Structure of the bacterial cell division determinant GpsB and its interaction with penicillin-binding proteins

Staphylococcus aureus FtsZ and PBP4 bind to the conformationally dynamic N-terminal domain of GpsB

In the Firmicutes phylum, GpsB is a membrane associated protein that coordinates peptidoglycan synthesis with cell growth and division. Although GpsB has been studied in several bacteria, the structure, function, and interactome of Staphylococcus aureus GpsB is largely uncharacterized. To address this knowledge gap, we solved the crystal structure of the N-terminal domain of S. aureus GpsB, which adopts an atypical, asymmetric dimer, and demonstrates major conformational flexibility that can be mapped to a hinge region formed by a three-residue insertion exclusive to Staphylococci. When this three-residue insertion is excised, its thermal stability increases, and the mutant no longer produces a previously reported lethal phenotype when overexpressed in Bacillus subtilis. In S. aureus, we show that these hinge mutants are less functional and speculate that the conformational flexibility imparted by the hinge region may serve as a dynamic switch to fine-tune the function of the GpsB complex and/or to promote interaction with its various partners. Furthermore, we provide the first biochemical, biophysical, and crystallographic evidence that the N-terminal domain of GpsB binds not only PBP4, but also FtsZ, through a conserved recognition motif located on their C-termini, thus coupling peptidoglycan synthesis to cell division. Taken together, the unique structure of S. aureus GpsB and its direct interaction with FtsZ/PBP4 provide deeper insight into the central role of GpsB in S. aureus cell division.

Anchors: A way for FtsZ filaments to stay membrane bound

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

Multisite Phosphorylation Regulates GpsB Function in Cephalosporin Resistance of Enterococcus faecalis

Enterococci are normal human commensals and major causes of hospital-acquired infections. Enterococcal infections can be difficult to treat because enterococci harbor intrinsic and acquired antibiotic resistance, such as resistance to cephalosporins. In Enterococcus faecalis, the transmembrane kinase IreK, a member of the bacterial PASTA kinase family, is essential for cephalosporin resistance. The activity of IreK is boosted by the cytoplasmic protein GpsB, which promotes IreK autophosphorylation and signaling to drive cephalosporin resistance. A previous phosphoproteomics study identified eight putative IreK-dependent phosphorylation sites on GpsB, but the functional importance of GpsB phosphorylation was unknown. Here we used genetic and biochemical approaches to define three sites of phosphorylation on GpsB that functionally impact IreK activity and cephalosporin resistance. Phosphorylation at two sites (S80 and T84) serves to impair the ability of GpsB to activate IreK in vivo, suggesting phosphorylation of these sites acts as a means of negative feedback for IreK. The third site of phosphorylation (T133) occurs in a segment of GpsB termed the C-terminal extension that is unique to enterococcal GpsB homologs. The C-terminal extension is highly mobile in solution, suggesting it is largely unstructured, and phosphorylation of T133 appears to enable efficient phosphorylation at S80 / T84. Overall our results are consistent with a model in which multisite phosphorylation of GpsB impairs its ability to activate IreK, thereby diminishing signal transduction through the IreK-dependent pathway and modulating phenotypic cephalosporin resistance.

Negative regulation of MurZ and MurA underlies the essentiality of GpsB- and StkP-mediated protein phosphorylation in Streptococcus pneumoniae D39

GpsB links peptidoglycan synthases to other proteins that determine the shape of the respiratory pathogen Streptococcus pneumoniae (pneumococcus; Spn) and other low-GC Gram-positive bacteria. GpsB is also required for phosphorylation of proteins by the essential StkP(Spn) Ser/Thr protein kinase. Here we report three classes of frequently arising chromosomal duplications (≈21-176 genes) containing murZ (MurZ-family homolog of MurA) or murA that suppress ΔgpsB or ΔstkP. These duplications arose from three different repeated sequences and demonstrate the facility of pneumococcus to modulate gene dosage of numerous genes. Overproduction of MurZ or MurA alone or overproduction of MurZ caused by ΔkhpAB mutations suppressed ΔgpsB or ΔstkP phenotypes to varying extents. ΔgpsB and ΔstkP were also suppressed by MurZ amino-acid changes distant from the active site, including one in commonly studied laboratory strains, and by truncation or deletion of the homolog of IreB(ReoM). Unlike in other Gram-positive bacteria, MurZ is predominant to MurA in pneumococcal cells. However, ΔgpsB and ΔstkP were not suppressed by ΔclpCP, which did not alter MurZ or MurA amounts. These results support a model in which regulation of MurZ and MurA activity, likely by IreB(Spn), is the only essential requirement for StkP-mediated protein phosphorylation in exponentially growing D39 pneumococcal cells.

Chromosomal Duplications of MurZ (MurA2) or MurA (MurA1), Amino Acid Substitutions in MurZ (MurA2), and Absence of KhpAB Obviate the Requirement for Protein Phosphorylation in Streptococcus pneumoniae D39

GpsB links peptidoglycan synthases to other proteins that determine the shape of the respiratory pathogen Streptococcus pneumoniae (pneumococcus; Spn ) and other low-GC Gram-positive bacteria. GpsB is also required for phosphorylation of proteins by the essential StkP( Spn ) Ser/Thr protein kinase. Here we report three classes of frequently arising chromosomal duplications (≈21-176 genes) containing murZ (MurZ-family homolog of MurA) or murA that suppress Δ gpsB or Δ stkP . These duplications arose from three different repeated sequences and demonstrate the facility of pneumococcus to modulate gene dosage of numerous genes. Overproduction of MurZ or MurA alone or overexpression of MurZ caused by Δ khpAB mutations suppressed Δ gpsB or Δ stkP phenotypes to varying extents. Δ gpsB and Δ stkP were also suppressed by MurZ amino-acid changes distant from the active site, including one in commonly studied laboratory strains, and by truncation or deletion of the homolog of IreB(ReoM). Unlike in other Gram-positive bacteria, MurZ is predominant to MurA in pneumococcal cells. However, Δ gpsB and Δ stkP were not suppressed by Δ clpCP , which did not alter MurZ or MurA amounts. These results support a model in which regulation of MurZ and MurA activity, likely by IreB( Spn ), is the only essential requirement for protein phosphorylation in exponentially growing D39 pneumococcal cells.

