Penicillin binding protein 3 of Staphylococcus aureus NCTC 8325-4 binds and activates human plasminogen.

BACKGROUND: Staphylococcus aureus is a versatile pathogen expressing a number of virulence-associated adhesive molecules. In a previous study, we generated in a secretion-competent Escherichia coli strain a library of random FLAG-tag positive (FTP) polypeptides of S. aureus. To identify adhesive proteins and gain additional knowledge on putative virulence factors of S. aureus, we here screened the FTP library against human serum proteins. FINDINGS: Staphylococcus aureus NCTC 8325-4, origin of the FTP library, adhered to immobilized plasminogen in vitro. In an enzyme-linked immunoassay a C-terminal part of penicillin binding protein 3 (PBP3), included in the FTP library, bound to immobilized plasminogen. We expressed and purified full-length PBP3 and its C-terminal fragments as recombinant proteins. In a time-resolved fluorometry-based assay the PBP3 polypeptides bound to immobilized plasminogen. The polypeptides enhanced formation of plasmin from plasminogen as analyzed by cleavage of a chromogenic plasmin substrate. CONCLUSIONS: The present findings, although preliminary, demonstrate reliably that S. aureus NCTC 8325-4 adheres to immobilized plasminogen in vitro and that the adhesion may be mediated by a C-terminal fragment of the PBP3 protein. The full length PBP3 and the penicillin binding C-terminal domain of PBP3 expressed as recombinant proteins bound plasminogen and activated plasminogen to plasmin. These phenomena were inhibited by the lysine analogue ε-aminocaproic acid suggesting that the binding is mediated by lysine residues. A detailed molecular description of surface molecules enhancing the virulence of S. aureus will aid in understanding of its pathogenicity and help in design of antibacterial drugs in the future.

A proteinaceous fraction of wheat bran may interfere in the attachment of enterotoxigenic E. coli K88 (F4+) to porcine epithelial cells.

Wheat bran (WB) from Triticum aestivum has many beneficial effects on human health. To the best of our knowledge, very little has been published about its ability to prevent pathogenic bacterial adhesion in the intestine. Here, a WB extract was fractionated using different strategies, and the obtained fractions were tested in different in vitro methodologies to evaluate their interference in the attachment of enterotoxigenic Escherichia coli (ETEC) K88 to intestinal porcine epithelial cells (IPEC-J2) with the aim of identifying the putative anti-adhesive molecules. It was found that a proteinaceous compound in the >300-kDa fraction mediates the recognition of ETEC K88 to IPEC-J2. Further fractionation of the >300-kDa sample by size-exclusion chromatography showed several proteins below 90 kDa, suggesting that the target protein belongs to a high-molecular-weight (MW) multi-component protein complex. The identification of some relevant excised bands was performed by mass spectrometry (MS) and mostly revealed the presence of various protease inhibitors (PIs) of low MW: Serpin-Z2B, Class II chitinase, endogenous alpha-amylase/subtilisin inhibitor and alpha-amylase/trypsin inhibitor CM3. Furthermore, an incubation of the WB extract with ETEC K88 allowed for the identification of a 7S storage protein globulin of wheat, Globulin 3 of 66 kDa, which may be one of the most firmly attached WB proteins to ETEC K88 cells. Further studies should be performed to gain an understanding of the molecular recognition of the blocking process that takes place. All gathered information can eventually pave the way for the development of novel anti-adhesion therapeutic agents to prevent bacterial pathogenesis.

Casein glycomacropeptide in the diet may reduce Escherichia coli attachment to the intestinal mucosa and increase the intestinal lactobacilli of early weaned piglets after an enterotoxigenic E. coli K88 challenge.

Casein glycomacropeptide (CGMP), a glycoprotein originating during cheese manufacture, has shown promising effects by promoting the growth of some beneficial bacteria in vitro, although its activity has not been well explored. The present study was designed to evaluate the effects of CGMP against enterotoxigenic Escherichia coli (ETEC) K88 in vitro (Trial 1) and in vivo (Trial 2). In Trial 1, increasing concentrations of CGMP (0, 0.5, 1.5 or 2.5 mg/ml) were tested regarding its ability to block the attachment of ETEC K88 to ileal mucosa tissues obtained from piglets. Increasing the concentration of CGMP resulted in a gradual decrease in ETEC K88 attachment to the epithelial surface. In Trial 2, seventy-two piglets were distributed in a 2 × 2 factorial combination including or omitting CGMP in the diet (control diet v. CGMP) and challenged or not with ETEC K88 (yes v. no). Inclusion of CGMP increased crude protein, ammonia and isoacid concentrations in colon digesta. CGMP also increased lactobacilli numbers in ileum and colon digesta, and reduced enterobacteria counts in mucosa scrapings and the percentage of villi with E. coli adherence measured by fluorescence in situ hybridisation. The inclusion of CGMP in the diets of challenged animals also prevented the increase of enterobacteria in ileal digesta. We can conclude that CGMP may improve gut health by diminishing the adhesion of ETEC K88 to the intestinal mucosa, by increasing the lactobacilli population in the intestine and by reducing the overgrowth of enterobacteria in the digestive tract of piglets after an ETEC K88 challenge.

