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

Coupling enterotoxigenic Escherichia coli heat-stable peptide toxin with 8-arm PEG enhances immunogenicity

Enterotoxigenic Escherichia coli (ETEC) strains, which produce the heat-stable enterotoxin (ST) either alone or in combination with the heat-labile enterotoxin, contribute to the bulk of the burden of child diarrheal disease in resource-limited countries and are associated with mortality. Developing an effective vaccine targeting ST presents challenges due to its potent enterotoxicity, non-immunogenicity, and the risk of autoimmune reaction stemming from its structural similarity to the human endogenous ligands, guanylin, and uroguanylin. This study aimed to assess a novel synthetic vaccine carrier platform employing a single chemical coupling step for making human ST (STh) immunogenic. Specifically, the method involved cross-linking STh to an 8-arm N-hydroxysuccinimide (NHS) ester-activated PEG cross-linker. A conjugate of STh with 8-arm structure was prepared, and its formation was confirmed through immunoblotting analysis. The impact of conjugation on STh epitopes was assessed using ELISAs with polyclonal and monoclonal antibodies targeting various epitopes of STh. Immunization of mice with the conjugate induced the production of anti-STh antibodies, exhibiting neutralizing activity against STh.

Therapeutically targeting guanylate cyclase-C: computational modeling of plecanatide, a uroguanylin analog

Plecanatide is a recently developed guanylate cyclase-C (GC-C) agonist and the first uroguanylin analog designed to treat chronic idiopathic constipation (CIC) and irritable bowel syndrome with constipation (IBS-C). GC-C receptors are found across the length of the intestines and are thought to play a key role in fluid regulation and electrolyte balance. Ligands of the GC-C receptor include endogenous agonists, uroguanylin and guanylin, as well as diarrheagenic, Escherichia coli heat-stable enterotoxins (ST). Plecanatide mimics uroguanylin in its 2 disulfide-bond structure and in its ability to activate GC-Cs in a pH-dependent manner, a feature associated with the presence of acid-sensing residues (Asp2 and Glu3). Linaclotide, a synthetic analog of STh (a 19 amino acid member of ST family), contains the enterotoxin's key structural elements, including the presence of three disulfide bonds. Linaclotide, like STh, activates GC-Cs in a pH-independent manner due to the absence of pH-sensing residues. In this study, molecular dynamics simulations compared the stability of plecanatide and linaclotide to STh. Three-dimensional structures of plecanatide at various protonation states (pH 2.0, 5.0, and 7.0) were simulated with GROMACS software. Deviations from ideal binding conformations were quantified using root mean square deviation values. Simulations of linaclotide revealed a rigid conformer most similar to STh. Plecanatide simulations retained the flexible, pH-dependent structure of uroguanylin. The most active conformers of plecanatide were found at pH 5.0, which is the pH found in the proximal small intestine. GC-C receptor activation in this region would stimulate intraluminal fluid secretion, potentially relieving symptoms associated with CIC and IBS-C.

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.

Class A β-lactamases as versatile scaffolds to create hybrid enzymes: applications from basic research to medicine

Designing hybrid proteins is a major aspect of protein engineering and covers a very wide range of applications from basic research to medical applications. This review focuses on the use of class A β-lactamases as versatile scaffolds to design hybrid enzymes (referred to as β-lactamase hybrid proteins, BHPs) in which an exogenous peptide, protein or fragment thereof is inserted at various permissive positions. We discuss how BHPs can be specifically designed to create bifunctional proteins, to produce and to characterize proteins that are otherwise difficult to express, to determine the epitope of specific antibodies, to generate antibodies against nonimmunogenic epitopes, and to better understand the structure/function relationship of proteins.

Constella™(EU)-Linzess™(USA): the last milestone in the long journey of the peptide linaclotide and its implications for the future of peptide drugs

Irritable bowel syndrome with constipation (IBS-C) and chronic idiopathic constipation (CIC) are highly prevalent gastrointestinal disorders associated with health, economical and social problems. Recently, after a long journey of preclinical studies and clinical trials, linaclotide, a first-in-class GC-C receptor peptide agonist, has received the approval in the USA and Europe for the treatment of IBS-C and CIC. This article provides an overview of clinical, economic and biological aspects of IBS-C and CIC and covers the current and emerging therapeutic agents for treating these conditions. Particularly, the pharmacodynamic and pharmacokinetic properties of linaclotide, a small, disulfide-rich peptide, and its implications in the future of peptide drug discovery and development are discussed.

Guanylin peptide family: history, interactions with ANP, and new pharmacological perspectives

The guanylin family of peptides has 3 subclasses of peptides containing either 3 intramolecular disulfide bonds found in bacterial heat-stable enterotoxins (ST), or 2 disulfides observed in guanylin and uroguanylin, or a single disulfide exemplified by lymphoguanylin. These peptides bind to and activate cell-surface receptors that have intrinsic guanylate cyclase (GC) activity. These hormones are synthesized in the intestine and released both luminally and into the circulation, and are also produced within the kidney. Stimulation of renal target cells by guanylin peptides in vivo or ex vivo elicits a long-lived diuresis, natriuresis, and kaliuresis by both cGMP-dependent and independent mechanisms. Uroguanylin may act as a hormone in a novel endocrine axis linking the digestive system and kidney as well as a paracrine system intrarenally to increase sodium excretion in the postprandial period. This highly integrated and redundant mechanism allows the organism to maintain sodium balance by eliminating excess sodium in the urine. In addition, small concentrations of the atrial natriuretic peptide (ANP) can synergize with low concentrations of both guanylin or uroguanylin, which do not induce natriuresis per se, to promote significant natriuresis. Interestingly, the activation of the particulate guanylate cyclase receptors by natriuretic peptides can promote relaxation of animal and human penile erectile tissue and increase intracavernosal pressure to induce penile erection. These peptides can be prototypes for new drugs to treat erectile dysfunction, especially in patients with endothelial and nitrergic dysfunction, such as in diabetes.

