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

The divisome is a self-enhancing machine in Escherichia coli and Caulobacter crescentus

During bacterial cytokinesis, polymers of the bacterial tubulin FtsZ coalesce into the Z ring to orchestrate divisome assembly and septal cell wall synthesis. Previous studies have found that Z ring condensation and stability is critical for successful cell division. However, how FtsZ filaments condense into a Z ring remains enigmatic and whether septal cell wall synthesis can feedback to the Z ring has not been investigated. Here, we show that FtsZ-associated proteins (Zaps) play important roles in Z ring condensation and stability, and discover septal cell wall synthesis as a novel player for Z ring condensation and stabilization in Escherichia coli and Caulobacter crescentus. Moreover, we find that the interaction between the Z ring membrane anchor, FtsA, and components of the septal cell wall synthetic complex are critical for septal cell wall synthesis-mediated Z ring condensation. Altogether, these findings suggest that the divisome is a self-enhancing machine in these two gram-negative bacteria, where the Z ring and the septal cell wall synthetic complex communicate with and reinforce each other to ensure robustness of cell division.

Unveiling the antibacterial mechanism of resveratrol against Aeromonas hydrophila through proteomics analysis

This investigation delves into elucidating the mechanism by which resveratrol (Res), a natural polyterpenoid renowned for its antimicrobial properties, exerts its effects on Aeromonas hydrophila, a ubiquitous waterborne pathogen. Our findings underscore the dose-dependent manifestation of resveratrol in exhibiting antibacterial and antibiofilm formation activities against A. hydrophila. Employing a Data-independent acquisition (DIA) based quantitative proteomics methodology, we systematically compared differentially expressed proteins in A. hydrophila subjected to varying concentrations of Res. Subsequent bioinformatics analyses revealed key proteins and pathways pivotal in resveratrol's antimicrobial action, encompassing oxidative stress, energy metabolism, and cell membrane integrity. Validation of the proteomics outcomes was meticulously conducted using the qPCR method at the mRNA level. Dynamic trend analysis unveiled alterations in biological processes, notably the correlation between the cell division-related protein ZapC and resveratrol content. Furthermore, scanning electron microscopy corroborated a significant elongation of A. hydrophila cells, affirming resveratrol's capability to inhibit cell division. In concert, resveratrol emerges as a participant in the cell membrane integrity pathway, biofilm formation, and potentially, the regulation of genes associated with cell division, resulting in morphological elongation. These revelations position resveratrol as a promising natural alternative to conventional antibiotics for treating A. hydrophila infections.

A dynamic duo: Understanding the roles of FtsZ and FtsA for Escherichia coli cell division through in vitro approaches

Bacteria divide by binary fission. The protein machine responsible for this process is the divisome, a transient assembly of more than 30 proteins in and on the surface of the cytoplasmic membrane. Together, they constrict the cell envelope and remodel the peptidoglycan layer to eventually split the cell into two. For Escherichia coli, most molecular players involved in this process have probably been identified, but obtaining the quantitative information needed for a mechanistic understanding can often not be achieved from experiments in vivo alone. Since the discovery of the Z-ring more than 30 years ago, in vitro reconstitution experiments have been crucial to shed light on molecular processes normally hidden in the complex environment of the living cell. In this review, we summarize how rebuilding the divisome from purified components - or at least parts of it - have been instrumental to obtain the detailed mechanistic understanding of the bacterial cell division machinery that we have today.

The divergent early divisome: is there a functional core?

The bacterial divisome is a complex nanomachine that drives cell division and separation. The essentiality of these processes leads to the assumption that proteins with core roles will be strictly conserved across all bacterial genomes. However, recent studies in diverse proteobacteria have revealed considerable variation in the early divisome compared with Escherichia coli. While some proteins are highly conserved, their specific functions and interacting partners vary. Meanwhile, different subphyla use clade-specific proteins with analogous functions. Thus, instead of focusing on gene conservation, we must also explore how key functions are maintained during early division by diverging protein networks. An enhanced awareness of these complex genetic networks will clarify the physical and evolutionary constraints of bacterial division.

Building the Bacterial Divisome at the Septum

Across living organisms, division is necessary for cell survival and passing heritable information to the next generation. For this reason, cell division is highly conserved among eukaryotes and prokaryotes. Among the most highly conserved cell division proteins in eukaryotes are tubulin and actin. Tubulin polymerizes to form microtubules, which assemble into cytoskeletal structures in eukaryotes, such as the mitotic spindle that pulls chromatids apart during mitosis. Actin polymerizes to form a morphological framework for the eukaryotic cell, or cytoskeleton, that undergoes reorganization during mitosis. In prokaryotes, two of the most highly conserved cell division proteins are the tubulin homolog FtsZ and the actin homolog FtsA. In this chapter, the functions of the essential bacterial cell division proteins FtsZ and FtsA and their roles in assembly of the divisome at the septum, the site of cell division, will be discussed. In most bacteria, including Escherichia coli, the tubulin homolog FtsZ polymerizes at midcell, and this step is crucial for recruitment of many other proteins to the division site. For this reason, both FtsZ abundance and polymerization are tightly regulated by a variety of proteins. The actin-like FtsA protein polymerizes and tethers FtsZ polymers to the cytoplasmic membrane. Additionally, FtsA interacts with later stage cell division proteins, which are essential for division and for building the new cell wall at the septum. Recent studies have investigated how actin-like polymerization of FtsA on the lipid membrane may impact division, and we will discuss this and other ways that division in bacteria is regulated through FtsZ and FtsA.

The cell division protein ZapE is targeted by the antibiotic aztreonam to induce cell filamentation in Escherichia coli

Bacterial division is mediated by a protein complex called the Z-ring, and Z-ring associated protein E (ZapE) is a Z-ring-associated protein that acts as its negative regulator. In the present study, we show that treatment of Escherichia coli with the antibiotic aztreonam stabilized the Z-ring, induced filamentation, and reduced viability, with similar phenotypes being observed in ZapE deletion strains. Aztreonam treatment decreased ZapE expression, and the overexpression of ZapE rescued filamentous morphology significantly and viability partially. However, overexpression of filamentous temperature sensitive I (FtsI), a known target of aztreonam, could not rescue the filamentation. Interestingly, overexpression of ZapE and FtsI together was able to rescue both filamentous morphology and cell viability. Using in silico and biochemical analyses, we show that aztreonam directly interacts with ZapE. Our study suggests that the inhibitory effects of aztreonam in E. coli could be mediated by targeting ZapE.

