The U(L)13 protein kinase and the infected cell type are determinants of posttranslational modification of ICP0

The herpes simplex virus infected-cell protein 0 (ICP0) acts as a promiscuous transactivator of genes introduced into eukaryotic cells by transfection or infection. The protein is highly posttranslationally modified by phosphorylation and nucleotidylylation. We have examined the electrophoretic mobility and phosphorylation of ICP0 in Vero and rabbit skin cells infected with wild-type virus or viruses from which the U(L)13 gene (Delta U(L)13) encoding a protein kinase or the alpha 22/U-S 1.5 genes (Delta alpha 22/Delta U-S 1.5) encoding putative transcriptional factors has been deleted. We report the following: (i) The accumulation of ICP0 and the electrophoretic mobility of ICP0 were dependent on the nature of the infected cell type and the presence of U(L)13. ICP0 encoded by wild-type virus accumulated to maximum levels earlier in infected Vero cells and its electrophoretic mobility was slower than that made in rabbit skin cells. In both Vero and rabbit skin cells infected with the Delta U(L)13 virus, the prevailing ICP0 form migrated faster than that accumulating in the corresponding cells infected with wild-type virus. (ii) The alteration in electrophoretic mobility of ICP0 made in cells infected with Delta U(L)13 virus was due to the absence of the U(L)13 protein and not to failure of posttranslational modification of Delta alpha 22/Delta U(S)1.5 proteins inasmuch as the mobility of ICP0 in cells infected with Delta alpha 22/Delta U(S)1.5 virus could not be differentiated from that of wild-type infected cells. (iii) ICP0 is extensively phosphorylated in infected cells even in the absence of U(L)13 protein. ICP0 is, however, a substrate for the U(L)13 kinase inasmuch as ICP0 was phosphorylated in mixtures of immune complexes of ICP0 and U(L)13. Complexes containing ICP0 only or infected cell lysate proteins reacting with preimmune serum from the rabbit immunized with U(L)13 protein failed to phosphorylate ICP0. (iv) In the absence of U(L)13, ICP22 is overproduced-an imbalance attributed to U(L)13. Thus, ICP22 regulates both the utilization of splice acceptor sites and the longevity of ICP0 mRNA (K. L Carter and B. Roizman, 1996, Proc. Natl. Acad. Sci. USA 93, 12535-12540); U(L)13 is involved in the posttranslational modification of ICP0 and is required for both posttranslational processing and control of abundance of ICP22. (C) 1997 Academic Press.