Transactivation by the human cytomegalovirus IE2 86-kilodalton protein requires a domain that binds to both the TATA box-binding protein and the retinoblastoma protein.

The human cytomegalovirus major immediate-early (IE) proteins play an indispensable role in regulating viral gene expression. One of these gene products, the IE2 86-kDa protein (IE2 86), is a potent activator of both homologous and heterologous promoters and can form a complex with a component of the basal transcription apparatus, the TATA box-binding protein (TBP). In this report, we show that when IE2 86 is expressed as a glutathione S-transferase (GST)-IE2 86 fusion protein, there are three independent regions that can interact with TBP and with another important cellular regulatory protein, the retinoblastoma gene product (RB). One of these three regions, as well as a domain at the carboxy terminus, contain consensus sites for casein kinase phosphorylation and negatively regulate binding of in vitro-translated IE2 86 to GST-TBP or GST-RB. The dimerization domain of IE2 86 must be present for the interaction of the in vitro-translated protein with GST-TBP and GST-RB. Analysis of IE2 86 mutants in vivo demonstrates that one of the strong binding regions is required for the protein to function as a transactivator. Our results also indicate that domains other than those that interact with TBP and RB are required for the activation function of this protein.