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.

Toxin mediated diarrhea in the 21 century: the pathophysiology of intestinal ion transport in the course of ETEC, V. cholerae and rotavirus infection

An estimated 4 billion episodes of diarrhea occur each year. As a result, 2–3 million children and 0.5–1 million adults succumb to the consequences of this major healthcare concern. The majority of these deaths can be attributed to toxin mediated diarrhea by infectious agents, such as E. coli, V. cholerae or Rotavirus. Our understanding of the pathophysiological processes underlying these infectious diseases has notably improved over the last years. This review will focus on the cellular mechanism of action of the most common enterotoxins and the latest specific therapeutic approaches that have been developed to contain their lethal effects.

Receptor guanylyl cyclase C (GC-C): regulation and signal transduction

Receptor guanylyl cyclase C (GC-C) is the target for the gastrointestinal hormones, guanylin, and uroguanylin as well as the bacterial heat-stable enterotoxins. The major site of expression of GC-C is in the gastrointestinal tract, although this receptor and its ligands play a role in ion secretion in other tissues as well. GC-C shares the domain organization seen in other members of the family of receptor guanylyl cyclases, though subtle differences highlight some of the unique features of GC-C. Gene knock outs in mice for GC-C or its ligands do not lead to embryonic lethality, but modulate responses of these mice to stable toxin peptides, dietary intake of salts, and development and differentiation of intestinal cells. It is clear that there is much to learn in future about the role of this evolutionarily conserved receptor, and its properties in intestinal and extra-intestinal tissues.

Characterization of the cattle serum antibody responses against TEM beta-lactamase and the nonimmunogenic Escherichia coli heat-stable enterotoxin (STaI)

In order to test the use of a subunit recombinant vaccine for its capacity to induce antibodies against the nonimmunogenic heat-stable enterotoxin STa from Escherichia coli and the TEM-1 beta-lactamase, cattle were immunized with a hybrid protein created by insertion of the STa sequence at position 197 of the TEM-1 beta-lactamase. Specific anti-STa IgG and IgG1 antibodies were detected at low levels, while no IgG2 antibodies were detected. In contrast, high levels of the different anti-TEM IgG subtypes were detected in cattle sera. In addition, beta-lactamase activity was inhibited by the sera. The presence of antibodies against STa and TEM-1 beta-lactamase was assessed in sera from 366 cattle taken from the field. No significant level of IgGs against the toxin or the TEM-1 was detected. A comparison of the antibody level between the immunized and the nonimmunized animals clearly demonstrated that STa was not able to induce a significant level of antibodies in the vaccinated animals. In contrast, a strong antibody response against TEM-1 beta-lactamase was demonstrated.

In vitro and in vivo Evaluation of 111In-labeled E. coli Heat-Stable Enterotoxin Analogs for Specific Targeting of Human Breast Cancers

Research into the interaction between the E. coli heat-stable enterotoxin (STh) and the guanylin receptor guanylate cyclase C (GC-C) has generated >100 synthetic analogs of the peptide, several of which have been investigated as imaging or therapeutic agents for colorectal cancers. The evidence presented here suggests that in addition to STh binding to GC-C expressing cell lines derived from human colon, STh also specifically binds to an as yet unidentified receptor expressed in high densities on the surface of cell lines derived from human breast cancers. In vitro whole-cell crosslinking studies using 125I-labeled F19-STh(1-19) demonstrate that the putative STh binding protein migrates as an approximately 120-125 kDa species by SDS-PAGE, significantly smaller than the glycosylated GC-C molecule found in the T84 human colon cancer cell line. RT-PCR using total RNA isolated from breast and colon cancer cell lines indicates that GC-C transcripts are undetectable in human breast cancer cell lines and abundant in human colon cancer cell lines. In vitro competitive binding studies using STh analogs and the estrogen receptor positive (ER+) T-47D cell line demonstrated IC50 values between 2.6 and 8.5 nM. Similar studies on the estrogen receptor negative (ER-) cell line MDA-MB-231 showed IC50's between 5.6 and 9.9 nM. Saturation binding analysis revealed receptor expression to fall between 40,000 and 120,000 sites per cell in these cell lines, receptor abundances equal to or greater than the abundance of GC-C in colorectal cancer cell lines. STh binding to these cells, although of similar affinity to STh binding to GC-C, is distinguishable from it on the basis of its ligand specificity. The characteristics of STh analogs as radiopharmaceutical agents were tested in an in vivo model utilizing T-47D human breast cancer cell xenografts in SCID mice. Clearance of STh analogs was rapid, primarily via renal excretion into the urine, with >85% ID excreted into the urine at 1 h p.i. Tumor uptake at 1 h p.i. in T-47D tumor cell xenografts was 0.67+/-0.23% ID/g, and was significantly decreased (p<0.05) upon co-administration of 4 mg/kg unlabeled STh. These results suggest that STh may find application for the imaging and treatment of breast cancer.

