Assembly of nucleocapsids with cytosolic and membrane-derived matrix proteins of vesicular stomatitis virus

Atomic model of vesicular stomatitis virus and mechanism of assembly

Like other negative-strand RNA viruses (NSVs) such as influenza and rabies, vesicular stomatitis virus (VSV) has a three-layered organization: a layer of matrix protein (M) resides between the glycoprotein (G)-studded membrane envelope and the nucleocapsid, which is composed of the nucleocapsid protein (N) and the encapsidated genomic RNA. Lack of in situ atomic structures of these viral components has limited mechanistic understanding of assembling the bullet-shaped virion. Here, by cryoEM and sub-particle reconstruction, we have determined the in situ structures of M and N inside VSV at 3.47 Å resolution. In the virion, N and M sites have a stoichiometry of 1:2. The in situ structures of both N and M differ from their crystal structures in their N-terminal segments and oligomerization loops. N-RNA, N-N, and N-M-M interactions govern the formation of the capsid. A double layer of M contributes to packaging of the helical nucleocapsid: the inner M (IM) joins neighboring turns of the N helix, while the outer M (OM) contacts G and the membrane envelope. The pseudo-crystalline organization of G is further mapped by cryoET. The mechanism of VSV assembly is delineated by the network interactions of these viral components.

Impact of Respiratory Syncytial Virus Infection on Host Functions: Implications for Antiviral Strategies

Respiratory syncytial virus (RSV) is one of the leading causes of viral respiratory tract infection in infants, the elderly, and the immunocompromised worldwide, causing more deaths each year than influenza. Years of research into RSV since its discovery over 60 yr ago have elucidated detailed mechanisms of the host-pathogen interface. RSV infection elicits widespread transcriptomic and proteomic changes, which both mediate the host innate and adaptive immune responses to infection, and reflect RSV's ability to circumvent the host stress responses, including stress granule formation, endoplasmic reticulum stress, oxidative stress, and programmed cell death. The combination of these events can severely impact on human lungs, resulting in airway remodeling and pathophysiology. The RSV membrane envelope glycoproteins (fusion F and attachment G), matrix (M) and nonstructural (NS) 1 and 2 proteins play key roles in modulating host cell functions to promote the infectious cycle. This review presents a comprehensive overview of how RSV impacts the host response to infection and how detailed knowledge of the mechanisms thereof can inform the development of new approaches to develop RSV vaccines and therapeutics.

Two second-site mutations compensate the engineered mutation of R7A in vesicular stomatitis virus nucleocapsid protein

The functional template for the transcription and replication of vesicular stomatitis virus (VSV) is genomic RNA encapsidated by nucleocapsid (N) protein. Previous studies showed that the amino acid R7 in the N-terminal arm of N is involved in N-N interaction in the N-RNA complex. In our study, the recombinant virus with mutation of R7A (rVSV(R7A)) in N was recovered, and the replication level of passage 1 (P1) of rVSV(R7A) was 1000 times lower than that of wild-type rVSV at 37°C. After eight passages, the replication level of P8 of rVSV(R7A) with two second-site mutations in the genome (T242 P in N protein and U7-U8 in G-L gene junction) was significantly higher than that of P1. Furthermore, we demonstrate that the mutation of either T242P or U7-U8 can compensate the effect caused by the mutation of R7A on the replication of rVSV(R7A). Therefore, we conclude that two second-site mutations both can compensated the engineered mutation of R7A in VSV N protein.

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.

