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 mediated by two classes of proteins: the retinoic acid receptors (RAR) and cellular 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 transcriptional activation of a reporter gene in COS-7 cells. The equilibrium dissociation constants of complexes of CRABP-I or CRABP-II with RA were found to differ by 2-fold. It is thus unlikely that the distinct effects of the two proteins on transactivation stem from differential ligand-binding affinities. 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 involves dissociation of the ligand from the binding protein, followed by association with the receptor. In contrast, movement of RA from CRABP-II to the receptor is facilitated by a mechanism that involves direct interactions between CRABP-II and RAR. These findings reveal a striking functional difference 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.
Dong
D
D
Cornell University, Division of Nutritional Sciences, Savage Hall, Ithaca, New York 14853, USA.
Ruuska
S E
SE
Levinthal
D J
DJ
Noy
N
N
D000818:Animals; D019556:COS Cells; D018168:Receptors, Retinoic Acid / Q000502:physiology*; D015533:Transcriptional Activation / Q000187:drug effects; D014212:Tretinoin / Q000494:pharmacology*
D018168:Receptors, Retinoic Acid; C082927:retinoic acid binding protein I, cellular; C082928:retinoic acid binding protein II, cellular; D014212:Tretinoin