Kostova Zlatka, Wolf Dieter H
Institut für Biochemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany.
J Cell Sci. 2005 Apr 1;118(Pt 7):1485-92. doi: 10.1242/jcs.01740. Epub 2005 Mar 15.
In the endoplasmic reticulum (ER), N-linked glycans (N-glycans) function as signals to recruit the lectin chaperones involved in protein folding, quality control and ER-associated degradation. We undertook a systematic study of the four N-glycans of mutated carboxypeptidase yscY (CPY*) to determine whether there are positional differences between the glycans in ER-associated degradation. We constructed hypoglycosylated CPY* variants containing one, two or three N-glycans in various combinations and studied their degradation kinetics. We found that the four carbohydrate chains on CPY* are not equal in their signaling function: presence of the Asn368-linked glycan is necessary and sufficient for efficient degradation of CPY*. We also analysed the involvement of the ER lectins Htm1p and Cne1p (yeast calnexin) in the glycan-based recognition process with respect to number and position of N-glycans. We observed that Htm1p function depends on the presence of N-glycans in general but that there is no positional preference for a particular glycan. Cne1p, however, is selective with respect to substrate, and participates in the quality control only of some underglycosylated variants. For cases in which both lectins are involved, Cne1p and Htm1p play competing roles in targeting the substrate for degradation: loss of Cne1p accelerates degradation, whereas loss of Htm1p stabilizes the substrate.
在内质网（ER）中，N-连接聚糖（N-聚糖）作为信号，招募参与蛋白质折叠、质量控制和内质网相关降解的凝集素伴侣。我们对突变的羧肽酶yscY（CPY*）的四种N-聚糖进行了系统研究，以确定这些聚糖在内质网相关降解中是否存在位置差异。我们构建了以各种组合包含一个、两个或三个N-聚糖的低糖基化CPY变体，并研究了它们的降解动力学。我们发现CPY上的四条糖链在信号功能上并不相同：Asn368连接的聚糖的存在对于CPY*的有效降解是必要且充分的。我们还分析了内质网凝集素Htm1p和Cne1p（酵母钙连蛋白）在基于聚糖的识别过程中对N-聚糖数量和位置的参与情况。我们观察到，一般来说，Htm1p的功能依赖于N-聚糖的存在，但对特定聚糖没有位置偏好。然而，Cne1p对底物具有选择性，仅参与一些低糖基化变体的质量控制。对于两种凝集素都参与的情况，Cne1p和Htm1p在将底物靶向降解方面发挥竞争作用：Cne1p的缺失加速降解，而Htm1p的缺失使底物稳定。