The cell cycle of Staphylococcus aureus: An updated review

As bacteria proliferate, DNA replication, chromosome segregation, cell wall synthesis, and cytokinesis occur concomitantly and need to be tightly regulated and coordinated. Although these cell cycle processes have been studied for decades, several mechanisms remain elusive, specifically in coccus-shaped cells such as Staphylococcus aureus. In recent years, major progress has been made in our understanding of how staphylococci divide, including new, fundamental insights into the mechanisms of cell wall synthesis and division site selection. Furthermore, several novel proteins and mechanisms involved in the regulation of replication initiation or progression of the cell cycle have been identified and partially characterized. In this review, we will summarize our current understanding of the cell cycle processes in the spheroid model bacterium S. aureus, with a focus on recent advances in the understanding of how these processes are regulated.

GpsB Promotes PASTA Kinase Signaling and Cephalosporin Resistance in Enterococcus faecalis

Enterococci are opportunistic pathogens that can cause severe bacterial infections. Treatment of these infections is challenging because enterococci possess intrinsic and acquired mechanisms of resistance to commonly used antibiotics, including cephalosporins. The transmembrane serine/threonine PASTA kinase, IreK, is an important determinant of enterococcal cephalosporin resistance. Upon exposure to cephalosporins, IreK becomes autophosphorylated, which stimulates its kinase activity to phosphorylate downstream substrates and drive cephalosporin resistance. However, the molecular mechanisms that modulate IreK autophosphorylation in response to cell wall stress, such as that induced by cephalosporins, remain unknown. A cytoplasmic protein, GpsB, promotes signaling by PASTA kinase homologs in other bacterial species, but the function of enterococcal GpsB has not been previously investigated. We used in vitro and in vivo approaches to test the hypothesis that enterococcal GpsB promotes IreK signaling in response to cephalosporins to drive cephalosporin resistance. We found that GpsB promotes IreK activity both in vivo and in vitro. This effect is required for cephalosporins to trigger IreK autophosphorylation and activation of an IreK-dependent signaling pathway, and thereby is also required for enterococcal intrinsic cephalosporin resistance. Moreover, analyses of GpsB mutants and a ΔireK gpsB double mutant suggest that GpsB has an additional function, beyond regulation of IreK activity, which is required for optimal growth and full cephalosporin resistance. Collectively, our data provide new insights into the mechanism of signal transduction by the PASTA kinase IreK and the mechanism of enterococcal intrinsic cephalosporin resistance. IMPORTANCE Enterococci are opportunistic pathogens that can cause severe bacterial infections. Treatment of these infections is challenging because enterococci possess intrinsic and acquired resistance to commonly used antibiotics. In particular, enterococci are intrinsically resistant to cephalosporin antibiotics, a trait that requires the activity of a transmembrane serine/threonine kinase, IreK, which belongs to the bacterial PASTA kinase family. The mechanisms by which PASTA kinases are regulated in cells are poorly understood. Here, we report that the cytoplasmic protein GpsB directly promotes IreK signaling in enterococci to drive cephalosporin resistance. Thus, we provide new insights into PASTA kinase regulation and control of enterococcal cephalosporin resistance, and suggest that GpsB could be a promising target for new therapeutics to disable cephalosporin resistance.

Listeria monocytogenes genes supporting growth under standard laboratory cultivation conditions and during macrophage infection

The Gram-positive bacterium Listeria monocytogenes occurs widespread in the environment and infects humans when ingested along with contaminated food. Such infections are particularly dangerous for risk group patients, for whom they represent a life-threatening disease. To invent novel strategies to control contamination and disease, it is important to identify those cellular processes that maintain pathogen growth inside and outside the host. Here, we have applied transposon insertion sequencing (Tn-Seq) to L. monocytogenes for the identification of such processes on a genome-wide scale. Our approach identified 394 open reading frames that are required for growth under standard laboratory conditions and 42 further genes, which become necessary during intracellular growth in macrophages. Most of these genes encode components of the translation machinery and act in chromosome-related processes, cell division, and biosynthesis of the cellular envelope. Several cofactor biosynthesis pathways and 29 genes with unknown functions are also required for growth, suggesting novel options for the development of antilisterial drugs. Among the genes specifically required during intracellular growth are known virulence factors, genes compensating intracellular auxotrophies, and several cell division genes. Our experiments also highlight the importance of PASTA kinase signaling for general viability and of glycine metabolism and chromosome segregation for efficient intracellular growth of L. monocytogenes.

Toward the Complete Functional Characterization of a Minimal Bacterial Proteome

Recently, we presented a whole-cell kinetic model of the genetically minimal bacterium JCVI-syn3A that described the coupled metabolic and genetic information processes and predicted behaviors emerging from the interactions among these networks. JCVI-syn3A is a genetically reduced bacterial cell that has the fewest number and smallest fraction of genes of unclear function, with approximately 90 of its 452 protein-coding genes (that is less than 20%) unannotated. Further characterization of unclear JCVI-syn3A genes strengthens the robustness and predictive power of cell modeling efforts and can lead to a deeper understanding of biophysical processes and pathways at the cell scale. Here, we apply computational analyses to elucidate the functions of the products of several essential but previously uncharacterized genes involved in integral cellular processes, particularly those directly affecting cell growth, division, and morphology. We also suggest directed wet-lab experiments informed by our analyses to further understand these "missing puzzle pieces" that are an essential part of the mosaic of biological interactions present in JCVI-syn3A. Our workflow leverages evolutionary sequence analysis, protein structure prediction, interactomics, and genome architecture to determine upgraded annotations. Additionally, we apply the structure prediction analysis component of our work to all 452 protein coding genes in JCVI-syn3A to expedite future functional annotation studies as well as the inverse mapping of the cell state to more physical models requiring all-atom or coarse-grained representations for all JCVI-syn3A proteins.

