Simplified purification method for Clostridium botulinum type E toxin

Purification and Characterization of Recombinant Botulinum Neurotoxin Serotype FA, Also Known as Serotype H

We have purified and characterized recombinant botulinum neurotoxin serotype FA (BoNT/FA). This protein has also been named as a new serotype (serotype H), but the classification has been controversial. A lack of well-characterized, highly pure material has been a roadblock to study. Here we report purification and characterization of enzymatically active, and of inactive nontoxic, recombinant forms of BoNT/FA as tractable alternatives to purifying this neurotoxin from native Clostridium botulinum. BoNT/FA cleaves the same intracellular target proteins as BoNT/F1 and other F serotype BoNTs; the intracellular targets are vesicle associated membrane proteins (VAMP) 1, 2 and 3. BoNT/FA cleaves the same site in VAMP-2 as BoNT/F5, which is different from the cleavage site of other F serotype BoNTs. BoNT/FA has slower enzyme kinetics than BoNT/F1 in a cell-free protease assay and is less potent at inhibiting ex vivo nerve-stimulated skeletal muscle contraction. In contrast, BoNT/FA is more potent at inhibiting neurotransmitter release from cultured neurons.

Molecular Assembly of Clostridium botulinum progenitor M complex of type E

Clostridium botulinum neurotoxin (BoNT) is released as a progenitor complex, in association with a non-toxic-non-hemagglutinin protein (NTNH) and other associated proteins. We have determined the crystal structure of M type Progenitor complex of botulinum neurotoxin E [PTC-E(M)], a heterodimer of BoNT and NTNH. The crystal structure reveals that the complex exists as a tight, interlocked heterodimer of BoNT and NTNH. The crystal structure explains the mechanism of molecular assembly of the complex and reveals several acidic clusters at the interface responsible for association at low acidic pH and disassociation at basic/neutral pH. The similarity of the general architecture between the PTC-E(M) and the previously determined PTC-A(M) strongly suggests that the progenitor M complexes of all botulinum serotypes may have similar molecular arrangement, although the neurotoxins apparently can take very different conformation when they are released from the M complex.

Holotoxin Activity of Botulinum Neurotoxin Subtype A4 Originating from a Nontoxigenic Clostridium botulinum Expression System

Clostridium botulinum subtype A4 neurotoxin (BoNT/A4) is naturally expressed in the dual-toxin-producing C. botulinum strain 657Ba at 100× lower titers than BoNT/B. In this study, we describe purification of recombinant BoNT/A4 (rBoNT/A4) expressed in a nonsporulating and nontoxigenic C. botulinum expression host strain. The rBoNT/A4 copurified with nontoxic toxin complex components provided in trans by the expression host and was proteolytically cleaved to the active dichain form. Activity of the recombinant BoNT/A4 in mice and in human neuronal cells was about 1,000-fold lower than that of BoNT/A1, and the recombinant BoNT/A4 was effectively neutralized by botulism heptavalent antitoxin. A previous report using recombinant truncated BoNT/A4 light chain (LC) expressed in Escherichia coli has indicated reduced stability and activity of BoNT/A4 LC compared to BoNT/A1 LC, which was surmounted by introduction of a single-amino-acid substitution, I264R. In order to determine whether this mutation would also affect the holotoxin activity of BoNT/A4, a recombinant full-length BoNT/A4 carrying this mutation as well as a second mutation predicted to increase solubility (L260F) was produced in the clostridial expression system. Comparative analyses of the in vitro, cellular, and in vivo activities of rBoNT/A4 and rBoNT/A4-L260F I264R showed 1,000-fold-lower activity than BoNT/A1 in both the mutated and nonmutated BoNT/A4. This indicates that these mutations do not alter the activity of BoNT/A4 holotoxin. In summary, a recombinant BoNT from a dual-toxin-producing strain was expressed and purified in an endogenous clostridial expression system, allowing analysis of this toxin.

