Analysis of the interaction of FtsZ with itself, GTP, and FtsA

Assembly of the Caulobacter cell division machine

Cytokinesis in Gram-negative bacteria is mediated by a multiprotein machine (the divisome) that invaginates and remodels the inner membrane, peptidoglycan and outer membrane. Understanding the order of divisome assembly would inform models of the interactions among its components and their respective functions. We leveraged the ability to isolate synchronous populations of Caulobacter crescentus cells to investigate assembly of the divisome and place the arrival of each component into functional context. Additionally, we investigated the genetic dependence of localization among divisome proteins and the cell cycle regulation of their transcript and protein levels to gain insight into the control mechanisms underlying their assembly. Our results revealed a picture of divisome assembly with unprecedented temporal resolution. Specifically, we observed (i) initial establishment of the division site, (ii) recruitment of early FtsZ-binding proteins, (iii) arrival of proteins involved in peptidoglycan remodelling, (iv) arrival of FtsA, (v) assembly of core divisome components, (vi) initiation of envelope invagination, (vii) recruitment of polar markers and cytoplasmic compartmentalization and (viii) cell separation. Our analysis revealed differences in divisome assembly among Caulobacter and other bacteria that establish a framework for identifying aspects of bacterial cytokinesis that are widely conserved from those that are more variable.

Identification of Escherichia coli ZapC (YcbW) as a component of the division apparatus that binds and bundles FtsZ polymers

Assembly of the cell division apparatus in bacteria starts with formation of the Z ring on the cytoplasmic face of the membrane. This process involves the accumulation of FtsZ polymers at midcell and their interaction with several FtsZ-binding proteins that collectively organize the polymers into a membrane-associated ring-like configuration. Three such proteins, FtsA, ZipA, and ZapA, have previously been identified in Escherichia coli. FtsA and ZipA are essential membrane-associated division proteins that help connect FtsZ polymers with the inner membrane. ZapA is a cytoplasmic protein that is not required for the fission process per se but contributes to its efficiency, likely by promoting lateral interactions between FtsZ protofilaments. We report the identification of YcbW (ZapC) as a fourth FtsZ-binding component of the Z ring in E. coli. Binding of ZapC promotes lateral interactions between FtsZ polymers and suppresses FtsZ GTPase activity. This and additional evidence indicate that, like ZapA, ZapC is a nonessential Z-ring component that contributes to the efficiency of the division process by stabilizing the polymeric form of FtsZ.

Imaging-based identification of a critical regulator of FtsZ protofilament curvature in Caulobacter

FtsZ is an essential bacterial GTPase that polymerizes at midcell, recruits the division machinery, and may generate constrictive forces necessary for cytokinesis. However, many of the mechanistic details underlying these functions are unknown. We sought to identify FtsZ-binding proteins that influence FtsZ function in Caulobacter crescentus. Here, we present a microscopy-based screen through which we discovered two FtsZ-binding proteins, FzlA and FzlC. FzlA is conserved in α-proteobacteria and was found to be functionally critical for cell division in Caulobacter. FzlA altered FtsZ structure both in vivo and in vitro, forming stable higher-order structures that were resistant to depolymerization by MipZ, a spatial determinant of FtsZ assembly. Electron microscopy revealed that FzlA organizes FtsZ protofilaments into striking helical bundles. The degree of curvature induced by FzlA depended on the nucleotide bound to FtsZ. Induction of FtsZ curvature by FzlA carries implications for regulating FtsZ function by modulating its superstructure.

Cross-linking FtsZ polymers into coherent Z rings

A key event in bacterial cytokinesis is the formation of the Z ring, which serves as a mechanical scaffold that recruits other cytokinetic proteins to establish functional divisomes. This scaffolding function of Z rings is essential throughout cytokinesis, but the underlying molecular interactions are poorly understood. Here we report that a widely conserved FtsZ binding protein, ZapA, has cytological, biochemical and biophysical properties that argue for the importance of cross-linking interactions between FtsZ polymers in the coherence of Z rings. Escherichia coli zapA null mutant cells have Z rings that are structurally looser and many helical precursors of Z rings fail to coalesce into coherent rings. Biophysical behaviour of FtsZ in the presence of ZapA reveals that ZapA not only bundles, but also cross-links FtsZ polymers, which makes it the first cross-linking protein of the bacterial cytoskeleton. Cross-linking in vitro occurs at the stoichiometry of FtsZ-ZapA interaction at the Z rings in vivo, where nearly all intracellular ZapA is dynamically associated. ZapA also stabilizes longitudinal bonds between FtsZ monomers since it promotes the polymerization of FtsZ mutants with lesions at the polymerization interface and since it reverses the inhibitory effects of SulA, a known antagonist of FtsZ longitudinal interactions.

Essential biological processes of an emerging pathogen: DNA replication, transcription, and cell division in Acinetobacter spp

Within the last 15 years, members of the bacterial genus Acinetobacter have risen from relative obscurity to be among the most important sources of hospital-acquired infections. The driving force for this has been the remarkable ability of these organisms to acquire antibiotic resistance determinants, with some strains now showing resistance to every antibiotic in clinical use. There is an urgent need for new antibacterial compounds to combat the threat imposed by Acinetobacter spp. and other intractable bacterial pathogens. The essential processes of chromosomal DNA replication, transcription, and cell division are attractive targets for the rational design of antimicrobial drugs. The goal of this review is to examine the wealth of genome sequence and gene knockout data now available for Acinetobacter spp., highlighting those aspects of essential systems that are most suitable as drug targets. Acinetobacter spp. show several key differences from other pathogenic gammaproteobacteria, particularly in global stress response pathways. The involvement of these pathways in short- and long-term antibiotic survival suggests that Acinetobacter spp. cope with antibiotic-induced stress differently from other microorganisms.

The complexity and evolution of the plastid-division machinery

Plastids are vital organelles, fulfilling important metabolic functions that greatly influence plant growth and productivity. In order to both regulate and harness the metabolic output of plastids, it is vital that the process of plastid division is carefully controlled. This is essential, not only to ensure persistence in dividing plant cells and that optimal numbers of plastids are obtained in specialized cell types, but also to allow the cell to act in response to developmental signals and environmental changes. How this control is exerted by the host nucleus has remained elusive. Plastids evolved by endosymbiosis and during the establishment of a permanent endosymbiosis they retained elements of the bacterial cell-division machinery. Through evolution the photosynthetic eukaryotes have increased dramatically in complexity, from single-cell green algae to multicellular non-vascular and vascular plants. Reflected with this is an increasing complexity of the division machinery and recent findings also suggest increasing complexity in the molecular mechanisms used by the host cell to control the process of plastid division. In the present paper, we explore the current understanding of the process of plastid division at the molecular and cellular level, with particular respect to the evolution of the division machinery and levels of control exerted on the process.

