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Using tunicamycin, we have investigated the role of glycoproteins in membrane transport. 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 Iibroblasts by tunicamycin or other inhibitors of glycosylation resulted in defective transport of glucose, uridine, and amino acid analogs (a-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, such as 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 molecular weights of 95,000 and 75,000 were induced by tunicamycin treatment. Our results indicate that glycoprotein glycosylation is required for membrane transport.The oligosaccharides on glycoproteins appear to play important roles in protein secretion (1), specific recognition of serum glycoproteins (2), protection of glycoproteins against proteolytic degradation (3), the insertion or proper orientation of glycoproteins in the plasma membrane (4, 5), and cell adhesion and morphology (4-6). Because the transport of metabolites across cell membranes is thought to be mediated via glycoprotein carriers (7), we examined whether inhibiting protein glycosylation would alter a number of plasma membrane enzymatic and nutrient transport processes.For these studies we have used tunicamycin, an inhibitor of the synthesis of N-acetylglucosaminyl pyrophosphoryl polyisoprenol (8-10). Because this reaction is required for the synthesis of the core sequence of N-glycosidically linked oligosaccharides, tunicamycin treatment results in synthesis of glycoproteins deficient in asparagine-linked oligosaccharides (10). -We present evidence that inhibition of protein glycosylation results in defective membrane transport and glucose metabolism in chick embryo fibroblasts (CEF). We demonstrate that the carbohydrate moieties of glycoproteins are required for the normal transport of metabolites across the plasma membrane and their subsequent metabolism. This effect might occur directly on carrier function or indirectly by inhibiting insertion of transport molecules into the membrane or by increasing their degradation rate (3). MATERIALS AND METHODSCell Culture. Secondary or tertiary CEF were grown for 24 hr in the presence or absence of tunicamycin (0.05 ,4g/ml or 50 nM) in 35-mm plastic tissue culture dishes (Costar) as described (11).Transport Assays. For uptake experiments, the conditioned medium was aspirated and fresh medium containing 0.05 ,g of tunicamycin per ml and the radiolabeled transport substrate was added. After incubation at 23°C for the ...

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