A mutated membrane protein of vesicular stomatitis virus has an abnormal distribution within the infected cell and causes defective budding

Characterization of a Slow-Migrating Component of the Rabies Virus Matrix Protein Strongly Associated with the Viral Glycoprotein

We investigated multiple forms of rabies virus matrix (M) protein. Under non-reducing electrophoretic conditions, we detected, in addition to major bands of monomer forms (23- and 24-kDa) of M protein, an M antigen-positive slow-migrating minor band (about 54 kDa) in both the virion and infected cells. Relative contents of the 54-kDa and monomer components in the virion were about 20-30% and 70-80% of the whole M protein, respectively, while the content of the 54-kDa component was smaller (about 10-20% of the total M protein) in the cell than in the virion. The 54-kDa components could be extracted from the infected cells with sodium deoxycholate, but they were quite resistant to extraction with 1% nonionic detergents by which most monomer components were solubilized. The 54-kDa component was precipitated more efficiently than the monomer by a monoclonal antibody (mAb; #3-9-16), which recognized a linear epitope located at the N-terminal of the M protein. The mAb #3-9-16 coprecipitated the viral glycoprotein (G), which was demonstrated to be due to strong association between the G and 54-kDa component of the M protein. Monomers and the 54-kDa polypeptide migrated to the same isoelectric point (pI) in twodimensional (2-D) gel electrophoresis, implicating that the 54-kDa component was composed of component(s) of the same pI as that of the M protein monomers. From these results, we conclude that the M antigen-positive 54-kDa polypeptide is a homodimer of M protein, taking an N-terminal-exposed conformation, and is strongly associated with the viral glycoprotein. Possible association with a membrane microdomain of the cell will be discussed.

Monoclonal Antibody #3-9-16 Recognizes One of the Two Isoforms of Rabies Virus Matrix Protein That Exposes Its N-Terminus on the Virion Surface

We investigated behaviors of the rabies virus matrix (M) protein using a monoclonal antibody (mAb), #3-9-16, that recognized a linear epitope located at the N-terminus of the protein. Based on the reactivity with this mAb, M proteins could be divided into at least two isoforms; an ordinary major form (Malpha) whose 3-9-16 epitope is hidden, and an N-terminal-exposed epitope-positive form (Mbeta). The Mbeta protein accounted for about 25-30% of the total M proteins in the virion, while its content in the cell ranged from 10 to 15% of total M protein. Fluorescent antibody (FA) staining showed that the Mbeta antigen distributed in the Golgi area where the colocalized viral glycoprotein antigen was also detected. Mbeta antigen was shown to be exposed on the surface of infected cells by both immunoprecipitation and FA staining with the mAb, whereby the cells might have become sensitive to the mAb-dependent complement-mediated cytolysis. Similarly, the Mbeta antigen was shown to be exposed on the virion surface, and the infectivity of the virus was destroyed by the mAb in the presence of a complement. Together with these results, we think that the M protein molecule takes either of two conformations, one (Mbeta) of which exposes the 3-9-16 epitope located in the N-terminal region of the M protein, that are also exposed on the surface of the virion and infected cells, whereby it might play a certain important role(s) in the virus replication process differently from the other form (Malpha), probably through its intimate association with the Golgi area and/or the cell membrane.

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.

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.

Escaping from the cell: assembly and budding of negative-strand RNA viruses

Negative-strand RNA virus particles are formed by a process that includes the assembly of viral components at the plasma membranes of infected cells and the subsequent release of particles by budding. Here, we review recent progress that has been made in understanding the mechanisms of negative-strand RNA virus assembly and bud- ding. Important topics for discussion include the key role played by the viral matrix proteins in assembly of viruses and viruslike particles, as well as roles played by additional viral components such as the viral glycoproteins. Various interactions that contribute to virus assembly are discussed, including interactions between matrix proteins and membranes, interactions between matrix proteins and glycoproteins, interactions between matrix proteins and nucleocapsids, and interactions that lead to matrix protein self-assembly. Selection of specific sites on plasma membranes to be used for virus assembly and budding is described, including the asymmetric budding of some viruses in polarized epithelial cells and assembly of viral components in lipid raft microdomains. Evidence for the involvement of cellular proteins in the late stages of rhabdovirus and filovirus budding is discussed as well as the possible involvement of similar host factors in the late stages of budding of other negative-strand RNA viruses.

