Activity of vesicular stomatitis virus M protein mutants in cell rounding is correlated with the ability to inhibit host gene expression and is not correlated with virus assembly function.

In addition to its role in virus assembly, the matrix (M) protein of vesicular stomatitis virus (VSV) is involved in virus-induced cell rounding and inhibition of host-directed gene expression. Previous experiments have shown that two M protein mutants genetically dissociate the ability of M protein to inhibit host-directed gene expression from its function in virus assembly. M protein from tsO82 virus is fully functional in virus assembly but defective in the inhibition of host-directed gene expression, while the MN1 deletion mutant, which lacks amino acids 4-21, inhibits host-directed gene expression but cannot function in virus assembly. Experiments presented here compared cell rounding induced by these two mutant M proteins to that of wt M protein. BHK cells were transfected with M protein mRNA transcribed in vitro, and the extent of cell rounding was evaluated at 24 hr posttransfection. The MN1 protein was nearly as effective as wt M protein in the induction of cell rounding, while tsO82 M protein expressed from transfected RNA was not able to induce cell rounding above that observed in negative controls without M protein, although it did cause BHK cells to have a less elongated shape. These results indicate that the ability of MN1 and tsO82 M proteins to induce cell rounding is not correlated with their virus assembly function. Instead the cell rounding activity of these mutants is correlated with their ability to inhibit host-directed gene expression. Previous data suggesting that these two cytopathic activities could be dissociated can be readily accounted for by quantitative differences in M protein expression required. Infection of either BHK cells or L cells with tsO82 virus induced cell rounding, although cell rounding was delayed relative to that following infection with wt VSV, suggesting that tsO82 M protein retains some cytopathic activity. The distribution of actin, vimentin, and tubulin in transfected cells was determined by fluorescence microscopy. In cells transfected with tsO82 M mRNA, these cytoskeletal elements were indistinguishable from those of negative control transfected cells. In cells rounded as a result of transfection with wt M or MN1 mRNA, actin-containing filaments were reorganized into a thick perinuclear ring but were not depolymerized. In contrast, tubulin and vimentin appeared to be diffusely distributed throughout the cytoplasm of rounded cells. These results support the idea that cell rounding induced by M protein results from the depolymerization of microtubules and/or intermediate filaments.