Aspects of eukaryotic-like signaling in Gram-positive cocci: a focus on virulence

Quantitative phosphoproteomics reveals the effect of baeSR and acrB genes on protein phosphorylation in Salmonella enterica serovar Typhimurium

The BaeSR two-component system and the AcrB efflux pump are closely associated with Salmonella resistance to antibiotics. However, the relationship between the two-component system, efflux pumps and protein phosphorylation of Salmonella is poorly understood. In this study, S. typhimurium ciprofloxacin-resistant strain CR, baeSR gene deletion strain CR^ΔbaeSR, acrB gene deletion strain CR^ΔacrB, and double gene deletion strain CR^{ΔbaeSRΔacrB} were used to explore phosphorylated proteins with significant difference, based on non-marker, quantitative phosphorylation modified proteomics technique. Consequently, 363 phosphosites of 213 phosphoproteins were identified in the four strains. More than 70% of the phosphosites were serine phosphorylation. In the CR^ΔbaeSR/CR, CR^ΔacrB/CR and CR^{ΔbaeSRΔacrB}/CR comparison groups, 36, 37 and 49 phosphosites were significantly altered, respectively. Bioinformatic analysis revealed that the main enrichment pathways of these differentially phosphorylated proteins were metabolic pathways, biosynthesis of antibiotics, phosphotransferase system (PTS), ABC transporters, and lipopolysaccharide biosynthesis. Furthermore, 21 differentially phosphorylated proteins were identified to be associated with antibiotic resistance. These results suggest that the BaeSR two-component system and the AcrB efflux pump affect the phosphorylation of proteins in S. typhimurium and may influence the drug resistance and virulence of S. typhimurium by affecting protein phosphorylation, providing a new idea to explore the mechanism of drug resistance in Salmonella.

Quantitative proteomic analysis reveals that serine/threonine kinase is involved in Streptococcus suis virulence and adaption to stress conditions

The eukaryotic-type serine/threonine kinase of Streptococcus suis serotype 2 (SS2) performs critical roles in bacterial pathogenesis. In this study, isobaric tags for relative and absolute quantification (iTRAQ) MS/MS were used to analyze the protein profiles of wild type strain SS2-1 and its isogenic STK deletion mutant (Δstk). A total of 281 significant differential proteins, including 147 up-regulated and 134 down-regulated proteins, were found in Δstk. Moreover, 69 virulence factors (VFs) among these 281 proteins were predicted by the Virulence Factor Database (VFDB), including 38 downregulated and 31 up-regulated proteins in Δstk, among which 15 down regulated VFs were known VFs of SS2. Among the down-regulated proteins, high temperature requirement A (HtrA), glutamine synthase (GlnA), ferrichrome ABC transporter substrate-binding protein FepB, and Zinc-binding protein AdcA are known to be involved in bacterial survival and/or nutrient and energy acquisition under adverse host conditions. Overall, our results indicate that STK regulates the expression of proteins involved in virulence of SS2 and its adaption to stress environments.

Evaluation of the β-barrel outer membrane protein VP1243 as a candidate antigen for a cross-protective vaccine against Vibrio infections

Vibrio parahaemolyticus is a Gram-negative halophilic bacterium that causes acute gastroenteritis after the consumption of contaminated food, wound infection, and seizures. Antibiotic therapy is the main method for controlling Vibrio infections, which inevitably leads to drug resistance. Therefore, a vaccine is urgently needed to avoid this problem. Outer membrane proteins (OMPs) play a pivotal role in the interaction between the host immune system and bacteria. VP1243 is an OMP of V. parahaemolyticus, and it possessed immunogenicity in our previous study. The present study found that VP1243 was widely distributed, highly conserved and possessed similar surface epitopes among the major Vibrio species. The protein stimulated a strong antibody response and induced cross-reactive immune responses in V. parahaemolyticus, V. alginolyticus and V. vulnificus. Notably, it provided 100% immune protection against lethal challenges by the three Vibrio species in mice immunized with VP1243. Efficient clearance of cells of the three Vibrio bacterial species was observed in immunized mice. These findings provide solid evidence for VP1243 as a promising candidate for the development of a versatile vaccine to protect against Vibrio infections.

