Characterization of nontoxic-nonhemagglutinin component of the two types of progenitor toxin (M and L) produced by Clostridium botulinum type D CB-16

Building-block architecture of botulinum toxin complex: Conformational changes provide insights into the hemagglutination ability of the complex

Clostridium botulinum produces the botulinum neurotoxin (BoNT). Previously, we provided evidence for the “building-block” model of botulinum toxin complex (TC). In this model, a single BoNT is associated with a single nontoxic nonhemagglutinin (NTNHA), yielding M-TC; three HA-70 molecules are attached and form M-TC/HA-70, and one to three “arms” of the HA-33/HA-17 trimer (two HA-33 and one HA-17) further bind to M-TC/HA-70 via HA-17 and HA-70 binding, yielding one-, two-, and three-arm L-TC. Of all TCs, only the three-arm L-TC caused hemagglutination. In this study, we determined the solution structures for the botulinum TCs using small-angle X-ray scattering (SAXS). The mature three-arm L-TC exhibited the shape of a “bird spreading its wings”, in contrast to the model having three “arms”, as revealed by transmission electron microscopy. SAXS images indicated that one of the three arms of the HA-33/HA-17 trimer bound to both HA-70 and BoNT. Taken together, these findings regarding the conformational changes in the building-block architecture of TC may explain why only three-arm L-TC exhibited hemagglutination.

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We examined the structures of botulinum TCs using SAXS.

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The mature three-arm L-TC exhibited the shape of a “bird spreading its wings”.

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One of the three arms of the HA-33/HA-17 trimer bound to both HA-70 and BoNT.

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The building-block architecture may explain hemagglutination by the three-arm L-TC.

Reversible Association of the Hemagglutinin Subcomplex, HA-33/HA-17 Trimer, with the Botulinum Toxin Complex

Botulinum neurotoxin (BoNT) associates with nontoxic proteins, either a nontoxic nonhemagglutinin (NTNHA) or the complex of NTNHA and hemagglutinin (HA), to form M- or L-toxin complexes (TCs). Single BoNT and NTNHA molecules are associated and form M-TC. A trimer of the 70-kDa HA protein (HA-70) attaches to the M-TC to form M-TC/HA-70. Further, 1-3 arm-like 33- and 17-kDa HA molecules (HA-33/HA-17 trimer), consisting of 1 HA-17 protein and 2 HA-33 proteins, can attach to the M-TC/HA-70 complex, yielding 1-, 2-, and 3-arm L-TC. In this study, the purified 1- and 2-arm L-TCs spontaneously converted into another L-TC species after acquiring the HA-33/HA-17 trimer from other TCs during long-term storage and freezing/thawing. Transmission electron microscopy analysis provided evidence of the formation of detached HA-33/HA-17 trimers in the purified TC preparation. These findings provide evidence of reversible association/dissociation of the M-TC/HA-70 complex with the HA-33/HA-17 trimers, as well as dynamic conversion of the quaternary structure of botulinum TC in culture.

Construction of "Toxin Complex" in a Mutant Serotype C Strain of Clostridium botulinum Harboring a Defective Neurotoxin Gene

A non-toxigenic mutant of the toxigenic serotype C Clostridium botulinum strain Stockholm (C-St), C-N71, does not produce the botulinum neurotoxin (BoNT). However, the original strain C-St produces botulinum toxin complex, in which BoNT is associated with non-toxic non-hemagglutinin (NTNHA) and three hemagglutinin proteins (HA-70, HA-33, and HA-17). Therefore, in this study, we aimed to elucidate the effects of bont gene knockout on the formation of the "toxin complex." Nucleotide sequence analysis revealed that a premature stop codon was introduced in the bont gene, whereas other genes were not affected by this mutation. Moreover, we successfully purified the "toxin complex" produced by C-N71. The "toxin complex" was identified as a mixture of NTNHA/HA-70/HA-17/HA-33 complexes with intact NTNHA or C-terminally truncated NTNHA, without BoNT. These results indicated that knockout of the bont gene does not affect the formation of the "toxin complex." Since the botulinum toxin complex has been shown to play an important role in oral toxin transport in the human and animal body, a non-neurotoxic "toxin complex" of C-N71 may be valuable for the development of an oral drug delivery system.

Data describing inhibitory profiles of sugars against hemagglutination by the botulinum toxin complex of Clostridium botulinum serotypes C and D

Serotype C and D of Clostridium botulinum produce botulinum toxin complex (TC), which is comprised of botulinum neurotoxin, nontoxic nonhemagglutinin, and hemagglutinins (HAs). The TC is capable of aggregating equine erythrocytes via interaction between one of the HAs, namely HA-33, and sugar chains on the cell surface. This hemagglutination is inhibited by specific sugars. In this data article, we used four TCs from serotype C and D strains. The hemagglutination-inhibiting effects of 18 sugars and 8 glycoproteins were studied. The purified TC was mixed with the sugar to enable binding of the sugar to the TC; then, the erythrocytes were added to the mixture. Specific binding between the sugar and TC resulted in inhibition of cell aggregation. Here, data illustrating the inhibitory effects of various sugars and glycoproteins against hemagglutination induced by TC of C. botulinum serotypes C and D are presented.

