Department of Infectious Disease, Imperial College, London, UK.
Mol Microbiol. 2021 Mar;115(3):412-424. doi: 10.1111/mmi.14664. Epub 2020 Dec 29.
The type II secretion system (T2SS) is a multi-protein complex used by many bacteria to move substrates across their cell membrane. Substrates released into the environment serve as local and long-range effectors that promote nutrient acquisition, biofilm formation, and pathogenicity. In both animals and plants, the T2SS is increasingly recognized as a key driver of virulence. The T2SS spans the bacterial cell envelope and extrudes substrates through an outer membrane secretin channel using a pseudopilus. An inner membrane assembly platform and a cytoplasmic motor controls pseudopilus assembly. This microreview focuses on the structure and mechanism of the T2SS. Advances in cryo-electron microscopy are enabling increasingly elaborate sub-complexes to be resolved. However, key questions remain regarding the mechanism of pseudopilus extension and retraction, and how this is coupled with the choreography of the substrate moving through the secretion system. The T2SS is part of an ancient type IV filament superfamily that may have been present within the last universal common ancestor (LUCA). Overall, mechanistic principles that underlie T2SS function have implication for other closely related systems such as the type IV and tight adherence pilus systems.
II 型分泌系统（T2SS）是许多细菌用来将底物穿过细胞膜的一种多蛋白复合物。释放到环境中的底物作为局部和远程效应物，促进营养物质的获取、生物膜的形成和致病性。在动物和植物中，T2SS 越来越被认为是毒力的关键驱动因素。T2SS 跨越细菌细胞包膜，并使用外膜分泌通道通过假菌毛将底物排出。内膜组装平台和细胞质马达控制假菌毛的组装。本综述重点介绍了 T2SS 的结构和机制。冷冻电子显微镜的进展使得能够解析越来越精细的亚复合物。然而，关于假菌毛延伸和缩回的机制，以及如何与通过分泌系统移动的底物的编排相耦合，仍然存在关键问题。T2SS 是古老的 IV 型细丝超家族的一部分，可能存在于最后一个普遍共同祖先（LUCA）中。总的来说，T2SS 功能的基础机械原理对其他密切相关的系统，如 IV 型和紧密附着菌毛系统，具有启示意义。
