Isolation of a prokaryotic plasmin receptor. Relationship to a plasminogen activator produced by the same micro-organism

Inhibition of Fibrinolysis by Streptococcal Phage LysinSM1

Expression of bacteriophage lysin_SM1 by Streptococcus oralis strain SF100 is thought to be important for the pathogenesis of infective endocarditis, due to its ability to mediate bacterial binding to fibrinogen. To better define the lysin_SM1 binding site on fibrinogen Aα, and to investigate the impact of binding on fibrinolysis, we examined the interaction of lysin_SM1 with a series of recombinant fibrinogen Aα variants. These studies revealed that lysin_SM1 binds the C-terminal region of fibrinogen Aα spanned by amino acid residues 534 to 610, with an affinity of equilibrium dissociation constant (K_D) of 3.23 × 10^-5 M. This binding site overlaps the known binding site for plasminogen, an inactive precursor of plasmin, which is a key protease responsible for degrading fibrin polymers. When tested in vitro, lysin_SM1 competitively inhibited plasminogen binding to the αC region of fibrinogen Aα. It also inhibited plasminogen-mediated fibrinolysis, as measured by thromboelastography (TEG). These results indicate that lysin_SM1 is a bi-functional virulence factor for streptococci, serving as both an adhesin and a plasminogen inhibitor. Thus, lysin_SM1 may facilitate the attachment of bacteria to fibrinogen on the surface of damaged cardiac valves and may also inhibit plasminogen-mediated lysis of infected thrombi (vegetations) on valve surfaces. IMPORTANCE The interaction of streptococci with human fibrinogen and platelets on damaged endocardium is a central event in the pathogenesis of infective endocarditis. Streptococcus oralis can bind platelets via the interaction of bacteriophage lysin_SM1 with fibrinogen on the platelet surface, and this process has been associated with increased virulence in an animal model of endocarditis. We now report that lysin_SM1 binds to the αC region of the human fibrinogen Aα chain. This interaction blocks plasminogen binding to fibrinogen and inhibits fibrinolysis. In vivo, this inhibition could prevent the lysis of infected vegetations, thereby promoting bacterial persistence and virulence.

Structural Biology and Protein Engineering of Thrombolytics

Myocardial infarction and ischemic stroke are the most frequent causes of death or disability worldwide. Due to their ability to dissolve blood clots, the thrombolytics are frequently used for their treatment. Improving the effectiveness of thrombolytics for clinical uses is of great interest. The knowledge of the multiple roles of the endogenous thrombolytics and the fibrinolytic system grows continuously. The effects of thrombolytics on the alteration of the nervous system and the regulation of the cell migration offer promising novel uses for treating neurodegenerative disorders or targeting cancer metastasis. However, secondary activities of thrombolytics may lead to life-threatening side-effects such as intracranial bleeding and neurotoxicity. Here we provide a structural biology perspective on various thrombolytic enzymes and their key properties: (i) effectiveness of clot lysis, (ii) affinity and specificity towards fibrin, (iii) biological half-life, (iv) mechanisms of activation/inhibition, and (v) risks of side effects. This information needs to be carefully considered while establishing protein engineering strategies aiming at the development of novel thrombolytics. Current trends and perspectives are discussed, including the screening for novel enzymes and small molecules, the enhancement of fibrin specificity by protein engineering, the suppression of interactions with native receptors, liposomal encapsulation and targeted release, the application of adjuvants, and the development of improved production systems.

Group A Streptococcus exploits human plasminogen for bacterial translocation across epithelial barrier via tricellular tight junctions

Group A Streptococcus (GAS) is a human-specific pathogen responsible for local suppurative and life-threatening invasive systemic diseases. Interaction of GAS with human plasminogen (PLG) is a salient characteristic for promoting their systemic dissemination. In the present study, a serotype M28 strain was found predominantly localized in tricellular tight junctions of epithelial cells cultured in the presence of PLG. Several lines of evidence indicated that interaction of PLG with tricellulin, a major component of tricellular tight junctions, is crucial for bacterial localization. A site-directed mutagenesis approach revealed that lysine residues at positions 217 and 252 within the extracellular loop of tricellulin play important roles in PLG-binding activity. Additionally, we demonstrated that PLG functions as a molecular bridge between tricellulin and streptococcal surface enolase (SEN). The wild type strain efficiently translocated across the epithelial monolayer, accompanied by cleavage of transmembrane junctional proteins. In contrast, amino acid substitutions in the PLG-binding motif of SEN markedly compromised those activities. Notably, the interaction of PLG with SEN was dependent on PLG species specificity, which influenced the efficiency of bacterial penetration. Our findings provide insight into the mechanism by which GAS exploits host PLG for acceleration of bacterial invasion into deeper tissues via tricellular tight junctions.

Preferential Acquisition and Activation of Plasminogen Glycoform II by PAM Positive Group A Streptococcal Isolates

