Biochemical and biological properties of Staphylococcal enterotoxin K

Staphylococcus aureus is an important human pathogen which is implicated in a wide variety of diseases. Major determinants of the virulence of this organism include extracellular virulence factors. Staphylococcal enterotoxins (SEs) are important causative agents in staphylococcal toxic shock syndrome and food poisoning. Our study identified a novel enterotoxin, SEK, and examined its biochemical and biological properties. SEK had a molecular weight of 26,000 and an experimentally determined pI of between 7.0 and 7.5. SEK was secreted by clinical isolates of S. aureus. We demonstrated that SEK had many of the biological activities associated with the SEs, including superantigenicity, pyrogenicity, the ability to enhance the lethal effect of endotoxin, and lethality in a rabbit model when administered by subcutaneous miniosmotic pump. Recombinant SEK was shown to stimulate human CD4(+) and CD8(+) T cells in a Vbeta-specific manner; T-cells bearing Vbeta 5.1, 5.2, and 6.7 were significantly stimulated to proliferate.

Structural evidence for the evolution of pyrogenic toxin superantigens

Pathogenic bacteria have evolved a wide variety of toxins to invade and attack host organisms. In particular, strains of the bacteria Staphylococcus aureus and Streptococcus pyogenes produce a family of pyrogenic toxin superantigens (PTSAgs) that can cause illness, e.g., toxic shock syndrome, or synergize with a number of other immune system disorders. The PTSAgs are all similar in size and have a conserved two-domain tertiary fold despite minimal amino acid sequence identity. The tertiary structure of PTSAg domain 1 is similar to the immunoglobulin binding motif of streptococcal proteins G and L. PTSAg domain 2 resembles members of the oligosaccharide/oligonucleotide binding fold family that includes the B subunits of the AB(5) heat-labile enterotoxins, cholera toxin, pertussis toxin, and verotoxin. The strong structural homology between the pyrogenic toxins and other bacterial proteins suggests that the PTSAgs evolved through the recombination of two smaller beta-strand motifs.

Structural and functional features of modified heat-stable toxins produced by enteropathogenic Klebsiella cells

Heat-stable enterotoxins (STs) are 18- or 19-amino acid peptides (STa or ST1) produced by enteropathogenic bacteria with small differences in their amino acid sequence and a highly conserved carboxy terminus. All STs contain a core of three disulfide bridges whose integrity is believed to be necessary for full biologic activity. We previously reported that strains of Klebsiella pneumoniae transformed by the plasmid pSLM004 produce a modified toxin not recognized by MAb raised against genuine Escherichia coli ST. Investigation of the chemical structure of the modified toxins revealed that three new toxins were present. These were purified to homogeneity by a series of sequential chromatography on reverse-phase columns using guanylate cyclase to monitor the enterotoxic activity during purification procedures. The sequence of the modified toxins was obtained by a combination of Edman degradation and mass spectrometry, showing that they are proteolytically processed forms of E. coli ST1b. In particular, toxin A-2 lacks the cysteine at position 18 and then is not able to form the disulfide bridge cysteine-10-cysteine-18. All three toxins showed the ability to stimulate guanylate cyclase and to elicit chloride secretion in Caco-2 cell monolayers mounted in Ussing chambers. Toxin A-1 and toxin B demonstrated greatly reduced immunoreactivity whereas toxin A-2 was not recognized at all in the ELISA. It is likely that the three modified toxins were generated by Klebsiella specific proteolytic processing of the original pretoxin. These results have important implications for the diagnosis and prevention of heat-stable toxin-induced diarrhea.

Immune modulation by the cholera-like enterotoxin B-subunits: from adjuvant to immunotherapeutic

Cholera toxin (Ctx) and its close relative, Escherichia coli heat-labile enterotoxin (Etx) have long been established as potent mucosal and systemic adjuvants. Problems arising from their inherent toxicity have, however, precluded human use. Here we describe findings which demonstrate that contrary to the established dogma the non-toxic B-subunit of Etx (EtxB) is a highly potent mucosal adjuvant capable of potentiating protective immunity to viral infection. The mechanisms which underlie this activity arise from an ability to trigger specific signaling processes in lymphocyte populations which modulate differentially their activation, differentiation and survival. The elucidation of these properties has led to the further use of EtxB as an agent capable of preventing the establishment of autoimmune diseases. The basis for these activities and their potential applicability to human therapies are discussed.

