The Unique Tryptophan Residue of the Vitamin D Receptor Is Critical for Ligand Binding and Transcriptional Activation

The human vitamin D receptor (hVDR) is a member of the nuclear receptor superfamily of transcriptional regulators. Here we show that tryptophan 286 of the hVDR is critical for ligand binding and transactivation of 1,25‐dihydroxyvitamin D 3 [1,25(OH) 2 D 3 ] target genes. Two mutants of the hVDR were produced, W286A and W286F, in which the tryptophan was replaced with an alanine or a phenylalanine, respectively. The W286A mutant did not bind 1,25(OH) 2 D 3 , interact with steroid receptor coactivator 1 (SRC‐1) in vitro, or activate transcription. Moreover, the W286A receptor did not heterodimerize in a ligand‐dependent manner with the human retinoid X receptor α (hRXRα). Although the W286F receptor heterodimerized with hRXRα, interacted with SRC‐1, and bound 1,25(OH) 2 D 3 , its capacity to transactivate was attenuated severely. Thus, tryptophan 286 of hVDR plays an important role in specific 1,25(OH) 2 D 3 ligand interaction and subsequently in hVDR/RXR interaction, SRC‐1 binding, and ligand‐dependent transactivation of 1,25(OH) 2 D 3 target genes. These results identify the first amino acid that is absolutely required for ligand binding in the VDR and further define the structure‐function relationship of 1,25(OH) 2 D 3 interaction with its receptor.