Binary fission of many prokaryotes as well as some eukaryotic organelles depends on the FtsZ protein, which self-assembles into a membrane-associated ring structure early in the division process. FtsZ is homologous to tubulin, the building block of the microtubule cytoskeleton in eukaryotes. Recent advances in genomics and cell-imaging techniques have paved the way for the remarkable progress in our understanding of fission in bacteria and organelles.Duplication of cells occurs by the division of a mother cell into two daughter cells. This process, known as cytokinesis, provides the force to split cells and is spatially regulated to faithfully partition the genetic material. The cytoskeleton has a crucial role in cytokinesis. In animal and fungal cells, a medial ring of actin and myosin, helped by other proteins, contracts to divide the cell. Plant cells use a cell plate, and are guided by actin filaments and microtubules. Prokaryotes, which include bacteria and archaea, possess homologues of eukaryotic cytoskeletal proteins. Most prokaryotes use a tubulin homologue, a protein known as FtsZ, to divide. Eukaryotic organelles such as chloroplasts and mitochondria evolved from bacteria, and all chloroplasts and some mitochondria use FtsZ to divide.FtsZ is thought to be the first protein to localize to the site of future division in bacteria 1 , and it assembles into what is known as the Z ring (FIG. 1). In Escherichia coli, the Z ring recruits at least ten other proteins, all of which are required for the progression and completion of cytokinesis 2 , as will be discussed below. Whereas the cytokinetic apparatus in eukaryotic cells and even organelles can be observed directly in stained thin sections by transmission electron microscopy, the Z ring and its associated factors are not detectable by these methods. This is probably because the protein machinery is largely membrane bound and resides in a densely populated cytoplasmic environment. However, the development of green fluorescent protein (GFP) fusion and improved immunofluorescence techniques over the past 10 years have been crucial in allowing the first direct visualization of many cellular components and their dynamics. For example, using GFP fusions, the dynamics of the Z ring and other components of the cell-division protein machinery can now be visualized in living bacterial cells. The combination of these new cytological tools with genomics and the Competing interests statementThe author declares no competing financial interests. HHS Public Access Author Manuscript Author ManuscriptAuthor ManuscriptAuthor Manuscript already powerful genetics of several model systems have spurred rapid progress in our understanding of the molecular cell biology of bacteria and eukaryotic organelles. This review will discuss how the recent revolutions in genomics and cell biology have provided new evolutionary and mechanistic insights into how bacterial cells and organelles divide. The FtsZ family of proteins Conservation of FtsZFtsZ is a highly conserved protein ...

In the current model for bacterial cell division, FtsZ protein forms a ring that marks the division plane, creating a cytoskeletal framework for the subsequent action of other proteins such as FtsA. This putative protein complex ultimately generates the division septum. Herein we report that FtsZ and FtsA proteins tagged with green f luorescent protein (GFP) colocalize to division-site ring-like structures in living bacterial cells in a visible space between the segregated nucleoids. Cells with higher levels of FtsZ-GFP or with FtsA-GFP plus excess wild-type FtsZ were inhibited for cell division and often exhibited bright f luorescent spiral tubules that spanned the length of the filamentous cells. This suggests that FtsZ may switch from a septation-competent localized ring to an unlocalized spiral under some conditions and that FtsA can bind to FtsZ in both conformations. FtsZ-GFP also formed nonproductive but localized aggregates at a higher concentration that could represent FtsZ nucleation sites. The general domain structure of FtsZ-GFP resembles that of tubulin, since the C terminus of FtsZ is not required for polymerization but may regulate polymerization state. The N-terminal portion of Rhizobium FtsZ polymerized in Escherichia coli and appeared to copolymerize with E. coli FtsZ, suggesting a degree of interspecies functional conservation. Analysis of several deletions of FtsA-GFP suggests that multiple segments of FtsA are important for its localization to the FtsZ ring.

The chemoreceptors of Escherichia coli localize to the cell poles and form a highly ordered array in concert with the CheA kinase and the CheW coupling factor. However, a high-resolution structure of the array has been lacking, and the molecular basis of array assembly has thus remained elusive. Here, we use cryoelectron tomography of flagellated E. coli minicells to derive a 3D map of the intact array. Docking of high-resolution structures into the 3D map provides a model of the core signaling complex, in which a CheA/CheW dimer bridges two adjacent receptor trimers via multiple hydrophobic interactions. A further, hitherto unknown, hydrophobic interaction between CheW and the homologous P5 domain of CheA in an adjacent core complex connects the complexes into an extended array. This architecture provides a structural basis for array formation and could explain the high sensitivity and cooperativity of chemotaxis signaling in E. coli.

