Maturation processing and characterization of streptopain

RopB-regulated SpeB cysteine protease degrades extracellular vesicles-associated streptolysin O and bacterial proteins from group A Streptococcus

Extracellular vesicles (EVs) can be released from gram-positive bacteria and would participate in the delivery of bacterial toxins. Streptococcus pyogenes (group A Streptococcus, GAS) is one of the most common pathogens of monomicrobial necrotizing fasciitis. Spontaneous inactivating mutation in the CovR/CovS two-component regulatory system is related to the increase of EVs production via an unknown mechanism. This study aimed to investigate whether the CovR/CovS-regulated RopB, the transcriptional regulator of GAS exoproteins, would participate in regulating EVs production. Results showed that the size, morphology, and number of EVs released from the wild-type strain and the ropB mutant were similar, suggesting RopB is not involved in controlling EVs production. Nonetheless, RopB-regulated SpeB protease degrades streptolysin O and bacterial proteins in EVs. Although SpeB has crucial roles in modulating protein composition in EVs, the SpeB-positive EVs failed to trigger HaCaT keratinocytes pyroptosis, suggesting that EVs did not deliver SpeB into keratinocytes or the amount of SpeB in EVs was not sufficient to trigger cell pyroptosis. Finally, we identified that EV-associated enolase was resistant to SpeB degradation, and therefore could be utilized as the internal control protein for verifying SLO degradation. This study revealed that RopB would participate in modulating protein composition in EVs via SpeB-dependent protein degradation and suggested that enolase is a potential internal marker for studying GAS EVs.

Group A Streptococcus Vaccine Targeting the Erythrogenic Toxins SpeA and SpeB Is Safe and Immunogenic in Rabbits and Does Not Induce Antibodies Associated with Autoimmunity

Group A streptococcus (GAS) is a global pathogen associated with significant morbidity and mortality for which there is currently no licensed vaccine. Vaccine development has been slow, mostly due to safety concerns regarding streptococcal antigens associated with autoimmunity and related complications. For a GAS vaccine to be safe, it must be ensured that the antigens used in the vaccine do not elicit an antibody response that can cross-react with host tissues. In this study, we evaluated the safety of our GAS vaccine candidate called VaxiStrep in New Zealand White rabbits. VaxiStrep is a recombinant fusion protein comprised of streptococcal pyrogenic exotoxin A (SpeA) and exotoxin B (SpeB), also known as erythrogenic toxins, adsorbed to an aluminum adjuvant. The vaccine elicited a robust immune response against the two toxins in the rabbits without any adverse events or toxicity. No signs of autoimmune pathology were detected in the rabbits' brains, hearts, and kidneys via immunohistochemistry, and serum antibodies did not cross-react with cardiac or neuronal tissue proteins associated with rheumatic heart disease or Sydenham chorea (SC). This study further confirms that VaxiStrep does not elicit autoantibodies and is safe to be tested in a first-in-human trial.

RopB represses the transcription of speB in the absence of SIP in group A Streptococcus

RopB is a quorum-sensing regulator that binds to the SpeB-inducing peptide (SIP) under acidic conditions. SIP is known to be degraded by the endopeptidase PepO, whose transcription is repressed by the CovR/CovS two-component regulatory system. Both SIP-bound RopB (RopB-SIP) and SIP-free RopB (apo-RopB) can bind to the speB promoter; however, only RopB-SIP activates speB transcription. In this study, we found that the SpeB expression was higher in the ropB mutant than in the SIP-inactivated (SIP*) mutant. Furthermore, the deletion of ropB in the SIP* mutant derepressed speB expression, suggesting that apo-RopB is a transcriptional repressor of speB Up-regulation of PepO in the covS mutant degraded SIP, resulting in the down-regulation of speB We demonstrate that deleting ropB in the covS mutant derepressed the speB expression, suggesting that the speB repression in this mutant was mediated not only by PepO-dependent SIP degradation but also by apo-RopB. These findings reveal a crosstalk between the CovR/CovS and RopB-SIP systems and redefine the role of RopB in regulating speB expression in group A Streptococcus.

C10Pred: A First Machine Learning Based Tool to Predict C10 Family Cysteine Peptidases Using Sequence-Derived Features

Streptococcus pyogenes, or group A Streptococcus (GAS), a gram-positive bacterium, is implicated in a wide range of clinical manifestations and life-threatening diseases. One of the key virulence factors of GAS is streptopain, a C10 family cysteine peptidase. Since its discovery, various homologs of streptopain have been reported from other bacterial species. With the increased affordability of sequencing, a significant increase in the number of potential C10 family-like sequences in the public databases is anticipated, posing a challenge in classifying such sequences. Sequence-similarity-based tools are the methods of choice to identify such streptopain-like sequences. However, these methods depend on some level of sequence similarity between the existing C10 family and the target sequences. Therefore, in this work, we propose a novel predictor, C10Pred, for the prediction of C10 peptidases using sequence-derived optimal features. C10Pred is a support vector machine (SVM) based model which is efficient in predicting C10 enzymes with an overall accuracy of 92.7% and Matthews’ correlation coefficient (MCC) value of 0.855 when tested on an independent dataset. We anticipate that C10Pred will serve as a handy tool to classify novel streptopain-like proteins belonging to the C10 family and offer essential information.

Proteolytic Profiling of Streptococcal Pyrogenic Exotoxin B (SpeB) by Complementary HPLC-MS Approaches

Streptococcal pyrogenic exotoxin B (SpeB) is a cysteine protease expressed during group A streptococcal infection that represents a major virulence factor. Although subject to several studies, its role during infection is still under debate, and its proteolytic properties remain insufficiently characterized. Here, we revisited this protease through a set of complementary approaches relying on state of-the-art HPLC-MS methods. After conceiving an efficient protocol to recombinantly express SpeB, the zymogen of the protease and its activation were characterized. Employing proteome-derived peptide libraries, a strong preference for hydrophobic and aromatic residues at P2 alongside negatively charged amino acids at P3′ to P6′ was revealed. To identify relevant in vivo substrates, native proteins were obtained from monocytic secretome and plasma to assess their cleavage under physiological conditions. Besides corroborating our findings concerning specificity, more than 200 cleaved proteins were identified, including proteins of the extracellular matrix, proteins of the immune system, and proteins involved in inflammation. Finally, the cleavage of IgG subclasses was studied in detail. This study precisely depicts the proteolytic properties of SpeB and provides a library of potential host substrates, including their exact cleavage positions, as a valuable source for further research to unravel the role of SpeB during streptococcal infection.

An Irreversible Inhibitor to Probe the Role of Streptococcus pyogenes Cysteine Protease SpeB in Evasion of Host Complement Defenses

Members of the CA class of cysteine proteases have multifaceted roles in physiology and virulence for many bacteria. Streptococcal pyrogenic exotoxin B (SpeB) is secreted by Streptococcus pyogenes and implicated in the pathogenesis of the bacterium through degradation of key human immune effector proteins. Here, we developed and characterized a clickable inhibitor, 2S-alkyne, based on X-ray crystallographic analysis and structure-activity relationships. Our SpeB probe showed irreversible enzyme inhibition in biochemical assays and labeled endogenous SpeB in cultured S. pyogenes supernatants. Importantly, application of 2S-alkyne decreased S. pyogenes survival in the presence of human neutrophils and supports the role of SpeB-mediated proteolysis as a mechanism to limit complement-mediated host defense. We posit that our SpeB inhibitor will be a useful chemical tool to regulate, label, and quantitate secreted cysteine proteases with SpeB-like activity in complex biological samples and a lead candidate for new therapeutics designed to sensitize S. pyogenes to host immune clearance.