Rhabdoviruses and apoptosis

Viral-induced apoptosis is recognized as a common method utilized by viruses to overcome the host. Recent evidence indicates that infection by rhabdoviruses such as vesicular stomatitis virus (VSV), spring viremia of carp virus (SVCV), and rabies virus results in apoptotic cell death. Similar morphological changes and host cell proteins are induced in cells infected with these different viruses; however, the viral proteins responsible for these changes vary. In addition, the molecular mechanism(s) utilized by these viruses to induce apoptosis are on the brink of discovery. This article serves to summarize our current understanding of the apoptotic process during rhabdovirus infection and to illustrate forthcoming areas of study in the field

Requirement for cyclophilin A for the replication of vesicular stomatitis virus New Jersey serotype

Several host proteins have been shown to play key roles in the life-cycle of vesicular stomatitis virus (VSV). We have identified an additional host protein, cyclophilin A (CypA), a chaperone protein possessing peptidyl cis-trans prolyl-isomerase activity, as one of the cellular factors required for VSV replication. Inhibition of the enzymatic activity of cellular CypA by cyclosporin A (CsA) or SDZ-211-811 resulted in a drastic inhibition of gene expression by VSV New Jersey (VSV-NJ) serotype, while these drugs had a significantly reduced effect on the genome expression of VSV Indiana (VSV-IND) serotype. Overexpression of a catalytically inactive mutant of CypA resulted in the reduction of VSV-NJ replication, suggesting a requirement for functional CypA for VSV-NJ infection. It was also shown that CypA interacted with the nucleocapsid (N) protein of VSV-NJ and VSV-IND in infected cells and was incorporated into the released virions of both serotypes. VSV-NJ utilized CypA for post-entry intracellular primary transcription, since inhibition of CypA with CsA reduced primary transcription of VSV-NJ by 85-90 %, whereas reduction for VSV-IND was only 10 %. Thus, it seems that cellular CypA binds to the N protein of both serotypes of VSV. However, it performs an obligatory function on the N protein activity of VSV-NJ, while its requirement is significantly less critical for VSV-IND N protein function. The different requirements for CypA by two serologically different viruses belonging to the same family has highlighted the utilization of specific host factors during their evolutionary lineages.

Role of M protein aggregation in defective assembly of temperature-sensitive M protein mutants of vesicular stomatitis virus

The goal of these experiments was to determine the steps in virus assembly that are defective at the nonpermissive temperature in temperature-sensitive (ts) matrix (M) protein mutants of vesicular stomatitis virus. It has been proposed that mutations in M protein either reduce the binding affinity for nucleocapsids or lead to aggregation, reducing the amount of M protein available for virus assembly. Cytosolic or membrane-derived M proteins from wild-type VSV and two ts M protein mutant viruses, tsM301 and tsO23, as well as a revertant of tsO23 virus, O23R1, were analyzed for binding to nucleocapsid-M protein (NCM) complexes and for M protein aggregation. The experiments presented here showed that ts M proteins synthesized at the nonpermissive temperature were capable of binding to nucleocapsids and that aggregation of ts M proteins did not reduce the amount of soluble M protein below the amount required for assembly of the O23R1 virus. Instead, the most pronounced defect in ts M proteins was in the ability of membrane-derived M proteins to be solubilized in the presence of the detergent Triton X-100. It is proposed that this detergent-insoluble form of M protein interferes with a step necessary to initiate assembly of NCM complexes. A similar detergent, Triton X-114, caused aggregation of membrane-derived wild-type M protein, disproving an earlier proposal that membrane-derived M protein behaves like an integral membrane protein in the presence of Triton X-114. Aggregation of wild-type M protein in the presence of Triton X-100 could be induced by incubation at 37 degrees C with a high-molecular-weight fraction isolated from uninfected cells by sucrose gradient centrifugation. These results implicate host components in inducing M protein aggregation.