In vitro and in vivo evaluation of 177Lu- and 90Y-labeled E. coli heat-stable enterotoxin for specific targeting of uroguanylin receptors on human colon cancers

The human E. coli heat-stable enterotoxin (ST(h), amino acid sequence N1SSNYCCELCCNPACTGCY19) binds specifically to the guanylate cyclase C (GC-C) receptor, which is present in high density on the apical surface of normal intestinal epithelial cells as well as on the surface of human colon cancer cells. Analogs of ST(h) are currently being used as vectors targeting human colon cancers. Previous studies in our laboratory have focused on development of 111Indium-labeled ST(h) analogs for in vivo imaging applications. Here, we extend the scope of this work to include targeting of the therapeutic radionuclides 90Y and 177Lu. The peptide DOTA-F19-ST(h)(1-19) was synthesized using conventional Fmoc-based solid-phase techniques and refolded in dilute aqueous solution. The peptide was purified by RP-HPLC and characterized by MALDI-TOF MS and in vitro receptor binding assay. The DOTA-conjugate was metallated with nonradioactive Lu(III)Cl3 and Y(III)Cl3, and IC50 values of 2.6+/-0.1 and 4.2+/-0.9 nM were determined for the Lu- and Y-labeled peptides, respectively. 177Lu(III)Cl3 and 90Y(III)Cl3 labeling yielded tracer preparations that were inseparable by C18 RP-HPLC, indicating that putative differences between Lu-, Y- and In coordination spheres are not observed in the context of labeled ST(h) peptides. In vivo biodistribution studies of the 177Lu-labeled peptide in severe combined immunodeficient (SCID) mice bearing T-84 human cancer tumor xenografts showed rapid clearance from the bloodstream, with >90 %ID in the urine at 1 h pi. Localization of the tracer within tumor xenografts was 1.86+/-0.91 %ID/g at 1 h pi, a value higher than for all other tissues with the exception of kidney (2.74+/-0.24 %ID/g). At 24 h pi, >98 %ID was excreted into the urine, and 0.35+/-0.23 %ID/g remained in tumor, again higher than in all other tissues except kidney (0.91+/-0.46 %ID/g). Biodistribution results at 24 h pi for the 90Y-labeled peptide mirrored those for the 177Lu analog, in agreement with the identical behavior of the labeled analogs by C18 RP-HPLC. These results demonstrate the ability of 177Lu- and 90Y-labeled ST(h) molecules to specifically target GC-C receptors expressed on T-84 human colon cancer cells.

Synthesis of a small, cysteine-rich, 29 amino acids long peptide in Mycoplasma pneumoniae

A 205-210 bases long, small RNA (MP200RNA) of Mycoplasma pneumoniae encodes an open reading frame (ORF pmp200) that has the potential to be translated into a 29 amino acids long peptide with nine cysteines. The expression of this peptide in M. pneumoniae was proven indirectly by constructing a gene fusion between the ORF pmp200 and mrfp1, the gene encoding the monomeric red fluorescent protein. The fusion construct was translated in M. pneumoniae. The corresponding fusion protein, with a molecular mass of approximately 35,000 Da, was isolated and the correct sequence was proven by Edman degradation and by mass spectrometry.

DNA vaccination for the priming of neutralizing antibodies against non-immunogenic STa enterotoxin from enterotoxigenic Escherichia coli

In order to test the use of DNA vaccination for its capacity to induce antibodies against the non-immunogenic heat-stable enterotoxin STa from Escherichia coli, BALB/c mice were immunized with plasmid DNA encoding hybrid proteins made by the insertion of wild type STa or insertion of the Cys6Ala, Cys17Ala and Cys6Ala-Cys17Ala STa mutants at positions 195 or 216 of the TEM-1 beta-lactamase. No STa specific antibodies could be detected after three plasmid injections, but a subsequent boost with native STa peptide was capable of inducing low levels of neutralizing antibodies, as tested in the suckling mouse assay. Highest STa specific responses were found in mice primed with the double mutated STa inserted in position 195. This plasmid induced highest T-cell responses to the TEM-1 protein, indicating that priming of helper T-cell responses to the carrier protein was essential. Mixed IgG1/IgG2a isotypes also reflected this T helper 1 type priming. Moreover, insertion into loop A of the TEM-1 carrier may be more suitable than insertion into loop B, because of reduced competition between carrier and hapten B cell responses.

A comparative molecular field analysis (COMFA) of the structural determinants of heat-stable enterotoxins mediating activation of guanylyl cyclase C

The heat-stable enterotoxin binds to and activates guanylyl cyclase C (GC-C), regulating fluid and electrolyte secretion in intestinal epithelial cells. A COMFA model was developed to predict the primary interactions between GC-C agonists and their receptor. This model predicts that the amide backbone of Cys(5)-Cys(6)-Glu(7)-Leu(8), the beta carbon atoms of Cys(5)-Cys(6), and the side chains of Pro(12), Ala(13), and Ala(15) comprise the primary interactions of GC-C agonists with the receptor surface.

