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

Plumbagin Prevents Secretory Diarrhea by Inhibiting CaCC and CFTR Channel Activities

Secretory diarrhea, which primarily originates through intestinal pathogens and viruses, is a health burden in many regions worldwide. Enterocyte Cl⁻ channels, as the final step in enterotoxin-induced fluid secretion, constitute an attractive class of targets for diarrhea therapy. Chloride channel inhibitors have become a new class of candidates for antisecretion and anti-intestinal motility agents. In the present study, we identified plumbagin as a transmembrane protein 16A (TMEM16A) inhibitor in a cell-based fluorescence-quenching assay, and the IC₅₀ value was ∼12.46 µM. Short-circuit current measurements showed that plumbagin reversibly inhibited the E_act-induced Cl⁻ current on the apical side of TMEM16A-transfected Fischer rat thyroid (FRT) cells with no significant effect on cytoplasmic Ca²⁺ signaling. Notably, plumbagin also inhibited the activity of intestinal epithelial calcium-activated chloride channel (CaCC) and cystic fibrosis transmembrane conductance regulator (CFTR) in both HT-29 cells and mouse colons, but had no effects on the activity of the Na⁺-K⁺ ATPase or K⁺ channels. In in vivo experiments, the administration of plumbagin reduced both Escherichia coli heat-stable enterotoxin (STa)- and cholera toxin (CT)-induced intestinal fluid secretion. In neonatal mouse models of CT- and rotavirus infection-induced diarrhea, 0.4 µg plumbagin inhibited secretory diarrhea by >40% and 50%, respectively, without affecting intestinal epithelial integrity or the rotaviral infection. In addition, plumbagin exerted inhibitory effects on the vasoactive intestinal peptide (VIP)-, prostaglandin E2 (PGE2)-, and 5-hydroxytryptamine (5-HT)-stimulated Cl⁻ currents. In the evaluations of intestinal motility, plumbagin significantly delayed intestinal motility and inhibited intestinal smooth muscle contractility without an evident impact on contractive frequency. Collectively, our results indicate that plumbagin inhibits both Ca²⁺- and cAMP-activated Cl⁻ channels, accounting for the mechanisms of plumbagin inhibition of chloride secretion and intestinal motility. Thus, plumbagin can be a lead compound in the treatment of CT-induced, Traveler’s, and rotaviral diarrhea, as well as other types of secretory diarrhea that result from excessive intestinal fluid secretion and increased intestinal peristalsis.

Molecular Determinants of Enterotoxigenic Escherichia coli Heat-Stable Toxin Secretion and Delivery

Enterotoxigenic Escherichia coli (ETEC), a heterogeneous diarrheal pathovar defined by production of heat-labile (LT) and/or heat-stable (ST) toxins, causes substantial morbidity among young children in the developing world. Studies demonstrating a major burden of ST-producing ETEC have focused interest on ST toxoids for ETEC vaccines. We examined fundamental aspects of ST biology using ETEC strain H10407, which carries estH and estP genes encoding STh and STp, respectively, in addition to eltAB genes responsible for LT. Here, we found that deletion of estH significantly diminished cyclic GMP (cGMP) activation in target epithelia, while deletion of estP had a surprisingly modest impact, and a dual estH estP mutant was not appreciably different from the estH mutant. However, we noted that either STh or STp recombinant peptides stimulated cGMP production and that the loss of estP was compensated by enhanced estH transcription. We also found that the TolC efflux protein was essential for toxin secretion and delivery, providing a potential avenue for efflux inhibitors in treatment of acute diarrheal illness. In addition, we demonstrated that the EtpA adhesin is required for optimal delivery of ST and that antibodies against either the adhesin or STh significantly impaired toxin delivery and cGMP activation in target T84 cells. Finally, we used FLAG epitope fusions to demonstrate that the STh propeptide sequence is secreted by ETEC, potentially providing additional epitopes for antibody neutralization. These studies collectively extend our understanding of ETEC pathogenesis and potentially inform additional avenues to mitigate disease by these common diarrheal pathogens.

Guanylate cyclase-C as a therapeutic target in gastrointestinal disorders

Functional gastrointestinal disorders (FGIDs) and IBDs are two of the most prevalent disorders of the GI tract and consume a significant proportion of healthcare resources. Recent studies have shown that membrane-bound guanylate cyclase-C (GC-C) receptors lining the GI tract may serve as novel therapeutic targets in the treatment of FGIDs and IBDs. GC-C receptor activation by its endogenous paracrine hormones uroguanylin and guanylin, and the resulting intracellular production of its downstream effector cyclic GMP, occurs in a pH-dependent manner and modulates key physiological functions. These include fluid and electrolyte homeostasis, maintenance of the intestinal barrier, anti-inflammatory activity and regulation of epithelial regeneration. Studies of the GC-C paracrine signalling axis have revealed the therapeutic potential of these receptors in treating GI disorders, including chronic idiopathic constipation and irritable bowel syndrome-constipation. This review focuses on the evolving understanding of GC-C function in health and disease, and strategies for translating these principles into new treatments for FGIDs and IBDs.

