A Single Amino Acid Substitution in B Subunit of Escherichia coli Enterotoxin Affects Its Oligomer Formation

Little heterogeneity among genes encoding heat-labile and heat-stable toxins of enterotoxigenic Escherichia coli strains isolated from diarrheal pigs

To examine whether the heat-labile enterotoxin gene in porcine enterotoxigenic Escherichia coli (ETEC) strains is as divergent as in human ETEC strains, we sequenced the heat-labile and heat-stable toxin genes from 52 and 33 porcine ETEC strains, respectively. We found that the STa gene is identical, that the LT gene has only two mutations in 4 (of 52) strains, and that both mutations cause a reduction in GM1 binding and toxicity.

Residues of heat-labile enterotoxin involved in bacterial cell surface binding

Enterotoxigenic Escherichia coli (ETEC) is a leading cause of traveler's diarrhea worldwide. One major virulence factor released by this pathogen is the heat-labile enterotoxin LT, which upsets the balance of electrolytes in the intestine. After export, LT binds to lipopolysaccharide (LPS) on the bacterial surface. Although the residues responsible for LT's binding to its host receptor are known, the portion of the toxin which mediates LPS binding has not been defined previously. Here, we describe mutations in LT that impair the binding of the toxin to the external surface of E. coli without altering holotoxin assembly. One mutation in particular, T47A, nearly abrogates surface binding without adversely affecting expression or secretion in ETEC. Interestingly, T47A is able to bind mutant E. coli expressing highly truncated forms of LPS, indicating that LT binding to wild-type LPS may be due primarily to association with an outer core sugar. Consequently, we have identified a region of LT distinct from the pocket involved in eukaryotic receptor binding that is responsible for binding to the surface of E. coli.

Development and evaluation of genotypic assays for the detection and characterization of enterotoxigenic Escherichia coli

We developed and evaluated a method to genotypically identify enterotoxigenic Escherichia coli (ETEC) and to characterize these organisms with respect to 18 of 21 known colonization factors (CFs). The method, which is based on polynucleotide DNA-DNA colony hybridization, includes a pooled toxin probe assay to identify ETEC, and individual probe assays to detect the enterotoxins STp, STh, and LT, and the CFs CFA/I, CS1-CS8, CS12-CS15, CS17-CS19, CS21, and CS22. We evaluated the pooled toxin probe assay during a cohort study of childhood diarrhea, and the individual probe assays against 33 reference strains and 92 clinical ETEC isolates. There was close to a complete agreement between the pooled toxin probe assay and the individual toxin probe assays, and between the individual CF probe assays and the corresponding phenotypic assays.

Mutational analysis of ganglioside GM(1)-binding ability, pentamer formation, and epitopes of cholera toxin B (CTB) subunits and CTB/heat-labile enterotoxin B subunit chimeras

Variants of cholera toxin B subunit (CTB) were made by bisulfite- and oligonucleotide-directed mutagenesis of the ctxB gene. Variants were screened by a radial passive immune hemolysis assay (RPIHA) for loss of binding to sheep erythrocytes (SRBC). Variant CTBs were characterized for the formation of immunoreactive pentamers, the ability to bind ganglioside GM(1) in vitro, and reactivity with a panel of monoclonal anti-CTB antibodies. Substitutions at eight positions (i.e., positions 22, 29, 36, 45, 64, 86, 93, and 100) greatly reduced the yield of immunoreactive CTB. RPIHA-negative substitution variants that formed immunoreactive pentamers were obtained for residues 12, 33, 36, 51, 52 + 54, 91, and 95. Tyrosine-12 was identified as a novel residue important for GM(1) binding since, among all of the novel variants isolated with altered RPIHA phenotypes, only CTB with aspartate substituted for tyrosine at position 12 failed to bind significantly to ganglioside GM(1) in vitro. In contrast, CTB variants with single substitutions for several other residues (Glu-51, Lys-91, and Ala-95) that participate in GM(1) binding, based on the crystal structure of CTB and the oligosaccharide of GM(1), were not appreciably altered in their ability to bind GM(1) in vitro, even though they showed altered RPIHA phenotypes and did not bind to SRBC. Hybrid B genes made by fusing ctxB and the related Escherichia coli heat-labile enterotoxin eltB genes at codon 56 produced CTB variants that had 7 or 12 heat-labile enterotoxin B residue substitutions in the amino or carboxyl halves of the monomer, respectively, each of which which also bound GM(1) as well as wild-type CTB. This collection of variant CTBs in which 47 of the 103 residues were substituted was used to map the epitopes of nine anti-CTB monoclonal antibodies (MAbs). Each MAb had a unique pattern of reactivity with the panel of CTB variants. Although no two of the epitopes recognized by different MAbs were identical, most of the single amino acid substitutions that altered the immunoreactivity of CTB affected more that one epitope. The tertiary structures of the epitopes of these anti-CTB MAbs are highly conformational and may involve structural elements both within and between CTB monomers. Substitution of valine for alanine at positions 10 and 46 had dramatic effects on the immunoreactivity of CTB, affecting epitopes recognized by eight or six MAbs, respectively.