GpsB Coordinates Cell Division and Cell Surface Decoration by Wall Teichoic Acids in Staphylococcus aureus

Bacterial cell division is a complex and highly regulated process requiring the coordination of many different proteins. Despite substantial work in model organisms, our understanding of the systems regulating cell division in noncanonical organisms, including critical human pathogens, is far from complete. One such organism is Staphylococcus aureus, a spherical bacterium that lacks known cell division regulatory proteins. Recent studies on GpsB, a protein conserved within the Firmicutes phylum, have provided insight into cell division regulation in S. aureus and other related organisms. It has been revealed that GpsB coordinates cell division and cell wall synthesis in multiple species. In S. aureus, we have previously shown that GpsB directly regulates FtsZ polymerization. In this study, using Bacillus subtilis as a tool, we isolated spontaneous suppressors that abrogate the lethality of S. aureus GpsB overproduction in B. subtilis. Through characterization, we identified several residues important for the function of GpsB. Furthermore, we discovered an additional role for GpsB in wall teichoic acid (WTA) biosynthesis in S. aureus. Specifically, we show that GpsB directly interacts with the WTA export protein TarG. We also identified a region in GpsB that is crucial for this interaction. Analysis of TarG localization in S. aureus suggests that WTA machinery is part of the divisome complex. Taken together, this research illustrates how GpsB performs an essential function in S. aureus by directly linking the tightly regulated cell cycle processes of cell division and WTA-mediated cell surface decoration. IMPORTANCE Cytokinesis in bacteria involves an intricate orchestration of several key cell division proteins and other factors involved in building a robust cell envelope. Presence of teichoic acids is a signature characteristic of the Gram-positive cell wall. By characterizing the role of Staphylococcus aureus GpsB, an essential cell division protein in this organism, we have uncovered an additional role for GpsB in wall teichoic acid (WTA) biosynthesis. We show that GpsB directly interacts with TarG of the WTA export complex. We also show that this function of GpsB may be conserved in other GpsB homologs as GpsB and the WTA exporter complex follow similar localization patterns. It has been suggested that WTA acts as a molecular signal to control the activity of autolytic enzymes, especially during the separation of conjoined daughter cells. Thus, our results reveal that GpsB, in addition to playing a role in cell division, may also help coordinate WTA biogenesis.

MurA escape mutations uncouple peptidoglycan biosynthesis from PrkA signaling

Gram-positive bacteria are protected by a thick mesh of peptidoglycan (PG) completely engulfing their cells. This PG network is the main component of the bacterial cell wall, it provides rigidity and acts as foundation for the attachment of other surface molecules. Biosynthesis of PG consumes a high amount of cellular resources and therefore requires careful adjustments to environmental conditions. An important switch in the control of PG biosynthesis of Listeria monocytogenes, a Gram-positive pathogen with a high infection fatality rate, is the serine/threonine protein kinase PrkA. A key substrate of this kinase is the small cytosolic protein ReoM. We have shown previously that ReoM phosphorylation regulates PG formation through control of MurA stability. MurA catalyzes the first step in PG biosynthesis and the current model suggests that phosphorylated ReoM prevents MurA degradation by the ClpCP protease. In contrast, conditions leading to ReoM dephosphorylation stimulate MurA degradation. How ReoM controls degradation of MurA and potential other substrates is not understood. Also, the individual contribution of the ~20 other known PrkA targets to PG biosynthesis regulation is unknown. We here present murA mutants which escape proteolytic degradation. The release of MurA from ClpCP-dependent proteolysis was able to activate PG biosynthesis and further enhanced the intrinsic cephalosporin resistance of L. monocytogenes. This latter effect required the RodA3/PBP B3 transglycosylase/transpeptidase pair. One murA escape mutation not only fully rescued an otherwise non-viable prkA mutant during growth in batch culture and inside macrophages but also overcompensated cephalosporin hypersensitivity. Our data collectively indicate that the main purpose of PrkA-mediated signaling in L. monocytogenes is control of MurA stability during standard laboratory growth conditions and intracellular growth in macrophages. These findings have important implications for the understanding of PG biosynthesis regulation and β-lactam resistance of L. monocytogenes and related Gram-positive bacteria.

Peptidoglycan (PG) is the main component of the bacterial cell wall and many of the PG synthesizing enzymes are antibiotic targets. We previously have discovered a new signaling route controlling PG production in the human pathogen Listeria monocytogenes. This route also determines the intrinsic resistance of L. monocytogenes against cephalosporins, a group of β-lactam antibiotics. Signaling involves PrkA, a membrane-embedded protein kinase, that is activated during cell wall stress to phosphorylate its target ReoM. Depending on its phosphorylation, ReoM activates or inactivates PG production by controlling the proteolytic stability of MurA, which catalyzes the first step in PG biosynthesis. MurA degradation depends on the ClpCP protease and we here have isolated murA mutations that escape this degradation. Using these mutants, we could show that regulation of PG biosynthesis through control of MurA stability is an important purpose of PrkA-mediated signaling in L. monocytogenes. Further experiments identified the transglycosylase RodA and the transpeptidase PBP B3 as additional downstream factors. Our results suggest that both proteins act together to translate the signals received by PrkA into adjustment of PG biosynthesis. These findings shed new light on the regulation of PG biosynthesis in Gram-positive bacteria with intrinsic β-lactam resistance.

