Mutational analysis of the carboxy-terminal (YGX)4 repeat domain of CpsD, an autophosphorylating tyrosine kinase required for capsule biosynthesis in Streptococcus pneumoniae

Thermophilic PHP Protein Tyrosine Phosphatases (Cap8C and Wzb) from Mesophilic Bacteria

Protein tyrosine phosphatases (PTPs) of the polymerase and histidinol phosphatase (PHP) superfamily with characteristic phosphatase activity dependent on divalent metal ions are found in many Gram-positive bacteria. Although members of this family are co-purified with metal ions, they still require the exogenous supply of metal ions for full activation. However, the specific roles these metal ions play during catalysis are yet to be well understood. Here, we report the metal ion requirement for phosphatase activities of S. aureus Cap8C and L. rhamnosus Wzb. AlphaFold-predicted structures of the two PTPs suggest that they are members of the PHP family. Like other PHP phosphatases, the two enzymes have a catalytic preference for Mn2+, Co2+ and Ni2+ ions. Cap8C and Wzb show an unusual thermophilic property with optimum activities over 75 °C. Consistent with this model, the activity-temperature profiles of the two enzymes are dependent on the divalent metal ion activating the enzyme.

RNA thermosensors facilitate Streptococcus pneumoniae and Haemophilus influenzae immune evasion

Bacterial meningitis is a major cause of death and disability in children worldwide. Two human restricted respiratory pathogens, Streptococcus pneumoniae and Haemophilus influenzae, are the major causative agents of bacterial meningitis, attributing to 200,000 deaths annually. These pathogens are often part of the nasopharyngeal microflora of healthy carriers. However, what factors elicit them to disseminate and cause invasive diseases, remain unknown. Elevated temperature and fever are hallmarks of inflammation triggered by infections and can act as warning signals to pathogens. Here, we investigate whether these respiratory pathogens can sense environmental temperature to evade host complement-mediated killing. We show that productions of two vital virulence factors and vaccine components, the polysaccharide capsules and factor H binding proteins, are temperature dependent, thus influencing serum/opsonophagocytic killing of the bacteria. We identify and characterise four novel RNA thermosensors in S. pneumoniae and H. influenzae, responsible for capsular biosynthesis and production of factor H binding proteins. Our data suggest that these bacteria might have independently co-evolved thermosensing abilities with different RNA sequences but distinct secondary structures to evade the immune system.

Decoding capsule synthesis in Streptococcus pneumoniae

Streptococcus pneumoniae synthesizes more than one hundred types of capsular polysaccharides (CPS). While the diversity of the enzymes and transporters involved is enormous, it is not limitless. In this review, we summarized the recent progress on elucidating the structure-function relationships of CPS, the mechanisms by which they are synthesized, how their synthesis is regulated, the host immune response against them, and the development of novel pneumococcal vaccines. Based on the genetic and structural information available, we generated provisional models of the CPS repeating units that remain unsolved. In addition, to facilitate cross-species comparisons and assignment of glycosyltransferases, we illustrated the biosynthetic pathways of the known CPS in a standardized format. Studying the intricate steps of pneumococcal CPS assembly promises to provide novel insights for drug and vaccine development as well as improve our understanding of related pathways in other species.

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.

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.

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.

Streptococcus pneumoniae Capsular Polysaccharide

The polysaccharide capsule of Streptococcus pneumoniae is the dominant surface structure of the organism and plays a critical role in virulence, principally by interfering with host opsonophagocytic clearance mechanisms. The capsule is the target of current pneumococcal vaccines, but there are 98 currently recognised polysaccharide serotypes and protection is strictly serotype-specific. Widespread use of these vaccines is driving changes in serotype prevalence in both carriage and disease. This chapter summarises current knowledge on the role of the capsule and its regulation in pathogenesis, the mechanisms of capsule synthesis, the genetic basis for serotype differences, and provides insights into how so many structurally distinct capsular serotypes have evolved. Such knowledge will inform ongoing refinement of pneumococcal vaccination strategies.

Adenylate kinase potentiates the capsular polysaccharide by modulating Cps2D in Streptococcus pneumoniae D39

Streptococcus pneumoniae is a polysaccharide-encapsulated bacterium. The capsule thickens during blood invasion compared with the thinner capsules observed in asymptomatic nasopharyngeal colonization. However, the underlying mechanism regulating differential CPS expression remains unclear. CPS synthesis requires energy that is supplied by ATP. Previously, we demonstrated a correlation between ATP levels and adenylate kinase in S. pneumoniae (SpAdK). A dose-dependent induction of SpAdK in serum was also reported. To meet medical needs, this study aimed to elucidate the role of SpAdK in the regulation of CPS production. CPS levels in S. pneumoniae type 2 (D39) increased proportionally with SpAdK levels, but they were not related to pneumococcal autolysis. Moreover, increased SpAdK levels resulted in increased total tyrosine kinase Cps2D levels and phosphorylated Cps2D, which is a regulator of CPS synthesis in the D39 strain. Our results also indicated that the SpAdK and Cps2D proteins interact in the presence of Mg-ATP. In addition, in silico analysis uncovered the mechanism behind this protein–protein interaction, suggesting that SpAdK binds with the Cps2D dimer. This established the importance of the ATP-binding domain of Cps2D. Taken together, the biophysical interaction between SpAdK and Cps2D plays an important role in enhancing Cps2D phosphorylation, which results in increased CPS synthesis.

A physical interaction between two key enzymes explains how the bacterium responsible for causing pneumococcal disease thickens its external capsule during infection of the bloodstream. A team led by Dong-Kwon Rhee from Sungkyunkwan University in Suwon, South Korea, studied strains of Streptococcus pneumoniae expressing varying levels of an enzyme that helps maintain the proper balance of cellular energy. They found that this enzyme stimulated the production of sugar chains that coat the outside of the bacterial capsule by binding and activating an intermediary enzyme involved in the synthesis of these sugar chains. Since the capsule is critical in warding off the human immune response, the findings suggest that drugs designed to disrupt the enzyme-mediated induction of capsule formation could help prevent or treat pneumococcal disease.

Comparative genomic analysis of ten clinical Streptococcus pneumoniae collected from a Malaysian hospital reveal 31 new unique drug-resistant SNPs using whole genome sequencing

Background

Streptococcus pneumoniae or pneumococcus is a leading cause of morbidity and mortality worldwide, specifically in relation to community-acquired pneumonia. Due to the overuse of antibiotics, S. pneumoniae has developed a high degree of resistance to a wide range of antibacterial drugs.

Methods

In this study, whole genome sequencing (WGS) was performed for 10 clinical strains of S. pneumoniae with different levels of sensitivity to standard antibiotics. The main objective was to investigate genetic changes associated with antibiotic resistance in S. pneumoniae.

Results

Our results showed that resistant isolates contain a higher number of non-synonymous single nucleotide polymorphisms (SNPs) as compared to susceptible isolates. We were able to identify SNPs that alter a single amino acid in many genes involved in virulence and capsular polysaccharide synthesis. In addition, 90 SNPs were only presented in the resistant isolates, and 31 SNPs were unique and had not been previously reported, suggesting that these unique SNPs could play a key role in altering the level of resistance to different antibiotics.

Conclusion

Whole genome sequencing is a powerful tool for comparing the full genome of multiple isolates, especially those closely related, and for analysing the variations found within antibiotic resistance genes that lead to differences in antibiotic sensitivity. We were able to identify specific mutations within virulence genes related to resistant isolates. These findings could provide insights into understanding the role of single nucleotide mutants in conferring drug resistance.

Electronic supplementary material

The online version of this article (10.1186/s12929-018-0414-8) contains supplementary material, which is available to authorized users.

