Direct observation of rotation and steps of the archaellum in the swimming halophilic archaeon Halobacterium salinarum

Motile archaea swim using a rotary filament, the archaellum, a surface appendage that resembles bacterial flagella structurally, but is homologous to bacterial type IV pili. Little is known about the mechanism by which archaella produce motility. To gain insights into this mechanism, we characterized archaellar function in the model organism Halobacterium salinarum. Three-dimensional tracking of quantum dots enabled visualization of the left-handed corkscrewing of archaea in detail. An advanced analysis method combined with total internal reflection fluorescence microscopy, termed cross-kymography, was developed and revealed a right-handed helical structure of archaella with a rotation speed of 23 ± 5Hz. Using these structural and kinetic parameters, we computationally reproduced the swimming and precession motion with a hydrodynamic model and estimated the archaellar motor torque to be 50pN nm. Finally, in a tethered-cell assay, we observed intermittent pauses during rotation with ∼36° or 60° intervals, which we speculate may be a unitary step consuming a single adenosine triphosphate molecule, which supplies chemical energy of 80pN nm when hydrolysed. From an estimate of the energy input as ten or six adenosine triphosphates per revolution, the efficiency of the motor is calculated to be ∼6-10%.