Animal Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is the most common cause of E. coli diarrhea in farm animals. ETEC are characterized by the ability to produce two types of virulence factors: adhesins that promote binding to specific enterocyte receptors for intestinal colonization and enterotoxins responsible for fluid secretion. The best-characterized adhesins are expressed in the context of fimbriae, such as the F4 (also designated K88), F5 (K99), F6 (987P), F17, and F18 fimbriae. Once established in the animal small intestine, ETEC produce enterotoxin(s) that lead to diarrhea. The enterotoxins belong to two major classes: heat-labile toxins that consist of one active and five binding subunits (LT), and heat-stable toxins that are small polypeptides (STa, STb, and EAST1). This review describes the disease and pathogenesis of animal ETEC, the corresponding virulence genes and protein products of these bacteria, their regulation and targets in animal hosts, as well as mechanisms of action. Furthermore, vaccines, inhibitors, probiotics, and the identification of potential new targets by genomics are presented in the context of animal ETEC.

Different assay conditions for detecting the production and release of heat-labile and heat-stable toxins in enterotoxigenic Escherichia coli isolates

Enterotoxigenic Escherichia coli (ETEC) produce heat-labile (LT) and/or heat-stable enterotoxins (ST). Despite that, the mechanism of action of both toxins are well known, there is great controversy in the literature concerning the in vitro production and release of LT and, for ST, no major concerns have been discussed. Furthermore, the majority of published papers describe the use of only one or a few ETEC isolates to define the production and release of these toxins, which hinders the detection of ETEC by phenotypic approaches. Thus, the present study was undertaken to obtain a better understanding of ST and LT toxin production and release under laboratory conditions. Accordingly, a collection of 90 LT-, ST-, and ST/LT-producing ETEC isolates was used to determine a protocol for toxin production and release aimed at ETEC detection. For this, we used previously raised anti-LT antibodies and the anti-ST monoclonal and polyclonal antibodies described herein. The presence of bile salts and the use of certain antibiotics improved ETEC toxin production/release. Triton X-100, as chemical treatment, proved to be an alternative method for toxin release. Consequently, a common protocol that can increase the production and release of LT and ST toxins could facilitate and enhance the sensitivity of diagnostic tests for ETEC using the raised and described antibodies in the present work.

Escherichia coli virulence factors

Escherichia coli was described in 1885 by a German pediatrician, Theodor Escherich, in the faeces of a child suffering diarrhoea. In 1893, a Danish veterinarian postulated that the E. coli species comprises different strains, some being pathogens, others not. Today the E. coli species is subdivided into several pathogenic strains causing different intestinal, urinary tract or internal infections and pathologies, in animal species and in humans. Since this congress topic is the interaction between E. coli and the mucosal immune system, the purpose of this manuscript is to present different classes of adhesins (fimbrial adhesins, afimbrial adhesins and outer membrane proteins), the type 3 secretion system, and some toxins (oligopeptide, AB, and RTX pore-forming toxins) produced by E. coli, that can directly interact with the epithelial cells of the intestinal, respiratory and urinary tracts.

Cure and curse: E. coli heat-stable enterotoxin and its receptor guanylyl cyclase C

Enterotoxigenic Escherichia coli (ETEC) associated diarrhea is responsible for roughly half a million deaths per year, the majority taking place in developing countries. The main agent responsible for these diseases is the bacterial heat-stable enterotoxin STa. STa is secreted by ETEC and after secretion binds to the intestinal receptor guanylyl cyclase C (GC-C), thus triggering a signaling cascade that eventually leads to the release of electrolytes and water in the intestine. Additionally, GC-C is a specific marker for colorectal carcinoma and STa is suggested to have an inhibitory effect on intestinal carcinogenesis. To understand the conformational events involved in ligand binding to GC-C and to devise therapeutic strategies to treat both diarrheal diseases and colorectal cancer, it is paramount to obtain structural information on the receptor ligand system. Here we summarize the currently available structural data and report on physiological consequences of STa binding to GC-C in intestinal epithelia and colorectal carcinoma cells.