Intracellular behavior of rabies virus matrix protein (M) is determined by the viral glycoprotein (G)

To investigate the nature and intracellular behavior of the matrix (M) protein of an avirulent strain (HEP-Flury) of rabies virus, we cloned and sequenced the cDNA of the protein. Using expression vectors pZIP-NeoSV(X)1 and pCDM8, the cDNA was transfected to animal cells (BHK-21 and COS-7) with or without coexpression of viral glycoprotein (G). When M protein alone was expressed in the cells, it displayed homogeneous distribution in the whole cell including the nucleus. In contrast, coexpression with G protein resulted in the abolishment of nuclear distribution of M antigen, and both of the antigens displayed a colocalized distribution in the cell, especially at the cellular membrane as seen in the virus-infected cells, while the distribution of G antigen was not affected by coexpressed M antigen. Immunoprecipitation studies revealed that M protein was coprecipitated with G protein by anti-G antibody, and vice versa, although cross-linking with dithiobis(succinimidyl propionate) was necessary for coprecipitation because of their easier dissociation in the presence of sodium deoxycholate. These results suggest that M protein intimately associates with G protein, which may affect or regulate the behavior (e.g., intracellular localization) of M protein. Studies with deletion mutants of M protein indicate that an internal region around the amino acids from 115 to 151 is essential for the M protein to preserve its binding ability to G protein.

The reovirus mutant tsA279 L2 gene is associated with generation of a spikeless core particle: implications for capsid assembly

Previous studies which used intertypic reassortants of the wild-type reovirus serotype 1 Lang and the temperature-sensitive (ts) serotype 3 mutant clone tsA279 identified two ts lesions; one lesion, in the M2 gene segment, was associated with defective transmembrane transport of restrictively assembled virions (P. R. Hazelton and K. M. Coombs, Virology 207:46-58, 1995). In the present study we show that the second lesion, in the L2 gene segment, which encodes the lambda2 protein, is associated with the accumulation of a core-like particle defective for the lambda2 pentameric spike. Physicochemical, biochemical, and immunological studies showed that these structures were deficient for genomic double-stranded RNA, the core spike protein lambda2, and the minor core protein micro2. Core particles with the lambda2 spike structure accumulated after temperature shift-down from a restrictive to a permissive temperature in the presence of cycloheximide. These data suggest the spike-deficient, core-like particle is an assembly intermediate in reovirus morphogenesis. The existence of this naturally occurring primary core structure suggests that the core proteins lambda1, lambda3, and sigma2 interact to initiate the process of virion capsid assembly through a dodecahedral mechanism. The next step in the proposed capsid assembly model would be the association of the minor core protein mu2, either preceding or collateral to the condensation of the lambda2 pentameric spike at the apices of the primary core structure. The assembly pathway of the reovirus double capsid is further elaborated when these observations are combined with structures identified in other studies.

Inducible and conditional inhibition of human immunodeficiency virus proviral expression by vesicular stomatitis virus matrix protein

