Cooperative stator assembly of bacterial flagellar motor mediated by rotation

Cooperativity has a central place in biological regulation, providing robust and highly-sensitive regulation. The bacterial flagellar motor implements autonomous torque regulation based on the stator’s dynamic structure; the stator units bind to and dissociate from the motor dynamically in response to environmental changes. However, the mechanism of this dynamic assembly is not fully understood. Here, we demonstrate the cooperativity in the stator assembly dynamics. The binding is slow at the stalled state, but externally forced rotation as well as driving by motor torque in either direction boosts the stator binding. Hence, once a stator unit binds, it drives the rotor and triggers the avalanche of succeeding bindings. This cooperative mechanism based on nonequilibrium allostery accords with the recently-proposed gear-type coupling between the rotor and stator. Ito and co-workers unravel how bacteria such as Salmonella switch gears with their flagellar driving machinery. External load triggers the dynamic remodeling of the molecular complex sustaining the torque, and the number of stator units is adapted in a non-trivial, cooperative manner.