Rapid in vitro assembly dynamics and subunit turnover of FtsZ demonstrated by fluorescence resonance energy transfer

Filamentous temperature sensitive mutant Z: a putative target to combat antibacterial resistance

In the pre-antibiotic era, common bacterial infections accounted for high mortality and morbidity. Moreover, the discovery of penicillin in 1928 marked the beginning of an antibiotic revolution, and this antibiotic era witnessed the discovery of many novel antibiotics, a golden era. However, the misuse or overuse of these antibiotics, natural resistance that existed even before the antibiotics were discovered, genetic variations in bacteria, natural selection, and acquisition of resistance from one species to another consistently increased the resistance to the existing antibacterial targets. Antibacterial resistance (ABR) is now becoming an ever-increasing concern jeopardizing global health. Henceforth, there is an urgent unmet need to discover novel compounds to combat ABR, which act through untapped pathways/mechanisms. Filamentous Temperature Sensitive mutant Z (FtsZ) is one such unique target, a tubulin homolog involved in developing a cytoskeletal framework for the cytokinetic ring. Additionally, its pivotal role in bacterial cell division and the lack of homologous structural protein in mammals makes it a potential antibacterial target for developing novel molecules. Approximately 2176 X-crystal structures of FtsZ were available, which initiated the research efforts to develop novel antibacterial agents. The literature has reported several natural, semisynthetic, peptides, and synthetic molecules as FtsZ inhibitors. This review provides valuable insights into the basic crystal structure of FtsZ, its inhibitors, and their inhibitory activities. This review also describes the available in vitro detection and quantification methods of FtsZ-drug complexes and the various approaches for determining drugs targeting FtsZ polymerization.

Understanding Supramolecular Assembly of Supercharged Proteins

Ordered supramolecular assemblies have recently been created using electrostatic interactions between oppositely charged proteins. Despite recent progress, the fundamental mechanisms governing the assembly of oppositely supercharged proteins are not fully understood. Here, we use a combination of experiments and computational modeling to systematically study the supramolecular assembly process for a series of oppositely supercharged green fluorescent protein variants. We show that net charge is a sufficient molecular descriptor to predict the interaction fate of oppositely charged proteins under a given set of solution conditions (e.g., ionic strength), but the assembled supramolecular structures critically depend on surface charge distributions. Interestingly, our results show that a large excess of charge is necessary to nucleate assembly and that charged residues not directly involved in interprotein interactions contribute to a substantial fraction (∼30%) of the interaction energy between oppositely charged proteins via long-range electrostatic interactions. Dynamic subunit exchange experiments further show that relatively small, 16-subunit assemblies of oppositely charged proteins have kinetic lifetimes on the order of ∼10-40 min, which is governed by protein composition and solution conditions. Broadly, our results inform how protein supercharging can be used to create different ordered supramolecular assemblies from a single parent protein building block.

Dendrimersome Synthetic Cells Harbor Cell Division Machinery of Bacteria

The integration of active cell machinery with synthetic building blocks is the bridge toward developing synthetic cells with biological functions and beyond. Self-replication is one of the most important tasks of living systems, and various complex machineries exist to execute it. In Escherichia coli, a contractile division ring is positioned to mid-cell by concentration oscillations of self-organizing proteins (MinCDE), where it severs membrane and cell wall. So far, the reconstitution of any cell division machinery has exclusively been tied to liposomes. Here, the reconstitution of a rudimentary bacterial divisome in fully synthetic bicomponent dendrimersomes is shown. By tuning the membrane composition, the interaction of biological machinery with synthetic membranes can be tailored to reproduce its dynamic behavior. This constitutes an important breakthrough in the assembly of synthetic cells with biological elements, as tuning of membrane-divisome interactions is the key to engineering emergent biological behavior from the bottom-up.

How Does the Spatial Confinement of FtsZ to a Membrane Surface Affect Its Polymerization Properties and Function?

FtsZ is the cytoskeletal protein that organizes the formation of the septal ring and orchestrates bacterial cell division. Its association to the membrane is essential for its function. In this mini-review I will address the question of how this association can interfere with the structure and dynamic properties of the filaments and argue that its dynamics could also remodel the underlying lipid membrane through its activity. Thus, lipid rearrangement might need to be considered when trying to understand FtsZ’s function. This new element could help understand how FtsZ assembly coordinates positioning and recruitment of the proteins forming the septal ring inside the cell with the activity of the machinery involved in peptidoglycan synthesis located in the periplasmic space.

How Do MinC-D Copolymers Act on Z-Ring Localization Regulation? A New Model of Bacillus subtilis Min System

Division site selection in rod-shaped bacteria is strictly regulated spatially by the Min system. Although many sophisticated studies, including in vitro recombination, have tried to explain these regulations, the precise mechanisms are still unclear. A previous model suggested that the concentration gradient of MinC, an FtsZ inhibitor, regulates the position of the Z-ring in the cell. In Escherichia coli, the oscillation of MinCDE proteins leads to a gradient of Min proteins with the average concentration being lowest in the middle and highest near the poles. In contrast to the Min system of E. coli, the Min system of Bacillus subtilis lacks MinE and exhibits a stable concentration distribution, which is regulated by the binding of DivIVA to the negative curvature membrane. The Min proteins first accumulate at the poles of the cell and relocalize near the division site when the membrane invagination begins. It is inconsistent with the previous model of high concentrations of MinC inhibiting Z-ring formation. Our preliminary data here using electron microscopy and light scattering technology reported that B. subtilis MinC (BsMinC) and MinD (BsMinD) also assembled into large straight copolymers in the presence of ATP, similar to the Min proteins of E. coli. Their assembly is fast and dominated by MinD concentration. When BsMinD is 5 μM, a clear light scattering signal can be observed even at 0.3 μM BsMinC. Here, we propose a new model based on the MinC-D copolymers. In our hypothesis, it is not the concentration gradient of MinC, but the MinC-D copolymer assembled in the region of high concentration MinD that plays a key role in the regulation of Z-ring positioning. In B. subtilis, the regions with high MinD concentration are initially at both ends of the cell and then appear at midcell when cell division began. MinC-D copolymer will polymerize and form a complex with MinJ and DivIVA. These complexes capture FtsZ protofilaments to prevent their diffusion away from the midcell and narrow the Z-ring in the middle of the cell.

Localization, Assembly, and Activation of the Escherichia coli Cell Division Machinery

Decades of research, much of it in Escherichia coli, have yielded a wealth of insight into bacterial cell division. Here, we provide an overview of the E. coli division machinery with an emphasis on recent findings. We begin with a short historical perspective into the discovery of FtsZ, the tubulin homolog that is essential for division in bacteria and archaea. We then discuss assembly of the divisome, an FtsZ-dependent multiprotein platform, at the midcell septal site. Not simply a scaffold, the dynamic properties of polymeric FtsZ ensure the efficient and uniform synthesis of septal peptidoglycan. Next, we describe the remodeling of the cell wall, invagination of the cell envelope, and disassembly of the division apparatus culminating in scission of the mother cell into two daughter cells. We conclude this review by highlighting some of the open questions in the cell division field, emphasizing that much remains to be discovered, even in an organism as extensively studied as E. coli.

