The crystal structure of exfoliative toxin B: a superantigen with enzymatic activity

Necrotic activity of ExhC from Mammaliicoccus sciuri is mediated by specific amino acid residues

Mammaliicoccus sciuri, a commensal and pathogenic bacterium of significant clinical and veterinary relevance, expresses exfoliative toxin C (ExhC), a specific glutamyl endopeptidase belonging to the chymotrypsin family as the principal virulence factor. However, unlike most members of this family, ETs are inactive against a wide range of substrates and possess exquisite specificity for desmoglein-1 (Dsg1), a cadherin-like adhesion molecule that is crucial to maintain tissue integrity, thereby preventing the separation of skin cells and the entry of pathogens. ExhC is of clinical importance since in addition to causing exfoliation in pigs and mice, it induces necrosis in multiple mammalian cell lines, a property not observed for other ETs. Previous experiments have implicated the ExhC79-128 fragment in causing necrosis. Site-directed mutagenesis of specific residues within this fragment were studied and led to the design of an ExhC variant containing four-point mutations (ExhCmut4) lacking necrotic potential but retaining nearly wild-type (wt) levels of enzymatic activity. Moreover, the determination of the ExhCwt and ExhCmut4 crystal structures identified the conformation in the necrosis-linked region. These results constitute an important step toward the understanding of the mechanisms underlying the necrotic and epidermolytic activity of ExhC.

Staphylococcal Enterotoxin B and C Mutants and Vaccine Toxoids

Three mutants individually of both staphylococcal enterotoxins B and C were prepared by site-specific mutagenesis of enterotoxin amino acids that contact host T lymphocyte immune cell receptor sites (N23A, Q210A, and N23A/Q210A); these amino acids are shared between the two enterotoxins, and mutations reduce the interaction with the variable part of the β-chain of the T lymphocyte receptor. The mutant proteins, as expressed in Staphylococcus aureus RN4220, lacked biological toxicity as measured by the loss of (i) stimulation of rabbit splenocyte proliferation, (ii) pyrogenicity, and (iii) the ability to enhance the lethality of endotoxin shock, compared to wild-type enterotoxins. In addition, the mutants were able to vaccinate rabbits against pyrogenicity, the enhancement of endotoxin shock, and lethality in a pneumonia model when animals were challenged with methicillin-resistant S. aureus. Three vaccine injections (one primary and two boosters) protected rabbits for at least 3.5 months postvaccination when challenged with wild-type enterotoxins (last time point tested). These mutant proteins have the potential to function as toxoid vaccines against these two causes of nonmenstrual toxic shock syndrome (TSS). IMPORTANCE Toxic shock syndrome toxin 1 (TSST-1) and staphylococcal enterotoxins B and C cause the majority of cases of staphylococcal toxic shock syndrome. Previously, vaccine toxoids of TSST-1 have been prepared. In this study, vaccine toxoids of enterotoxins B and C were prepared. The toxoids lost biological toxicity but were able to vaccinate rabbits against lethal TSS.

Three-Dimensional Structure Characterization and Inhibition Study of Exfoliative Toxin D From Staphylococcus aureus

The Staphylococcus aureus exfoliative toxins (ETs) are the main toxins that produce staphylococcal scalded skin syndrome (SSSS), an abscess skin disorder. The victims of the disease are usually newborns and kids, as well as grown-up people. Five ETs namely, exfoliative toxins A, B, C, D, and E have been identified in S. aureus. The three-dimensional (3D) structure of exfoliative toxins A, B, C and E is known, while that of exfoliative toxin D (ETD) is still unknown. In this work, we have predicted the 3D structure of ETD using protein modeling techniques (software used for 3D structure modeling comprising the MODELLER 9v19 program, SWISS-Model, and I-TESSER). The validation of the build model was done using PROCHECK (Ramachandran plot), ERRAT2, and Verify 3D programs. The results from 3D modeling show that the build model was of good quality as indicated by a GMQE score of 0.88 and by 91.1% amino acid residues in the most favored region of the Ramachandran plot, the ERRAT2 quality factor of 90.1%, and a verify3D score of >0.2 for 99.59% of amino acid residues. The 3D structure analysis indicates that the overall structure of ETD is similar to the chymotrypsin-like serine protease fold. The structure is composed of 13 β-strands and seven α-helices that fold into two well-defined six-strand β-barrels whose axes are roughly perpendicular to each other. The active site residues include histidine-97, aspartic acid-147, and serine-221. This represents the first structure report of ETD. Structural comparison with the other ETs shows some differences, particularly in the loop region, which also change the overall surface charge of these toxins. This may convey variable substrate specificity to these toxins. The inhibition of these toxins by natural (2S albumin and flocculating proteins from Moringa oleifera seeds) and synthetic inhibitors (suramin) was also carried out in this study. The results from docking indicate that the inhibitors bind near the C-terminal domain which may restrict the movement of this domain and may halt the access of the substrate to the active site of this enzyme. Molecular dynamic simulation was performed to see the effect of inhibitor binding to the enzyme. This work will further elucidate the structure–function relationship of this enzyme. The inhibition of this enzyme will lead to a new treatment for SSSS.

Role of the I16-D194 ionic interaction in the trypsin fold

Activity in trypsin-like proteases is the result of proteolytic cleavage at R15 followed by an ionic interaction that ensues between the new N terminus of I16 and the side chain of the highly conserved D194. This mechanism of activation, first proposed by Huber and Bode, organizes the oxyanion hole and primary specificity pocket for substrate binding and catalysis. Using the clotting protease thrombin as a relevant model, we unravel contributions of the I16-D194 ionic interaction to Na⁺ binding, stability of the transition state and the allosteric E*-E equilibrium of the trypsin fold. The I16T mutation abolishes the I16-D194 interaction and compromises the architecture of the oxyanion hole. The D194A mutation also abrogates the I16-D194 interaction but, surprisingly, has no effect on the architecture of the oxyanion hole that remains intact through a new H-bond established between G43 and G193. In both mutants, loss of the I16-D194 ionic interaction compromises Na⁺ binding, reduces stability of the transition state, collapses the 215–217 segment into the primary specific pocket and abrogates the allosteric E*-E equilibrium in favor of a rigid conformation that binds ligand at the active site according to a simple lock-and-key mechanism. These findings refine the structural role of the I16-D194 ionic interaction in the Huber-Bode mechanism of activation and reveal a functional linkage with the allosteric properties of the trypsin fold like Na⁺ binding and the E*-E equilibrium.

