Apparent cooperative assembly of the bacterial cell division protein FtsZ demonstrated by isothermal titration calorimetry

The assembly dynamics of FtsZ, a prokaryotic homolog of tubulin, are important for their role in bacterial cytokinesis. Here we used isothermal titration calorimetry (ITC) to measure the heat of FtsZ self-association under various conditions. The measurements were designed to test whether FtsZ protofilaments are assembled by an isodesmic (linear aggregates in which each bond has an identical equilibrium constant) or a cooperative (aggregates only become stable after forming a oligomeric nucleus) assembly process. The isodesmic model can fit the assembly in GDP closely but cannot fit the assembly in GTP. FtsZ-GTP without Mg²⁺ exhibits an apparent critical concentration, which is indicative of cooperative assembly, near 2.9 μM. With 2.5 mM Mg²⁺ (which allows FtsZ to hydrolyze GTP) the critical concentration is reduced 10-fold to ∼0.31 μM. Both with and without Mg²⁺ there is no evidence for assembly below the critical concentration, but there is an abrupt transition to full assembly above. The ITC data are highly suggestive of a cooperative assembly, although this is difficult to reconcile with the 1-subunit-thick protofilaments observed by electron microscopy.