The effect of antagonists on the conformational exchange of the retinoid X receptor alpha ligand-binding domain.

The effect of retinoid X receptor (RXR) antagonists on the conformational exchange of the RXR ligand-binding domain (LBD) remains poorly characterized. To address this question, we used nuclear magnetic resonance spectroscopy to compare the chemical shift perturbations induced by RXR antagonists and agonists on the RXRalpha LBD when partnered with itself as a homodimer and as the heterodimeric partner with the peroxisome proliferator-activated receptor gamma (PPARgamma) LBD. Chemical shift mapping on the crystal structure showed that agonist binding abolished a line-broadening effect caused by a conformational exchange on backbone amide signals for residues in helix H3 and other regions of either the homo- or hetero-dimer, whereas binding of antagonists with similar binding affinities failed to do so. A lineshape analysis of a glucocorticoid receptor-interacting protein 1 NR box 2 coactivator peptide showed that the antagonists enhanced peptide binding to the RXRalpha LBD homodimer, but to a lesser extent than that enhanced by the agonists. This was further supported by a lineshape analysis of the RXR C-terminal residue, threonine 462 (T462) in the homodimer but not in the heterodimer. Contrary to the agonists, the antagonists failed to abolish a line-broadening effect caused by a conformational exchange on the T462 signal corresponding to the RXRalpha LBD-antagonist-peptide ternary complex. These results suggest that the antagonists lack the ability of the agonists to shift the equilibrium of multiple RXRalpha LBD conformations in favor of a compact state, and that a PPARgamma LBD-agonist complex can prevent the antagonist from enhancing the RXRalpha LBD-coactivator binding interaction.