Control by potassium of the size distribution of Escherichia coli FtsZ polymers is independent of GTPase activity.

The influence of potassium content (at neutral pH and millimolar Mg(2+)) on the size distribution of FtsZ polymers formed in the presence of constantly replenished GTP under steady-state conditions was studied by a combination of biophysical methods. The size of the GTP-FtsZ polymers decreased with lower potassium concentration, in contrast with the increase in the mass of the GDP-FtsZ oligomers, whereas no effect was observed on FtsZ GTPase activity and critical concentration of polymerization. Remarkably, the concerted formation of a narrow size distribution of GTP-FtsZ polymers previously observed at high salt concentration was maintained in all KCl concentrations tested. Polymers induced with guanosine 5'-(α,β-methylene)triphosphate, a slowly hydrolyzable analog of GTP, became larger and polydisperse as the potassium concentration was decreased. Our results suggest that the potassium dependence of the GTP-FtsZ polymer size may be related to changes in the subunit turnover rate that are independent of the GTP hydrolysis rate. The formation of a narrow size distribution of FtsZ polymers under very different solution conditions indicates that it is an inherent feature of FtsZ, not observed in other filament-forming proteins, with potential implications in the structural organization of the functional Z-ring.