The guanylyl cyclase family at Y2K

During the 1980s the purification, cloning, and expression of various forms of guanylyl cyclase (GC) revealed that they served as receptors for extracellular signals. Seven membrane forms, which presumably exist as homodimers, and four subunits of apparent heterodimers (commonly referred to as the soluble forms) are known, but in animals such as nematodes, much larger numbers of GCs are expressed. The number of transmembrane segments (none, one, or multiple) divide the GC family into three groups. Those with no or one transmembrane segment bind nitric oxide/carbon monoxide (NO/CO) or peptides. There are no known ligands for the multiple transmembrane segment class of GCs. Mutational and structural analyses support a model where catalysis requires a shared substrate binding site between the subunits, whether homomeric or heteromeric in nature. Because some cyclases or cyclase ligand genes lack specific GC inhibitors, disruption of either has been used to define the functions of individual cyclases, as well as to define human genetic disease counterparts.

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.

Structural and functional features of modified heat-stable toxins produced by enteropathogenic Klebsiella cells

Heat-stable enterotoxins (STs) are 18- or 19-amino acid peptides (STa or ST1) produced by enteropathogenic bacteria with small differences in their amino acid sequence and a highly conserved carboxy terminus. All STs contain a core of three disulfide bridges whose integrity is believed to be necessary for full biologic activity. We previously reported that strains of Klebsiella pneumoniae transformed by the plasmid pSLM004 produce a modified toxin not recognized by MAb raised against genuine Escherichia coli ST. Investigation of the chemical structure of the modified toxins revealed that three new toxins were present. These were purified to homogeneity by a series of sequential chromatography on reverse-phase columns using guanylate cyclase to monitor the enterotoxic activity during purification procedures. The sequence of the modified toxins was obtained by a combination of Edman degradation and mass spectrometry, showing that they are proteolytically processed forms of E. coli ST1b. In particular, toxin A-2 lacks the cysteine at position 18 and then is not able to form the disulfide bridge cysteine-10-cysteine-18. All three toxins showed the ability to stimulate guanylate cyclase and to elicit chloride secretion in Caco-2 cell monolayers mounted in Ussing chambers. Toxin A-1 and toxin B demonstrated greatly reduced immunoreactivity whereas toxin A-2 was not recognized at all in the ELISA. It is likely that the three modified toxins were generated by Klebsiella specific proteolytic processing of the original pretoxin. These results have important implications for the diagnosis and prevention of heat-stable toxin-induced diarrhea.

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.

Cross-talk between enteric pathogens and the intestine

Enteric pathogens finely regulate the expression of virulence genes in reply to stimuli generated by the intestinal environment. This minireview focuses on recently discovered strategies developed by enteric bacteria to cause intestinal secretion through the elaboration of factors that share structure and function with specific host counterparts. Such bacterial antigens appear to interfere largely with the epithelial cell signalling that physiologically regulates the numerous and, as yet not fully elucidated, mechanisms controlling both the transcellular and the paracellular secretion pathways. Heat-stable enterotoxins (STs) elaborated by enterotoxigenic Escherichia coli and the enteroaggregative E. coli enterotoxin (EAST1) are both typical examples of enteric toxins that activate the transcellular secretion pathway by mimicking guanylin, the endogenous modulator of cGMP signalling. Alternative strategies have been developed by Salmonella to induce intestinal secretion through the elaboration of a factor (SopB) that resembles at least two of the host cell 4-phosphatases, enzymes that activate the Ca-dependent transcellular secretion pathway. Finally, Vibrio cholerae has developed innovative tactics to activate the paracellular secretion pathway through the elaboration of Zonula occludens toxin (Zot), a factor that mimics a recently described physiological modulator of intercellular tight junctions.

Guanylin regulatory peptides: structures, biological activities mediated by cyclic GMP and pathobiology

The guanylin family of bioactive peptides consists of three endogenous peptides, including guanylin, uroguanylin and lymphoguanylin, and one exogenous peptide toxin produced by enteric bacteria. These small cysteine-rich peptides activate cell-surface receptors, which have intrinsic guanylate cyclase activity, thus modulating cellular function via the intracellular second messenger, cyclic GMP. Membrane guanylate cyclase-C is an intestinal receptor for guanylin and uroguanylin that is responsible for stimulation of Cl- and HCO3- secretion into the intestinal lumen. Guanylin and uroguanylin are produced within the intestinal mucosa to serve in a paracrine mechanism for regulation of intestinal fluid and electrolyte secretion. Enteric bacteria secrete peptide toxin mimics of uroguanylin and guanylin that activate the intestinal receptors in an uncontrolled fashion to produce secretory diarrhea. Opossum kidney guanylate cyclase is a key receptor in the kidney that may be responsible for the diuretic and natriuretic actions of uroguanylin in vivo. Uroguanylin serves in an endocrine axis linking the intestine and kidney where its natriuretic and diuretic actions contribute to the maintenance of Na+ balance following oral ingestion of NaCl. Lymphoguanylin is highly expressed in the kidney and myocardium where this unique peptide may act locally to regulate cyclic GMP levels in target cells. Lymphoguanylin is also produced in cells of the lymphoid-immune system where other physiological functions may be influenced by intracellular cyclic GMP. Observations of nature are providing insights into cellular mechanisms involving guanylin peptides in intestinal diseases such as colon cancer and diarrhea and in chronic renal diseases or cardiac disorders such as congestive heart failure where guanylin and/or uroguanylin levels in the circulation and/or urine are pathologically elevated. Guanylin peptides are clearly involved in the regulation of salt and water homeostasis, but new findings indicate that these novel peptides have diverse physiological roles in addition to those previously documented for control of intestinal and renal function.

