Evolution and structure of two ADP-ribosylation enterotoxins, Escherichia coli heat-labile toxin and cholera toxin

Impaired mucociliary motility enhances antigen-specific nasal IgA immune responses to a cholera toxin-based nasal vaccine

Nasal mucosal tissues are equipped with physical barriers, mucus and cilia, on their surface. The mucus layer captures inhaled materials, and the cilia remove the inhaled materials from the epithelial layer by asymmetrical beating. The effect of nasal physical barriers on the vaccine efficacy remains to be investigated. Tubulin tyrosine ligase-like family, member 1 (Ttll1) is an essential enzyme for appropriate movement of the cilia on respiratory epithelium, and its deficiency (Ttll1-KO) leads to mucus accumulation in the nasal cavity. Here, when mice were intra-nasally immunized with pneumococcal surface protein A (PspA, as vaccine antigen) together with cholera toxin (CT, as mucosal adjuvant), Ttll1-KO mice showed higher levels of PspA-specific IgA in the nasal wash and increased numbers of PspA-specific IgA-producing plasma cells in the nasal passages when compared with Ttll1 hetero (He) mice. Mucus removal by N-acetylcysteine did not affect the enhanced immune responses in Ttll1-KO mice versus Ttll1-He mice. Immunohistological and flow cytometry analyses revealed that retention time of PspA in the nasal cavity in Ttll1-KO mice was longer than that in Ttll1-He mice. Consistently, uptake of PspA by dendritic cells was higher in the nasopharynx-associated lymphoid tissue (NALT) of Ttll1-KO mice than that of Ttll1-He mice. These results indicate that the ciliary function of removing vaccine antigen from the NALT epithelial layer is a critical determinant of the efficacy of nasal vaccine.

Toxins Utilize the Endoplasmic Reticulum-Associated Protein Degradation Pathway in Their Intoxication Process

Several bacterial and plant AB-toxins are delivered by retrograde vesicular transport to the endoplasmic reticulum (ER), where the enzymatically active A subunit is disassembled from the holotoxin and transported to the cytosol. In this process, toxins subvert the ER-associated degradation (ERAD) pathway. ERAD is an important part of cellular regulatory mechanism that targets misfolded proteins to the ER channels, prior to their retrotranslocation to the cytosol, ubiquitination and subsequent degradation by a protein-degrading complex, the proteasome. In this article, we present an overview of current understanding of the ERAD-dependent transport of AB-toxins to the cytosol. We describe important components of ERAD and discuss their significance for toxin transport. Toxin recognition and disassembly in the ER, transport through ER translocons and finally cytosolic events that instead of overall proteasomal degradation provide proper folding and cytotoxic activity of AB-toxins are discussed as well. We also comment on recent reports presenting medical applications for toxin transport through the ER channels.

Heat-labile enterotoxin: beyond G(m1) binding

Enterotoxigenic Escherichia coli (ETEC) is a significant source of morbidity and mortality worldwide. One major virulence factor released by ETEC is the heat-labile enterotoxin LT, which is structurally and functionally similar to cholera toxin. LT consists of five B subunits carrying a single catalytically active A subunit. LTB binds the monosialoganglioside G_M1, the toxin’s host receptor, but interactions with A-type blood sugars and E. coli lipopolysaccharide have also been identified within the past decade. Here, we review the regulation, assembly, and binding properties of the LT B-subunit pentamer and discuss the possible roles of its numerous molecular interactions.

Cholera toxin: A paradigm for multi-functional engagement of cellular mechanisms (Review)

Cholera toxin (Ctx) from Vibrio cholerae and its closely related homologue, heat-labile enterotoxin (Etx) from Escherichia coli have become superb tools for illuminating pathways of cellular trafficking and immune cell function. These bacterial protein toxins should be viewed as conglomerates of highly evolved, multi-functional elements equipped to engage the trafficking and signalling machineries of cells. Ctx and Etx are members of a larger family of A-B toxins of bacterial (and plant) origin that are comprised of structurally and functionally distinct enzymatically active A and receptor-binding B sub-units or domains. Intoxication of mammalian cells by Ctx and Etx involves B pentamer-mediated receptor binding and entry into a vesicular pathway, followed by translocation of the enzymatic A1 domain of the A sub-unit into the target cell cytosol, where covalent modification of intracellular targets leads to activation of adenylate cyclase and a sequence of events culminating in life-threatening diarrhoeal disease. Importantly, Ctx and Etx also have the capacity to induce a wide spectrum of remarkable immunological processes. With respect to the latter, it has been found that these toxins activate signalling pathways that modulate the immune system. This review explores the complexities of the cellular interactions that are engaged by these bacterial protein toxins, and highlights some of the new insights to have recently emerged.

"Multivalent" saccharides: development of new approaches for inhibiting the effects of glycosphingolipid-binding pathogens

A number of diseases are initiated by the adherence of viruses, bacteria, or bacterial toxins to cell surface carbohydrates, a number of which are components of glycosphingolipids (GSLs). Studies of the binding of lectins indicated that many adhered weakly to monomeric carbohydrate ligands. The seminal observation that lectins adhered more strongly to a ligand with multiple carbohydrate binding sites initiated a plethora of studies designed to identify effective "multivalent" carbohydrate ligands for pathogens expressing multiple carbohydrate-binding sites. In addition to more completely defining ligand specificity of the carbohydrate-binding pathogen, identification of "multivalent" carbohydrate ligands has led to studies of their efficacy as pathogen inhibitors. This commentary focuses on pathogens that recognize the carbohydrate portion of GSLs. Because many GSL-binding pathogens have been shown to bind "multivalent" saccharides, approaches for identifying and preparing them as well as methods for characterizing their effectiveness as ligands are reviewed. Also discussed are areas of promise that should be investigated and pitfalls that might be encountered in the development of "multivalent" saccharides as pharmacologic agents.

