Abstract
Human cytomegalovirus (HCMV) is a kind of high prevalence herpesvirus, and the immediate-early protein 2 (IE2) is one of the critical regulatory viral proteins of HCMV. In this study, we demonstrate that IE2 regulates transcriptional process by affecting common transcription cofactors on both cellular and viral promoters. Various viral proteins can target cellular transcriptional regulator to modulate transcription in which histone acetyltransferases (HATs) serve as a key common target for viral proteins. The immediate-early 2 protein (IE2) of human cytomegalovirus (HCMV) interacts with the tumor suppressor p53, and downregulates its functions by unknown mechanisms. In our study, we demonstrate that IE2 influences the p53 functions through inhibiting the acetyltransferase activity of the p53 coactivators, p300 and CREB-binding protein (CBP), which in turn inhibits functions of both histones and p53. IE2 can directly interact with p300 and CBP, and the minimal HAT inactivation region on IE2 contains the N-terminal 98 amino acids. It also decreases the level of local acetylated histones on p53-dependent promoters, such that the in vivo DNA binding ability of p53 is influenced. However, the mutant IE2 proteins that lack the HAT inhibition region do not have similar function. In addition, the p53 acetylation site mutant, K320/373/382R, retains both DNA binding and promoter transactivation activity in vivo and these effects are suppressed by IE2 as well. On the other hand, we have also observed that only wild-type IE2 has an antiapoptotic effect. Together with these finding, our results imply that HCMV IE2 represses p53-dependent gene activation through inhibiting p300/CBP-mediated local histone acetylation and that IE2 may have oncogenic activity. In addition, IE2 plays an important role in viral replication. To enter lytic cycle, HCMV requires full activation of the HCMV major immediate-early promoter (MIEP) whose activity is shown to be autorepressed by the IE2 in vitro, with unknown mechanism and physiological significance. Here we demonstrate that IE2 mediates local chromatin structure remodeling via recruitment of distinct coregulators to MIEP in HCMV latently or lytically infected cells. IE2-binding to MIEP in HCMV non-permissive cells results in inhibition of the MIEP activity in a cis-repression sequence (crs)- and histone deacetylase-dependent manner. In cells latently infected by HCMV, IE2 binding to crs recruits histone 3 K9 methylatransferases as well as the Mi-2 corepressor complex which involves distinct activities such as ATP-dependent chromatin remodeling, histone deacetylation and binding to methylated CpG. Consistently, IE2 also mediates local DNA methylation. In contrast, IE2 recruits coactivator PCAF to MIEP in HCMV lytically infected cells. Taken together, these studies indicate that IE2 recruits specific chromatin coregulators to regulate the activity of MIEP and this may potentially play an important role to determinate whether HCMV enter latency or lytic cycle.