Cellular microbiology: can we learn cell physiology from microorganisms?

Cellular microbiology is a new discipline that is emerging at the interface between cell biology and microbiology. The application of molecular techniques to the study of bacterial pathogenesis has made possible discoveries that are changing the way scientists view the bacterium-host interaction. Today, research on the molecular basis of the pathogenesis of infective diarrheal diseases of necessity transcends established boundaries between cell biology, bacteriology, intestinal pathophysiology, and immunology. The use of microbial pathogens to address questions in cell physiology is just now yielding promising applications and striking results.

Lymphoguanylin: cloning and characterization of a unique member of the guanylin peptide family

Guanylin and uroguanylin are small peptides containing two disulfide bonds that activate membrane guanylate cyclase-receptors in the intestine, kidney and other epithelia. Hybridization assays with a uroguanylin complementary DNA (cDNA) detected uroguanylin-like messenger RNAs (mRNAs) in the opossum spleen and testis, but these transcripts are larger than uroguanylin mRNAs. RT of RNA from spleen to produce cDNAs for amplification in the PCR followed by cloning and sequencing revealed a novel lymphoid-derived cDNA containing an open reading frame encoding a 109-amino acid polypeptide. This protein shares 84% and 40% of its residues with preprouroguanylin and preproguanylin, respectively. A 15-amino acid, uroguanylin-like peptide occurs at the COOH-terminus of the precursor polypeptide. However, this peptide is unique in having only three cysteine residues. We named the gene and its peptide product lymphoguanylin because the source of the first cDNA isolated was spleen and its mRNA is expressed in all of the lymphoid tissues tested. A 15-amino acid form of lymphoguanylin containing a single disulfide bond was synthesized that activates the guanylate cyclase receptors of human T84 intestinal and opossum kidney (OK) cells, although with less potency than uroguanylin and guanylin. Northern and/or RT-PCR assays detected lymphoguanylin mRNA transcripts in many tissues and organs of opossums, including those within the lymphoid/immune, cardiovascular/renal, reproductive, and central nervous organ systems. Lymphoguanylin joins guanylin and uroguanylin in a growing family of peptide agonists that activate transmembrane guanylate cyclase receptors, thus influencing target cell function via the intracellular second messenger, cGMP.

One peptide, two topologies: structure and interconversion dynamics of human uroguanylin isomers

The peptide hormone uroguanylin stimulates chloride secretion via activation of intestinal guanylyl cyclase C (GC-C). It is characterized by two disulfide bonds in a 1-3/2-4 pattern that causes the existence of two topological stereoisomers of which only one induces intracellular cGMP elevation. To obtain an unambiguous structure-function relationship of the isomers, we determined the solution structure of the separated uroguanylin isoforms using NMR spectroscopy. Both isomers adopt well-defined structures that correspond to those of the isomers of the related peptide guanylin. Furthermore, the structure of the GC-C-activating uroguanylin isomer A closely resembles the structure of the agonistic Escherichia coli heat-stable enterotoxin. Compared with guanylin isomers, the conformational interconversion of uroguanylin isomers is retarded significantly. As judged from chromatography and NMR spectroscopy, both uroguanylin isoforms are stable at low temperatures, but are subject to a slow pH-dependent mutual isomerization at 37 degrees C with an equilibrium isomer ratio of approximately 1:1. The conformational exchange is most likely under the sterical control of the carboxy-terminal leucine. These results imply that GC-C is activated by ligands exhibiting the molecular framework corresponding to the structure of uroguanylin isomer A.

Synthesis, biological activity and isomerism of guanylate cyclase C-activating peptides guanylin and uroguanylin

Recently, the peptides guanylin and uroguanylin were identified as endogenous ligands of the membrane-bound guanylate cyclase C (GC-C) that is mainly expressed in the intestinal epithelium. In the present study, bioactive guanylin and uroguanylin have been prepared by solid-phase methodology using Fmoc/HBTU chemistry. The two disulfide bonds with relative 1/3 and 2/4 connectivity have been introduced selectively by air oxidation of thiol groups and iodine treatment of Cys(Acm) residues. Using this strategy, several sequential derivatives were prepared. Temperature-dependent HPLC characterization of the bioactive products revealed that guanylin-related peptides exist as a mixture of two compounds. The isoforms are interconverted within approximately 90 min, which prevents their separate characterization. This effect was not detected for uroguanylin-like peptides. Synthetic peptides were tested for their potential to activate GC-C in cultured human colon carcinoma cells (T84), known to express high levels of GC-C. The results obtained show that both disulfide bonds are necessary for GC-C activation. The presence of the amino-terminally neighboring residues of Cys104 for guanylin and Cys100 for uroguanylin has been found to be essential for GC-C stimulation. Unexpectedly, a hybrid peptide obtained from substitution of the central tripeptide AYA of guanylin by the tripeptide VNV of uroguanylin was not bioactive.

