FtsA mutants impaired for self-interaction bypass ZipA suggesting a model in which FtsA's self-interaction competes with its ability to recruit downstream division proteins

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 ClpX chaperone and a hypermorphic FtsA variant with impaired self-interaction are mutually compensatory for coordinating Staphylococcus aureus cell division

Bacterial cell division requires the coordinated assembly and disassembly of a large protein complex called the divisome; however, the exact role of molecular chaperones in this critical process remains unclear. We here provide genetic evidence that ClpX unfoldase activity is a determinant for proper coordination of bacterial cell division by showing the growth defect of a Staphylococcus aureus clpX mutant is rescued by a spontaneously acquired G325V substitution in the ATP-binding domain of the essential FtsA cell division protein. The polymerization state of FtsA is thought to control initiation of bacterial septum synthesis and, while restoring the aberrant FtsA dynamics in clpX cells, the FtsAG325V variant displayed reduced ability to interact with itself and other cell division proteins. In wild-type cells, the ftsAG325V allele shared phenotypes with Escherichia coli superfission ftsA mutants and accelerated the cell cycle, increased the risk of daughter cell lysis, and conferred sensitivity to heat and antibiotics inhibiting cell wall synthesis. Strikingly, lethality was mitigated by spontaneous mutations that inactivate ClpX. Taken together, our results suggest that ClpX promotes septum synthesis by antagonizing FtsA interactions and illuminates the critical role of a protein unfoldase in coordinating bacterial cell division.

Insights into the assembly and regulation of the bacterial divisome

The ability to split one cell into two is fundamental to all life, and many bacteria can accomplish this feat several times per hour with high accuracy. Most bacteria call on an ancient homologue of tubulin, called FtsZ, to localize and organize the cell division machinery, the divisome, into a ring-like structure at the cell midpoint. The divisome includes numerous other proteins, often including an actin homologue (FtsA), that interact with each other at the cytoplasmic membrane. Once assembled, the protein complexes that comprise the dynamic divisome coordinate membrane constriction with synthesis of a division septum, but only after overcoming checkpoints mediated by specialized protein-protein interactions. In this Review, we summarize the most recent evidence showing how the divisome proteins of Escherichia coli assemble at the cell midpoint, interact with each other and regulate activation of septum synthesis. We also briefly discuss the potential of divisome proteins as novel antibiotic targets.

Anchors: A way for FtsZ filaments to stay membrane bound

Most bacteria use the tubulin homolog FtsZ to organize their cell division. FtsZ polymers initially assemble into mobile complexes that circle around a ring-like structure at the cell midpoint, followed by the recruitment of other proteins that will constrict the cytoplasmic membrane and synthesize septal peptidoglycan to divide the cell. Despite the need for FtsZ polymers to associate with the membrane, FtsZ lacks intrinsic membrane binding ability. Consequently, FtsZ polymers have evolved to interact with the membrane through adaptor proteins that both bind FtsZ and the membrane. Here, we discuss recent progress in understanding the functions of these FtsZ membrane tethers. Some, such as FtsA and SepF, are widely conserved and assemble into varied oligomeric structures bound to the membrane through an amphipathic helix. Other less-conserved proteins, such as EzrA and ZipA, have transmembrane domains, make extended structures, and seem to bind to FtsZ through two separate interactions. This review emphasizes that most FtsZs use multiple membrane tethers with overlapping functions, which not only attach FtsZ polymers to the membrane but also organize them in specific higher-order structures that can optimize cell division activity. We discuss gaps in our knowledge of these concepts and how future studies can address them.

Gene-Directed FtsZ Ring Assembly Generates Constricted Liposomes with Stable Membrane Necks

Mimicking bacterial cell division in well-defined cell-free systems has the potential to elucidate the minimal set of proteins required for cytoskeletal formation, membrane constriction, and final abscission. Membrane-anchored FtsZ polymers are often regarded as a sufficient system to realize this chain of events. By using purified FtsZ and its membrane-binding protein FtsA or the gain-of-function mutant FtsA* expressed in PURE (Protein synthesis Using Reconstituted Elements) from a DNA template, it is shown in this study that cytoskeletal structures are formed, and yield constricted liposomes exhibiting various morphologies. However, the resulting buds remain attached to the parental liposome by a narrow membrane neck. No division events can be monitored even after long-time tracking by fluorescence microscopy, nor when the osmolarity of the external solution is increased. The results provide evidence that reconstituted FtsA-FtsZ proto-rings coating the membrane necks are too stable to enable abscission. The prospect of combining a DNA-encoded FtsZ system with assisting mechanisms to achieve synthetic cell division is discussed.

Construction and Characterization of Functional FtsA Sandwich Fusions for Studies of FtsA Localization and Dynamics during Escherichia coli Cell Division

FtsA, a homolog of actin, is essential for cell division of Escherichia coli and is widely conserved among many bacteria. FtsA helps to tether polymers of the bacterial tubulin homolog FtsZ to the cytoplasmic membrane as part of the cytokinetic Z ring. GFP fusions to FtsA have illuminated FtsA's localization in live E. coli, but these fusions have not been fully functional and required the presence of the native FtsA. Here, we characterize "sandwich" fusions of E. coli FtsA to either mCherry or msfGFP that are functional for cell division and exhibit fluorescent rings at midcell that persist throughout constriction until cell separation. FtsA within the Z ring moved circumferentially like FtsZ, and FtsA outside the rings formed highly dynamic patches at the membrane. Notably, both FtsA-mCherry^sw and FtsA-msfGFP^sw acted as mild hypermorphs, as they were not toxic when overproduced, bypassed the essential cell division protein ZipA, and suppressed several thermosensitive fts alleles, although not as effectively as the prototypical hypermorph FtsA*. Overall, our results indicate that fluorescent FtsA sandwich fusions can be used as the sole FtsA in E. coli and thus should shed new light on FtsA dynamics during the cell division cycle in this model system. IMPORTANCE FtsA is a key conserved cell division protein, and E. coli is the most well studied model system for bacterial cell division. One obstacle to full understanding of this process is the lack of a fully functional fluorescent reporter for FtsA in vivo. Here, we describe a fluorescent fusion to E. coli FtsA that promotes efficient cell division in the absence of the native FtsA and can be used to monitor FtsA dynamics during cell division.