Conformational divergence in the HA-33/HA-17 trimer of serotype C and D botulinum toxin complex

Clostridium botulinum produces a large toxin complex (L-TC) comprising botulinum neurotoxin associated with auxiliary nontoxic proteins. A complex of 33- and 17-kDa hemagglutinins (an HA-33/HA-17 trimer) enhances L-TC transport across the intestinal epithelial cell layer via binding HA-33 to a sugar on the cell surface. At least two subtypes of serotype C/D HA-33 exhibit differing preferences for the sugars sialic acid and galactose. Here, we compared the three-dimensional structures of the galactose-binding HA-33 and HA-33/HA-17 trimers produced by the C-Yoichi strain. Comparisons of serotype C/D HA-33 sequences reveal a variable region with relatively low sequence similarity across the C. botulinum strains; the variability of this region may influence the manner of sugar-recognition by HA-33. Crystal structures of sialic acid- and galactose-binding HA-33 are broadly similar in appearance. However, small-angle X-ray scattering revealed distinct solution structures for HA-33/HA-17 trimers. A structural change in the C-terminal variable region of HA-33 might cause a dramatic shift in the conformation and sugar-recognition mode of HA-33/HA-17 trimer.

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.

Structure of a bimodular botulinum neurotoxin complex provides insights into its oral toxicity

Botulinum neurotoxins (BoNTs) are produced by Clostridium botulinum and cause the fatal disease botulism, a flaccid paralysis of the muscle. BoNTs are released together with several auxiliary proteins as progenitor toxin complexes (PTCs) to become highly potent oral poisons. Here, we report the structure of a ∼760 kDa 14-subunit large PTC of serotype A (L-PTC/A) and reveal insight into its absorption mechanism. Using a combination of X-ray crystallography, electron microscopy, and functional studies, we found that L-PTC/A consists of two structurally and functionally independent sub-complexes. A hetero-dimeric 290 kDa complex protects BoNT, while a hetero-dodecameric 470 kDa complex facilitates its absorption in the harsh environment of the gastrointestinal tract. BoNT absorption is mediated by nine glycan-binding sites on the dodecameric sub-complex that forms multivalent interactions with carbohydrate receptors on intestinal epithelial cells. We identified monosaccharides that blocked oral BoNT intoxication in mice, which suggests a new strategy for the development of preventive countermeasures for BoNTs based on carbohydrate receptor mimicry.

Food-borne botulinum neurotoxin (BoNT) poisoning results in fatal muscle paralysis. But how can BoNT–a large protein released by the bacteria clostridia–survive the hostile gastrointestinal (GI) tract to gain access to neurons that control muscle contraction? Here, we report the complete structure of a bimodular ∼760 kDa BoNT/A large progenitor toxin complex (L-PTC), which is composed of BoNT and four non-toxic bacterial proteins. The architecture of this bacterial machinery mimics an Apollo lunar module, whereby the “ascent stage” (a ∼290 kDa module) protects BoNT from destruction in the GI tract and the 3-arm “descent stage” (a ∼470 kDa module) mediates absorption of BoNT by binding to host carbohydrate receptors in the small intestine. This new finding has helped us identify the carbohydrate-binding sites and the monosaccharide IPTG as a prototypical oral inhibitor, which extends survival following lethal BoNT/A intoxication of mice. Hence, pre-treatment with small molecule inhibitors based on carbohydrate receptor mimicry can provide temporary protection against BoNT entry into the circulation.

Purification and characterization of nontoxic protein complex from serotype D 4947 botulinum toxin complex

The large-sized botulinum toxin complex (L-TC) is composed of botulinum neurotoxin (BoNT) and nontoxic proteins, e.g. nontoxic nonhemagglutinin (NTNHA) and three types of hemagglutinins (HAs; HA-33, HA-17 and HA-70). The nontoxic proteins play a critical role in L-TC oral toxicity by protecting the BoNT in the digestive tract, and facilitating absorption of the L-TC across the intestinal wall. Under alkaline conditions, the L-TC separates into BoNT and the nontoxic protein complex (NC). In this study, we established a two-step procedure to yield highly pure NC from the L-TC produced by Clostridium botulinum serotype D strain 4947 in which the NC was isolated from the L-TC by gel filtration under alkaline conditions followed by immunoprecipitation with an anti-BoNT antibody to remove contaminating BoNT from the NC fraction. Western blotting and electrophoretic analysis showed that the highly purified NC fraction had only very slight or no BoNT contamination. In addition, the purified NC fraction showed no intraperitoneal (ip) toxicity to mice at a dose of 38 ng per animal whereas the L-TC exhibited an ip median lethal dose of 0.38 ng per mouse. The highly purified NC displayed the same hemagglutination titer as the L-TC. The NC, as well as the L-TC, demonstrated cell binding and monolayer transport in the rat intestinal epithelial cell line IEC-6. Consequently, the highly purified NC can function as a "delivery vehicle" even without the BoNT.

Crystallization and preliminary X-ray analysis of the Clostridium botulinum type D nontoxic nonhaemagglutinin

Clostridium botulinum produces botulinum neurotoxin (BoNT) as a large toxin complex assembled with nontoxic nonhaemagglutinin (NTNHA) and/or haemagglutinin components. Complex formation with NTNHA is considered to be critical in eliciting food poisoning because the complex shields the BoNT from the harsh conditions in the digestive tract. In the present study, NTNHA was expressed in Escherichia coli and crystallized. Diffraction data were collected to 3.9 Å resolution. The crystal belonged to the trigonal space group P321 or P3(1)21/P3(2)21, with unit-cell parameters a = b = 147.85, c = 229.74 Å. The structure of NTNHA will provide insight into the assembly mechanism that produces the unique BoNT-NTNHA complex.

Expression and stability of the nontoxic component of the botulinum toxin complex

Clostridium botulinum produces botulinum neurotoxin (BoNT) as a large toxin complex associated with nontoxic-nonhemagglutinin (NTNHA) and/or hemagglutinin components. In the present study, high-level expression of full-length (1197 amino acids) rNTNHA from C. botulinum serotype D strain 4947 (D-4947) was achieved in an Escherichia coli system. Spontaneous nicking of the rNTNHA at a specific site was observed during long-term incubation in the presence of protease inhibitors; this was also observed in natural NTNHA. The rNTNHA assembled with isolated D-4947 BoNT with molar ratio 1:1 to form a toxin complex. The reconstituted toxin complex exhibited dramatic resistance to proteolysis by pepsin or trypsin at high concentrations, despite the fact that the isolated BoNT and rNTNHA proteins were both easily degraded. We provide definitive evidence that NTNHA plays a crucial role in protecting BoNT, which is an oral toxin, from digestion by proteases common in the stomach and intestine.