Role of Streptococcus pneumoniae OM001 operon in capsular polysaccharide production, virulence and survival in human saliva

Streptococcus pneumoniae is the leading cause of community-acquired pneumonia in all ages worldwide, and with ever-increasing antibiotic resistance, the understanding of its pathogenesis and spread is as important as ever. Recently, we reported the presence of a Low Molecular Weight Tyrosine Phosphatase (LMWPTP) Spd1837 in the pneumococcus. This protein is encoded in an operon, OM001 with two other genes, with previous work implicating this operon as important for pneumococcal virulence. Thus, we set out to investigate the role of the individual genes in the operon during pneumococcal pathogenesis. As LMWPTPs play a major role in capsular polysaccharide (CPS) biosynthesis in many bacteria, we tested the effect of mutating spd1837 and its adjacent genes, spd1836 and spd1838 on CPS levels. Our results suggest that individual deletion of the genes, including the LMWPTP, did not modulate CPS levels, in multiple conditions, and in different strain backgrounds. Following in vivo studies, Spd1836 was identified as a novel virulence factor during pneumococcal invasive disease, in both the lungs and blood, with this protein alone responsible for the effects of operon’s role in virulence. We also showed that a deletion in spd1836, spd1838 or the overall OM001 operon reduced survival in human saliva during the conditions that mimic transmission compared to the wildtype strain. With studies suggesting that survival in human saliva may be important for transmission, this study identifies Spd1836 and Spd1838 as transmission factors, potentially facilitating the spread of the pneumococcus from person to person. Overall, this study hopes to further our understanding of the bacterial transmission that precedes disease and outbreaks.

Proteomic variation and diversity in clinical Streptococcus pneumoniae isolates from invasive and non-invasive sites

Streptococcus pneumoniae is responsible for a variety of invasive and non-invasive human infections. There are over 90 serotypes of S. pneumoniae differing in their ability to adapt to the different niches within the host. Two-dimensional gel electrophoresis was used to discriminate clinical S. pneumoniae isolates recovered from either blood cultures (invasive site isolates) or other sites, including sputum, tracheal aspirate, ear, eye and skin swabs (non-invasive site isolates). Global protein expression profiles for five invasive site and six non-invasive site isolates representing five different serotypes (serotypes 4, 6, 9, 14 and 23) were obtained for each isolate and combined into a single data set using Progenesis SameSpots^™ software. One-hundred and eighty six protein spots (39% of the protein spots in the dataset) differed significantly (ANOVA, p<0.05) in abundance between the invasive site (101 upregulated protein spots) and non-invasive site (85 upregulated protein spots) isolates. Correlations between the bacterial proteomes and their sites of isolation were determined by Principal Component Analysis (PCA) using the significantly different protein spots. Out of the 186 variable protein spots, 105 exhibited a serotype-associated pattern of variability. The expression of the remaining 81 protein spots was concluded to be uniquely linked to the site of bacterial isolation. Mass spectrometry was used to identify selected protein spots that showed either constant or differential abundance levels. The identified proteins had a diverse range of functions including, capsule biogenesis, DNA repair, protein deglycation, translation, stress response and virulence as well as amino acid, carbohydrate, lipid and nucleotide metabolism. These findings provide insight on the proteins that contribute towards the adaptation of the bacteria to different sites within the host.

GidA, a tRNA Modification Enzyme, Contributes to the Growth, and Virulence of Streptococcus suis Serotype 2

Glucose-inhibited division protein (GidA), is a tRNA modification enzyme functioning together with MnmE in the addition of a carboxymethylaminomethyl group to position 5 of the anticodon wobble uridine of tRNA. Here, we report a GidA homolog from a Chinese isolate SC-19 of the zoonotic Streptococcus suis serotype 2 (SS2). gidA disruption led to a defective growth, increased capsule thickness, and reduced hemolytic activity. Moreover, the gidA deletion mutant (ΔgidA) displayed reduced mortality and bacterial loads in mice, reduced ability of adhesion to and invasion in epithelial cells, and increased sensitivity to phagocytosis. The iTRAQ analysis identified 372 differentially expressed (182 up- and 190 down-regulated) proteins in ΔgidA and SC-19. Numerous DNA replication, cell division, and virulence associated proteins were downregulated, whereas many capsule synthesis enzymes were upregulated by gidA disruption. This is consistent with the phenotypes of the mutant. Thus, GidA is a translational regulator that plays an important role in the growth, cell division, capsule biosynthesis, and virulence of SS2. Our findings provide new insight into the regulatory function of GidA in bacterial pathogens.

Tyrosine Phosphorylation and Dephosphorylation in Burkholderia cenocepacia Affect Biofilm Formation, Growth under Nutritional Deprivation, and Pathogenicity

Burkholderia cenocepacia, a member of the B. cepacia complex (Bcc), is an opportunistic pathogen causing serious chronic infections in patients with cystic fibrosis. Tyrosine phosphorylation has emerged as an important posttranslational modification modulating the physiology and pathogenicity of Bcc bacteria. Here, we investigated the predicted bacterial tyrosine kinases BCAM1331 and BceF and the low-molecular-weight protein tyrosine phosphatases BCAM0208, BceD, and BCAL2200 of B. cenocepacia K56-2. We show that BCAM1331, BceF, BCAM0208, and BceD contribute to biofilm formation, while BCAL2200 is required for growth under nutrient-limited conditions. Multiple deletions of either tyrosine kinase or low-molecular-weight protein tyrosine phosphatase genes resulted in the attenuation of B. cenocepacia intramacrophage survival and reduced pathogenicity in the Galleria mellonella larval infection model. Experimental evidence indicates that BCAM1331 displays reduced tyrosine autophosphorylation activity compared to that of BceF. With the artificial substrate p-nitrophenyl phosphate, the phosphatase activities of the three low-molecular-weight protein tyrosine phosphatases demonstrated similar kinetic parameters. However, only BCAM0208 and BceD could dephosphorylate BceF. Further, BCAL2200 became tyrosine phosphorylated in vivo and catalyzed its autodephosphorylation. Together, our data suggest that despite having similar biochemical activities, low-molecular-weight protein tyrosine phosphatases and tyrosine kinases have both overlapping and specific roles in the physiology of B. cenocepacia.

Autophosphorylation of the Bacterial Tyrosine-Kinase CpsD Connects Capsule Synthesis with the Cell Cycle in Streptococcus pneumoniae

Bacterial capsular polysaccharides (CPS) are produced by a multi-protein membrane complex, in which a particular type of tyrosine-autokinases named BY-kinases, regulate their polymerization and export. However, our understanding of the role of BY-kinases in these processes remains incomplete. In the human pathogen Streptococcus pneumoniae, the BY-kinase CpsD localizes at the division site and participates in the proper assembly of the capsule. In this study, we show that the cytoplasmic C-terminal end of the transmembrane protein CpsC is required for CpsD autophosphorylation and localization at mid-cell. Importantly, we demonstrate that the CpsC/CpsD complex captures the polysaccharide polymerase CpsH at the division site. Together with the finding that capsule is not produced at the division site in cpsD and cpsC mutants, these data show that CPS production occurs exclusively at mid-cell and is tightly dependent on CpsD interaction with CpsC. Next, we have analyzed the impact of CpsD phosphorylation on CPS production. We show that dephosphorylation of CpsD induces defective capsule production at the septum together with aberrant cell elongation and nucleoid defects. We observe that the cell division protein FtsZ assembles and localizes properly although cell constriction is impaired. DAPI staining together with localization of the histone-like protein HlpA further show that chromosome replication and/or segregation is defective suggesting that CpsD autophosphorylation interferes with these processes thus resulting in cell constriction defects and cell elongation. We show that CpsD shares structural homology with ParA-like ATPases and that it interacts with the chromosome partitioning protein ParB. Total internal reflection fluorescence microscopy imaging demonstrates that CpsD phosphorylation modulates the mobility of ParB. These data support a model in which phosphorylation of CpsD acts as a signaling system coordinating CPS synthesis with chromosome segregation to ensure that daughter cells are properly wrapped in CPS.

Bacteria utilize a multi-protein membrane complex to synthesize and export the polysaccharide capsule that conceals and covers the cell. In bacterial pathogens, the capsule protects the cell form opsonophagocytosis and complement-mediated killing. The mechanisms allowing the bacterial cell to maintain this protective capsule during cell growth and division remain unknown. The capsule assembly machinery encompasses a particular type of tyrosine-kinases found only in bacteria, which are called BY-kinases. These kinases are involved in the regulation of several cellular functions including polysaccharide capsule production. Studying the role of BY-kinase represents thus an interesting approach to decipher the mechanisms of capsule synthesis and export. Here, we study the role of the BY-kinase CpsD in the human pathogen Streptococcus pneumoniae. We show that CpsD plays a dual function in the pneumococcus. Indeed, CpsD captures the capsule assembly machinery at the site of division, but we also show that CpsD coordinates capsule production with the cell cycle by interacting with the chromosome segregation system. These features provide a simple mechanism to cover the complete surface of the pneumococcal daughter cells. This finding further opens a new view of the function of BY-kinases in the bacterial cell notably in localizing protein complexes in subcellular regions over the cell cycle.

