8.4 The Rotary Bacterial Flagellar Motor

Abstract

Bacterial cell envelopes often contain a flagellar motor – a reversible rotary nanomachine with an approximate diameter of 45nm – that allows cells to swim. Power is provided by the movement of H+ or Na+ down the electrochemical gradients across the cytoplasmic membrane, often termed the proton motive force or sodium motive force. A helical filament is rotated by each motor at several hundred revolutions per second. In many species, the motor switches direction stochastically; switching rates are controlled by a network of sensory and signaling proteins. The first direct observation, approximately 40 years ago, of the function of a single molecular motor was of the bacterial flagellar motor. Nevertheless, due to the large size and complexity of the motor, much remains to be discovered about this nanomachine, particularly the many structural details of the torque-generating mechanism. This chapter summarizes what has been learned about the structure and function of the motor with a focus on recent observations, particularly those obtained using single molecule techniques.