Structural regions of ERalpha critical for synergistic transcriptional responses contain co-factor interacting surfaces

Most highly estrogen responsive genes are synergistically activated by multiple copies of estrogen responsive elements (EREs) capable of binding to the estrogen receptor (ER). We examined here the structural features of the receptor necessary to interact with co-regulatory proteins and to produce a synergistic pattern of activation from multiple EREs. Using full length and truncated variants of ERalpha, we show in transfected mammalian cells that although the carboxyl (AF-2) and the amino (AF-1) terminal activation domains are functionally integrated to induce transcription, AF-1 is critical for mediating synergy. Partial characterization of AF-1 sub-domains revealed that both Box-1 and Box-2 regions (amino acids 41-64 and 87-108, respectively) are essential for a synergistic response to estrogen. We show that members of the p160 family of co-factors and TIF-1 interact with the AF-2 domain of ERalpha. We also found that TIF-2, a member of the p160 family, can interact with the Box-1 region of AF-1. Apparently, structural regions required for the ability of ERalpha to induce transcription synergistically from tandem ERE sequences are also critical for the interaction of the receptor with the co-regulatory proteins.