[Structure and function of botulinum toxin]

Botulinum toxins (types A to G) inhibit the release of acetylcholine at the neuromuscular junction. These toxins are produced as progenitor toxins of large molecular sizes of 12S (M toxin), 16S (L toxin) and 19S (LL toxin) in culture supernatants. Three different molecular forms have been demonstrated in botulinum type A toxin. L and M toxins are recognized in botulinum type C and D toxins. Type E toxin is exclusively composed of M toxin. In an alkaline condition, M and L toxins dissociate into neurotoxin and nontoxic components. Nontoxic components consist of nontoxic-nonhemagultinin component (nontoxic-nonHA) and hemagultinin (HA). M toxin is made up by association of neurotoxin with nontoxic-nonHA, and L toxin is formed by conjugation of M toxin with HA. HA also consists of several subcomponents. These genes with related functions (progenitor toxin) are closely grouped as operon on the chromosome. Nontoxic-nonHA gene is located only 17 bp (type C) or 27 bp (type E) upstream of the neurotoxin gene. Both genes may be transcribed (right-ward transcription) by a polycistronic mRNA species initiated from a promoter located in the 5'-untranslated region of the nontoxic-nonHA gene. The construction of HA subcomponent genes (HA-33, HA-17, HA-25 and HA-53) also appears operon structure. The gene cluster related HA is located 262 bp upstream of nontoxic-nonHA gene of type C and transcribed (left-ward transcription) by the same mRNA from the 5'-noncoding region of HA-33 gene. Botulinum neurotoxin undergoes cleavage to form a dichain molecule linked through a disulphide bond. The heavy chain correlates with the binding of toxin to peripheral synapses, and the light chain is associated with the intracellular activity of blocking of acetylcholine release. Fifty amino acids in C-terminal region of type C toxin is essential for the binding activity of toxin to the target cells. However, the binding efficiency of type C toxin is not antagonized by the other type of botulinum toxins because of low homology of this binding domain of type C toxin to other types. Furthermore, five highly homologous regions are found in light chain among seven neurotoxins. One of these homologous regions, sequence HEL-H--, shows strong similarity with the active site of zinc-proteases. The inhibition of acetylcholine release is associated with this protease activity which selectively cleaves the synaptic vesicle membrane proteins. These target membrane proteins are key components of the synaptic vesicle docking and fusion.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular construction of Clostridium botulinum type C progenitor toxin and its gene organization

The 16S progenitor toxin of Clostridium botulinum type C is made up by conjugation of a neurotoxin with nontoxic components designated as nontoxic-nonHA and hemagglutinin (HA). The HA was found to be composed of subcomponents having 53, 33, 22-23, and 17 kDa molecular masses. Since we previously determined the whole nucleotide sequences of the genes for neurotoxin, nontoxic-nonHA, and HA-33, the cloning and nucleotide sequencing of the genes for the remaining HA subcomponents were performed. Two open reading frames coding for 16.7 kDa (HA-17) and 70.6 kDa proteins were identified. The N-terminal amino acid sequences of HA-53 and HA-22-23 indicated that the 70.6 kDa protein is split into 53 and the 22-23 kDa proteins after translation and that the 22-23 kDa protein consists of at least four proteins showing slightly different molecular weights.

Secondary structural predictions for the clostridial neurotoxins

The primary structures of a family of ten clostridial neurotoxins have recently been deduced yet little information is presently available concerning their secondary or tertiary structures. Because the overall similarity percentage of multiply aligned sequences is high, the secondary structures of these metalloendopeptidases are also expected to be conserved. The neural net program, PHD (Rost and Sander, Proc. Natl. Acad. Sci. USA 90:7558-7562, 1993), predicted that the secondary structures of the neurotoxins were indeed conserved in both single and multiple sequence modes of analysis. Predictions for the amounts of helical, extended, and loop states from the single sequence analyses were consistent with previously published data from circular dichroism studies on some of these neurotoxins. In the single analysis mode, only the aligned regions were predicted to show conservation of the three-state structure. In contrast, the multiple sequence analysis predicted that a conserved state (variable loops) also exists in non-aligned regions. Alignments with the primary structure of the prototypic metalloendopeptidase thermolysin showed that about 25% of the residues within this enzyme are similar to those in the neurotoxins. A comparison of thermolysin's known secondary structure with the predictions from this study showed that about 80% of thermolysin's residues could be structurally aligned with those in the neurotoxins. These predictions provide the necessary framework to build a homologous low-resolution tertiary structure of the neurotoxin active site that will be essential in the development of synthetic inhibitors.

The light chain of botulinum neurotoxin forms channels in a lipid membrane

The ability of botulinum neurotoxin and its isolated subunits, the heavy and light chains, to bind to a lipid membrane and to form channels in the membrane was examined. At pH 4.0, the neurotoxin caused aggregation of calcein-containing liposomes, providing evidence of binding of the neurotoxin to the surface of the outer lipid membrane. Aggregation was followed by the release of calcein, as a result of the formation of channels. The heavy chain evoked the same responses as those of the neurotoxin. The light chain did not cause aggregation of the liposomes but did evoke the release of calcein. The channel-forming ability of the light chain appeared to be higher than that of the neurotoxin or the heavy chain. This novel property of the light chain may help us to understand the mechanism of action of botulinum neurotoxin.