Discovery and remodeling of Vibrio natriegens as a microbial platform for efficient formic acid biorefinery

Formic acid (FA) has emerged as a promising one-carbon feedstock for biorefinery. However, developing efficient microbial hosts for economically competitive FA utilization remains a grand challenge. Here, we discover that the bacterium Vibrio natriegens has exceptional FA tolerance and metabolic capacity natively. This bacterium is remodeled by rewiring the serine cycle and the TCA cycle, resulting in a non-native closed loop (S-TCA) which as a powerful metabolic sink, in combination with laboratory evolution, enables rapid emergence of synthetic strains with significantly improved FA-utilizing ability. Further introduction of a foreign indigoidine-forming pathway into the synthetic V. natriegens strain leads to the production of 29.0 g · L-1 indigoidine and consumption of 165.3 g · L-1 formate within 72 h, achieving a formate consumption rate of 2.3 g · L-1 · h-1. This work provides an important microbial chassis as well as design rules to develop industrially viable microorganisms for FA biorefinery.

Pan-Genomics of Escherichia albertii for Antibiotic Resistance Profiling in Different Genome Fractions and Natural Product Mediated Intervention: In Silico Approach

Escherichia albertii is an emerging, enteric pathogen of significance. It was first isolated in 2003 from a pediatric diarrheal sample from Bangladesh. In this study, a comprehensive in silico strategy was followed to first list out antibiotic-resistant genes from core, accessory and unique genome fractions of 95 available genomes of E. albertii. Then, 56 drug targets were identified from the core essential genome. Finally, ZipA, an essential cell division protein that stabilizes the FtsZ protofilaments by cross-linking them and serves as a cytoplasmic membrane anchor for the Z ring, was selected for further downstream processing. It was computationally modeled using a threading approach, followed by virtual screening of two phytochemical libraries, Ayurvedic (n = 2103 compounds) and Traditional Chinese Medicine (n = 36,043 compounds). ADMET profiling, followed by PBPK modeling in the central body compartment, in a population of 250 non-diseased, 250 cirrhotic and 250 renally impaired people was attempted. ZINC85624912 from Chinese medicinal library showed the highest bioavailability and plasma retention. This is the first attempt to simulate the fate of natural products in the body through PBPK. Dynamics simulation of 20 ns for the top three compounds from both libraries was also performed to validate the stability of the compounds. The obtained information from the current study could aid wet-lab scientists to work on the scaffold of screened drug-like compounds from natural resources and could be useful in our quest for therapy against antibiotic-resistant E. albertii.

Cell Cycle-Dependent Recruitment of FtsN to the Divisome in Escherichia coli

Cell division in Escherichia coli starts with the formation of an FtsZ protofilament network at midcell, the Z ring. However, only after a considerable lag period does the cell start to form a midcell constriction. The onset of constriction depends upon the arrival of so-called late divisome proteins, among which, FtsN is the last essential one. The timing and dependency of FtsN arrival to the divisome, along with genetic evidence, suggests it triggers cell division. In this study, we used high-throughput fluorescence microscopy to determine the arrival of FtsN and the early divisome protein ZapA to midcell at a single-cell level during the cell cycle. Our data show while the recruitment of ZapA/FtsZ is gradual in the cell cycle, recruitment of FtsN is rapid and begins at about the onset of constriction. At this time, the fraction of ZapA/FtsZ in the Z ring approaches its peak value. We also find a second increase in FtsN recruitment to the divisome, which begins once the amount of ZapA/FtsZ at midcell starts decreasing. Increasing hypermorphic FtsA* (FtsA R286W), but not FtsA, accelerates FtsN recruitment but not constriction. This finding is consistent with FtsA* recruiting FtsN with some other divisome component being rate-limiting for constriction under these conditions. Finally, our data support the recently proposed idea that ZapA/FtsZ and FtsN are part of physically separate complexes in midcell throughout the whole septation process. IMPORTANCE Cell division in most bacteria starts with the formation of an FtsZ protofilament network at midcell, the Z ring. However, cells only start to constrict after a considerable lag. A factor thought to trigger the onset of constriction in Escherichia coli is FtsN, which is the last essential protein to be recruited to the Z ring. Using a high-throughput quantitative fluorescence microscopy, we determine the cell cycle-dependent recruitment of FtsN to the Z ring. Our data show rapid accumulation of FtsN to the Z ring about a quarter of the cell cycle after the formation of the Z ring. This initial wave is followed by another increase in FtsN recruitment once the FtsZ protofilament network starts to disassemble. The presence of FtsA* accelerates FtsN recruitment to the Z ring but does not lead to earlier constrictions. Our data furthermore suggest FtsZ and FtsN are part of physically separate complexes throughout the division process.

Early midcell localization of Escherichia coli PBP4 supports the function of peptidoglycan amidases

Insertion of new material into the Escherichia coli peptidoglycan (PG) sacculus between the cytoplasmic membrane and the outer membrane requires a well-organized balance between synthetic and hydrolytic activities to maintain cell shape and avoid lysis. Since most bacteria carry multiple enzymes carrying the same type of PG hydrolytic activity, we know little about the specific function of given enzymes. Here we show that the DD-carboxy/endopeptidase PBP4 localizes in a PBP1A/LpoA and FtsEX dependent fashion at midcell during septal PG synthesis. Midcell localization of PBP4 requires its non-catalytic domain 3 of unknown function, but not the activity of PBP4 or FtsE. Microscale thermophoresis with isolated proteins shows that PBP4 interacts with NlpI and the FtsEX-interacting protein EnvC, an activator of amidases AmiA and AmiB, which are needed to generate denuded glycan strands to recruit the initiator of septal PG synthesis, FtsN. The domain 3 of PBP4 is needed for the interaction with NlpI and EnvC, but not PBP1A or LpoA. In vivo crosslinking experiments confirm the interaction of PBP4 with PBP1A and LpoA. We propose that the interaction of PBP4 with EnvC, whilst not absolutely necessary for mid-cell recruitment of either protein, coordinates the activities of PBP4 and the amidases, which affects the formation of denuded glycan strands that attract FtsN. Consistent with this model, we found that the divisome assembly at midcell was premature in cells lacking PBP4, illustrating how the complexity of interactions affect the timing of cell division initiation.

Peptidoglycan biosynthesis is a major target for antibacterials. The covalently closed peptidoglycan mesh, called sacculus, protects the bacterium from lysis due to its turgor. Sacculus growth is facilitated by the balanced activities of synthases and hydrolases, and disturbing this balance leads to cell lysis and bacterial death. Because of the large number and possible redundant functions of peptidoglycan hydrolases, it has been difficult to decipher their individual functions. In this paper we show that the DD-endopeptidase PBP4 localizes at midcell during septal peptidoglycan synthesis in Escherichia coli and is involved in the timing of the assembly and activation of the division machinery. This shows that inhibition of certain hydrolases could weaken the cells and might enhance antibiotic action.

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.

