The destruction of one pilus by PilT releases enough energy to rein in the bacterium.


Many gram-negative bacteria, including the human pathogen that causes gonorrhea, use type IV pili for attachment to host cells. Retraction of the pilus is strong enough to propel a bacterium along cells. Berenike Maier, Michael Sheetz (Columbia University, New York, NY), and colleagues have now measured just how much force is released by pilus retraction, a process known to require the PilT ATPase. Their findings make PilT the strongest known molecular motor.The strength of pilus retraction was measured by its ability to pull an attached bead from a laser trap of varying stiffness. Retraction frequency was proportional to PilT concentration, indicating that PilT is the molecular motor powering retraction. The retraction of one pilus generated forces in excess of 100 pN, between 2 and 20 times that released by kinesin and polymerases.

PilT's exceptional strength may stem from its unusual mode of action. Unlike most motors, which move bidirectionally along filaments, PilT's action was irreversible. PilT apparently powered retraction by dissociating pilus subunits into the bacterial membrane. The dissociated subunits may be unable to reassemble directly into the filament. PilT belongs to an ATPase family that works in hexameric form and may therefore hydrolyze up to six ATP molecules for each pilus subunit released. This would generate several times more energy than is required for disassembly. As yet, no analogous motors have been found in eukaryotes. If they are prokaryote-specific, PilT-like motors may be convenient targets for drugs to treat certain bacterial infections. ▪


Maier, B., et al.
Proc. Natl. Acad. Sci. USA.