Glycoprotein synthesis in the human retina: localization of the lipid intermediate pathway

Retinal tissue from human eye donors was incubated with [3H]-labeled mannose, glucosamine, fucose, and leucine in the presence and absence of tunicamycin, a selective inhibitor of dolichol-dependent glycoprotein biosynthesis. The incorporation of labeled substrates was examined by quantitative light microscopic autoradiography and biochemical methods. [3H]-Mannose and [3H]-fucose were predominantly incorporated into the photoreceptor layer, while [3H]-glucosamine and [3H]-leucine labeled the entire retinal expanse. Tunicamycin caused a marked and selective reduction in the incorporation of [3H]-mannose and [3H]-glucosamine in the photoreceptor layer (especially the inner segments) without affecting the relative distribution of labeled products derived from [3H]-fucose or [3H]-leucine. All three [3H]-labeled sugars were incorporated preferentially into rod inner segments, relative to cones. Dual-label experiments with [3H]-sugars and [14C]-leucine revealed that tunicamycin selectively inhibited the incorporation of [3H]-mannose, but not the other substrates, into total retinal TCA-precipitable material. This inhibition was not due to decreased mannose uptake by retinal cells. Detergent-solubilized retinas were analyzed by polyacrylamide gel electrophoresis and fluorography. The results indicated that each labeled substrate was incorporated into a variety of glyoproteins, including a component having the electrophoretic mobility of opsin. These results suggest that the lipid intermediate pathway of glycoprotein synthesis is localized preferentially to the photoreceptor layer of the retina, and may subserve the biosynthesis of rod cell glycoproteins (e.g. opsin).