Characterisation of cholera toxin by liquid chromatography--electrospray mass spectrometry

Cholera toxin, one of the toxins that may be generated by various strains of the bacterium Vibrio cholerae, can be considered as a substance possibly used in biological warfare. The possibilities of characterising the toxin by liquid chromatography electrospray mass spectrometry (LC-ES-MS) were investigated. The toxin can be detected by flow-injection (FIA) ES-MS of a dialysed solution and observation of the charge envelope signals of its A-unit and B-chain protein; sufficient information for identification by the molecular mass of either protein could be obtained for quantities in the order of 10 fmol. Confirmatory analysis was carried out by 2-mercaptoethanol reduction and FIA-ES-MS detection of the product proteins or by tryptic digest LC-ES-MS with ion chromatogram detection of most of the tryptic fragments of the A-unit and B-chain from the singly, doubly or triply charged ion signals. The confirmatory tryptic digest LC-ES-MS analysis could be achieved with quantities as low as 1 pmol. Possible biovariations in the toxin can mostly be determined by sequencing, where the amino acid composition of tryptic fragments of the A1-chain, T5 and T15, and of the B-chain, T1, T4 and T5, cover all known biovariations. Partial sequencing of cholera toxin, originating from a classical strain, O1/569B, was achieved by LC-ES-MS/MS of most tryptic fragments larger than three amino acid residues.

Nicking sites in a subunit of cholera toxin and Escherichia coli heat-labile enterotoxin for Vibrio cholerae hemagglutinin/protease

We analyzed the nicking site of the A subunit of Escherichia coli heat-labile enterotoxin for hemagglutinin/protease produced by Vibrio cholerae non-O1 (NAG-HA/P). The determined nicking site was the Thr193-Ile194 junction, which was distinct from that for a protease of V. cholerae (Ichinose et al., European Journal of Epidemiology 8, 743-747, 1992). We further analyzed proteolytic cleavage by NAG-HA/P of a synthetic peptide corresponding to the nicking region of cholera toxin A subunit and determined the cleavage site to be preferentially between Ser194 and Met195, and in addition between Ser193 and Ser194.

The role of ADP-ribosylation and G(M1)-binding activity in the mucosal immunogenicity and adjuvanticity of the Escherichia coli heat-labile enterotoxin and Vibrio cholerae cholera toxin

The mucosal route of vaccination has attracted a great deal of attention recently. Not only is mucosal application of vaccines, for example, orally or intranasally, particularly convenient, it also offers the possibility to induce locally produced and secreted S-IgA antibodies in addition to systemic IgG antibodies. These IgA antibodies are known to play a key role in protection against pathogens that invade the host through mucosal surfaces. Induction of such responses is not readily achieved by currently used vaccination strategies, which generally involve intramuscular or subcutaneous injection with inactivated pathogens or antigens thereof. For the induction of a mucosal immune response, the vaccine needs to be applied locally. However, local vaccination with non-replicating antigens is usually ineffective and may result in tolerance unless a mucosal immunoadjuvant is included. The most potent mucosal immunoadjuvants known to date are probably cholera toxin (CT) and the closely related Escherichia coli heat-labile enterotoxin (LT). Although CT and LT have become standard adjuvants for experimental mucosal vaccines, the intrinsic toxicity has thus far precluded their use as adjuvants for human vaccine formulations. In the present review, the mucosal immunogenic and adjuvant properties of LT and CT are described, with special emphasis on the functional role of the individual subunits on their immune-stimulatory properties.

Mutations in the A subunit affect yield, stability, and protease sensitivity of nontoxic derivatives of heat-labile enterotoxin

Heat-labile toxin (LT) is a protein related to cholera toxin, produced by enterotoxigenic Escherichia coli strains, that is organized as an AB5 complex. A number of nontoxic derivatives of LT, useful for new or improved vaccines against diarrheal diseases or as mucosal adjuvants, have been constructed by site-directed mutagenesis. Here we have studied the biochemical properties of the nontoxic mutants LT-K7 (Arg-7-->Lys), LT-D53 (Val-53-->Asp), LT-K63 (Ser-63-->Lys), LT-K97 (Val-97-->Lys), LT-K104 (Tyr-104-->Lys), LT-K114 (Ser-114-->Lys), and LT-K7/K97 (Arg-7-->Lys and Val-97-->Lys). We have found that mutations in the A subunit may have profound effects on the ability to form the AB5 structure and on the stability and trypsin sensitivity of the purified proteins. Unstable mutants, during long-term storage at 4 degrees C, showed a decrease in the amount of the assembled protein in solution and a parallel appearance of soluble monomeric B subunit. This finding suggests that the stability of the B pentamer is influenced by the A subunit which is associated with it. Among the seven nontoxic mutants tested, LT-K63 was found to be efficient in AB5 production, extremely stable during storage, resistant to proteolytic attack, and very immunogenic. In conclusion, LT-K63 is a good candidate for the development of antidiarrheal vaccines and mucosal adjuvants.

Molecular mechanisms of action of bacterial exotoxins

Toxins are one of the inventive strategies that bacteria have developed in order to survive. As virulence factors, they play a major role in the pathogenesis of infectious diseases. Recent discoveries have once more highlighted the effectiveness of these precisely adjusted bacterial weapons. Furthermore, toxins have become an invaluable tool in the investigation of fundamental cell processes, including regulation of cellular functions by various G proteins, cytoskeletal dynamics and neural transmission. In this review, the bacterial toxins are presented in a rational classification based on the molecular mechanisms of action.

