ABSTRACT
Most of the biological effects of 1,25-dihydroxyvitamin D(3) (hormone D) are mediated through the nuclear vitamin D receptor (VDR). Hormone binding induces conformational changes in VDR that enable the receptor to activate gene transcription. It is known that residues S237 and R274 form hydrogen bonds with the 1-hydroxyl group of hormone D, while residues Y143 and S278, and residues H305 and H397 form hydrogen bonds with the 3-hydroxyl and the 25-hydroxyl groups of the hormone. A series of VDR mutations were constructed (S237A, R274A, R274Q, Y143F, Y143A, S278A, H305A, and H397F; double mutants: S237A/R274A, Y143F/S278A, Y143A/S278A, and H305A/H397F). The relative binding affinities of the wild-type and variant VDRs were assessed. All of the mutants except H397F resulted in lower binding affinity compared to wild-type VDR. Binding to hormone was barely detectable in Y143F, H305A, and H305A/H397F mutants, and undetectable in mutants R274A, R274Q, Y143A, S237A/R274A, and Y143A/S278A, indicating the importance of these residues. Ability to activate gene transcription was also assessed. All of the VDR mutants, except the single mutant S278A, required higher doses of hormone D for half-maximal response. Defining the role of hormone D-VDR binding will lead to a better understanding of the vitamin D signal transduction pathway.