A hidden state in the turnover of a functioning membrane protein complex.

Most membrane proteins exist in complexes that carry out critical cellular functions and exhibit rich dynamics. The bacterial flagellar motor, a large membrane-spanning ion-driven rotary motor that propels the bacteria to swim, provides a canonical example. Rotation of the motor is driven by multiple torque-generating units (stators). Turnover of the stators has been shown previously, demonstrating the exchange of stator units between the motor and a membrane pool. But the details of the turnover kinetics remain unclear. Here, we directly measured the kinetics of stator turnover in individual motors via analysis of a large dataset of long-term high-resolution recordings of motor speed at high load. We found that the dwell time distribution of the stator units exhibits a multi-exponential shape, suggesting the existence of a hidden state in the turnover of the stators.