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

Update on Non-Interchangeability of Botulinum Neurotoxin Products

The growing use of botulinum neurotoxins (BoNTs) for medical and aesthetic purposes has led to the development and marketing of an increasing number of BoNT products. Given that BoNTs are biological medications, their characteristics are heavily influenced by their manufacturing methods, leading to unique products with distinct clinical characteristics. The manufacturing and formulation processes for each BoNT are proprietary, including the potency determination of reference standards and other features of the assays used to measure unit potency. As a result of these differences, units of BoNT products are not interchangeable or convertible using dose ratios. The intrinsic, product-level differences among BoNTs are compounded by differences in the injected tissues, which are innervated by different nerve fiber types (e.g., motor, sensory, and/or autonomic nerves) and require unique dosing and injection sites that are particularly evident when treating complex therapeutic and aesthetic conditions. It is also difficult to compare across studies due to inherent differences in patient populations and trial methods, necessitating attention to study details underlying each outcome reported. Ultimately, each BoNT possesses a unique clinical profile for which unit doses and injection paradigms must be determined individually for each indication. This practice will help minimize unexpected adverse events and maximize efficacy, duration, and patient satisfaction. With this approach, BoNT is poised to continue as a unique tool for achieving individual goals for an increasing number of medical and aesthetic indications.

Complete subunit structure of serotype C and D botulinum progenitor toxin complex induces vacuolation in the specific epithelial cell line

Clostridium botulinum produces seven botulinum neurotoxin (BoNT) serotypes. In nature, BoNT exists as a part of the progenitor toxin complex (PTC) through associations with neurotoxin associated proteins (NAPs), including nontoxic nonhemagglutinin and hemagglutinin (HA) complex, consists of HA-70, HA-17 and HA-33. Because PTC displays higher oral toxicity than pure BoNTs, NAPs play a critical role in food poisoning. In a previous study, we demonstrated that the NAP complex in mature large-sized PTC (L-PTC) from serotypes C and D concomitantly induced cell death and cytoplasmic vacuolation in the rat intestinal epithelial cell line IEC-6. Here, we found that the serotype D NAP complex induces only cytoplasmic vacuolation in the normal rat kidney cell line NRK-52E without reducing cell viability. NAP complexes from serotype A and B L-PTCs did not affect cell viability or cytoplasmic vacuolation in IEC-6 and NRK-52E cells. Furthermore, we assessed the effect of immature L-PTCs with fewer HA-33/HA-17 trimers (two HA-33 and one HA-17) than mature L-PTCs on cell viability and cytoplasmic vacuolation in IEC-6 and NRK-52E cells. As a result, mature L-PTCs with the maximum number of HA-33/HA-17 trimers displayed the greatest potency. Consequently, the reduction in cell viability and vacuolation induction are related to the number of HA-33/HA-17 trimers in PTC. The discovery of an epithelial cell model where botulinum PTC specifically induces vacuolization may help clarify the unknown cytotoxicity of PTC, which plays an important role in the trans-epithelial transport of the toxin.

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.

Botulinum Neurotoxins: Biology, Pharmacology, and Toxicology

The study of botulinum neurotoxins (BoNT) is rapidly progressing in many aspects. Novel BoNTs are being discovered owing to next generation sequencing, but their biologic and pharmacological properties remain largely unknown. The molecular structure of the large protein complexes that the toxin forms with accessory proteins, which are included in some BoNT type A1 and B1 pharmacological preparations, have been determined. By far the largest effort has been dedicated to the testing and validation of BoNTs as therapeutic agents in an ever increasing number of applications, including pain therapy. BoNT type A1 has been also exploited in a variety of cosmetic treatments, alone or in combination with other agents, and this specific market has reached the size of the one dedicated to the treatment of medical syndromes. The pharmacological properties and mode of action of BoNTs have shed light on general principles of neuronal transport and protein-protein interactions and are stimulating basic science studies. Moreover, the wide array of BoNTs discovered and to be discovered and the production of recombinant BoNTs endowed with specific properties suggest novel uses in therapeutics with increasing disease/symptom specifity. These recent developments are reviewed here to provide an updated picture of the biologic mechanism of action of BoNTs, of their increasing use in pharmacology and in cosmetics, and of their toxicology.

