Published Ahead of Print 17 November 2010.
2011, 85(3):1158. DOI: 10.1128/JVI.01369-10.
Thary Jacob, Céline Van den Broeke and Herman W.
Viral Serine/Threonine Protein Kinases
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JOURNAL OF VIROLOGY, Feb. 2011, p. 1158–1173
Copyright © 2011, American Society for Microbiology. All Rights Reserved.
Vol. 85, No. 3
Viral Serine/Threonine Protein Kinases?
Thary Jacob,† Ce ´line Van den Broeke,† and Herman W. Favoreel*
Department of Virology, Parasitology, and Immunology, Faculty of Veterinary Medicine, Ghent University, Ghent, Belgium
Phosphorylation represents one the most abundant and important posttranslational modifications of pro-
teins, including viral proteins. Virus-encoded serine/threonine protein kinases appear to be a feature that is
unique to large DNA viruses. Although the importance of these kinases for virus replication in cell culture is
variable, they invariably play important roles in virus virulence. The current review provides an overview of the
different viral serine/threonine protein kinases of several large DNA viruses and discusses their function,
importance, and potential as antiviral drug targets.
Protein kinases are characterized by their potential to cata-
lyze the transfer of a phosphate group from a nucleoside
triphosphate (generally ATP) to an amino acid residue of a
protein substrate. This phosphorylation usually results in a
functional change of the target protein by interfering with its
enzymatic activity, cellular location, and/or association with
other proteins. Depending on the specificity of the substrate
amino acids, protein kinases are subdivided in serine/threonine
(S/T) or tyrosine (Tyr) kinases. In addition, rare dual-specific-
ity kinases have also been identified. Protein kinases phosphor-
ylate either a serine, a threonine, or a tyrosine residue when it
is present in a specific stretch of other amino acids, known as
the consensus sequence. The catalytic core of protein kinases
consists of 12 conserved subdomains that fold into a common
structure. Several strongly conserved residues were identified
as indispensable for kinase activity, such as the conserved ly-
sine in subdomain VIb, which is critical for ATP binding, and
the conserved aspartate in the catalytic loop (subdomain VIb)
(77). These residues are frequently used as targets for mu-
tagenesis in the construction of kinase-negative mutants.
The human genome contains about 500 protein kinase
genes, constituting approximately 2% of all human genes
(127). Up to 30% of all human proteins may be modified by
kinase activity, and kinases are known to regulate the majority
of cellular pathways, especially those involved in signal trans-
Since viruses have evolved complex interactions with their
host and often mimic cellular proteins in order to usurp the
cellular machinery for their own benefit, it might not be sur-
prising that several viruses encode protein kinases. In the late
1970s, the first reports speculating about the existence of viral
kinases were published (40, 206); it was only 10 years later that
the first viral kinases were identified (115, 117). Over the last
few years, several new insights have been gained concerning
the often pleiotropic functions of viral protein kinases.
Both serine/threonine and tyrosine viral protein kinases
have been identified. The best-characterized viral tyrosine ki-
nases are the oncogenic tyrosine kinases encoded by acute
transforming retroviruses. These kinases are captured forms of
normal cellular genes (proto-oncogenes), and this incorpora-
tion may actually be considered an “accident de parcours” dur-
ing virus replication, typically rendering the virus replication
defective. These viral tyrosine kinases will not be covered in
this review but have been covered elsewhere (126). Viral
serine/threonine kinases, on the other hand, often display little
homology to any of the known cellular kinases. They appear to
be encoded exclusively by large DNA viruses and form the
subject of the current review.
Herpesviruses are large, enveloped viruses with a linear,
double-stranded DNA genome that varies in size from 125 to
245 kb. The majority of herpesviruses have been classified in
one of three subfamilies: the alpha-, beta-, and gammaherpes-
viruses. Eight human herpesviruses have been described: the
alphaherpesviruses herpes simplex virus 1 (HSV-1) and HSV-2
and varicella-zoster virus (VZV), the betaherpesviruses human
cytomegalovirus (HCMV) and human herpesvirus 6 (HHV-6)
and HHV-7, and the gammaherpesviruses Epstein-Barr virus
(EBV) and Kaposi’s sarcoma-associated herpesvirus (KSHV).
Invariably, primary infections are followed by lifelong per-
sistence. Specific circumstances may trigger reactivation, with
or without viral shedding and/or clinical symptoms. Disease
symptoms vary greatly, and include cold sores (HSV-1), genital
lesions (HSV), chickenpox (VZV), shingles (VZV), congenital
disease (HCMV), posttransplant diseases (HCMV), roseola
(HHV-6, -7), infectious mononucleosis (EBV, HCMV), lym-
phoproliferative disorders (EBV), and Kaposi’s sarcoma
All herpesviruses encode serine/threonine kinases. Two
types of conserved herpesvirus serine/threonine kinases
have been described. One type is conserved over the three
different subfamilies. The prototypical example of this ki-
nase is the UL13 kinase of HSV. These conserved kinases
will be designated conserved herpesvirus protein kinases
* Corresponding author. Mailing address: Department of Virology,
Parasitology, and Immunology, Faculty of Veterinary Medicine, Ghent
University, Salisburylaan 133, 9820 Merelbeke, Belgium. Phone: 32 9
264 73 74. Fax: 32 9 264 74 95. E-mail: Herman.Favoreel@UGent.be.
† These authors contributed equally.
?Published ahead of print on 17 November 2010.
on August 14, 2013 by UNIVERSITEIT GENT/UZGENT
(CHPK) throughout the text. Another type of herpesvirus
protein kinases, exemplified by the US3 kinase of HSV, is
conserved in the alphaherpesvirus subfamily. In addition,
serine/threonine protein kinase activity has been suggested
to be present in the large subunit of the ribonucleotide
reductase of HSV-2.
CHPK. Orthologs of CHPK include UL13 of HSV, ORF47
of VZV, UL97 of HCMV, U69 of HHV-6, BGLF4 of EBV,
and ORF36 of KSHV and murine herpesvirus 68 (MHV-68).
