Load-Dependent Assembly of the Bacterial Flagellar Motor

Abstract

It is becoming clear that the bacterial flagellar motor output is important not only for bacterial locomotion but also for mediating the transition from liquid to surface living. The output of the flagellar motor changes with the mechanical load placed on it by the external environment: at a higher load, the motor runs more slowly and produces higher torque. Here we show that the number of torque-generating units bound to the flagellar motor also depends on the external mechanical load, with fewer stators at lower loads. Stalled motors contained at least as many stators as rotating motors at high load, indicating that rotation is unnecessary for stator binding. Mutant stators incapable of generating torque could not be detected around the motor. We speculate that a component of the bacterial flagellar motor senses external load and mediates the strength of stator binding to the rest of the motor.

A Delicate Nanoscale Motor Made by Nature—The Bacterial Flagellar Motor

“…Lele et al showed here that the stators themselves dynamically respond to load, and this response was consistently observed even when chemotaxis and rotational switching was removed—it is the stators themselves that engage or disengage with changes in load. The similar observation has also been reported by Tipping et al58 This is presumably because at low load only few stators are required to drive rotation and additional stators simply waste ions 64. However, the mechanism for force‐sensing of the stators remains unknown.…”

Osmotaxis in Escherichia coli through changes in motor speed

“…The BFM has been shown to act as a mechanosensor, increasing the number of stator units in response to higher loads [Lele et al, 2013, Tipping et al, 2013. As the viscosity of the medium rises with addition of sucrose, the speed increase we observed could be due to the incorporation of additional stator units.…”

“…In majority of the traces no obvious steps are observed, suggesting that the increase in motor speed could be due to the increase in PMF with full set of stators present. Here we note that stator engagement with the rotor has been reported as torque dependent [Tipping et al, 2013], where in absence of torque (motor rotation) stators disengage from the rotor. We would then expect stator resurrection during the Recovery Phase shown in SI Appendix Fig.…”

“…The large increase in motor speed post osmotic shock could be due to increases in PMF, possibly through alterations in cell metabolism; or, as an alternative explanation, due to the increase in number of stator units through mechanosensation [Lele et al, 2013,Tipping et al, 2013 or adaptive motor remodeling [Yuan et al, 2012]. The BFM has been shown to act as a mechanosensor, increasing the number of stator units in response to higher loads [Lele et al, 2013, Tipping et al, 2013.…”

Diverse high-torque bacterial flagellar motors assemble wider stator rings using a conserved protein scaffold

“…The enteric flagellar motor has been suggested to incorporate ∼11 stator complexes that interact with the outer lobe of the C-ring in the cytoplasm and with the peptidoglycan and the P-ring (13). The number of stator complexes incorporated into the enteric motor is proportional to the load on the motor (35,36). Because blotting during the sample vitrification process for cryo-tomographic imaging necessitated imaging in low-viscosity growth medium, we anticipated that a low motor load would result in low stator-complex occupancy and that our subtomogram averaging procedure would not resolve stator complexes.…”