Plasminogen (Plg) circulates in the host as two predominant glycoforms. Glycoform I Plg (GI-Plg) contains glycosylation sites at Asn289 and Thr346, whereas glycoform II Plg (GII-Plg) is exclusively glycosylated at Thr346. Surface plasmon resonance experiments demonstrated that Plg binding group A streptococcal M protein (PAM) exhibits comparative equal affinity for GI- and GII-Plg in the "closed" conformation (for GII-Plg, KD = 27.4 nM; for GI-Plg, KD = 37.0 nM). When Plg was in the "open" conformation, PAM exhibited an 11-fold increase in affinity for GII-Plg (KD = 2.8 nM) compared with that for GI-Plg (KD = 33.2 nM). The interaction of PAM with Plg is believed to be mediated by lysine binding sites within kringle (KR) 2 of Plg. PAM-GI-Plg interactions were fully inhibited with 100 mM lysine analogue ε-aminocaproic acid (εACA), whereas PAM-GII-Plg interactions were shown to be weakened but not inhibited in the presence of 400 mM εACA. In contrast, binding to the KR1-3 domains of GII-Plg (angiostatin) by PAM was completely inhibited in the presence 5 mM εACA. Along with PAM, emm pattern D GAS isolates express a phenotypically distinct SK variant (type 2b SK) that requires Plg ligands such as PAM to activate Plg. Type 2b SK was able to generate an active site and activate GII-Plg at a rate significantly higher than that of GI-Plg when bound to PAM. Taken together, these data suggest that GAS selectively recruits and activates GII-Plg. Furthermore, we propose that the interaction between PAM and Plg may be partially mediated by a secondary binding site outside of KR2, affected by glycosylation at Asn289.

Modulation of the coagulation system during severe streptococcal disease

Haemostasis is maintained by a tightly regulated coagulation system that comprises platelets, procoagulant proteins, and anticoagulant proteins. During the local and systemic response to bacterial infection, the coagulation system becomes activated, and contributes to the pathophysiological response to infection. The significant human pathogen, Streptococcus pyogenes has multiple strategies to modulate coagulation. This can range from systemic activation of the intrinsic and extrinsic pathway of coagulation to local stimulation of fibrinolysis. Such diverse effects on this host system imply a finely tuned host-bacteria interaction. The molecular mechanisms that underlie this modulation of the coagulation system are discussed in this review.

Plasmin is essential in preventing periodontitis in mice

Periodontitis involves bacterial infection, inflammation of the periodontium, degradation of gum tissue, and alveolar bone resorption, which eventually leads to loss of teeth. To study the role of the broad-spectrum protease plasmin in periodontitis, we examined the oral health of plasminogen (Plg)-deficient mice. In wild-type mice, the periodontium was unaffected at all time points studied; in Plg-deficient mice, periodontitis progressed rapidly, within 20 weeks. Morphological study results of Plg-deficient mice revealed detachment of gingival tissue, resorption of the cementum layer, formation of necrotic tissue, and severe alveolar bone degradation. IHC staining showed massive infiltration of neutrophils in the periodontal tissues. Interestingly, doubly deficient mice, lacking both tissue- and urokinase-type plasminogen activators, developed periodontal disease similar to that in Plg-deficient mice; however, mice lacking only tissue- or urokinase-type plasminogen activator remained healthy. Supplementation by injection of Plg-deficient mice with human plasminogen for 10 days led to necrotic tissue absorption, inflammation subsidence, and full regeneration of gum tissues. Notably, there was also partial regrowth of degraded alveolar bone. Taken together, our results show that plasminogen is essential for the maintenance of a healthy periodontium and plays an important role in combating the spontaneous development of chronic periodontitis. Moreover, reversal to healthy status after supplementation of Plg-deficient mice with plasminogen suggests the possibility of using plasminogen for therapy of periodontal diseases.

Streptococcus pyogenes M49 plasminogen/plasmin binding facilitates keratinocyte invasion via integrin-integrin-linked kinase (ILK) pathways and protects from macrophage killing

The entry into epithelial cells and the prevention of primary immune responses are a prerequisite for a successful colonization and subsequent infection of the human host by Streptococcus pyogenes (group A streptococci, GAS). Here, we demonstrate that interaction of GAS with plasminogen promotes an integrin-mediated internalization of the bacteria into keratinocytes, which is independent from the serine protease activity of potentially generated plasmin. α(1)β(1)- and α(5)β(1)-integrins were identified as the major keratinocyte receptors involved in this process. Inhibition of integrin-linked kinase (ILK) expression by siRNA silencing or blocking of PI3K and Akt with specific inhibitors, reduced the GAS M49-plasminogen/plasmin-mediated invasion of keratinocytes. In addition, blocking of actin polymerization significantly reduced GAS internalization into keratinocytes. Altogether, these results provide a first model of plasminogen-mediated GAS invasion into keratinocytes. Furthermore, we demonstrate that plasminogen binding protects the bacteria against macrophage killing.

Borrelia burgdorferi infection-associated surface proteins ErpP, ErpA, and ErpC bind human plasminogen

Host-derived plasmin plays a critical role in mammalian infection by Borrelia burgdorferi. The Lyme disease spirochete expresses several plasminogen-binding proteins. Bound plasminogen is converted to the serine protease plasmin and thereby may facilitate the bacterium's dissemination throughout the host by degrading extracellular matrix. In this work, we demonstrate plasminogen binding by three highly similar borrelial outer surface proteins, ErpP, ErpA, and ErpC, all of which are expressed during mammalian infection. Extensive characterization of ErpP demonstrated that this protein bound in a dose-dependent manner to lysine binding site I of plasminogen. Removal of three lysine residues from the carboxy terminus of ErpP significantly reduced binding of plasminogen, and the presence of a lysine analog, epsilon-aminocaproic acid, inhibited the ErpP-plasminogen interaction, thus strongly pointing to a primary role for lysine residues in plasminogen binding. Ionic interactions are not required in ErpP binding of plasminogen, as addition of excess NaCl or the polyanion heparin did not have any significant effect on binding. Plasminogen bound to ErpP could be converted to the active enzyme, plasmin. The three plasminogen-binding Erp proteins can also bind the host complement regulator factor H. Plasminogen and factor H bound simultaneously and did not compete for binding to ErpP, indicating separate binding sites for both host ligands and the ability of the borrelial surface proteins to bind both host proteins.