The Streptococcal Protease SpeB Antagonizes the Biofilms of the Human Pathogen Staphylococcus aureus USA300 through Cleavage of the Staphylococcal SdrC Protein

Streptococcus pyogenes, or group A Streptococcus (GAS), is both a pathogen and an asymptomatic colonizer of human hosts and produces a large number of surface-expressed and secreted factors that contribute to a variety of infection outcomes. The GAS-secreted cysteine protease SpeB has been well studied for its effects on the human host; however, despite its broad proteolytic activity, studies on how this factor is utilized in polymicrobial environments are lacking. Here, we utilized various forms of SpeB protease to evaluate its antimicrobial and antibiofilm properties against the clinically important human colonizer Staphylococcus aureus, which occupies niches similar to those of GAS. For our investigation, we used a skin-tropic GAS strain, AP53CovS+, and its isogenic ΔspeB mutant to compare the production and activity of native SpeB protease. We also generated active and inactive forms of recombinant purified SpeB for functional studies. We demonstrate that SpeB exhibits potent biofilm disruption activity at multiple stages of S. aureus biofilm formation. We hypothesized that the surface-expressed adhesin SdrC in S. aureus was cleaved by SpeB, which contributed to the observed biofilm disruption. Indeed, we found that SpeB cleaved recombinant SdrC in vitro and in the context of the full S. aureus biofilm. Our results suggest an understudied role for the broadly proteolytic SpeB as an important factor for GAS colonization and competition with other microorganisms in its niche.IMPORTANCEStreptococcus pyogenes (GAS) causes a range of diseases in humans, ranging from mild to severe, and produces many virulence factors in order to be a successful pathogen. One factor produced by many GAS strains is the protease SpeB, which has been studied for its ability to cleave and degrade human proteins, an important factor in GAS pathogenesis. An understudied aspect of SpeB is the manner in which its broad proteolytic activity affects other microorganisms that co-occupy niches similar to that of GAS. The significance of the research reported herein is the demonstration that SpeB can degrade the biofilms of the human pathogen Staphylococcus aureus, which has important implications for how SpeB may be utilized by GAS to successfully compete in a polymicrobial environment.

Cytotoxicity and Survival Fitness of Invasive covS Mutant of Group A Streptococcus in Phagocytic Cells

Group A streptococci (GAS) with spontaneous mutations in the CovR/CovS regulatory system are more invasive and related to severe manifestations. GAS can replicate inside phagocytic cells; therefore, phagocytic cells could serve as the niche to select invasive covS mutants. Nonetheless, the encapsulated covS mutant is resistant to phagocytosis. The fate of intracellular covS mutant in phagocytic cells and whether the intracellular covS mutant contributes to invasive infections are unclear. In this study, capsule-deficient (cap^-) strains were utilized to study how intracellular bacteria interacted with phagocytic cells. Results from the competitive infection model showed that the cap^-covS mutant had better survival fitness than the cap^- wild-type strain in the PMA-activated U937 cells. In addition, the cap^-covS mutant caused more cell damages than the cap^- wild-type strain and encapsulated covS mutant. Furthermore, treatments with infected cells with clindamycin to inhibit the intracellular bacteria growth was more effective to reduce bacterial toxicity than utilized penicillin to kill the extracellular bacteria. These results not only suggest that the covS mutant could be selected from the intracellular niche of phagocytic cells but also indicating that inactivating or killing intracellular GAS may be critical to prevent invasive infection.

Deletion of the L-Lactate Dehydrogenase Gene ldh in Streptococcus pyogenes Leads to a Loss of SpeB Activity and a Hypovirulent Phenotype

Streptococcus pyogenes uses lactic acid fermentation for the generation of ATP. Here, we analyzed the impact of a deletion of the L-lactate dehydrogenase gene ldh on the virulence of S. pyogenes M49. While the ldh deletion does not cause a general growth deficiency in laboratory media, the growth in human blood and plasma is significantly hampered. The ldh deletion strain is furthermore less virulent in a Galleria mellonella infection model. We show that the ldh deletion leads to a decrease in the activity of the cysteine protease SpeB, an important secreted virulence factor of S. pyogenes. The reduced SpeB activity is caused by a hampered autocatalytic activation of the SpeB zymogen into the mature SpeB. The missing SpeB activity furthermore leads to increased plasmin activation and a reduced activation of the contact system on the surface of S. pyogenes. All these effects can be reversed when ldh is reintroduced into the mutant via a plasmid. The results demonstrate a previously unappreciated role for LDH in modulation of SpeB maturation.

Comparative pan genome analysis of oral Prevotella species implicated in periodontitis

Prevotella is part of the oral bacterial community implicated in periodontitis. Pan genome analyses of eight oral Prevotella species, P. dentalis, P. enoeca, P. fusca, P. melaninogenica, P. denticola, P. intermedia 17, P. intermedia 17-2 and P. sp. oral taxon 299 are presented in this study. Analysis of the Prevotella pan genome revealed features such as secretion systems, resistance to oxidative stress and clustered regularly interspaced short palindromic repeat (CRISPR)-Cas systems that enable the bacteria to adapt to the oral environment. We identified the presence of type VI secretion system (T6SS) in P. fusca and P. intermedia strains. For some VgrG and Hcp proteins which were not part of the core T6SS loci, we used gene neighborhood analysis and identified putative effector proteins and putative polyimmunity loci in P. fusca and polymorphic toxin systems in P. intermedia strains. Earlier studies have identified the presence of Por secretion system (PorSS) in P. gingivalis, P. melaninogenica and P. intermedia. We noted the presence of their homologs in six other oral Prevotella studied here. We suggest that in Prevotella, PorSS is used to secrete cysteine proteases such as interpain and C-terminal domain containing proteins with a "Por_secre_tail" domain. We identified subtype I-B CRISPR-Cas system in P. enoeca. Putative CRISPR-Cas system subtypes for 37 oral Prevotella and 30 non-oral Prevotella species were also predicted. Further, we performed a BLASTp search of the Prevotella proteins which are also conserved in the red-complex pathogens, against the human proteome to identify potential broad-spectrum drug targets. In summary, the use of a pan genome approach enabled identification of secretion systems and defense mechanisms in Prevotella that confer adaptation to the oral cavity.

Streptococcal pyrogenic exotoxin B inhibits apoptotic cell clearance by macrophages through protein S cleavage

Clearance of apoptotic cells by macrophages plays an important role in maintaining tissue homeostasis. Previous study indicated that streptococcal pyrogenic exotoxin B (SPE B) reduces phagocytic activity in group A streptococcus (GAS) infection. Here, we demonstrate that SPE B causes an inhibitory effect on protein S-mediated phagocytosis. In the presence of SPE B, serum- and purified protein S-mediated phagocytosis of apoptotic cells were significantly inhibited. The binding abilities of protein S to apoptotic cells were decreased by treatment with SPE B. Bacterial culture supernatants from GAS NZ131 strain also caused a reduction of protein S binding to apoptotic cells, but speB mutant strain did not. SPE B directly cleaved protein S in vitro and in vivo, whereas a lower level of cleavage occurred in mice infected with a speB isogenic mutant strain. SPE B-mediated initial cleavage of protein S caused a disruption of phagocytosis, and also resulted in a loss of binding ability of protein S-associated C4b-binding protein to apoptotic cells. Taken together, these results suggest a novel pathogenic role of SPE B that initiates protein S degradation followed by the inhibition of apoptotic cell clearance by macrophages.

