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

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

Assembly dynamics of FtsZ rings in Bacillus subtilis and Escherichia coli and effects of FtsZ-regulating proteins

FtsZ is the major cytoskeletal component of the bacterial cell division machinery. It forms a ring-shaped structure (the Z ring) that constricts as the bacterium divides. Previous in vivo experiments with green fluorescent protein-labeled FtsZ and fluorescence recovery after photobleaching have shown that the Escherichia coli Z ring is extremely dynamic, continually remodeling itself with a half time of 30 s, similar to microtubules in the mitotic spindle. In the present work, under different experimental conditions, we have found that the half time for fluorescence recovery of E. coli Z rings is even shorter (approximately 9 s). As before, the turnover appears to be coupled to GTP hydrolysis, since the mutant FtsZ84 protein, with reduced GTPase in vitro, showed an approximately 3-fold longer half time. We have also extended the studies to Bacillus subtilis and found that this species exhibits equally rapid dynamics of the Z ring (half time, approximately 8 s). Interestingly, null mutations of the FtsZ-regulating proteins ZapA, EzrA, and MinCD had only modest effects on the assembly dynamics. This suggests that these proteins do not directly regulate FtsZ subunit exchange in and out of polymers. In B. subtilis, only 30 to 35% of the FtsZ protein was in the Z ring, from which we conclude that a Z ring only 2 or 3 protofilaments thick can function for cell division.     

In vivo organization of the FtsZ‐ring by ZapA and ZapB revealed by quantitative super‐resolution microscopy

In most bacterial cells, cell division is dependent on the polymerization of the FtsZ protein to form a ring‐like structure (Z‐ring) at the midcell. Despite its essential role, the molecular architecture of the Z‐ring remains elusive. In this work we examine the roles of two FtsZ‐associated proteins, ZapA and ZapB, in the assembly dynamics and structure of the Z‐ring in Escherichia coli cells. In cells deleted of zapA or zapB, we observed abnormal septa and highly dynamic FtsZ structures. While details of these FtsZ structures are difficult to discern under conventional fluorescence microscopy, single‐molecule‐based super‐resolution imaging method Photoactivated Localization Microscopy (PALM) reveals that these FtsZ structures arise from disordered arrangements of FtsZ clusters. Quantitative analysis finds these clusters are larger and comprise more molecules than a single FtsZ protofilament, and likely represent a distinct polymeric species that is inherent to the assembly pathway of the Z‐ring. Furthermore, we find these clusters are not due to the loss of ZapB–MatP interaction in ΔzapA and ΔzapB cells. Our results suggest that the main function of ZapA and ZapB in vivo may not be to promote the association of individual protofilaments but to align FtsZ clusters that consist of multiple FtsZ protofilaments.     

Bacterial division: the fellowship of the ring

The Z ring, composed of the tubulin homolog FtsZ, is essential for bacterial cell division. Recently a new protein, ZapA, has been discovered that localizes to the Z ring and stabilizes it, probably by promoting the bundling of FtsZ protofilaments.     

Spatial resolution of two bacterial cell division proteins: ZapA recruits ZapB to the inner face of the Z‐ring

FtsZ, the essential regulator of bacterial cell division, is a dynamic cytoskeletal protein that forms helices that condense into the Z‐ring prior to division. Two small coiled‐coil proteins, ZapA and ZapB, are both recruited early to the Z‐ring. We show here that ZapB is recruited to the Z‐ring by ZapA. A direct interaction between ZapA and ZapB is supported by bacterial two‐hybrid and in vitro interaction assays. Using high‐resolution 3‐D reconstruction microscopy, we find that, surprisingly, ZapB is located inside the Z‐ring in virtually all cells investigated. We propose a molecular model in which ZapA increases lateral interactions between FtsZ proto‐filaments and ZapB mediates further stabilization of this interaction by cross‐linking ZapA molecules bound to adjacent FtsZ proto‐filaments. Gene deletion and complementation assays show that ZapB can mitigate cell division and Z‐ring assembly defects even in the absence of ZapA, raising the possibility that ZapB stimulates Z‐ring assembly by two different mechanisms.     