The Stress-Active Cell Division Protein ZapE Alters FtsZ Filament Architecture to Facilitate Division in Escherichia coli

During pathogenic infections, bacterial cells experience environmental stress conditions, including low oxygen and thermal stress. Bacterial cells proliferate during infection and divide by a mechanism characterized by the assembly of a large cytoskeletal structure at the division site called the Z-ring. The major protein constituting the Z-ring is FtsZ, a tubulin homolog and GTPase that utilizes the nucleotide to assemble into dynamic polymers. In Escherichia coli, many cell division proteins interact with FtsZ and modulate Z-ring assembly, while others direct cell wall insertion and peptidoglycan remodeling. Here, we show that ZapE, an ATPase that accumulates during late constriction, directly interacts with FtsZ and phospholipids in vitro. In the presence of adenosine triphosphate (ATP), ZapE induces bundling of GTP-induced FtsZ polymers; however, ZapE also binds FtsZ in the absence of GTP. The ZapE mutant protein ZapE(K84A), which is defective for ATP hydrolysis, also interacts with FtsZ and induces FtsZ filament bundling. In vivo, cultures of zapE deletion cells contain a low percentage of filamentous cells, suggesting that they have a modest division defect; however, they are able to grow when exposed to stress, such as high temperature and limited oxygen. When combined with the chromosomal deletion of minC, which encodes an FtsZ disassembly factor, ΔzapE ΔminC cells experience growth delays that slow proliferation at high temperature and prevent recovery. This synthetic slow growth phenotype after exposure to stress suggests that ZapE may function to ensure proliferation during and after stress, and this is exacerbated when cells are also deleted for minC. Expression of either ZapE or ZapE(K84A) complements the aberrant growth phenotypes in vivo suggesting that the division-associated role of ZapE does not require ZapE ATP hydrolysis. These results support that ZapE is a stress-regulated cell division protein that interacts directly with FtsZ and phospholipids, promoting growth and division after exposure to environmental stress.

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.

ZapG (YhcB/DUF1043), a novel cell division protein in gamma-proteobacteria linking the Z-ring to septal peptidoglycan synthesis

YhcB, a poorly understood protein conserved across gamma-proteobacteria, contains a domain of unknown function (DUF1043) and an N-terminal transmembrane domain. Here, we used an integrated approach including X-ray crystallography, genetics, and molecular biology to investigate the function and structure of YhcB. The Escherichia coli yhcB KO strain does not grow at 45 °C and is hypersensitive to cell wall–acting antibiotics, even in the stationary phase. The deletion of yhcB leads to filamentation, abnormal FtsZ ring formation, and aberrant septum development. The Z-ring is essential for the positioning of the septa and the initiation of cell division. We found that YhcB interacts with proteins of the divisome (e.g., FtsI, FtsQ) and elongasome (e.g., RodZ, RodA). Seven of these interactions are also conserved in Yersinia pestis and/or Vibrio cholerae. Furthermore, we mapped the amino acid residues likely involved in the interactions of YhcB with FtsI and RodZ. The 2.8 Å crystal structure of the cytosolic domain of Haemophilus ducreyi YhcB shows a unique tetrameric α-helical coiled-coil structure likely to be involved in linking the Z-ring to the septal peptidoglycan-synthesizing complexes. In summary, YhcB is a conserved and conditionally essential protein that plays a role in cell division and consequently affects envelope biogenesis. Based on these findings, we propose to rename YhcB to ZapG (Z-ring-associated protein G). This study will serve as a starting point for future studies on this protein family and on how cells transit from exponential to stationary survival.

Streptococcus suis MsmK: Novel Cell Division Protein Interacting with FtsZ and Maintaining Cell Shape

Bacteria of different shapes have adopted distinct mechanisms to faithfully coordinate morphogenesis and segregate their chromosomes prior to cell division. Despite recent focuses and advances, the mechanism of cell division in ovococci remains largely unknown. Streptococcus suis, a major zoonotic pathogen that causes problems in human health and in the global swine industry, is an elongated and ellipsoid bacterium that undergoes successive parallel splitting perpendicular to its long axis. Studies on cell cycle processes in this bacterium are limited. Here, we report that MsmK (multiple sugar metabolism protein K), an ATPase that contributes to the transport of multiple carbohydrates, has a novel role as a cell division protein in S. suis. MsmK can display ATPase and GTPase activities, interact with FtsZ via the N terminus of MsmK, and promote the bundling of FtsZ protofilaments in a GTP-dependent manner in vitro. Deletion of the C-terminal region or the Walker A or B motif affects the affinity between MsmK and FtsZ and decreases the ability of MsmK to promote FtsZ protofilament bundling. MsmK can form a complex with FtsZ in vivo, and its absence is not lethal but results in long chains and short, occasionally anuclear daughter cells. Superresolution microscopy revealed that the lack of MsmK in cells leads to normal septal peptidoglycan walls in mother cells but disturbed cell elongation and peripheral peptidoglycan synthesis. In summary, MsmK is a novel cell division protein that maintains cell shape and is involved in the synthesis of the peripheral cell wall.

IMPORTANCE Bacterial cell division is a highly ordered process regulated in time and space and is a potential target for the development of antimicrobial drugs. Bacteria of distinct shapes depend on different cell division mechanisms, but the mechanisms used by ovococci remain largely unknown. Here, we focused on the zoonotic pathogen Streptococcus suis and identified a novel cell division protein named MsmK, which acts as an ATPase of the ATP-binding cassette-type carbohydrate transport system. MsmK has GTPase and ATPase activities. In vitro protein assays showed that MsmK interacts with FtsZ and promotes FtsZ protofilament bundling that relies on GTP. Superresolution microscopy revealed that MsmK maintains cell shape and is involved in peripheral peptidoglycan synthesis. Knowledge of the multiple functions of MsmK may broaden our understanding of known cell division processes. Further studies in this area will elucidate how bacteria can faithfully and continually multiply in a constantly changing environment.

Simulations of Proposed Mechanisms of FtsZ-Driven Cell Constriction

FtsZ is thought to generate constrictive force to divide the cell, possibly via one of two predominant models in the field. In one, FtsZ filaments overlap to form complete rings which constrict as filaments slide past each other to maximize lateral contact.

To divide, bacteria must constrict their membranes against significant force from turgor pressure. A tubulin homolog, FtsZ, is thought to drive constriction, but how FtsZ filaments might generate constrictive force in the absence of motor proteins is not well understood. There are two predominant models in the field. In one, FtsZ filaments overlap to form complete rings around the circumference of the cell, and attractive forces cause filaments to slide past each other to maximize lateral contact. In the other, filaments exert force on the membrane by a GTP-hydrolysis-induced switch in conformation from straight to bent. Here, we developed software, ZCONSTRICT, for quantitative three-dimensional (3D) simulations of Gram-negative bacterial cell division to test these two models and identify critical conditions required for them to work. We find that the avidity of any kind of lateral interactions quickly halts the sliding of filaments, so a mechanism such as depolymerization or treadmilling is required to sustain constriction by filament sliding. For filament bending, we find that a mechanism such as the presence of a rigid linker is required to constrain bending to within the division plane and maintain the distance observed in vivo between the filaments and the membrane. Of these two models, only the filament bending model is consistent with our lab’s recent observation of constriction associated with a single, short FtsZ filament.

