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 functions (AFs) responsible for its transcriptional activity. One of these, activation 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 superfamily. Recently, proteins interacting with the HBD of several nuclear receptors have been cloned. One of these proteins, glucocorticoid receptor interacting protein (GRIP-1), has been shown to interact with ER and was originally hypothesized to mediate its transcriptional activity through AF-2. However, we find in this study that the transcriptional activity of ER, containing mutations in the AF-2 core sequence, can be enhanced by coexpression of the coactivator GRIP-I, suggesting that this protein may not rely solely on the AF-2 domain for interaction. We propose, therefore, that the HBD of ER either contains multiple binding sites that are necessary for association with GRIP-I or, alternatively, that this coactivator contacts the receptor in an undetermined region within the HBD. Importantly, these studies demonstrate also that mutations or deletion of AF-2 alter the ligand pharmacology of the receptor such that ER loses the ability to discriminate between agonists and antagonists. Interestingly, on these mutant receptors GRIP-I still functions as a coactivator independent of the nature of the bound ligand. It is likely, therefore, that the C-terminal AF-2 domain may function as a molecular switch allowing the wild-type receptor to discriminate between agonists and antagonists as well as providing a surface with which associated proteins can interact.