Mutations in the 1,25-dihydroxyvitamin D-3 receptor identifying C-terminal amino acids required for transcriptional activation that are functionally dissociated from hormone binding, heterodimeric DNA binding, and interaction with basal transcription factor IIB, in vitro

To investigate a potential ligand-dependent transcriptional activation domain (AF-2) in the C-terminal region of the human vitamin D receptor (hVDR), two conserved residues, Leu-417 and Glu-420, were replaced with alanines by site-directed mutagenesis (L417A and E420A). Transcriptional activation in response to 1,25-dihydroxyvitamin D-3 (1,25-(OH)(2)D-3) was virtually eliminated when either point mutant was transfected into several mammalian cell lines. Furthermore, both mutants exhibited a dominant negative phenotype when expressed in COS-7 cells. Scatchard analysis at 4 degrees C and a ligand-dependent DNA binding assay at 25 degrees C revealed essentially normal 1,25-(OH)(2)D-3 binding for the mutant hVDRs, which were also equivalent to native receptor in associating with the rat osteocalcin vitamin D responsive element as a presumed heterodimer with retinoid X receptor. Glutathione S-transferase-human transcription factor IIB (TFIIB) fusion protein linked to Sepharose equally coprecipitated the wild-type hVDR and the AF-2 mutants. These data implicate amino acids Leu-417 and Glu-420, residing in a putative alpha-helical region at the extreme C terminus of hVDR, as critical in the mechanism of 1,25-(OH)(2)D-3-stimulated transcription, likely mediating an interaction with a coactivator(s) or a component of the basal transcriptional machinery distinct from TFIIB.