Besides its role in viral assembly, the vesicular stomatitis virus (VSV) matrix (M) protein causes cytopathic effects such as cell rounding (D. Blondel, G. G. Harmison, and M. Schubert, J. Virol. 64:1716-1725, 1990). DNA cotransfection assays demonstrated that VSV M protein was able to inhibit the transcription of a reporter gene (B. L. Black and D. S. Lyles, J. Virol. 66:4058-4064, 1992). We have confirmed these observations by using cotransfections with an infectious clone of human immunodeficiency virus type 1 (HIV-1) and found that the amino-terminal 32 amino acids of M protein which are essential for viral assembly were not required for this inhibition. For the study of the potential role of M protein in the shutoff of transcription from chromosomal DNA, we have isolated stable HeLa T4 cell lines which encode either a wild-type or a temperature-sensitive (ts) VSV M gene under control of the HIV-1 long terminal repeat promoter. Transcription of the M mRNA was transactivated after HIV-1 infections. A cell line which encodes the wild-type M protein was nonpermissive for either HIV-1 or HIV-2. A cell line that encodes the ts M gene was transfected with the infectious HIV-1 DNA or was infected with HIV-1 or HIV-2. In all cases, at 32 degrees C, the permissive temperature for M protein, the cells were nonpermissive for HIV replication. At 40 degrees C, the ts M protein was nonfunctional and both HIV-1 and HIV-2 were able to replicate at high levels. A comparison of the amounts of proviral HIV-1 DNAs and HIV-1 mRNAs at 10 and 36 h after HIV-1 infection demonstrated that proviral insertion had not been prevented by M protein and that the block in HIV-1 replication was at the level of proviral expression. The severe reduction of HIV-1 proviral transcripts demonstrates that the VSV M protein alone can inhibit expression from chromosomal DNA. These results strongly support the hypothesis that the VSV M protein is involved in the shutoff of host cell transcription. M protein was able to attenuate HIV-1 infections and protect the cell population from HIV-1 pathogenesis. The temperature-dependent switch from a persistent to a lytic HIV-1 infection in the presence of ts M protein could be useful for studies of HIV-1 replication and pathogenesis.

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.

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.

Sites of in vivo phosphorylation of vesicular stomatitis virus matrix protein

We mapped the in vivo phosphorylation sites for the matrix (M) protein of the Orsay and San Juan strains of vesicular stomatitis virus, Indiana serotype, using limited proteolysis and phosphoamino acid analysis. M protein was solubilized from 32P-labeled virions by using detergent and high-salt conditions, then treated with either trypsin or Staphylococcus aureus V8 protease, and analyzed by polyacrylamide gel electrophoresis and autoradiography to determine which fragments contained phosphate residues. The M protein fragment extending from amino acid 20 to the carboxy terminus contained approximately 70% of the control 32P label, while the fragment extending from amino acid 35 to the carboxy terminus had only trace amounts of label. These data indicate that the major phosphorylation site was between amino acids 20 and 34 in the Orsay strain M protein. Phosphoamino acid analysis of M protein by thin-layer electrophoresis showed the presence of phosphothreonine and phosphoserine and that phosphothreonine continued to be released after prolonged vapor-phase acid hydrolysis. These data identify Thr-31 as the primary in vivo phosphate acceptor for M protein of the Orsay strain of vesicular stomatitis virus. The San Juan strain M protein has serine at position 32, which may also be an important phosphate acceptor. In addition, phosphorylation at Ser-2, -3, or -17 occurs to a greater extent in the San Juan strain M protein than in the Orsay strain M protein. The subcellular distribution of phosphorylated M protein was investigated to determine a probable intracellular site(s) of phosphorylation. Phosphorylated M protein was associated primarily with cellular membranes, suggesting phosphorylation by a membrane-associated kinase. Virion M protein was phosphorylated to a greater extent than membrane-bound M protein, indicating that M protein phosphorylation occurs at a late stage in virus assembly. Phosphorylation of wild-type and temperature-sensitive mutant M protein was studied in vivo at the nonpermissive temperature. The data show that phosphorylated M protein was detected only in wild-type virus-infected cells and virions, suggesting that association with nucleocapsids may be required for M protein phosphorylation or that misfolding of mutant M protein at the nonpermissive temperature prevents phosphorylation.

Insertion of the human immunodeficiency virus CD4 receptor into the envelope of vesicular stomatitis virus particles

