Structural features of the binding site of cholera toxin inferred from fluorescence measurements

Analysis of Nanoconfined Protein Dielectric Signals Using Charged Amino Acid Network Models

Protein slow motions involving collective molecular fluctuations on the timescale of microseconds to seconds are difficult to measure and not well understood despite being essential to sustain protein folding and protein function. Broadband dielectric spectroscopy (BDS) is one of the most powerful experimental techniques to monitor, over a broad frequency and temperature range, the molecular dynamics of soft matter through the orientational polarisation of permanent dipole moments that are generated by the chemical structure and morphological organisation of matter. Its typical frequency range goes from 107 Hz down to 10−3 Hz, being thus suitable for investigations on slow motions in proteins. Moreover, BDS has the advantage of providing direct experimental access to molecular fluctuations taking place on different length-scales, from local to cooperative dipolar motions. The unfolding of the cholera toxin B pentamer (CtxB5) after thermal treatment for 3h at 80°C is investigated by BDS under nanoconfined and dehydrated conditions. From the X-ray structure of the toxin pentamer, network-based models are used to infer the toxin dipoles present in the native state and to compute their stability and dielectric properties. Network analyses highlight three domains with distinct dielectric and stability properties that support a model where the toxin unfolds into three conformations after the treatment at 80°C. This novel integrative approach offers some perspective into the investigation of the relation between local perturbations (e.g. mutation, thermal treatment) and larger scale protein conformational changes. It might help ranking protein sequence variants according to their respective scale of dynamics perturbations.

Cholera toxin B subunits assemble into pentamers--proposition of a fly-casting mechanism

The cholera toxin B pentamer (CtxB₅), which belongs to the AB₅ toxin family, is used as a model study for protein assembly. The effect of the pH on the reassembly of the toxin was investigated using immunochemical, electrophoretic and spectroscopic methods. Three pH-dependent steps were identified during the toxin reassembly: (i) acquisition of a fully assembly-competent fold by the CtxB monomer, (ii) association of CtxB monomer into oligomers, (iii) acquisition of the native fold by the CtxB pentamer. The results show that CtxB₅ and the related heat labile enterotoxin LTB₅ have distinct mechanisms of assembly despite sharing high sequence identity (84%) and almost identical atomic structures. The difference can be pinpointed to four histidines which are spread along the protein sequence and may act together. Thus, most of the toxin B amino acids appear negligible for the assembly, raising the possibility that assembly is driven by a small network of amino acids instead of involving all of them.

Deletion mutations in N-terminal alpha1 helix render heat labile enterotoxin B subunit susceptible to degradation

Heat-labile enterotoxin (LT) from enterotoxigenic Escherichia coli is a heterohexameric protein consisting of an enzymatically active A subunit, LTA, and a carrier pentameric B subunit, LTB. It is clear from the crystal structure of LTB that the N-terminal alpha1 helix lies outside the core structure. However, the function of the N-terminal alpha1 helix of LTB is unknown. The present work was carried out to investigate the effect of site-directed mutagenesis of the alpha1 helix on LTB synthesis. Six amino acids (PQSITE) located at positions 2-7 from the N terminus, including 4 aa from the alpha1 helix, were deleted by site-directed mutagenesis. The deletion resulted in complete inhibition of LTB expression in E. coli when expressed along with its signal sequence. A single amino acid deletion within the alpha1 helix also resulted in loss of expression. However, a single amino acid deletion outside the alpha1 helix did not affect LTB synthesis. Mutant proteins, whose synthesis was not detected in vivo, could be successfully translated in vitro by using the coupled transcription-translation system. Immunoblot analysis, Northern blot analysis, and in vitro transcription-translation data collectively indicate that the lack of synthesis of the mutant proteins is caused by the immediate degradation of the expressed product by cellular proteases rather than by faulty translation of mutant LTB mRNA. Coexpression of the LTA could not rescue the degradation of LTB mutants.

