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. The rotation of the bacterial flagellar motor is generated by the interaction between two parts in the motor, the rotor and the stator. The rotor is composed of a central rod and surrounding rings, and the stator is a unit that works as an energy convertor to couple ion influx to rotation of the rotor. The Na+-driven flagellar motor in Vibrio alginolyticus , whose rotation speed is faster than that of any H+-driven motor, contains PomA/PomB as stator proteins and some additional ring-like structures, the T-, H-, and O-rings, that are not found in H+-driven motors. Assembly of the PomA/PomB stators into the motor depends on the environmental Na+ concentration. In the presence of sodium, the periplasmic region of the stator changes its conformation to an activated form that binds to peptidoglycan, and charged residues in the cytoplasmic region interact with charged residues in the rotor protein FliG to generate rotational force. FliG dynamically changes its conformation to switch the rotational direction of the motor in response to the chemotactic signal transferred from the other rotor component, FliM, that binds to the chemotaxis response regulator protein CheY. [ABSTRACT FROM AUTHOR]