Interaction of Escherichia coli heat-stable enterotoxin B with cultured human intestinal epithelial cells

Binding of Escherichia coli heat-stable enterotoxin B (STb) to the human intestinal epithelial cell lines T84 and HT29 and to polarized T84 cells was studied to define the initial interaction of this peptide toxin with target cells. Equilibrium and competitive binding isotherms showed that 125I-STb bound specifically to T84 and HT29 cells; however, the toxin-epithelial cell interactions could be characterized by low-affinity binding (< or = 10(5) M(-1)) to a high number of binding sites (> or = 10(6) per cell). STb binding to T84 and HT29 cells as a function of 125I-STb concentration did not approach saturation at levels well above the effective biological concentration of STb for fluid secretion. Treatment of the 125I-STb-bound T84 and HT29 cells with an acidic saline solution to remove surface-bound toxin revealed that only approximately 55% +/- 10% of 125I-STb could be removed by this treatment at 4 degrees C, suggesting that approximately half of the bound STb was stably associated with the plasma membrane and/or internalized into the cytoplasm. Similar results were obtained when binding and internalization experiments were conducted at 22 and 37 degrees C. Immunofluorescence studies demonstrated that the strongest signal for STb appeared in the plasma membrane even after acid treatment. Toxin-treated cells also displayed diffuse cytoplasmic staining, indicating that once cell bound, STb did not appear to preferentially associate with membrane vesicles or cellular organelles. Binding and subsequent internalization of 125I-STb were not affected by treatment of the cells with trypsin, endoglycosidase F/peptide N-glycosidase F, Vibrio cholerae neuraminidase, tunicamycin, or 5 mM sodium chlorate, which blocks sulfation of surface proteoglycans. In addition, the internalization process was not altered by preincubation of the cells with the cytoskeleton inhibitors cytochalasin D and colchicine or cellular perturbants (i.e., 0.45 M sucrose and 5 mM sodium azide), indicating that cell surface proteins or carbohydrates did not function as STb receptors. The binding of 125I-STb to polarized T84 cells was also examined, and the total and nonspecific binding isotherms were found to overlap, indicating that the apical surface of polarized T84 cells did not contain a specific receptor for STb. In comparison to undifferentiated cells, twice the amount of bound STb (approximately 80% +/- 10%) was removable from polarized T84 cells after treatment with acidic solution. The percentage of surface-bound STb to polarized T84 cells did not vary significantly with the transepithelial electrical resistance of the cells or when STb was applied basolaterally. Together, our results indicate that STb binds with relatively low affinity to the plasma membrane of cultured intestinal epithelial cells and polarized T84 cells, probably to membrane lipids, and becomes stably associated with the lipid bilayer. The fact that a significant portion of the bound STb becomes free in the cytoplasm, even at a low temperature, suggests that the bound toxin may directly traverse the membrane bilayer.

Detection of the enteroaggregative Escherichia coli heat-stable enterotoxin 1 gene sequences in enterotoxigenic E. coli strains pathogenic for humans

The sequence of the enteroaggregative Escherichia coli enterotoxin 1 ( EAST1) gene was present in most (or all) strains of human-colonizing enterotoxigenic E. coli (ETEC) with colonization factor antigen II (CFA/II) or CFA/IV (CS6). The EAST1 gene was also strongly associated with PCF09+ ETEC or CFA/I+ ETEC elaborating heat-liable enterotoxin (and heat-stable enterotoxin I). In contrast, CFA/I+ ETEC elaborating heat-stable enterotoxin I, CFA/III+ ETEC, or CS17+ ETEC exhibited very weak or no associated. E. coli from healthy volunteers had no EAST1 gene sequence. A CFA/I+ ETEC strain (H10407) possessed multiple copies of the EAST1 gene on the CFA/I-encoding plasmid and chromosome. In one CFA/II+ ETEC strain, the EAST1 gene was present on the CFA/II-encoding plasmid. The EAST1 gene sequences of the CFA/I+ and CFA/II+ ETEC strains were identical to each other and 99.1% homologous to the reported gene sequence of enteroaggregative E. coli. The data indicate that the EAST1 gene is distributed among ETEC strains with a case of the presence of multiple copies in a single cell and that this distribution is associated with the adherence factor type.

Contribution of individual disulfide bonds to biological action of Escherichia coli heat-stable enterotoxin B