Trapping of a spiral-like intermediate of the bacterial cytokinetic protein FtsZ

The earliest stage in bacterial cell division is the formation of a ring, composed of the tubulin-like protein FtsZ, at the division site. Tight spatial and temporal regulation of Z-ring formation is required to ensure that division occurs precisely at midcell between two replicated chromosomes. However, the mechanism of Z-ring formation and its regulation in vivo remain unresolved. Here we identify the defect of an interesting temperature-sensitive ftsZ mutant (ts1) of Bacillus subtilis. At the nonpermissive temperature, the mutant protein, FtsZ(Ts1), assembles into spiral-like structures between chromosomes. When shifted back down to the permissive temperature, functional Z rings form and division resumes. Our observations support a model in which Z-ring formation at the division site arises from reorganization of a long cytoskeletal spiral form of FtsZ and suggest that the FtsZ(Ts1) protein is captured as a shorter spiral-forming intermediate that is unable to complete this reorganization step. The ts1 mutant is likely to be very valuable in revealing how FtsZ assembles into a ring and how this occurs precisely at the division site.     

Cross-linking FtsZ polymers into coherent Z rings

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

A conserved coiled‐coil protein pair focuses the cytokinetic Z‐ring in Caulobacter crescentus

The cytoskeletal GTPase FtsZ assembles at midcell, recruits the division machinery and directs envelope invagination for bacterial cytokinesis. ZapA, a conserved FtsZ‐binding protein, promotes Z‐ring stability and efficient division through a mechanism that is not fully understood. Here, we investigated the function of ZapA in Caulobacter crescentus. We found that ZapA is encoded in an operon with a small coiled‐coil protein we named ZauP. ZapA and ZauP co‐localized at the division site and were each required for efficient division. ZapA interacted directly with both FtsZ and ZauP. Neither ZapA nor ZauP influenced FtsZ dynamics or bundling, in vitro, however. Z‐rings were diffuse in cells lacking zapA or zauP and, conversely, FtsZ was enriched at midcell in cells overproducing ZapA and ZauP. Additionally, FtsZ persisted at the poles longer when ZapA and ZauP were overproduced, and frequently colocalized with MipZ, a negative regulator of FtsZ polymerization. We propose that ZapA and ZauP promote efficient cytokinesis by stabilizing the midcell Z‐ring through a bundling‐independent mechanism. The zauPzapA operon is present in diverse Gram‐negative bacteria, indicating a common mechanism for Z‐ring assembly.     

FtsZ-ZapA-ZapB interactome of Escherichia coli

Bacterial cell division relies on the formation and contraction of the Z ring, coordinated and regulated by a dynamic protein complex called the divisome. The cell division factor ZapA interacts directly with FtsZ and thereby increases FtsZ protofilament association and Z-ring stability. Here, we investigated ZapB interaction with ZapA and its effect on Z-ring formation and FtsZ protofilament bundling. The combination of the ftsZ84 allele that encodes an FtsZ variant that polymerizes inefficiently with a zapB null mutant resulted in a synthetic defective phenotype. Overproduction of ZapA led to the formation of aberrant FtsZ helical structures and delocalization of ZapB. The N-terminal end of ZapB was essential for ZapB-ZapA interaction, and amino acid changes close to the N terminus of ZapB exhibited reduced interaction with ZapA. Sedimentation assays showed that ZapB interacts strongly with ZapA and reduces ZapA's interaction with FtsZ in vitro. The morphology of the structures formed by ZapA and ZapB together was similar to the cables formed by ZapB in the presence of CaCl(2), a known ZapB bundling agent. The in vivo and in vitro data support a model in which ZapA interacts strongly with ZapB and the ZapA-ZapB interaction is favored over ZapA-FtsZ.     

Identification of ZapD as a cell division factor that promotes the assembly of FtsZ in Escherichia coli

The tubulin homolog FtsZ forms a polymeric membrane-associated ring structure (Z ring) at midcell that establishes the site of division and provides an essential framework for the localization of a multiprotein molecular machine that promotes division in Escherichia coli. A number of regulatory proteins interact with FtsZ and modulate FtsZ assembly/disassembly processes, ensuring the spatiotemporal integrity of cytokinesis. The Z-associated proteins (ZapA, ZapB, and ZapC) belong to a group of FtsZ-regulatory proteins that exhibit functionally redundant roles in stabilizing FtsZ-ring assembly by binding and bundling polymeric FtsZ at midcell. In this study, we report the identification of ZapD (YacF) as a member of the E. coli midcell division machinery. Genetics and cell biological evidence indicate that ZapD requires FtsZ but not other downstream division proteins for localizing to midcell, where it promotes FtsZ-ring assembly via molecular mechanisms that overlap with ZapA. Biochemical evidence indicates that ZapD directly interacts with FtsZ and promotes bundling of FtsZ protofilaments. Similarly to ZapA, ZapB, and ZapC, ZapD is dispensable for division and therefore belongs to the growing group of FtsZ-associated proteins in E. coli that aid in the overall fitness of the division process.     