Staphylococcus aureus Secreted Toxins and Extracellular Enzymes

Staphylococcus aureus is a formidable pathogen capable of causing infections in different sites of the body in a variety of vertebrate animals, including humans and livestock. A major contribution to the success of S. aureus as a pathogen is the plethora of virulence factors that manipulate the host's innate and adaptive immune responses. Many of these immune modulating virulence factors are secreted toxins, cofactors for activating host zymogens, and exoenzymes. Secreted toxins such as pore-forming toxins and superantigens are highly inflammatory and can cause leukocyte cell death by cytolysis and clonal deletion, respectively. Coagulases and staphylokinases are cofactors that hijack the host's coagulation system. Exoenzymes, including nucleases and proteases, cleave and inactivate various immune defense and surveillance molecules, such as complement factors, antimicrobial peptides, and surface receptors that are important for leukocyte chemotaxis. Additionally, some of these secreted toxins and exoenzymes can cause disruption of endothelial and epithelial barriers through cell lysis and cleavage of junction proteins. A unique feature when examining the repertoire of S. aureus secreted virulence factors is the apparent functional redundancy exhibited by the majority of the toxins and exoenzymes. However, closer examination of each virulence factor revealed that each has unique properties that have important functional consequences. This chapter provides a brief overview of our current understanding of the major secreted virulence factors critical for S. aureus pathogenesis.

Interaction of host and Staphylococcus aureus protease-system regulates virulence and pathogenicity

Staphylococcus aureus causes various health care- and community-associated infections as well as certain chronic TH2 driven inflammatory diseases. It is a potent pathogen with serious virulence and associated high morbidity. Severe pathogenicity is accredited to the S. aureus secreted virulence factors such as proteases and host protease modulators. These virulence factors promote adhesion and invasion of bacteria through damage of tight junction barrier and keratinocytes. They inhibit activation and transmigration of various immune cells such as neutrophils (and neutrophil proteases) to evade opsono-phagocytosis and intracellular bacterial killing. Additionally, they protect the bacteria from extracellular killing by disrupting integrity of extracellular matrix. Platelet activation and agglutination is also impaired by these factors. They also block the classical as well as alternative pathways of complement activation and assist in spread of infection through blood and tissue. As these factors are exquisite factors of S. aureus mediated disease development, we have focused on review of diversification of various protease-system associated virulence factors, their structural building, diverse role in disease development and available therapeutic counter measures. This review summarises the role of protease-associated virulence factors during invasion and progression of disease.

Staphylococcus aureus Manipulates Innate Immunity through Own and Host-Expressed Proteases

Neutrophils, complement system and skin collectively represent the main elements of the innate immune system, the first line of defense of the host against many common microorganisms. Bacterial pathogens have evolved strategies to counteract all these defense activities. Specifically, Staphylococcus aureus, a major human pathogen, secretes a variety of immune evasion molecules including proteases, which cleave components of the innate immune system or disrupt the integrity of extracellular matrix and intercellular connections of tissues. Additionally, S. aureus secretes proteins that can activate host zymogens which, in turn, target specific defense components. Secreted proteins can also inhibit the anti-bacterial function of neutrophils or complement system proteases, potentiating S. aureus chances of survival. Here, we review the current understanding of these proteases and modulators of host proteases in the functioning of innate immunity and describe the importance of these mechanisms in the pathology of staphylococcal diseases.

Direct and indirect mechanisms of KLK4 inhibition revealed by structure and dynamics

The kallikrein-related peptidase (KLK) family of proteases is involved in many aspects of human health and disease. One member of this family, KLK4, has been implicated in cancer development and metastasis. Understanding mechanisms of inactivation are critical to developing selective KLK4 inhibitors. We have determined the X-ray crystal structures of KLK4 in complex with both sunflower trypsin inhibitor-1 (SFTI-1) and a rationally designed SFTI-1 derivative to atomic (~1 Å) resolution, as well as with bound nickel. These structures offer a structural rationalization for the potency and selectivity of these inhibitors, and together with MD simulation and computational analysis, reveal a dynamic pathway between the metal binding exosite and the active site, providing key details of a previously proposed allosteric mode of inhibition. Collectively, this work provides insight into both direct and indirect mechanisms of inhibition for KLK4 that have broad implications for the enzymology of the serine protease superfamily, and may potentially be exploited for the design of therapeutic inhibitors.

Autocatalytic activation of a thermostable glutamyl endopeptidase capable of hydrolyzing proteins at high temperatures

Glutamyl endopeptidases (GSEs) specifically hydrolyze peptide bonds formed by α-carboxyl groups of Glu and Asp residues. We cloned the gene for a thermophilic GSE (designated TS-GSE) from Thermoactinomyces sp. CDF. A proform of TS-GSE that contained a 61-amino acid N-terminal propeptide and a 218-amino acid mature domain was produced in Escherichia coli. We found that the proform possessed two processing sites and was capable of autocatalytic activation via multiple pathways. The N-terminal propeptide could be autoprocessed at the Glu^-1-Ser¹ bond to directly generate the mature enzyme. It could also be autoprocessed at the Glu^-12-Lys^-11 bond to yield an intermediate, which was then converted into the mature form after removal of the remaining part of the propeptide. The segment surrounding the two processing sites was flexible, which allowed the proform and the intermediate form to be trans-processed into the mature form by either active TS-GSE or heterogeneous proteases. Deletion analysis revealed that the N-terminal propeptide is important for the correct folding and maturation of TS-GSE. The propeptide, even its last 11-amino acid peptide segment, could inhibit the activity of its cognate mature domain. The mature TS-GSE displayed a temperature optimum of 85 °C and retained approximately 90 % of its original activity after incubation at 70 °C for 6 h, representing the most thermostable GSE reported to date. Mutational analysis suggested that the disulfide bonds Cys³²-Cys⁴⁸ and Cys¹⁸⁰-Cys¹⁸³ cumulatively contributed to the thermostability of TS-GSE.

Crystal structure of Staphylococcus aureus exfoliative toxin D-like protein: Structural basis for the high specificity of exfoliative toxins

Exfoliative toxins are serine proteases secreted by Staphylococcus aureus that are associated with toxin-mediated staphylococcal syndromes. To date, four different serotypes of exfoliative toxins have been identified and 3 of them (ETA, ETB, and ETD) are linked to human infection. Among these toxins, only the ETD structure remained unknown, limiting our understanding of the structural determinants for the functional differentiation between these toxins. We recently identified an ETD-like protein associated to S. aureus strains involved in mild mastitis in sheep. The crystal structure of this ETD-like protein was determined at 1.95 Å resolution and the structural analysis provide insights into the oligomerization, stability and specificity and enabled a comprehensive structural comparison with ETA and ETB. Despite the highly conserved molecular architecture, significant differences in the composition of the loops and in both the N- and C-terminal α-helices seem to define ETD-like specificity. Molecular dynamics simulations indicate that these regions defining ET specificity present different degrees of flexibility and may undergo conformational changes upon substrate recognition and binding. DLS and AUC experiments indicated that the ETD-like is monomeric in solution whereas it is present as a dimer in the asymmetric unit indicating that oligomerization is not related to functional differentiation among these toxins. Differential scanning calorimetry and circular dichroism assays demonstrated an endothermic transition centered at 52 °C, and an exothermic aggregation in temperatures up to 64 °C. All these together provide insights about the mode of action of a toxin often secreted in syndromes that are not associated with either ETA or ETB.

