Motor torque measurement of Halobacterium salinarum archaellar suggests a general model for ATP-driven rotary motors.

It is unknown how the archaellum—the rotary propeller used by Archaea for motility—works. To further understand the molecular mechanism by which the hexameric ATPase motor protein FlaI drives rotation of the membrane-embedded archaellar motor, we determined motor torque by imposition of various loads on Halobacterium salinarum archaella. Markers of different sizes were attached to single archaella, and their trajectories were quantified using three-dimensional tracking and high-speed recording. We show that rotation slows as the viscous drag of markers increases, but torque remains constant at 160 pN·nm independent of rotation speed. Notably, the estimated work done in a single rotation is twice the expected energy that would come from hydrolysis of six ATP molecules in the hexamer, indicating that more ATP molecules are required for one rotation of archaellum. To reconcile the apparent contradiction, we suggest a new and general model for the mechanism of ATP-driven rotary motors. Seiji Iwata et al. report protocols for visualizing and measuring the torque of the rotary propeller (archaellar) used by the Archea Halobacterium salinarum. They find that the estimated work done in a single rotation is more than expected and propose a new model for the mechanism of ATP-driven rotary motors.