Characterization of the Interaction between the Matrix Protein of Vesicular Stomatitis Virus and the Immunoproteasome Subunit LMP2

The matrix protein (M) of vesicular stomatitis virus (VSV) is involved in virus assembly, budding, gene regulation, and cellular pathogenesis. Using a yeast two-hybrid system, the M globular domain was shown to interact with LMP2, a catalytic subunit of the immunoproteasome (which replaces the standard proteasome catalytic subunit PSMB6). The interaction was validated by coimmunoprecipitation of M and LMP2 in VSV-infected cells. The sites of interaction were characterized. A single mutation of M (I96A) which significantly impairs the interaction between M and LMP2 was identified. We also show that M preferentially binds to the inactive precursor of LMP2 (bearing an N-terminal propeptide which is cleaved upon LMP2 maturation). Furthermore, taking advantage of a sequence alignment between LMP2 and its proteasome homolog, PSMB6 (which does not bind to M), we identified a mutation (L45R) in the S1 pocket where the protein substrate binds prior to cleavage and a second one (D17A) of a conserved residue essential for the catalytic activity, resulting in a reduction of the level of binding to M. The combination of both mutations abolishes the interaction. Taken together, our data indicate that M binds to LMP2 before its incorporation into the immunoproteasome. As the immunoproteasome promotes the generation of major histocompatibility complex (MHC) class I-compatible peptides, a feature which favors the recognition and the elimination of infected cells by CD8 T cells, we suggest that M, by interfering with the immunoproteasome assembly, has evolved a mechanism that allows infected cells to escape detection and elimination by the immune system.

IMPORTANCE

The immunoproteasome promotes the generation of MHC class I-compatible peptides, a feature which favors the recognition and the elimination of infected cells by CD8 T cells. Here, we report on the association of vesicular stomatitis virus (VSV) matrix protein (M) with LMP2, one of the immunoproteasome-specific catalytic subunits. M preferentially binds to the LMP2 inactive precursor. The M-binding site on LMP2 is facing inwards in the immunoproteasome and is therefore not accessible to M after its assembly. Hence, M binds to LMP2 before its incorporation into the immunoproteasome. We suggest that VSV M, by interfering with the immunoproteasome assembly, has evolved a mechanism that allows infected cells to escape detection and elimination by the immune system. Modulating this M-induced immunoproteasome impairment might be relevant in order to optimize VSV for oncolytic virotherapy.

Assembly and budding of negative-strand RNA viruses

Assembly of negative-strand RNA viruses occurs by budding from host plasma membranes. The budding process involves association of the viral core or nucleocapsid with a region of cellular membrane that will become the virus budding site, which contains the envelope glycoproteins and matrix protein. This region of membrane then buds out and pinches off to become the virus envelope. This review will address the questions of what are the mechanisms that bring the nucleocapsid and envelope glycoproteins together to form the virus budding site, and how does this lead to release of progeny virions? Recent evidence supports the idea that viral envelope glycoproteins and matrix proteins are organized into membrane microdomains that coalesce to form virus budding sites. There has also been substantial progress in understanding the last step in virus release, referred to as the "late budding function," which often involves host proteins of the vacuolar protein sorting apparatus. Key questions are raised as to the mechanism of the initial steps in formation of virus budding sites: How are membrane microdomains brought together and how are nucleocapsids selected for incorporation into these budding sites, particularly in the case of viruses for which genome RNA sequences are important for envelopment of nucleocapsids?

Phenotypes of vesicular stomatitis virus mutants with mutations in the PSAP motif of the matrix protein

Vesicular stomatitis virus (VSV) matrix protein (M) has a flexible amino-terminal part that recruits cellular partners. It contains a dynamin-binding site that is required for efficient virus assembly, and two motifs, 24PPPY27 and 37PSAP40, that constitute potential late domains. Late domains are present in proteins of several enveloped viruses and are involved in the ultimate step of the budding process (i.e. fission between viral and cellular membranes). In baby hamster kidney (BHK)-21 cells, it has been demonstrated that the 24PPPY27 motif binds the Nedd4 (neuronal precursor cell-expressed developmentally downregulated 4) E3 ubiquitin ligase for efficient virus budding and that the 37PSAP40 motif, although conserved among M proteins of vesiculoviruses, does not possess late-domain activity. In this study, we have re-examined the contribution of the PSAP motif to VSV budding. First, we demonstrate that VSV M indeed binds TSG101 [tumour susceptibility gene 101; a component of the ESCRT1 (endosomal sorting complex required for transport 1)] through its PSAP motif. Second, we analysed the phenotype of several recombinant mutants. We show that a double mutant with point mutations in both the PSAP and the PPPY motifs is impaired compared with a single mutant in the PPPY motif, indicating that the PSAP motif partially compensates for the lack of the PPPY motif. Mutants’ phenotypes depend on cell lines: in CERA (chicken embryo-related, Alger clone) cells, a recombinant virus with a single mutation in the PSAP motif was impaired compared with the wild type, and a mutant with a single mutation in the dynamin-binding motif was much less impaired in Vero cells than in BSR (clones of BHK-21) cells. These results have implications for the VSV budding pathway that will be discussed.