Assembly of the B subunit pentamer of Escherichia coli heat-labile enterotoxin. Kinetics and molecular basis of rate-limiting steps in vitro

The B subunits of Escherichia coli heat-labile enterotoxin (EtxB) and cholera toxin (CtxB) assemble in vivo into exceptionally stable homopentameric complexes, which maintain their quaternary structure in a range of conditions that would normally be expected to cause protein denaturation. Recently, we showed that the simultaneous protonation of two of the COOH-terminal carboxylates in pentameric EtxB was required to cause its disassembly at pH values below 2.0 (Ruddock, L., Ruston, S. P., Kelly, S. M., Price, N. C., Freedman, R. B., and Hirst, T. R.(1995) J. Biol. Chem. 270, 29953-29958). Here, we investigate the influence of environmental parameters on the kinetics of reassembly of acid-generated EtxB monomers in vitro. Such monomers were found to undergo a further acid-mediated conformational change, with an activation energy of 76 +/- 2 J.mol-1.K-1, consistent with isomerization of the cis-proline residue at position 93, and which prevented subsequent EtxB reassembly. By using rapid neutralization of acid-generated monomers, a high proportion of the B-subunits adopted an assembly-competent conformation, which resulted in up to 75% of the protein reassembling into a stable pentameric complex, indistinguishable from native EtxB pentamers. The rate-limiting step in reassembly, over a concentration range of 50-200 microg/ml, was shown to be due to an intramolecular event, which exhibited a pH dependence with a pKa of 7.0. Modification of EtxB with amine-specific probes revealed that the protonation state of the NH2-terminal alanine residue was responsible for the pH dependence of reassembly. The implications of these findings for the biogenesis of Escherichia coli enterotoxin and related enterotoxins in vivo, are considered.

Construction, purification and immunogenicity of antigen-antibody-LTB complexes

An oligonucleotide, encoding a short epitope peptide tag, termed Pk, was inserted at the 3'-end of the gene coding B-subunit of Escherichia coli heat-labile enterotoxin (LTB). The presence of the Pk epitope on LTB-Pk was used to construct novel macromolecular assemblies comprising LTB-Pk, an anti-Pk mAb, (mAb SV5-P-k) and Pk-linked recombinant SIV proteins. The 1:1:1 stoichiometry of such complexes was ensured by binding LTB-Pk to one arm of mAb SV5-P-k and an SIV-Pk antigen to the other arm of the antibody. Such SIV-mAb-LTB macromolecular complexes bound to GM1-ganglioside in vitro, and when immunized systemically into mice were highly immunogenic, inducing both humoral and cell-mediated responses to the recombinant SIV antigens.

Characterization of an internal permissive site in the cholera toxin B-subunit and insertion of epitopes from human immunodeficiency virus-1, hepatitis B virus and enterotoxigenic Escherichia coli

We previously described the construction of novel hybrid proteins based on the B-subunit of cholera toxin (CTB) [Bäckström et al., Gene 149 (1994) 211-217], in which a neutralizing B-cell epitope from the third variable (V3) loop in the envelope glycoprotein gp120 from human immunodeficiency virus type 1 (HIV-1) was inserted within a surface-exposed region between amino acids (aa) 55 and 64. The resulting protein retained properties of native CTB and could induce strong anti-CTB antibody (Ab) responses, but the inserted gp120 epitope was only modestly immunogenic. In this study, the potential use of this internal permissive site in CTB for the insertion of heterologous epitopes has been further investigated. Six additional plasmids were constructed encoding HIV::CTB hybrid proteins with ten to fourteen aa from the V3 loop of gp120 genetically inserted at different positions between aa 52 and 65, with deletions of different CTB aa. Plasmids encoding proteins with peptides inserted between aa 53 and 64 in CTB gave rise to stable proteins which reacted with CTB-specific monoclonal antibodies (mAb) and bound to GM1 gangliosides (GM1), indicating that insertions between these positions do not drastically alter the conformation or the receptor-binding properties of native CTB. Plasmids were also constructed encoding CTB hybrid proteins which had either an 11-aa peptide from hepatitis B virus (HBV) pre-S(2) or one of two peptides related to the heat-stable toxin (STa) of enterotoxigenic Escherichia coli inserted between aa 55 and 64 of CTB. This resulted in the production of HBV::CTB or ST::CTB hybrid proteins and illustrated that the internal permissive site can be used for insertion of peptides of varying aa composition. The reactivity of the inserted epitopes with epitope-specific mAb in GM1-ELISA and immunoblots varied greatly between hybrid proteins and depended on the position in CTB and the aa composition of the inserted peptides. Despite these differences, all the HIV::CTB, ST::CTB and HBV::CTB hybrid proteins could induce low, but significant, levels of serum Ab in mice against gp120, STa or pre-S(2), in addition to strong serum Ab responses against CTB. The Ab response against the internally inserted gp120 peptide was similar to that against the same peptide fused to the N-terminus of CTB, indicating that internally placed peptides had similar immunogenicity to the same peptides added terminally.

