Characterization of the retinoid orphan-related receptor-α coactivator binding interface: A structural basis for ligand-independent transcription

The retinoid orphan-related receptor-α (RORα) is a member of the ROR subfamily of orphan receptors and acts as a constitutive activator of transcription in the absence of exogenous ligands. To understand the basis of this activity, we constructed a homology model of RORα using the closely related TRβ as a template. Molecular modeling suggested that bulky hydrophobic side chains occupy the RORα ligand cavity leaving a small but distinct cavity that may be involved in receptor stabilization. This model was subject to docking simulation with a receptor-interacting peptide from the steroid receptor coactivator, GR-interacting protein-1, which delineated a coactivator binding surface consisting of the signature motif spanning helices 3-5 and helix 12 [activation function 2 (AF2)]. Probing this surface with scanning alanine mutagenesis showed structural and functional equivalence between homologous residues of RORα and TRβ. This was surprising (given that RORα is a ligand-independent activator, whereas TRβ has an absolute requirement for ligand) and prompted us to use molecular modeling to identify differences between RORα and TRβ in the way that the AF2 helix interacts with the rest of the receptor. Modeling high-lighted a nonconserved amino acid in helix 11 of RORα (Phe491) and a short-length of 3.10 helix at the N terminus of AF2 which we suggest 1) ensures that AF2 is locked permanently in the holoconformation described for other liganded receptors and thus 2) enables ligand-independent recruitment of coactivators. Consistent with this, mutation of RORα Phe491 to either methionine or alanine (methionine is the homologous residue in TRβ), reduced and ablated transcriptional activation and recruitment of coactivators, respectively. Furthermore, we were able to reconstitute transcriptional activity for both a deletion mutant of RORα lacking AF2, and Phe491Met, by overexpression of a GAL-AF2 fusion protein, demonstrating ligand-independent recruitment of AF2 and a role for Phe491 in recruiting AF2.