Heat-stable enterotoxin of enterotoxigenic Escherichia coli as a vaccine target

Enterotoxigenic Escherichia coli (ETEC) is responsible for 280 million to 400 million episodes of diarrhea and about 380,000 deaths annually. Epidemiological data suggest that ETEC strains which secrete heat-stable toxin (ST), alone or in combination with heat-labile toxin (LT), induce the most severe disease among children in developing countries. This makes ST an attractive target for inclusion in an ETEC vaccine. ST is released upon colonization of the small intestine and activates the guanylate cyclase C receptor, causing profuse diarrhea. To generate a successful toxoid, ST must be made immunogenic and nontoxic. Due to its small size, ST is nonimmunogenic in its natural form but becomes immunogenic when coupled to an appropriate large-molecular-weight carrier. This has been successfully achieved with several carriers, using either chemical conjugation or recombinant fusion techniques. Coupling of ST to a carrier may reduce toxicity, but further reduction by mutagenesis is desired to obtain a safe vaccine. More than 30 ST mutants with effects on toxicity have been reported. Some of these mutants, however, have lost the ability to elicit neutralizing immune responses to the native toxin. Due to the small size of ST, separating toxicity from antigenicity is a particular challenge that must be met. Another obstacle to vaccine development is possible cross-reactivity between anti-ST antibodies and the endogenous ligands guanylin and uroguanylin, caused by structural similarity to ST. Here we review the molecular and biological properties of ST and discuss strategies for developing an ETEC vaccine that incorporates immunogenic and nontoxic derivatives of the ST toxin.

Enterotoxigenic Escherichia coli (ETEC): a recurring decimal in infants' and travelers' diarrhea

Enterotoxigenic Escherichia coli (ETEC) is an important cause of diarrhea in infants and in travelers from developed to underdeveloped countries, especially in regions of poor sanitation. The ETEC are acquired by the ingestion of contaminated food and water, and adults living in endemic areas develop immunity. The disease condition manifests as a minor discomfort to a severe cholera-like syndrome and requires colonization by the microorganism and the elaboration of one or more enterotoxins. The ETEC attach to the epithelial cells of the gastrointestinal tract and release substances that affect the normal functioning of the tract, thereby resulting in diarrhea, and subsequently millions of deaths everyday, particularly in children. The prevention of the spread of this strain of diarrheagenic E. coli depends on ensuring appropriate sanitary measures; hand-washing and proper preparation of food; chlorination of water supplies; and appropriate sewage treatment and disposal. Parenteral or oral fluid and electrolyte replacement is used to prevent dehydration, and broad-spectrum antibiotics are used in chronic or life-threatening cases, but in most cases, should be avoided because of severe side effects.

Contribution of defined amino acid residues to the immunogenicity of recombinant Escherichia coli heat-stable enterotoxin fusion proteins

We investigated whether the toxicity-associated receptor-binding domain of the non-immunogenic Escherichia coli heat-stable enterotoxin (STh) as a fusion with a carrier protein and the inclusion of an appropriate spacer are critical factors for eliciting antibody responses against the native toxin. The immunological properties of three toxic and one non-toxic fusion proteins, consisting of STh N-terminally joined to the C-terminus of the major subunit ClpG of E. coli CS31A fimbriae, were compared. In contrast to the non-toxic hybrid STh with glycine and leucine simultaneously substituted for the receptor-interacting Pro(13) and Ala(14) amino acids, the toxic chimeras responded by producing high serum levels of anti-STh antibodies in immunized animals. On the other hand, only the toxic ClpG-STh construct with the natural peptide 47KSGPESM(53) of Pro-STh as spacer stimulated STh-neutralizing responses against both native toxin and enterotoxigenic live E. coli cells. Altogether, these findings suggest a close relationship between conformational similarity to the native structure of STh and the ability to elicit specific antibody responses against STh.

Full capacity of recombinant Escherichia coli heat-stable enterotoxin fusion proteins for extracellular secretion, antigenicity, disulfide bond formation, and activity