Mycobacterium tuberculosis FtsZ requires at least one arginine residue at the C-terminal end for polymerization in vitro

We examined whether C-terminal residues of soluble recombinant FtsZ of Mycobacterium tuberculosis (MtFtsZ) have any role in MtFtsZ polymerization in vitro. MtFtsZ-deltaC1, which lacks C-terminal extreme Arg residue (underlined in the C-terminal extreme stretch of 13 residues, DDDDVDVPPFMRR), but retaining the penultimate Arg residue (DDDDVDVPPFMR), polymerizes like full-length MtFtsZ in vitro. However, MtFtsZ-deltaC2 that lacks both the Arg residues at the C-terminus (DDDDVDVPPFM), neither polymerizes at pH 6.5 nor forms even single- or double-stranded filaments at pH 7.7 in the presence of 10 mM CaCl(2). Neither replacement of the penultimate Arg residue, in the C-terminal Arg deletion mutant DDDDVDVPPFMR, with Lys or His or Ala or Asp (DDDDVDVPPFMK/H/A/D) enabled polymerization. Although MtFtsZ-deltaC2 showed secondary and tertiary structural changes, which might have affected polymerization, GTPase activity of MtFtsZ-deltaC2 was comparable to that of MtFtsZ. These data suggest that MtFtsZ requires an Arg residue as the extreme C-terminal residue for polymerization in vitro. The polypeptide segment containing C-terminal 67 residues, whose coordinates were absent from MtFtsZ crystal structure, was modeled on tubulin and MtFtsZ dimers. Possibilities for the influence of the C-terminal Arg residues on the stability of the dimer and thereby on MtFtsZ polymerization have been discussed.

ClpXP protease degrades the cytoskeletal protein, FtsZ, and modulates FtsZ polymer dynamics

FtsZ is the major cytoskeletal protein in bacteria and a tubulin homologue. It polymerizes and forms a ring where constriction occurs to divide the cell. We found that FtsZ is degraded by E. coli ClpXP, an ATP-dependent protease. In vitro, ClpXP degrades both FtsZ protomers and polymers; however, polymerized FtsZ is degraded more rapidly than the monomer. Deletion analysis shows that the N-terminal domain of ClpX is important for polymer recognition and that the FtsZ C terminus contains a ClpX recognition signal. In vivo, FtsZ is turned over slower in a clpX deletion mutant compared with a WT strain. Overexpression of ClpXP results in increased FtsZ degradation and filamentation of cells. These results suggest that ClpXP may participate in cell division by modulating the equilibrium between free and polymeric FtsZ via degradation of FtsZ filaments and protomers.

The conserved C-terminal tail of FtsZ is required for the septal localization and division inhibitory activity of MinC(C)/MinD

The Escherichia coli Min system contributes to spatial regulation of cytokinesis by preventing assembly of the Z ring away from midcell. MinC is a cell division inhibitor whose activity is spatially regulated by MinD and MinE. MinC has two functional domains of similar size, both of which have division inhibitory activity in the proper context. However, the molecular mechanism of the inhibitory action of either domain is not very clear. Here, we report that the septal localization and division inhibitory activity of MinC(C)/MinD requires the conserved C-terminal tail of FtsZ. This tail also mediates interaction with two essential division proteins, ZipA and FtsA, to link FtsZ polymers to the membrane. Overproduction of MinC(C)/MinD displaces FtsA from the Z ring and eventually disrupts the Z ring, probably because it also displaces ZipA. These results support a model for the division inhibitory action of MinC/MinD. MinC/MinD binds to ZipA and FtsA decorated FtsZ polymers located at the membrane through the MinC(C)/MinD-FtsZ interaction. This binding displaces FtsA and/or ZipA, and more importantly, positions MinC(N) near the FtsZ polymers making it a more effective inhibitor.

Adenine nucleotide-dependent regulation of assembly of bacterial tubulin-like FtsZ by a hypermorph of bacterial actin-like FtsA

Cytokinesis in bacteria depends upon the contractile Z ring, which is composed of dynamic polymers of the tubulin homolog FtsZ as well as other membrane-associated proteins such as FtsA, a homolog of actin that is required for membrane attachment of the Z ring and its subsequent constriction. Here we show that a previously characterized hypermorphic mutant FtsA (FtsA*) partially disassembled FtsZ polymers in vitro. This effect was strictly dependent on ATP or ADP binding to FtsA* and occurred at substoichiometric levels relative to FtsZ, similar to cellular levels. Nucleotide-bound FtsA* did not affect FtsZ GTPase activity or the critical concentration for FtsZ assembly but was able to disassemble preformed FtsZ polymers, suggesting that FtsA* acts on FtsZ polymers. Microscopic examination of the inhibited FtsZ polymers revealed a transition from long, straight polymers and polymer bundles to mainly short, curved protofilaments. These results indicate that a bacterial actin, when activated by adenine nucleotides, can modify the length distribution of bacterial tubulin polymers, analogous to the effects of actin-depolymerizing factor/cofilin on F-actin.

SepF increases the assembly and bundling of FtsZ polymers and stabilizes FtsZ protofilaments by binding along its length

SepF (Septum Forming) protein has been recently identified through genetic studies, and it has been suggested to be involved in the division of Bacillus subtilis cells. We have purified functional B. subtilis SepF from the inclusion bodies overexpressed in Escherichia coli. Far-UV circular dichroism and fluorescence spectroscopic analysis involving the extrinsic fluorescent probe 1-anilinonaphthalene-8-sulfonic acid suggested that the purified SepF had characteristics of folded proteins. SepF was found to promote the assembly and bundling of FtsZ protofilaments using three complimentary techniques, namely 90 degrees light scattering, sedimentation, and transmission electron microscopy. SepF also decreased the critical concentration of FtsZ assembly, prevented the dilution-induced disassembly of FtsZ protofilaments, and suppressed the GTPase activity of FtsZ. Further, thick bundles of FtsZ protofilaments were observed using fluorescein isothiocyanate-labeled SepF (FITC-SepF). Interestingly, FITC-SepF was found to be uniformly distributed along the length of the FtsZ protofilaments, suggesting that SepF copolymerizes with FtsZ. SepF formed a stable complex with FtsZ, as evident from the gel filtration analysis. Using a C-terminal tail truncated FtsZ (FtsZDelta16) and a C-terminal synthetic peptide of B. subtilis FtsZ (366-382); we provided evidence indicating that SepF binds primarily to the C-terminal tail of FtsZ. The present work in concert with the available in vivo data support a model in which SepF plays an important role in regulating the assembly dynamics of the divisome complex; therefore, it may have an important role in bacterial cell division.

Cell-division inhibitors: new insights for future antibiotics

The growing problem of antibiotic resistance has been exacerbated by the use of new drugs that are merely variants of older overused antibiotics. While it is naive to expect to restrain the spread of resistance without controlling antibacterial usage, the desperate need for drugs with novel targets has been recognized by health organizations, industry and academia alike. The wealth of knowledge available about the bacterial cell-division pathway has aided target-driven approaches to identify novel inhibitors. Here, we discuss the therapeutic potential of inhibiting bacterial cell division, and review the progress made in this exciting new area of antibacterial discovery.

