ENHANCEMENT OF ESTROGEN-RECEPTOR TRANSCRIPTIONAL ACTIVITY BY THE COACTIVATOR GRIP-1 HIGHLIGHTS THE ROLE OF ACTIVATION FUNCTION-2 IN DETERMINING ESTROGEN-RECEPTOR PHARMACOLOGY

The human estrogen receptor (ER) contains two major activation functio ns (AFs) responsible for its transcriptional activity. One of these, a ctivation function 2 (AF-2), located within the hormone-binding domain (HBD), has been shown to mediate the ligand-dependent transcriptional activity of ER as well as other members of the nuclear receptor super family. Recently, proteins interacting with the HBD of several nuclear receptors have been cloned. One of these proteins, glucocorticoid rec eptor interacting protein (GRIP-1), has been shown to interact with ER and was originally hypothesized to mediate its transcriptional activi ty through AF-2. However, we find in this study that the transcription al activity of ER, containing mutations in the AF-2 core sequence, can be enhanced by coexpression of the coactivator GRIP-I, suggesting tha t this protein may not rely solely on the AF-2 domain for interaction. We propose, therefore, that the HBD of ER either contains multiple bi nding sites that are necessary for association with GRIP-I or, alterna tively, that this coactivator contacts the receptor in an undetermined region within the HBD. Importantly, these studies demonstrate also th at mutations or deletion of AF-2 alter the ligand pharmacology of the receptor such that ER loses the ability to discriminate between agonis ts and antagonists. Interestingly, on these mutant receptors GRIP-I st ill functions as a coactivator independent of the nature of the bound ligand. It is likely, therefore, that the C-terminal AF-2 domain may f unction as a molecular switch allowing the wild-type receptor to discr iminate between agonists and antagonists as well as providing a surfac e with which associated proteins can interact.