Fig 1. Nucleation and growth of FtsZ filaments into rings on SLBs.

(A) Representative snapshots from a time-lapse experiment displaying different stages of ring formation. Images were taken every 10 s using TIRF illumination (YFP channel). Frames correspond to the times (min) indicated after the addition of GTP and Mg²⁺. (B) Polar clockwise growth of a single FtsZ from a nucleation point. Growth seems to occur stepwise and depend on the accessibility of small filaments nearby (panel 1 and 2). Lower local protein density (white arrow) correlates with a higher flexibility of the polymer (panels 7–12). Breakage occurs primarily in trailing regions (15). After about 3 min, a primitive ring made of 3 distinct short filaments is exhibited (17). (C) Directional filament gliding via treadmilling. Fragmentation or depolymerization destabilizes the trailing (“older”) edge as shown in the kymograph (d-labeled white line). Images in (B) and (C) were taken every 2 s, and the scale bar represents 500 nm. Further details are under “Results.” Movies reconstructed from the whole collection of images can be found in supporting information S2 and S3 Movies. GTP, guanosine triphosphate; SLB, supported lipid bilayer; TIRF, total internal reflection fluorescence; YFP, yellow fluorescent protein.