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.     

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.     

Structural and Functional Analyses Reveal Insights into the Molecular Properties of the Escherichia coli Z Ring Stabilizing Protein,ZapC

In Escherichia coli cell division is driven by the tubulin-like GTPase, FtsZ, which forms the cytokinetic Z-ring. The Z-ring serves as a dynamic platform for the assembly of the multiprotein divisome, which catalyzes membrane cleavage to create equal daughter cells. Several proteins effect FtsZ assembly, thereby providing spatiotemporal control over cell division. One important class of FtsZ interacting/regulatory proteins is the Z-ring-associated proteins, Zaps, which typically modulate Z-ring formation by increasing lateral interactions between FtsZ protofilaments. Strikingly, these Zap proteins show no discernable sequence similarity, suggesting that they likely harbor distinct structures and mechanisms. The 19.8-kDa ZapC in particular shows no homology to any known protein. To gain insight into ZapC function, we determined its structure to 2.15 Å and performed genetic and biochemical studies. ZapC is a monomer composed of two domains, an N-terminal α/β region and a C-terminal twisted β barrel-like domain. The structure contains two pockets, one on each domain. The N-domain pocket is lined with residues previously implicated to be important for ZapC function as an FtsZ bundler. The adjacent C-domain pocket contains a hydrophobic center surrounded by conserved basic residues. Mutagenesis analyses indicate that this pocket is critical for FtsZ binding. An extensive FtsZ binding surface is consistent with the fact that, unlike many FtsZ regulators, ZapC binds the large FtsZ globular core rather than C-terminal tail, and the presence of two adjacent pockets suggests possible mechanisms for ZapC-mediated FtsZ bundling.     

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.     

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.     

Localization of PBP3 in Caulobacter crescentus is highly dynamic and largely relies on its functional transpeptidase domain

In rod-shaped bacteria, septal peptidoglycan synthesis involves the late recruitment of the ftsI gene product (PBP3 in Escherichia coli) to the FtsZ ring. We show that in Caulobacter crescentus, PBP3 accumulates at the new pole at the beginning of the cell cycle. Fluorescence recovery after photobleaching experiments reveal that polar PBP3 molecules are, constantly and independently of FtsZ, replaced by those present in the cellular pool, implying that polar PBP3 is not a remnant of the previous division. By the time cell constriction is initiated, all PBP3 polar accumulation has disappeared in favour of an FtsZ-dependent localization near midcell, consistent with PBP3 function in cell division. Kymograph analysis of time-lapse experiments shows that the recruitment of PBP3 to the FtsZ ring is progressive and initiated very early on, shortly after FtsZ ring formation and well before cell constriction starts. Accumulation of PBP3 near midcell is also highly dynamic with a rapid exchange of PBP3 molecules between midcell and cellular pools. Localization of PBP3 at both midcell and pole appears multifactorial, primarily requiring the catalytic site of PBP3. Collectively, our results suggest a role for PBP3 in pole morphogenesis and provide new insights into the process of peptidoglycan assembly during division.     

RefZ Facilitates the Switch from Medial to Polar Division during Spore Formation in Bacillus subtilis

During sporulation, Bacillus subtilis redeploys the division protein FtsZ from midcell to the cell poles, ultimately generating an asymmetric septum. Here, we describe a sporulation-induced protein, RefZ, that facilitates the switch from a medial to a polar FtsZ ring placement. The artificial expression of RefZ during vegetative growth converts FtsZ rings into FtsZ spirals, arcs, and foci, leading to filamentation and lysis. Mutations in FtsZ specifically suppress RefZ-dependent division inhibition, suggesting that RefZ may target FtsZ. During sporulation, cells lacking RefZ are delayed in polar FtsZ ring formation, spending more time in the medial and transition stages of FtsZ ring assembly. A RefZ-green fluorescent protein (GFP) fusion localizes in weak polar foci at the onset of sporulation and as a brighter midcell focus at the time of polar division. RefZ has a TetR DNA binding motif, and point mutations in the putative recognition helix disrupt focus formation and abrogate cell division inhibition. Finally, chromatin immunoprecipitation assays identified sites of RefZ enrichment in the origin region and near the terminus. Collectively, these data support a model in which RefZ helps promote the switch from medial to polar division and is guided by the organization of the chromosome. Models in which RefZ acts as an activator of FtsZ ring assembly near the cell poles or as an inhibitor of the transient medial ring at midcell are discussed.     