Staphylococcus pseudintermedius exfoliative toxin EXI selectively digests canine desmoglein 1 and causes subcorneal clefts in canine epidermis

Staphylococcal exfoliative toxins are known to digest desmoglein (Dsg) 1, a desmosomal cell-cell adhesion molecule, thus causing intraepidermal splitting in human bullous impetigo, staphylococcal scalded skin syndrome and swine exudative epidermitis. Recently, a novel exfoliative toxin gene (exi), whose sequence shares significant homology with previously identified exfoliative toxins, was isolated from Staphylococcus pseudintermedius. Little is known about the pathogenic involvement of this toxin in canine pustular diseases such as impetigo. The aim of this study was to determine whether EXI, the product of the exi gene, digests canine Dsg1 and causes intraepidermal splitting in canine skin. An exi gene was isolated from chromosomal DNA of an S. pseudintermedius strain obtained from a pustule of a dog with impetigo, and was used to produce a recombinant EXI by Escherichia coli expression. When purified recombinant EXI was injected intradermally into normal dogs, it caused the development of vesicles or erosions with superficial epidermal splitting. In addition, the EXI abolished immunofluorescence for Dsg1, but not for Dsg3, at the injection sites. Moreover, the EXI directly degraded baculovirus-secreted recombinant extracellular domains of canine Dsg1, but not that of canine Dsg3, in vitro. The EXI also degraded mouse Dsg1α and swine Dsg1, but not human Dsg1, mouse Dsg1β and Dsg1γ. Conversely, recombinant SIET, previously designated as S. intermedius exfoliative toxin, did not cause intraepidermal splitting or degradation of any Dsgs. These findings indicate that EXI has a proteolytic activity that digests canine Dsg1, and this characteristic might be involved in the pathogenesis of intraepidermal splitting in canine impetigo.

The molecular logic of pemphigus and impetigo: the desmoglein story

Desmosomes are intercellular adhesive junctions of epithelial cells that contain two major transmembrane components, desmogleins (Dsg) and desmocollins; these are both cadherin-type cell-cell adhesion molecules. Pemphigus is an autoimmune blistering disease caused by IgG autoantibodies that target Dsg1 and Dsg3 in pemphigus foliaceus and pemphigus vulgaris respectively. Bullous impetigo is a common and highly contagious superficial skin infection caused by Staphylococcus aureus. Staphylococcal scalded skin syndrome (SSSS) is a generalized form of bullous impetigo. The blisters in bullous impetigo and SSSS are induced by exfoliative toxin that specifically cleaves Dsg1. Clinical and microscopic localization of blisters in pemphigus, bullous impetigo and SSSS are logically explained at the molecular level by the desmoglein compensation theory; the similarity of lesions among these diseases is underscored by a similar pathogenesis.

Exfoliative toxins of Staphylococcus aureus

Staphylococcus aureus is an important pathogen of humans and livestock. It causes a diverse array of diseases, ranging from relatively harmless localized skin infections to life-threatening systemic conditions. Among multiple virulence factors, staphylococci secrete several exotoxins directly associated with particular disease symptoms. These include toxic shock syndrome toxin 1 (TSST-1), enterotoxins, and exfoliative toxins (ETs). The latter are particularly interesting as the sole agents responsible for staphylococcal scalded skin syndrome (SSSS), a disease predominantly affecting infants and characterized by the loss of superficial skin layers, dehydration, and secondary infections. The molecular basis of the clinical symptoms of SSSS is well understood. ETs are serine proteases with high substrate specificity, which selectively recognize and hydrolyze desmosomal proteins in the skin. The fascinating road leading to the discovery of ETs as the agents responsible for SSSS and the characterization of the molecular mechanism of their action, including recent advances in the field, are reviewed in this article.

Neonate twin with staphylococcal scalded skin syndrome from a renal source

OBJECTIVE

To understand the underlying mechanism of exfoliative toxins causing staphylococcal scalded skin syndrome or Ritter's Disease that predominantly affects newborns and infants, although it is sometimes found in adults. Staphylococcal scalded skin syndrome is typically diagnosed by the characteristic fluid-filled bullae together with superficial skin loss. A histopathological diagnosis may be made by looking for subcorneal acantholytic cleavage with minimal inflammation on biopsy, although this is not normally required. Exfoliative toxin A and B are both responsible for the "acantholytic" infection of Staphylococcus aureus as they target desmoglein-1 leading to loss of cell-to-cell cohesion and subsequent spread of infection. Other factors produced by S. aureus can cause a myriad of other problems including neutralization of antimicrobial peptides, inactivation of neutrophils, proteolysis, T-cell anergy, and immunosuppression.

DESIGN

Individual care report.

SETTING

Pediatric intensive care unit.

PATIENT

We describe a normal male infant who was born at term and developed 100% total body surface area staphylococcal scalded skin syndrome on the 14 day of life with associated renal sepsis.

INTERVENTIONS

After cultures from the lesions, bloodstream, and urine were obtained, intravenous Vancomycin and Ceftriaxone were commenced. The initial lesions increased in size over a 36-hr period to cover the entire body surface; this was associated with a decline in hemodynamic status.

MEASUREMENTS AND MAIN RESULTS

Cultures from the urine and blood grew coagulase-positive S. aureus. An ultrasound scan revealed bilateral pyonephroses, which necessitated the placement of percutaneous nephrostomies with subsequent decompression of the collecting system.

CONCLUSIONS

After the decompression hemodynamic status stabilized and over the ensuing 10 days, the patient made a full recovery with no scarring. No similar lesions were noticed on the infant's twin brother. We discuss the recent developments in understanding the underlying mechanism of exfoliative toxins causing staphylococcal scalded skin syndrome, review current treatment guidelines, and outline the need for new therapeutic options.

Autoimmune and infectious skin diseases that target desmogleins

Desmosomes are intercellular adhesive junctions of epithelial cells that contain two major transmembrane components, the desmogleins (Dsg) and desmocollins (Dsc), which are cadherin-type cell-cell adhesion molecules and are anchored to intermediate filaments of keratin through interactions with plakoglobin and desmoplakin. Desmosomes play an important role in maintaining the proper structure and barrier function of the epidermis and mucous epithelia. Four Dsg isoforms have been identified to date, Dsg1-Dsg4, and are involved in several skin and heart diseases. Dsg1 and Dsg3 are the two major Dsg isoforms in the skin and mucous membranes, and are targeted by IgG autoantibodies in pemphigus, an autoimmune disease of the skin and mucous membranes. Dsg1 is also targeted by exfoliative toxin (ET) released by Staphylococcus aureus in the infectious skin diseases bullous impetigo and staphylococcal scalded skin syndrome (SSSS). ET is a unique serine protease that shows lock and key specificity to Dsg1. Dsg2 is expressed in all tissues possessing desmosomes, including simple epithelia and myocardia, and mutations in this gene are responsible for arrhythmogenic right ventricular cardiomyopathy/dysplasia. Dsg4 plays an important adhesive role mainly in hair follicles, and Dsg4 mutations cause abnormal hair development. Recently, an active disease model for pemphigus was generated by a unique approach using autoantigen-deficient mice that do not acquire tolerance against the defective autoantigen. Adoptive transfer of Dsg3(-/-) lymphocytes into mice expressing Dsg3 induces stable anti-Dsg3 IgG production with development of the pemphigus phenotype. This mouse model is a valuable tool with which to investigate immunological mechanisms of harmful IgG autoantibody production in pemphigus. Further investigation of desmoglein molecules will continue to provide insight into the unsolved pathophysiological mechanisms of diseases and aid in the development of novel therapeutic strategies with minimal side effects.

