Jakob C A, Burda P, Roth J, Aebi M
Division of Cell and Molecular Pathology, Department of Pathology, University of Zürich, CH-8091 Zürich, Switzerland.
J Cell Biol. 1998 Sep 7;142(5):1223-33. doi: 10.1083/jcb.142.5.1223.
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 alpha1,2 glucose and the first alpha1,3 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 Man8GlcNAc2 oligosaccharide. Reduced rate of degradation was mainly observed for misfolded CPY bearing Man6GlcNAc2, Man7GlcNAc2 and Man9GlcNAc2 oligosaccharides, whereas Man8GlcNAc2 and, to a lesser extent, Man5GlcNAc2 oligosaccharides supported degradation. These results suggest a role for the Man8GlcNAc2 oligosaccharide in the degradation process. They may indicate the presence of a Man8GlcNAc2-binding lectin involved in targeting of misfolded glycoproteins to degradation in S. cerevisiae.
在酿酒酵母中，N-连接寡糖的转移紧接着是内质网定位糖苷酶的修剪。我们分析了特定寡糖结构对错误折叠的羧肽酶Y（CPY）降解的影响。通过研究体内的修剪反应，我们发现葡萄糖苷酶I和葡萄糖苷酶II分别快速去除末端α1,2葡萄糖和第一个α1,3葡萄糖，而甘露糖苷酶I切割甘露糖则较慢。正确折叠的CPY的运输和成熟不依赖于寡糖结构。然而，错误折叠的CPY的降解依赖于特定的修剪步骤。与带有正确修剪的Man8GlcNAc2寡糖的错误折叠CPY相比，含有葡萄糖残基的N-连接寡糖的错误折叠CPY的降解效率较低。主要观察到带有Man6GlcNAc2、Man7GlcNAc2和Man9GlcNAc2寡糖的错误折叠CPY的降解速率降低，而Man8GlcNAc2以及程度较轻的Man5GlcNAc2寡糖支持降解。这些结果表明Man8GlcNAc2寡糖在降解过程中起作用。它们可能表明存在一种Man8GlcNAc2结合凝集素，参与将错误折叠的糖蛋白靶向酿酒酵母中的降解。