Molecular basis of activation of endopeptidase activity of botulinum neurotoxin type E

Botulinum neurotoxins (BoNTs) are a group of large proteins that are responsible for the clinical syndrome of botulism. The seven immunologically distinct serotypes of BoNTs (A-G), each produced by various strains of Clostridium botulinum, act on the neuromuscular junction by blocking the release of the neurotransmitter acetylcholine, thereby resulting in flaccid muscle paralysis. BoNTs are synthesized as single inactive polypeptide chains that are cleaved by endogenous or exogenous proteases to generate the active dichain form of the toxin. Nicking of the single chain BoNT/E to the dichain form is associated with 100-fold increase in toxicity. Here we investigated the activation mechanism of botulinum neurotoxin type E upon nicking and subsequent reduction of disulfide bond. It was observed that nicking of BoNT/E significantly enhances its endopeptidase activity and that at the physiological temperature of 37 degrees C the reduced form of nicked BoNT/E adopts a dynamically flexible conformation resulting from the exposure of hydrophobic segments and facilitating optimal cleavage of its substrate SNAP-25. Such reduction-induced increase in the flexibility of the polypeptide folding provides a rationale for the mechanism of BoNT/E endopeptidase against its intracellular substrate, SNAP-25, and complements current understanding of the mechanistics of interaction between the substrate and BoNT endopeptidase.

Expression of the Clostridium botulinum A2 neurotoxin gene cluster proteins and characterization of the A2 complex

Clostridium botulinum subtype A2 possesses a botulinum neurotoxin type A (BoNT/A) gene cluster consisting of an orfX cluster containing open reading frames (ORFs) of unknown functions. To better understand the association between the BoNT/A2 complex proteins, first, the orfX cluster proteins (ORFX1, ORFX3, P47, and the middle part of NTNH) from C. botulinum A2 strain Kyoto F and NTNH of A1 strain ATCC 3502 were expressed by using either an Escherichia coli or a C. botulinum expression system. Polyclonal antibodies against individual orfX cluster proteins were prepared by immunizing a rabbit and mice against the expressed proteins. Antibodies were then utilized as probes to determine which of the A2 orfX cluster genes were expressed in the native A2 culture. N-terminal protein sequencing was also employed to specifically detect ORFX2. Results showed that all of the neurotoxin cluster proteins, except ORFX1, were expressed in the A2 culture. A BoNT/A2 toxin complex (TC) was purified which showed that C. botulinum A2 formed a medium-size (300-kDa) TC composed of BoNT/A2 and NTNH without any of the other OrfX cluster proteins. NTNH subtype-specific immunoreactivity was also discovered, allowing for the differentiation of subtypes based on cluster proteins associated with BoNT.

A new scaleable method for the purification of botulinum neurotoxin type E

Botulinum neurotoxins belong to the most toxic substances in nature. Well-known as a food poisoning agent for almost two centuries, the beneficial aspects of this bacterial metabolite were rediscovered about 30 years ago. These toxins, which are produced by the anaerobic bacterium Clostridium botulinum are nowadays used to treat a variety of neuro-muscular disorders. The increased demand requires techniques for the production and purification of these toxins on an industrial scale. The method described herein is based on filtration and chromatography procedures only. Precipitation, centrifugation and dialysis steps were consequently excluded to develop a protocol, which can easily be scaled up from the laboratory purification to industrial needs. About 4 mg of BoNT/E were purified from a 10-L batch culture corresponding to an overall recovery of approximately 14%.

Isolation and characterization of a neutralizing antibody specific to internalization domain of Clostridium botulinum neurotoxin type B

Botulinum neurotoxins (BoNTs), the causative agents for life-threatening human disease botulism, have been recognized as biological warfare agents. In this study, a neutralizing mouse monoclonal antibody against botulinum neurotoxin serotype B (BoNT/B), named BTBH-N1, was developed from mice immunized with BoNT/B toxoid without non-toxic components, which are generally associated with the toxin. Western blot analysis, using recombinant toxin fragments containing light (L), N-terminal half of heavy (HN) and C-terminal half of heavy chains, indicated that BTBH-N1 recognizes linear epitopes located on the HN domain. An in vivo neutralization assay with mice, was conducted to characterize the neutralization capacity of the BTBH-N1. Only 10 microg of BTBH-N1 completely neutralized 20 units (1 unit = one 50% lethal dose) of BoNT/B. Even though the Mab (up to 100 microg) failed to protect mice challenged with 100 units, it significantly prolonged the time to death in a dose dependent manner. BTBH-N1, the first neutralizing antibody against BoNT/B, could be further developed as effective biological therapeutics for preventing and treating botulism, as well as other diseases caused by BoNT/B.