Modeling a unit cell: crystallographic refinement procedure using the biomolecular MD simulation platform Amber

A procedure has been developed for the refinement of crystallographic protein structures based on the biomolecular simulation program Amber. The procedure constructs a model representing a crystal unit cell, which generally contains multiple protein molecules and is fully hydrated with TIP3P water. Periodic boundary conditions are applied to the cell in order to emulate the crystal lattice. The refinement is conducted in the form of a specially designed short molecular-dynamics run controlled by the Amber ff14SB force field and the maximum-likelihood potential that encodes the structure-factor-based restraints. The new Amber-based refinement procedure has been tested on a set of 84 protein structures. In most cases, the new procedure led to appreciably lower R free values compared with those reported in the original PDB depositions or obtained by means of the industry-standard phenix.refine program. In particular, the new method has the edge in refining low-accuracy scrambled models. It has also been successful in refining a number of molecular-replacement models, including one with an r.m.s.d. of 2.15 Å. In addition, Amber-refined structures consistently show superior MolProbity scores. The new approach offers a highly realistic representation of protein-protein interactions in the crystal, as well as of protein-water interactions. It also offers a realistic representation of protein crystal dynamics (akin to ensemble-refinement schemes). Importantly, the method fully utilizes the information from the available diffraction data, while relying on state-of-the-art molecular-dynamics modeling to assist with those elements of the structure that do not diffract well (for example mobile loops or side chains). Finally, it should be noted that the protocol employs no tunable parameters, and the calculations can be conducted in a matter of several hours on desktop computers equipped with graphical processing units or using a designated web service.

Lytic transglycosylase MltG cleaves in nascent peptidoglycan and produces short glycan strands

Bacteria encase their cytoplasmic membrane with peptidoglycan (PG) to maintain the shape of the cell and protect it from bursting. The enlargement of the PG layer is facilitated by the coordinated activities of PG synthesising and -cleaving enzymes. In Escherichia coli, the cytoplasmic membrane-bound lytic transglycosylase MltG associates with PG synthases and was suggested to terminate the polymerisation of PG glycan strands. Using pull-down and surface plasmon resonance, we detected interactions between MltG from Bacillus subtilis and two PG synthases; the class A PBP1 and the class B PBP2B. Using in vitro PG synthesis assays with radio-labelled or fluorophore-labelled B. subtilis-type and/or E. coli-type lipid II, we showed that both, BsMltG and EcMltG, are lytic tranglycosylases and that their activity is higher during ongoing glycan strand polymerisation. MltG competed with the transpeptidase activity of class A PBPs, but had no effect on their glycosyltransferase activity, and produced glycan strands with a length of 7 disaccharide units from cleavage in the nascent strands. We hypothesize that MltG cleaves the nascent strands to produce short glycan strands that are used in the cell for a yet unknown process.

Linking the Peptidoglycan Synthesis Protein Complex with Asymmetric Cell Division during Bacillus subtilis Sporulation

Peptidoglycan is generally considered one of the main determinants of cell shape in bacteria. In rod-shaped bacteria, cell elongation requires peptidoglycan synthesis to lengthen the cell wall. In addition, peptidoglycan is synthesized at the division septum during cell division. Sporulation of Bacillus subtilis begins with an asymmetric cell division. Formation of the sporulation septum requires almost the same set of proteins as the vegetative septum; however, these two septa are significantly different. In addition to their differences in localization, the sporulation septum is thinner and it contains SpoIIE, a crucial sporulation specific protein. Here we show that peptidoglycan biosynthesis is linked to the cell division machinery during sporulation septum formation. We detected a direct interaction between SpoIIE and GpsB and found that both proteins co-localize during the early stages of asymmetric septum formation. We propose that SpoIIE is part of a multi-protein complex which includes GpsB, other division proteins and peptidoglycan synthesis proteins, and could provide a link between the peptidoglycan synthesis machinery and the complex morphological changes required for forespore formation during B. subtilis sporulation.

PrkA controls peptidoglycan biosynthesis through the essential phosphorylation of ReoM

Peptidoglycan (PG) is the main component of bacterial cell walls and the target for many antibiotics. PG biosynthesis is tightly coordinated with cell wall growth and turnover, and many of these control activities depend upon PASTA-domain containing eukaryotic-like serine/threonine protein kinases (PASTA-eSTK) that sense PG fragments. However, only a few PG biosynthetic enzymes are direct kinase substrates. Here, we identify the conserved ReoM protein as a novel PASTA-eSTK substrate in the Gram-positive pathogen Listeria monocytogenes. Our data show that the phosphorylation of ReoM is essential as it controls ClpCP-dependent proteolytic degradation of the essential enzyme MurA, which catalyses the first committed step in PG biosynthesis. We also identify ReoY as a second novel factor required for degradation of ClpCP substrates. Collectively, our data imply that the first committed step of PG biosynthesis is activated through control of ClpCP protease activity in response to signals of PG homeostasis imbalance.

Effect of membrane composition on DivIVA-membrane interaction

DivIVA is a crucial membrane-binding protein that helps to localize other proteins to negatively curved membranes at cellular poles and division septa in Gram-positive bacteria. The N-terminal domain of DivIVA is responsible for membrane binding. However, to which lipids the domain binds or how it recognizes the membrane negative curvature remains elusive. Using computer simulations, we demonstrate that the N-terminal domain of Streptomyces coelicolor DivIVA adsorbs to membranes with affinity and orientation dependent on the lipid composition. The domain interacts non-specifically with lipid phosphates via its arginine-rich tip and the strongest interaction is with cardiolipin. Moreover, we observed a specific attraction between a negatively charged side patch of the domain and ethanolamine lipids, which addition caused the change of the domain orientation from perpendicular to parallel alignment to the membrane plane. Similar but less electrostatically dependent behavior was observed for the N-terminal domain of Bacillus subtilis. The domain propensity for lipids which prefer negatively curved membranes could be a mechanism for the cellular localization of DivIVA protein.

