Mutations in the PPPY motif of vesicular stomatitis virus matrix protein reduce virus budding by inhibiting a late step in virion release

The N terminus of the matrix (M) protein of vesicular stomatitis virus (VSV ) and of other rhabdoviruses contains a highly conserved PPPY sequence (or PY motif) similar to the late (L) domains in the Gag proteins of some retro viruses. These L domains in retroviral Gag proteins are required for effici ent release of virus particles. In this report, we show that mutations in t he PPPY sequence of the VSV M protein reduce virus yield by blocking a late stage in virus budding. We also observed a delay in the ability of mutant viruses to cause inhibition of host gene expression compared to wild-type ( WT) VSV, The effect of PY mutations on virus budding appears to be due to a block at a stage just prior to virion release, since electron microscopic examination of PPPA mutant-infected cells showed a large number of assemble d virions at the plasma membrane trapped in the process of budding. Deletio n of the glycoprotein (G) in addition to these mutations further reduced th e virus yield to less than 1% of WT levels, and very few particles were ass embled at the cell surface. This observation suggested that G protein aids in the initial stage of budding, presumably during the formation of the bud site. Overall, our results confirm that the PPPY sequence of the VSV M pro tein possesses L domain activity analogous to that of the retroviral Gag pr oteins.