Bacterial type III secretion machines are widely used to inject virulence proteins into eukaryotic host cells. These secretion machines are evolutionarily related to bacterial flagella and consist of a large cytoplasmic complex, a transmembrane basal body, and an extracellular needle. The cytoplasmic complex forms a sorting platform essential for effector selection and needle assembly, but it remains largely uncharacterized. Here we use high-throughput cryoelectron tomography (cryo-ET) to visualize intact machines in a virulent Shigella flexneri strain genetically modified to produce minicells capable of interaction with host cells. A high-resolution in situ structure of the intact machine determined by subtomogram averaging reveals the cytoplasmic sorting platform, which consists of a central hub and six spokes, with a pod-like structure at the terminus of each spoke. Molecular modeling of wild-type and mutant machines allowed us to propose a model of the sorting platform in which the hub consists mainly of a hexamer of the Spa47 ATPase, whereas the MxiN protein comprises the spokes and the Spa33 protein forms the pods. Multiple contacts among those components are essential to align the Spa47 ATPase with the central channel of the MxiA protein export gate to form a unique nanomachine. The molecular architecture of the Shigella type III secretion machine and its sorting platform provide the structural foundation for further dissecting the mechanisms underlying type III secretion and pathogenesis and also highlight the major structural distinctions from bacterial flagella.nanomachine | injectisome | pathogen-host interaction | cryo-electron tomography | protein secretion

Preface Bacteria must divide in order to increase in number and colonize their niche. Binary fission is the most widespread means of bacterial cell division, but even this relatively simple mechanism displays many variations on a theme. In most bacteria, the tubulin homolog FtsZ assembles into a ring structure (Z ring) at the site of cytokinesis and recruits additional proteins to form a large protein machine (divisome) that spans the membrane. Here we discuss current insights into the regulation of Z ring assembly and how the divisome drives membrane invagination and septal cell wall growth while still flexibly responding to various cellular inputs.

SummaryTo understand further the role of the nucleoid and the min system in selection of the cell division site, we examined FtsZ localization in Escherichia coli cells lacking MinCDE and in parC mutants defective in chromosome segregation. More than one FtsZ ring was sometimes found in the gaps between nucleoids in min mutant filaments. These multiple FtsZ rings were more apparent in longer cells; double or triple rings were often found in the nucleoid-free gaps in ftsI min and ftsA min double mutant filaments. Introducing a parC mutation into the ftsA min double mutant allowed the nucleoid-free gaps to become significantly longer. These gaps often contained dramatic clusters of FtsZ rings. In contrast, filaments of the ftsA parC double mutant, which contained active MinCDE, assembled only one or two rings in most of the large nucleoidfree gaps. These results suggest that all positions along the cell length are competent for FtsZ ring assembly, not just sites at mid-cell or at the poles. Consistent with previous results, unsegregated nucleoids also correlated with a lack of FtsZ localization. A model is proposed in which both the inhibitor y effect of the nucleoid and the regulation by MinCDE ensure that cells divide precisely at the midpoint.

ZipA and FtsA are recruited independently to the FtsZ cytokinetic ring (Z ring) and are essential for cell division of Escherichia coli. The molecular role of FtsA in cell division is unknown; however, ZipA is thought to stabilize the Z ring, anchor it to the membrane, and recruit downstream cell division proteins. Here we demonstrate that the requirement for ZipA can be bypassed completely by a single alteration in a conserved residue of FtsA (FtsA*). Cells with ftsA* in single copy in place of WT ftsA or with ftsA* alone on a multicopy plasmid divide mostly normally, whether they are zipA؉ or zipA؊. Experiments with ftsQAZ and ftsQA*Z on multicopy plasmids indicate that ftsQAZ͞zipA؉ and ftsQA*Z͞zipA؊ cells divide fairly normally, whereas ftsQAZ͞zipA؊ cells divide poorly and ftsQA*Z͞zipA؉ cells display a phenotype that suggests their septa are unusually stable. In support of the idea that ftsA* stabilizes Z rings, single-copy ftsA* confers resistance to excess MinC, which destabilizes Z rings. The inhibitory effect of excess ZipA on division is also suppressed by ftsA*. These results suggest that the molecular mechanism of the FtsA* bypass is to stabilize FtsZ assembly via a parallel pathway and that FtsA* can replace the multiple functions of ZipA. This is an example of a complete functional replacement of an essential prokaryotic cell division protein by another and may explain why most bacteria can divide without an obvious ZipA homolog.

FtsZ, a tubulin‐like GTPase that forms a dynamic ring marking the division plane of prokaryotic cells, is essential for cytokinesis. It is not known what triggers FtsZ ring assembly. In this work, we use a FtsZ–green fluorescent protein (Gfp) chimera to assay FtsZ assembly over time by using fluorescence microscopy. We show that FtsZ polymers can assemble dynamically in solution in a GTP‐dependent manner. Initially, FtsZ nucleation centers grow into aster‐like structures that dramatically resemble microtubule organizing centers. As assembly proceeds further, protofilament bundles emanating from different asters interconnect, mimicking the closure of the FtsZ ring in vivo. Surprisingly, millimolar levels of Ca2+ promote FtsZ dynamic assembly. FtsZ can undergo repeated GTP‐dependent assembly and disassembly in solution by sequential addition and removal of Ca2+. In addition, GTP binding and hydrolysis by FtsZ are regulated by Ca2+ concentration. Although the concentration of Ca2+ required for FtsZ assembly in vitro is high, its clear and specific effect on FtsZ dynamics suggests the possibility that Ca2+ may have a role in regulating FtsZ ring assembly in the cell.