Structural basis for the coordination of cell division with the synthesis of the bacterial cell envelope

Bacteria are surrounded by a complex cell envelope made up of one or two membranes supplemented with a layer of peptidoglycan (PG). The envelope is responsible for the protection of bacteria against lysis in their oft-unpredictable environments and it contributes to cell integrity, morphology, signaling, nutrient/small-molecule transport, and, in the case of pathogenic bacteria, host-pathogen interactions and virulence. The cell envelope requires considerable remodeling during cell division in order to produce genetically identical progeny. Several proteinaceous machines are responsible for the homeostasis of the cell envelope and their activities must be kept coordinated in order to ensure the remodeling of the envelope is temporally and spatially regulated correctly during multiple cycles of cell division and growth. This review aims to highlight the complexity of the components of the cell envelope, but focusses specifically on the molecular apparatuses involved in the synthesis of the PG wall, and the degree of cross talk necessary between the cell division and the cell wall remodeling machineries to coordinate PG remodeling during division. The current understanding of many of the proteins discussed here has relied on structural studies, and this review concentrates particularly on this structural work.

¡vIVA la DivIVA!

Reproduction in the bacterial kingdom predominantly occurs through binary fission-a process in which one parental cell is divided into two similarly sized daughter cells. How cell division, in conjunction with cell elongation and chromosome segregation, is orchestrated by a multitude of proteins has been an active area of research spanning the past few decades. Together, the monumental endeavors of multiple laboratories have identified several cell division and cell shape regulators as well as their underlying regulatory mechanisms in rod-shaped Escherichia coli and Bacillus subtilis, which serve as model organisms for Gram-negative and Gram-positive bacteria, respectively. Yet our understanding of bacterial cell division and morphology regulation is far from complete, especially in noncanonical and non-rod-shaped organisms. In this review, we focus on two proteins that are highly conserved in Gram-positive organisms, DivIVA and its homolog GpsB, and attempt to summarize the recent advances in this area of research and discuss their various roles in cell division, cell growth, and chromosome segregation in addition to their interactome and posttranslational regulation.

Structural basis for interaction of DivIVA/GpsB proteins with their ligands

DivIVA proteins and their GpsB homologues are late cell division proteins found in Gram-positive bacteria. DivIVA/GpsB proteins associate with the inner leaflet of the cytosolic membrane and act as scaffolds for other proteins required for cell growth and division. DivIVA/GpsB proteins comprise an N-terminal lipid-binding domain for membrane association fused to C-terminal domains supporting oligomerization. Despite sharing the same domain organization, DivIVA and GpsB serve different cellular functions: DivIVA plays diverse roles in division site selection, chromosome segregation and controlling peptidoglycan homeostasis, whereas GpsB contributes to the spatiotemporal control of penicillin-binding protein activity. The crystal structures of the lipid-binding domains of DivIVA from Bacillus subtilis and GpsB from several species share a fold unique to this group of proteins, whereas the C-terminal domains of DivIVA and GpsB are radically different. A number of pivotal features identified from the crystal structures explain the functional differences between the proteins. Herein we discuss these structural and functional relationships and recent advances in our understanding of how DivIVA/GpsB proteins bind and recruit their interaction partners, knowledge that might be useful for future structure-based DivIVA/GpsB inhibitor design.

The Cell Wall of Streptococcus pneumoniae

Streptococcus pneumoniae has a complex cell wall that plays key roles in cell shape maintenance, growth and cell division, and interactions with components of the human host. The peptidoglycan has a heterogeneous composition with more than 50 subunits (muropeptides)-products of several peptidoglycan-modifying enzymes. The amidation of glutamate residues in the stem peptide is needed for efficient peptide cross-linking, and peptides with a dipeptide branch prevail in some beta-lactam-resistant strains. The glycan strands are modified by deacetylation of N-acetylglucosamine residues and O-acetylation of N-acetylmuramic acid residues, and both modifications contribute to pneumococcal resistance to lysozyme. The glycan strands carry covalently attached wall teichoic acid and capsular polysaccharide. Pneumococci are unique in that the wall teichoic acid and lipoteichoic acid contain the same unusually complex repeating units decorated with phosphoryl choline residues, which anchor the choline-binding proteins. The structures of lipoteichoic acid and the attachment site of wall teichoic acid to peptidoglycan have recently been revised. During growth, pneumococci assemble their cell walls at midcell in coordinated rounds of cell elongation and division, leading to the typical ovococcal cell shape. Cell wall growth depends on the cytoskeletal FtsA and FtsZ proteins and is regulated by several morphogenesis proteins that also show patterns of dynamic localization at midcell. Some of the key regulators are phosphorylated by StkP and dephosphorylated by PhpP to facilitate robust selection of the division site and plane and to maintain cell shape.

Deciphering the Role of a SLOG Superfamily Protein YpsA in Gram-Positive Bacteria

Bacteria adapt to different environments by regulating cell division and several conditions that modulate cell division have been documented. Understanding how bacteria transduce environmental signals to control cell division is critical in understanding the global network of cell division regulation. In this article we describe a role for Bacillus subtilis YpsA, an uncharacterized protein of the SLOG superfamily of nucleotide and ligand-binding proteins, in cell division. We observed that YpsA provides protection against oxidative stress as cells lacking ypsA show increased susceptibility to hydrogen peroxide treatment. We found that the increased expression of ypsA leads to filamentation and disruption of the assembly of FtsZ, the tubulin-like essential protein that marks the sites of cell division in B. subtilis. We also showed that YpsA-mediated filamentation is linked to the growth rate. Using site-directed mutagenesis, we targeted several conserved residues and generated YpsA variants that are no longer able to inhibit cell division. Finally, we show that the role of YpsA is possibly conserved in Firmicutes, as overproduction of YpsA in Staphylococcus aureus also impairs cell division.