Enveloped virus particles carrying the human immunodeficiency virus (HIV) CD4 receptor may potentially be employed in a targeted antiviral approach. The mechanisms for efficient insertion and the requirements for the functionality of foreign glycoproteins within viral envelopes, however, have not been elucidated. Conditions for efficient insertion of foreign glycoproteins into the vesicular stomatitis virus (VSV) envelope were first established by inserting the wild-type envelope glycoprotein (G) of VSV expressed by a vaccinia virus recombinant. To determine whether the transmembrane and cytoplasmic portions of the VSV G protein were required for insertion of the HIV receptor, a chimeric CD4/G glycoprotein gene was constructed and a vaccinia virus recombinant which expresses the fused CD4/G gene was isolated. The chimeric CD4/G protein was functional as shown in a syncytium-forming assay in HeLa cells as demonstrated by coexpression with a vaccinia virus recombinant expressing the HIV envelope protein. The CD4/G protein was efficiently inserted into the envelope of VSV, and the virus particles retained their infectivity even after specific immunoprecipitation experiments with monoclonal anti-CD4 antibodies. Expression of the normal CD4 protein also led to insertion of the receptor into the envelope of VSV particles. The efficiency of CD4 insertion was similar to that of CD4/G, with approximately 60 molecules of CD4/G or CD4 per virus particle compared with 1,200 molecules of VSV G protein. Considering that (i) the amount of VSV G protein in the cell extract was fivefold higher than for either CD4 or CD4/G and (ii) VSV G protein is inserted as a trimer (CD4 is a monomer), the insertion of VSV G protein was not significantly preferred over CD4 or CD4/G, if at all. We conclude that the efficiency of CD4 or CD4/G insertion appears dependent on the concentration of the glycoprotein rather than on specific selection of these glycoproteins during viral assembly.

Structure of the gene encoding the M1 protein of sonchus yellow net virus

The gene encoding the M1 protein of sonchus yellow net virus (SYNV), a plant rhabdovirus, has been sequenced and identified by Western blot analysis of SYNV proteins using antibodies directed against a fusion protein derived from a portion of the sequenced gene. The M1 gene is positioned between nucleotides 4039 and 5109 relative to the 3' end of the viral RNA and is the fourth gene from the 3' end of the genome. The 1071-nucleotide (nt) M1 gene lies between a putative nonstructural gene of unknown function and the gene encoding the glycoprotein and is bordered on either side by the same GG intergenic dinucleotide that separates other genes in the SYNV genome. The M1 mRNA (scRNA 6) consists of a 71-nt untranslated region at the 5' terminus followed by an 858-nt open reading frame (ORF) capable of encoding a protein with a calculated molecular weight of 31,779. The amino acid sequence deduced from this ORF is not highly homologous to those of other rhabdovirus matrix proteins, but has some localized regions of similarity. The UGA codon that terminates the M1 ORF is followed by a 3' untranslated region of 142 nt. The viral RNA (minus-sense) sequence corresponding to the extreme 3' end of the mRNA contains a 9-nt tract (3'-AUUGUUUUU-5') that is identical to the sequences at the termini of other SYNV genes.

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

The membrane-reactive, photoactivatable probe 125I-TID [3-(trifluoromethyl)-3-(m-[125I]iodophenyl)-3H-diazirine] was found to label the M protein of vesicular stomatitis virus about 40% as much as G protein in intact virions, in agreement with labeling studies with other probes. By analyzing limited tryptic digestion and specific chemical cleavage products, the label was essentially entirely localized within the first 19, and probably within the first 5 to 10, amino acid residues at the N terminus, identifying this short amphipathic segment as the likely site of interaction of M protein with the viral bilayer.

Role of matrix protein in cytopathogenesis of vesicular stomatitis virus

The matrix (M) protein of vesicular stomatitis virus (VSV) plays an important structural role in viral assembly, and it also has a regulatory role in viral transcription. We demonstrate here that the M protein has an additional function. It causes visible cytopathic effects (CPE), as evidenced by the typical rounding of polygonal cells after VSV infection. We have analyzed a temperature-sensitive mutant of the M protein of VSV (tsG33) which is defective in viral assembly and which fails to cause morphological changes of the cells after infection at the nonpermissive temperature (40 degrees C). Interestingly, this defect in viral assembly as well as the CPE were reversible. Microinjection of antisense oligonucleotides which specifically inhibit M protein translation also inhibited the occurrence of CPE. Most importantly, when cells were transfected with a cDNA encoding the temperature-sensitive M protein of tsG33, no CPE was observed at the nonpermissive temperature. However, when these cells were shifted to the permissive temperature (32 degrees C), they rounded up and detached from the dish. These results demonstrate that M protein in the absence of the other viral proteins causes rounding of the cells, probably through a disorganization of the cytoskeleton. The absence of CPE at the nonpermissive temperature is correlated with an abnormal dotted staining pattern of M in these cells, suggesting that the mutant M protein may self-aggregate or associate with membranes rather than interact with cytoskeletal elements.

