A bifunctional ATPase drives tad pilus extension and retraction

Molecular motors convert chemical energy directly into mechanical work1 and are found in all domains of life2. These motors are critical to intracellular transport3, motility4,5, macromolecular protein assembly3,6, and many essential processes7. A wide-spread class of related bacterial motors drive the dynamic activity of extracellular fibers, such as type IV pili (T4P), that are extended and retracted using so-called secretion motor ATPases. Among these, the tight adherence (tad) pili are critical for surface sensing, surface attachment, and biofilm formation8-10. How tad pili undergo dynamic cycles of extension and retraction8 despite lacking a dedicated retraction motor ATPase has remained a mystery. Here we find that a bifunctional pilus motor ATPase, CpaF, drives both activities through ATP hydrolysis. Specifically, we show that mutations within the ATP hydrolysis active site of Caulobacter crescentus CpaF result in a correlated reduction in the rates of extension and retraction. Moreover, a decrease in the rate of ATP hydrolysis directly scales with a decrease in the force of retraction and reduced dynamics in these CpaF mutants. This mechanism of motor protein bifunctionality extends to another genus of tad-bearing bacteria. In contrast, the T4aP subclass of pili possess dedicated extension and retraction motor ATPase paralogs. We show that these processes are uncoupled using a slow ATP hydrolysis mutation in the extension ATPase of competence T4aP of Vibrio cholerae that decreases the rate of extension but has no effect on the rate of retraction. Thus, a single motor ATPase is able to drive the bidirectional processes of pilus fiber extension and retraction.