  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.
