Rewiring the Pneumococcal Cell Cycle with Serine/Threonine- and Tyrosine-kinases

Molecular dissection of the chromosome partitioning protein RocS and regulation by phosphorylation

Chromosome segregation in bacteria is a critical process ensuring that each daughter cell receives an accurate copy of the genetic material during cell division. Active segregation factors, such as the ParABS system or SMC complexes, are usually essential for this process, but they are surprisingly dispensable in Streptococcus pneumoniae. Rather, chromosome segregation in S. pneumoniae relies on the protein Regulator of Chromosome Segregation (RocS), although the molecular mechanisms involved remain elusive. By combining genetics, in vivo imaging, and biochemical approaches, we dissected the molecular features of RocS involved in chromosome segregation. We investigated the respective functions of the three RocS domains, specifically the C-terminal amphipathic helix (AH), the N-terminal DNA-binding domain (DBD), and the coiled-coil domain (CCD) separating the AH and the DBD. Notably, we found that a single AH is not sufficient for membrane binding and that RocS requires prior oligomerization to interact with the membrane. We further demonstrated that this self-interaction was driven by the N-terminal part of the CCD. On the other hand, we revealed that the C-terminal part of the CCD corresponds to a domain of unknown function (DUF 536) and is defined by three conserved glutamines, which play a crucial role in RocS-mediated chromosome segregation. Finally, we showed that the DBD is phosphorylated by the unique serine-threonine kinase of S. pneumoniae StkP and that mimicking this phosphorylation abrogated RocS binding to DNA. Overall, this study offers new insights into chromosome segregation in Streptococci and paves the way for a deeper understanding of RocS-like proteins in other bacteria.IMPORTANCEBacteria have evolved a variety of mechanisms to properly segregate their genetic material during cell division. In this study, we performed a molecular dissection of the chromosome partitioning protein Regulator of Chromosome Segregation (RocS), a pillar element of chromosome segregation in S. pneumoniae that is also generally conserved in the Streptococcaceae family. Our systematic investigation sheds light on the molecular features required for successful pneumococcal chromosome segregation and the regulation of RocS by phosphorylation. In addition, our study also revealed that RocS shares functional domains with the Par protein, involved in an atypical plasmid segregation system. Therefore, we expect that our findings may serve to extend our understanding of RocS and RocS-like proteins while broadening the repertoire of partitioning systems used in bacteria.

Streptococcus pneumoniae extracellular vesicles aggravate alveolar epithelial barrier disruption via autophagic degradation of OCLN (occludin)

Streptococcus pneumoniae (S. pneumoniae) represents a major human bacterial pathogen leading to high morbidity and mortality in children and the elderly. Recent research emphasizes the role of extracellular vesicles (EVs) in bacterial pathogenicity. However, the contribution of S. pneumoniae EVs (pEVs) to host-microbe interactions has remained unclear. Here, we observed that S. pneumoniae infections in mice led to severe lung injuries and alveolar epithelial barrier (AEB) dysfunction. Infections of S. pneumoniae reduced the protein expression of tight junction protein OCLN (occludin) and activated macroautophagy/autophagy in lung tissues of mice and A549 cells. Mechanically, S. pneumoniae induced autophagosomal degradation of OCLN leading to AEB impairment in the A549 monolayer. S. pneumoniae released the pEVs that could be internalized by alveolar epithelial cells. Through proteomics, we profiled the cargo proteins inside pEVs and found that these pEVs contained many virulence factors, among which we identified a eukaryotic-like serine-threonine kinase protein StkP. The internalized StkP could induce the phosphorylation of BECN1 (beclin 1) at Ser93 and Ser96 sites, initiating autophagy and resulting in autophagy-dependent OCLN degradation and AEB dysfunction. Finally, the deletion of stkP in S. pneumoniae completely protected infected mice from death, significantly alleviated OCLN degradation in vivo, and largely abolished the AEB disruption caused by pEVs in vitro. Overall, our results suggested that pEVs played a crucial role in the spread of S. pneumoniae virulence factors. The cargo protein StkP in pEVs could communicate with host target proteins and even hijack the BECN1 autophagy initiation pathway, contributing to AEB disruption and bacterial pathogenicity.Abbreviations: AEB: alveolarepithelial barrier; AECs: alveolar epithelial cells; ATG16L1: autophagy related 16 like 1; ATP:adenosine 5'-triphosphate; BafA1: bafilomycin A1; BBB: blood-brain barrier; CFU: colony-forming unit; co-IP: co-immunoprecipitation; CQ:chloroquine; CTRL: control; DiO: 3,3'-dioctadecylox-acarbocyanineperchlorate; DOX: doxycycline; DTT: dithiothreitol; ECIS: electricalcell-substrate impedance sensing; eGFP: enhanced green fluorescentprotein; ermR: erythromycin-resistance expression cassette; Ery: erythromycin; eSTKs: eukaryotic-like serine-threoninekinases; EVs: extracellular vesicles; HA: hemagglutinin; H&E: hematoxylin and eosin; HsLC3B: human LC3B; hpi: hours post-infection; IP: immunoprecipitation; KD: knockdown; KO: knockout; LAMP1: lysosomal associated membrane protein 1; LC/MS: liquid chromatography-mass spectrometry; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MVs: membranevesicles; NC:negative control; NETs:neutrophil extracellular traps; OD: optical density; OMVs: outer membrane vesicles; PBS: phosphate-buffered saline; pEVs: S.pneumoniaeextracellular vesicles; protK: proteinase K; Rapa: rapamycin; RNAi: RNA interference; S.aureus: Staphylococcusaureus; SNF:supernatant fluid; sgRNA: single guide RNA; S.pneumoniae: Streptococcuspneumoniae; S.suis: Streptococcussuis; TEER: trans-epithelium electrical resistance; moi: multiplicity ofinfection; TEM:transmission electron microscope; TJproteins: tight junction proteins; TJP1/ZO-1: tight junction protein1; TSA: tryptic soy agar; WB: western blot; WT: wild-type.