A Novel Z-Ring Associated Protein ZapA-Like Protein (PA5407) From Pseudomonas aeruginosa Promotes FtsZ to Form Double Filaments

Bacterial cell division is initiated by the assembly of the contraction ring (Z-ring), which consists of the self-assembled FtsZ protofilaments and dozens of other associate proteins. ZapA, a regulatory protein found in almost all bacteria, stabilizes FtsZ protofilaments to form bundles and enhances the Z-ring condensation. Here, we reported that another small protein from Pseudomonas aeruginosa, ZapA-Like protein (ZapAL; PA5407), is a new FtsZ associated protein. ZapAL exists in many Pseudomonas species and shares only 20% sequence identity to ZapA. ZapAL interacts with FtsZ and induces FtsZ to form long straight double filaments; in comparison, ZapA promotes long bundles with multiple FtsZ filaments. ZapAL has only a mild effect on GTPase activity of FtsZ, which is reduced by around 26% when 10 μM ZapAL is added in the solution. However, to study their assembly dynamics using light-scattering assay, we found that FtsZ-ZapAL double filament is stable and no depolymerization process is observed, which is different from ZapA. Further research found that ZapA and ZapL are likely to form heterodimers. The bundles formed by the mixture of FtsZ-ZapA-ZapAL will depolymerize after GTP is hydrolyzed. Consistent with ZapAL interaction with FtsZ in vitro, the expression of ZapAL-GFP was observed as a narrow band or spots in the middle of the cells, suggesting that it is a component of bacterial division machinery. Similar to ZapA, ZapAL is also not essential for bacterial cell division. Little changes were observed when zapAL gene was deleted, or overexpressed under normal conditions; however, overexpression of ZapAL caused zapA-deficient cells to grow approximately two times longer, showing a mild bacterial division defect. Although we still do not know the exact physiological roles of ZapAL, our results suggest that ZapAL is a novel Z-ring associate protein, which may work together with ZapA to stabilize the FtsZ protofilament and Z-ring structure.

The Arabidopsis thaliana chloroplast division protein FtsZ1 counterbalances FtsZ2 filament stability in vitro

Bacterial cell and chloroplast division are driven by a contractile “Z ring” composed of the tubulin-like cytoskeletal GTPase FtsZ. Unlike bacterial Z rings, which consist of a single FtsZ, the chloroplast Z ring in plants is composed of two FtsZ proteins, FtsZ1 and FtsZ2. Both are required for chloroplast division in vivo, but their biochemical relationship is poorly understood. We used GTPase assays, light scattering, transmission electron microscopy, and sedimentation assays to investigate the assembly behavior of purified Arabidopsis thaliana (At) FtsZ1 and AtFtsZ2 both individually and together. Both proteins exhibited GTPase activity. AtFtsZ2 assembled relatively quickly, forming protofilament bundles that were exceptionally stable, as indicated by their sustained assembly and slow disassembly. AtFtsZ1 did not form detectable protofilaments on its own. When mixed with AtFtsZ2, AtFtsZ1 reduced the extent and rate of AtFtsZ2 assembly, consistent with its previously demonstrated ability to promote protofilament subunit turnover in living cells. Mixing the two FtsZ proteins did not increase the overall GTPase activity, indicating that the effect of AtFtsZ1 on AtFtsZ2 assembly was not due to a stimulation of GTPase activity. However, the GTPase activity of AtFtsZ1 was required to reduce AtFtsZ2 assembly. Truncated forms of AtFtsZ1 and AtFtsZ2 consisting of only their conserved core regions largely recapitulated the behaviors of the full-length proteins. Our in vitro findings provide evidence that FtsZ1 counterbalances the stability of FtsZ2 filaments in the regulation of chloroplast Z-ring dynamics and suggest that restraining FtsZ2 self-assembly is a critical function of FtsZ1 in chloroplasts.

FtsZ Interactions and Biomolecular Condensates as Potential Targets for New Antibiotics

FtsZ is an essential and central protein for cell division in most bacteria. Because of its ability to organize into dynamic polymers at the cell membrane and recruit other protein partners to form a “divisome”, FtsZ is a leading target in the quest for new antibacterial compounds. Strategies to potentially arrest the essential and tightly regulated cell division process include perturbing FtsZ’s ability to interact with itself and other divisome proteins. Here, we discuss the available methodologies to screen for and characterize those interactions. In addition to assays that measure protein-ligand interactions in solution, we also discuss the use of minimal membrane systems and cell-like compartments to better approximate the native bacterial cell environment and hence provide a more accurate assessment of a candidate compound’s potential in vivo effect. We particularly focus on ways to measure and inhibit under-explored interactions between FtsZ and partner proteins. Finally, we discuss recent evidence that FtsZ forms biomolecular condensates in vitro, and the potential implications of these assemblies in bacterial resistance to antibiotic treatment.

Assembly properties of bacterial tubulin homolog FtsZ regulated by the positive regulator protein ZipA and ZapA from Pseudomonas aeruginosa

Bacterial tubulin homolog FtsZ self-assembles into dynamic protofilaments, which forms the scaffold for the contractile ring (Z-ring) to achieve bacterial cell division. Here, we study the biochemical properties of FtsZ from Pseudomonas aeruginosa (PaFtsZ) and the effects of its two positive regulator proteins, ZipA and ZapA. Similar to Escherichia coli FtsZ, PaFtsZ had a strong GTPase activity, ~ 7.8 GTP min^-1 FtsZ^-1 at pH 7.5, and assembled into mainly short single filaments in vitro. However, PaFtsZ protofilaments were mixtures of straight and “intermediate-curved” (100–300 nm diameter) in pH 7.5 solution and formed some bundles in pH 6.5 solution. The effects of ZipA on PaFtsZ assembly varied with pH. In pH 6.5 buffer ZipA induced PaFtsZ to form large bundles. In pH 7.5 buffer PaFtsZ-ZipA protofilaments were not bundled, but ZipA enhanced PaFtsZ assembly and promoted more curved filaments. Comparable to ZapA from other bacterial species, ZapA from P. aeruginosa induced PaFtsZ protofilaments to associate into long straight loose bundles and/or sheets at both pH 6.5 and pH 7.5, which had little effect on the GTPase activity of PaFtsZ. These results provide us further information that ZipA functions as an enhancer of FtsZ curved filaments, while ZapA works as a stabilizer of FtsZ straight filaments.

A key bacterial cytoskeletal cell division protein FtsZ as a novel therapeutic antibacterial drug target

Nowadays, the emergence of multidrug-resistant bacterial strains initiates the urgent need for the elucidation of the new drug targets for the discovery of antimicrobial drugs. Filamenting temperature-sensitive mutant Z (FtsZ), a eukaryotic tubulin homolog, is a GTP-dependent prokaryotic cytoskeletal protein and is conserved among most bacterial strains. In vitro studies revealed that FtsZ self-assembles into dynamic protofilaments or bundles and forms a dynamic Z-ring at the center of the cell in vivo, leading to septation and consequent cell division. Speculations on the ability of FtsZ in the blockage of cell division make FtsZ a highly attractive target for developing novel antibiotics. Researchers have been working on synthetic molecules and natural products as inhibitors of FtsZ. Accumulating data suggest that FtsZ may provide the platform for the development of novel antibiotics. In this review, we summarize recent advances in the properties of FtsZ protein and bacterial cell division, as well as in the development of FtsZ inhibitors.