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.     

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

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

Modeling FtsZ ring formation in the bacterial cell-anisotropic aggregation via mutual interactions of polymer rods

The cytoskeletal protein FtsZ polymerizes to a ring structure (Z ring) at the inner cytoplasmic membrane that marks the future division site and scaffolds the division machinery in many bacterial species. FtsZ is known to polymerize in the presence of GTP into single-stranded protofilaments. In vivo, FtsZ polymers become associated with the cytoplasmic membrane via interaction with the membrane-binding proteins FtsA and ZipA. The FtsZ ring structure is highly dynamic and undergoes constantly polymerization and depolymerization processes and exchange with the cytoplasmic pool. In this theoretical study, we consider a scenario of Z ring self-organization via self-enhanced attachment of FtsZ polymers due to end-to-end interactions and lateral interactions of FtsZ polymers on the membrane. With the assumption of exclusively circumferential polymer orientations, we derive coarse-grained equations for the dynamics of the pool of cytoplasmic and membrane-bound FtsZ. To capture stochastic effects expected in the system due to low particle numbers, we simulate our computational model using a Gillespie-type algorithm. We obtain ring- and arc-shaped aggregations of FtsZ polymers on the membrane as a function of monomer numbers in the cell. In particular, our model predicts the number of FtsZ rings forming in the cell as a function of cell geometry and FtsZ concentration. We also calculate the time of FtsZ ring localization to the midplane in the presence of Min oscillations. Finally, we demonstrate that the assumptions and results of our model are confirmed by 3D reconstructions of fluorescently-labeled FtsZ structures in E. coli that we obtained.     

Backbone and side chain NMR assignments of <Emphasis Type="Italic">Geobacillus stearothermophilus</Emphasis> ZapA allow identification of residues that mediate the interaction of ZapA with FtsZ

Bacterial division begins with the formation of a contractile protein ring at midcell, which constricts the bacterial envelope to generate two daughter cells. The central component of the division ring is FtsZ, a tubulin-like protein capable of self-assembling into filaments which further associate into a higher order structure known as the Z ring. Proteins that bind to FtsZ play a crucial role in the formation and regulation of the Z ring. One such protein is ZapA, a widely conserved 21 kDa homodimeric protein that associates with FtsZ filaments and promotes their bundling. Although ZapA was discovered more than a decade ago, the structural details of its interaction with FtsZ remain unknown. In this work, backbone and side chain NMR assignments for the Geobacillus stearothermophilus ZapA homodimer are described. We titrated FtsZ into ¹⁵N²H-ZapA and mapped ZapA residues whose resonances are perturbed upon FtsZ binding. This information provides a structural understanding of the interaction between FtsZ and ZapA.     

MinC spatially controls bacterial cytokinesis by antagonizing the scaffolding function of FtsZ

BACKGROUND: Cytokinesis in bacteria is mediated by a cytokinetic ring, termed the Z ring, which forms a scaffold for recruitment of other cell-division proteins. The Z ring is composed of FtsZ filaments, but their organization in the Z ring is poorly understood. In Escherichia coli, the Min system contributes to the spatial regulation of cytokinesis by preventing the assembly of the Z ring away from midcell. The effector of the Min system, MinC, inhibits Z ring assembly by a mechanism that is not clear. RESULTS: Here, we report that MinC controls the scaffolding function of FtsZ by antagonizing the mechanical integrity of FtsZ structures. Specifically, MinC antagonizes the ability of FtsZ filaments to be in a solid-like gel state. MinC is a modular protein whose two domains (MinC(C) and MinC(N)) synergize to inhibit FtsZ function. MinC(C) interacts directly with FtsZ polymers to target MinC to Z rings. MinC(C) also prevents lateral interactions between FtsZ filaments, an activity that seems to be unique among cytoskeletal proteins. Because MinC(C) is inhibitory in vivo, it suggests that lateral interactions between FtsZ filaments are important for the structural integrity of the Z ring. MinC(N) contributes to MinC activity by weakening the longitudinal bonds between FtsZ molecules in a filament leading to a loss of polymer rigidity and consequent polymer shortening. On the basis of our results, we develop the first computational model of the Z ring and study the effects of MinC. CONCLUSIONS: Control over the scaffolding activity of FtsZ probably represents a universal regulatory mechanism of bacterial cytokinesis.     

