Evidence for role of glycoprotein carbohydrates in membrane transport: specific inhibition by tunicamycin.

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Abstract
Using tunicamycin, the role of glycoproteins in membrane transport was investigated. Tunicamycin is a glucosamine-containing antibiotic that specifically inhibits dolichol pyrophosphate-mediated glycosylation of asparaginyl residues of glycoproteins. Inhibition of protein glycosylation in chick embryo fibroblasts by tunicamycin or other inhibitors of glycosylation resulted in defective transport of glucose, uridine and amino acid analogs (.alpha.-aminoisobutyrate and cycloleucine). The defect in glucose transport is accompanied by decreased glucose metabolism, as determined by rates of CO2 and lactate production. In contrast, tunicamycin treatment did not affect other membrane-associated processes, e.g., secretion of fibronectin and procollagen, uptake of glucose by passive diffusion, Na+/K+ ATPase and adenylate cyclase activities or stimulation of adenylate cyclase by prostaglandin and cholera toxin. Two glucose/glycosylation-regulated membrane proteins with apparent subunit MW of 95,000 and 75,000 were induced by tunicamycin treatment. Apparently glycoprotein glycosylation is required for membrane transport.