Tubulin-like protofilaments in Ca2+-induced FtsZ sheets

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Abstract
The 40 kDa protein FtsZ is a major septum‐forming component of bacterial cell 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 filaments, in a GTP‐dependent manner. Together with limited sequence homology, the occurrence of the tubulin signature motif in FtsZ and a similar three‐dimensional structure, this leads to the conclusion that FtsZ is the bacterial tubulin homologue. We have polymerized FtsZ1 from Methanococcus jannaschii in the presence of millimolar concentrations of Ca2+ ions to produce two‐dimensional crystals of plane group P2221. Most of the protein precipitates and forms filaments ∼23.0 nm in diameter. A three‐dimensional reconstruction of tilted micrographs of FtsZ sheets in negative stain between 0 and 60° 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 protofilamant structure very similar to that of tubulin protofilaments.

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