The autonomous transactivation domain in helix H3 of the vitamin D receptor is required for transactivation and coactivator interaction

A ligand-inducible transactivation function (AF-2) exists in the extreme carboxyl terminus of the vitamin D receptor (VDR) that is essential for 1 alpha,25-dihydroxyvitamin D-3 (1,25-(OH)(2)D-3)-activated transcription and p160 coactivator interaction. Crystallographic data of related nuclear receptors suggest that binding of 1,26-(OH)(2)D-3 by VDR induces conformational changes in the ligand-binding domain (LBD), the most striking of which is a packing of the AF-2 helix onto the LED adjacent to helices H3 and H4. In this study, a panel of VDR helix H3 mutants was generated, and residues in helix H3 that are important for ligand-activated transcription by the full-length VDR were identified. In particular, one mutant (VDR (Y236A)) exhibited normal ligand binding and heterodimerization with the retinoid X receptor (RXR) but was transcriptionally inactive. Yeast two-hybrid studies and in vitro protein interaction assays demonstrated that VDR (Y236A) was selectively impaired in interaction with AF-2-interacting coactivator proteins such as SRC-1 and GRIP-1. These data indicate an importance of helix H3 in the mechanism of VDR-mediated transcription, and they support the concept that helix HS functions in concert with the AF-2 domain to form a transactivation surface for binding the p160 class of nuclear receptor coactivators.