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Journal of Virology, July 2004, p. 7823-7827, Vol. 78, No. 14
0022-538X/04/$08.00+0 DOI: 10.1128/JVI.78.14.7823-7827.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Functional Analysis of Late-Budding Domain Activity Associated with the PSAP Motif within the Vesicular Stomatitis Virus M Protein
Takashi Irie,1 Jillian M. Licata,1 Himangi R. Jayakar,2 Michael A. Whitt,2,3 Peter Bell,4 and Ronald N. Harty1*
Department of Pathobiology, School of Veterinary Medicine,1
Cell Morphology Core, Gene Therapy Program, Division of Medical Genetics, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104,4
GTx, Inc.,2
Department of Molecular Sciences, University of Tennessee Health Science Center, Memphis, Tennessee 381633
Received 21 January 2004/
Accepted 24 February 2004

ABSTRACT
A PPPY motif within the M protein of vesicular stomatitis virus
(VSV) functions as a late-budding domain (L-domain); however,
L-domain activity has yet to be associated with a downstream
PSAP motif. VSV recombinants with mutations in the PPPY and/or
PSAP motif were recovered by reverse genetics and examined for
growth kinetics, plaque size, and budding efficiency by electron
microscopy. Results indicate that unlike the PPPY motif, the
PSAP motif alone does not possess L-domain activity. Finally,
the insertion of the human immunodeficiency virus type 1 p6
L-domain and flanking sequences into the PSAP region of M protein
rescued budding of a PPPY mutant of VSV to wild-type levels.

