Retinoic acid receptors: structural basis for coregulator interaction and exchange.

In the form of heterodimers with retinoid X receptors (RXRs), retinoic acid receptors (RARs) are master regulators of gene expression in humans and important drug targets. They act as ligand-dependent transcription factors that regulate a large variety of gene networks controlling cell growth, differentiation, survival and death. The biological functions of RARs rely on a dynamic series of coregulator exchanges controlled by ligand binding. Unliganded RARs exert a repressor activity by interacting with transcriptional corepressors which themselves serve as docking platforms for the recruitment of histone deacetylases that impose a higher order structure on chromatin which is not permissive to gene transcription. Upon ligand binding, the receptor undergoes conformational changes inducing corepressor release and the recruitment of coactivators with histone acetylase activities allowing chromatin decompaction and gene transcription. In the following, we review the structural determinants of the interaction between RAR and either type of coregulators both at the level of the individual receptor and in the context of the RAR-RXR heterodimers. We also discuss the molecular details of the fine tuning of these associations by the various pharmacological classes of ligands.