Inhibition of streptococcal pyrogenic exotoxin B using allicin from garlic

Streptococcal pyrogenic exotoxin B (SpeB) is an important virulence factor of group A streptococci (GAS) and inactivation of SpeB results in the significantly decreased virulence of the bacterium. The protein is secreted as an inactive zymogen of 40 KDa (SpeBz) and undergoes proteolytic truncation to result in a 28 KDa mature active protease (SpeBm). In this study the effect of allicin on the proteolytic activity of SpeBm was evaluated using azocasein assay. Allicin neutralized the SpeBm proteolytic activity in a concentration dependent manner (IC50 = 15.71 ± 0.45 μg/ml). The loss of activity was completely reversed by subsequent treatment with a reducing agent, dithiothreitol (DTT; 10 mM final concentration), suggesting that allicin likely inhibits the SpeBm by forming a disulfide linkage with an active thiol group in its active site. This mechanism of action was further confirmed with the fact that DTT did not reverse the SpeBm activity in the presence of E-64, a cysteine protease-specific inhibitor, which works specially by forming a thioether linkage with free sulfhydryl groups in enzymes active site. The MIC of allicin against GAS was found to be 32 μg/ml. Exposure of GAS culture to allicin (25 μg/ml) inhibited maturation of SpeBz to the SpeBm. In conclusion, the results of this study suggest that allicin inhibits the maturation of SpeBz and proteolytic activity of SpeBm and could be a potential therapeutic agent for the treatment of GAS infections.

Highly efficient recombinant production and purification of streptococcal cysteine protease streptopain with increased enzymatic activity

Streptococcus pyogenes produces the cysteine protease streptopain (SpeB) as a critical virulence factor for pathogenesis. Despite having first been described seventy years ago, this protease still holds mysteries which are being investigated today. Streptopain can cleave a wide range of human proteins, including immunoglobulins, the complement activation system, chemokines, and structural proteins. Due to the broad activity of streptopain, it has been challenging to elucidate the functional results of its action and precise mechanisms for its contribution to S. pyogenes pathogenesis. To better study streptopain, several expression and purification schemes have been developed. These methods originally involved isolation from S. pyogenes culture but were more recently expanded to include recombinant Escherichia coli expression systems. While substantially easier to implement, the latter recombinant approach can prove challenging to reproduce, often resulting in mostly insoluble protein and poor purification yields. After extensive optimization of a wide range of expression and purification conditions, we applied the autoinduction method of protein expression and developed a two-step column purification scheme that reliably produces large amounts of purified soluble and highly active streptopain. This method reproducibly yielded 3 mg of streptopain from 50 mL of expression culture at >95% purity, with an activity of 5306 ± 315 U/mg, and no remaining affinity tags or artifacts from recombinant expression. This improved method therefore enables the facile production of the important virulence factor streptopain at higher yields, with no purification scars that might bias functional studies, and with an 8.1-fold increased enzymatic activity compared to previously described procedures.

Application of the C3-binding motif of streptococcal pyrogenic exotoxin B to protect mice from invasive group a streptococcal infection

Group A streptococcus (GAS) is an important human pathogen that produces several extracellular exotoxins to facilitate invasion and infection. Streptococcal pyrogenic exotoxin B (SPE B) has been demonstrated to be an important virulence factor of GAS. Our previous studies indicate that SPE B cleaves complement 3 (C3) and inhibits the activation of complement pathways. In this study, we constructed and expressed recombinant fragments of SPE B to examine the C3-binding site of SPE B. Using enzyme-linked immunosorbent assays and pull-down assays, we found that the C-terminal domain, containing amino-acid residues 345–398, of SPE B was the major binding site of human serum C3. We further identified a major, Ala³⁷⁶-Pro³⁹⁸, and a minor C3-binding motif, Gly³⁴⁶-Gly³⁶⁰, that both mediated the binding of C3 complement. Immunization with the C3-binding motifs protected mice against challenge with a lethal dose of non-invasive M49 strain GAS but not invasive M1 strains. To achieve higher efficiency against invasive M1 GAS infection, a combination of synthetic peptides derived from C-terminal epitope of streptolysin S (SLSpp) and from the major C3-binding motif of SPE B (PP6, Ala³⁷⁶-Pro³⁹⁸) was used to elicit specific immune response to those two important streptococcal exotoxins. Death rates and the severity of skin lesions decreased significantly in PP6/SLSpp-immunized mice that were infected with invasive M1 strains of GAS. These results indicate a combination of the C3-binding motif of SPE B and the protective epitope of SLS could be used as a subunit vaccine against invasive M1 strains group A streptococcal infection.

Metal-mediated modulation of streptococcal cysteine protease activity and its biological implications

Streptococcal cysteine protease (SpeB), the major secreted protease produced by group A streptococcus (GAS), cleaves both host and bacterial proteins and contributes importantly to the pathogenesis of invasive GAS infections. Modulation of SpeB expression and/or its activity during invasive GAS infections has been shown to affect bacterial virulence and infection severity. Expression of SpeB is regulated by the GAS CovR-CovS two-component regulatory system, and we demonstrated that bacteria with mutations in the CovR-CovS two-component regulatory system are selected for during localized GAS infections and that these bacteria lack SpeB expression and exhibit a hypervirulent phenotype. Additionally, in a separate study, we showed that expression of SpeB can also be modulated by human transferrin- and/or lactoferrin-mediated iron chelation. Accordingly, the goal of this study was to investigate the possible roles of iron and other metals in modulating SpeB expression and/or activity in a manner that would potentiate bacterial virulence. Here, we report that the divalent metals zinc and copper inhibit SpeB activity at the posttranslational level. Utilizing online metal-binding site prediction servers, we identified two putative metal-binding sites in SpeB, one of which involves the catalytic-dyad residues (47)Cys and (195)His. Based on our findings, we propose that zinc and/or copper availability in the bacterial microenvironment can modulate the proteolytic activity of SpeB in a manner that preserves the integrity of several other virulence factors essential for bacterial survival and dissemination within the host and thereby may exacerbate the severity of invasive GAS infections.

The streptococcal cysteine protease SpeB is not a natural immunoglobulin-cleaving enzyme

The human bacterial pathogen Streptococcus pyogenes has developed a broad variety of virulence mechanisms to evade the actions of the host immune defense. One of the best-characterized factors is the streptococcal cysteine protease SpeB, an important multifunctional protease that contributes to group A streptococcal pathogenesis in vivo. Among many suggested activities, SpeB has been described to degrade various human plasma proteins, including immunoglobulins (Igs). In this study, we show that SpeB has no Ig-cleaving activity under physiological conditions and that only Igs in a reduced state, i.e., semimonomeric molecules, are cleaved and degraded by SpeB. Since reducing conditions outside eukaryotic cells have to be considered nonphysiological and IgG in a reduced state lacks biological effector functions, we conclude that SpeB does not contribute to S. pyogenes virulence through the proteolytic degradation of Igs.