A highly sensitive immuno-polymerase chain reaction assay for Clostridium botulinum neurotoxin type A

Our goal was to develop a sensitive method for detecting Clostridium botulinum neurotoxin type A (BoNT/A). We were able to detect BoNT/A in the femtogram (10(-15)g) range using an indirect immuno-polymerase chain reaction (immuno-PCR) assay and an indirect sandwich immuno-PCR assay. For the indirect immuno-PCR assay, enzyme-linked immunosorbent assay (ELISA) plates were coated with BoNT/A that was recognized by anti-BoNT/A monoclonal antibody. For the indirect sandwich immuno-PCR assay, the monoclonal antibody was immobilized on ELISA plates for detecting BoNT/A that was recognized by its polyclonal antibodies. Reporter DNA was prepared by PCR amplification using biotinylated 5'-primers, and it was coupled with biotinylated antibodies through streptavidin. In order to increase sensitivity and reduce background noise, the amounts of reporter DNA (ranging from 50 fg to 50 ng) and streptavidin (ranging from 0.125 ng to 8 ng) were optimized. Using the optimized concentration of reporter DNA and streptavidin, both indirect and indirect sandwich immuno-PCR assays detected BoNT/A as low as 50 fg. These results are a 10(5)-fold improvement over conventional indirect ELISA and indirect sandwich ELISA methods. The assays we developed are currently the most sensitive methods for detecting BoNT/A.

Botulinum neurotoxin types B and E: purification, limited proteolysis by endoproteinase Glu-C and pepsin, and comparison of their identified cleaved sites relative to the three-dimensional structure of type A neurotoxin

Botulinum neurotoxin (NT) serotypes B and E are approximately 150 kDa proteins. Isolated from the liquid culture of Clostridium botulinum the NT type E is a single chain protein while the NT type B, from the proteolytic strain of the bacteria, is a mixture of dichain (nicked within a disulfide loop located about one-third the way from the N-terminus to the C-terminus) protein and its precursor single-chain protein. Endoproteinase Glu-C (EC 3.4.21.19) and pepsin (EC 3.4.23.1) were used for controlled digestion of NT types B and E; the amino acid residues flanking many of the cleavable peptide bonds were identified and the corresponding proteolytic fragments partially characterized. Chemical identification of 82 and 108 residues of types B and E NT, respectively, revealed that the residue 738 and 1098 in type E NT, identified as Leu and Asn, respectively, differ from those deduced from nucleotide sequences. Several fragments overlapped spanning various segments of the NT's functional domains; they appear to have potential for structure-function studies of the NT. The cleavage sites were compared with the previously determined proteolyzed sites on NT types A and E. The cleavage sites of the NT types A, B and E, all exposed on the protein surface, were scrutinized in the context of the three-dimensional structure of crystallized NT type A [Lacy, D.B., Stevens, R.C., 1999. J. Mol. Biol. 291, 1091-1104]. Detailed procedures for isolation of pure NT types B and E in large quantities (average yield 92 and 62 mg, respectively) suitable for crystallization are reported.

