Regulation of Staphylococcus aureus pathogenesis via target of RNAIII-activating Protein (TRAP)

Staphylococcus aureus can cause disease through the production of toxins. Toxin production is autoinduced by the protein RNAIII-activating protein (RAP) and by the autoinducing peptide (AIP), and is inhibited by RNAIII-inhibiting peptide (RIP) and by inhibitory AIPs. RAP has been shown to be a useful vaccine target site, and RIP and inhibitory AIPs as therapeutic molecules to prevent and suppress S. aureus infections. Development of therapeutic strategies based on these molecules has been hindered by a lack of knowledge of the molecular mechanisms by which they activate or inhibit virulence. Here, we show that RAP specifically induces the phosphorylation of a novel 21-kDa protein, whereas RIP inhibits its phosphorylation. This protein was termed target of RAP (TRAP). The synthesis of the virulence regulatory molecule, RNAIII, is not activated by RAP in the trap mutant strain, suggesting that RAP activates RNAIII synthesis via TRAP. Phosphoamino acid analysis shows that TRAP is histidine-phosphorylated, suggesting that TRAP may be a sensor of RAP. AIPs up-regulate the synthesis of RNAIII also in trap mutant strains, suggesting that TRAP and AIPs activate RNAIII synthesis via distinct signal transduction pathways. Furthermore, TRAP phosphorylation is down-regulated in the presence of AIP, suggesting that a network of signal transduction pathways regulate S. aureus pathogenesis.     

Inhibition of Staphylococcus aureus pathogenesis in vitro and in vivo by RAP-binding peptides

Staphylococcus aureus cause many diseases by producing toxins, whose synthesis is regulated by quorum-sensing mechanisms. S. aureus secretes a protein termed RNAIII activating protein (RAP) which autoinduces toxin production via the phosphorylation of is target protein TRAP. Mice vaccinated with RAP were protected from S. aureus infection, suggesting that RAP is an useful target for selecting potential therapeutic molecules to inhibit S. aureus pathogenesis. We show here that RAP (native and recombinant) was used to select RAP-binding peptides (RBPs) from a random 12-mer phage-displayed peptide library. Two RBPs were shown to inhibit RNAIII production in vitro (used a marker for pathogenesis). The peptide WPFAHWPWQYPR, which had the strongest inhibitory activity, was chemically synthesized and also expressed in Escherichia coli as a GST-fusion. Both synthetic peptide and GST-fusion peptide decreased RNAIII levels in a dose-dependent manner. The GST-fusion peptide was also shown to protect mice from a S. aureus infection in vivo (tested in a murine cutaneous S. aureus infection model). Our results suggest the potential use of RAP-binding proteins in treating clinical S. aureus infections.     

Characterization of RAP,a quorum sensing activator of Staphylococcus aureus

Staphylococcus aureus are Gram-positive bacteria and cause diverse serious diseases in humans and animals through the production of toxins. The production of toxins is regulated by quorum sensing mechanisms, where proteins such as RNAIII activating protein (RAP) are secreted by the bacteria and induce virulence. Antibodies to RAP have been shown to protect mice from infection, but the molecular structure of RAP was not known and hindered vaccine development. To characterize RAP, recombinant protein was made and tested for its ability to induce genes important for pathogenesis (agr). In addition, monoclonal antibodies were produced to identify its cellular localization. Results shown here indicate that RAP is a 277-aa protein that is an ortholog of the ribosomal protein L2. Like the native molecule, recombinant RAP activates the production of RNAIII (encoded by agr). Using RAP specific monoclonal antibodies we demonstrate that RAP is continuously secreted and while RAP is expressed also in other bacteria (like Staphylococcus epidermidis, Staphylococcus xylosus and Escherichia coli), it is secreted to the culture medium only by S. aureus. Our results show that the ribosomal protein L2 has an extraribosomal function and that when secreted RAP acts as an autoinducer of virulence to regulate S. aureus pathogenesis.     