Determining antibody-binding site of streptococcal pyrogenic exotoxin B to protect mice from group a streptococcus infection

Streptococcal pyrogenic exotoxin B (SPE B), a cysteine protease, is an important virulence factor in group A streptococcal (GAS) infection. SPE B binds and cleaves antibody isotypes and further impairs the immune system by inhibiting complement activation. In this study, we examined the antibody-binding site of SPE B and used it to block SPE B actions during GAS infection. We constructed different segments of the spe B gene and induced them to express different recombinant fragments of SPE B. Using an enzyme-linked immunosorbent assay (ELISA), we found that residues 345–398 of the C-terminal domain of SPE B (rSPE B_345–398), but not the N-terminal domain, was the major binding site for antibody isotypes. Using a competitive ELISA, we also found that rSPE B_345–398 bound to the Fc portion of IgG. The in vitro functional assays indicate that rSPE B_345–398 not only interfered with cleavage of antibody isotypes but also interfered with SPE B-induced inhibition of complement activation. Immunization of BALB/c mice using rSPE B_345–398 was able to induce production of a high titer of anti-rSPE B_345–398 antibodies and efficiently protected mice from GAS-induced death. These findings suggest that SPE B uses its C-terminal domain to bind the Fc portion of IgG and that immunization of mice with this binding domain (rSPE B_345–398) could protect mice from GAS infection.

Dynamics and functional differences between dendroaspin and rhodostomin: insights into protein scaffolds in integrin recognition

Dendroaspin (Den) and rhodostomin (Rho) are snake venom proteins containing a PRGDMP motif. Although Den and Rho have different 3D structures, they are highly potent integrin inhibitors. To study their structure, function, and dynamics relationships, we expressed Den and Rho in Pichia pastoris. The recombinant Den and Rho inhibited platelet aggregation with the K(I) values of 149.8 and 83.2 nM. Cell adhesion analysis showed that Den was 3.7 times less active than Rho when inhibiting the integrin αIIbβ3 and 2.5 times less active when inhibiting the integrin αvβ3. In contrast, Den and Rho were similarly active when inhibiting the integrin α5β1 with the IC₅₀ values of 239.8 and 256.8 nM. NMR analysis showed that recombinant Den and Rho have different 3D conformations for their arginyl-glycyl-aspartic acid (RGD) motif. However, the comparison with Rho showed that the docking of Den into integrin αvβ3 resulted in a similar number of contacts. Analysis of the dynamic properties of the RGD loop in Den and Rho showed that they also had different dynamic properties. These results demonstrate that protein scaffolds affect the function, structure, and dynamics of their RGD motif.

Ultrahigh and high resolution structures and mutational analysis of monomeric Streptococcus pyogenes SpeB reveal a functional role for the glycine-rich C-terminal loop

Cysteine protease SpeB is secreted from Streptococcus pyogenes and has been studied as a potential virulence factor since its identification almost 70 years ago. Here, we report the crystal structures of apo mature SpeB to 1.06 Å resolution as well as complexes with the general cysteine protease inhibitor trans-epoxysuccinyl-l-leucylamido(4-guanidino)butane and a novel substrate mimetic peptide inhibitor. These structures uncover conformational changes associated with maturation of SpeB from the inactive zymogen to its active form and identify the residues required for substrate binding. With the use of a newly developed fluorogenic tripeptide substrate to measure SpeB activity, we determined IC(50) values for trans-epoxysuccinyl-l-leucylamido(4-guanidino)butane and our new peptide inhibitor and the effects of mutations within the C-terminal active site loop. The structures and mutational analysis suggest that the conformational movements of the glycine-rich C-terminal loop are important for the recognition and recruitment of biological substrates and release of hydrolyzed products.

Effect of P to A mutation of the N-terminal residue adjacent to the Rgd motif on rhodostomin: importance of dynamics in integrin recognition

Rhodostomin (Rho) is an RGD protein that specifically inhibits integrins. We found that Rho mutants with the P48A mutation 4.4-11.5 times more actively inhibited integrin α5β1. Structural analysis showed that they have a similar 3D conformation for the RGD loop. Docking analysis also showed no difference between their interactions with integrin α5β1. However, the backbone dynamics of RGD residues were different. The values of the R(2) relaxation parameter for Rho residues R49 and D51 were 39% and 54% higher than those of the P48A mutant, which caused differences in S(2), R(ex), and τ(e). The S(2) values of the P48A mutant residues R49, G50, and D51 were 29%, 14%, and 28% lower than those of Rho. The R(ex) values of Rho residues R49 and D51 were 0.91 s(-1) and 1.42 s(-1); however, no R(ex) was found for those of the P48A mutant. The τ(e) values of Rho residues R49 and D51 were 9.5 and 5.1 times lower than those of P48A mutant. Mutational study showed that integrin α5β1 prefers its ligands to contain (G/A)RGD but not PRGD sequences for binding. These results demonstrate that the N-terminal proline residue adjacent to the RGD motif affect its function and dynamics, which suggests that the dynamic properties of the RGD motif may be important in Rho's interaction with integrin α5β1.

Localization of the Clostridium difficile cysteine protease Cwp84 and insights into its maturation process

Clostridium difficile is a nosocomial pathogen involved in antibiotic-associated diarrhea. C. difficile expresses a cysteine protease, Cwp84, which has been shown to degrade some proteins of the extracellular matrix and play a role in the maturation of the precursor of the S-layer proteins. We sought to analyze the localization and the maturation process of this protease. Two identifiable forms of the protease were found to be associated in the bacteria: a form of ∼80 kDa and a cleaved one of 47 kDa, identified as the mature protease. They were found mainly in the bacterial cell surface fractions and weakly in the extracellular fraction. The 80-kDa protein was noncovalently associated with the S-layer proteins, while the 47-kDa form was found to be tightly associated with the underlying cell wall. Our data supported that the anchoring of the Cwp84 47-kDa form is presumably due to a reassociation of the secreted protein. Moreover, we showed that the complete maturation of the recombinant protein Cwp84(30-803) is a sequential process beginning at the C-terminal end, followed by one or more cleavages at the N-terminal end. The processing sites of recombinant Cwp84 are likely to be residues Ser-92 and Lys-518. No proteolytic activity was detected with the mature recombinant protease Cwp84(92-518) (47 kDa). In contrast, a fragment including the propeptide (Cwp84(30-518)) displayed proteolytic activity on azocasein and fibronectin. These results showed that Cwp84 is processed essentially at the bacterial cell surface and that its different forms may display different proteolytic activities.