A Unified Model for Treadmilling and Nucleation of Single-Stranded FtsZ Protofilaments

Bacterial cell division is tightly coupled to the dynamic behavior of FtsZ, a tubulin homolog. Recent experimental work in vitro and in vivo has attributed FtsZ's assembly dynamics to treadmilling, in which subunits add to the bottom and dissociate from the top of protofilaments. However, the molecular mechanisms producing treadmilling have yet to be characterized and quantified. We have developed a Monte Carlo model for FtsZ assembly that explains treadmilling, and also explains assembly nucleation by the same mechanisms. A key element of the model is a conformational change from R (relaxed), which is highly favored for monomers, to T (tense), which is favored for subunits in a protofilament. This model was created in MATLAB. Kinetic parameters were converted to probabilities of execution during a single, small time step. These were used to stochastically determine FtsZ dynamics. Our model is able to accurately describe the results of several in vitro and in vivo studies for a variety of FtsZ flavors. With standard conditions, the model FtsZ polymerized and produced protofilaments that treadmilled at 23 nm/s, hydrolyzed GTP at 3.6-3.7 GTP min^-1 FtsZ^-1, and had an average length of 30-40 subunits, all similar to experimental results. Adding a bottom capper resulted in shorter protofilaments and higher GTPase, similar to the effect of the known bottom capper protein MciZ. The model could match nucleation kinetics of several flavors of FtsZ using the same parameters as treadmilling and varying only the R to T transition of monomers.

A stochastic model for dynamics of FtsZ filaments and the formation of Z -ring

Understanding the mechanisms responsible for the formation and growth of FtsZ polymers and their subsequent formation of the Z -ring is important for gaining insight into the cell division in prokaryotic cells. In this work, we present a minimal stochastic model that qualitatively reproduces in vitro observations of polymerization, formation of dynamic contractile ring that is stable for a long time and depolymerization shown by FtsZ polymer filaments. In this stochastic model, we explore different mechanisms for ring breaking and hydrolysis. In addition to hydrolysis, which is known to regulate the dynamics of other tubulin polymers like microtubules, we find that the presence of the ring allows for an additional mechanism for regulating the dynamics of FtsZ polymers. Ring breaking dynamics in the presence of hydrolysis naturally induce rescue and catastrophe events in this model irrespective of the mechanism of hydrolysis.

The speed of FtsZ treadmilling is tightly regulated by membrane binding

As one of the key elements in bacterial cell division, the cytoskeletal protein FtsZ appears to be highly involved in circumferential treadmilling along the inner membrane, yielding circular vortices when transferred to flat membranes. However, it remains unclear how a membrane-targeted protein can produce these dynamics. Here, we dissect the roles of membrane binding, GTPase activity, and the unstructured C-terminal linker on the treadmilling of a chimera FtsZ protein through in vitro reconstitution of different FtsZ-YFP-mts variants on supported membranes. In summary, our results suggest substantial robustness of dynamic vortex formation, where only significant mutations, resulting in abolished membrane binding or compromised lateral interactions, are detrimental for the generation of treadmilling rings. In addition to GTPase activity, which directly affects treadmilling dynamics, we found a striking correlation of membrane binding with treadmilling speed as a result of changing the MTS on our chimera proteins. This discovery leads to the hypothesis that the in vivo existence of two alternative tether proteins for FtsZ could be a mechanism for controlling FtsZ treadmilling.

Insights into the Structure, Function, and Dynamics of the Bacterial Cytokinetic FtsZ-Ring

The FtsZ protein is a highly conserved bacterial tubulin homolog. In vivo, the functional form of FtsZ is the polymeric, ring-like structure (Z-ring) assembled at the future division site during cell division. While it is clear that the Z-ring plays an essential role in orchestrating cytokinesis, precisely what its functions are and how these functions are achieved remain elusive. In this article, we review what we have learned during the past decade about the Z-ring's structure, function, and dynamics, with a particular focus on insights generated by recent high-resolution imaging and single-molecule analyses. We suggest that the major function of the Z-ring is to govern nascent cell pole morphogenesis by directing the spatiotemporal distribution of septal cell wall remodeling enzymes through the Z-ring's GTP hydrolysis-dependent treadmilling dynamics. In this role, FtsZ functions in cell division as the counterpart of the cell shape-determining actin homolog MreB in cell elongation.

Dissecting the Functional Contributions of the Intrinsically Disordered C-terminal Tail of Bacillus subtilis FtsZ

FtsZ is a bacterial GTPase that is central to the spatial and temporal control of cell division. It is a filament-forming enzyme that encompasses a well-folded core domain and a disordered C-terminal tail (CTT). The CTT is essential for ensuring proper assembly of the cytokinetic ring, and its deletion leads to mis-localization of FtsZ, aberrant assembly, and cell death. In this work, we dissect the contributions of modules within the disordered CTT to assembly and enzymatic activity of Bacillus subtilis FtsZ (Bs-FtsZ). The CTT features a hypervariable C-terminal linker (CTL) and a conserved C-terminal peptide (CTP). Our in vitro studies show that the CTL weakens the driving forces for forming single-stranded active polymers and suppresses lateral associations of these polymers, whereas the CTP promotes the formation of alternative assemblies. Accordingly, in full-length Bs-FtsZ, the CTL acts as a spacer that spatially separates the CTP sticker from the core, thus ensuring filament formation through core-driven polymerization and lateral associations through CTP-mediated interactions. We also find that the CTL weakens GTP binding while enhancing the catalytic rate, whereas the CTP has opposite effects. The joint contributions of the CTL and CTP make Bs-FtsZ, an enzyme that is only half as efficient as a truncated version that lacks the CTT. Overall, our data suggest that the CTT acts as an auto-regulator of Bs-FtsZ assembly and as an auto-inhibitor of enzymatic activity. Based on our results, we propose hypotheses regarding the hypervariability of CTLs and compare FtsZs to other bacterial proteins with tethered IDRs.

Transient Membrane-Linked FtsZ Assemblies Precede Z-Ring Formation in Escherichia coli

During the early stages of cytokinesis, FtsZ protofilaments form a ring-like structure, the Z-ring, in most bacterial species. This cytoskeletal scaffold recruits downstream proteins essential for septal cell wall synthesis. Despite progress in understanding the dynamic nature of the Z-ring and its role in coordinating septal cell wall synthesis, the early stages of protofilament formation and subsequent assembly into the Z-ring are still not understood. Here we investigate a sequence of assembly steps that lead to the formation of the Z-ring in Escherichia coli using high temporal and spatial resolution imaging. Our data show that formation of the Z-ring is preceded by transient membrane-linked FtsZ assemblies. These assemblies form after attachment of short cytosolic protofilaments, which we estimate to be less than 20 monomers long, to the membrane. The attachments occur at random locations along the length of the cell. The filaments treadmill and show periods of rapid growth and shrinkage. Their dynamic properties imply that protofilaments are bundled in these assemblies. Furthermore, we establish that the size of assemblies is sensitively controlled by the availability of FtsZ molecules and by the presence of ZapA proteins. The latter has been implicated in cross-linking the protofilaments. The likely function of these dynamic FtsZ assemblies is to sample the cell surface for the proper location for the Z-ring.