We have successfully used the major subunit ClpG of Escherichia coli CS31A fimbriae as an antigenic and immunogenic exposure-delivery vector for various heterologous peptides varying in nature and length. However, the ability of ClpG as a carrier to maintain in vitro and in vivo the native biological properties of passenger peptide has not yet been reported. To address this possibility, we genetically fused peptides containing all or part of the E. coli human heat-stable enterotoxin (STh) sequence to the amino or carboxyl ends of ClpG. Using antibodies to the ClpG and STh portions for detecting the hybrids; AMS (4-acetamido-4'-maleimidylstilbene-2, 2'-disulfonate), a potent free thiol-trapping reagent, for determining the redox state of STh in the fusion; and the suckling mouse assay for enterotoxicity, we demonstrated that all ClpG-STh proteins were secreted in vitro and in vivo outside the E. coli cells in a heat-stable active oxidized (disulfide-bonded) form. Indeed, in contrast to many earlier studies, blocking the natural NH(2) or COOH extremities of STh had, in all cases, no drastic effect on cell release and toxin activity. Only antigenicity of STh C-terminally extended with ClpG was strongly affected in a conformation-dependent manner. These results suggest that the STh activity was not altered by the chimeric structure, and therefore that, like the natural toxin, STh in the fusion had a spatial structure flexible enough to be compatible with secretion and enterotoxicity (folding and STh receptor recognition). Our study also indicates that disulfide bonds were essential for enterotoxicity but not for release, that spontaneous oxidation by molecular oxygen occurred in vitro in the medium, and that the E. coli cell-bound toxin activity in vivo resulted from an effective export processing of hybrids and not cell lysis. None of the ClpG-STh subunits formed hybrid CS31A-STh fimbriae at the cell surface of E. coli, and a strong decrease in the toxin activity was observed in the absence of CS31A helper proteins. In fact, chimeras translocated across the outer membrane as a free folded monomer once they were guided into the periplasm by the ClpG leader peptide through the CS31A-dependent secretory pathway. In summary, ClpG appears highly attractive as a carrier reporter protein for basic and applied research through the engineering of E. coli for culture supernatant delivery of an active cysteine-containing protein, such as the heat-stable enterotoxin.

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

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

Diarrheagenic Escherichia coli

Escherichia coli is the predominant nonpathogenic facultative flora of the human intestine. Some E. coli strains, however, have developed the ability to cause disease of the gastrointestinal, urinary, or central nervous system in even the most robust human hosts. Diarrheagenic strains of E. coli can be divided into at least six different categories with corresponding distinct pathogenic schemes. Taken together, these organisms probably represent the most common cause of pediatric diarrhea worldwide. Several distinct clinical syndromes accompany infection with diarrheagenic E. coli categories, including traveler's diarrhea (enterotoxigenic E. coli), hemorrhagic colitis and hemolytic-uremic syndrome (enterohemorrhagic E. coli), persistent diarrhea (enteroaggregative E. coli), and watery diarrhea of infants (entero-pathogenic E. coli). This review discusses the current level of understanding of the pathogenesis of the diarrheagenic E. coli strains and describes how their pathogenic schemes underlie the clinical manifestations, diagnostic approach, and epidemiologic investigation of these important pathogens.

Extracellular secretion of Escherichia coli heat-stable enterotoxin I across the outer membrane

Escherichia coli heat-stable enterotoxin Ip (STIp) is an extracellular toxin consisting of 18 amino acid residues that is synthesized as a precursor of pre (amino acid residues 1 to 19), pro (amino acid residues 20 to 54), and mature (amino acid residues 55 to 72) regions. The precursor synthesized in the cytoplasm is translocated across the inner membrane by the general export pathway consisting of Sec proteins. The pre region functions as a leader peptide and is cleaved during translocation. However, it remains unknown how the resulting peptide (pro-mature peptide) translocates across the outer membrane. In this study, we investigated the structure of the STIp that passes through the outer membrane to determine how it translocates through the outer membrane. The results showed that the pro region is cleaved in the periplasmic space. The generated peptide becomes the mature form of STIp, which happens to have disulfide bonds, which then passes through the outer membrane. We also showed that STIp with a carboxy-terminal peptide consisting of 3 amino acid residues passes through the outer membrane, whereas STIp with a peptide composed of 37 residues does not. Amino acid analysis of mutant STIp purified from culture supernatant revealed that the peptide composed of 37 amino acid residues was cleaved into fragments of 5 amino acid residues. In addition, analyses of STIps with a mutation at the cysteine residue and the dsbA mutant strain revealed that the formation of an intramolecular disulfide bond within STIp is not absolutely required for the mature region of STIp to pass through the outer membrane.

Amino acid residues in the pro region of Escherichia coli heat-stable enterotoxin I that affect efficiency of translocation across the inner membrane

