Bacterial Expression and Characterization of the Ligand-Binding Domain of the Vitamin D Receptor

The ligand-binding domain of the rat vitamin D receptor (amino acids 115–423) was expressed as an amino-terminal His-tagged protein in a bacterial expression system and purified over Ni-nitrilotriacetic acid resin and a Mono S column. The purified protein bound its ligand, 1,25-dihydroxyvitamin D3, with high affinity, similar to that of the full-length protein. Saturation of the protein with ligand quenched 90% of the tryptophan fluorescence, consistent with the purified protein being uniformly able to bind ligand. Addition of ligand produced no change in the tryptophan fluorescence lifetime, suggesting static quenching as the mechanism of fluorescence decrease. The near-UV circular dichroism spectrum showed a large increase in signal following the addition of ligand, consistent with a change in the environment of aromatic amino acid side chains. The far-UV circular dichroism spectrum was consistent with a protein of high α-helical content. Sedimentation equilibrium experiments demonstrated that the protein formed higher-order complexes, and the distribution of the protein among these complexes was significantly shifted by addition of ligand.