The morphogenic protein CopD controls the spatio-temporal dynamics of PBP1a and PBP2b in Streptococcus pneumoniae

IMPORTANCE

Penicillin-binding proteins (PBPs) are essential for proper bacterial cell division and morphogenesis. The genome of Streptococcus pneumoniae encodes for two class B PBPs (PBP2x and 2b), which are required for the assembly of the peptidoglycan framework and three class A PBPs (PBP1a, 1b and 2a), which remodel the peptidoglycan mesh during cell division. Therefore, their activities should be finely regulated in space and time to generate the pneumococcal ovoid cell shape. To date, two proteins, CozE and MacP, are known to regulate the function of PBP1a and PBP2a, respectively. In this study, we describe a novel regulator (CopD) that acts on both PBP1a and PBP2b. These findings provide valuable information for understanding bacterial cell division. Furthermore, knowing that ß-lactam antibiotic resistance often arises from PBP mutations, the characterization of such a regulator represents a promising opportunity to develop new strategies to resensitize resistant strains.

The choline-binding proteins PspA, PspC, and LytA of Streptococcus pneumoniae and their interaction with human endothelial and red blood cells

Streptococcus pneumoniae is a Gram-positive opportunistic pathogen that can colonize the upper respiratory tract. It is a leading cause of a wide range of infectious diseases, including community-acquired pneumonia and meningitis. Pneumococcal infections cause 1-2 million deaths per year, most of which occur in developing countries. Here, we focused on three choline-binding proteins (CBPs), i.e., PspC, PspA, and LytA. These pneumococcal proteins have different surface-exposed regions but share related choline-binding anchors. These surface-exposed pneumococcal proteins are in direct contact with host cells and have diverse functions. We explored the role of the three CBPs on adhesion and pathogenicity in a human host by performing relevant imaging and functional analyses, such as electron microscopy, confocal laser scanning microscopy, and functional quantitative assays, targeting biofilm formation and the hemolytic capacity of S. pneumoniae. In vitro biofilm formation assays and electron microscopy experiments were used to examine the ability of knockout mutant strains lacking the lytA, pspC, or pspA genes to adhere to surfaces. We found that LytA plays an important role in robust synthesis of the biofilm matrix. PspA and PspC appeared crucial for the hemolytic effects of S. pneumoniae on human red blood cells. Furthermore, all knockout mutants caused less damage to endothelial cells than wild-type bacteria, highlighting the significance of each CPB for the overall pathogenicity of S. pneumoniae. Hence, in addition to their structural function within the cell wall of S. pneumoniae, each of these three surface-exposed CBPs controls or mediates multiple steps during bacterial pathogenesis.

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.

Molecular basis of the final step of cell division in Streptococcus pneumoniae

Bacterial cell-wall hydrolases must be tightly regulated during bacterial cell division to prevent aberrant cell lysis and to allow final separation of viable daughter cells. In a multidisciplinary work, we disclose the molecular dialogue between the cell-wall hydrolase LytB, wall teichoic acids, and the eukaryotic-like protein kinase StkP in Streptococcus pneumoniae. After characterizing the peptidoglycan recognition mode by the catalytic domain of LytB, we further demonstrate that LytB possesses a modular organization allowing the specific binding to wall teichoic acids and to the protein kinase StkP. Structural and cellular studies notably reveal that the temporal and spatial localization of LytB is governed by the interaction between specific modules of LytB and the final PASTA domain of StkP. Our data collectively provide a comprehensive understanding of how LytB performs final separation of daughter cells and highlights the regulatory role of eukaryotic-like kinases on lytic machineries in the last step of cell division in streptococci.

A link between STK signalling and capsular polysaccharide synthesis in Streptococcus suis

Synthesis of capsular polysaccharide (CPS), an important virulence factor of pathogenic bacteria, is modulated by the CpsBCD phosphoregulatory system in Streptococcus. Serine/threonine kinases (STKs, e.g. Stk1) can also regulate CPS synthesis, but the underlying mechanisms are unclear. Here, we identify a protein (CcpS) that is phosphorylated by Stk1 and modulates the activity of phosphatase CpsB in Streptococcus suis, thus linking Stk1 to CPS synthesis. The crystal structure of CcpS shows an intrinsically disordered region at its N-terminus, including two threonine residues that are phosphorylated by Stk1. The activity of phosphatase CpsB is inhibited when bound to non-phosphorylated CcpS. Thus, CcpS modulates the activity of phosphatase CpsB thereby altering CpsD phosphorylation, which in turn modulates the expression of the Wzx-Wzy pathway and thus CPS production.

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.

A multiomics analysis of direct interkingdom dynamics between influenza A virus and Streptococcus pneumoniae uncovers host-independent changes to bacterial virulence fitness

BACKGROUND

For almost a century, it has been recognized that influenza A virus (IAV) infection can promote the development of secondary bacterial infections (SBI) mainly caused by Streptococcus pneumoniae (Spn). Recent observations have shown that IAV is able to directly bind to the surface of Spn. To gain a foundational understanding of how direct IAV-Spn interaction alters bacterial biological fitness we employed combinatorial multiomic and molecular approaches.

RESULTS

Here we show IAV significantly remodels the global transcriptome, proteome and phosphoproteome profiles of Spn independently of host effectors. We identified Spn surface proteins that interact with IAV proteins (hemagglutinin, nucleoprotein, and neuraminidase). In addition, IAV was found to directly modulate expression of Spn virulence determinants such as pneumococcal surface protein A, pneumolysin, and factors associated with antimicrobial resistance among many others. Metabolic pathways were significantly altered leading to changes in Spn growth rate. IAV was also found to drive Spn capsule shedding and the release of pneumococcal surface proteins. Released proteins were found to be involved in evasion of innate immune responses and actively reduced human complement hemolytic and opsonizing activity. IAV also led to phosphorylation changes in Spn proteins associated with metabolism and bacterial virulence. Validation of proteomic data showed significant changes in Spn galactose and glucose metabolism. Furthermore, supplementation with galactose rescued bacterial growth and promoted bacterial invasion, while glucose supplementation led to enhanced pneumolysin production and lung cell apoptosis.

