Binding of p53 to the KIX domain of CREB binding protein. A potential link to human T-cell leukemia virus, type I-associated leukemogenesis.

The pleiotropic cellular coactivator CREB binding protein (CBP) plays a critical role in supporting p53-dependent tumor suppressor functions. p53 has been shown to directly interact with a carboxyl-terminal region of CBP for recruitment of the coactivator to p53-responsive genes. In this report, we identify the KIX domain as a new p53 contact point on CBP. We show that both recombinant and endogenous forms of p53 specifically interact with KIX. We demonstrate that the activation domain of p53 participates in KIX binding and provide evidence showing that this interaction is critical for p53 transactivation function. The human T-cell leukemia virus, type-I-encoded oncoprotein Tax is a well established repressor of p53 transcription function. Like p53, Tax also binds to KIX. The finding that both transcription factors bind to a common region of CBP suggests that coactivator competition may account for the observed repression. We demonstrate reciprocal repression between Tax and p53 in transient transfection assays, supporting the idea of intracellular coactivator competition. We biochemically confirm coactivator competition by directly showing that both transcription factors bind to KIX in a mutually exclusive fashion. These data provide molecular evidence for the observed intracellular competition and suggest that Tax inhibits p53 function by abrogating a novel p53-KIX interaction. Thus, Tax competition for the p53-KIX complex may be a pivotal event in the human T-cell leukemia virus, type I transformation pathway.