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

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 U(s)3 protein kinase. HEp-2 cells infected with the d120 mutant express predominantly alpha proteins. Studies on these proteins revealed the accumulation of a M-r 37,500 protein that reacted with antibody directed against the carboxyl-terminal domain of ICP22. We report that the M-r 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 M-r 37,500 protein was blocked in cells transduced with the U(s)3 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 M-r 37,500 protein with respect to electrophoretic mobility. We conclude that the cellular apoptotic response targets at least one viral protein for destruction. (C) 2003 Elsevier Science (USA).