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A tiny bacterial flagellar motor rotates in both counter-clockwise (CCW) andclockwise (CW) rotational directions. The most important measurablecharacteristic of the flagellar motor is its torque versus angular speedrelationship in CCW or CW modes, which is found to be non-symmetrical with eachother, and still, such a phenomenon is not clearly understood.Here, we explainthis asymmetry through a mechanistic model based on the detailed torqueanalysis for the rotation of the motor and the revolutionary as well asspinning motion of the filament and bead. We find out that the asymmetryresults from the conformational changing of the hook due to rotationalswitching, rather than any non-symmetric changes in the potential of mean forcegenerated by the stator-rotor interactions. In CCW mode, when the hook remainsbend and flexible, the revolution motion predominates and the restoring torquein this motion, originated due to drag, governs the shape of the torque-speedcurve. However, in CW mode, spinning motion dominates as the hook becomesstraight and rigid, and the linear torque-speed relation arises due to therestoring torque for the drag corresponding to this motion. Our study indicatesthe significant role of the hook's conformational change upon the biologicalfunctions of the motor and paves the way for further experimental explorationon the structural origin of such asymmetry.

A mechanistic model for the asymmetric torque-speed relationships of a bacterial flagellar motor