Gut A, Kappeler F, Hyka N, Balda M S, Hauri H P, Matter K
Department of Cell Biology, University of Geneva, Geneva, Switzerland.
EMBO J. 1998 Apr 1;17(7):1919-29. doi: 10.1093/emboj/17.7.1919.
Polarized expression of most epithelial plasma membrane proteins is achieved by selective transport from the Golgi apparatus or from endosomes to a specific cell surface domain. In Madin-Darby canine kidney (MDCK) cells, basolateral sorting generally depends on distinct cytoplasmic targeting determinants. Inactivation of these signals often resulted in apical expression, suggesting that apical transport of transmembrane proteins occurs either by default or is mediated by widely distributed characteristics of membrane glycoproteins. We tested the hypothesis of N-linked carbohydrates acting as apical targeting signals using three different membrane proteins. The first two are normally not glycosylated and the third one is a glycoprotein. In all three cases, N-linked carbohydrates were clearly able to mediate apical targeting and transport. Cell surface transport of proteins containing cytoplasmic basolateral targeting determinants was not significantly affected by N-linked sugars. In the absence of glycosylation and a basolateral sorting signal, the reporter proteins accumulated in the Golgi complex of MDCK as well as CHO cells, indicating that efficient transport from the Golgi apparatus to the cell surface is signal-mediated in polarized and non-polarized cells.
大多数上皮细胞质膜蛋白的极化表达是通过从高尔基体或内体选择性转运至特定的细胞表面区域来实现的。在犬肾上皮（MDCK）细胞中，基底外侧分选通常依赖于独特的细胞质靶向决定因素。这些信号的失活常常导致顶端表达，这表明跨膜蛋白的顶端转运要么是默认发生的，要么是由膜糖蛋白广泛分布的特征介导的。我们使用三种不同的膜蛋白测试了N-连接碳水化合物作为顶端靶向信号的假说。前两种蛋白通常不进行糖基化，第三种是糖蛋白。在所有三种情况下，N-连接碳水化合物显然都能够介导顶端靶向和转运。含有细胞质基底外侧靶向决定因素的蛋白质的细胞表面转运不受N-连接糖的显著影响。在没有糖基化和基底外侧分选信号的情况下，报告蛋白在MDCK以及CHO细胞的高尔基体复合物中积累，表明从高尔基体到细胞表面的有效转运在极化和非极化细胞中都是信号介导的。