TEXT
Late-budding domains (L-domains) have been identified in the
matrix or matrix-like proteins of several different RNA virus
families (for a review, see reference
7). L-domains are thought
to facilitate virus budding by interacting with and usurping
specific host proteins or machinery during late stages of virus
replication. The PPPY (PY) motif within the vesicular stomatitis
virus (VSV) M protein has been shown to function as an L-domain
(
4,
11,
14). For example, mutations that disrupt the PY motif
result in the defective budding of both the M protein alone
and the infectious virus (
4,
11,
14). The PY motif of VSV has
been shown to mediate interactions in vitro with WW-containing
cellular ubiquitin ligases, such as Nedd4 (
9,
11). Such virus-host
interactions have been postulated to occur in vivo to promote
the efficient budding of virus from infected cells.
It has been noted recently that many of the RNA viruses that possess functional L-domains may also possess redundant or secondary L-domain motifs. For example, a bipartite L-domain has recently been identified in the p6 Gag protein of human immunodeficiency virus type 1 (HIV-1) (17). In addition, the VP40 protein of Ebola virus possesses two functional, overlapping L-domains (16). A potential secondary L-domain having the core consensus sequence of PSAP (PS) is also present in the M protein of VSV (Indiana serotype) just downstream of the PY motif. The PS motif is similar in sequence to functional L-domains identified in HIV-1 and Ebola virus (1, 6, 10, 12, 16-19, 22). Since mutations that disrupt the PY motif of VSV M never completely abolished budding (14), a possible role for PS as a secondary L-domain was hypothesized.
To determine whether the PS motif within M protein possesses L-domain activity and contributes to the budding of VSV, we introduced changes into the PS sequence within the full-length cDNA clone of VSV (Fig. 1). In addition to the PY>AAPA mutant described previously (14), we constructed a second mutant in which the PY motif was mutated to four alanines (PY>A4) (Fig. 1). A similar four-alanine substitution for the PSAP motif yielded the PS>A4 mutant (Fig. 1). Two additional constructs in which the PS motif was converted to a PTAP (PT) motif were generated (Fig. 1). The S-to-T substitution was engineered into both the wild-type (wt) VSV M protein background to yield the PS>PT mutant and the PY>A4 background to yield the PYPS>A4PT mutant (Fig. 1). The serine-to-threonine substitution was constructed, since functional L-domains within HIV-1 p6 Gag and Ebola virus VP40 have the core sequence PT rather than PS (7). All mutations introduced into the genomic cDNA were confirmed by automated DNA sequencing. All cDNA constructs (Fig. 1) were employed in the VSV reverse-genetics system (15, 24), and all constructs yielded infectious viruses. The resultant VSV recombinants were plaque purified two times, and the introduced mutations were confirmed by reverse transcription-PCR followed by automated DNA sequencing.
To determine the effect of PS mutations on virus replication,
recombinants were assayed for growth kinetics in cell culture
(Fig.
2). BHK-21 cells were infected with the indicated viruses
at a multiplicity of infection (MOI) of 10. Culture medium was
harvested at 2, 4, 6, 8, and 10 h postinfection (p.i.), and
viral titers were determined by duplicate plaque assays with
BHK-21 cells (Fig.
2). wt VSV served as a positive control,
and the PY>AAPA recombinant served as an L-domain-defective
control (Fig.
2A), as reported previously (
14). As expected,
titers of the PY>AAPA mutant were reduced by 1 to 2 logs
compared to those obtained for wt VSV (Fig.
2A). Similarly,
titers of the PY>A4 recombinant were virtually identical
to those of the PY>AAPA recombinant, confirming that the
substitution of alanines for the PY L-domain resulted in reduced
virus yield (Fig.
2A). Unlike with the PY mutants, titers of
the PS>A4 recombinant were on average only twofold lower
than those of wt VSV (Fig.
2B). These data suggest that the
disruption of the PS motif does not impair the budding of VSV
as severely as does disruption of the PY motif.
We next sought to determine whether a serine-to-threonine (PS>PT)
change would alter the growth kinetics of VSV (Fig.
2C). Viral
titers of the PS>PT recombinant were virtually indistinguishable
from those of wt VSV and were slightly higher (twofold) than
those of the PS>A4 recombinant (compare Fig.
2B and C). Although
the PS>PT recombinant virus replicated to titers virtually
identical to those of wt VSV, the presence of PT was not sufficient
to rescue budding of a PY mutant virus to wt levels (Fig.
2C).
Indeed, titers of the PYPS>A4PT recombinant remained an average
of 1 to 2 logs lower than those of wt VSV (Fig.
2C).
We next examined the profiles of virion proteins encoded by wt virus and the various VSV recombinants. Virions were harvested from infected BHK-21 cells and lysed in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) sample buffer. Virion proteins were fractionated by SDS-PAGE and visualized by staining with Coomassie brilliant blue (Fig. 3). The quantity of virion proteins correlated well with titers obtained in the growth curve assays. For example, the level of M protein present in wt VSV virions was >10-fold higher than in the PY>AAPA and PY>A4 mutants (Fig. 3A, lanes 1 to 3). In addition, the level of M protein present in PS>A4 virions was approximately twofold lower than that in wt VSV (Fig. 3A, compare lanes 1 and 4), whereas the amount of M protein present in the PS>PT mutant was virtually identical to that in wt virions (Fig. 3A, compare lanes 1 and 5). Finally, the amount of virion proteins present in the PYPS>A4PT mutant was identical to that in both the PY>AAPA and PY>A4 mutants (Fig. 3A, lane 6). The amounts of M protein present in cells infected with wt VSV and all recombinants were equivalent, indicating that mutations in the PY and PS motifs did not disrupt viral protein synthesis.
Electron microscopy was used previously to demonstrate conclusively
that PY mutants of VSV were defective in the ability to separate
from the plasma membrane of infected cells (
14). The ability
of the PS>A4 mutant to separate from infected cells was assessed
by electron microscopy (Fig.
4A). Briefly, BHK-21 cells were
infected with either wt VSV or the PS>A4 mutant at an MOI
of 3.0. At 6 h p.i., cells were washed extensively and resuspended
in 1.0% low-melting-point agarose in 1
x phosphate-buffered saline.
