BINDING OF RETINOL IN BOTH RETINOID-BINDING SITES OF INTERPHOTORECEPTOR RETINOID-BINDING PROTEIN (IRBP) IS STABILIZED MAINLY BY HYDROPHOBICINTERACTIONS

Interphotoreceptor retinoid-binding protein (IRBP) is an ocular protei n which is believed to participate in the visual cycle by mediating tr ansport of retinoids between pigment epithelium and photoreceptor cell s, The molecular mechanism underlying the ability of IRBP to target pa rticular retinoids to the specific cells that are their sites of actio n and metabolism is not completely clear, and little information is av ailable regarding the structure of the protein's multiple ligand-bindi ng sites, IRBP possesses two retinoid-binding sites, and it was report ed that binding of the visual chromophore, Il cis-retinal, in one of t hese sites, but not in the other, is tightly regulated by another IRBP ligand, docosahexaenoic acid (Chen, Y., Houghton, L. A., Brenna, J. T ., and Noy, N. (1996) J. Biol. Chem. 271, 20507), The two sites are th us functionally distinct, Here, the thermodynamic parameters governing the interactions of retinol with the IRBP retinoid-binding sites were measured, The data demonstrate that the interactions of retinol with both sites are stabilized mainly by hydrophobic interactions, and that the hydroxyl head group of retinol is not involved in formation of pr otein-ligand complexes, Nevertheless, the data indicate that the two s ites are structurally distinct, and that binding of retinol in them oc curs by remarkably different modes of interactions.