei during telophase. Detailed analysis with the electron microscope has revealed that UsnRNPs are found in association with nascent hn RNP fibrils, indicating an early association between UsnRNPs and pre-mRNA. It is not clear whether this indicates a very early start to RNA processing events following transcription, or whether it is simply a reflection of an early stage in the assembly of splicing complexes. The major conclusion from these electron microscopic studies is however that the bulk of UsnRNPs are not associated with short-lived hnRNP. This enables me to finish this summary with a question for the future. What are the bulk of the UsnRNPs in the nucleus doing? Are they simply waiting to be used in RNA processing, or do they perform other functions? A much more detailed knowledge of nuclear structure and function will be required before this can be answered satisfactorily.
All eukaryotic cells contain multiple small nuclear RNAs, designated U-sn At present ten U-snRNAs have been identified; they account for about 1% total mammalian cellular RNA. Of these, the U1 to U8 snRNAs have been sively characterized. All the U-snRNAs contain a 5′-cap structure consis a blocked 5′-terminal pyrophosphate linkage; except for U6 RNA, a tri guanosine-containing “cap” is the 5′-end. These U-snRNAs are confined nucleus and are metabolieally stable. They are synthesized by RNA polyII. U6 snRNA, which has a non-nucleotide cap structure, is synthesized by RNA polymerase III. All the U-snRNAs are present in the cells as small ribonucleopro- tein particles (snRNPs), that are metabolically stable.
RNA synthesis was studied during oogenesis of the South-African clawed toadXenopus laevis. Seven classes of oocytes were isolated from immature and from mature females. The RNA extracted from these oocytes was analyzed by filtration on Sephadex G-100 and by centrifugation in sucrose gradients. The previtellogenic oocytes (average diameter : 50 to 150 μ) were found to contain very large amounts of D-RNA and of low molecular weight RNA. The latter comprises two classes of molecules whose properties are those of 5 s RNA and of transfer RNA. In the oocytes of 150 μ, D-RNA, 5 s RNA and transfer RNA formed 10%, 40% and 35%, respectively, of the total optical density. The vitellogenic oocytes (average diameter : 375-1100 μ) synthesized mostly D-RNA and 28 s + 18 s RNA. The latter classes of RNA accumulated very rapidly in large oocytes since they made up more than 90% of the RNA present in mature eggs. D-RNA synthesized by the large oocytes sedimented in sucrose gradients as two distinct peaks (40 s and 24 s). The synthesis of the different RNA classes is therefore not at all coordinate during oogenesis. Transfer RNA and 5 s RNA are produced in excess in small oocytes. These classes of RNA are probably not utilized immediately, but stored for further use. RNA synthesis was found to be very different in large oocytes (average diameter: 375-1100 μ) taken from females which have laid eggs since 1 week and in oocytes from females which have not laid eggs since more than 2 months. Differences were noticed in the labeling rate of RNA, and in the sedimentation profile of newly synthesized RNA.
Publisher Summary This chapter discusses the enzymatic mechanism of the individual capping reactions and the organization of the functional domains, within the relevant enzymes. Greatest attention is devoted to two model systems, Vaccinia virus and Saccharomyces cerevisim, in which biochemistry, molecular genetics, and protein engineering have fueled considerable progress, since the past review of ribonucleic acid (RNA) capping in this series. It also discusses the genetic studies in yeast that shed light on the physiologic role of the cap in eukaryotic RNA metabolism. Capping may serve to protect the messenger RNA (mRNA) from nucleolytic degradation. Elucidation of the mechanism and potential regulation of cap formation is, thus, pertinent to the understanding of eukaryotic gene expression. Capping occurs, by a series of three enzymatic reactions, in which the 5’ triphosphate terminus of a primary transcript is first cleaved to a diphosphateterminated RNA by RNA triphosphatase, then capped with GMP by RNA guanylyltransferase, and methylated at the N7 position of guanine by RNA (guanine-7) methyltransferase.
The export of mRNA from the nucleus to the cytoplasm is an essential step in the expression of genetic information in eukaryotes. It is an energy-dependent process and involves transport across the nuclear pores. It requires both cis-acting ribonucleoprotein particle signals and specific trans-acting factors. Although much remains to be learned, recent information has begun to define this pathway at both the cellular and biochemical levels and indicates that it is used as a key regulatory step by several viruses.