Aurantimycin resistance genes contribute to survival of Listeria monocytogenes during life in the environment

Bacteria can cope with toxic compounds such as antibiotics by inducing genes for their detoxification. A common detoxification strategy is compound excretion by ATP-binding cassette (ABC) transporters, which are synthesized upon compound contact. We previously identified the multidrug resistance ABC transporter LieAB in Listeria monocytogenes, a Gram-positive bacterium that occurs ubiquitously in the environment, but also causes severe infections in humans upon ingestion. Expression of the lieAB genes is strongly induced in cells lacking the PadR-type transcriptional repressor LftR, but compounds leading to relief of this repression in wild-type cells were not known. Using RNA-Seq and promoter-lacZ fusions, we demonstrate highly specific repression of the lieAB and lftRS promoters through LftR. Screening of a natural compound library yielded the depsipeptide aurantimycin A - synthesized by the soil-dwelling Streptomyces aurantiacus - as the first known naturally occurring inducer of lieAB expression. Genetic and phenotypic experiments concordantly show that aurantimycin A is a substrate of the LieAB transporter and thus, lftRS and lieAB represent the first known genetic module conferring and regulating aurantimycin A resistance. Collectively, these genes may support the survival of L. monocytogenes when it comes into contact with antibiotic-producing bacteria in the soil.

The cell cycle regulator GpsB functions as cytosolic adaptor for multiple cell wall enzymes

Bacterial growth and cell division requires precise spatiotemporal regulation of the synthesis and remodelling of the peptidoglycan layer that surrounds the cytoplasmic membrane. GpsB is a cytosolic protein that affects cell wall synthesis by binding cytoplasmic mini-domains of peptidoglycan synthases to ensure their correct subcellular localisation. Here, we describe critical structural features for the interaction of GpsB with peptidoglycan synthases from three bacterial species (Bacillus subtilis, Listeria monocytogenes and Streptococcus pneumoniae) and suggest their importance for cell wall growth and viability in L. monocytogenes and S. pneumoniae. We use these structural motifs to identify novel partners of GpsB in B. subtilis and extend the members of the GpsB interactome in all three bacterial species. Our results support that GpsB functions as an adaptor protein that mediates the interaction between membrane proteins, scaffolding proteins, signalling proteins and enzymes to generate larger protein complexes at specific sites in a bacterial cell cycle-dependent manner.

A Listeria monocytogenes ST2 clone lacking chitinase ChiB from an outbreak of non-invasive gastroenteritis

An outbreak with a remarkable Listeria monocytogenes clone causing 163 cases of non-invasive listeriosis occurred in Germany in 2015. Core genome multi locus sequence typing grouped non-invasive outbreak isolates and isolates obtained from related food samples into a single cluster, but clearly separated genetically close isolates obtained from invasive listeriosis cases. A comparative genomic approach identified a premature stop codon in the chiB gene, encoding one of the two L. monocytogenes chitinases, which clustered with disease outcome. Correction of this premature stop codon in one representative gastroenteritis outbreak isolate restored chitinase production, but effects in infection experiments were not found. While the exact role of chitinases in virulence of L. monocytogenes is still not fully understood, our results now clearly show that ChiB-derived activity is not required to establish L. monocytogenes gastroenteritis in humans. This limits a possible role of ChiB in human listeriosis to later steps of the infection.

An essential Staphylococcus aureus cell division protein directly regulates FtsZ dynamics

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

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

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

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

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

Bacterial Strategies to Preserve Cell Wall Integrity Against Environmental Threats

Bacterial cells are surrounded by an exoskeleton-like structure, the cell wall, composed primarily of the peptidoglycan (PG) sacculus. This structure is made up of glycan strands cross-linked by short peptides generating a covalent mesh that shapes bacteria and prevents their lysis due to their high internal osmotic pressure. Even though the PG is virtually universal in bacteria, there is a notable degree of diversity in its chemical structure. Modifications in both the sugars and peptides are known to be instrumental for bacteria to cope with diverse environmental challenges. In this review, we summarize and discuss the cell wall strategies to withstand biotic and abiotic environmental insults such as the effect of antibiotics targeting cell wall enzymes, predatory PG hydrolytic proteins, and PG signaling systems. Finally we will discuss the opportunities that species-specific PG variability might open to develop antimicrobial therapies.

Phosphorylation-dependent activation of the cell wall synthase PBP2a in Streptococcus pneumoniae by MacP

Most bacterial cells are surrounded by an essential cell wall composed of the net-like heteropolymer peptidoglycan (PG). Growth and division of bacteria are intimately linked to the expansion of the PG meshwork and the construction of a cell wall septum that separates the nascent daughter cells. Class A penicillin-binding proteins (aPBPs) are a major family of PG synthases that build the wall matrix. Given their central role in cell wall assembly and importance as drug targets, surprisingly little is known about how the activity of aPBPs is controlled to properly coordinate cell growth and division. Here, we report the identification of MacP (SPD_0876) as a membrane-anchored cofactor of PBP2a, an aPBP synthase of the Gram-positive pathogen Streptococcus pneumoniae We show that MacP localizes to the division site of S. pneumoniae, forms a complex with PBP2a, and is required for the in vivo activity of the synthase. Importantly, MacP was also found to be a substrate for the kinase StkP, a global cell cycle regulator. Although StkP has been implicated in controlling the balance between the elongation and septation modes of cell wall synthesis, none of its substrates are known to modulate PG synthetic activity. Here we show that a phosphoablative substitution in MacP that blocks StkP-mediated phosphorylation prevents PBP2a activity without affecting the MacP-PBP2a interaction. Our results thus reveal a direct connection between PG synthase function and the control of cell morphogenesis by the StkP regulatory network.