A dipeptide metalloendoprotease substrate completely blocks the response of cells in culture to cholera toxin

Prior exposure (15 min at 37 degrees C) of several cell types (Vero, SH-SY5Y neuroblastoma, human intestinal epithelial T84) to 3 mm N-benzoyloxycarbonyl-Gly-Phe-amide (Cbz-Gly-Phe-NH(2)), a competitive substrate for metalloendoproteases, completely suppressed cholera toxin (CT)-induced intracellular cAMP accumulation. The specificity of the inhibitory effect was demonstrated by the complete lack of effect of the dipeptide Cbz-Gly-Gly-NH(2), an inactive analogue of Cbz-Gly-Phe-NH(2). The effect was reversible and dose- (IC(50) as low as 0.2 mm depending on the cell type) and time-dependent. Adding Cbz-Gly-Phe-NH(2) during the lag phase caused a diminution of its inhibitory effect similar to that observed with brefeldin A (BFA). Whereas the dipeptide completely suppressed the CT-induced adenylate cyclase (AC) activity, a direct effect on AC is unlikely since the elevation of intracellular cAMP by forskolin was only slightly reduced. The A(1) peptide of CT and NAD(+) activated the AC to the same extent in membranes from control and Cbz-Gly-Phe-NH(2)-treated cells or when Cbz-Gly-Phe-NH(2) was added directly to the assay. The inhibitory effects of suboptimal amounts of Cbz-Gly-Phe-NH(2) and BFA were not additive pointing to a similar mode of action of the two substances. However, Madin-Darby canine kidney cells of which the Golgi structure is BFA-resistant were not resistant to the inhibitory action of Cbz-Gly-Phe-NH(2) on CT cytotoxicity. Several lines of evidence indicate that a perturbation of intracellular Ca(2+) homeostasis by Cbz-Gly-Phe-NH(2) is not responsible for the inhibitory effect of the dipeptide. The dipeptide had also no effect on the binding of (125)I-CT to cells and even increased its intracellular internalization. In contrast with BFA, Cbz-Gly-Phe-NH(2) did not completely suppress the formation of the catalytically active A(1) fragment from bound CT. The data are compatible with a role of metalloendoprotease activity in the intracellular trafficking and processing of CT, although other mechanisms of action of Cbz-Gly-Phe-NH(2) cannot be excluded.

A single mutation disrupts the pH-dependent dimerization of glycinamide ribonucleotide transformylase

Monomeric GART reversibly associates into a dimeric form as a function of decreasing solution pH. The transition is consistent with a three-proton transfer reaction with an apparent pKa near 7. We now report that a single mutation, which replaces a glutamic acid at position 70 in the dimer interface with alanine (E70A), disrupts the pH-dependent dimerization of GART based on dynamic light scattering and gel filtration studies. A comparison of data obtained from UV-absorbance difference spectroscopy for both the wild-type and mutant forms of GART indicates that a tyrosine residue(s) undergoes a change in solvent exposure over the pH range 6.55 to 8.19. A conformational change in tertiary structure that accompanies dimerization accounts for 60% of the observed optical difference, while the remaining 40% can be attributed to a pH-dependent process unrelated to dimerization. In addition, fluorescence studies of the mutant protein indicate that a pH-dependent change in tryptophan fluorescence exhibited by the wild-type protein is unrelated to quaternary structural changes and is likely a result of simple fluorescence quenching by nearby protonated histidine side-chains. Taken together, our results indicate that a single amino acid change at the dimer interface is sufficient to interrupt the highly specific, pH-dependent assembly reaction of GART, although pH-dependent conformational changes present in the wild-type protein also occur in E70A GART. This work is a first application of structure-based site-directed mutagenesis to the analysis of this pH-dependent assembly reaction.

A pH-dependent conformational change in the B-subunit pentamer of Escherichia coli heat-labile enterotoxin: structural basis and possible functional role for a conserved feature of the AB5 toxin family

The non-covalently associated B-subunit moieties of AB5 toxins, such as cholera toxin and related diarrheagenic enterotoxins, exhibit exceptional pH stability and remain pentameric at pH values as low as 2.0. Here, we investigate the structural basis of a pH-dependent conformational change which occurs within the B5 structure of Escherichia coli heat-labile enterotoxin (EtxB) at around pH 5.0. The use of far-UV CD and fluorescence spectroscopy showed that EtxB pentamers undergo a fully reversible pH-dependent conformational change with a pKa of 4.9 +/- 0.1 (R2 = 0.999) or 5.13 +/- 0.01 (R2 = 0.999), respectively. This renders the pentamer susceptible to SDS-mediated disassembly and decreases its thermal stability by 18 degrees C. A comparison of the pH-dependence of the structural change in EtxB5, with that of a mutant containing a Ser substitution at His 57, revealed that the pKa of the conformational change was shifted from ca. 5.1 to 4.4. This finding suggests that protonation of the imidazole side chain of His 57 might facilitate disruption of a spatially adjacent salt bridge, located between Glu 51 and Lys 91 in each B-subunit, thus triggering the conformational change in the pentameric structure. The pH-dependent conformational change was found to be inhibited when B-subunits bound to monosialoganglioside, GMI; and to have no effect on the stability of interaction between A- and B-subunits within the AB5 complex. This suggests that the conformational change is unlikely to have a direct involvement in toxicity. Conservation of the pH-dependent conformational change in the AB5 toxin family, combined with the potential exposure of the hydrophobic core of beta-barrel in the monomeric units, leads to the proposal that the conformational change may be the common feature that ensures the secretion of these proteins from the Vibrionaceae.