Quaternary structure of botulinum and tetanus neurotoxins as probed by chemical cross-linking and native gel electrophoresis

Botulinum and tetanus neurotoxins are water-soluble proteins (mol. wt 150,000) produced by Clostridium botulinum and Clostridium tetani, respectively. It is believed that these neurotoxins, once internalized via receptor-mediated endocytosis, form membrane channels in order to traverse the endosomal membrane and enter the cytoplasm of the nerve terminal. Investigation of the associative properties between neurotoxin molecules could yield an understanding of this channel formation. That is, an association between neurotoxin monomers could result in an oligomeric form of the neurotoxin necessary for assembly of a channel through the hydrophobic interior of the endosomal membrane, thereby allowing passage of the neurotoxin or its active fragment through the resulting pore. Based on the native gel electrophoresis and chemical cross-linking experiments, tetanus neurotoxin exists as a dimer and a trimer, type A botulinum neurotoxin exists as a dimer, trimer, and a larger species, type E botulinum neurotoxin exists as a monomer and dimer, and type B botulinum neurotoxin appears to exist as a dimer in aqueous solution. The results imply that quaternary structures of these neurotoxins may play an important role in their mode of action during neuronal poisoning.

Inhibition of phagocytosis by rainbow trout (Oncorhynchus mykiss) macrophages by botulinum C2 toxin and its trypsinized component II

Botulinum C2 toxin (C2T) is composed of two nonlinked protein components, components I and II. The toxin reconstituted with component I and trypsinized component II inhibited phagocytosis of rainbow trout (Oncorhynchus mykiss) and mouse macrophages. Inhibition in both cell types was observed over a range of toxin concentrations that were not toxic to the cells. Because cytoplasmic action of rainbow trout macrophages is ADP-ribosylated by component I, the inhibition of phagocytosis in trout cells by C2T is probably due to inactivation of cytoplasmic actin. Moreover, phagocytosis by trout cells was also inhibited in a dose-dependent manner by trypsinized component II alone, and did not cause cell death. The present results show that the macrophages of aquatic vertebrates are susceptible to C2T, and that trypsinized component II elicits a novel biological activity by binding to the cell membrane of the macrophages.

Conserved structure of genes encoding components of botulinum neurotoxin complex M and the sequence of the gene coding for the nontoxic component in nonproteolytic Clostridium botulinum type F

For investigation of the genes of proteins associated in vivo with botulinum neurotoxin (BoNT), polymerase chain reaction (PCR) experiments were carried out with oligonucleotide primers designed to regions of the nontoxic-nonhemagglutinin (NTNH) gene of Clostridium botulinum type C. The primers were used to amplify a DNA fragment from genomic DNA of C. botulinum types A, B, E, F, G and toxigenic strains of Clostridium barati and Clostridium butyricum. The amplified product from all of these strains hybridized with an internal oligonucleotide probe, whereas all nontoxigenic clostridia tested gave no PCR product and showed no reaction with the probe. The NTNH gene was shown to be located upstream of the gene encoding BoNT, thereby revealing a conserved structure for genes encoding the proteins of the M complex of the progenitor botulinum toxin in these organisms. The sequence of the NTNH gene of nonproteolytic C. botulinum type F was determined by PCR amplification and sequencing of overlapping cloned fragments. NTNH/F showed 71% and 61% identity with NTNH of C. botulinum type E and type C respectively.

Clostridial neurotoxins as tools to investigate the molecular events of neurotransmitter release

The clostridial neurotoxins responsible for tetanus and botulism are eight different proteins, composed of two disulfide-linked polypeptide chains. They bind specifically to the presynaptic membrane via the heavy chain, while the light chain enters the cytosol of the neurons, where it displays a zinc-endopeptidase activity directed to proteins of the neuroexocytosis apparatus. Tetanus neurotoxin and botulinum neurotoxin serotypes B, D, F and G cleave specifically and at single different peptide bonds VAMP/synaptobrevin, a component of small synaptic vesicles. In contrast, the other neurotoxins catalyze the hydrolysis of proteins of the presynaptic membrane. Serotypes A and E of botulinum neurotoxin cleave SNAP-25, at different sites located within the carboxyl-terminus, while the specific target of serotype C is syntaxin.