After chilling on ice, the cells in small agarose blocks were
fixed overnight in 2.5% glutaraldehyde-2.0% paraformaldehyde
in 0.1 M sodium cacodylate buffer (pH 7.4), washed in cacodylate
buffer, and incubated in 2.0% OsO
4 for 2 h. The samples were
then stained for 1 h with 1.0% uranyl acetate in 150 mM maleate
buffer (pH 6.0), washed, dehydrated with ethanol, and finally
embedded in resin (LX-112; Ladd Research Industries). Ultrathin
sections (80 nm thick) were stained with uranyl acetate and
lead citrate according to standard protocols, and samples were
examined with a Philips CM-100 transmission electron microscope.
As expected, wt VSV was found to separate efficiently from cellular
membranes with the typical bullet morphology (Fig.
4A). Similarly,
the PS>A4 mutant was also observed to separate efficiently
as a bullet-shaped virion. A representative micrograph for both
wt VSV and the PS>A4 mutant is shown in Fig.
4A. Examination
of numerous micrographs revealed no significant quantitative
or qualitative evidence to suggest that the PS>A4 mutant
was defective in budding.
The PY mutants of VSV were previously reported to exhibit a
small-plaque phenotype which correlated with their budding defect
(
14). To determine whether PS mutants displayed a similar small-plaque
phenotype, the average plaque size of each recombinant virus
was measured (Fig.
4B). Briefly, BHK-21 cells in 100-mm-diameter
dishes were infected to yield approximately 50 plaques per dish.
At 24 h p.i., cells were stained with crystal violet, and the
areas of 10 plaques on each plate were measured with NIH Image
1.52 software. As expected, the average plaque size of the PY
mutants was approximately 50% smaller than that observed for
wt VSV (Fig.
4B). In contrast, the PS>A4 and PS>PT mutants
yielded plaques that were virtually identical in size to those
of wt VSV (Fig.
4B). Finally, the small-plaque phenotype remained
associated with the PYPS>A4PT mutant. Thus, the small-plaque
phenotype correlated precisely with those recombinants that
exhibited a defect in budding due to mutations in the PY motif.
Taken together, these data strongly suggest that the PS motif within VSV M does not possess L-domain activity similar to that of the PY motif. Attempts to recover a double mutant having four alanine substitutions in both the PY and PS motifs are currently under way. Mutations that disrupt both the PY and PS regions of M may be too disruptive to normal M protein structure and/or function. In an attempt to address this possibility, we engineered and rescued a recombinant virus (M6) containing the known functional L-domain and flanking sequences of the p6 Gag protein of HIV-1 in place of the PS motif of VSV M protein (Fig. 5A). Moreover, the p6 L-domain and flanking sequences were inserted into the PY>A4 background (Fig. 5A). Interestingly, the presence of the HIV-1 p6 L-domain was sufficient to rescue the budding defect of the PY>A4 mutant and thus allow the M6 recombinant virus to achieve titers identical to those of wt VSV (Fig. 5B). These data indicate that alterations of the PS core sequence and surrounding amino acids are not lethal and that the insertion of a heterologous, functional L-domain at this site can effectively restore the L-domain activity absent in the PY>A4 mutant. Furthermore, these data indicate that amino acids flanking the PT core motif must contribute to the restoration of budding to the M6 recombinant virus. Thus, the recovery of the M6 recombinant lends further support to our conclusion that the PS region of wt VSV M protein does not possess significant L-domain activity.
The possibility that the PS region is important for either the
structure or functions of M protein other than budding remains.
It is also possible that the PS region may play a role in virus
replication in cells derived from animal hosts (e.g., equine
or bovine cells) or during VSV replication in insect vectors
(e.g., mosquito or sandfly) at low temperatures. The replication
of wt and PS mutant viruses in bovine, equine, mosquito, and
sandfly cell lines maintained at various temperatures is currently
being examined (T. Irie and R. N. Harty, unpublished data).
The PT- and PS-type L-domains identified in the p6 Gag and VP40 proteins of HIV-1 and Ebola virus, respectively, have been shown to interact with host protein TSG101 (2, 3, 8, 16, 18, 20, 21, 23). This PT/PS-TSG101 interaction appears to be important for the efficient budding of both HIV-1 and Ebola virus. Indeed, the overexpression of the N-terminal region of TSG101 or the depletion of TSG101 by small interfering RNA resulted in a decrease in the budding efficiency of p6 Gag and VP40 (5, 16, 18, 21). In contrast, the expression of small interfering RNA specific for TSG101 did not significantly reduce the budding of wt VSV M protein but did reduce the budding of a chimeric M protein containing the PT-type L-domain of VP40 (12a). The fact that the PYPS>A4PT mutant remained defective in budding is consistent with our notion that VSV buds in a TSG101-independent manner. It will now be of interest to determine whether the M6 recombinant virus, for example, has been redirected into the HIV-1 budding pathway and as a result become sensitive to the depletion of host TSG101.
The precise mechanisms by which viral L-domains interact with host proteins to promote budding during late stages of replication remain unclear. The VSV model system and VSV recombinants represent ideal tools to help elucidate these mechanisms and to understand the biological relevance of these virus-host interactions. Further understanding of virus budding will hopefully lead to the development of novel therapeutics to block this late stage of replication.

ACKNOWLEDGMENTS
We thank Shiho Irie for technical assistance, Jason Paragas
for critical reading of the manuscript, and all members of the
Harty lab for fruitful discussions.
This work was supported in part by NIH grant AI46499 to R.N.H. J.M.L. is supported by NIH training grant AI-07324.

FOOTNOTES
* Corresponding author. Mailing address: Department of Pathobiology, School of Veterinary Medicine, University of Pennsylvania, 3800 Spruce St., Philadelphia, PA 19104-6049. Phone: (215) 573-4485. Fax: (215) 898-7887. E-mail:
rharty{at}vet.upenn.edu.


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Journal of Virology, July 2004, p. 7823-7827, Vol. 78, No. 14
0022-538X/04/$08.00+0 DOI: 10.1128/JVI.78.14.7823-7827.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
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