Functional analysis of conserved motifs in a bacterial tyrosine kinase, BtkB, from Myxococcus xanthus

Myxococcus xanthus has two bacterial protein-tyrosine (BY) kinases, BtkA and BtkB. Autophosphorylation in C-terminal tyrosine-rich clusters and poly(Glu, Tyr) kinase activities of cytoplasmic catalytic domains of BtkA and BtkB were activated by the intracellular juxtamembrane regions of the second transmembrane helices. Protein kinase activity against poly(Glu, Tyr) of cytoplasmic fragment of BtkB (CF-BtkB) containing an activator region was not inhibited by serine/threonine protein kinase inhibitors. However, addition of tyrosine protein kinase inhibitors, genistein and 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2), at a concentration of 0.2 mM, inhibited the CF-BtkB kinase activity by 20 and 64%, respectively. A CF-BtkB mutant constructed by replacing all C-terminal tyrosine residues with phenylalanines, did not undergo autophosphorylation. Further, this mutation did not significantly affect poly(Glu, Tyr) kinase activity, suggesting that M. xanthus BtkB kinase activity is not dependent on autophosphorylation in the C-terminal tyrosine cluster. A conserved motif (ExxRxxR) of BY kinases is involved in the self-association of catalytic domains of BY kinases, necessary to accomplish trans-phosphorylation. An ExxRxxR motif mutant of CF-BtkB led to loss of autophosphorylation and poly(Glu, Tyr) kinase activities. These observations provide insights into the regulation mechanism of M. xanthus BY kinase activity.

Mutations in pneumococcal cpsE generated via in vitro serial passaging reveal a potential mechanism of reduced encapsulation utilized by a conjunctival isolate

The polysaccharide capsule of Streptococcus pneumoniae is required for nasopharyngeal colonization and for invasive disease in the lungs, blood, and meninges. In contrast, the vast majority of conjunctival isolates are acapsular. The first serotype-specific gene in the capsule operon, cpsE, encodes the initiating glycosyltransferase and is one of the few serotype-specific genes that can tolerate null mutations. This report characterizes a spontaneously arising TIGR4 mutant exhibiting a reduced capsule, caused by a 6-nucleotide duplication in cpsE which results in duplication of Ala and Ile at positions 45 and 46. This strain (AI45dup) possessed more exposed phosphorylcholine and was hypersusceptible to C3 complement deposition compared to the wild type. Accordingly, the mutant was significantly better at forming abiotic biofilms and binding epithelial cells in vitro but was avirulent in a sepsis model. In vitro serial passaging of the wild-type strain failed to reproduce the AI45dup mutation but instead led to a variety of mutants with reduced capsule harboring single nucleotide polymorphisms (SNPs) in cpsE. A single passage in the sepsis model after high-dose inoculation readily yielded revertants of AI45dup with restored wild-type capsule level, but the majority of SNP alleles of cpsE could not revert, suppress, or bypass. Analysis of cpsE in conjunctival isolates revealed a strain with a single missense mutation at amino acid position 377, which was responsible for reduced encapsulation. This study supports the hypothesis that spontaneous, nonreverting mutations in cpsE serve as a form of adaptive mutation by providing a selective advantage to S. pneumoniae in niches where expression of capsule is detrimental.

IMPORTANCE

While the capsule of Streptococcus pneumoniae is required for colonization and invasive disease, most conjunctival isolates are acapsular by virtue of deletion of the entire capsular operon. We show that spontaneous acapsular mutants isolated in vitro harbor mostly nonrevertible single nucleotide polymorphism (SNP) null mutations in cpsE, encoding the initiating glycosyltransferase. From a small collection of acapsular conjunctival isolates, we identified one strain with a complete capsular operon but containing a SNP in cpsE that we show is responsible for the acapsular phenotype. We propose that acapsular conjunctival isolates may arise initially from such nonreverting SNP null mutations in cpsE, which can be followed later by deletion of portions or all of the cps operon.

A tyrosine phosphorylation switch controls the interaction between the transmembrane modulator protein Wzd and the tyrosine kinase Wze of Lactobacillus rhamnosus

Background

One proposed mechanism for assembly of secreted heteropolysaccharides by many Gram positive bacteria relies on the coordinated action of a polymerization complex through reversible phosphorylation events. The role of the tyrosine protein kinase transmembrane modulator is, however, not well understood.

Results

The protein sequences deduced from the wzb, wzd and wze genes from Lactobacillus rhamnosus ATCC 9595 and RW-9595 M contain motifs also found in corresponding proteins CpsB, CpsC and CpsD from Streptococcus pneumoniae D39 (serotype 2). Use of an anti-phosphotyrosine antibody demonstrated that both Wzd and Wze can be found in tyrosine phosphorylated form. When tyrosine 266 was mutated to phenylalanine, WzdY266F showed slightly less phosphorylated protein than those produced by using eight other tyrosine mutated Wzd genes, when expressed along with Wze and Wzb in Lactococcus lactis subsp. cremoris MG1363. In order to demonstrate the importance of ATP for the interactions among these proteins, native and fusion Wzb, Wzd and Wze proteins were expressed and purified from Escherichia coli cultures. The modulator protein, Wzd, binds with the phosphotyrosine kinase Wze, irrespective of its phosphorylation status. However, Wze attained a higher phosphorylation level after interacting with phosphorylated Wzd in the presence of 10 mM ATP. This highly phosphorylated Wze did not remain in close association with phosphorylated Wzd.

Conclusion

The Wze tyrosine kinase protein of Lactobacillus rhamnosus thus carries out tyrosine phosphorylation of Wzd in addition to auto- and trans- phosphorylation of the kinase itself.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0371-2) contains supplementary material, which is available to authorized users.

Streptococcus agalactiae capsule polymer length and attachment is determined by the proteins CpsABCD

The production of capsular polysaccharides (CPS) or secreted exopolysaccharides is ubiquitous in bacteria, and the Wzy pathway constitutes a prototypical mechanism to produce these structures. Despite the differences in polysaccharide composition among species, a group of proteins involved in this pathway is well conserved. Streptococcus agalactiae (group B Streptococcus; GBS) produces a CPS that represents the main virulence factor of the bacterium and is a prime target in current vaccine development. We used this human pathogen to investigate the roles and potential interdependencies of the conserved proteins CpsABCD encoded in the cps operon, by developing knock-out and functional mutant strains. The mutant strains were examined for CPS quantity, size, and attachment to the cell surface as well as CpsD phosphorylation. We observed that CpsB, -C, and -D compose a phosphoregulatory system where the CpsD autokinase phosphorylates its C-terminal tyrosines in a CpsC-dependent manner. These Tyr residues are also the target of the cognate CpsB phosphatase. An interaction between CpsD and CpsC was observed, and the phosphorylation state of CpsD influenced the subsequent action of CpsC. The CpsC extracellular domain appeared necessary for the production of high molecular weight polysaccharides by influencing CpsA-mediated attachment of the CPS to the bacterial cell surface. In conclusion, although having no impact on cps transcription or the synthesis of the basal repeating unit, we suggest that these proteins are fine-tuning the last steps of CPS biosynthesis (i.e. the balance between polymerization and attachment to the cell wall).

Impact of the exopolysaccharide layer on biofilms, adhesion and resistance to stress in Lactobacillus johnsonii FI9785

BACKGROUND

The bacterial cell surface is a crucial factor in cell-cell and cell-host interactions. Lactobacillus johnsonii FI9785 produces an exopolysaccharide (EPS) layer whose quantity and composition is altered in mutants that harbour genetic changes in their eps gene clusters. We have assessed the effect of changes in EPS production on cell surface characteristics that may affect the ability of L. johnsonii to colonise the poultry host and exclude pathogens.

