Why do we need a three-dimensional architecture of the ligand-binding domain of the nuclear 1α,25-dihydroxyvitamin D3 receptor?

Highly specific binding of 1 alpha ,25-dihydroxyvitamin D-3 (l alpha ,25(QH)(2)D-3) by vitamin D receptor (VDR), a nuclear transcriptional factor, activates a genomic mechanism that is manifested in the multiple biologic properties of 1 alpha .25(OH)(2)D-3. Numerous synthetic analogs of 1 alpha ,25(OH)(2)D-3 have been employed to study the interaction between 1 alpha ,25(OH)(2)D-3 and VDR, and to identify structural markers in 1 alpha ,25(OH)(2)D-3 that are important for VDR-binding. On the other hand the three-dimensional structure of VDR remained elusive till very recently. In the present study we employed affinity labeling (by 1 alpha ,25-dihydroxyvitamin D-3-3-bromoacetate, 1 alpha ,25(OH)(2)D-3-3-BE) of VDR to identify C-288 as the anchoring residue for the 3-hydroxyl group of 1 alpha ,75(OH)(2)D-3 inside the ligand-binding domain of VDR (VDR-LBD). In addition we carried out mutation/hormone-binding analyses to determine the importance of M-284 and W-286 toward hormone binding. We incorporated this information with the three-dimensional structure of the LED of progesterone receptor to develop a homology-extension model of VDR-LBD. This model identified several amino acid residues as ligand-contact points inside the LED. Mutational and hormone-binding analyses of these residues verified the structure-functional authenticity of this model, in comparison with the crystal structure of VDR, bound to 1 alpha ,25(OH)(2)D-3. (C) 2001 Elsevier Science Inc. All rights reserved.