Pet toxin from enteroaggregative Escherichia coli produces cellular damage associated with fodrin disruption

Pet toxin is a serine protease from enteroaggregative Escherichia coli which has been described as causing enterotoxic and cytotoxic effects. In this paper we show that Pet produces spectrin and fodrin (nonerythroid spectrin) disruption. Using purified erythrocyte membranes treated with Pet toxin, we observed degradation of alpha- and beta-spectrin chains; this effect was dose and time dependent, and a 120-kDa protein fraction was observed as a breakdown product. Spectrin degradation and production of the 120-kDa subproduct were confirmed using specific antibodies against the alpha- and beta-spectrin chains. The same degradation effect was observed in alpha-fodrin from epithelial HEp-2 cells, both in purified cell membranes and in cultured cells which had been held in suspension for 36 h; these effects were confirmed using antifodrin rabbit antibodies. The spectrin and fodrin degradation caused by Pet is related to the Pet serine protease motif. Fluorescence and light microscopy of HEp-2 Pet-treated cells showed morphological alterations, which were associated with irregular distribution of fodrin in situ. Spectrin and fodrin degradation by Pet toxin were inhibited by anti-Pet antibodies and by phenylmethylsulfonyl fluoride. A site-directed Pet mutant, which had been shown to abolish the enterotoxic and cytotoxic effects of Pet, was unable to degrade spectrin in erythrocyte membranes or purified spectrin or fodrin in epithelial cell assays. This is a new system of cellular damage identified in bacterial toxins which includes the internalization of the protease, induction of some unknown intermediate signaling steps, and finally the fodrin degradation to destroy the cell.

Staphylococcal enterotoxins

Staphylococcus aureus is a major human pathogen that produces a wide array of toxins, thus causing various types of disease symptoms. Staphylococcal enterotoxins (SEs), a family of nine major serological types of heat stable enterotoxins, are a leading cause of gastroenteritis resulting from consumption of contaminated food. In addition, SEs are powerful superantigens that stimulate non-specific T-cell proliferation. SEs share close phylogenetic relationships, with similar structures and activities. Here we review the structure and function of each known enterotoxin.

The crystal structure of staphylococcal enterotoxin H: implications for binding properties to MHC class II and TcR molecules

The X-ray structure of the superantigen staphylococcal enterotoxin H (SEH) has been determined at 1.69 A resolution. In this paper we present two structures of zinc-free SEH (apoSEH) and one zinc-loaded form of SEH (ZnSEH). SEH exhibits the conventional superantigen (SAg) fold with two characteristic domains. In ZnSEH one zinc ion per SEH molecule is bound to the C-terminal beta-sheet in the region implicated for major histocompatibility complex class II (MHC class II) binding in SEA, SED and SEE. Surprisingly, the zinc ion has only two ligating amino acid residues His206 and Asp208. The other ligands to the zinc ion are two water molecules. An extensive packing interaction between two symmetry-related molecules in the crystal, 834 A(2)/molecule, forms a cavity that buries the zinc ions of the molecules. This dimer-like interaction is found in two crystal forms. Nevertheless, zinc-dependent dimerisation is not observed in solution, as seen in the case of SED. A unique feature of SEH as compared to other staphylococcal enterotoxins is a large negatively charged surface close to the Zn(2+) site. The interaction of SEH with MHC class II is the strongest known among the staphylococcal enterotoxins. However, SEH seems to lack a SEB-like MHC class II binding site, since the side-chain properties of structurally equivalent amino acid residues in SEH and those in SEB-binding MHC class II differ dramatically. There is also a structural flexibility between the domains of SEH. The domains of two apoSEH structures are related by a 5 degrees rotation leading to at most 3 A difference in C(alpha) positions. Since the T-cell receptor probably interacts with both domains, SEH by this rotation may modulate its binding to different TcR Vbeta-chains.

Characterization and distribution of a new enterotoxin-related superantigen produced by Staphylococcus aureus

Staphylococcal enterotoxins (SEs) are a family of structurally related pyrogenic exotoxins consisting of the five prototypic SEs (types A to E) and three newly characterized SEs (types G to I) produced by Staphylococcus aureus (S. aureus). They also work as superantigens and cause food poisoning and shock symptoms in humans. In this study, we cloned a new variant gene of the seg and characterized its superantigenic properties and distribution among the clinical isolates of S. aureus. The gene encodes a 233 amino acid protein which is highly homologous to SEG (97.7%). The variant SEG (SEGv) expressed by the cloned gene exerted mitogenic activity on human peripheral blood mononuclear cells at the concentration of 100 pg/ml. T cells bearing Vbeta3, 12, 13.1, 13.2, 14 and 15 were preferentially expanded after stimulation with the recombinant protein. The mRNA of the variant seg gene was detected in the total RNA of the organisms bearing this gene. By PCR, 27 out of 48 clinical isolates of S. aureus (56%) possessed either the seg or variant seg gene. These findings suggest that SEG, or SEGv, is one of the most frequently produced superantigen exotoxins by S. aureus and may participate in the inflammatory process of the host by activating a distinct set of Vbeta families of T cells.

