Superresolution Investigation of In Vivo Structure and Dynamics of E. coli FtsZ Structures during Cytokinesis

Abstract

Control of E. coli cell division is achieved by a network of proteins that organize cytokinesis in space and time. FtsZ, a tubulin-like GTPase that forms head-to-tail protofilaments, localizes to the midcell and assembles into a ring-like structure known as the Z-ring. The Z-ring serves as an essential scaffold to recruit all other division proteins and generates contractile force for cytokinesis, but its supramolecular structure and contraction mechanism remain unknown. We have used photoactivated localization microscopy (PALM) to characterize the in vivo structural dynamics of the Z-ring in E. coli at a spatial resolution of ∼35 nm.Superresolution imaging revealed that the Z-ring is ∼110nm wide and is composed of a loose 3D bundle of FtsZ protofilaments that overlap with each other in both the longitudinal and radial directions in the cell. These 110nm rings were observed in either a single-ring conformation at midcell or in a multiple-ring conformation reminiscent of a tight helix. The tight helix conformation was very rarely observed in cells with a visible septation, suggesting that the Z-ring fluctuates between the single-ring and tight helix conformation before cytokinesis begins, but then adopts only the single-ring conformation during constriction. We also observed non-ringlike FtsZ structures emanating from the midcell in cells at later stages of constriction, indicating Z-ring disassembly. These disassembling structures were observed in cells with 200-600nm septum diameters, suggesting that Z-ring disassembly occurs gradually throughout the constriction process. We also observed that the bandwidth distribution of the Z-ring did not change with varying expression levels, cell size, or during the progression of cytokinesis, suggesting that the factors that regulate Z-ring thickness are not affected by FtsZ concentration or changes in cell shape.