Structural and functional characterization of SplA, an exclusively specific protease of Staphylococcus aureus

Staphylococcus aureus is a dangerous human pathogen whose antibiotic resistance is steadily increasing and no efficient vaccine is as yet available. This serious threat drives extensive studies on staphylococcal physiology and pathogenicity pathways, especially virulence factors. Spl (serine protease-like) proteins encoded by an operon containing up to six genes are a good example of poorly characterized secreted proteins probably involved in virulence. In the present study, we describe an efficient heterologous expression system for SplA and detailed biochemical and structural characterization of the recombinant SplA protease. The enzyme shares a significant sequence homology to V8 protease and epidermolytic toxins which are well documented staphylococcal virulence factors. SplA has a very narrow substrate specificity apparently imposed by the precise recognition of three amino acid residues positioned N-terminal to the hydrolysed peptide bond. To explain determinants of this extended specificity we resolve the crystal structure of SplA and define the consensus model of substrate binding. Furthermore we demonstrate that artificial N-terminal elongation of mature SplA mimicking a naturally present signal peptide abolishes enzymatic activity. The probable physiological role of the process is discussed. Of interest, even though precise N-terminal trimming is a common regulatory mechanism among S1 family enzymes, the crystal structure of SplA reveals novel significantly different mechanistic details.

Enzymatic activity of the Staphylococcus aureus SplB serine protease is induced by substrates containing the sequence Trp-Glu-Leu-Gln

Proteases are of significant importance for the virulence of Staphylococcus aureus. Nevertheless, their subset, the serine protease-like proteins, remains poorly characterized. Here presented is an investigation of SplB protease catalytic activity revealing that the enzyme possesses exquisite specificity and only cleaves efficiently after the sequence Trp-Glu-Leu-Gln. To understand the molecular basis for such selectivity, we solved the three-dimensional structure of SplB to 1.8 A. Modeling of substrate binding to the protease demonstrated that selectivity relies in part on a canonical specificity pockets-based mechanism. Significantly, the conformation of residues that ordinarily form the oxyanion hole, an essential structural element of the catalytic machinery of serine proteases, is not canonical in the SplB structure. We postulate that within SplB, the oxyanion hole is only formed upon docking of a substrate containing the consensus sequence motif. It is suggested that this unusual activation mechanism is used in parallel with classical determinants to further limit enzyme specificity. Finally, to guide future development, we attempt to point at likely physiological substrates and thus the role of SplB in staphylococcal physiology.

High Resolution Structures of p-Aminobenzamidine- and Benzamidine-VIIa/Soluble Tissue Factor

Factor VIIa (FVIIa) consists of a gamma-carboxyglutamic acid (Gla) domain, two epidermal growth factor-like domains, and a protease domain. FVIIa binds seven Ca(2+) ions in the Gla, one in the EGF1, and one in the protease domain. However, blood contains both Ca(2+) and Mg(2+), and the Ca(2+) sites in FVIIa that could be specifically occupied by Mg(2+) are unknown. Furthermore, FVIIa contains a Na(+) and two Zn(2+) sites, but ligands for these cations are undefined. We obtained p-aminobenzamidine-VIIa/soluble tissue factor (sTF) crystals under conditions containing Ca(2+), Mg(2+), Na(+), and Zn(2+). The crystal diffracted to 1.8A resolution, and the final structure has an R-factor of 19.8%. In this structure, the Gla domain has four Ca(2+) and three bound Mg(2+). The EGF1 domain contains one Ca(2+) site, and the protease domain contains one Ca(2+), one Na(+), and two Zn(2+) sites. (45)Ca(2+) binding in the presence/absence of Mg(2+) to FVIIa, Gla-domainless FVIIa, and prothrombin fragment 1 supports the crystal data. Furthermore, unlike in other serine proteases, the amide N of Gly(193) in FVIIa points away from the oxyanion hole in this structure. Importantly, the oxyanion hole is also absent in the benzamidine-FVIIa/sTF structure at 1.87A resolution. However, soaking benzamidine-FVIIa/sTF crystals with d-Phe-Pro-Arg-chloromethyl ketone results in benzamidine displacement, d-Phe-Pro-Arg incorporation, and oxyanion hole formation by a flip of the 192-193 peptide bond in FVIIa. Thus, it is the substrate and not the TF binding that induces oxyanion hole formation and functional active site geometry in FVIIa. Absence of oxyanion hole is unusual and has biologic implications for FVIIa macromolecular substrate specificity and catalysis.

Conformational lability in serine protease active sites: structures of hepatocyte growth factor activator (HGFA) alone and with the inhibitory domain from HGFA inhibitor-1B

Hepatocyte growth factor activator (HGFA) is a serine protease that converts hepatocyte growth factor (HGF) into its active form. When activated HGF binds its cognate receptor Met, cellular signals lead to cell growth, differentiation, and migration, activities which promote tissue regeneration in liver, kidney and skin. Intervention in the conversion of HGF to its active form has the potential to provide therapeutic benefit where HGF/Met activity is associated with tumorigenesis. To help identify ways to moderate HGF/Met effects, we have determined the molecular structure of the protease domain of HGFA. The structure we determined, at 2.7 A resolution, with no pseudo-substrate or inhibitor bound is characterized by an unconventional conformation of key residues in the enzyme active site. In order to find whether this apparently non-enzymatically competent arrangement would persist in the presence of a strongly-interacting inhibitor, we also have determined, at 2.6 A resolution, the X-ray structure of HGFA complexed with the first Kunitz domain (KD1) from the physiological inhibitor hepatocyte growth factor activator inhibitor 1B (HAI-1B). In this complex we observe a rearranged substrate binding cleft that closely mirrors the cleft of other serine proteases, suggesting an extreme conformational dynamism. We also characterize the inhibition of 16 serine proteases by KD1, finding that the previously reported enzyme specificity of the intact extracellular region of HAI-1B resides in KD1 alone. We find that HGFA, matriptase, hepsin, plasma kallikrein and trypsin are potently inhibited, and use the complex structure to rationalize the structural basis of these results.