CONCLUSIONS

Here we demonstrate that IAV can directly modulate Spn biology without the requirement of host effectors and support the notion that inter-kingdom interactions between human viruses and commensal pathobionts can promote bacterial pathogenesis and microbiome dysbiosis.

BY-kinases: Protein tyrosine kinases like no other

BY-kinases (for bacterial tyrosine kinases) constitute a family of protein tyrosine kinases that are highly conserved in the bacterial kingdom and occur most commonly as essential components of multicomponent assemblies responsible for the biosynthesis, polymerization, and export of complex polysaccharides involved in biofilm or capsule formation. BY-kinase function has been attributed to a cyclic process involving formation of an oligomeric species, its disassembly into constituent monomers, and subsequent reassembly, depending on the overall phosphorylation level of a C-terminal cluster of tyrosine residues. However, the relationship of this process to the active/inactive states of the enzyme and the mechanism of its integration into the polysaccharide production machinery remain unclear. Here, we synthesize the substantial body of biochemical, cell-biological, structural, and computational data, acquired over the nearly 3 decades since the discovery of BY-kinases, to suggest means by which they fulfill their physiological function. We propose a mechanism involving temporal coordination of the assembly/disassembly process with the autokinase activity of the enzyme and its ability to be dephosphorylated by its counteracting phosphatase. We speculate that this temporal control enables BY-kinases to function as molecular timers that coordinate the diverse processes involved in the synthesis, polymerization, and export of complex sugar derivatives. We suggest that BY-kinases, which deploy distinctive catalytic domains resembling P-loop nucleoside triphosphatases, have uniquely adapted this ancient fold to drive functional processes through exquisite spatiotemporal control over protein-protein interactions and conformational changes. It is our hope that the hypotheses proposed here will facilitate future experiments targeting these unique protein kinases.

Effect of amino acids on free exopolysaccharide biosynthesis by Streptococcus thermophilus 937 in chemically defined medium

Free exopolysaccharide (f-EPS) produced by Streptococcus thermophilus improves the texture and functionality of fermented dairy foods. Our previous study showed a major improvement in f-EPS production of Strep. thermophilus 937 by increasing the concentrations of histidine, isoleucine, and glutamate to 15 mM in an optimized chemically defined medium. The aim of this study was to elucidate the effect of His, Ile, and Glu on the growth, f-EPS biosynthesis pathway, and carbohydrate metabolism profiles of Strep. thermophilus 937. The growth kinetics; transcript levels of key genes in the EPS biosynthesis pathway; enzyme activity involved in sugar nucleotide synthesis; concentrations of lactic acid, lactose, and galactose; and extracellular and intracellular pH were analyzed in chemically defined media with different initial histidine, isoleucine, and glutamate concentrations. The results showed that f-EPS production and viable cell counts of Strep. thermophilus 937 increased 2-fold after the concentrations of His, Ile, and Glu were increased. Additionally, increasing the concentrations of His, Ile, and Glu upregulated transcription of EPS biosynthesis genes and increased the activity of key enzymes in sugar nucleotide synthesis. Moreover, the consumption of lactose increased and secretion of galactose decreased, indicating that increasing the concentration of His, Ile, and Glu could enhance f-EPS production by maintaining viable cell counts, promoting sugar nucleotide synthesis, and increasing the transcript levels of the eps gene cluster. Our results provide a better understanding of the effect of AA on EPS biosynthesis in Strep. thermophilus.

Protein phosphatase SppA regulates apical growth and dephosphorylates cell polarity determinant DivIVA in Streptomyces coelicolor

Members of the Actinobacteria, including mycobacteria and streptomycetes, exhibit a distinctive mode of polar growth, with cell wall synthesis occurring in zones at cell poles and directed by the essential cell polarity determinant DivIVA. Streptomyces coelicolor modulates polar growth via the Ser/Thr protein kinase AfsK, which phosphorylates DivIVA. Here, we show that the phosphoprotein phosphatase SppA has strong effects on polar growth and cell shape and that it reverses the AfsK-mediated phosphorylation of DivIVA. SppA affects hyphal branching and the rate of tip extension. The sppA mutant hyphae also exhibit a high frequency of spontaneous growth arrests, indicating problems with maintenance of tip extension. The phenotypic effects are partially suppressed in an afsK sppA double mutant, indicating that AfsK and SppA to some extent share target proteins. Strains with a nonphosphorylatable mutant DivIVA confirm that the effect of afsK on hyphal branching during normal growth is mediated by DivIVA phosphorylation. However, the phenotypic effects of sppA deletion are independent of DivIVA phosphorylation and must be mediated via other substrates. This study adds a PPP-family protein phosphatase to the proteins involved in the control of polar growth and cell shape determination in S. coelicolor.

Comparative Phenotypic, Proteomic, and Phosphoproteomic Analysis Reveals Different Roles of Serine/Threonine Phosphatase and Kinase in the Growth, Cell Division, and Pathogenicity of Streptococcus suis

Eukaryote-like serine/threonine kinases (STKs) and cognate phosphatases (STPs) comprise an important regulatory system in many bacterial pathogens. The complexity of this regulatory system has not been fully understood due to the presence of multiple STKs/STPs in many bacteria and their multiple substrates involved in many different physiological and pathogenetic processes. Streptococci are the best materials for the study due to a single copy of the gene encoding STK and its cognate STP. Although several studies have been done to investigate the roles of STK and STP in zoonotic Streptococcus suis, respectively, few studies were performed on the coordinated regulatory roles of this system. In this study, we carried out a systemic study on STK/STP in S. suis by using a comparative phenotypic, proteomic, and phosphoproteomic analysis. Mouse infection assays revealed that STK played a much more important role in S. suis pathogenesis than STP. The ∆stk and ∆stp∆stk strains, but not ∆stp, showed severe growth retardation. Moreover, both ∆stp and ∆stk strains displayed defects in cell division, but they were abnormal in different ways. The comparative proteomics and phosphoproteomics revealed that deletion of stk or stp had a significant influence on protein expression. Interestingly, more virulence factors were found to be downregulated in ∆stk than ∆stp. In ∆stk strain, a substantial number of the proteins with a reduced phosphorylation level were involved in cell division, energy metabolism, and protein translation. However, only a few proteins showed increased phosphorylation in ∆stp, which also included some proteins related to cell division. Collectively, our results show that both STP and STK are critical regulatory proteins for S. suis and that STK seems to play more important roles in growth, cell division, and pathogenesis.