Bacterial divisome protein FtsA forms curved antiparallel double filaments when binding to FtsN

During bacterial cell division, filaments of tubulin-like FtsZ form the Z-ring, which is the cytoplasmic scaffold for divisome assembly. In Escherichia coli, the actin homologue FtsA anchors the Z-ring to the membrane and recruits divisome components, including bitopic FtsN. FtsN regulates the periplasmic peptidoglycan synthase FtsWI. To characterize how FtsA regulates FtsN, we applied electron microscopy to show that E. coli FtsA forms antiparallel double filaments on lipid monolayers when bound to the cytoplasmic tail of FtsN. Using X-ray crystallography, we demonstrate that Vibrio maritimus FtsA crystallizes as an equivalent double filament. We identified an FtsA-FtsN interaction site in the IA-IC interdomain cleft of FtsA using X-ray crystallography and confirmed that FtsA forms double filaments in vivo by site-specific cysteine cross-linking. FtsA-FtsN double filaments reconstituted in or on liposomes prefer negative Gaussian curvature, like those of MreB, the actin-like protein of the elongasome. We propose that curved antiparallel FtsA double filaments together with treadmilling FtsZ filaments organize septal peptidoglycan synthesis in the division plane.

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.

Min waves without MinC can pattern FtsA-anchored FtsZ filaments on model membranes

Although the essential proteins that drive bacterial cytokinesis have been identified, the precise mechanisms by which they dynamically interact to enable symmetrical division are largely unknown. In Escherichia coli, cell division begins with the formation of a proto-ring composed of FtsZ and its membrane-tethering proteins FtsA and ZipA. In the broadly proposed molecular scenario for ring positioning, Min waves composed of MinD and MinE distribute the FtsZ-polymerization inhibitor MinC away from mid-cell, where the Z-ring can form. Therefore, MinC is believed to be an essential element connecting the Min and FtsZ subsystems. Here, by combining cell-free protein synthesis with planar lipid membranes and microdroplets, we demonstrate that MinDE drive the formation of dynamic, antiphase patterns of FtsA-anchored FtsZ filaments even in the absence of MinC. These results suggest that Z-ring positioning may be achieved with a more minimal set of proteins than previously envisaged, providing a fresh perspective about synthetic cell division.

Cell-free protein synthesis of bacterial cytokinesis factors reveals that MinDE surface waves regulate FtsA-anchored FtsZ filaments in time and space independently of MinC.

In vitro reconstitution of Escherichia coli divisome activation

The actin-homologue FtsA is essential for E. coli cell division, as it links FtsZ filaments in the Z-ring to transmembrane proteins. FtsA is thought to initiate cell constriction by switching from an inactive polymeric to an active monomeric conformation, which recruits downstream proteins and stabilizes the Z-ring. However, direct biochemical evidence for this mechanism is missing. Here, we use reconstitution experiments and quantitative fluorescence microscopy to study divisome activation in vitro. By comparing wild-type FtsA with FtsA R286W, we find that this hyperactive mutant outperforms FtsA WT in replicating FtsZ treadmilling dynamics, FtsZ filament stabilization and recruitment of FtsN. We could attribute these differences to a faster exchange and denser packing of FtsA R286W below FtsZ filaments. Using FRET microscopy, we also find that FtsN binding promotes FtsA self-interaction. We propose that in the active divisome FtsA and FtsN exist as a dynamic copolymer that follows treadmilling filaments of FtsZ.

In E. coli, FtsA and FtsZ control the place and time of cell division. Here, the authors use in vitro experiments to show how FtsA can follow FtsZ treadmilling and that downstream proteins form dynamic copolymers with FtsA to initiate division.

An Updated Model of the Divisome: Regulation of the Septal Peptidoglycan Synthesis Machinery by the Divisome

The synthesis of a peptidoglycan septum is a fundamental part of bacterial fission and is driven by a multiprotein dynamic complex called the divisome. FtsW and FtsI are essential proteins that synthesize the peptidoglycan septum and are controlled by the regulatory FtsBLQ subcomplex and the activator FtsN. However, their mode of regulation has not yet been uncovered in detail. Understanding this process in detail may enable the development of new compounds to combat the rise in antibiotic resistance. In this review, recent data on the regulation of septal peptidoglycan synthesis is summarized and discussed. Based on structural models and the collected data, multiple putative interactions within FtsWI and with regulators are uncovered. This elaborates on and supports an earlier proposed model that describes active and inactive conformations of the septal peptidoglycan synthesis complex that are stabilized by these interactions. Furthermore, a new model on the spatial organization of the newly synthesized peptidoglycan and the synthesis complex is presented. Overall, the updated model proposes a balance between several allosteric interactions that determine the state of septal peptidoglycan synthesis.

Assembly and architecture of Escherichia coli divisome proteins FtsA and FtsZ

During Escherichia coli cell division, an intracellular complex of cell division proteins known as the Z-ring assembles at midcell during early division and serves as the site of constriction. While the predominant protein in the Z-ring is the widely conserved tubulin homolog FtsZ, the actin homolog FtsA tethers the Z-ring scaffold to the cytoplasmic membrane by binding to FtsZ. While FtsZ is known to function as a dynamic, polymerized GTPase, the assembly state of its partner, FtsA, and the role of ATP are still unclear. We report that a substitution mutation in the FtsA ATP-binding site impairs ATP hydrolysis, phospholipid vesicle remodeling in vitro, and Z-ring assembly in vivo. We demonstrate by transmission electron microscopy and Förster Resonance Energy Transfer that a truncated FtsA variant, FtsA(ΔMTS) lacking a C-terminal membrane targeting sequence, self assembles into ATP-dependent filaments. These filaments coassemble with FtsZ polymers but are destabilized by unassembled FtsZ. These findings suggest a model wherein ATP binding drives FtsA polymerization and membrane remodeling at the lipid surface, and FtsA polymerization is coregulated with FtsZ polymerization. We conclude that the coordinated assembly of FtsZ and FtsA polymers may serve as a key checkpoint in division that triggers cell wall synthesis and division progression.

