Location and amino acid sequence around the ADP-ribosylation site in the cholera toxin active subunit A1

ADP-ribosylation factor 6 acts as an allosteric activator for the folded but not disordered cholera toxin A1 polypeptide

The catalytic A1 subunit of cholera toxin (CTA1) has a disordered structure at 37°C. An interaction with host factors must therefore place CTA1 in a folded conformation for the modification of its Gsα target which resides in a lipid raft environment. Host ADP-ribosylation factors (ARFs) act as in vitro allosteric activators of CTA1, but the molecular events of this process are not fully characterized. Isotope-edited Fourier transform infrared spectroscopy monitored ARF6-induced structural changes to CTA1, which were correlated to changes in CTA1 activity. We found ARF6 prevents the thermal disordering of structured CTA1 and stimulates the activity of stabilized CTA1 over a range of temperatures. Yet ARF6 alone did not promote the refolding of disordered CTA1 to an active state. Instead, lipid rafts shifted disordered CTA1 to a folded conformation with a basal level of activity that could be further stimulated by ARF6. Thus, ARF alone is unable to activate disordered CTA1 at physiological temperature: additional host factors such as lipid rafts place CTA1 in the folded conformation required for its ARF-mediated activation. Interaction with ARF is required for in vivo toxin activity, as enzymatically active CTA1 mutants that cannot be further stimulated by ARF6 fail to intoxicate cultured cells.

The A subunit of Escherichia coli heat-labile enterotoxin functions as a mucosal adjuvant and promotes IgG2a, IgA, and Th17 responses to vaccine antigens

Enterotoxigenic Escherichia coli (ETEC) produces both heat-labile (LT) and heat-stable (ST) enterotoxins and is a major cause of diarrhea in infants in developing countries and in travelers to those regions. In addition to inducing fluid secretion, LT is a powerful mucosal adjuvant capable of promoting immune responses to coadministered antigens. In this study, we examined purified A subunit to further understand the toxicity and adjuvanticity of LT. Purified A subunit was enzymatically active but sensitive to proteolytic degradation and unable to bind gangliosides, and even in the presence of admixed B subunit, it displayed low cyclic AMP (cAMP) induction and no enterotoxicity. Thus, the AB5 structure plays a key role in protecting the A subunit from proteolytic degradation and in delivering the enzymatic signals required for secretion. In contrast, the A subunit alone was capable of activating dendritic cells and enhanced immune responses to multiple antigens following intranasal immunization; therefore, unlike toxicity, LT adjuvanticity is not dependent on the AB5 holotoxin structure or the presence of the B subunit. However, immune responses were maximal when signals were received from both subunits either in an AB5 structure or with A and B admixed. Furthermore, the quality of the immune response (i.e., IgG1/IgG2 balance and mucosal IgA and IL-17 secretion) was determined by the presence of an A subunit, revealing for the first time induction of Th17 responses with the A subunit alone. These results have important implications for understanding ETEC pathogenesis, unraveling immunologic responses induced by LT-based adjuvants, and developing new mucosal vaccines.

Structure and function of cholera toxin and the related Escherichia coli heat-labile enterotoxin

Cholera and the related Escherichia coli-associated diarrheal disease are important problems confronting Third World nations and any area where water supplies can become contaminated. The disease is extremely debilitating and may be fatal in the absence of treatment. Symptoms are caused by the action of cholera toxin, secreted by the bacterium Vibrio cholerae, or by a closely related heat-labile enterotoxin, produced by Escherichia coli, that causes a milder, more common traveler's diarrhea. Both toxins bind receptors in intestinal epithelial cells and insert an enzymatic subunit that modifies a G protein associated with the adenylate cyclase complex. The consequent stimulated production of cyclic AMP, or other factors such as increased synthesis of prostaglandins by intoxicated cells, initiates a metabolic cascade that results in the excessive secretion of fluid and electrolytes characteristic of the disease. The toxins have a very high degree of structural and functional homology and may be evolutionarily related. Several effective new vaccine formulations have been developed and tested, and a growing family of endogenous cofactors is being discovered in eukaryotic cells. The recent elucidation of the three-dimensional structure of the heat-labile enterotoxin has provided an opportunity to examine and compare the correlations between structure and function of the two toxins. This information may improve our understanding of the disease process itself, as well as illuminate the role of the toxin in studies of signal transduction and G-protein function.