IMPORTANCE FtsZ is thought to generate constrictive force to divide the cell, possibly via one of two predominant models in the field. In one, FtsZ filaments overlap to form complete rings which constrict as filaments slide past each other to maximize lateral contact. In the other, filaments exert force on the membrane by switching conformation from straight to bent. Here, we developed software, ZCONSTRICT, for three-dimensional (3D) simulations to test these two models. We find that a mechanism such as depolymerization or treadmilling are required to sustain constriction by filament sliding. For filament bending, we find that a mechanism that constrains bending to within the division plane is required to maintain the distance observed in vivo between the filaments and the membrane.

Molecular interactions and their predictive roles in cell pole determination in bacteria

Bacterial cell cycle is divided into well-coordinated phases; chromosome duplication and segregation, cell elongation, septum formation, and cytokinesis. The temporal separation of these phases depends upon the growth rates and doubling time in different bacteria. The entire process of cell division starts with the assembly of divisome complex at mid-cell position followed by constriction of the cell wall and septum formation. In the mapping of mid-cell position for septum formation, the gradient of oscillating Min proteins across the poles plays a pivotal role in several bacteria genus. The cues in the cell that defines the poles and plane of cell division are not fully characterized in cocci. Recent studies have shed some lights on molecular interactions at the poles and the underlying mechanisms involved in pole determination in non-cocci. In this review, we have brought forth recent findings on these aspects together, which would suggest a model to explain the mechanisms of pole determination in rod shaped bacteria and could be extrapolated as a working model in cocci.

Bacterial Vivisection: How Fluorescence-Based Imaging Techniques Shed a Light on the Inner Workings of Bacteria

The rise in fluorescence-based imaging techniques over the past 3 decades has improved the ability of researchers to scrutinize live cell biology at increased spatial and temporal resolution. In microbiology, these real-time vivisections structurally changed the view on the bacterial cell away from the "watery bag of enzymes" paradigm toward the perspective that these organisms are as complex as their eukaryotic counterparts. Capitalizing on the enormous potential of (time-lapse) fluorescence microscopy and the ever-extending pallet of corresponding probes, initial breakthroughs were made in unraveling the localization of proteins and monitoring real-time gene expression. However, later it became clear that the potential of this technique extends much further, paving the way for a focus-shift from observing single events within bacterial cells or populations to obtaining a more global picture at the intra- and intercellular level. In this review, we outline the current state of the art in fluorescence-based vivisection of bacteria and provide an overview of important case studies to exemplify how to use or combine different strategies to gain detailed information on the cell's physiology. The manuscript therefore consists of two separate (but interconnected) parts that can be read and consulted individually. The first part focuses on the fluorescent probe pallet and provides a perspective on modern methodologies for microscopy using these tools. The second section of the review takes the reader on a tour through the bacterial cell from cytoplasm to outer shell, describing strategies and methods to highlight architectural features and overall dynamics within cells.

DrpB (YedR) Is a Nonessential Cell Division Protein in Escherichia coli

A thorough understanding of bacterial cell division requires identifying and characterizing all of the proteins that participate in this process. Our discovery of DrpB brings us one step closer to this goal in E. coli.

We report that the small Escherichia coli membrane protein DrpB (formerly YedR) is involved in cell division. We discovered DrpB in a screen for multicopy suppressors of a ΔftsEX mutation that prevents divisome assembly when cells are plated on low ionic strength medium, such as lysogeny broth without NaCl. Characterization of DrpB revealed that (i) translation initiates at an ATG annotated as codon 22 rather than the GTG annotated as codon 1, (ii) DrpB localizes to the septal ring when cells are grown in medium of low ionic strength but localization is greatly reduced in medium of high ionic strength, (iii) overproduction of DrpB in a ΔftsEX mutant background improves recruitment of the septal peptidoglycan synthase FtsI, implying multicopy suppression works by rescuing septal ring assembly, (iv) a ΔdrpB mutant divides quite normally, but a ΔdrpB ΔdedD double mutant has a strong division and viability defect, albeit only in medium of high ionic strength, and (v) DrpB homologs are found in E. coli and a few closely related enteric bacteria, but not outside this group. In sum, DrpB is a poorly conserved nonessential division protein that improves the efficiency of cytokinesis under suboptimal conditions. Proteins like DrpB are likely to be a widespread feature of the bacterial cell division apparatus, but they are easily overlooked because mutants lack obvious shape defects.

IMPORTANCE A thorough understanding of bacterial cell division requires identifying and characterizing all of the proteins that participate in this process. Our discovery of DrpB brings us one step closer to this goal in E. coli.

Cell division protein FtsZ: from structure and mechanism to antibiotic target

Antimicrobial resistance to virtually all clinically applied antibiotic classes severely limits the available options to treat bacterial infections. Hence, there is an urgent need to develop and evaluate new antibiotics and targets with resistance-breaking properties. Bacterial cell division has emerged as a new antibiotic target pathway to counteract multidrug-resistant pathogens. New approaches in antibiotic discovery and bacterial cell biology helped to identify compounds that either directly interact with the major cell division protein FtsZ, thereby perturbing the function and dynamics of the cell division machinery, or affect the structural integrity of FtsZ by inducing its degradation. The impressive antimicrobial activities and resistance-breaking properties of certain compounds validate the inhibition of bacterial cell division as a promising strategy for antibiotic intervention.

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.

pH-dependent activation of cytokinesis modulates Escherichia coli cell size

Cell size is a complex trait, derived from both genetic and environmental factors. Environmental determinants of bacterial cell size identified to date primarily target assembly of cytosolic components of the cell division machinery. Whether certain environmental cues also impact cell size through changes in the assembly or activity of extracytoplasmic division proteins remains an open question. Here, we identify extracellular pH as a modulator of cell division and a significant determinant of cell size across evolutionarily distant bacterial species. In the Gram-negative model organism Escherichia coli, our data indicate environmental pH impacts the length at which cells divide by altering the ability of the terminal cell division protein FtsN to localize to the cytokinetic ring where it activates division. Acidic environments lead to enrichment of FtsN at the septum and activation of division at a reduced cell length. Alkaline pH inhibits FtsN localization and suppresses division activation. Altogether, our work reveals a previously unappreciated role for pH in bacterial cell size control.

