Stator pool size shapes the mechanosensitivity of the bacterial flagellar motor

The bacterial flagellar motor adapts to mechanical load by adding or removing stator units, which can either be inactive in the inner membrane (the stator pool) or attached to the motor in a torque-generating configuration. This adaptation is well understood, but what sets the load range over which the motor responds remains unclear. Building on a previous two-state model of stator binding, we derive how the on-rate depends on the diffusing stator pool and find that it increases in proportion to pool size. The model predicts that pool size governs motor occupancy at low load but not at high load, where the motor saturates regardless of how many stators are available. We confirmed this prediction in Escherichia coli : raising stator expression increased swimming speed and single-motor rotation rate at low load. This dependence on expression weakened as load rose and was undetectable at high load. The model further predicts that larger pools make the motor mechanosensitive at lower loads, which our measurements of load-induced occupancy change support. Mechanosensitivity is therefore not fixed by motor architecture alone, but also determined by stator abundance, which cells can set through expression to position their mechanosensory range.

Using a statistical mechanics model of stator binding, we predict how flagellar motor output depends on mechanical load and on the number of stators a cell expresses. Combining the model with measurements of swimming cells and single motors, we find that the size of the stator pool sets the range of load over which the motor is mechanosensitive. Mechanosensitivity is therefore a property the cell can tune through stator expression, not a fixed feature of the motor, a principle that may extend to other force-responsive molecular complexes assembled from a limited pool of components.