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. 2021 Jan 27;12:609. doi: 10.1038/s41467-020-20873-y

Fig. 2. An FtsZ treadmilling-mediated Brownian ratchet mechanism drives FtsI’s directional movement.

Fig. 2

a Phase diagram depicting the dependence of FtsI’s persistent end-tracking (blue-shaded region) on FtsI’s diffusion constant and FtsI–FtsZ binding potential. b A representative simulated trajectory of an FtsI molecule persistently end-tracking with a treadmilling FtsZ filament. Inset: zoomed-in view of a boxed region of the trajectory. The model parameters for this simulation are: FtsZ’s treadmilling speed VZ = 25 nm/s, FtsI diffusion constant D = 0.04 μm2/s, FtsZ–FtsI binding potential U = 10 kBT, and the simulation time step = 5 × 10−6 s. The full trajectory is plotted every 5 × 10−2 s, and the zoomed-in inset is plotted every 5 × 10−6 s. c FtsI’s directional speed tightly couples with FtsZ’s treadmilling speed. Each of the data points was the average of >80 independent stochastic simulation trajectories using the segments that undergo directional movement. FtsZ’s treadmilling speed was fixed within each trajectory but varied across the ensemble following a Gaussian distribution with a standard deviation (SD) of 30%, in line with the experimental measurements in FtsZ WT and GTPase mutants13,16.