Structure of m-carboxyphenyl-alpha-D-galactopyranoside complexed to heat-labile enterotoxin at 1.3 A resolution: surprising variations in ligand-binding modes

In the quest to develop drugs against traveller's diarrhoea and cholera, the structure of the B pentamer of heat-labile enterotoxin (LT) complexed with a new receptor-binding antagonist, m-carboxyphenyl-alpha-D-galactopyranoside, has been determined. The high resolution obtained for this structure allowed anisotropic refinement of the model. It was also now possible to confirm at a near-atomic resolution the structural similarity between the B subunits of LT and the closely related cholera toxin (CT), including the similarity in deviations of planarity of the same peptide unit in LT and CT. The structure of the LT complex clearly revealed different conformations for the m--carboxyphenyl moiety of the ligand in the five B subunits of LT, while the binding modes of the well defined galactopyranoside moieties were identical. In two binding sites the m-carboxyphenyl moiety displayed no significant electron density, demonstrating significant flexibility of this moiety. In a third binding site the m-carboxyphenyl moiety could be modelled unambiguously into the density. The two remaining binding sites were involved in crystal packing contacts and the density for the ligands in these two binding sites clearly revealed different binding modes, of which one conformation was identical to and one completely different from the conformation of m-carboxyphenyl-galactopyranoside in the third subunit. The multiple binding modes observed in the crystal may represent the ensemble of conformations of m-carboxyphenyl-alpha-D-galactopyranoside complexed to LT in solution.

Extracellular DsbA-insensitive folding of Escherichia coli heat-stable enterotoxin STa in vitro

To study the folding of human Escherichia coli heat-stable enterotoxin STh, we used the major protein subunit of CS31A fimbriae (ClpG) as a marker of STh secretion and a provider of a signal peptide. We established that STh genetically fused to the N or C terminus of ClpG was able to mobilize ClpG to the culture supernatant while still retaining full enterotoxicity. These features indicate that the STh activity was not altered by the chimeric structure and suggest that spatial conformation of STh in the fusion is close to that of the native toxin, thus permitting recognition and activation of the intestinal STh receptor in vivo. In contrast to other studies, we showed that disulfide bond formation did not occur in the periplasm through the DsbA pathway and that there was no correlation between DsbA and secretion, folding, or activity. This discrepancy was not attributable to the chimeric nature of STh since there was no effect of dsbA or dsbB mutations on secretion and activity of recombinant STh from which ClpG had been deleted. Periplasmic and lysate fractions of dsbA(+) and dsbA(-) cells did not have any STh activity. In addition, the STh chimera was exclusively found in an inactive reduced form intracellularly and in an active oxidized form extracellularly, irrespective of the dsbA background. Subsequently, a time course experiment in regard to the secretion of STh from both dsbA(+) and dsbA(-) cells indicated that the enterotoxin activity (proper folding) in the extracellular milieu increased with time. Overall, these findings provide evidence that STa toxins can be cell-released in an unfolded state before being completely disulfide-bonded outside the cell.

Superantigen antagonist protects against lethal shock and defines a new domain for T-cell activation

Superantigens trigger an excessive cellular immune response, leading to toxic shock. We have designed a peptide antagonist that inhibits superantigen-induced expression of human genes for interleukin-2, gamma interferon and tumor necrosis factor-b, which are cytokines that mediate shock. The peptide shows homology to a b-strand-hinge-a-helix domain that is structurally conserved in superantigens, yet is remote from known binding sites for the major histocompatibility class II molecule and T-cell receptor. Superantigens depend on this domain for T-cell activation. The peptide protected mice against lethal challenge with staphylococcal and streptococcal superantigens. Moreover, it rescued mice undergoing toxic shock. Surviving mice rapidly developed protective antibodies against superantigen that rendered them resistant to further lethal challenges, even with different superantigens. Thus, the lethal effect of superantigens can be blocked with a peptide antagonist that inhibits their action at the beginning of the toxicity cascade, before activation of T cells takes place.

