The co-repressor hairless protects RORalpha orphan nuclear receptor from proteasome-mediated degradation.

RORalpha is a constitutively active orphan nuclear receptor essential for cerebellar development and is previously shown to regulate genes involved in both myogenesis and adipogenesis. The transcriptional activity of RORalpha is dependent on the presence of a ubiquitous ligand and can be abolished by interaction with Hairless (Hr), a ligand-oblivious nuclear receptor co-repressor. In this study, we first demonstrate that RORalpha is a short-lived protein and that treatment with the MG-132 proteasome inhibitor results in the accumulation of ubiquitin-conjugated receptor and inhibition of transcription. These data show that RORalpha transcriptional activity and degradation are intrinsically linked. In addition, the introduction of inactivation mutations in the ligand-binding pocket and co-regulator-binding surface of RORalpha significantly increases protein stability, indicating that ligand and/or co-regulator binding perpetuates RORalpha degradation. Strikingly, expression of the co-repressor Hr results in the stabilization of RORalpha because of an inhibition of proteasome-mediated degradation of the receptor. Stabilization of RORalpha by Hr requires intact nuclear receptor recognition LXXLL motifs within Hr. Interestingly, the co-repressor nuclear receptor co-repressor (NCoR) has no effect on RORalpha protein turnover. This study shows that stabilization of RORalpha is an essential component of Hr-mediated repression and suggests a molecular mechanism to achieve transcriptional repression by a liganded receptor-co-repressor complex.