The TRAP/Mediator coactivator complex interacts directly with estrogen receptors alpha and beta through the TRAP220 subunit and directly enhances estrogen receptor function in vitro.

Target gene activation by nuclear hormone receptors, including estrogen receptors (ERs), is thought to be mediated by a variety of interacting cofactors. Here we identify a number of nuclear extract-derived proteins that interact with immobilized ER ligand binding domains in a 17beta-estradiol-dependent manner. The most prominent of these are components of the thyroid hormone receptor-associated protein (TRAP)/Mediator coactivator complex, which interacts with ERalpha and ERbeta in both unfractionated nuclear extracts and purified form. Studies with extracts from TRAP220(-/-) fibroblasts reveal that these interactions depend on TRAP220, a TRAP/Mediator subunit previously shown to interact with ER and other nuclear receptors in a ligand-dependent manner. The physiological relevance of the in vitro interaction is documented further by the isolation of an ERalpha-TRAP/Mediator complex from cultured cells expressing an epitope-tagged ERalpha. Finally, the complete TRAP/Mediator complex is shown to enhance ER function directly in a highly purified cell-free transcription system. These studies firmly establish a direct role for TRAP/Mediator, through TRAP220, in ER function.