JOURNAL OF VIROLOGY, Sept. 2009, p. 8998–9001
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
Vol. 83, No. 17
Differential Rates of Protein Folding and Cellular Trafficking for the
Hendra Virus F and G Proteins: Implications for
F-G Complex Formation?
Shannon D. Whitman, Everett Clinton Smith, and Rebecca Ellis Dutch*
Department of Molecular and Cellular Biochemistry, University of Kentucky, Lexington, Kentucky 40536
Received 25 February 2009/Accepted 5 June 2009
Hendra virus F protein-promoted membrane fusion requires the presence of the viral attachment protein,
G. However, events leading to the association of these glycoproteins remain unclear. Results presented here
demonstrate that Hendra virus G undergoes slower secretory pathway trafficking than is observed for Hendra
virus F. This slowed trafficking is not dependent on the G protein cytoplasmic tail, the presence of the G
receptor ephrin B2, or interaction with other viral proteins. Instead, Hendra virus G was found to undergo
intrinsically slow oligomerization within the endoplasmic reticulum. These results suggest that the critical F-G
interactions occur only after the initial steps of synthesis and cellular transport.
The Henipavirus genus of the paramyxovirus family com-
prises two recently emerged, zoonotic pathogens. Hendra vi-
rus, first identified in Australia in 1994, caused respiratory
illness in and the subsequent death of over one dozen horses
and two of the three humans infected (12, 21, 25). Nipah virus
led to an outbreak of respiratory and encephalitic illnesses in
Malaysia in 1999, affecting both swine and humans and leading
to fatality in 105 of the 265 human cases (11). Additional
periodic outbreaks of infections with these viruses have oc-
curred (11), and evidence indicates human-to-human transmis-
sion of Nipah virus in at least one outbreak (16). Henipavirus,
like many paramyxoviruses, requires the presence of two sur-
face glycoproteins for virus-cell and cell-cell fusion (8, 9, 36):
the fusion protein, F, which mediates the membrane fusion
event, and the attachment protein, G, which binds cellular
receptors ephrin B2 (6, 22) and ephrin B3 (23) and which is
required for F-mediated membrane fusion. Interactions be-
tween the fusion and attachment proteins of a number of
paramyxoviruses have been observed previously (30, 33, 35),
and interactions between the henipavirus F and G proteins
have been demonstrated by coimmunoprecipitation (1–5, 18).
However, important questions remain concerning the timing of
these interactions and the mechanism by which the attachment
protein regulates F-mediated fusion. Results from studies of
measles virus (30), Newcastle disease virus (33), and human
parainfluenza virus (35) have suggested that the initial inter-
action between the two glycoproteins occurs within the endo-
plasmic reticulum (ER) at the time of synthesis, potentially
allowing the attachment protein to hold the F protein in its
prefusion conformation. In contrast, the retention of the para-
influenza virus type 5 (PIV5) F protein in the ER does not lead
to the retention of the PIV5 attachment protein (29), suggest-
ing that interaction between these glycoproteins does not occur
soon after synthesis. Recently, henipavirus F proteins have
been shown to undergo processing through a complex intra-
cellular trafficking pathway, with expression on the cell surface
in a nonfusogenic precursor form (F0), subsequent endocyto-
sis, cleavage by cathepsin L into the fusogenic (F1?F2) form,
and retrafficking to the plasma membrane (10, 19, 26–28).
While the half-life (t1/2) of F protein uncleaved by cathepsin L
is approximately 2 h (28), results from initial studies of Hendra
virus G trafficking indicate much slower trafficking of this pro-
tein through the secretory pathway (37), a result inconsistent
with the formation of an F-G complex in the ER.