Disruption of the epithelial barrier by botulinum haemagglutinin (HA) proteins – differences in cell tropism and the mechanism of action between HA proteins of types A or B, and HA proteins of type C

Orally ingested botulinum neurotoxin (BoNT) causes food-borne botulism, but BoNT must pass through the gut lining and enter the bloodstream. We have previously found that type B haemagglutinin (HA) proteins in the toxin complex play an important role in the intestinal absorption of BoNT by disrupting the paracellular barrier of the intestinal epithelium, and therefore facilitating the transepithelial delivery of BoNT. Here, we show that type A HA proteins in the toxin complex have a similar disruptive activity and a greater potency than type B HA proteins in the human intestinal epithelial cell lines Caco-2 and T84 and in the canine kidney epithelial cell line MDCK I. In contrast, type C HA proteins in the toxin complex (up to 300 nM) have no detectable effect on the paracellular barrier in these human cell lines, but do show a barrier-disrupting activity and potent cytotoxicity in MDCK I. These findings may indicate that type A and B HA proteins contribute to the development of food-borne botulism, at least in humans, by facilitating the intestinal transepithelial delivery of BoNTs, and that the relative inability of type C HA proteins to disrupt the paracellular barrier of the human intestinal epithelium is one of the reasons for the relative absence of food-borne human botulism caused by type C BoNT.

Molecular characterization of the protease from Clostridium botulinum serotype C responsible for nicking in botulinum neurotoxin complex

A protease was purified from the culture medium of Clostridium botulinum serotype C strain Stockholm (C-St). The purified protease belonged to the cysteine protease family based on assays for enzyme inhibitors, activators and kinetic parameters. The protease formed a binary complex consisting of 41- and 17-kDa proteins held together non-covalently. The DNA sequence encoding the protease gene was shown to be a single open reading frame of 1593 nucleotides, predicting 530 amino acid residues including a signal peptide. The N-terminal region of the native enzyme underwent further proteolytic modification after processing by a signal peptidase. The protease introduced intermolecular cleavage into an intact single chain botulinum neurotoxin (BoNT) at a specific site. Homology modeling and docking simulation of C-St BoNT and C-St protease demonstrated that the specific nicking-site of the BoNT appears to fit into the deep pocket in the active site of the protease.

Effect of Nicking the C-terminal Region of the Clostridium botulinum Serotype D Neurotoxin Heavy Chain on its Toxicity and Molecular Properties

A unique strain of Clostridium botulinum serotype D 4947 produces toxin complexes that are composed of un-nicked components, including a neurotoxin (BoNT) and auxiliary proteins. This BoNT showed aberrant elution upon Superdex gel filtration, indicating a much lower molecular weight, due to hydrophobic interaction with the column. Limited trypsin proteolysis of BoNT produces two nicks; first nick yielded a BoNT 50 kDa light chain disulfide linked to a 100 kDa heavy chain (Hc), and a second nick arose in Hc C-terminal 10 kDa. The second nick occurred in the putative binding domain of the BoNT molecule and induced alterations in its secondary structure, leading to a significant reduction of mouse toxicity in comparison with that of the fully-activated singly nicked BoNT. These results help to clarify the role of the C-terminal half of the Hc in the oral toxicity of single-chain and more complex forms of BoNT.

Quantitative detection of gene expression and toxin complex produced by Clostridium botulinum serotype D strain 4947

Botulinum toxin is produced by Clostridium botulinum as a large toxin complex (L-TC) non-covalently assembled with a neurotoxin (NT), a non-toxic non-hemagglutinin (NTNHA) and hemagglutinin subcomponents (HA-70, HA-33, and HA-17). In this study, the gene expressions of five individual L-TC components were examined by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) in C. botulinum serotype D strain 4947 (D-4947) during cell growth. Transcripts for the five component genes were successfully detected in the mid-exponential growth phase (6.5 h), reaching a maximum at the early stationary growth phase (12 h). The ratio of the mRNA transcripts of nt and ntnha was approximately 1:1, suggesting that nt and ntnha are bicistronically transcribed. On the other hand, the transcript levels of the ha genes were several-fold higher than those of nt and ntnha, although the mRNA transcript level of ha-33 was less than the other two ha subcomponent genes. The results based on qRT-PCR indicate that a shortage of HA-33 among the proteins associated with botulinum TC could explain the production by D-4947 of other smaller-sized L-TCs (610, 540 and 410 kDa) with fewer HA-33 molecules than the mature 650 kDa L-TC. Western blot analysis demonstrated that TC species in cell lysate were initially observed in the mid-exponential phase, while extracellular TCs were detected subsequently in the early stationary phase.