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.

"Non-Toxic" Proteins of the Botulinum Toxin Complex Exert In-vivo Toxicity

The botulinum neurotoxin (BoNT) causes muscle paralysis and is the most potent toxin in nature. BoNT is associated with a complex of auxiliary “Non-Toxic” proteins, which constitute a large-sized toxin complex (L-TC). However, here we report that the “Non-Toxic” complex of serotype D botulinum L-TC, when administered to rats, exerts in-vivo toxicity on small-intestinal villi. Moreover, Serotype C and D of the “Non-Toxic” complex, but not BoNT, induced vacuole-formation in a rat intestinal epithelial cell line (IEC-6), resulting in cell death. Our results suggest that the vacuole was formed in a manner distinct from the mechanism by which Helicobacter pylori vacuolating toxin (VacA) and Vibrio cholerae haemolysin induce vacuolation. We therefore hypothesise that the serotype C and D botulinum toxin complex is a functional hybrid of the neurotoxin and vacuolating toxin (VT) which arose from horizontal gene transfer from an ancestral BoNT-producing bacterium to a hypothetical VT-producing bacterium.

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.

New Elements To Consider When Modeling the Hazards Associated with Botulinum Neurotoxin in Food

Botulinum neurotoxins (BoNTs) produced by the anaerobic bacterium Clostridium botulinum are the most potent biological substances known to mankind. BoNTs are the agents responsible for botulism, a rare condition affecting the neuromuscular junction and causing a spectrum of diseases ranging from mild cranial nerve palsies to acute respiratory failure and death. BoNTs are a potential biowarfare threat and a public health hazard, since outbreaks of foodborne botulism are caused by the ingestion of preformed BoNTs in food. Currently, mathematical models relating to the hazards associated with C. botulinum, which are largely empirical, make major contributions to botulinum risk assessment. Evaluated using statistical techniques, these models simulate the response of the bacterium to environmental conditions. Though empirical models have been successfully incorporated into risk assessments to support food safety decision making, this process includes significant uncertainties so that relevant decision making is frequently conservative and inflexible. Progression involves encoding into the models cellular processes at a molecular level, especially the details of the genetic and molecular machinery. This addition drives the connection between biological mechanisms and botulism risk assessment and hazard management strategies. This review brings together elements currently described in the literature that will be useful in building quantitative models of C. botulinum neurotoxin production. Subsequently, it outlines how the established form of modeling could be extended to include these new elements. Ultimately, this can offer further contributions to risk assessments to support food safety decision making.

The long journey of botulinum neurotoxins into the synapse

Botulinum neurotoxins (BoNT) cause the disease botulism, a flaccid paralysis of the muscle. They are also very effective, widely used medicines applied locally in sub-nanogram quantities. BoNTs are released together with several non-toxic, associated proteins as progenitor toxin complexes (PCT) by Clostridium botulinum to become highly potent oral poisons ingested via contaminated food. They block the neurotransmission in susceptible animals and humans already in nanogram quantities due to their specific ability to enter motoneurons and to cleave only selected neuronal proteins involved in neuroexocytosis. BoNTs have developed a sophisticated strategy to passage the gastrointestinal tract and to be absorbed in the intestine of the host to finally attack neurons. A non-toxic non-hemagglutinin (NTNHA) forms a binary complex with BoNT to protect it from gastrointestinal degradation. This binary M-PTC is one component of the bi-modular 14-subunit ∼760 kDa large progenitor toxin complex. The other component is the structurally and functionally independent dodecameric hemagglutinin (HA) complex which facilitates the absorption on the intestinal epithelium by glycan binding. Subsequent to its transcytosis the HA complex disrupts the tight junction of the intestinal barrier from the basolateral side by binding to E-cadherin. Now, the L-PTC can also enter the circulation by paracellular routes in much larger quantities. From here, the dissociated BoNTs reach the neuromuscular junction and accumulate via interaction with polysialo gangliosides, complex glycolipids, on motoneurons at the neuromuscular junction. Subsequently, additional specific binding to luminal segments of synaptic vesicles proteins like SV2 and synaptotagmin leads to their uptake. Finally, the neurotoxins shut down the synaptic vesicle cycle, which they had exploited before to enter their target cells, via specific cleavage of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, which constitute the core components of the cellular membrane fusion machinery.