C-Terminal extension of a plant cysteine protease modulates proteolytic activity through a partial inhibitory mechanism

The amino acid sequence of ervatamin-C, a thermostable cysteine protease from a tropical plant, revealed an additional 24-amino-acid extension at its C-terminus (CT). The role of this extension peptide in zymogen activation, catalytic activity, folding and stability of the protease is reported. For this study, we expressed two recombinant forms of the protease in Escherichia coli, one retaining the CT-extension and the other with it truncated. The enzyme with the extension shows autocatalytic zymogen activation at a higher pH of 8.0, whereas deletion of the extension results in a more active form of the enzyme. This CT-extension was not found to be cleaved during autocatalysis or by limited proteolysis by different external proteases. Molecular modeling and simulation studies revealed that the CT-extension blocks some of the substrate-binding unprimed subsites including the specificity-determining subsite (S2) of the enzyme and thereby partially occludes accessibility of the substrates to the active site, which also corroborates the experimental observations. The CT-extension in the model structure shows tight packing with the catalytic domain of the enzyme, mediated by strong hydrophobic and H-bond interactions, thus restricting accessibility of its cleavage sites to the protease itself or to the external proteases. Kinetic stability analyses (T(50) and t(1/2) ) and refolding experiments show similar thermal stability and refolding efficiency for both forms. These data suggest that the CT-extension has an inhibitory role in the proteolytic activity of ervatamin-C but does not have a major role either in stabilizing the enzyme or in its folding mechanism.

From transcription to activation: how group A streptococcus, the flesh-eating pathogen, regulates SpeB cysteine protease production

Streptococcal pyrogenic exotoxin B (SpeB) is a protease secreted by group A streptococci and known to degrade a wide range of host and GAS proteins in vitro. Although the role of SpeB in GAS infection is debated, recent evidence has conclusively demonstrated that SpeB is critical for the pathogenesis of severe invasive disease caused by GAS. Genetic inactivation of the speB gene results in significantly decreased virulence in a necrotizing fasciitis model of infection. Production of fully active SpeB by GAS is extremely complex. Following transcription and translation the SpeB protein is secreted as an inactive zymogen, which is autocatalytically processed through a series of intermediates to form an active protease. Each step from transcription to protease activation is tightly controlled and regulated by the bacterial cell reflecting the critical role played by this virulence factor in GAS infection. Here we review the molecular aspects of SpeB production by GAS from transcription to activation and the multiple layers of control involved.

Molecular mimicry between streptococcal pyrogenic exotoxin B and endothelial cells

Molecular mimicry between group A streptococcus and host antigens has important roles in the development of post-streptococcal sequelae, including glomerulonephritis and rheumatic heart disease (RHD). The etiology of RHD involves host cross-reactivity with M proteins and carbohydrate antigens. In this study, we show that anti-streptococcal pyrogenic exotoxin B (SPE B) antibodies exhibited characteristics of autoantibodies, which cross-react with endothelial cells. Immunoglobulin G (IgG) deposition and complement activation were observed in the heart valve of SPE B-immunized mice. In addition, apoptosis in the heart valve was detected in SPE B-immunized mice. An anti-SPE B monoclonal antibody (mAb) 10G showed cross-reactivity with human microvascular endothelial (HMEC-1) cells and mouse valve endothelial cells. Passive immunization with mAb 10G also caused IgG deposition, complement activation, and apoptotic cell death in the mouse heart valve. We conducted peptide array and ELISA using synthetic peptides to identify the SPE B antigenic epitope recognized by mAb 10G. Results showed that the major epitope of mAb 10G is localized to amino-acid residues 296-310 of SPE B (P7-8). The cross-reactivity of mAb 10G with endothelial cells was inhibited using P7-8 peptides for competition. These results suggest that anti-SPE B antibodies cross-react with endothelial cells, and that a dominant epitope is located within the amino-acid residues 296-310 of SPE B. Moreover, we found that mAb 10G can also bind to N-acetyl-β-D-glucosamine (GlcNAc) conjugated with bovine serum albumin (BSA), but not to BSA or M1 protein. Competition assay showed that the binding activity of mAb 10G with GlcNAc-BSA and P7-8 of SPE B was inhibited by pretreatment with GlcNAc-BSA or P7-8 peptides. Therefore, our results suggest that conformational molecular mimicry may exist between SPE B and GlcNAc.

Bax is upregulated by p53 signal pathway in the SPE B-induced apoptosis

We identify integrin α(v)β(3) and Fas as receptors for the streptococcal pyrogenic exotoxin B (SPE B), and G308S (SPE B mutant, glycine at residue 308 is changed to serine), which interacts with Fas only, in our previous study. Here, we explore the signal pathways that regulate proapoptotic protein expression after SPE B stimulation. We find that both SPE B and G308S can stimulate the serine phosphorylation of p53, and p53 phosphorylation is inhibited by the anti-Fas antibody but not by anti-α(V)β(3) antibody. p38 inhibitor and siRNA decrease the activation and translocation of p53 into the nucleus, which executes its transcription activity. These results indicate that after SPE B treatment, p53 is activated and p38 is the upstream of p53. p38 siRNA also decreases the binding of p53 to the bax promoter and interferes with the association of p53 and STAT1. p53, p38, and STAT1 siRNAs downregulate SPE B-induced Bax expression. This shows that SPE B activates the bax promoter via p38/p53 signal pathways through the Fas receptor, and that STAT1 acts as a coactivator of p53. In addition, p38 and p53 siRNAs inhibit SPE B-induced apoptosis. This is consistent with the findings that SPE B upregulates Bax expression through p38/p53 signal pathways that enhance cell apoptosis.

The IL-8 Production by Streptococcal Pyrogenic Exotoxin B

We have previously identified αvβ3 and Fas as receptors for the streptococcal pyrogenic exotoxin B (SPE B), and G308S, a mutant of SPE B with RSD motif, which interacts with Fas only. This study aims to evaluate how SPE B interacts with cells to induce the production of IL-8. Our results showed that following exposure to SPE B or G308S, the levels of IL-8 protein and mRNA were increased and the increase was inhibited by the addition of anti-Fas antibody, suggesting that the increased production of IL-8 by SPE B is mediated through Fas receptor. In the presence of G308S, the association of FADD and procaspase 8, and activation of NF-κB were also detected. The application of siRNA of FADD and of procaspase 8 could inhibit the NF-κB activity. The proteolytic activity of caspase 8 was required for the NF-κB activity. Further studies showed that G308S could increase the phosphorylation of ERK and the translocation of NF-κB into the nucleus, and the inhibition of ERK phosphorylation decreased the IL-8 production, mRNA expression and activation of NF-κB. In addition, siRNA of procaspase 8 could inhibit the G308S-induced cleavage of MEKK1, binding of MEKK1 to caspase 8, activation of ERK and the NF-κB activity. Taken together, the production of IL-8 by SPE B in A549 cells is mediated by Fas, and followed by the activation of FADD, caspase 8, MEKK1, ERK and NF-κB.

Procaspase 8 and Bax are up-regulated by distinct pathways in Streptococcal pyrogenic exotoxin B-induced apoptosis

We have previously identified integrin alpha(v)beta(3) and Fas as receptors for the streptococcal pyrogenic exotoxin B (SPE B), and G308S, a mutant of SPE B that binds to Fas only. In the current study we found that after binding to alpha(v)beta(3), SPE B stimulated the tyrosine phosphorylation of JAK2 and STAT1. STAT1 tyrosine phosphorylation was inhibited by a JAK2 inhibitor, AG490, short interfering RNA (siRNA) silencing of JAK2, and anti-alpha(V)beta(3) antibody. AG490 also decreased the binding of tyrosine-phosphorylated STAT1 to the procaspase 8 promoter, decreasing procaspase 8 expression, suggesting that SPE B up-regulates procaspase 8 expression via the JAK2/STAT1 pathway. Alternatively, both SPE B and G308S increased STAT1 phosphorylation at serine 727, which was inhibited by anti-Fas antibody, a p38 inhibitor, SB203580, and siRNA silencing of p38. In addition, SPE B and G308S increased binding of serine-phosphorylated STAT1 to the Bax promoter and Bax expression, which was decreased by SB203580. SPE B and G308S-stimulated Bax expression was also inhibited by anti-Fas antibody. These findings suggest that Fas mediate SPE B-induced Bax expression through p38. Silencing of JAK2 or p38 by siRNA blocked procaspase 8 expression, whereas only p38 siRNA decreased Bax expression. Furthermore, JAK2 inhibition and p38 inhibition reduced SPE B-induced apoptosis, but only p38 inhibition blocked G308S-induced apoptosis.

