A screen for yeast mutants with defects in the dolichol-mediated pathway for N-glycosylation

Dolichol in the form of dolichyl phosphate participates in the synthesis of N- and O-linked glycoproteins and phosphatidylinositol-linked proteins in the yeast Saccharomyces cerevisiae. In this organism, as well as in higher eukaryotes, a number of the enzymes in the polyisoprenoid and glycoprotein biosynthetic pathways have not been identified. In this study, we have developed a convenient, highly sensitive assay that uses one of the end products of the dolichylphosphate synthetic pathway, oligosaccharide-diphosphodolichol, and a 125I-labeled peptide substrate for N-linked glycosylation to screen a collection of temperature-sensitive yeast mutants for defects in protein glycosylation. By using a combination of biochemical and genetic procedures, the defective mutants were grouped into three categories: those containing defects in dolichyl-phosphate synthesis (class 1), lipid-linked oligosaccharide assembly (class 2), or oligosaccharide transferase activity (class 3). Among the mutants identified by this screen were sec59 (which encodes dolichol kinase) and a mutant that affects the activity of the ALG1-encoded mannosyltransferase that forms dolichol-PP-(GlcNAc)2Man1. Of particular interest was a mutant that exhibits a temperature-sensitive defect in oligosaccharide transferase activity. This mutant, meg1 (microsomal protein essential for glycosylation 1) assembles a complete oligosaccharide chain and, therefore, is likely to be a class 3 mutant. We report the cloning of MEG1, the gene that rescues the oligosaccharide transferase activity defect in this mutant. A number of criteria distinguish this gene from previously described genes in this pathway.