GENETIC TAILORING OF N-LINKED OLIGOSACCHARIDES - THE ROLE OF GLUCOSE RESIDUES IN GLYCOPROTEIN PROCESSING OF SACCHAROMYCES-CEREVISIAE IN-VIVO

In higher eukaryotes a quality control system monitoring the folding s tate of glycoproteins is located in the ER and is composed of the prot eins calnexin, calreticulin, glucosidase II, and UDP-glucose: glycopro tein glucosyltransferase. It is believed that the innermost glucose re sidue of the N-linked oligosaccharide of a glycoprotein serves as a ta g in this control system and therefore performs an important function in the protein folding pathway, To address this function, we construct ed Saccharomyces cerevisiae strains which contain nonglucosylated (G0) , monoglucosylated (G1), or diglucosylated (G2) glycoproteins in the E R and used these strains to study the role of glucose residues in the ER processing of glycoproteins, These alterations of the oligosacchari de structure did not result in a growth phenotype, but the induction o f the unfolded protein response upon treatment with DTT was much highe r in G0 and G2 strains as compared to wild-type and G1 strains, Our re sults provide lit vivo evidence that the G1 oligosaccharide is an acti ve oligosaccharide structure in the ER glycoprotein processing pathway of S. cerevisiae. Furthermore, by analyzing N-linked oligosaccharides of the constructed strains we can directly show that no general glyco protein glucosyltransferase exists in S. cerevisiae.