DNA vaccination using the fragment C of botulinum neurotoxin type A provided protective immunity in mice

Botulinum neurotoxin (BoNT) is one of the most toxic substances known to produce severe neuromuscular paralysis. The currently used vaccine is prepared mainly from biohazardous toxins. Thus, we studied an alternative method and demonstrated that DNA immunization provided sufficient protection against botulism in a murine model. A plasmid of pBoNT/A-Hc, which encodes the fragment C gene of type A botulinum neurotoxin, was constructed and fused with an Igkappa leader sequence under the control of a human cytomegalovirus promoter. After 10 cycles of DNA inoculation with this plasmid, mice survived lethal doses of type A botulinum neurotoxin challenges. Immunized mice also elicited cross-protection to the challenges of type E botulinum neurotoxin. This is the first study demonstrating the potential use of DNA vaccination for botulinum neurotoxins.

Covalent structure of botulinum neurotoxin type E: location of sulfhydryl groups, and disulfide bridges and identification of C-termini of light and heavy chains

Botulinum neurotoxin (NT) serotype E is synthesized by Clostridium botulinum as an approximately 150-kDa single-chain polypeptide of 1252 amino acid residues of which 8 are Cys residues [Puolet et al. (1992); Biochem. Biophys. Res. Commun. 183, 107-113]. The posttranslational processing of the gene product removes only the initiating methionine. A very narrow segment of this 1251-residue-long mature protein--at one-third the distance from the N-terminus (between residues Lys 418 and Arg 421)--is highly sensitive to proteases, such as trypsin. The single-chain NT easily undergoes an exogenous posttranslational modification by trypsin; residues 419-421 (Gly-Ile-Arg) are excised. The proteolytically processed NT is a dichain protein in which Pro 1-Lys 418 constitute the approximately 50-kDa light chain, Lys 422-Lys 1251 constitute the approximately 100-kDa heavy chain; Cys 411-Cys 425 and Cys 1196-Cys 1237 form the interchain and intrachain disulfide bonds, respectively; the other four Cys residues at positions 25, 346, 941, and 1035 remain as free sulfhydryl groups. The approximately 150-kDa dichain NT, and separated light and heavy chains, were fragmented with CNBr and endoproteases (pepsin and clostripain); some of these fragments were carboxymethylated with iodoacetamide (with or without 14C label) before and after fragmentation. The fragments were separated and analyzed for amino acid compositions and sequences by Edman degradation to determine the complete covalent structure of the dichain type E NT. A total of 208 amino acid residues, i.e., 16.5% of the entire protein's sequence deduced from nucleotide sequence, was identified. Direct chemical identification of these amino acids was in complete agreement with that deduced from nucleotide sequence.

Physicochemical and immunological characterization of the type E botulinum neurotoxin binding protein purified from Clostridium botulinum

Type E botulinum neurotoxin is produced by Clostridium botulinum along with a neurotoxin binding protein which helps protect the neurotoxin from adverse pH, temperature, and proteolytic conditions. The neurotoxin binding protein has been purified as a 118-kDa protein. Secondary structure content of the neurotoxin binding protein as revealed by far-UV circular dichroism spectroscopy was 19% alpha-helix, 50% beta-sheets, 28% random coils, and 3% beta-turns. This compared to 22% alpha-helix, 44% beta-sheets, 34% random coils, and no beta-turns of the type E botulinum neurotoxin. The complex of the two proteins revealed 25% alpha-helix, 45% beta-sheets, 27% random coils, and 3% beta-turns, suggesting a significant alteration at least in the alpha-helical folding of the two proteins upon their interaction. Tyrosine topography is altered considerably (28%) when the neurotoxin and its binding protein are separated, indicating strong interaction between the two proteins. Gel filtration results suggested that type E neurotoxin binding protein clearly complexes with type E neurotoxin. The interaction is favored at low pH as indicated by an initial binding rate of 8.4 min-1 at pH 5.7 compared to 4.0 min-1 at pH 7.5 as determined using a fiber optic-based biosensor. The neurotoxin and its binding protein apparently are of equivalent antigenicity, as both reacted equally on enzyme-linked immunosorbent assay to polyclonal antibodies raised against the toxoid of their complex.

