Figure 4: Z ring condensation is required for cell division.
a Z ring condensation in control cells. Each pair of images shows a newly formed Z ring that has not yet condensed (left), and the same Z ring after condensation (right). Representative images from 4 replicates. b Left: Z ring condensation during the cell cycle. Top: Average intensity projections of Z rings from normalized time points over the cell cycle. Bottom: Z ring width over the cell cycle, measured as the full width at half maximum of the average intensity projections. Time from Z ring formation to Z ring disassembly (defined as the first and last frames in which the Z ring could be detected) was normalized for each cell. Shading: bootstrapped standard error. Right: Z ring width, total intensity, and maximum intensity over the cell cycle, normalized and plotted on the same axis. N = 760 cell cycles. c The FtsZ(K86E) mutant rescues the ΔezrA ΔzapA synthetic lethal condition and partially restores Z ring morphology, as seen in representative Z ring images from at least two replicates each (left) and width distributions (right). ↓ indicates depletion. d Pbp2B dynamics in ΔZBPs. Kymographs were drawn at the Z rings indicated above. 4 replicates were obtained. e Left and centre: Colocalization of Pbp2B with FtsZ and colocalization of FDAA labelling with FtsZ in control cells (left) and ΔZBPs cells (centre), from at least two replicates for each condition. Right: Amount of Pbp2B and FDAA labelling at the division site, measured by fluorescence intensity. N > 1000 for each condition. For each box plot, the white line indicates the median, the box extends to the 25th and 75th percentiles, and the whiskers indicate 1.5x interquartile range. P-values were obtained from a two-sided t-test; **** indicates p<0.0001, and p-values are included in parenthesis. f Left: At the start of the cell division process, FtsZ filaments treadmill around the cell circumference at midcell. Centre: Stationary ZBPs transiently bind to FtsZ filaments to condense the Z ring. Right: ZBP-driven bundling of FtsZ filaments may also function during cytokinesis, where crowding may induce inward membrane deformations, both concentrating cell wall synthesis to the Z ring and orienting it to divide the cell in two. Scale bars: horizontal: 2 μm, vertical: 1 min.