Lettuce necrotic yellows cytorhabdovirus protein localization and interaction map, and comparison with nucleorhabdoviruses

Lettuce necrotic yellows virus (LNYV), Sonchus yellow net virus (SYNV) and Potato yellow dwarf virus (PYDV) are members of the family Rhabdoviridae that infect plants. LNYV is a cytorhabdovirus that replicates in the cytoplasm, while SYNV and PYDV are nucleorhabdoviruses that replicate in the nuclei of infected cells. LNYV and SYNV share a similar genome organization with a gene order of nucleoprotein (N), phosphoprotein (P), putative movement protein (Mv), matrix protein (M), glycoprotein (G) and polymerase (L). PYDV contains an additional predicted gene of unknown function located between N and P. In order to gain insight into the associations of viral structural and non-structural proteins and the mechanisms by which they may function, we constructed protein localization and interaction maps. Subcellular localization was determined by transiently expressing the viral proteins fused to green or red fluorescent protein in leaf epidermal cells of Nicotiana benthamiana. Protein interactions were tested in planta by using bimolecular fluorescence complementation. All three viruses showed Mv to be localized at the cell periphery and the G protein to be membrane associated. Comparing the interaction maps revealed that only the N-P and M-M interactions are common to all three viruses. Associations unique to only one virus include P-M for LNYV, G-Mv for SYNV and M-Mv, M-G and N-M for PYDV. The cognate N-P proteins of all three viruses interacted and exhibited characteristic changes in localization when co-expressed.

Potential of vesicular stomatitis virus as an oncolytic therapy for recurrent and drug-resistant ovarian cancer

In the last decade, we have gained significant understanding of the mechanism by which vesicular stomatitis virus (VSV) specifically kills cancer cells. Dysregulation of translation and defective innate immunity are both thought to contribute to VSV oncolysis. Safety and efficacy are important objectives to consider in evaluating VSV as a therapy for malignant disease. Ongoing efforts may enable VSV virotherapy to be considered in the near future to treat drug-resistant ovarian cancer when other options have been exhausted. In this article, we review the development of VSV as a potential therapeutic approach for recurrent or drug-resistant ovarian cancer.

The protease-sensitive loop of the vesicular stomatitis virus matrix protein is involved in virus assembly and protein translation

To study the contribution of the protease-sensitive loop of the VSV M protein in virus assembly we recovered recombinant VSV (rVSV) with mutations in this region and examined virus replication. Mutations in the highly conserved LXD motif (aa 123-125) resulted in reduced virion budding, reduced virus titers and enhanced M protein exchange with M-ribonucleocapsid complexes (M-RNPs), suggesting that the mutant M proteins were less tightly associated with RNP skeletons. In addition, viral protein synthesis began to decrease at 4h post-infection (hpi) and was reduced by ~80% at 8 hpi for the mutant rVSV-D125A. The reduced protein synthesis was not due to decreased VSV replication or transcription; however, translation of a reporter gene with an EMCV IRES was not reduced, suggesting that cap-dependent, but not cap-independent translation initiation was affected in rVSV-D125A infected cells. These results indicate that the LXD motif is involved in both virus assembly and VSV protein translation.