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.     

Dynamic Interaction of the Escherichia coli Cell Division ZipA and FtsZ Proteins Evidenced in Nanodiscs

The full-length ZipA protein from Escherichia coli, one of the essential components of the division proto-ring that provides membrane tethering to the septation FtsZ protein, has been incorporated in single copy into nanodiscs formed by a membrane scaffold protein encircling an E. coli phospholipid mixture. This is an acellular system that reproduces the assembly of part of the cell division components. ZipA contained in nanodiscs (Nd-ZipA) retains the ability to interact with FtsZ oligomers and with FtsZ polymers. Interactions with FtsZ occur at similar strengths as those involved in the binding of the soluble form of ZipA, lacking the transmembrane region, suggesting that the transmembrane region of ZipA has little influence on the formation of the ZipA·FtsZ complex. Peptides containing partial sequences of the C terminus of FtsZ compete with FtsZ polymers for binding to Nd-ZipA. The affinity of Nd-ZipA for the FtsZ polymer formed with GTP or GMPCPP (a slowly hydrolyzable analog of GTP) is moderate (micromolar range) and of similar magnitude as for FtsZ-GDP oligomers. Polymerization does not stabilize the binding of FtsZ to ZipA. This supports the role of ZipA as a passive anchoring device for the proto-ring with little implication, if any, in the regulation of its assembly. Furthermore, it indicates that the tethering of FtsZ to the membrane shows sufficient plasticity to allow for its release from noncentral regions of the cytoplasmic membrane and its subsequent relocation to midcell when demanded by the assembly of a division ring.     

A novel membrane anchor for FtsZ is linked to cell wall hydrolysis in Caulobacter crescentus

In most bacteria, the tubulin‐like GTPase FtsZ forms an annulus at midcell (the Z‐ring) which recruits the division machinery and regulates cell wall remodeling. Although both activities require membrane attachment of FtsZ, few membrane anchors have been characterized. FtsA is considered to be the primary membrane tether for FtsZ in bacteria, however in Caulobacter crescentus, FtsA arrives at midcell after stable Z‐ring assembly and early FtsZ‐directed cell wall synthesis. We hypothesized that additional proteins tether FtsZ to the membrane and demonstrate that in C. crescentus, FzlC is one such membrane anchor. FzlC associates with membranes directly in vivo and in vitro and recruits FtsZ to membranes in vitro. As for most known membrane anchors, the C‐terminal peptide of FtsZ is required for its recruitment to membranes by FzlC in vitro and midcell recruitment of FzlC in cells. In vivo, overproduction of FzlC causes cytokinesis defects whereas deletion of fzlC causes synthetic defects with dipM, ftsE and amiC mutants, implicating FzlC in cell wall hydrolysis. Our characterization of FzlC as a novel membrane anchor for FtsZ expands our understanding of FtsZ regulators and establishes a role for membrane‐anchored FtsZ in the regulation of cell wall hydrolysis.     

Diverse paths to midcell: assembly of the bacterial cell division machinery

At the heart of bacterial cell division is a dynamic ring-like structure of polymers of the tubulin homologue FtsZ. This ring forms a scaffold for assembly of at least ten additional proteins at midcell, the majority of which are likely to be involved in remodeling the peptidoglycan cell wall at the division site. Together with FtsZ, these proteins are thought to form a cell division complex, or divisome. In Escherichia coli, the components of the divisome are recruited to midcell according to a strikingly linear hierarchy that predicts a step-wise assembly pathway. However, recent studies have revealed unexpected complexity in the assembly steps, indicating that the apparent linearity does not necessarily reflect a temporal order. The signals used to recruit cell division proteins to midcell are diverse and include regulated self-assembly, protein-protein interactions, and the recognition of specific septal peptidoglycan substrates. There is also evidence for a complex web of interactions among these proteins and at least one distinct subcomplex of cell division proteins has been defined, which is conserved among E. coli, Bacillus subtilis and Streptococcus pneumoniae.     

