The bacterial flagellum is a complex self-assembling nanomachine that confers motility to the cell. Despite great variation across species, all flagella are ultimately constructed from a helical propeller that is attached to a motor embedded in the inner membrane. The motor consists of a series of stator units surrounding a central rotor made up of two ring complexes, the MS-ring and the C-ring. Despite many studies, high-resolution structural information is still lacking for the MS-ring of the rotor, and proposed mismatches in stoichiometry between the two rings have long provided a source of confusion for the field. Here, we present structures of the Salmonella MS-ring, revealing a high level of variation in inter- and intrachain symmetry that provides a structural explanation for the ability of the MS-ring to function as a complex and elegant interface between the two main functions of the flagellum—protein secretion and rotation.

细菌鞭毛是一种复杂的自组装纳米机器，可赋予细胞运动性。尽管物种之间存在很大差异，但所有鞭毛最终都是由一个螺旋桨构成的，该螺旋桨连接到嵌入内膜的电机上。该电机由一系列定子单元组成，这些定子单元围绕着由两个环复合体（MS 环和 C 环）组成的中央转子。尽管进行了许多研究，但转子的 MS 环仍然缺乏高分辨率的结构信息，并且两个环之间的化学计量不匹配问题长期以来一直为该领域提供混乱的根源。在这里，我们介绍沙门氏菌的结构MS 环，揭示了链间和链内对称性的高度变化，这为 MS 环作为鞭毛的两个主要功能（蛋白质分泌和旋转）之间复杂而优雅的界面的能力提供了结构解释.