Essential structure for full enterotoxigenic activity of heat-stable enterotoxin produced by enterotoxigenic Escherichia coli

Nucleotide sequence of yst, the Yersinia enterocolitica gene encoding the heat-stable enterotoxin, and prevalence of the gene among pathogenic and nonpathogenic yersiniae

The gene encoding the heat-stable enterotoxin (yst) was cloned from the chromosome of Yersinia enterocolitica W1024 (serotype O:9), and the nucleotide sequence was determined. The yst gene encodes a 71-amino-acid polypeptide. The C-terminal 30 amino acids of the predicted protein exactly correspond to the amino acid sequence of the toxin extracted from culture supernatants (T. Takao, N. Tominaga, and Y. Shimonishi, Biochem. Biophys. Res. Commun. 125:845-851, 1984). The N-terminal 18 amino acids have the properties of a signal sequence. The central 22 residues are removed during or after the secretion process. This organization in three domains (Pre, Pro, and mature Yst) resembles that of the enterotoxin STa of Escherichia coli. The degree of conservation between the E. coli and Y. enterocolitica toxins is much lower in the Pre and the Pro domains than in the mature proteins. The mature toxin of Y. enterocolitica is much larger than that of E. coli, but the active domain appears to be highly conserved. The yst gene of Y. enterocolitica introduced in E. coli K-12 directed the secretion of an active toxin. The cloned yst gene was used as an epidemiological probe among a collection of 174 strains representative of all Yersinia species except Yersinia pestis and numerous Y. enterocolitica subgroups. In Y. enterocolitica, there was a clear-cut difference between pathogenic and nonpathogenic strains: 89 of 89 pathogenic and none of 51 nonpathogenic strains contained yst-homologous DNA, suggesting that Yst is involved in pathogenesis. Among the other Yersinia species, only four strains of Yersinia kristensenii had DNA homologous to yst.

Two precursors of the heat-stable enterotoxin of Escherichia coli: evidence of extracellular processing

Expression of the gene of the methanol-soluble, heat-stable enterotoxin of Escherichia coli (STA) allowed the identification by SDS-PAGE of a cell-associated 7500 Dalton STA-related peptide; when similar experiments were performed with a phosphate buffer SDS-PAGE system, an additional Mr 9800 band became apparent. The 9800 Dalton form, pre-pro-STA, accumulated as an intracellular species when the experiments were performed in the presence of the proton ionophore CCCP (carbonylcyanide m-chlorophenylhydrazone); by pulse-chase experiments, it was shown that pre-pro-STA became a periplasmic Mr 7500 pro-STA and this form was chased to the culture supernatant; periplasmic and extracellular pro-STA showed the same electrophoretic mobility. A short time after the pulse, pro-STA was converted extracellularly to mature STA (Mr 4500). It is proposed that STA is synthesized as pre-pro-STA, a 72-amino-acid peptide that is subsequently cleaved between amino acids 19 and 20 as it is translocated across the inner membrane. The resulting 53-amino-acid pro-STA is first detected in the periplasm and is then secreted to the culture supernatant. Pro-STA is cleaved extracellularly to yield mature STA (Mr 4500).

Different crosslinking agents identify distinctly different putative Escherichia coli heat-stable enterotoxin rat intestinal cell receptor proteins

The receptor for heat-stable enterotoxins (ST) produced by Escherichia coli and related organisms is located in the brush border region of intestinal villus cells. Heterobifunctional and homobifunctional crosslinkers were used to covalently couple 125I-ST to rat intestinal cell brush border membrane proteins. Experimental conditions during ligand binding and subsequent crosslinking significantly influence the efficiency of crosslinking, and the number of peptides specifically crosslinked to the 125I-ST. Multiple proteins efficiently coupled to 125I-ST with agents that can couple through the ST amino terminus. The crosslinker 1-ethyl-3-(dimethylaminopropyl)carbodiimide (EDC), which can react with the carboxy terminus of the ST, covalently crosslinked 125I-ST to a single protein with an apparent Mr of 125,000-130,000, larger than the proteins identified using longer crosslinkers. Each of the proteins identified by crosslinking migrate with the same retention time on gel filtration after solubilization, with an approximate molecular size of 150,000-200,000.