Developmentally regulated association of plastid division protein FtsZ1 with thylakoid membranes in Arabidopsis thaliana

FtsZ is a key protein involved in bacterial and organellar division. Bacteria have only one ftsZ gene, while chlorophytes (higher plants and green alga) have two distinct FtsZ gene families, named FtsZ1 and FtsZ2. This raises the question of why chloroplasts in these organisms need distinct FtsZ proteins to divide. In order to unravel new functions associated with FtsZ proteins, we have identified and characterized an Arabidopsis thaliana FtsZ1 loss-of-function mutant. ftsZ1-knockout mutants are impeded in chloroplast division, and division is restored when FtsZ1 is expressed at a low level. FtsZ1-overexpressing plants show a drastic inhibition of chloroplast division. Chloroplast morphology is altered in ftsZ1, with chloroplasts having abnormalities in the thylakoid membrane network. Overexpression of FtsZ1 also induced defects in thylakoid organization with an increased network of twisting thylakoids and larger grana. We show that FtsZ1, in addition to being present in the stroma, is tightly associated with the thylakoid fraction. This association is developmentally regulated since FtsZ1 is found in the thylakoid fraction of young developing plant leaves but not in mature and old plant leaves. Our results suggest that plastid division protein FtsZ1 may have a function during leaf development in thylakoid organization, thus highlighting new functions for green plastid FtsZ.

Effects of mutations in Arabidopsis FtsZ1 on plastid division, FtsZ ring formation and positioning, and FtsZ filament morphology in vivo

In plants, chloroplast division FtsZ proteins have diverged into two families, FtsZ1 and FtsZ2. FtsZ1 is more divergent from its bacterial counterparts and lacks a C-terminal motif conserved in most other FtsZs. To begin investigating FtsZ1 structure-function relationships, we first identified a T-DNA insertion mutation in the single FtsZ1 gene in Arabidopsis thaliana, AtFtsZ1-1. Homozygotes null for FtsZ1, though impaired in chloroplast division, could be isolated and set seed normally, indicating that FtsZ1 is not essential for viability. We then mapped five additional atftsZ1-1 alleles onto an FtsZ1 structural model and characterized chloroplast morphologies, FtsZ protein levels and FtsZ filament morphologies in young and mature leaves of the corresponding mutants. atftsZ1-1(G267R), atftsZ1-1(R298Q) and atftsZ1-1(Delta404-433) exhibit reduced FtsZ1 accumulation but wild-type FtsZ2 levels. The semi-dominant atftsZ1-1(G267R) mutation caused the most severe phenotype, altering a conserved residue in the predicted T7 loop. atftsZ1-1(G267R) protein accumulates normally in young leaves but is not detected in rings or filaments. atftsZ1-1(R298Q) has midplastid FtsZ1-containing rings in young leaves, indicating that R298 is not critical for ring formation or positioning despite its conservation. atftsZ1-1(D159N) and atftsZ1-1(G366A) both have overly long, sometimes spiral-like FtsZ filaments, suggesting that FtsZ dynamics are altered in these mutants. However, atftsZ1-1(D159N) exhibits loss of proper midplastid FtsZ positioning while atftsZ1-1(G366A) does not. Finally, truncation of the FtsZ1 C-terminus in atftsZ1-1(Delta404-433) impairs chloroplast division somewhat but does not prevent midplastid Z ring formation. These alleles will facilitate understanding of how the in vitro biochemical properties of FtsZ1 are related to its in vivo function.

Treadmilling of a prokaryotic tubulin-like protein, TubZ, required for plasmid stability in Bacillus thuringiensis

Prokaryotes rely on a distant tubulin homolog, FtsZ, for assembling the cytokinetic ring essential for cell division, but are otherwise generally thought to lack tubulin-like polymers that participate in processes such as DNA segregation. Here we characterize a protein (TubZ) from the Bacillus thuringiensis virulence plasmid pBtoxis, which is a member of the tubulin/FtsZ GTPase superfamily but is only distantly related to both FtsZ and tubulin. TubZ assembles dynamic, linear polymers that exhibit directional polymerization with plus and minus ends, movement by treadmilling, and a critical concentration for assembly. A point mutation (D269A) that alters a highly conserved catalytic residue within the T7 loop completely eliminates treadmilling and allows the formation of stable polymers at a much lower protein concentration than the wild-type protein. When expressed in trans, TubZ(D269A) coassembles with wild-type TubZ and significantly reduces the stability of pBtoxis, demonstrating a direct correlation between TubZ dynamics and plasmid maintenance. The tubZ gene is in an operon with tubR, which encodes a putative DNA-binding protein that regulates TubZ levels. Our results suggest that TubZ is representative of a novel class of prokaryotic cytoskeletal proteins important for plasmid stability that diverged long ago from the ancient tubulin/FtsZ ancestor.

Identification of a region of FtsA required for interaction with FtsZ

The assembly of the Z ring is the earliest step in bacterial cell division. In Escherichia coli this assembly requires either FtsA or ZipA which bind to a conserved, C-terminal 17 amino acid motif in FtsZ and to the membrane. The FtsZ-ZipA interaction is well characterized; however, nothing is known about the region of FtsA involved in the interaction with FtsZ even though the FtsA-FtsZ interaction is nearly ubiquitous in Eubacteria. FtsA is proposed to bind to the membrane through its conserved C-terminal amphiphatic helix before efficiently interacting with FtsZ. Based upon this model we designed a genetic screen to identify mutants specifically impaired for the FtsA-FtsZ interaction. The mutants obtained retain the ability to be targeted to the membrane but fail to be recruited to the Z ring or interact with FtsZ in the yeast two-hybrid system. These mutants do not complement an ftsA-depletion strain. Through this approach we have identified a region of FtsA containing some invariant residues which is required for binding to FtsZ. The results support our model that FtsA is targeted to the membrane before it interacts with FtsZ and demonstrates that this interaction plays an essential role in E. coli cell division.

A new assembly pathway for the cytokinetic Z ring from a dynamic helical structure in vegetatively growing cells of Bacillus subtilis

The earliest event in bacterial cell division is the formation of a Z ring, composed of the tubulin-like FtsZ protein, at the division site at midcell. This ring then recruits several other division proteins and together they drive the formation of a division septum between two replicated chromosomes. Here we show that, in addition to forming a cytokinetic ring, FtsZ localizes in a helical-like pattern in vegetatively growing cells of Bacillus subtilis. FtsZ moves rapidly within this helix-like structure. Examination of FtsZ localization in individual live cells undergoing a single cell cycle suggests a new assembly mechanism for Z ring formation that involves a cell cycle-mediated multistep remodelling of FtsZ polymers. Our observations suggest that initially FtsZ localizes in a helical pattern, with movement of FtsZ within this structure occurring along the entire length of the cell. Next, movement of FtsZ in a helical-like pattern is restricted to a central region of the cell. Finally the FtsZ ring forms precisely at midcell. We further show that another division protein, FtsA, shown to interact with FtsZ prior to Z ring formation in B. subtilis, also localizes to similar helical patterns in vegetatively growing cells.