A selective, slow binding inhibitor of factor VIIa binds to a nonstandard active site conformation and attenuates thrombus formation in vivo

The serine protease factor VIIa (FVIIa) in complex with its cellular cofactor tissue factor (TF) initiates the blood coagulation reactions. TF.FVIIa is also implicated in thrombosis-related disorders and constitutes an appealing therapeutic target for treatment of cardiovascular diseases. To this end, we generated the FVIIa active site inhibitor G17905, which displayed great potency toward TF.FVIIa (Ki = 0.35 +/- 0.11 nM). G17905 did not appreciably inhibit 12 of the 14 examined trypsin-like serine proteases, consistent with its TF.FVIIa-specific activity in clotting assays. The crystal structure of the FVIIa.G17905 complex provides insight into the molecular basis of the high selectivity. It shows that, compared with other serine proteases, FVIIa is uniquely equipped to accommodate conformational disturbances in the Gln217-Gly219 region caused by the ortho-hydroxy group of the inhibitor's aminobenzamidine moiety located in the S1 recognition pocket. Moreover, the structure revealed a novel, nonstandard conformation of FVIIa active site in the region of the oxyanion hole, a "flipped" Lys192-Gly193 peptide bond. Macromolecular substrate activation assays demonstrated that G17905 is a noncompetitive, slow-binding inhibitor. Nevertheless, G17905 effectively inhibited thrombus formation in a baboon arterio-venous shunt model, reducing platelet and fibrin deposition by approximately 70% at 0.4 mg/kg + 0.1 mg/kg/min infusion. Therefore, the in vitro potency of G17905, characterized by slow binding kinetics, correlated with efficacious antithrombotic activity in vivo.

Treatment of staphylococcal scalded skin syndrome

Humans are a natural reservoir for Staphylococcal aureus. Colonization begins soon after birth and predisposes to infection. S. aureus is one of the most common causes of skin infection, giving rise to folliculitis, furunculosis, carbuncles, ecthyma, impetigo, cellulitis and abscesses. In addition, S. aureus may cause a number of toxin-mediated life-threatening diseases, including staphylococcal scalded skin syndrome (SSSS). Epidermolytic toxins released by certain S. aureus strains cause SSSS by cleaving the epidermal cell adhesion molecule, desmogelin-1, resulting in superficial skin erosion. Recent experiments have revealed similarities in the pathophysiology of SSSS and pemphigus foliaceus, an autoimmune disorder that is characterized by antibodies targeting the same epidermal attachment protein. SSSS typically affects neonates and infants but may also occur in predisposed adults. It is painful and distressing for the patient and parents, although most cases respond to antibiotic treatment. Mortality is low in infants but can be as high as 67% in adults, and is dependent on the extent of skin involvement and the comorbid state. Thus, the management of adults who develop SSSS remains a major therapeutic challenge. The antibody response against the toxins neutralizes their effect and prevents recurrence or limits the effects to the area of infection, which is known as bullous impetigo.

Desmosomes and disease: pemphigus and bullous impetigo

Desmosomal cadherins are the pathophysiologic targets of autoimmune or toxin-mediated disruption in the human diseases pemphigus and bullous impetigo (including its generalized form, called staphylococcal scalded skin syndrome). Experiments exploiting the production of both pathogenic and nonpathogenic antidesmoglein antibodies in pemphigus patients' sera have afforded data that make an invaluable contribution towards identifying the functional domains of the desmogleins involved in intercellular adhesion. Conformational epitopes of antidesmoglein autoantibodies in pemphigus patients' sera and the specific cleavage site of desmoglein 1 by exfoliative toxin have been identified, implicating the N-terminal extracellular domains of the desmogleins as critical regions for controlling intercellular adhesion. Furthermore, the development of active autoimmune mouse models for pemphigus allows in vivo characterization of the disease and its pathogenesis. These studies offer new insight into the potential mechanisms of acantholysis in pemphigus and staphylococcal-associated blistering disease, with implications for the role of desmogleins in desmosomal structure and function.

Crystal and Solution Structures of a Superantigen from Yersinia pseudotuberculosis Reveal a Jelly-Roll Fold

Superantigens are a class of microbial proteins with the ability to excessively activate T cells by binding to the T cell receptor. The staphylococcal and streptococcal superantigens are closely related in structure and possess an N-terminal domain that resembles an OB fold and a C-terminal domain similar to a beta-grasp fold. Yersinia pseudotuberculosis produces superantigens, YPMa, YPMb, and YPMc, which have no significant amino acid similarity to other proteins. We have determined the crystal and solution structures of YPMa, which show that the protein has a jelly-roll fold. The closest structural neighbors to YPMa are viral capsid proteins and members of the tumor necrosis factor superfamily. In the crystal structure, YPMa packs as a trimer, another feature shared with viral capsid proteins and TNF superfamily proteins. However, in solution YPMa behaves as a monomer, and any functional relevance of the trimer observed in the crystals is yet to be established.

Enzymatic and Molecular Characteristics of the Efficiency and Specificity of Exfoliative Toxin Cleavage of Desmoglein 1

Exfoliative toxins (ETs) from Staphylococcus aureus blister the superficial epidermis by hydrolyzing a single peptide bond, Glu381-Gly382, located between extracellular domains 3 and 4 of desmoglein 1 (Dsg1). Enzyme activity is dependent on the calcium-stabilized structure of Dsg1. Here we further define the characteristics of this cleavage. Kinetic studies monitoring the cleavage of Dsg1 by ETA, ETB, and ETD demonstrated kcat/Km values of 2-6 x 10(4) m(-1) s(-1), suggesting very efficient proteolysis. Proteolysis by ETA was not efficiently inhibited by broad spectrum serine protease inhibitors, suggesting that the enzyme cleavage site may be inactive or inaccessible before specific binding to its substrate. Using truncated mutants of human Dsg1 and chimeric molecules between human Dsg1 and either human Dsg3 or canine Dsg1, we show that for cleavage, human-specific amino acids from Dsg1 are necessary in extracellular domain 3 upstream of the scissile bond. If these residues are canine rather than human, ETA binds, but does not cleave, canine Dsg1. These data suggest that the exquisite specificity and efficiency of ETA may depend on the enzyme's binding upstream of the cleavage site with a very specific fit, like a key in a lock.

Understanding the mechanism of action of the exfoliative toxins of Staphylococcus aureus

The exfoliative toxins of Staphylococcus aureus are responsible for the staphylococcal scalded skin syndrome, a blistering skin disorder that particularly affects infants and young children, as well as adults with underlying disease. Their three-dimensional structure is similar to other glutamate-specific trypsin-like serine proteases with two substrate-binding domains and a serine-histidine-aspartate catalytic triad that forms the active site. However, unlike other serine proteases, the exfoliative toxins possess a highly charged N-terminal alpha-helix and a unique orientation of a critical peptide bond, which blocks the active site of the toxins so that, in their native state, they do not possess any significant enzymatic activity. The target for the toxins has recently been identified as desmoglein-1, a desmosomal glycoprotein which plays an important role in maintaining cell-to-cell adhesion in the superficial epidermis. It is speculated that binding of the N-terminal alpha-helix to desmoglein-1 results in a conformation change that opens the active site of the toxin to cleave the extracellular domain of desmoglein-1 between the third and fourth domains, resulting in disruption of intercellular adhesion and formation of superficial blisters. Elucidating the mechanism of action of the toxins and identifying desmoglein-1 as their specific epidermal substrate has not only given us an insight into the pathogenesis of the staphylococcal scalded skin syndrome, but also provided us with useful information on normal skin physiology and the pathogenesis of other toxin-mediated diseases. It is hoped that this knowledge will lead to development of rapid screening and diagnostic tests, and new antitoxin strategies for the treatment and prevention of the staphylococcal scalded skin syndrome in the near future.