Characterization by Western blotting of mouse intestinal glycoproteins bound by Escherichia coli heat-labile enterotoxin type I

Escherichia coli heat-labile enterotoxin type I (LT-I)-binding galactoproteins, which were not recognized by cholera toxin, were detected in intestinal epithelial cells of BALB/c mouse by Western blotting. Inhibitory studies using lectins and modifications of sugar chain suggest that LT-I recognizes certain mucin-type sugar chains containing the terminal Galbeta1-3GalNAc sugar sequence in the galactoproteins. The terminal sugar sequence is identical to that of GM1 ganglioside, the well-documented functional receptor for LT-I.

The heat-labile enterotoxin of Escherichia coli binds to polylactosaminoglycan-containing receptors in CaCo-2 human intestinal epithelial cells

The E. coli type I heat-labile enterotoxin (LT-I) shares considerable functional, structural, and immunological homology with cholera toxin (CT). Although the ganglioside GM1 is the sole receptor for CT, LT-I also appears to utilize additional, unique receptors on intestinal cells not recognized by CT. We characterized this second class of LT-I receptors using the human intestinal epithelial cell line, CaCo-2. CaCo-2 cells bound 8-fold more LT-I than CT, and some of these additional LT-I receptors appeared to be functional, as CT-B only partially inhibited LT-I activity at concentrations that completely inhibited CT activity. Membranes from unlabeled or [3H]galactose-labeled cells were incubated with toxin B subunits and extracted with Triton X-100, and the solubilized toxin B-receptor complexes were immunoabsorbed with anti-B bound to protein A-Sepharose. When organic extracts of the complexes were separated by thin-layer chromatography and overlayed with [125I]toxin, both toxins were found to bind only GM1. Separation of the complexes from [3H]galactose-labeled membranes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a series of galactoproteins specifically recognized by LT-I but not by CT. Similar proteins were detected on Western blots probed with [125I]toxin. LT-I activity on intact cells and binding to membranes and the above galactoproteins were enhanced by neuraminidase treatment even in the presence of CT-B. beta-1,4-Galactosidase and endo-beta-1,4-galactosidase, but not beta-1,3-galactosidase, significantly reduced LT-I binding. LT-I binding to fetuin and transferrin exhibited a similar glycosidase sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Probing the structure-activity relationship of Escherichia coli LT-A by site-directed mutagenesis

Computer analysis of the crystallographic structure of the A subunit of Escherichia coli heat-labile toxin (LT) was used to predict residues involved in NAD binding, catalysis and toxicity. Following site-directed mutagenesis, the mutants obtained could be divided into three groups. The first group contained fully assembled, non-toxic new molecules containing mutations of single amino acids such as Val-53-->Glu or Asp, Ser-63-->Lys, Val-97-->Lys, Tyr-104-->Lys or Asp, and Ser-114-->Lys or Glu. This group also included mutations in amino acids such as Arg-7, Glu-110 and Glu-112 that were already known to be important for enzymatic activity. The second group was formed by mutations that caused the collapse or prevented the assembly of the A subunit: Leu-41-->Phe, Ala-45-->Tyr or Glu, Val-53-->Tyr, Val-60-->Gly, Ser-68-->Pro, His-70-->Pro, Val-97-->Tyr and Ser-114-->Tyr. The third group contained those molecules that maintained a wild-type level of toxicity in spite of the mutations introduced: Arg-54-->Lys or Ala, Tyr-59-->Met, Ser-68-->Lys, Ala-72-->Arg, His or Asp and Arg-192-->Asn. The results provide a further understanding of the structure-function of the active site and new, non-toxic mutants that may be useful for the development of vaccines against diarrhoeal diseases.

Escherichia coli heat-labile enterotoxin binds to glycosylated proteins with lactose by amino carbonyl reaction

The binding of Escherichia coli heat-labile enterotoxin (LT) type I to glycosylated proteins with lactose (Gal beta 1-4Glc) by amino carbonyl reaction was studied by the Western blot assay and by the microtiter well binding assay. LT bound to a lactose-alpha-lactalbumin amino carbonyl product (Lac-LA), whereas cholera toxin did not. The binding ability of Lac-LA was abolished by beta-galactosidase treatment, indicating that the terminal galactose is essential for the binding of LT. The binding of LT to Lac-LA was inhibited by galactose and lactose, and most effectively inhibited by lactulose (Gal beta 1-4Fru), which is a structural analog of the Amadori rearrangement product of the amino carbonyl reaction between lactose and an epsilon-amino group of a lysine residue (lactuloselysine). The results suggest that LT recognizes the portion of lactuloselysine in Lac-LA. LT also bound to a melibiose (Gal alpha 1-6Glc)-alpha-lactalbumin amino carbonyl product (Mel-LA), but the binding ability of Mel-LA was weaker than that of Lac-LA, suggesting that the beta 1-4 linked terminal galactose is dispensable but preferable for the binding. Furthermore, LT bound to the amino carbonyl products of lactose with beta-lactoglobulin, caseins, bovine serum albumin, and ovalbumin. These results indicate that LT binds to the amino carbonyl products between proteins and sugars containing the terminal galactose, such as lactose.

