Sasamura Takeshi, Ishikawa Hiroyuki O, Sasaki Nobuo, Higashi Syunsuke, Kanai Maiko, Nakao Shiho, Ayukawa Tomonori, Aigaki Toshiro, Noda Katsuhisa, Miyoshi Eiji, Taniguchi Naoyuki, Matsuno Kenji
Precursory Research for Embryonic Science and Technology (PRESTO Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama, Japan.
Development. 2007 Apr;134(7):1347-56. doi: 10.1242/dev.02811. Epub 2007 Feb 28.
Notch is a transmembrane receptor that mediates the cell-cell interactions necessary for many cell-fate decisions. Endocytic trafficking of Notch plays important roles in the activation and downregulation of this receptor. A Drosophila O-FucT-1 homolog, encoded by O-fut1, catalyzes the O-fucosylation of Notch, a modification essential for Notch signaling and ligand binding. It was recently proposed that O-fut1 acts as a chaperon for Notch in the endoplasmic reticulum and is required for Notch to exit the endoplasmic reticulum. Here, we report that O-fut1 has additional functions in the endocytic transportation of Notch. O-fut1 was indispensable for the constitutive transportation of Notch from the plasma membrane to the early endosome, which we show was independent of the O-fucosyltransferase activity of O-fut1. We also found that O-fut1 promoted the turnover of Notch, which consequently downregulated Notch signaling. O-fut1 formed a stable complex with the extracellular domain of Notch. In addition, O-fut1 protein added to conditioned medium and endocytosed was sufficient to rescue normal Notch transportation to the early endosome in O-fut1 knockdown cells. Thus, an extracellular interaction between Notch and O-fut1 is essential for the normal endocytic transportation of Notch. We propose that O-fut1 is the first example, except for ligands, of a molecule that is required extracellularly for receptor transportation by endocytosis.
Notch是一种跨膜受体，介导许多细胞命运决定所必需的细胞间相互作用。Notch的内吞运输在该受体的激活和下调中发挥重要作用。由O-fut1编码的果蝇O-FucT-1同源物催化Notch的O-岩藻糖基化，这是Notch信号传导和配体结合所必需的修饰。最近有人提出，O-fut1在内质网中作为Notch的伴侣，是Notch离开内质网所必需的。在此，我们报道O-fut1在Notch的内吞运输中具有额外功能。O-fut1对于Notch从质膜到早期内体的组成型运输是不可或缺的，我们发现这与O-fut1的O-岩藻糖基转移酶活性无关。我们还发现O-fut1促进了Notch的周转，从而下调了Notch信号传导。O-fut1与Notch的细胞外结构域形成稳定复合物。此外，添加到条件培养基中并被内吞的O-fut1蛋白足以挽救O-fut1敲低细胞中Notch向早期内体的正常运输。因此，Notch与O-fut1之间的细胞外相互作用对于Notch的正常内吞运输至关重要。我们提出，除配体之外，O-fut1是第一个在内吞作用中受体运输在细胞外需要的分子实例。