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 Escherichia coli Outer Membrane β-Barrel Assembly Machinery (BAM) Crosstalks with the Divisome

The BAM is a macromolecular machine responsible for the folding and the insertion of integral proteins into the outer membrane of diderm Gram-negative bacteria. In Escherichia coli, it consists of a transmembrane β-barrel subunit, BamA, and four outer membrane lipoproteins (BamB-E). Using BAM-specific antibodies, in E. coli cells, the complex is shown to localize in the lateral wall in foci. The machinery was shown to be enriched at midcell with specific cell cycle timing. The inhibition of septation by aztreonam did not alter the BAM midcell localization substantially. Furthermore, the absence of late cell division proteins at midcell did not impact BAM timing or localization. These results imply that the BAM enrichment at the site of constriction does not require an active cell division machinery. Expression of the Tre1 toxin, which impairs the FtsZ filamentation and therefore midcell localization, resulted in the complete loss of BAM midcell enrichment. A similar effect was observed for YidC, which is involved in the membrane insertion of cell division proteins in the inner membrane. The presence of the Z-ring is needed for preseptal peptidoglycan (PG) synthesis. As BAM was shown to be embedded in the PG layer, it is possible that BAM is inserted preferentially simultaneously with de novo PG synthesis to facilitate the insertion of OMPs in the newly synthesized outer membrane.

FtsA acts through FtsW to promote cell wall synthesis during cell division in Escherichia coli

In Escherichia coli, FtsQLB is required to recruit the essential septal peptidoglycan (sPG) synthase FtsWI to FtsA, which tethers FtsZ filaments to the membrane. The arrival of FtsN switches FtsQLB in the periplasm and FtsA in the cytoplasm from a recruitment role to active forms that synergize to activate FtsWI. Genetic evidence indicates that the active form of FtsQLB has an altered conformation with an exposed domain of FtsL that acts on FtsI to activate FtsW. However, how FtsA contributes to the activation of FtsW is not clear, as it could promote the conformational change in FtsQLB or act directly on FtsW. Here, we show that the overexpression of an activated FtsA (FtsA*) bypasses FtsQ, indicating it can compensate for FtsQ's recruitment function. Consistent with this, FtsA* also rescued FtsL and FtsB mutants deficient in FtsW recruitment. FtsA* also rescued an FtsL mutant unable to deliver the periplasmic signal from FtsN, consistent with FtsA* acting on FtsW. In support of this, an FtsW mutant was isolated that was rescued by an activated FtsQLB but not by FtsA*, indicating it was specifically defective in activation by FtsA. Our results suggest that in response to FtsN, the active form of FtsA acts on FtsW in the cytoplasm and synergizes with the active form of FtsQLB acting on FtsI in the periplasm to activate FtsWI to carry out sPG synthesis.

Bacterial cell proliferation: from molecules to cells

Bacterial cell proliferation is highly efficient, both because bacteria grow fast and multiply with a low failure rate. This efficiency is underpinned by the robustness of the cell cycle and its synchronization with cell growth and cytokinesis. Recent advances in bacterial cell biology brought about by single-cell physiology in microfluidic chambers suggest a series of simple phenomenological models at the cellular scale, coupling cell size and growth with the cell cycle. We contrast the apparent simplicity of these mechanisms based on the addition of a constant size between cell cycle events (e.g. two consecutive initiation of DNA replication or cell division) with the complexity of the underlying regulatory networks. Beyond the paradigm of cell cycle checkpoints, the coordination between the DNA and division cycles and cell growth is largely mediated by a wealth of other mechanisms. We propose our perspective on these mechanisms, through the prism of the known crosstalk between DNA replication and segregation, cell division and cell growth or size. We argue that the precise knowledge of these molecular mechanisms is critical to integrate the diverse layers of controls at different time and space scales into synthetic and verifiable models.

Natural flavonoid morin showed anti-bacterial activity against Vibrio cholera after binding with cell division protein FtsA near ATP binding site

BACKGROUND

Increasing antibiotic-resistance in bacterial strains has boosted the need to find new targets for drug delivery. FtsA, a major bacterial divisome protein can be a potent novel drug-target.

METHODS AND RESULTS

This study finds, morin (3,5,7,2',4'-pentahydroxyflavone), a bio-available flavonoid, had anti-bacterial activities against Vibrio cholerae, IC₅₀ (50 μM) and MIC (150 μM). Morin (2 mM) kills ~20% of human lung fibroblast (WI38) and human intestinal epithelial (HIEC-6) cells in 24 h in-vitro. Fluorescence studies showed morin binds to VcFtsA (FtsA of V. cholerae) with a K_d of 4.68 ± 0.4 μM, inhibiting the protein's polymerization by 72 ± 7% at 25 μM concentration. Morin also affected VcFtsA's ATPase activity, recording ~80% reduction at 20 μM concentration. The in-silico binding study indicated binding sites of morin and ATP on VcFtsA had overlapping amino acids. Mant-ATP, a fluorescent ATP-derivative, showed increased fluorescence on binding to VcFtsA in absence of morin, but in its presence, Mant-ATP fluorescence decreased. VcFtsA-S40A mutant protein did not bind to morin.

CONCLUSIONS

VcFtsA-morin interaction inhibits the polymerization of the protein by affecting its ATPase activity. The destabilized VcFtsA assembly in-turn affected the cell division in V. cholerae, yielding an elongated morphology.

GENERAL SIGNIFICANCE

Collectively, these findings explore the anti-bacterial effect of morin on V. cholerae cells targeting VcFtsA, encouraging it to become a potent anti-bacterial agent. Low cytotoxicity of morin against human cells (host) is therapeutically advantageous. This study will also help in synthesizing novel derivatives that can target VcFtsA more efficiently.

FtsZ dynamics in bacterial division: What, how, and why?

Bacterial cell division is orchestrated by the divisome, a protein complex centered on the tubulin homolog FtsZ. FtsZ polymerizes into a dynamic ring that defines the division site, recruits downstream proteins, and directs peptidoglycan synthesis to drive constriction. Recent studies have documented treadmilling of FtsZ polymer clusters both in cells and in vitro. Emerging evidence suggests that FtsZ dynamics are regulated largely by intrinsic properties of FtsZ itself and by the membrane anchoring protein FtsA. Although FtsZ dynamics are broadly required for Z-ring assembly, their role(s) during constriction may vary among bacterial species. These recent advances set the stage for future studies to investigate how FtsZ dynamics are physically and/or functionally coupled to peptidoglycan metabolic enzymes to direct efficient division.

