Diversification of division mechanisms in endospore-forming bacteria revealed by analyses of peptidoglycan synthesis in Clostridioides difficile

The bacterial enzymes FtsW and FtsI, encoded in the highly conserved dcw gene cluster, are considered to be universally essential for the synthesis of septal peptidoglycan (PG) during cell division. Here, we show that the pathogen Clostridioides difficile lacks a canonical FtsW/FtsI pair, and its dcw-encoded PG synthases have undergone a specialization to fulfill sporulation-specific roles, including synthesizing septal PG during the sporulation-specific mode of cell division. Although these enzymes are directly regulated by canonical divisome components during this process, dcw-encoded PG synthases and their divisome regulators are dispensable for cell division during normal growth. Instead, C. difficile uses a bifunctional class A penicillin-binding protein as the core divisome PG synthase, revealing a previously unreported role for this class of enzymes. Our findings support that the emergence of endosporulation in the Firmicutes phylum facilitated the functional repurposing of cell division factors. Moreover, they indicate that C. difficile, and likely other clostridia, assemble a distinct divisome that therefore may represent a unique target for therapeutic interventions.

Analyses of cell wall synthesis in Clostridioides difficile reveal a diversification in cell division mechanisms in endospore-forming bacteria

Current models of bacterial cell division assume that the core synthases of the multiprotein divisome complex, FtsW-FtsI, are the primary drivers of septal peptidoglycan (PG) synthesis. These enzymes are typically encoded in the highly conserved division and cell wall (dcw) cluster and are considered to be universally essential for cell division. Here, we combine bioinformatics analyses with functional characterization in the pathogen Clostridioides difficile to show that dcw-encoded PG synthases have undergone a surprising specialization in the sole endospore-forming phylum, Firmicutes, to fulfill sporulation-specific roles. We describe a novel role for these enzymes in synthesizing septal PG during the sporulation-specific mode of cell division in C. difficile. Although these enzymes are directly regulated by canonical divisome components during this process, dcw-encoded PG synthases and their divisome regulators are unexpectedly dispensable for cell division during normal growth. Instead, C. difficile uses its sole bifunctional class A penicillin-binding protein (aPBP) to drive cell division, revealing a previously unreported role for this class of PG synthases as the core divisome enzyme. Collectively, our findings reveal how the emergence of endosporulation in the Firmicutes phylum was a key driver for the functional repurposing of an otherwise universally conserved cellular process such as cell division. Moreover, they indicate that C. difficile, and likely other clostridia, assemble a divisome that differs markedly from previously studied bacteria, thus representing an attractive, unique target for therapeutic purposes.

Bacterial division ring stabilizing ZapA versus destabilizing SlmA modulate FtsZ switching between biomolecular condensates and polymers

Cytokinesis is a fundamental process for bacterial survival and proliferation, involving the formation of a ring by filaments of the GTPase FtsZ, spatio-temporally regulated through the coordinated action of several factors. The mechanisms of this regulation remain largely unsolved, but the inhibition of FtsZ polymerization by the nucleoid occlusion factor SlmA and filament stabilization by the widely conserved cross-linking protein ZapA are known to play key roles. It was recently described that FtsZ, SlmA and its target DNA sequences (SlmA-binding sequence (SBS)) form phase-separated biomolecular condensates, a type of structure associated with cellular compartmentalization and resistance to stress. Using biochemical reconstitution and orthogonal biophysical approaches, we show that FtsZ-SlmA-SBS condensates captured ZapA in crowding conditions and when encapsulated inside cell-like microfluidics microdroplets. We found that, through non-competitive binding, the nucleotide-dependent FtsZ condensate/polymer interconversion was regulated by the ZapA/SlmA ratio. This suggests a highly concentration-responsive tuning of the interconversion that favours FtsZ polymer stabilization by ZapA under conditions mimicking intracellular crowding. These results highlight the importance of biomolecular condensates as concentration hubs for bacterial division factors, which can provide clues to their role in cell function and bacterial survival of stress conditions, such as those generated by antibiotic treatment.