The target of RNAIII-activating protein (TRAP) from Staphylococcus aureus: purification, crystallization and preliminary X-ray analysis

The target of the RNAIII-activating protein (TRAP) is a 21 kDa protein in which phosphorylation is activated by the RNAIII-activating protein (RAP), which causes an increase in RNAII and RNAIII synthesis and the production of the virulence factors. In an attempt to examine the structural role of TRAP in the signal transduction pathway, TRAP from Staphylococcus aureus was overexpressed, purified and crystallized using PEG 8000 and 5% Jeffamine M600 (pH 7.0), as precipitants by hanging-drop vapour diffusion methods at 287 K. The crystals belong to the orthorhombic space group, P2(1)2(1)2(1), with unit cell parameters of a=39.68, b=50.41, c=85.45 A. There is one monomer of TRAP per crystallographic asymmetric unit with a crystal volume per protein mass (V(M)) of 2.06 A(3) Da(-1) and a solvent content of 40.3%. A complete data set diffracting to 1.9 A resolution was collected from a single crystal at 100 K using a synchrotron-radiation source.     

C-terminus of TRAP in Staphylococcus can enhance the activity of lysozyme and lysostaphin

In Staphylococcus aureus , the target of RNAIII activating protein (TRAP) is a membrane-associated protein whose C-terminus can be used as a vaccine to provide protection against staphylococcal infection. Here, we show for the first time by surface plasmon resonance and enzyme-linked immunosorbent assay that TRAP can specifically bind lysozyme and lysostaphin through its C-terminus (amino acids 155–167) and enhance lysozomal activities in vitro . It was also found that the traP mutant strain is more resistant to lysostaphin than wild-type. Our previous data showed that the C-terminus of TRAP might be extracellular. So our results suggested that the C-terminus of TRAP could act as the specific targeting protein of the lysozyme/lysostaphin on the S. aureus cell wall and the biological significance of the interaction might be to facilitate lysozyme/lysostaphin-mediated cell lysis.     

Quorum sensing in Staphylococci is regulated via phosphorylation of three conserved histidine residues

Staphylococcus aureus cause infections by producing toxins, a process regulated by cell-cell communication (quorum sensing) through the histidine-phosphorylation of the target of RNAIII-activating protein (TRAP). We show here that TRAP is highly conserved in staphylococci and contains three completely conserved histidine residues (His-66, His-79, His-154) that are phosphorylated and essential for its activity. This was tested by constructing a TRAP(-) strain with each of the conserved histidine residues changed to alanine by site-directed mutagenesis. All mutants were tested for pathogenesis in vitro (expression of RNAIII and hemolytic activity) and in vivo (murine cellulitis model). Results show that RNAIII is not expressed in the TRAP(-) strain, that it is non hemolytic, and that it does not cause disease in vivo. These pathogenic phenotypes could be rescued in the strain containing the recovered traP, confirming the importance of TRAP in S. aureus pathogenesis. The phosphorylation of TRAP mutated in any of the conserved histidine residues was significantly reduced, and mutants defective in any one of these residues were non-pathogenic in vitro or in vivo, whereas those mutated in a non-conserved histidine residue (His-124) were as pathogenic as the wild type. These results confirm the importance of the three conserved histidine residues in TRAP activity. The phosphorylation pattern, structure, and gene organization of TRAP deviates from signaling molecules known to date, suggesting that TRAP belongs to a novel class of signal transducers.     

Improved protective efficacy of a chimeric Staphylococcus aureus vaccine candidate iron‐regulated surface determinant B (N126–P361)‐target of RNAIII activating protein in mice

The pathogen Staphylococcus aureus causes a wide range of serious infections, necessitating urgent development of a vaccine against this organism. However, currently developed vaccines are relatively ineffective because of the limited antigenic component that is contained in the vaccine formulations. To develop an effective S. aureus candidate vaccine, overlapping PCR was used to add the truncated immunodominant antigen iron‐regulated surface determinant B (IsdB)_{(N126–P361)} (tIsdB) to the N‐terminal of intact antigen target of RNAIII activating protein (TRAP) and thus construct a tIsdB‐TRAP chimera. The humoral and cellular immune responses against tIsdB‐TRAP were compared with those against single or combined formulations. tIsdB‐TRAP elicited significantly stronger humoral responses in mice (P < 0.05). As to cellular immune responses in mice, the tIsdB‐TRAP group resulted in a greater IL‐4 response than did other groups (P < 0.05). Greater amounts of IL‐2 and IFN‐γ were found in the tIsdB‐TRAP group. Mouse challenge also showed that tIsdB‐TRAP provided better protection against S. aureus than did the control groups. These results suggest that this chimeric protein may be a promising pathogen target for further vaccine development.     