The Pneumococcal Divisome: Dynamic Control of Streptococcus pneumoniae Cell Division

Cell division in Streptococcus pneumoniae (pneumococcus) is performed and regulated by a protein complex consisting of at least 14 different protein elements; known as the divisome. Recent findings have advanced our understanding of the molecular events surrounding this process and have provided new understanding of the mechanisms that occur during the division of pneumococcus. This review will provide an overview of the key protein complexes and how they are involved in cell division. We will discuss the interaction of proteins in the divisome complex that underpin the control mechanisms for cell division and cell wall synthesis and remodelling that are required in S. pneumoniae, including the involvement of virulence factors and capsular polysaccharides.

Streptococcus suis MsmK: Novel Cell Division Protein Interacting with FtsZ and Maintaining Cell Shape

Bacteria of different shapes have adopted distinct mechanisms to faithfully coordinate morphogenesis and segregate their chromosomes prior to cell division. Despite recent focuses and advances, the mechanism of cell division in ovococci remains largely unknown. Streptococcus suis, a major zoonotic pathogen that causes problems in human health and in the global swine industry, is an elongated and ellipsoid bacterium that undergoes successive parallel splitting perpendicular to its long axis. Studies on cell cycle processes in this bacterium are limited. Here, we report that MsmK (multiple sugar metabolism protein K), an ATPase that contributes to the transport of multiple carbohydrates, has a novel role as a cell division protein in S. suis. MsmK can display ATPase and GTPase activities, interact with FtsZ via the N terminus of MsmK, and promote the bundling of FtsZ protofilaments in a GTP-dependent manner in vitro. Deletion of the C-terminal region or the Walker A or B motif affects the affinity between MsmK and FtsZ and decreases the ability of MsmK to promote FtsZ protofilament bundling. MsmK can form a complex with FtsZ in vivo, and its absence is not lethal but results in long chains and short, occasionally anuclear daughter cells. Superresolution microscopy revealed that the lack of MsmK in cells leads to normal septal peptidoglycan walls in mother cells but disturbed cell elongation and peripheral peptidoglycan synthesis. In summary, MsmK is a novel cell division protein that maintains cell shape and is involved in the synthesis of the peripheral cell wall.

IMPORTANCE Bacterial cell division is a highly ordered process regulated in time and space and is a potential target for the development of antimicrobial drugs. Bacteria of distinct shapes depend on different cell division mechanisms, but the mechanisms used by ovococci remain largely unknown. Here, we focused on the zoonotic pathogen Streptococcus suis and identified a novel cell division protein named MsmK, which acts as an ATPase of the ATP-binding cassette-type carbohydrate transport system. MsmK has GTPase and ATPase activities. In vitro protein assays showed that MsmK interacts with FtsZ and promotes FtsZ protofilament bundling that relies on GTP. Superresolution microscopy revealed that MsmK maintains cell shape and is involved in peripheral peptidoglycan synthesis. Knowledge of the multiple functions of MsmK may broaden our understanding of known cell division processes. Further studies in this area will elucidate how bacteria can faithfully and continually multiply in a constantly changing environment.

Recent progress in our understanding of peptidoglycan assembly in Firmicutes

Years of intense research have shown that the assembly of peptidoglycan, the extracellular mesh-like polymer surrounding the bacterial cell, is incredibly complex. It requires a suite of reactions catalyzed by dynamic macromolecular protein complexes whose localization and activity should be finely regulated in space and time. In this review, we focus on the main developments reported over the last five years for the assembly of peptidoglycan in Firmicutes, a bacterial phylum that comprises monoderm bacteria and that encompasses well studied bacterial models with different cell shapes and lifestyles.

A CozE Homolog Contributes to Cell Size Homeostasis of Streptococcus pneumoniae

Control of peptidoglycan assembly is critical to maintain bacterial cell size and morphology. Penicillin-binding proteins (PBPs) are crucial enzymes for the polymerization of the glycan strand and/or their cross-linking via peptide branches. Over the last few years, it has become clear that PBP activity and localization can be regulated by specific cognate regulators. The first regulator of PBP activity in Gram-positive bacteria was discovered in the human pathogen Streptococcus pneumoniae This regulator, named CozE, controls the activity of the bifunctional PBP1a to promote cell elongation and achieve a proper cell morphology. In this work, we studied a previously undescribed CozE homolog in the pneumococcus, which we named CozEb. This protein displays the same membrane organization as CozE but is much more widely conserved among Streptococcaceae genomes. Interestingly, cozEb deletion results in cells that are smaller than their wild-type counterparts, which is the opposite effect of cozE deletion. Furthermore, double deletion of cozE and cozEb results in poor viability and exacerbated cell shape defects. Coimmunoprecipitation further showed that CozEb is part of the same complex as CozE and PBP1a. However, although we confirmed that CozE is required for septal localization of PBP1a, the absence of CozEb has no effect on PBP1a localization. Nevertheless, we found that the overexpression of CozEb can compensate for the absence of CozE in all our assays. Altogether, our results show that the interplay between PBP1a and the cell size regulators CozE and CozEb is required for the maintenance of pneumococcal cell size and shape.IMPORTANCE Penicillin-binding proteins (PBPs), the proteins catalyzing the last steps of peptidoglycan assembly, are critical for bacteria to maintain cell size, shape, and integrity. PBPs are consequently attractive targets for antibiotics. Resistance to antibiotics in Streptococcus pneumoniae (the pneumococcus) are often associated with mutations in the PBPs. In this work, we describe a new protein, CozEb, controlling the cell size of pneumococcus. CozEb is a highly conserved integral membrane protein that works together with other proteins to regulate PBPs and peptidoglycan synthesis. Deciphering the intricate mechanisms by which the pneumococcus controls peptidoglycan assembly might allow the design of innovative anti-infective strategies, for example, by resensitizing resistant strains to PBP-targeting antibiotics.