Bacteria are constantly under assault from endogenous and environmental stressors. To ensure viability and reproductive fitness, many bacteria alter their growth and replication in response to stressful conditions. Previous work from many groups has identified regulatory mechanisms linking cell division with nutrient availability and metabolic state. However, comparatively little is known about how the cell division machinery responds to physical and chemical cues in the environment. Here, we identify a fundamental property of the extracellular environment—environmental pH—as a significant contributor to bacterial cell size. Our genetic and cytological data indicate pH-dependent changes in E. coli cell size are in part due to differential localization of the cell division activator FtsN across pH environments. Increased abundance of FtsN at midcell in acidic environments promotes cell division at a reduced cell volume, while decreased abundance of FtsN at midcell in alkaline environments effectively delays cell division until a larger size is reached. Altogether, our work identifies pH as an environmental determinant of E. coli cell division and illuminates FtsN recruitment as a mediator of cell size.

Cooperative ordering of treadmilling filaments in cytoskeletal networks of FtsZ and its crosslinker ZapA

During bacterial cell division, the tubulin-homolog FtsZ forms a ring-like structure at the center of the cell. This Z-ring not only organizes the division machinery, but treadmilling of FtsZ filaments was also found to play a key role in distributing proteins at the division site. What regulates the architecture, dynamics and stability of the Z-ring is currently unknown, but FtsZ-associated proteins are known to play an important role. Here, using an in vitro reconstitution approach, we studied how the well-conserved protein ZapA affects FtsZ treadmilling and filament organization into large-scale patterns. Using high-resolution fluorescence microscopy and quantitative image analysis, we found that ZapA cooperatively increases the spatial order of the filament network, but binds only transiently to FtsZ filaments and has no effect on filament length and treadmilling velocity. Together, our data provides a model for how FtsZ-associated proteins can increase the precision and stability of the bacterial cell division machinery in a switch-like manner.

The Z-ring, constituted of the tubulin homolog FtsZ protein, plays an essential role for bacterial cell division. Here the authors use an in vitro reconstitution approach to determine how the regulatory protein ZapA affects FtsZ treadmilling and filament organization into large-scale patterns.

Regulation of cytokinesis: FtsZ and its accessory proteins

Bacterial cell division is a highly controlled process regulated accurately by a diverse array of proteins spatially and temporally working together. Among these proteins, FtsZ is recognized as a cytoskeleton protein because it can assemble into a ring-like structure called Z-ring at midcell. Z-ring recruits downstream proteins, thus forming a multiprotein complex termed the divisome. When the Z-ring scaffold is established and the divisome matures, peptidoglycan (PG) biosynthesis and chromosome segregation are triggered. In this review, we focus on multiple interactions between FtsZ and its accessory proteins in bacterial cell cytokinesis, including FtsZ localization, Z-ring formation and stabilization, PG biosynthesis, and chromosome segregation. Understanding the interactions among these proteins may help discover superior targets on treating bacterial infectious diseases.

Division-site localization of RodZ is required for efficient Z ring formation in Escherichia coli

Bacteria such as Escherichia coli must coordinate cell elongation and cell division. Elongation is regulated by an elongasome complex containing MreB actin and the transmembrane protein RodZ, which regulates assembly of MreB, whereas division is regulated by a divisome complex containing FtsZ tubulin. These complexes were previously thought to function separately. However, MreB has been shown to directly interact with FtsZ to switch to cell division from cell elongation, indicating that these complexes collaborate to regulate both processes. Here, we investigated the role of RodZ in the regulation of cell division. RodZ localized to the division site in an FtsZ-dependent manner. We also found that division-site localization of MreB was dependent on RodZ. Formation of a Z ring was delayed by deletion of rodZ, suggesting that division-site localization of RodZ facilitated the formation or stabilization of the Z ring during early cell division. Thus, RodZ functions to regulate MreB assembly during cell elongation and facilitates the formation of the Z ring during cell division in E. coli.

An integrative view of cell cycle control in Escherichia coli

Bacterial proliferation depends on the cells' capability to proceed through consecutive rounds of the cell cycle. The cell cycle consists of a series of events during which cells grow, copy their genome, partition the duplicated DNA into different cell halves and, ultimately, divide to produce two newly formed daughter cells. Cell cycle control is of the utmost importance to maintain the correct order of events and safeguard the integrity of the cell and its genomic information. This review covers insights into the regulation of individual key cell cycle events in Escherichia coli. The control of initiation of DNA replication, chromosome segregation and cell division is discussed. Furthermore, we highlight connections between these processes. Although detailed mechanistic insight into these connections is largely still emerging, it is clear that the different processes of the bacterial cell cycle are coordinated to one another. This careful coordination of events ensures that every daughter cell ends up with one complete and intact copy of the genome, which is vital for bacterial survival.

An essential Staphylococcus aureus cell division protein directly regulates FtsZ dynamics

Binary fission has been well studied in rod-shaped bacteria, but the mechanisms underlying cell division in spherical bacteria are poorly understood. Rod-shaped bacteria harbor regulatory proteins that place and remodel the division machinery during cytokinesis. In the spherical human pathogen Staphylococcus aureus, we found that the essential protein GpsB localizes to mid-cell during cell division and co-constricts with the division machinery. Depletion of GpsB arrested cell division and led to cell lysis, whereas overproduction of GpsB inhibited cell division and led to the formation of enlarged cells. We report that S. aureus GpsB, unlike other Firmicutes GpsB orthologs, directly interacts with the core divisome component FtsZ. GpsB bundles and organizes FtsZ filaments and also stimulates the GTPase activity of FtsZ. We propose that GpsB orchestrates the initial stabilization of the Z-ring at the onset of cell division and participates in the subsequent remodeling of the divisome during cytokinesis.

A bacterium called Staphylococcus aureus causes many infections in humans, especially in hospital patients with weakened immune systems. These infections are generally treated with drugs known as antibiotics that interact with specific proteins in the bacteria to kill the cells, or stop them from growing. However, some S. aureus infections are resistant to the antibiotics currently available so there is a need to develop new drugs that target different bacterial proteins.

Bacteria multiply by dividing to make identical copies of themselves. When a bacterium is preparing to divide, filaments made of a protein called FtsZ form a ring at the site where the cell will split. Many other proteins are involved in controlling how and when a cell divides. For example, several species of bacteria harbor a dispensable cell division protein called GpsB. In at least one organism, it helps to maintain the proper shape of the cell during cell division. In S. aureus, though, GpsB is essential for cells to survive and could therefore be a potential target for new antibiotics. However, its role in S. aureus has not been studied.

Eswara et al. have now used genetic and biochemical approaches to study the S. aureus form of the GpsB protein. The experiments show that GpsB moves to the middle of S. aureus cells just before they begin to divide and binds directly to FtsZ. This helps to secure the position of FtsZ across the middle of the cell and activates the protein so that the cell can begin to divide into two. In cells that produce too much GpsB, the FtsZ proteins become active too early, leading to the cells growing larger and larger until they burst.

