Ultrastructural localization of lectin binding sites on the surface of retinal photoreceptors and pigment epithelium

Abstract

The ferritin-lectin complexes of Ricinus communis agglutinin (RCA-120), wheat germ agglutinin (WGA), concanavalin A (con A), soybean agglutinin (SBA) and fucose binding protein (FBP) have been used to study the distribution of sugars along the plasma membranes of frog photoreceptor and pigment epithelial cells. The tissues were fixed prior to labeling in order to prevent the lectin-induced rearrangement of binding sites. The plasma membrane of different regions of the rod and cone photoreceptor cells bound each lectin to a different extent. In all cases, the outer segment plasma membrane bound more of the ferritin-lectin complex than did the inner segment plasma membrane. This might reflect the specific differentiation of the outer segment plasma membranes, related to their role in the visual process. The intensity of binding of each lectin to the pigment epithelium plasma membrane was similar to its binding to the rod outer segment plasma membrane. In cone outer segments, where one side of the disk membrane is accessible to the incubation medium, direct binding of lectin to the disk edges, as well as to the plasma membrane, was seen. Cone outer segments bound more of the α-D-N-acetylgalactosamine and D-galactose specific lectin, SBA, than did rod outer segments. No differences were observed in the binding of ferritin-lectin complexes to rod outer segments from dark or light adapted frogs.