Solution structure and backbone dynamics of streptopain: insight into diverse substrate specificity

Streptococcal pyrogenic exotoxin B (SPE B) is a cysteine protease expressed by Streptococcus pyogenes. The D9N, G163S, G163S/A172S, and G239D mutant proteins were expressed to study the effect of the allelic variants on their protease activity. In contrast to other mutants, the G239D mutant was approximately 12-fold less active. The Gly-239 residue is located within the C-terminal S230-G239 region, which cannot be observed in the x-ray structure. The three-dimensional structure and backbone dynamics of the 28-kDa mature SPE B (mSPE B) were determined. Unlike the x-ray structure of the 40-kDa zymogen SPE B (proSPE B), we observed the interactions between the C-terminal loop and the active site residues in mSPE B. The structural differences between mSPE B and proSPE B were the conformation of the C-terminal loop and the orientation of the catalytic His-195 residue, suggesting that activation and inactivation of SPE B is involved in the His-195 side-chain rotation. Dynamics analysis of mSPE B and the mSPE B/inhibitor complexes showed that the catalytic and C-terminal loops were the most flexible regions with low order parameter values of 0.5 to 0.8 and exhibited the motion on the ps/ns timescale. These findings suggest that the flexible C-terminal loop of SPE B may play an important role in controlling the substrate binding, resulting in its broad substrate specificity.

Streptococcal pyrogenic exotoxin B-induced apoptosis in A549 cells is mediated through alpha(v)beta(3) integrin and Fas

Our previous work suggested that streptococcal pyrogenic exotoxin (SPE) B-induced apoptosis is mediated through a receptor-like mechanism. In this study, we have identified alpha(v)beta(3) and Fas as the SPE B receptors for this function. The SPE B fragment without the RGD motif and G308S, a SPE B mutant with the RSD motif, induced less apoptosis than did native SPE B, suggesting that the RGD motif is critical for SPE B-induced apoptosis. Fluorescein isothiocyanate-SPE B binding assays and immunoprecipitation analysis showed that SPE B specifically interacted with alpha(v)beta(3). Anti-alpha(v)beta(3) antibody partially inhibited SPE B-induced apoptosis but had no effect on G308S-induced apoptosis. In addition, Fas binding to SPE B was verified in an affinity column and an immunoprecipitation analysis. Anti-Fas antibody inhibited SPE B- and G308S-induced apoptosis in a dose-dependent manner, suggesting that Fas-mediated SPE B-induced apoptosis also occurs RGD independently. Both anti-alpha(v)beta(3) and anti-Fas antibodies synergistically inhibited SPE B-induced apoptosis. The apoptotic cascades were activated by SPE B and G308S, with a little delay by the latter. After SPE B binding, the cell surface level of alpha(v)beta(3), but not of Fas, was decreased. The decreased alpha(v)beta(3) level was restored by treatment with the proteasome inhibitor MG132, suggesting a SPE B-mediated endocytosis of integrin alpha(v)beta(3) via the ubiquitin-proteasome system. Taken together, our results demonstrate that SPE B-induced apoptosis is mediated through alpha(v)beta(3) integrin and Fas in a synergistic manner.

Degradation of complement 3 by streptococcal pyrogenic exotoxin B inhibits complement activation and neutrophil opsonophagocytosis

Streptococcal pyrogenic exotoxin B (SPE B), a cysteine protease, is an important virulence factor in group A streptococcus (GAS) infection. The inhibition of phagocytic activity by SPE B may help prevent bacteria from being ingested. In this study, we examined the mechanism SPE B uses to enable bacteria to resist opsonophagocytosis. Using an enzyme-linked immunosorbent assay, we found that SPE B-treated serum impaired the activation of the classical, the lectin, and the alternative complement pathways. In contrast, C192S, a SPE B mutant lacking protease activity, had no effect on complement activation. Further study showed that cleavage of serum C3 by SPE B, but not C192S, blocked zymosan-induced production of reactive oxygen species in neutrophils as a result of decreased deposition of C3 fragments on the zymosan surface. Reconstitution of C3 into SPE B-treated serum unblocked zymosan-mediated neutrophil activation dose dependently. SPE B-treated, but not C192S-treated, serum also impaired opsonization of C3 fragments on the surface of GAS strain A20. Moreover, the amount of C3 fragments on the A20 cell surface, a SPE B-producing strain, was less than that on its isogenic mutant strain, SW507, after opsonization with normal serum. A20 opsonized with SPE B-treated serum was more resistant to neutrophil killing than A20 opsonized with normal serum, and SPE B-mediated resistance was C3 dependent. These results suggest a novel SPE B mechanism, one which degrades serum C3 and enables GAS to resist complement damage and opsonophagocytosis.

A new autocatalytic activation mechanism for cysteine proteases revealed by Prevotella intermedia interpain A

Prevotella intermedia is a major periodontopathogen contributing to human gingivitis and periodontitis. Such pathogens release proteases as virulence factors that cause deterrence of host defenses and tissue destruction. A new cysteine protease from the cysteine-histidine-dyad class, interpain A, was studied in its zymogenic and self-processed mature forms. The latter consists of a bivalved moiety made up by two subdomains. In the structure of a catalytic cysteine-to-alanine zymogen variant, the right subdomain interacts with an unusual prodomain, thus contributing to latency. Unlike the catalytic cysteine residue, already in its competent conformation in the zymogen, the catalytic histidine is swung out from its active conformation and trapped in a cage shaped by a backing helix, a zymogenic hairpin, and a latency flap in the zymogen. Dramatic rearrangement of up to 20A of these elements triggered by a tryptophan switch occurs during activation and accounts for a new activation mechanism for proteolytic enzymes. These findings can be extrapolated to related potentially pathogenic cysteine proteases such as Streprococcus pyogenes SpeB and Porphyromonas gingivalis periodontain.

Streptococcal pyrogenic exotoxin B antibodies in a mouse model of glomerulonephritis

Streptococcal pyrogenic exotoxin B is an extracellular cysteine protease. Only nephritis-associated strains of group A streptococci secrete this protease and this may be involved in the pathogenesis of post-streptococcal glomerulonephritis. Mice were actively immunized with a recombinant protease inactive exotoxin B mutant or passively immunized with exotoxin B antibody. Characteristics of glomerulonephritis were measured using histology, immunoglobulin deposition, complement activation, cell infiltration, and proteinuria. None of the mice given bovine serum albumin or exotoxin A as controls showed any marked changes. Immunoglobulin deposition, complement activation, and leukocyte infiltration occurred only in the glomeruli of exotoxin B-hyperimmunized mice. One particular anti-exotoxin B monoclonal antibody, 10G, was cross-reactive with kidney endothelial cells and it caused kidney injury and proteinuria when infused into mice. This cross-reactivity may be involved in the pathogenesis of glomerulonephritis following group A streptococcal infection.