Four molecules of the 33 kDa haemagglutinin component of the Clostridium botulinum serotype C and D toxin complexes are required to aggregate erythrocytes

Normally, large-sized botulinum toxin complexes (L-TC) of serotype C and D are composed of a single neurotoxin, a single non-toxic non-haemagglutinin, two HA-70 molecules, four HA-33 molecules and four HA-17 molecules that assemble to form a 650 kDa L-TC. The 540 and 610 kDa TC species (designated here as L-TC2and L-TC3, respectively) were purified in addition to the 650 kDa L-TC from the culture supernatants of serotype D strains (D-4947 and D-CB16) and serotype C strains (C-6814 and C-Yoichi). The 650 kDa L-TC from D-4947, D-CB16 and C-6814 showed haemagglutination and erythrocyte-binding activity, but their L-TC2and L-TC3species had only binding activity. In contrast, every TC species from C-Yoichi having the C-terminally truncated variant of HA-33 exhibited neither haemagglutination activity nor erythrocyte-binding activity. Four strain-specific HA-33/HA-17 complexes were isolated from the 650 kDa L-TC of each strain. The 650 kDa HA-hybrid L-TCs were reconstituted by various combinations of isolated HA-33/HA-17 complexes and haemagglutination-negative L-TC2or L-TC3from each strain. HA-hybrid 650 kDa L-TC, including at least one HA-33/HA-17 complex derived from C-Yoichi, lost haemagglutination activity, leading to the conclusion that the binding of four HA-33 molecules is required for haemagglutination activity of botulinum L-TC. The results of the modelling approach indicated that the structure of a variant C-Yoichi HA-33 molecule reveals clear deformation of theβ-trefoil domain responsible for the carbohydrate recognition site.

The Structure of the Neurotoxin-associated Protein HA33/A from Clostridium botulinum Suggests a Reoccurring β-Trefoil Fold in the Progenitor Toxin Complex

The hemagglutinating protein HA33 from Clostridium botulinum is associated with the large botulinum neurotoxin secreted complexes and is critical in toxin protection, internalization, and possibly activation. We report the crystal structure of serotype A HA33 (HA33/A) at 1.5 A resolution that contains a unique domain organization and a carbohydrate recognition site. In addition, sequence alignments of the other toxin complex components, including the neurotoxin BoNT/A, hemagglutinating protein HA17/A, and non-toxic non-hemagglutinating protein NTNHA/A, suggests that most of the toxin complex consists of a reoccurring beta-trefoil fold.

Characterization of toxin complex produced by a unique strain of Clostridium botulinum serotype D 4947

A unique strain of Clostridium botulinum, serotype D 4947 (D-4947), produces a considerable amount of a 650 kDa toxin complex (L-TC) and a small amount of a 280 kDa M-TC, a 540 kDa TC, and a 610 kDa TC. The complexes are composed of only un-nicked components, including neurotoxin (NT), nontoxic nonhemagglutinin (NTNHA) and hemagglutinin subcomponents (HA-70, HA-33 and HA-17). Unlike other NTs from all serotype strains, separation of D-4947 NT from L-TC, except for M-TC, during chromatography required highly alkaline conditions around pH 8.8. The separated NT and NTNHA/HAs complex can be reconstituted to L-TC that is indistinguishable from the parent L-TC with respect to toxicity, hemagglutination activity and gel filtration profile. The isoelectric points of NT and NTNHA/HAs were close together depending on the number of HA-33/17 molecules. We have established a new method to separate the unique D-4947 NT from the complex, which will yield valuable information on structure of botulinum toxin.

Molecular characterization of binding subcomponents of Clostridium botulinum type C progenitor toxin for intestinal epithelial cells and erythrocytes

Clostridium botulinum type C 16S progenitor toxin consists of a neurotoxin (NTX), a non-toxic non-HA (NTNH), and a haemagglutinin (HA). The HA acts as an adhesin, allowing the 16S toxin to bind to intestinal epithelial cells and erythrocytes. In type C, these bindings are dependent on sialic acid. The HA consists of four distinct subcomponents designated HA1, HA2, HA3a and HA3b. To identify the binding subcomponent(s) of HA of type C 16S toxin, all of the HA-subcomponents and some of their precursor forms were produced as recombinant proteins fused to glutathione S-transferase (GST). These proteins were evaluated for their capacity to adhere to intestinal epithelial cells of guinea pig and human erythrocytes. GST-HA1, GST-HA3b and GST-HA3 (a precursor form of HA3a and HA3b) bound intestinal epithelial cells and erythrocytes, whereas GST alone, GST-HA2 and GST-HA3a did not. GST-HA3b and GST-HA3 showed neuraminidase-sensitive binding to the intestinal epithelial cells and erythrocytes, whereas GST-HA1 showed neuraminidase-insensitive binding. TLC binding assay revealed that GST-HA3b and GST-HA3 recognized sialosylparagloboside (SPG) and GM3 in the ganglioside fraction of the erythrocytes, like native type C 16S toxin [Inoue, K. et al. (1999). Microbiology 145, 2533-2542]. On the other hand, GST-HA1 recognized paragloboside (PG; an asialo- derivative of SPG) in addition to SPG and GM3. Deletion mutant analyses of GST-HA3b showed that the C-terminal region of HA3b is important for its binding activity. Based on these data, it is concluded that the HA component contains two distinct carbohydrate-binding subcomponents, HA1 and HA3b, which recognize carbohydrates in different specificities.

Characterisation of botulinum toxins type C, D, E, and F by matrix-assisted laser desorption ionisation and electrospray mass spectrometry

