Arg(278), but not Lys(229) or Lys(236), plays an important role in the binding of retinoic acid by retinoic acid receptor gamma

The diverse biological actions of retinoic acid (RA) are mediated by retino ic acid receptors (RAR alpha, beta and gamma) and retinoid X receptors (RXR alpha beta, and gamma), Although the Ligand-binding domains of RARs share the same novel folding pattern, many RAR subtype-specific retinoids have be en synthesized indicating that the ligand-binding pocket of each RAR subtyp e has unique features. Previously we have demonstrated the importance for R A binding and RA-dependent transactivation of Arg(276) of RAR alpha alone a nd in RAR beta Arg(269) in conjunction with Lys(220). In this study, we hav e examined the role of the homologous amino acid residues (Lys(229) and Arg (278)) in RAR gamma for these activities. Like RAR alpha but dissimilar to RAR beta, Arg(278) in RAR gamma alone was found to play an important role i n RA binding and RA-dependent transactivation. Since Lys(236) in RAR gamma was suggested from the crystal structure of holo-RAR gamma to interact with RA, we also examined its role and that of its homologs in RAR alpha and RA R beta. Despite the suggestion from the crystal structure, neither Lys(236) nor its homologs in RAR alpha and RAR beta play a role in the binding of R A or RA-dependent transactivation. It is likely that Lys(236) in RAR gamma and its homologs in RAR alpha and RAR beta are solvent exposed rather than pointing into the RA-binding pocket.