Institute for Biology-Bacterial Physiology, Humboldt-Universität zu Berlin, Berlin, Germany.
Max Planck Unit for the Science of Pathogens, Berlin, Germany.
Nat Commun. 2021 Jun 28;12(1):3999. doi: 10.1038/s41467-021-24226-1.
Type-III secretion systems (T3SSs) of the bacterial flagellum and the evolutionarily related injectisome are capable of translocating proteins with a remarkable speed of several thousand amino acids per second. Here, we investigate how T3SSs are able to transport proteins at such a high rate while preventing the leakage of small molecules. Our mutational and evolutionary analyses demonstrate that an ensemble of conserved methionine residues at the cytoplasmic side of the T3SS channel create a deformable gasket (M-gasket) around fast-moving substrates undergoing export. The unique physicochemical features of the M-gasket are crucial to preserve the membrane barrier, to accommodate local conformational changes during active secretion, and to maintain stability of the secretion pore in cooperation with a plug domain (R-plug) and a network of salt-bridges. The conservation of the M-gasket, R-plug, and salt-bridge network suggests a universal mechanism by which the membrane integrity is maintained during high-speed protein translocation in all T3SSs.
III 型分泌系统（T3SS）的细菌鞭毛和进化相关的注射装置能够以每秒几千个氨基酸的惊人速度转运蛋白质。在这里，我们研究了 T3SS 如何能够以如此高的速度运输蛋白质，同时防止小分子泄漏。我们的突变和进化分析表明，T3SS 通道细胞质侧的一组保守蛋氨酸残基在经历出口的快速移动底物周围形成可变形的垫圈（M-垫圈）。M-垫圈的独特物理化学特性对于维持膜屏障、适应主动分泌过程中的局部构象变化以及与塞子结构域（R-塞子）和盐桥网络合作维持分泌孔的稳定性至关重要。M-垫圈、R-塞子和盐桥网络的保守性表明，在所有 T3SS 中，高速蛋白质转运过程中保持膜完整性的一种通用机制。
