Sommer Bernhard, Oprins Adrian, Rabouille Catherine, Munro Sean
Medical Research Council Laboratory of Molecular Biology, Cambridge CB2 2QH, England, UK.
J Cell Biol. 2005 Jun 20;169(6):953-63. doi: 10.1083/jcb.200411053. Epub 2005 Jun 13.
The exocyst is an octameric complex required for polarized secretion. Some components of the exocyst are found on the plasma membrane, whereas others are recruited to Golgi membranes, suggesting that exocyst assembly tethers vesicles to their site of fusion. We have found that in Drosophila melanogaster oocytes the majority of the exocyst component Sec5 is unexpectedly present in clathrin-coated pits and vesicles at the plasma membrane. In oocytes, the major substrate for clathrin-dependent endocytosis is the vitellogenin receptor Yolkless. A truncation mutant of Sec5 (sec5(E13)) allows the formation of normally sized oocytes but with greatly reduced yolk uptake. We find that in sec5(E13) oocytes Yolkless accumulates aberrantly in late endocytic compartments, indicating a defect in the endocytic cycling of the receptor. An analogous truncation of the yeast SEC5 gene results in normal secretion but a temperature-sensitive defect in endocytic recycling. Thus, the exocyst may act in both Golgi to plasma membrane traffic and endocytic cycling, and hence in oocytes is recruited to clathrin-coated pits to facilitate the rapid recycling of Yolkless.
外泌体是极化分泌所需的八聚体复合物。外泌体的一些成分存在于质膜上，而其他成分则被募集到高尔基体膜上，这表明外泌体组装将囊泡系在其融合位点。我们发现在果蝇卵母细胞中，外泌体成分Sec5的大部分意外地存在于质膜上的网格蛋白包被小窝和囊泡中。在卵母细胞中，网格蛋白依赖性内吞作用的主要底物是卵黄蛋白原受体无黄卵。Sec5的截短突变体（sec5(E13)）允许形成正常大小的卵母细胞，但卵黄摄取大大减少。我们发现在sec5(E13)卵母细胞中，无黄卵异常积聚在晚期内吞区室中，表明受体的内吞循环存在缺陷。酵母SEC5基因的类似截短导致正常分泌，但在内吞再循环中存在温度敏感缺陷。因此，外泌体可能在高尔基体到质膜的运输以及内吞循环中起作用，因此在卵母细胞中被募集到网格蛋白包被小窝，以促进无黄卵的快速再循环。