Immune response with biodegradable nanospheres and alum: studies in rabbits using staphylococcal enterotoxin B-toxoid

In this study, the adjuvant effect of the sustained release biodegradable nanospheres (100-150 nm in diameter) has been compared with alum. Nanospheres were formulated using a biodegradable polylactic polyglycolic acid copolymer (PLGA, 50:50) containing Staphylococcal Enterotoxin B (SEB) toxoid as a model vaccine antigen. Systemic immune response of the nanospheres containing toxoid was studied in rabbits by subcutaneous immunization. The data demonstrated that approximately 30% of the toxoid activity was lost following its encapsulation into nanospheres. Under in vitro conditions, nanospheres demonstrated sustained release of the toxoid. However, only 20% of the antigenic toxoid was released over the first 2 weeks of the release study. Immunization of animals with equal doses of toxoid, either using nanospheres or alum induced a comparable systemic immune response (IgG, IgM and IgA). The immune response reached a maximum level at 7 weeks post-immunization, which then gradually declined with time. The booster dose of toxoid at 19 weeks, either using alum or nanospheres induced similar immune response in both the groups, but was greater than the primary immune response. The studies, thus, suggest that biodegradable nanospheres could be used as a vaccine adjuvant.

A new biological agent for treatment of Shiga toxigenic Escherichia coli infections and dysentery in humans

Gastrointestinal disease caused by Shiga toxin-producing bacteria (such as Escherichia coli O157:H7 and Shigella dysenteriae) is often complicated by life-threatening toxin-induced systemic sequelae, including hemolytic-uremic syndrome. Such infections can now be diagnosed very early in the course of the disease, but at present no effective therapeutic intervention is possible. Here, we constructed a recombinant bacterium that displayed a Shiga toxin receptor mimic on its surface, and it adsorbed and neutralized Shiga toxins with very high efficiency. Moreover, oral administration of the recombinant bacterium completely protected mice from challenge with an otherwise 100%-fatal dose of Shiga toxigenic E. coli. Thus, the bacterium shows great promise as a 'probiotic' treatment for Shiga toxigenic E. coli infections and dysentery.

Novel carbohydrate binding site recognizing blood group A and B determinants in a hybrid of cholera toxin and Escherichia coli heat-labile enterotoxin B-subunits

The B-subunits of cholera toxin (CTB) and Escherichia coli heat-labile enterotoxin (LTB) are structurally and functionally related. However, the carbohydrate binding specificities of the two proteins differ. While both CTB and LTB bind to the GM1 ganglioside, LTB also binds to N-acetyllactosamine-terminated glycoconjugates. The structural basis of the differences in carbohydrate recognition has been investigated by a systematic exchange of amino acids between LTB and CTB. Thereby, a CTB/LTB hybrid with a gain-of-function mutation resulting in recognition of blood group A and B determinants was obtained. Glycosphingolipid binding assays showed a specific binding of this hybrid B-subunit, but not CTB or LTB, to slowly migrating non-acid glycosphingolipids of human and animal small intestinal epithelium. A binding-active glycosphingolipid isolated from cat intestinal epithelium was characterized by mass spectrometry and proton NMR as GalNAcalpha3(Fucalpha2)Galbeta4(Fucalpha3)Glc NAcbeta3Galbeta4Glc NAcbeta3Galbeta4Glcbeta1Cer. Comparison with reference glycosphingolipids showed that the minimum binding epitope recognized by the CTB/LTB hybrid was Galalpha3(Fucalpha2)Galbeta4(Fucalpha3)GlcNAc beta. The blood group A and B determinants bind to a novel carbohydrate binding site located at the top of the B-subunit interfaces, distinct from the GM1 binding site, as found by docking and molecular dynamics simulations.

Defining a novel domain of staphylococcal toxic shock syndrome toxin-1 critical for major histocompatibility complex class II binding, superantigenic activity, and lethality

