Assembly processes leading to the formation of the Z-ring in Escherichia coli

In most bacterial species, FtsZ protofilaments form a ring-like structure, the Z-ring, during the early stages of cell division. The sequence of steps on how FtsZ protofilaments assemble into the Z-ring is not yet understood. Here, we investigate how this energy-driven phase transition occurs in Escherichia coli model using high temporal and spatial resolution imaging and computer modeling. Our data show that the formation of the Z-ring is preceded by a period when transient membrane-linked FtsZ assemblies form throughout the cell body. The filaments in transient assemblies treadmill. In addition to treadmilling, the assemblies also show periods of rapid growth and shrinkage, whose speed can be five-fold faster than expected from a simple treadmilling model. The data also indicate that protofilaments in the transient assemblies are bundled and that the assembly size depends sensitively on the concentration of FtsZ. The latter suggests that transient assemblies compete for the available FtsZ pool, and the most stable assembly becomes the functional Z-ring. To test these ideas further, we have been developing a molecular dynamics simulation that implements the assembly in a 3D cell setting.