Identification and characterization of ZapC, a stabilizer of the FtsZ ring in Escherichia coli

In Escherichia coli, spatiotemporal control of cell division occurs at the level of the assembly/disassembly process of the essential cytoskeletal protein FtsZ. A number of regulators interact with FtsZ and modulate the dynamics of the assembled FtsZ ring at the midcell division site. In this article, we report the identification of an FtsZ stabilizer, ZapC (Z-associated protein C), in a protein localization screen conducted with E. coli. ZapC colocalizes with FtsZ at midcell and interacts directly with FtsZ, as determined by a protein-protein interaction assay in yeast. Cells lacking or overexpressing ZapC are slightly elongated and have aberrant FtsZ ring morphologies indicative of a role for ZapC in FtsZ regulation. We also demonstrate the ability of purified ZapC to promote lateral bundling of FtsZ in a sedimentation reaction visualized by transmission electron microscopy. While ZapC lacks sequence similarity with other nonessential FtsZ regulators, ZapA and ZapB, all three Zap proteins appear to play an important role in FtsZ regulation during rapid growth. Taken together, our results suggest a key role for lateral bundling of the midcell FtsZ polymers in maintaining FtsZ ring stability during division.     

FtsZ polymer-bundling by the Escherichia coli ZapA orthologue, YgfE, involves a conformational change in bound GTP

Cell division is a fundamental process for both eukaryotic and prokaryotic cells. In bacteria, cell division is driven by a dynamic, ring-shaped, cytoskeletal element (the Z-ring) made up of polymers of the tubulin-like protein FtsZ. It is thought that lateral associations between FtsZ polymers are important for function of the Z-ring in vivo, and that these interactions are regulated by accessory cell division proteins such as ZipA, EzrA and ZapA. We demonstrate that the putative Escherichia coli ZapA orthologue, YgfE, exists in a dimer/tetramer equilibrium in solution, binds to FtsZ polymers, strongly promotes FtsZ polymer bundling and is a potent inhibitor of the FtsZ GTPase activity. We use linear dichroism, a technique that allows structure analysis of molecules within linear polymers, to reveal a specific conformational change in GTP bound to FtsZ polymers, upon bundling by YgfE. We show that the consequences of FtsZ polymer bundling by YgfE and divalent cations are very similar in terms of GTPase activity, bundle morphology and GTP orientation and therefore propose that this conformational change in bound GTP reveals a general mechanism of FtsZ bundling.     

The effect of MinC on FtsZ polymerization is pH dependent and can be counteracted by ZapA

The min system prevents polar cell division in bacteria. Here, the biochemical characterization of the interaction of MinC and FtsZ from a Gram-positive bacterium, Bacillus subtilis, is reported. B. subtilis MinC inhibits FtsZ polymerization in a pH-dependent manner by preventing the formation of lateral associations between filaments. The inhibitory effect of MinC on FtsZ polymerization is counteracted by the presence of ZapA, a protein that promotes FtsZ filament bundling.     

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

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

Negative-stain electron microscopy of inside-out FtsZ rings reconstituted on artificial membrane tubules show ribbons of protofilaments

FtsZ, the primary cytoskeletal element of the Z ring, which constricts to divide bacteria, assembles into short, one-stranded filaments in vitro. These must be further assembled to make the Z ring in bacteria. Conventional electron microscopy (EM) has failed to image the Z ring or resolve its substructure. Here we describe a procedure that enabled us to image reconstructed, inside-out FtsZ rings by negative-stain EM, revealing the arrangement of filaments. We took advantage of a unique lipid that spontaneously forms 500 nm diameter tubules in solution. We optimized conditions for Z-ring assembly with fluorescence light microscopy and then prepared specimens for negative-stain EM. Reconstituted FtsZ rings, encircling the tubules, were clearly resolved. The rings appeared as ribbons of filaments packed side by side with virtually no space between neighboring filaments. The rings were separated by variable expanses of empty tubule as seen by light microscopy or EM. The width varied considerably from one ring to another, but each ring maintained a constant width around its circumference. The inside-out FtsZ rings moved back and forth along the tubules and exchanged subunits with solution, similarly to Z rings reconstituted outside or inside tubular liposomes. FtsZ from Escherichia coli and Mycobacterium tuberculosis assembled rings of similar structure, suggesting a universal structure across bacterial species. Previous models for the Z ring in bacteria have favored a structure of widely scattered filaments that are not in contact. The ribbon structure that we discovered here for reconstituted inside-out FtsZ rings provides what to our knowledge is new evidence that the Z ring in bacteria may involve lateral association of protofilaments.