Comparative in silico analysis of ftsZ gene from different bacteria reveals the preference for core set of codons in coding sequence structuring and secondary structural elements determination

The deluge of sequence information in the recent times provide us with an excellent opportunity to compare organisms on a large genomic scale. In this study we have tried to decipher the variation in the gene organization and structuring of a vital bacterial gene called ftsZ which codes for an integral component of the bacterial cell division, the FtsZ protein. FtsZ is homologous to tubulin protein and has been found to be ubiquitous in eubacteria. FtsZ is showing increasing promise as a target for antibacterial drug discovery. Our study of ftsZ protein from 143 different bacterial species spanning a wider range of morphological and physiological type demonstrates that the ftsZ gene of about ninety three percent of the organisms show relatively biased codon usage profile and significant GC deviation from their genomic GC content. Comparative codon usage analysis of ftsZ and a core housekeeping gene rpoB demonstrated that codon usage pattern of ftsZ CDS is shaped by natural selection to a large extent and mimics that of a housekeeping gene. We have also detected a tendency among the different organisms to utilize a core set of codons in structuring the ftsZ coding sequence. We observed that the compositional frequency of the amino acid serine in the FtsZ protein appears to be a indicator of the bacterial lifestyle. Our meticulous analysis of the ftsZ gene linked with the corresponding FtsZ protein show that there is a bias towards the use of specific synonymous codons particularly in the helix and strand regions of the multi-domain FtsZ protein. Overall our findings suggest that in an indispensable and vital protein such as FtsZ, there is an inherent tendency to maintain form for optimized performance in spite of the extrinsic variability in coding features.

Quantification of Predictive Uncertainty in Models of FtsZ ring assembly in Escherichia coli

Quantitative predictions of FtsZ protein polymerization are essential for understanding the self-regulating mechanisms in biochemical systems. Due to structural complexity and parametric uncertainty, existing kinetic models remain incomplete and their predictions error-prone. To address such challenges, we perform probabilistic uncertainty quantification and global sensitivity analysis of the concentrations of various protein species predicted with a recent FtsZ protein polymerization model. Our results yield a ranked list of modeling shortcomings that can be improved in order to develop more accurate predictions and more realistic representations of key mechanisms of such biochemical systems and their response to changes in internal or external conditions. Our conclusions and improvement recommendations can be extended to other kinetics models.

Assembly properties of the bacterial tubulin homolog FtsZ from the cyanobacterium Synechocystis sp. PCC 6803

The tubulin homolog FtsZ is the major cytoskeletal protein in the bacterial cell division machinery, conserved in almost all bacteria, archaea, and chloroplasts. Bacterial FtsZ assembles spontaneously into single protofilaments, sheets, and bundles in vitro, and it also accumulates at the site of division early during cell division, where it forms a dynamic protein complex, the contractile ring or Z-ring. The biochemical properties of FtsZ proteins from many bacteria have been studied, but comparable insights into FtsZs from cyanobacteria are limited. Here, using EM and light-scattering assays, we studied the biochemical and assembly properties of SyFtsZ, the FtsZ protein from the cyanobacterial strain Synechocystis sp. PCC 6803. SyFtsZ had a slow GTPase activity of ∼0.4 GTP/FtsZ molecule/min and assembled into thick, straight protofilament bundles and curved bundles, designated toroids. The assembly of SyFtsZ in the presence of GTP occurred in two stages. The first stage consisted of the assembly of single-stranded straight protofilaments and opened circles; in the second stage, the protofilaments associated into straight protofilament bundles and toroids. In addition to these assemblies, we also observed highly curved oligomers and minirings after GTP hydrolysis or in the presence of excess GDP. The three types of protofilaments of SyFtsZ observed here provide support for the hypothesis that a constriction force due to curved protofilaments bends the membrane. In summary, our findings indicate that, unlike other bacterial FtsZ, SyFtsZ assembles into thick protofilament bundles. This bundling is similar to that of chloroplast FtsZ, consistent with its origin in cyanobacteria.

Surface Orientation and Binding Strength Modulate Shape of FtsZ on Lipid Surfaces

We have used a simple model system to test the prediction that surface attachment strength of filaments presenting a torsion would affect their shape and properties. FtsZ from E. coli containing one cysteine in position 2 was covalently attached to a lipid bilayer containing maleimide lipids either in their head group (to simulate tight attachment) or at the end of a polyethylene glycol molecule attached to the head group (to simulate loose binding). We found that filaments tightly attached grew straight, growing from both ends, until they formed a two-dimensional lattice. Further monomer additions to their sides generated a dense layer of oriented filaments that fully covered the lipid membrane. After this point the surface became unstable and the bilayer detached from the surface. Filaments with a loose binding were initially curved and later evolved into straight thicker bundles that destabilized the membrane after reaching a certain surface density. Previously described theoretical models of FtsZ filament assembly on surfaces that include lateral interactions, spontaneous curvature, torsion, anchoring to the membrane, relative geometry of the surface and the filament ‘living-polymer’ condition in the presence of guanosine triphosphate (GTP) can offer some clues about the driving forces inducing these filament rearrangements.

Bacterial cell division: modeling FtsZ assembly and force generation from single filament experimental data

The bacterial cytoskeletal protein FtsZ binds and hydrolyzes GTP, self-aggregates into dynamic filaments and guides the assembly of the septal ring on the inner side of the membrane at midcell. This ring constricts the cell during division and is present in most bacteria. Despite exhaustive studies undertaken in the last 25 years after its discovery, we do not yet know the mechanism by which this GTP-dependent self-aggregating protein exerts force on the underlying membrane. This paper reviews recent experiments and theoretical models proposed to explain FtsZ filament dynamic assembly and force generation. It highlights how recent observations of single filaments on reconstituted model systems and computational modeling are contributing to develop new multiscale models that stress the importance of previously overlooked elements as monomer internal flexibility, filament twist and flexible anchoring to the cell membrane. These elements contribute to understand the rich behavior of these GTP consuming dynamic filaments on surfaces. The aim of this review is 2-fold: (1) to summarize recent multiscale models and their implications to understand the molecular mechanism of FtsZ assembly and force generation and (2) to update theoreticians with recent experimental results.

Species- and C-terminal linker-dependent variations in the dynamic behavior of FtsZ on membranes in vitro

Bacterial cell division requires the assembly of FtsZ protofilaments into a dynamic structure called the 'Z-ring'. The Z-ring recruits the division machinery and directs local cell wall remodeling for constriction. The organization and dynamics of protofilaments within the Z-ring coordinate local cell wall synthesis during cell constriction, but their regulation is largely unknown. The disordered C-terminal linker (CTL) region of Caulobacter crescentus FtsZ (CcFtsZ) regulates polymer structure and turnover in solution in vitro, and regulates Z-ring structure and activity of cell wall enzymes in vivo. To investigate the contributions of the CTL to the polymerization properties of FtsZ on its physiological platform, the cell membrane, we reconstituted CcFtsZ polymerization on supported lipid bilayers (SLB) and visualized polymer dynamics and structure using total internal reflection fluorescence microscopy. Unlike Escherichia coli FtsZ protofilaments that organized into large, bundled patterns, CcFtsZ protofilaments assembled into small, dynamic clusters on SLBs. Moreover, CcFtsZ lacking its CTL formed large networks of straight filament bundles that underwent slower turnover than the dynamic clusters of wildtype FtsZ. Our in vitro characterization provides novel insights into species- and CTL-dependent differences between FtsZ assembly properties that are relevant to Z-ring assembly and function on membranes in vivo.

