LIGAND MODULATES THE CONVERSION OF DNA-BOUND VITAMIN D-3 RECEPTOR (VDR) HOMODIMERS INTO VDR-RETINOID-X RECEPTOR HETERODIMERS

Protein dimerization facilitates cooperative, high-affinity interactio ns with DNA. Nuclear hormone receptors, for example, bind either as ho modimers or as heterodimers with retinoid X receptors (RXR) to half-si te repeats that are stabilized by protein-protein interactions mediate d by residues,within both the DNA- and ligand-binding domains. In vivo , ligand binding among the subfamily of steroid receptors unmasks the nuclear localization and DNA-binding domains from a complex with auxil iary factors such as the heat shock proteins. However, the role of lig and is less clear among nuclear receptors, since they are constitutive ly localized to the nucleus and are presumably associated with DNA in the absence of ligand. In this study, we have begun to explore the rol e of the ligand in vitamin D-3 receptor (VDR) function by examining it s effect on receptor homodimer and heterodimer formation. Our results demonstrate that VDR is a monomer in solution; VDR binding to a specif ic DNA element leads to the formation of a homodimeric complex through a monomeric intermediate. We find that 1,25-dihydroxyvitamin D-3, the ligand for VDR, decreases the amount of the DNA-bound VDR homodimer c omplex. It does so by significantly decreasing the rate of conversion of DNA-bound monomer to homodimer and at the same time enhancing the d issociation of the dimeric complex. This effectively stabilizes the bo und monomeric species, which in turn serves to favor the formation of a VDR-RXR heterodimer. The ligand for RXR, 9-cis retinoic acid, has th e opposite effect of destabilizing the heterodimeric-DNA complex. Thes e results may explain how a nuclear receptor can bind DNA constitutive ly but still act to regulate transcription in a fully hormone-dependen t manner.