Role of carbohydrates in protein secretion and turnover: effects of tunicamycin on the major cell surface glycoprotein of chick embryo fibroblasts

Using tunicamycin, we have investigated the role of glycosylation in the biosynthesis, processing and turnover of CSP, the major cell surface glycoprotein of chick embryo fibroblasts (CEF). This antibiotic specifically inhibits glycosylation mediated by dolichol pyrophosphate and consequently inhibits the glycosylation of asparaginyl residues of glycoproteins. Tunicamycin inhibited the incorporation of 3H-mannose into CSP by 92--98% and 14C-glucosamine by 84--96%, whereas total protein synthesis was decreased by only 15--45%. Tunicamycin treatment decreased total amounts of CSP by approximately 50--65%, with equal decreases in CSP occurring on the cell surface and in culture medium, whereas intracellular pools of CSP were not substantially affected. In contrast to CSP, three other membrane-associated proteins of apparent molecular weights 75,000, 95,000 and 150,000 daltons were found in increased amounts. Procollagen secretion was not inhibited by tunicamycin. Both procollagen and CSP secretion into culture medium were also not increased in AD6, a glycosylation-deficient, mutant mouse 3T3 cell line compared to wild-type cells. We examined the mechanism of the decrease in CSP after tunicamycin treatment. The rate of CSP biosynthesis as measured by pulse-labeling with 14C-leucine was not altered. Tunicamycin had only a slight effect on the initial times and rates of CSP appearance on the cell surface; some apparent intracellular redistribution of CSP was detected by immunofluorescence. The major effect of tunicamycin treatment was to accelerate the rate of degradation of CSP 2--3 fold. This increase is sufficient to account for the observed decreases after tunicamycin treatment. Our results suggest that carbohydrates may not be essential for CSP or procollagen synthesis, intracellular processing and secretion, but that carbohydrates may help stabilize CSP against proteolytic degradation.