Monomer of the B subunit of heat-labile enterotoxin from enterotoxigenic Escherichia coli has little ability to bind to GM1 ganglioside compared to its coligenoid

Coligenoid, composed of the B subunit of heat-labile enterotoxin from enterotoxigenic Escherichia coli, was separated into monomers in the presence of 2% propionic acid containing 6 M urea (pH 3.8). Monomers equilibrated against 0.75% or 0.5% propionic acid containing 3 M urea (pH 3.8) did not reassemble into coligenoid. Complexes of GM1 ganglioside and coligenoid in these buffers were detected by SDS-polyacrylamide gel electrophoresis, but those of the GM1 ganglioside and monomers were not. The binding ability of monomer to GM1 ganglioside in these buffers was about 1% of that of normal coligenoid by GM1-enzyme-linked immunosorbent assay. Moreover, monomers in these buffers reassembled into coligenoid by buffering against original TEAN buffer, and the binding ability of the resulting coligenoid to GM1 ganglioside was identical to that of native coligenoid. These data suggest that although coligenoid formation is important for the receptor binding of the B subunit, little binding ability to GM1 ganglioside remains in monomer of the B subunit.

Isolation of mutant toxins of Vibrio parahaemolyticus hemolysin by in vitro mutagenesis

Thermostable direct hemolysin produced by Vibrio parahaemolyticus is a major virulence factor of the organism. The hemolysin has a variety of biological activities such as lethality to mice, cytotoxicity to cultured cells, cardiotoxicity, and fluid accumulating activity in rabbit ileal loop test. In this study, we attempted to isolate less hemolytic mutant toxins of the thermostable direct hemolysin to use them for analysis of mode of action of the hemolysin. Six mutant toxins were obtained by in vitro mutagenesis of the cloned gene for the hemolysin. Characterization of the mutant toxins demonstrated that single amino acid substitutions at Gly62, Trp65, Thr67, Gly86, Glu116 and Glu138 resulted in a loss or lowering of the hemolytic activity. Two of the mutant toxins inhibited hemolysis by wild-type toxin on rabbit blood agar plates, while their hemolytic activity was below the detectable level. These mutant toxins would be useful for identifying the as yet unknown receptor for the hemolysin on the target cell membrane.

Interaction of a cholera toxin derivative containing a reduced number of receptor binding sites with intact cells in culture

Hybrid CTB (hCTB), having only one or two functional binding sites, has been constructed from two chemically inactivated derivatives of CTB. One inactive derivative consisted of CTB formylated in the lone Trp-88 of each beta-chain (fCTB), whereas the other inactive derivative consisted of CTB specifically succinylated in three amino groups located in or near the receptor binding site (sssCTB). hCTB, fCTB and sssCTB were able to reassociate with CTA and form the corresponding holotoxins hCT, fCT and sssCT as measured by gel filtration chromatography. In contrast to fCT and sssCT, hCT could increase the cAMP content of intact Vero cells in a time- and dose-dependent way: concentrations as low as a few nanograms of hCT per milliliter caused a significant increase in the intracellular cAMP level. The maximal cAMP level induced by hCT (1 microgram/ml) was, however, more than 2-fold lower than that elicited by its native counterpart. At saturating ligand concentrations and at 37 degrees C, the lag periods and rates of CT and hCT induced cAMP accumulation were essentially the same. Treatment of Vero and HeLa cells with GM1 did not affect their difference in response to CT and hCT. When Vero cells treated with hCT were incubated for longer periods of time, a further slow accumulation of cAMP occurred until after about 20 h cAMP levels of cells exposed to CT or hCT were essentially the same. In contrast to Vero and HeLa cells, human skin fibroblasts exhibited an almost identical response to CT as well as to hCT. Acidotropic agents such as chloroquine and monensin affected the CT and hCT induced increase in cAMP content of Vero cells, fibroblasts and GM1 treated Hela cells in a similar way. The results are consistent with the view that CT receptor recognition domains are shared between adjacent beta-chains, that pentavalent binding appears not to be essential for cytotoxicity and that in the cell types studied intracellular processing of CT, hCT is involved.