To more closely examine the trafficking of the henipavirus
glycoproteins, endoglycosidase H (endo H) analysis was used
as a marker for trafficking time to the medial-Golgi compart-
ment. We previously reported that wild-type (wt) Hendra virus
G protein becomes endo H resistant, with a t1/2of between 2
and 3 h (37), suggesting slow trafficking through the secretory
pathway. Porotto et al. (31) described a mutant G protein
lacking the first 32 residues of the cytoplasmic tail (G-?32),
which showed enhanced fusion promotion. This Hendra virus
G-?32 mutant also exhibited higher overall expression than the
wt Hendra virus G (S. D. Whitman and R. E. Dutch, unpub-
lished results). To determine if this mutant exhibited altered
trafficking kinetics, Vero cells transfected with pCAGGS-Hen-
dra G-?32 were examined by pulse-chase analysis followed by
endo H treatment as described previously (37). Trafficking
kinetics similar to those of wt G were observed, as G-?32
became endo H resistant, with a t1/2of approximately 2 to 3 h
(Fig. 1A). In contrast, endo H analysis of Hendra virus F
revealed a resistant population of F0within 30 min (Fig. 1B),
with the majority of F converted to either an F0endo H-
resistant form or to the F1cleaved form by 2 h. The cleavage
of F0was observed concomitantly with the appearance of two
partially endo H-resistant F1bands, suggestive of differential
complex sugar additions within the Golgi compartment. These
data confirm that Hendra virus F and G traffic through the
secretory pathway at different rates, with Hendra virus G traf-
ficking unaffected by the 32-amino-acid deletion. Endo H anal-
* Corresponding author. Mailing address: Department of Molecular
and Cellular Biochemistry, University of Kentucky, College of Medi-
cine, Biomedical Biological Sciences Research Building, 741 South
Limestone, Lexington, KY 40536-0509. Phone: (859) 323-1795. Fax:
(859) 323-1037. E-mail: email@example.com.
?Published ahead of print on 24 June 2009.
ysis of the Nipah virus G dimeric form (Fig. 1C), for which the
mobility shift after endo H treatment was most apparent, mir-
rors that of wt Hendra virus G, suggesting that slow trafficking
through the secretory pathway is a property of both henipavi-
rus G proteins.
Slow trafficking of Hendra virus G through the secretory
pathway may be caused by interactions with other viral pro-
teins or with its receptor ephrin B2. Chinese hamster ovary
(CHO) cells do not express ephrin B2 (39) and thus were
utilized to examine Hendra virus G trafficking in the absence of
its cognate receptor. When Hendra virus G-?32 was expressed
in CHO cells, an endo H-resistant population appeared at 2 h
(Fig. 2A) and was found to have increased at subsequent time
points, consistent with results obtained using Vero cells. These
data suggest that the low trafficking rate of Hendra virus G is
not due to association with ephrin B2 and is not cell type
specific. Coexpression of either the Hendra virus matrix (M)
protein or the F protein with G-?32 did not alter trafficking
kinetics, indicating that an interaction with either M or F is not
responsible for the low rate of G-?32 trafficking (Whitman and
Lysine-rich motifs (KKXX or KXKXX) have been shown
previously to be involved in the retention of type I glycopro-
teins in the ER (17), while motifs containing multiple arginines
have been implicated in the retention of type II integral mem-
brane proteins (20, 32). The type II Hendra and Nipah virus G
proteins contain no and one arginine residue in their cytoplas-
mic domains, respectively, but do have several lysine-rich mo-
tifs. To determine if these motifs facilitated ER retention of
Hendra virus G, thus resulting in a lowered trafficking rate, an
additional mutant (Hendra virus G-?41) with the removal of
the first 41 amino acids of the cytoplasmic tail, deleting both
KKXX and KXKXX motifs and the majority of the cytoplas-
mic tail, was constructed. Consistent with the results from endo
H analyses of wt Hendra virus G and Hendra virus G-?32, an
endo H-resistant form of Hendra virus G-?41 appeared after
only 2 h (Fig. 2B), suggesting that Hendra virus G is not
retained in the ER by the putative ER retention motifs KKXX
and KXKXX or by any other sequence in the cytoplasmic tail.