Host-cell specificity and transcytosis of nontoxic nonhemagglutinin protein of botulinum neurotoxin serotype D

Serotype D botulinum toxin (BoNT) complex (TC), a causative agent of foodborne botulism in animals, traverses the gastrointestinal tract and circulation, eventually becoming localized in neuromuscular junctions, where the serotype D BoNT cleaves SNARE substrate synaptobrevin II involved in neurotransmitter release. During this process, BoNT must pass through cells, thus from the intestinal lumen to the cells of the intestinal tract and blood vessels. The botulinum TC is formed by association of the BoNT with at least one nontoxic protein, which may be a nontoxic nonhemagglutinin (NTNHA). In this work, we examined the binding and transcytosis of serotype D NTNHA protein in epithelial and endothelial cells to clarify the role played by the protein in toxin delivery. Our studies showed that NTNHA bound to and transcytosed across rat intestinal epithelial (IEC-6) and bovine aortic endothelial (BAEC) cells. While NTNHA also bound to canine renal (MDCK) or human colon carcinoma (Caco-2) cells, but it did not traverse across MDCK or Caco-2 cells. Such specificity of NTNHA protein transcytosis may explain why only some animals are sensitive to botulinum toxin. The sensitivity depends on the toxin serotype in play, and the route of toxin delivery.

Crystallization and preliminary X-ray analysis of a novel haemagglutinin component of the toxin complex of serotype C Clostridium botulinum

The botulinum toxin complex, the causative agent of botulism, passes through the intestinal wall via sugar-chain-dependent cell binding of a haemagglutinin of 33 kDa molecular weight (HA-33). The amino-acid sequence of the C-terminal half of HA-33 of the serotype C strain Yoichi (C-Yoichi) shares only 46% identity with those of the major serotype C strains. Additionally, C-Yoichi HA-33 exhibits a unique sugar-binding specificity. In the present work, C-Yoichi HA-33 was expressed in Escherichia coli and crystallized. Diffraction data were collected at a resolution of 2.2 Å. The crystals belonged to space group R3. The complete detailed protein structure will yield insight into how the unique HA-33 protein recognizes sugar moieties.

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.

Sugar-induced conformational change found in the HA-33/HA-17 trimer of the botulinum toxin complex

Large-sized botulinum toxin complex (L-TC) is formed by conjugation of neurotoxin, nontoxic nonhemagglutinin and hemagglutinin (HA) complex. The HA complex is formed by association of three HA-70 molecules and three HA-33/HA-17 trimers, comprised of a single HA-17 and two HA-33 proteins. The HA-33/HA-17 trimer isolated from serotype D L-TC has the ability to bind to and penetrate through the intestinal epithelial cell monolayer in a sialic acid-dependent manner, and thus it plays an important role in toxin delivery through the intestinal cell wall. In this study, we determined the solution structure of the HA-33/HA-17 trimer by using small-angle X-ray scattering (SAXS). The SAXS image of HA-33/HA-17 exhibited broadly similar appearance to the crystal image of the complex. On the other hand, in the presence of N-acetylneuraminic acid, glucose and galactose, the solution structure of the HA-33/HA-17 trimer was drastically altered compared to the structure in the absence of the sugars. Sugar-induced structural change of the HA-33/HA-17 trimer may contribute to cell binding and subsequent transport across the intestinal cell layer.

Botulinum toxin complex increases paracellular permeability in intestinal epithelial cells via activation of p38 mitogen-activated protein kinase

Clostridium botulinum produces a large toxin complex (L-TC) that increases paracellular permeability in intestinal epithelial cells by a mechanism that remains unclear. Here, we show that mitogen-activated protein kinases (MAPKs) are involved in this permeability increase. Paracellular permeability was measured by FITC-dextran flux through a monolayer of rat intestinal epithelial IEC-6 cells, and MAPK activation was estimated from western blots. L-TC of C. botulinum serotype D strain 4947 increased paracellular dextran flux and activated extracellular signal-regulated kinase (ERK), p38, but not c-Jun N-terminal kinase (JNK) in IEC-6 cells. The permeability increase induced by L-TC was abrogated by the p38 inhibitor SB203580. These results indicate that L-TC increases paracellular permeability by activating p38, but not JNK and ERK.