Inhibition by the protein kinase inhibitors, isoquinolinesulfonamides, of fluid accumulation induced by Escherichia coli heat-stable enterotoxin, 8-bromo-cGMP and 8-bromo-cAMP in suckling mice

The effects of isoquinolinesulfonamides, which inhibit protein kinase, on fluid accumulations induced by heat-stable enterotoxin of enterotoxigenic Escherichia coli (STh), 8-bromo-cGMP and 8-bromo-cAMP in suckling mice were studied. Both N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide (H-8) and N-(2-aminoethyl)-5-isoquinolinesulfonamide (H-9) inhibited the fluid accumulation induced by STh. Fluid accumulation induced by four mouse units of STh (four-fold the minimum effective dose, 10 ng) was completely inhibited by 0.4 mumol of H-8 or H-9. H-8 and H-9 also inhibited fluid accumulation induced by 8-bromo-cGMP. On the other hand, H-8 and H-9 only partially inhibited fluid accumulation in suckling mice induced by 8-bromo-cAMP, probably because their affinities to cAMP-dependent protein kinase were lower than their affinities to cGMP-dependent protein kinase. From these results, it is concluded that the activation of cGMP-dependent protein kinase by increase in cGMP by ST or by 8-bromo-cGMP, and very probably the activation of cAMP-dependent protein kinase by increase in cAMP by cholera enterotoxin and heat-labile enterotoxin of enterotoxigenic E. coli and by 8-bromo-cAMP are necessary steps in signal transduction following increases in concentrations of cGMP and cAMP in intestinal brush border cells.

Detection with synthetic oligonucleotide probes of nucleotide sequence variations in the genes encoding enterotoxins of Escherichia coli

We examined variations in the genes encoding heat-stable enterotoxin (ST) and heat-labile enterotoxin (LT) in 88 strains of Escherichia coli isolated from individuals with traveler's diarrhea to find suitable sequences for use as oligonucleotide probes. Four oligonucleotide probes of the gene encoding ST of human origin (STIb or STh), one oligonucleotide probe of the gene encoding ST of porcine origin (STIa or STp), and three oligonucleotide probes of the gene encoding LT of human origin (LTIh) were used in DNA colony hybridization tests. In 15 of 22 strains possessing the STh gene and 28 of 42 strains producing LT, the sequences of all regions tested were identical to the published sequences. One region in the STh gene examined with a 18-mer probe was relatively well conserved and was shown to be closely associated with the enterotoxicity of the E. coli strains in suckling mice. This oligonucleotide, however, hybridized with strains of Vibrio cholerae O1, V. parahaemolyticus, and Yersinia enterocolitica that gave negative results in the suckling mouse assay.

Influence of a glycine or proline substitution on the functional properties of a 14-amino-acid analog of Escherichia coli heat-stable enterotoxin

Analogs of Escherichia coli heat-stable enterotoxin (ST) differing in chain length or the presence of turn-forming residues were assessed for binding to receptors, activation of particulate guanylate cyclase, and stimulation of secretion in suckling mice. These analogs included the native 18-amino-acid peptide (ST), the 14-amino-acid carboxy terminus of this native peptide with a proline at position 12 (ST[5-18]proline), and the 14-amino-acid carboxy terminus in which the proline at position 12 was substituted with glycine (ST[5-18]glycine). Each analog bound to the receptor in a dose-dependent fashion, completely displacing [125I]ST in competitive binding assays. However, their potencies differed significantly: ST demonstrated the highest affinity (inhibition constant [Ki], 10(-9) M), followed by ST[5-18]proline (Ki, 10(-7) M) and ST[5-18]glycine (Ki, 10(-6) M). Similarly, these peptides maximally activated particulate guanylate cyclase and stimulated intestinal secretion in suckling mice. Their rank order of potency in these assays was similar to that described for receptor binding: ST greater than ST[5-18]proline greater than ST[5-18]glycine. These data demonstrate that the full peptide structure is not absolutely required for pharmacological, biochemical, or biological activity. However, the four amino-terminal residues contribute significantly to the potency of these peptides. In addition, the turn imposed by the proline residue at position 12 is not absolutely required for receptor occupancy or activation of the biochemical cascade that results in intestinal secretion. However, it significantly increases the potency of the toxin. These data illustrate the importance of primary and secondary structures to the biochemical, pharmacological, and physiological activities of the ST produced by E. coli.