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.

FtsK and SpoIIIE, coordinators of chromosome segregation and envelope remodeling in bacteria

The translocation of DNA during bacterial cytokinesis is mediated by the SpoIIIE/FtsK family of proteins. These proteins ensure efficient chromosome segregation into sister cells by ATP-driven translocation of DNA and they control chromosome dimer resolution. How FtsK/SpoIIIE mediate chromosome translocation during cytokinesis in Gram-positive and Gram-negative organisms has been the subject of debate. Studies on FtsK in Escherichia coli, and recent work on SpoIIIE in Bacillus subtilis, have identified interactions between each translocase and the division machinery, supporting the idea that SpoIIIE and FtsK coordinate the final steps of cytokinesis with completion of chromosome segregation. Here we summarize and discuss the view that SpoIIIE and FtsK play similar roles in coordinating cytokinesis with chromosome segregation, during growth and differentiation.

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

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

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.

A novel whole-cell biosensor of Pseudomonas aeruginosa to monitor the expression of quorum sensing genes

A novel whole-cell biosensor was developed to noninvasively and simultaneously monitor the in situ genetic activities of the four quorum sensing (QS) networks in Pseudomonas aeruginosa PAO1, including the las, rhl, pqs, and iqs systems. P. aeruginosa PAO1 is a model bacterium for studies of biofilm and pathogenesis while both processes are closely controlled by the QS systems. This biosensor worked well by selectively monitoring the expression of one representative gene from each network. In the biosensor, the promoter regions of lasI, rhlI, pqsA, and ambB (QS genes) controlled the fluorescent reporter genes of Turbo YFP, mTag BFP2, mNEON Green, and E2-Orange, respectively. The biosensor was successful in monitoring the impact of an important environmental factor, salt stress, on the genetic regulation of QS networks. High salt concentrations (≥ 20 g·L^-1) significantly downregulated rhlI, pqsA, and ambB after the biosensor was incubated for 17 h to 18 h at 37 °C, resulting in slow bacterial growth.

The distinctive cell division interactome of Neisseria gonorrhoeae

BACKGROUND

Bacterial cell division is an essential process driven by the formation of a Z-ring structure, as a cytoskeletal scaffold at the mid-cell, followed by the recruitment of various proteins which form the divisome. The cell division interactome reflects the complement of different interactions between all divisome proteins. To date, only two cell division interactomes have been characterized, in Escherichia coli and in Streptococcus pneumoniae. The cell divison proteins encoded by Neisseria gonorrhoeae include FtsZ, FtsA, ZipA, FtsK, FtsQ, FtsI, FtsW, and FtsN. The purpose of the present study was to characterize the cell division interactome of N. gonorrhoeae using several different methods to identify protein-protein interactions. We also characterized the specific subdomains of FtsA implicated in interactions with FtsZ, FtsQ, FtsN and FtsW.

RESULTS

Using a combination of bacterial two-hybrid (B2H), glutathione S-transferase (GST) pull-down assays, and surface plasmon resonance (SPR), nine interactions were observed among the eight gonococcal cell division proteins tested. ZipA did not interact with any other cell division proteins. Comparisons of the N. gonorrhoeae cell division interactome with the published interactomes from E. coli and S. pneumoniae indicated that FtsA-FtsZ and FtsZ-FtsK interactions were common to all three species. FtsA-FtsW and FtsK-FtsN interactions were only present in N. gonorrhoeae. The 2A and 2B subdomains of FtsA_Ng were involved in interactions with FtsQ, FtsZ, and FtsN, and the 2A subdomain was involved in interaction with FtsW.

CONCLUSIONS

Results from this research indicate that N. gonorrhoeae has a distinctive cell division interactome as compared with other microorganisms.