Thrombin-activatable Fibrinolysis Inhibitor Binds to Streptococcus pyogenes by Interacting with Collagen-like Proteins A and B

Regulation of proteolysis is a critical element of the host immune system and plays an important role in the induction of pro- and anti-inflammatory reactions in response to infection. Some bacterial species take advantage of these processes and recruit host proteinases to their surface in order to counteract the host attack. Here we show that Thrombin-activatable Fibrinolysis Inhibitor (TAFI), a zinc-dependent procarboxypeptidase, binds to the surface of group A streptococci of an M41 serotype. The interaction is mediated by the streptococcal collagen-like surface proteins A and B (SclA and SclB), and the streptococcal-associated TAFI is then processed at the bacterial surface via plasmin and thrombin-thrombomodulin. These findings suggest an important role for TAFI in the modulation of host responses by streptococci.

Pathogenesis of poststreptococcal glomerulonephritis a century after Clemens von Pirquet

Considerable insight has been gained into the etiopathogenesis of poststreptococcal glomerulonephritis since the landmark theoretical construct of Clemens von Pirquet postulated that disease-causing immune complexes were responsible for the nephritis that followed scarlet fever. Over the years, molecular mimicry between streptococcal products and renal components, autoimmune reactivity and several streptococcal antigens have been extensively studied. Recent investigations assign a critical role to both in situ formation and deposition of circulating immune complexes that would trigger a variety of effector mechanisms. Glomerular plasmin-binding activity of streptococcal glyceraldehyde-3-phosphate-dehydrogenase may play a role in nephritogenicity and streptococcal pyrogenic exotoxin B and its zymogen precursor may be the long-sought nephritogenic antigen.

The plasminogen-binding group A streptococcal M protein-related protein Prp binds plasminogen via arginine and histidine residues

The migration of the human pathogen Streptococcus pyogenes (group A streptococcus) from localized to deep tissue sites may result in severe invasive disease, and sequestration of the host zymogen plasminogen appears crucial for virulence. Here, we describe a novel plasminogen-binding M protein, the plasminogen-binding group A streptococcal M protein (PAM)-related protein (Prp). Prp is phylogenetically distinct from previously described plasminogen-binding M proteins of group A, C, and G streptococci. While competition experiments indicate that Prp binds plasminogen with a lower affinity than PAM (50% effective concentration = 0.34 microM), Prp nonetheless binds plasminogen with high affinity and at physiologically relevant concentrations of plasminogen (K(d) = 7.8 nM). Site-directed mutagenesis of the putative plasminogen binding site indicates that unlike the majority of plasminogen receptors, Prp does not interact with plasminogen exclusively via lysine residues. Mutagenesis to alanine of lysine residues Lys(96) and Lys(101) reduced but did not abrogate plasminogen binding by Prp. Plasminogen binding was abolished only with the additional mutagenesis of Arg(107) and His(108) to alanine. Furthermore, mutagenesis of Arg(107) and His(108) abolished plasminogen binding by Prp despite the presence of Lys(96) and Lys(101) in the binding site. Thus, binding to plasminogen via arginine and histidine residues appears to be a conserved mechanism among plasminogen-binding M proteins.

Enhanced activation of bound plasminogen on Staphylococcus aureus by staphylokinase

Activation of plasminogen (plg) to plasmin by the staphylococcal activator, staphylokinase (SAK), is effectively regulated by the circulating inhibitor, alpha2-antiplasmin (alpha2AP). Here it is demonstrated that intact Staphylococcus aureus cells and solubilized staphylococcal cell wall proteins not only protected SAK-promoted plg activation against the inhibitory effect of alpha2AP but also enhanced the activation. The findings suggest that the surface-associated plg activation by SAK may have an important physiological function in helping staphylococci in tissue dissemination. Amino acid sequencing of tryptic peptides originating from the 59-, 56- and 43-kDa proteins, isolated as putative plg-binding proteins, identified them as staphylococcal inosine 5'-monophosphate dehydrogenase, alpha-enolase, and ribonucleotide reductase subunit 2, respectively.

Proteolysis and its regulation at the surface of Streptococcus pyogenes

Pathogenic bacteria often produce proteinases that are believed to be involved in virulence. Moreover, several host defence systems depend on proteolysis, demonstrating that proteolysis and its regulation play an important role during bacterial infections. Here, we discuss how proteolytical events are regulated at the surface of Streptococcus pyogenes during infection with this important human pathogen. Streptococcus pyogenes produces proteinases, and host proteinases are produced and released as a result of the infection. Streptococcus pyogenes also recruits host proteinase inhibitors to its surface, suggesting that proteolysis is tightly regulated at the bacterial surface. We propose that the initial phase of a S. pyogenes infection is characterized by inhibition of proteolysis and complement activity at the bacterial surface. This is achieved mainly through binding of host proteinase inhibitors and complement regulatory proteins to bacterial surface proteins. In a later phase of the infection, massive proteolytic activity will release bacterial surface proteins and degrade human tissues, thus facilitating bacterial spread. These proteolytic events are regulated both temporally and spatially, and should influence virulence and the outcome of S. pyogenes infections.

