Immunocytochemical study of the intracellular localization of M protein of vesicular stomatitis virus

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 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.

Double-layered membrane vesicles released from mammalian cells infected with Sendai virus expressing the matrix protein of vesicular stomatitis virus

The matrix (M) protein of vesicular stomatitis virus (VSV) was reported to form vesicles on the cell surface and subsequently to be released into the cultured medium when expressed from cDNA by virus vectors. To further investigate VSV M activity, we generated a recombinant Sendai virus (SeV) expressing the VSV M protein (SeV-M(VSV)). When cells were infected with SeV-M(VSV), VSV M was found abundantly in the culture medium. Electron microscopy demonstrated the budding of two-membraned vesicles (>/= 0.8 microm in diameter) from the infected cells. The outer membrane of the vesicle was derived from the plasma membrane and the inner one possibly derived from the membrane of an intracellular vesicle. Immuno-gold labeling showed that VSV M was exclusively located in a double-layered region. The released membranes were divided into three parts: the VSV M vesicles with SeV F and HN glycoproteins, SeV particles, and vesicles associated with the cytosolic components. The last abundantly contained phosphorylated SeV matrix (M) protein, which is not released in a usual SeV infection. Furthermore the VSV M protein expressed without using a virus vector was efficiently released into the culture medium. These results suggest that the VSV M protein has a budding activity per se and that SeV proteins are passively involved in the release of VSV M.

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

During budding of vesicular stomatitis virus (VSV), the viral matrix (M) protein binds the viral nucleocapsid to the host plasma membrane and condenses the nucleocapsid into the tightly coiled nucleocapsid-M protein (NCM) complex observed in virions. In infected cells, the viral M protein exists mostly as a soluble molecule in the cytoplasm, and a small amount is bound to the plasma membrane. Despite the high concentrations of M protein and intracellular nucleocapsids in the cytoplasm, they are not associated with each other except at the sites of budding. The experiments presented here address the question of why M protein and nucleocapsids associate with each other only at the plasma membrane but not in the cytoplasm of infected cells. An assay for exchange of soluble M protein into NCM complexes in vitro was used to show that both cytosolic and membrane-derived M proteins bound to virion NCM complexes with affinities similar to that observed for virion M protein, indicating that both cytosolic and membrane-derived M proteins are competent for virus assembly. However, neither cytosolic nor membrane-derived M protein bound to intracellular nucleocapsids with the same high affinity observed for virion NCM complexes. Cytosolic M protein was able to bind intracellular nucleocapsids, but with an affinity approximately eightfold less than that observed in virion NCM complexes. Membrane-derived M protein exhibited little or no binding activity for intracellular nucleocapsids. These data indicate that intracellular nucleocapsids, and not intracellular M proteins, need to undergo an assembly-initiating event in order to assemble into an NCM complex. Since neither membrane-derived nor cytosolic M protein could initiate high-affinity binding to intracellular nucleocapsids, the results suggest that another viral or host factor is required for assembly of the NCM complex observed in virions.

Freeze-fracture immunocytochemistry for intracellular localization of serotonin in mast cells stimulated with compound 48/80

Changes of intracellular localization of serotonin in rat mast cells were examined by freeze-fracture immunocytochemistry, to prevent the translocation of the serotonin antigen. Rat peritoneal cells including mast cells were stimulated in vitro with compound 48/80, at 17 degrees C for 0, 30 or 60 s for exocytosis to occur. The mast cells were fixed, quickly frozen and freeze-fractured to expose the antigen on the fractured surface. They were immunostained with serotonin antibody, and the immunoreactions on the fractured surface were examined on ultrathin sections by electron microscopy. Unstimulated mast cells exhibited serotonin localization mostly in each intragranular matrix. In contrast, mast cells stimulated for 30 s exhibited increased serotonin in their intergranular cytoplasm. Mast cells showed more distinct immunoreactions in the cytoplasm where degranulation would be promoted after 60 s. It is suggested that intracellular release of serotonin occurred in the stimulated mast cells.

Immunoelectron microscopic localization of fibronectin in cultured human keratinocytes

We investigated the ultrastructural localization of fibronectin (FN) in cultured human keratinocytes using an improved method of peroxidase-immunoelectron microscopy. This method enabled us to visualize the precise localization of FN within the cells while preserving the morphology. FN was localized in the protein synthetic and secretory organelles, including the rough-surfaced endoplasmic reticulum, Golgi complex, multivesicular bodies and perinuclear space. It was also detected in the extracellular space, on small regions of the villous projections of cell membranes at the site of secretion and at cell-substratum contact sites. These findings confirm that human keratinocytes synthesize, secrete and deposit FN in the pericellular matrix.

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.

Membrane fusion of enveloped viruses: especially a matter of proteins

To infect mammalian cells, enveloped viruses have to deposit their nucleocapsids into the cytoplasm of a host cell. Membrane fusion represents a key element in this entry mechanism. The fusion activity resides in specific, virally encoded membrane glycoproteins. Some molecular properties of these fusion proteins will be briefly described. These properties will then be correlated to the ability of a virus to fuse with target membranes, and to induce cell-cell fusion. Some molecular and physical parameters affecting virus fusion--at the level of either viral or target membrane or both--and the significance of modelling virus fusion by using synthetic peptides resembling viral fusion peptides, will also be discussed.

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.