Localization of the membrane-associated region of vesicular stomatitis virus M protein at the N terminus, using the hydrophobic, photoreactive probe 125I-TID

Compensatory mutations in the matrix protein of viral hemorrhagic septicemia virus (VHSV) genotype IVa in response to artificial mutation of two amino acids (D62A E181A)

The matrix (M) protein of rhabdoviruses locates between the inner line of the viral envelope and the nucleocapsids core and plays an important role in viral replication. In the present study, we aimed to rescue a mutant of VHSV genotype IVa that has artificial mutations in the M protein (M-D62A E181A). However, most rescued recombinant viruses unexpectedly showed non-targeted secondary mutations in the M protein. Therefore, this study was conducted to know whether the targeted artificial mutation can lead to specific non-targeted secondary mutations in the M protein and whether the secondary mutations are compensatory for the targeted artificial mutations. Experiments were conducted to rescue three kinds of M protein mutants (rVHSV-M-D62A, -E181A, and -D62A E181A), and rVHSV-M-E181A and rVHSV-M-D62A E181A without the secondary mutations were rescued only from IRF-9 gene-knockout EPC cells. Recombinant VHSVs having only targeted mutation(s) (rVHSV-M-D62A, -E181A, and -D62A E181A) showed slower CPE progression and retarded growth compared to rVHSV-wild. Although the sites of secondary mutations were changed in every transfection experiment to generate recombinant VHSVs, the positions of the secondary mutations were not random. Some amino acid residues in the M protein showed more frequent mutations than others, and the changed amino acid residues were always the same. EPC cells infected with rVHSV-M-D62A E181A showed significantly higher type I interferon response and NF-κB activity, and the inhibitory activity against type I interferon response and NF-κB activity in other recombinant VHSVs having secondary mutations in M gene were similar to those of rVHSV-wild. In conclusion, the present results showed that VHSV actively responded to the artificial mutation of M protein through the secondary mutations, and those secondary mutations occurred when the artificial mutations were deleterious to viral replication and protein stability. Furthermore, most secondary mutations in recombinant viruses compensated for the deleterious effect of the engineered mutations.

Visualizing molecular interactions that determine assembly of a bullet-shaped vesicular stomatitis virus particle

Vesicular stomatitis virus (VSV) is a negative-strand RNA virus with a non-segmented genome, closely related to rabies virus. Both have characteristic bullet-like shapes. We report the structure of intact, infectious VSV particles determined by cryogenic electron microscopy. By compensating for polymorphism among viral particles with computational classification, we obtained a reconstruction of the shaft ("trunk") at 3.5 Å resolution, with lower resolution for the rounded tip. The ribonucleoprotein (RNP), genomic RNA complexed with nucleoprotein (N), curls into a dome-like structure with about eight gradually expanding turns before transitioning into the regular helical trunk. Two layers of matrix (M) protein link the RNP with the membrane. Radial inter-layer subunit contacts are fixed within single RNA-N-M1-M2 modules, but flexible lateral and axial interactions allow assembly of polymorphic virions. Together with published structures of recombinant N in various states, our results suggest a mechanism for membrane-coupled self-assembly of VSV and its relatives.

Components and Architecture of the Rhabdovirus Ribonucleoprotein Complex

Rhabdoviruses, as single-stranded, negative-sense RNA viruses within the order Mononegavirales, are characterised by bullet-shaped or bacteroid particles that contain a helical ribonucleoprotein complex (RNP). Here, we review the components of the RNP and its higher-order structural assembly.

The Matrix Protein of a Plant Rhabdovirus Mediates Superinfection Exclusion by Inhibiting Viral Transcription

Superinfection exclusion (SIE) or cross-protection phenomena have been documented for plant viruses for nearly a century and are widespread among taxonomically diverse viruses, but little information is available about SIE of plant negative-strand RNA viruses. Here, we demonstrate that SIE by sonchus yellow net nucleorhabdovirus virus (SYNV) is mediated by the viral matrix (M) protein, a multifunctional protein involved in transcription regulation, virion assembly, and virus budding. We show that fluorescent protein-tagged SYNV variants display mutual exclusion/cross-protection in Nicotiana benthamiana plants. Transient expression of the SYNV M protein, but not other viral proteins, interfered with SYNV local infections. In addition, SYNV M deletion mutants failed to exclude superinfection by wild-type SYNV. An SYNV minireplicon reporter gene expression assay showed that the M protein inhibited viral transcription. However, M protein mutants with weakened nuclear localization signals (NLS) and deficient nuclear interactions with the SYNV nucleocapsid protein were unable to suppress transcription. Moreover, SYNV with M NLS mutations exhibited compromised SIE against wild-type SYNV. From these data, we propose that M protein accumulating in nuclei with primary SYNV infections either coils or prevents uncoiling of nucleocapsids released by the superinfecting SYNV virions and suppresses transcription of superinfecting genomes, thereby preventing superinfection. Our model suggests that the rhabdovirus M protein regulates the transition from replication to virion assembly and renders the infected cells nonpermissive for secondary infections.IMPORTANCE Superinfection exclusion (SIE) is a widespread phenomenon in which an established virus infection prevents reinfection by closely related viruses. Understanding the mechanisms governing SIE will not only advance our basic knowledge of virus infection cycles but may also lead to improved design of antiviral measures. Despite the significance of SIE, our knowledge about viral SIE determinants and their modes of actions remain limited. In this study, we show that sonchus yellow net virus (SYNV) SIE is mediated by the viral matrix (M) protein. During primary infections, accumulation of M protein in infected nuclei results in coiling of genomic nucleocapsids and suppression of viral transcription. Consequently, nucleocapsids released by potential superinfectors are sequestered and are unable to initiate new infections. Our data suggest that SYNV SIE is caused by M protein-mediated transition from replication to virion assembly and that this process prevents secondary infections.

