Abstract
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.
Highlights
It is unknown how the archaellum—the rotary propeller used by Archaea for motility—works
The estimated work done in a single rotation was higher than the expected energy input that would come from the hexameric FlaI architecture, suggesting a model for the mechanism of archaellar motor rotation involving hydrolysis of more than one ATP molecule per FlaI subunit per full rotation
Rotation rate varied depending on marker diameter (Fig. 2d), decreasing as size increased (Fig. 2e), indicating that the archaellar motor in H. salinarum has an upper limit to its rotation rate of ∼25 Hz, as an extrapolation in Fig. 2e, which was reduced by the viscous drag of the attached beads
Summary
It is unknown how the archaellum—the rotary propeller used by Archaea for motility—works. No other archaellar components have been implicated as energy-transducing proteins, indicating that FlaI powers archaellar rotation alone, with a catalytic cycle involving sequential hydrolysis of six ATP molecules. Despite these biochemical insights, little is known of the physics of archaellar rotation. We found that archaellar torque remains constant at 160 pN·nm independent of rotation speeds between 0.5 and 30 Hz. Unexpectedly, the estimated work done in a single rotation was higher than the expected energy input that would come from the hexameric FlaI architecture, suggesting a model for the mechanism of archaellar motor rotation involving hydrolysis of more than one ATP molecule per FlaI subunit per full rotation
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