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.

Glutamic acid-112 of the A subunit of heat-labile enterotoxin from enterotoxigenic Escherichia coli is important for ADP-ribosyltransferase activity

A mutant strain of enterotoxigenic Escherichia coli (E. coli pTUH 6A) produced an abnormal heat-labile enterotoxin (LT), the A subunit of which has a single amino acid substitution at position 112 (Glu-112 to Lys-112). As already reported, this mutant LT had no ileal loop and vascular permeability activities [(1990) J. Biol. Chem. 265, 22520-22525]. In this paper we report that the mutant LT showed no CHO cell elongation activity and did not activate adenylate cyclase of target cells. Moreover, no ADP-ribosyltransferase activity was detected in the mutant LT. It is concluded that the amino acid substitution at position 112 abolished the ADP-ribosyltransferase activity of the A subunit and this leads to the loss of toxic activities of LT.

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.

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.

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.

Cholera toxin A subunit: functional sites correlated with regions of secondary structure

The A subunit of cholera toxin contains the ADP-ribosyltransferase activity in its major constituent polypeptide A1 (Mr 23,000) which is responsible for the elevation of cAMP typically observed with most mammalian cell types after exposure to the toxin. The primary structure of the A subunit, recently established by sequence analyses, is presented and used as the basis for the secondary structure prediction according to the method of Chou and Fasman. The results indicated the presence of 27% alpha-helix, 25% beta-structure, 12% beta-turn, and 36% random coil. The majority of the beta-structure consisted of six strands located in the NH2-terminal portion of the molecule (residues 33-106) covering one-half of the region corresponding to the A1 polypeptide portion. The beta-sheet domain led immediately into the active site region characterized by the alternating structures of beta-pleated sheet and alpha-helix (residues 95-140) similar to that reported for other NAD+ binding proteins. The presence of this structural feature in the region was confirmed by the use of another predictive method (J. Garnier et al., J. Mol. Biol. 1978, 120, 97-120). In addition, two regions (residues 14-18 and 200-214), previously identified to contain binding sites for the B subunit as evidenced by chemical modification and monoclonal antibody studies, were found to be in alpha-helix configuration.

The primary structure of the COOH-terminal half of cholera toxin subunit A1 containing the ADP-ribosylation site

The sequence of 96 amino acid residues from the COOH-terminus of the active subunit of cholera toxin, A1, has been determined as (sequence; see text) This is the largest fragment obtained by BrCN cleavage of the subunit A1 (Mr 23,000), and has previously been indicated to contain the active site for the adenylate cyclase-stimulating activity. Unequivocal identification of the COOH-terminal structure was achieved by separation and analysis of the terminal peptide after the specific chemical cleavage at the only cysteine residue in A1 polypeptide. The site of self ADP-ribosylation in the A1 subunit [C. Y. Lai, Q.-C. Xia, and P.T. Salotra (1983) Biochem. Biophys. Res. Commun. 116, 341-348] has now been identified as Arg-50 of this peptide, 46 residues removed from the COOH-terminus. The cysteine that forms disulfide bridge to A2 subunit in the holotoxin is at position 91.

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

Nucleotide sequence comparisons of the heat-labile enterotoxin (LTh) genes of E. coli pathogenic for humans with cholera toxin (CT) genes suggest that the two toxin genes have evolved from a common ancestry by a series of single base changes, while conserving the catalytic fragment A1 (ADP-ribose transferase). Based on the local hydrophilicity profiles of LTh and CT peptides, a transmembrane segment appears to be present in A1 in both toxins.