Generation, characterization and application of monoclonal antibodies against matrix protein of hirame novirhabdovirus (HIRRV) in flounder

Hirame novirhabdovirus (HIRRV) causes severe disease in fish cultures, resulting in great economic loss in Asia and Europe. In this study, the matrix protein (M) of HIRRV was recombinantly expressed as the immunogen to produce monoclonal antibodies (MAbs) using hybridoma cell fusion technology, and 3 MAbs were produced and characterized by indirect ELISA, Western blotting and immunofluorescence assay (IFA). Western blotting and mass spectrometric analysis showed that the MAbs could specifically react with the nature M protein of HIRRV. The MAbs were employed to detect virions in HIRRV-infected epithelioma papulosum cyprini (EPC) cells and flounder Paralichthys olivaceus by IFA and immunohistochemistry (IHC). In the virus-infected EPC cells, the virions were mainly located in the cytoplasm, whereas in flounder, HIRRV was present in all 10 tested tissues, and the positive signals in spleen, head-kidney and heart were higher than in other tissues, consistent with the results obtained by RT-PCR. Moreover, strong positive signals were observed in the endothelial cells of blood vessels, but only the leukocytes were infected by HIRRV in the whole blood cells. These results indicate that the high susceptibility to HIRRV of leukocytes and endothelial cells may facilitate the spread of HIRRV and finally cause systemic infection in flounder. This study provides a foundation for further studies on rapid diagnosis of HIRRV and its infection mechanisms.

Tracking the Fate of Genetically Distinct Vesicular Stomatitis Virus Matrix Proteins Highlights the Role for Late Domains in Assembly

Vesicular stomatitis virus (VSV) assembly requires condensation of the viral ribonucleoprotein (RNP) core with the matrix protein (M) during budding from the plasma membrane. The RNP core comprises the negative-sense genomic RNA completely coated by the nucleocapsid protein (N) and associated by a phosphoprotein (P) with the large polymerase protein (L). To study the assembly of single viral particles, we tagged M and P with fluorescent proteins. We selected from a library of viruses with insertions in the M gene a replication-competent virus containing a fluorescent M and combined that with our previously described virus containing fluorescent P. Virus particles containing those fusions maintained the same bullet shape appearance as wild-type VSV but had a modest increase in particle length, reflecting the increased genome size. Imaging of the released particles revealed a variation in the amount of M and P assembled into the virions, consistent with a flexible packaging mechanism. We used the recombinants to further study the importance of the late domains in M, which serve to recruit the endosomal sorting complex required for transport (ESCRT) machinery during budding. Mutations in late domains resulted in the accumulation of virions that failed to pinch off from the plasma membrane. Imaging of single virions released from cells that were coinfected with M tagged with enhanced green fluorescent protein and M tagged with mCherry variants in which the late domains of one virus were inactivated by mutation showed a strong bias against the incorporation of the late-domain mutant into the released virions. In contrast, the intracellular expression and membrane association of the two variants were unaltered. These studies provide new tools for imaging particle assembly and enhance our resolution of existing models for assembly of VSV.

IMPORTANCE

Assembly of vesicular stomatitis virus (VSV) particles requires the separate trafficking of the viral replication machinery, a matrix protein (M) and a glycoprotein, to the plasma membrane. The matrix protein contains a motif termed a "late domain" that engages the host endosomal sorting complex required for transport (ESCRT) machinery to facilitate the release of viral particles. Inactivation of the late domains through mutation results in the accumulation of virions arrested at the point of release. In the study described here, we developed new tools to study VSV assembly by fusing fluorescent proteins to M and to a constituent of the replication machinery, the phosphoprotein (P). We used those tools to show that the late domains of M are required for efficient incorporation into viral particles and that the particles contain a variable quantity of M and P.

Everything You Always Wanted to Know About Rabies Virus (But Were Afraid to Ask)

The cultural impact of rabies, the fatal neurological disease caused by infection with rabies virus, registers throughout recorded history. Although rabies has been the subject of large-scale public health interventions, chiefly through vaccination efforts, the disease continues to take the lives of about 40,000-70,000 people per year, roughly 40% of whom are children. Most of these deaths occur in resource-poor countries, where lack of infrastructure prevents timely reporting and postexposure prophylaxis and the ubiquity of domestic and wild animal hosts makes eradication unlikely. Moreover, although the disease is rarer than other human infections such as influenza, the prognosis following a bite from a rabid animal is poor: There is currently no effective treatment that will save the life of a symptomatic rabies patient. This review focuses on the major unanswered research questions related to rabies virus pathogenesis, especially those connecting the disease progression of rabies with the complex dysfunction caused by the virus in infected cells. The recent applications of cutting-edge research strategies to this question are described in detail.

Rabies virus assembly and budding

Rabies virus (RABV) and other negative-strand RNA viruses are the causes of serious diseases in humans and animals worldwide. Assembly and budding are important late events in the replication cycles of these negative-strand RNA viruses that have received much attention in the past decade. Indeed, important insights into the molecular mechanisms by which rhabdoviral proteins usurp and/or interact with host proteins to promote efficient virion assembly and egress has greatly enhanced our understanding of the budding process. Assembly/budding of rhabdoviruses is driven largely by the matrix (M) protein. RABV M protein contains a late budding domain that mediates the recruitment of host proteins linked to the vacuolar protein sorting pathway of the cell to facilitate virus-cell separation. This chapter summarizes our current knowledge of the roles that both RABV M protein and interacting host proteins play during the budding process.