Design of Antibacterial Agents: Alkyl Dihydroxybenzoates against Xanthomonas citri subsp. citri

Xanthomonas citri subsp. citri (Xcc) causes citrus canker, affecting sweet orange-producing areas around the world. The current chemical treatment available for this disease is based on cupric compounds. For this reason, the objective of this study was to design antibacterial agents. In order to do this, we analyzed the anti-Xcc activity of 36 alkyl dihydroxybenzoates and we found 14 active compounds. Among them, three esters with the lowest minimum inhibitory concentration values were selected; compounds 4 (52 μM), 16 (80 μM) and 28 (88 μM). Our study demonstrated that alkyl dihydroxybenzoates cause a delay in the exponential phase. The permeability capacity of alkyl dihydroxybenzoates in a quarter of MIC was compared to nisin (positive control). Compound 28 was the most effective (93.8), compared to compound 16 (41.3) and compound 4 (13.9) by percentage values. Finally, all three compounds showed inhibition of FtsZ GTPase activity, and promoted changes in protofilaments, leading to depolymerization, which prevents bacterial cell division. In conclusion, heptyl dihydroxybenzoates (compounds 4, 16 and 28) are promising anti-Xcc agents which may serve as an alternative for the control of citrus canker.

FtsZ filaments have the opposite kinetic polarity of microtubules

FtsZ is the ancestral homolog of tubulin and assembles into the Z ring that organizes the division machinery to drive cell division in most bacteria. In contrast to tubulin that assembles into 13 stranded microtubules that undergo dynamic instability, FtsZ assembles into single-stranded filaments that treadmill to distribute the peptidoglycan synthetic machinery at the septum. Here, using longitudinal interface mutants of FtsZ, we demonstrate that the kinetic polarity of FtsZ filaments is opposite to that of microtubules. A conformational switch accompanying the assembly of FtsZ generates the kinetic polarity of FtsZ filaments, which explains the toxicity of interface mutants that function as a capper and reveals the mechanism of cooperative assembly. This approach can also be employed to determine the kinetic polarity of other filament-forming proteins.

Lateral interactions between protofilaments of the bacterial tubulin homolog FtsZ are essential for cell division

The prokaryotic tubulin homolog FtsZ polymerizes into protofilaments, which further assemble into higher-order structures at future division sites to form the Z-ring, a dynamic structure essential for bacterial cell division. The precise nature of interactions between FtsZ protofilaments that organize the Z-ring and their physiological significance remain enigmatic. In this study, we solved two crystallographic structures of a pair of FtsZ protofilaments, and demonstrated that they assemble in an antiparallel manner through the formation of two different inter-protofilament lateral interfaces. Our in vivo photocrosslinking studies confirmed that such lateral interactions occur in living cells, and disruption of the lateral interactions rendered cells unable to divide. The inherently weak lateral interactions enable FtsZ protofilaments to self-organize into a dynamic Z-ring. These results have fundamental implications for our understanding of bacterial cell division and for developing antibiotics that target this key process.

Treadmilling analysis reveals new insights into dynamic FtsZ ring architecture

FtsZ, the primary protein of the bacterial Z ring guiding cell division, has been recently shown to engage in intriguing treadmilling dynamics along the circumference of the division plane. When coreconstituted in vitro with FtsA, one of its natural membrane anchors, on flat supported membranes, these proteins assemble into dynamic chiral vortices compatible with treadmilling of curved polar filaments. Replacing FtsA by a membrane-targeting sequence (mts) to FtsZ, we have discovered conditions for the formation of dynamic rings, showing that the phenomenon is intrinsic to FtsZ. Ring formation is only observed for a narrow range of protein concentrations at the bilayer, which is highly modulated by free Mg²⁺ and depends upon guanosine triphosphate (GTP) hydrolysis. Interestingly, the direction of rotation can be reversed by switching the mts from the C-terminus to the N-terminus of the protein, implying that the filament attachment must have a perpendicular component to both curvature and polarity. Remarkably, this chirality switch concurs with previously shown inward or outward membrane deformations by the respective FtsZ mutants. Our results lead us to suggest an intrinsic helicity of FtsZ filaments with more than one direction of curvature, supporting earlier hypotheses and experimental evidence.

FtsZ is a tubulin homologue and the primary protein of the bacterial Z ring that guides cell division. In vivo, but also in reconstituted systems, FtsZ shows an intriguing treadmilling dynamic along circular tracks of approximately 1 micrometer in diameter. In cells, this treadmilling along the circumference of the division site is suggested to dynamically guide peptidoglycan—and thus new cell wall—synthesis. In vitro, when reconstituted along with its membrane adaptor FtsA on flat supported membranes, FtsZ self-organizes into similarly treadmilling vortices as observed in vivo but with a clear chirality. With the aim of thoroughly investigating these dynamics, revealing the origin of chirality, and potentially relating it to a membrane-transforming ability of FtsZ, we reconstituted different membrane-targeted mutants of FtsZ on flat membranes. In this minimized system, we found that dynamic ring formation is an intrinsic feature of FtsZ without the need of any other protein. However, self-organization into dynamic treadmilling only occurs within a specific protein, cation, and guanosine triphosphate (GTP) concentration range. Our work led us to propose that the observed chirality of FtsZ treadmilling may be explained by an inherent helical character of the filaments with more than one direction of curvature.

The chloroplast division protein ARC6 acts to inhibit disassembly of GDP-bound FtsZ2

Chloroplasts host photosynthesis and fulfill other metabolic functions that are essential to plant life. They have to divide by binary fission to maintain their numbers throughout cycles of cell division. Chloroplast division is achieved by a complex ring-shaped division machinery located on both the inner (stromal) and the outer (cytosolic) side of the chloroplast envelope. The inner division ring (termed the Z ring) is formed by the assembly of tubulin-like FtsZ1 and FtsZ2 proteins. ARC6 is a key chloroplast division protein that interacts with the Z ring. ARC6 spans the inner envelope membrane, is known to stabilize or maintain the Z ring, and anchors the Z ring to the inner membrane through interaction with FtsZ2. The underlying mechanism of Z ring stabilization is not well-understood. Here, biochemical and structural characterization of ARC6 was conducted using light scattering, sedimentation, and light and transmission EM. The recombinant protein was purified as a dimer. The results indicated that a truncated form of ARC6 (tARC6), representing the stromal portion of ARC6, affects FtsZ2 assembly without forming higher-order structures and exerts its effect via FtsZ2 dynamics. tARC6 prevented GDP-induced FtsZ2 disassembly and caused a significant net increase in FtsZ2 assembly when GDP was present. Single particle analysis and 3D reconstruction were performed to elucidate the structural basis of ARC6 activity. Together, the data reveal that a dimeric form of tARC6 binds to FtsZ2 filaments and does not increase FtsZ polymerization rates but rather inhibits GDP-associated FtsZ2 disassembly.

Efficient models of polymerization applied to FtsZ ring assembly in Escherichia coli

High protein concentrations complicate modeling of polymer assembly kinetics by introducing structural complexity and a large variety of protein forms. We present a modeling approach that achieves orders of magnitude speed-up by replacing distributions of lengths and widths with their average counterparts and by introducing a hierarchical classification of species and reactions into sets. We have used this model to study FtsZ ring assembly in Escherichia coli The model's prediction of key features of the ring formation, such as time to reach the steady state, total concentration of FtsZ species in the ring, total concentration of monomers, and average dimensions of filaments and bundles, are all in agreement with the experimentally observed values. Besides validating our model against the in vivo observations, this study fills some knowledge gaps by proposing a specific structure of the ring, describing the influence of the total concentration in short and long kinetics processes, determining some characteristic mechanisms in polymer assembly regulation, and providing insights about the role of ZapA proteins, critical components for both positioning and stability of the ring.

