In vitro Phosphorylation of NS Protein by the L Protein of Vesicular Stomatitis virus

Abstract

The structural proteins L and NS of vesicular stomatitis virus were obtained from purified viral ribonucleoprotein complex followed by phosphocellulose column chromatography and assayed for protein kinase activity using [.gamma.-32P]ATP as the phosphate donor. The fractions containing purified L protein phosphorylated NS protein in vitro. 8-Azido-ATP, a photoreactive analog of ATP, was also used as the phosphate donor for phosphorylation of NS protein by the L protein. In the presence of UV light, only L protein was specifically cross-linked with 8-azido-[.gamma.-32P]ATP. In the absence of UV light 8-azido ATP did not inhibit RNA synthesis in vitro. Thus, 8-azido-ATP was probably bound to the kinase site and phosphorylation of NS protein was evidently mediated by the L protein. Exogenous phosphate acceptor proteins such as phosvitin and casein were also phosphorylated by the L protein fraction. However, addition of an excess of phosvitin failed to compete with the phosphorylation of NS by L, indicating that the protein kinase activity possessed higher affinity for NS. The phosphorylation of NS was strongly inhibited by photoreaction of L protein with 8-azido-ATP with concomitant inhibition of transcription in vitro. These results suggest that phosphorylation of NS protein by L may have a role in the regulation of the virus genome transcription in vitro.