Structure and Energy-Conversion Mechanism of the Bacterial Na⁺-Driven Flagellar Motor

Many bacteria swim by means of flagella that are rotated by a nanoscale motor embedded in the cell membrane. Torque is generated by the interaction between ion-conducting membrane proteins that comprise the stator and ring-shaped structures that form the rotor. Although the structure and function of the motor have been extensively studied, many mysteries remain, including the force-generation mechanism, the path of ion flow through the stator, the activation mechanism of the stator, and the mechanism of switching between clockwise (CW) and counterclockwise (CCW) rotation. We summarize recent knowledge of the Na⁺-driven flagellar motor, especially the Vibrio polar motor that rotates much faster than the H⁺-driven motor and provides a useful model system for examining comparative aspects of flagellar function.