Tunicamycin blocks the incorporation of opsin into retinal rod outer segment membranes.

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Abstract
Isolated frog retinas were incubated with radiolabeled glycoprotein precursors in the presence or absence of tunicamycin (TM), a selective inhibitor of protein N-glycosylation. In dual-label incubations, TM inhibited the incorporation of [3H]mannose into total retina Cl3CCOOH-precipitable material by 85% relative to controls, whereas incorporation of [14C]Leu was not significantly affected. In a companion single-label incubation, TM blocked the incorporation of [3H]Leu into rod outer segment (ROS) membrane Cl3CCOOH-precipitable material by 95% relative to controls. When retinas were labeled with [35S]Met, fluorograms of NaDodSO4[sodium dodecyl sulfate]/polyacrylamide gels from control retinas and ROS membranes exhibited a heavily labeled component (apparent MW .apprxeq. 37,000) which had the electrophoretic and antigenic properties of opsin, the rod visual pigment apoglycoprotein. TM-treated retinas exhibited a substantially reduced labeling of the MW 37,000 component and incorporation of label into a component (apparent MW .apprxeq. 32,000) not found in control retinas, which exhibited the electrophoretic and antigenic behavior of nonglycosylated opsin. ROS membranes isolated from TM-treated retinas contained neither the MW 37,000 nor the MW 32,000 radiolabeled species. Light-microscopic autoradiograms of retinas incubated with [3H]Leu in the absence of TM exhibited bands of Ag grains at the base of ROS, indicative of new membrane assembly. No such bands were observed in autoradiograms of TM-treated retinas. Apparently glycosylation of opsin is required for its incorporation into ROS membranes.