Colony immunoblot assay of botulinal toxin

Botulinal neurotoxin in and around colonies of Clostridium botulinum types A, B, and E and of toxigenic Clostridium butyricum was detected by an enzyme-linked immunoassay procedure whereby the toxin was transferred from the agar medium to a nitrocellulose support and the immobilized toxin was probed with type-specific antibodies. The method identified the toxin types of the colonies grown from a mixed inoculum of C. botulinum serotypes. The specificity of the antitoxins for type A and B toxins was improved by adsorption of the antitoxins with the antigens of heterologous type cultures.

Calcium-dependent release of norepinephrine from permeabilized PC12 cells is inhibited by approximately 48 and approximately 112 kDa fragments of botulinum neurotoxin type E

Permeabilized PC12 cells exhibit a Ca(2+)-stimulated norepinephrine secretory pathway which is sensitive to botulinum neurotoxin serotypes A, B and E [Lomneth R., Martin T.F.J. and DasGupta B. R. (1991) J. Neurochem. 57: 1413-1421]. Two novel amino terminal fragments of the 150 kDa neurotoxin serotype E (approximately 112 and 48 kDa), produced by digestion with pepsin, were tested in permeabilized PC12 cells. The intracellular inhibitory activity of the approximately 112 kDa amino terminal fragment, like that of the 150 kDa neurotoxin, was progressively enhanced after trypsinization and dithiothreitol reduction. The approximately 50 kDa C-terminal half of the heavy chain therefore does not contribute to the enhancement of inhibitory activity. The approximately 48 kDa amino terminal light chain-like fragment completely inhibited release of norepinephrine, with an IC50 = 500 pM (more potent than the light chain isolated after digestion with trypsin) not requiring reduction with dithiothreitol. These results clarify the molecular basis of activation of neurotoxin by trypsin and dithiothreitol.

Pepsin fragmentation of botulinum type E neurotoxin: isolation and characterization of 112, 48, 46, and 16 kD fragments

Controlled digestion of approximately 150 kD single chain botulinum type E neurotoxin with pepsin at pH 6.0 produced 112, 48, 46, and 16 kD fragments. These were chromatographically purified; their locations in the approximately 1300 amino acid residue long neurotoxin were determined by identifying the amino terminal 10 residues of 112 and 48 kD fragments, 50 residues of 46 kD fragment, and 59 residues of 16 kD fragment. The 48 and 112 kD fragments contain the N-terminal segment of the neurotoxin (i.e., residue no. 1 to approximately 425 and 1 to approximately 990, respectively), the 46 kD fragment corresponds to approximately 407 residues of the C-terminal region, and the 16 kD fragment contains the approximately 140 residues from a segment nearer to the C-terminus. The 48 kD fragment is similar to the approximately 50 kD N-terminal light chain of the approximately 150 kD dichain neurotoxin, which is generated by tryptic cleavage of the approximately 150 kD single chain neurotoxin, and is separated from the approximately 100 kD C-terminal heavy chain by dithiothreitol (DTT) reduction of an intrachain disulfide bond in the presence of 2 M urea (Sathyamoorthy and DasGupta, J. Biol. Chem. 260, 10461, 1985). The pepsin-generated 48 kD fragment, unlike the light chain, was isolated without exposure to DTT and urea. The single chain 112 kD fragment following trypsin digestion yielded 48 and 60 kD fragments that were separable after DTT reduction of the intrachain disulfide which links them. The N-terminal residues of the smaller fragment were identical to that of the single chain 150 kD neurotoxin; the single chain 112 kD fragment is therefore the neurotoxin minus the approximately 50 kD C-terminal half of the heavy chain. The biological activities of the 48 and 112 kD fragments can be demonstrated in permeabilized PC12 cells (Lomneth et al., J. Neurochem. 57, 1413, 1991); they inhibit norepinephrine release.