Glycoprotein-dependent acidification of vesicular stomatitis virus enhances release of matrix protein

To study vesicular stomatitis virus (VSV) entry and uncoating, we generated a recombinant VSV encoding a matrix (M) protein containing a C-terminal tetracysteine Lumio tag (rVSV-ML) that could be fluorescently labeled using biarsenical compounds. Quantitative confocal microscopy showed that there is a transient loss of fluorescence at early times after the initiation of endocytosis of rVSV-ML-Green (rVSV-MLG) virions, which did not occur when cells were treated with bafilomycin A1. The reduction in fluorescence occurred 5 to 10 min postentry, followed by a steady increase in fluorescence intensity from 15 to 60 min postentry. A similar loss of fluorescence was observed in vitro when virions were exposed to acidic pH. The reduction in fluorescence required G protein since "bald" DeltaG-MLG particles did not show a similar loss of fluorescence at low pH. Based on the pH-dependent fluorescence properties of Lumio Green, we hypothesize that the loss of fluorescence of rVSV-MLG virions during virus entry is due to a G ectodomain-dependent acidification of the virion interior. Biochemical analysis indicated that low pH also resulted in an enhancement of M protein dissociation from partially permeabilized, but otherwise intact, wild-type virions. From these data we propose that low-pH conformational changes in G protein promote acidification of the virus interior, which facilitates the release of M from ribonucleoprotein particles during uncoating.

Rhabdovirus matrix protein structures reveal a novel mode of self-association

The matrix (M) proteins of rhabdoviruses are multifunctional proteins essential for virus maturation and budding that also regulate the expression of viral and host proteins. We have solved the structures of M from the vesicular stomatitis virus serotype New Jersey (genus: Vesiculovirus) and from Lagos bat virus (genus: Lyssavirus), revealing that both share a common fold despite sharing no identifiable sequence homology. Strikingly, in both structures a stretch of residues from the otherwise-disordered N terminus of a crystallographically adjacent molecule is observed binding to a hydrophobic cavity on the surface of the protein, thereby forming non-covalent linear polymers of M in the crystals. While the overall topology of the interaction is conserved between the two structures, the molecular details of the interactions are completely different. The observed interactions provide a compelling model for the flexible self-assembly of the matrix protein during virion morphogenesis and may also modulate interactions with host proteins.

Vesicular stomatitis virus matrix protein mutations that affect association with host membranes and viral nucleocapsids

Viral matrix (M) proteins bind the nucleoprotein core (nucleocapsid) to host membranes during the process of virus assembly by budding. Previous studies using truncated M proteins had implicated the N-terminal 50 amino acids of the vesicular stomatitis virus M protein in binding both membranes and nucleocapsids and a sequence from amino acids 75-106 as an additional membrane binding region. Structure-based mutations were introduced into these two regions, and their effects on membrane association and incorporation into nucleocapsid-M protein complexes were determined using quantitative assays. The results confirmed that the N terminus of M protein is involved in association with plasma membranes as well as nucleocapsids, although these two activities were differentially affected by individual mutations. Mutations in the 75-106 region affected incorporation into nucleocapsid-M complexes but had only minor effects on association with membranes. The ability of site-specific mutant M proteins to complement growth of temperature-sensitive M mutant virus did not correlate well with the ability to associate with membranes or nucleocapsids, suggesting that complementation involves an additional activity of M protein. Mutants with similar abilities to associate with membranes and nucleocapsids but differing in complementation activity were incorporated into infectious cDNA clones. Infectious virus was repeatedly recovered containing mutant M proteins capable of complementation but was never recovered with mutant M proteins that lacked complementation activity, providing further evidence for a separate activity of M protein that is essential for virus replication.

Role of residues 121 to 124 of vesicular stomatitis virus matrix protein in virus assembly and virus-host interaction

The recent solution of the crystal structure of a fragment of the vesicular stomatitis virus matrix (M) protein suggested that amino acids 121 to 124, located on a solvent-exposed loop of the protein, are important for M protein self-association and association with membranes. These residues were mutated from the hydrophobic AVLA sequence to the polar sequence DKQQ. Expression and purification of this mutant from bacteria showed that it was structurally stable and that the mutant M protein had self-association kinetics similar to those of the wild-type M protein. Analysis of the membrane association of M protein in the context of infection with isogenic recombinant viruses showed that both wild-type and mutant M proteins associated with membranes to the same extent. Virus expressing the mutant M protein did show an approximately threefold-lower binding affinity of M protein for nucleocapsid-M complexes. In contrast to the relatively minor effects of the M protein mutation on virus assembly, the mutant virus exhibited growth restriction in MDBK but not BHK cells, a slower induction of apoptosis, and lower viral-protein synthesis. Despite translating less viral protein, the mutant virus produced more viral mRNA, showing that the mutant virus could not effectively promote viral translation. These results demonstrate that the 121-to-124 region of the VSV M protein plays a minor role in virus assembly but is involved in virus-host interactions and VSV replication by augmenting viral-mRNA translation.