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.

Cell-free biogenesis of bacterial division proto-rings that can constrict liposomes

A major challenge towards the realization of an autonomous synthetic cell resides in the encoding of a division machinery in a genetic programme. In the bacterial cell cycle, the assembly of cytoskeletal proteins into a ring defines the division site. At the onset of the formation of the Escherichia coli divisome, a proto-ring consisting of FtsZ and its membrane-recruiting proteins takes place. Here, we show that FtsA-FtsZ ring-like structures driven by cell-free gene expression can be reconstituted on planar membranes and inside liposome compartments. Such cytoskeletal structures are found to constrict the liposome, generating elongated membrane necks and budding vesicles. Additional expression of the FtsZ cross-linker protein ZapA yields more rigid FtsZ bundles that attach to the membrane but fail to produce budding spots or necks in liposomes. These results demonstrate that gene-directed protein synthesis and assembly of membrane-constricting FtsZ-rings can be combined in a liposome-based artificial cell.

Godino et al. show that FtsA-FtsZ ring-like structures driven by cell-free gene expression can be reconstituted on planar membranes and inside liposome compartments. These cytoskeletal structures constrict the liposome, generating elongated membrane necks and budding vesicles. This study represents a step forward to realizing genetic programming of synthetic cell division.

Roles of ATP Hydrolysis by FtsEX and Interaction with FtsA in Regulation of Septal Peptidoglycan Synthesis and Hydrolysis

In Escherichia coli, FtsEX coordinates peptidoglycan (PG) synthesis and hydrolysis at the septum. It acts on FtsA in the cytoplasm to promote recruitment of septal PG synthetases and recruits EnvC, an activator of septal PG hydrolases, in the periplasm. Following recruitment, ATP hydrolysis by FtsEX is thought to regulate both PG synthesis and hydrolysis, but how it does this is not well understood. Here, we show that an ATPase mutant of FtsEX blocks septal PG synthesis similarly to cephalexin, suggesting that ATP hydrolysis by FtsEX is required throughout septation. Using mutants that uncouple the roles of FtsEX in septal PG synthesis and hydrolysis, we find that recruitment of EnvC to the septum by FtsEX, but not ATP hydrolysis, is required to promote cell separation when the NlpD-mediated cell separation system is present. However, ATP hydrolysis by FtsEX becomes necessary for efficient cell separation when the NlpD system is inactivated, suggesting that the ATPase activity of FtsEX is required for optimal activity of EnvC. Importantly, under conditions that suppress the role of FtsEX in cell division, disruption of the FtsEX-FtsA interaction delays cell separation, highlighting the importance of this interaction in coupling the cell separation system with the septal PG synthetic complex.IMPORTANCE Cytokinesis in Gram-negative bacteria requires coordinated invagination of the three layers of the cell envelope; otherwise, cells become sensitive to hydrophobic antibiotics and can even undergo cell lysis. In E. coli, the ABC transporter FtsEX couples the synthesis and hydrolysis of the stress-bearing peptidoglycan layer at the septum by interacting with FtsA and EnvC, respectively. ATP hydrolysis by FtsEX is critical for its function, but the reason why is not clear. Here, we find that in the absence of ATP hydrolysis, FtsEX blocks septal PG synthesis similarly to cephalexin. However, an FtsEX ATPase mutant, under conditions where it cannot block division, rescues ftsEX phenotypes as long as a partially redundant cell separation system is present. Furthermore, we find that the FtsEX-FtsA interaction is important for efficient cell separation.

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.

Bacterial cell division at a glance

Bacterial cell division is initiated by the midcell assembly of polymers of the tubulin-like GTPase FtsZ. The FtsZ ring (Z-ring) is a discontinuous structure made of dynamic patches of FtsZ that undergo treadmilling motion. Roughly a dozen additional essential proteins are recruited to the division site by the dynamic Z-ring scaffold and subsequently activate cell wall synthesis to drive cell envelope constriction during division. In this Cell Science at a Glance article and the accompanying poster, we summarize our understanding of the assembly and activation of the bacterial cell division machinery. We introduce polymerization properties of FtsZ and discuss our current knowledge of divisome assembly and activation. We further highlight the intimate relationship between the structure and dynamics of FtsZ and the movement and activity of cell wall synthases at the division site, before concluding with a perspective on the most important open questions on bacterial cell division.

FtsA Regulates Z-Ring Morphology and Cell Wall Metabolism in an FtsZ C-Terminal Linker-Dependent Manner in Caulobacter crescentus

Bacterial cell division requires the assembly of a multiprotein division machinery, or divisome, that remodels the cell envelope to cause constriction. The cytoskeletal protein FtsZ forms a ringlike scaffold for the divisome at the incipient division site. FtsZ has three major regions: a conserved GTPase domain that polymerizes into protofilaments on binding GTP, a C-terminal conserved peptide (CTC) required for binding membrane-anchoring proteins, and a C-terminal linker (CTL) region of varied length and low sequence conservation. Recently, we demonstrated that the CTL regulates FtsZ polymerization properties in vitro and Z-ring structure and cell wall metabolism in vivo In Caulobacter crescentus, an FtsZ variant lacking the CTL (designated ΔCTL) can recruit all known divisome members and drive local cell wall synthesis but has dominant lethal effects on cell wall metabolism. To understand the underlying mechanism of the CTL-dependent regulation of cell wall metabolism, we expressed chimeras of FtsZ domains from C. crescentus and Escherichia coli and observed that the E. coli GTPase domain fused to the C. crescentus CTC phenocopies C. crescentus ΔCTL. By investigating the contributions of FtsZ-binding partners, we identified variants of FtsA, a known membrane anchor for FtsZ, that delay or exacerbate the ΔCTL phenotype. Additionally, we observed that the ΔCTL protein forms extended helical structures in vivo upon FtsA overproduction. We propose that misregulation downstream of defective ΔCTL assembly is propagated through the interaction between the CTC and FtsA. Overall, our study provides mechanistic insights into the CTL-dependent regulation of cell wall enzymes downstream of FtsZ polymerization.IMPORTANCE Bacterial cell division is essential and requires the recruitment and regulation of a complex network of proteins needed to initiate and guide constriction and cytokinesis. FtsZ serves as a master regulator for this process, and its function is highly dependent on both its assembly into the canonical Z ring and interactions with protein binding partners, all of which results in the activation of enzymes that remodel the cell wall to drive constriction. Using mutants of FtsZ, we have elaborated on the role of its C-terminal linker domain in regulating Z-ring stability and dynamics, as well as the requirement for its conserved C-terminal domain and interaction with the membrane-anchoring protein FtsA for regulating the process of cell wall remodeling for constriction.