High-throughput CRISPRi phenotyping identifies new essential genes in Streptococcus pneumoniae

Genome‐wide screens have discovered a large set of essential genes in the opportunistic human pathogen Streptococcus pneumoniae. However, the functions of many essential genes are still unknown, hampering vaccine development and drug discovery. Based on results from transposon sequencing (Tn‐seq), we refined the list of essential genes in S. pneumoniae serotype 2 strain D39. Next, we created a knockdown library targeting 348 potentially essential genes by CRISPR interference (CRISPRi) and show a growth phenotype for 254 of them (73%). Using high‐content microscopy screening, we searched for essential genes of unknown function with clear phenotypes in cell morphology upon CRISPRi‐based depletion. We show that SPD_1416 and SPD_1417 (renamed to MurT and GatD, respectively) are essential for peptidoglycan synthesis, and that SPD_1198 and SPD_1197 (renamed to TarP and TarQ, respectively) are responsible for the polymerization of teichoic acid (TA) precursors. This knowledge enabled us to reconstruct the unique pneumococcal TA biosynthetic pathway. CRISPRi was also employed to unravel the role of the essential Clp‐proteolytic system in regulation of competence development, and we show that ClpX is the essential ATPase responsible for ClpP‐dependent repression of competence. The CRISPRi library provides a valuable tool for characterization of pneumococcal genes and pathways and revealed several promising antibiotic targets.

The bacterial divisome: ready for its close-up

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

A multi-layered protein network stabilizes the Escherichia coli FtsZ-ring and modulates constriction dynamics

The prokaryotic tubulin homolog, FtsZ, forms a ring-like structure (FtsZ-ring) at midcell. The FtsZ-ring establishes the division plane and enables the assembly of the macromolecular division machinery (divisome). Although many molecular components of the divisome have been identified and their interactions extensively characterized, the spatial organization of these proteins within the divisome is unclear. Consequently, the physical mechanisms that drive divisome assembly, maintenance, and constriction remain elusive. Here we applied single-molecule based superresolution imaging, combined with genetic and biophysical investigations, to reveal the spatial organization of cellular structures formed by four important divisome proteins in E. coli: FtsZ, ZapA, ZapB and MatP. We show that these interacting proteins are arranged into a multi-layered protein network extending from the cell membrane to the chromosome, each with unique structural and dynamic properties. Further, we find that this protein network stabilizes the FtsZ-ring, and unexpectedly, slows down cell constriction, suggesting a new, unrecognized role for this network in bacterial cell division. Our results provide new insight into the structure and function of the divisome, and highlight the importance of coordinated cell constriction and chromosome segregation.

Bacterial cell division is a highly regulated process that must be coordinated with other cellular processes (i.e. DNA replication and chromosome segregation) to promote faithful reproduction. In Escherichia coli, this regulation is most often mediated through the polymerization of the prokaryotic tubulin homolog, FtsZ, which forms a ring-like structure (FtsZ-ring) at midcell. The establishment of the FtsZ-ring marks the site of division and enables the assembly of the macromolecular division machinery (divisome). Here we applied single-molecule based superresolution imaging to reveal the three-dimensional structure of FtsZ in the context of its regulatory proteins: ZapA, ZapB and MatP. We found that these four proteins exist in a multi-layered network that extends from the cell membrane to the chromosome. This layered organization not only helps to stabilize the FtsZ-ring, but also serves to coordinate division with DNA status by influencing constriction rate. Our results not only provide a comprehensive view of the divisome, but also allow new insight to be garnered regarding the structure and function of the divisome.

The function of the transmembrane and cytoplasmic domains of pneumococcal penicillin-binding proteins 2x and 2b extends beyond that of simple anchoring devices