Escherichia coli heat-stable enterotoxin Ip (STIp), which is a typical extracellular toxin consisting of 18 amino acid residues, is synthesized as a precursor consisting of pre (amino acid residues 1 to 19), pro (amino acid residues 20 to 54), and mature (amino acid residues 55 to 72) regions. Though the pre region functions as a conventional leader peptide that guides the following region to cross the inner membrane, the role of the pro region in the maturation pathway remains to be elucidated. We previously indicated that the sequence from residues 29 to 38 in the pro region increases the efficiency of STI translocation across the inner membrane (H. Yamanaka, Y. Fuke, S. Hitotsubashi, Y. Fujii, and K. Okamoto, Microbiol. Immunol. 37:195-205, 1993). We therefore examined the amino acid residues in the sequence that are responsible for this function. We substituted several amino acid residues in the sequence by means of oligonucleotide-directed site-specific mutagenesis. We then evaluated the effect of the substitution on the efficiency of STI translocation across the inner membrane by determining the enterotoxic activity of the culture supernatant, the amount of a fusion protein consisting of STI and nuclease A released into the periplasm, and the amount of the labeled ST released into the periplasm after pulse-labeling with [35S]cysteine. Substitution of the charged amino acid residues at positions 29 to 31 (K-E-K) with hydrophobic (I-V-L, F-W-F, or F-W-Q) or basic (K-K-K) residues significantly reduced these values in every assay. In contrast, the substitution of these amino acid residues with acidic amino acid residues (E-E-E) increased these values in all assays. This means that the negative charge near position 30 is important for STI to translocate efficiently across the inner membrane. A similar substitution of lysine residues at positions 37 and 38 showed that they are not involved in the translocation of STI across the inner membrane.

The structure of Escherichia coli heat-stable enterotoxin b by nuclear magnetic resonance and circular dichroism

The heat-stable enterotoxin b (STb) is secreted by enterotoxigenic Escherichia coli that cause secretory diarrhea in animals and humans. It is a 48-amino acid peptide containing two disulfide bridges, between residues 10 and 48 and 21 and 36, which are crucial for its biological activity. Here, we report the solution structure of STb determined by two- and three-dimensional NMR methods. Approximate interproton distances derived from NOE data were used to construct structures of STb using distance-geometry and simulated annealing procedures. The NMR-derived structure shows that STb is helical between residues 10 and 22 and residues 38 and 44. The helical structure in the region 10-22 is amphipathic and exposes several polar residues to the solvent, some of which have been shown to be important in determining the toxicity of STb. The hydrophobic residues on the opposite face of this helix make contacts with the hydrophobic residues of the C-terminal helix. The loop region between residues 21 and 36 has another cluster of hydrophobic residues and exposes Arg 29 and Asp 30, which have been shown to be important for intestinal secretory activity. CD studies show that reduction of disulfide bridges results in a dramatic loss of structure, which correlates with loss of function. Reduced STb adopts a predominantly random-coil conformation. Chromatographic measurements of concentrations of native, fully reduced, and single-disulfide species in equilibrium mixtures of STb in redox buffers indicate that the formation of the two disulfide bonds in STb is only moderately cooperative. Similar measurements in the presence of 8 M urea suggest that the native secondary structure significantly stabilizes the disulfide bonds.

Characterization of a highly toxic, large molecular size heat-stable enterotoxin produced by a clinical isolate of Yersinia enterocolitica

A novel heat-stable enterotoxin (ST) designated as Y-STc was purified to homogeneity from the culture supernatant of a pathogenic strain of Yersinia enterocolitica serotype O3 and its amino acid sequence was determined. The mature Y-STc was found to consist of 53 amino acid residues, which includes the putative pro-sequence. The molecular weight of Y-STc was 5638 and constituted the largest molecular size in the family of currently known STs. The minimum effective dose of purified Y-STc in the suckling mouse assay was 0.6 ng (0.0 pmol), indicating that, despite the long sequence, Y-STc is the most toxic in the ST family.

Maturation pathway of Escherichia coli heat-stable enterotoxin I: requirement of DsbA for disulfide bond formation

The Escherichia coli heat-stable enterotoxin STp is synthesized as a precursor consisting of pre, pro and mature regions. Mature STp is released into the culture supernatant and is composed of 18-amino-acid resides which contain three intramolecular disulfide bonds. The involvement of DsbA in the formation of the disulfide bonds of STp was examined in this study. A dsbA mutant was transformed with a plasmid harboring the STp gene, and the ST activity was significantly lower than that of the parent strain harboring the same plasmid. Furthermore, purified DsbA induced the conversion of synthetic STp peptide (inactive form) to the active form and increased the ST activity of the culture supernatant derived from the dsbA transformants. These results showed that DsbA directly catalyzes the formation of the disulfide bonds of STp. DsbA is located in periplasmic space, where STp is released as an intermediate form consisting of pro and mature regions. To examine the effect of the pro region on the action of DsbA, we replaced the cysteine residue at position 39 and tested the effect in vivo. The substitution caused a significant decrease of ST activity in the culture supernatant, the accumulation of inactive ST in periplasmic space, and an alteration in the cleavage site of the intermediate of STp. We conclude that Cys-39 is important for recognition by the processing enzymes required for the maturation of STp.