Characterization of hybrid toxins produced in Escherichia coli by assembly of A and B polypeptides from type I and type II heat-labile enterotoxins

The genes encoding the individual A and B polypeptides of the type I enterotoxin LTp-I and type II enterotoxins LT-IIa and LT-IIb were cloned and tested for complementation in Escherichia coli. Each gene encoding an A polypeptide was cloned into pACYC184, and each gene encoding a B polypeptide was cloned into the compatible plasmid Bluescript KS+. In addition, operon fusions representing all combinations of A and B genes were constructed in Bluescript KS+. Extracts from strains of E. coli expressing each combination of A and B genes, either from compatible plasmids or from operon fusions, were tested for immunoreactive holotoxin by radioimmunoassays and for toxicity by Y1 adrenal cell assays. Biologically active holotoxin was detected in each case, but the toxicity of extracts containing the hybrid toxins was usually less than that of extracts containing the wild-type holotoxins. The ganglioside-binding activity of each holotoxin was tested, and in each case, the B polypeptide determined the ganglioside-binding specificity. The A and B polypeptides of the type II heat-labile enterotoxins were also shown to form holotoxin in vitro without exposure to denaturing conditions, in contrast to the polypeptides of the type I enterotoxins that failed to form holotoxin in vitro under comparable conditions. These findings suggest that type I and type II enterotoxins have conserved structural features that permit their A and B polypeptides to form hybrid holotoxins, although the B polypeptides of the type I and type II enterotoxins have very little amino acid sequence homology.

Molecular genetic analysis of ganglioside GD1b-binding activity of Escherichia coli type IIa heat-labile enterotoxin by use of random and site-directed mutagenesis

Mutagenesis of the B-subunit gene of Escherichia coli heat-labile enterotoxin LT-IIa was performed in vitro with sodium bisulfite. Mutants were screened initially by radial passive immune hemolysis assays for loss of binding to erythrocytes. Mutant B polypeptides were characterized for immunoreactivity; for binding to gangliosides GD1b, GD1a, and GM1; for formation of holotoxin; and for biological activity. Mutant alleles that determined altered binding specificities were sequenced. Three such mutant alleles encoded Thr-to-Ile substitutions at residues 13, 14, and 34 in the mature B polypeptide of LT-IIa. Each mutant protein failed to bind to ganglioside GD1b, although the Ile-14 mutant retained the ability to bind to ganglioside GM1. Site-specific mutagenesis was used to construct mutants with various amino acid substitutions at residue 13, 14, or 34. Only those mutant proteins with Ser substituted for Thr at position 13, 14, or 34 retained the ability to bind to ganglioside GD1b, thereby suggesting a role for the hydroxyl group of Thr or Ser in ganglioside GD1b binding.

Detection of a heat-labile enterotoxin gene in enterotoxigenic Escherichia coli by densitometric evaluation using highly specific enzyme-linked oligonucleotide probes

Two alkaline phosphatase-conjugated 24-mer oligonucleotide probes were developed to detect the heat-labile enterotoxin gene in enterotoxigenic Escherichia coli. Probes were antisense codon sequences, which are transcribed into mRNA, of the heat-labile enterotoxin gene of enterotoxigenic Escherichia coli of human origin. Using dot-blot hybridization, probes were tested with 100 clinical isolates and evaluated by a reflectance-type densitometer. Results agreed very well with those of an immunological test, the Biken test, and a 32P-labelled recombinant DNA probe. The oligonucleotide probes did not react with nucleic acids prepared from other diarrhoeagenic bacterial pathogens. Thus, the alkaline phosphatase-conjugated oligonucleotide probes seem to be highly sensitive and specific for detection of heat-labile enterotoxin-producing enterotoxigenic Escherichia coli. Moreover, the results indicate a potential usefulness for densitometric evaluation of DNA hybridization.

Site-specific mutagenesis of the catalytic subunit of cholera toxin: substituting lysine for arginine 7 causes loss of activity

Cholera and pertussis toxins each contain a subunit with ADP-ribosyltransferase activity, sharing a region of nearly identical amino acid sequence near the NH2 terminus. Previous investigations have shown that substitution of a lysine residue for Arg-9 in the catalytic A subunit of pertussis toxin substantially eliminates its enzyme activity. We now report that substitution of lysine for the position-equivalent Arg-7 of cholera toxin subunit A leads to a similar loss of catalytic activity. This result suggests a correlation of function with structure between the sequence-related cholera and pertussis toxin A subunits and may contribute to the design of a vaccine containing an enzymatically inert analog of cholera toxin.

Cloning, sequencing, and expression of a gene encoding a 100-kilodalton mosquitocidal toxin from Bacillus sphaericus SSII-1

A cosmid library was prepared from a partial BamHI digest of total DNA from Bacillus sphaericus SSII-1. Two hundred fifty Escherichia coli clones were screened for toxicity against larvae of the mosquito Culex quinquefasciatus. One toxic clone, designated pKF2, was chosen for further study. Two toxic subclones, designated pXP33 and pXP34, obtained by ligating PstI-derived fragments of pKF2 into pUC18, contained the same 3.8-kb fragment, but in opposite orientations. Sequence analysis revealed the presence of an open reading frame corresponding to a 100-kDa protein and the 3' end of a further open reading frame having significant homology to open reading frames of transposons Tn501 and Tn21. The sequence of the SSII-1 toxin was compared with those of known toxins and was found to show regional homology to those of ADP-ribosyltransferase toxins. The distribution of the toxin gene among other B. sphaericus strains was examined.

