PIAS1 enhances SUMO-1 modification and the transactivation activity of the major immediate-early IE2 protein of human cytomegalovirus

The protein inhibitor of activated STAT1 (PIAS1), known to be a small ubiquitin-like modifier (SUMO) E3 ligase, was found to interact with the human cytomegalovirus IE2 protein. We found that the sumoylation of IE2 was markedly enhanced by wild-type PIAS1 but not by a mutant containing a Cys to Ser substitution at position 351 (C351S) within the RING finger-like domain. In target reporter gene assays, wild-type PIAS1, but not the C351S mutant, enhanced the IE2-mediated transactivations of viral polymerase promoter and cellular cyclin E promoter and this augmentation required the intact sumoylation sites of IE2. Our results suggest that PIAS1 acts as a SUMO E3 ligase toward IE2 and that it may regulate the transactivation function of IE2. To our knowledge, IE2 is the first viral target found to be regulated by a SUMO E3 ligase.