Tubulin-like protofilaments in Ca2+-induced FtsZ sheets

The 40 kDa protein FtsZ is a major septum-forming component of bacterial ce ll division. Early during cytokinesis at midcell, FtsZ forms a cytokinetic ring that constricts as septation progresses. FtsZ has a high propensity to polymerize in vitro into various structures, including sheets and filament s, in a GTP-dependent manner. Together with limited sequence homology, the occurrence of the tubulin signature motif in FtsZ and a similar three-dimen sional structure, this leads to the conclusion that FtsZ is the bacterial t ubulin homologue, We have polymerized FtsZ1 from Methanococcus jannaschii i n the presence of millimolar concentrations of Ca2+ ions to produce two-dim ensional crystals of plane group P222(1), Most of the protein precipitates and forms filaments similar to 23.0 nm in diameter. A three-dimensional rec onstruction of tilted micrographs of FtsZ sheets in negative stain between 0 and 60 degrees shows protofilaments of FtsZ running along the sheet axis. Pairs of parallel FtsZ protofilaments associate in an antiparallel fashion to form a two-dimensional sheet. The antiparallel arrangement is believed to generate flat sheets instead of the curved filaments seen in other FtsZ polymers. Together with the subunit spacing along the protofilament axis, a fitting of the FtsZ crystal structure into the reconstruction suggests a p rotofilamant structure very similar to that of tubulin protofilaments.