Inhibition of cholera toxin binding to membrane receptors by pig gastric mucin-derived glycopeptides: differential effect depending on the ABO blood group antigenic determinants

The capacity of pig gastric mucin-derived glycopeptides to interfere with the binding of cholera toxin (CT) to membrane receptors was studied. Two types of glycopeptide preparations with or without human blood group A antigenic activity were assayed for comparison in a system in which the target for the toxin was rat erythrocyte ghosts. Blood group A-active glycopeptides (A+ glycopeptides) were more potent inhibitors for the toxin binding than those lacking group A activity (A- glycopeptides). The mean values of the 50% inhibitory dose revealed that the A+ glycopeptide preparations were 6.6-fold-more potent inhibitors than the A- ones (P less than 0.001). The inhibitory capacity of the different A+ glycopeptide preparations was not directly proportional to the group A antigenic titer. The A+ glycopeptides showed a higher capacity than the A- glycopeptides to interact with the toxin as revealed by CT-glycopeptide complex formation, which could be detected by Sephacryl S-400 chromatography. This result suggests that glycopeptide inhibition of CT binding to the erythrocyte ghosts is mediated by a competition between the GM1 receptors and the glycopeptides for the toxin. The differential effect between both types of glycoconjugates was independent of the way of measuring the amount of glycopeptides used (dry weight, carbohydrate or protein content). The existence in the gastrointestinal tract of mucins not carrying or carrying different ABO blood group determinants, which could behave as more or less potent inhibitors of CT binding to membrane receptors, may help to explain the relationship between ABO blood groups and severity of cholera.

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.

Cloning, nucleotide sequence, and hybridization studies of the type IIb heat-labile enterotoxin gene of Escherichia coli

Type IIb heat-labile enterotoxin (LT-IIb) is produced by Escherichia coli 41. Restriction fragments of total cell DNA from strain 41 were cloned into a cosmid vector, and one cosmid clone that encoded LT-IIb was identified. The genes for LT-IIb were subcloned into a variety of plasmids, expressed in minicells, sequenced, and compared with the structural genes for other members of the Vibrio cholerae-E. coli enterotoxin family. The A subunits of these toxins all have similar ADP-ribosyltransferase activity. The A genes of LT-IIa and LT-IIb exhibited 71% DNA sequence homology with each other and 55 to 57% homology with the A genes of cholera toxin (CT) and the type I enterotoxins of E. coli (LTh-I and LTp-I). The A subunits of the heat-labile enterotoxins also have limited homology with other ADP-ribosylating toxins, including pertussis toxin, diphtheria toxin, and Pseudomonas aeruginosa exotoxin A. The B subunits of LT-IIa and LT-IIb differ from each other and from type I enterotoxins in their carbohydrate-binding specificities. The B genes of LT-IIa and LT-IIb were 66% homologous, but neither had significant homology with the B genes of CT, LTh-I, and LTp-I. The A subunit genes for the type I and type II enterotoxins represent distinct branches of an evolutionary tree, and the divergence between the A subunit genes of LT-IIa and LT-IIb is greater than that between CT and LT-I. In contrast, it has not yet been possible to demonstrate an evolutionary relationship between the B subunits of type I and type II heat-labile enterotoxins. Hybridization studies with DNA from independently isolated LT-II producing strains of E. coli also suggested that additional variants of LT-II exist.

Cloning and nucleotide sequence of the gene (trh) encoding the hemolysin related to the thermostable direct hemolysin of Vibrio parahaemolyticus

Vibrio parahaemolyticus isolates derived from an outbreak of gastroenteritis in the Republic of Maldives did not have the genetic potential to produce the thermostable direct hemolysin, but one such isolate produced a hemolysin immunologically related to the thermostable direct hemolysin (T. Honda, Y. Ni, and T. Miwatani, Infect. Immun. 56:61-965, 1988). The Maldives isolates hybridized with the DNA probe for the gene encoding the thermostable direct hemolysin (the tdh gene) under reduced stringencies. A DNA fragment containing the probe-reactive nucleotide sequence was isolated from a selected strain and cloned into pBR322 in Escherichia coli. A clone producing the thermostable direct hemolysin-related hemolysin was obtained by screening with hemolysis assays and by an immunological assay. Nucleotide sequence analysis of the cloned DNA fragment revealed that the gene encoding the thermostable direct hemolysin-related hemolysin (the trh gene), like the tdh gene, encoded the hemolysin subunit composed of 189 amino acid residues. The trh gene had significant nucleotide sequence homology with the tdh gene (68.4% with the tdh1 gene copy and 68.6% with the tdh2 gene copy). The amino acid sequences of the hemolysin subunits deduced from the nucleotide sequences of the trh gene and tdh gene were homologous (61.9% homology with the tdh1-encoded subunit and 63.0% homology with the tdh2-encoded subunit) and contained the two cysteine residues to form an intrachain bond at the same positions, and their possible conformations appeared to be similar as determined by hydrophobicity-hydrophilicity analysis and a secondary structure prediction. The trh and tdh genes may have had a common ancestor and may have evolved by single-base changes so that they maintained the fundamental architecture of the molecules.

His-426 of the Pseudomonas aeruginosa exotoxin A is required for ADP-ribosylation of elongation factor II

Exotoxin A (ETA) is recognized as the most toxic product associated with the opportunistic pathogen Pseudomonas aeruginosa. Identification of the amino acids in the polypeptide sequence that are required for toxin activity is critical for vaccine development. By defining the nucleotide sequence of the structural gene of a mutant that encodes an enzymatically inactive ETA (CRM 66), we identified an essential amino acid (His-426), which is involved in the ADP-ribosyltransferase activity associated with functional ETA. A monoclonal antibody that inhibits ETA enzymatic activity in vitro fails to react with ETA variants that have a His 426----Tyr substitution. Several mono-ADP-ribosylating toxins, including diphtheria and pertussis toxins, within the primary amino acid sequences carry a histidine residue that is conserved in spacing and in location with respect to other critical residues. Analysis of the three-dimensional structure of ETA revealed that His-426 is not associated with the proposed NAD+ binding site. These findings should be useful for the design and construction of toxin vaccines.

