The switching dynamics of the bacterial flagellar motor

TL;DR

This paper introduces a statistical-mechanical model of bacterial flagellar motor switching, linking conformational states, torque, and filament dynamics to explain observed switching times.

Contribution

The model integrates rotor conformational states, stator interactions, and filament polymorphic transitions to explain switching dynamics and times.

Findings

Switching frequency depends on load and torque.

Model predicts characteristic switching times matching experimental data.

Filament polymorphic transitions influence load buildup after switching.

Abstract

Many swimming bacteria are propelled by flagellar motors that stochastically switch between the clockwise and counterclockwise rotation direction. While the switching dynamics are one of the most important characteristics of flagellar motors, the mechanisms that control switching are poorly understood. We present a statistical-mechanical model of the flagellar rotary motor, which consists of a number of stator proteins that drive the rotation of a ring of rotor proteins, which in turn drives the rotation of a flagellar filament. At the heart of our model is the assumption that the rotor protein complex can exist in two conformational states corresponding to the two respective rotation directions, and that switching between these states depends on interactions with the stator proteins. This naturally couples the switching dynamics to the rotation dynamics, making the switch sensitive to torque and speed. Another key element of our model is that after a switching event, it takes time for the load to build up, due to polymorphic transitions of the filament. Our model predicts that this slow relaxation dynamics of the filament, in combination with the load dependence of the switching frequency, leads to a characteristic switching time, in agreement with recent observations.