Characterization of the neurotoxin isolated from a Clostridium baratii strain implicated in infant botulism

Botulism is widely known to result from ingestion of food containing botulinum neurotoxin produced in situ by certain strains of Clostridium botulinum. Infant botulism caused by C. botulinum, unlike the food-borne intoxication, is the toxicoinfectious form of botulism (S. S. Arnon, p. 331-345, in G. E. Lewis, ed., Biomedical Aspects of Botulism, 1981). The strain of Clostridium baratii implicated in infant botulism produced a neurotoxin that was neutralized with antiserum for botulinum neurotoxin serotype F (J. D. Hall, L. M. McCroskey, B. J. Pincomb, and C. L. Hatheway, J. Clin. Microbiol. 21:654-655, 1985). We developed a procedure to culture the toxigenic C. baratii (strain 6341) in dialysis bags and a simple purification scheme (precipitation of 900-ml culture supernatant with ammonium sulfate and two anion-exchange chromatographic steps at pH 5.5 and 8.0) that yielded up to 150 micrograms of purified neurotoxin. It is an approximately 140-kDa single-chain protein and has the following sequence of amino acid residues at the N terminus: Pro-Val-Asn-Ile-Asn-Asn-Phe-Asn-Tyr-Asn-Asp-Pro-Ile-Asn-Asn-Thr-Thr-Ile- Leu. Comparison of this amino acid sequence with those of the botulinum neurotoxin serotypes A, B, and E showed 40 to 50% identical residues in comparable positions. The specific toxicity of the neurotoxin, approximately 2 x 10(6) 50% lethal doses for mice per mg of protein injected, was not enhanced significantly by mild trypsinization, although the protease cleaved the neurotoxin within a disulfide loop that generated at least two primary fragments, approximately 47 and approximately 86 kDa, that remained linked by an interchain disulfide. These two fragments resembled the light and heavy chains of the well-characterized neurotoxin serotypes A, B, C, D, E, and F produced by C. botulinum.

Clostridium botulinum types A, B, C1, and E produce proteins with or without hemagglutinating activity: do they share common amino acid sequences and genes?

Clostridium botulinum produce the antigenically distinct 150 kD neurotoxin serotypes (e.g., A, B, C1, and E) and simultaneously proteins, A Hn+, B Hn+, C Hn+, and E Hn-, that have high, low, and no hemagglutinating activity. A Hn+ and B Hn+ are serologically cross-reactive. A Hn+, B Hn+, and C Hn+ found as large aggregates (900-220 kD) can be dissociated on SDS-PAGE into multiple subunits, the smallest for A Hn+, B Hn+ is 17 kD and 27 kD for C Hn+. The 116 kD E Hn- does not aggregate. We determined the sequences of 10-33 amino terminal residues of the 17, 21.5, 35, and 57 kD subunits of A Hn+ and B Hn+. Each of these subunits have unique sequences, indicating that the larger units studies are not homomers or heteromers of smaller units. The subunits of A Hn+ and B Hn+ of comparable size have striking sequence identity (e.g., 21.5 kD subunits from the two are identical and 57 kD subunits have 80% identity). In vitro proteolysis of 116 kD E Hn- with different proteases did not impart hemagglutinating activity to the fragments. The 116 kD E Hn- and one of its proteolytic fragments (87 kD) were partially sequenced. Sixty-two base pairs downstream from the termination codon of the cloned 33 kD subunit of C Hn+, there is an initiation codon followed by an open reading frame for at least 34 amino acid residues (Tsuzuki et al., 1990). The derived amino acid sequence of this open reading frame, we found, has 73-84% sequence identity with those of the 17 kD subunits of A Hn+ and B Hn+ and significant identity with the N-terminal of E Hn-. These highly conserved sequences show existence of genetic linkage among the Hn+ and Hn- proteins.