In a follow-up of the earlier characterisation of botulinum toxins type A and B (BTxA and BTxB) by mass spectrometry (MS), types C, D, E, and F (BTxC, BTxD, BTxE, BTxF) were now investigated. Botulinum toxins are extremely neurotoxic bacterial toxins, likely to be used as biological warfare agent. Biologically active BTxC, BTxD, BTxE, and BTxF are comprised of a protein complex of the respective neurotoxins with non-toxic non-haemagglutinin (NTNH) and, sometimes, specific haemagglutinins (HA). These protein complexes were observed in mass spectrometric identification. The BTxC complex, from Clostridium botulinum strain 003-9, consisted of a 'type C1 and D mosaic' toxin similar to that of type C strain 6813, a non-toxic non-hemagglutinating and a 33 kDa hemagglutinating (HA-33) component similar to those of strain C-Stockholm, and an exoenzyme C3 of which the sequence was in full agreement with the known genetic sequence of strain 003-9. The BTxD complex, from C. botulinum strain CB-16, consisted of a neurotoxin with the observed sequence identical with that of type D strain BVD/-3 and of an NTNH with the observed sequence identical with that of type C strain C-Yoichi. Remarkably, the observed protein sequence of CB-16 NTNH differed by one amino acid from the known gene sequence: L859 instead of F859. The BTxE complex, from a C. botulinum isolated from herring sprats, consisted of the neurotoxin with an observed sequence identical with that from strain NCTC 11219 and an NTNH similar to that from type E strain Mashike (1 amino acid difference with observed sequence). BTxF, from C. botulinum strain Langeland (NCTC 10281), consisted of the neurotoxin and an NTNH; observed sequences from both proteins were in agreement with the gene sequence known from strain Langeland. As with BTxA and BTxB, matrix-assisted laser desorption/ionisation (MALDI) MS provided provisional identification from trypsin digest peptide maps and liquid chromatography-electrospray (tandem) mass spectrometry (LC-ES MS) afforded unequivocal identification from amino acid sequence information of digest peptides obtained in trypsin digestion.

Purification of fully activated Clostridium botulinum serotype B toxin for treatment of patients with dystonia

Clostridium botulinum serotype B toxins 12S and 16S were separated by using a beta-lactose gel column at pH 6.0; toxin 12S passed through the column, whereas toxin 16S bound to the column and eluted with lactose. The fully activated neurotoxin was obtained by applying the trypsin-treated 16S toxin on the same column at pH 8.0; the neurotoxin passed through the column, whereas remaining nontoxic components bound to the column. The toxicity of this purified fully activated neurotoxin was retained for a long period by addition of albumin in the preparation.

Mucosal immunisation with Clostridium botulinum type C 16 S toxoid and its non-toxic component

Clostridium botulinum types C and D produce a 16 S (500 kDa) toxin that is formed by conjugation of neurotoxin with a non-toxic component (nonTox). The amino acid sequences of type C and D nonTox components are almost identical. In a previous report it was proposed that nonTox is necessary for the effective absorption of the toxin from the small intestine. This suggested the hypothesis that mucosal immunity against nonTox in the small intestine might prevent the absorption of both C- and D-16 S toxins. The nonTox was purified from a mutant strain, (C)-N71, that does not produce neurotoxin. This nonTox or detoxified C-16 S toxin were mixed with adjuvant (a mutant form of heat-labile toxin of Escherichia coli), and inoculated into mice via the nasal or oral route, or both. The mice inoculated nasally four times with nonTox or toxoid produced high levels of antibodies (including IgA) against the immunogens, both in intestinal fluids and sera. When these nonTox-immunised mice were challenged orally with 2 and 20 oral minimum lethal doses (MLD) of C- or D-16 S toxins, the same results were obtained with both C and D; the mice survived after challenge with 2 MLD of either C or D but were killed by 20 MLD of either toxin although the time to death was significantly longer than in the control non-immunised mice. These results indicate that the local anti-nonTox antibodies reduce absorption of both C- and D-16 S toxins from the small intestine. The C-16 S toxoid-immunised mice showed similar behaviour with type D toxin challenge, probably due to the same mechanism, but were protected against 20 MLD of C-16 S toxin.

Spontaneous nicking in the nontoxic-nonhemagglutinin component of the Clostridium botulinum toxin complex

The nontoxic-nonhemagglutinin (NTNHA) component, in both isolated form and the neurotoxin (NT)/NTNHA complexed form, was prepared protease-free from toxin complexes produced by Clostridium botulinum type D strain 4947. NTNHA in both preparations was found to be spontaneously converted to the nicked NTNHA form leading to 15- and 115-kDa fragments with the excision of several amino acid residues at specific sites on SDS-PAGE during long-term incubation, while that of the NT/NTNHA/hemagglutinin complexed form remained unnicked single-chain polypeptides under the same conditions. Considering that the NTNHA preparation contained small amounts of the nicked form of NTNHA and the addition of trypsin accelerated the cleavage, it is speculated that a nicked form of NTNHA remaining after the purification and/or NTNHA itself catalyzes the cleavage of intact NTNHA.

In vitro reconstitution of the Clostridium botulinum type D progenitor toxin

Clostridium botulinum type D strain 4947 produces two different sizes of progenitor toxins (M and L) as intact forms without proteolytic processing. The M toxin is composed of neurotoxin (NT) and nontoxic-nonhemagglutinin (NTNHA), whereas the L toxin is composed of the M toxin and hemagglutinin (HA) subcomponents (HA-70, HA-17, and HA-33). The HA-70 subcomponent and the HA-33/17 complex were isolated from the L toxin to near homogeneity by chromatography in the presence of denaturing agents. We were able to demonstrate, for the first time, in vitro reconstitution of the L toxin formed by mixing purified M toxin, HA-70, and HA-33/17. The properties of reconstituted and native L toxins are indistinguishable with respect to their gel filtration profiles, native-PAGE profiles, hemagglutination activity, binding activity to erythrocytes, and oral toxicity to mice. M toxin, which contained nicked NTNHA prepared by treatment with trypsin, could no longer be reconstituted to the L toxin with HA subcomponents, whereas the L toxin treated with proteases was not degraded into M toxin and HA subcomponents. We conclude that the M toxin forms first by assembly of NT with NTNHA and is subsequently converted to the L toxin by assembly with HA-70 and HA-33/17.

