Role of noncovalent SUMO binding by the human cytomegalovirus IE2 transactivator in lytic growth.

The 86-kDa immediate-early 2 (IE2) protein of human cytomegalovirus (HCMV) is a promiscuous transactivator essential for viral gene expression. IE2 is covalently modified by SUMO at two lysine residues (K175 and K180) and also interacts noncovalently with SUMO. Although SUMOylation of IE2 has been shown to enhance its transactivation activity, the role of SUMO binding is not clear. Here we showed that SUMO binding by IE2 is necessary for its efficient transactivation function and for viral growth. IE2 bound physically to SUMO-1 through a SUMO-interacting motif (SIM). Mutations in SIM (mSIM) or in both SUMOylation sites and SIM (KR/mSIM), significantly reduced IE2 transactivation effects on viral early promoters. The replication of IE2 SIM mutant viruses (mSIM or KR/mSIM) was severely depressed in normal human fibroblasts. Analysis of viral growth curves revealed that the replication defect of the mSIM virus correlated with low-level accumulation of SUMO-modified IE2 and of viral early and late proteins. Importantly, both the formation of viral transcription domains and the association of IE2 with viral promoters in infected cells were significantly reduced in IE2 SIM mutant virus infection. Furthermore, IE2 was found to interact with the SUMO-modified form of TATA-binding protein (TBP)-associated factor 12 (TAF12), a component of the TFIID complex, in a SIM-dependent manner, and this interaction enhanced the transactivation activity of IE2. Our data demonstrate that the interaction of IE2 with SUMO-modified proteins plays an important role for the progression of the HCMV lytic cycle, and they suggest a novel viral mechanism utilizing the cellular SUMO system.