Endo H data strongly indicated differential rates of traffick-
ing through the secretory pathway for Hendra virus G and
Hendra virus F, suggesting that interactions between these two
proteins likely occur subsequent to transport to the cell sur-
face. To verify the different trafficking kinetics of these two
proteins, the rates at which Hendra virus F and G appear on
the cell surface were determined by using surface biotinylation,
with the more highly expressed mutant Hendra virus G-?32
utilized in these experiments to facilitate visualization of the
surface population. Hendra virus F was present on the cell
surface as F0at the completion of the 30-min labeling (Fig. 3A)
(time zero), with F1?F2appearing on the surface at approxi-
mately 2 h (Fig. 3A), consistent with the complex trafficking
pathway observed for F. In contrast, the majority of Hendra
virus G-?32 arrived on the cell surface after the 2-h time point
(Fig. 3B). Similar cell surface profiles were observed when the
proteins were coexpressed (Fig. 3C). These data confirm the
differential trafficking rates and suggest that Hendra virus F
and G do not interact within the secretory pathway but traffic
independently to the cell surface.
Exit out of the ER is a carefully controlled process, allowing
only properly folded proteins to be transported to the Golgi
FIG. 1. Endo H digestion indicates differential rates of trafficking
for the henipavirus F and G proteins. (A) Vero cells were transfected
with pCAGGS-Hendra G-?32, and 24 h posttransfection, the cells
were labeled for 30 min and chased for various times and Hendra virus
G was immunoprecipitated using an antibody directed to a soluble
form of G (7). Endo H digestion was performed as described previ-
ously (37). Proteins were separated via sodium dodecyl sulfate-poly-
acrylamide gel electrophoresis and visualized using the Typhoon im-
aging system. (B) Vero cells were transfected with pCAGGS-Hendra
F. Twenty-four hours later, the cells were labeled for 30 min. Hendra
virus F was immunoprecipitated with a Hendra virus F-specific anti-
body (28) and subjected to endo H treatment. (C) Pulse-chase analysis
of Vero cells transfected with pCAGGS-Nipah G was performed.
Immunoprecipitation with antibody to a soluble form of G (7) and
endo H analysis were performed as described previously. R and S
denote the endo H-resistant and -sensitive species, respectively. ?,
present; ?, absent.
FIG. 2. Endo H analysis indicates that slow trafficking through the
secretory pathway is not dependent on ephrin B2 or on sequences
present in the Hendra virus G cytoplasmic tail. (A) CHO cells express-
ing Hendra virus G-?32 were metabolically labeled with35S for 30 min
and chased for the times indicated. Following immunoprecipitation
and endo H analysis, proteins were separated by sodium dodecyl sul-
fate-polyacrylamide gel electrophoresis and visualized via the Typhoon
imaging system. (B) Pulse-chase analysis of Vero cells expressing the
Hendra virus G-?41 tail was performed, followed by immunoprecipi-
tation and endo H analysis. R and S denote the endo H-resistant and
-sensitive species, respectively. ?, present; ?, absent.