Mutation in the pssZ Gene Negatively Impacts Exopolysaccharide Synthesis, Surface Properties, and Symbiosis of Rhizobium leguminosarum bv. trifolii with Clover

Rhizobium leguminosarum bv. trifolii is a soil bacterium capable of establishing a nitrogen-fixing symbiosis with clover plants (Trifolium spp.). This bacterium secretes large amounts of acidic exopolysaccharide (EPS), which plays an essential role in the symbiotic interaction with the host plant. This polymer is biosynthesized by a multi-enzymatic complex located in the bacterial inner membrane, whose components are encoded by a large chromosomal gene cluster, called Pss-I. In this study, we characterize R. leguminosarum bv. trifolii strain Rt297 that harbors a Tn5 transposon insertion located in the pssZ gene from the Pss-I region. This gene codes for a protein that shares high identity with bacterial serine/threonine protein phosphatases. We demonstrated that the pssZ mutation causes pleiotropic effects in rhizobial cells. Strain Rt297 exhibited several physiological and symbiotic defects, such as lack of EPS production, reduced growth kinetics and motility, altered cell-surface properties, and failure to infect the host plant. These data indicate that the protein encoded by the pssZ gene is indispensable for EPS synthesis, but also required for proper functioning of R. leguminosarum bv. trifolii cells.

Autophosphorylation Mechanism of the Ser/Thr Kinase Stk1 From Staphylococcus aureus

The eukaryotic-like Ser/Thr kinase Stk1 is crucial for virulence, cell wall biosynthesis, and drug susceptibility in methicillin-resistant Staphylococcus aureus (S. aureus) (MRSA). Importantly, MRSA lacking Stk1 become sensitive to β-lactam antibiotics, implying that Stk1 could be an alternative target for combination therapy. However, the autophosphorylation mechanism of Stk1 remains elusive. Using a phosphoproteomic study, we identified six in vivo phosphorylated activation loop residues (Ser159, Thr161, Ser162, Thr164, Thr166, and Thr172) of Stk1, which are also phosphorylated in vitro. We further showed that cis autophosphorylation of Thr172 in the GT/S motif is essential for self-activation and kinase activity of Stk1 kinase domain (Stk1-KD), whereas the trans autophosphorylation of other activation loop serines/threonines are required for the optimal kinase activity of Stk1-KD. Moreover, substitution of the activation loop serines/threonines impaired in vivo autophosphorylation activity of kinase variants, while T172A and T172D variants were unable to autophosphorylate in the cellular content, underlining the essential role of Thr172 for Stk1 activity in vivo. This study provides insights into molecular basis for regulation of Stk1 activity from S. aureus.

Serine/threonine protein kinase PpkA contributes to the adaptation and virulence in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a leading cause of nosocomial infections worldwide and has become a serious public health threat, which is attributed to a large extent to its extraordinary environmental adaptability and diverse virulence factors that result in infection and progression of pathogenesis. The eukaryote-type serine/threonine (Ser/Thr) protein kinases, known for playing major regulatory roles in eukaryotes, have been demonstrated to play a central role in regulating various bacterial cellular processes via catalyzing protein phosphorylation. Although PpkA, a Ser/Thr protein kinase first identified in P. aeruginosa, has been implicated in association with bacterial virulence, little is known about the protein. Therefore, in this study, to assess the potential role of PpkA in the regulation of P. aeruginosa environmental adaptation and virulence, variations of biofilm formation, pyocyanin production, tolerance to stress, cell invasion and plant virulence were determined in wild type PAO1, ppkA gene-deleted and complemented mutant strains. Our results indicate that the mutant strain lacking ppkA exhibited a significant decrease of biofilm formation and pyocyanin production, less tolerance to oxidative and osmotic stresses, inefficient invasion of host cells and a reduction of bacterial virulence. These findings provide new insight into the regulation of various cellular processes by PpkA; this is an important mechanism for adaptation and virulence in P. aeruginosa.

Regulation of transcription by eukaryotic-like serine-threonine kinases and phosphatases in Gram-positive bacterial pathogens

Bacterial eukaryotic-like serine threonine kinases (eSTKs) and serine threonine phosphatases (eSTPs) have emerged as important signaling elements that are indispensable for pathogenesis. Differing considerably from their histidine kinase counterparts, few eSTK genes are encoded within the average bacterial genome, and their targets are pleiotropic in nature instead of exclusive. The growing list of important eSTK/P substrates includes proteins involved in translation, cell division, peptidoglycan synthesis, antibiotic tolerance, resistance to innate immunity and control of virulence factors. Recently it has come to light that eSTK/Ps also directly modulate transcriptional machinery in many microbial pathogens. This novel form of regulation is now emerging as an additional means by which bacteria can alter their transcriptomes in response to host-specific environmental stimuli. Here we focus on the ability of eSTKs and eSTPs in Gram-positive bacterial pathogens to directly modulate transcription, the known mechanistic outcomes of these modifications, and their roles as an added layer of complexity in controlling targeted RNA synthesis to enhance virulence potential.

