Degradation of misfolded endoplasmic reticulum glycoproteins in Saccharomyces cerevisiae is determined by a specific oligosaccharide structure

In Saccharomyces cerevisiae, transfer of N-linked oligosaccharides is immediately followed by trimming of ER-localized glycosidases. We analyzed the influence of specific oligosaccharide structures for degradation of misfolded carboxypeptidase Y (CPY). By studying the trimming reactions in vivo, we found that removal of the terminal alpha 1,2 glucose and the first alpha 1,3 glucose by glucosidase I and glucosidase II respectively, occurred rapidly, whereas mannose cleavage by mannosidase I was slow. Transport and maturation of correctly folded CPY was not dependent on oligosaccharide structure. However, degradation of misfolded CPY was dependent on specific trimming steps. Degradation of misfolded CPY with N-linked oligosaccharides containing glucose residues was less efficient compared with misfolded CPY bearing the correctly trimmed Man(8)GlcNAc(2) oligosaccharide. Reduced rate of degradation was mainly observed for misfolded CPY bearing Man(6)GlcNAc(2), Man(7)GlcNAc(2) and Man(9)GlcNA(2) oligosaccharides, whereas Man(8)GlcNAc(2) and, to a lesser extent, Man(5)GlcNAc(2) oligosaccharides supported degradation. These results suggest a role for the Man(8)GlcNAc(3) oligosaccharide in the degradation process. They may indicate the presence of a Man(8)GlcNA(2)-binding lectin involved in targeting of misfolded glycoproteins to degradation in S. cerevisiae.