Many bacteria use the flagellum for locomotion and chemotaxis. Its bidirectional rotation is driven by a membrane-embedded motor, which uses energy from the transmembrane ion gradient to generate torque at the interface between stator units and rotor. The structural organization of the stator unit (MotAB), its conformational changes upon ion transport, and how these changes power rotation of the flagellum remain unknown. Here, we present ~3 Å-resolution cryoelectron microscopy reconstructions of the stator unit in different functional states. We show that the stator unit consists of a dimer of MotB surrounded by a pentamer of MotA. Combining structural data with mutagenesis and functional studies, we identify key residues involved in torque generation and present a detailed mechanistic model for motor function and switching of rotational direction.

许多细菌利用鞭毛进行运动和趋化。它的双向旋转由嵌入膜的电机驱动，该电机使用来自跨膜离子梯度的能量在定子单元和转子之间的界面处产生扭矩。定子单元 (MotAB) 的结构组织、离子传输时的构象变化以及这些变化如何推动鞭毛旋转仍然未知。在这里，我们展示了不同功能状态下定子单元的~3 Å 分辨率冷冻电子显微镜重建。我们表明定子单元由 MotB 的二聚体组成，周围是 MotA 的五聚体。将结构数据与诱变和功能研究相结合，我们确定了参与扭矩产生的关键残基，并提出了运动功能和旋转方向切换的详细机械模型。