RESULTS

Analysis of physicochemical cell surface characteristics reflected by Zeta potential and adhesion to hexadecane showed that an increase in EPS gave a less negative, more hydrophilic surface and reduced autoaggregation. Autoaggregation was significantly higher in mutants that have reduced EPS, indicating that EPS can mask surface structures responsible for cell-cell interactions. EPS also affected biofilm formation, but here the quantity of EPS produced was not the only determinant. A reduction in EPS production increased bacterial adhesion to chicken gut explants, but made the bacteria less able to survive some stresses.

CONCLUSIONS

This study showed that manipulation of EPS production in L. johnsonii FI9785 can affect properties which may improve its performance as a competitive exclusion agent, but that positive changes in adhesion may be compromised by a reduction in the ability to survive stress.

Membrane Translocation and Assembly of Sugar Polymer Precursors

Bacterial polysaccharides play an essential role in cell viability, virulence, and evasion of host defenses. Although the polysaccharides themselves are highly diverse, the pathways by which bacteria synthesize these essential polymers are conserved in both Gram-negative and Gram-positive organisms. By utilizing a lipid linker, a series of glycosyltransferases and integral membrane proteins act in concert to synthesize capsular polysaccharide, teichoic acid, and teichuronic acid. The pathways used to produce these molecules are the Wzx/Wzy-dependent, the ABC-transporter-dependent, and the synthase-dependent pathways. This chapter will cover the initiation, synthesis of the various polysaccharides on the cytoplasmic face of the membrane using nucleotide sugar precursors, and export of the nascent chain from the cytoplasm to the extracellular milieu. As microbial glycobiology is an emerging field in Gram-positive bacteria research, parallels will be drawn to the more widely studied polysaccharide biosynthesis systems in Gram-negative species in order to provide greater understanding of these biologically significant molecules.

The role of bacterial protein tyrosine phosphatases in the regulation of the biosynthesis of secreted polysaccharides

SIGNIFICANCE

Tyrosine phosphorylation and associated protein tyrosine phosphatases are gaining prominence as critical mechanisms in the regulation of fundamental processes in a wide variety of bacteria. In particular, these phosphatases have been associated with the control of the biosynthesis of capsular polysaccharides and extracellular polysaccharides, critically important virulence factors for bacteria.

RECENT ADVANCES

Deletion and overexpression of the phosphatases result in altered polysaccharide biosynthesis in a range of bacteria. The recent structures of associated auto-phosphorylating tyrosine kinases have suggested that the phosphatases may be critical for the cycling of the kinases between monomers and higher order oligomers.

CRITICAL ISSUES

Additional substrates of the phosphatases apart from cognate kinases are currently being identified. These are likely to be critical to our understanding of the mechanism by which polysaccharide biosynthesis is regulated.

FUTURE DIRECTIONS

Ultimately, these protein tyrosine phosphatases are an attractive target for the development of novel antimicrobials. This is particularly the case for the polymerase and histidinol phosphatase family, which is predominantly found in bacteria. Furthermore, the determination of bacterial tyrosine phosphoproteomes will likely help to uncover the fundamental roles, mechanism, and critical importance of these phosphatases in a wide range of bacteria.

Streptococcus pneumoniae phosphotyrosine phosphatase CpsB and alterations in capsule production resulting from changes in oxygen availability

Streptococcus pneumoniae produces a protective capsular polysaccharide whose production must be modulated for bacterial survival within various host niches. Capsule production is affected in part by a phosphoregulatory system comprised of CpsB, CpsC, and CpsD. Here, we found that growth of serotype 2 strain D39 under conditions of increased oxygen availability resulted in decreased capsule levels concurrent with an ∼5-fold increase in Cps2B-mediated phosphatase activity. The change in Cps2B phosphatase activity did not result from alterations in the levels of either the cps2B transcript or the Cps2B protein. Recombinant Cps2B expressed in Escherichia coli similarly exhibited increased phosphatase activity under conditions of high-oxygen growth. S. pneumoniae D39 derivatives with defined deletion or point mutations in cps2B demonstrated reduced phosphatase activity with corresponding increases in levels of Cps2D tyrosine phosphorylation. There was, however, no correlation between these phenotypes and the level of capsule production. During growth under reduced-oxygen conditions, the Cps2B protein was essential for parental levels of capsule, but phosphatase activity alone could be eliminated without an effect on capsule. Under increased-oxygen conditions, deletion of cps2B did not affect capsule levels. These results indicate that neither Cps2B phosphatase activity nor Cps2D phosphorylation levels per se are determinants of capsule levels, whereas the Cps2B protein is important for capsule production during growth under conditions of reduced but not enhanced oxygen availability. Roles for factors outside the capsule locus, possible interactions between capsule regulatory proteins, and links to other cellular processes are also suggested by the results described in this study.

The protein tyrosine kinases EpsB and PtkA differentially affect biofilm formation in Bacillus subtilis

The Gram-positive soil bacterium Bacillus subtilis is able to choose between motile and sessile lifestyles. The sessile way of life, also referred to as biofilm, depends on the formation of an extracellular polysaccharide matrix and some extracellular proteins. Moreover, a significant proportion of cells in a biofilm form spores. The first two genes of the 15-gene operon for extracellular polysaccharide synthesis, epsA and epsB, encode a putative transmembrane modulator protein and a putative protein tyrosine kinase, respectively, with similarity to the TkmA/PtkA modulator/kinase couple. Here we show that the putative kinase EpsB is required for the formation of structured biofilms. However, an epsB mutant is still able to form biofilms. As shown previously, a ptkA mutant is also partially defective in biofilm formation, but this defect is related to spore formation in the biofilm. The absence of both kinases resulted in a complete loss of biofilm formation. Thus, EpsB and PtkA fulfil complementary functions in biofilm formation. The activity of bacterial protein tyrosine kinases depends on their interaction with modulator proteins. Our results demonstrate the specific interaction between the putative kinase EpsB and its modulator protein EpsA and suggest that EpsB activity is stimulated by its modulator EpsA.

Structure and biosynthesis of two exopolysaccharides produced by Lactobacillus johnsonii FI9785

Exopolysaccharides were isolated and purified from Lactobacillus johnsonii FI9785, which has previously been shown to act as a competitive exclusion agent to control Clostridium perfringens in poultry. Structural analysis by NMR spectroscopy revealed that L. johnsonii FI9785 can produce two types of exopolysaccharide: EPS-1 is a branched dextran with the unusual feature that every backbone residue is substituted with a 2-linked glucose unit, and EPS-2 was shown to have a repeating unit with the following structure: -6)-α-Glcp-(1-3)-β-Glcp-(1-5)-β-Galf-(1-6)-α-Glcp-(1-4)-β-Galp-(1-4)-β-Glcp-(1-. Sites on both polysaccharides were partially occupied by substituent groups: 1-phosphoglycerol and O-acetyl groups in EPS-1 and a single O-acetyl group in EPS-2. Analysis of a deletion mutant (ΔepsE) lacking the putative priming glycosyltransferase gene located within a predicted eps gene cluster revealed that the mutant could produce EPS-1 but not EPS-2, indicating that epsE is essential for the biosynthesis of EPS-2. Atomic force microscopy confirmed the localization of galactose residues on the exterior of wild type cells and their absence in the ΔepsE mutant. EPS2 was found to adopt a random coil structural conformation. Deletion of the entire 14-kb eps cluster resulted in an acapsular mutant phenotype that was not able to produce either EPS-2 or EPS-1. Alterations in the cell surface properties of the EPS-specific mutants were demonstrated by differences in binding of an anti-wild type L. johnsonii antibody. These findings provide insights into the biosynthesis and structures of novel exopolysaccharides produced by L. johnsonii FI9785, which are likely to play an important role in biofilm formation, protection against harsh environment of the gut, and colonization of the host.

