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 al pha) ligand-binding pocket (LBP) generate constitutive, ligand-binding -competent mutants with structural and functional characteristics simi lar to those of agonist-bound wild-type RXR, Modelling of the mouse RX R alpha F318A LBP suggests that, like agonist binding, the mutation di srupts a cluster of van der Waals interactions that maintains helix H1 1 in the apo-receptor location, thereby shifting the thermodynamic equ ilibrium 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 ot her hand, efficiently contributes to synergistic transactivation withi n NGFI-B-RXR heterodimers. RAR mutants disabled for corepressor bindin g and/or lacking a functional AF-2 activation domain, do not relieve R XR 'silencing', Not only RAR agonists, but also the RAR antagonist BMS 614 induce conformational changes allowing RXR to exert constitutive ( RXR alpha F318A) or agonist-induced (wild-type RXR) activity in hetero dimers. Interestingly, the RXR alpha F318A constitutive activity gener ated within heterodimers in the presence of BMS614 requires the integr ity 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 transcription ally responsive to agonists.