Mycobacterium tuberculosis cells growing in macrophages are filamentous and deficient in FtsZ rings

FtsZ, a bacterial homolog of tubulin, forms a structural element called the FtsZ ring (Z ring) at the predivisional midcell site and sets up a scaffold for the assembly of other cell division proteins. The genetic aspects of FtsZ-catalyzed cell division and its assembly dynamics in Mycobacterium tuberculosis are unknown. Here, with an M. tuberculosis strain containing FtsZ(TB) tagged with green fluorescent protein as the sole source of FtsZ, we examined FtsZ structures under various growth conditions. We found that midcell Z rings are present in approximately 11% of actively growing cells, suggesting that the low frequency of Z rings is reflective of their slow growth rate. Next, we showed that SRI-3072, a reported FtsZ(TB) inhibitor, disrupted Z-ring assembly and inhibited cell division and growth of M. tuberculosis. We also showed that M. tuberculosis cells grown in macrophages are filamentous and that only a small fraction had midcell Z rings. The majority of filamentous cells contained nonring, spiral-like FtsZ structures along their entire length. The levels of FtsZ in bacteria grown in macrophages or in broth were comparable, suggesting that Z-ring formation at midcell sites was compromised during intracellular growth. Our results suggest that the intraphagosomal milieu alters the expression of M. tuberculosis genes affecting Z-ring formation and thereby cell division.     

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

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

Interactions between heterologous FtsA and FtsZ proteins at the FtsZ ring.

FtsZ and FtsA are essential for cell division in Escherichia coli and colocalize to the septal ring. One approach to determine what regions of FtsA and FtsZ are important for their interaction is to identify in vivo interactions between FtsA and FtsZ from different species. As a first step, the ftsA genes of Rhizobium meliloti and Agrobacterium tumefaciens were isolated and characterized. In addition, an FtsZ homolog that shared the unusual C-terminal extension of R. meliloti FtsZ1 was found in A. tumefaciens. In order to visualize their localization in cells, we tagged these proteins with green fluorescent protein (GFP). When R. meliloti FtsZ1-GFP or A. tumefaciens FtsZ-GFP was expressed at low levels in E. coli, they specifically localized only to the E. coli FtsZ ring, possibly by coassembly. When A. tumefaciens FtsA-GFP or R. meliloti FtsA-GFP was expressed in E. coli, they failed to localize detectably to the E. coli FtsZ ring. However, when R. meliloti FtsZ1 was coexpressed with them, fluorescence localized to a band at the midcell division site, strongly suggesting that FtsA from either A. tumefaciens or R. meliloti can bind directly to its cognate FtsZ. As expected, GFP-tagged FtsZ1 and FtsA from either R. meliloti or A. tumefaciens localized to the division site in A. tumefaciens cells. Therefore, the 61 amino acid changes between A. tumefaciens FtsA and R. meliloti FtsA do not prevent their direct interaction with FtsZ1 from either species, suggesting that those residues are not essential for protein-protein contacts. Moreover, the failure of the two non-E. coli FtsA derivatives to interact strongly with E. coli FtsZ in this in vivo system unless their cognate FtsZ was also present suggests that FtsA-FtsZ interactions have coevolved and that the residues which differ between the E. coli proteins and those of the two other species may be important for specific interactions.     

Cell wall growth during elongation and division: one ring to bind them?

The role of the cell division protein FtsZ in bacterial cell wall (CW) synthesis is believed to be restricted to localizing proteins involved in the synthesis of the septal wall. In this issue of Molecular Microbiology, the groups of Christine Jacobs-Wagner and Waldemar Vollmer provide compelling evidence that in Caulobacter crescentus, FtsZ plays an additional role in CW synthesis in non-dividing cells. During elongation (cell growth) FtsZ is responsible for the incorporation of CW material in a zone at the midcell by recruiting MurG, a protein involved in peptidoglycan (PG) precursor synthesis. This resembles earlier findings of FtsZ mediated PG synthesis activity in Escherichia coli. A role of FtsZ in PG synthesis during elongation forces a rethink of the current model of CW synthesis in rod-shaped bacteria.     