In 1989, by using sequencing assays, Chee and colleagues
identified a gene encoding a putative protein kinase that
appeared to be conserved in the herpesvirus family, since it
was present in the alphaherpesviruses HSV-1 and VZV, the
betaherpesviruses HCMV and HHV-6, and the gammaher-
pesvirus EBV (35). The corresponding protein was con-
firmed a few years later to display serine protein kinase
activity for the CHPK of the alphaherpesviruses VZV and
porcine pseudorabies virus (PRV) (54, 159). In vitro studies
using cell cultures and primary cells showed that the impor-
tance of CHPK for virus growth in vitro varies from crucial
for viral replication to unimportant, depending on both the
virus and the cell (45, 184, 185). In vivo studies, however,
invariably point to an important role of the kinase in virus
virulence (45, 86, 150, 180). Although the kinase is con-
served among herpesviruses, CHPK from different herpes-
virus subfamilies show considerable variation in amino acid
sequence. Although some sequence preferences for sub-
strate specificity have been described for individual CHPK,
there is no consensus phosphorylation sequence for all
CHPK (10, 30, 95, 96, 100).
Despite these differences, partial functional conservation
has been reported (193). Characteristics and functions of
CHPK that appear to be conserved to some extent include
its autophosphorylating activity, tegument incorporation,
phosphorylation of cellular elongation factor 1 delta (EF-
1?), evasion of the interferon (IFN) response, and phospho-
rylation of ganciclovir. Specific subfamily- and virus-specific
functions have also been described. Several of the CHPK
funtions are shown in Fig. 1, and different substrates are
shown in Table 1.
FIG. 1. Some of the major functions associated with the conserved herpesvirus protein kinase (CHPK). Red lines indicate functions that are
conserved over the three herpesvirus subfamilies. Green lines indicate functions that are common to beta- and gammaherpesviruses. Blue and
black lines indicate functions that have been reported only for alpha- and betaherpesviruses, respectively.
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(i) Autophosphorylation and tegument incorporation. Like
many cellular serine/threonine kinases, all herpesviral CHPK
orthologs appear to possess both autophosphorylation and
transphosphorylation activities (95). Autophosphorylation was
first demonstrated for the VZV CHPK by immunoprecipita-
tion experiments with infected cells and by protein kinase
The importance of autophosphorylation for the activity of
CHPK orthologs is not entirely clear. For example, there have
been conflicting reports on whether the autophosphorylating
activity of HCMV CHPK is required for phosphorylation of
exogenous substrates, such as histone 2B (10, 129, 141). Auto-
phosphorylation has been reported to be important for the
ability of the KSHV CHPK to activate JNK signaling (76).
All CHPK orthologs of HSV, VZV, HCMV, and EBV are
incorporated into the virus particle (7, 168, 219). More specif-
ically, the viral kinase is located in the tegument, a protein-
aceous layer in herpesvirus virions between the capsid and the
(ii) Phosphorylation of EF-1?. The translation elongation
factor 1 delta (EF-1?) consists of two forms, a hypo- and a
hyperphosphorylated form. In 1997, Kawaguchi et al. reported
that whereas the hyperphosphorylated form of EF-1? cells was
a minor species in mock-infected cells, it became the predom-
inant form in cells infected with HSV-1 (94). One year later,
the same group identified the HSV-1 CHPK as the kinase
responsible for the observed EF-1? hyperphosphorylation (98).
Hyperphosphorylation of EF-1? was subsequently observed for
members of the three different herpesvirus subfamilies, and
the HCMV CHPK ortholog could compensate for the HSV
CHPK to induce EF-1? hyperphosphorylation in HSV-1-in-
fected cells (97). EF-1? is a subunit of EF-1, a complex of
proteins which mediate the elongation of polypeptide chains
during translation of mRNA. Although the physiological role
of EF-1? hyperphosphorylation is not known, this function of
CHPK is likely to regulate and/or stimulate the translation
process in infected cells.
The cellular cyclin-dependent kinase cdc2 also phosphory-
lates EF-1?, and research on the interaction of CHPK with
EF-1? provided a first clue that CHPK orthologs to some
extent mimic the function and substrate specificity of cdc2. It
was demonstrated that different CHPK phosphorylate EF-1?
TABLE 1. Human CHPK and their cellular and viral substrates
HSV-1UL13 IRF-3 (82)
RNA polymerase II (125)
gE, gI (161)
ICP22/Us1.5 (184, 185)
UL13 (46, 47)
HSV-2 UL13Lamin A/C, limited (31)
VZV ORF47 Akt (186)ORF32 (187)
HCMV UL97Lamin A/C (130)
Rb tumor suppressor (81, 181)
RNA polymerase II (12)
Histone H2B (141, 210)
UL44 (109, 129)
HHV-6/HHV-7 U69Lamin A/C (111)
Rb tumor suppressor (111)
EBVBGLF4Lamin A/C (118)
Rb tumor suppressor (111)
IRF-3 (82, 225)
KSHV ORF36Lamin A/C (111)
Rb tumor suppressor (111)
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at the same amino acid residue (Ser-133) as for cdc2 (8, 96).
Other features of CHPK, discussed below, also point toward a
significant functional similarity between CHPK and cyclin-de-
pendent kinases. However, Cano-Monreal et al. reported that
the CHPK of the alphaherpesvirus HSV-2 did not phosphor-
ylate standard cdc2 peptide substrates in vitro, suggesting that
this similarity is less obvious in alphaherpesviruses (30). In
support of this are the findings that all human beta- and gam-
maherpesvirus CHPK, except for the KSHV CHPK, are able
to rescue a G1-to-S cell cycle defect in Saccharomyces cerevisiae
lacking cyclin-dependent kinase function, whereas alphaher-
pesvirus CHPK are unable to do so (81, 111).
(iii) Nuclear egress. Thus far, for all CHPK, a fraction has
been described to be present in the nucleus of infected cells
(47, 49, 71, 72, 84, 143, 169, 226), indicating that the kinase may
play important, conserved roles in the nuclear phase of infec-
A function that appears to be largely conserved is a role of
CHPK in nuclear egress of progeny capsids. Herpesvirus cap-
sids assemble in the nucleus and, because of their relatively
large size, need to (locally) disrupt the dense meshwork of the
nuclear lamina to gain access to the nuclear envelope to con-
tinue the process of virion assembly and egress. Disassembly
and reassembly of the nuclear lamina, a key process in cell
division, occurs through phosphorylation and dephosphoryla-
tion of lamins, e.g., by cyclin-dependent kinases and protein
kinase C. Phosphorylation of lamins during herpesvirus infec-
tion appears to involve both cellular kinases, such as protein
kinase C (144, 155, 170), but also the viral CHPK kinases. This
is best documented for beta- and gammaherpesvirus CHPK.
