Structural basis of torque generation in the bi-directional bacterial flagellar motor

The flagellar stator unit is an oligomeric complex of two membrane proteins (MotA5B2) that powers bi-directional rotation of the bacterial flagellum. Harnessing the ion motive force across the cytoplasmic membrane, the stator unit operates as a miniature rotary motor itself to provide torque for rotation of the flagellum. Recent cryo-electron microscopic (cryo-EM) structures of the stator unit provided novel insights into its assembly, function, and subunit stoichiometry, revealing the ion flux pathway and the torque generation mechanism. Furthermore, in situ cryo-electron tomography (cryo-ET) studies revealed unprecedented details of the interactions between stator unit and rotor. In this review, we summarize recent advances in our understanding of the structure and function of the flagellar stator unit, torque generation, and directional switching of the motor. The bacterial flagellum is a supramolecular machine essential for locomotion and virulence of many bacteria and comprises a long filament connected through a hook to a cell-enveloped embedded basal body.The basal body comprises a bi-directional rotary motor energized by the stator units that surround it; the stator unit harnesses the electrochemical gradient of ions across the cytoplasmic membrane to generate torque.Cryo-electron microscopic (cryo-EM) structures of the stator unit revealed its stoichiometry (a MotA pentamer surrounding a MotB dimer), its autoinhibition mechanism, the ion flux pathway, and the conformational changes upon protonation driving MotA rotation.Phosphorylated chemotaxis signaling protein CheY-P binds to the C-ring of the flagellar motor, inducing a conformational change that alters the interaction between C-ring and stator units, switching the rotational direction of the flagellar motor from counterclockwise to clockwise.