Transport of the botulinum neurotoxin-associating protein, nontoxic nonhemagglutinin, across the rat small intestinal epithelial cell monolayer

Botulinum neurotoxin (BoNT) associates with nontoxic nonhemagglutinin (NTNHA) yielding a complex in culture. BoNT and NTNHA have similar domain organizations, implying that they share common functions, although this remains unclear. Here, we examined cell monolayer transport of serotype D NTNHA in the rat intestinal epithelial cell line IEC-6. NTNHA and BoNT both bound to the cell and were transported across the cell layer. NTNHA contains a QXW motif and a β-trefoil fold, both common in sugar chain-recognizing proteins, whereas the QXW motif is absent in all BoNT serotypes. This could explain the distinct sugar chain-recognizing properties of NTNHA and BoNT.

Assembly and function of the botulinum neurotoxin progenitor complex

Botulinum neurotoxins (BoNTs) are among the most poisonous substances known to man, but paradoxically, BoNT-containing medicines and cosmetics have been used with great success in the clinic. Accidental BoNT poisoning mainly occurs through oral ingestion of food contaminated with Clostridium botulinum. BoNTs are naturally produced in the form of progenitor toxin complexes (PTCs), which are high molecular weight (up to ~900 kDa) multiprotein complexes composed of BoNT and several non-toxic neurotoxin-associated proteins (NAPs). NAPs protect the inherently fragile BoNTs against the hostile environment of the gastrointestinal (GI) tract and help BoNTs pass through the intestinal epithelial barrier before they are released into the general circulation. These events are essential for ingested BoNTs to gain access to motoneurons, where they inhibit neurotransmitter release and cause muscle paralysis. In this review, we discuss the structural basis for assembly of NAPs and BoNT into the PTC that protects BoNT and facilitate its delivery into the bloodstream.

Small-angle X-ray scattering reveals structural dynamics of the botulinum neurotoxin associating protein, nontoxic nonhemagglutinin

In cell culture supernatants, the botulinum neurotoxin (BoNT) exists as part of a toxin complex (TC) in which nontoxic nonhemagglutinin (NTNHA) and/or hemagglutinins (HAs) are assembled onto the BoNT. A series of investigations indicated that formation of the TC is vital for delivery of the toxin to nerve cells through the digestive tract. In the assembly process, BoNT binds to NTNHA yielding M-TC, and it then matures into L-TC by further association with the HAs via NTNHA in the M-TC. Here, we report a crystal structure of the NTNHA from Clostridium botulinum serotype D strain 4947. Additionally, we performed small-angle X-ray scattering (SAXS) analysis of the NTNHA and the M-TC to elucidate the solution structure. The crystal structure of D-4947 NTNHA revealed that BoNT and NTNHA share a closely related structure consisting of three domains. The SAXS image indicated that, even though the N-terminal two-thirds of the NTNHA molecule had an apparently similar conformation in both the crystal and solution structures, the C-terminal third of the molecule showed a more extended structure in the SAXS image than that seen in the crystallographic image. The discrepancy between the crystal and solution structures implies a high flexibility of the C-terminal third domain of NTNHA, which is involved in binding to BoNT. Structural dynamics of the NTNHA molecule revealed by SAXS may explain its binding to BoNT to form the BoNT/NTNHA complex.

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.

Interaction of botulinum toxin with the epithelial barrier

Botulinum neurotoxin (BoNT) is a protein toxin (approximately 150 kDa), which possesses a metalloprotease activity. Food-borne botulism is manifested when BoNT is absorbed from the digestive tract to the blood stream and enters the peripheral nerves, where the toxin cleaves core proteins of the neuroexocytosis apparatus and elicits the inhibition of neurotransmitter release. The initial obstacle to orally ingested BoNT entering the body is the epithelial barrier of the digestive tract. Recent cell biology and molecular biology studies are beginning to elucidate the mechanism by which this large protein toxin crosses the epithelial barrier. In this review, we provide an overview of the structural features of botulinum toxins (BoNT and BoNT complex) and the interaction of these toxins with the epithelial barrier.

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.