Plastid division: evolution, mechanism and complexity

BACKGROUND

The continuity of chloroplasts is maintained by division of pre-existing chloroplasts. Chloroplasts originated as bacterial endosymbionts; however, the majority of bacterial division factors are absent from chloroplasts and the eukaryotic host has added several new components. For example, the ftsZ gene has been duplicated and modified, and the Min system has retained MinE and MinD but lost MinC, acquiring at least one new component ARC3. Further, the mechanism has evolved to include two members of the dynamin protein family, ARC5 and FZL, and plastid-dividing (PD) rings were most probably added by the eukaryotic host.

SCOPE

Deciphering how the division of plastids is coordinated and controlled by nuclear-encoded factors is key to our understanding of this important biological process. Through a number of molecular-genetic and biochemical approaches, it is evident that FtsZ initiates plastid division where the coordinated action of MinD and MinE ensures correct FtsZ (Z)-ring placement. Although the classical FtsZ antagonist MinC does not exist in plants, ARC3 may fulfil this role. Together with other prokaryotic-derived proteins such as ARC6 and GC1 and key eukaryotic-derived proteins such as ARC5 and FZL, these proteins make up a sophisticated division machinery. The regulation of plastid division in a cellular context is largely unknown; however, recent microarray data shed light on this. Here the current understanding of the mechanism of chloroplast division in higher plants is reviewed with an emphasis on how recent findings are beginning to shape our understanding of the function and evolution of the components.

CONCLUSIONS

Extrapolation from the mechanism of bacterial cell division provides valuable clues as to how the chloroplast division process is achieved in plant cells. However, it is becoming increasingly clear that the highly regulated mechanism of plastid division within the host cell has led to the evolution of features unique to the plastid division process.

Plastid division coordination across a double-membraned structure

Chloroplasts still retain components of the bacterial cell division machinery and research over the past decade has led to an understanding of how these stromal division proteins assemble and function as a complex chloroplast division machinery. However, during evolution plant chloroplasts have acquired a number of cytosolic division proteins, indicating that unlike the cyanobacterial ancestors of plastids, chloroplast division in higher plants require a second division machinery located on the chloroplast outer envelope membrane. Here we review the current understanding of the stromal and cytosolic plastid division machineries and speculate how two protein machineries coordinate their activities across a double-membraned structure.

Interaction between cell division proteins FtsE and FtsZ

FtsE and FtsX, which are widely conserved homologs of ABC transporters and interact with each other, have important but unknown functions in bacterial cell division. Coimmunoprecipitation of Escherichia coli cell extracts revealed that a functional FLAG-tagged version of FtsE, the putative ATP-binding component, interacts with FtsZ, the bacterial tubulin homolog required to assemble the cytokinetic Z ring and recruit the components of the divisome. This interaction is independent of FtsX, the predicted membrane component of the ABC transporter, which has been shown previously to interact with FtsE. The interaction also occurred independently of FtsA or ZipA, two other E. coli cell division proteins that interact with FtsZ. In addition, FtsZ copurified with FLAG-FtsE. Surprisingly, the conserved C-terminal tail of FtsZ, which interacts with other cell division proteins, such as FtsA and ZipA, was dispensable for interaction with FtsE. In support of a direct interaction with FtsZ, targeting of a green fluorescent protein (GFP)-FtsE fusion to Z rings required FtsZ, but not FtsA. Although GFP-FtsE failed to target Z rings in the absence of ZipA, its localization was restored in the presence of the ftsA* bypass suppressor, indicating that the requirement for ZipA is indirect. Coexpression of FLAG-FtsE and FtsX under certain conditions resulted in efficient formation of minicells, also consistent with an FtsE-FtsZ interaction and with the idea that FtsE and FtsX regulate the activity of the divisome.

Dynamic filaments of the bacterial cytoskeleton

Bacterial cells contain a variety of structural filamentous proteins necessary for the spatial regulation of cell shape, cell division, and chromosome segregation, analogous to the eukaryotic cytoskeletal proteins. The molecular mechanisms by which these proteins function are beginning to be revealed, and these proteins show numerous three-dimensional structural features and biochemical properties similar to those of eukaryotic actin and tubulin, revealing their evolutionary relationship. Recent technological advances have illuminated links between cell division and chromosome segregation, suggesting a higher complexity and organization of the bacterial cell than was previously thought.

Filamentous temperature-sensitive Z (FtsZ) isoforms specifically interact in the chloroplasts and in the cytosol of Physcomitrella patens

Plant filamentous temperature-sensitive Z (FtsZ) proteins have been reported to be involved in biological processes related to plastids. However, the precise functions of distinct isoforms are still elusive. Here, the intracellular localization of the FtsZ1-1 isoform in a moss, Physcomitrella patens, was examined. Furthermore, the in vivo interaction behaviour of four distinct FtsZ isoforms was investigated. Localization studies of green fluorescent protein (GFP)-tagged FtsZ1-1 and fluorescence resonance energy transfer (FRET) analyses employing all dual combinations of four FtsZ isoforms were performed in transient protoplast transformation assays. FtsZ1-1 is localized to network structures inside the chloroplasts and exerts influence on plastid division. Interactions between FtsZ isoforms occur in distinct ordered structures in the chloroplasts as well as in the cytosol. The results expand the view of the involvement of Physcomitrella FtsZ proteins in chloroplast and cell division. It is concluded that duplication and diversification of ftsZ genes during plant evolution were the main prerequisites for the successful remodelling and integration of the prokaryotic FtsZ-dependent division mechanism into the cellular machineries of distinct complex processes in plants.

Mechanism of regulation of prokaryotic tubulin-like GTPase FtsZ by membrane protein EzrA

At initiation of cell division, FtsZ, a tubulin-like GTPase, assembles into a so-called Z-ring structure at the site of division. The formation of Z ring is negatively regulated by EzrA, which ensures only one ring at the midcell per cell cycle. The mechanism leading to the negative regulation of Z-ring formation by EzrA has been analyzed. Our data reveal that the interaction between EzrA and FtsZ not only reduces the GTP-binding ability of FtsZ but also accelerates the rate of GTP hydrolysis, both of which are unfavorable for the polymerization of FtsZ. Moreover, the acceleration in rate of GTP hydrolysis by EzrA is attributed to stabilization of the transition state for GTP hydrolysis and reduction in the affinity of GDP for FtsZ. Clearly, EzrA is able to modify the GTP hydrolysis cycle of FtsZ. On the basis of these results, a model for how EzrA acts to negatively regulate Z-ring formation is proposed.

FtsZ from divergent foreign bacteria can function for cell division in Escherichia coli

FtsZs from Mycoplasma pulmonis (MpuFtsZ) and Bacillus subtilis (BsFtsZ) are only 46% and 53% identical in amino acid sequence to FtsZ from Escherichia coli (EcFtsZ). In the present study we show that MpuFtsZ and BsFtsZ can function for cell division in E. coli provided we make two modifications. First, we replaced their C-terminal tails with that from E. coli, giving the foreign FtsZ the binding site for E. coli FtsA and ZipA. Second, we selected for mutations in the E. coli genome that facilitated division by the foreign FtsZs. These suppressor strains arose at a relatively high frequency of 10(-3) to 10(-5), suggesting that they involve loss-of-function mutations in multigene pathways. These pathways may be negative regulators of FtsZ or structural pathways that facilitate division by slightly defective FtsZ. Related suppressor strains were obtained for EcFtsZ containing certain point mutations or insertions of yellow fluorescent protein. The ability of highly divergent FtsZs to function for division in E. coli is consistent with a two-part mechanism. FtsZ assembles the Z ring, and perhaps generates the constriction force, through self interactions; the downstream division proteins remodel the peptidoglycan wall by interacting with each other and the wall. The C-terminal peptide of FtsZ, which binds FtsA, provides the link between FtsZ assembly and peptidoglycan remodeling.

