L7 PROTEIN IS A COREGULATOR OF VITAMIN-D-RECEPTOR RETINOID-X RECEPTOR-MEDIATED TRANSACTIVATION

The vitamin D receptor (VDR) heterodimerizes with the retinoid X recep tor (RXR) and requires additional protein-protein interactions to regu late the expression of target genes. Using the yeast two-hybrid system , we identified the previously described protein L7, that specifically interacted with the VDR in the presence of vitamin D. Deletion analys is indicated, that the N-terminus of L7, which harbours a basic region leucine zipper like domain, mediated interaction with the VDR. Bindin g assays with purified GST-L7 demonstrated, that L7 specifically pulle d down the VDR, that was either expressed in yeast or endogenously con tained in the cell line U937. Interestingly, L7 inhibited ligand-depen dent VDR-RXR heterodimerization, when constitutively expressed in yeas t. We also demonstrate that L7 repressed binding of VDR-RXR heterodime rs to a vitamin D response element. Surprisingly, L7 recruited RXR to the same response element in the presence of 9-cis retinoic acid. Liga nd-dependent protein-protein interaction in the yeast two-hybrid syste m confirmed, that binding of L7 also was targeted at the RXR. Our data suggest,that protein L7 is a coregulator of VDR-RXR mediated transact ivation of genes, that modulates transcriptional activity by interferi ng with binding of the receptors to genomic enhancer elements. (C) 199 8 Wiley-Liss, Inc.