Indeed, both the CHPK of EBV and that of HCMV have been
reported to phosphorylate lamins (75, 118, 130), and lack of
these kinases is correlated with accumulation of capsids in the
nucleus (109, 180, 233). Interestingly, the CHPK of both EBV
and HCMV phosphorylate lamins on residues that are also
substrates for phosphorylation by cdc2 (75, 118), further high-
lighting the similarities in substrate specificity between the
beta- and gammaherpesvirus CHPK and cdc2. Recently, lamin
A/C phosphorylation was confirmed for all human beta- and
A role for CHPK of alphaherpesviruses in nuclear egress is
less clear, possibly because these viruses encode a second viral
kinase, US3, which appears to fulfill at least part of this func-
tion (see below). The CHPK of HSV-1 is still thought to have
some function in nuclear egress, albeit indirectly, by phosphor-
ylating the viral US3 kinase and thereby regulating the func-
tion of US3 in nuclear egress (91). In support of a combined
role of CHPK and US3, deletion of both in the genome of the
porcine alphaherpesvirus PRV resulted in strongly reduced
production of infectious virus (54). A direct effect of alphaher-
pesvirus CHPK on lamins and the nuclear lamina is either
absent or very limited. A recent study reported a failure to
detect phosphorylation of lamin A or disruption of the nuclear
lamina by HSV-1 and VZV CHPK (111). For HSV-2 CHPK,
a noticeable, but limited, effect on lamin phosphorylation and
nuclear lamina integrity was observed, in line with an earlier
report (31, 111).
(iv) Virus replication. Besides their role in viral nuclear
egress, CHPK have been reported to play other functions in
the nucleus that may affect virus replication efficiency.
In 2008, two groups independently showed that the HCMV
CHPK directly phosphorylates and thereby inactivates the ret-
inoblastoma tumor suppressor (Rb), again similar to one of the
functions of cdc2 (81, 181). Recently, Rb phosphorylation was
demonstrated for all human beta- and gammaherpesvirus
CHPK, but not the alphaherpesvirus counterparts (111). Nev-
ertheless, the HSV-1 CHPK has also been reported to affect
regulation of the cell cycle via an increase in cellular cdc2
HSV-1 CHPK also promotes the expression of a subset of
viral genes, including ICP22 and several late genes (2, 184,
185), although this effect appears to be independent of the
kinase activity of the protein (207). Interestingly, the CHPK of
the betaherpesvirus HCMV could compensate for the loss of
HSV CHPK with regard to its role in viral gene expression
(162), demonstrating that there is some functional overlap
between alpha- and betaherpesvirus CHPK members.
Deletion of the CHPK in HCMV results in reduced viral
gene expression (109, 233). In vitro kinase assays showed that
the HCMV CHPK is able to directly phosphorylate RNA poly-
merase II, which is also phosphorylated in HCMV-infected
cells (12). Inhibition of this phosphorylation by the addition of
roscovitine resulted in defective immediate-early gene expres-
sion, suggesting a causal relationship between RNA polymer-
ase II phosphorylation and increased viral gene expression
(205). However, it is unclear whether the effect of roscovitine
can be attributed solely to the CHPK. Roscovitine is a purine
derivative that also inhibits several cellular cyclin-dependent
kinases by competing with ATP binding, and as such is being
used in clinical trials as a treatment for various tumors (110,
Reduced viral gene expression observed in the absence of
the HCMV CHPK has also been attributed to the modest
reduction in viral DNA accumulation observed with a CHPK-
deleted virus (109, 233). This defect in DNA accumulation may
correlate with the finding that this CHPK phosphorylates the
viral DNA polymerase processivity factor UL44, an essential
component of the replication complex (109, 129). Like the
HCMV CHPK, the CHPK of EBV phosphorylates the DNA
processivity factor BMRF1, which is homologous to HCMV
UL44 (36, 65). Phosphorylation of BMRF1 may affect its trans-
activation activity (234). EBV CHPK also phosphorylates the
EBV coactivator EBNA-LP, one of the EBV proteins involved
in B-cell transformation. Phosphorylation of EBNA-LP is im-
portant for its function, but phosphorylation does not critically
rely on CHPK, since cellular cdc2 is able to phosphorylate
EBNA-LP on the same residue (93).
Effects of CHPK on viral gene expression may also be re-
lated to phosphorylation of histones. In vitro, HCMV CHPK
phosphorylates histone H2B, which has also been hypothesized
to affect gene expression (11). More recently, CHPK of EBV
and MHV-68 were found to induce phosphorylation of histone
H2AX, in contrast to the CHPK counterparts of the related
virus KSHV and the more distantly related virus HSV (209).
Phosphorylated H2AX is involved in the cellular DNA damage
response (28). Lack of MHV-68 CHPK or H2AX resulted in
inefficient MHV-68 replication in primary macrophages, indi-
cating that a virus-activated DNA damage response facilitates
MHV-68 lytic replication (209). Recently, the lack of CHPK or
H2AX was also correlated with decreased levels of MHV-68
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latency in mice (210). Clearly, more research is needed to
clarify the role of CHPK in viral gene expression, DNA repli-
cation, and viral pathogenesis.
(v) Evasion of the interferon response. A potential role for
CHPK in evasion of the IFN response was first suggested for
HSV-1. In 2001, Shibaki et al. reported that a CHPK-null virus
was rapidly cleared upon intraperitoneal inoculation of mice.
The authors showed that this correlated with increased levels
of type I IFN induced by the mutant virus and greater sensi-
tivity of the mutant toward the antiviral effects of type I IFN
(200). Later, HSV-1 was found to induce the expression of a
negative regulator of IFN, suppressor of cytokine signaling 3
(SOCS-3). Deletion of the CHPK resulted in less efficient
induction of SOCS-3 (236).
Recently, the involvement of CHPK orthologs in counter-
acting the interferon response was further substantiated and
was found to be conserved over the three different herpesvirus
subfamilies. The CHPK of HSV-1, HCMV, EBV, KSHV, and
MHV-68 were all found to subvert the type I IFN response by
suppressing the activity of interferon regulatory factor 3
(IRF-3) (82, 225). The viral kinase interacts with activated
IRF-3, thereby interfering with IRF-3 recruitment to the IFN
promoter (82, 225). Although the interaction of EBV CHPK
with IRF-3 leads to phosphorylation of IRF-3 (225), the kinase
activity of the CHPK does not appear to be absolutely required
for its effect on the IFN response (82).
(vi) Phosphorylation of viral tegument and membrane pro-
teins. CHPK-mediated phosphorylation of tegument proteins
(other than the CHPK itself) and envelope proteins is best
documented for alphaherpesviruses.
