FliI6-FliJ molecular motor assists with unfolding in the type III secretion export apparatus

TL;DR

This study models the FliI6-FliJ molecular motor to understand its role in assisting the unfolding of flagellar proteins during bacterial flagella assembly, revealing its kinetic properties and potential modes of action.

Contribution

We introduce a two-state model for the FliI6-FliJ motor, quantifying its velocity, power output, and stall torque, and propose its function in lowering energy barriers for protein unfolding.

Findings

Average angular velocity ~9.0 rps

Maximum power output ~42 kBT/s

Stall torque ~3 kBT/rad

Abstract

The role of rotational molecular motors of the ATP synthase class is integral to the metabolism of cells. Yet the function of FliI6-FliJ complex - homologous to the F1 ATPase motor - within the flagellar export apparatus remains unclear. We use a simple two-state model adapted from studies of linear molecular motors to identify key features of this motor. The two states are the 'locked' ground state where the FliJ coiled coil filament experiences fluctuations in an asymmetric torsional potential, and a 'free' excited state in which FliJ undergoes rotational diffusion. Michaelis-Menten kinetics was used to treat transitions between these two states, and obtain the average angular velocity of the FliJ filament within the FliI6 stator: ~9.0 rps. The motor was then studied under external counter torque conditions in order to ascertain its maximal power output: Pmax ~42 kBT/s, and the stall torque: ~3 kBT/rad. Two modes of action within the flagellar export apparatus are proposed, in which the motor performs useful work either by continuously 'grinding' through the resistive environment, or by exerting equal and opposite stall force on it. In both cases, the resistance is provided by flagellin subunits entering the flagellar export channel prior to their unfolding. We therefore propose that the function of the FliI6-FliJ complex is to lower the energy barrier and therefore assist in unfolding of the flagellar proteins before feeding them into the transport channel.