A tyrosine phosphoregulatory system controls exopolysaccharide biosynthesis and biofilm formation in Vibrio cholerae

Production of an extracellular matrix is essential for biofilm formation, as this matrix both secures and protects the cells it encases. Mechanisms underlying production and assembly of matrices are poorly understood. Vibrio cholerae, relies heavily on biofilm formation for survival, infectivity, and transmission. Biofilm formation requires Vibrio polysaccharide (VPS), which is produced by vps gene-products, yet the function of these products remains unknown. Here, we demonstrate that the vps gene-products vpsO and vpsU encode respectively for a tyrosine kinase and a cognate tyrosine phosphatase. Collectively, VpsO and VpsU act as a tyrosine phosphoregulatory system to modulate VPS production. We present structures of VpsU and the kinase domain of VpsO, and we report observed autocatalytic tyrosine phosphorylation of the VpsO C-terminal tail. The position and amount of tyrosine phosphorylation in the VpsO C-terminal tail represses VPS production and biofilm formation through a mechanism involving the modulation of VpsO oligomerization. We found that tyrosine phosphorylation enhances stability of VpsO. Regulation of VpsO phosphorylation by the phosphatase VpsU is vital for maintaining native VPS levels. This study provides new insights into the mechanism and regulation of VPS production and establishes general principles of biofilm matrix production and its inhibition.

The biofilm life style protects microbes from a plethora of harm, to increase their survival and pathogenicity. Exopolysaccharides are the essential glue of the microbial biofilm matrix, and loss of this glue negates biofilm formation and renders cells more sensitive to antimicrobial agents. Here, we show that a tyrosine phosphoregulatory system controls the biosynthesis and abundance of Vibrio exopolysaccharide (VPS), an essential biofilm component of the pathogen Vibrio cholerae. The phosphorylation state of the tyrosine autokinase VpsO, mediated by the tyrosine phosphatase VpsU, directly modulates VPS production and also affects the kinase’s own degradation, to regulate VPS production. This study provides new insights into the mechanisms of V. cholerae biofilm formation and consequently ways to combat pathogens more broadly, due to conservation of tyrosine phosphoregulatory systems among exopolysaccharide producing bacteria.

Exopolysaccharides From Streptococcus thermophilus ST538 Modulate the Antiviral Innate Immune Response in Porcine Intestinal Epitheliocytes

It was reported that exopolysaccharides (EPSs) from lactobacilli are able to differentially modulate mucosal antiviral immunity. Although research has described the ability of EPSs derived from Streptococcus thermophilus to modulate the mucosal immune system, their impact on antiviral immunity was less explored. In this work, we investigated the capacity of the EPS-producing S. thermophilus ST538 to modulate the innate antiviral immune response triggered by the activation of the Toll-like receptor 3 (TLR3) in porcine intestinal epitheliocytes (PIE cells). Moreover, in order to study the immunomodulatory potential of S. thermophilus ST538 EPS, we successfully developed two mutant strains through the knockout of the epsB or epsC genes. High-performance liquid chromatography and scanning electron microscopy studies demonstrated that the wild type (WT) strain produced as high as 595 μg/ml of EPS in the skim milk medium, while none of the mutant strains (S. thermophilus ΔepsB and ΔepsC) were able to produce EPS. Studies in PIE cells demonstrated that the EPS of S. thermophilus ST538 is able to significantly improve the expression of interferon β (IFN-β), interleukin 6 (IL-6), and C-X-C motif chemokine 10 (CXCL10) in response to TLR3 stimulation. The role of EPS in the modulation of antiviral immune response in PIE cells was confirmed by comparative studies of cell free culture supernatants and fermented skim milks obtained from S. thermophilus ΔepsB and ΔepsC. These results suggest that S. thermophilus ST538 could be used as an immunobiotic strain for the development of new immunologically functional foods, which might contribute to improve resistance against viral infections.

The N-terminal domain of rhamnosyltransferase EpsF influences exopolysaccharide chain length determination in Streptococcus thermophilus 05-34

Glycosyltransferases are key enzymes involved in the assembly of repeating units of exopolysaccharides (EPS). A glycosyltransferase generally consists of the N-terminal and the C-terminal domain, however, the functional role of these domains in EPS biosynthesis remains largely unknown. In this study, homologous overexpression was employed to investigate the effects of EpsF_N, a truncated form of rhamnosyltransferase EpsF with only the N-terminal domain, on EPS biosynthesis in Streptococcus thermophilus 05-34. Reverse transcription qPCR and Western blotting analysis confirmed the successful expression of epsF_N in 05-34 at the transcription and translation level, respectively. Further analysis showed that the monosaccharide composition and yield of EPS were not affected by the overexpression of epsF_N, whereas the molecular mass decreased by 5-fold. Accordingly, the transcription levels of genes involved in EPS biosynthesis, including chain-length determination gene epsC, were down-regulated by 5- to 6-fold. These results indicated that the N-terminal domain of EpsF alone could influence the molecular mass of EPS, probably via lowering the concentration of sugar precursors, which may lead to decreased expression of genes responsible for chain-length determination.