Staphylococcal toxic shock syndrome toxin-1 (TSST-1) is implicated in the pathogenesis of superantigen-mediated shock. We previously identified TSST-1 residues G31/S32 to be important for major histocompatibility complex (MHC) class II binding, as well as superantigenic and lethal activities. However, the site-directed TSST-1 mutant toxin, G31R, could still induce mitogenesis and low-level TNF alpha secretion, suggesting that additional MHC class II binding sites other than G31/S32 may exist. In the current study, a TSST-1-neutralizing monoclonal antibody, MAb5, was found to inhibit TSST-1 binding to human peripheral blood mononuclear cells, neutralize TSST-1-induced mitogenesis and cytokine secretion, and protect against TSST-1-induced lethality in vivo. Epitope mapping revealed that MAb5 bound to TSST-1 residues 51-56 (T(51-56); 51YYSPAF56). Peptide T(51-56) was synthesized and found to also inhibit TSST-1 binding to human monocytes as well as TSST-1-induced mitogenesis, cytokine secretion, and lethality in vivo. This T(51-56) epitope, located within the beta 3/beta 4 loop, and the previously identified G31/S32 epitope, within the beta 1/beta 2 loop of TSST-1, are separated within the primary sequence, but spatially juxtaposed to each other. Collectively, these findings suggest that a discontinuous epitope comprising of regions within both the beta 1/beta 2 and beta 3/beta 4 loops, are critical for MHC class II binding, and the consequent superantigenic and lethal activities of TSST-1.

[Study on the physicochemical properties of Campylobacter jejuni enterotoxin]

Precipitate of Campylobacter jejuni cytotonic enterotoxin(CE) performed in an 80% saturated solution of ammonium sulfateit indicated that there were some little molecular proteins except the 68 kD main band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis(SDS-PAGE), whereas the eluate from GM1 ganglioside affinity column chromatography exhibited only one 68 kD band on SDS-PAGE. The results suggest that CE mainly be consisted of 68 kD protein. The toxin is heat-labile, pH dependent and resistant to trypsin, It could be completely inactivated by heating at either 56 degrees C and 60 degrees C for 30 min or 100 degrees C for 15 min. The activity was maximum at pH 6.0 and was completely inactivate at pH 3.0 and pH 9.0, and rapidly reduced after storage over 3 d at 4 degrees C. The anti-LT serum could completely inhibited the activity of CE.

The spectral and thermodynamic properties of staphylococcal enterotoxin A, E, and variants suggest that structural modifications are important to control their function

The superantigens staphylococcal enterotoxin A and E (SEA and SEE) can activate a large number of T-cells. SEA and SEE have approximately 80% sequence identity but show some differences in their biological function. Here, the two superantigens and analogues were characterized biophysically. SEE was shown to have a substantially higher thermal stability than SEA. Both SEA and SEE were thermally stabilized by 0.1 mM Zn(2+) compared with Zn(2+)-reduced conditions achieved using 1 mM EDTA or specific replacements that affect Zn(2+) coordination. The higher stability of SEE was only partly caused by the T-cell receptor (TCR) binding regions, whereas regions in the vicinity of the major histocompatibility complex class II binding sites affected the stability to a greater extent. SEE exhibited a biphasic denaturation between pH 5.0-6.5, influenced by residues in the TCR binding regions. Interestingly, enzyme-linked immunosorbent assay, isoelectric focusing, and circular dichroism analysis indicated that conformational changes had occurred in the SEA/E chimerical constructs relative to SEA and SEE. Thus, it is proposed that the Zn(2+) binding site is very important for the stability and potency of SEA and SEE, whereas residues in the TCR binding site have a substantial influence on the molecular conformation to control specificity and function.

N-terminus of mature heat-labile enterotoxin chain B is critical for its extracellular secretion in Vibrio cholerae

The effects of addition of a few amino acids to the amino- and carboxy-terminal regions of the mature portion of the heat-labile enterotoxin chain B (LTB) of Escherichia coli on protein export, secretion and assembly were investigated. In E. coli, LTB (secretory protein) with or without the extension at the N- or C-terminus accumulated in the periplasmic fraction. For Vibrio cholerae, LTB with the extension at the C-terminus was exported to the periplasm followed by secretion to the extracellular milieu. However, LTB with the N-terminus extension was exported to the periplasm only. Our findings suggest that in the case of V. cholerae, the N-terminus of the mature LTB plays an important role in its secretion to the extracellular milieu.

Lectin-mediated transport of nanoparticles across Caco-2 and OK cells

Recent experiments by a number of workers have suggested that it may be possible to use various targeting molecules, which bind to the intestinal epithelium, to promote the uptake and transport of nanoparticles from the intestine to the circulation. We have used commercial nanoparticles to examine the effect of size, density and inhibitors on uptake of lectin-coated nanoparticles by epithelial cells. The degree of uptake was most influenced by the density of lectin on the particle, with size and type of lectin being less important. Uptake could be inhibited by the presence of specific sugars or free lectin. These studies should provide a good basis for the design of targetable biodegradable drug-loadable particles suitable for oral delivery.