Detecting protein-protein interactions in vesicular stomatitis virus using a cytoplasmic yeast two hybrid system

Protein-protein interactions play an important role in many virus-encoded functions and in virus-host interactions. While a "classical" yeast two-hybrid system (Y2H) is one of the most common techniques to detect such interactions, it has a number of limitations, including a requirement for the proteins of interest to be relocated to the nucleus. Modified Y2H, such as the Sos recruitment system (SRS), which detect interactions occurring in the cytoplasm rather than the nucleus, allow proteins from viruses replicating in the cytoplasm to be tested in a more natural context. In this study, a SRS was used to detect interactions involving proteins from vesicular stomatitis virus (VSV), a prototypic non-segmented negative strand RNA (NNS) virus. All five full-length VSV proteins, as well as several truncated proteins, were screened against each other. Using the SRS, most interactions demonstrated previously involving VSV phosphoprotein, nucleocapsid (N) and large polymerase proteins were confirmed independently, while difficulties were encountered using the membrane associated matrix and glycoproteins. A human cDNA library was also screened against VSV N protein and one cellular protein, SFRS18, was identified which interacted with N in this context. The system presented can be redesigned easily for studies in other less tractable NNS viruses.

The matrix protein of vesicular stomatitis virus binds dynamin for efficient viral assembly

Matrix proteins (M) direct the process of assembly and budding of viruses belonging to the Mononegavirales order. Using the two-hybrid system, the amino-terminal part of vesicular stomatitis virus (VSV) M was shown to interact with dynamin pleckstrin homology domain. This interaction was confirmed by coimmunoprecipitation of both proteins in cells transfected by a plasmid encoding a c-myc-tagged dynamin and infected by VSV. A role for dynamin in the viral cycle (in addition to its role in virion endocytosis) was suggested by the fact that a late stage of the viral cycle was sensitive to dynasore. By alanine scanning, we identified a single mutation of M protein that abolished this interaction and reduced virus yield. The adaptation of mutant virus (M.L4A) occurred rapidly, allowing the isolation of revertants, among which the M protein, despite having an amino acid sequence distinct from that of the wild type, recovered a significant level of interaction with dynamin. This proved that the mutant phenotype was due to the loss of interaction between M and dynamin. The infectious cycle of the mutant virus M.L4A was blocked at a late stage, resulting in a quasi-absence of bullet-shaped viruses in the process of budding at the cell membrane. This was associated with an accumulation of nucleocapsids at the periphery of the cell and a different pattern of VSV glycoprotein localization. Finally, we showed that M-dynamin interaction affects clathrin-dependent endocytosis. Our study suggests that hijacking the endocytic pathway might be an important feature for enveloped virus assembly and budding at the plasma membrane.

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.

Biarsenical labeling of vesicular stomatitis virus encoding tetracysteine-tagged m protein allows dynamic imaging of m protein and virus uncoating in infected cells

A recombinant vesicular stomatitis virus (VSV-PeGFP-M-MmRFP) encoding enhanced green fluorescent protein fused in frame with P (PeGFP) in place of P and a fusion matrix protein (monomeric red fluorescent protein fused in frame at the carboxy terminus of M [MmRFP]) at the G-L gene junction, in addition to wild-type (wt) M protein in its normal location, was recovered, but the MmRFP was not incorporated into the virions. Subsequently, we generated recombinant viruses (VSV-PeGFP-DeltaM-Mtc and VSV-DeltaM-Mtc) encoding M protein with a carboxy-terminal tetracysteine tag (Mtc) in place of the M protein. These recombinant viruses incorporated Mtc at levels similar to M in wt VSV, demonstrating recovery of infectious rhabdoviruses encoding and incorporating a tagged M protein. Virions released from cells infected with VSV-PeGFP-DeltaM-Mtc and labeled with the biarsenical red dye (ReAsH) were dually fluorescent, fluorescing green due to incorporation of PeGFP in the nucleocapsids and red due to incorporation of ReAsH-labeled Mtc in the viral envelope. Transport and subsequent association of M protein with the plasma membrane were shown to be independent of microtubules. Sequential labeling of VSV-DeltaM-Mtc-infected cells with the biarsenical dyes ReAsH and FlAsH (green) revealed that newly synthesized M protein reaches the plasma membrane in less than 30 min and continues to accumulate there for up to 2 1/2 hours. Using dually fluorescent VSV, we determined that following adsorption at the plasma membrane, the time taken by one-half of the virus particles to enter cells and to uncoat their nucleocapsids in the cytoplasm is approximately 28 min.

