Prokaryotic cell division is one of the most fundamental processes in biology, but the dynamics and mechanics are far from being understood. In many bacteria, FtsZ, a tubulin homologue assembles into a ring-like structure – Z-ring at precisely the middle of the cell. This accurate site selection is dependent on the Min proteins. Min D and MinE self-organise into waves in vitro, and oscillate pole to pole in vivo. MinC is thought to couple the Min oscillations to FtsZ by direct interaction. The mechanism of inhibitory action of MinC on FtsZ assembly is not known. Critical to the understanding of regulation of FtsZ by MinC and other proteins and its probable role in force generation is the organisation, structure and the dynamics of the Z-ring. Current models of the FtsZ filament organization in the Z-ring argue between two different structures – (i) short overlapping protofilaments with lateral interactions and (ii) few long annealed protofilaments with or without lateral contacts.
Our observations of the characteristics of polymerization and turnover studies using fluorescence microscopy suggest that the FtsZ filament is a continuous and irresolute bundle. The results are consistent with a structure where the turnover happens throughout, and any specialised structure resulting in a GTP cap like structure can be ruled out. We show that the turnover rates and hydrolysis rates are similar arguing for a model in which subunit leaves as soon as it hydrolyses GTP. On the basis of crystal structures, we cloned the N-terminal of FtsZ, which acts as a C-terminal end capping fragment and is able to interact with monomers. The end-capping fragment, NZ can disassemble the FtsZ polymers, without influencing the GTPase activity, offering a comparable standard for the activity of MinC. On the basis of our observations, we propose a model on how MinC can disassemble FtsZ polymers. Furthermore, our data shows that the Min CDE system is sufficient to cause spatial regulation of FtsZ provided FtsZ is dynamic.
How the Z-ring takes the form of a functional helical or ring-like structure remains unclear. Extensive modelling approaches have tried to explain the ring formation and force generation. Previous studies have qualitatively shown bending of liposome membranes by FtsZ filaments. We hypothesised that the presumably intrinsically curved filaments should respond to pre-curved substrates, and the alignment should be quantifiable. This should ascertain whether or not FtsZ has intrinsic curvature and/or actively induces any force. Thus, we investigated how FtsZ filaments respond to a range of curvatures, which mimic different stages of the division process.
Our results show that the FtsZ filaments possess intrinsic curvatures as well as spontaneous twist. This facilitates the formation of Z-ring by utilizing geometrical cues. Our results are in agreement with consistent helical FtsZ polymers observed in vivo by Cryo-EM or super resolution microscopy. The alignment of filaments over a range of curvature suggests that the filaments have considerable flexibility, which strongly suggests reconsidering possible mechanisms of force generation. Moreover, the developed assay constitutes a valuable platform to further study proteins involved in modifying curvature of FtsZ filaments.
In summary, by reconstituting the FtsZ filament in vitro, we have elucidated the nature of FtsZ filaments. The dynamics of FtsZ filaments allows them to be inhibited by MinC, thus cooperating with the Min waves. The presence of intrinsic curvature and twist facilitates their formation into a ring necessary for the cell to carry out cytokinesis