The unique serine/threonine phosphatase from the minimal bacterium Mycoplasma synoviae: biochemical characterization and metal dependence

Serine/threonine protein phosphatases have been described in many pathogenic bacteria as essential enzymes involved in phosphorylation-dependent signal transduction pathways and frequently associated with the virulence of these organisms. An inspection of Mycoplasma synoviae genome revealed the presence of a gene (prpC) encoding a putative protein phosphatase of the protein phosphatase 2C (PP2C) subfamily. Here, we report a complete biochemical characterization of M. synoviae phosphatase (PrpC) and the particular role of metal ions in the structure-function relationship of this enzyme. PrpC amino acid sequence analysis revealed that all the residues involved in the dinuclear metal center and the putative third metal ion-coordinating residues, conserved in PP2C phosphatases, are present in PrpC. PrpC is a monomeric protein able to dephosphorylate phospho-substrates with Mn(2+) ions' dependence. Thermal stability analysis demonstrated the enzyme stability at mild temperatures and the influence of Mn(2+) ions in this property. Mass spectrometry analysis suggested that three metal ions bind to PrpC, two of which with an apparent high-affinity constant. Mutational analysis of the putative third metal-coordinating residues, Asp122 and Arg164, revealed that these variants exhibited a weaker binding of manganese ions, and that both mutations affected PrpC phosphatase activity. According to these results, PrpC is a metal-dependent protein phosphatase member with an improved stability in the holo form and with Asp122, possibly implicated in the third metal-binding site, essential to catalytic activity.

Contribution of eukaryotic-type serine/threonine kinase to stress response and virulence of Streptococcus suis

Streptococcus suis serotype 2 (SS2) is an important swine and human pathogen responsible for septicemia and meningitis. The bacterial homologues of eukaryotic-type serine/threonine kinases (ESTKs) have been reported to play critical roles in various cellular processes. To investigate the role of STK in SS2, an isogenic stk mutant strain (Δstk) and a complemented strain (CΔstk) were constructed. The Δstk showed a significant decrease in adherence to HEp-2 cells, compared with the wild-type strain, and a reduced survival ratio in whole blood. In addition, the Δstk exhibited a notable reduced tolerance of environmental stresses including high temperature, acidic pH, oxidative stress, and high osmolarity. More importantly, the Δstk was attenuated in both the CD1 mouse and piglet models of infection. The results of quantitative reverse transcription-PCR (qRT-PCR) analysis indicated that the expressions of a few genes involving in adherence, stress response and virulence were clearly decreased in the Δstk mutant strain. Our data suggest that SsSTK is required for virulence and stress response in SS2.

Two unique phosphorylation-driven signaling pathways crosstalk in Staphylococcus aureus to modulate the cell-wall charge: Stk1/Stp1 meets GraSR

The Stk1/Stp1 and GraSR signal-transduction pathways are two distinct pathways in Staphylococcus aureus that rely on a reversible phosphorylation process in transducing external stimuli intracellularly. Stk1/Stp1 is an eukaryote-like Ser/Thr kinase phosphatase pair involved in purine biosynthesis, cell-wall metabolism, and autolysis. GraSR is a two-component system involved in resistance to cationic antimicrobial peptides. Both systems are implicated in S. aureus virulence and resistance to cell-wall inhibitors. Our study shows that the response regulator protein GraR undergoes phosphorylation by Stk1 at three threonine residues in the DNA-binding domain. Phosphorylation by Stk1 depends on the structural integrity of GraR as well as the amino acid sequences flanking the phosphorylation sites. Its homologue in Bacillus subtilis , BceR, which harbors two of the three phosphorylation sites in GraR, does not undergo Stk1-dependent phosphorylation. GraR is involved in regulation of the dltABCD operon, the gene products of which add the d-Ala on wall teichoic acid (WTA). Investigation of WTA isolated from the S. aureus RN6390 ΔgraR strain by NMR spectroscopy showed a clear negative effect that graR deletion has on the d-Ala content of WTA. Moreover, complementation of ΔgraR mutant with graR lacking the Stk1 phosphorylation sites mirrors this effect. These findings provide evidence that GraR is a target of Stk1 in vivo and suggest that modification of WTA by d-Ala is modulated by Stk1. The crosstalk between these two otherwise independent signaling pathways may facilitate S. aureus interaction with its environment to modulate processes such as cell growth and division and virulence.