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.

Plasma membrane microdomains containing vesicular stomatitis virus M protein are separate from microdomains containing G protein and nucleocapsids

Immunogold electron microscopy and analysis were used to determine the organization of the major structural proteins of vesicular stomatitis virus (VSV) during virus assembly. We determined that matrix protein (M protein) partitions into plasma membrane microdomains in VSV-infected cells as well as in transfected cells expressing M protein. The sizes of the M-protein-containing microdomains outside the virus budding sites (50 to 100 nm) were smaller than those at sites of virus budding (approximately 560 nm). Glycoprotein (G protein) and M protein microdomains were not colocalized in the plasma membrane outside the virus budding sites, nor was M protein colocalized with microdomains containing the host protein CD4, which efficiently forms pseudotypes with VSV envelopes. These results suggest that separate membrane microdomains containing either viral or host proteins cluster or merge to form virus budding sites. We also determined whether G protein or M protein was colocalized with VSV nucleocapsid protein (N protein) outside the budding sites. Viral nucleocapsids were observed to cluster in regions of the cytoplasm close to the plasma membrane. Membrane-associated N protein was colocalized with G protein in regions of plasma membrane of approximately 600 nm. In contrast to the case for G protein, M protein was not colocalized with these areas of nucleocapsid accumulation. These results suggest a new model of virus assembly in which an interaction of VSV nucleocapsids with G-protein-containing microdomains is a precursor to the formation of viral budding sites.

Role of intermolecular interactions of vesicular stomatitis virus nucleoprotein in RNA encapsidation

The crystal structure of the vesicular stomatitis virus nucleoprotein (N) in complex with RNA reveals extensive and specific intermolecular interactions among the N molecules in the 10-member oligomer. What roles these interactions play in encapsidating RNA was studied by mutagenesis of the N protein. Three N mutants intended for disruption of the intermolecular interactions were designed and coexpressed with the phosphoprotein (P) in an Escherichia coli system previously described (T. J. Green et al., J. Virol. 74:9515-9524, 2000). Mutants N (Delta1-22), N (Delta347-352), and N (320-324, (Ala)(5)) lost RNA encapsidation and oligomerization but still bound with P. Another mutant, N (Ser290-->Trp), was able to form a stable ring-like N oligomer and bind with the P protein but was no longer able to encapsidate RNA. The crystal structure of N (Ser290-->Trp) at 2.8 A resolution showed that this mutant can maintain all the same intermolecular interactions as the wild-type N except for a slight unwinding of the N-terminal lobe. These results suggest that the intermolecular contacts among the N molecules are required for encapsidation of the viral RNA.

Pseudotypes of vesicular stomatitis virus with CD4 formed by clustering of membrane microdomains during budding

