The type II secretion system (T2SS) is a multiprotein envelope-spanning assembly that translocates a wide range of virulence factors, enzymes and effectors through the outer membrane of many Gram-negative bacteria1-3. Here, using electron cryotomography and subtomogram averaging methods, we reveal the in vivo structure of an intact T2SS imaged within the human pathogen Legionella pneumophila. Although the T2SS has only limited sequence and component homology with the evolutionarily related type IV pilus (T4P) system4,5, we show that their overall architectures are remarkably similar. Despite similarities, there are also differences, including, for example, that the T2SS-ATPase complex is usually present but disengaged from the inner membrane, the T2SS has a much longer periplasmic vestibule and it has a short-lived flexible pseudopilus. Placing atomic models of the components into our electron cryotomography map produced a complete architectural model of the intact T2SS that provides insights into the structure and function of its components, its position within the cell envelope and the interactions between its different subcomplexes.

中文翻译：

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通过电子冷冻断层扫描显示军团菌 II 型分泌系统的体内结构。

II 型分泌系统 (T2SS) 是一种多蛋白包膜跨越组件，可通过许多革兰氏阴性细菌 1-3 的外膜转运多种毒力因子、酶和效应物。在这里，我们使用电子冷冻断层扫描和亚断层图像平均方法，揭示了在人类病原体嗜肺军团菌内成像的完整 T2SS 的体内结构。尽管 T2SS 与进化相关的 IV 型菌毛 (T4P) 系统仅具有有限的序列和组件同源性 4,5，但我们表明它们的整体架构非常相似。尽管有相似之处，但也存在差异，例如，T2SS-ATPase 复合物通常存在但与内膜脱离，T2SS 具有更长的周质前庭，并且具有短寿命的柔性假菌毛。