FtsZ Protofilaments Use A Hinge-Opening Mechanism For Constrictive Force Generation

The bacterial cytokinesis is initiated by the constriction of Z-ring at the center of a cell, formed by the major cytoskeletal protein FtsZ(Filamentous temperature sensitive mutant Z), a GTPase which can self-assemble into protofilaments. During the division, the Z-ring exerts a constrictive force on the membrane by utilizing the chemical energy of GTP (guanosine triphosphate) hydrolysis, with the diameter contracting till the division completion. Although multiple crystal structures of FtsZ of various species in different nucleotide states as monomers or protofilaments are available, little is known about the structural mechanism underlying a "hydrolysis-mediated" conformational change.The program is aimed to address the crucial problem that how Z-ring can transform the chemical energy from the GTP hydrolysis to the constrictive force for the cell division in the prokaryotic cytokinesis field. The MtbFtsZ-GDP subunits in the determined structure arrange longitudinally in a head-to-hail manner and assemble into two continuous polymers that intertwine to from a left-handed, antiparallel, double-stranded structure. They designed a series of interfacial mutations based on the structure, with in vivo complement assay, GTPase assay and in vitro EM study demonstrating the mutations indeed disrupt the relative functions of FtsZ. Through these studies, they prove that the longitudinal interface is important for FtsZ function and highlight a hydrolysis-dependent conformational switch at the T3loop that leads to longitudinal bending between subunits, which can generate sufficient force to drive cytokinesis.More specifically, through the superposition between the straight SaFtsZ protofilaments and the curved MtbFtsZ protofilaments mentioned, they discover that GTP y-phosphate stabilizes the T3loop conformation in a compact state(tension or T state)in the straight filaments while a relaxed conformation(Relax state or R state) in our GDP-bound curved structure. Because both β-and γ-phosphates are negatively charged, the release of γ-phosphates by hydrolysis may trigger the T3loop conformational transition from the T state to R state, driving the hinge-opening event around the pivot point(L269in MtbFtsZ). The chemical energy from the GTP hydrolysis thus can transform to the constrictive force for the Z-ring contraction.Additionally, FtsZ contains a flexible C-terminus that binds FtsA, which is attached to the membrane through its C-terminal amphipathic helix. Thus, in the context of FtsZ division function, the straight-to-curved transition can exert constrictive forces on the envelope.This study addresses the mechanism that how Z-ring can transform the chemical energy from the GTP hydrolysis to the constrictive force for the cell division in the prokaryotic cytokinesis field. What’s more, the results can provide the exact and straightforward structural information for the design of new antibiotic drug targeting the bacteria division protein.