Calcium-dependent conformation of desmoglein 1 is required for its cleavage by exfoliative toxin

In bullous impetigo, Staphylococcus aureus spreads under the stratum corneum of skin by elaboration of exfoliative toxin, which hydrolyzes only one peptide bond in a highly structured calcium-binding domain of desmoglein 1, resulting in loss of its function. We investigated the basis of this exquisite specificity. Exfoliative toxin cannot cleave desmoglein 1 pretreated at 56 degrees C or higher or at low or high pH, suggesting that the proper conformation of desmoglein 1 is critical for its cleavage. Because cleavage occurs in an area of desmoglein 1 stabilized by calcium, we determined if the conformation necessary for cleavage is calcium-dependent. Depletion of calcium from desmoglein 1 completely inhibited its cleavage by exfoliative toxin, even after calcium was added back. A change in conformation of desmoglein 1 by calcium depletion was shown, with immunofluorescence and enzyme-linked immunoassay, by loss of binding of PF sera, which recognize conformational epitopes. This change in conformation was confirmed by tryptophan fluorometry and circular dichroism, and was irreversible with repletion of calcium. These data suggest that the specificity of exfoliative toxin cleavage of desmoglein 1 resides not only in simple amino acid sequences but also in its calcium-dependent conformation.

Staphylococcal epidermolysins

PURPOSE OF REVIEW

Staphylococcal epidermolysins are the major causative toxins of bullous impetigo and staphylococcal scalded skin syndrome. This disease is characterized by the splitting of the epidermis between two cell layers resulting in exfoliation. It predominantly affects newborn babies and exposes them to secondary infections. This leads to the risk of epidemics, especially in nurseries. With only an experimental model which consists of skin injections in newborn mice and the recent determination of three-dimensional structures, the essential function of these toxins remained controversial, split between that of specific proteases and that of superantigens.

RECENT FINDINGS

Staphylococcal epidermolysins now constitute a family of toxins, with the recent characterizations of two new serotypes: ETC and ETD. They may be secreted by sensitive or methicillin-resistant strains. Four molecules were also identified in Staphylococcus hyicus responsible for exudative epidermitis in swine. While different observations suggested a proteolytic action to these toxins, the histological parallel made with pemphigus foliaceus greatly helped in the characterization of the targets for epidermolysins ETA, ETB, ETD: desmoglein-1, a desmosome-constitutive protein, and incidentally melanocyte-stimulating hormones, which accounts for the blisters observed clinically.

SUMMARY

The growing complexity in staphylococcal toxins has to be taken into account both for their association with diseases and for diagnosis purposes. Even though cases of staphylococcal scalded skin syndrome in adults are rare, they raise further questions about the pathogenic features of the disease such as individual sensitivity and distribution of the toxins into the body.

Desmoglein as a target in autoimmunity and infection

Clinical phenotypes of most diseases are complex. However, once the mechanism behind the scene is clarified, the nature shows amazing beauty. There is a simple logic behind a complex disease. The exact molecular mechanism of the blister formation in staphylococcal scalded skin syndrome (SSSS) remained to be elucidated for 3 decades since exfoliative toxin was discovered by Melish and Glasgow in 1970. A knowledge accumulated to understand the pathogenesis of pemphigus and cell-cell adhesion of keratinocytes led us to solve this question. Desmoglein 1, which is a cadherin type cell-cell adhesion molecule in desmosomes, is targeted in two different skin diseases, pemphigus foliaceus, and SSSS. In pemphigus foliaceus IgG autoantibodies are developed against desmoglein 1 and inhibit its adhesive function with resultant blister formation in the superficial epidermis. In SSSS, exfoliative toxin produced by Staphylococcus aureus specifically binds and cleaves desmoglein 1 with resultant blister formation at the identical site.

Identification of the Staphylococcus aureus etd pathogenicity island which encodes a novel exfoliative toxin, ETD, and EDIN-B

We identified a novel pathogenicity island in Staphylococcus aureus which contains open reading frames (ORFs) similar to the exfoliative toxin (ET) gene, glutamyl endopeptidase gene, and edin-B gene in tandem and the phage resistance gene, flanked by hsdM, hsdS (restriction and modification system), and IS256. The protein encoded by the ET-like gene showed 40, 59, and 68% amino acid sequence identities with exfoliative toxin A (ETA), exfoliative toxin B (ETB), and Staphylococcus hyicus ETB (ShETB), respectively. When injected into neonatal mice, the recombinant protein derived from the ET-like gene induced exfoliation of the skin with loss of cell-to-cell adhesion in the upper part of the epidermis as observed in histological examinations, just as was found in neonatal mice injected with ETA or ETB. Western blot analysis indicated that the recombinant protein is serologically distinct from ETA and ETB. Therefore, the product encoded by this new ORF is a new ET member produced by S. aureus and is termed ETD. ETD did not induce blisters in 1-day-old chickens. In the skins of mice injected with ETD, cell surface staining of desmoglein 1 (Dsg1), a cadherin type cell-to-cell adhesion molecule in desmosomes, was abolished without affecting that of desmoglein 3 (Dsg3). Furthermore, in vitro incubation of the recombinant extracellular domains of Dsg1 and Dsg3 with the recombinant protein demonstrated that both mouse and human Dsg1, but not Dsg3, were directly cleaved in a dose-dependent manner. These results demonstrate that ETD and ETA induce blister formation by identical pathophysiological mechanisms. Clinical strains positive for edin-B were suggested to be clonally associated, and all edin-B-positive strains tested were positive for etd. Among 18 etd-positive strains, 12 produced ETD extracellularly. Interestingly, these strains are mainly isolated from other sources of infections and not from patients with bullous impetigo or staphylococcal scalded-skin syndrome. This strongly suggests that ETD might play a pathogenic role in a broader spectrum of bacterial infections than previously considered.

Molecular mechanisms of blister formation in bullous impetigo and staphylococcal scalded skin syndrome

Bullous impetigo due to Staphylococcus aureus is one of the most common bacterial infections of man, and its generalized form, staphylococcal scalded skin syndrome (SSSS), is a frequent manifestation of staphylococcal epidemics in neonatal nurseries. Both diseases are mediated by exfoliative toxins (ETs), which show exquisite pathologic specificity in blistering only the superficial epidermis. We show that these toxins act as serine proteases with extremely focused molecular specificity to cleave mouse and human desmoglein 1 (Dsg1) once after glutamic acid residue 381 between extracellular domains 3 and 4. Mutation of the predicted catalytically active serine to alanine completely inhibits cleavage. The mutated ETs bind specifically to Dsg1 by immunofluorescence colocalization and by coimmunoprecipitation. Thus, ETs, through specific recognition and proteolytic cleavage of one structurally critical peptide bond in an adhesion molecule, cause its dysfunction and allow S. aureus to spread under the stratum corneum, the main barrier of the skin, explaining how, although they circulate through the entire body in SSSS, they cause pathology only in the superficial epidermis.