Proteomic Investigation Uncovers Potential Targets and Target Sites of Pneumococcal Serine-Threonine Kinase StkP and Phosphatase PhpP

Like eukaryotes, different bacterial species express one or more Ser/Thr kinases and phosphatases that operate in various signaling networks by catalyzing phosphorylation and dephosphorylation of proteins that can immediately regulate biochemical pathways by altering protein function. The human pathogen Streptococcus pneumoniae encodes a single Ser/Thr kinase-phosphatase couple known as StkP-PhpP, which has shown to be crucial in the regulation of cell wall synthesis and cell division. In this study, we applied proteomics to further understand the physiological role of pneumococcal PhpP and StkP with an emphasis on phosphorylation events on Ser and Thr residues. Therefore, the proteome of the non-encapsulated D39 strain (WT), a kinase (ΔstkP), and phosphatase mutant (ΔphpP) were compared in a mass spectrometry based label-free quantification experiment. Results show that a loss of function of PhpP causes an increased abundance of proteins in the phosphate uptake system Pst. Quantitative proteomic data demonstrated an effect of StkP and PhpP on the two-component systems ComDE, LiaRS, CiaRH, and VicRK. To obtain further information on the function, targets and target sites of PhpP and StkP we combined the advantages of phosphopeptide enrichment using titanium dioxide and spectral library based data evaluation for sensitive detection of changes in the phosphoproteome of the wild type and the mutant strains. According to the role of StkP in cell division we identified several proteins involved in cell wall synthesis and cell division that are apparently phosphorylated by StkP. Unlike StkP, the physiological function of the co-expressed PhpP is poorly understood. For the first time we were able to provide a list of previously unknown putative targets of PhpP. Under these new putative targets of PhpP are, among others, five proteins with direct involvement in cell division (DivIVA, GpsB) and peptidoglycan biosynthesis (MltG, MreC, MacP).

Mechanisms of DNA Uptake by Naturally Competent Bacteria

Transformation is a widespread mechanism of horizontal gene transfer in bacteria. DNA uptake to the periplasmic compartment requires a DNA-uptake pilus and the DNA-binding protein ComEA. In the gram-negative bacteria, DNA is first pulled toward the outer membrane by retraction of the pilus and then taken up by binding to periplasmic ComEA, acting as a Brownian ratchet to prevent backward diffusion. A similar mechanism probably operates in the gram-positive bacteria as well, but these systems have been less well characterized. Transport, defined as movement of a single strand of transforming DNA to the cytosol, requires the channel protein ComEC. Although less is understood about this process, it may be driven by proton symport. In this review we also describe various phenomena that are coordinated with the expression of competence for transformation, such as fratricide, the kin-discriminatory killing of neighboring cells, and competence-mediated growth arrest.

¡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.

Tyrosine Phosphorylation as a Widespread Regulatory Mechanism in Prokaryotes

Phosphorylation events modify bacterial and archaeal proteomes, imparting cells with rapid and reversible responses to specific environmental stimuli or niches. Phosphorylated proteins are generally modified at one or more serine, threonine, or tyrosine residues. Within the last ten years, increasing numbers of global phosphoproteomic surveys of prokaryote species have revealed an abundance of tyrosine-phosphorylated proteins. In some cases, novel phosphorylation-dependent regulatory paradigms for cell division, gene transcription, and protein translation have been identified, suggesting that a wide scope of prokaryotic physiology remains to be characterized. Recent observations of bacterial proteins with putative phosphotyrosine binding pockets or Src homology 2 (SH2)-like domains suggest the presence of phosphotyrosine-dependent protein interaction networks. Here in this minireview, we focus on protein tyrosine phosphorylation, a posttranslational modification once thought to be rare in prokaryotes but which has emerged as an important regulatory facet in microbial biology.

The Tyrosine-Autokinase UbK Is Required for Proper Cell Growth and Cell Morphology of Streptococcus pneumoniae

Protein phosphorylation is a key post-translational modification required for many cellular functions of the bacterial cell. Recently, we identified a new protein-kinase, named UbK, in Bacillus subtilis that belongs to a new family of protein-kinases widespread in bacteria. In this study, we analyze the function of UbK in Streptococcus pneumoniae. We show that UbK displays a tyrosine-kinase activity and autophosphorylates on a unique tyrosine in vivo. To get insights into its cellular role, we constructed a set of pneumococcal ubk mutants. Using conventional and electron microscopy, we show that the ubk deficient strain, as well as an ubk catalytic dead mutant, display both severe cell-growth and cell-morphology defects. The same defects are observed with a mutant mimicking permanent phosphorylation of UbK whereas they are not detected for a mutant mimicking defective autophosphorylation of UbK. Moreover, we find that UbK phosphorylation promotes its ability to hydrolyze ATP. These observations show that the hydrolysis of ATP by UbK serves not only for its autophosphorylation but also for a distinct purpose essential for the optimal cell growth and cell-morphogenesis of the pneumococcus. We thus propose a model in which the autophosphorylation/dephosphorylation of UbK regulates its cellular function through a negative feedback loop.

EzrA, a cell shape regulator contributing to biofilm formation and competitiveness in Streptococcus mutans

Bacterial cell division is initiated by tubulin homologue FtsZ that assembles into a ring structure at mid-cell to facilitate cytokinesis. EzrA has been identified to be implicated in FtsZ-ring dynamics and cell wall biosynthesis during cell division of Bacillus subtilis and Staphylococcus aureus, the model rod and cocci. However, its role in pathogenic streptococci remains largely unknown. Here, the role of EzrA was investigated in Streptococcus mutans, the primary etiological agent of human dental caries, by constructing an ezrA in-frame deletion mutant. Our data showed that the ezrA mutant was slow-growing with a shortened length and extended width round cell shape compared to the wild type, indicating a delay in cell division with abnormalities of peptidoglycan biosynthesis. Additionally, FtsZ irregularly localized in dividing ezrA mutant cells forming angled division planes, potentially contributing to an aberrant cell shape. Furthermore, investigation using single-species cariogenic biofilm model revealed that deletion of ezrA resulted in defective biofilm formation with less extracellular polysaccharides and altered three-dimensional biofilm architecture. Unexpectedly, in a dual-species ecological model, the ezrA mutant exhibited substantially lower tolerance for H₂ O₂ and reduced competitiveness against one commensal species, Streptococcus sanguinis. Taken together, these results demonstrate that EzrA plays a key role in regulating cell division and maintaining a normal morphology in S. mutans and is required for its robust biofilm formation/interspecies competition. Therefore, EzrA protein represents a potential therapeutic target in the development of drugs controlling dental caries and other biofilm-related diseases.

