Glucose removal from N-linked oligosaccharides is required for efficient maturation of certain secretory glycoproteins from the rough endoplasmic reticulum to the Golgi complex.

Abstract

1-Deoxynojirimycin is a specific inhibitor of glucosidases I and II, the 1st enzymes that process N-linked oligosaccharides after their transfer to polypeptides in the rough endoplasmic reticulum. In a pulse-chase experiment, 1-deoxynojirimycin greatly reduced the rate of secretion of .alpha.1-antitrypsin and .alpha.1-antichymotrypsin by human hepatoma HepG2 cells, but had marginal effects on secretion of the glycoproteins C3 and transferrin, or of albumin. As judged by equilibrium gradient centrifugation, 1-deoxynojirimycin caused .alpha.1-antitrypsin and .alpha.1-antichymotrypsin to accumulate in the rough endoplasmic reticulum. The oligosaccharides on cell-associated .alpha.1-antitrypsin and .alpha.1-antichymotrypsin synthesized in the presence of 1-deoxynojirimycin, remained sensitive to endoglycosidase H and most likely had the structure Glu1-3Man9GlcNAc2. Tunicamycin, an antibiotic that inhibits addition of N-linked oligosaccharide units to glycoproteins, had a similar differential effect on secretion of these proteins. Swainsonine, an inhibitor of the Golgi enzyme .alpha.-mannosidase II, had no effect on the rates of protein secretion, although the proteins were in this case secreted with an abnormal N-linked, partially complex, oligosaccharide. The movement of .alpha.1-antitrypsin and .alpha.1-antichymotrypsin from the rough endoplasmic reticulum to the Golgi evidently requires that the N-linked oligosaccharides be processed to at least the Man9GlcNAc2 form; this oligosaccharide possibly forms part of the recognition site of a transport receptor for certain secretory proteins.