The fate of SPE B after internalization and its implication in SPEB-induced apoptosis

After streptococcal pyrogenic exotoxin B (SPE B) induces apoptosis, its fate is unknown. Using confocal time-course microscopy at 37 degrees C, we detected green fluorescence 20 min after adding FITC-SPE B. Orange fluorescence, an indication of co-localization of SPE B with lysosomes which were labeled with a red fluorescent probe, was maximal at 40 min and absent by 60 min. SPE B was co-precipitated with clathrin, which is consistent with endocytotic involvement. Western blotting assay also indicated that uptake of SPE B was maximal at 40 min and disappeared after 60 min. However, in the presence of chloroquine, a lysosome inhibitor, the uptake of SPE B was not detectable. The disappearance of TCA-precipitated FITC-SPE B was parallel to the appearance of TCA soluble FITC-SPE B; in the presence of chloroquine, however, no SPE B degradation occurred. Chloroquine increased the level of SPE B-induced apoptosis by inhibiting the degradation of SPE B. These results suggest that the internalization and degradation of SPE B in cells may be a host defense system that removes toxic substances by sacrificing the exposed cells.

Abrogation of streptococcal pyrogenic exotoxin B-mediated suppression of phagocytosis in U937 cells by Cordyceps sinensis mycelium via production of cytokines

Streptococcal pyrogenic exotoxin B (SPE B) is a virulent factor in group A streptococcal infection. We previously showed that SPE B reduced phagocytosis in human monocytic U937 cells. Here we show that the mycelium extract of Cordyceps sinensis (CS), a Chinese immunomodulatory herbal medicine, increased phagocytosis in U937 cells. Neither heat nor trypsin pretreatment prevented CS extract from causing this increase. Further studies indicated that SPE B-mediated suppression of U937 cell phagocytic activity was abrogated by CS extract. Factors in the conditioned medium from CS-extract-treated U937 cells were responsible for blocking the SPE B-mediated suppression of phagocytosis. Heating the conditioned medium eliminated the increase, which suggested that the U937-cell protein products augmented phagocytosis. Analyzing cytokine mRNA expression of U937 cells revealed increases in interferon-gamma (IFN-gamma), interleukin (IL)-12 p35 and p40, and tumor necrosis factor-alpha (TNF-alpha), but not in IL-1beta, IL-6, or IL-8. Treating U937 cells with anti-IFN-gamma, IL-12, and TNF-alpha antibodies also eliminated the conditioned medium-induced increase in phagocytosis. Taken together, SPE B inhibited phagocytosis, but CS mycelium extract abrogated this inhibition by causing cytokine production.

Streptococcal mitogenic exotoxin, SmeZ, is the most susceptible M1T1 streptococcal superantigen to degradation by the streptococcal cysteine protease, SpeB

Superantigens (SAgs) play an important role in the pathogenesis of severe invasive infections caused by Group A Streptococcus (GAS). We had shown earlier that the expression of streptococcal cysteine protease SpeB results in partial loss of the immune-stimulating activity of the native secreted GAS SAgs, namely the streptococcal pyrogenic exotoxins produced by the globally disseminated M1T1 GAS strain, associated with invasive infections worldwide. In this study, we examined the susceptibility of each of the M1T1 recombinant SAgs to degradation by rSpeB. Whereas SmeZ was degraded completely within 30 min of incubation with rSpeB, SpeG, and SpeA were more resistant and SpeJ was completely unaffected by the proteolytic effects of this protease. Proteomic analyses demonstrated that the order of susceptibility of the M1T1 SAgs to SpeB proteolysis is unaltered when they are present in a mixture that reflects their native physiological status. As expected, the degradation of SmeZ abolished its immune stimulatory activity. In silico sequence disorder and structural analyses revealed that SmeZ, unlike the three other structurally related SAgs, possesses a putative SpeB cleavage site within an area of the protein likely to be exposed to the surface. The study provides evidence for the effect of subtle structural differences between highly similar SAgs on their biological activity.

Identification and characterization of bicistronic speB and prsA gene expression in the group A Streptococcus

Severe, invasive group A streptococcal infections have reemerged worldwide, and extracellular toxins, including streptococcal pyrogenic exotoxin B (SpeB), have been implicated in pathogenesis. The genetic regulation of SpeB is not fully understood, and the mechanisms involved in the processing of the protoxin to its enzymatically active form have not been definitively established. The present work demonstrated that the genes encoding SpeB (speB) and a peptidyl-prolyl isomerase (prsA) constitute an operon with transcription initiated from two promoters upstream of speB. Further, the speB-prsA operon was transcribed as a bicistronic mRNA. This finding is in contrast to the generally accepted notion that speB is transcribed only as a monocistronic gene. In addition, prsA has its own promoter, and transcription from this promoter starts in early log phase, prior to the transcription of speB. Genomic disruption of prsA decreased the production of enzymatically active SpeB but not the level of the pro-SpeB zymogen. Taken together, these results demonstrate that prsA is required for production of fully mature, enzymatically active SpeB.

Aqueous nickel-nitrilotriacetate modified Fe(3)O(4)-NH(3)(+) nanoparticles for protein purification and cell targeting

A comprehensive totally aqueous phase synthesis of nickel-nitrilotriacetate (Ni-NTA) modified superparamagnetic Fe(3)O(4) nanoparticles is presented. The Fe(3)O(4)-NTA-Ni nanoparticles are able to perform efficient and specific purification of 6-His tagged proteins from crude cell lysates, as evidenced by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot analysis. The average binding capacity, as demonstrated by streptopain (M(W) 42 kDa), is 0.23 mg/mg (protein/Fe(3)O(4)-NTA-Ni). Considering the high affinity and specificity of the binding between hexahistidine motif and Ni-NTA, Ni-NTA modified nanoparticles could act as a module to carry 6-His tagged proteins on the particle surface with molecular orientation control, since only the 6-His domain could be attached. These modularly designed functional nanoparticles enhance cancer cell targeting, as supported by the in vitro receptor mediated targeting assay using RGD-4C-6-His fusion peptide. The nanoparticles show no significant hemolysis for human blood and could be investigated further for their in vivo functional imaging applications.

Solution structure of gamma-bungarotoxin: the functional significance of amino acid residues flanking the RGD motif in integrin binding

Gamma-bungarotoxin, a snake venom protein isolated from Bungarus multicinctus, contains 68 amino acids, including 10 cysteine residues and a TAVRGDGP sequence at positions 30-37. The solution structure of gamma-bungarotoxin has been determined by nuclear magnetic resonance (NMR) spectroscopy. The structure is similar to that of the short-chain neurotoxins that contain three loops extending from a disulfide-bridged core. The tripeptide Arg-Gly-Asp (RGD) sequence is located at the apex of the flexible loop and is similar to that of other RGD-containing proteins. However, gamma-bungarotoxin only inhibits platelet aggregations with an IC50 of 34 microM. To understand its weak activity in inhibiting platelet aggregation, we mutated the RGD loop sequences of rhodostomin, a potent platelet aggregation inhibitor, from RIPRGDMP to TAVRGDGP, resulting in a 196-fold decrease in activity. In addition, the average Calpha-to-Calpha distance between R33 and G36 of gamma-bungarotoxin is 6.02 A, i.e., shorter than that of other RGD-containing proteins that range from 6.55 to 7.46 A. These results suggested that the amino acid residues flanking the RGD motif might control the width of the RGD loop. This structural difference may be responsible for its decrease in platelet aggregation inhibition compared with other RGD-containing proteins.