Heat-stable enterotoxins (STs) of Escherichia coli are peptides which alter normal gut physiology by stimulating the loss of water and electrolytes. The action of heat-stable toxin B (STb) is associated with an increase in levels of lumenal 5-hydroxytryptamine and prostaglandin E2, known mediators of intestinal secretion. In addition, the toxin is responsible for elevation of cytosolic calcium ion levels in cultured cells. STb is a 48-amino-acid basic peptide containing four cysteine residues and two disulfide bonds. Previous work indicates that disulfide bonds are required for intestinal secretory activity, and yet the relative contribution of the two bonds to toxin stability and action is presently unclear. Site-directed mutagenesis was used to alter the cysteine residues of STb to assess the role of the individual disulfide bonds in toxin activity. Our results indicate that loss of a single disulfide bond was sufficient to abolish the intestinal secretory and G protein-coupled calcium ion influx activities associated with STb toxicity. Loss of toxin action was not a function of increased sensitivity of STb mutants to proteolysis, since mutant toxins displayed proteolytic decay rates equivalent to that of wild-type STb. Circular dichroism spectroscopy of mutant STb toxins indicated that single-disulfide-bond elimination did not apparently affect the toxin secondary structure of one mutant, STbC33S,C71S. In contrast, the alpha-helical content of the other disulfide bond mutant, STbC44S,C59G, was significantly altered, as was that of reduced and alkylated authentic STb. Since both Cys-Cys mutant STbs were completely nontoxic, the absence of biological activity cannot be explained by dramatic secondary structural changes alone; keys to the conformational requirements for STb toxicity undoubtedly reside in the three-dimensional structure of this peptide.

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.

Modeling of the three-dimensional structure of polypeptides in solution using potential-scaled/hot-solute molecular dynamics

We present here an efficient and accurate procedure for modeling of the three-dimensional structures of polypeptides in the explicit solvent water based on molecular dynamics calculations. Using the toxic domain analog of heat-stable enterotoxin as a model peptide, we examined the utilities of two molecular dynamics techniques with the system containing the explicit solvent. One is the potential-scaled molecular dynamics that had been designed for effective conformational analyses of biomolecules with the explicit solvent water by partially scaling down the potential energies involved in the solute molecules. The other is the variation of Berendsen's weak coupling method (referred to as "hot-solute" method), in which only the solute of the system is heated to a high temperature while the solvent is kept at a normal temperature. Each method successfully increased the rate of folding of the peptides, and the most effective was a combination of the two methods. Moreover, the final structure obtained via cooling process successfully reproduced the experimentally known structure from the extended amino acid sequence using only the distance restraints representing three disulfide bonds in the peptide. Additional distance restraints derived from some of the NOE cross peaks accelerated the folding of the peptide, but gave almost the same structure as in the case without these additional restraints. Because a similar calculation without the explicit solvent could not reproduce the known structure, it is suggested that the explicit solvent water could play an important role in the modeling. The methods presented here have the potential for accurate modeling even when less experimental information was available.

Characterization and partial purification of the human receptor for the heat-stable enterotoxin

The receptor for the Escherichia coli heat-stable enterotoxin has been characterized and partially purified from the T84 human colonic cell line. Using a novel mutant heat-stable enterotoxin peptide as a radioligand (the C-terminal tyrosine residue is replaced by phenylalanine in the mutant), a single class of high-affinity receptor sites was detected in T84 cells, with a Kd of 0.1 nM, similar in affinity to the receptor described in human intestinal tissue. The receptor was solubilised from T84 cell membranes and affinity cross-linking of the solubilised preparation indicated that a single species of M(r) 160,000 served as the receptor. Freshly solubilised preparations of the receptor retained heat-stable enterotoxin-activable guanylyl cyclase activity. Purification of the receptor was achieved through sequential affinity chromatography on GTP--epoxy-Sepharose and wheat-germ-agglutinin columns resulting in purification of the receptor by 3000 fold. The heat-stable enterotoxin-binding characteristics of the receptor were unchanged during the purification and silver staining of the purified receptor preparation indicated a band of M(r) 160,000, which was specifically cross-linked to the 125I-labeled mutant peptide. The purified receptor retained guanylyl cyclase activity, but the activity was not stimulated on addition of human heat-stable enterotoxin, suggesting that accessory structural factors may be involved in the activation of the guanylyl cyclase/receptor.

The Escherichia coli heat-stable enterotoxin is a long-lived superagonist of guanylin

The mechanism by which bacterial heat-stable enterotoxins (ST I STA) cause diarrhea in humans and animals has been linked to the activation of an intestinal membrane-bound guanylate cyclase. Guanylin, a recently discovered rat intestinal peptide, is homologous in structure to ST I and can activate guanylate cyclase present on the human colonic carcinoma cell line T84. To directly test the mechanistic association of guanylate cyclase activation with diarrhea, we synthesized guanylin and a guanylin analog termed N9P10 guanylin and compared their biological activities with those of a synthetic ST I analog, termed ST Ib(6-18). We report that guanylin is able to inhibit the binding of a radiolabeled ST I analog to rat intestinal cells but causes diarrhea in infant mice only at doses at least 4 orders of magnitude higher than that of ST Ib(6-18). In contrast, N9P10 guanylin was enterotoxic in mice at much lower doses than guanylin but proved to be a weaker inhibitor of radiolabeled ST I than guanylin in the receptor binding assay. The pattern of guanylate cyclase activation observed for ST Ib(6-18) and the two guanylin analogs parallels the results observed in the receptor binding assay rather than those observed in the diarrheal assay. Treatment of guanylin with chymotrypsin or lumenal fluid derived from newborn mouse intestines resulted in a rapid loss of binding activity. Together, these results suggest that ST I enterotoxins may represent a class of long-lived superagonists of guanylin.

