STRUCTURAL DETERMINANTS OF THE LIGAND-BINDING SITE OF THE HUMAN RETINOIC ACID RECEPTOR-ALPHA

The ligand-dependent transactivating properties of retinoic acid recep tors are controlled through a complex structure at the C-terminus of t hese proteins, commonly referred to as the hormone binding domain. Thi s domain is involved not only in ligand recognition but also in protei n-protein interactions such as homo- and heterodimerization processes. To identify more precisely regions of the human all-trans-retinoic ac id receptor alpha (hRAR alpha) that are involved in Ligand binding, we constructed a series of deletion mutants of this molecule and overexp ressed them in bacteria. We found that the C-terminal part of the D do main (amino acids 186-198) was necessary for ligand binding. The F dom ain and the 10 C-terminal amino acids of the E domain were dispensable for high-affinity binding of various natural and synthetic retinoids. A further deletion to position 403 resulted in a moderate decrease in affinity for all-trans-(ATRA) and 9-cis-retinoic acids, whereas the b inding of two RAR alpha-specific ligands (Am80 and Am580) was abolishe d. In addition, hRAR alpha and the minimal hormone binding domain (ami no acids 186-410) bound ATRA with a positive, cooperative mechanism. T his behavior was not observed with CD367, a conformationally restricte d synthetic retinoid. The positive cooperativity could be correlated w ith stable ATRA binding to RAR homodimers, whose formation was trigger ed by ligand. In the same conditions, only monomeric CD367-RAR alpha c omplexes were detected. These data indicate that ligand binding to hRA R alpha requires the presence of part of the D domain, whereas the C-t erminal end of the E domain is involved in more subtle ligand recognit ion processes, They also clearly suggest that structurally distinct re tinoids interact differently with the Ligand-binding site of this rece ptor.