Solubility of vesicular stomatitis virus M protein in the cytosol of infected cells or isolated from virions

The peripheral membrane M protein of vesicular stomatitis virus purified by detergent extraction of virions and ion-exchange chromatography was determined to be a monomer in the absence of detergent at high salt concentrations. Reduction of the ionic strength below 0.2 M resulted in a rapid aggregation of M protein. This self-association was reversible by the detergent Triton X-100 even in low salt. However, aggregation was not reversible by high salt concentration alone. M protein is initially synthesized as a soluble protein in the cytosol of infected cells, thus raising the question of how the solubility of M protein is maintained at physiological ionic strength. Addition of radiolabeled M protein purified from virions to unlabeled cytosol from either infected or uninfected cells inhibited the self-association reaction. Cytosolic fractions from infected or uninfected cells were equally effective at preventing the self-association of M protein. Self-association could also be prevented by an irrelevant protein such as bovine serum albumin. Sedimentation velocity analysis indicated that most of the newly synthesized M protein is monomeric, suggesting that the solubility of M protein in the cytosol is maintained by either low-affinity interaction with macromolecules in the cytosol or interaction of a small population of M-protein molecules with cytosolic components.

Distribution of M protein and nucleocapsid protein of vesicular stomatitis virus in infected cell plasma membranes

The association of M protein and the nucleocapsid (N) protein of vesicular stomatitis virus (VSV) with the cytoplasmic surface of plasma membranes prepared from infected cells was examined by double label immunofluorescence. M protein in association with the cytoplasmic surface of the plasma membrane was distributed in two distinct labeling patterns. Punctate labeling of M protein in the plasma membrane was observed in association with corresponding labeling for the nucleocapsid protein. Diffusely labeled M protein was distributed in areas of the plasma membrane that were devoid of any detectable labeling for the nucleocapsid protein. Similar results were obtained with two different cell types at 4 h and later times postinfection. The diffuse label for M protein was present in membranes prepared from cells infected with a temperature-sensitive M protein mutant at the nonpermissive temperature, but neither the punctate label for M protein nor labeling for the nucleocapsid protein was observed. Upon shift to permissive temperature, both the punctate label for M protein and labeling for the nucleocapsid protein began to reappear in membranes prepared from cells infected with the M protein mutant. These results indicate that M protein can associate with the plasma membrane without prior binding to nucleocapsids and that association of functional M protein with the plasma membrane is required for the stable association of nucleocapsids with the membrane during the process of viral budding.

The M protein of vesicular stomatitis virus associates specifically with the basolateral membranes of polarized epithelial cells independently of the G protein

Using monoclonal antibodies and indirect immunofluorescence microscopy, we investigated the distribution of the M protein in situ in vesicular stomatitis virus-(VSV) infected MDCK cells. M protein was observed free in the cytoplasm and associated with the plasma membrane. Using the ts045 mutant of VSV to uncouple the synthesis and transport of the VSV G protein we demonstrated that this distribution was not related to the presence of G protein on the cell surface. Sections of epon-embedded infected cells labeled with antibody to the M protein and processed for indirect horseradish peroxidase immunocytochemistry revealed that the M protein was associated specifically with the basolateral plasma membrane. The G and M proteins of VSV have therefore evolved features which bring them independently to the basolateral membrane of polarized epithelial cells and allow virus to bud specifically from that membrane.

Vesicular stomatitis virus M protein in the nuclei of infected cells

The M protein of vesicular stomatitis virus (VSV) was localized in the nuclei and cytoplasm of VSV-infected cells by subcellular fractionation and immunofluorescence microscopy. Nuclei isolated from VSV-infected Friend erythroleukemia cells were fractionated into a nuclear membrane and a nucleoplasm fraction by DNase digestion and differential centrifugation. G protein was present in the membrane fraction, and M protein was present in the nucleoplasm fraction. Immunofluorescence detection of M protein in the nucleus required that fixed cells be permeabilized with higher concentrations of detergent than were required for detection of M protein in the cytoplasm of VSV-infected BHK cells.