The CHPK of VZV is essential for infection of differentiated
human T cells and skin xenografts in SCID-hu mice (150). The
VZV CHPK phosphorylates the tegument proteins IE62 and
IE63 (101, 102, 160), and phosphorylation of IE62 has been
correlated with CHPK-mediated virulence in SCID-hu mice.
IE62 is the major immediate-early transactivating protein, and
the interaction between ORF47 and IE62, rather than IE62
phosphorylation per se, appears to be a pivotal element in
CHPK-mediated virulence in SCID-hu mice (19).
The VZV CHPK also phosphorylates the viral glycoprotein
gE (99). Kenyon et al. reported that ORF47-mediated phos-
phorylation of gE may be associated with the surprising in-
crease in virus spread observed for a CHPK-null VZV recom-
binant in MeWo cells. Phosphorylation by the CHPK
redirected gE to the plasma membrane instead of the trans-
Golgi network, where herpesvirus budding is thought to occur,
possibly explaining the suppressive effect on viral spread (99).
The HSV CHPK also phosphorylates gE and its complexing
partner gI (159). In addition, the CHPK of HSV phosphoryl-
ates the tegument proteins VP13/14 and VP22, which may
facilitate their release from the capsid during entry (45, 70,
152). Alphaherpesvirus CHPK-mediated phosphorylation of
tegument and/or cellular proteins in combination with phos-
phorylation by the alphaherpesvirus kinase US3 (see below)
may regulate the directionality of microtubule-based virus
transport in neuronal axons (39). HSV CHPK also phospho-
rylates the virion host shutoff protein (UL41), a viral tegument
protein involved in shutting down host translation. Inactivation
of CHPK resulted in the production of virions that did not
possess host shutoff activity, although they did incorporate
For HCMV, the CHPK interacts with and phosphorylates
the tegument protein pp65 in vitro (88). Although the biolog-
ical consequences of this phosphorylation are unclear, in the
absence of CHPK activity, HCMV-infected cells produce large
nuclear aggregates that contain substantial quantities of pp65
(179). The association of HCMV CPHK with pp65 most likely
explains the protein kinase activity associated with immuno-
precipitates of pp65 (25, 88), although pp65 itself has also been
suggested to display kinase activity (201, 235).
(vii) Phosphorylation of GCV. The remarkable success of
the nucleoside analogue acyclovir (ACV) in antiviral therapy
against HSV and VZV is based on its selectivity: the viral
thymidine kinase converts ACV to its monophosphate form
and cellular kinases then further process it to its active triphos-
phate form. Ganciclovir (GCV) was synthesized in 1980 at
Syntex Laboratories as an analogue of acyclovir with an im-
proved structural resemblance to natural nucleosides. GCV
turned out to display potent antiviral activity against HCMV.
This was somewhat surprising, since HCMV does not encode a
thymidine kinase-like enzyme. In 1992, the CHPK of HCMV
was found to be responsible for the conversion of GCV to its
monophosphate form (123, 202). Hence, the HCMV CHPK
possesses the unusual ability to act as both a protein kinase and
a nucleoside kinase. The lack of homology to any known nu-
cleoside kinase and the inability of the HCMV CHPK to phos-
phorylate natural nucleosides indicate that it is not a natural
nucleoside kinase (143). Besides GCV, this CHPK also phos-
phorylates ACV and penciclovir (238). The ability of CHPK
orthologs to phosphorylate GCV is conserved with various
levels of efficiency in some, but not all, betaherpesviruses and
gammaherpesviruses, but apparently not in alphaherpesviruses
(4, 29, 136, 142, 193, 222).
US3 Protein kinase orthologs. In the second part of the
1980s, the US3 orthologs of HSV-1 and -2 and VZV were
reported to show homology to eukaryotic protein kinases (133)
and soon thereafter, by using antisera and a kinase assay, HSV
US3 was identified as a bona fide viral protein kinase (67). The
US3 kinase is conserved throughout the alphaherpesvirus sub-
family but is not present in other herpesvirus genomes. The
consensus target sequence for US3 of the porcine alphaher-
pesvirus PRV was characterized as RnX(S/T)ZZ, where n is
greater than or equal to 2; X can be absent or Arg, Ala, Val,
Pro, or Ser; and Z can be any amino acid except an acidic
residue (116, 183) and was found to be broadly similar for
HSV-1 and -2 and VZV (48, 58, 183) and the cellular protein
kinase A (17, 89). Recent findings indicate that HSV-1 US3 is
more promiscuous than previously thought and suggest the
existence of more substrates than originally predicted (153).
US3 autophosphorylation at a site that corresponds to serine
147 in HSV-1 has been reported to affect US3 function and
localization in both HSV-1 and HSV-2 (90, 151). Several pos-
sible US3 phosphorylation substrates have been proposed, but
few have been confirmed as biologically relevant thus far.
Based on studies showing that recombinant US3 mutant
viruses have modestly impaired growth properties in cell cul-
tures but are severely attenuated in animal models, US3 was
identified as a positive regulator of viral replication and viral
pathogenicity (134, 191, 218). Several lines of evidence suggest
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the involvement of US3 in a variety of functions, some of which
are conserved among the alphaherpesviruses, while others ap-
pear to be unique for a specific virus. The kinase activity of
US3 was shown to be crucial for most of its functions. Different
functions of US3 are shown in Fig. 2.
(i) Nuclear egress. In 1995, a first report was published
pointing at the involvement of US3 in nuclear egress (221).
Alphaherpesviruses, and herpesviruses in general, use a unique
system to transport progeny nucleocapsids out of the nucleus
and into the cytoplasm. Nucleocapsids undergo primary envel-
opment by budding in the inner nuclear membrane, followed
by fusion with the outer nuclear membrane, releasing the
virion into the cytoplasm (reviewed in reference 140). In cells
infected with US3-null PRV, HSV-1, and Marek’s disease virus
(MDV), virions aggregate aberrantly within the perinuclear
space in large invaginations (105, 191, 198, 221), suggesting a
conserved role for the US3 kinase in the de-envelopment step
during nuclear egress. This defect in capsid nuclear export is,
however, not absolute, since extracellular virus titers are typi-
cally only mildly reduced in the absence of US3 (39, 191, 194,
Since these initial reports, US3 has been implicated in dif-
ferent steps of the nuclear egress pathway. First, lamin A/C
and emerin, key elements of the nuclear lamina network, can
be phosphorylated by HSV-1 US3 (114, 153). This leads to the
disruption of the nuclear lamina, which represents a barrier for
the virions to reach the inner nuclear membrane. Second,
infection with US3-null HSV-1 or PRV results in an altered
FIG. 2. Major functions associated with US3 kinase orthologs of alphaherpesviruses. Proteins in blue represent US3 substrates that may be
involved in the different functions of US3.
