Matter K, Balda M S
Département de Biologie Cellulaire, Université de Genève, Sciences III, 30, 1211 Genève-4, Switzerland.
J Cell Sci. 1998 Feb;111 ( Pt 4):511-9. doi: 10.1242/jcs.111.4.511.
Tight junctions form a morphological and physical border between the apical and the basolateral cell surface domains of epithelial cells; hence assembly of tight junctions could occur from both of the two plasma membrane domains. We show here that the C-terminal cytoplasmic domain of occludin, the only known transmembrane protein of tight junctions, was sufficient to mediate basolateral expression of a chimeric protein. Since this chimera was transported directly to the basolateral membrane during biosynthesis, the C-terminal domain of occludin contains a basolateral targeting signal. Additionally, the C-terminal domain of occludin was also able to mediate endocytosis. Thus, the C-terminal cytoplasmic domain appears to govern intracellular transport of occludin. To test whether the basolateral membrane is an obligatory intermediate in transport of occludin to tight junctions, we analyzed the expression of occludin molecules rendered unable to efficiently integrate into tight junctions by the introduction of N-linked glycosylation sites into the two extracellular loops. Indeed, glycosylated occludin accumulated in the basolateral membrane, supporting a model in which the biogenesis of tight junctions occurs from this cell-surface domain.
紧密连接在上皮细胞的顶端和基底外侧细胞表面结构域之间形成形态学和物理边界；因此，紧密连接可以从两个质膜结构域中的任何一个组装而成。我们在此表明，紧密连接中唯一已知的跨膜蛋白闭合蛋白的C末端胞质结构域足以介导嵌合蛋白的基底外侧表达。由于这种嵌合体在生物合成过程中直接转运到基底外侧膜，闭合蛋白的C末端结构域包含一个基底外侧靶向信号。此外，闭合蛋白的C末端结构域也能够介导内吞作用。因此，C末端胞质结构域似乎控制着闭合蛋白的细胞内运输。为了测试基底外侧膜是否是闭合蛋白运输到紧密连接过程中的一个必需中间体，我们分析了通过在两个细胞外环引入N-连接糖基化位点而无法有效整合到紧密连接中的闭合蛋白分子的表达。事实上，糖基化的闭合蛋白在基底外侧膜中积累，支持了一种紧密连接生物发生从这个细胞表面结构域开始的模型。
