Single proton tight coupling in the bacterial flagellar motor

The bacterial flagellar motor is the largest and most complex biological rotary machine that exerts a torque of up to about 1000 pN to propel the swimming of flagellated bacteria. It is embedded in the cell membrane and consists of a 40 nm rotor and about 11 stators. Each stator unit, a torque generating protein complex, is driven by the proton motive force, a proton electrochemical gradient across the inner membrane. However, despite much progress, we lack sufficient evidence of how the ion flow is coupled to motor rotation. Here, we measured the motor speed as a function of the number of stators and found that the number of stators is linearly proportional to the motor speed. Our measurement shows that each stator passes about 24 ions per revolution, indicating that each proton flow can generate torque to drive the motor rotation about 14 degrees which is consistent with 26-fold periodic due to 26 FliG subunits. This result shows that the fixed number of ions yields a constant motor rotation independent of the number of stators and motor speed, indicating proton tight coupling between torque generation and proton flux.