Assembly and Activation of the MotA/B Proton Channel Complex of the Proton-Driven Flagellar Motor of Salmonella enterica

Salmonella enterica can swim by rotating multiple flagella, which arise randomly over the cell surface (Fig. 1A). The flagellum consists of at least three parts: the basal body, the hook, and the filament. The basal body is embedded within the cell membranes and acts as a bidirectional rotary motor powered by an electrochemical potential gradient of protons across the cytoplasmic membrane (Fig. 1B, C). The hook and filament extend outwards in the cell exterior. The filament acts as a helical propeller. The hook exists between the basal body and filament and functions as a universal joint to smoothly transmit torque produced by the motor to the filament. When the motors rotate in counterclockwise direction, the cells can swim smoothly. By quick reversal rotation of the motor to clockwise direction, the cells tumble and change their swimming direction to move toward more favorable environments (Fig. 2) (Berg, 2003; Blair, 2003; Minamino et al., 2008).