The findings of Eswara et al. reveal that GpsB plays a different role in S. aureus cells than in some other species of bacteria. Further studies into such differences could help researchers to develop new antibiotics, as well as improving our understanding of why bacteria are so diverse.

Gain-of-function variants of FtsA form diverse oligomeric structures on lipids and enhance FtsZ protofilament bundling

Escherichia coli requires FtsZ, FtsA and ZipA proteins for early stages of cell division, the latter two tethering FtsZ polymers to the cytoplasmic membrane. Hypermorphic mutants of FtsA such as FtsA* (R286W) map to the FtsA self-interaction interface and can bypass the need for ZipA. Purified FtsA forms closed minirings on lipid monolayers that antagonize bundling of FtsZ protofilaments, whereas FtsA* forms smaller oligomeric arcs that enable bundling. Here, we examined three additional FtsA*-like mutant proteins for their ability to form oligomers on lipid monolayers and bundle FtsZ. Surprisingly, all three formed distinct structures ranging from mostly arcs (T249M), a mixture of minirings, arcs and straight filaments (Y139D) or short straight double filaments (G50E). All three could form filament sheets at higher concentrations with added ATP. Despite forming these diverse structures, all three mutant proteins acted like FtsA* to enable FtsZ protofilament bundling on lipid monolayers. Synthesis of the FtsA*-like proteins in vivo suppressed the toxic effects of a bundling-defective FtsZ, exacerbated effects of a hyper-bundled FtsZ, and rescued some thermosensitive cell division alleles. Together, the data suggest that conversion of FtsA minirings into any type of non-miniring oligomer can promote progression of cytokinesis through FtsZ bundling and other mechanisms.

Escherichia coli ZipA Organizes FtsZ Polymers into Dynamic Ring-Like Protofilament Structures

ZipA is an essential cell division protein in Escherichia coli. Together with FtsA, ZipA tethers dynamic polymers of FtsZ to the cytoplasmic membrane, and these polymers are required to guide synthesis of the cell division septum. This dynamic behavior of FtsZ has been reconstituted on planar lipid surfaces in vitro, visible as GTP-dependent chiral vortices several hundred nanometers in diameter, when anchored by FtsA or when fused to an artificial membrane binding domain. However, these dynamics largely vanish when ZipA is used to tether FtsZ polymers to lipids at high surface densities. This, along with some in vitro studies in solution, has led to the prevailing notion that ZipA reduces FtsZ dynamics by enhancing bundling of FtsZ filaments. Here, we show that this is not the case. When lower, more physiological levels of the soluble, cytoplasmic domain of ZipA (sZipA) were attached to lipids, FtsZ assembled into highly dynamic vortices similar to those assembled with FtsA or other membrane anchors. Notably, at either high or low surface densities, ZipA did not stimulate lateral interactions between FtsZ protofilaments. We also used E. coli mutants that are either deficient or proficient in FtsZ bundling to provide evidence that ZipA does not directly promote bundling of FtsZ filaments in vivo. Together, our results suggest that ZipA does not dampen FtsZ dynamics as previously thought, and instead may act as a passive membrane attachment for FtsZ filaments as they treadmill.

Bacterial cells use a membrane-attached ring of proteins to mark and guide formation of a division septum at midcell that forms a wall separating the two daughter cells and allows cells to divide. The key protein in this ring is FtsZ, a homolog of tubulin that forms dynamic polymers. Here, we use electron microscopy and confocal fluorescence imaging to show that one of the proteins required to attach FtsZ polymers to the membrane during E. coli cell division, ZipA, can promote dynamic swirls of FtsZ on a lipid surface in vitro. Importantly, these swirls are observed only when ZipA is present at low, physiologically relevant surface densities. Although ZipA has been thought to enhance bundling of FtsZ polymers, we find little evidence for bundling in vitro. In addition, we present several lines of in vivo evidence indicating that ZipA does not act to directly bundle FtsZ polymers.

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.

FzlA, an essential regulator of FtsZ filament curvature, controls constriction rate during Caulobacter division

During bacterial division, polymers of the tubulin-like GTPase FtsZ assemble at midcell to form the cytokinetic Z-ring, which coordinates peptidoglycan (PG) remodeling and envelope constriction. Curvature of FtsZ filaments promotes membrane deformation in vitro, but its role in division in vivo remains undefined. Inside cells, FtsZ directs PG insertion at the division plane, though it is unclear how FtsZ structure and dynamics are mechanistically coupled to PG metabolism. Here we study FzlA, a division protein that stabilizes highly curved FtsZ filaments, as a tool for assessing the contribution of FtsZ filament curvature to constriction. We show that in Caulobacter crescentus, FzlA must bind to FtsZ for division to occur and that FzlA-mediated FtsZ curvature is correlated with efficient division. We observed that FzlA influences constriction rate, and that this activity is associated with its ability to bind and curve FtsZ polymers. Further, we found that a slowly constricting fzlA mutant strain develops 'pointy' poles, suggesting that FzlA influences the relative contributions of radial versus longitudinal PG insertion at the septum. These findings implicate FzlA as a critical coordinator of envelope constriction through its interaction with FtsZ and suggest a functional link between FtsZ curvature and efficient constriction in C. crescentus.

The hypermorph FtsA* protein has an in vivo role in relieving the Escherichia coli proto-ring block caused by excess ZapC

Assembly of the proto-ring, formed by the essential FtsZ, FtsA and ZipA proteins, and its progression into a divisome, are essential events for Escherichia coli division. ZapC is a cytoplasmic protein that belongs to a group of non-essential components that assist FtsZ during proto-ring assembly. Any overproduction of these proteins leads to faulty FtsZ-rings, resulting in a cell division block. We show that ZapC overproduction can be counteracted by an excess of the ZipA-independent hypermorph FtsA* mutant, but not by similar amounts of wild type FtsA+. An excess of FtsA+ allowed regular spacing of the ZapC-blocked FtsZ-rings, but failed to promote recruitment of the late-assembling proteins FtsQ, FtsK and FtsN and therefore, to activate constriction. In contrast, overproduction of FtsA*, besides allowing correct FtsZ-ring localization at midcell, restored the ability of FtsQ, FtsK and FtsN to be incorporated into active divisomes.

