Ligand-independent recruitment of SRC-1 to estrogen receptor β through phosphorylation of activation function AF-1

The estrogen receptors (ERs) alpha and beta possess a constitutive N-terminal activation function (AF-1) whose activity can be modulated by kinase signaling pathways. We demonstrate here that phosphorylation of AF-1 by MAP kinase (MAPK) leads to the recruitment of steroid receptor coactivator-1 (SRC-1) by ERP in vitro. Enhancement of the interaction between SRC-1 and ER beta AF-1 is also observed in vivo in cells either treated with EGF or expressing activated Ras. Two serine residues in ER beta AF-1, of which one is contained within a motif present in other steroid receptors, are critical for physical interaction with SRC-1 and transcriptional activation. Our results establish a role for nuclear receptor phosphorylation in the recruitment of SRC-1 and provide a molecular basis for ligand-independent activation by ER beta via the MAPK pathway.