Ligand-dependent activation of transcription by nuclear receptors (NRs) is mediated by interactions with coactivators. Receptor agonists promote coactivator binding, and antagonists block coactivator binding. Here we report the crystal structure of the human estrogen receptor alpha (hER alpha) ligand-binding domain (LBD) bound to both the agonist diethylstilbestrol (DES) and a peptide derived from the NR box II region of the coactivator GRIP1 and the crystal structure of the hER alpha LBD bound to the selective antagonist 4-hydroxytamoxifen (OHT). In the DES-LBD-peptide complex, the peptide binds as a short alpha helix to a hydrophobic groove on the surface of the LBD. In the OHT-LBD complex, helix 12 occludes the coactivator recognition groove by mimicking the interactions of the NR box peptide with the LBD. These structures reveal the two distinct mechanisms by which structural features of OHT promote this "autoinhibitory" helix 12 conformation.

The R,R enantiomer of 5,11-cis-diethyl-5,6,11,12-tetrahydrochrysene-2,8-diol (THC) exerts opposite effects on the transcriptional activity of the two estrogen receptor (ER) subtypes, ER alpha and ER beta. THC acts as an ER alpha agonist and as an ER beta antagonist. We have determined the crystal structures of the ER alpha ligand binding domain (LBD) bound to both THC and a fragment of the transcriptional coactivator GRIP1, and the ER beta LBD bound to THC. THC stabilizes a conformation of the ER alpha LBD that permits coactivator association and a conformation of the ER beta LBD that prevents coactivator association. A comparison of the two structures, taken together with functional data, reveals that THC does not act on ER beta through the same mechanisms used by other known ER antagonists. Instead, THC antagonizes ER beta through a novel mechanism we term 'passive antagonism'.

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