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 highly conserved 'C' region or DNA binding domain (DBD). The conserved 'DE' region or ligand binding domain (LED) of the RXRs is functionally complex, mediating dimerization and a ligand-dependent activation function (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, however, cell-specific steroid receptor-mediated trans-activation is a poorly understood phenomenon, We utilized the GAL4 hybrid assay system and have demonstrated that RXR gamma contains two functional domains in the AB and DE regions that activate transcription in a ligand-independent and -dependent manner respectively, The functions of the AB (AF-1) and DE (AF-2) domains were regulated by cAMP-dependent protein kinases, 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 acids 1 and 103 and contained multiple motifs that were targets of cAMP-dependent protein kinases, Transfection analyses in non-muscle and myogenic cells clearly demonstrated that: (i) the AF-1 of RXR gamma functions in a muscle-specific manner and is required for optimal ligand-dependent trans-activation from an RXRE; (ii) RXR gamma trans-activates more efficiently in a myogenic background.