RocS drives chromosome segregation and nucleoid protection in Streptococcus pneumoniae

Chromosome segregation in bacteria is poorly understood outside some prominent model strains^1-5 and even less is known about how it is coordinated with other cellular processes. This is the case for the opportunistic human pathogen Streptococcus pneumoniae (the pneumococcus)⁶, which lacks the Min and the nucleoid occlusion systems⁷, and possesses only an incomplete chromosome partitioning Par(A)BS system, in which ParA is absent⁸. The bacterial tyrosine kinase⁹ CpsD, which is required for capsule production, was previously found to interfere with chromosome segregation¹⁰. Here, we identify a protein of unknown function that interacts with CpsD and drives chromosome segregation. RocS (Regulator of Chromosome Segregation) is a membrane-bound protein that interacts with both DNA and the chromosome partitioning protein ParB to properly segregate the origin of replication region to new daughter cells. In addition, we show that RocS interacts with the cell division protein FtsZ and hinders cell division. Altogether, this work reveals that RocS is the cornerstone of a nucleoid protection system ensuring proper chromosome segregation and cell division in coordination with the biogenesis of the protective capsular layer.

Transcriptomic Insights Into the Growth Phase- and Sugar-Associated Changes in the Exopolysaccharide Production of a High EPS-Producing Streptococcus thermophilus ASCC 1275

In a previous study, incorporation of high exopolysaccharide (EPS) producing dairy starter bacterium Streptococcus thermophilus ASCC 1275 was found to improve functionality of low fat mozzarella cheese and yogurt. This bacterium in its eps gene cluster has a unique pair of chain length determining genes, epsC- epsD, when compared to other sequenced S. thermophilus strains. Hence, the aim of this study was to understand the regulatory mechanism of EPS production in this bacterium using transcriptomic analysis to provide opportunities to improve the yield of EPS. As sugars are considered as one of the major determinants of EPS production, after preliminary screening, we selected three sugars, glucose, sucrose and lactose to identify the EPS producing mechanism of this bacterium in M17 medium. Complete RNA-seq analysis was performed using Illumina HiSeq 2000 sequencing system on S. thermophilus 1275 grown in three different sugars at two-time points, 5 h (log phase) and 10 h (stationary phase) to recognize the genes involved in sugar uptake, UDP-sugar formation, EPS assembly and export of EPS outside the bacterial cell. S. thermophilus 1275 was found to produce high amount of EPS (∼430 mg/L) in sucrose (1%) supplemented M17 medium when compared to other two sugars. Differential gene expression analysis revealed the involvement of phosphoenolpyruvate phosphotransferase system (PEP-PTS) for glucose and sucrose uptake, and lacS gene for lactose uptake. The pathways for the formation of UDP-glucose and UDP-galactose were highly upregulated in all the three sugars. In the presence of sucrose, eps1C1D2C2D were found to be highly expressed which refers to high EPS production. Protein homology study suggested the presence of Wzx/Wzy-dependent EPS synthesis and transport pathway in this bacterium. KEGG pathway and COG functional enrichment analysis were also performed to support the result. This is the first report providing the transcriptomic insights into the EPS production mechanism of a common dairy bacterium, S. thermophilus.

Crosstalk between the serine/threonine kinase StkP and the response regulator ComE controls the stress response and intracellular survival of Streptococcus pneumoniae

Streptococcus pneumoniae is an opportunistic human bacterial pathogen that usually colonizes the upper respiratory tract, but the invasion and survival mechanism in respiratory epithelial cells remains elusive. Previously, we described that acidic stress-induced lysis (ASIL) and intracellular survival are controlled by ComE through a yet unknown activation mechanism under acidic conditions, which is independent of the ComD histidine kinase that activates this response regulator for competence development at pH 7.8. Here, we demonstrate that the serine/threonine kinase StkP is essential for ASIL, and show that StkP phosphorylates ComE at Thr¹²⁸. Molecular dynamic simulations predicted that Thr¹²⁸-phosphorylation induces conformational changes on ComE’s DNA-binding domain. Using nonphosphorylatable (ComE^T128A) and phosphomimetic (ComE^T128E) proteins, we confirmed that Thr¹²⁸-phosphorylation increased the DNA-binding affinity of ComE. The non-phosphorylated form of ComE interacted more strongly with StkP than the phosphomimetic form at acidic pH, suggesting that pH facilitated crosstalk. To identify the ComE-regulated genes under acidic conditions, a comparative transcriptomic analysis was performed between the comE^T128A and wt strains, and differential expression of 104 genes involved in different cellular processes was detected, suggesting that the StkP/ComE pathway induced global changes in response to acidic stress. In the comE^T128A mutant, the repression of spxB and sodA correlated with decreased H₂O₂ production, whereas the reduced expression of murN correlated with an increased resistance to cell wall antibiotic-induced lysis, compatible with cell wall alterations. In the comE^T128A mutant, ASIL was blocked and acid tolerance response was higher compared to the wt strain. These phenotypes, accompanied with low H₂O₂ production_, are likely responsible for the increased survival in pneumocytes of the comE^T128A mutant. We propose that the StkP/ComE pathway controls the stress response, thus affecting the intracellular survival of S. pneumoniae in pneumocytes, one of the first barriers that this pathogen must cross to establish an infection.