Role of streptolysin O in a mouse model of invasive group A streptococcal disease

Many of the virulence factors that have been characterized for group A streptococci (GAS) are not expressed in all clinical isolates. One putative virulence factor that is present among most is streptolysin O (Slo), a protein with well-characterized cytolytic activity for many eukaryotic cells types. In other bacterial pathogens, proteins homologous to Slo have been shown to be essential for virulence, but the role of Slo in GAS had not been previously examined. To investigate the role of Slo in GAS virulence, we examined both revertible and stable slo mutants in a mouse model of invasive disease. When the revertible slo mutant was used to infect mice, the reversion frequency of bacteria isolated from the wounds and spleens of infected animals was more than 100 times that of the inoculum, indicating that there was selective pressure in the animal for Slo(+) GAS. Experiments with the stable slo mutant demonstrated that Slo was not necessary for the formation of necrotic lesions, nor was it necessary for escape from the lesion to cause disseminated infection. Bacteria were present in the spleens of 50% of the mice that survived infection with the stable slo mutant, indicating that dissemination of Slo(-) GAS does not always cause disease. Finally, mice infected with the stable slo mutant exhibited a significant decrease in mortality rates compared to mice infected with wild-type GAS (P < 0.05), indicating that Slo plays an important role in GAS virulence.

The potential role for nephritis-associated plasmin receptor in acute poststreptococcal glomerulonephritis

Immunoglobulin G from a patient convalescing from acute poststreptococcal glomerulonephritis (APSGN) bound specific antigenic sites in early APSGN glomeruli. A streptococcal cytoplasmic antigen (preabsorbing antigen, PA-Ag), could selectively preabsorb fluorescein isothiocyanate (FITC)-labeled IgG and prevented glomerular staining. The antigen was purified and identified as an M(r) approximately 43,000 protein with a pI of 4.7 that strongly activated complement C3 (N. Yoshizawa, S. Oshima, I. Sagel, J. Shimizu, and G. Treser, 1992, J. Immunol. 148, 3110-3116). In the present study, a nephritogenic antigen was purified by affinity chromatography using APSGN IgG-immobilized Sepharose followed by chromatography on an anion-exchange resin. Purification was monitored by ELISA and Western blotting using the binding characteristics of the specific antibodies present in APSGN serum. The molecular weight of the purified antigen, named nephritis-associated plasmin receptor (NAPlr), was an M(r) approximately 43,000 protein and the internal amino acid sequence was found to be homologous to those of the plasmin receptor (Plr) of group A streptococci strain 64/14 and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) from Bacillus subtilis. The purified NAPlr exhibited GAPDH activity and plasmin(ogen) binding activity. Using FITC-labeled rabbit anti-NAPlr, the antigen was found to be present in the glomeruli of 22 of 22 patients in the early stage of APSGN. Bacterial Plr was also demonstrated in human APSGN glomeruli for the first time using monoclonal antibody to the recombinant Plr protein. Antibody to NAPlr was found in the sera of 46 of 50 (92%) patients within 3 months of onset. These results led us to speculate that NAPlr bound to the glomeruli may contribute to the pathogenesis of APSGN via plasmin and complement activation.

Use of the plasminogen activation system by microorganisms

The use of host-derived PAS components by invasive bacteria is an increasingly recognized mechanism for acquisition of extracellular proteolytic activity. This overview summarizes the pertinent contributions to this field and is divided into three parts: (1) the PAS, (2) the interaction of bacteria that produce their own plasminogen activators with the host's PAS, and (3) the interaction of bacteria that do not produce their own plasminogen activators but use plasminogen activators supplied by the host. The significance of these mechanisms in relation to the invasive potentials of the various organisms is discussed.

Crystal structure of streptokinase beta-domain

Streptokinase, a 47 kDa secreted protein of hemolytic strains of streptococci, is a human plasminogen activator and contains three structural domains linked by flexible loops. We describe here the crystal structure of the isolated streptokinase middle (SKbeta) domain determined at 2.4 A resolution. Among the functionally important structural features is a putative binding site for a kringle domain of plasminogen located at the tip of a fully exposed hairpin loop. The distribution of genetically conserved residues of SKbeta is strongly correlated with their functions. The extensive interface of the SKbeta dimer suggests that such dimers may also exist in solution for free SKbeta.

Expression of different group A streptococcal M proteins in an isogenic background demonstrates diversity in adherence to and invasion of eukaryotic cells

The M protein of group A streptococcus (GAS) is considered to be a major virulence factor because it renders GAS resistant to phagocytosis and allows bacterial growth in human blood. There are more than 80 known serotypes of M proteins, and protective opsonic antibodies produced during disease in humans are serotype specific. M proteins also mediate bacterial adherence to epithelial cells of skin and pharynx. GAS strains vary in the genomic organization of the mga regulon, which contains the genes encoding M and M-like proteins and other virulence factors. This diversity of organization makes it difficult to assess virulence of M proteins of different serotypes, unless they can be expressed in an isogenic background. Here, we express M proteins of different serotypes in the M protein- and protein F1-deficient GAS strain, SAM2, which also lacks M-like proteins. Genes encoding M proteins of different serotypes (emmXs) have been integrated into the SAM2 chromosome in frame with the emm6.1 promoter and its mga regulon, resulting in similar levels of emmX expression. Although SAM2 exhibits a very low level of adherence to and invasion of HEp-2 and HaCaT cells, a SAM2-derived strain expressing M6 protein adheres to and invades both cell types. In contrast, the isogenic strain expressing M18 protein adheres to both cell types, but invades with a very low efficiency. A strain expressing M3 protein adheres to both types of cells, but its invasion of HEp-2 cells is serum dependent. A GAS strain expressing M6 protein does not compete with the isogenic strain expressing M18 protein for adherence to or invasion of HaCaT cells. We conclude that M proteins of different serotypes recognize different repertoires of receptors on the surfaces of eukaryotic cells.