Kinetics of acid-mediated disassembly of the B subunit pentamer of Escherichia coli heat-labile enterotoxin. Molecular basis of pH stability

The B-subunit pentamer of Escherichia coli heat-labile enterotoxin (EtxB) is highly stable, maintaining its quaternary structure in a range of conditions that would normally be expected to cause protein denaturation. In this paper the structural stability of EtxB has been studied as a function of pH by electrophoretic, immunochemical, and spectroscopic techniques. Disassembly of the cyclic pentameric structure of human EtxB occurs only below pH 2. As determined by changes in intrinsic fluorescence this process follows first-order kinetics, with the rate constant for disassembly being proportional to the square of the H+ ion concentration, and with an activation energy of 155 kJ mol-1. A C-terminal deletion mutant, hEtxB214, similarly shows first-order kinetics for disassembly but with a higher pH threshold, resulting in disassembly being seen at pH 3.4 and below. These findings are consistent with the rate-limiting step for disassembly of human EtxB being the simultaneous disruption of two interfaces by protonation of two C-terminal carboxylates. By comparison, disassembly of the B-subunit of cholera toxin (CtxB), a protein which shows 80% sequence identity with EtxB, exhibits a much lower stability to acid conditions; with disassembly of CtxB occurring below pH 3.9, with an activation energy of 81 kJ mol-1. Reasons for the observed differences in acid stability are discussed, and the implications of these findings to the development of oral vaccines using EtxB and CtxB are considered.

Regeneration of active receptor recognition domains on the B subunit of cholera toxin by formation of hybrids from chemically inactivated derivatives

In order to test the hypothesis that binding sites of cholera toxin for its receptor, the monosialoganglioside GM1, are shared between adjacent beta-polypeptide chains, two inactive chemical derivatives of the B subunit of cholera toxin (CTB) were prepared and were subsequently used for the construction of hybrid CTB pentamers. One inactive derivative consisted of CTB specifically modified in the single essential Trp-88 residue of each beta-chain. This residue was modified by formylation, a treatment preserving the structural integrity of CTB. The other inactive derivative consisted of CTB specifically succinylated in three amino groups located in or near the receptor binding site. Using [1,4-14C]succinic anhydride for the site-specific succinylation and analysis of radiolabeled tryptic fragments of S-carboxymethylated [14C]sssCTB revealed that the amino groups specifically modified were the alpha-amino group of Thr-1 and the epsilon-amino groups of respectively Lys-34 and Lys-91. Upon submitting equal amounts of formylated CTB and site-specific succinylated CTB to a denaturation-renaturation cycle, hybrid pentamers were formed which in contrast to the parental compounds were able to bind GM1. The affinity of hybrid CTB for GM1, as estimated by a competitive solid-phase radiobinding assay was unexpectedly high and only 2.5-fold lower than that of its native counterpart. The number of active binding sites on hybrid CTB was determined from: (i) titration with the oligosaccharide moiety of GM1 (oligo-GM1) and monitoring the reversal of the Trp fluorescence quenching by iodide ions and (ii) rapid gel filtration over a superdex HR column of a mixture of hybrid CTB and an excess of 3H-labeled oligo-GM1. The data are in agreement with the formation of one active binding per four reconstituted binding sites in hybrid CTB, which is consistent with a random association of CTB monomers during the denaturation-renaturation cycle.

Nucleotide sequence analysis of the CT operon of the Vibrio cholerae classical strain 569B

The complete nucleotide sequence of the Vibrio cholerae classical strain 569B was determined. The results prove the exactness of the amino acid CT B sequence published by Takao et al. (1985, Eur. J. Biochem. 146, 503-508). A comparison is made with already reported CT genes.

