Structural Insights into Corepressor Recognition by Antagonist-bound Estrogen Receptors

Abstract

Direct recruitment of transcriptional corepressors to estrogen receptors (ER) is thought to contribute to the tissue-specific effects of clinically important ER antagonists. Here, we present the crystal structures of two affinity-selected peptides in complex with antagonist-bound ERα ligand-binding domain. Both peptides adopt helical conformations, bind along the activation function 2 coregulator interaction surface, and mimic corepressor (CoRNR) sequence motif binding. Peptide binding is weak in a wild-type context but significantly enhanced by removal of ER helix 12. This region contains a previously unrecognized CoRNR motif that is able to compete with corepressors for binding to activation function 2, thereby providing a structural explanation for the poor ability of ER to directly interact with classical corepressors. Furthermore, the ability of other sequence motifs to mimic corepressor binding raises the possibility that coregulators do not necessarily require CoRNR motifs for direct recruitment to antagonist-bound ER. Direct recruitment of transcriptional corepressors to estrogen receptors (ER) is thought to contribute to the tissue-specific effects of clinically important ER antagonists. Here, we present the crystal structures of two affinity-selected peptides in complex with antagonist-bound ERα ligand-binding domain. Both peptides adopt helical conformations, bind along the activation function 2 coregulator interaction surface, and mimic corepressor (CoRNR) sequence motif binding. Peptide binding is weak in a wild-type context but significantly enhanced by removal of ER helix 12. This region contains a previously unrecognized CoRNR motif that is able to compete with corepressors for binding to activation function 2, thereby providing a structural explanation for the poor ability of ER to directly interact with classical corepressors. Furthermore, the ability of other sequence motifs to mimic corepressor binding raises the possibility that coregulators do not necessarily require CoRNR motifs for direct recruitment to antagonist-bound ER. The biological significance of corepressor recruitment by nuclear receptors (NRs) 5The abbreviations used are: NR, nuclear receptor; AF, activation function; CoR, corepressor; ER, estrogen receptor; LBD, ligand-binding domain; OHT, 4-hydroxytamoxifen; RAL, raloxifene; SERM, selective estrogen receptor modulator; SPR, surface plasmon resonance; SMRT, silencing mediator of retinoid and thyroid receptors; PPAR, peroxisome proliferators activated receptor. in gene repression is well documented (reviewed in Ref. 1Rosenfeld M.G. Lunyak V.V. Glass C.K. Genes Dev. 2006; 20: 1405-1428Crossref PubMed Scopus (782) Google Scholar). Known NR corepressors have been isolated in complexes together with histone deacetylases, which facilitate gene repression through deacetylation of histone tails (2Huang E.Y. Zhang J. Miska E.A. Guenther M.G. Kouzarides T. Lazar M.A. Genes Dev. 2000; 14: 45-54PubMed Google Scholar). Even though the components of several complexes have been identified, less is known about the processes controlling their recruitment by NRs. Unlike most NRs, the estrogen receptor (ER) is unusual in that it does not appear to be repressed in the absence of hormone; consequently, the importance of NR corepressors in ER-mediated transcriptional signaling remains controversial. However, a number of recent studies have demonstrated that both agonist- and antagonist-bound ERs are able to recruit a variety of proteins that can repress its activity (3Dobrzycka K.M. Townson S.M. Jiang S. Oesterreich S. Endocr.-Relat. Cancer. 2003; 10: 517-536Crossref PubMed Scopus (110) Google Scholar). Differential coregulator recruitment is also known to contribute to the tissue-specific effects of selective ER modulators (SERMs), a therapeutically important class of ER ligands that exhibit characteristics of both estrogens and anti-estrogens depending on the tissue (4Katzenellenbogen B.S. Katzenellenbogen J.A.