Comparative molecular topography of botulinum neurotoxins from Clostridium butyricum and Clostridium botulinum type E

Production of botulinum-like neurotoxin by a non-Clostridium botulinum organism has profound implications in the epidemiology of the disease botulism. Molecular topography of the approximately 150 kDa neurotoxic protein produced by Clostridium butyricum (strain 5839) and its activation kinetics were examined and compared with a serologically related botulinum neurotoxin produced by C. botulinum type E to further characterize the butyricum neurotoxin. Botulinum neurotoxin was fully activated within 30 min of incubation with trypsin, whereas butyricum neurotoxin achieved maximum activation within 5 min of incubation. Molecular topography of the two neurotoxins was analyzed in terms of secondary structures and the surface accessibilities of the polypeptide domains containing aromatic amino acids. The secondary structure parameters of the butyricum neurotoxin (alpha-helix 22%, beta-sheet 41% and random coil 37%), as estimated from the far ultraviolet circular dichroic spectra, appeared similar to that of botulinum neurotoxin. (Singh, B.R. and DasGupta, B.R., (1989) Mol. Cell. Biochem. 86, 87). Second derivative ultraviolet spectral analysis revealed 37 and 41 Tyr residues exposed on the surface of butyricum and botulinum neurotoxins, respectively, suggesting a differential surface accessibility of polypeptide segments containing Tyr residues. Fluorescent Trp residues in both the botulinum type E and butyricum neurotoxins were in a relatively hydrophobic environment as indicated by the blue-shifted emission maxima (334 nm). About half of the fluorescent Trp residues of both proteins were accessible to acrylamide, a neutral fluorescence quencher, and appeared to be in a similar molecular environment. The ionic surface probe, I-, quenched the Trp fluorescence of botulinum significantly, but not that of butyricum neurotoxin. Thus, a considerable number of fluorescent Trp residues were apparently located on the surface of the botulinum, but not on that of the butyricum neurotoxin. Botulinum and butyricum neurotoxins, indistinguishable by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, migrated differently in the absence of sodium dodecyl sulfate suggesting difference(s) in their surface charge distribution. These results provide the first report of the secondary and tertiary structure parameters of the neurotoxin produced by a non-botulinum species and comparison of the molecular topography of the neurotoxin with the antigenically related botulinum neurotoxin type E.

Immunological characterization of Clostridium butyricum neurotoxin and its trypsin-induced fragment by use of monoclonal antibodies against Clostridium botulinum type E neurotoxin

We examined the reactivities of Clostridium butyricum neurotoxin to nine monoclonal antibodies against Clostridium botulinum type E neurotoxin which recognize the light chain or the amino-terminal half (H-1 fragment) or the carboxyl-terminal half (H-2 fragment) of the heavy chain of botulinum neurotoxin. Butyricum neurotoxin and its derived chains did not react to two of four monoclonal antibodies recognizing the light chain, one of three recognizing the H-1 fragment, and one of two recognizing the H-2 fragment. The results indicate that the immunological difference between the two neurotoxins is not attributable to a particular portion of the toxin molecule. The fragment of butyricum neurotoxin obtained by prolonged tryptic treatment was found to comprise the light chain and H-1 fragment linked together by a disulfide bond.

Structural analysis of botulinum neurotoxin types A and E in aqueous and nonpolar solvents by Fourier transform infrared, second derivative UV absorption, and circular dichroic spectroscopies

Two pharmacologically similar but antigenetically distinct botulinum neurotoxins, types A and E with a 1000-fold difference in their toxicity, were examined for nonpolar solvent-induced changes in secondary structures and polypeptide foldings to understand their structural differences and their comparative responsiveness/susceptibility to solvent perturbation. Analysis of far UV circular dichroic spectra in aqueous buffer for types A and E neurotoxins yielded the following: the alpha-helix contents were 27 and 20%; the beta-sheets were 36 and 44%, the beta-turns were 6.0 and 0%, and the random coils were 31 and 36%, respectively. Fourier transform infrared spectra, obtained by using attenuated total reflection technique, indicated high content of alpha-helix and beta-pleated sheet structures for both neurotoxins as judged by strong bands at 1651 and 1633 cm-1 in the amide I frequency region and bands at 1314 and 1245 cm-1 in the amide III frequency region. The peak height ratio of 1314 and 1245 cm-1 bands, suggests that the type A neurotoxin has slightly higher alpha-helical content than the type E neurotoxin. These observations are consistent with the secondary structures estimated from far UV circular dichroic spectra. Fourier transform infrared spectra of the neurotoxins, exposed to methanol, showed sharp increases of the 1651 cm-1 band and a significant increase in the height of the 1314 cm-1 band, suggesting increases in the alpha-helical contents of the proteins. The changes were more in the type A than in the type E neurotoxin. The changes were reversible upon reexposure of the proteins to the aqueous buffer. Second derivative absorption spectroscopy demonstrated that methanol also induced changes in the degree of Tyr exposure to solvent. The results are discussed in terms of structural differences between the single and dichain neurotoxins and in terms of their mode of action.