Two dynamin-like proteins stabilize FtsZ rings during Streptomyces sporulation

During sporulation, the filamentous bacteria Streptomyces undergo a massive cell division event in which the synthesis of ladders of sporulation septa convert multigenomic hyphae into chains of unigenomic spores. This process requires cytokinetic Z-rings formed by the bacterial tubulin homolog FtsZ, and the stabilization of the newly formed Z-rings is crucial for completion of septum synthesis. Here we show that two dynamin-like proteins, DynA and DynB, play critical roles in this process. Dynamins are a family of large, multidomain GTPases involved in key cellular processes in eukaryotes, including vesicle trafficking and organelle division. Many bacterial genomes encode dynamin-like proteins, but the biological function of these proteins has remained largely enigmatic. Using a cell biological approach, we show that the two Streptomyces dynamins specifically localize to sporulation septa in an FtsZ-dependent manner. Moreover, dynamin mutants have a cell division defect due to the decreased stability of sporulation-specific Z-rings, as demonstrated by kymographs derived from time-lapse images of FtsZ ladder formation. This defect causes the premature disassembly of individual Z-rings, leading to the frequent abortion of septum synthesis, which in turn results in the production of long spore-like compartments with multiple chromosomes. Two-hybrid analysis revealed that the dynamins are part of the cell division machinery and that they mediate their effects on Z-ring stability during developmentally controlled cell division via a network of protein-protein interactions involving DynA, DynB, FtsZ, SepF, SepF2, and the FtsZ-positioning protein SsgB.

Assembly and activation of the Escherichia coli divisome

Cell division in Escherichia coli is mediated by a large protein complex called the divisome. Most of the divisome proteins have been identified, but how they assemble onto the Z ring scaffold to form the divisome and work together to synthesize the septum is not well understood. In this review, we summarize the latest findings on divisome assembly and activation as well as provide our perspective on how these two processes might be regulated.

ZapA and ZapB form an FtsZ-independent structure at midcell

Cell division in Escherichia coli begins with the polymerization of FtsZ into a ring-like structure, the Z-ring, at midcell. All other division proteins are thought to require the Z-ring for recruitment to the future division site. Here, it is reported that the Z-ring associated proteins ZapA and ZapB form FtsZ-independent structures at midcell. Upon Z-ring disruption by the FtsZ polymerization antagonist SulA, ZapA remained at midcell as a cloud-like accumulation. Using ZapA(N60Y), a variant defective for interaction with FtsZ, it was established that these ZapA structures form without a connection to the Z-ring. Furthermore, midcell accumulations of GFP-ZapA(N60Y) often preceded Z-rings at midcell and required ZapB to assemble, suggesting that ZapB polymers form the foundation of these structures. In the absence of MatP, a DNA-binding protein that links ZapB to the chromosomal terminus region, cloud-like ZapA structures still formed but failed to track with the chromosome terminus and did not consistently precede FtsZ at midcell. Taken together, the results suggest that FtsZ-independent structures of ZapA-ZapB provide additional positional cues for Z-ring formation and may help coordinate its assembly with chromosome replication and segregation.

Structure of the Z Ring-associated Protein, ZapD, Bound to the C-terminal Domain of the Tubulin-like Protein, FtsZ, Suggests Mechanism of Z Ring Stabilization through FtsZ Cross-linking

Cell division in most bacteria is mediated by the tubulin-like FtsZ protein, which polymerizes in a GTP-dependent manner to form the cytokinetic Z ring. A diverse repertoire of FtsZ-binding proteins affects FtsZ localization and polymerization to ensure correct Z ring formation. Many of these proteins bind the C-terminal domain (CTD) of FtsZ, which serves as a hub for FtsZ regulation. FtsZ ring-associated proteins, ZapA-D (Zaps), are important FtsZ regulatory proteins that stabilize FtsZ assembly and enhance Z ring formation by increasing lateral assembly of FtsZ protofilaments, which then form the Z ring. There are no structures of a Zap protein bound to FtsZ; therefore, how these proteins affect FtsZ polymerization has been unclear. Recent data showed ZapD binds specifically to the FtsZ CTD. Thus, to obtain insight into the ZapD-CTD interaction and how it may mediate FtsZ protofilament assembly, we determined the Escherichia coli ZapD-FtsZ CTD structure to 2.67 Å resolution. The structure shows that the CTD docks within a hydrophobic cleft in the ZapD helical domain and adopts an unusual structure composed of two turns of helix separated by a proline kink. FtsZ CTD residue Phe-377 inserts into the ZapD pocket, anchoring the CTD in place and permitting hydrophobic contacts between FtsZ residues Ile-374, Pro-375, and Leu-378 with ZapD residues Leu-74, Trp-77, Leu-91, and Leu-174. The structural findings were supported by mutagenesis coupled with biochemical and in vivo studies. The combined data suggest that ZapD acts as a molecular cross-linking reagent between FtsZ protofilaments to enhance FtsZ assembly.

FtsZ-ring Architecture and Its Control by MinCD

In bacteria and archaea, the most widespread cell division system is based on the tubulin homologue FtsZ protein, whose filaments form the cytokinetic Z-ring. FtsZ filaments are tethered to the membrane by anchors such as FtsA and SepF and are regulated by accessory proteins. One such set of proteins is responsible for Z-ring's spatiotemporal regulation, essential for the production of two equal-sized daughter cells. Here, we describe how our still partial understanding of the FtsZ-based cell division process has been progressed by visualising near-atomic structures of Z-rings and complexes that control Z-ring positioning in cells, most notably the MinCDE and Noc systems that act by negatively regulating FtsZ filaments. We summarise available data and how they inform mechanistic models for the cell division process.

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.

WhmD promotes the assembly of Mycobacterium smegmatis FtsZ: A possible role of WhmD in bacterial cell division

WhmD is considered to have a role in the septation and division of Mycobacterium smegmatis cells. Since FtsZ is the central protein of the septum, we determined the effect of WhmD on the assembly of Mycobacterium smegmatis FtsZ (MsFtsZ) in vitro. WhmD increased both the rate and extent of the assembly of MsFtsZ in vitro. WhmD also increased the amount of polymerized MsFtsZ as evident from a sedimentation assay. Further, the assembly promoting activity of WhmD occurred in the presence of GTP. MsFtsZ polymerized to form thin filaments in the absence of WhmD while MsFtsZ formed thick filaments in the presence of WhmD suggesting that WhmD enhanced the bundling of MsFtsZ filaments. Interestingly, WhmD neither suppressed the dilution-induced disassembly of FtsZ filaments nor significantly altered the GTPase activity of FtsZ. Using size exclusion chromatography, circular dichroism and fluorescence spectroscopy, WhmD was found to bind to MsFtsZ in vitro. The results showed that WhmD can promote the assembly of FtsZ and indicated that WhmD may play a role in the division of M. smegmatis cells by assisting the polymerization of FtsZ.

Beyond force generation: Why is a dynamic ring of FtsZ polymers essential for bacterial cytokinesis?

We propose that the essential function of the most highly conserved protein in bacterial cytokinesis, FtsZ, is not to generate a mechanical force to drive cell division. Rather, we suggest that FtsZ acts as a signal-processing hub to coordinate cell wall synthesis at the division septum with a diverse array of cellular processes, ensuring that the cell divides smoothly at the correct time and place, and with the correct septum morphology. Here, we explore how the polymerization properties of FtsZ, which have been widely attributed to force generation, can also be advantageous in this signal processing role. We suggest mechanisms by which FtsZ senses and integrates both mechanical and biochemical signals, and conclude by proposing experiments to investigate how FtsZ contributes to the remarkable spatial and temporal precision of bacterial cytokinesis.