Many plasma membrane components are organized into detergent-resistant membrane microdomains referred to as lipid rafts. However, there is much less information about the organization of membrane components into microdomains outside of lipid rafts. Furthermore, there are few approaches to determine whether different membrane components are colocalized in microdomains as small as lipid rafts. We have previously described a new method of determining the extent of organization of proteins into membrane microdomains by analyzing the distribution of pairwise distances between immunogold particles in immunoelectron micrographs. We used this method to analyze the microdomains involved in the incorporation of the T-cell antigen CD4 into the envelope of vesicular stomatitis virus (VSV). In cells infected with a recombinant virus that expresses CD4 from the viral genome, both CD4 and the VSV envelope glycoprotein (G protein) were found in detergent-soluble (nonraft) membrane fractions. However, analysis of the distribution of CD4 and G protein in plasma membranes by immunoelectron microscopy showed that both were organized into membrane microdomains of similar sizes, approximately 100 to 150 nm. In regions of plasma membrane outside of virus budding sites, CD4 and G protein were present in separate membrane microdomains, as shown by double-label immunoelectron microscopy data. However, virus budding occurred from membrane microdomains that contained both G protein and CD4, and extended to approximately 300 nm, indicating that VSV pseudotype formation with CD4 occurs by clustering of G protein- and CD4-containing microdomains.

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.

Sequence and structure relatedness of matrix protein of human respiratory syncytial virus with matrix proteins of other negative-sense RNA viruses

Matrix proteins of viruses within the order Mononegavirales have similar functions and play important roles in virus assembly. Protein sequence alignment, phylogenetic tree derivation, hydropathy profiles and secondary structure prediction were performed on selected matrix protein sequences, using human respiratory syncytial virus matrix protein as the reference. No general conservation of primary, secondary or tertiary structure was found, except for a broad similarity in the hydropathy pattern correlating with the fact that all the proteins studied are membrane-associated. Interestingly, the matrix proteins of Ebola virus and human respiratory syncytial virus shared secondary structure homology.

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.

Rabies virus matrix protein regulates the balance of virus transcription and replication

RNA synthesis by negative-strand RNA viruses (NSVs) involves transcription of subgenomic mRNAs and replication of ribonucleoprotein complexes. In this study, the envelope matrix (M) protein of rabies virus (RV) was identified as a factor which inhibits transcription and stimulates replication. Transcription, but not replication, of RV minigenomes or of full-length RV was decreased by expression of heterologous M. Since RV assembly involving M and the glycoprotein G renders virus synthetically quiescent, an RV was generated with the M and G genes substituted by placeholders. Surprisingly, RNA synthesis by this recombinant was characterized not only by an increased transcription rate but also by a diminished accumulation of replication products. This phenotype was reversed in a dose-dependent manner by providing M in trans, showing that M is a replication-stimulatory factor. The role of M in determining the balance of replication and transcription was further exploited by generation of a recombinant RV with attenuated M expression, which is highly active in transcription. Regulation of RNA synthesis by matrix proteins may represent a general mechanism of nonsegmented NSVs, which is probably obscured by the silencing activity of M during virus assembly.

Ability of the matrix protein of vesicular stomatitis virus to suppress beta interferon gene expression is genetically correlated with the inhibition of host RNA and protein synthesis

The vesicular stomatitis virus (VSV) matrix (M) protein plays a major role in the virus-induced inhibition of host gene expression. It has been proposed that the inhibition of host gene expression by M protein is responsible for suppressing activation of host interferon gene expression. Most wild-type (wt) strains of VSV induce little if any interferon gene expression. Interferon-inducing mutants of VSV have been isolated previously, many of which contain mutations in their M proteins. However, it was not known whether these M protein mutations were responsible for the interferon-inducing phenotype of these viruses. Alternatively, mutations in other genes besides the M gene may enhance the ability of VSV to induce interferons. These hypotheses were tested by transfecting cells with mRNA expressing wt and mutant M proteins in the absence of other viral components and determining their ability to inhibit interferon gene expression. The M protein mutations were the M51R mutation originally found in the tsO82 and T1026R1 mutant viruses, the double substitution V221F and S226R found in the TP3 mutant virus, and the triple substitution E213A, V221F, and S226R found in the TP2 mutant virus. wt M proteins suppressed expression of luciferase from the simian virus 40 promoter and from the beta interferon (IFN-beta) promoter, while M proteins of interferon-inducing viruses were unable to inhibit luciferase expression from either promoter. The M genes of the interferon-inducing mutants of VSV were incorporated into the wt background of a recombinant VSV infectious cDNA clone. The resulting recombinant viruses were tested for their ability to activate interferon gene expression and for their ability to inhibit host RNA and protein synthesis. Each of the recombinant viruses containing M protein mutations induced expression of a luciferase reporter gene driven by the IFN-beta promoter and induced production of interferon bioactivity more effectively than viruses containing wt M proteins. Furthermore, the M protein mutant viruses were defective in their ability to inhibit both host RNA synthesis and host protein synthesis. These data support the idea that wt M protein suppresses interferon gene expression through the general inhibition of host RNA and protein synthesis.

