Abstract
The vitamin D receptor (VDR) heterodimerizes with retinoid X receptors (RXR) on many vitamin D-responsive promoter elements, suggesting that this complex is the active factor in vitamin D-mediated transcription. However, the mechanism of transcriptional regulation following VDR-RXR binding to DNA is not well characterized. Using a yeast two-hybrid protein interaction assay, we demonstrate that VDR forms specific protein: protein contacts with the basal transcription factor TFIIB. Deletion analysis indicated that the carboxyl-terminal ligand binding domain of VDR interacted with a 43-residue amino-terminal domain in TFIIB. The interaction with TFIIB showed selectivity for the ligand binding domain of VDR as similar regions of RXR alpha or of retinoic acid receptor alpha did not couple with TFIIB. Binding assays with purified proteins showed a direct interaction between VDR and TFIIB in vitro. These data suggest a mechanism for VDR-dependent transcription in which protein contacts between VDR and TFIIB may impart regulatory information to the transcription preinitiation complex.
Highlights
The vitamin D receptor (VDR) heterodimerizes with retinoid X receptors (RXR) on many vitamin D-responsive promoter elements, suggesting that this complex is the active factor in vitamin D-mediated transcription
In one current hypothesis, 1,25(OH)2Da-dependent gene expression is mediated by heterodimeric binding of VDR and RXR to the vitamin D-responsive element with the heterodimer serving as the functional transcription factor
The basal transcription factor TFIIB is important in this regard
Summary
The vitamin D receptor (VDR) heterodimerizes with retinoid X receptors (RXR) on many vitamin D-responsive promoter elements, suggesting that this complex is the active factor in vitamin D-mediated transcription. The VDR is a member of the superfamily of nuclear receptors for steroid hormones, thyroid hormone, and retinoids As such a member, the VDR acts as a ligand-induced transcription factor that binds to specific DNA elements in vitamin D-responsive genes and influences the rate of transcription [1]. Purified VDR does not bind to authentic vitamin D-responsive elements with high affinity unless an additional nuclear protein, designated NAF or RAF (for nuclear or receptor auxiliary factor) is present [2,3,4]. In one current hypothesis, 1,25(OH)2Da-dependent gene expression is mediated by heterodimeric binding of VDR and RXR to the vitamin D-responsive element with the heterodimer serving as the functional transcription factor
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