Department of Biochemistry, McGill University, Montreal, Quebec H3G 1Y6, Canada.
Semin Cell Dev Biol. 2010 Jul;21(5):500-11. doi: 10.1016/j.semcdb.2010.03.006. Epub 2010 Mar 25.
The mechanism, in molecular terms of protein quality control, specifically of how the cell recognizes and discriminates misfolded proteins, remains a challenge. In the secretory pathway the folding status of glycoproteins passing through the endoplasmic reticulum is marked by the composition of the N-glycan. The different glycoforms are recognized by specialized lectins. The folding sensor UGGT acts as an unusual molecular chaperone and covalently modifies the Man9 N-glycan of a misfolded protein by adding a glucose moiety and converts it to Glc1Man9 that rebinds the lectin calnexin. However, further links between the folding status of a glycoprotein and the composition of the N-glycan are unclear. There is little unequivocal evidence for other proteins in the ER recognizing the N-glycan and also acting as molecular chaperones. Nevertheless, based upon a few examples, we suggest that this function is carried out by individual proteins in several different complexes. Thus, calnexin binds the protein disulfide isomerase ERp57, that acts upon Glc1Man9 glycoproteins. In another example the protein disulfide isomerase ERdj5 binds specifically to EDEM (which is probably a mannosidase) and a lectin OS9, and reduces the disulfide bonds of bound glycoproteins destined for ERAD. Thus the glycan recognition is performed by a lectin and the chaperone function performed by a specific partner protein that can recognize misfolded proteins. We predict that this will be a common arrangement of proteins in the ER and that members of protein foldase families such as PDI and PPI will bind specifically to lectins in the ER. Molecular chaperones BiP and GRp94 will assist in the folding of proteins bound in these complexes as well as in the folding of non-glycoproteins.
蛋白质质量控制的分子机制,特别是细胞如何识别和区分错误折叠的蛋白质,仍然是一个挑战。在分泌途径中,穿过内质网的糖蛋白的折叠状态由 N-聚糖的组成来标记。不同的糖型被专门的凝集素识别。折叠传感器 UGGT 作为一种不寻常的分子伴侣,通过添加一个葡萄糖部分共价修饰错误折叠蛋白的 Man9 N-聚糖,并将其转化为 Glc1Man9,从而重新结合凝集素 calnexin。然而,糖蛋白的折叠状态与 N-聚糖的组成之间的进一步联系尚不清楚。几乎没有明确的证据表明内质网中的其他蛋白质能够识别 N-聚糖并作为分子伴侣发挥作用。尽管如此,基于少数几个例子,我们认为这种功能是由几个不同复合物中的单个蛋白质来执行的。因此,calnexin 结合蛋白质二硫键异构酶 ERp57,后者作用于 Glc1Man9 糖蛋白。在另一个例子中,蛋白质二硫键异构酶 ERdj5 特异性结合 EDEM(可能是一种甘露糖苷酶)和凝集素 OS9,并还原结合的糖蛋白的二硫键,这些糖蛋白注定要进行内质网相关降解。因此,糖的识别是由凝集素完成的,而伴侣功能是由能够识别错误折叠蛋白质的特定伴侣蛋白完成的。我们预测,这将是内质网中蛋白质的常见排列方式,并且像 PDI 和 PPI 这样的蛋白质折叠酶家族的成员将特异性地结合内质网中的凝集素。分子伴侣 BiP 和 GRp94 将协助这些复合物中结合的蛋白质以及非糖蛋白的折叠。