De Novo Synthesis of Basal Bacterial Cell Division Proteins FtsZ, FtsA, and ZipA Inside Giant Vesicles

Cell division is the most dynamic event in the cell cycle. Recently, efforts have been made to reconstruct it using the individual component proteins to obtain a better understanding of the process of self-reproduction of cells. However, such reconstruction studies are frequently hampered by difficulties in preparing membrane-associated proteins. Here we demonstrate a de novo synthesis approach based on a cell-free translation system. Genes for fundamental cell division proteins, FtsZ, FtsA, and ZipA, were expressed inside the lipid compartment of giant vesicles (GVs). The synthesized proteins showed polymerization, membrane localization, and eventually membrane deformation. Notably, we found that this morphological change of the vesicle is forced by only FtsZ and ZipA, which form clusters on the membrane at the vesicle interior. Our cell-free approach provides a platform for studying protein dynamics associated with lipid membrane and paves the way to create a synthetic cell that undergoes self-reproduction.

Imaging Bacterial Cell Wall Biosynthesis

Peptidoglycan is an essential component of the cell wall that protects bacteria from environmental stress. A carefully coordinated biosynthesis of peptidoglycan during cell elongation and division is required for cell viability. This biosynthesis involves sophisticated enzyme machineries that dynamically synthesize, remodel, and degrade peptidoglycan. However, when and where bacteria build peptidoglycan, and how this is coordinated with cell growth, have been long-standing questions in the field. The improvement of microscopy techniques has provided powerful approaches to study peptidoglycan biosynthesis with high spatiotemporal resolution. Recent development of molecular probes further accelerated the growth of the field, which has advanced our knowledge of peptidoglycan biosynthesis dynamics and mechanisms. Here, we review the technologies for imaging the bacterial cell wall and its biosynthesis activity. We focus on the applications of fluorescent d-amino acids, a newly developed type of probe, to visualize and study peptidoglycan synthesis and dynamics, and we provide direction for prospective research.

Exploration of cell division times during bacterial cytokinesis

Filamenting temperature-sensitive mutant Z (FtsZ), an essential cell division protein in bacteria, has recently emerged as an important and exploitable antibacterial target. The perturbation of FtsZ assembly is found to have an effect on cell cytokinesis and cell survival. Cell division time is an important physical parameter in cell cytokinesis. Here, the theoretical framework that has been developed by combining a phase field model for rod-shaped cells with a kinetic description for FtsZ ring maintenance is extended to explore cell division times during bacterial cytokinesis. The cell division times of around 72 s in the numerical studies have the same magnitude as the division time of several minutes observed physiologically. The dependence of the cell division time on parameters such as the initial state of rod-shaped cells and various kinetic rates of FtsZ assembly dynamics is thoroughly investigated. The theoretical analysis of the relations between the cell division time and these parameters is found to coincide well with the numerical calculated results.

Treadmilling by FtsZ filaments drives peptidoglycan synthesis and bacterial cell division

The mechanism by which bacteria divide is not well understood. Cell division is mediated by filaments of FtsZ and FtsA (FtsAZ) that recruit septal peptidoglycan-synthesizing enzymes to the division site. To understand how these components coordinate to divide cells, we visualized their movements relative to the dynamics of cell wall synthesis during cytokinesis. We found that the division septum was built at discrete sites that moved around the division plane. FtsAZ filaments treadmilled circumferentially around the division ring and drove the motions of the peptidoglycan-synthesizing enzymes. The FtsZ treadmilling rate controlled both the rate of peptidoglycan synthesis and cell division. Thus, FtsZ treadmilling guides the progressive insertion of new cell wall by building increasingly smaller concentric rings of peptidoglycan to divide the cell.

ZipA and FtsA* stabilize FtsZ-GDP miniring structures

The cytokinetic division ring of Escherichia coli comprises filaments of FtsZ tethered to the membrane by FtsA and ZipA. Previous results suggested that ZipA is a Z-ring stabilizer, since in vitro experiments it is shown that ZipA enhanced FtsZ assembly and caused the filaments to bundles. However, this function of ZipA has been challenged by recent studies. First, ZipA-induced FtsZ bundling was not significant at pH greater than 7. Second, some FtsA mutants, such as FtsA* were able to bypass the need of ZipA. We reinvestigated the interaction of FtsZ with ZipA in vitro. We found that ZipA not only stabilized and bundled straight filaments of FtsZ-GTP, but also stabilized the highly curved filaments and miniring structures formed by FtsZ-GDP. FtsA* had a similar stabilization of FtsZ-GDP minirings. Our results suggest that ZipA and FtsA* may contribute to constriction by stabilizing this miniring conformation.

Efficient Multiscale Models of Polymer Assembly

Protein polymerization and bundling play a central role in cell physiology. Predictive modeling of these processes remains an open challenge, especially when the proteins involved become large and their concentrations high. We present an effective kinetics model of filament formation, bundling, and depolymerization after GTP hydrolysis, which involves a relatively small number of species and reactions, and remains robust over a wide range of concentrations and timescales. We apply this general model to study assembly of FtsZ protein, a basic element in the division process of prokaryotic cells such as Escherichia coli, Bacillus subtilis, or Caulobacter crescentus. This analysis demonstrates that our model outperforms its counterparts in terms of both accuracy and computational efficiency. Because our model comprises only 17 ordinary differential equations, its computational cost is orders-of-magnitude smaller than the current alternatives consisting of up to 1000 ordinary differential equations. It also provides, to our knowledge, a new insight into the characteristics and functioning of FtsZ proteins at high concentrations. The simplicity and versatility of our model render it a powerful computational tool, which can be used either as a standalone descriptor of other biopolymers' assembly or as a component in more complete kinetic models.

The Chloroplast Tubulin Homologs FtsZA and FtsZB from the Red Alga Galdieria sulphuraria Co-assemble into Dynamic Filaments

FtsZ is a homolog of eukaryotic tubulin and is present in almost all bacteria and many archaea, where it is the major cytoskeletal protein in the Z ring, required for cell division. Unlike some other cell organelles of prokaryotic origin, chloroplasts have retained FtsZ as an essential component of the division machinery. However, chloroplast FtsZs have been challenging to study because they are difficult to express and purify. To this end, we have used a FATT tag expression system to produce as soluble proteins the two chloroplast FtsZs from Galdieria sulphuraria, a thermophilic red alga. GsFtsZA and GsFtsZB assembled individually in the presence of GTP, forming large bundles of protofilaments. GsFtsZA also assembled in the presence of GDP, the first member of the FtsZ/tubulin superfamily to do so. Mixtures of GsFtsZA and GsFtsZB assembled protofilament bundles and hydrolyzed GTP at a rate approximately equal to the sum of their individual rates, suggesting a random co-assembly. GsFtsZA assembly by itself in limiting GTP gave polymers that remained stable for a prolonged time. However, when GsFtsZB was added, the co-polymers disassembled with enhanced kinetics, suggesting that the GsFtsZB regulates and enhances disassembly dynamics. GsFtsZA-mts (where mts is a membrane-targeting amphipathic helix) formed Z ring-like helices when expressed in Escherichia coli Co-expression of GsFtsZB (without an mts) gave co-assembly of both into similar helices. In summary, we provide biochemical evidence that GsFtsZA assembles as the primary scaffold of the chloroplast Z ring and that GsFtsZB co-assembly enhances polymer disassembly and dynamics.

