A concentration of fucosylated glycoconjugates at the base of cone outer segments: quantitative electron microscope...

Rod photoreceptors show a discrete band of labeled molecules at the outer segment base shortly after the administration of radioactive protein precursors (Young, 1967). The band signifies the insertion of radiolabeled protein, primarily opsin, into rod disc membrane (Basinger, Bok and Hall, 1976). In contrast, the autoradiographic labeling pattern in cones is characteristically diffuse (Young, 1971), although cones as well as rods are now thought to replace their disc membranes continually (Anderson, Fisher, and Steinberg, 1978). Using grain count analysis in conjunction with electron microscope autoradiography, we demonstrate for the first time that the initial cone outer segment (COS) labeling pattern in the retinas of cone-dominant rodents is not random or diffuse after intraocular injection of [3H]-fucose--a specific glycoprotein precursor. Thirty minutes after [3H]-fucose injection, the Golgi complex is heavily labeled in both rods and cones. At 1.5 hr, labeling first appears in the periciliary region and in the basal portion of the COS. We detected a basal concentration of fucosylated molecules up to 12 hr after injection; thereafter, we found no differential distribution of labeled product molecules in COS. In rods, we observed no band of labeled protein at any of the time points sampled. The low level of labeling in rods is uniformly distributed between the basal and apical portion of the outer segments. Although the identity of the fucosylated material remains unknown, the early labeling of the Golgi complex--followed by labeling over the periciliary region and the basal portion of the outer segment--suggests that newly synthesized membrane protein is transported from sites of synthesis in the inner segment and inserted into disc membrane at the COS base. However, in contrast to rods, the protein is apparently capable of diffusing longitudinally throughout the interconnected membrane network.