Comparison of the carbohydrate-binding specificities of cholera toxin and Escherichia coli heat-labile enterotoxins LTh-I, LT-IIa, and LT-IIb

The heat-labile enterotoxins of Vibrio cholerae and Escherichia coli are related in structure and function. They are oligomers consisting of A and B polypeptide subunits. They bind to gangliosides, and they activate adenylate cyclase. The toxins form two antigenically distinct groups; members of each group cross-react but are not necessarily identical. Serogroup I includes cholera toxin (CT) and type I heat-labile enterotoxin (LT-I) of E. coli. LTh-I and LTp-I are antigenic variants of LT-I produced by strains of E. coli from humans and pigs, respectively. Serogroup II contains the type II heat-labile enterotoxin (LT-II) of E. coli. Two antigenic variants designated LT-IIa and LT-IIb have been described. The binding of CT, LTh-I, LT-IIa, and LT-IIb to gangliosides was analyzed by immunostaining thin-layer chromatograms and by solid-phase radioimmunoassay. The four toxins have different glycolipid-binding specificities. LTh-I and CT bind strongly to ganglioside GM1 and less strongly to ganglioside GD1b. However, LTh-I, unlike CT, also binds weakly to GM2 and asialo GM1. LTh-I, like CT, probably binds to the terminal sugar sequence Gal beta 1-3GalNAc beta 1-4(NeuAc alpha 2-3)Gal . . ., where GalNAc is N-acetylgalactosamine and NeuAc is N-acetylneuraminic acid. LT-IIa probably binds to the same sugar sequence to which CT and LTh-I bind, with the additional contribution to binding of a second NeuAc as in GD1b and GD2. Also, LT-IIa must bind the Gal beta 1-3GalNAc . . . sequence in such a way that its binding is relatively unaffected by attachment of NeuAc to the terminal galactose residue as in GD1a, GT1b, and GQ1b. LT-IIb probably binds to the terminal sugar sequence NeuAc alpha 2-3Gal beta 1-4GalNAc . . ., as it binds to gangliosides GD1a and GT1b but not to GM1.

Effect of substitution of glycine for arginine at position 146 of the A1 subunit on biological activity of Escherichia coli heat-labile enterotoxin

The ADP-ribosyltransferase activity of polypeptide A1 of cholera toxin and that of Escherichia coli heat-labile enterotoxin (LT) are primarily responsible for the toxic activities of these toxins. Since the amino acid sequences of the two A1 polypeptides are very similar, their functional mechanisms are considered to be the same. Arg-146 of polypeptide A1 is thought to be involved in the active site, because this amino acid of cholera toxin has been identified as the site of self-ADP-ribosylation. However, the exact role of Arg-146 and the significance of self-ADP-ribosylation in toxicity remain unclear. We substituted Arg-146 of polypeptide A1 of LT with Gly by oligonucleotide-directed mutagenesis and examined the biological property of the resultant mutant LT. The substitution changed the mobility of subunit A on sodium dodecyl sulfate-polyacrylamide gel but did not reduce the vascular permeability activity of LT. This result indicates that Arg-146 is not absolutely required for toxic activity and that LT can express its toxic activity without self-ADP-ribosylation at Arg-146.

Cloning and expression of the Salmonella enterotoxin gene

This report examines the genetic basis for Salmonella typhimurium Q1 enterotoxin production. A 918-base-pair XbaI-HincII fragment of plasmid pJM17, composed of cholera toxin (CT) coding sequences (ctxAB), was used as a gene probe. With this probe, the S. typhimurium enterotoxin was identified on a 6.3-kilobase EcoRI-PstI fragment of chromosomal DNA from plasmidless strain Q1. We cloned this 6.3-kilobase fragment into Escherichia coli RR1. The genetic map of the cloned Salmonella enterotoxin (stx) gene was similar but not identical to the CT and E. coli heat-labile enterotoxin genes. By using synthetic oligonucleotides derived from the sequences of CT subunits A (ctxA) and B (ctxB), it was revealed that there were some conserved regions of DNA encoding the enterotoxins of strain Q1 and Vibrio cholerae. Expression of the cloned stx gene in minicells and subsequent Western blot (immunoblot) analysis with CT antitoxin demonstrated that the Salmonella enterotoxin had two or more subunits with molecular sizes of 45, 26, and 12 kilodaltons. Crude cell lysates of E. coli RR1(pCHP4), containing the cloned Salmonella enterotoxin gene, elicited fluid secretion in ligated rabbit intestinal loops and firm induration in rabbit skin. Both of these enterotoxic responses were neutralized by antisera specific for CT. Mucosal tissue from positive intestinal loops contained elevated levels of cyclic AMP. These data suggest some evolutionary relatedness between the enterotoxin genes of S. typhimurium and V. cholerae.

Genetics of type IIa heat-labile enterotoxin of Escherichia coli: operon fusions, nucleotide sequence, and hybridization studies

Operon fusions for the Escherichia coli heat-labile enterotoxin type IIa (LT-IIa) operon were isolated and characterized. The LT-IIa genes are organized in a transcriptional unit similar to those of cholera toxin (CT) and the closely related E. coli heat-labile toxin type I (LT-I, with subtypes LTh-I and LTp-I). The nucleotide sequence of the LT-IIa genes was determined and compared with the sequences of LTh-I and CT. The A subunit gene of LT-IIa was found to be 57% homologous with the A subunit gene of LTh-I and 55% homologous with the A gene of CT. Most of the homology derived from the region of the A gene which encodes the A1 fragment. The B gene of LT-IIa was not homologous with the B gene of LTh-I or CT. DNA probes containing various portions of the LT-IIa genes and adjacent sequences were used for hybridization studies with restriction endonuclease fragments of DNA from a collection of LT-II-producing strains. These studies showed that a probe containing much of the A subunit gene hybridized well to DNA from the various strains, but a probe for the B subunit gene did not.

