Early steps in processing of yeast glycoproteins.

N-linked oligosaccharides have been examined on glycoproteins accumulated in yeast mutants that are blocked at successive stages in the secretory pathway, and in a new mutant, gls1-1, deficient in removal of glucose from N-linked core oligosaccharides, but not blocked in secretion. Oligosaccharides on invertase, a secreted protein, and carboxypeptidase Y, a vacuolar protein, are matured normally in the gls1 mutant but retain three glucoses/carbohydrate chain. The gls1 mutation is recessive and extracts of mutant cells are inactive in release of labeled glucose from core oligosaccharides. The mutant thus lacks glucosidase I activity but could also be deficient in the other core oligosaccharide glucosidase. When transport from the endoplasmic reticulum is blocked in sec18, N-linked oligosaccharides accumulate with a size corresponding to Man8GlcNAc2 when the normal GLS1 allele is present, and Glc3Man8GlcNAc2 in the gls1 mutant. From this we infer that all glucose units are removed prior to glycoprotein transport from the endoplasmic reticulum.