Motor properties of PilT-independent type 4 pilus retraction in gonococci
Bacterial type 4 pili (T4P) belong to the strongest molecular machines. The gonococcal T4P retraction ATPase PilT supports forces exceeding 100 pN during T4P retraction. Here, we address the question whether gonococcal T4P retract in the absence of PilT. We show that pilT deletion strains indeed retract their T4P but the maximum force is reduced to 5 pN. Similarly, the speed of T4P retraction is lower by orders of magnitude compared to T4P retraction driven by PilT. Deleting the pilT paralogues pilU and pilT2 in the{Delta} pilT background did not inhibit T4P retraction, indicating that the PilT-like proteins do not compensate for PilT. Furthermore, we show that depletion of proton motive force did not inhibit pilT-independent T4P retraction. We conclude that the retraction ATPase is not essential for gonococcal T4P retraction. However, the force generated in the absence of PilT is too low to support important functions of T4P including twitching motility, fluidization of colonies, or induction of host cell response.\n\nIMPORTANCEBacterial type 4 pili (T4P) have been termed the swiss army knive of bacteria because they perform numerous functions including host cell interaction, twitching motility, colony formation, DNA uptake, protein secretion, and surface sensing. The pilus fibre continuously elongates or retracts and these dynamics are functionally important. Curiously, only a subset of T4P systems employs T4P retraction ATPases to power T4P retraction. Here we show that one of the strongest T4P machines, the gonococcal T4P, retracts without a retraction ATPase. Biophysical characterization reveal strongly reduced force and speed compared to retraction with ATPase. We propose that bacteria encode for retraction ATPases when T4P have to generate high force supporting functions like twitching motility, triggering host cell response, or fluidizing colonies.