Site-directed mutagenesis of streptococcal plasmin receptor protein (Plr) identifies the C-terminal Lys334 as essential for plasmin binding, but mutation of the plr gene does not reduce plasmin binding to group A streptococci

Plasmin(ogen) binding is a common property of many pathogenic bacteria including group A streptococci. Previous analysis of a putative plasmin receptor protein, Plr, from the group A streptococcal strain 64/14 revealed that it is a glyceraldehyde-3-phosphate dehydrogenase and that the plr gene is present on the chromosome as a single copy. This study continues the functional characterization of Plr as a plasmin receptor. Attempts at insertional inactivation of the plr gene suggested that this single-copy gene may be essential for cell viability. Therefore, an alternative strategy was applied to manipulate this gene in vivo. Site-directed mutagenesis of Plr revealed that a C-terminal lysyl residue is required for wild-type levels of plasmin binding. Mutated Plr proteins expressed in Escherichia coli demonstrated reduced plasmin-binding activity yet retained glyceraldehyde-3-phosphate dehydrogenase activity. A novel integration vector was constructed to precisely replace the wild-type copy of the plr gene with these mutations. Isogenic streptococcal strains expressing altered Plr bound equivalent amounts of plasmin as wild-type streptococci. These data suggest that Plr does not function as a unique plasmin receptor, and underscore the need to identify other plasmin-binding structures on group A streptococci and to assess the importance of the plasminogen system in pathogenesis by inactivation of plasminogen activators and the use of appropriate animal models.

Interaction of a group A Streptococcus within human plasma results in assembly of a surface plasminogen activator that contributes to occupancy of surface plasmin-binding structures

Group A streptococcal isolate 187061 incubated in human plasma or serum reconstituted with fibrinogen but not plasminogen-depleted plasma or serum alone acquired a surface plasminogen activator activity. Assembly of the surface plasminogen activator was inhibited by the presence of neutralizing antibodies to streptokinase. Once assembled, the bacterial-associated plasminogen activator could generate plasmin when incubated in human plasminogen, plasmin or serum which could bind to bacterial surface plasmin-binding structures despite the presence of host physiological inhibitors. These studies provide evidence that the pathways by which group A isolates interact with human plasmin(ogen) are potentially linked and may provide a mechanism for bacteria to acquire host enzymatic activity efficiently in the infected host.

The interaction of Streptococcus dysgalactiae with plasmin and plasminogen

The activation of plasminogen and the binding of plasmin by bacteria may have many effects which promote infection. The occurrence of such activities in streptococci is well documented; however, these are yet to be demonstrated for S. dysgalactiae. Consequently, the ability of this bacterium to activate mammalian plasminogen and bind either plasmin or its zymogen was investigated. Activation of bovine plasminogen was dependent on both the strain and the growth medium used for cultivation. Eighteen strain were able to activate bovine and ovine plasminogen and some of these also activated plasminogen from the horse, rabbit and pig. None activated human plasminogen and one strain (CE127) did not activate plasminogen from any source. Tricine-SDS PAGE and zymographic analysis of culture supernatants showed that bovine plasminogen was activated by four out of six strains at two locations corresponding to 16 kDa and 10 kDa. Following the growth of five strains in the presence of bovine plasminogen, all but strain CE127 bound high levels of plasmin activity. In contrast, following growth in human plasminogen none of the strains exhibited bound plasmin activity although all could bind human plasmin directly. All strains were also able to bind bovine and human plasminogen in such a way as to allow its activation by urokinase. We conclude that S. dysgalactiae is capable of activating mammalian plasminogen in a species-specific fashion and that the bacterium is also capable of binding plasmin and plasminogen with an apparent preference for bovine plasmin over human plasmin and/or plasminogen from either species.

Molecular co-operation between protein PAM and streptokinase for plasmin acquisition by Streptococcus pyogenes

Bacterial surface-associated plasmin formation is believed to contribute to invasion, although the underlying molecular mechanisms are poorly understood. To define the components necessary for plasmin generation on group A streptococci we used strain AP53 which exposes an M-like protein ("PAM") that contains a plasminogen-binding sequence with two 13-amino acid residues long tandem repeats (a1 and a2). Utilizing an Escherichia coli-streptococcal shuttle vector, we replaced a 29-residue long sequence segment of Arp4, an M-like protein that does not bind plasminogen, with a single (a1) or the combined a1a2 repeats of PAM. When expressed in E. coli, the purified chimeric Arp/PAM proteins both bound plasminogen, as well as plasmin, and when used to transform group A streptococcal strains lacking the plasminogen-binding ability, transformants with the Arp/PAM constructs efficiently bound plasminogen. Moreover, when grown in the presence of plasminogen, both Arp/PAM- and PAM-expressing streptococci acquired surface-bound plasmin. In contrast, plasminogen activation failed to occur on PAM- and Arp/PAM-expressing streptococci carrying an inactivated streptokinase gene: this block was overcome by exogenous streptokinase. Together, these results provide evidence for an unusual co-operation between a surface-bound protein, PAM, and a secreted protein, streptokinase, resulting in bacterial acquisition of a host protease that is likely to spur parasite invasion of host tissues.

Group A streptococcal isolate 64/14 expresses surface plasmin-binding structures in addition to Plr

A recombinant plasmin receptor (Plr) gene product originally cloned from group A streptococcal isolate 64/14 was analysed for its ability to bind plasmin(ogen) and to account for all the surface plasmin-binding properties of streptococcal isolate 64/14. Functional analysis of recombinant Plr demonstrated that the protein exhibited equal reactivity with human Lys-plasmin and Lys-plasminogen, but significantly lower reactivity with Glu-plasminogen. Plasmin-binding was both inhibitable and elutable by lysine or lysine analogs, and active plasmin bound to recombinant Plr was not neutralized by alpha 2-antiplasmin. Thus, the plasmin-binding properties of recombinant Plr correlated with the plasmin-binding phenotype of the intact streptococcal isolate 64/14. In addition, fluid-phase recombinant Plr could completely inhibit binding of plasmin to either immobilized recombinant Plr or group A streptococcal isolate 64/14 with equal efficiency, indicating that surface-expressed Plr could account for all the plasmin-binding properties of the intact organism. An IgM monoclonal antibody to recombinant Plr that specifically recognized a surface structure on streptococcal isolate 64/14 significantly inhibited the binding of plasmin to the recombinant protein; however, the antibody was not successful at inhibiting plasmin-binding to the intact bacteria, indicating the presence of other plasmin-binding structures on the bacterial surface in addition to Plr.

