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 (RARα, -β, and -γ) and retinoid X receptors (RXRα, -β, and -γ). 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 Arg276 in RARα and Arg278 in RARγ; however, in RARβ Arg269 functions in conjunction with Lys220. Here we have examined the role of the hydroxyl group of RARγ Ser289 and its homologous amino acid residues in RARα (Ser287) and RARβ (Ser280) alone and in conjunction with their respective RARγ Arg278 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 RARα and RARγ this Ser appears to play a small role in conjunction with Arg276 and Arg278, respectively, for these activities. Alternatively, strong synergism was observed in RARβ between Ser280 and Arg269 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 RARβ is different from that of RARα and RARγ.