Spontaneous mutation reveals influence of exopolysaccharide on Lactobacillus johnsonii surface characteristics

As a competitive exclusion agent, Lactobacillus johnsonii FI9785 has been shown to prevent the colonization of selected pathogenic bacteria from the chicken gastrointestinal tract. During growth of the bacterium a rare but consistent emergence of an altered phenotype was noted, generating smooth colonies in contrast to the wild type rough form. A smooth colony variant was isolated and two-dimensional gel analysis of both strains revealed a protein spot with different migration properties in the two phenotypes. The spot in both gels was identified as a putative tyrosine kinase (EpsC), associated with a predicted exopolysaccharide gene cluster. Sequencing of the epsC gene from the smooth mutant revealed a single substitution (G to A) in the coding strand, resulting in the amino acid change D88N in the corresponding gene product. A native plasmid of L. johnsonii was engineered to produce a novel vector for constitutive expression and this was used to demonstrate that expression of the wild type epsC gene in the smooth mutant produced a reversion to the rough colony phenotype. Both the mutant and epsC complemented strains had increased levels of exopolysaccharides compared to the wild type strain, indicating that the rough phenotype is not solely associated with the quantity of exopolysaccharide. Another gene in the cluster, epsE, that encoded a putative undecaprenyl-phosphate galactosephosphotransferase, was deleted in order to investigate its role in exopolysaccharide biosynthesis. The ΔepsE strain exhibited a large increase in cell aggregation and a reduction in exopolysaccharide content, while plasmid complementation of epsE restored the wild type phenotype. Flow cytometry showed that the wild type and derivative strains exhibited clear differences in their adhesive ability to HT29 monolayers in tissue culture, demonstrating an impact of EPS on surface properties and bacteria-host interactions.

Impaired capsular polysaccharide is relevant to enhanced biofilm formation and lower virulence in Streptococcus pneumoniae

Streptococcus pneumoniae has been reported to form biofilms. Many different surface molecules, including capsular polysaccharide (CPS), may play a fundamental role in pneumococcal biofilm development. We designed a CPS mutant, TIGR4cps4D(-), from the TIGR4 strain and detected enhanced biofilm formation. The pathogenic diversities of the mutant were also investigated with the in vitro expression levels of pavA, lytA, IgA1, piaA, psaA, ply, and spxB. The mean OD595 of TIGR4cps4D(-) biofilm was 1.77 and 1.74, whereas that of TIGR4 was 0.76 and 0.33 on day 1 and day 2, respectively. Scanning electron microscopy and confocal laser scanning microscopy showed TIGR4cps4D(-) formed a biofilm that was significantly thicker than that formed by TIGR4 (~12.22 vs. ~6.29 μm). Compared to TIGR4, the gene expression of lytA, IgA1, and, psaA in TIGR4cps4D(-) was 1.9 × 10(-5)-, 2.4 × 10(-5)-, and 3.2 × 10(-3) fold lower under the planktonic condition, and 1.9 × 10(-5)- and 9.7 × 10(-5) fold lower in biofilms, respectively. Furthermore, TIGR4cps4D(-) seemed to induce less cell death, compared to the results of TIGR4 (21.38 vs. 33.47 %, after a 5-h exposure; P < 0.05). Our data indicate that impaired pneumococcal CPS may increase biofilm formation and be involved in inhibition of virulence, possibly by influencing the gene expression.

Evolution of the capsular operon of Streptococcus iniae in response to vaccination

Streptococcus iniae causes severe septicemia and meningitis in farmed fish and is also occasionally zoonotic. Vaccination against S. iniae is problematic, with frequent breakdown of protection in vaccinated fish. The major protective antigens in S. iniae are the polysaccharides of the capsule, which are essential for virulence. Capsular biosynthesis is driven and regulated by a 21-kb operon comprising up to 20 genes. In a long-term study, we have sequenced the capsular operon of strains that have been used in autogenous vaccines across Australia and compared it with the capsular operon sequences of strains subsequently isolated from infected vaccinated fish. Intriguingly, strains isolated from vaccinated fish that subsequently become infected have coding mutations that are confined to a limited number of genes in the cps operon, with the remainder of the genes in the operon remaining stable. Mutations in strains in diseased vaccinated fish occur in key genes in the capsular operon that are associated with polysaccharide configuration (cpsG) and with regulation of biosynthesis (cpsD and cpsE). This, along with high ratios of nonsynonymous to synonymous mutations within the cps genes, suggests that immune response directed predominantly against capsular polysaccharide may be driving evolution in a very specific set of genes in the operon. From these data, it may be possible to design a simple polyvalent vaccine with a greater operational life span than the current monovalent killed bacterins.

Chemical inhibition of bacterial protein tyrosine phosphatase suppresses capsule production

Capsule polysaccharide is a major virulence factor for a wide range of bacterial pathogens, including Streptococcus pneumoniae. The biosynthesis of Wzy-dependent capsules in both gram-negative and -positive bacteria is regulated by a system involving a protein tyrosine phosphatase (PTP) and a protein tyrosine kinase. However, how the system functions is still controversial. In Streptococcus pneumoniae, a major human pathogen, the system is present in all but 2 of the 93 serotypes found to date. In order to study this regulation further, we performed a screen to find inhibitors of the phosphatase, CpsB. This led to the observation that a recently discovered marine sponge metabolite, fascioquinol E, inhibited CpsB phosphatase activity both in vitro and in vivo at concentrations that did not affect the growth of the bacteria. This inhibition resulted in decreased capsule synthesis in D39 and Type 1 S. pneumoniae. Furthermore, concentrations of Fascioquinol E that inhibited capsule also lead to increased attachment of pneumococci to a macrophage cell line, suggesting that this compound would inhibit the virulence of the pathogen. Interestingly, this compound also inhibited the phosphatase activity of the structurally unrelated gram-negative PTP, Wzb, which belongs to separate family of protein tyrosine phosphatases. Furthermore, incubation with Klebsiella pneumoniae, which contains a homologous phosphatase, resulted in decreased capsule synthesis. Taken together, these data provide evidence that PTPs are critical for Wzy-dependent capsule production across a spectrum of bacteria, and as such represents a valuable new molecular target for the development of anti-virulence antibacterials.

Tyrosine phosphorylation and bacterial virulence

Protein phosphorylation on tyrosine has emerged as a key device in the control of numerous cellular functions in bacteria. In this article, we review the structure and function of bacterial tyrosine kinases and phosphatases. Phosphorylation is catalyzed by autophosphorylating adenosine triphosphate-dependent enzymes (bacterial tyrosine (BY) kinases) that are characterized by the presence of Walker motifs. The reverse reaction is catalyzed by three classes of enzymes: the eukaryotic-like phosphatases (PTPs) and dual-specific phosphatases; the low molecular weight protein-tyrosine phosphatases (LMW-PTPs); and the polymerase–histidinol phosphatases (PHP). Many BY kinases and tyrosine phosphatases can utilize host cell proteins as substrates, thereby contributing to bacterial pathogenicity. Bacterial tyrosine phosphorylation/dephosphorylation is also involved in biofilm formation and community development. The Porphyromonas gingivalis tyrosine phosphatase Ltp1 is involved in a restraint pathway that regulates heterotypic community development with Streptococcus gordonii. Ltp1 is upregulated by contact with S. gordonii and Ltp1 activity controls adhesin expression and levels of the interspecies signal AI-2.

Intraspecific and interspecific interactions among proteins regulating exopolysaccharide synthesis in Streptococcus thermophilus, Streptococcus iniae, and Lactococcus lactis subsp. cremoris and the assessment of potential lateral gene transfer

Using the yeast two-hybrid system, intraspecific protein interactions were detected in Streptococcus iniae and Lactococcus lactis subsp. cremoris between the transmembrane activation protein (CpsC and EpsA, respectively) and the protein tyrosine kinase (CpsD and EpsB, respectively), between two protein tyrosine kinases, and between the protein tyrosine kinase and the phosphotyrosine phosphatase (CpsB and EpsC, respectively). For each of these intraspecific interactions, interspecific interactions were also detected when one protein was from S. iniae and the other was from Streptococcus thermophilus . Interactions were also observed between two protein tyrosine kinases when one protein was from either of the Streptococcus species and the other from L. lactis subsp. cremoris. The results and sequence comparisons performed in this study support the conclusion that interactions among the components of the tyrosine kinase - phosphatase regulatory system are conserved in the order Lactobacillales and that interspecific genetic exchanges of the genes that encode these proteins have the potential to form functional recombinants. A better understanding of intraspecific and interspecific protein interactions involved in regulating exopolysaccharide biosynthesis may facilitate construction of improved strains for industrial uses as well as identification of factors needed to form functional regulatory complexes in naturally occurring recombinants.