Staphylococcus aureus strains differ in their in vitro responsiveness to human urokinase: evidence that methicillin-resistant strains are predominately nonresponsive to the growth-enhancing effects of urokinase

Clinical isolates of Staphylococcus aureus were found to exhibit strain-specific heterogeneity to the growth-enhancing effects of human urokinase (UK), a proteinase with plasminogen activator activity. Nine out of fourteen (64%) methicillin-sensitive strains of S. aureus were responsive to UK in "in vitro" cultures. In contrast, 3/29 (10%) methicillin-resistant strains were responsive to the proteinase. When only strains isolated from western Canada were considered, 6/11 methicillin-sensitive strains and 1/26 methicillin-resistant strains were responsive to UK. The single western Canadian methicillin-resistant strain (strain 456) responsive to UK was one of two isolated from the same patient, indicating that the two strains were phenotypically different. Strain 456, resistant to 32 micrograms methicillin/mL, was responsive to as little as 50 U UK/mL and enhancement of growth was evident by 9 h of incubation at 37 degrees C. This growth enhancement was specific to UK and not duplicated by equivalent concentrations of other proteins (bovine serum albumin, trypsin, plasminogen). The results presented indicate differences in the frequency of the UK-responsive phenotype between methicillin-sensitive and -resistant S. aureus. These findings indicate that the UK phenotype of S. aureus may have utility in both phenotyping clinical isolates, as well as providing insights into the regulation of growth in this clinically important organism.

Epidermal growth factor-binding protein in Mycobacterium avium and Mycobacterium tuberculosis: a possible role in the mechanism of infection

Epidermal growth factor (EGF) is a potent mitogen for a variety of eukaryotic cells. EGF is found in a number of tissues and is prevalent in necrotic tissues and granulomata. The biological effect of EGF on mammalian cells is initiated by the binding to a specific receptor. Both Mycobacterium avium and Mycobacterium tuberculosis cause lung infections and localized or disseminated disease in both patients without AIDS and those with AIDS. Histopathologic studies show necrosis in the lung, liver, and splenic tissues of patients with disseminated mycobacterial infection. In the course of experiments to examine the effect of growth factors on macrophages, it was observed that M. avium and M. tuberculosis but not Mycobacterium smegmatis cultured in the presence of 5, 50, or 500 ng of EGF per ml grew significantly faster than mycobacteria cultured in the absence of EGF. 125I-EGF was found to bind to M. avium and M. tuberculosis, and the binding was competitively inhibited by unlabeled EGF. A receptor for EGF was identified on mycobacteria. Incubation of mycobacteria with EGF prior to infection of macrophage monolayers resulted in faster bacterial growth within macrophages compared with that of mycobacteria not incubated with EGF. EGF-binding protein was cloned and expressed in Escherichia coli, and subsequently the protein was purified and the N-terminal amino acids were sequenced. These results suggest that EGF is a growth factor for pathogenic mycobacteria in granulomatous tissues and within macrophages and might enhance growth rates of both intracellular and extracellular mycobacteria in the site of infection.

Cloning, sequencing and functional overexpression of the Streptococcus equisimilis H46A gapC gene encoding a glyceraldehyde-3-phosphate dehydrogenase that also functions as a plasmin(ogen)-binding protein. Purification and biochemical characterization of the protein

We previously identified DNA sequences involved in the function of the complex promoter of the streptokinase gene from Streptococcus equisimilis H46A, a human serogroup C strain known to express this gene at a high level. As a prerequisite to understanding possible mechanisms that control the balance between the plasminogen activating and plasmin(ogen) binding capacities of H46A, we describe here its gapC gene encoding glyceraldehyde-3-phosphate dehydrogenase (GraP-DH, EC 1.2.1.12), a glycolytic enzyme apparently transported to the cell surface where it functions as a plasmin(ogen).binding protein. The gapC gene was cloned and sequenced and found to code for a 336-amino-acid polypeptide (approximately 35.9 kDa) exhibiting 94.9% sequence identity to the Plr protein from Streptococcus pyogenes shown by others to be capable of plasmin binding [Lottenberg, R., Broder, C. C., Boyle, M. D., Kain, S. J., Schroeder, B. L. & Curtiss, R. III (1992) J. Bacteriol. 174, 5204-5210]. To study the properties of the GapC protein, its gene was inducibly overexpressed in Escherichia coli from QIAexpress expression plasmids to yield the authentic GapC or (His)6GapC carrying a hexahistidyl N-terminus to permit affinity purification. Both proteins were functionally active, exhibiting specific GraP-DH activities of about 80 kat/mol (approximately 130 U/mg) after purification. Their binding parameters [association (ka) and dissociation (kd) rate constants, and equlibrium dissociation constants (Kd = kd/ka)] for the interaction with human Gluplasminogen and plasmin were determined by real-time biospecific interaction analysis using the Pharmacia BIAcore instrument. For comparative purposes, the commercial GraP-DH from Bacillus stearothermophilus (BstGraP-DH), a nonpathogenic organism, was included in these experiments. The Kd values for binding of plasminogen to GapC, (His)6GapC and BstGraP-DH were 220 nM, 260 nM and 520 nM, respectively, as compared to 25 nM, 17 nM and 98 nM, respectively, for the binding to plasmin. These data show that both the zymogen and active enzyme possess low-affinity binding sites for the gapC gene product and that the hexahistidyl terminus does not affect its function. Prior limited treatment with plasmin enhanced the subsequent plasminogen binding capacity of all three GraP-DHs, presumably by the exposure of new C-terminal lysine residues for binding to the zymogen.