Membrane deformations induced by the matrix protein of vesicular stomatitis virus in a minimal system

The matrix (M) protein of vesicular stomatitis virus plays a key role in both assembly and budding of progeny virions. In vitro experiments have shown a strong propensity of M protein to bind to vesicles containing negatively charged phospholipids. In vivo, it has also been demonstrated that recruitment of some cellular proteins by M protein is required for efficient virus budding and release of newly synthesized virions in the extracellular medium. The ability of M protein to deform target membranes in vitro was investigated in this study. It was shown that incubation of purified M protein with giant unilamellar vesicles results in the formation of patches of M protein at their surface, followed by deformations of the membrane toward the inside of the vesicle, which could be observed in phase-contrast microscopy. This provides the first evidence that M protein alone is able to impose the correct budding curvature on the membrane. Using confocal microscopy, patches of M protein that colocalized with negatively charged lipid domains a few minutes after vesicle injection were observed. After a longer incubation period, membrane deformations appeared in these domains. At this time, a strict colocalization of M protein, negatively charged lipids and membrane deformation was observed. The influence on this process of the basic N-terminal part of the protein and of the previously identified hydrophobic loop has also been investigated. Interestingly, the final fission event has never been observed in our experimental system, indicating that other partners are required for this step.

Utilization of homotypic and heterotypic proteins of vesicular stomatitis virus by defective interfering particle genomes for RNA replication and virion assembly: implications for the mechanism of homologous viral interference

Defective interfering (DI) particles of Indiana serotype of vesicular stomatitis virus (VSV(Ind)) are capable of interfering with the replication of both homotypic VSV(Ind) and heterotypic New Jersey serotype (VSV(NJ)) standard virus. In contrast, DI particles from VSV(NJ) do not interfere with the replication of VSV(Ind) standard virus but do interfere with VSV(NJ) replication. The differences in the interfering activities of VSV(Ind) DI particles and VSV(NJ) DI particles against heterotypic standard virus were investigated. We examined the utilization of homotypic and heterotypic VSV proteins by DI particle genomic RNAs for replication and maturation into infectious DI particles. Here we show that the RNA-nucleocapsid protein (N) complex of one serotype does not utilize the polymerase complex (P and L) of the other serotype for RNA synthesis, while DI particle genomic RNAs of both serotypes can utilize the N, P, and L proteins of either serotype without serotypic restriction but with differing efficiencies as long as all three proteins are derived from the same serotype. The genomic RNAs of VSV(Ind) DI particles assembled and matured into DI particles by using either homotypic or heterotypic viral proteins. In contrast, VSV(NJ) DI particles could assemble only with homotypic VSV(NJ) viral proteins, although the genomic RNAs of VSV(NJ) DI particles could be replicated by using heterotypic VSV(Ind) N, P, and L proteins. Thus, we concluded that both efficient RNA replication and assembly of DI particles are required for the heterotypic interference by VSV DI particles.

Rhabdovirus assembly and budding

Rhabdoviruses are a diverse, widely-distributed group of enveloped viruses that assemble and bud from the plasma membrane of host cells. Recent advances in the identification of domains on both the envelope glycoprotein and the matrix protein of rhabdoviruses that contribute to virus assembly and release have allowed us to refine current models of rhabdovirus budding and to describe in better detail the interplay between both viral and cellular components involved in the budding process. In this review we discuss the steps involved in rhabdovirus assembly beginning with genome encapsidation and the association of nucleocapsid-matrix protein pre-assembly complexes with the inner leaflet of the plasma membrane, how condensation of these complexes may occur, how microdomains containing the envelope glycoprotein facilitate bud site formation, and how multiple forms of the matrix protein may participate in virion extrusion and release.

Transport of the intracisternal A-type particle Gag polyprotein to the endoplasmic reticulum is mediated by the signal recognition particle

Intracisternal A-type particles (IAP) are defective endogenous retroviruses that accumulate in the endoplasmic reticulum (ER) of rodent cells. The enveloped particles are produced by assembly and budding of IAP Gag polyproteins at the ER membrane. In this study, we analyzed the specific ER transport of the Gag polyprotein of the IAP element MIA14. To this end, we performed in vitro translation of Gag in the presence of microsomal membranes or synthetic proteoliposomes followed by membrane sedimentation or flotation. ER binding of IAP Gag occurred mostly cotranslationally, and Gag polyproteins interacted specifically with proteoliposomes containing only signal recognition particle (SRP) receptor and the Sec61p complex, which form the minimal ER translocation apparatus. The direct participation of SRP in ER targeting of IAP Gag was demonstrated in cross-linking and immunoprecipitation experiments. The IAP polyprotein was not translocated into the ER; it was found to be tightly associated with the cytoplasmic side of the ER membrane but did not behave as an integral membrane protein. Substituting the functional signal peptide of preprolactin for the hydrophobic sequence at the N terminus of IAP Gag also did not result in translocation of the chimeric protein into the ER lumen, and grafting the IAP hydrophobic sequence onto preprolactin failed to yield luminal transport as well. These results suggest that the N-terminal hydrophobic region of the IAP Gag polyprotein functions as a transport signal which mediates SRP-dependent ER targeting, but polyprotein translocation or integration into the membrane is prevented by the signal sequence itself and by additional regions of Gag.

Crystal structure of vesicular stomatitis virus matrix protein

The vesicular stomatitis virus (VSV) matrix protein (M) interacts with cellular membranes, self-associates and plays a major role in virus assembly and budding. We present the crystallographic structure, determined at 1.96 A resolution, of a soluble thermolysin resistant core of VSV M. The fold is a new fold shared by the other vesiculovirus matrix proteins. The structure accounts for the loss of stability of M temperature-sensitive mutants deficient in budding, and reveals a flexible loop protruding from the globular core that is important for self-assembly. Membrane floatation shows that, together with the M lysine-rich N-terminal peptide, a second domain of the protein is involved in membrane binding. Indeed, the structure reveals a hydrophobic surface located close to the hydrophobic loop and surrounded by conserved basic residues that may constitute this domain. Lastly, comparison of the negative-stranded virus matrix proteins with retrovirus Gag proteins suggests that the flexible link between their major membrane binding domain and the rest of the structure is a common feature shared by these proteins involved in budding and virus assembly.

