DISTINCT MODES OF INTERACTION OF THE RETINOIC ACID RECEPTOR-ALPHA WITH NATURAL AND SYNTHETIC RETINOIDS

Retinoids regulate key cellular processes through their binding to the ir cognate nuclear receptors, RARs and RXRs. Synthetic ligands mimic m ost of their biological effects and alteration of their chemical struc ture confers selectivity for RAR isotypes alpha, beta or gamma. In thi s study, we have examined the contribution of a domain (L box) of hRAR alpha located at the C-terminus of the ligand binding domain (LBD), b etween helices H11 and H12, to the ligand binding activity of this rec eptor. By site-directed mutagenesis, we demonstrate that, in the absen ce of the ligand-dependent activation domain 2 (AF2-AD), the receptor discriminates between classes of structurally distinct retinoids. This property was lost in the presence of the AF2-AD domain, evidencing ma jor structural transitions in this part of the receptor. We propose th at ligand binding occurs in two steps: first, the ligand interacts wit h the LBD in its opened, hole-receptor conformation in which the L box plays a crucial role in defining the ligand binding repertoire of hRA R alpha; secondly, the LED adopts its closed. conformation in which th e ligand interacts with the receptor mostly through its carboxylic moi ety. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.