Effect of tunicamycin on the glycosylation of rhodopsin

The incorporation of [ 3H]glucosamine, [ 3H]mannose, and [ 35S]methionine into rhodopsin was investigated in retinas which had been incubated in the presence and absence of the antibiotic, tunicamycin. In its presence, the incorporation of glucosamine was inhibited 70% and mannose, 96% compared to controls. In the presence of tunicamycin the attachment of glucosamine to core-region sites was virtually eliminated. The formation of unglycosylated rhodopsin was also indicated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and concanavalin A-Sepharose chromatography. These findings are consistent with the participation of the lipid-linked pathway in the glycosylation of this well-characterized intrinsic glycoprotein of the membranes of the disk of the rod outer segment. As indicated by the incorporation of [ 35S]methionine, the synthesis of rhodopsin apoprotein was inhibited by a much lesser amount. This suggests that the glycosylation of rhodopsin is not required for its insertion into the disk membrane.