Cleavage of vesicular stomatitis virus matrix protein prevents self-association and leads to crystallization

The matrix protein (M) of vesicular stomatitis virus is responsible for the budding of newly formed virions out of host cells. In vitro, it has been shown to self-associate, a property that may be related to the role of M in virus assembly but also prevents crystallization. Using limited proteolysis by thermolysin, we have isolated and characterized two soluble fragments of the protein that remain noncovalently associated. The digestion product does not self-associate nor is it recruited in aggregates formed by intact M molecules. These results identify a peptide, located at the surface of the protein and disorganized by thermolysin cleavage, responsible for M self-association. The thermolysin-resistant core of M has been crystallized and the crystals diffract to 2-A resolution.

Mutations in the PPPY motif of vesicular stomatitis virus matrix protein reduce virus budding by inhibiting a late step in virion release

The N terminus of the matrix (M) protein of vesicular stomatitis virus (VSV) and of other rhabdoviruses contains a highly conserved PPPY sequence (or PY motif) similar to the late (L) domains in the Gag proteins of some retroviruses. These L domains in retroviral Gag proteins are required for efficient release of virus particles. In this report, we show that mutations in the PPPY sequence of the VSV M protein reduce virus yield by blocking a late stage in virus budding. We also observed a delay in the ability of mutant viruses to cause inhibition of host gene expression compared to wild-type (WT) VSV. The effect of PY mutations on virus budding appears to be due to a block at a stage just prior to virion release, since electron microscopic examination of PPPA mutant-infected cells showed a large number of assembled virions at the plasma membrane trapped in the process of budding. Deletion of the glycoprotein (G) in addition to these mutations further reduced the virus yield to less than 1% of WT levels, and very few particles were assembled at the cell surface. This observation suggested that G protein aids in the initial stage of budding, presumably during the formation of the bud site. Overall, our results confirm that the PPPY sequence of the VSV M protein possesses L domain activity analogous to that of the retroviral Gag proteins.

Vesicular stomatitis virus matrix protein inhibits host cell gene expression by targeting the nucleoporin Nup98

Vesicular stomatitis virus matrix protein (VSV M) has been shown to inhibit both transcription and nucleocytoplasmic transport. We have isolated a mutant form of M, termed M(D), lacking both inhibitory activities. HeLa cells expressing M, but not M(D), accumulate polyadenylated RNAs within the nucleus. Concomitantly, a fraction of M, but not of the M(D) mutant, localizes at the nuclear rim. Additionally, the nucleoporin Nup98 specifically interacts with M but not with M(D). In Nup98(-/-) cells, both the levels of M at the nuclear envelope and its inhibitory effects on host cell-directed expression of reporter genes were significantly reduced. Together, our data demonstrate that VSV M inhibits host cell gene expression by targeting a nucleoporin and primarily blocking nuclear export.

Matrix protein of Chandipura virus inhibits transcription from an RNA polymerase II promoter

Chandipura virus (CHPV) is a Vesiculovirus, related to, but phylogenetically distinct from, vesicular stomatitis virus (VSV). The matrix protein of VSV, as well as its role in virus assembly, inhibits the transcription from promoters for host RNA polymerases I and II. Cloning and expression of the matrix protein of CHPV in human cells showed that this protein is also functional in its inhibitory effect on transcription of a reporter gene from the cytomegalovirus immediate-early promoter, despite sharing only 28% amino acid sequence identity with the matrix protein of VSV.

Late domain function identified in the vesicular stomatitis virus M protein by use of rhabdovirus-retrovirus chimeras

Little is known about the mechanisms used by enveloped viruses to separate themselves from the cell surface at the final step of budding. However, small sequences in the Gag proteins of several retroviruses (L domains) have been implicated in this process. A sequence has been identified in the M proteins of rhabdoviruses that closely resembles the PPPPY motif in the L domain of Rous sarcoma virus (RSV), an avian retrovirus. To evaluate whether the PPPY sequence in vesicular stomatitis virus (VSV) M protein has an activity analogous to that of the retroviral sequence, M-Gag chimeras were characterized. The N-terminal 74 amino acids of the VSV (Indiana) M protein, including the PPPY motif, was able to replace the L domain of RSV Gag and allow the assembly and release of virus-like particles. Alanine substitutions in the VSV PPPY motif severely compromised the budding activity of this hybrid protein but not that of another chimera which also contained the RSV PPPPY sequence. We conclude that this VSV sequence is functionally homologous to the RSV L domain in promoting virus particle release, making this the first example of such an activity in a virus other than a retrovirus. Both the RSV and VSV motifs have been shown to interact in vitro with certain cellular proteins that contain a WW interaction module, suggesting that the L domains are sites of interaction with unknown host machinery involved in virus release.

