Department of Cell and Developmental Biology, Biocenter, University of Würzburg, 97074 Würzburg, Germany.
Department of Biochemistry and Cell Biology, Max Perutz Labs, University of Vienna, Vienna Biocenter (VBC), 1030 Vienna, Austria.
J Cell Sci. 2023 Oct 15;136(20). doi: 10.1242/jcs.261548. Epub 2023 Oct 26.
All endocytosis and exocytosis in the African trypanosome Trypanosoma brucei occurs at a single subdomain of the plasma membrane. This subdomain, the flagellar pocket, is a small vase-shaped invagination containing the root of the single flagellum of the cell. Several cytoskeleton-associated multiprotein complexes are coiled around the neck of the flagellar pocket on its cytoplasmic face. One of these, the hook complex, was proposed to affect macromolecule entry into the flagellar pocket lumen. In previous work, knockdown of T. brucei (Tb)MORN1, a hook complex component, resulted in larger cargo being unable to enter the flagellar pocket. In this study, the hook complex component TbSmee1 was characterised in bloodstream form T. brucei and found to be essential for cell viability. TbSmee1 knockdown resulted in flagellar pocket enlargement and impaired access to the flagellar pocket membrane by surface-bound cargo, similar to depletion of TbMORN1. Unexpectedly, inhibition of endocytosis by knockdown of clathrin phenocopied TbSmee1 knockdown, suggesting that endocytic activity itself is a prerequisite for the entry of surface-bound cargo into the flagellar pocket.
在非洲锥虫（Trypanosoma brucei）中，所有的内吞作用和外排作用都发生在质膜的一个单一亚域。这个亚域称为鞭毛口袋，是一个含有细胞单鞭毛根部的小花瓶状凹陷。几个与细胞骨架相关的多蛋白复合物在鞭毛口袋的细胞质面上围绕其颈部盘旋。其中之一，钩复合物，被认为会影响大分子进入鞭毛口袋腔。在之前的工作中，敲低非洲锥虫（Tb）MORN1（一种钩复合物成分）会导致较大的货物无法进入鞭毛口袋。在这项研究中，鞭毛口袋复合物成分 TbSmee1 在血液阶段的非洲锥虫中得到了表征，发现它对细胞活力是必需的。TbSmee1 的敲低导致鞭毛口袋扩大，并损害了表面结合货物进入鞭毛口袋膜的通道，这与 TbMORN1 的耗尽相似。出乎意料的是，通过敲低网格蛋白抑制内吞作用，类似于 TbSmee1 敲低，这表明内吞作用本身是表面结合货物进入鞭毛口袋的前提条件。