Conformational changes associated with the nicking and activation of botulinum neurotoxin type E

Secondary and tertiary structural parameters of type E botulinum neurotoxin in the unactivated single-chain and activated two-chain (i.e., after proteolytic cleavage) forms were analyzed using circular dichroism, derivative absorption and fluorescence spectroscopy. The estimated secondary structures (22 and 20% alpha-helix, 44 and 44% beta-pleated sheets, and 34 and 36% random coils for the single- and two-chain neurotoxins, respectively) indicated that virtually no change occurred upon nicking of the single-chain neurotoxin. About 57% of the 70 Tyr residues were exposed in the single-chain form, which increased to 62% in the two-chain form. Fluorescence quenching experiments with neutral, anionic and cationic quenchers indicated that about 40% of the maximum accessible fluorescent Trp residues were exposed on the surface of the single-chain neurotoxin as compared to only 20% in the case of the two-chain neurotoxin. Acrylamide was the most effective quencher with a fraction accessibility of 0.56 and 0.48 of maximum accessible Trp fluorescence residues in the single and two-chain forms of the neurotoxin, respectively. Native polyacrylamide gel electrophoresis of the two forms of the neurotoxin revealed greater mobility for the two chain form. This indicates that the surface charges in the single-chain neurotoxin were altered upon nicking. These observations suggest that nicking of the single-chain type E neurotoxin results in refolding and redistribution of the surface charges of the neurotoxin.

Botulinum neurotoxin type E fragmented with endoproteinase Lys-C reveals the site trypsin nicks and homology with tetanus neurotoxin

Botulinum neurotoxin type E, a 150 kDa single chain protein, cleaved with endoproteinase Lys-C yielded 113, 73, and 50 kDa fragments. The N-terminal sequence of the 113 kDa fragment, Gly-Ile-Arg-Lys-Ser-Ile-Cys-Ile, overlaps the N-terminal sequence, Lys-Ser-Ile-Cys-Ile, of the 103 kDa heavy chain produced by nicking the neurotoxin with trypsin. The -Arg-Lys- bond is therefore the site on the single chain type E NT where trypsin nicks generating the 50 kDa light and 103 kDa heavy chains of the dichain NT. The sequence of the first 50 N-terminal residues of the 73 kDa fragment were determined. This fragment is a segment of the heavy chain; 50% of the 50 residues are present in identical positions in a similar segment of the heavy chain of tetanus neurotoxin.

Comparison of toxins of Clostridium butyricum and Clostridium botulinum type E

The toxin of Clostridium butyricum strains isolated from two infants with botulism is neutralized by antitoxin for type E botulinum toxin. This toxin and that of a C. botulinum type E strain were purified by the same protocol. Both toxins were Mr 145,000 proteins which, when activated with trypsin, were composed of an H subunit of Mr 105,000 and an L subunit of Mr 50,000. The activated specific toxicity of purified butyricum toxin based on an intravenous assay was 2 X 10(8) mouse 50% lethal doses (LD50s)/mg of protein, but that based on an intraperitoneal assay was 7 X 10(7) LD50s/mg, compared with 6 X 10(7) LD50s/mg for type E toxin as determined by both methods. Immunodiffusion tests with antitoxin raised with type E toxin indicated that the two toxins were serologically very similar except for a spur formed by type E toxin. The close similarities of the two toxins suggest that toxigenic C. butyricum could arise when a wild-type strain, which is normally nontoxigenic, acquires the toxin gene of a C. botulinum type E strain.