Genetic Dissection of DivIVA Functions in Listeria monocytogenes

DivIVA is a membrane binding protein that clusters at curved membrane regions, such as the cell poles and the membrane invaginations occurring during cell division. DivIVA proteins recruit many other proteins to these subcellular sites through direct protein-protein interactions. DivIVA-dependent functions are typically associated with cell growth and division, even though species-specific differences in the spectrum of DivIVA functions and their causative interaction partners exist. DivIVA from the Gram-positive human pathogen Listeria monocytogenes has at least three different functions. In this bacterium, DivIVA is required for precise positioning of the septum at midcell, it contributes to the secretion of autolysins required for the breakdown of peptidoglycan at the septum after the completion of cell division, and it is essential for flagellar motility. While the DivIVA interaction partners for control of division site selection are well established, the proteins connecting DivIVA with autolysin secretion or swarming motility are completely unknown. We set out to identify divIVA alleles in which these three DivIVA functions could be separated, since the question of the degree to which the three functions of L. monocytogenes DivIVA are interlinked could not be answered before. Here, we identify such alleles, and our results show that division site selection, autolysin secretion, and swarming represent three discrete pathways that are independently influenced by DivIVA. These findings provide the required basis for the identification of DivIVA interaction partners controlling autolysin secretion and swarming in the future.IMPORTANCE DivIVA of the pathogenic bacterium Listeria monocytogenes is a central scaffold protein that influences at least three different cellular processes, namely, cell division, protein secretion, and bacterial motility. How DivIVA coordinates these rather unrelated processes is not known. We here identify variants of L. monocytogenes DivIVA, in which these functions are separated from each other. These results have important implications for the models explaining how DivIVA interacts with other proteins.

Absence of the KhpA and KhpB (JAG/EloR) RNA-binding proteins suppresses the requirement for PBP2b by overproduction of FtsA in Streptococcus pneumoniae D39

Suppressor mutations were isolated that obviate the requirement for essential PBP2b in peripheral elongation of peptidoglycan from the midcells of dividing Streptococcus pneumoniae D39 background cells. One suppressor was in a gene encoding a single KH-domain protein (KhpA). ΔkhpA suppresses deletions in most, but not all (mltG), genes involved in peripheral PG synthesis and in the gpsB regulatory gene. ΔkhpA mutations reduce growth rate, decrease cell size, minimally affect shape and induce expression of the WalRK cell-wall stress regulon. Reciprocal co-immunoprecipitations show that KhpA forms a complex in cells with another KH-domain protein (KhpB/JAG/EloR). ΔkhpA and ΔkhpB mutants phenocopy each other exactly, consistent with a direct interaction. RNA-immunoprecipitation showed that KhpA/KhpB bind an overlapping set of RNAs in cells. Phosphorylation of KhpB reported previously does not affect KhpB function in the D39 progenitor background. A chromosome duplication implicated FtsA overproduction in Δpbp2b suppression. We show that cellular FtsA concentration is negatively regulated by KhpA/B at the post-transcriptional level and that FtsA overproduction is necessary and sufficient for suppression of Δpbp2b. However, increased FtsA only partially accounts for the phenotypes of ΔkhpA mutants. Together, these results suggest that multimeric KhpA/B may function as a pleiotropic RNA chaperone controlling pneumococcal cell division.

Metabolism Shapes the Cell

More than 5 decades of work support the idea that cell envelope synthesis, including the inward growth of cell division, is tightly coordinated with DNA replication and protein synthesis through central metabolism. Remarkably, no unifying model exists to account for how these fundamentally disparate processes are functionally coupled. Recent studies demonstrate that proteins involved in carbohydrate and nitrogen metabolism can moonlight as direct regulators of cell division, coordinate cell division and DNA replication, and even suppress defects in DNA replication. In this minireview, we focus on studies illustrating the intimate link between metabolism and regulation of peptidoglycan (PG) synthesis during growth and division, and we identify the following three recurring themes. (i) Nutrient availability, not growth rate, is the primary determinant of cell size. (ii) The degree of gluconeogenic flux is likely to have a profound impact on the metabolites available for cell envelope synthesis, so growth medium selection is a critical consideration when designing and interpreting experiments related to morphogenesis. (iii) Perturbations in pathways relying on commonly shared and limiting metabolites, like undecaprenyl phosphate (Und-P), can lead to pleotropic phenotypes in unrelated pathways.

Regulation of bacterial cell wall growth

During growth and propagation, a bacterial cell enlarges and subsequently divides its peptidoglycan (PG) sacculus, a continuous mesh-like layer that encases the cell membrane to confer mechanical strength and morphological robustness. The mechanism of sacculus growth, how it is regulated and how it is coordinated with other cellular processes is poorly understood. In this article, we will discuss briefly the current knowledge of how cell wall synthesis is regulated, on multiple levels, from both sides of the cytoplasmic membrane. According to the current knowledge, cytosolic scaffolding proteins connect PG synthases with cytoskeletal elements, and protein phosphorylation regulates cell wall growth in Gram-positive species. PG-active enzymes engage in multiple protein-protein interactions within PG synthesis multienzyme complexes, and some of the interactions modulate activities. PG synthesis is also regulated by central metabolism, and by PG maturation through the action of PG hydrolytic enzymes. Only now are we beginning to appreciate how these multiple levels of regulating PG synthesis enable the cell to propagate robustly with a defined cell shape under different and variable growth conditions.

