Ligand affinity, homodimerization, and ligand-induced secondary structuralchange of the human vitamin D receptor

The intrinsic tryptophan fluorescence signal of the full-length nuclear rec eptor hVDR was used to directly determine the dissociation constants, K-d, Of two ligands yielding K-d, = 32 nM for 1 alpha ,25(OH)(2)D-3 and K-d = 32 2 nM for 25(OH)D-3. Ligand binding was accompanied by a conformational chan ge in the a-helical part of hVDR as revealed by CD spectroscopy. In additio n, the presence of calcitriol was found to be a necessary prerequisite for the homodimerisation of hVDR which was monitored using fluorescence anisotr opy. We conclude that the observed ligand-induced structural change of hVDR is conditional for dimerisation of the protein. (C) 2001 Academic Press.