A new FtsZ-interacting protein, YlmF, complements the activity of FtsA during progression of cell division in Bacillus subtilis

The assembly of ring-like structures, composed of FtsZ proteins (i.e. the Z ring), is the earliest and most essential process in bacterial cytokinesis. It has been shown that this process is directly regulated by the FtsZ-binding proteins, FtsA, ZapA, and EzrA, in Bacillus subtilis. In this study, protein complexes that are involved in Z-ring formation were chemically cross-linked in vivo, purified by affinity chromatography, and analysed by mass spectrometry. Analysis of the results identified YlmF as a new component of the FtsZ complex. Yeast two-hybrid analysis and fluorescence microscopy of YFP-YlmF in B. subtilis cells indicated YlmF localizes to the division site in an FtsZ-dependent manner. A single disruption of YlmF resulted in a slight elongation of cells; however, simultaneous inactivation of both YlmF and FtsA showed synthetic lethality caused by complete blockage of cell division due to the defect in Z-ring formation. In contrast, the ftsA-null mutant phenotype, caused by inefficient Z-ring formation, could be complemented by overexpression of YlmF. These results suggest that YlmF has an overlapping function with FtsA in stimulating the formation of Z rings in B. subtilis.

Premature targeting of cell division proteins to midcell reveals hierarchies of protein interactions involved in divisome assembly

In order to divide, the bacterium Escherichia coli must assemble a set of at least 10 essential proteins at the nascent division site. These proteins localize to midcell according to a linear hierarchy, suggesting that cell division proteins are added to the nascent divisome in strict sequence. We previously described a method, 'premature targeting', which allows us to target a protein directly to the division site independently of other cell division proteins normally required for its localization at midcell. By systematically applying this method to probe the recruitment of and associations among late cell division proteins, we show that this linear assembly model is likely incorrect. Rather, we find that the assembly of most of the late proteins can occur independently of 'upstream' proteins. Further, most late proteins, when prematurely targeted to midcell, can back-recruit upstream proteins in the reverse of the predicted pathway. Together these observations indicate that the late proteins, with the notable exception of the last protein in the pathway, FtsN, are associated in a hierarchical set of protein complexes. Based on these observations we present a revised model for assembly of the E. coli division apparatus.

Probing the domain structure of FtsZ by random truncation and insertion of GFP

Random transposon-mediated mutagenesis has been used to create truncations and insertions of green fluorescent protein (GFP), and Venus-yellow fluorescent protein (YFP), in Escherichia coli FtsZ. Sixteen unique insertions were obtained, and one of them, in the poorly conserved C-terminal spacer, was functional for cell division with the Venus-YFP insert. The insertion of enhanced GFP (eGFP) at this same site was not functional; Venus-YFP was found to be superior to eGFP in other respects too. Testing the constructs for dominant negative effects led to the following general conclusion. The N-terminal domain, aa 1-195, is an independently folding domain that can poison Z-ring function when expressed without a functional C-terminal domain. The effects were weak, requiring expression of the mutant at 3-5 times the level of wild-type FtsZ. The C-terminal domain, aa 195-383, was also independently folding, but had no activity in vivo. The differential activity of the N- and C-terminal domains suggests that FtsZ protofilament assembly is directional, with subunits adding primarily at the bottom of the protofilament. Directional assembly could occur by either a treadmilling or a dynamic instability mechanism.

Bacterial cell division: the mechanism and its precison

The recent development of cell biology techniques for bacteria to allow visualization of fundamental processes in time and space, and their use in synchronous populations of cells, has resulted in a dramatic increase in our understanding of cell division and its regulation in these tiny cells. The first stage of cell division is the formation of a Z ring, composed of a polymerized tubulin-like protein, FtsZ, at the division site precisely at midcell. Several membrane-associated division proteins are then recruited to this ring to form a complex, the divisome, which causes invagination of the cell envelope layers to form a division septum. The Z ring marks the future division site, and the timing of assembly and positioning of this structure are important in determining where and when division will take place in the cell. Z ring assembly is controlled by many factors including negative regulatory mechanisms such as Min and nucleoid occlusion that influence Z ring positioning and FtsZ accessory proteins that bind to FtsZ directly and modulate its polymerization behavior. The replication status of the cell also influences the positioning of the Z ring, which may allow the tight coordination between DNA replication and cell division required to produce two identical newborn cells.

Cell division in Bacillus subtilis: FtsZ and FtsA association is Z-ring independent, and FtsA is required for efficient midcell Z-Ring assembly

The earliest stage in cell division in bacteria is the assembly of a Z ring at the division site at midcell. Other division proteins are also recruited to this site to orchestrate the septation process. FtsA is a cytosolic division protein that interacts directly with FtsZ. Its function remains unknown. It is generally believed that FtsA localization to the division site occurs immediately after Z-ring formation or concomitantly with it and that FtsA is responsible for recruiting the later-assembling membrane-bound division proteins to the division site. Here, we report the development of an in vivo chemical cross-linking assay to examine the association between FtsZ and FtsA in Bacillus subtilis cells. We subsequently use this assay in a synchronous cell cycle to show that these two proteins can interact prior to Z-ring formation. We further show that in a B. subtilis strain containing an ftsA deletion, FtsZ localized at regular intervals along the filament but the majority of Z rings were abnormal. FtsA in this organism is therefore critical for the efficient formation of functional Z rings. This is the first report of abnormal Z-ring formation resulting from the loss of a single septation protein. These results suggest that in this organism, and perhaps others, FtsA ensures recruitment of the membrane-bound division proteins by ensuring correct formation of the Z ring.

Coordination of DNA replication and cell division in CyanobacteriaMicrocystis aeruginosa

Little is known about the cyanobacterial cell cycle. When either nalidixic acid or hydroxyurea was added to a synchronized culture of Microcystis aeruginosa to block DNA replication, cell division did not occur. Furthermore, transcription of the essential cell division gene, ftsZ was repressed. After DNA replication, ftsZ transcription, as well as cell division, was not affected by hydroxyl urea, suggesting that the DNA replication and cell division of M. aeruginosa are coordinated and that this coordination is partly controlled by ftsZ transcription depending on DNA replication.

