A mutation mimicking ligand-induced conformational change yields a constitutive RXR that senses allosteric effects in heterodimers

Mutations of a single residue in the retinoid X receptor alpha (RXR alpha) ligand-binding pocket (LBP) generate constitutive, ligand-binding-competent mutants with structural and functional characteristics similar to those of agonist-bound wild-type RXR, Modelling of the mouse RXR alpha F318A LBP suggests that, like agonist binding, the mutation disrupts a cluster of van der Waals interactions that maintains helix H11 in the apo-receptor location, thereby shifting the thermodynamic equilibrium to the hole form. Heterodimerization with some apo-receptors (retinoic acid, thyroid hormone and vitamin D-3 receptors) results in 'silencing' of RXR alpha F318A constitutive activity, which, on the other hand, efficiently contributes to synergistic transactivation within NGFI-B-RXR heterodimers. RAR mutants disabled for corepressor binding and/or lacking a functional AF-2 activation domain, do not relieve RXR 'silencing', Not only RAR agonists, but also the RAR antagonist BMS614 induce conformational changes allowing RXR to exert constitutive (RXR alpha F318A) or agonist-induced (wild-type RXR) activity in heterodimers. Interestingly, the RXR alpha F318A constitutive activity generated within heterodimers in the presence of BMS614 requires the integrity of both RXR and RAR AF-2 domains. These observations suggest that, within RXR-RAR heterodimers, RAR can adopt a structure distinct from that of the active holo-RAR, thus allowing RXR to become transcriptionally responsive to agonists.