The bacterial divisome: ready for its close-up

Bacterial cells divide by targeting a transmembrane protein machine to the division site and regulating its assembly and disassembly so that cytokinesis occurs at the correct time in the cell cycle. The structure and dynamics of this machine (divisome) in bacterial model systems are coming more clearly into focus, thanks to incisive cell biology methods in combination with biochemical and genetic approaches. The main conserved structural element of the machine is the tubulin homologue FtsZ, which assembles into a circumferential ring at the division site that is stabilized and anchored to the inner surface of the cytoplasmic membrane by FtsZ-binding proteins. Once this ring is in place, it recruits a series of transmembrane proteins that ultimately trigger cytokinesis. This review will survey the methods used to characterize the structure of the bacterial divisome, focusing mainly on the Escherichia coli model system, as well as the challenges that remain. These methods include recent super-resolution microscopy, cryo-electron tomography and synthetic reconstitution.

Identification of putative Z-ring-associated proteins, involved in cell division in human pathogenic bacteria Helicobacter pylori

Cell division in bacteria is initiated by FtsZ, which forms a Z ring at the middle of the cell, between the nucleoids. The Z ring is stabilized by Z ring-associated proteins (Zaps), which crosslink the FtsZ filaments and provide strength. The deletion of Zaps leads to the elongation phenotype with an abnormal Z ring. The components of cell division in Helicobacter pylori are similar to other gram negative bacteria except for the absence of few components including Zaps. Here, we used HHsearch to identify homologs of the missing cell division proteins and got potential hits for ZapA and ZapB, as well as for few other cell division proteins. We further validated the function of the putative ZapA homolog by genetic complementation, immuno-colocalization and biochemical analysis.

Beyond the Discovery Void: New targets for antibacterial compounds

Antibiotics save many lives, but their efficacy is under threat: overprescription, population growth, and global travel all contribute to the rapid origination and spread of resistant strains. Exacerbating this threat is the fact that no new major classes of antibiotics have been discovered in the last 30 years: this is the "discovery void." We discuss the traditional molecular targets of antibiotics as well as putative novel targets.

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.

Characterization of the FtsZ C-Terminal Variable (CTV) Region in Z-Ring Assembly and Interaction with the Z-Ring Stabilizer ZapD in E. coli Cytokinesis

Polymerization of a ring-like cytoskeletal structure, the Z-ring, at midcell is a highly conserved feature in virtually all bacteria. The Z-ring is composed of short protofilaments of the tubulin homolog FtsZ, randomly arranged and held together through lateral interactions. In vitro, lateral associations between FtsZ protofilaments are stabilized by crowding agents, high concentrations of divalent cations, or in some cases, low pH. In vivo, the last 4–10 amino acid residues at the C-terminus of FtsZ (the C-terminal variable region, CTV) have been implicated in mediating lateral associations between FtsZ protofilaments through charge shielding. Multiple Z-ring associated proteins (Zaps), also promote lateral interactions between FtsZ protofilaments to stabilize the FtsZ ring in vivo. Here we characterize the complementary role/s of the CTV of E. coli FtsZ and the FtsZ-ring stabilizing protein ZapD, in FtsZ assembly. We show that the net charge of the FtsZ CTV not only affects FtsZ protofilament bundling, confirming earlier observations, but likely also the length of the FtsZ protofilaments in vitro. The CTV residues also have important consequences for Z-ring assembly and interaction with ZapD in the cell. ZapD requires the FtsZ CTV region for interaction with FtsZ in vitro and for localization to midcell in vivo. Our data suggest a mechanism in which the CTV residues, particularly K380, facilitate a conformation for the conserved carboxy-terminal residues in FtsZ, that lie immediately N-terminal to the CTV, to enable optimal contact with ZapD. Further, phylogenetic analyses suggest a correlation between the nature of FtsZ CTV residues and the presence of ZapD in the β- γ-proteobacterial species.

Splitsville: structural and functional insights into the dynamic bacterial Z ring

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

ClpXP and ClpAP control the Escherichia coli division protein ZapC by proteolysis

The bacterial FtsZ-ring is an essential cytokinetic structure under tight spatiotemporal regulation. In Escherichia coli, FtsZ polymerization and assembly into the Z-ring is controlled on multiple levels through interactions with positive and negative regulators. Among these regulatory factors are ZapC, a Z-ring stabilizer, and the conserved protease ClpXP, which has been shown to degrade FtsZ protofilaments in preference to FtsZ monomers. Here we report that ZapC and ClpX interact in a protein-protein interaction assay, and that ZapC is degraded in a ClpXP-dependent manner in vivo. The SspB adaptor protein is not required for targeting ZapC to the ClpXP proteolytic machinery. A mutation disrupting the zapC ssrA-like sequence (zapCDD) stabilizes ZapC consistent with a reduction in ClpXP-mediated ZapC degradation. ZapCDD retains the ability to interact with FtsZ and to promote bundling in vitro indicating that WT ZapC contains discrete FtsZ and ClpX recognition motifs. Additionally, ClpAP complexes are sufficient for degradation of ZapC in the absence of ClpX in vivo. Further, chromosomal expression of zapCDD suppresses filamentation of the temperature-sensitive ftsZ84 mutant, confirming the role of ZapC as a Z-ring stabilizer. Lastly, changes in ClpXP and ZapC levels lead to cell division effects, likely through their roles in modulating FtsZ assembly dynamics. Taken together, our results indicate that the Z-ring stabilizer ZapC is a substrate of both ClpXP and ClpAP in vivo. Our data also point to a more complex regulatory circuit that integrates FtsZ, ClpXP and ZapC in achieving Z-ring stability in E. coli and related species.

Defining the rate-limiting processes of bacterial cytokinesis

Bacterial cytokinesis is accomplished by the essential 'divisome' machinery. The most widely conserved divisome component, FtsZ, is a tubulin homolog that polymerizes into the 'FtsZ-ring' ('Z-ring'). Previous in vitro studies suggest that Z-ring contraction serves as a major constrictive force generator to limit the progression of cytokinesis. Here, we applied quantitative superresolution imaging to examine whether and how Z-ring contraction limits the rate of septum closure during cytokinesis in Escherichia coli cells. Surprisingly, septum closure rate was robust to substantial changes in all Z-ring properties proposed to be coupled to force generation: FtsZ's GTPase activity, Z-ring density, and the timing of Z-ring assembly and disassembly. Instead, the rate was limited by the activity of an essential cell wall synthesis enzyme and further modulated by a physical divisome-chromosome coupling. These results challenge a Z-ring-centric view of bacterial cytokinesis and identify cell wall synthesis and chromosome segregation as limiting processes of cytokinesis.