Plastid division is mediated by combinatorial assembly of plastid division proteins

Plastids arise by division from pre-existing organelles, and with the recent characterization of several new components of plastid division our understanding of the division process in higher plants has improved dramatically. However, it is still not known how these different protein components act together during division. Here we analyse protein-protein interactions between all known stromal plastid division proteins. Using a combination of quantitative yeast two-hybrid assays, in planta co-localization studies, fluorescence resonance energy transfer and bimolecular fluorescence complementation assays we show that these proteins do not act in isolation but rather in protein complexes to govern appropriate plastid division. We have previously shown that AtMinD1 forms functional homodimers and we show here that in addition to homodimerization AtMinD1 also interacts with AtMinE1. Furthermore, AtMinE1 has the ability to homodimerize. We also demonstrate that proteins from both FtsZ families (AtFtsZ1-1 and AtFtsZ2-1) not only interact with themselves but also with each other, and we show that these interactions are not dependent on correct Z-ring formation. Further to this we demonstrate that ARC6 specifically interacts with the core domain of AtFtsZ2-1, but not with AtFtsZ1-1, providing in planta evidence for a functional difference between the two FtsZ protein families in plants. Our studies have enabled us to construct a meaningful intraplastidic protein-protein interaction map of all known stromal plastid division proteins in Arabidopsis.

Cell division in cocci: localization and properties of the Streptococcus pneumoniae FtsA protein

We studied the cytological and biochemical properties of the FtsA protein of Streptococcus pneumoniae. FtsA is a widespread bacterial cell division protein that belongs to the actin superfamily. In Escherichia coli and Bacillus subtilis, FtsA localizes to the septal ring after FtsZ, but its exact role in septation is not known. In S. pneumoniae, we found that, during exponential growth, the protein localizes to the nascent septa, at the equatorial zones of the dividing cells, where an average of 2200 FtsA molecules per cell are present. Likewise, FtsZ was found to localize with the same pattern and to be present at an average of 3000 molecules per cell. Consistent with the colocalization, FtsA was found to interact with FtsZ and with itself. Purified FtsA is able to bind several nucleotides, the affinity being highest for adenosine triphosphate (ATP), and lower for other triphosphates and diphosphates. The protein polymerizes in vitro, in a nucleotide-dependent manner, forming long corkscrew-like helixes, composed of 2 + 2 paired protofilaments. No nucleotide hydrolytic activity was detected. Consistent with the absence of an ATPase activity, the polymers are highly stable and not dynamic. These results suggest that the FtsA protein could also polymerize in vivo and the polymers participate in septation.

Tethering the Z ring to the membrane through a conserved membrane targeting sequence in FtsA

The cytokinetic Z ring is required for bacterial cell division. It consists of polymers of FtsZ, the bacterial ancestor of eukaryotic tubulin, linked to the cytoplasmic membrane. Formation of a Z ring in Escherichia coli occurs as long as one of two proteins, ZipA or FtsA, is present. Both of these proteins bind FtsZ suggesting that they might function to tether FtsZ filaments to the membrane. Although ZipA has a transmembrane domain and therefore can function as a membrane anchor, interaction of FtsA with the membrane has not been explored. In this study we demonstrate that FtsA, which is structurally related to eukaryotic actin, has a conserved C-terminal amphipathic helix that is essential for FtsA function. It is required to target FtsA to the membrane and subsequently to the Z ring. As FtsA is much more widely conserved in bacteria than ZipA, it is likely that FtsA serves as the principal membrane anchor for the Z ring.

Maturation of the Escherichia coli divisome occurs in two steps

Cell division proteins FtsZ (FtsA, ZipA, ZapA), FtsE/X, FtsK, FtsQ, FtsL/B, FtsW, PBP3, FtsN and AmiC localize at mid cell in Escherichia coli in an interdependent order as listed. To investigate whether this reflects a time dependent maturation of the divisome, the average cell age at which FtsZ, FtsQ, FtsW, PBP3 and FtsN arrive at their destination was determined by immuno- and GFP-fluorescence microscopy of steady state grown cells at a variety of growth rates. Consistently, a time delay of 14-21 min, depending on the growth rate, between Z-ring formation and the mid cell recruitment of proteins down stream of FtsK was found. We suggest a two-step model for bacterial division in which the Z-ring is involved in the switch from cylindrical to polar peptidoglycan synthesis, whereas the much later localizing cell division proteins are responsible for the modification of the envelope shape into that of two new poles.

Bacterial cell division and the septal ring

Cell division in bacteria is mediated by the septal ring, a collection of about a dozen (known) proteins that localize to the division site, where they direct assembly of the division septum. The foundation of the septal ring is a polymer of the tubulin-like protein FtsZ. Recently, experiments using fluorescence recovery after photobleaching have revealed that the Z ring is extremely dynamic. FtsZ subunits exchange in and out of the ring on a time scale of seconds even while the overall morphology of the ring appears static. These findings, together with in vitro studies of purified FtsZ, suggest that the rate-limiting step in turnover of FtsZ polymers is GTP hydrolysis. Another component of the septal ring, FtsK, is involved in coordinating chromosome segregation with cell division. Recent studies have revealed that FtsK is a DNA translocase that facilitates decatenation of sister chromosomes by TopIV and resolution of chromosome dimers by the XerCD recombinase. Finally, two murein hydrolases, AmiC and EnvC, have been shown to localize to the septal ring of Escherichia coli, where they play an important role in separation of daughter cells.

ftsZ mutations affecting cell division frequency, placement and morphology in Bacillus subtilis

A key event in cytokinesis in bacteria is the assembly of the essential division protein FtsZ into ring-like structures at the nascent division site. FtsZ is the prokaryotic homologue of tubulin, and is found in nearly all bacteria. In vitro, FtsZ polymerizes in the presence of GTP to form higher-ordered polymers. FtsZ consists of two domains, with the GTP-binding site located in the N-terminal domain. The less-conserved C-terminal domain contains residues important for GTP hydrolysis, but its overall function is still unclear. This paper reports the development of a simple strategy to generate mutations in the essential division gene ftsZ. Nine novel and viable ftsZ mutants of Bacillus subtilis are described. Eight of the mutations would affect the C-terminus of FtsZ. The collection of mutants exhibits a range of morphological phenotypes, ranging from normal to highly filamentous cells; some produce minicells, or divide in a twisted configuration; one mutation has a temperature-sensitive effect specifically impairing sporulation. The sites of the amino acid changes generated by the mutations could be informative about FtsZ function and its protein–protein interactions.

Interaction network among Escherichia coli membrane proteins involved in cell division as revealed by bacterial two-hybrid analysis

Formation of the Escherichia coli division septum is catalyzed by a number of essential proteins (named Fts) that assemble into a ring-like structure at the future division site. Several of these Fts proteins are intrinsic transmembrane proteins whose functions are largely unknown. Although these proteins appear to be recruited to the division site in a hierarchical order, the molecular interactions underlying the assembly of the cell division machinery remain mostly unspecified. In the present study, we used a bacterial two-hybrid system based on interaction-mediated reconstitution of a cyclic AMP (cAMP) signaling cascade to unravel the molecular basis of septum assembly by analyzing the protein interaction network among E. coli cell division proteins. Our results indicate that the Fts proteins are connected to one another through multiple interactions. A deletion mapping analysis carried out with two of these proteins, FtsQ and FtsI, revealed that different regions of the polypeptides are involved in their associations with their partners. Furthermore, we showed that the association between two Fts hybrid proteins could be modulated by the coexpression of a third Fts partner. Altogether, these data suggest that the cell division machinery assembly is driven by the cooperative association among the different Fts proteins to form a dynamic multiprotein structure at the septum site. In addition, our study shows that the cAMP-based two-hybrid system is particularly appropriate for analyzing molecular interactions between membrane proteins.