Predicting the redox state and secondary structure of cysteine residues in proteins using NMR chemical shifts

We report 2D cluster analyses of 1Halpha, 1HN, 13Calpha, and 13C' versus 13Cbeta NMR chemical shifts (CSs) that can be used to predict the redox state and secondary structure of cysteine residues in proteins. A database of cysteine 1Halpha, 1Hbeta2, 1Hbeta3, 1HN, 13Calpha, 13Cbeta, 13C', and 15NH CSs as a function of secondary structure and redox state was constructed from BioMagResBank entries. One-dimensional statistical analysis showed that cysteine 1Halpha, 1HN, 13Calpha, 13C', and 15NH CSs reflected the secondary structure, and that cysteine Cbeta CS is extremely sensitive to the redox state. In contrast, cysteine 1Hbeta CS was not correlated with its redox state or secondary structure. Two-dimensional cluster analysis revealed that 2D Calpha/Cbeta, C'/Cbeta, HNu/Cbeta, and Halpha/Cbeta clusters were helpful in distinguishing both the redox state and secondary structure of cysteine residues. Based on these results, we derived rules using a score matrix to predict the redox state and secondary structure of cysteines using their CSs. The score matrix predicts the redox state and secondary structure of cysteine residues in proteins with approximately 90% accuracy. This suggests that the redox state and secondary structure of cysteine residues in peptides and proteins can be obtained from their CSs without recourse to nuclear Overhauser effect measurements.

Streptococcal pyrogenic exotoxin B cleaves properdin and inhibits complement-mediated opsonophagocytosis

Streptococcal pyrogenic exotoxin B (SPE B), a cysteine protease, is an important virulence factor in group A streptococcal (GAS) infection. The reduction of phagocytic activity by SPE B may help prevent bacteria from being ingested. In this study, we investigated the mechanism SPE B uses to enable bacteria to resist opsonophagocytosis. Using Western blotting and an affinity column immobilized with SPE B, we found that both SPE B and C192S, an SPE B mutant lacking protease activity, bound to serum properdin, and that SPE B, but not C192S, degraded serum properdin. Further study showed that SPE B-treated, but not C192S-treated, serum blocked the alternative complement pathway. Reconstitution of properdin into SPE B-treated serum unblocked the alternative pathway. GAS opsonized with SPE B-treated serum was more resistant to neutrophil killing than GAS opsonized with C192S-treated or normal serum. These results suggest that a novel SPE B mechanism, one which degrades serum properdin, enables GAS to resist opsonophagocytosis.

Fighting an enemy within: cytoplasmic inhibitors of bacterial cysteine proteases

The genes encoding secreted, broad-spectrum activity cysteine proteases of Staphylococcus spp. (staphopains) and Streptococcus pyogenes (streptopain, SpeB) are genetically linked to genes encoding cytoplasmic inhibitors. While staphopain inhibitors have lipocalin-like folds, streptopain is inhibited by a protein bearing the scaffold of the enzyme profragment. Bioinformatic analysis of other prokaryotic genomes has revealed that two more species may utilize this same genetic arrangement to control streptopain-like proteases with lipocalin-like inhibitors, while three other species may employ a C-terminally located domain that resembles the profragment. This apparently represents a novel system that bacteria use to control the intracellular activity of their proteases.

Cordyceps sinensis mycelium protects mice from group A streptococcal infection

Group A streptococcus (GAS) infection can cause severe invasive diseases, including necrotizing fasciitis and streptococcal toxic shock syndrome. Cordyceps sinensis, a Chinese herbal medicine, is an immunomodulator. In this study the air-pouch bacterial inoculation model was used to investigate the protective efficacy of C. sinensis mycelium extract against GAS infection. Force-feeding mice with C. sinensis mycelium extract for 3 consecutive days before GAS infection increased the survival rate and reduced local skin-tissue injury compared with mice fed PBS. Bacterial numbers in the air pouch exudates from C. sinensis-treated mice were lower than those from PBS-treated mice. Blood and organs in PBS-treated mice showed bacterial dissemination, but those in C. sinensis-treated mice did not. Three days of pretreatment with C. sinensis extract followed by C. sinensis treatment every other day after GAS infection resulted in 100% survival. The post-GAS-infection levels of aspartate aminotransferase, alanine aminotransferase and blood urea nitrogen in the sera of C. sinensis-treated mice were lower than those of PBS-treated mice. Taken together, these results show that C. sinensis mycelium extract protects by decreasing bacterial growth and dissemination, thereby increasing mouse survival rate. IL-12 and IFN-gamma expression and macrophage phagocytic activity also increased after C. sinensis treatment.

Streptococcal pyrogenic exotoxin B causes mitochondria damage to polymorphonuclear cells preventing phagocytosis of group A streptococcus

The streptococcal pyrogenic exotoxin B (SpeB) is known to be involved in group A streptococcus (GAS) survival in blood, but the detailed mechanism is not clear. For clarification of this issue, speB isogenic mutants of strains M6 and M49 were constructed by using an integrational plasmid and confirmed by Southern blot analysis. The resistance to phagocytosis of wild-type strains and their speB isogenic mutants was analyzed. The results demonstrated a five-fold increase in phagocytosis of speB mutants compared to that of wild-type strains in whole blood, but no significant difference in plasma. To further clarify whether this effect is due to a functional SpeB protein, recombinant SpeB (r-SpeB) and a SpeB mutant protein lacking proteinase activity (r-C192S) were purified and incubated with a speB mutant in whole blood. The results showed a two- to threefold increase in resistance to phagocytosis when the M6 speB mutant was incubated with r-SpeB, but not with r-C192S. Incubation with the wild-type strain, speB mutant, or the r-SpeB protein did not affect the total cell number of polymorphonuclear (PMN) cells in whole blood under laboratory conditions. However, the PMN cells' mitochondria showed decreasing dehydrogenase activity and loss of membrane potential after r-SpeB treatment. These data indicate that SpeB could cause the mitochondria damage to the PMN cells, preventing immune clearance at an early infectious stage.

The pro-sequence domain of streptopain directs the folding of the mature enzyme

The cysteine endopeptidase streptopain, an extracellular enzyme from pathogenic Streptococcus pyogenes, is synthesized as a precursor containing an NH2-terminal pro-sequence. The pro-sequence of streptopain was expressed in Escherichia coli and subjected to structural and functional investigation. Heat-induced denaturation of the pro-sequence studied using circular dichroism spectroscopy revealed that it forms a compact structure and represents an independently folded domain. The isolated pro-sequence exhibits high affinity towards mature streptopain and associates with its cognate enzyme by forming an equimolar complex. Refolding of denatured streptopain in the presence of pro-sequence in vitro facilitated recovery of active enzyme. Expression of the mature streptopain in E. coli either alone, or in trans with its pro-sequence as an independent polypeptide, led to the formation of insoluble protein aggregates or functionally active enzyme, respectively. These results demonstrate that the pro-sequence domain acts as an intramolecular chaperone that directs the correct folding of the mature streptopain.