A proline-rich motif within the matrix protein of vesicular stomatitis virus and rabies virus interacts with WW domains of cellular proteins: implications for viral budding

The matrix (M) protein of rhabdoviruses has been shown to play a key role in virus assembly and budding; however, the precise mechanism by which M mediates these processes remains unclear. We have associated a highly conserved, proline-rich motif (PPxY or PY motif, where P denotes proline, Y represents tyrosine, and x denotes any amino acid) of rhabdoviral M proteins with a possible role in budding mediated by the M protein. Point mutations that disrupt the PY motif of the M protein of vesicular stomatitis virus (VSV) have no obvious effect on membrane localization of M but instead lead to a decrease in the amount of M protein released from cells in a functional budding assay. Interestingly, the PPxY sequence within rhabdoviral M proteins is identical to that of the ligand which interacts with WW domains of cellular proteins. Indeed, results from two in vitro binding assays demonstrate that amino acids 17 through 33 and 29 through 44, which contain the PY motifs of VSV and rabies virus M proteins, respectively, mediate interactions with WW domains of specific cellular proteins. Point mutations that disrupt the consensus PY motif of VSV or rabies virus M protein result in a significant decrease in their ability to interact with the WW domains. These properties of the PY motif of rhabdovirus M proteins are strikingly analogous to those of the late (L) budding domain identified in the gag-specific protein p2b of Rous sarcoma virus. Thus, it is possible that rhabdoviruses may usurp host proteins to facilitate the budding process and that late stages in the budding process of rhabdoviruses and retroviruses may have features in common.

Virus maturation by budding

Enveloped viruses mature by budding at cellular membranes. It has been generally thought that this process is driven by interactions between the viral transmembrane proteins and the internal virion components (core, capsid, or nucleocapsid). This model was particularly applicable to alphaviruses, which require both spike proteins and a nucleocapsid for budding. However, genetic studies have clearly shown that the retrovirus core protein, i.e., the Gag protein, is able to form enveloped particles by itself. Also, budding of negative-strand RNA viruses (rhabdoviruses, orthomyxoviruses, and paramyxoviruses) seems to be accomplished mainly by internal components, most probably the matrix protein, since the spike proteins are not absolutely required for budding of these viruses either. In contrast, budding of coronavirus particles can occur in the absence of the nucleocapsid and appears to require two membrane proteins only. Biochemical and structural data suggest that the proteins, which play a key role in budding, drive this process by forming a three-dimensional (cage-like) protein lattice at the surface of or within the membrane. Similarly, recent electron microscopic studies revealed that the alphavirus spike proteins are also engaged in extensive lateral interactions, forming a dense protein shell at the outer surface of the viral envelope. On the basis of these data, we propose that the budding of enveloped viruses in general is governed by lateral interactions between peripheral or integral membrane proteins. This new concept also provides answers to the question of how viral and cellular membrane proteins are sorted during budding. In addition, it has implications for the mechanism by which the virion is uncoated during virus entry.

Rabies virus M protein expressed in Escherichia coli and its regulatory role in virion-associated transcriptase activity

Rabies virus M protein was expressed in Escherichia coli in the form of a fusion protein with maltose binding protein (MBP) and purified by amylose affinity column chromatography after extraction. In order to investigate the possible regulatory role of M protein in viral transcription, an assay system for rabies virion-associated transcriptase activity was established by using the ribonucleoprotein (RNP) cores prepared from purified virions. Analysis of the products of the transcription assay system showed that the products are sensitive to RNase and are positive-strand RNA. Addition of the fusion protein to the system after cleavage with a proteinase Factor Xa (FXa), which cleaves the fusion protein into the M protein and MBP, resulted in an efficient and dose-dependent inhibition of the transcription. Furthermore, addition to the system of anti-M protein monoclonal antibody significantly restored the transcription. Control experiments with the same transcription assaying system using rabies virus nucleoprotein expressed as a fusion protein with MBP and cleaved with FXa did not result in an inhibition of the transcription. These results suggest that the M protein of rabies virus has the property to down-regulate virion-associated transcription.

Aggregation of VSV M protein is reversible and mediated by nucleation sites: implications for viral assembly

Purified M protein of VSV has been reported to aggregate at low NaCl concentration. Using light scattering, analytical centrifugation, and electron microscopy (EM), we have studied this phenomenon. Our results demonstrate that self aggregation of M protein can be reversed by increasing the salt concentration. Below 250 mM NaCl, there is an equilibrium between aggregates and monomeric M protein. Most importantly, we demonstrate that aggregation only occurs in the presence of nucleation sites and that these sites are sensitive to trypsin. We have found conditions under which these nucleation sites can be eliminated, after which M remains soluble even at low salt concentration. Finally, using EM, we show that the aggregates of purified M protein share common structural aspects with the previously described internal "cigar" around which the nucleocapsid is wrapped. These new results help to explain why M is a soluble protein in the cytoplasm of the infected cell just up to the moment that it is integrated into the budding virion.

Membrane-binding domains and cytopathogenesis of the matrix protein of vesicular stomatitis virus

The membrane-binding affinity of the matrix (M) protein of vesicular stomatitis virus (VSV) was examined by comparing the cellular distribution of wild-type (wt) virus M protein with that of temperature-sensitive (ts) and deletion mutants probed by indirect fluorescent-antibody staining and fractionation of infected or plasmid-transfected CV1 cells. The M-gene mutant tsO23 caused cytopathic rounding of cells infected at permissive temperature but not of cells at the nonpermissive temperature; wt VSV also causes rounding, which prohibits study of M protein distribution by fluorescent-antibody staining. Little or no M protein can be detected in the plasma membrane of cells infected with tsO23 at the nonpermissive temperature, whereas approximately 20% of the M protein colocalized with the membrane fraction of cells infected with tsO23 at the permissive temperature. Cells transfected with a plasmid expressing intact 229-amino-acid wt M protein (M1-229) exhibited cytopathic cell rounding and actin filament dissolution, whereas cells retained normal polygonal morphology and actin filaments when transfected with plasmids expressing M proteins truncated to the first 74 N-terminal amino acids (M1-74) or deleted of the first 50 amino acids (M51-229) or amino acids 1 to 50 and 75 to 106 (M51-74/107-229). Truncated proteins M1-74 and M51-229 were readily detectable in the plasma membrane and cytosol of transfected cells as determined by both fluorescent-antibody staining and cell fractionation, as was the plasmid-expressed intact wt M protein. However, the expressed doubly deleted protein M51-74/107-229 could not be detected in plasma membrane by fluorescent-antibody staining or by cell fractionation, suggesting the presence of two membrane-binding sites spanning the region of amino acids 1 to 50 and amino acids 75 to 106 of the VSV M protein. These in vivo data were confirmed by an in vitro binding assay in which intact M protein and its deletion mutants were reconstituted in high- or low-ionic-strength buffers with synthetic membranes in the form of sonicated unilammelar vesicles. The results of these experiments appear to confirm the presence of two membrane-binding sites on the VSV M protein, one binding peripherally by electrostatic forces at the highly charged NH2 terminus and the other stably binding membrane integration of hydrophobic amino acids and located by a hydropathy plot between amino acids 88 and 119.