Premature targeting of a cell division protein to midcell allows dissection of divisome assembly in Escherichia coli

Cell division in Escherichia coli requires the recruitment of at least 10 essential proteins to the bacterial midcell. Recruitment of these proteins follows a largely linear dependency pathway in which depletion of one cell division protein leads to the absence from the division site of "downstream" proteins in the pathway. Analysis of events that underlie this pathway is complicated by the fact that a protein's ability to recruit "downstream" proteins is dependent on its own recruitment by "upstream" proteins. Hence, one cannot separate the individual contributions of various upstream proteins to any specific recruitment step. Here we present a method--premature targeting--for bypassing the normal localization requirements of a cell division protein and apply it to FtsQ, a protein recruited midway through the pathway. We fused FtsQ to the FtsZ-binding protein ZapA such that FtsQ was targeted to FtsZ rings independently of proteins FtsA and FtsK, which are normally required for FtsQ localization. Analysis of the resulting ZapA-FtsQ fusion suggests that FtsQ associates with a large complex of cell division proteins and that premature targeting of FtsQ can restore localization of this complex under conditions in which neither FtsQ nor the associated proteins would normally be localized.

New temperature-sensitive alleles of ftsZ in Escherichia coli

We isolated five new temperature-sensitive alleles of the essential cell division gene ftsZ in Escherichia coli, using P1-mediated, localized mutagenesis. The five resulting single amino acid changes (Gly109-->Ser109 for ftsZ6460, Ala129-->Thr129 for ftsZ972, Val157-->Met157 for ftsZ2066, Pro203-->Leu203 for ftsZ9124, and Ala239-->Val239 for ftsZ2863) are distributed throughout the FtsZ core region, and all confer a lethal cell division block at the nonpermissive temperature of 42 degrees C. In each case the division block is associated with loss of Z-ring formation such that fewer than 2% of cells show Z rings at 42 degrees C. The ftsZ9124 and ftsZ6460 mutations are of particular interest since both result in abnormal Z-ring formation at 30 degrees C and therefore cause significant defects in FtsZ polymerization, even at the permissive temperature. Neither purified FtsZ9124 nor purified FtsZ6460 exhibited polymerization when it was assayed by light scattering or electron microscopy, even in the presence of calcium or DEAE-dextran. Hence, both mutations also cause defects in FtsZ polymerization in vitro. Interestingly, FtsZ9124 has detectable GTPase activity, although the activity is significantly reduced compared to that of the wild-type FtsZ protein. We demonstrate here that unlike expression of ftsZ84, multicopy expression of the ftsZ6460, ftsZ972, and ftsZ9124 alleles does not complement the respective lethalities at the nonpermissive temperature. In addition, all five new mutant FtsZ proteins are stable at 42 degrees C. Therefore, the novel isolates carrying single ftsZ(Ts) point mutations, which are the only such strains obtained since isolation of the classical ftsZ84 mutation, offer significant opportunities for further genetic characterization of FtsZ and its role in cell division.

Cell cycle-dependent abundance, stability and localization of FtsA and FtsQ in Caulobacter crescentus

Coordination between cell division and DNA replication is ensured by checkpoints that act through proteins required for cell division. Following a block in DNA replication, transcription of the cell division progression genes ftsA and ftsQ is prevented in Caulobacter crescentus. One requirement for this checkpoint is that FtsA and/or FtsQ should be limiting for division in the next cell cycle. We show that the number of FtsA and FtsQ molecules fluctuates such that their concentration is low in swarmer and stalked cells, peaks in pre-divisional cells, and then dramatically decreases after cell division. Despite constitutive expression from an inducible promoter, FtsA and FtsQ levels still vary during the cell cycle, and the half-life of FtsA increases from 13 min in swarmer cells to 55 min in stalked cell types, confirming cell type-specific degradation. The post-division degradation of FtsA and FtsQ in swarmer cells reduces their concentration to 7% and 10% of their maximal level, respectively, strongly suggesting that de novo synthesis of both proteins is required for each division cycle. The localization of FtsA and FtsQ is also cell type-specific. FtsA and FtsQ are recruited to the midcell during a short period in late pre-divisional cells, consistent with the demonstrated requirement of FtsA for late stages of cell division. As previously reported for FtsZ, constitutive expression of FtsA causes cell division defects. These results indicate that the tight control of FtsA, and probably FtsQ, by cell cycle transcription, proteolysis, and localization are critical for optimal cell division and the coordination of cell division with the DNA replication cycle.

Two types of FtsZ proteins in mitochondria and red-lineage chloroplasts: the duplication of FtsZ is implicated in endosymbiosis

The ancestors of plastids and mitochondria were once free-living bacteria that became organelles as a result of endosymbiosis. According to this theory, a key bacterial division protein, FtsZ, plays a role in plastid division in algae and plants as well as in mitochondrial division in lower eukaryotes. Recent studies have shown that organelle division is a process that combines features derived from the bacterial division system with features contributed by host eukaryotic cells. Two nonredundant versions of FtsZ, FtsZ1 and FtsZ2, have been identified in green-lineage plastids, whereas most bacteria have a single ftsZ gene. To examine whether there is also more than one type of FtsZ in red-lineage chloroplasts (red algal chloroplasts and chloroplasts that originated from the secondary endosymbiosis of red algae) and in mitochondria, we obtained FtsZ sequences from the complete sequence of the primitive red alga Cyanidioschyzon merolae and the draft sequence of the stramenopile (heterokont) Thalassiosira pseudonana. Phylogenetic analyses that included known FtsZ proteins identified two types of chloroplast FtsZ in red algae (FtsZA and FtsZB) and stramenopiles (FtsZA and FtsZC). These analyses also showed that FtsZB emerged after the red and green lineages diverged, while FtsZC arose by the duplication of an ftsZA gene that in turn descended from a red alga engulfed by the ancestor of stramenopiles. A comparison of the predicted proteins showed that like bacterial FtsZ and green-lineage FtsZ2, FtsZA has a short conserved C-termmal sequence (the C-terminal core domain), whereas FtsZB and FtsZC, like the green-lineage FtsZ1, lack this sequence. In addition, the Cyanidioschyzon and Dictyostelium genomes encode two types of mitochondrial FtsZ proteins, one of which lacks the C-terminal variable domain. These results suggest that the acquisition of an additional FtsZ protein with a modified C terminus was common to the primary and secondary endosymbioses that produced plastids and that this also occurred during the establishment of mitochondria, presumably to regulate the multiplication of these organelles.

