Infected cell protein No. 22 is subject to proteolytic cleavage by caspases activated by a mutant that induces apoptosis

Abstract

Earlier reports have shown that the d120 mutant of herpes simplex virus 1 lacking both copies of the gene encoding the infected cells protein No. 4 (ICP4) induces apoptosis in a variety of cell lines. The programmed cell death induced by this mutant is blocked by overexpression of Bcl-2 or by transduction of infected cells with the gene encoding the viral US3 protein kinase. HEp-2 cells infected with the d120 mutant express predominantly α proteins. Studies on these proteins revealed the accumulation of a Mr 37,500 protein that reacted with antibody directed against the carboxyl-terminal domain of ICP22. We report that the Mr 37,500 protein is a product of the proteolytic cleavage of ICP22 by a caspase activated by the d120 mutant. Thus the accumulation of the Mr 37,500 protein was blocked in cells transduced with the US3 protein kinase, in cells overexpressing Bcl-2, or in infected cells treated with the general caspase inhibitor zVAD-fmk. Exposure of ICP22 made in wild-type virus-infected cells to caspase 3 yielded two polypeptides, of which one could not be differentiated from the Mr 37,500 protein with respect to electrophoretic mobility. We conclude that the cellular apoptotic response targets at least one viral protein for destruction. © 2003 Elsevier Science (USA).