Transactivation activity of the human cytomegalovirus IE2 protein occurs at steps subsequent to TATA box-binding protein recruitment

The IE2 protein of human cytomegalovirus transactivates viral and cellular promoters through a wide variety of cis-elements, but the mechanism of its action has not been well characterized. Here, IE2-Sp1 synergy and IE2-TATA box-binding protein (TBP) interaction are examined by artificial recruitmen t of either Sp1 or TBP to the promoter. It was found that IE2 could coopera te with DNA-bound Sp1.A 117 amino acid glutamine-rich fragment of Sp1, whic h can interact with Drosophila TAF(II)110 and human TAF(II)130, was suffici ent for the augmentation of IE2-driven transactivation. In binding assays i n vitro, IE2 interacted directly with the C-terminal region of Sp1,which co ntains the zinc finger DNA-binding domain, but not with its transactivation domain, suggesting that synergy between IE2 and the transactivation domain of Sp1 might be mediated by other proteins such as TAF or TBP. It was also found that TBP recruitment to the promoter markedly increased IE2-mediated transactivation. Thus, IE2 acts synergistically with DNA-bound Sp1 and DNA -bound TBP. These results suggest that, in human cytomegalovirus IE2 transa ctivation, Sp1 functions at an early step such as recruitment of TBP and IE 2 acts to accelerate rate-limiting steps after TBP recruitment.