Extracellular proteases of Staphylococcus spp

Bacterial proteases secreted into an infected host may exhibit a wide range of pathogenic potentials. Staphylococci, in particular Staphylococcus aureus, are known to produce several extracellular proteases, including serine-, cysteine- and metalloenzymes. Their insensitivity to most human plasma protease inhibitors and, even more, the ability to inactivate some of these make the proteases potentially harmful. Indeed, several recent studies have shown that staphylococcal proteases are able to interact with the host defense mechanisms and tissue components as well as to modify other pathogen-derived virulence factors. A tight, cell density-dependent control of proteolytic activity expression, similar to that of the well-defined virulence determinants, further suggests the role of staphylococcal proteases in the infection process. Consistently, alterations in coordinated expression of extracellular proteins markedly diminished the virulence. However, despite these data and the fact that a strain deficient in sspABC operon coding for serine (sspA) and cysteine (sspB) proteases was highly attenuated in virulence in the animal infection model, it was impossible to unambiguously demonstrate the importance of any particular protease as a virulence factor. Therefore, it can be assumed that the orchestrated expression and interaction of a variety of extracellular and cell surface proteins rather than any particular one is responsible for the staphylococcal pathogenicity and that the proteases apparently play an important role in this complex process. Such redundant mechanism is very well suited for promoting the survival of staphylococci under diverse environmental conditions encountered in the infected host.

A novel method for rapid production and purification of exfoliative toxin A of Staphylococcus aureus

The exfoliative toxins of Staphylococcus aureus are the causative agents of the scalded-skin syndrome. Previously described methods of toxin production and purification require large quantities of culture medium, take a long time and often produce low yields of toxin. A novel method of toxin production and purification using a dialysis sac to separate the culture medium from the staphylococci is described. This method produces up to 12 mg of crude toxin per ml of bacterial cell culture bathing the surface of the dialysis sac within 36 h and almost 10 mg of purified toxin per ml of cell culture within 3 days, in contrast to previous procedures that took over a week to produce 0.1-1.0 mg ml(-1) crude toxin and less than 0.01 mg ml(-1) purified toxin. This rapid method of toxin production should speed up future research into the pathogenesis of the staphylococcal scalded-skin syndrome.

Staphylococcal exfoliative toxin B specifically cleaves desmoglein 1

Staphylococcal scalded skin syndrome and its localized form, bullous impetigo, show superficial epidermal blister formation caused by exfoliative toxin A or B produced by Staphylococcus aureus. Recently we have demonstrated that exfoliative toxin A specifically cleaves desmoglein 1, a desmosomal adhesion molecule, that when inactivated results in blisters. In this study we determine the target molecule for exfoliative toxin B. Exfoliative toxin B injected in neonatal mice caused superficial epidermal blisters, abolished cell surface staining of desmoglein 1, and degraded desmoglein 1 without affecting desmoglein 3 or E-cadherin. When adenovirus-transduced cultured keratinocytes expressing exogenous mouse desmoglein 1 or desmoglein 3 were incubated with exfoliative toxin B, desmoglein 1, but not desmoglein 3, was cleaved. Furthermore, cell surface staining of desmoglein 1, but not that of desmoglein 3, was abolished when cryosections of normal human skin were incubated with exfoliative toxin B, suggesting that living cells were not necessary for exfoliative toxin B cleavage of desmoglein 1. Finally, in vitro incubation of the recombinant extracellular domains of desmoglein 1 and desmoglein 3 with exfoliative toxin B demonstrated that both mouse and human desmoglein 1, but not desmoglein 3, were directly cleaved by exfoliative toxin B in a dose-dependent fashion. These findings demonstrate that exfoliative toxin A and exfoliative toxin B cause blister formation in staphylococcal scalded skin syndrome and bullous impetigo by identical molecular pathophysiologic mechanisms.

Staphylococcus aureus isolated in cases of impetigo produces both epidermolysin A or B and LukE-LukD in 78% of 131 retrospective and prospective cases

Clinical symptoms of impetigo and staphylococcal scalded skin syndrome may not only be expressed as the splitting of cell layers within the epidermis but are often accompanied by some localized inflammation. Toxin patterns of Staphylococcus aureus isolates originating from patients with impetigo and also from those with other primary and secondary skin infections in a retrospective isolate collection in France and a prospective isolate collection in French Guiana revealed a significant association (75% of the cases studied) of impetigo with production of at least one of the epidermolysins A and B and the bicomponent leucotoxin LukE-LukD (P < 0.001). However, most of the isolates were able to produce one of the nonubiquitous enterotoxins. Pulsed-field gel electrophoresis (PFGE) of genomic DNA hydrolyzed with SmaI showed a polymorphism of the two groups of isolates despite the fact that endemic clones were suspected in French Guiana and France. The combination of toxin patterns with PFGE fingerprinting may provide further discrimination among isolates defined in a given cluster or a given pulsotype and account for a specific virulence. The new association of toxins with a clinical syndrome may reveal principles of the pathological process.

Complete nucleotide sequence of a Staphylococcus aureus exfoliative toxin B plasmid and identification of a novel ADP-ribosyltransferase, EDIN-C

The complete nucleotide sequence of pETB, a 38.2-kb Staphylococcus aureus plasmid encoding the exfoliative toxin B (ETB), was determined. A total of 50 open reading frames were identified on the plasmid genome and, among these, 32 showed sequence similarity to known proteins. pETB contains three copies of IS257, which divide the pETB genome into three regions: (i) a cadmium resistance operon-containing region, (ii) a lantibiotic production gene-containing region, and (iii) the remaining part where genes for plasmid replication and/or maintenance are dispersed. In the third region, genes of various kinds of functions are present among the replication- and maintenance-related genes. They include two virulence-related genes, the etb gene and a gene encoding a novel ADP-ribosyltransferase closely related to EDIN, which belongs to the C3 family of ADP-ribosyltransferases modifying Rho GTPases. They also include genes for a cell wall-anchoring surface protein and a phage resistance protein. Based on the determined sequence of pETB, the genome structures of etb-bearing plasmids (ETB plasmids) from various clinical isolates were analyzed by the PCR scanning method. The data indicate that, although the ETB plasmids are highly heterogeneous in genome size, the fundamental genome organization is well conserved. The size variation of the plasmid is mainly attributed to defined regions which may be hot spots for gene shuffling.