The biosynthesis of cell-wall peptidoglycan is a complex process that involves six different penicillin-binding proteins (PBPs) in Streptococcus pneumoniae. Two of these, PBP2x and PBP2b, are monofunctional transpeptidases that catalyse the formation of peptide cross-links between adjacent glycan strands. Both of them are bitopic membrane proteins with a small cytoplasmic and a large extracellular domain. PBP2x and PBP2b are essential for septal and peripheral peptidoglycan synthesis, respectively. Although several studies have investigated the properties of their extracellular catalytic domains, it is not known whether the role of their N-terminal non-catalytic domains extends beyond that of being simple anchoring devices. We therefore decided to use reciprocal domain swapping and mutational analysis to gain more information about the biological function of the membrane anchors and cytoplasmic tails of PBP2x and PBP2b. In the case of PBP2x both domains are essential, but neither the membrane anchor nor the cytoplasmic domain of PBP2x appear to serve as major localization signals. Instead, our results suggest that they are involved in interactions with other components of the divisome. Mutations of conserved amino acids in the cytoplasmic domain of PBP2x resulted in loss of function, underlining the importance of this region. The cytoplasmic domain of PBP2b could be swapped with the corresponding domain from PBP2x, whereas replacement of the PBP2b transmembrane domain with the corresponding PBP2x domain gave rise to slow-growing cells with grossly abnormal morphology. When both domains were exchanged simultaneously the cells were no longer viable.

SpyAD, a moonlighting protein of group A Streptococcus contributing to bacterial division and host cell adhesion

Group A streptococcus (GAS) is a human pathogen causing a wide repertoire of mild and severe diseases for which no vaccine is yet available. We recently reported the identification of three protein antigens that in combination conferred wide protection against GAS infection in mice. Here we focused our attention on the characterization of one of these three antigens, Spy0269, a highly conserved, surface-exposed, and immunogenic protein of unknown function. Deletion of the spy0269 gene in a GAS M1 isolate resulted in very long bacterial chains, which is indicative of an impaired capacity of the knockout mutant to properly divide. Confocal microscopy and immunoprecipitation experiments demonstrated that the protein was mainly localized at the cell septum and could interact in vitro with the cell division protein FtsZ, leading us to hypothesize that Spy0269 is a member of the GAS divisome machinery. Predicted structural domains and sequence homologies with known streptococcal adhesins suggested that this antigen could also play a role in mediating GAS interaction with host cells. This hypothesis was confirmed by showing that recombinant Spy0269 could bind to mammalian epithelial cells in vitro and that Lactococcus lactis expressing Spy0269 on its cell surface could adhere to mammalian cells in vitro and to mice nasal mucosa in vivo. On the basis of these data, we believe that Spy0269 is involved both in bacterial cell division and in adhesion to host cells and we propose to rename this multifunctional moonlighting protein as SpyAD (Streptococcus pyogenes Adhesion and Division protein).

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.

From models to pathogens: how much have we learned about Streptococcus pneumoniae cell division?

Streptococcus pneumoniae is an oval-shaped Gram-positive coccus that lives in intimate association with its human host, both as a commensal and pathogen. The seriousness of pneumococcal infections and the spread of multi-drug resistant strains call for new lines of intervention. Bacterial cell division is an attractive target to develop antimicrobial drugs. This review discusses the recent advances in understanding S. pneumoniae growth and division, in comparison with the best studied rod-shaped models, Escherichia coli and Bacillus subtilis. To maintain their shape, these bacteria propagate by peripheral and septal peptidoglycan synthesis, involving proteins that assemble into distinct complexes called the elongasome and the divisome, respectively. Many of these proteins are conserved in S. pneumoniae, supporting the notion that the ovococcal shape is also achieved by rounds of elongation and division. Importantly, S. pneumoniae and close relatives with similar morphology differ in several aspects from the model rods. Overall, the data support a model in which a single large machinery, containing both the peripheral and septal peptidoglycan synthesis complexes, assembles at midcell and governs growth and division. The mechanisms generating the ovococcal or coccal shape in lactic-acid bacteria have likely evolved by gene reduction from a rod-shaped ancestor of the same group.

