U(S)3 protein kinase of herpes simplex virus 1 blocks caspase 3 activation induced by the products of U(S)1.5 and U(L)13 genes and modulates expression of transduced U(S)1.5 open reading frame in a cell type-specific manner.

The coding domain of the herpes simplex virus type 1 (HSV-1) alpha22 gene encodes two proteins, the 420-amino-acid infected-cell protein 22 (ICP22) and U(S)1.5, a protein colinear with the carboxyl-terminal domain of ICP22. In HSV-1-infected cells, ICP22 and U(S)1.5 are extensively modified by the U(L)13 and U(S)3 viral protein kinases. In this report, we show that in contrast to other viral proteins defined by their properties as alpha proteins, U(S)1.5 becomes detectable and accumulated only at late times after infection. Moreover, significantly more U(S)1.5 protein accumulated in cells infected with a mutant lacking the U(L)13 gene than in cells infected with wild-type virus. To define the role of viral protein kinases on the accumulation of U(S)1.5 protein, rabbit skin cells or Vero cells were exposed to recombinant baculoviruses that expressed U(S)1.5, U(L)13, or U(S)3 proteins under a human cytomegalovirus immediate-early promoter. The results were as follows. (i) Accumulation of the U(S)1.5 protein was reduced by concurrent expression of the U(L)13 protein kinase and augmented by concurrent expression of the U(S)3 protein kinase. The magnitude of the reduction or increase in the accumulation of the U(S)1.5 protein was cell type dependent. The effect of U(L)13 kinase appears to be specific inasmuch as it did not affect the accumulation of glycoprotein D in cells doubly infected by recombinant baculoviruses expressing these genes. (ii) The reduction in accumulation of the U(S)1.5 protein was partially due to proteasome-dependent degradation. (iii) Both U(S)1.5 and U(L)13 proteins activated caspase 3, indicative of programmed cell death. (iv) Concurrent expression of the U(S)3 protein kinase blocked activation of caspase 3. The results are concordant with those published elsewhere (J. Munger and B. Roizman, Proc. Natl. Acad. Sci. USA 98:10410-10415, 2001) that the U(S)3 protein kinase can block apoptosis by degradation or posttranslational modification of BAD.