The ionic channels formed by cholera toxin in planar bilayer lipid membranes are entirely attributable to its B-subunit

The interaction of cholera toxin with planar bilayer lipid membranes (BLM) at low pH results in the formation of ionic channels, the conductance of which can be directly measured in voltage-clamp experiments. It is found that the B-subunit of cholera toxin (CT-B) also is able to induce ionic channels in BLM whereas the A-subunit is not able to do it. The increase of pH inhibited the channel-forming activity of CT-B. The investigation of pH-dependences of both the conductance and the cation-anion selectivity of the CT-B channel allowed us to suggest that the water pore of this channel is confined to the B-subunit of cholera toxin. The effective diameter of the CT-B channels water pores was directly measured in BLM and is equal to 2.1 +/- 0.2 nm. The channels formed by whole toxin and its B-subunit exhibit voltage-dependent activity. We believe these channels are relevant to the mode of action of cholera toxin and especially to the endosomal pathway of the A-subunit into cells.

Structure-function analysis of protein active sites with anti-idiotypic antibody

Antigen and internal image-bearing anti-idiotypic antibody, owing to potential differences in size and chemical nature, need not necessarily demonstrate identical binding specificities. Such differences, termed "dissociability," may be exploited in structure-function analysis of receptor-ligand interaction to identify functionally important amino acid residues, define receptor class, or distinguish receptor conformation. In this sense, ligand and the anti-idiotypes they elicit constitute alternative and complementary probes of protein active sites.

Anti-idiotypic antibodies as probes of protein active sites: application to cholera toxin subunit B

Since Jerne proposed a "network" theory of immune regulation, the properties of anti-idiotypic antibodies (anti-IdAb) have been investigated widely. Anti-IdAb raised against antibodies to a variety of ligands have been shown to bind the ligands' receptors. Thus, the combining site of an anti-IdAb may contain information regarding the three-dimensional structure of an antigen. However, this remarkable property of "internal imagery" has not been exploited for structural investigation at the molecular level. In the present report, a monoclonal "auto"-anti-IdAb was raised against ganglioside GM1 (a cell-surface glycolipid that binds cholera toxin) and was shown to crossreact with the B subunit of cholera toxin. This antibody was presumed to recognize amino acid residues located within the GM1 binding domain. To identify these residues, the antibody was screened against homologous toxins purified from enterotoxigenic strains of Escherichia coli and chimeric peptides produced by recombinant methods. Amino acid variation at position 4 from the N terminus of these proteins was found to disrupt antibody binding. Since the toxins and chimera are all closely related in structure and function, the residue at position 4 (an asparagine in cholera toxin B subunit) appears to be in the epitope of the antibody and, by implication, in the GM1 binding site. Of particular significance, this structural detail could not be deduced with GM1 alone. It would seem that ligand and anti-ligand anti-IdAb encode similar stereochemical information but do so with different "chemical alphabets," giving rise to distinct binding specificities.

The effect of pH on the conformation and stability of the structure of plant toxin-ricin

The effect of pH on the conformation of ricin and its A- and B-chains has been studied by measuring their intrinsic fluorescence. At pH 5.0 and 7.5, the structural stability of toxin and subunits was estimated according to the denaturing action of guanidine hydrochloride. It was demonstrated that the fluorescence of native toxin and catalytic A-subunit does not depend significantly on pH in the range pH 3-8, whereas ricin B-chain undergoes a structural transition at pH less than 5.0. The structural stability of ricin and isolated chains differs significantly at pH 7.5 and 5.0; the structural stability of ricin and the A-chain increases, whereas that of the B-chain decreases.

Two-dimensional crystals of cholera toxin B-subunit-receptor complexes: projected structure at 17-A resolution

The B subunit of cholera toxin forms two-dimensional crystals when bound to its membrane receptor, ganglioside GM1, in phospholipid layers. A rectangular crystal lattice gives diffraction extending to 15-A resolution in negative stain, and image-processing of electron micrographs reveals a ring of five protein densities. The diameter of the central hole and the outer diameter of the ring are about 20 and 60 A, respectively. These data are consistent with a pentameric, doughnut-shaped structure of the B subunit that lies flat on a membrane surface. A hexagonal crystal lattice is obtained as well, and results of image processing and chemical crosslinking allow two interpretations: the B subunit may exist in both pentameric and hexameric forms or, more likely, the hexagonal lattice may represent a disordered or liquid crystalline form, in which a pentamer undergoes rotational averaging about its 5-fold axis.