FtsZ Constriction Force - Curved Protofilaments Bending Membranes

FtsZ assembles in vitro into protofilaments (pfs) that are one subunit thick and ~50 subunits long. In vivo these pfs assemble further into the Z ring, which, along with accessory division proteins, constricts to divide the cell. We have reconstituted Z rings in liposomes in vitro, using pure FtsZ that was modified with a membrane targeting sequence to directly bind the membrane. This FtsZ-mts assembled Z rings and constricted the liposomes without any accessory proteins. We proposed that the force for constriction was generated by a conformational change from straight to curved pfs. Evidence supporting this mechanism came from switching the membrane tether to the opposite side of the pf. These switched-tether pfs assembled "inside-out" Z rings, and squeezed the liposomes from the outside, as expected for the bending model. We propose three steps for the full process of cytokinesis: (a) pf bending generates a constriction force on the inner membrane, but the rigid peptidoglycan wall initially prevents any invagination; (b) downstream proteins associate to the Z ring and remodel the peptidoglycan, permitting it to follow the constricting FtsZ to a diameter of ~250 nm; the final steps of closure of the septum and membrane fusion are achieved by excess membrane synthesis and membrane fluctuations.

E. coli Cell Cycle Machinery

Cytokinesis in E. coli is organized by a cytoskeletal element designated the Z ring. The Z ring is formed at midcell by the coalescence of FtsZ filaments tethered to the membrane by interaction of FtsZ's conserved C-terminal peptide (CCTP) with two membrane-associated proteins, FtsA and ZipA. Although interaction between an FtsZ monomer and either of these proteins is of low affinity, high affinity is achieved through avidity - polymerization linked CCTPs interacting with the membrane tethers. The placement of the Z ring at midcell is ensured by antagonists of FtsZ polymerization that are positioned within the cell and target FtsZ filaments through the CCTP. The placement of the ring is reinforced by a protein network that extends from the terminus (Ter) region of the chromosome to the Z ring. Once the Z ring is established, additional proteins are recruited through interaction with FtsA, to form the divisome. The assembled divisome is then activated by FtsN to carry out septal peptidoglycan synthesis, with a dynamic Z ring serving as a guide for septum formation. As the septum forms, the cell wall is split by spatially regulated hydrolases and the outer membrane invaginates in step with the aid of a transenvelope complex to yield progeny cells.

Structure and Mutational Analyses of Escherichia coli ZapD Reveal Charged Residues Involved in FtsZ Filament Bundling

Bacterial cell division is an essential and highly coordinated process. It requires the polymerization of the tubulin homologue FtsZ to form a dynamic ring (Z-ring) at midcell. Z-ring formation relies on a group of FtsZ-associated proteins (Zap) for stability throughout the process of division. In Escherichia coli, there are currently five Zap proteins (ZapA through ZapE), of which four (ZapA, ZapB, ZapC, and ZapD) are small soluble proteins that act to bind and bundle FtsZ filaments. In particular, ZapD forms a functional dimer and interacts with the C-terminal tail of FtsZ, but little is known about its structure and mechanism of action. Here, we present the crystal structure of Escherichia coli ZapD and show it forms a symmetrical dimer with centrally located α-helices flanked by β-sheet domains. Based on the structure of ZapD and its chemical cross-linking to FtsZ, we targeted nine charged ZapD residues for modification by site-directed mutagenesis. Using in vitro FtsZ sedimentation assays, we show that residues R56, R221, and R225 are important for bundling FtsZ filaments, while transmission electron microscopy revealed that altering these residues results in different FtsZ bundle morphology compared to those of filaments bundled with wild-type ZapD. ZapD residue R116 also showed altered FtsZ bundle morphology but levels of FtsZ bundling similar to that of wild-type ZapD. Together, these results reveal that ZapD residues R116, R221, and R225 likely participate in forming a positively charged binding pocket that is critical for bundling FtsZ filaments.

IMPORTANCE

Z-ring assembly underpins the formation of the essential cell division complex known as the divisome and is required for recruitment of downstream cell division proteins. ZapD is one of several proteins in E. coli that associates with the Z-ring to promote FtsZ bundling and aids in the overall fitness of the division process. In the present study, we describe the dimeric structure of E. coli ZapD and identify residues that are critical for FtsZ bundling. Together, these results advance our understanding about the formation and dynamics of the Z-ring prior to bacterial cell division.

Functional Analysis of the Chloroplast Division Complex Using Schizosaccharomyces pombe as a Heterologous Expression System

Chloroplast division is driven by a macromolecular complex that assembles at the midplastid. The FtsZ ring (Z ring) is the central structure in this complex, and is composed of the functionally distinct cytoskeletal proteins FtsZ1 and FtsZ2. Recent studies in the heterologous Schizosaccharomyces pombe system showed that Arabidopsis FtsZ1 and FtsZ2 filaments have distinct assembly and turnover characteristics. To further analyze these FtsZs, we employed this system to compare the assembly and dynamic properties of FtsZ1 and FtsZ2 lacking their N- and/or C-termini with those of their full-length counterparts. Our data provide evidence that the N-terminus of FtsZ2 is critical for its structural dominance over FtsZ1, and that the N- and C-termini promote polymer bundling and turnover of both FtsZs and contribute to their distinct behaviors. We also assessed how ARC6 affects FtsZ2 filament dynamics, and found that it interacts with and stabilizes FtsZ2 filaments in S. pombe independent of its presumed Z-ring tethering function in planta. Finally, we generated FtsZ1-FtsZ2 coexpression constructs to facilitate reconstitution of more complex interaction networks. Our experiments yield new insight into factors influencing FtsZ behavior and highlight the utility of S. pombe for analyzing chloroplast FtsZs and their assembly regulators.

NDK Interacts with FtsZ and Converts GDP to GTP to Trigger FtsZ Polymerisation--A Novel Role for NDK

Introduction

Nucleoside diphosphate kinase (NDK), conserved across bacteria to humans, synthesises NTP from NDP and ATP. The eukaryotic homologue, the NDPK, uses ATP to phosphorylate the tubulin-bound GDP to GTP for tubulin polymerisation. The bacterial cytokinetic protein FtsZ, which is the tubulin homologue, also uses GTP for polymerisation. Therefore, we examined whether NDK can interact with FtsZ to convert FtsZ-bound GDP and/or free GDP to GTP to trigger FtsZ polymerisation.

Methods

Recombinant and native NDK and FtsZ proteins of Mycobacterium smegmatis and Mycobacterium tuberculosis were used as the experimental samples. FtsZ polymersation was monitored using 90° light scattering and FtsZ polymer pelleting assays. The γ³²P-GTP synthesised by NDK from GDP and γ³²P-ATP was detected using thin layer chromatography and quantitated using phosphorimager. The FtsZ bound ³²P-GTP was quantitated using phosphorimager, after UV-crosslinking, followed by SDS-PAGE. The NDK-FtsZ interaction was determined using Ni²⁺-NTA-pulldown assay and co-immunoprecipitation of the recombinant and native proteins in vitro and ex vivo, respectively.

Results

NDK triggered instantaneous polymerisation of GDP-precharged recombinant FtsZ in the presence of ATP, similar to the polymerisation of recombinant FtsZ (not GDP-precharged) upon the direct addition of GTP. Similarly, NDK triggered polymerisation of recombinant FtsZ (not GDP-precharged) in the presence of free GDP and ATP as well. Mutant NDK, partially deficient in GTP synthesis from ATP and GDP, triggered low level of polymerisation of MsFtsZ, but not of MtFtsZ. As characteristic of NDK’s NTP substrate non-specificity, it used CTP, TTP, and UTP also to convert GDP to GTP, to trigger FtsZ polymerisation. The NDK of one mycobacterial species could trigger the polymerisation of the FtsZ of another mycobacterial species. Both the recombinant and the native NDK and FtsZ showed interaction with each other in vitro and ex vivo, alluding to the possibility of direct phosphorylation of FtsZ-bound GDP by NDK.