A long-acting heat-stable enterotoxin analog of enterotoxigenic Escherichia coli with a single D-amino acid

A heat-stable enterotoxin (STp) consisting of 18 amino acid residues including 6 half-cystine residues is produced by a porcine strain of enterotoxigenic Escherichia coli. Analogs of STp with replacements of single residues at each from positions 5 to 17 by the corresponding D-amino acid residue were synthesized by a solid-phase method. Of these analogs, [D-Cys5]-STp[5-17] showed the same biological properties as STp[5-17]. Moreover, its activity to cause fluid accumulation in suckling mouse lasts more than 24 hours, whereas the activity of STp[5-17] decreases after 6-10 hours. These results indicate that the action of the analog [D-Cys5]-STp[5-17] is strongly agonistic to that of the native ST.

Opossum kidney contains a functional receptor for the Escherichia coli heat-stable enterotoxin

The Escherichia coli heat-stable enterotoxin (ST1 or STa) binds to specific receptors on mammalian intestinal brush border membranes, and stimulates guanylate cyclase in those membranes. We have found a similar signal transduction system in brush border membranes prepared from kidney cortex of the American opossum (Didelphis virginiana, and in a cell line (OK cell) derived from that tissue. Activation of guanylate cyclase by ST1 is therefore not limited to intestinal cells. Furthermore, since it is unlikely that ST1 which is produced in the intestinal lumen, would have access to kidney receptors, this suggests the existence of an endogenous peptide resembling ST1, at least in marsupials.

Rectification of two Escherichia coli heat-stable enterotoxin allele sequences and lack of biological effect of changing the carboxy-terminal tyrosine to histidine

Resequencing estA3, an allele of the methanol-soluble heat-stable enterotoxin of Escherichia coli showed that the proline triplet 19 is in fact an alanine codon; thus, estA alleles 3 and 4 were shown to be identical. Resequencing has also shown that the carboxy terminus of another allele, estA2, is not the previously inferred histidine triplet but the same tyrosine codon reported for all other estA alleles. The improperly inferred histidine codon was used in constructions to fuse estA2 to the B subunit of the heat-labile enterotoxin gene, and the fused gene products as well as three amino acid insertional mutants containing histidine-72 were not efficiently secreted. We show that the defective secretion is not due to histidine as a carboxy-terminal residue, since site-directed mutagenesis of wild-type tyrosine-72 to histidine did not influence the localization of the activity of the methanol-soluble heat-stable enterotoxin.

Citrobacter freundii produces an 18-amino-acid heat-stable enterotoxin identical to the 18-amino-acid Escherichia coli heat-stable enterotoxin (ST Ia)

We purified and sequenced the heat-stable enterotoxin produced by Citrobacter freundii. The toxin was detected during purification by reaction with monoclonal antibody to Escherichia coli heat-stable enterotoxin. The C. freundii toxin amino acid sequence was identical to that of the 18-amino-acid heat-stable enterotoxin (STa) produced by toxigenic E. coli.

Importance of disulfide bridges in the structure and activity of Escherichia coli enterotoxin ST1b

A 13-amino acid sequence of the Escherichia coli heat-stable enterotoxin ST1b encodes its receptor-binding and diarrheal functions. This sequence includes six cysteines involved in three intramolecular disulfide bridges. To determine the importance of disulfide bridges to the biological activity of ST1b, we synthesized 15 analogues of the tridecapeptide representing all possible replacements of two of the six cysteines by alanines. Only 2 analogues--namely, A6,11ST1b-(6-18) and A10,18ST1b(6-18)--could inhibit the binding of a radiolabeled analogue of ST1b to rat intestinal cells. The purified peptides were, respectively, 4200 and 130 times less effective as inhibitors than ST1b(6-18), the sequence that includes all six cysteines. In addition, both peptides produce diarrhea when given orally to suckling mice. These analogues share in common only two cysteines (Cys-7 and Cys-15), suggesting that four cysteines, two of which are Cys-7 and Cys-15, are necessary for activity. A pattern of disulfide linkages is proposed where Cys-7 is paired to Cys-15, Cys-6 to Cys-11, and Cys-10 to Cys-18, the preceding disulfide bridges being ranked in descending order of importance in terms of their respective contribution to the activity of the enterotoxin. Using this disulfide bridge arrangement and constraints derived from NMR spectroscopy, we propose a folding pattern for the toxic domain of ST1b.