Peptide Linkers within the Essential FtsZ Membrane Tethers ZipA and FtsA Are Nonessential for Cell Division

Bacteria such as Escherichia coli divide by organizing filaments of FtsZ, a tubulin homolog that assembles into dynamic treadmilling membrane-associated protein filaments at the cell midpoint. FtsA and ZipA proteins are required to tether these filaments to the inner face of the cytoplasmic membrane, and loss of either tether is lethal. ZipA from E. coli and other closely related species harbors a long linker region that connects the essential N-terminal transmembrane domain to the C-terminal globular FtsZ-binding domain, and part of this linker includes a P/Q-rich peptide that is predicted to be intrinsically disordered. We found unexpectedly that several large deletions of the ZipA linker region, including the entire P/Q rich peptide, had no effect on cell division under normal conditions. However, we found that the loss of the P/Q region made cells more resistant to excess levels of FtsA and more sensitive to conditions that displaced FtsA from FtsZ. FtsA also harbors a short ∼20-residue peptide linker that connects the main globular domain with the C-terminal amphipathic helix that is important for membrane binding. In analogy with ZipA, deletion of 11 of the central residues in the FtsA linker had little effect on FtsA function in cell division.IMPORTANCE Escherichia coli cells divide using a cytokinetic ring composed of polymers of the tubulin-like FtsZ. To function properly, these polymers must attach to the inner surface of the cytoplasmic membrane via two essential membrane-associated tethers, FtsA and ZipA. Both FtsA and ZipA contain peptide linkers that connect their membrane-binding domains with their FtsZ-binding domains. Although they are presumed to be crucial for cell division activity, the importance of these linkers has not yet been rigorously tested. Here, we show that large segments of these linkers can be removed with few consequences for cell division, although several subtle defects were uncovered. Our results suggest that ZipA, in particular, can function in cell division without an extended linker.

FtsA G50E mutant suppresses the essential requirement for FtsK during bacterial cell division in Escherichia coli

In Escherichia coli, the N-terminal domain of the essential protein FtsK (FtsK_N) is proposed to modulate septum formation through the formation of dynamic and essential protein interactions with both the Z-ring and late-stage division machinery. Using genomic mutagenesis, complementation analysis, and in vitro pull-down assays, we aimed to identify protein interaction partners of FtsK essential to its function during division. Here, we identified the cytoplasmic Z-ring membrane anchoring protein FtsA as a direct protein-protein interaction partner of FtsK. Random genomic mutagenesis of an ftsK temperature-sensitive strain of E. coli revealed an FtsA point mutation (G50E) that is able to fully restore normal cell growth and morphology, and further targeted site-directed mutagenesis of FtsA revealed several other point mutations capable of fully suppressing the essential requirement for functional FtsK. Together, this provides insight into a potential novel co-complex formed between these components during division and suggests FtsA may directly impact FtsK function.

Roles of FtsEX in cell division

FtsEX is a member of a small subclass of ABC transporters that uses mechano-transmission to perform work in the periplasm. FtsEX controls periplasmic peptidoglycan (PG) hydrolase activities in many Gram negative and positive organisms to ensure the safe separation of daughter cells during division. In these organisms FtsEX localizes to the Z ring and uses its ATPase activity to regulate its periplasmic effectors. In Escherichia coli, FtsEX also participates in building the divisome and coordinates PG synthesis with PG hydrolysis. This review discusses studies that are beginning to elucidate the mechanisms of FtsEX's various roles in cell division.

How FtsEX localizes to the Z ring and interacts with FtsA to regulate cell division

In Escherichia coli, FtsEX, a member of the ABC transporter superfamily, is involved in regulating the assembly and activation of the divisome to couple cell wall synthesis to cell wall hydrolysis at the septum. Genetic studies indicate FtsEX acts on FtsA to begin the recruitment of the downstream division proteins but blocks septal PG synthesis until a signal is received that divisome assembly is complete. However, the details of how FtsEX localizes to the Z ring and how it interacts with FtsA are not clear. Our results show that recruitment of FtsE and FtsX is codependent and suggest that the FtsEX complex is recruited through FtsE interacting with the conserved tail of FtsZ (CCTP), thus adding FtsEX to a growing list of proteins that interacts with the CCTP of FtsZ. Furthermore, we find that the N-terminus of FtsX is not required for FtsEX localization to the Z ring but is required for its functions in cell division indicating that it interacts with FtsA. Taken together, these results suggest that FtsEX first interacts with FtsZ to localize to the Z ring and then interacts with FtsA to promote divisome assembly and regulate septal PG synthesis.

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.

At the Heart of Bacterial Cytokinesis: The Z Ring

Bacterial cell division is mediated by the divisome which is organized by the Z ring, a cytoskeletal element formed by the polymerization of the tubulin homologue FtsZ. Despite billions of years of bacterial evolution, the Z ring is nearly universal among bacteria that have a cell wall and divide by binary fission. Recent studies have revealed the mechanism of cooperative assembly of FtsZ and that the Z ring consists of patches of FtsZ filaments tethered to the membrane that treadmill to distribute the septal biosynthetic machinery. Here, we summarize these advances and discuss questions raised by these new findings.