Regulation of prokaryotic gene expression by eukaryotic-like enzymes

A growing body of evidence indicates that serine/threonine kinases (STKs) and phosphatases (STPs) regulate gene expression in prokaryotic organisms. As prokaryotic STKs and STPs are not DNA binding proteins, regulation of gene expression is accomplished through post-translational modification of their targets. These include two-component response regulators, DNA binding proteins and proteins that mediate transcription and translation. This review summarizes our current understanding of how STKs and STPs mediate gene expression in prokaryotes. Further studies to identify environmental signals that trigger the signaling cascade and elucidation of mechanisms that regulate crosstalk between eukaryotic-like signaling enzymes, two-component systems, and components of the transcriptional and translational machinery will facilitate a greater understanding of prokaryotic gene regulation.

Common regulators of virulence in streptococci

Streptococcal species are a diverse group of bacteria which can be found in animals and humans. Their interactions with host organisms can vary from commensal to pathogenic. Many of the pathogenic species are causative agents of severe, invasive infections in their hosts, accounting for a high burden of morbidity and mortality, associated with high economic costs in industry and health care. Among them, Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus pneumoniae, and Streptococcus suis are discussed here. An environmentally stimulated and tightly controlled expression of their virulence factors is of utmost importance for their pathogenic potential. Thus, the most universal and widespread regulators from the classes of stand-alone transcriptional regulators, two-component signal transduction systems (TCS), eukaryotic-like serine/threonine kinases, and small noncoding RNAs are the topic of this chapter. The regulatory levels are reviewed with respect to function, activity, and their role in pathogenesis. Understanding of and interfering with transcriptional regulation mechanisms and networks is a promising basis for the development of novel anti-infective therapies.

Serine/threonine phosphatase Stp1 mediates post-transcriptional regulation of hemolysin, autolysis, and virulence of group B Streptococcus

Elucidating how serine/threonine phosphatases regulate kinase function and bacterial virulence is critical for our ability to combat these infections. Group B streptococci (GBS) are β-hemolytic Gram-positive bacteria that cause invasive infections in humans. To adapt to environmental changes, GBS encodes signaling mechanisms comprising two component systems and eukaryotic-like enzymes. We have previously described the importance of the serine/threonine kinase Stk1 to GBS pathogenesis. However, how the presence or absence of the cognate serine/threonine phosphatase Stp1 affects Stk1 function and GBS virulence is not known. Here, we show that GBS deficient only in Stp1 expression are markedly reduced for their ability to cause systemic infections, exhibit decreased β-hemolysin/cytolysin activity, and show increased sensitivity to autolysis. Although transcription of genes important for β-hemolysin/cytolysin expression and export is similar to the wild type (WT), 294 genes (excluding stp1) showed altered expression in the stp1 mutant and included autolysin genes. Furthermore, phosphopeptide enrichment analysis identified that 35 serine/threonine phosphopeptides, corresponding to 27 proteins, were unique to the stp1 mutant. This included phosphorylation of ATP synthase, DNA and RNA helicases, and proteins important for cell division and protein synthesis. Collectively, our results indicate that Stp1 is important for appropriate regulation of Stk1 function, hemolysin activity, autolysis, and GBS virulence.

Activation of a prophage-encoded tyrosine kinase by a heterologous infecting phage results in a self-inflicted abortive infection

Bacteria in their struggle for survival have evolved or acquired defences against attacking phage. However, phage often contribute to this defence through mechanisms in which a prophage protects the bacterial population from attack by another, often unrelated, phage. The 933W prophage, which carries Shiga toxin genes that enhance pathogenicity of enterohaemorrhagic Escherichia coli strain O157:H7, also carries the stk gene encoding a eukaryotic-like tyrosine kinase that excludes (aborts) infection by phage HK97. This exclusion requires the kinase activity of Stk. Little, if any, protein tyrosine phosphorylation can be detected in a 933W lysogen prior to infection with HK97, while extensive Stk-mediated tyrosine phosphorylation is evident following infection. This includes autophosphorylation that stabilizes Stk protein from degradation. Although increased levels of Stk are found following HK97 infection, these higher levels are not necessary or sufficient for exclusion or protein phosphorylation. An HK97 open reading frame, orf41, is necessary for exclusion and Stk kinase activity. We hypothesize that interaction with gp41 stimulates Stk kinase activity. Exclusion of HK97 appears to be specific since other phages tested, λ, φ80, H-19B, λ-P22dis and T4rII, were not excluded. Infection of the 933W lysogen with a non-excluded phage fails to induce Stk-determined phosphorylation.