US3 protein kinase of HSV-1 cycles between the cytoplasm and nucleus and interacts with programmed cell death protein 4 (PDCD4) to block apoptosis

The US3 protein kinase of herpes simplex virus 1 plays a key role in blocking apoptosis induced by viral gene products or exogenous agents. The US3 protein kinase is similar to protein kinase A with respect to substrate range and specificity. We report that in the yeast two-hybrid system a domain of US3 essential for antiapoptotic activity reacted with programmed cell death protein 4 (PDCD4). We report that US3 interacts with PDCD4, that PDCD4 is posttranslationally modified in infected cells both in a US3-dependent and -independent fashion, and that depletion of PDCD4 by siRNA blocked apoptosis induced by a Δα4 mutant virus. In infected cells, PDCD4 accumulates in the nucleus, whereas US3 accumulates in the cytoplasm. Studies designed to elucidate the convergence of these proteins led to the discovery that US3 protein kinase cycles between the nucleus and cytoplasm and that US3 retains PDCD4 in infected cell nuclei.