Mode of disulfide bond formation of a heat-stable enterotoxin (STh) produced by a human strain of enterotoxigenic Escherichia coli

To determine the modes of three disulfide linkages in the heat-stable enterotoxin (STh) produced by a human strain of enterotoxigenic Escherichia coli, we synthesized STh(6-18), which consists of 13 amino acid residues and has the same intramolecular disulfide linkages as native STh [(1985) FEBS Lett. 181, 138-142], by stepwise and selective formation of disulfide bonds using different types of removable protecting groups for the Cys residues. Synthesis of the peptide with different modes of disulfide bond formation provided three peptides consistent with standard STh(6-18) in their physicochemical and biological properties, thereby indicating that the disulfide bonds in STh(6-18) are Cys-Cys-Glu-Leu-Cys-Cys-Asn-Pro-Ala-Cys-Thr-Gly-Cys.

Immunisation of volunteers with a synthetic peptide vaccine for enterotoxigenic Escherichia coli

A completely synthetically produced peptide vaccine, consisting of the 18-aminoacid Escherichia coli heat-stable toxin and the 26-aminoacid epitope of the heat-labile toxin B subunit, was given orally to thirteen volunteers. It raised antitoxin titres to both toxin components four-fold in serum samples and seven-fold in jejunal aspirates over preimmunisation control titres. Jejunal aspirates taken after immunisation from vaccinees, but not controls, neutralised the secretory activity of both toxins in appropriate biological assays. These findings show that synthetically produced vaccines are immunogenic in human beings. The peptide used is a promising vaccine for diarrhoeal disease caused by enterotoxigenic strains of E coli.

Design of a photoreactive analogue of the Escherichia coli heat-stable enterotoxin STIb: use in identifying its receptor on rat brush border membranes

The Escherichia coli heat-stable enterotoxin, STIb was prepared by solid-phase peptide synthesis and purified to homogeneity by high-pressure liquid chromatography. This analogue was iodinated and shown to bind specifically to rat intestinal membranes. The radiolabeled peptide was derivatized at the amino terminus with the photoreactive heterobifunctional crosslinking agent N-hydroxysuccinimidyl p-benzoylbenzoate. This photoreactive probe also exhibited binding specificity. It was mixed with rat intestinal brush border membranes and photolyzed in the presence or absence of excess unlabeled STIb. Polyacrylamide gel electrophoresis performed in the presence of sodium dodecyl sulfate and 2-mercaptoethanol indicated that the peptide probe was crosslinked specifically to two molecular species of 57 and 75 kDa. One or both of these molecules appear to constitute the enterotoxin receptor or to be in close proximity to it.

Amino acid sequence of heat-stable enterotoxin produced by Vibrio cholerae non-01

The amino acid sequence of heat-stable enterotoxin, produced by Vibrio cholerae non-01 and isolated from its culture supernatant, was determined by both Edman degradation of native and reductively carboxymethylated enterotoxin and also a combination of fast atom bombardment mass spectrometry and carboxypeptidase Y digestion of native enterotoxin to be as follows: Ile-Asp-Cys-Cys-Glu-Ile-Cys-Cys-Asn-Pro-Ala-Cys-Phe-Gly-Cys-Leu-Asn. This sequence is very similar, but not identical, to those of heat-stable enterotoxins produced by enterotoxigenic Escherichia coli and Yersinia enterocolitica.

Isolation, primary structure and synthesis of heat-stable enterotoxin produced by Yersinia enterocolitica

Five heat-stable enterotoxins were isolated from the culture supernatant of Yersinia enterocolitica and purified to homogeneity by DEAE-Sephacel and high-performance liquid chromatographies. They caused acute fluid accumulation in the intestine of suckling mice. The amino acid sequence of one of the enterotoxins was determined to be Ser-Ser-Asp-Trp-Asp-Cys-Cys-Asp-Val-Cys-Cys-Asn-Pro-Ala-Cys-Ala-Gly-Cys, by Edman degradation of its pyridylethylated derivative and a combination of fast atom bombardment mass spectrometry and carboxypeptidase B digestion. This structure was unambiguously confirmed by chemical synthesis. The other enterotoxins had longer or shorter amino acid sequences at their N termini, but the same sequence at their C termini. The six half-cystine residues formed intramolecular disulfide linkages, as shown by measurement of the molecular masses of the enterotoxins by fast atom bombardment mass spectrometry. The sequence of 13 amino acid residues at the C terminus showed similarity to those of heat-stable enterotoxins isolated from enterotoxigenic Escherichia coli [Aimoto, S. et al. (1982) Eur. J. Biochem. 129, 257-263; Takao, T. et al. (1983) FEBS Lett. 152, 1-5] suggesting that these similar sequences are related to the common biological and immunological properties of enterotoxins produced by Y. enterocolitica and enterotoxigenic E. coli.