Abstract
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 Mg(2+) exhibits an apparent critical concentration, which is indicative of cooperative assembly, near 2.9 microm. With 2.5 mm Mg(2+) (which allows FtsZ to hydrolyze GTP) the critical concentration is reduced 10-fold to approximately 0.31 microm. Both with and without Mg(2+) 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.
Highlights
Isodesmic assembly was demonstrated for glutamate dehydrogenase [19, 20] and -lactoglobulin [21], and the thermodynamic principles of the assembly were developed
The isothermal titration calorimetry (ITC) data are highly suggestive of a cooperative assembly, this is difficult to reconcile with the 1-subunit-thick protofilaments observed by electron microscopy
One of the most distinctive predictions of the isodesmic assembly model is that the association constant for each interface should be very high (KA ϭ 3.3 ϫ 108 MϪ1 (KD ϭ 3 nM)) for assembly in GTP [16]
Summary
Isodesmic assembly was demonstrated for glutamate dehydrogenase [19, 20] and -lactoglobulin [21], and the thermodynamic principles of the assembly were developed. The assembly of glutamate dehydrogenase and -lactoglobin are probably not physiologically important, and the GDP-tubulin polymer is probably only transiently important during microtubule disassembly. In contrast to isodesmic assembly, all physiologically relevant filaments studied so far, in particular tubulin and actin, assemble in a cooperative manner. In contrast, produces a population of relatively short filaments, in which fragmentation of every interface in the middle is equivalent to dissociation of a subunit from the end. There is a sharp transition from disassembly to assembly as the pool of free subunits rises above Cc [24] This sharp transition at the Cc, coupled with unfavorable nucleation, results in a population of very long polymers, in equilibrium with monomers at a concentration Cc. Assembly and disassembly at the ends can be finely controlled by the concentration of free subunits. Several studies have suggested that FtsZ assembly is cooperative based on the observation of an apparent critical concentration.
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