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distribution of the viral UL34 and UL31 proteins, both of
which are crucial regulators of primary envelopment of nucleo-
capsids at the nuclear membrane, from a roughly continuous
distribution to a distribution in discrete aggregates in the inner
nuclear membrane (105, 190). This relocalization of the envel-
opment machinery was recently found to be regulated by phos-
phorylation of the N terminus of UL31 by HSV-1 US3 (154).
A third potential involvement of US3 in nuclear egress has also
been put forward. HSV-1 US3 phosphorylates the cytoplasmic
domain of the envelope glycoprotein gB (83). The gB protein
is one of the viral proteins that is essential to mediate fusion
between envelope and host membrane during viral entry. For
HSV-1, gB, together with gH, has been suggested to be in-
volved in the fusion between the primary enveloped virion in
the perinuclear space and the outer nuclear membrane (232).
Recent evidence indicates that phosphorylation of gB by US3
drives this process (232). The latter may not be a conserved
function of alphaherpesviruses since, for PRV, neither gB nor
gH appears to be present at the nuclear membranes and nei-
ther protein functions in the nuclear egress of virions (74, 104).
(ii) Antiapoptotic activity. US3 possesses antiapoptotic
properties in HSV, PRV, and MDV, but possibly not in bovine
herpesvirus 1 (BoHV-1) (9, 69, 119, 197, 204). The kinase
activity of HSV-1, PRV, and MDV US3 is necessary for this
US3 function (33, 52), and several phosphorylation targets
implicated in the HSV-1 US3-mediated suppression of apop-
tosis have been identified, including Bad, Bid, and procaspase
3, pointing at the involvement of US3 in different antiapoptotic
pathways (18, 32). This may explain how US3 is able to inhibit
apoptosis induced by very diverse apoptotic stimuli, including
herpesvirus infection itself, cytotoxic T lymphocytes and gran-
zyme B, overexpression of BclX, staurosporine, and sorbitol (9,
32, 33, 78, 87, 119, 157, 165).
(iii) Actin rearrangements. The US3 kinase induces drastic
rearrangements of the cytoskeleton, including disassembly of
actin stress fibers and the formation of actin- and microtubule-
containing cellular projections, which are implicated in inter-
cellular spread (63). The kinase activity of US3 is essential for
these processes in HSV-2, PRV, and BoHV-1, but not in
MDV, suggesting different or multiple cytoskeleton-affecting
activities of US3 (27, 66, 156, 197, 215). Recent evidence with
PRV US3 indicates that the kinase interferes with actin-con-
trolling Rho-GTPase signaling to cause the cytoskeletal alter-
ations. PRV US3 phosphorylates and thereby activates p21-
activated kinases (PAKs), key regulators of Rho GTPase
(iv) Gene expression. Over the last couple of years, it has
become evident that US3 also affects gene expression. US3
orthologs of HSV-1, HSV-2, PRV, and VZV phosphorylate
HDAC1 and -2, thereby inhibiting the deacetylation of his-
tones, which otherwise silence gene expression (151, 176, 223,
224). US3-mediated phosphorylation of HDAC2 occurs at a
C-terminally located conserved serine residue (224). HSV-1
US3 is able to phosphorylate HDAC-1 and -2 directly (177,
178), although there are indications that VZV, PRV, and
HSV-1 phosphorylate HDAC indirectly by activating a cellular
kinase pathway (223, 224). For PRV, hyperphosphorylation of
HDAC2 was still observed in cells infected with a US3-null
recombinant (224). This is in contrast to findings for HSV-1
and VZV and indicates virus-specific US3-independent mech-
anisms of HDAC hyperphosphorylation. Inhibition of HDAC
activity increased plaquing efficiency of US3-null virus for PRV
and VZV, but not for HSV-1, pointing to virus- and cell-
dependent differences in the functional significance of US3-
mediated HDAC modification (223, 224).
An additional role in gene expression has been described for
the US3 ortholog of VZV, ORF66. Both ORF66 and the
CHPK of VZV have a unique viral substrate, IE62, a nuclear
transcription regulatory protein. After phosphorylation by
ORF66, IE62 accumulates in the cytoplasm, where it is incor-
porated in newly formed virions. This points at a role for the
VZV US3 ortholog in reducing nuclear import of IE62 and
thereby regulating gene expression (58, 60).
(v) Immune evasion. Apart from their ability to protect cells
from apoptotic cell death, US3 orthologs have been linked to
other aspects of evasion of the host immune response. The
VZV ORF66 was reported to be involved in downregulating
the surface expression of the major histocompatibility complex
class I (MHC-I) in VZV-infected cells and ORF66-transfected
cells (1, 59), suggesting a role for ORF66 in immune evasion.
The kinase activity of ORF66 protein kinase was beneficial but
not absolutely required for MHC-I downregulation (59). Re-
cently, PRV US3 was also shown to be able to downregulate
MHC-I surface expression. However, this ability was highly
cell-type dependent. The underlying mechanism is unclear but
appears different from that for VZV ORF66 since transfection
of PRV US3 did not affect MHC-I downregulation (53). Still,
the potential effect of US3 orthologs on MHC-I surface ex-
pression appears to be cell-type dependent and not always very
robust; therefore, further investigation may be needed to de-
termine its importance.
