1] Centre for Cancer Biomedicine, Faculty of Medicine, University of Oslo, Montebello, N-0379 Oslo, Norway [2] Department of Molecular Cell Biology, Institute for Cancer Research, Oslo University Hospital, Montebello, N-0379 Oslo, Norway.
Institute of Medical Biology, University of Tromsø - The Arctic University of Norway, N-9037 Tromsø, Norway.
Nature. 2015 Apr 9;520(7546):234-8. doi: 10.1038/nature14359.
The main organelles of the secretory and endocytic pathways--the endoplasmic reticulum (ER) and endosomes, respectively--are connected through contact sites whose numbers increase as endosomes mature. One function of such sites is to enable dephosphorylation of the cytosolic tails of endosomal signalling receptors by an ER-associated phosphatase, whereas others serve to negatively control the association of endosomes with the minus-end-directed microtubule motor dynein or mediate endosome fission. Cholesterol transfer and Ca(2+) exchange have been proposed as additional functions of such sites. However, the compositions, activities and regulations of ER-endosome contact sites remain incompletely understood. Here we show in human and rat cell lines that protrudin, an ER protein that promotes protrusion and neurite outgrowth, forms contact sites with late endosomes (LEs) via coincident detection of the small GTPase RAB7 and phosphatidylinositol 3-phosphate (PtdIns(3)P). These contact sites mediate transfer of the microtubule motor kinesin 1 from protrudin to the motor adaptor FYCO1 on LEs. Repeated LE-ER contacts promote microtubule-dependent translocation of LEs to the cell periphery and subsequent synaptotagmin-VII-dependent fusion with the plasma membrane. Such fusion induces outgrowth of protrusions and neurites, which requires the abilities of protrudin and FYCO1 to interact with LEs and kinesin 1. Thus, protrudin-containing ER-LE contact sites are platforms for kinesin-1 loading onto LEs, and kinesin-1-mediated translocation of LEs to the plasma membrane, fuelled by repeated ER contacts, promotes protrusion and neurite outgrowth.
分泌和内吞途径的主要细胞器——内质网（ER）和内体，分别通过接触位点相连，这些接触位点的数量随着内体的成熟而增加。这些位点的一个功能是使 ER 相关磷酸酶能够去磷酸化内体信号受体的胞质尾部，而其他功能则是负调控内体与微管动力蛋白 dynein 的结合，或介导内体分裂。胆固醇转移和 Ca(2+)交换被认为是这些位点的另外的功能。然而，ER-内体接触位点的组成、活性和调节仍不完全清楚。在这里，我们在人源和大鼠细胞系中表明，突起蛋白 protrudin 通过小 GTPase RAB7 和磷脂酰肌醇 3-磷酸（PtdIns(3)P）的同时检测，与晚期内体（LE）形成接触位点。这些接触位点介导微管马达 kinesin 1 从 protrudin 转移到 LE 上的马达衔接蛋白 FYCO1。重复的 LE-ER 接触促进微管依赖性的 LE 向细胞边缘的易位，随后与质膜上的突触结合蛋白 VII 依赖性融合。这种融合诱导突起和神经突的生长，这需要 protrudin 和 FYCO1 与 LE 和 kinesin 1 相互作用的能力。因此，含有 protrudin 的 ER-LE 接触位点是将 kinesin 1 加载到 LE 上的平台，并且由重复的 ER 接触提供动力的 LE 向质膜的 kinesin-1 介导的易位，促进了突起和神经突的生长。
