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Bacterial type IV pili are critical for diverse biological processes including horizontal gene transfer, surface sensing, biofilm formation, adherence, motility, and virulence. These dynamic appendages extend and retract from the cell surface. In many type IVa pilus systems, extension occurs through the action of an extension ATPase, often called PilB, while optimal retraction requires the action of a retraction ATPase, PilT. Many type IVa systems also encode a homolog of PilT called PilU. However, the function of this protein has remained unclear because  pilU  mutants exhibit inconsistent phenotypes among type IV pilus systems and because it is relatively understudied compared to PilT. Here, we study the type IVa competence pilus of  Vibrio cholerae  as a model system to define the role of PilU. We show that the ATPase activity of PilU is critical for pilus retraction in PilT Walker A and/or Walker B mutants. PilU does not, however, contribute to pilus retraction in Δ pilT  strains. Thus, these data suggest that PilU is a  bona fide  retraction ATPase that supports pilus retraction in a PilT-dependent manner. We also found that a Δ pilU  mutant exhibited a reduction in the force of retraction suggesting that PilU is important for generating maximal retraction forces. Additional  in vitro  and  in vivo  data show that PilT and PilU act as independent homo-hexamers that may form a complex to facilitate pilus retraction. Finally, we demonstrate that the role of PilU as a PilT-dependent retraction ATPase is conserved in  Acinetobacter baylyi , suggesting that the role of PilU described here may be broadly applicable to other type IVa pilus systems.

PilT and PilU are homohexameric ATPases that coordinate to retract type IVa pili