Genetic analysis of the capsular polysaccharide synthesis locus in 15 Streptococcus suis serotypes

The capsular polysaccharide (CPS) synthesis locus of 13 Streptococcus suis serotypes (serotype 1, 3, 4, 5, 7, 8, 9, 10, 14, 19, 23, 25 and 1/2) was sequenced and compared with that of serotype 2 and 16. The CPS synthesis locus of these 15 serotypes falls into two genetic groups. The locus is located on the chromosome between orfZ and aroA. All the translated proteins in the CPS synthesis locus were clustered into 127 homology groups using the tribemcl algorithm. The general organization of the locus suggested that the CPS of S. suis could be synthesized by the Wzy-dependent pathway. The capsule of serotypes 3, 4, 5, 7, 9, 10, 19 and 23 was predicted to be amino-polysaccharide. Sialic acid was predicted to be present in the capsule of serotypes 1, 2, 14, 16 and 1/2. The characteristics of the CPS synthesis locus suggest that some genes may have been imported into S. suis (or their ancestors) on multiple occasions from different and unknown sources.

Protein-protein interactions among the components of the biosynthetic machinery responsible for exopolysaccharide production in Streptococcus thermophilus MR-1C

AIM

This study identified protein-protein interactions among the biosynthetic machinery responsible for exopolysaccharide (EPS) production in Streptococcus thermophilus MR-1C.

METHODS AND RESULTS

Protein-protein interactions were investigated using the yeast two-hybrid system. A strong protein-protein interaction was detected between the transmembrane activation protein Wzd and the protein tyrosine kinase Wze. Weaker protein-protein interactions were detected between two duplicate Wze proteins and between Wze and the phosphotyrosine phosphatase Wzh. Protein-protein interactions involving a Wzd/Wze fusion protein and Wzd and Wze may indicate that these proteins form multi-protein complexes. All combinations of the Wzh, Wzd, Wze, Wzg (regulation), CpsE (glycosyl-1-phosphate transferase), CpsS (polymerization), CpsL (unknown), CpsW (regulation) and CpsU (membrane translocation) were analysed for protein-protein interactions but no additional interactions were discovered using the yeast two-hybrid system.

CONCLUSIONS

Interactions among the phosphotyrosine phosphatase, tyrosine kinase, and transmembrane activation protein are important in the regulation of capsule biosynthesis in Strep. thermophilus MR-1C.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides some valuable insight into the organization and interactions between the many proteins involved in EPS production. A better understanding of this process may facilitate the genetic manipulation of capsule production to impart desirable properties to dairy starter cultures.

Post-translational control of Bacillus subtilis biofilm formation mediated by tyrosine phosphorylation

A biofilm is a complex community of cells enveloped in a self-produced polymeric matrix. Entry into a biofilm is exquisitely controlled at the level of transcription and in the Gram-positive organism Bacillus subtilis it requires the concerted efforts of three major transcription factors. Here, we demonstrate that in addition to transcriptional control, B. subtilis utilizes post-translational modifications to control biofilm formation; specifically through phosphorylation of tyrosine residues. Through our work we have assigned novel roles during biofilm formation to two proteins; the protein tyrosine kinase PtkA and the protein tyrosine phosphatase PtpZ. Furthermore by introducing amino acid point mutations within the catalytic domains of PtkA and PtpZ we have identified that the kinase and phosphatase activities, respectively, are essential for function. PtkA contains a conserved C-terminal tyrosine cluster that is the site of autophosphorylation; however, our in vivo analysis demonstrates that this domain is not required during biofilm formation. With the aim of identifying the target(s) of PtkA controlled during biofilm formation we used a systematic mutagenesis approach but, despite extensive efforts, it remained elusive. Our findings highlight the complexity of biofilm development by revealing an additional level of regulation in the form of protein tyrosine phosphorylation.

Production of succinoglycan polymer in Sinorhizobium meliloti is affected by SMb21506 and requires the N-terminal domain of ExoP

The protein tyrosine kinase ExoP, consisting of an N-terminal periplasmic and a C-terminal cytoplasmic domain, is important for polymerization of the exopolysaccharide succinoglycan (EPS I) in Sinorhizobium meliloti. We analyzed the contribution of the ExoP paralogs ExoP2 and SMb21506 to the production of the high molecular weight (HMW) form of EPS I. ExoP2, though not contributing to EPS I or lipopolysaccharide biosynthesis, showed increased expression at high osmolarity and was expressed in Medicago sativa nodules, suggesting an involvement in the synthesis of an as-yet-unidentified polysaccharide. Furthermore, a mutation in SMb21506 affected the production of HMW EPS I, particularly in the absence of the C-terminal ExoP domain. High salinity induced the production of HMW EPS I by the wild type and mutants whereas high osmolarity had the opposite effect. It was shown that ExoP localizes at the inner membrane of S. meliloti cells. Tyrosine phosphorylation of the C-terminal domain was strongly increased by amino acid substitutions in the polysaccharide co-polymerase motif (formerly proline-rich motif) located in the N-terminal domain, suggesting that this phosphorylation could be modulated by conformational changes of the N-terminal domain. Moreover, deletion of a coiled-coil motif present in the N-terminal domain abolished phosphorylation and EPS I production and, consequently, the ability to nodulate M. sativa.

Crystal structures of Wzb of Escherichia coli and CpsB of Streptococcus pneumoniae, representatives of two families of tyrosine phosphatases that regulate capsule assembly

Many Gram-positive and Gram-negative bacteria utilize polysaccharide surface layers called capsules to evade the immune system; consequently, the synthesis and export of the capsule are a potential therapeutic target. In Escherichia coli K-30, the integral membrane tyrosine autokinase Wzc and the cognate phosphatase Wzb have been shown to be key for both synthesis and assembly of capsular polysaccharides. In the Gram-positive bacterium Streptococcus pneumoniae, the CpsCD complex is analogous to Wzc and the phosphatase CpsB is the corresponding cognate phosphatase. The phosphatases are known to dephosphorylate their corresponding autokinases, yet despite their functional equivalence, they share no sequence homology. We present the structure of Wzb in complex with phosphate and high-resolution structures of apo-CpsB and a phosphate-complexed CpsB. We show that both proteins are active toward Wzc and thereby demonstrate that CpsB is not specific for CpsCD. CpsB is a novel enzyme and represents the first solved structure of a tyrosine phosphatase from a Gram-positive bacterium. Wzb and CpsB have completely different structures, suggesting that they must operate by very different mechanisms. Although the mechanism of Wzb can be inferred from previous studies, CpsB appears to have a tyrosine phosphatase mechanism not observed before. We propose a chemical mechanism for CpsB based on site-directed mutagenesis and structural data.

Pneumococcal capsular polysaccharide structure predicts serotype prevalence

There are 91 known capsular serotypes of Streptococcus pneumoniae. The nasopharyngeal carriage prevalence of particular serotypes is relatively stable worldwide, but the host and bacterial factors that maintain these patterns are poorly understood. Given the possibility of serotype replacement following vaccination against seven clinically important serotypes, it is increasingly important to understand these factors. We hypothesized that the biochemical structure of the capsular polysaccharides could influence the degree of encapsulation of different serotypes, their susceptibility to killing by neutrophils, and ultimately their success during nasopharyngeal carriage. We sought to measure biological differences among capsular serotypes that may account for epidemiological patterns. Using an in vitro assay with both isogenic capsule-switch variants and clinical carriage isolates, we found an association between increased carriage prevalence and resistance to non-opsonic neutrophil-mediated killing, and serotypes that were resistant to neutrophil-mediated killing tended to be more heavily encapsulated, as determined by FITC-dextran exclusion. Next, we identified a link between polysaccharide structure and carriage prevalence. Significantly, non-vaccine serotypes that have become common in vaccinated populations tend to be those with fewer carbons per repeat unit and low energy expended per repeat unit, suggesting a novel biological principle to explain patterns of serotype replacement. More prevalent serotypes are more heavily encapsulated and more resistant to neutrophil-mediated killing, and these phenotypes are associated with the structure of the capsular polysaccharide, suggesting a direct relationship between polysaccharide biochemistry and the success of a serotype during nasopharyngeal carriage and potentially providing a method for predicting serotype replacement.

