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Bacterial cell division ends with septation, the constriction of the cell wall and cell membranes that leads to the formation of two daughter cells *RF 1,2*.During septation, FtsZ, a protein of relative molecular mass 40,000 which is ubiquitous in eubacteria and is also found in archaea and chloroplasts [3], localizes early at the division site to form a ring-shaped septum. This septum is required for the mechanochemical process of membrane constriction [4]. FtsZ is a GTPase [5,6] with weak sequence homology to tubulins [7]. The nature of FtsZ polymers in vivo is unknown, but FtsZ can form tubules, sheets and minirings in vitro [8,9]. Here we report the crystal structure at 2.8 Angstrom resolution of recombinant FtsZ from the hyperthermophilic methanogen Methanococcus jannaschii. FtsZ has two domains, one of which is a GTPase domain with a fold related to one found in the proteins p21ras and elongation factor EF-Tu. The carboxy-terminal domain, whose function is unknown, is a four-stranded beta-sheet tilted by 90 degrees against the beta-sheet of the GTPase domain. The two domains are arranged around a central helix. GDP binding is different from that typically found in GTPases and involves four phosphate-binding loops and a sugar-binding loop in the first domain, with guanine being recognized by residues in the central connecting helix. The three-dimensional structure of FtsZ is similar to the structure of alpha- and beta-tubulin [10].

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