Structural determinants of the BRCA1 : estrogen receptor interaction

Previously, we showed that the BRCA1 protein interacts directly and functionally with estrogen receptor-alpha (ER-α), resulting in the inhibition of estradiol (E2)-stimulated ER-α transcriptional activity. The interaction sites were mapped to the N-terminus of BRCA1 (within amino acids (aa) 1–302) and the ligand-binding domain/activation function-2 (LBD/AF-2) region (within aa 282–420) of ER-α. In this study, we have further characterized the structure/function relationship for the BRCA1 : ER-α interaction. We found that the N-terminal RING domain (aa 20–64) is not required for the BRCA1 : ER-α interaction. We identified two separate contact points for ER-α, one within aa 1–100 and the other within aa 100–200 of BRCA1; and we showed that each of these BRCA1 peptides interacts with BRCA1 in vitro and in vivo. By using different fragments of the BRCA1 N-terminus, we found that aa 67–100 and 101–133 are required for the interaction with ER-α, but that aa 1–67 and 134–302 are dispensible. Previously, we showed that BRCA1 aa 1–302 does not inhibit E2-stimulated ER-α transcriptional activity but does bind to ER-α and acts as a dominant negative inhibitor of the full-length BRCA1 protein. Somewhat surprisingly, we found that BRCA1 aa 1–100 and BRCA1 aa 101–200 (but not aa 201–300) each inhibited ER-α activity, although not as efficiently as full-length BRCA1. Mutations within an HIV Rev-like nuclear export signal that resembles a nuclear receptor corepressor motif (aa 86–95) impaired the ability of both truncated (aa 1–100) and full-length (aa 1–1863) BRCA1 proteins to interact with and/or repress ER-α activity. Based on these findings, a partial BRCA1 : ER-α three-dimensional structure is proposed. The implications of these findings for understanding the BRCA1 : ER-α interaction are discussed.