Differential distribution of opsin in the plasma membrane of frog photoreceptors: an immunocytochemical study.

Opsin molecules on the surface of frog photoreceptors were visualized by immunocytochemistry at the ultrastructural level. Isolated retinas were immersed in biotinyl-antibody to bovine opsin followed by avidin-ferritin conjugates. Anti-opsin bound to the plasma membrane and to the surface of the most basal discs of red rod outer segments. Inner segment plasma membranes of red rod photoreceptors were devoid of anti-opsin label except for the apical plasma membrane in the region of the recently described periciliary ridge complex. The connecting cilium surface from its base at the periciliary region to the site of new disc evagination was almost free of anti-opsin binding, an observation in consonance with prior studies of thin sectioned retinas embedded in glutaraldehyde cross-linked bovine serum albumin. These results indicate that the continuous plasma membrane of photoreceptors is highly polarized. Opsin, which is free to diffuse throughout the outer segment plasma membrane and along the discs, does not back-diffuse onto the inner segment plasma membrane. The periciliary ridge complex and the base of the connecting cilium are possible sites of restriction of opsin mobility. This study also has provided new insight into the molecular structure of frog visual pigments. Frog green rod and cone outer and inner segment plasma membranes were not labeled by this sheep antiserum to bovine opsin. In contrast, discs of green ROS and the lamellae of some cones were labeled when these antibodies were applied to albumin embedded thin sections of frog retinas. Apparently, only internal or intramembraneous domains of green ROS and cone visual pigments were recognized by this antibody while both internal and extracellular domain(s) of red ROS opsin were reactive.