Conservation of conformational dynamics across prokaryotic actins

The actin family of cytoskeletal proteins is essential to the physiology of virtually all archaea, bacteria, and eukaryotes. While X-ray crystallography and electron microscopy have revealed structural homologies among actin-family proteins, these techniques cannot probe molecular-scale conformational dynamics. Here, we use all-atom molecular dynamic simulations to reveal conserved dynamical behaviors in four prokaryotic actin homologs: MreB, FtsA, ParM, and crenactin. We demonstrate that the majority of the conformational dynamics of prokaryotic actins can be explained by treating the four subdomains as rigid bodies. MreB, ParM, and FtsA monomers exhibited nucleotide-dependent dihedral and opening angles, while crenactin monomer dynamics were nucleotide-independent. We further show that the opening angle of ParM is sensitive to a specific interaction between subdomains. Steered molecular dynamics simulations of MreB, FtsA, and crenactin dimers revealed that changes in subunit dihedral angle lead to intersubunit bending or twist, suggesting a conserved mechanism for regulating filament structure. Taken together, our results provide molecular-scale insights into the nucleotide and polymerization dependencies of the structure of prokaryotic actins, suggesting mechanisms for how these structural features are linked to their diverse functions.

Simulations are a critical tool for uncovering the molecular mechanisms underlying biological form and function. Here, we use molecular-dynamics simulations to identify common and specific dynamical behaviors in four prokaryotic homologs of actin, a cytoskeletal protein that plays important roles in cellular structure and division in eukaryotes. The four actin homologs have diverse functions including cell division, cell shape maintenance, and DNA segmentation. Dihedral angles and opening angles in monomers of bacterial MreB, FtsA, and ParM were all sensitive to whether the subunit was bound to ATP or ADP, unlike in the archaeal homolog crenactin. In simulations of MreB, FtsA, and crenactin dimers, changes in subunit dihedral angle led to bending or twisting in filaments of these proteins, suggesting a mechanism for regulating the properties of large filaments. Taken together, our simulations set the stage for understanding and exploiting structure-function relationships of prokaryotic cytoskeletons.

Synthetic Minimal Cell: Self-Reproduction of the Boundary Layer

A critical aspect in the bottom-up construction of a synthetic minimal cell is to develop an entity that is capable of self-reproduction. A key role in this process is the expansion and division of the boundary layer that surrounds the compartment, a process in which content loss has to be avoided and the barrier function maintained. Here, we describe the latest developments regarding self-reproduction of a boundary layer with a focus on the growth and division of phospholipid-based membranes in the context of a synthetic minimal cell.

Roles of the DedD Protein in Escherichia coli Cell Constriction

Two key tasks of the bacterial septal-ring (SR) machinery during cell constriction are the generation of an inward-growing annulus of septal peptidoglycan (sPG) and the concomitant splitting of its outer edge into two layers of polar PG that will be inherited by the two new cell ends. FtsN is an essential SR protein that helps trigger the active constriction phase in Escherichia coli by inducing a self-enhancing cycle of processes that includes both sPG synthesis and splitting and that we refer to as the sPG loop. DedD is an SR protein that resembles FtsN in several ways. Both are bitopic inner membrane proteins with small N-terminal cytoplasmic parts and larger periplasmic parts that terminate with a SPOR domain. Though absence of DedD normally causes a mild cell-chaining phenotype, the protein is essential for division and survival of cells with limited FtsN activity. Here, we find that a small N-terminal portion of DedD (^NDedD; DedD^1-54) is required and sufficient to suppress ΔdedD-associated division phenotypes, and we identify residues within its transmembrane domain that are particularly critical to DedD function. Further analyses indicate that DedD and FtsN act in parallel to promote sPG synthesis, possibly by engaging different parts of the FtsBLQ subcomplex to induce a conformation that permits and/or stimulates the activity of sPG synthase complexes composed of FtsW, FtsI (PBP3), and associated proteins. We propose that, like FtsN, DedD promotes cell fission by stimulating sPG synthesis, as well as by providing positive feedback to the sPG loop.IMPORTANCE Cell division (cytokinesis) is a fundamental biological process that is incompletely understood for any organism. Division of bacterial cells relies on a ring-like machinery called the septal ring or divisome that assembles along the circumference of the mother cell at the site where constriction eventually occurs. In the well-studied bacterium Escherichia coli, this machinery contains over 30 distinct proteins. We identify functionally important parts of one of these proteins, DedD, and present evidence supporting a role for DedD in helping to induce and/or sustain a self-enhancing cycle of processes that are executed by fellow septal-ring proteins and that drive the active constriction phase of the cell division cycle.

Direct Interaction between the Two Z Ring Membrane Anchors FtsA and ZipA

The initiation of Escherichia coli cell division requires three proteins, FtsZ, FtsA, and ZipA, which assemble in a dynamic ring-like structure at midcell. Along with the transmembrane protein ZipA, the actin-like FtsA helps to tether treadmilling polymers of tubulin-like FtsZ to the membrane. In addition to forming homo-oligomers, FtsA and ZipA interact directly with the C-terminal conserved domain of FtsZ. Gain-of-function mutants of FtsA are deficient in forming oligomers and can bypass the need for ZipA, suggesting that ZipA may normally function to disrupt FtsA oligomers, although no direct interaction between FtsA and ZipA has been reported. Here, we use in vivo cross-linking to show that FtsA and ZipA indeed interact directly. We identify the exposed surface of FtsA helix 7, which also participates in binding to ATP through its internal surface, as a key interface needed for the interaction with ZipA. This interaction suggests that FtsZ's membrane tethers may regulate each other's activities.IMPORTANCE To divide, most bacteria first construct a protein machine at the plane of division and then recruit the machinery that will synthesize the division septum. In Escherichia coli, this first stage involves the assembly of FtsZ polymers at midcell, which directly bind to membrane-associated proteins FtsA and ZipA to form a discontinuous ring structure. Although FtsZ directly binds both FtsA and ZipA, it is unclear why FtsZ requires two separate membrane tethers. Here, we uncover a new direct interaction between the tethers, which involves a helix within FtsA that is adjacent to its ATP binding pocket. Our findings imply that in addition to their known roles as FtsZ membrane anchors, FtsA and ZipA may regulate each other's structure and function.

Z-ring membrane anchors associate with cell wall synthases to initiate bacterial cell division

During the transition from elongation to septation, Escherichia coli establishes a ring-like peptidoglycan growth zone at the future division site. This preseptal peptidoglycan synthesis does not require the cell division-specific peptidoglycan transpeptidase PBP3 or most of the other cell division proteins, but it does require FtsZ, its membrane-anchor ZipA and at least one of the bi-functional transglycosylase-transpeptidases, PBP1A or PBP1B. Here we show that PBP1A and PBP1B interact with ZipA and localise to preseptal sites in cells with inhibited PBP3. ZipA stimulates the glycosyltransferase activity of PBP1A. The membrane-anchored cell division protein FtsN localises at preseptal sites and stimulates both activities of PBP1B. Genes zipA and ftsN can be individually deleted in ftsA* mutant cells, but the simultaneous depletion of both proteins is lethal and cells do not establish preseptal sites. Our data support a model according to which ZipA and FtsN-FtsA have semi-redundant roles in connecting the cytosolic FtsZ ring with the membrane-anchored peptidoglycan synthases during the preseptal phase of envelope growth.

