Human vitamin D receptor (hVDR) fused to glutathione S-transferase was utilized to detect a VDR-interacting protein (VIP) of approximately 170 kDa, VIP170 is expressed in osteoblast-like ROS 17/2.8 cells and, to a lesser extent, in COS-7 and HeLa cells. VIP170 may be a coactivator because it interacts only with 1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) ligand-bound hVDR and because a mutation (E420A) in the activation function-2 (AF-2) of hVDR abolishes both receptor-mediated transactivation and VIP170 binding, Unlike L254G hVDR, a heterodimerization mutant with an intact AF-2, the E420A mutant is only partially attenuated in its association with the retinoid X receptor (RXR) DNA-binding partner, Finally, the ability of overexpressed hVDR to squelch glucocorticoid receptor-mediated transactivation is lost in both the L254G and E420A mutants. These results suggest that several protein-protein interactions, including VDR association with RXR and VIP170, are required for stabilization of a multimeric complex that transduces the signal for 1,25(OH)(2)D-3-elicited transactivation.