The UL13 Protein Kinase and the Infected Cell Type Are Determinants of Posttranslational Modification of ICP0

Abstract

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 UL13 gene (ΔUL13) encoding a protein kinase or the α22/US1.5 genes (Δα22/ΔUS1.5) 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 UL13. 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 ΔUL13 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 ΔUL13 virus was due to the absence of the UL13 protein and not to failure of posttranslational modification of Δα22/ΔUS1.5 proteins inasmuch as the mobility of ICP0 in cells infected with Δα22/ΔUS1.5 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 UL13 protein. ICP0 is, however, a substrate for the UL13 kinase inasmuch as ICP0 was phosphorylated in mixtures of immune complexes of ICP0 and UL13. Complexes containing ICP0 only or infected cell lysate proteins reacting with preimmune serum from the rabbit immunized with UL13 protein failed to phosphorylate ICP0. (iv) In the absence of UL13, ICP22 is overproduced—an imbalance attributed to UL13. 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. USA93, 12535–12540); UL13 is involved in the posttranslational modification of ICP0 and is required for both posttranslational processing and control of abundance of ICP22.