A biophysical mechanism for non-equilibrium cooperativity in the bacterial flagellar motor

The bacterial flagellar motor is an adaptive, macromolecular machine. By switching its rotation direction in response to external cues, flagellar motors enable bacteria to navigate up chemical gradients and perform other functions. Switching responses to chemical inputs are extremely sensitive, with a Hill coefficient of at least 10. Experiments have demonstrated that the rotation switching decision is driven out of equilibrium, but the biophysical mechanism of this driving and its consequences are unknown. Here, we propose a theoretical model for how mechanical forces on subunits of the motor could coordinate their dynamics, giving rise to non-equilibrium, highly cooperative switching responses. We unpack the predictions of this mechanism using simulations and propose experiments to test it. More broadly, this work highlights a nonequilibrium role for mechanics in coordinating spatially-separated components.