Identification of AP-2 as an interactive target of Rb and a regulator of the G1/S control element of the hamster histone H3.2 promoter.

Previous studies have established that a 32 bp cis -regulatory region, referred to as the H3core spanning -241 to -210 of the hamster histone H3.2 promoter, is critical for its G1/S-phase induction of transcription. Here we report that the transcription factor AP-2 is a major component of the protein complex which interacts with the H3core from hamster nuclear extracts. In search of the control mechanism(s) whereby AP-2 can mediate cell cycle regulation of the histone H3.2 promoter, we found that AP-2 can physically interact with the retinoblastoma (Rb) tumor suppressor protein in vitro , and when over-expressed, can also associate with Rb in vivo . Importantly, in contrast to the majority of Rb binding proteins, the C-terminal domain of Rb alone is sufficient for its interaction with AP-2. Using a reporter gene system linking tandem copies of the H3core to a heterologous minimal promoter, we demonstrated that over-expression of AP-2 proteins results in transactivation of the reporter gene through the H3core in a sequence-specific but orientation-independent manner. Additionally, this stimulative effect was suppressed by co-expression of Rb. Thus, AP-2, through its physical and functional interaction with Rb, may contribute to the cell cycle regulation of its target genes.