Mechanical Force-Based Regulation of Protein Assemblies

The bacterial flagellar motor detects the presence of surfaces and undergoes structural remodeling (Lele et al., PNAS 2013). In response to mechanical cues, the stator builds itself by recruiting additional force-generators. However, the mechanism for such load-dependent self-assembly is currently unknown. Here, we tested a hypothesis that the amount of force generated by each stator-unit modulates its association with the rotor. To do this, we measured stator-binding in mutants strains in which the motors reportedly develop lower torque compared to wild-type motors. Our measurements are consistent with the notion that a unit binds stronger when delivering a higher force to the rotor. An analytical model based on a catch-bond type mechanism was developed that incorporated an exponential dependence of the off-rate for individual units on the force delivered to the rotor. The model provided accurate fits to measurements of stator-rotor binding over a range of loads. It is possible that the tensile forces that develop when a stator-unit delivers a high torque uncover cryptic binding sites that stabilize the stator-rotor association. Our results represent the first steps towards establishing a plausible mechanism for mechanical force-based regulation of the flagellar stator.