Distinct roles for cellular retinoic acid-binding proteins I and II in regulating signaling by retinoic acid

The pleiotropic effects of retinoic acid (RA) in mammalian cells are mediat ed by two classes of proteins: the retinoic acid receptors (RAR) and cellul ar retinoic acid-binding proteins (CRABP-I and CRABP-II), Here we show that expression of CRABP-II, but not CRABP-I, markedly enhanced RAR-mediated tr anscriptional activation of a reporter gene in COS-7 cells. The equilibrium dissociation constants of complexes of CRABP-I or CRABP-II with RA were fo und to differ by 2-fold. It is thus unlikely that the distinct effects of t he two proteins on transactivation stem from differential ligand-binding af finities. The mechanisms by which RA transfers from the CRABPs to RAR were thus investigated directly. The rate constant for movement of RA from CRABP -II, but not from CRABP-I, to RAR strongly depended on the concentration of the acceptor. The data suggest that transfer of RA from CRABP-I to RAR inv olves dissociation of the ligand from the binding protein, followed by asso ciation with the receptor. In contrast, movement of RA from CRABP-II to the receptor is facilitated by a mechanism that involves direct interactions b etween CRABP-II and RAR, These findings reveal a striking functional differ ence between CRABP-I and CRABP-II, and point at a novel mechanism by which the transcriptional activity of RA can be regulated by CRABP-II.