Rebuilding the initial steps of bacterial cell division

For a long time it was assumed that a cytoskeleton would be exclusive to eukaryotes. However, proteins related to actin and tubulin also exist in bacteria, where they provide key structural components coordinating a number cellular functions.

HFSP Long-Term Fellow Martin Loose and colleagues
authored on Mon, 10 February 2014

Bacterial cytokinesis, for example, is commonly initiated by the formation of the Z-ring: an annular cytoskeletal structure, which assembles at the middle of a rod-shaped cell. This cytoskeletal ring organizes the formation of the so-called divisome, a complex protein machinery, which ultimately performs cell division. The primary component of the Z-ring is FtsZ, a tubulin superfamily GTPase, which is recruited to the cytoplasmic membrane by the actin-related protein FtsA. Because of their crucial role during cell division, both proteins are highly conserved during evolution. Despite their importance, how FtsZ and FtsA assemble into a cytoskeletal structure on the membrane was not known.

Figure: Left: In Escherichia coli, FtsZ and FtsA form a ring at the center of the cell initiating cell division. Yellow dashed line indicates the outline of the cell. Right: These two proteins form dynamic rings of similar size when incubated on a supported membrane in vitro.

Using a bottom-up, in vitro reconstitution approach we developed an experimental assay to rebuild the initial steps of bacterial cell division, based on purified components from the bacterium Escherichia coli and fluorescence microscopy. To our surprise, we found that FtsZ and FtsA self-organized into complex cytoskeletal patterns when incubated on a flat membrane, such as fast-moving filament bundles and chirally rotating rings (see Figure).

How do FtsZ and FtsA form these rapidly reorganizing patterns? Using biochemical assays and perturbation experiments, we found that the rapid rearrangements of the filament network correspond to treadmilling FtsZ filaments and a dual, antagonistic role of FtsA: recruitment of FtsZ filaments to the membrane and a negative regulation of filament organization.

In the cell, this kind of dynamic reorganization of FtsZ filaments could be important for repositioning the Z-ring during its assembly, furthermore it could also allow the ring to adapt to the decreasing diameter of the constricting division septum.

In summary, our findings not only provide a novel model for the initial steps of bacterial cell division but also illustrate a new way in which dynamic polymers can self-organize into large-scale structures.

Reference

The bacterial cell division proteins FtsA and FtsZ self-organize into dynamic cytoskeletal patterns. Loose M, Mitchison TJ. Nat Cell Biol. 2014 Jan;16(1):38-46. doi: 10.1038/ncb2885. Epub 2013 Dec 8.

Pubmed link