Differential role of homologous positively charged amino acid residues for ligand binding in retinoic acid receptor α compared with retinoic acid receptor β

The diverse biological actions of retinoic acid (RA) are mediated by retinoic acid receptors (RARs) and retinoid X receptors. Although it has been suggested that the ligand binding domains (LBDs) of RARs share the same novel folding pattern, many RAR subtype-specific agonists and antagonists have been synthesized demonstrating that the LBD of each RAR subtype has unique features. We have examined the role of several positively charged amino acid residues located in the LBD of RARα in RA binding. These results are compared with previously published data for the homologous mutations in RARβ. Lys²²⁷ of RARα does not appear to be important for RA binding or RA-dependent transactivation, whereas the homologous residue in RARβ, Lys²²⁰, plays an important synergistic role with Arg²⁶⁹ in these two activities. In addition, Arg²⁷⁶ of RARα, like its homologous residue Arg²⁶⁹ of RARβ, was found to play an important role in the binding of RA most likely by interacting with the carboxylate group of RA. However, the orientation of and electronic environment associated with Arg²⁷⁶ in RARα appears to be different from that of Arg²⁶⁹ in RARβ, thus contributing to the uniqueness of the ligand binding pocket of each receptor.