Iron starvation causes release from the group A streptococcus of the ADP-ribosylating protein called plasmin receptor or surface glyceraldehyde-3-phosphate-dehydrogenase

In many pathogenic bacteria, iron starvation serves as an environmental signal that triggers the expression of virulence factors, many of which are found on the cell surface or secreted into the culture supernatant. Using the chelating agent nitrilotriacetic acid, we have established conditions for iron starvation of the important human pathogen Streptococcus pyogenes (the group A streptococcus) and determined that iron limitation results in the specific appearance of several new proteins in the culture supernatant. One of these supernatant proteins is the ADP-ribosylating protein known as streptococcal plasmin receptor (Plr) or as the streptococcal surface glyceraldehyde-3-phosphate-dehydrogenase because of its other activities. Upon iron starvation, Plr is specifically released into the culture supernatant in a time-dependent manner, and its appearance in the supernatant is not accompanied by induction of plr mRNA synthesis. Release of Plr from the bacteria may be important for the virulence of group A streptococci and the manifestation of diseases.

Plasmin stimulates expression of endothelin-1 mRNA and endothelin-1 release in vascular endothelial cells

Incubation of cultured porcine aortic endothelial cells (ECs) with plasmin resulted in a significant and concentration-dependent increase in endothelin-1 (ET-1) release from the cells. This increasing effect was completely inhibited by aprotinin but not by tranexamic acid, thereby suggesting that the plasmin-induced stimulation of ET-1 release requires the catalytic site but not the lysine binding site, in plasmin molecule. Plasmin stimulated the expression of prepro ET-1 mRNA in ECs. Actinomycin D chase experiments suggested that enhanced stability of ET-T mRNA could not account for the above plasmin-induced stimulation. It is likely that plasmin potentiates the endothelial ET-1 production, probably by the stimulation of ET-1 gene transcription. It remains to be seen whether the endothelial ET-1 production is enhanced after the thrombolytic therapy.

Analysis of the interaction of group A streptococci with fibrinogen, streptokinase and plasminogen

Group A streptococci demonstrate a number of distinct ways to interact with the human fibrinolytic system to acquire unregulatable cell-surface enzymatic activity. Interactions between bacteria, fibrinogen, streptokinase and plasminogen resulted in acquisition of cell-associated enzymatic activity that can lyse fibrin clots despite the presence of the major physiological plasmin inhibitor, alpha 2-antiplasmin. Western blot analysis of extracted streptococcal surface proteins suggested that binding of fibrinogen to M or M-related proteins mediated the capture of streptokinase-plasminogen complexes to the bacteria. The enzymatic complex formed by reaction of bacteria with fibrinogen, streptokinase and plasminogen was found to be more stable in human plasma than pre-formed plasmin bound directly to the same bacteria strain.

Streptokinase activity among group A streptococci in relation to streptokinase genotype, plasminogen binding, and disease manifestations

Certain genotypic variants of streptokinase (ska) of beta-hemolytic streptococci group A have been associated with acute post-streptococcal glomerulonephritis (APSGN). In our earlier studies on strains isolated from Ethiopian children with various streptococcal disease manifestation, we reported an even distribution of streptokinase genotypes with no association to disease patterns. Considering the possibility that strains could differ in their ability to secrete the protein, levels of streptokinase activity in culture supernatants of these strains were determined by a plasminogen activation assay using a synthetic tripeptide, H-D-valyl-leucyl-lysin-p-nitroaniline, as a substrate. Of the 53 streptococcal group A strains, ten (19%), which belonged to genotype ska4 and ska8, did not activate human plasminogen. These strains did not activate bovine, sheep, horse, rabbit or porcine plasminogens either. They represented at least five M protein and non-typeable serotypes, and were characterized by high human plasminogen binding activity. Six of the 53 strains (11%) harbouring genotype ska3 and ska7 showed low levels of human plasminogen activation. Strains of ska1 and ska2, 37/53, activated human plasminogen at a higher level (p < 0.005). Levels of plasminogen activation were not significantly different among the ska1 and ska2 strains associated with various streptococcal disease manifestations. Antibody levels against streptokinase were higher (p < 0.05) in convalescent sera from acute rheumatic fever and APSGN patients in comparison with sera from other patient categories and healthy controls. Streptokinase genotype and in vitro streptokinase production do not correlate directly to streptococcal disease manifestation, indicating a probable significance of additional streptococcal and/or host factors in the initiation of APSGN.

Plasminogen, absorbed by Escherichia coli expressing curli or by Salmonella enteritidis expressing thin aggregative fimbriae, can be activated by simultaneously captured tissue-type plasminogen activator (t-PA)

Curli are fimbrial structures expressed by Escherichia coli that specifically interact with matrix proteins such as fibronectin and laminin. Similar structures are also expressed by Salmonella enteritidis and have been denoted thin aggregative fimbriae. Bacteria expressing curli and thin aggregative fimbriae were found to bind radiolabelled plasminogen as well as the tissue-type plasminogen activator (t-PA). By contrast, E. coli carrying a gene locus with an insertionally inactivated chromosomal curlin subunit were unable to bind the two human proteins. The purified subunit polypeptides of curli and thin aggregative fimbriae bound plasminogen and t-PA with high affinity (1 x 10(8) to 2 x 10(8) M-1). The binding of plasminogen and t-PA to curli-expressing E. coli was only partially inhibited by fibronectin and laminin. Plasminogen absorbed from human plasma by curli-expressing E. coli was readily converted to plasmin by t-PA; both plasmin and t-PA were functionally active when bound to the bacteria. A simultaneous binding of fibrinolytic proteins and matrix proteins to fimbriae of E. coli and S. enteritidis could provide these pathogens with both adhesive and invasive properties.