Streptococcus pneumoniae, or pneumococcus, is an important pathogen worldwide and causes a wide range of diseases, mostly in young children and the elderly. There are 91 serotypes of pneumococcus, each of which produces a unique polysaccharide, called the capsule, that attaches to the bacterial surface and prevents it from being cleared by the host. The serotypes differ greatly in their prevalence in the human population. There is currently a vaccine, effective in infancy, which targets seven clinically important serotypes, but several types not covered by the vaccine are beginning to increase in carriage frequency. As a result, it is critical to understand why some serotypes are frequently carried in the human population while others are not. In this study, we find that the high-prevalence serotypes tend to be more heavily encapsulated and more resistant to killing by neutrophils. Significantly, we find that the biochemical properties of the different polysaccharides can be used to predict their carriage frequency both before and after introduction of the vaccine. These results provide a biologically plausible explanation for differences in prevalence between serotypes.

Tyrosine-kinases in bacteria: from a matter of controversy to the status of key regulatory enzymes

When considering protein phosphorylation in bacteria, phosphorylation of aspartic acid and histidine residues mediated by the two-component systems is the first to spring to mind. And yet other phosphorylation systems have been described in bacteria in the past 20 years including eukaryotic-like serine/threonine kinases and more recently tyrosine-kinases. Among the latter, a peculiar type is widespread among bacteria, but not in higher organisms. These enzymes possess unique structural features defining thus a new family of enzymes termed Bacterial tyrosine kinases (BY-kinases). BY-kinases have been shown to be mainly involved in polysaccharide production, but their ability to phosphorylate endogenous substrates indicates that they participate in the regulation of other functions of the bacterial cell. Recent advances in mass spectrometry based phosphoproteomics provided lists of many new phosphotyrosine-proteins, indicating that BY-kinases may be involved in regulating a large array of other cellular functions. One may expect that in a near future, tyrosine phosphorylation will turn out to be one of the key regulatory processes in the bacterial cell and will yield new insights into the understanding of its physiology.

Structural basis for the regulation mechanism of the tyrosine kinase CapB from Staphylococcus aureus

Bacteria were thought to be devoid of tyrosine-phosphorylating enzymes. However, several tyrosine kinases without similarity to their eukaryotic counterparts have recently been identified in bacteria. They are involved in many physiological processes, but their accurate functions remain poorly understood due to slow progress in their structural characterization. They have been best characterized as copolymerases involved in the synthesis and export of extracellular polysaccharides. These compounds play critical roles in the virulence of pathogenic bacteria, and bacterial tyrosine kinases can thus be considered as potential therapeutic targets. Here, we present the crystal structures of the phosphorylated and unphosphorylated states of the tyrosine kinase CapB from the human pathogen Staphylococcus aureus together with the activator domain of its cognate transmembrane modulator CapA. This first high-resolution structure of a bacterial tyrosine kinase reveals a 230-kDa ring-shaped octamer that dissociates upon intermolecular autophosphorylation. These observations provide a molecular basis for the regulation mechanism of the bacterial tyrosine kinases and give insights into their copolymerase function.

An idiosyncratic new class of bacterial enzymes, bacterial tyrosine-kinases (BY-kinases), has been characterized. These enzymes, which are involved in an increasing number of physiological processes ranging from stress resistance to pathogenicity, share no sequence similarities with eukaryotic kinases, and their function remains largely unknown. They have nevertheless been described to undergo autophosphorylation on a C-terminal tyrosine cluster and to phosphorylate endogenous protein substrates. We describe here the first crystal structure of a bacterial tyrosine kinase, namely CapB from the pathogen Staphylococcus aureus, in complex with the cytoplasmic domain of the transmembrane stimulatory protein CapA. Our data explain the activation mechanism of CapB by CapA and allow us to propose a regulatory mechanism based on intermolecular autophosphorylation. These results also give new insights onto the phosphorylation of the endogenous substrate CapO, an enzyme involved in the synthesis of polysaccharide precursors. CapA and CapB, among others, are involved as copolymerases in the synthesis of extracellular polysaccharides that are thought to be potent virulence factors. Thus, these structural data provide the basis for designing specific inhibitors for these enzymes, which constitute an original and attractive target for the development of new drugs to treat infectious diseases.

Structural analysis of a conserved bacterial tyrosine kinase fromStaphylococcus aureus provides the basis for deciphering its regulatory mechanism, leading to a model for its implication in extracellular polysaccharide synthesis.

Tyrosine phosphorylation of the UDP-glucose dehydrogenase of Escherichia coli is at the crossroads of colanic acid synthesis and polymyxin resistance

BACKGROUND

In recent years, an idiosyncratic new class of bacterial enzymes, named BY-kinases, has been shown to catalyze protein-tyrosine phosphorylation. These enzymes share no structural and functional similarities with their eukaryotic counterparts and, to date, only few substrates of BY-kinases have been characterized. BY-kinases have been shown to participate in various physiological processes. Nevertheless, we are at a very early stage of defining their importance in the bacterial cell. In Escherichia coli, two BY-kinases, Wzc and Etk, have been characterized biochemically. Wzc has been shown to phosphorylate the UDP-glucose dehydrogenase Ugd in vitro. Not only is Ugd involved in the biosynthesis of extracellular polysaccharides, but also in the production of UDP-4-amino-4-deoxy-L-arabinose, a compound that renders E. coli resistant to cationic antimicrobial peptides.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we studied the role of Ugd phosphorylation. We first confirmed in vivo the phosphorylation of Ugd by Wzc and we demonstrated that Ugd is also phosphorylated by Etk, the other BY-kinase identified in E. coli. Tyrosine 71 (Tyr71) was characterized as the Ugd site phosphorylated by both Wzc and Etk. The regulatory role of Tyr71 phosphorylation on Ugd activity was then assessed and Tyr71 mutation was found to prevent Ugd activation by phosphorylation. Further, Ugd phosphorylation by Wzc or Etk was shown to serve distinct physiological purposes. Phosphorylation of Ugd by Wzc was found to participate in the regulation of the amount of the exopolysaccharide colanic acid, whereas Etk-mediated Ugd phosphorylation appeared to participate in the resistance of E. coli to the antibiotic polymyxin.

CONCLUSIONS/SIGNIFICANCE

Ugd phosphorylation seems to be at the junction between two distinct biosynthetic pathways, illustrating the regulatory potential of tyrosine phosphorylation in bacterial physiology.

The role of Streptococcus pneumoniae virulence factors in host respiratory colonization and disease

Streptococcus pneumoniae is a Gram-positive bacterial pathogen that colonizes the mucosal surfaces of the host nasopharynx and upper airway. Through a combination of virulence-factor activity and an ability to evade the early components of the host immune response, this organism can spread from the upper respiratory tract to the sterile regions of the lower respiratory tract, which leads to pneumonia. In this Review, we describe how S. pneumoniae uses its armamentarium of virulence factors to colonize the upper and lower respiratory tracts of the host and cause disease.

Characterization and site-directed mutagenesis of Wzb, an O-phosphatase from Lactobacillus rhamnosus

Background

Reversible phosphorylation events within a polymerisation complex have been proposed to modulate capsular polysaccharide synthesis in Streptococcus pneumoniae. Similar phosphatase and kinase genes are present in the exopolysaccharide (EPS) biosynthesis loci of numerous lactic acid bacteria genomes.

Results

The protein sequence deduced from the wzb gene in Lactobacillus rhamnosus ATCC 9595 reveals four motifs of the polymerase and histidinol phosphatase (PHP) superfamily of prokaryotic O-phosphatases. Native and modified His-tag fusion Wzb proteins were purified from Escherichia coli cultures. Extracts showed phosphatase activity towards tyrosine-containing peptides. The purified fusion protein Wzb was active on p-nitrophenyl-phosphate (pNPP), with an optimal activity in presence of bovine serum albumin (BSA 1%) at pH 7.3 and a temperature of 75°C. At 50°C, residual activity decreased to 10 %. Copper ions were essential for phosphatase activity, which was significantly increased by addition of cobalt. Mutated fusion Wzb proteins exhibited reduced phosphatase activity on p-nitrophenyl-phosphate. However, one variant (C6S) showed close to 20% increase in phosphatase activity.

Conclusion

These characteristics reveal significant differences with the manganese-dependent CpsB protein tyrosine phosphatase described for Streptococcus pneumoniae as well as with the polysaccharide-related phosphatases of Gram negative bacteria.