VOL. 83, 2009NOTES 8999
compartment (15). Thus, slow folding kinetics for Hendra virus
G may explain the delayed appearance of an endo H-resistant
population. Unlike Hendra virus F, which folds as a trimer (13,
14), Hendra virus G is a tetramer composed of two disulfide-
linked dimers (7). To examine folding kinetics, cross-linking
analysis (13) was performed at various time points post-meta-
bolic labeling. By the end of the 30-min labeling, the majority
of Hendra virus F was folded into a trimeric state which could
be stabilized by the addition of a cross-linker (Fig. 4A), and
little of the F protein was present in the monomeric form after
the addition of the cross-linker. No further increases in trimers
were subsequently observed, suggesting very rapid oligomer-
ization of Hendra virus F. Rapid oligomerization of paramyxo-
virus fusion proteins is consistent with the prefusion structure
(38), in which monomers are tightly folded together to form
the trimeric unit. The presence of a dimeric species with the
addition of a cross-linker is due likely to incomplete cross-
linking and does not represent an intermediate conformational
population. The folding of Hendra virus G into a tetramer,
however, occurs at a lower rate than that of F. In the absence
of a cross-linker, monomeric G is present until 2 h postlabel-
ing, suggesting that the formation of the disulfide-linked dimer
is intrinsically slow (Fig. 4B). The formation of the disulfide-
linked dimer occurs at a rate similar to that of tetramer for-
mation, as the majority of G can be cross-linked to a tetramer
by the 2-h time point. Tetramerization of other paramyxovirus
attachment proteins, such as PIV5 hemagglutinin-neuramini-
dase, occurs more rapidly, with t1/2s for these proteins of 25 to
30 min (24), contrasting greatly with the t1/2of 1 to 2 h ob-
served for Hendra virus G. These results suggest that, unlike
other paramyxovirus attachment proteins, Hendra virus G un-
dergoes very slow tetramerization and that the slow trafficking
of this protein through the secretory pathway is likely a direct
reflection of the low intrinsic rate of folding and oligomeriza-
While the majority of paramyxovirus F proteins require their
homotypic attachment protein for fusogenic activity, the role
of the attachment protein in controlling F protein function
remains unclear. The attachment protein has been proposed to
hold the F protein in its prefusion conformation until receptor
binding occurs (34), and research from several systems has
suggested that this F protein-attachment protein interaction
occurs in the ER during initial protein folding (30, 33, 35).
Data from experiments presented here demonstrating differ-
ential rates of oligomerization and secretory pathway transport
for Hendra virus F and G strongly indicate that the association
of the newly synthesized proteins does not occur in the ER but
that they instead traffic independently through the secretory
pathway. Thus, at least for the henipaviruses, F-G interactions
are unlikely to play a role in preventing premature triggering of
the newly synthesized F protein. An alternative model for
paramyxovirus fusion has suggested that attachment protein-
fusion protein interactions occur only after the attachment
protein binds the receptor. However, analysis of henipavirus G
and F mutants suggests that F-G avidity inversely correlates
with fusion (2, 3, 5), and Hendra virus G protein mutants
deficient in receptor binding also lose the ability to coimmu-
FIG. 3. Analysis of cell surface populations confirms the differen-
tial trafficking rates of the Hendra virus F and G proteins. (A) Vero
cells were transfected with pCAGGS-Hendra F. Twenty-four hours
later, the cells were metabolically labeled with
chased for the indicated times. Analysis of biotinylated surface pro-
teins was performed as described previously (37). (B) Surface biotiny-
lation of Vero cells expressing Hendra virus G-?32 was performed,
and samples were analyzed as described previously (37). (C) Biotiny-
lation analyses of Vero cells expressing both Hendra virus F and
Hendra virus G-?32 were performed as described previously.
35S for 30 min and
FIG. 4. Cross-linking analysis of Hendra virus glycoproteins indi-
cates slow tetramerization of the Hendra virus G protein. (A) Vero
cells expressing Hendra virus F were metabolically labeled for 30 min
and chased for the indicated times. Cross-linking with DTSSP [3,3?-
dithiobis(sulfosuccinimidyl propionate)] was performed as described
previously (14). (B) Vero cells expressing Hendra virus G-?32 were
labeled for 30 min and chased for the indicated times, and cross-linking
analysis with DTSSP was performed as described previously (14). ?,
present; ?, absent.
noprecipitate Hendra virus F (4). These data support a model Download full-text
in which F-G interactions occur prior to receptor binding, with
the subsequent G-ephrin B2 interaction leading to the release
of the F-G interaction and the triggering of fusion. Taken
together, these results support a model in which the henipavi-
rus F and G associate only after trafficking to the cell surface.
The mechanisms by which this interaction is promoted and/or
regulated represent an exciting area of future research.
We thank Lin-fa Wang (Australian Animal Health Laboratory) for
the Hendra and Nipah virus F and G plasmids and Christopher Broder
(Uniformed Services University) for antisera to the henipavirus G
protein. We are also grateful to members of the Dutch lab for critically
reviewing the manuscript.
This study was supported by NIAID grant A151517 to R.E.D.
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