For VZV- and PRV-infected cells, the lack of US3 reduced
the capacity of the virus to interfere with induction of the IFN
signaling pathway following exposure to IFN (175, 195). Re-
cently, Peri et al. reported that US3-null HSV-1 induces a
strong activation of IRF-3 and type I IFN mRNA expression
and that HSV-1 US3 downregulates the intracellular expres-
sion of TLR3 and the type I interferon-inducible protein MxA,
which is known to posses antiviral activity (171). In addition,
HSV-1 US3 phosphorylates the gamma interferon (IFN-?)
receptor, which interferes with IFN-?-dependent gene expres-
Ribonucleotide reductase. Alphaherpesviruses, like other
herpesviruses, encode a ribonucleotide reductase that is in-
volved in generating deoxyribonucleotides. Therefore, these
viruses do not depend on cellular S-phase enzymes, allowing
them to replicate in nondividing cells, including neuronal cells
(34). The ribonucleotide reductase consists of two subunits, R1
and R2. The large R1 subunit (ICP10) of the HSV-2 ribonu-
cleotide reductase has been suggested to contain protein ki-
nase activity, which may be responsible for the antiapoptotic
activity associated with ICP10 (173). Inhibition of apoptosis by
ICP10 involves a c-Raf-1-dependent mechanism and induction
of the antiapoptotic protein Bag-1 by the activated ERK
survival pathway. ICP10 is also responsible for an increased
activation/stability of the transcription factor CREB and
stabilization of the antiapoptotic protein Bcl-2 (172). This
antiapoptotic function could be exploited in cancer therapy,
based on the observation that an HSV-2-based oncolytic virus
with a deletion of the putative protein kinase domain of the
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on August 14, 2013 by UNIVERSITEIT GENT/UZGENT
ICP10 gene is a potent inducer of apoptotic death in tumor
cells (41, 68). Although ICP10 indisputably displays antiapop-
totic activity, there has been some controversy as to whether
the protein contains intrinsic protein kinase activity. Based on
biochemical studies, it has been suggested that the observed
kinase activity is caused by contaminating cellular kinases, in
particular casein kinase II (34, 44, 112).
The Poxviridae family can be subdivided into the Ento-
mopoxvirinae, which infect insects, and the Chordopoxvirinae,
which infect vertebrates. All poxviruses are double-stranded
DNA viruses with an envelope and a very large genome, rang-
ing from 130 to 360 kb. All poxviruses encode serine/threonine
kinases. Unlike the vast majority of DNA viruses, poxviruses
replicate in the cytoplasm since they encode all necessary rep-
lication enzymes and therefore do not rely on cellular nuclear
enzymes. Poxviruses share this property with all members of a
proposed clade of very large DNA viruses of eukaryotes, the
nucleo-cytoplasmic large DNA viruses (NCLDV). Poxviruses
and African swine fever virus are the only NCLDV with mam-
malian hosts (106). Several poxviruses have been reported to
infect humans. Most notable is the smallpox virus (variola
virus), for which humans are the only known natural host.
Smallpox has caused millions of deaths and has had a major
impact on human history (132). Although smallpox was erad-
icated in 1980, the virus is still considered to be a threat as a
potential bioterrorist weapon (6). Other poxviruses able to
infect humans include vaccinia virus (origin unknown; used for
immunization against smallpox), cowpox virus (causes ulcer-
ative lesions on the hands of dairy workers), molluscum con-
tagiosum virus (causes infectious warty papules of the skin),
monkeypox virus (causes rare smallpox disease of children in
central Africa), pseudocowpox virus (causes nonulcerating
nodules on the hands of dairy workers), and Orf virus (causes
single lesions on the skin of people handling sheep and goats).
Major functions associated with conserved poxviral kinases are
shown in Fig. 3.
B1 kinase. Almost 30 years ago, a wide array of temperature-
sensitive mutants of vaccinia virus, the Condit collection, was
constructed (42, 43). The ts2 and ts25 mutants were found to be
arrested at the stage of DNA replication, and the mutations
were located in the B1 gene (42, 189). Sequencing of this
region of the genome revealed that the B1 gene displays strong
homology to serine/threonine kinases (80, 212). Soon thereaf-
ter, B1 was indeed confirmed to encode a catalytically active
viral serine/threonine kinase of 34 kDa, which was expressed
early in infection, associated with cytoplasmic virus factories,
and incorporated as a minor component in the virion (14, 122).
Interestingly, based on sequence homology, identification of
B1 resulted in the subsequent discovery of a novel class of
cellular serine/threonine kinases that share up to 40% amino
acid identity with B1, the vaccinia virus B1 kinase-related ki-
nases (VRKs) (158, 163). VRK orthologs are present in mam-
FIG. 3. Major functions associated with B1 and F10 kinases of poxviruses. Proteins in orange represent phosphorylation substrates that may
be involved in the different functions of B1 and F10.
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on August 14, 2013 by UNIVERSITEIT GENT/UZGENT
mals, fruit flies, and nematodes, but apparently not in yeast.
They reside predominantly in the nucleus, where they fulfill
several crucial roles in successful DNA replication, mitosis,
and gene expression (reviewed in reference 103).
Nearly all known poxviruses encode B1 orthologs; the ex-
ception is molluscum contagiosum virus (199). Although not
proven as of yet, it is possible that the molluscum contagiosum
virus is able to compensate for the lack of B1 by usurping
related host kinases, such as VRKs (108). In line with this idea,
expression of the VRK1 genes of humans and, to a lesser
extent, mice in the B1-defective ts2 mutant of vaccinia virus
rescues the inhibition of DNA replication, virus production,
and plaque formation observed with this mutant (24).
The phenotypes associated with temperature-sensitive mu-
tants of B1 show that the kinase is critically involved in at least
two key stages of the virus cycle: DNA replication and produc-
tion of intermediate viral proteins (108, 189).
(i) DNA replication. In 2007, a paper by Wiebe and Trakt-
man provided fascinating insights into the mechanism of B1-
mediated vaccinia virus DNA replication and, at the same
time, uncovered a novel aspect of the innate, cellular antiviral
response (229). The paper showed that B1 acts on barrier-to-
autointegration factor (BAF). BAF is a DNA-binding protein,
known to participate in nuclear reorganization and reassembly
during mitosis by bridging chromatin to the nuclear membrane
(reviewed in reference 128). The B1-related VRKs had already
previously been shown to be able to phosphorylate BAF,
thereby abrogating the DNA-binding abilities of BAF (164). In
cells infected with vaccinia virus, but not with the B1-defective
ts2 mutant, BAF phosphorylation was increased and BAF was
prevented from association with virus DNA replication sites.
Importantly, RNA interference-mediated silencing of BAF
rescued the defect in virus growth of the ts2 mutant. Hence,
cytoplasmic BAF likely constitutes a novel component of the
innate viral response, acting by binding and thereby sequester-
ing foreign, cytoplasmic DNA. B1 allows vaccinia virus to over-
come this defense barrier of the cell by phosphorylating BAF,
thereby interfering with its DNA-binding abilities.
