FtsZ treadmilling is essential for Z-ring condensation and septal constriction initiation in Bacillus subtilis cell division

Despite the central role of division in bacterial physiology, how division proteins work together as a nanoscale machine to divide the cell remains poorly understood. Cell division by cell wall synthesis proteins is guided by the cytoskeleton protein FtsZ, which assembles at mid-cell as a dense Z-ring formed of treadmilling filaments1,2. However, although FtsZ treadmilling is essential for cell division, the function of FtsZ treadmilling remains unclear2-5. Here, we systematically resolve the function of FtsZ treadmilling across each stage of division in the Gram-positive model organism Bacillus subtilis using a novel combination of nanofabrication, advanced microscopy, and microfluidics to measure the division-protein dynamics in live cells with ultrahigh sensitivity. We find that FtsZ treadmilling has two essential functions: mediating condensation of diffuse FtsZ filaments into a dense Z-ring, and initiating constriction by guiding septal cell wall synthesis. After constriction initiation, FtsZ treadmilling has a dispensable function in accelerating septal constriction rate. Our results show that FtsZ treadmilling is critical for assembling and initiating the bacterial cell division machine.