Interphotoreceptor retinoid-binding protein contains three retinoid binding sites

Interphotoreceptor retinoid binding protein (IRBP), the major soluble prote in component of the interphotoreceptor matrix, is believed to participate i n the visual cycle by transporting retinoids between retinal pigment epithe lium and photoreceptor cells in the eye. IRBP can associate with several ch emical and isomeric forms of retinoids but displays the highest affinity to wards the retinoids that are important in the visual cycle, 11-cis-retinal and all-trans-retinol. It was previously reported that IRBP can associate w ith 2 mol of all-trans-retinol or 2 mol of 11-cis-retinal per mol of protei n. One of the retinoid binding sites, termed 'site 1', was found to display a broad ligand selectivity and to bind either all-trans-retinol or 11-cis- retinal with similar affinities. Here, the retinoid-binding properties of I RBP were further examined. The data demonstrate that IRBP contains three di stinct retinoid binding sites. The promiscuous 'site 1', and two additional sites with a stricter selectivity. One of the latter sites appears to be s elective towards all-trans-retinol, while the other is specific for 11-cis- retinal. (C) 2001 Academic Press.