The interplay of ClpXP with the cell division machinery in Escherichia coli

ClpXP is a two-component protease composed of ClpX, an ATP-dependent chaperone that recognizes and unfolds specific substrates, and ClpP, a serine protease. One ClpXP substrate in Escherichia coli is FtsZ, which is essential for cell division. FtsZ polymerizes and forms the FtsZ ring at midcell, where division occurs. To investigate the role of ClpXP in cell division, we examined the effects of clpX and clpP deletions in several strains that are defective for cell division. Together, our results suggested that ClpXP modulates cell division through degradation of FtsZ and possibly other cell division components that function downstream of FtsZ ring assembly. In the ftsZ84 strain, which is temperature sensitive for filamentation due to a mutation in ftsZ, we observed that deletion of clpX or clpP suppresses filamentation and reduces FtsZ84 degradation. These results are consistent with ClpXP playing a role in cell division by modulating the level of FtsZ through degradation. In another division-defective strain, ΔminC, the additional deletion of clpX or clpP delays cell division and exacerbates filamentation. Our results demonstrate that ClpXP modulates division in cells lacking MinC by a mechanism that requires ATP-dependent degradation. However, antibiotic chase experiments in vivo indicate that FtsZ degradation is slower in the ΔminC strain than in the wild type, suggesting there may be another cell division component degraded by ClpXP. Taken together these studies suggest that ClpXP may degrade multiple cell division proteins, thereby modulating the precise balance of the components required for division.     

The division inhibitor EzrA contains a seven-residue patch required for maintaining the dynamic nature of the medial FtsZ ring

The essential cytoskeletal protein FtsZ assembles into a ring-like structure at the nascent division site and serves as a scaffold for the assembly of the prokaryotic division machinery. We previously characterized EzrA as an inhibitor of FtsZ assembly in Bacillus subtilis. EzrA interacts directly with FtsZ to prevent aberrant FtsZ assembly and cytokinesis at cell poles. EzrA also concentrates at the cytokinetic ring in an FtsZ-dependent manner, although its precise role at this position is not known. Here, we identified a conserved patch of amino acids in the EzrA C terminus that is essential for localization to the FtsZ ring. Mutations in this patch (designated the “QNR patch”) abolish EzrA localization to midcell but do not significantly affect EzrA's ability to inhibit FtsZ assembly at cell poles. ezrA QNR patch mutant cells exhibit stabilized FtsZ assembly at midcell and are significantly longer than wild-type cells, despite lacking extra FtsZ rings. These results indicate that EzrA has two distinct activities in vivo: (i) preventing aberrant FtsZ ring formation at cell poles through inhibition of de novo FtsZ assembly and (ii) maintaining proper FtsZ assembly dynamics within the medial FtsZ ring, thereby rendering it sensitive to the factors responsible for coordinating cell growth and cell division.     

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.     

Analysis of MinC reveals two independent domains involved in interaction with MinD and FtsZ

In Escherichia coli FtsZ assembles into a Z ring at midcell while assembly at polar sites is prevented by the min system. MinC, a component of this system, is an inhibitor of FtsZ assembly that is positioned within the cell by interaction with MinDE. In this study we found that MinC consists of two functional domains connected by a short linker. When fused to MalE the N-terminal domain is able to inhibit cell division and prevent FtsZ assembly in vitro. The C-terminal domain interacts with MinD, and expression in wild-type cells as a MalE fusion disrupts min function, resulting in a minicell phenotype. We also find that MinC is an oligomer, probably a dimer. Although the C-terminal domain is clearly sufficient for oligomerization, the N-terminal domain also promotes oligomerization. These results demonstrate that MinC consists of two independently functioning domains: an N-terminal domain capable of inhibiting FtsZ assembly and a C-terminal domain responsible for localization of MinC through interaction with MinD. The fusion of these two independent domains is required to achieve topological regulation of Z ring assembly.     

PomZ,a ParA‐like protein,regulates Z‐ring formation and cell division in Myxococcus xanthus

Accurate positioning of the division site is essential to generate appropriately sized daughter cells with the correct chromosome number. In bacteria, division generally depends on assembly of the tubulin homologue FtsZ into the Z‐ring at the division site. Here, we show that lack of the ParA‐like protein PomZ in Myxococcus xanthus resulted in division defects with the formation of chromosome‐free minicells and filamentous cells. Lack of PomZ also caused reduced formation of Z‐rings and incorrect positioning of the few Z‐rings formed. PomZ localization is cell cycle regulated, and PomZ accumulates at the division site at midcell after chromosome segregation but prior to FtsZ as well as in the absence of FtsZ. FtsZ displayed cooperative GTP hydrolysis in vitro but did not form detectable filaments in vitro. PomZ interacted with FtsZ in M. xanthus cell extracts. These data show that PomZ is important for Z‐ring formation and is a spatial regulator of Z‐ring formation and cell division. The cell cycle‐dependent localization of PomZ at midcell provides a mechanism for coupling cell cycle progression and Z‐ring formation. Moreover, the data suggest that PomZ is part of a system that recruits FtsZ to midcell, thereby, restricting Z‐ring formation to this position.