Functional properties of pro region of Escherichia coli heat-stable enterotoxin

Escherichia coli heat-stable enterotoxin Ip (STp) is synthesized as the 72-amino-acid residue precursor consisting of three regions: pre region (amino acid residues 1 to 19), pro region (amino acid residues 20 to 54), and mature ST (mST) region (amino acid residues 55 to 72). We examined the role of the pro sequence of STp in enterotoxigenicity of a strain by deleting the gene fragment encoding amino acids 22 to 57. This deletion caused a remarkable reduction of its enterotoxic activity of culture supernatant. In order to analyze the sequence responsible for the function of the pro region, two additional deletion mutants were made. The deletion of the sequence covering amino acids 29 to 38, which is conserved in all sequences of ST reported, brought about a significant reduction of enterotoxic activity but the deletion of the non-conserved sequence (amino acids 40 to 53) did not. This result shows that conserved sequence is mainly responsible for the function. Subsequently, to examine the mechanism of action of the pro region, plasmids carrying DNA sequences of hybrid proteins consisting of pre-pro-nuclease, pre-mST-nuclease, pre-pro-mST-nuclease and pre-pro-nuclease-mST were constructed. Amino acid sequence determination and SDS-polyacrylamide gel analysis revealed that these fusion proteins were cleaved between pre sequence and pro sequence during secretion and the cleaved fusion proteins were accumulated in periplasmic space. But the amount of hybrid protein accumulated in the periplasmic space varied among the strains. That is, the amount of the pre-pro-nuclease gene product that accumulated in the periplasmic space was the highest of all fusion gene products. These results indicate that the existence of the mST region strongly interferes with the translocation of the gene product into the periplasmic space and that the pro region functions to guide the mST region into the periplasmic space.

Recombinant enterotoxins as vaccines against Escherichia coli-mediated diarrhoea

A fusion protein, comprising the B subunit of the heat-labile enterotoxin and a portion of the precursor to the heat-stable enterotoxin of Escherichia coli, has been created by recombinant genetic techniques. It is exported successfully to the bacterial periplasm and assembles into pentamers which retain the ability to bind to GM1 ganglioside. Native toxin epitopes are displayed and the molecule can be easily purified from periplasmic extracts of cells expressing the gene fusion. Although the protein carries the natural sequence of the heat-stable enterotoxin, it is greatly attenuated in toxicity. Systemic immunization of mice or oral administration of the fusion elicits antibody responses against both classes of E. coli enterotoxin.

Secretion of the STA3 heat-stable enterotoxin of Escherichia coli: extracellular delivery of Pro-STA is accomplished by either Pro or STA

The methanol-soluble, heat-stable enterotoxin of Escherichia coli is a protease-resistant extracellular peptide which is synthesized as a 72-amino-acid precursor Pre-Pro-STA3. The specific roles of Pre (19 amino acids), Pro (34 amino acids) and STA3 (19 amino acids) in the secretion process were studied by functionally deleting each of the three domains. Deletion of the Pre signal sequence resulted in a short-lived cell-associated molecule with an M(r) equivalent to that of Pro-STA3. Deletion of Pro (i.e., Pre-STA3) resulted in the rapid extracellular accumulation of STA3; the periplasmic intermediate found in the secretion of the wild-type toxin was undetected. Deletion of the STA3 domain resulted in a cell-associated Pre-Pro peptide; with time this form converted to periplasmic Pro which later became extracellular. When DNA encoding either STA3, by itself, or Pro-STA3 (lacking the signal peptide) was expressed, these peptides were degraded intracellularly, with no periplasmic or extracellular forms detected. The results presented demonstrate that the signal peptide (Pre) is essential even for the export of small peptides to the periplasm, and that its absence causes the STA3 domain to become susceptible to intracellular proteases. The rapid degradation of intracellular STA3 indicates that its proteolytic resistance is acquired in a compartment other than the cytoplasm. The results also show that after the Pre domain is proteolytically cleaved from Pre-STA3 and Pre-Pro, the STA3 and Pro peptides can exit to the culture supernatant.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of Yersinia enterocolitica Yst toxin in experimental infection of young rabbits

We constructed a Yst-negative mutant of Yersinia enterocolitica W1024 by reverse genetics, and we compared the virulence of the yst+ and yst isogenic strains in an experimental oral infection of the young rabbit. The rabbits infected with the yst+ strain suffered from the diarrhea and lost weight, and most of them died. By contrast, the occurrence of diarrhea, weight loss, and death in the group of rabbits infected with the yst mutant was as low as that in the group of uninfected rabbits. Bacteria from both strains were excreted in the feces and induced a serum antibody response against Yop proteins. The yst mutant disappeared more rapidly from the feces. We conclude that the enterotoxin Yst is a major factor involved in the Y. enterocolitica-associated diarrhea in the young rabbit. Given the similarity with the symptoms observed for children, this result suggests that Yst could also be an important factor in diarrhea in young children infected with Y. enterocolitica.