Suppression of temperature-sensitive assembly mutants of heat-labile enterotoxin B subunits

Deletions or substitutions of amino acids at the carboxyl-terminus of the heat-labile enterotoxin B subunit (EtxB) affect its assembly into pentamers in a temperature-dependent manner. At 42 degrees C, the mutations prevent the B subunits from achieving their final pentameric structure resulting in membrane association of the monomers. However, mutant B subunits produced at 30 degrees C assemble, in the periplasm, into pentamers that remain stable when transferred to 42 degrees C, indicating that the mutant pentamers are stable under conditions where their formation is inhibited. The mutant pentamers are, similarly to wild-type pentamers, SDS-resistant and stable, in vitro, at temperatures up to 65 degrees C. This suggests that although the C-terminal amino acids are part of the subunit interface, they appear not to contribute significantly to the stability of the final pentameric complex, but are instead essential for the formation or stabilization of an assembly intermediate in the pentamerization process. Single second site mutations suppress the assembly defect of mutant EtxB191.5, which carries substitutions at its C-terminus. The Thr-->Ile replacement at position 75 in the alpha 2-helix probably restores the van der Waals contact between residues 75 and 101, which had been greatly reduced by the Met-->Leu substitution at position 101 in the beta 6-strand of EtxB191.5. Interaction between the alpha 2-helix and beta 6-strand which contains the C-terminus probably stabilizes a conformation essential for assembly and is therefore required for the formation of pentamers.

Mapping epitopic regions of cholera toxin B-subunit protein

Continuous overlapping synthetic hexapeptides representing the entire 103 amino acid sequence of the immunodominant B-subunit protein of cholera enterotoxin were used to examine reactivities of a variety of antisera in attempts to detect and define sequence-related (continuous) antigenic regions. The validity of the methods was established by the reactions of polyclonal antisera raised against longer synthetic peptides with appropriate synthetic hexapeptides. An unexpected cross-reaction is attributed to the presence of three identical amino acids (Gln16-Ile17-His18)--although in different order (Gln56-His57-Ile58)--in two parts of the B-subunit chain. Adsorption studies using polyclonal rabbit antisera revealed that, in many instances, denatured B-subunit protein more effectively removed reactivity with hexapeptides than did the native protein. Native holotoxin was more effective than native B-subunit. Sera from human cholera convalescents gave diffuse patterns of reactivity with synthetic hexapeptides--primarily against regions of reactive hexapeptides rather than with clearly defined continuous epitopes. Among many epitopic regions encountered, a strongly reactive tetramer, Ser-Gln-His-Ile (SQHI), was discovered in a highly conserved region, residues 55-58, of the B-subunit amino acid sequence. Adsorption studies revealed that this epitope is apparently exposed on the surface of the native protein. Amino acid substitution revealed the essentiality of Gln and His residues to this epitope. Gly54 was not part of the epitope but substitution of acidic residues Glu and Asp for Gly eliminated reactivity with antibody. The results suggest that continuous epitopes may contribute to the antigenicity of the native toxin protein and may be potentially useful for development of a peptide vaccine.

Analysis of structure and function of the B subunit of cholera toxin by the use of site-directed mutagenesis

Oligonucleotide-directed mutagenesis of ctxB was used to produce mutants of cholera toxin B subunit (CT-B) altered at residues Cys-9, Gly-33, Lys-34, Arg-35, Cys-86 and Trp-88. Mutants were identified phenotypically by radial passive immune haemolysis assays and genotypically by colony hybridization with specific oligonucleotide probes. Mutant CT-B polypeptides were characterized for immunoreactivity, binding to ganglioside GM1, ability to associate with the A subunit, ability to form holotoxin, and biological activity. Amino acid substitutions that caused decreased binding of mutant CT-B to ganglioside GM1 and abolished toxicity included negatively charged or large hydrophobic residues for Gly-33 and negatively or positively charged residues for Trp-88. Substitution of lysine or arginine for Gly-33 did not affect immunoreactivity or GM1-binding activity of CT-B but abolished or reduced toxicity of the mutant holotoxins, respectively. Substitutions of Glu or Asp for Arg-35 interfered with formation of holotoxin, but none of the observed substitutions for Lys-34 or Arg-35 affected binding of CT-B to GM1. The Cys-9, Cys-86 and Trp-88 residues were important for establishing or maintaining the native conformation of CT-B or protecting the CT-B polypeptide from rapid degradation in vivo.

Crystal structure of a cholera toxin-related heat-labile enterotoxin from E. coli

Examination of the structure of Escherichia coli heat-labile enterotoxin in the AB5 complex at a resolution of 2.3A reveals that the doughnut-shaped B pentamer binds the enzymatic A subunit using a hairpin of the A2 fragment, through a highly charged central pore. Putative ganglioside GM1-binding sites on the B subunits are more than 20A removed from the membrane-crossing A1 subunit. This ADP-ribosylating (A1) fragment of the toxin has structural homology with the catalytic region of exotoxin A and hence also to diphtheria toxin.