X-ray crystal structures of the estrogen-related receptor-γ ligand binding domain in three functional states reveal the molecular basis of small molecule regulation

X-ray crystal structures of the ligand binding domain (LBD) of the estrogen-related receptor-gamma (ERR gamma) were determined that describe this receptor in three distinct states: unliganded, inverse agonist bound, and agonist bound. Two structures were solved for the unliganded state, the ERR gamma LBD alone, and in complex with a coregulator peptide representing a portion of receptor interacting protein140(RIP140). No significant differences were seen between these structures that both exhibited the conformation of ERR gamma seen in studies with other coactivators. Two structures were obtained describing the inverse agonist-bound state, the ERR gamma LBD with 4-hydroxytamoxifen (4-OHT), and the ERR gamma LBD with 4-OHT and a peptide representing a portion of the silencing mediator of retinoid and thyroid hormone action protein (SMRT). The 4-OHT structure was similar to other reported inverse agonist bound structures, showing reorientation of phenylalanine 435 and a displacement of the AF-2 helix relative to the unliganded structures with little other rearrangement occurring. No significant changes to the LBD appear to be induced by peptide binding with the addition of the SMRT peptide to the ERR gamma plus 4-OHT complex. The observed agonist-bound state contains the ERR gamma LBD, a ligand (GSK4716), and the RIP140 peptide and reveals an unexpected rearrangement of the phenol-binding residues. Thermal stability studies show that agonist binding leads to global stabilization of the ligand binding domain. In contrast to the conventional mechanism of nuclear receptor ligand activation, activation of ERR gamma by GSK4716 does not appear to involve a major rearrangement or significant stabilization of the C-terminal helix.