Ligand-based histochemical localization and capture of cells expressing heat-stable enterotoxin receptors

The heat stable enterotoxins (ST) of enterotoxigenic Escherichia coli (ETEC) cause diarrhoea by binding specific intestinal receptors. Precise histochemical localization of ST receptors could provide more information about the pathophysiology of secretory diarrhoea and the role of ST receptors in normal biology. To accomplish this, we quantitatively coupled biotin to the N-terminus of ST1b using biotin-X-X-N-hydroxysuccinimide ester. The derivatized toxin (BST) has an apparent Kd of 11.7 +/- 10 nM for rat brush border receptors. We used BST in an affinity panning cell-capture system, to validate its ability to discriminate between receptor-positive and receptor-negative cells. Cell lines expressing ST receptors (human colon carcinoma T84, and COS cells transfected with guanylyl cyclase-C (GC-C) ST receptor cDNA) were captured to streptavidin and anti-biotin-coated plates with high efficiency and specificity. This system provides a novel approach to screening cells for the presence of unique ST-binding proteins. BST was then used with streptavidin-gold to demonstrate the cellular topography of ST receptors at the light microscopic level. Villus enterocytes were intensely stained, but only a faint signal was observed in upper crypts of rat small intestine. Thus, a gradient of increasing receptor density was seen as upper crypt cells matured into villus enterocytes. Higher magnification revealed that ST receptors are concentrated at the apical aspect of villus enterocytes. Recently, guanylin, a putative endogenous ligand for ST receptors, has been localized to Paneth cells, at the base of intestinal crypts. Thus, ST receptors are concentrated in villus enterocytes, while guanylin appears to be produced at the base of the crypts.(ABSTRACT TRUNCATED AT 250 WORDS)

Enteroaggregative Escherichia coli heat-stable enterotoxin 1 represents another subfamily of E. coli heat-stable toxin

Enteroaggregative Escherichia coli (EAggEC) are associated with persistent diarrhea in young children. Some of these organisms produce a low-molecular-weight, heat-stable, plasmid-encoded enterotoxin that has been named EAggEC heat-stable enterotoxin 1 (EAST1). We have cloned a 4.4-kb DNA fragment from the virulence plasmid of prototype EAggEC strain 17-2, which expresses enterotoxic activity as measured by electrogenic response in Ussing chambers mounted with rabbit ileal tissue. DNA-sequence analysis of this fragment identified an open reading frame (ORF) encoding a cysteine-rich polypeptide of 38 amino acids (M(r), 4100). Insertional and deletional mutations in this ORF resulted in loss of enterotoxic activity. The ORF was cloned into a T7 expression vector, and postinduction culture filtrates exhibited enterotoxic activity and increased ileal tissue cGMP levels. A synthetic peptide consisting of predicted amino acid residues 8-29 also showed enterotoxic activity. These data indicate that this ORF, named astA (EAggEC heat-stable enterotoxin), represents the EAST1 structural gene. EAST1 shows significant homology with the enterotoxic domain of heat-stable enterotoxin a (STa) of enterotoxigenic E. coli and with guanylin, a mammalian analog of STa. Unlike STa, which requires six cysteines and three disulfide linkages for full biological activity, both EAST1 and guanylin contain four cysteine residues. Based on the cGMP data and the sequence homology to STa and guanylin, it is predicted that EAST1 stimulates the particulate form of guanylate cyclase through the same receptor-binding region as STa and guanylin.

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)

Purification and characterization of Escherichia coli heat-stable enterotoxin II

Escherichia coli heat-stable enterotoxin II (STII) was purified to homogeneity by successive column chromatographies from the culture supernatant of a strain harboring the plasmid encoding the STII gene. The purified STII evoked a secretory response in the suckling mouse assay and ligated rat intestinal loop assay in the presence of protease inhibitor, but the response was not observed in the absence of the inhibitor. Analyses of the peptide by the Edman degradation method and fast atom bombardment mass spectrometry revealed that purified STII is composed of 48 amino acid residues and that its amino acid sequence was identical to the 48 carboxy-terminal amino acids of STII predicted from the DNA sequence (C. H. Lee, S. L. Mosely, H. W. Moon, S. C. Whipp, C. L. Gyles, and M. So, Infect. Immun. 42:264-268, 1983). STII has four cysteine residues which form two intramolecular disulfide bonds. Two disulfide bonds were determined to be formed between Cys-10-Cys-48 and Cys-21-Cys-36 by analyzing tryptic hydrolysates of STII.

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.

Conformations of disulfide bridges in proteins

The conformational characteristics of disulfide bridges in proteins have been analyzed using a dataset of 22 protein structures, available at a resolution of less than or equal to 2.0 A, containing a total of 72 disulfide crosslinks. The parameters used in the analysis include (phi, psi) values at Cys residues, bridge dihedral angles chi ss, chi i1, chi j1, chi i2, and chi j2, the distances C alpha i-C alpha j and C beta i-C beta j between the C alpha and C beta atoms of Cys(i) and Cys(j). Eight families of bridge conformations with three or more occurrences have been identified on the basis of these stereochemical parameters. The most populated family corresponds to the "left handed spiral" identified earlier by Richardson [1981) Adv. Protein Chem. 34, 167-330). Disulfide bridging across antiparallel extended strands is observed in alpha-lytic protease, crambin, and beta-trypsin and this structure is shown to be very similar to those obtained in small cystine peptides. Solvent accessible surface area calculations show that the overwhelming majority of disulfide bridges are inaccessible to solvent.

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.

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

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

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

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