Role of C-terminal region of HA-33 component of botulinum toxin in hemagglutination

Using SDS-PAGE, we found that one subcomponent, hemagglutinin (HA-33), from the Clostridium botulinum progenitor toxin of type D strain 1873 and type C strain Yoichi had slightly smaller molecular sizes than those of type C and D reference strains, but other components did not. Based on N- and C-terminal sequence analyses of HA-33, a deletion of 31 amino acid residues from the C-terminus at a specific site was observed in the HA-33 proteins of both strains. The progenitor toxins from both strains showed poor hemagglutination activities, titers of 2(1) or less, which were much lower than titers from the reference strains (2(6)), and did not bind to erythrocytes. These results suggest strongly that the short C-terminal region of the HA-33 plays an essential role in the hemagglutination activity of the botulinum progenitor toxin. Additionally, a sequence motif search predicted that the C-terminal region of HA-33 has a carbohydrate-recognition subdomain.

Characterization and reconstitution of functional hemagglutinin of the Clostridium botulinum type C progenitor toxin

The purified progenitor toxin of Clostridium botulinum type C strain 6814 (C-6814) forms a large complex composed of 150-kDa neurotoxin (NT), 130-kDa nontoxic-nonhemagglutinin (NTNHA), and hemagglutinin (HA) components. The HA component consisted of a mixture of several subcomponents with molecular masses of 70, 55, 33, 26-21 and 17 kDa. We isolated the HA subcomponents from the progenitor toxin by chromatography in the presence of denaturants. The isolated HA subcomponents, designated as i-HA-33, i-HA-55, i-HA-70 and i-HA-33/17, were nearly homogeneous on SDS/PAGE, but the HA-17 and HA-26-21 components were not purified. Some HA subcomponents, designated as f-HA-33 and f-HA-33/17 complex, existed free of the progenitor toxin in the culture medium and they were separately purified. Every HA subcomponent so far isolated shows binding activity to erythrocytes. The hemagglutination activities of each HA subcomponent had a titer of 25 for the f-HA-33/17 complex, and below 23 for the other f- and i-HA subcomponents, while the parent progenitor L toxin was 28. The reconstitution of various combinations of f- and i-HA subcomponents was attempted via mixing and tested for hemagglutination activity. When the i-HA-33/17 complex and i-HA-55 were mixed, the hemagglutination activity was recovered to a titer of 29, which was slightly higher than that of the parent toxin. These data imply that a combination of at least HA-33, -17 and -55 subcomponents is required for full hemagglutination activity of the botulinum progenitor toxin, but each single HA subcomponent shows weak or no aggregation of erythrocytes.

Characterization of nicking of the nontoxic-nonhemagglutinin components of Clostridium botulinum types C and D progenitor toxin

Clostridium botulinum C and D strains produce two types of progenitor toxins, M and L. Previously we reported that a 130-kDa nontoxic-nonhemagglutinin (NTNHA) component of the M toxin produced by type D strain CB16 was nicked at a unique site, leading to a 15-kDa N-terminal fragment and a 115-kDa C-terminal fragment. In this study, we identified the amino acid sequences around the nicking sites in the NTNHAs of the M toxins produced by C. botulinum type C and D strains by analysis of their C-terminal and N-terminal sequences and mass spectrometry. The C-terminus of the 15-kDa fragments was identified as Lys127 from these strains, indicating that a bacterial trypsin-like protease is responsible for the nicking. The 115-kDa fragment had mixtures of three different N-terminal amino acid sequences beginning with Leu135, Val139, and Ser141, indicating that 7-13 amino acid residues were deleted from the nicking site. The sequence beginning with Leu135 would also suggest cleavage by a trypsin-like protease, while the other two N-terminal amino acid sequences beginning with Val139 and Ser141 would imply proteolysis by an unknown protease. The nicked NTNHA forms a binary complex of two fragments that could not be separated without sodium dodecyl sulfate.

Identification and characterization of functional subunits of Clostridium botulinum type A progenitor toxin involved in binding to intestinal microvilli and erythrocytes

Clostridium botulinum type A hemagglutinin-positive progenitor toxin consists of three distinct components: neurotoxin (NTX), hemagglutinin (HA), and non-toxic non-HA (NTNH). The HA consists of four subcomponents designated HA1, 2, 3a and 3b. By employing purified toxin and GST-fusion proteins of each HA subcomponent, we found that the HA-positive progenitor toxin, GST-HA1 and GST-HA3b bind to human erythrocytes and microvilli of guinea pig upper small intestinal sections. The HA-positive progenitor toxin and GST-HA1 bind via galactose moieties, GST-HA3b binds via sialic acid moieties. GST-2 and GST-3a showed no detectable binding.

Dichain structure of botulinum neurotoxin: identification of cleavage sites in types C, D, and F neurotoxin molecules

Botulinum neurotoxin (NT) is synthesized by Clostridium botulinum as about a 150-kDa single-chain polypeptide. Posttranslational modification by bacterial or exogenous proteases yielded dichain structure which formed a disulfide loop connecting a 50-kDa light chain (Lc) and 100-kDa heavy chain (Hc). We determined amino acid sequences around cleavage sites in the loop region of botulinum NTs produced by type C strain Stockholm, type D strain CB16, and type F strain Oslo by analysis of the C-terminal sequence of Lc and the N-terminal sequence of Hc. Cleavage was found at one or two sites at Arg444/Ser445 and Lys449/Thr450 for type C, and Lys442/Asn443 and Arg445/Asp446 for type D, respectively. In culture fluid of mildly proteolytic strains of type C and D, therefore, NT exists as a mixture of at least three forms of nicked dichain molecules. The NT of type F proteolytic strain Oslo showed the Arg435 as a C-terminal residue of Lc and Ala440 as an N-terminal residue of Hc, indicating that the bacterial protease cuts twice (Arg435/Lys436 and Lys439/Ala440), with excision of four amino acid residues. The location of cleavage and number of amino acid residue excisions in the loop region could be explained by the degree of exposure of amino acid residues on the surface of the molecule, which was predicted as surface probability from the amino acid sequence. In addition, the observed correlation may also be adapted to the cleavage sites of the other botulinum toxin types, A, B, E, and G.

