Serine 232 and methionine 272 define the ligand binding pocket in retinoic acid receptor subtypes

The transcriptional response mediated by retinoic acid involves a complex series of events beginning with ligand recognition by a nuclear receptor. To dissect the amino acid contacts important for receptor-specific ligand recognition, a series of retinoic acid receptor (RAR) mutants were constructed. Transcriptional studies revealed that serine 232 (Ser(232)) in RAR alpha and methionine 272 (Met(272)) in RAR gamma are critical residues for the recognition of their respective receptor-selective analogs. The identification of these key amino acids in the ligand binding pocket is confirmed by the reported crystal structure of RAR gamma. Interestingly, the serine at position 232 in RAR alpha gives an explanation for the observed differences in the affinity of the naturally occurring ligand, all-trans-retinoic acid (t-RA), in this receptor compared with that for the other receptors, since hydrogen bonding would not be permitted between the hydroxyl of serine and the hydrophobic linker of t-RA. Using this model, a molecular mechanism for the transcriptional antagonism of a synthetic analog is suggested that involves an alteration in the structure of the receptor protein in the region around the AF2 domain in helix 12.