Proteins FtsZ, ZipA, and either PBP1A or PBP1B are required for the synthesis of preseptal peptidoglycan at the future cell division site in E. coli. Here, Pazos et al. provide evidence that ZipA and FtsA-FtsN connect the cytosolic FtsZ ring with the membrane-anchored PBPs.

Antimicrobial peptides from C-terminal amphipathic region of E. coli FtsA

Antimicrobial peptides constitute an indispensable component of innate immune system in organisms ranging from bacteria to man. Despite this, peptides lag far behind the conventional antibiotics in treating infections. The menace of multidrug-resistant bacteria, however, has revived the antimicrobial peptide research. We reasoned that the membrane-binding regions of bacterial proteins could be purposed to combat them. Here, we identify potent antimicrobial peptides from the C-terminal amphipathic helix of E. coli FtsA protein. The 11 and 13-residue peptides exhibited activity against E. coli, gentamicin-resistant MRSA, and C. albicans. The activity is little affected by the presence of salt and divalent cations. The peptides preferentially bind to the negatively-charged membranes as indicated by tryptophan fluorescence studies. The peptides permeabilize the E. coli outer and inner membranes at very promising concentrations suggesting membrane-disruption as one of the mechanisms of killing.

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.

Disruption of divisome assembly rescued by FtsN-FtsA interaction in Escherichia coli

Cell division requires the assembly of a protein complex called the divisome. The divisome assembles in a hierarchical manner, with FtsA functioning as a hub to connect the Z-ring with the rest of the divisome and FtsN arriving last to activate the machine to synthesize peptidoglycan. FtsEX arrives as the Z-ring forms and acts on FtsA to initiate recruitment of the other divisome components. In the absence of FtsEX, recruitment is blocked; however, a multitude of conditions allow FtsEX to be bypassed. Here, we find that all such FtsEX bypass conditions, as well as the bypass of FtsK, depend upon the interaction of FtsN with FtsA, which promotes the back-recruitment of the late components of the divisome. Furthermore, our results suggest that these bypass conditions enhance the weak interaction of FtsN with FtsA and its periplasmic partners so that the divisome proteins are brought to the Z-ring when the normal hierarchical pathway is disrupted.

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.

Unite to divide: Oligomerization of tubulin and actin homologs regulates initiation of bacterial cell division

To generate two cells from one, bacteria such as Escherichia coli use a complex of membrane-embedded proteins called the divisome that synthesize the division septum. The initial stage of cytokinesis requires a tubulin homolog, FtsZ, which forms polymers that treadmill around the cell circumference. The attachment of these polymers to the cytoplasmic membrane requires an actin homolog, FtsA, which also forms dynamic polymers that directly bind to FtsZ. Recent evidence indicates that FtsA and FtsZ regulate each other’s oligomeric state in E. coli to control the progression of cytokinesis, including the recruitment of septum synthesis proteins. In this review, we focus on recent advances in our understanding of protein-protein association between FtsZ and FtsA in the initial stages of divisome function, mainly in the well-characterized E. coli system.

Cell Division in genus Corynebacterium: protein-protein interaction and molecular docking of SepF and FtsZ in the understanding of cytokinesis in pathogenic species

The genus Corynebacterium includes species of great importance in medical, veterinary and biotechnological fields. The genus-specific families (PLfams) from PATRIC have been used to observe conserved proteins associated to all species. Our results showed a large number of conserved proteins that are associated with the cellular division process. Was not observe in our results other proteins like FtsA and ZapA that interact with FtsZ. Our findings point that SepF overlaps the function of this proteins explored by molecular docking, protein-protein interaction and sequence analysis. Transcriptomic analysis showed that these two (Sepf and FtsZ) proteins can be expressed in different conditions together. The work presents novelties on molecules participating in the cell division event, from the interaction of FtsZ and SepF, as new therapeutic targets.

FtsA reshapes membrane architecture and remodels the Z-ring in Escherichia coli

Cell division in prokaryotes initiates with assembly of the Z-ring at midcell, which, in Escherichia coli, is tethered to the inner leaflet of the cytoplasmic membrane through a direct interaction with FtsA, a widely conserved actin homolog. The Z-ring is comprised of polymers of tubulin-like FtsZ and has been suggested to provide the force for constriction. Here, we demonstrate that FtsA exerts force on membranes causing redistribution of membrane architecture, robustly hydrolyzes ATP and directly engages FtsZ polymers in a reconstituted system. Phospholipid reorganization by FtsA occurs rapidly and is mediated by insertion of a C-terminal membrane targeting sequence (MTS) into the bilayer and further promoted by a nucleotide-dependent conformational change relayed to the MTS. FtsA also recruits FtsZ to phospholipid vesicles via a direct interaction with the FtsZ C-terminus and regulates FtsZ assembly kinetics. These results implicate the actin homolog FtsA in establishment of a Z-ring scaffold, while directly remodeling the membrane and provide mechanistic insight into localized cell wall remodeling, invagination and constriction at the onset of division.