Molecular composition of progenitor toxin produced by Clostridium botulinum type C strain 6813

The molecular composition of the purified progenitor toxin produced by a Clostridium botulinum type C strain 6813 (C-6813) was analyzed. The strain produced two types of progenitor toxins (M and L). Purified L toxin is formed by conjugation of the M toxin (composed of a neurotoxin and a non-toxic nonhemagglutinin) with additional hemagglutinin (HA) components. The dual cleavage sites at loop region of the dichain structure neurotoxin were identified between Arg444-Ser445 and Lys449-Thr450 by the analyses of C-terminal of the light chain and N-terminal of the heavy chain. Analysis of partial amino acid sequences of fragments generated by limited proteolysis of the neurotoxin has shown to that the neurotoxin protein produced by C-6813 was a hybrid molecule composed of type C and D neurotoxins as previously reported. HA components consist of a mixture of several subcomponents with molecular weights of 70-, 55-, 33-, 26 through 21- and 17-kDa. The N-terminal amino acid sequences of 70-, 55-, and 26 through 21-kDa proteins indicated that the 70-kDa protein was intact HA-70 gene product, and other 55- and 26 through 21-kDa proteins were derived from the 70-kDa protein by modification with proteolysis after translation of HA-70 gene. Furthermore, several amino acid differences were exhibited in the amino acid sequence as compared with the deduced sequence from the nucleotide sequence of the HA-70 gene which was common among type C (strains C-St and C-468) and D progenitor toxins (strains D-CB16 and D-1873).

Characterization of haemagglutinin activity of Clostridium botulinum type C and D 16S toxins, and one subcomponent of haemagglutinin (HA1)

The 16S toxin and one subcomponent of haemagglutinin (HA), designated HA1, were purified from a type D culture of Clostridium botulinum by a newly established procedure, and their HA activities as well as that of purified type C 16S toxin were characterized. SDS-PAGE analysis indicated that the free HA1 forms a polymer with a molecular mass of approximately 200 kDa. Type C and D 16S toxins agglutinated human erythrocytes in the same manner. Their HA titres were dramatically reduced by employing erythrocytes that had been previously treated with neuraminidase, papain or proteinase K, and were inhibited by the addition of N-acetylneuraminic acid to the reaction mixtures. In a direct-binding test to glycolipids such as SPG (NeuAc alpha2-3Gal beta1-4GlcNAc beta1-3Gal beta1-4Glc beta1-Cer) and GM3 (NeuAc alpha2-3Gal beta1-4Glc beta1-Cer), and glycoproteins such as glycophorin A and/or B prepared from the erythrocytes, both toxins bound to sialylglycolipids and sialoglycoproteins, but bound to neither neutral glycolipids nor asialoglycoproteins. On the basis of these results, it was concluded that type C and D 165 toxins bind to erythrocytes through N-acetylneuraminic acid. HA1 showed no haemagglutination activity, although it did bind to sialylglycolipids. We therefore speculate that binding to glycoproteins rather than to glycolipids may be important in causing haemagglutination by type C and D 16S toxins.

TetR is a positive regulator of the tetanus toxin gene in Clostridium tetani and is homologous to botR

The TetR gene immediately upstream from the tetanus toxin (TeTx) gene was characterized. It encodes a 21,562-Da protein which is related (50 to 65% identity) to the equivalent genes (botR) in Clostridium botulinum. TetR has the feature of a DNA binding protein with a basic pI (9.53). It contains a helix-turn-helix motif and shows 29% identity with other putative regulatory genes in Clostridium, i.e., uviA from C. perfringens and txeR from C. difficile. We report for the first time the transformation of C. tetani by electroporation, which permitted us to investigate the function of tetR. Overexpression of tetR in C. tetani induced an increase in TeTx production and in the level of the corresponding mRNA. This indicates that TetR is a transcriptional activator of the TeTx gene. Overexpression of botR/A (60% identity with TetR at the amino acid level) in C. tetani induced an increase in TeTx production comparable to that for overexpression of tetR. However, botR/C (50% identity with TetR at the amino acid level) was less efficient. This supports that TetR positively regulates the TeTx gene in C. tetani and that a conserved mechanism of regulation of the neurotoxin genes is involved in C. tetani and C. botulinum.

Molecular composition of the 16S toxin produced by a Clostridium botulinum type D strain, 1873

The 16S toxin was purified from a Clostridium botulinum type D strain 1873 (D-1873). Furthermore, the entire nucleotide sequences of the genes coding for the 16S toxin were determined. It became clear that the purified D-1873 16S toxin consists of neurotoxin, nontoxic nonhemagglutinin (NTNH), and hemagglutinin (HA), and that HA consists of four subcomponents, HA1, HA2, HA3a, and HA3b, the same as type D strain CB16 (D-CB16) 16S toxin. The nucleotide sequences of the nontoxic components of these two strains were also found to be identical except for several bases. However, the culture supernatant and the purified 16S toxin of D-1873 showed little HA activity, unlike D-CB16, though the fractions successively eluted after the D-1873 16S toxin peak from an SP-Toyopearl 650S column showed a low level of HA activity. The main difference between D-1873 and D-CB16 HA molecules was the mobility of the HA1 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Therefore it was presumed that the loss of HA activity of D-1873 16S toxin might be caused by the differences of processing HA after the translation.

botR/A is a positive regulator of botulinum neurotoxin and associated non-toxin protein genes in Clostridium botulinum A