Site-specific mutations in vectors that express antigenic and temperature-sensitive phenotypes of the M gene of vesicular stomatitis virus

Full-length cDNA copies of mRNAs coding for the matrix (M) proteins of vesicular stomatitis virus and its mutant tsO23(III) were cloned in pBSM13- (BlueScribe). The authenticity of these clones was demonstrated by restriction enzyme mapping, DNA sequencing, and in vitro transcription and translation to identify the two M proteins by Western immunoblotting with epitope-specific monoclonal antibodies. Site-directed mutants were constructed by primer extension of synthetic oligodeoxynucleotides with one or two nucleotide changes to alter the glycine at amino acid 21 of the wild-type (wt) M gene to glutamic acid, alanine, or proline. Similarly, a revertant was created in the M gene of mutant tsO23 by a Glu-21----Gly substitution. A series of wt- and mutant-M-gene chimeras was also constructed to create mutant and revertant clones with Leu----Phe and His----Tyr alterations at amino acids 111 and 227, respectively. We then moved the wt and tsO23 M genes and their site-specific mutants and chimeras cloned in pBSM13- into the eucaryotic expression vector pTF7 directed by the T7 bacteriophage RNA polymerase of the vaccinia virus recombinant vTF1-6,2. Western blot analysis of the M proteins transiently expressed in CV-1 cells by plasmids carrying M genes altered at amino acid 21 revealed that the critical antigenic determinant (epitope 1) is expressed only by the Gly-21 M protein and not by Glu-21, Ala-21, or Pro-21 M proteins. Of particular interest is an apparent conformational change, evidenced by slightly but significantly retarded electrophoretic migration, in plasmid-expressed M proteins with amino acids substituted for glycine at position 21. The glutamic acid at position 21 of tsO23 is not responsible for its temperature-sensitive phenotype, because a tsO23 revertant plasmid with glycine substituted at position 21 fails to rescue tsO23 virus in cells infected at the restrictive temperature; conversely, plasmids expressing wt M protein with substitutions of glutamic acid, alanine, or proline at position 21 are just as effective in marker rescue of tsO23 as is the Gly-21 wt M protein. Marker rescue experiments with wt- and mutant-M-gene chimeras support the hypothesis of K. Morita, R. Vanderoef, and J. Lenard (J. Virol. 61:256-263, 1987) that the temperature-sensitive phenotype of tsO23 is due to a phenylalanine substituted for leucine at amino acid 111, rather than the His-227----Tyr substitution or the Gly-21----Glu substitution, which independently accounts for the loss of epitope 1 in the mutant M protein of tsO23.(ABSTRACT TRUNCATED AT 400 WORDS)

Direct adverse effects of Sendai virus DI particles on virus budding and on M protein fate and stability

Upon infections of BHK cells with a mixture of Sendai standard and defective interfering (DI) viruses (mixed virus infection), viral budding was found to be restricted by factors ranging from 5 to more than 20. The reduced viral budding correlated with a high intracellular M protein turnover. M appeared to be degraded shortly after its synthesis, and seemed not to be able to self-associate in a stable way under the plasma membrane as it did in St virus-infected cells. These data, added to the previous findings that infection with DI particles allowed infected cell survival and favored the cell-surface turnover of the hemagglutinin-neuraminidase protein, led to the hypothesis that DI genomes directly act by preventing the stable formation inside the cells of a viral structure composed of M/HN/nucleocapsids. When involved in this structure M would be protected from degradation and HN would be stably anchored in the plasma membrane. Formation of this structure would be necessary for viral budding and would be damaging for the cells. Comparison with results published by other authors shows that such a model is consistent with other data. It can integrate, as well, data obtained in the analysis of mutant viruses involved in persistence.