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.

Rhabdoviruses and the cellular ubiquitin-proteasome system: a budding interaction

The matrix (M) proteins of vesicular stomatitis virus (VSV) and rabies virus (RV) play a key role in both assembly and budding of progeny virions. A PPPY motif (PY motif or late-budding domain) is conserved in the M proteins of VSV and RV. These PY motifs are important for virus budding and for mediating interactions with specific cellular proteins containing WW domains. The PY motif and flanking sequences of the M protein of VSV were used as bait to screen a mouse embryo cDNA library for cellular interactors. The mouse Nedd4 protein, a membrane-localized ubiquitin ligase containing multiple WW domains, was identified from this screen. Ubiquitin ligase Rsp5, the yeast homolog of Nedd4, was able to interact both physically and functionally with full-length VSV M protein in a PY-dependent manner. Indeed, the VSV M protein was multiubiquitinated by Rsp5 in an in vitro ubiquitination assay. To demonstrate further that ubiquitin may be involved in the budding process of rhabdoviruses, proteasome inhibitors (e.g., MG132) were used to decrease the level of free ubiquitin in VSV- and RV-infected cells. Viral titers measured from MG132-treated cells were reproducibly 10- to 20-fold lower than those measured from untreated control cells, suggesting that free ubiquitin is important for efficient virus budding. Last, release of a VSV PY mutant was not inhibited in the presence of MG132, signifying that the functional L domain of VSV is required for the inhibitory effect exhibited by MG132. These data suggest that the cellular ubiquitin-proteasome machinery is involved in the budding process of VSV and RV.

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.

Versatility of the accessory C proteins of Sendai virus: contribution to virus assembly as an additional role

The P/C mRNA of Sendai virus (SeV) encodes a nested set of accessory proteins, C', C, Y1, and Y2, referred to collectively as C proteins, using the +1 frame relative to the open reading frame of phospho (P) protein and initiation codons at different positions. The C proteins appear to be basically nonstructural proteins as they are found abundantly in infected cells but greatly underrepresented in the virions. We previously created a 4C(-) SeV, which expresses none of the four C proteins, and concluded that the C proteins are categorically nonessential gene products but greatly contribute to viral full replication and infectivity (A. Kurotani et al., Genes Cells 3:111-124, 1998). Here, we further characterized the 4C(-) virus multiplication in cultured cells. The viral protein and mRNA synthesis was enhanced with the mutant virus relative to the parental wild-type (WT) SeV. However, the viral yields were greatly reduced. In addition, the 4C(-) virions appeared to be highly anomalous in size, shape, and sedimentation profile in a sucrose gradient and exhibited the ratios of infectivity to hemagglutination units significantly lower than those of the WT. In the WT infected cells, C proteins appeared to colocalize almost perfectly with the matrix (M) proteins, pretty well with an external envelope glycoprotein (hemagglutinin-neuraminidase [HN]), and very poorly with the internal P protein. In the absence of C proteins, there was a significant delay of the incorporation of M protein and both of the envelope proteins, HN and fusion (F) proteins, into progeny virions. These results strongly suggest that the accessory and basically nonstructural C proteins are critically required in the SeV assembly process. This role of C proteins was further found to be independent of their recently discovered function to counteract the antiviral action of interferon-alpha/beta. SeV C proteins thus appear to be quite versatile.