Immunocytochemical localization of vesicular stomatitis virus proteins N and NS with monoclonal antibodies

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

Two temperature-sensitive (ts) mutants of the M protein of vesicular stomatitis virus (tsG31 and tsG33) are defective in viral assembly, but the exact nature of this defect is not known. When infected cells are switched from nonpermissive (40 degrees C) to permissive (32 degrees C) temperatures in the presence of cycloheximide, tsG33 virus release increased by 100-fold, whereas tsG31 release increased only by 10-fold. Thus, the tsG33 defect is more reversible than that of tsG31. Therefore, we investigated how the altered synthesis and cellular distribution of tsG33 M protein correlates with the viral assembly defect. At 32 degrees C tsG33 M protein is stained diffusely in the cell cytoplasm and later at the budding sites. In contrast, at 40 degrees C the mutant M protein formed unusual aggregates mostly located in the perinuclear regions of virus-infected cells and partially colocalized with G protein in this region. In temperature shift-down experiments, M can be disaggregated and used to some extent for nucleocapsid coiling and budding, which correlates with the virus titer increase. M aggregates also formed after shift-up from 32 to 40 degrees C, indicating a complete dependence of M aggregation on the temperature. Biochemical analysis with sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting revealed that at 40 degrees C M protein is detected exclusively in pellet fractions (nuclear and cytoskeleton components), whereas at 32 degrees C M protein is mainly in the cytoplasmic soluble fractions. Furthermore, when the temperature is raised from 32 to 40 degrees C, the distribution of M protein tends to shift from the soluble to the pellet and cytoskeletal fractions. Electron micrographs of immunoperoxidase-labeled M protein showed that at 40 degrees C M aggregates are often associated with the outer nuclear membranes as well as with vesicular structures. No nucleocapsid coiling was observed in these cells, whereas coiling and budding were seen at 32 degrees C in cells where M protein was partly associated with the plasma membrane. We suggest that the tsG33 M protein mutation may produce a reversible conformational alteration which causes M protein to aggregate at 40 degrees C, therefore inhibiting the proper association of M protein with nucleocapsids and budding membranes.

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

The purpose of this paper is to describe the immunocytochemical localization of M protein of vesicular stomatitis virus (VSV) in infected cells. Vero cells, MDBK cells, Swiss 3T3 cells, and BHK cells were examined at various times after infection. For immunofluorescent staining, the cells were fixed with PLP fixative and then treated with 0.05% Triton X-100 before incubation with antibodies. Three hours after infection, M protein exhibited diffuse immunostaining throughout the cytoplasm and later accumulated along the cell membrane. The localization of M protein differed from the granular localization of the nucleocapsid N protein of VSV in the cytoplasm. For electron microscopy, the cells were fixed first in a mixture of 2% paraformaldehyde and 0.05% glutaraldehyde and then with PLP fixative, this being followed by treatment with 0.05% saponin. They were then immunostained using the immunoperoxidase method. The M protein was found to be distributed throughout the cytoplasm and later under the cell membrane, especially at virus budding sites. We also used postembedding immunostaining and freeze-fracture immunostaining to avoid the translocation of M protein caused by the detergent treatment. These techniques confirmed our previous results. Our findings are consistent with the view that the M protein of VSV is synthesized on free ribosomes and is then associated with the cell membrane where viral assembly may occur.

Immunocytochemical study on the cytoplasmic side of cell membranes infected with vesicular stomatitis virus by quick-freezing and deep-etching replica method

Synthesized N protein of vesicular stomatitis virus (VSV) is associated with replicated viral genomes in the infected cells. The cytoplasmic side of cell membranes was examined by quick-freezing and deep-etching replica method, in order to clarify the localization of VSV genomes. Control or infected monolayer Vero cells were fixed in 2% paraformaldehyde, scraped and centrifuged to make pellets. A drop of the cell pellet was put between two glass coverslips, which were coated with 3-aminopropyl triethoxy silane and glutaraldehyde. The cells were consequently split open and postfixed in the mixture of glutaraldehyde and paraformaldehyde. Some inside-out cell membranes on the coverslips were immunostained with anti-N monoclonal antibody directly coupled to gold particles. Others were immunostained with anti-N monoclonal antibody and rabbit anti-mouse IgG coupled to peroxidase and fixed again in glutaraldehyde. They were incubated in diaminobenzidine and hydrogen peroxide solution for 1 min. All of them were infiltrated with 10% methanol in distilled water and quickly frozen in a mixture of isopentane and propane cooled by liquid nitrogen. Such preparations were deep-etched and shadowed by platinum and carbon. Although many cell organelles were found to be associated with the cytoplasmic side of cell membranes in the normal Vero cells, few cell organelles were attached to it in the infected cells. On the contrary, special strand structures were identified, which could be immunostained with anti-N monoclonal antibody. It is concluded that platinum replicas have sufficient resolution to identify the VSV genomes coated with N protein and that these nucleocapsids can be associated with the cytoplasmic side of cell membranes in the infected cells.