Murein (peptidoglycan) binding property of the essential cell division protein FtsN from Escherichia coli

The binding of the essential cell division protein FtsN of Escherichia coli to the murein (peptidoglycan) sacculus was studied. Soluble truncated variants of FtsN, including the complete periplasmic part of the protein as well as a variant containing only the C-terminal 77 amino acids, did bind to purified murein sacculi isolated from wild-type cells. FtsN variants lacking this C-terminal region showed reduced or no binding to murein. Binding of FtsN was severely reduced when tested against sacculi isolated either from filamentous cells with blocked cell division or from chain-forming cells of a triple amidase mutant. Binding experiments with radioactively labeled murein digestion products revealed that the longer murein glycan strands (>25 disaccharide units) showed a specific affinity to FtsN, but neither muropeptides, peptides, nor short glycan fragments bound to FtsN. In vivo FtsN could be cross-linked to murein with the soluble disulfide bridge containing cross-linker DTSSP. Less FtsN, but similar amounts of OmpA, was cross-linked to murein of filamentous or of chain-forming cells compared to levels in wild-type cells. Expression of truncated FtsN variants in cells depleted in full-length FtsN revealed that the presence of the C-terminal murein-binding domain was not required for cell division under laboratory conditions. FtsN was present in 3,000 to 6,000 copies per cell in exponentially growing wild-type E. coli MC1061. We discuss the possibilities that the binding of FtsN to murein during cell division might either stabilize the septal region or might have a function unrelated to cell division.

Structure of Mycobacterium tuberculosis FtsZ reveals unexpected, G protein-like conformational switches

We report three crystal structures of the Mycobacterium tuberculosis cell division protein FtsZ, as the citrate, GDP, and GTPgammaS complexes, determined at 1.89, 2.60, and 2.08A resolution. MtbFtsZ crystallized as a tight, laterally oriented dimer distinct from the longitudinal polymer observed for alphabeta-tubulin. Mutational data on Escherichia coli FtsZ suggest that this dimer interface is important for proper protofilament and "Z-ring" assembly and function. An alpha-to-beta secondary structure conformational switch at the dimer interface is spatially analogous to, and has many of the hallmarks of, the Switch I conformational changes exhibited by G-proteins upon activation. The presence of a gamma-phosphate in the FtsZ active site modulates the conformation of the "tubulin" loop T3 (spatially analogous to the G-protein Switch II); T3 switching upon gamma-phosphate ligation is directly coupled to the alpha-to-beta switch by steric overlap. The dual conformational switches observed here for the first time in an FtsZ link GTP binding and hydrolysis to FtsZ (and tubulin) lateral assembly and Z-ring contraction, and they are suggestive of an underappreciated functional analogy between FtsZ, tubulin and G-proteins.

Structural determinants required to target penicillin-binding protein 3 to the septum of Escherichia coli

In Escherichia coli, cell division is mediated by the concerted action of about 12 proteins that assemble at the division site to presumably form a complex called the divisome. Among these essential division proteins, the multimodular class B penicillin-binding protein 3 (PBP3), which is specifically involved in septal peptidoglycan synthesis, consists of a short intracellular M1-R23 peptide fused to a F24-L39 membrane anchor that is linked via a G40-S70 peptide to an R71-I236 noncatalytic module itself linked to a D237-V577 catalytic penicillin-binding module. On the basis of localization analyses of PBP3 mutants fused to green fluorescent protein by fluorescence microscopy, it appears that the first 56 amino acid residues of PBP3 containing the membrane anchor and the G40-E56 peptide contain the structural determinants required to target the protein to the cell division site and that none of the putative protein interaction sites present in the noncatalytic module are essential for the positioning of the protein to the division site. Based on the effects of increasing production of FtsQ or FtsW on the division of cells expressing PBP3 mutants, it is suggested that these proteins could interact. We postulate that FtsQ could play a role in regulating the assembly of these division proteins at the division site and the activity of the peptidoglycan assembly machineries within the divisome.

The crystal structure of ZapA and its modulation of FtsZ polymerisation

FtsZ is part of a mid-cell cytokinetic structure termed the Z-ring that recruits a hierarchy of fission related proteins early in the bacterial cell cycle. The widely conserved ZapA has been shown to interact with FtsZ, to drive its polymerisation and to promote FtsZ filament bundling thereby contributing to the spatio-temporal tuning of the Z-ring. Here, we show the crystal structure of ZapA (11.6 kDa) from Pseudomonas aeruginosa at 2.8 A resolution. The electron density reveals two dimers associating via an extensive C-terminal coiled-coil protrusion to form an elongated anti-parallel tetramer. In solution, ZapA exists in a dimer-tetramer equilibrium that is strongly correlated with concentration. An increase in concentration promotes formation of the higher oligomeric state. The dimer is postulated to be the predominant physiological species although the tetramer could become significant if, as FtsZ is integrated into the Z-ring and is cross-linked, the local concentration of the dimer becomes sufficiently high. We also show that ZapA binds FtsZ with an approximate 1:1 molar stoichiometry and that this interaction provokes dramatic FtsZ polymerisation and inter-filament association as well as yielding filaments, single or bundled, more stable and resistant to collapse. Whilst in vitro dynamics of FtsZ are well characterised, its in vivo arrangement within the ultra-structural architecture of the Z-ring is yet to be determined despite being fundamental to cell division. The ZapA dimer has single 2-fold symmetry whilst the bipolar tetramer displays triple 2-fold symmetry. Given the symmetry of these ZapA oligomers and the polar nature of FtsZ filaments, the structure of ZapA carries novel implications for the inherent architecture of the Z-ring in vivo.

Role of two essential domains of Escherichia coli FtsA in localization and progression of the division ring

The FtsA protein is a member of the actin superfamily that localizes to the bacterial septal ring during cell division. Deletions of domain 1C or the S12 and S13 beta-strands in domain 2B of the Escherichia coli FtsA, previously postulated to be involved in dimerization, result in partially active proteins that do not allow the normal progression of septation. The truncated FtsA protein lacking domain 1C (FtsADelta1C) localizes in correctly placed division rings, together with FtsZ and ZipA, but does not interact with other FtsA molecules in the yeast two-hybrid assay, and fails to recruit FtsQ and FtsN into the division ring. The rings containing FtsADelta1C are therefore incomplete and do not support division. The production of high levels of FtsADelta1C causes filamentation, an effect that has been reported to result as well from the imbalance between FtsA+ and FtsZ+ molecules. These data indicate that the domain 1C of FtsA participates in the interaction of the protein with other FtsA molecules and with the other proteins that are incorporated at later stages of ring assembly, and is not involved in the interaction with FtsZ and the localization of FtsA to the septal ring. The deletion of the S12-S13 strands of domain 2B generates a protein (FtsADeltaS12-13) that retains the ability to interact with FtsA+. When the mutated protein is expressed at wild-type levels, it localizes into division rings and recruits FtsQ and FtsN, but it fails to sustain septation at normal levels resulting in filamentation. A fivefold overexpression of FtsADeltaS12-13 produces short cells that have normal division rings, but also cells with polar localization of the mutated protein, and cells with rings at abnormal positions that result in the production of a fraction (15%) of small nucleoid-free cells. The S12-S13 strands of domain 2B are not essential for septation, but affect the localization of the division ring.