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 coreprescor proteins. The product of the thyroid hormone-regulated mammalian gene hairless (Hr) was recently shown to function as a thyroid hormone receptor coreprescor. 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 RORa 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 RORa activity is ligand-dependent, suggesting that coreprescor 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 RORa ligand binding domain, indicating that Hr-mediated repression is not simply through displacement of coactivators. Remarkably, the specificity of Hr coreprescor 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 coreprescor. These results suggest a novel molecular mechanism for coreprescor action and demonstrate that the AF-2 helix can play a dynamic role in controlling coreprescor as well as coactivator interactions. The interaction of Hr with RORa provides direct evidence for the convergence of thyroid hormone and RORalpha-mediated pathways in cerebellar development.