Calcium transport affinity, ion competition and cholera toxin effects on cytosolic Ca concentration

The physiological relevance of an apparent ionophore activity of cholera toxin towards Ca2+ has been examined in several different systems designed to measure affinity, specificity, rates of ion transfer, and effects on intracellular ion concentrations. Half-maximal transfer rates across porcine jejunal brush-border vesicles were obtained at a concentration of 0.20 microM Ca2+. When examined in the presence of competing ions the transfer process was blocked by very low concentrations of La3+ or Cd2+, Sr2+, Ba2+ and Mg2+ were relatively inefficient competitors for Ca2+ transport mediated by cholera toxin. The relative affinities observed would be compatible with a selectivity for Ca2+ transfer at physiological ion concentrations, as well as an inhibition of this ionophore activity by recognized antagonists of cholera toxin such as lanthanum ions. Entry rates of Ca2+ into brush-border vesicles exposed to cholera toxin were large enough to accelerate the collapse of a Ca2+ gradient generated by endogenous Ca, Mg-ATPase activity. The treatment of isolated jejunal enterocytes with cholera toxin caused a significant elevation in cytosolic Ca2+ concentrations as measured by Quin-2 fluorescence. This effect was specifically prevented by prior exposure of the cholera toxin to excess ganglioside GM1. We conclude that cholera toxin has many of the properties required for promoting transmembranes Ca2+ movement in membrane vesicles and appears to be an effective Ca2+ ionophore in isolated mammalian cells.

Evolutionary origin of pathogenic determinants in enterotoxigenic Escherichia coli and Vibrio cholerae O1

Three families of the evolutionarily related pathogenic determinants in enterotoxigenic Escherichia coli and Vibrio cholerae O1, a family of cholera enterotoxin (CT) and heat-labile enterotoxin (LT) including CT, LTh, and LTp, a family of heat-stable enterotoxin I (STI) including STIa and STIb, and a family of K88 enteroadhesion fimbriae including K88ab, K88ac, and K88ad were analyzed for synonymous (silent) nucleotide substitutions by using the gene nucleotide sequences of earlier reports and the LTp gene nucleotide sequence presented in this paper. The data suggested that the divergences between LT and CT and between STIa and STIb occurred in the remote past, whereas those between LTh and LTp and between members of the K88 family occurred very recently. We concluded that the LT gene is a foreign gene that has been acquired by E. coli to form an enteropathogen. This provides evolutionary evidence of species-to-species transfer of pathogenic determinants in procaryotes.

Variation in chemical properties and antigenic determinants among type II heat-labile enterotoxins of Escherichia coli

Type II heat-labile enterotoxin (LT-II) from Escherichia coli 41 was purified and compared with prototype LT-II encoded by genes from E. coli SA53. Both toxins were oligomeric proteins consisting of polypeptides A (Mr, 28,000) and B (Mr, 11,800). The A polypeptides were cleaved by trypsin into fragments A1 (Mr, 21,000) and A2 (Mr, about 7,000). These two toxins were shown to belong to two different subclasses of LT-II. We propose to designate the prototype toxin LT-IIa and the new variant LT-IIb. The pI of LT-IIb was between 5.2 and 5.6, significantly lower than the pI of 6.8 for LT-IIa, and the behavior of LT-IIb during purification differed significantly from that of LT-IIa. The toxic dose of unnicked LT-IIb in the Y1 adrenal-cell assay was 94 pg, but trypsin-treated, nicked LT-IIb was toxic at about 3 pg. In contrast, the toxic dose of LT-IIa was previously shown to be 0.5 to 1 pg for several preparations that varied from unnicked to partially nicked, and treatment with trypsin was not required for full toxicity. The titer of LT-II antiserum in neutralization tests was 100-fold greater against LT-IIa than against LT-IIb. In immunodiffusion tests, LT-IIa and LT-IIb gave a reaction of partial identity. In a radioimmunobinding assay, the titer of LT-IIa antiserum against homologous LT-IIa was approximately 10-fold greater than against LT-IIb. The cholera-E. coli family of heat-labile enterotoxins has been divided into serogroup I, which includes cholera toxin and the antigenic variants of E. coli heat-labile toxin designated LTh-I and LTp-I, and serogroup II, which includes LT-IIa and LT-IIb. The type I and type II toxins do not cross-react in neutralization or immunodiffusion tests. By using very sensitive radioimmunobinding assays, it was possible to demonstrate common antigenic determinants between the type I and type II toxins. However, the titers of antibodies in hyperimmune sera that recognized these common determinants were very low.

Purification and characterization of type II heat-labile enterotoxin of Escherichia coli

Type II heat-labile enterotoxin (LT-II) of Escherichia coli has several biologic activities similar to cholera toxin (CT) and E. coli type I heat-labile enterotoxin (LT-I), but it is not neutralized by antiserum prepared against CT or LT-I. LT-II was purified from E. coli SA53 and from E. coli HB101(pCP3837), a strain that contains the cloned LT-II genes in a hybrid plasmid and produces up to 600 times more LT-II than does SA53. Purification involved sonic disruption of bacterial cells, ammonium sulfate fractionation, chromatography on Affi-Gel Blue, chromatofocusing, and gel filtration on Sephadex G-100. The LT-II purified to apparent homogeneity from HB101(pCP3837) had an isoelectric point of 6.8, induced increased vascular permeability in rabbit intracutaneous tests, caused rounding of cultured Y1 adrenal cells accompanied by increased intracellular cyclic AMP, and was 25 to 50 times more potent than CT or LT-I in the Y1 adrenal-cell assay. In contrast, purified LT-II did not cause secretion in ligated rabbit ileal segments at doses corresponding to CT controls that gave strongly positive reactions. LT-II was composed of two different polypeptides with MrS of 28,000 (A) and 11,800 (B); treatment of LT-II with trypsin cleaved the A polypeptide to fragments A1 (Mr, 21,000) and A2 (Mr, 7,000). The activity of LT-II was not blocked by ganglioside GM1 at concentrations that inactivated LT-I or CT. Antiserum against the LT-II from E. coli HB101(pCP3837) completely neutralized purified LT-II and the LT-II in crude extracts of SA53, but it did not neutralize purified LT-I or CT.

