Identification of a membrane protein binding the retinol in retinal pigment epithelium

Abstract

AVAILABLE experimental evidence indicates the primary role of retinal pigment epithelium (PE) in the uptake of retinol from the extravascular space of the choriocapillaris¹ and possibly in its transfer to and from the photoreceptors². While the transport mechanism of retinol in plasma by the retinol-binding protein (RBP) is known in detail^3,4, only partial information is available on how circulating retinol is transferred from its carrier protein to PE. It has been shown⁵ recently that iodinated human RBP binds specifically to isolated bovine PE and autoradiographic investigations⁶ have suggested that the binding is confined to the choroidal surface of these cells. We have previously demonstrated⁷ that isolated human and bovine PE actively binds ³H-retinol only if this is present in the incubation medium as a complex with human RBP, and suggested that the interaction of circulating RBP with the cell membranes of PE is specific (G.M. and F.G., unpublished observations), and essential, for the active transport of retinol in these cells against a concentration gradient. We have now identified, in the plasma membranes of bovine PE incubated in the presence of ³H-retinol-RBP, a binding protein for retinol; some evidence is presented which suggests that this membrane protein is different from plasma RBP.