RNAIII inhibiting peptide (RIP) inhibits agr-regulated toxin production

Staphylococcal enterotoxins (SEs) are emetic toxins that cause food poisoning. SEs also function as powerful pyrogenic toxin superantigens that stimulate non-specific T-cell proliferation. Together with the hemolysins, SEs have been largely implicated as virulence factors in multiple infection models. Recent biochemical and genetic analyses have demonstrated that production of some of these toxins is partially regulated by quorum sensing mechanisms where proteins and peptides activate the accessory gene regulator (agr). Because toxin production is central to bacterial pathogenesis, therapeutic strategies alternative to antibiotics, and based on rational interference of the quorum sensing systems involved, are currently being developed. This approach would lead to repression of toxin production and, thus, to disease prevention. Here we provide evidence to conclude that synthetic analogs of the RNAIII inhibiting peptide (RIP) and antibodies to its target molecule TRAP function in vitro as efficient suppressors of agr-regulated exotoxin production by Staphylococcus aureus.     

Translation of RNAIII, the Staphylococcus aureus agr regulatory RNA molecule, can be activated by a 3'-end deletion

Abstract RNAIII, an RNA molecule shown to encode δ-hemolysin and independently to regulate toxin synthesis in Staphylococcus aureus , is transcribed at the mid-exponential phase of growth, while its target genes are activated 2 h later, at the post-exponential phase of growth. We show here that the translation of RNAIII to the 26-amino acid peptide δ-hemolysin is delayed by 1 h, and that this delay is abolished when the 3'-end of this molecule is deleted. We suggest that structural changes of RNAIII to a translatable form of the molecule precede its regulation of target gene expression.     

The production of extracellular proteins is regulated by ribonuclease III via two different pathways in Staphylococcus aureus

Staphylococcus aureus ribonuclease III belongs to the enzyme family known to degrade double-stranded RNAs. It has previously been reported that RNase III cannot influence cell growth but regulates virulence gene expression in S. aureus. Here we constructed an RNase III inactivation mutant (Δrnc) from S. aureus 8325-4. It was found that the extracellular proteins of Δrnc were decreased. Furthermore, we explored how RNase III regulated the production of the extracellular proteins in S. aureus. We found during the lag phase of the bacterial growth cycle RNase III could influence the extracellular protein secretion via regulating the expression of secY2, one component of accessory secretory (sec) pathway. After S. aureus cells grew to exponential phase, RNase III can regulate the expression of extracellular proteins by affecting the level of RNAIII. Further investigation showed that the mRNA stability of secY2 and RNAIII was affected by RNase III. Our results suggest that RNase III could regulate the pathogenicity of S. aureus by influencing the level of extracellular proteins via two different ways respectively at different growth phases.     

trans-Complementation of a Staphylococcus aureus agr Mutant by Staphylococcus lugdunensis agr RNAIII

RNAIII from Staphylococcus lugdunensis (RNAIII-sl) in a Staphylococcus aureus agr mutant partially restored the Agr phenotype. A chimeric construct consisting of the 5′ end of RNAIII-sl and the 3′ end of RNAIII from S. aureus restored the Agr phenotype to a greater extent, suggesting the presence of independent regulatory domains.     

Probing the structure of RNAIII, the Staphylococcus aureus agr regulatory RNA, and identification of the RNA domain involved in repression of protein A expression

RNAIII, a 514-nt RNA molecule, regulates the expression of many Staphylococcus aureus genes encoding exoproteins and cell-wall-associated proteins. We have studied the structure of RNAIII in solution, using a combination of chemical and enzymatic probes. A model of the secondary structure was derived from experimental data with the help of computer simulation of RNA folding. The model contains 14 hairpin structures connected by unpaired nucleotides. The data also point to three helices formed by distant nucleotides that close off structural domains. This model was generally compatible with the results of in vivo probing experiments with dimethylsulfate in late exponential-phase cultures. Toe-printing experiments revealed that the ribosome binding site of hld, which is encoded by RNAIII, was accessible to the Escherichia coli 30S ribosomal subunit, suggesting that the in vitro structure represented a translatable form of RNAIII. We also found that, within the 3' end of RNAIII, the conserved hairpin 13 and the terminator form an intrinsic structural domain that exerts specific regulatory activity on protein A gene expression.