Non-hydrolysable GTP-gamma-S stabilizes the FtsZ polymer in a GDP-bound state

FtsZ, a tubulin homologue, forms a cytokinetic ring at the site of cell div ision in prokaryotes. The ring is thought to consist of polymers that assem ble in a strictly GTP-dependent way. GTP, but not guanosine-5'-O-(3-thiotri phosphate) (GTP-gamma-S), has been shown to induce polymerization of FtsZ, whereas in vitro Ca2+ is known to inhibit the GTP hydrolysis activity of Ft sZ. We have studied FtsZ dynamics at limiting GTP concentrations in the pre sence of 10 mM Ca2+. GTP and its non-hydrolysable analogue GTP-gamma-S bind FtsZ with similar affinity, whereas the non-hydrolysable analogue guanylyl -imidodiphosphate (GMP-PNP) is a poor substrate. Preformed FtsZ polymers ca n be stabilized by GTP-gamma-S and are destabilized by GDP. As more than 95 % of the nucleotide associated with the FtsZ polymer is in the GDP form, it is concluded that GTP hydrolysis by itself does not trigger FtsZ polymer d isassembly. Strikingly, GTP-gamma-S exchanges only a small portion of the F tsZ polymer-bound GDP. These data suggest that FtsZ polymers are stabilized by a small fraction of GTP-containing FtsZ subunits. These subunits may be located either throughout the polymer or at the polymer ends, forming a GT P cap similar to tubulin.