Interactions of normal and mutant vesicular stomatitis virus matrix proteins with the plasma membrane and nucleocapsids

We demonstrated recently that a fraction of the matrix (M) protein of vesicular stomatitis virus (VSV) binds tightly to cellular membranes in vivo when expressed in the absence of other VSV proteins. This membrane-associated M protein was functional in binding purified VSV nucleocapsids in vitro. Here we show that the membrane-associated M protein is largely associated with a membrane fraction having the density of plasma membranes, indicating membrane specificity in the binding. In addition, we analyzed truncated forms of M protein to identify regions responsible for membrane association and nucleocapsid binding. Truncated M protein lacking the amino-terminal basic domain still associated with cellular membranes, although not as tightly as wild-type M protein, and could not bind nucleocapsids. In contrast, deletion of the carboxy-terminal 14 amino acids did not disrupt stable membrane association or nucleocapsid interaction. These results suggest that the amino terminus of M protein either interacts directly with membranes and nucleocapsids or stabilizes a conformation that is required for M protein to mediate both of these interactions.

Effect of vesicular stomatitis virus matrix protein on host-directed translation in vivo

Vesicular stomatitis virus infection causes a rapid and potent inhibition of both host transcription and translation. Recently, the viral matrix (M) protein was shown to inhibit host-directed transcription in vivo in the absence of any other viral component (B. L. Black and D. S. Lyles, J. Virol. 66:4058-4064, 1992). The goal of this study was to determine the effect of M protein on host-directed translation. In vitro-transcribed mRNAs encoding M protein and chloramphenicol acetyltransferase (CAT) were cotransfected into BHK cells to determine the effect of M protein expression on translation of CAT mRNA. The results presented here show that M protein did not inhibit host-directed translation of CAT mRNA. On the contrary, this study gave the unexpected result that M protein actually stimulated host-directed translation under the same conditions in which it potently inhibited host-directed transcription. Under these conditions, the combined effect on host gene expression was a greater-than-20-fold inhibition. Furthermore, the enhancement of host translation mediated by M protein was genetically correlated with M protein's ability to inhibit host transcription. Thus, the results of this study establish that M protein does not inhibit host protein synthesis under the same conditions in which it potently inhibits host transcription and suggest that the inhibition of transcription and that of translation by vesicular stomatitis virus require separate viral gene products.

Vesicular stomatitis virus M protein may be inside the ribonucleocapsid coil

Vesicular stomatitis virus is an enveloped virus with an external glycoprotein G and a nucleocapsid that form, together with the M protein, a tight helically coiled structure: the skeleton. Negative staining and immunoelectron microscopy studies on skeleton preparations were performed to determine the localization of the M protein. These studies have resulted in a new model for the structure of rhabdoviruses in which the nucleocapsid is wound around a core containing the M protein. This model predicts contact between M and lipid only at the extreme ends of the skeleton, which is confirmed by skeleton-liposome binding studies.

The role of vesicular stomatitis virus matrix protein in inhibition of host-directed gene expression is genetically separable from its function in virus assembly

Recently, the vesicular stomatitis virus matrix (M) protein has been shown to be capable of inhibition of host cell-directed transcription in the absence of other viral components (B. L. Black and D. S. Lyles, J. Virol. 66:4058-4064, 1992). M protein is a major structural protein that is known to play a critical role in virus assembly by binding the helical ribonucleoprotein core of the virus to the cytoplasmic surface of the cell plasma membrane during budding. In this study, two M protein mutants were tested to determine whether the inhibition of host transcription by M protein is an indirect effect of its function in virus assembly or whether it represents an independent function of M protein. The mutant M protein of the conditionally temperature-sensitive (ts) vesicular stomatitis virus mutant, tsO82, was found to be defective in its ability to inhibit host-directed gene expression, as shown by its inability to inhibit expression of a cotransfected target gene encoding chloramphenicol acetyltransferase. The ability of the tsO82 M protein to function in virus assembly was similar to that of wild-type M protein, as shown by its ability to complement the group III ts M protein mutant, tsO23. Another mutant, MN1, which lacks amino acids 4 to 21 of M protein demonstrated that the abilities of M protein to inhibit chloramphenicol acetyltransferase gene expression and to localize to the nucleus were unaffected by deletion of this lysine-rich amino-terminal region but that the ability to function in virus assembly was ablated. Thus, the two M protein mutants examined in this study exhibited complementary phenotypes: tsO82 M protein functioned in virus assembly but was defective in inhibition of host-directed gene expression, while MN1 M protein functioned in inhibiting gene expression but was unable to function in virus assembly. These data demonstrate that the role of M protein in inhibition of host transcription can be separated genetically from its role in virus assembly.

