Critical Role of Tyrosine 277 in the Ligand-Binding and Transactivating Properties of Retinoic Acid Receptor α

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Abstract
Retinoic acid receptors specifically bind all-trans-retinoic acid (RA) and function as RA-inducible transcriptional regulatory factors. Binding of RA to RARα, β, and γ is sensitive to nitration with tetranitromethane, a tyrosine-specific modifying reagent. To identify tyrosine residue(s) that are important for RA binding, we carried out chemical modification experiments with purified RARα ligand-binding domain (RARα-LBD) subjected to partial acid hydrolysis and selective proteolysis. The chemically modified peptides containing each of the three Tyr residues present in the RARα-LBD sequence were then analyzed and identified by high-performance liquid chromatography coupled to electrospray ionization mass spectrometry (HPLC/ESI-MS). We found that RA binding to RARα-LBD protected Tyr277-containing peptides from nitration. Protection of Tyr277 could result either from direct masking by the bound ligand or from ligand-induced changes in receptor conformation and tyrosine accessibility. The role of Tyr residues was further documented by site directed mutagenesis using three site-specific RARα mutants: Y208A, Y277A, and Y362A. The affinity for RA of these mutant receptors was in the range of that of the wild-type protein, except for the Y277A receptor mutant, which displays a 15−20-fold reduction in affinity and transactivation activity for RA. Whereas mutation of Tyr277 into alanine had a variable effect on different agonists and antagonists binding, it caused a dramatic decrease of retinoid-dependent transactivation activity. This later effect was also observed with mutation of Tyr277 into phenylalanine. It is unlikely that major conformational changes are responsible for the lower affinity of RA binding and RA-dependent transactivation since these mutants displayed wild-type dimerization and DNA-binding activities. Limited proteolysis revealed that upon ligand binding, the Y277A mutant induced a conformational change slightly different from that obtained with the wild-type protein. These data could suggest that Tyr277 play a critical role in the ligand-induced conformational changes required for the activation of RARα.