Role of Ser289 in RARγ and its homologous amino acid residue of RARα and RARβ in the binding of retinoic acid

The biological actions of retinoic acid (RA) are mediated by retinoic acid receptors (RAR alpha, -beta, and -gamma) and retinoid X receptors (RXR alpha, -beta, and -gamma). Although the ligand-binding domains of RARs and RXRs have been suggested to share the same novel folding pattern, the ligand-binding pockets of each of the retinoid receptors must have unique structural features since it has been possible to develop RAR subtype-selective and RXR-selective retinoids, We have previously demonstrated the importance for RA binding and RA-dependent transactivation of Arg(276) in RAR alpha and Arg(278) in RAR gamma; however, in RAR beta Arg(269) functions in conjunction with Lys(220). Here we have examined the role of the hydroxyl group of RAR gamma Ser(289) and its homologous amino acid residues in RAR gamma (Ser(287)) and RAR beta (Ser(280)) alone and in conjunction with their respective RAR gamma Arg(278) homologs for RA binding and RA-dependent transactivation activity. The hydroxyl group of this Ser in all three RARs was found by itself not to be important for RA binding and RA-dependent transactivation activity. However, in RAR alpha and RAR gamma this Ser appears to play a small role in conjunction with Arg(276) and Arg(278), respectively, for these activities. Alternatively, strong synergism was observed in RAR beta between Ser(280) and Arg(269) for RA-binding and RA-dependent transactivation activity. This provides further evidence that the mechanism of interaction between the carboxylate group of retinoids and the amino acid residues in the ligand binding pocket of RAR beta is different from that of RAR alpha and RAR gamma. (C) 2000 Academic Press.