(ii) Production of intermediate viral proteins. The mecha-
nism underlying the role of B1 in expression of viral interme-
diate proteins (108) is less clear. Likely important in this re-
spect is that B1 phosphorylates viral late gene transcription
factor 4 (VLTF-4; H5) (15, 16). Although H5 is a late gene
transcription factor, B1-mediated phosphorylation of H5 may
also affect intermediate protein synthesis, since H5 is the only
VLTF that is expressed prior to DNA replication (107). Since
H5 is also involved in transcription elongation (via interaction
with A18 and G2), DNA replication (via interaction with A20),
and virion morphogenesis, phosphorylation by B1 may also
affect these aspects of the virus cycle (23, 50, 131). However,
arguing against a direct involvement of B1-mediated H5 phos-
phorylation in DNA replication is the finding that rescue of
DNA replication in the ts2 mutant by expression of the B1-like
VRK1 was not associated with H5 phosphorylation (24). An
alternative role for the B1/H5 interaction may lie in evasion of
the innate antiviral response, since both proteins appear to be
involved in inhibiting CD1d-mediated antigen presentation to
natural killer T (NKT) cells (228).
Besides the effects of B1 on BAF and H5, the viral kinase
has been shown to phosphorylate two ribosomal proteins, Sa
and S2, which may have consequences on the efficiency of viral
versus cellular translation (13). In vitro kinase assays have
shown that the A30 protein, which is involved in virion mor-
phogenesis, may also be a B1 substrate (139).
F10 kinase. In 1994, a second kinase, encoded by the F10
gene, was identified in the vaccinia virus genome. The F10
kinase has a molecular weight of 50 kDa, and as is the case for
B1, the kinase is incorporated into virions and is essential for
viability of the virus. The majority of kinase activity packaged
in the vaccinia virion core is the product of the F10 gene (121).
F10 is strongly conserved in poxvirus genomes, with orthologs
occurring not only in chordopoxviruses but also in entomopox-
viruses (57). Unlike B1, F10 shows very little similarity to
cellular protein kinases and has a very atypical structure, which
probably explains the relatively late discovery of this viral ki-
nase. Kinase subdomains II (ATP-binding pocket) and VI (cat-
alytic loop) are the only typical kinase domains that are obvi-
ously recognizable in F10 (182). F10 has been characterized as
a dual-specificity kinase, a relatively rare type of kinase that is
able to phosphorylate not only serine/threonine but also ty-
rosine residues (51).
Experiments using temperature-sensitive mutants from the
Condit collection showed that lack of F10 did not prevent
genome replication or gene expression, but halted the virus at
the earliest points of virion morphogenesis (213, 227). Subse-
quent studies revealed that F10 phosphorylates, and thereby
influences several key viral factors involved in wrapping of
electron-dense virosomes by membrane crescents to form the
immature virion. F10 phosphorylates two viral membrane pro-
teins that are critical in early morphogenesis, A14 and A17,
which is thought to be required to form the membranes asso-
ciated with immature virions (20, 51, 138). Together with viral
proteins A30 and G7, F10 makes part of a viral assembly
complex, which stimulates F10-mediated A17 phosphorylation
(203). Additional components of this viral assembly complex
were subsequently identified and include J1, A15, D2, and D3
(37, 203). A30, G7, and F10 itself are phosphorylated in an
F10-mediated manner in this complex (139, 203). In vitro ki-
nase assays confirmed A30 as a potential substrate for F10,
whereas G7 was not phosphorylated by F10 in vitro, leaving the
possibility that F10-mediated G7 phosphorylation occurs via a
cellular kinase or occurs only in vivo (139).
In vitro kinase assays showed that F10 can phosphorylate
E8R and thereby abrogates the DNA-binding capacity of this
protein (55). Although there are indications that F10-mediated
E8R phosphorylation may occur inside virus particles, thereby
potentially facilitating DNA exit from the core during virus
entry, this remains to be formally demonstrated (55).
Members of the family Baculoviridae are large, double-
stranded, enveloped DNA viruses that infect mainly insects. In
1983, Miller and coworkers reported the presence of serine/
threonine kinase activity associated with both the extracellular
and the occluded forms of a baculovirus (145). Until now, only
one protein kinase gene has been identified in all baculovirus
genomes sequenced: PK1. Sequences of PK1 of baculoviruses
Spodoptera litura nucleopolyhedrovirus (SpltNPV) (148), Cho-
ristoneura fumiferana granulovirus (ChfuGV) (73), Lymantria
on August 14, 2013 by UNIVERSITEIT GENT/UZGENT
dispar nuclear polyhedrosis virus (LdNPV) (22), Helicoverpa
armigera single nucleopolyhedrovirus (HaSNVP) (237), and
Autographa californica nucleopolyhedrovirus (AcNPV) (188)
were analyzed and were found to contain typical serine/threo-
nine kinase domains. Recombinant PK1 of AcNPV, SpltNPV,
and LdNPV phosphorylates exogenous substrates, such as hi-
stone H1 and myelin basic protein (MBP) (22, 148, 188). Based
on genome sequence analyses, the existence of a viral protein
kinase in Anagrapha falcifera multinuclear polyhedrosis virus
(AfMNPV) and Neodiprion lecontei nucleopolyhedrovirus
(NeleNPV) was also predicted (64, 113). Baculovirus kinase
activity was first suggested to be essential for release of viral
DNA from virions during entry (230, 231). More recently, PK1
has been shown to regulate transcription from very late pro-
moters, such as the polyhedrin (polh) and p10 promoters. This
was first reported in a study of the temperature-sensitive
AcNPV mutant (62) and was recently confirmed for both Ac-
NPV and SpltNPV (146–149). AcNPV PK1 also plays a role in
the phosphorylation of the LEF8 protein, which is a compo-
nent of the very late gene transcription initiation complex
Apart from the above-mentioned viruses, viral kinases have
been described or predicted based on homology to kinase
domains in several other large DNA viral families, including
the asfavirus, nimavirus, phycodnavirus, iridovirus, and mim-
ivirus families. In irido- and phycodnavirus families, multiple
copies of protein kinase genes were identified (85, 211). A
comparison of nine sequenced iridoviruses revealed the pres-
ence of two conserved S/T kinases (56).
Phylogenetic trees have been constructed based on the pox-,
herpes-, asfa-, nima-, phycodna-, irido-, and baculovirus pro-
tein kinase sequences to obtain new insights in the evolution of
the PK genes and/or taxonomic classification of viruses (124,
217), which confirm broadly the evolutionary relationships of
the different virus families. Although the existence and appar-
ent conservation of viral protein kinases in the above-men-
tioned viral families point toward potentially important roles
during their replication, further research is needed to address
their exact biological roles.