Essential and non-essential interactions in interactome networks: the Escherichia coli division proteins FtsQ-FtsN interaction

The Escherichia coli division protein FtsQ, which plays a central role in the septosome assembly, interacts with several protein partners of the division machinery. Its interaction with FtsB and FtsL allows the formation of the trimeric complex connecting the early cytoplasmic cell division proteins with the late, essentially periplasmic, ones. Little is known about the interactions that FtsQ contracts with other divisome components, besides the fact that all are localized in its periplasmic domain. In this domain, two independent subdomains, both involved in FtsQ, FtsI and FtsN interactions, were also identified. The study of FtsQ interaction-defective mutants constituted a basis to investigate the biological significance of its interactions. However, in the case of interactions where two independent sites are involved, mutation(s) in one domain can be suppressed by the presence of the still-functional second interaction region. To ascertain the biological role of these interactions, it is therefore necessary to select double mutants, where both sites are impaired. This paper describes the behaviour of FtsQ double mutants that have lost the ability to interact with FtsN, which is the last component in the hierarchy of divisome assembly, and is necessary to guarantee its stability and function.

Mycobacterium tuberculosis and Clostridium difficille interactomes: demonstration of rapid development of computational system for bacterial interactome prediction

Background

Protein-protein interaction (PPI) networks (interactomes) of most organisms, except for some model organisms, are largely unknown. Experimental methods including high-throughput techniques are highly resource intensive. Therefore, computational discovery of PPIs can accelerate biological discovery by presenting "most-promising" pairs of proteins that are likely to interact. For many bacteria, genome sequence, and thereby genomic context of proteomes, is readily available; additionally, for some of these proteomes, localization and functional annotations are also available, but interactomes are not available. We present here a method for rapid development of computational system to predict interactome of bacterial proteomes. While other studies have presented methods to transfer interologs across species, here, we propose transfer of computational models to benefit from cross-species annotations, thereby predicting many more novel interactions even in the absence of interologs. Mycobacterium tuberculosis (Mtb) and Clostridium difficile (CD) have been used to demonstrate the work.

Results

We developed a random forest classifier over features derived from Gene Ontology annotations and genetic context scores provided by STRING database for predicting Mtb and CD interactions independently. The Mtb classifier gave a precision of 94% and a recall of 23% on a held out test set. The Mtb model was then run on all the 8 million protein pairs of the Mtb proteome, resulting in 708 new interactions (at 94% expected precision) or 1,595 new interactions at 80% expected precision. The CD classifier gave a precision of 90% and a recall of 16% on a held out test set. The CD model was run on all the 8 million protein pairs of the CD proteome, resulting in 143 new interactions (at 90% expected precision) or 580 new interactions (at 80% expected precision). We also compared the overlap of predictions of our method with STRING database interactions for CD and Mtb and also with interactions identified recently by a bacterial 2-hybrid system for Mtb. To demonstrate the utility of transfer of computational models, we made use of the developed Mtb model and used it to predict CD protein-pairs. The cross species model thus developed yielded a precision of 88% at a recall of 8%. To demonstrate transfer of features from other organisms in the absence of feature-based and interaction-based information, we transferred missing feature values from Mtb orthologs into the CD data. In transferring this data from orthologs (not interologs), we showed that a large number of interactions can be predicted.

Conclusions

Rapid discovery of (partial) bacterial interactome can be made by using existing set of GO and STRING features associated with the organisms. We can make use of cross-species interactome development, when there are not even sufficient known interactions to develop a computational prediction system. Computational model of well-studied organism(s) can be employed to make the initial interactome prediction for the target organism. We have also demonstrated successfully, that annotations can be transferred from orthologs in well-studied organisms enabling accurate predictions for organisms with no annotations. These approaches can serve as building blocks to address the challenges associated with feature coverage, missing interactions towards rapid interactome discovery for bacterial organisms.

Availability

The predictions for all Mtb and CD proteins are made available at: http://severus.dbmi.pitt.edu/TB and http://severus.dbmi.pitt.edu/CD respectively for browsing as well as for download.

Peptidoglycan assembly machines: the biochemical evidence

To make progress in understanding peptidoglycan metabolism, we will reconstitute in vitro the assembly process and the molecular machineries that carry out this formidable task. We review here the reports of isolation of complexes comprising penicillin-binding proteins (PBPs), the enzymes that synthesize the peptidoglycan from its lipid-linked precursor.