Purification of the STB enterotoxin of Escherichia coli and the role of selected amino acids on its secretion, stability and toxicity

The methanol-insoluble heat-stable enterotoxin of Escherichia coli (STB) was purified and characterized by automated Edman degradation and tryptic peptide analysis. The amino-terminal residue, Ser-24, confirmed that the first 23 amino acids inferred from the gene sequence were removed during translocation through the E. coli inner membrane. Tryptic peptide analysis coupled with automated Edman degradation revealed that disulphide bonds are formed between residues Cys-33 and Cys-71 and between Cys-44 and Cys-59. Oligonucleotide-directed mutagenesis performed on the STB gene demonstrated that disulphide bond formation does not precede translocation of the polypeptide through the inner membrane and that disulphide bridge formation is a periplasmic event; apparently, elimination of either of two disulphides of STB renders the molecule susceptible to periplasmic proteolysis. In addition, a loop defined by the Cys-44-Cys-59 bond contains at least two amino acids (Arg-52 and Asp-53) required for STB toxic activity.

Protein secretion in bacteria

Most secretory proteins are synthesized as precursors with an amino-terminal signal peptide. Genetic identification of proteins essential for signal peptide dependent translocation to the Escherichia coli periplasm has led to the biochemical dissection of the secretion pathway. Additional mechanisms exist in Gram-negative bacteria for protein secretion to the extracellular environment.

Construction of a bifunctional Escherichia coli heat-stable enterotoxin (STb)-alkaline phosphatase fusion protein

A fusion between the genes encoding the Escherichia coli STb heat-stable enterotoxin (estB) and alkaline phosphatase (phoA) was constructed, and the expressed protein product was characterized. The STb-alkaline phosphatase protein (STb-PhoA) had an apparent molecular mass of 50,000 daltons and was detected with both monoclonal anti-alkaline phosphatase and polyclonal anti-STb antibodies. Expression of the gene fusion resulted in high-level production of alkaline phosphatase activity, indicating that STb-PhoA was processed and exported into the periplasm of the E. coli host strain. Amino acid sequence analysis of the hybrid protein yielded the sequence Ser-Thr-Gln-Ser-Asn-Lys-Lys, indicating that STb-PhoA was processed during export in a fashion identical to that of native STb (Y. M. Kupersztoch, K. Tachias, C. R. Moomaw, L. A. Dreyfus, R. G. Urban, C. Slaughter, and S. Whipp, J. Bacteriol. 172: 2427-2432, 1990). STb-PhoA was purified from an expressed bacterial lysate by preparative isoelectric focusing. In a rat ligated intestinal loop model, purified STb-PhoA induced highly significant (P less than 0.002) fluid secretion. In addition, the specific activity of STb-PhoA was nearly identical to that of purified STb. Thus, the STb-PhoA hybrid protein represents a readily obtainable source of biologically active (STb) enterotoxin that may prove useful in studies to determine the mode of toxin action.

High-level production of Escherichia coli STb heat-stable enterotoxin and quantification by a direct enzyme-linked immunosorbent assay

A convenient and sensitive enzyme-linked immunosorbent assay (ELISA) for the STb heat-stable enterotoxin of Escherichia coli was developed and used to quantify STb production by strains with a high level of expression. Based on an antigenic profile of the secreted form of STb, a synthetic peptide (STb3-27) spanning the major predicted epitope was synthesized, coupled to keyhole limpet hemocyanin, and used to immunize rabbits. Anti-STb3-27 antibodies were affinity purified on a synthetic peptide-Sepharose 4B column and used in a direct-binding STb ELISA. Based on a highly purified form of toxin as a standard, the ELISA detected as little as 1 to 2 ng of STb from crude culture filtrates. ELISA data revealed that natural STb-producing strains elaborate little STb in defined-medium cultures relative to that elaborated by a recombinant strain harboring a cloned copy of the estB gene. Replacement of the endogenous STb promoter with any of several highly active promoters, including a bacteriophage T7 promoter, a beta-galactosidase promoter, and a tryptophan-beta-galactosidase hybrid (tac) promoter, increased the yield of STb 10- to 20-fold over levels obtained by an E. coli strain harboring the recombinant estB gene. The high level of STb antigen detected by the ELISA correlated with intestinal secretory activity. The combination of a convenient assay and effective hyperproduction of STb will serve as a basis for a large-scale toxin purification strategy.

