Are sugars involved in the binding of rhodopsin-membranes by the retinal pigment epithelium?

The binding by the retinal pigment epithelium (RPE) cells of the embryonic chick, in culture, of recombinant membranes composed of phosphatidylcholine and bovine rhodopsin (rhodopsin-liposomes [RL]) was investigated to explore the influence that the carbohydrate groups of rhodopsin might have on this process. The reaction was quantitated by measuring the amount of RL associated with the cells using a radioimmunoassay for rhodopsin. The process was saturated at about 8 microM RL (calculated as rhodopsin-equivalents), and a two- to threefold greater rate and extent of binding was observed at 37 degrees C as compared with 4 degrees C. The presence of up to 30 X 10(3)-fold molar excess over RL of mannose and N-acetylglucosamine, the sugars present on rhodopsin, as well as other saccharides, had little or no effect on the binding. These observation indicate that lectin-like recognition involving the sugar groups of rhodopsin is not involved in the binding of rhodopsin-containing membranes by the RPE cells. In contrast, exogenously added concanavalin A and wheat germ agglutinin enhanced the binding, an effect which was blocked by appropriate sugars.