Streptococcus pneumoniae is a major human pathogen and is the causal agent of otitis (media) and sinusitis. It is also responsible for severe infections such as bacteremia, pneumonia, and meningitis, associated with 2 million annual deaths. Although this bacterium is part of the human nasopharynx commensal microbiota, it can become a pathogen and cross the epithelial cell barrier to establishing infections of varying intensity. Although S. pneumoniae is considered to be a typical extracellular pathogen, transient intracellular life forms have been found in eukaryotic cells, suggesting a putative survival mechanism. Here, we report that the serine-threonine kinase StkP was able to phosphorylate the response regulator ComE to control different cellular processes in response to environmental stress. Moreover, the phosphorylation of ComE on Thr¹²⁸, and the consequent conformational and functional changes resulting from this event, extended the current knowledge of molecular activation mechanisms of response regulators. In this report, we provide evidence for the regulatory control exerted by the StkP/ComE pathway on acid-induced autolysis (associated with pneumolysin release), the acid tolerance response, and H₂O₂ production to modulate tissue damage and intracellular survival, which are ultimately linked to pneumococcal pathogenesis.

A programmed cell division delay preserves genome integrity during natural genetic transformation in Streptococcus pneumoniae

Competence for genetic transformation is a differentiation program during which exogenous DNA is imported into the cell and integrated into the chromosome. In Streptococcus pneumoniae, competence develops transiently and synchronously in all cells during exponential phase, and is accompanied by a pause in growth. Here, we reveal that this pause is linked to the cell cycle. At least two parallel pathways impair peptidoglycan synthesis in competent cells. Single-cell analyses demonstrate that ComM, a membrane protein induced during competence, inhibits both initiation of cell division and final constriction of the cytokinetic ring. Competence also interferes with the activity of the serine/threonine kinase StkP, the central regulator of pneumococcal cell division. We further present evidence that the ComM-mediated delay in division preserves genomic integrity during transformation. We propose that cell division arrest is programmed in competent pneumococcal cells to ensure that transformation is complete before resumption of cell division, to provide this pathogen with the maximum potential for genetic diversity and adaptation.

In Streptococcus pneumoniae, competence for genetic transformation is accompanied by a pause in growth. Here, Bergé et al. show that this pause is linked to the cell cycle via at least two pathways that impair peptidoglycan synthesis and preserve genomic integrity during transformation.

Identification of EloR (Spr1851) as a regulator of cell elongation in Streptococcus pneumoniae

In a screen for mutations suppressing the lethal loss of PBP2b in Streptococcus pneumoniae we identified Spr1851 (named EloR), a cytoplasmic protein of unknown function whose inactivation removed the requirement for PBP2b as well as RodA. It follows from this that EloR and the two elongasome proteins must be part of the same functional network. This network also includes StkP, as this serine/threonine kinase phosphorylates EloR on threonine 89 (T89). We found that ΔeloR cells, and cells expressing the phosphoablative form of EloR (EloR^T89A ), are significantly shorter than wild-type cells. Furthermore, the phosphomimetic form of EloR (EloR^T89E ) is not tolerated unless the cell in addition acquires a truncated MreC or non-functional RodZ protein. By itself, truncation of MreC as well as inactivation of RodZ gives rise to less elongated cells, demonstrating that the stress exerted by the phosphomimetic form of EloR is relieved by suppressor mutations that reduce or abolish the activity of the elongasome. Of note, it was also found that loss of elongasome activity caused by truncation of MreC elicits increased StkP-mediated phosphorylation of EloR. Together, the results support a model in which phosphorylation of EloR stimulates cell elongation, while dephosphorylation has an inhibitory effect.

Chromosome segregation drives division site selection in Streptococcus pneumoniae

Accurate spatial and temporal positioning of the tubulin-like protein FtsZ is key for proper bacterial cell division. Streptococcus pneumoniae (pneumococcus) is an oval-shaped, symmetrically dividing opportunistic human pathogen lacking the canonical systems for division site control (nucleoid occlusion and the Min-system). Recently, the early division protein MapZ was identified and implicated in pneumococcal division site selection. We show that MapZ is important for proper division plane selection; thus, the question remains as to what drives pneumococcal division site selection. By mapping the cell cycle in detail, we show that directly after replication both chromosomal origin regions localize to the future cell division sites, before FtsZ. Interestingly, Z-ring formation occurs coincidently with initiation of DNA replication. Perturbing the longitudinal chromosomal organization by mutating the condensin SMC, by CRISPR/Cas9-mediated chromosome cutting, or by poisoning DNA decatenation resulted in mistiming of MapZ and FtsZ positioning and subsequent cell elongation. Together, we demonstrate an intimate relationship between DNA replication, chromosome segregation, and division site selection in the pneumococcus, providing a simple way to ensure equally sized daughter cells.

Evidence that pneumococcal WalK is regulated by StkP through protein-protein interaction

WalRK is the only two-component regulatory system essential for viability in Streptococcus pneumoniae. Despite its importance, the biological role of this system is not well understood. However, previous studies have shown that it has a crucial role in controlling pneumococcal cell division. Considerable efforts have been made to understand how the WalRK system is regulated, but no signal(s) sensed by the WalK histidine kinase has been identified so far. Here, we provide evidence that the serine/threonine protein kinase StkP modulates the activity of WalK through direct protein-protein interaction, suggesting that this interaction is one of the signals sensed by WalK. In most low-G+C content Gram-positive bacteria, WalK orthologues are attached to the cytoplasmic membrane via two transmembrane segments separated by a large extracellular loop believed to function as a sensor domain. In contrast, members of the genus Streptococcus have WalK histidine kinases that are anchored to the cytoplasmic membrane by a single transmembrane segment. It has been a long-standing question whether this segment only serves as a membrane anchor or if it also functions as a signal-sensing domain. Our data strongly support the latter, i.e. that the transmembrane segment senses signals that regulate the activity of WalK.

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.