Identification of a Clostridium perfringens enterotoxin region required for large complex formation and cytotoxicity by random mutagenesis

Clostridium perfringens enterotoxin (CPE), a single polypeptide of 319 amino acids, has a unique multistep mechanism of action. In the first step, CPE binds to claudin proteins and/or a 50-kDa eukaryotic membrane protein receptor, forming a small ( approximately 90-kDa) complex. This small complex apparently then associates with a 70-kDa eukaryotic membrane protein, resulting in formation of a large complex that induces the onset of membrane permeability alterations. To better define the boundaries of CPE functional regions and to identify specific amino acid residues involved in various steps of CPE action, in this study we subjected the cloned cpe gene to random mutagenesis in XL-1 Red strains of Escherichia coli. Seven CPE random mutants with reduced cytotoxicity for Vero cells were phenotypically characterized for the ability to complete each step in CPE action. Five of these seven recombinant CPE (rCPE) random mutants (G49D, S59L, R116S, R137G, and S167P) exhibited binding characteristics similar to those of rCPE or native CPE, while the Y310C and W226Stop mutants showed reduced binding and no binding, respectively, to brush border membranes. Interestingly, two completely nontoxic mutants (G49D and S59L) were able to bind and form small complex but they did not mediate any detectable large complex formation. Another strongly attenuated mutant, R116S, formed reduced amounts of an anomalously migrating large complex. Collectively, these results provide further support for large complex formation being an essential step in CPE action and also identify the CPE region ranging from residues approximately 45 to 116 as important for large complex formation. Finally, we also report that limited removal of extreme N-terminal CPE sequences, which may occur in vivo during disease, stimulates cytotoxic activity by enhancing large complex formation.

Determination of the binding site on the extracellular domain of guanylyl cyclase C to heat-stable enterotoxin

Guanylyl cyclase C, one of the family of membrane-bound guanylyl cyclases, consists of an extracellular domain and an intracellular domain, which are connected by a single transmembrane polypeptide. The extracellular domain binds unique small polypeptides with high specificity, which include the endogenous peptide hormones, guanylin and uroguanylin, as well as an exogenous enterotoxigenic peptide, heat-stable enterotoxin, secreted by pathogenic Escherichia coli. Information on this specific binding is propagated into the intracellular domain, followed by the synthesis of cGMP, a second messenger that regulates a variety of intracellular physiological processes. This study reports the design of a photoaffinity labeled analog of heat-stable enterotoxin (biotinyl-(AC(5))(2)-[Gly(4), Pap(11)]STp(4-17)), which incorporates a Pap residue (p-azidophenylalanine) at position 11 and a biotin moiety at the N terminus, and the use of this analog to determine the ligand-binding region of the extracellular domain of guanylyl cyclase C. The endoproteinase Lys-C digestion of the extracellular domain, which was covalently labeled by this ligand, and mass spectrometric analyses of the digest revealed that the ligand specifically binds to the region (residue 387 to residue 393) of guanylyl cyclase C. This region is localized close to the transmembrane portion of guanylyl cyclase C on the external cellular surface. This result was further confirmed by characterization of site-directed mutants of guanylyl cyclase C in which each amino acid residue was substituted by an Ala residue instead of residues normally located in the region. This experiment provides the first direct demonstration of the ligand-binding site of guanylyl cyclase C and will contribute toward an understanding of the receptor recognition of a ligand and the modeling of the interaction of the receptor and its ligand at the molecular level.

Diarrhea induction by rotavirus NSP4 in the homologous mouse model system

Comparison of the NSP4 amino acid sequences from 31 strains of mammalian rotaviruses revealed the presence of four distinct NSP4 alleles; i.e., the Wa, KUN, AU-1, and EW alleles. The EW allele consists only of NSP4s from murine rotavirus strains and is divergent from other NSP4 alleles from the evolutionary perspective. There have been conflicting reports regarding the enterotoxigenic activity of NSP4 in the mouse model system; heterologous simian and porcine rotavirus NSP4s function as an enterotoxin in mice, while a homologous EC NSP4 does not play a dominant role as an enterotoxin in the cystic fibrosis conductance regulator knockout mice. To further examine the enterotoxigenic activity of NSP4, we expressed in Escherichia coli a recombinant protein consisting of glutathione S-transferase and amino acid residues 86-175 of the EW NSP4. We found that this fusion protein caused diarrhea in the majority (8/14) of 5- to 6-day-old CD1 mice. This study confirmed and extended that group A rotavirus NSP4s were able to induce diarrhea in neonatal mice and had an enterotoxigenic activity.