Control of cell division in Streptococcus pneumoniae by the conserved Ser/Thr protein kinase StkP

How the human pathogen Streptococcus pneumoniae coordinates cell-wall synthesis during growth and division to achieve its characteristic oval shape is poorly understood. The conserved eukaryotic-type Ser/Thr kinase of S. pneumoniae, StkP, previously was reported to phosphorylate the cell-division protein DivIVA. Consistent with a role in cell division, GFP-StkP and its cognate phosphatase, GFP-PhpP, both localize to the division site. StkP localization depends on its penicillin-binding protein and Ser/Thr-associated domains that likely sense uncross-linked peptidoglycan, because StkP and PhpP delocalize in the presence of antibiotics that target the latest stages of cell-wall biosynthesis and in cells that have stopped dividing. Time-lapse microscopy shows that StkP displays an intermediate timing of recruitment to midcell: StkP arrives shortly after FtsA but before DivIVA. Furthermore, StkP remains at midcell longer than FtsA, until division is complete. Cells mutated for stkP are perturbed in cell-wall synthesis and display elongated morphologies with multiple, often unconstricted, FtsA and DivIVA rings. The data show that StkP plays an important role in regulating cell-wall synthesis and controls correct septum progression and closure. Overall, our results indicate that StkP signals information about the cell-wall status to key cell-division proteins and in this way acts as a regulator of cell division.

Multiple essential roles for EzrA in cell division of Staphylococcus aureus

In Bacillus subtilis, EzrA is involved in preventing aberrant formation of FtsZ rings and has also been implicated in the localization cycle of Pbp1. We have identified the orthologue of EzrA in Staphylococcus aureus to be essential for growth and cell division in this organism. Phenotypic analyses following titration of EzrA levels in S. aureus have shown that the protein is required for peptidoglycan synthesis as well as for assembly of the divisome at the midcell and cytokinesis. Protein interaction studies revealed that EzrA forms a complex with both the cytoplasmic components of the division machinery and those with periplasmic domains, suggesting that EzrA may be a scaffold molecule permitting the assembly of the division complex and forming an interface between the cytoplasmic cytoskeletal element FtsZ and the peptidoglycan biosynthetic apparatus active in the periplasm.

Advances in understanding E. coli cell fission

Much of what we know about cytokinesis in bacteria has come from studies with Escherichia coli, and efforts to comprehensively understand this fundamental process in this organism continue to intensify. Major recent advances include in vitro assembly of a membrane-tethered version of FtsZ into contractile rings in lipid tubules, in vitro dynamic patterning of the Min proteins and a deeper understanding of how they direct assembly of the FtsZ-ring to midcell, the elucidation of structures, biochemical activities and interactions of other key components of the cell fission machinery, and the uncovering of additional components of this machinery with often redundant but important roles in invagination of the three cell envelope layers.

Evidence from artificial septal targeting and site-directed mutagenesis that residues in the extracytoplasmic β domain of DivIB mediate its interaction with the divisomal transpeptidase PBP 2B

Bacterial cytokinesis is achieved through the coordinated action of a multiprotein complex known as the divisome. The Escherichia coli divisome is comprised of at least 10 essential proteins whose individual functions are mostly unknown. Most divisomal proteins have multiple binding partners, making it difficult to pinpoint epitopes that mediate pairwise interactions between these proteins. We recently introduced an artificial septal targeting approach that allows the interaction between pairs of proteins to be studied in vivo without the complications introduced by other interacting proteins (C. Robichon, G. F. King, N. W. Goehring, and J. Beckwith, J. Bacteriol. 190:6048-6059, 2008). We have used this approach to perform a molecular dissection of the interaction between Bacillus subtilis DivIB and the divisomal transpeptidase PBP 2B, and we demonstrate that this interaction is mediated exclusively through the extracytoplasmic domains of these proteins. Artificial septal targeting in combination with mutagenesis experiments revealed that the C-terminal region of the β domain of DivIB is critical for its interaction with PBP 2B. These findings are consistent with previously defined loss-of-function point mutations in DivIB as well as the recent demonstration that the β domain of DivIB mediates its interaction with the FtsL-DivIC heterodimer. These new results have allowed us to construct a model of the DivIB/PBP 2B/FtsL/DivIC quaternary complex that strongly implicates DivIB, FtsL, and DivIC in modulating the transpeptidase activity of PBP 2B.