Suppression and synthetic-lethal genetic relationships of ΔgpsB mutations indicate that GpsB mediates protein phosphorylation and penicillin-binding protein interactions in Streptococcus pneumoniae D39

GpsB regulatory protein and StkP protein kinase have been proposed as molecular switches that balance septal and peripheral (side-wall like) peptidoglycan (PG) synthesis in Streptococcus pneumoniae (pneumococcus); yet, mechanisms of this switching remain unknown. We report that ΔdivIVA mutations are not epistatic to ΔgpsB division-protein mutations in progenitor D39 and related genetic backgrounds; nor is GpsB required for StkP localization or FDAA labeling at septal division rings. However, we confirm that reduction of GpsB amount leads to decreased protein phosphorylation by StkP and report that the essentiality of ΔgpsB mutations is suppressed by inactivation of PhpP protein phosphatase, which concomitantly restores protein phosphorylation levels. ΔgpsB mutations are also suppressed by other classes of mutations, including one that eliminates protein phosphorylation and may alter division. Moreover, ΔgpsB mutations are synthetically lethal with Δpbp1a, but not Δpbp2a or Δpbp1b mutations, suggesting GpsB activation of PBP2a activity. Consistent with this result, co-IP experiments showed that GpsB complexes with EzrA, StkP, PBP2a, PBP2b and MreC in pneumococcal cells. Furthermore, depletion of GpsB prevents PBP2x migration to septal centers. These results support a model in which GpsB negatively regulates peripheral PG synthesis by PBP2b and positively regulates septal ring closure through its interactions with StkP-PBP2x.

The GpsB files: the truth is out there

Peptidoglycan (PG), an essential stress-bearing component of the bacterial cell wall, is synthesised by penicillin binding proteins (PBPs). PG synthesis at the cell division septum is necessary for constructing new poles of progeny cells, and cells cannot elongate without inserting new PG in the side-wall. The cell division regulator GpsB appears to co-ordinate PG synthesis at the septum during division and at the side-wall during elongation in rod-shaped and ovococcoid Gram-positive bacteria. How the control over PG synthesis is exerted is unknown. In this issue of Molecular Microbiology, Rued et al. show that in pneumococci GpsB forms complexes with PBP2a and PBP2b, and that deletion or depletion of GpsB prevents closure of the septal ring that in itself is PBP2x-dependent. Loss of GpsB can be suppressed by spontaneous mutations, including within the gene encoding the only PP2C Ser/Thr phosphatase in Streptococcus pneumoniae, indicating that GpsB plays a key - but unknown - role in protein phosphorylation in pneumococci. Rued et al. combine phenotypic and genotypic analyses of mutant strains that suggest discrepancies in the literature concerning GpsB might have arisen from accumulation of unidentified suppressors, highlighting the importance and power of strain validation and whole genome sequencing in this context.

Suppressor Mutations Linking gpsB with the First Committed Step of Peptidoglycan Biosynthesis in Listeria monocytogenes

The cell division protein GpsB is a regulator of the penicillin binding protein A1 (PBP A1) in the Gram-positive human pathogen Listeria monocytogenes Penicillin binding proteins mediate the last two steps of peptidoglycan biosynthesis as they polymerize and cross-link peptidoglycan strands, the main components of the bacterial cell wall. It is not known what other processes are controlled by GpsB. L. monocytogenes gpsB mutants are unable to grow at 42°C, but we observed that spontaneous suppressors correcting this defect arise on agar plates with high frequency. We here describe a first set of gpsB suppressors that mapped to the clpC and murZ genes. While ClpC is the ATPase component of the Clp protease, MurZ is a paralogue of the listerial UDP-N-acetylglucosamine (UDP-GlcNAc) 1-carboxyvinyltransferase MurA. Both enzymes catalyze the enolpyruvyl transfer from phosphoenolpyruvate to UDP-GlcNAc, representing the first committed step of peptidoglycan biosynthesis. We confirmed that clean deletion of the clpC or murZ gene suppressed the ΔgpsB phenotype. It turned out that the absence of either gene leads to accumulation of MurA, and we show that artificial overexpression of MurA alone was sufficient for suppression. Inactivation of other UDP-GlcNAc-consuming pathways also suppressed the heat-sensitive growth of the ΔgpsB mutant, suggesting that an increased influx of precursor molecules into peptidoglycan biosynthesis can compensate for the lack of GpsB. Our results support a model according to which PBP A1 becomes misregulated and thus toxic in the absence of GpsB due to unproductive consumption of cell wall precursor molecules.

IMPORTANCE

The late cell division protein GpsB is important for cell wall biosynthesis in Gram-positive bacteria. GpsB of the human pathogen L. monocytogenes interacts with one of the key enzymes of this pathway, penicillin binding protein A1 (PBP A1), and influences its activity. PBP A1 catalyzes the last two steps of cell wall biosynthesis, but it is unknown how GpsB controls PBP A1. We observed that a L. monocytogenes gpsB mutant forms spontaneous suppressors and have mapped their mutations to genes mediating and influencing the first step of cell wall biosynthesis, likely stimulating the influx of metabolites into this pathway. We assume that GpsB is important to ensure productive incorporation of cell wall precursors into the peptidoglycan sacculus by PBP A1.

Subunit Arrangement in GpsB, a Regulator of Cell Wall Biosynthesis

GpsB, a key regulator of cell division in Gram-positive bacteria, interacts with a key peptidoglycan synthase at the cell division septum, the penicillin binding protein PBP1 (a.k.a. PonA). Bacillus subtilis GpsB has been reported to interact with other components of the cell division machinery, including EzrA, MreC, and PrkC. In this study, we report an analysis of the arrangement of subunits in Listeria monocytogenes GpsB by small-angle X-ray scattering. The resulting model has an elongated shape with residues critical for interaction with PBP1 and the cell membrane clustered at one end of the molecule. Mutations that destabilize the hexameric assembly of the wild-type protein have a gpsB null phenotype, indicating that oligomerization is critical for the correct function of GpsB. We suggest a model in which a single GpsB hexamer can interact with multiple PBP1 molecules and can therefore influence the arrangement of PBP1 molecules within the cell division machinery, a dynamic multiprotein complex called the divisome, consistent with a role for GpsB in modulating the synthesis of the cell wall.