Novel mechanism of nuclear receptor corepressor interaction dictated by activation function 2 helix determinants.

Transcriptional regulation by nuclear receptors is controlled by the concerted action of coactivator and corepressor proteins. The product of the thyroid hormone-regulated mammalian gene hairless (Hr) was recently shown to function as a thyroid hormone receptor corepressor. Here we report that Hr acts as a potent repressor of transcriptional activation by RORalpha, an orphan nuclear receptor essential for cerebellar development. In contrast to other corepressor-nuclear receptor interactions, Hr binding to RORalpha is mediated by two LXXLL-containing motifs, a mechanism associated with coactivator interaction. Mutagenesis of conserved amino acids in the ligand binding domain indicates that RORalpha activity is ligand-dependent, suggesting that corepressor activity is maintained in the presence of ligand. Despite similar recognition helices shared with coactivators, Hr does not compete for the same molecular determinants at the surface of the RORalpha ligand binding domain, indicating that Hr-mediated repression is not simply through displacement of coactivators. Remarkably, the specificity of Hr corepressor action can be transferred to a retinoic acid receptor by exchanging the activation function 2 (AF-2) helix. Repression of the chimeric receptor is observed in the presence of retinoic acid, demonstrating that in this context, Hr is indeed a ligand-oblivious nuclear receptor corepressor. These results suggest a novel molecular mechanism for corepressor action and demonstrate that the AF-2 helix can play a dynamic role in controlling corepressor as well as coactivator interactions. The interaction of Hr with RORalpha provides direct evidence for the convergence of thyroid hormone and RORalpha-mediated pathways in cerebellar development.