Treadmilling FtsZ polymers drive the directional movement of sPG-synthesis enzymes via a Brownian ratchet mechanism

FtsZ, a highly conserved bacterial tubulin GTPase homolog, is a central component of the cell division machinery in nearly all walled bacteria. FtsZ polymerizes at the future division site and recruits greater than 30 proteins to assemble into a macromolecular complex termed the divisome. Many of these divisome proteins are involved in septal cell wall peptidoglycan (sPG) synthesis. Recent studies found that FtsZ polymers undergo GTP hydrolysis-coupled treadmilling dynamics along the circumference the division site, driving the processive movement of sPG synthesis enzymes. How FtsZs treadmilling drives the directional transport of sPG enzymes and what its precise role is in bacterial cell division are unknown. Combining theoretical modeling and experimental testing, we show that FtsZs treadmilling drives the directional movement of sPG-synthesis enzymes via a Brownian ratchet mechanism, where the shrinking end of FtsZ polymers introduces an asymmetry to rectify diffusions of single sPG enzymes into persistent end-tracking movement. Furthermore, we show that the processivity of this directional movement is dependent on the binding potential between FtsZ and the enzyme, and hinges on the balance between the enzymes diffusion and FtsZs treadmilling speed. This interplay could provide a mechanism to control the level of available enzymes for active sPG synthesis both in time and space, explaining the distinct roles of FtsZ treadmilling in modulating cell wall constriction rate observed in different bacterial species.