Pertussis toxin gene: nucleotide sequence and genetic organization

The current pertussis vaccines, although efficacious, in some instances produce undesirable side effects. Molecular engineering of pertussis toxin, the major protective antigen, could provide a safer, new generation of vaccines against whooping cough. As a first critical step in the development of such a vaccine, the complete nucleotide sequence of the pertussis toxin gene was determined and the amino acid sequences of the individual subunits were deduced. All five subunits are coded by closely linked cistrons. A promoter-like structure was found in the 5'-flanking region, suggesting that the toxin is expressed through a polycistronic messenger RNA. The order of the cistrons is S1, S2, S4, S5, and S3. All subunits contain signal peptides of variable length. The calculated molecular weights of the mature subunits are 26,024 for S1, 21,924 for S2, 21,873 for S3, 12,058 for S4, and 11,013 for S5. Subunits S2 and S3 share 70% amino acid homology and 75% nucleotide homology. Subunit S1 contains two regions of eight amino acids homologous to analogous regions in the A subunit of both cholera and Escherichia coli heat labile toxins.

Molecular cloning of pertussis toxin genes

We have cloned a 4.5 kb EcoRI/BamHI DNA fragment from Bordetella pertussis which contains at least two genes responsible for expression of pertussis toxin. The S4 subunit of the toxin was isolated by high pressure liquid chromatography and the NH2-terminal amino acid sequence determined. Using a mixed synthetic oligonucleotide probe designed by reverse translation of a portion of the protein sequence, a cloned DNA fragment was identified which contains the coding information for at least the S4 structural subunit of the toxin. Sequence analyses indicate that the mature protein is derived by proteolytic cleavage of a precursor molecule. Southern blot analyses of Tn5-induced B. pertussis toxin-deficient mutants show that the Tn5 DNA is inserted 1.3 kb downstream from the S4 subunit gene. This second gene could code for another subunit required for assembly of the mature toxin or a non-structural transport protein, possibly in the same polycistronic operon. The molecular cloning of pertussis toxin genes provides the basis for development of a safer recombinant "new generation" vaccine for whooping cough.

Comparison between enterotoxic activity and methanol solubility in heat-stable enterotoxins (STa and STb) from Escherichia coli of human, porcine and bovine origins

We have investigated the enterotoxic activity of culture filtrates and their methanol extracted fractions from 10 ST (STa or STb) producing Escherichia coli strains from human, porcine and bovine origin, in the infant mouse test (IMT) as well as in the rabbit intestinal loop test (RILT). Unconcentrated culture filtrates and methanol-soluble fractions from the eight STa-producing strains were positive in the IMT while methanol-insoluble fractions obtained from these STa-positive strains, like methanol-soluble and -insoluble fractions from the two strains producing only STb, lacked activity in the IMT. Unconcentrated culture filtrates from all ST-producing strains were unable to cause fluid accumulation in the rabbit ligated intestinal loops after 6 h incubation. When this material, concentrated 5-fold, was tested again, the culture filtrates and methanol-soluble fractions from all STa-producing strains yielded strongly positive fluid accumulation in the RILT, whereas culture filtrates and their methanol extracted fractions from the strains producing only STb, like methanol-insoluble fractions from four STa-producing strains, caused slight fluid secretion in the rabbit intestinal loops.

Gene expression in the polycistronic operons of Escherichia coli heat-labile toxin and cholera toxin: a new model of translational control

A new model is proposed based on the suggestion that stable local secondary structures of mRNA interfere with ribosome movement on mRNA and consequently reduce the translation rate. This model accounts for a different level of translation for each cistron in the polycistronic mRNA of Escherichia coli heat-labile toxin (LT) and cholera toxin. We also conclude that the mRNA secondary structures have been conserved during the evolution of the toxin genes for its functional importance.

Mechanism of toxin secretion by Vibrio cholerae investigated in strains harboring plasmids that encode heat-labile enterotoxins of Escherichia coli

A genetically engineered Vibrio cholerae strain from which the cholera toxin genes had previously been deleted was used as a host in which to study the expression and secretion of related toxins and their subunits. Recombinant plasmids encoding heat-labile enterotoxins (LTs) from Escherichia coli of human and porcine origin were expressed in the V. cholerae host, and this resulted in the secretion of the LTs into the extracellular milieu. The secreted LTs were isolated and it was found that the A subunits of human and porcine LT were "unnicked" polypeptides, which indicates that nicking is not obligatory for toxin secretion. V. cholerae strains were also constructed that harbored plasmids encoding either the A or the B subunits of human LT (A+B-, or A-B+). Approximately 90% of the B subunits were secreted from the A-B+ strain, while all of the A subunits expressed by the A+B- strain remained cell associated. This implies that strains synthesizing both subunits assemble the A and B subunits prior to their secretion. We propose that the entry of the toxin into the secretory step of the export pathway is mediated by a secretory apparatus that recognizes structural domains within the B subunit of LT.