FtsQ interaction mutants: a way to identify new antibacterial targets

FtsQ is a highly conserved component of the divisome that plays a central role in the assembly of early and late cell division proteins. The biological activity of this protein is still largely unknown, but its ability to interact with many components of the divisome was described by both two-hybrid assays and co-immunoprecipitation experiments. This paper describes the behaviour of ftsQ point mutants, created by random mutagenesis without regard to their phenotype, in which FtsQ is impaired in its ability to interact with its Escherichia coli division partners. Our results allow the identification of FtsQ residues involved in the interaction with other partner proteins and the determination of the biological significance of these interactions. The knowledge derived by this study could constitute not only the basis for understanding how these proteins assemble in the divisome, but also a starting point for the design of new antibacterial drugs that disrupt the bacterial division machinery.

ZipN, an FtsA-like orchestrator of divisome assembly in the model cyanobacterium Synechocystis PCC6803

We pursued the characterization of the divisome of the spherical-celled cyanobacterium Synechocystis PCC6803, through deletion, site-directed mutagenesis, GFP tagging, two-hybrid and co-immunoprecipitation assays. We presently report that the DivIVA-like protein Cdv3 is essential to both cell growth and division, whereas the AmiC, AmpH, FtsE, FtsN, SpoIID, YlmD, YlmE and YlmG proteins are dispensable. With the exception of the self-interacting protein YlmD, none of these dispensable factors appeared to interact with ZipN, the crucial cytokinetic factor we previously characterized. By contrast, we found that ZipN interacts with itself and the self-interacting protein Cdv3, as well as with all other crucial cytokinetic factors we previously characterized, namely: FtsZ, FtsI, FtsQ, SepF and ZipS. We also identified ZipN amino acids selectively involved in ZipN interaction with one of its following partners, Cdv3, FtsQ or SepF. Finally, we found no direct interaction between Cdv3, SepF and ZipS. Collectively, these results indicate that ZipN is a central player of divisome assembly in cyanobacteria, similarly to the FtsA protein of E. coli that is absent in cyanobacteria and chloroplast.

Characterization of the Synechocystis strain PCC 6803 penicillin-binding proteins and cytokinetic proteins FtsQ and FtsW and their network of interactions with ZipN

Because very little is known about cell division in noncylindrical bacteria and cyanobacteria, we investigated 10 putative cytokinetic proteins in the unicellular spherical cyanobacterium Synechocystis strain PCC 6803. Concerning the eight penicillin-binding proteins (PBPs), which define three classes, we found that Synechocystis can survive in the absence of one but not two PBPs of either class A or class C, whereas the unique class B PBP (also termed FtsI) is indispensable. Furthermore, we showed that all three classes of PBPs are required for normal cell size. Similarly, the putative FtsQ and FtsW proteins appeared to be required for viability and normal cell size. We also used a suitable bacterial two-hybrid system to characterize the interaction web among the eight PBPs, FtsQ, and FtsW, as well as ZipN, the crucial FtsZ partner that occurs only in cyanobacteria and plant chloroplasts. We showed that FtsI, FtsQ, and ZipN are self-interacting proteins and that both FtsI and FtsQ interact with class A PBPs, as well as with ZipN. Collectively, these findings indicate that ZipN, in interacting with FtsZ and both FtsI and FtQ, plays a similar role to the Escherichia coli FtsA protein, which is missing in cyanobacteria and chloroplasts.