The N-terminal domain of transcription factor IIB is required for direct interaction with the vitamin D receptor and participates in vitamin D- mediated transcription

The interaction of the vitamin D receptor (VDR) with transcription factor IIB (TFIIB) represents a potential physical link between the VDR-DNA complex and the transcription preinitiation complex. However, the functional relevance of the VDR-TFIIB interaction in vitamin D-mediated transcription is not well understood. In the present study, we used site-directed mutagenesis to demonstrate that the structural integrity of the amino-terminal zinc finger of TFIIB is essential for VDR-TFIIB complex formation. Altering the putative zinc-coordinating residues (C15, C34, C37, or H18) to serines abolished TFIIB interaction with the VDR as assessed in a yeast two-hybrid system and by in vitro protein interaction assays. This N-terminal, VDR- interactive domain functioned as a selective, dominant-negative inhibitor of vitamin D-mediated transcription. Expressing amino acids 1-124 of human TFIIB (N-TFIIB) in COS-7 cells or in osteoblastic ROS17/2.8 cells effectively suppressed 1,25-dihydroxyvitamin D₃ (1,25-(OH)₂D₃)-induced transcription, but had no effect on basal or glucocorticoid-activated transcription. A mutant N-terminal domain [N-TFIIB(C34S:C37S)] that does not interact with VDR had no impact on 1,25(OH)₂D₃-induced transcription. Interestingly, both in vitro and in vivo protein interaction assays showed that the VDR-TFIIB protein complex was disrupted by the 1,25-(OH)₂D₃ ligand. Mechanistically, these data establish a functional role for the N terminus of TFIIB in VDR- mediated transcription, and they allude to a role for unliganded VDR in targeting TFIIB to the promoter regions of vitamin D-responsive target genes.