FtsZ

Abstract: Bacterial cell division ends with septation, the constriction of the cell wall and cell membranes that leads to the formation of two daughter cells. During septation, FtsZ, a protein of relative molecular mass 40,000 which is ubiquitous in eubacteria and is also found in archaea and chloroplasts, localizes early at the division site to form a ring-shaped septum. This septum is required for the mechanochemical process of membrane constriction. FtsZ is a GTPase with weak sequence homology to tubulins. The nature of FtsZ polymers in vivo is unknown, but FtsZ can form tubules, sheets and minirings in vitro. Here we report the crystal structure at 2.8 A 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.

Abstract: Bacterial cell division is orchestrated by a tubulin homologue, FtsZ, which polymerizes to form a ring-like structure that is both a scaffold for the assembly of the bacterial cytokinetic machinery and, at least in part, a source of the energy for constriction. FtsZ assembly is tightly regulated, and a diverse repertoire of accessory proteins contributes to the formation of a functional division machine that is responsive to cell cycle status and environmental stress. In this Review, we describe the interaction of these proteins with FtsZ and discuss recent advances in our understanding of Z ring assembly.

Abstract: It was thought until recently that bacteria lack the actin or tubulin filament networks that organize eukaryotic cytoplasm. However, we show here that the bacterial MreB protein assembles into filaments with a subunit repeat similar to that of F-actin-the physiological polymer of eukaryotic actin. By elucidating the MreB crystal structure we demonstrate that MreB and actin are very similar in three dimensions. Moreover, the crystals contain protofilaments, allowing visualization of actin-like strands at atomic resolution. The structure of the MreB protofilament is in remarkably good agreement with the model for F-actin, showing that the proteins assemble in identical orientations. The actin-like properties of MreB explain the finding that MreB forms large fibrous spirals under the cell membrane of rod-shaped cells, where they are involved in cell-shape determination. Thus, prokaryotes are now known to possess homologues both of tubulin, namely FtsZ, and of actin.

Abstract: Xanthomonas citri (Xac) is the causal agent of citrus canker, a disease that affects citrus crops and causes economic impact worldwide. To further characterize cell division in this plant pathogen, we investigated the role of the protein MinC in cell division, chromosome segregation, and peptidoglycan incorporation by deleting the gene minC using allele exchange. Xac with minC deleted exhibited the classic Δmin phenotype observed in other bacteria deleted for min components: minicells and short filamentation. In addition we noticed the formation of branches, which is similar to what was previously described for Escherichia coli deleted for either min or for several low molecular weight penicillin-binding proteins (PBPs). The branching phenotype was medium dependent and probably linked to gluconeogenic growth. We complemented the minC gene by integrating gfp-minC into the amy locus. Xac complemented strains displayed a wild-type phenotype. In addition, GFP-MinC oscillated from pole to pole, similar to MinCD oscillations observed in E. coli and more recently in Synechococcus elongatus. Further investigation of the branching phenotype revealed that in branching cells nucleoid organization, divisome formation and peptidoglycan incorporation were disrupted.