Temperature-induced changes in the lipopolysaccharide of Yersinia pestis affect plasminogen activation by the pla surface protease.

The Pla surface protease of Yersinia pestis activates human plasminogen and is a central virulence factor in bubonic and pneumonic plague. Pla is a transmembrane beta-barrel protein and member of the omptin family of outer membrane proteases which require bound lipopolysaccharide (LPS) to be proteolytically active. Plasminogen activation and autoprocessing of Pla were dramatically higher in Y. pestis cells grown at 37 degrees C than in cells grown at 20 degrees C; the difference in enzymatic activity by far exceeded the increase in the cellular content of the Pla protein. Y. pestis modifies its LPS structure in response to growth temperature. We purified His(6)-Pla under denaturing conditions and compared various LPS types for their capacity to enhance plasmin formation by His(6)-Pla solubilized in detergent. Reactivation of His(6)-Pla was higher with Y. pestis LPSs isolated from bacteria grown at 37 degrees C than with LPSs from cells grown at 25 degrees C. Lack of O antigens and the presence of the outer core region as well as a lowered level of acylation in LPS were found to enhance the Pla-LPS interaction. Genetic substitution of arginine 138, which is part of a three-dimensional protein motif for binding to lipid A phosphates, decreased both the enzymatic activity of His(6)-Pla and the amount of Pla in Y. pestis cells, suggesting the importance of the Pla-lipid A phosphate interaction. The temperature-induced changes in LPS are known to help Y. pestis to avoid innate immune responses, and our results strongly suggest that they also potentiate Pla-mediated proteolysis.

Using every trick in the book: the Pla surface protease of Yersinia pestis.

The Pla surface protease of Yersinia pestis, encoded by the Y. pestis-specific plasmid pPCP1, is a versatile virulence factor. In vivo studies have shown that Pla is essential in the establishment of bubonic plague, and in vitro studies have demonstrated various putative virulence functions for the Pla molecule. Pla is a surface protease of the omptin family, and its proteolytic targets include the abundant, circulating human zymogen plasminogen, which is activated by Pla to the serine protease plasmin. Plasmin is important in cell migration, and Pla also proteolytically inactivates the main circulating inhibitor of plasmin, alpha2-antiplasmin. Pla also is an adhesin with affinity for laminin, a major glycoprotein of mammalian basement membranes, which is degraded by plasmin but not by Pla. Together, these functions create uncontrolled plasmin proteolysis targeted at tissue barriers. Other proteolytic targets for Pla include complement proteins. Pla also mediates bacterial invasion into human endothelial cell lines; the adhesive and invasive charateristics of Pla can be genetically dissected from its proteolytic activity. Pla is a 10-stranded antiparallel beta-barrel with five surface-exposed short loops, where the catalytic residues are oriented inwards at the top of the beta-barrel. The sequence of Pla contains a three-dimensional motif for protein binding to lipid A of the lipopolysaccharide. Indeed, the proteolytic activity of Pla requires rough lipopolysaccharide but is sterically inhibited by the O antigen in smooth LPS, which may be the selective advantage of the loss of O antigen in Y. pestis. Members of the omptin family are highly similar in structure but differ in functions and virulence association. The catalytic residues of omptins are conserved, but the variable substrate specificities in proteolysis by Pla and other omptins are dictated by the amino acid sequences near or at the surface loops, and hence reflect differences in substrate binding. The closest orthologs of Pla are PgtE of Salmonella and Epo of Erwinia, which functionally differ from Pla. Pla gives a model of how a horizontally transferred protein fold can diverge into a powerful virulence factor through adaptive mutations.

Lack of O-antigen is essential for plasminogen activation by Yersinia pestis and Salmonella enterica.

The O-antigen of lipopolysaccharide (LPS) is a virulence factor in enterobacterial infections, and the advantage of its genetic loss in the lethal pathogen Yersinia pestis has remained unresolved. Y. pestis and Salmonella enterica express beta-barrel surface proteases of the omptin family that activate human plasminogen. Plasminogen activation is central in pathogenesis of plague but has not, however, been found to be important in diarrhoeal disease. We observed that the presence of O-antigen repeats on wild-type or recombinant S. enterica, Yersinia pseudotuberculosis or Escherichia coli prevents plasminogen activation by PgtE of S. enterica and Pla of Y. pestis; the O-antigen did not affect incorporation of the omptins into the bacterial outer membrane. Purified His6-Pla was successfully reconstituted with rough LPS but remained inactive after reconstitution with smooth LPS. Expression of smooth LPS prevented Pla-mediated adhesion of recombinant E. coli to basement membrane as well as invasion into human endothelial cells. Similarly, the presence of an O-antigen prevented PgtE-mediated bacterial adhesion to basement membrane. Substitution of Arg-138 and Arg-171 of the motif for protein binding to lipid A 4'-phosphate abolished proteolytic activity but not membrane translocation of PgtE, indicating dependence of omptin activity on a specific interaction with lipid A. The results suggest that Pla and PgtE require LPS for activity and that the O-antigen sterically prevents recognition of large-molecular-weight substrates. Loss of O-antigen facilitates Pla functions and invasiveness of Y. pestis; on the other hand, smooth LPS renders plasminogen activator cryptic in S. enterica.