Retinoic acid receptor-beta (RAR-beta) specifically binds retinoic acid (RA) and functions as a RA-inducible transcriptional regulatory factor. Simultaneous mutation of Arg269 and Lys220 of RAR-beta to Ala results in a dramatic reduction in both transactivation and affinity for RA along with creating a RA concentration-dependent dominant negative mutant. In this report, we found that mutation of these two amino acid residues singly and simultaneously to Gln results in mutant RAR-beta s, each displaying a more dramatic reduction in transactivation and affinity for RA than their corresponding Ala mutant, with the R269Q more profoundly affected than K220Q. Furthermore, we examined both the Ala and Gln mutants for their ability to transactivate and bind two other retinoids with different functional end groups (all-trans-retinol and all-trans-retinal). Mutation of Lys220 to either an Ala or a Gln favors transactivation and binding of retinal, while mutation of either Lys220 or Arg269 to Gln favors retinol transactivation and binding. Taken together, these results suggest that Arg269 and Lys220 lie within the ligand binding pocket of RAR-beta and Lys220 lie within the ligand binding pocket of RAR-beta and that these two amino acid residues play an important role in determining retinoid specificity most likely by directly interacting with the carboxylate group of RA.

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