Conclusion

Irrespective of the bacterial species, NDK interacts with FtsZ in vitro and ex vivo and, through the synthesis of GTP from FtsZ-bound GDP and/or free GDP, and ATP (CTP/TTP/UTP), triggers FtsZ polymerisation. The possible biological context of this novel activity of NDK is presented.

Mutations in the bacterial cell division protein FtsZ highlight the role of GTP binding and longitudinal subunit interactions in assembly and function

Background

Assembly of the tubulin-like GTPase, FtsZ, at the future division site initiates the process of bacterial cytokinesis. The FtsZ ring serves as a platform for assembly of the division machinery and constricts at the leading edge of the invaginating septum during cytokinesis. In vitro, FtsZ assembles in a GTP-dependent manner, forming straight filaments that curve upon GTP hydrolysis. FtsZ binds but cannot hydrolyze GTP as a monomer. Instead, the active site for GTP hydrolysis is formed at the monomer-monomer interface upon dimerization. While the dynamics of GTP hydrolysis and assembly have been extensively studied in vitro, significantly less is known about the role of GTP binding and hydrolysis in vivo. ftsZ84, a GTPase defective allele of Escherichia coli ftsZ, provides a striking example of the disconnect between in vivo and in vitro FtsZ assembly.

Results

Although ftsZ84 mutants are defective for FtsZ ring formation and division under nonpermissive conditions, they are near wild type for ring formation and division under permissive conditions. In vitro, however, purified FtsZ84 is defective in GTP binding, hydrolysis and assembly under standard reaction conditions. To clarify the nature of the FtsZ84 assembly defect, we isolated and characterized three intragenic suppressors of ftsZ84. All three suppressor mutations increased the apparent affinity of FtsZ84 for GTP, consistent with improved subunit-subunit interactions along the longitudinal interface. Although kinetic analysis indicates that the suppressor mutations increase the affinity of FtsZ84 for GTP, all three exhibit reduced rates of GTP hydrolysis and fail to support assembly in vitro.

Conclusion

Together, our data suggest that FtsZ, and potentially other enzymes whose assembly is similarly regulated, can compensate for defects in catalysis through increases in substrate binding and subunit-subunit interactions. In addition, these results highlight the dichotomy between commonly used in vitro assembly conditions and FtsZ ring formation in the complex intracellular milieu.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0544-z) contains supplementary material, which is available to authorized users.

The Cell Division Protein FtsZ from Streptococcus pneumoniae Exhibits a GTPase Activity Delay

The cell division protein FtsZ assembles in vitro by a mechanism of cooperative association dependent on GTP, monovalent cations, and Mg(2+). We have analyzed the GTPase activity and assembly dynamics of Streptococcus pneumoniae FtsZ (SpnFtsZ). SpnFtsZ assembled in an apparently cooperative process, with a higher critical concentration than values reported for other FtsZ proteins. It sedimented in the presence of GTP as a high molecular mass polymer with a well defined size and tended to form double-stranded filaments in electron microscope preparations. GTPase activity depended on K(+) and Mg(2+) and was inhibited by Na(+). GTP hydrolysis exhibited a delay that included a lag phase followed by a GTP hydrolysis activation step, until reaction reached the GTPase rate. The lag phase was not found in polymer assembly, suggesting a transition from an initial non-GTP-hydrolyzing polymer that switches to a GTP-hydrolyzing polymer, supporting models that explain FtsZ polymer cooperativity.

Remodeling of the Z-Ring Nanostructure during the Streptococcus pneumoniae Cell Cycle Revealed by Photoactivated Localization Microscopy

Ovococci form a morphological group that includes several human pathogens (enterococci and streptococci). Their shape results from two modes of cell wall insertion, one allowing division and one allowing elongation. Both cell wall synthesis modes rely on a single cytoskeletal protein, FtsZ. Despite the central role of FtsZ in ovococci, a detailed view of the in vivo nanostructure of ovococcal Z-rings has been lacking thus far, limiting our understanding of their assembly and architecture. We have developed the use of photoactivated localization microscopy (PALM) in the ovococcus human pathogen Streptococcus pneumoniae by engineering spDendra2, a photoconvertible fluorescent protein optimized for this bacterium. Labeling of endogenously expressed FtsZ with spDendra2 revealed the remodeling of the Z-ring’s morphology during the division cycle at the nanoscale level. We show that changes in the ring’s axial thickness and in the clustering propensity of FtsZ correlate with the advancement of the cell cycle. In addition, we observe double-ring substructures suggestive of short-lived intermediates that may form upon initiation of septal cell wall synthesis. These data are integrated into a model describing the architecture and the remodeling of the Z-ring during the cell cycle of ovococci.

The Gram-positive human pathogen S. pneumoniae is responsible for 1.6 million deaths per year worldwide and is increasingly resistant to various antibiotics. FtsZ is a cytoskeletal protein polymerizing at midcell into a ring-like structure called the Z-ring. FtsZ is a promising new antimicrobial target, as its inhibition leads to cell death. A precise view of the Z-ring architecture in vivo is essential to understand the mode of action of inhibitory drugs (see T. den Blaauwen, J. M. Andreu, and O. Monasterio, Bioorg Chem 55:27–38, 2014, doi:10.1016/j.bioorg.2014.03.007, for a review on FtsZ inhibitors). This is notably true in ovococcoid bacteria like S. pneumoniae, in which FtsZ is the only known cytoskeletal protein. We have used superresolution microscopy to obtain molecular details of the pneumococcus Z-ring that have so far been inaccessible with conventional microscopy. This study provides a nanoscale description of the Z-ring architecture and remodeling during the division of ovococci.

Cell age dependent concentration of Escherichia coli divisome proteins analyzed with ImageJ and ObjectJ

The rod-shaped Gram-negative bacterium Escherichia coli multiplies by elongation followed by binary fission. Longitudinal growth of the cell envelope and synthesis of the new poles are organized by two protein complexes called elongasome and divisome, respectively. We have analyzed the spatio-temporal localization patterns of many of these morphogenetic proteins by immunolabeling the wild type strain MC4100 grown to steady state in minimal glucose medium at 28°C. This allowed the direct comparison of morphogenetic protein localization patterns as a function of cell age as imaged by phase contrast and fluorescence wide field microscopy. Under steady state conditions the age distribution of the cells is constant and is directly correlated to cell length. To quantify cell size and protein localization parameters in 1000s of labeled cells, we developed ‘Coli-Inspector,’ which is a project running under ImageJ with the plugin ‘ObjectJ.’ ObjectJ organizes image-analysis tasks using an integrated approach with the flexibility to produce different output formats from existing markers such as intensity data and geometrical parameters. ObjectJ supports the combination of automatic and interactive methods giving the user complete control over the method of image analysis and data collection, with visual inspection tools for quick elimination of artifacts. Coli-inspector was used to sort the cells according to division cycle cell age and to analyze the spatio-temporal localization pattern of each protein. A unique dataset has been created on the concentration and position of the proteins during the cell cycle. We show for the first time that a subset of morphogenetic proteins have a constant cellular concentration during the cell division cycle whereas another set exhibits a cell division cycle dependent concentration variation. Using the number of proteins present at midcell, the stoichiometry of the divisome is discussed.