Capturing host plasmin(ogen): a common mechanism for invasive pathogens?

Plasmin is a potent enzyme that can dissolve blood clots and degrade extracellular matrix proteins. A number of pathogenic bacteria produce plasminogen activators. Many of these organisms can also bind plasmin(ogen) to surface receptors and protect the active enzyme from physiological inhibition. Cell-surface localization of plasmin may be a common mechanism used by bacteria to facilitate movement through normal tissue barriers.

Association between expression of immunoglobulin G-binding proteins by group A streptococci and virulence in a mouse skin infection model

In this study, we developed a mouse model of skin infection to test the association between expression of immunoglobulin-binding proteins by and infectivity of group A streptococci. Group A streptococci capable of crossing tissue barriers and establishing a lethal systemic infection in mice showed a higher level of immunoglobulin-binding protein expression. The group A streptococci recovered from the spleen of a mouse that died following a skin infection were found to be more virulent when injected into the skin of naive mice. Together, these results suggest that immunoglobulin-binding protein expression by group A streptococci correlates with their ability to establish invasive skin infections.

Polymorphism of the streptokinase gene: implications for the pathogenesis of post-streptococcal glomerulonephritis

Recent studies of streptokinase genes from epidemiologically and clinically defined streptococci of groups A, C and G have provided evidence of the polymorphism of the streptokinase locus in the chromosome of pathogenic streptococci. This review considers genetic and pathogenetic data suggesting that there exists a causal relationship between nephritis strain-associated streptokinase production and the initial stages of post-streptococcal glomerulonephritis (PSGN). Currently available sequence information allows to recognize, in the middle of the streptokinase molecule, a major variable region, V1, of about 70 amino acid residues in which sequence identity drops to below 50% when the proteins from nephritogenic and non-nephritogenic strains are compared. The V1 regions, although showing microheterogeneity within either protein category, appear to be more hydrophobic and possess a higher content of ordered secondary structures in the "nephritogenic" molecules. As a working hypothesis, they may be considered the nephrotropic domain(s) with which streptokinases from nephritogenic strains bind to glomerular structures and activate plasminogen in situ, thus triggering the cascade of proteolytic processes leading to PSGN.

Human plasmin induces a receptor-mediated arachidonate release coupled with G proteins in endothelial cells

Treatment of cultured bovine carotid artery endothelial cells with 10(-7) M plasmin increased arachidonate release coupled with the increase in prostacyclin production. The stimulatory effect of plasmin on arachidonate release could be divided into the early and late phases according to its calcium dependency and pertussis toxin sensitivity. The early phase of plasmin-induced arachidonate release was a calcium-dependent and pertussis toxin-sensitive response, which was observed within 20 min after plasmin treatment. The late phase was a calcium-independent and pertussis toxin-insensitive response, which was induced gradually from 20 to 60 min. Induction of the early phase of plasmin's effect required both the lysine binding and catalytic sites in plasmin molecule because it was inhibited either by the binding antagonist tranexamic acid or by the serine protease inhibitor aprotinin. Guanosine 5'-O-(2-thiotriphosphate) potentiated the effect of plasmin in permeabilized or nonpermeabilized cells, indicating that the early phase effect was mediated by a pertussis toxin-sensitive guanosine 5'-triphosphate (GTP)-binding protein. The late phase of plasmin's effect was due to the catalytic activity because it was inhibited by aprotinin but not by tranexamic acid. Microplasmin structurally having the catalytic sites induced a similar late phase effect. Plasmin did not elicit the metabolism of phosphatidyl polyphosphoinositides. These studies demonstrate that the activation of phospholipase A2, which results in arachidonate release, in the early phase of plasmin's effect is a receptor-mediation via GTP-binding protein that is not coupled through phospholipase C activation.

Cloning, sequence analysis, and expression in Escherichia coli of a streptococcal plasmin receptor

Plasmin(ogen) receptors are expressed by many gram-positive and gram-negative bacteria. We previously isolated a plasmin receptor from a pathogenic group A streptococcal strain (C. C. Broder, R. Lottenberg, G. O. von Mering, K. H. Johnston, and M. D. P. Boyle, J. Biol. Chem. 266:4922-4928, 1991). The gene encoding this plasmin receptor, plr, was isolated from a lambda gt11 library of chromosomal DNA from group A streptococcal strain 64/14 by screening plaques with antibodies raised against the purified streptococcal plasmin receptor protein. The gene was subcloned by using a low-copy-number plasmid and stably expressed in Escherichia coli, resulting in the production of an immunoreactive and functional receptor protein. The DNA sequence of the gene contained an open reading frame encoding 335 amino acids with a predicted molecular weight of 35,787. Upstream of the open reading frame, putative promoter and ribosomal binding site sequences were identified. The experimentally derived amino acid sequences of the N terminus and three cyanogen bromide fragments of the purified streptococcal plasmin receptor protein corresponded to the predicted sequence encoded by plr. The deduced amino acid sequence for the plasmin receptor protein revealed significant similarity (39 to 54% identical amino acid residues) to glyceraldehyde 3-phosphate dehydrogenases.