Suppression of a Thermosensitive zipA Cell Division Mutant by Altering Amino Acid Metabolism

ZipA is essential for cell division in Escherichia coli, acting early in the process to anchor polymers of FtsZ to the cytoplasmic membrane. Along with FtsA, FtsZ and ZipA form a proto-ring at midcell that recruits additional proteins to eventually build the division septum. Cells carrying the thermosensitive zipA1 allele divide fairly normally at 30°C in rich medium but cease dividing at temperatures above 34°C, forming long filaments. In a search for suppressors of the zipA1 allele, we found that deletions of specific genes involved in amino acid biosynthesis could partially rescue cell growth and division at 34°C or 37°C but not at 42°C. Notably, although a diverse group of amino acid biosynthesis gene deletions could partially rescue the growth of zipA1 cells at 34°C, only deletions of genes related to the biosynthesis of threonine, glycine, serine, and methionine could rescue growth at 37°C. Adding exogenous pyridoxal 5-phosphate (PLP), a cofactor for many of the enzymes affected by this study, partially suppressed zipA1 mutant thermosensitivity. For many of the deletions, PLP had an additive rescuing effect on the zipA1 mutant. Moreover, added PLP partially suppressed the thermosensitivity of ftsQ and ftsK mutants and weakly suppressed an ftsI mutant, but it failed to suppress ftsA or ftsZ thermosensitive mutants. Along with the ability of a deletion of metC to partially suppress the ftsK mutant, our results suggest that perturbations of amino acid metabolic pathways, particularly those that redirect the flow of carbon away from the synthesis of threonine, glycine, or methionine, are able to partially rescue some cell division defects.IMPORTANCE Cell division of bacteria, such as Escherichia coli, is essential for their successful colonization. It is becoming increasingly clear that nutritional status and central metabolism can affect bacterial size and shape; for example, a metabolic enzyme (OpgH) can moonlight as a regulator of FtsZ, an essential cell division protein. Here, we demonstrate a link between amino acid metabolism and ZipA, another essential cell division protein that binds directly to FtsZ and tethers it to the cytoplasmic membrane. Our evidence suggests that altering flux through the methionine-threonine-glycine-serine pathways and supplementing with the enzyme cofactor pyridoxal-5-phosphate can partially compensate for an otherwise lethal defect in ZipA, as well as several other cell division proteins.

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.

Escherichia coli FtsA forms lipid-bound minirings that antagonize lateral interactions between FtsZ protofilaments

Most bacteria divide using a protein machine called the divisome that spans the cytoplasmic membrane. Key divisome proteins on the membrane’s cytoplasmic side include tubulin-like FtsZ, which forms GTP-dependent protofilaments, and actin-like FtsA, which tethers FtsZ to the membrane. Here we present genetic evidence that in Escherichia coli, FtsA antagonizes FtsZ protofilament bundling in vivo. We then show that purified FtsA does not form straight polymers on lipid monolayers as expected, but instead assembles into dodecameric minirings, often in hexameric arrays. When coassembled with FtsZ on lipid monolayers, these FtsA minirings appear to guide FtsZ to form long, often parallel, but unbundled protofilaments, whereas a mutant of FtsZ (FtsZ*) with stronger lateral interactions remains bundled. In contrast, a hypermorphic mutant of FtsA (FtsA*) forms mainly arcs instead of minirings and enhances lateral interactions between FtsZ protofilaments. Based on these results, we propose that FtsA antagonizes lateral interactions between FtsZ protofilaments, and that the oligomeric state of FtsA may influence FtsZ higher-order structure and divisome function.

The actin-like protein FtsA and the tubulin-like protein FtsZ play crucial roles during cell division in most bacteria. Here, the authors show that FtsA forms minirings on lipid monolayers, and present evidence supporting that its oligomeric state modulates the bundling of FtsZ protofilaments.

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.

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.

Roles of the Essential Protein FtsA in Cell Growth and Division in Streptococcus pneumoniae

Streptococcus pneumoniae is an ovoid-shaped Gram-positive bacterium that grows by carrying out peripheral and septal peptidoglycan (PG) synthesis, analogous to model bacilli, such as Escherichia coli and Bacillus subtilis In the model bacilli, FtsZ and FtsA proteins assemble into a ring at midcell and are dedicated to septal PG synthesis but not peripheral PG synthesis; hence, inactivation of FtsZ or FtsA results in long filamentous cells unable to divide. Here, we demonstrate that FtsA and FtsZ colocalize at midcell in S. pneumoniae and that partial depletion of FtsA perturbs septum synthesis, resulting in elongated cells with multiple FtsZ rings that fail to complete septation. Unexpectedly, complete depletion of FtsA resulted in the delocalization of FtsZ rings and ultimately cell ballooning and lysis. In contrast, depletion or deletion of gpsB and sepF, which in B. subtilis are synthetically lethal with ftsA, resulted in enlarged and elongated cells with multiple FtsZ rings, with deletion of sepF mimicking partial depletion of FtsA. Notably, cell ballooning was not observed, consistent with later recruitment of these proteins to midcell after Z-ring assembly. The overproduction of FtsA stimulates septation and suppresses the cell division defects caused by the deletion of sepF and gpsB under some conditions, supporting the notion that FtsA shares overlapping functions with GpsB and SepF at later steps in the division process. Our results indicate that, in S. pneumoniae, both GpsB and SepF are involved in septal PG synthesis, whereas FtsA and FtsZ coordinate both peripheral and septal PG synthesis and are codependent for localization at midcell.IMPORTANCEStreptococcus pneumoniae (pneumococcus) is a clinically important human pathogen for which more therapies against unexploited essential targets, like cell growth and division proteins, are needed. Pneumococcus is an ovoid-shaped Gram-positive bacterium with cell growth and division properties that have important distinctions from those of rod-shaped bacteria. Gaining insights into these processes can thus provide valuable information to develop novel antimicrobials. Whereas rods use distinctly localized protein machines at different cellular locations to synthesize peripheral and septal peptidoglycans, we present evidence that S. pneumoniae organizes these two machines at a single location in the middle of dividing cells. Here, we focus on the properties of the actin-like protein FtsA as an essential orchestrator of peripheral and septal growth in this bacterium.

Reconstitution of Protein Dynamics Involved in Bacterial Cell Division

Even simple cells like bacteria have precisely regulated cellular anatomies, which allow them to grow, divide and to respond to internal or external cues with high fidelity. How spatial and temporal intracellular organization in prokaryotic cells is achieved and maintained on the basis of locally interacting proteins still remains largely a mystery. Bulk biochemical assays with purified components and in vivo experiments help us to approach key cellular processes from two opposite ends, in terms of minimal and maximal complexity. However, to understand how cellular phenomena emerge, that are more than the sum of their parts, we have to assemble cellular subsystems step by step from the bottom up. Here, we review recent in vitro reconstitution experiments with proteins of the bacterial cell division machinery and illustrate how they help to shed light on fundamental cellular mechanisms that constitute spatiotemporal order and regulate cell division.