Fig. 6.
Model for FtsZ assembly that accounts for the toxicity of FtsZL272E. FtsZ with GTP bound exists in two conformations, open and closed, with the closed form preferred. Above the critical concentration, two subunits in the open form combine to generate a nucleus, and additional subunits in the closed form add to the bottom end with the energy of binding causing a conformational switch (snap into the open form) to produce a bottom end with high affinity for another GTP-bound subunit. GTP hydrolysis follows assembly resulting in subunits disassociating from the top end, leading to treadmilling. FtsZL272E adds to the bottom end and snaps into the open form, but subsequent subunit addition is prevented, leading to disassembly of the filament. As subunits disassemble, they return to the closed form and rapidly undergo exchange to the GTP form.