The genes of the botulinum neurotoxin A (BoNT) complex are clustered in a locus consisting of two divergent polycistronic operons, one containing the non-toxic, non-haemagglutinin (NTNH) component and bontA genes, the other containing the haemagglutinin (HA) component genes. The two operons are separated by a gene (botR/A, previously called orf21) encoding a 21 kDa protein. A recombinant Clostridium botulinum A strain that overexpresses botR/A was constructed by electroporating strain 62 with the vector pAT19 containing botR/A under the control of its own promoter. The transformed strain produced more BoNT/A and associated non-toxic proteins (ANTPs) and the corresponding mRNAs than the non-transformed strain. Partial inhibition of botR/A by antisense mRNA resulted in lower levels of BoNT/A, NTNH and HA70 and the levels of the corresponding mRNAs. Gel mobility shift assays and immunoprecipitations showed that BotR/A bound to the DNA promoter region upstream from the two BoNT/A complex operons. These results show that botR/A activated transcription of the genes encoding BoNT/A and ANTPs in C. botulinum A by interacting directly with the region promoter, and that the homologous genes in C. botulinum B, C and D presumably have the same function.

Gene arrangement in the upstream region of Clostridium botulinum type E and Clostridium butyricum BL6340 progenitor toxin genes is different from that of other types

The cluster of genes encoding the botulinum progenitor toxin and the upstream region including p21 and p47 were divided into three different gene arrangements (class I-III). To determine the gene similarity of the type E neurotoxin (BoNT/E) complex to other types, the gene organization in the upstream region of the nontoxic-nonhemagglutinin gene (ntnh) was investigated in chromosomal DNA from Clostridium botulinum type E strain Iwanai and C. butyricum strain BL6340. The gene cluster of type E progenitor toxin (Iwanai and BL6340) was similar to those of type F and type A (from infant botulism in Japan), but not to those of types A, B, and C. Though genes for the hemagglutinin component and P21 were not discovered, genes encoding P47, NTNH, and BoNT were found in type E strain Iwanai and C. butyricum strain BL6340. However, the genes of ORF-X1 (435 bp) and ORF-X2 (partially sequenced) were present just upstream of that of P47. The orientation of these genes was in inverted direction to that of p47. The gene cluster of type E progenitor toxin (Iwanai and BL6340) is, therefore, a specific arrangement (class IV) among the genes encoding components of the BoNT complex.

The haemagglutinin of Clostridium botulinum type C progenitor toxin plays an essential role in binding of toxin to the epithelial cells of guinea pig small intestine, leading to the efficient absorption of the toxin

Binding of the purified type C 7S (neurotoxin), 12S and 16S botulinum toxins to epithelial cells of ligated small intestine or colon of the guinea pig (in vivo test) and to pre-fixed gastrointestinal tissue sections (in vitro test) was analysed. The 16S toxin bound intensely to the microvilli of epithelial cells of the small intestine in both in vivo and in vitro tests, but did not bind to cells of the stomach or colon. The neurotoxin and 12S toxin did not bind to epithelial cells of the small intestine or to cells of the stomach or colon. Absorption of the toxins was assessed by determining the toxin titre in the sera of guinea pigs 6-8 h after the intra-intestinal administration of the toxins. When the 16S toxin [1 x 10(5) minimum lethal dose (MLD)] was injected, 200-660 MLD ml-1 was detected in the sera, whereas when the 12S toxin (2 x 10(5) MLD) or 7S toxin (2 x 10(5) MLD) was injected, little toxin activity was detected in the sera. Therefore, the haemagglutinin of type C 16S toxin is apparently very important in the binding and absorption of botulinum toxin in the small intestine.

Molecular composition of Clostridium botulinum type A progenitor toxins

The molecular composition of progenitor toxins produced by a Clostridium botulinum type A strain (A-NIH) was analyzed. The strain produced three types of progenitor toxins (19 S, 16 S, and 12 S) as reported previously. Purified 19 S and 16 S toxins demonstrated the same banding profiles on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), indicating that they consist of the same protein components. The nontoxic components of the 19 S and 16 S toxins are a nontoxic non-hemagglutinin (HA) (molecular mass, 120 kDa) and HA. HA could be fractionated into five subcomponents with molecular masses of 52, 35, 20, 19, and 15 kDa in the presence of 2-mercaptoethanol. The molar ratios of neurotoxins, nontoxic non-HAs, and each HA subcomponent of the 19 S and 16 S toxins showed that only HA-35 of the 19 S toxin was approximately twice the size of that of the 16 S toxin, suggesting that the 19 S toxin is a dimer of the 16 S toxin cross-linked by the 35-kDa subcomponent. The nontoxic non-HA of the 12 S toxin, but not those of the 19 S and 16 S toxins, demonstrated two bands with molecular masses of 106 and 13 kDa on SDS-PAGE with or without 2-mercaptoethanol. It was concluded from the N-terminal amino acid sequences that 106- and 13-kDa proteins were generated by a cleavage of whole nontoxic non-HA. This may explain why the 12 S and 16 S (and 19 S) toxins exist in the same culture. We also found that the HA and its 35-kDa subcomponent exist in a free state in the culture fluid along with three types of progenitor toxins.

Molecular characterization of two forms of nontoxic-nonhemagglutinin components of Clostridium botulinum type A progenitor toxins

The entire sequences of the type A nontoxic-nonhemagglutinin gene and an adjacent open reading frame designated as orf 22-a, which are located between the neurotoxin and the HA-35 genes were determined. SDS-PAGE and N-terminal amino acid sequence analyses of the purified type A progenitor toxins (12S, 16S and 19S) indicate that the nontoxic-nonhemagglutinins of 16S and 19S are single peptides of approximately 120k, but that of 12S has a cleavage at the site between Pro-144 and Phe-145 of this protein.