Heterologous expression of the pneumococcal serotype 14 polysaccharide in Lactococcus lactis requires lactococcal epsABC regulatory genes

The pneumococcal serotype 14 polysaccharide was produced in Lactococcus lactis by coexpressing pneumococcal polysaccharide type 14-specific genes (cpsFGHIJKL(14)) with the lactococcal regulatory and priming glucosyltransferase-encoding genes specific for B40 polysaccharide (epsABCD(B40)). The polysaccharide produced by Lactococcus was secreted in the medium, simplifying downstream processing and polysaccharide isolation from culture broth.

UDP-Acetyl-Mannosamine Dehydrogenase Is an Endogenous Protein Substrate of Staphylococcus aureus Protein-Tyrosine Kinase Activity

The in silico analysis of the amino acid sequences deduced from the complete genome sequence of Staphylococcus aureus suggested the presence of two protein tyrosine kinase activities, each split into two distinct polypeptides, respectively Cap5A1/Cap5B1 and Cap5A2/Cap5B2, like in some other Gram-positive bacteria. To check this prediction, the corresponding genes were cloned and overexpressed, and the four corresponding proteins were purified by affinity chromatography and assayed for phosphorylating activity in vitro. Individually, none of them was found to autophosphorylate. However, when Cap5B2 was incubated in the presence of Cap5A2 or, with a larger efficiency, in the presence of Cap5A1, this protein exhibited intensive autokinase activity, occurring selectively at tyrosine residues. On the other hand, whatever the protein combination assayed, Cap5B1 did not present any phosphorylating activity. In search of a possible role for the phosphorylation reaction mediated by Cap5B2, an endogenous substrate of this kinase was characterized. This substrate, termed Cap5O, is the enzyme UDP-acetyl-mannosamine dehydrogenase involved in the cascade of reactions leading to the synthesis of the bacterial capsule. It represents the first endogenous substrate for a tyrosine kinase activity so far identified in S. aureus. The analysis of its dehydrogenase activity showed that it was positively controlled by its phosphorylation at tyrosine.

PelC is a Pseudomonas aeruginosa outer membrane lipoprotein of the OMA family of proteins involved in exopolysaccharide transport

Pseudomonas aeruginosa is a gram-negative bacterium, opportunistic pathogen, which causes severe acute or chronic infections, as is the case with cystic fibrosis patients. Chronic infections are frequently accompanied by the development of the bacterial population into a specialized community called biofilm. The pelA-G gene cluster of P. aeruginosa has been shown to be involved in pellicle production and biofilm formation. The pel genes have been proposed to contribute to the formation of the exopolysaccharide-containing pellicle. However, the function and the subcellular localization of the seven different Pel proteins are poorly understood. Based on bioinformatics analysis, we have previously considered that PelF is a putative glycosyltransferase (GT4 family), whereas PelG is a Wzx-like polysaccharide transporter from the PST family. In this study we have further characterized the PelC protein. We have shown that PelC is an outer membrane lipoprotein. The N-terminal signal peptide of the PelC lipoprotein is sufficient to target the protein into the membranes. However, by constructing various PelC hybrid proteins we also proposed that efficient and functional outer membrane insertion of PelC requires not only the signal peptide and the lipid modification, but also requires the C-terminal domain of PelC. Because the gene encoding the outer membrane lipoprotein PelC is part of a putative gene cluster involved in exopolysaccharide biogenesis, we suggest that PelC is a new member of the outer membrane auxiliary (OMA) family of lipoprotein whose Wza, involved in Escherichia coli capsular polysaccharide transport, is an archetype.

Modulation of adherence, invasion, and tumor necrosis factor alpha secretion during the early stages of infection by Streptococcus pneumoniae ClpL

Heat shock proteins (HSPs) play a pivotal role as chaperones in the folding of native and denatured proteins and can help pathogens penetrate host defenses. However, the underlying mechanism(s) of modulation of virulence by HSPs has not been fully determined. In this study, the role of the chaperone ClpL in the pathogenicity of Streptococcus pneumoniae was assessed. A clpL mutant adhered to and invaded nasopharyngeal or lung cells much more efficiently than the wild type adhered to and invaded these cells in vitro, as well as in vivo, although it produced the same amount of capsular polysaccharide. However, the level of secretion of tumor necrosis factor alpha (TNF-alpha) from macrophages infected with the clpL mutant was significantly lower than the level of secretion elicited by the wild type during the early stages of infection. Interestingly, treatment of the human lung epithelial carcinoma A549 and murine macrophage RAW 264.7 cell lines with cytochalasin D, an inhibitor of actin polymerization, increased adherence of the mutant to the host cells. In contrast, cytochalasin D treatment of RAW 264.7 cells decreased TNF-alpha secretion after infection with either the wild type or the mutant. However, pretreatment of cell lines with the actin polymerization activator jasplakinolide reversed these phenotypes. These findings indicate, for the first time, that the ClpL chaperone represses adherence of S. pneumoniae to host cells and induces secretion of TNF-alpha via a mechanism dependent upon actin polymerization during the initial infection stage.

Influence of tyrosine-kinase Wzc activity on colanic acid production in Escherichia coli K12 cells

Bacterial tyrosine-kinases have been demonstrated to participate in the regulation of capsule polysaccharides (CPS) and exopolysaccharides (EPS) production and export. However, discrepant data have been reported on the molecular mechanism responsible for this regulation depending on the bacterial species analyzed. Special attention was previously paid to the tyrosine-kinase Wzc(ca) of Escherichia coli K-12, which is involved in the production of the exopolysaccharide, colanic acid, and autophosphorylates by using a cooperative two-step process. In this work, we took advantage of these observations to investigate in further detail the effect of Wzc(ca) phosphorylation on the colanic acid production. First, it is shown that expression of the phosphorylated form of Wzc prevents production of colanic acid whereas expression of the non-phosphorylated form allows biosynthesis of this exopolysaccharide. However, we provide evidence that, in the latter case, the size distribution of the colanic acid polymer is less scattered than in the case of the wild-type strain expressing both phosphorylated and non-phosphorylated forms of Wzc. It is then demonstrated that colanic acid production is not merely regulated by an on/off mechanism and that, instead, both phosphorylated and non-phosphorylated forms of Wzc are required to promote colanic acid synthesis. Moreover, a series of data suggests that besides the involvement of phosphorylated and non-phosphorylated forms of Wzc in the production of colanic acid, two particular regions of this kinase play as such an important role in the synthesis of this exopolysaccharide: a proline-rich domain located in the N-terminal part of Wzc(ca), and a tyrosine cluster present in the C-terminal portion of the enzyme. Furthermore, considering that polysaccharides are known to facilitate bacterial resistance to certain environmental stresses, it is shown that the resistance of E. coli to desiccation is directly connected with the phosphorylation state of Wzc(ca).

Functional analysis of Burkholderia cepacia genes bceD and bceF, encoding a phosphotyrosine phosphatase and a tyrosine autokinase, respectively: role in exopolysaccharide biosynthesis and biofilm formation

The biosynthesis of the exopolysaccharide (EPS) cepacian by Burkholderia cepacia complex strains requires the 16.2-kb bce cluster of genes. Two of the clustered genes, bceD and bceF, code for two proteins homologous to phosphotyrosine phosphatases and tyrosine kinases, respectively. We show experimental evidence indicating that BceF is phosphorylated on tyrosine and that the conserved lysine residue present at position 563 in the Walker A ATP-binding motif is required for this autophosphorylation. It was also proved that BceD is capable of dephosphorylating the phosphorylated BceF. Using the artificial substrate p-nitrophenyl phosphate (PNPP), BceD exhibited a V(max) of 8.8 mumol of PNPP min(-1) mg(-1) and a K(m) of 3.7 mM PNPP at 30 degrees C. The disruption of bceF resulted in the abolishment of cepacian accumulation in the culture medium, but 75% of the parental strain's EPS production yield was still registered for the bceD mutant. The exopolysaccharide produced by the bceD mutant led to less viscous solutions and exhibited the same degree of acetylation as the wild-type cepacian, suggesting a lower molecular mass for this mutant biopolymer. The size of the biofilm produced in vitro by bceD and bceF mutant strains is smaller than the size of the biofilm formed by the parental strain, and this phenotype was confirmed by complementation assays, indicating that BceD and BceF play a role in the establishment of biofilms of maximal size.