CHARACTERIZATION OF THE AB (AF-1) REGION IN THE MUSCLE-SPECIFIC RETINOID-X-RECEPTOR-GAMMA - EVIDENCE THAT THE AF-1 REGION FUNCTIONS IN A CELL-SPECIFIC MANNER

The retinoid X receptors alpha-, beta- and gamma- (RXRs) share a highl y conserved 'C' region or DNA binding domain (DBD). The conserved 'DE' region or ligand binding domain (LED) of the RXRs is functionally com plex, mediating dimerization and a ligand-dependent activation functio n (AF-2), The AB or N-terminal region of the RXRs is poorly conserved and encodes a ligand-independent activation function (AF-1), RXR gamma mRNA is preferentially expressed in skeletal and cardiac muscle, howe ver, cell-specific steroid receptor-mediated trans-activation is a poo rly understood phenomenon, We utilized the GAL4 hybrid assay system an d have demonstrated that RXR gamma contains two functional domains in the AB and DE regions that activate transcription in a ligand-independ ent and -dependent manner respectively, The functions of the AB (AF-1) and DE (AF-2) domains were regulated by cAMP-dependent protein kinase s, furthermore, the function of AF-2 in the LED was activated by 8-Br- cAMP, independent of 9-cis-retinoic acid treatment, Deletion analysis demonstrated that the AF-1 of RXR gamma, is located between amino acid s 1 and 103 and contained multiple motifs that were targets of cAMP-de pendent protein kinases, Transfection analyses in non-muscle and myoge nic cells clearly demonstrated that: (i) the AF-1 of RXR gamma functio ns in a muscle-specific manner and is required for optimal ligand-depe ndent trans-activation from an RXRE; (ii) RXR gamma trans-activates mo re efficiently in a myogenic background.