Toxin levels in serum correlate with the development of staphylococcal scalded skin syndrome in a murine model

Staphylococcal scalded skin syndrome (SSSS) is an exfoliative dermatitis that results from infection with exfoliative toxin-producing Staphylococcus aureus. SSSS is seen primarily in infants and children. Here we ask if there is a specific maturation process that protects healthy adults from this syndrome. For these studies, an active recombinant exfoliative toxin A (rETA) was used in a neonatal mouse model. A time course generated on the susceptibility to the toxin as a function of mouse age indicated that BALB/c mice developed the characteristic symptoms of SSSS until day 7 of life. Between day 7 and day 8 of life there was a dramatic decrease in susceptibility, such that mice at day 9 of life were resistant to the effects of the toxin. This time course corresponds approximately to the time needed for maturation of the adaptive immune response, and SSSS in adults is often identified with immunocompromised states. Therefore, mice deficient in this response were examined. Adult mice thymectomized at birth and adult SCID mice did not develop the symptoms of SSSS after injection with the toxin, indicating that the adaptive immune response is not responsible for the lack of susceptibility observed in the older mice. SSSS in adults is also associated with renal disorders, suggesting that levels of toxin in serum are important in the development of the disease. rETA was not cleared as efficiently from the serum of 1-day-old mice compared to clearance from 10-day-old mice. Ten-day-old mice were given repeated injections of toxin so that the maximal level of toxin was maintained for a sustained period of time, and exfoliation occurred in these mice. Thus, whereas the adaptive immune response is not needed for protection of adult mice from SSSS, efficient clearance of the toxin from the bloodstream is a critical factor.

Development and evaluation of detection systems for staphylococcal exfoliative toxin A responsible for scalded-skin syndrome

Staphylococcal scalded-skin syndrome is usually diagnosed clinically by its characteristic exfoliating rash. Isolation of Staphylococcus aureus from the patient further supports the diagnosis. Several detection systems have been developed to determine whether the isolated strain produces exfoliative toxin, but none are routinely available in hospital laboratories. In a novel approach, we used computer models to predict the structure of the exfoliative toxins based on other serine proteases and to identify surface epitopes for the production of antibodies that specifically bound the exfoliative toxin A (ETA) serotype. Several rapid immunologically based diagnostic tests for ETA were developed with these antibodies and compared with existing systems. Our results showed that Western blot analysis using these antibodies was in complete correlation with PCR, which has been validated against the "gold standard" mouse model. On the other hand, the double-antibody enzyme-linked immunosorbent assay (ELISA) and Ouchterlony immunodiffusion assay gave unacceptably high false-positive results due to interference by staphylococcal protein A. This problem was successfully overcome by the development of a F(ab')(2) fragment ELISA, which was rapid and reproducible and was as sensitive and specific as PCR and Western blot analysis. The F(ab')(2) fragment ELISA is superior to existing diagnostic systems because it is quantitative, which may be related to the severity of the condition, and can detect amounts of exfoliative toxin in the picogram range directly from serum. This is the first detection system with the potential to confirm the diagnosis of staphylococcal scalded-skin syndrome from a routine blood test within 3 h of presentation.

Recombinant Staphylococcus aureus exfoliative toxins are not bacterial superantigens

Staphylococcal scalded-skin syndrome is an exfoliative dermatitis characterized by the separation of the epidermis at the stratum granulosum. This disruption is mediated by one of two Staphylococcus aureus exotoxins, exfoliative toxins A and B (ETA and ETB). Both ETA and ETB have been reported to be bacterial superantigens. A controversy exists, however, as other data indicate that these exotoxins are not superantigens. Here we demonstrate that recombinant exfoliative toxins produced in Escherichia coli do not act as T-cell mitogens and thus are not bacterial superantigens. These data fit the clinical profile of the disease, which is not associated with the classic symptoms of a superantigen-mediated syndrome.

Staphylococcal exfoliative toxins cleave alpha- and beta-melanocyte-stimulating hormones

The staphylococcal exfoliative toxins (ETs) A and B (ETA and ETB) are 27-kDa exotoxins produced by certain strains of Staphylococcus aureus and are the causative agents of staphylococcal scalded-skin syndrome. The crystal structures of the ETs strongly indicate that the proteins are members of the serine protease family of enzymes, although protease activity until now has not yet been conclusively demonstrated. Here, we show that the peptide beta-melanocyte-stimulating hormone (beta-MSH) is cleaved by ETA and that both ETA and ETB are capable of cleaving alpha-MSH. Both toxins exhibit cleavage at specific glutamic acid residues in MSH peptides. Moreover, biologically inactive mutants of ETA were incapable of cleaving beta-MSH.

Structural similarities and differences in Staphylococcus aureus exfoliative toxins A and B as revealed by their crystal structures

Staphylococcal aureus epidermolytic toxins (ETs) A and B are responsible for the induction of staphylococcal scalded skin syndrome, a disease of neonates and young children. The clinical features of this syndrome vary from localized blisters to severe exfoliation affecting most of the body surface. Comparison of the crystal structures of two subtypes of ETs-rETA (at 2.0 A resolution), rETB (at 2.8 A resolution), and an active site variant of rETA, Ser195Ala at 2.0 A resolution has demonstrated that their overall topology resembles that of a "trypsin-like" serine protease, but with significant differences at the N- and C-termini and loop regions. The details of the catalytic site in both ET structures are very similar to those in glutamate-specific serine proteases, suggesting a common catalytic mechanism. However, the "oxyanion hole," which is part of the catalytic sites of glutamate specific serine proteases, is in the closed or inactive conformation for rETA, yet in the open or active conformation for rETB. The ETs contain a unique amphipathic helix at the N-terminus, and it appears to be involved in optimizing the conformation of the catalytic site residues. Determination of the structure of the rETA catalytic site variant, Ser195Ala, showed no significant perturbation at the active site, establishing that the loss of biological and esterolytic activity can be attributed solely to disruption of the catalytic serine residue. Finally, the crystal structure of ETs, together with biochemical data and mutagenesis studies, strongly confirms the classification of these molecules as "serine proteases" rather than "superantigens."

Mutational analysis of the superantigen staphylococcal exfoliative toxin A (ETA)

Exfoliative toxin A (ETA) is known to be a causative agent of staphylococcal scalded skin syndrome (SSSS). Although relatively little is known about exactly how the exfoliative toxins (ETs) cause SSSS, much has been discovered recently that may help elucidate the mechanism(s) by which ETA exhibits activities such as lymphocyte mitogenicity and epidermolytic activity. Here, we have shown that highly purified ETA does have T lymphocyte mitogenic activity in that wild-type ETA induced T cell proliferation whereas several single amino acid mutants lacked significant activity. Neither wild-type ETA nor any single amino acid mutants were proteolytic for a casein substrate, yet esterase activity was detected in wild-type ETA and several mutants, but eliminated in other mutants. A mutation in aa 164 (Asp to Ala) showed a 9-fold increase in esterase activity as well. Finally, we correlated esterase activity with epidermolytic activity. All mutants that lost esterase activity also lost epidermolytic activity. Conversely, mutants that retained esterase activity also retained exfoliative activity, implicating serine protease or serine protease-like activity in the causation of SSSS. Moreover, the mutants that displayed markedly reduced T cell superantigenic activity retained their epidermolytic activity (although some of these mutants required higher doses of toxin to cause disease), which suggests an ancillary role for this activity in SSSS causation.