SERINE-232 AND METHIONINE-272 DEFINE THE LIGAND-BINDING POCKET IN RETINOIC ACID RECEPTOR SUBTYPES

The transcriptional response mediated by retinoic acid involves a comp lex series of events beginning with ligand recognition by a nuclear re ceptor. To dissect the amino acid contacts important for receptor-spec ific ligand recognition, a series of retinoic acid receptor (RAR) muta nts were constructed. Transcriptional studies revealed that serine 232 (Ser(232)) in RAR alpha and methionine 272 (Met(272)) in RAR gamma ar e critical residues for the recognition of their respective receptor-s elective analogs. The identification of these key amino acids in the l igand binding pocket is confirmed by the reported crystal structure of RAR gamma. Interestingly, the serine at position 232 in RAR alpha giv es an explanation for the observed differences in the affinity of the naturally occurring ligand, all-trans-retinoic acid (t-RA), in this re ceptor compared with that for the other receptors, since hydrogen bond ing would not be permitted between the hydroxyl of serine and the hydr ophobic linker of t-RA. Using this model, a molecular mechanism for th e transcriptional antagonism of a synthetic analog is suggested that i nvolves an alteration in the structure of the receptor protein in the region around the AF2 domain in helix 12.