Photodynamic inactivation of infectivity of human immunodeficiency virus and other enveloped viruses using hypericin and rose bengal: inhibition of fusion and syncytia formation

The mechanism of the antiviral activity of hypericin was characterized and compared with that of rose bengal. Both compounds inactivate enveloped (but not unenveloped) viruses upon illumination by visible light. Human immunodeficiency and vesicular stomatitis viruses were photodynamically inactivated by both dyes at nanomolar concentrations. Photodynamic inactivation of fusion (hemolysis) by vesicular stomatitis, influenza, and Sendai viruses was induced by both dyes under similar conditions (e.g., I50 = 20-50 nM for vesicular stomatitis virus), suggesting that loss of infectivity resulted from inactivation of fusion. Syncytium formation, between cells activated to express human immunodeficiency virus gp120 on their surfaces and CD4+ cells, was inhibited by illumination in the presence of 1 microM hypericin. Hypericin and rose bengal thus exert similar virucidal effects. Both presumably act by the same mechanism--namely, the inactivation of the viral fusion function by singlet oxygen produced upon illumination. The implications of this photodynamic antiviral action for the potential therapeutic usefulness of both hypericin and rose bengal are discussed.

Membrane association of functional vesicular stomatitis virus matrix protein in vivo

The matrix (M) protein of vesicular stomatitis virus (VSV) is a major structural component of the virion which is generally believed to bridge between the membrane envelope and the ribonucleocapsid (RNP) core. To investigate the interaction of M protein with cellular membranes in the absence of other VSV proteins, we examined its distribution by subcellular fractionation after expression in HeLa cells. Approximately 90% of M protein, expressed without other viral proteins, was soluble, whereas the remaining 10% was tightly associated with membranes. A similar distribution in VSV-infected cells has been observed previously. Conditions known to release peripherally associated membrane proteins did not detach M protein from isolated membranes. Membrane-associated M protein was soluble in the detergent Triton X-114, whereas soluble M protein was not, suggesting a chemical or conformational difference between the two forms. Membranes containing associated M protein were able to bind RNP cores, whereas membranes lacking M protein were not. We suggest that this membrane-bound M fraction constitutes a functional subset of M protein molecules required for the attachment of RNP cores to membranes during normal virus budding.

Intracellular distribution of input vesicular stomatitis virus proteins after uncoating

We have examined the fate of input viral proteins following the uncoating of vesicular stomatitis virus (VSV) by immunofluorescence microscopy, immunoelectron microscopy, and cell fractionation. VSV was adsorbed to BHK cells and allowed to become internalized in the presence of 100 mM NH4Cl; the NH4Cl was then removed to initiate synchronized uncoating. The three major structural proteins of VSV, the matrix protein (M), the nucleocapsid protein (N), and the glycoprotein (G), were each distributed uniquely after uncoating. Immunofluorescence microscopy showed that both G and N proteins retained a punctate distribution, whereas M protein was diffusely distributed throughout the cytoplasm, suggesting that it had become soluble. Immunoelectron microscopy showed that N protein was found in clusters (presumably in intact nucleocapsids) associated with the cell cytoskeleton and in unfused virions in endosomes and lysosomes. M protein was found diffusely distributed throughout the cytoplasm and also in endosomes and lysosomes. G protein was found only in association with endosomes and lysosomes after uncoating. Electrophoretic analysis of the high-speed cytosol fraction from infected cells showed that it contained chiefly M protein. The amount of M protein in the cytosol increased continuously during 90 min of uncoating, confirming its solubilization during uncoating. M protein was not covalently modified by phosphorylation upon uncoating, as evidenced by its mobility on nonequilibrium pH gradient gel electrophoresis. We suggest that those nucleocapsids associating with the cytoskeleton after uncoating may represent the sites of primary viral transcription.

Sequences of the vesicular stomatitis virus matrix protein involved in binding to nucleocapsids

The purpose of these experiments was to study the physical structure of the nucleocapsid-M protein complex of vesicular stomatitis virus by analysis of nucleocapsid binding by wild-type and mutant M proteins and by limited proteolysis. We used the temperature-sensitive M protein mutant tsO23 and six temperature-stable revertants of tsO23 to test the effect of sequence changes on M protein binding to the nucleocapsid as a function of NaCl concentration. The results showed that M proteins from wild-type, mutant, and three of the revertant viruses had similar NaCl titration curves, while the curve for M proteins from the other three revertants differed significantly. The altered NaCl dependence of M protein was correlated with a single amino acid substitution from Phe to Leu at position 111 compared with the original temperature-sensitive mutant and was not correlated with a substitution of Gly to Glu at position 21 in tsO23 and the revertants. To determine whether protease cleavage sites in the M protein were protected by interaction with the nucleocapsid, nucleocapsid-M protein complexes were subjected to limited proteolysis with trypsin, chymotrypsin, or Staphylococcus aureus V8 protease. The initial trypsin and chymotrypsin cleavage sites, located after amino acids 19 and 20, respectively, were as accessible to proteases when M protein was bound to the nucleocapsid as when it was purified, indicating that this region of the protein does not interact directly with the nucleocapsid. Furthermore, trypsin or chymotrypsin treatment released the M protein fragments from the nucleocapsid, presumably due to conformational changes following proteolysis. V8 protease cleaved the M protein at position 34 or 50, producing two distinct fragments. The M protein fragment produced by V8 protease cleavage at position 34 remained associated with the nucleocapsid, while the fragment produced by cleavage at position 50 was released from the nucleocapsid. These results suggest that the amino-terminal region of the M protein around amino acid 20 does not interact directly with the nucleocapsid and that conformational changes resulting from single-amino-acid substitutions at other sites in the M protein are important for this interaction.