One of the curious aspects of viral serine/threonine kinases
is that they appear to be encoded exclusively by large and
evolutionarily old DNA viruses, such as herpesviruses, poxvi-
ruses, and baculoviruses.
Trying to explain this requires some speculation. One pos-
sibility would be that viral kinases merely are an “optional
feature” of viruses, encoded only by those viral species that
have a large enough genome to be able to afford them. Alter-
natively, they may be evolutionary relics, present in many of
the evolutionarily ancient, complex DNA viruses and selected
away in viruses of more recent origin. The fact that all studied
viral kinases constitute important virulence factors argues
against this possibility in favor of a third possibility: viral
serine/threonine kinases may actually be one of the key ele-
ments of these old, large DNA viruses and may have aided
them in still being around after millions of years of host evo-
Indeed, evolutionarily very distinct members of the herpes-
viruses, with natural hosts ranging from koi to humans, are
predicted to encode viral kinases (5). Throughout the tremen-
dous evolution of their hosts, herpesviruses have successfully
adapted themselves and kept their ability to establish lifelong
infections. Apparently, viral kinases were not selected away in
the evolutionary process but instead may have aided in the
adaptation of these viruses to the increasing complexity of their
Intrinsic to the limited size of their genome, in order to be
able to replicate and spread in a host, viruses have to do a lot
with a little. Hence, it comes as no surprise that many viral
proteins are multifunctional. This is especially true for viral
kinases, which have the ability to phosphorylate, and thereby
alter, several viral and cellular targets, which may result in a
plethora of different effects. Some of these effects are inten-
tional and beneficial for viral replication, and others may ac-
tually be side effects that are biologically irrelevant and thereby
artifactual. One of the main tasks in the field of viral kinase
research will be to better discriminate these possibilities. This
is especially challenging since the major roles of viral kinases in
pathogenesis do not appear to translate easily to obvious phe-
notypes in vitro. Indeed, as indicated above, many kinases,
including the herpesvirus kinases, are often not absolutely re-
quired for growth in cell culture, although lack of viral kinase
activity results in strongly reduced virulence. One important
issue may be that, in contrast to the typical immortalized cell
cultures, many of the viral target cells in vivo comprise highly
differentiated, nonproliferating cells. These in vivo target cells
are therefore less suitable for viral replication, which may
increase the importance of viral kinases, especially since many
of these kinases have been reported to promote viral replica-
tion and gene expression. As an example to support this idea,
mutation of the CHPK kinase of VZV does not substantially
affect viral growth in cell culture, whereas it is essential for
replication in human T cells and human thymus/liver xe-
nografts in SCID-hu mice (150). Also indicative of the caution
needed when interpreting results from immortalized cell lines
is that SV40 T antigens have been reported to induce lamin
A/C phosphorylation in 293T cells, thereby compensating for
the defect in viral egress observed for a CHPK-null EBV re-
combinant in 293 cells (137).
In order to meet the need to better define the biological
importance of individual functions of viral kinases, several
approaches may be followed. Studies of important primary
target cells and/or organ cultures may highlight which kinase-
dependent phenotypes manifest themselves in these cells.
Since many of the kinase targets are cellular proteins, homol-
ogous studies, using primary cells of the natural host of the
virus, will be particularly informative. In addition, a search for
mutations in the viral kinases that abrogate their interaction
with specific, but not all, substrates may be vital in revealing the
importance of individual functions of viral kinases. Alterna-
tively, complementing kinase deficiency with cellular or viral
factors that restore only selected functions of the kinase should
also yield important data in this respect. For example, replac-
ing the US3 gene in alphaherpesviruses with specific antiapop-
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totic factors will allow us to better define the importance of the
antiapoptotic activity of this kinase in virus biology.
There are several indications that viral serine/threonine pro-
tein kinases may be promising future targets for the develop-
ment of antiviral drugs. First, although sometimes not associ-
ated with obvious phenotypes in cell culture, as described
above, all viral protein kinases identified appear to play im-
portant and frequently essential roles in virus virulence. Sec-
ond, and perhaps equally important, most viral serine/threo-
nine protein kinases are evolutionarily very distinct from
cellular protein kinases, which should facilitate the selection of
drugs that do not interfere with cellular kinase activity.
A first proof of principle that viral kinases may represent
interesting drug targets is maribavir. Maribavir is a drug di-
rected against the UL97 kinase of HCMV and displays potent
antiviral activity against HCMV (21). It is important to keep in
mind that the drug of preference used against HCMV is GCV
and that UL97 plays an important role in activating GCV by
phosphorylating it to its monophosphate form (123, 202).
Hence, the two treatment strategies may interfere with each
other (38, 61). Maribavir successfully passed phase I and phase
II clinical trials (214). Although this demonstrates the poten-
tial of viral kinases as therapeutic targets, unfortunately,
maribavir very recently did not pass phase III clinical trials
conducted by Viropharma. Despite this mishap, viral serine/
threonine protein kinases should still be considered one of the
promising avenues for the design of novel antiviral drugs (3).
One example may be the B1 kinase, which is of critical impor-
tance in the poxvirus replication cycle. A first indication for the
potential of B1 as a target for antiviral therapy was demon-
strated, since interfering RNAs that target B1 effectively re-
duced vaccinia and monkeypox virus yields and plaques in in
vitro assays, especially when administered together with the
proven antiviral drug cidofovir (220).
An aspect of viral kinases that may warrant further research
is their effect on cellular signaling networks and cascades.
Phosphorylation and dephosphorylation arguably represent
the most important regulatory switches of signaling proteins,
affecting virtually every aspect of the biology of a cell and the
host organism (174). Up to now, relatively little information
has been available on the potential effect of viral serine/threo-
nine kinases on cellular signaling cascades. This is even more
relevant when considering that most, if not all, of these viral
kinases are incorporated in the virus particle. Although largely
unexplored at this time, there is thus the possibility that viral
kinases may affect cellular signaling cascades at the very early
stages of infection, during viral entry, thereby preparing and
perhaps steering the otherwise hostile cellular environment to
their own benefit and needs.
Obviously, there is a need for more research on viral protein
kinases to fully address their invariably multifunctional and
pivotal roles in viral replication, spread, and immune evasion.
This information, together with rational drug design strategies,
is needed to explore the therapeutic potential of these viral
Work by the authors has been supported by grants from the Fonds
(grants G.0196.06 and G.0835.09) and a concerted research action
from the special research fund of Ghent University. C.V.D.B. is sup-
ported by a postdoctoral grant by the FWO-Vlaanderen.
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