Synthesis of Escherichia coli heat-stable enterotoxin STp as a pre-pro form and role of the pro sequence in secretion

Escherichia coli heat-stable enterotoxin STp is presumed from its DNA sequence to be synthesized in vivo as a 72-amino-acid residue precursor that is cleaved to generate mature STp consisting of the 18 carboxy-terminal amino acid residues. There are two methionine residues in the inferred STp sequence in addition to the methionine residue at position 1. In order to confirm production of the STp 72-amino-acid residue precursor, we substituted the additional methionine residues by oligonucleotide-directed site-specific mutagenesis. Since these substitutions did not cause a significant change in STp production, it can be concluded that STp is normally synthesized as the 72-amino-acid residue precursor. The length of the STp precursor indicated the existence of a pro sequence between the signal peptide and the mature protein. In order to identify the pro sequence and determine its role in protein secretion, deletion and fusion proteins were made. A deletion mutant in which the gene fragment encoding amino acid residues 22 to 53 of STp was removed was made. STp activity was found in the culture supernatant of cells. Amino acid sequence analysis of the purified STp deletion mutant revealed that the pro sequence encompasses amino acid residues 20 to 54. A hybrid protein consisting of STp amino acids 1 to 53 fused in frame from residue 53 to nuclease A was not secreted into the culture supernatant. These results indicate that the pro sequence does not function to guide periplasmic protein into the extracellular milieu.

Secretion of methanol-insoluble heat-stable enterotoxin (STB): energy- and secA-dependent conversion of pre-STB to an intermediate indistinguishable from the extracellular toxin

The methanol-insoluble, heat-stable enterotoxin of Escherichia coli synthesized by clinical strains or strains that harbor the cloned gene was shown to be an extracellular polypeptide. The toxin (STB) was first detected as an 8,100-Mr precursor (pre-STB) that was converted to a transiently cell-associated 5,200-Mr form. Proteolytic conversion of pre-STB to STB was shown to be inhibited by the proton motive force uncoupler carbonyl cyanide m-chlorophenylhydrazone and did not occur in a secA background. After STB was detected as a cell-associated molecule, an extracellular form with identical electrophoretic mobility became apparent. The results suggest that there is no proteolytic processing during the mobilization of STB from the periplasm to the culture supernatant. The determined amino acid sequence of STB coincides fully with the 48 carboxy-terminal amino acids inferred from the DNA sequence. The 23 amino-terminal residues inferred from the DNA sequence were absent in the mature toxin.

Export and processing analysis of a fusion between the extracellular heat-stable enterotoxin and the periplasmic B subunit of the heat-labile enterotoxin in Escherichia coli

As an initial approach in the study of the mechanism of secretion of the extracellular heat-stable enterotoxin of Escherichia coli (STA), and in order to use this polypeptide as an extracellular carrier we previously constructed a fusion between the complete STA toxin (pre-pro-STA) and the mature B subunit of the periplasmic heat-labile enterotoxin (LTB); the resulting STA-LTB hybrid was not secreted to the extracellular environment, and cells expressing the hybrid lysed at temperatures above 35 degrees C. In this work we have established that the hybrid is initially detected as pre-pro-STA-LTB and converted to pro-STA-LTB, which lacks the 19 amino acids that share the properties of a signal peptide; the sequenced 17 amino-terminal residues of pro-STA-LTB defined the processing site of pre-pro-STA-LTB at pro-3phe-2ala-1 decreases gln+1. This process was sensitive to an energy uncoupler (CCCP) and was correlated with translocation of pro-STA-LTB across the inner membrane. Additionally, we are able to show that although pre-pro-STA-LTB is processed at 37 degrees C and 29 degrees C, it is more efficiently processed at the latter temperature. At 37 degrees C, pro-STA-LTB was poorly released into the periplasm, resulting in accumulation of this protein, pre-pro-STA-LTB, and pre-beta-lactamase in the inner membrane, and in cell lysis. In contrast, at 29 degrees C pro-STA-LTB was localized in the periplasm and in the inner membrane, and pre-pro-STA-LTB and pre-beta-lactamase did not accumulate; however, translocation of periplasmic pro-STA-LTB across the outer membrane still did not occur, and a second processing step that would eliminate the pro segment from pro-STA-LTB was never observed. Thus, the fusion of pre-pro-STA and LTB resulted in a polypeptide that, while incompatible with secretion to the extracellular medium, is exported to the periplasm in a temperature-conditional fashion. This latter observation is consistent with an STA secretion pathway whereby pre-pro-STA is first processed to periplasmic pro-STA by the removal of a 19-amino-acid signal peptide.