Matrix protein and another viral component contribute to induction of apoptosis in cells infected with vesicular stomatitis virus

The induction of apoptosis in host cells is a prominent cytopathic effect o f vesicular stomatitis virus (VSV) infection. The viral matrix (M) protein is responsible for several important cytopathic effects, including the inhi bition of host gene expression and the induction of cell rounding in VSV-in fected cells. This raises the question of whether M protein is also involve d in the induction of apoptosis. HeLa or BHK cells were transfected with M mRNA to determine whether M protein induces apoptosis when expressed in the absence of other viral components. Expression of M protein induced apoptot ic morphological changes and activated caspase-3 in both cell types, indica ting that M protein induces apoptosis in the absence of other viral compone nts. An M protein containing a point mutation that renders it defective in the inhibition of host gene expression (M51R mutation) activated little, if any, caspase-3, while a deletion mutant lacking amino acids 4 to 21 that i s defective in the virus assembly function but fully functional in the inhi bition of host gene expression was as effective as wild-type (wt) M protein in activating caspase-3. To determine whether M protein influences the ind uction of apoptosis in the context of a virus infection, the M51R M protein mutation was incorporated onto a wt background by using a recombinant infe ctious cDNA clone (rM51R-M virus). The timing of the induction of apoptosis by rM51R-M virus was compared to that by the corresponding recombinant wt (rwt) virus and to that by tsO82 virus, the mutant virus in which the M51R mutation was originally identified. In HeLa cells, rwt virus induced apopto sis faster than did rM51R-M virus, demonstrating a role for M protein in th e induction of apoptosis. In contrast to the results obtained with HeLa cel ls, rwt virus induced apoptosis more slowly than did rM51R-M virus in BHK c ells. This indicates that a viral component other than M protein contribute s to induction of apoptosis in BHK cells and that wt M protein acts to dela y induction of apoptosis by the other viral component. tsO82 virus induced apoptosis more rapidly than did rM51R-M virus in both HeLa and BHK cells. T hese two viruses contain the same point mutation in their M proteins, sugge sting that sequence differences in genes other than that for M protein affe ct their rates of induction of apoptosis.