Abstract
Earlier studies have shown that ICP22 and the U(L)13 protein kinase but not the U(S)3 kinase are required for optimal expression of a subset of late (gamma(2)) genes exemplified by U(L)38, U(L)41, and U(S)11. In primate cells, ICP22 mediates the disappearance of inactive isoforms of cdc2 and degradation of cyclins A and B1. Active cdc2 acquires a new partner, the viral DNA synthesis processivity factor U(L)42. The cdc2-U(L)42 complex recruits and phosphorylates topoisomerase IIalpha for efficient expression of the gamma(2) genes listed above. In uninfected cells, the cdc25C phosphatase activates cdc2 by removing two inhibitory phosphates. The accompanying report shows that in the absence of cdc25C, the rate of degradation of cyclin B1 is similar to that occurring in infected wild-type mouse embryo fibroblast cells but the levels of cdc2 increase, and the accumulation of a subset of late proteins and virus yields are reduced. This report links ICP22 with cdc25C. We show that in infected cells, ICP22 and U(S)3 protein kinase mediate the phosphorylation of cdc25C at its C-terminal domain. In in vitro assays with purified components, both U(L)13 and U(S)3 viral kinases phosphorylate cdc25C and ICP22. cdc25C also interacts with cdc2. However, in infected cells, the ability of cdc25C to activate cdc2 by dephosphorylation of the inactive cdc2 protein is reduced. Coupled with the phosphorylation of cdc25C by the U(S)3 kinase, the results raise the possibility that herpes simplex virus 1 diverts cdc25C to perform functions other than those performed in uninfected cells.