Mechanism of action of tetanus and botulinum neurotoxins

The clostridial neurotoxins responsible for tetanus and botulism are metallo-proteases that enter nerve cells and block neurotransmitter release via zinc-dependent cleavage of protein components of the neuroexocytosis apparatus. Tetanus neurotoxin (TeNT) binds to the presynaptic membrane of the neuromuscular junction and is internalized and transported retroaxonally to the spinal cord. Whilst TeNT causes spastic paralysis by acting on the spinal inhibitory interneurons, the seven serotypes of botulinum neurotoxins (BoNT) induce a flaccid paralysis because they intoxicate the neuromuscular junction. TeNT and BoNT serotypes B, D, F and G specifically cleave VAMP/synaptobrevin, a membrane protein of small synaptic vesicles, at different single peptide bonds. Proteins of the presynaptic membrane are specifically attacked by the other BoNTs: serotypes A and E cleave SNAP-25 at two different sites located within the carboxyl terminus, whereas the specific target of serotype C is syntaxin.

Botulinum neurotoxin type G proteolyses the Ala81-Ala82 bond of rat synaptobrevin 2

Tetanus toxin and the botulinum neurotoxins types A to F inhibit neurotransmitter release from presynaptic nerve endings by selectively proteolysing the synaptic proteins synaptobrevin, syntaxin, or SNAP-25. Here, we show that botulinum toxin type G cleaves rat synaptobrevin 2 between Ala81 and Ala82, a peptide bond that differs from those attacked by tetanus toxin and the botulinal toxins types B, D, and F. Synaptobrevin isoforms carrying a Gly in the P1 position are poor substrates. Analyses of N-terminal deletion mutants of rat synaptobrevin 2 showed that a substrate starting at Leu54 is cleaved efficiently, whereas substrates beginning at Leu60 or Phe77 are cleaved partially or not at all, respectively.

Tetanus and botulinum neurotoxins are zinc proteases specific for components of the neuroexocytosis apparatus

Tetanus and botulinum neurotoxins bind to nerve cells, penetrate the cytosol, and block neurotransmitter release. Comparison of their amino-acid sequences shows the presence of the highly conserved His-Glu-x-x-His zinc-binding motif of zinc-endopeptidases (HExxH). Atomic absorption measurements of clostridial neurotoxins show the presence of one atom of zinc/toxin molecule bound to the light chain. The toxin-bound zinc ion is essential for the neurotoxins inhibition of neurotransmitter release in Aplysia neurons injected with the toxins. Phosphoramidon, a very specific inhibitor of zinc-endopeptidases, blocks the intracellular activity of the clostridial neurotoxins. Highly purified preparations of the light chain of tetanus and botulinum B and F neurotoxins cleaved specifically VAMP/synaptobrevin, an integral membrane protein of small synaptic vesicles, both in vivo and in vitro. From these studies, it can be concluded that the clostridial neurotoxins responsible for tetanus and botulism block neuroexocytosis via the proteolytic cleavage of specific components of the neuroexocytotic machinery.

[Molecular mechanism of action of tetanus toxin and botulinum neurotoxins]

Tetanus toxin and botulinum neurotoxins are di-chain proteins of 150 kD molecular weight. They are produced by bacteria of the Clostridium genus. These toxins act on the nervous system by inhibiting neurotransmitter release (glycine and GABA in the case of tetanus toxin; acetylcholine in the case of botulinum neurotoxins) thus inducing the spastic or flaccid paralysis that characterizes tetanus and botulism, respectively. Their cellular mechanism of action involves three main steps, namely binding to the neurone membrane, internalization and intracellular blockade of the release mechanism for neurotransmitters. Membrane acceptors for these toxins are not yet fully identified; they would consist of membrane gangliosides and proteins. The internalization step would be achieved by endocytosis. Recent findings show that both binding and internalization are mediated only by the heavy chain of the toxins whereas the intracellular blockade of neurotransmitter release involves their light chain alone. The light chain has been identified as a zinc metalloprotease and its substrates would be proteins involved in the neurotransmitter release mechanism. The target of tetanus toxin and of botulinum neurotoxin type B is VAMP/synaptobrevin, a membrane protein of the synaptic vesicles of nerve cell terminals.

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

Botulinum neurotoxin Type A is synthesized by Clostridium botulinum as a approximately 150 kD single chain polypeptide. The posttranslational processing of the 1296 amino acid residue long gene product involves removal of the initiating methionine, formation of disulfide bridges, and limited proteolysis (nicking) by the bacterial protease(s). The mature dichain neurotoxin is made of a approximately 50-kD light chain and a approximately 100-kD heavy chain connected by a disulfide bridge. DNA derived amino acid sequence predicted a total of 9 Cys residues (Binz et al., 1990, J. Biol. Chem. 265, 9153-9158; Thompson et al., 1990, Eur. J. Biochem. 189, 73-81). Treatment of the dichain neurotoxin, dissolved in 6 M guanidine. HCl, with 4-vinylpyridine converted 5 Cys residues into S-pyridylethyl cysteine residues; but alkylation after mercaptolysis converted all 9 Cys residues in the S-pyridylethylated form. After confirming the predicted number of Cys residues by amino acid analysis, the positions of the 5 Cys residues carrying sulfhydryl groups and the 4 involved in disulfide bridges were determined by comparing the elution patterns in reversed-phase HPLC of the cyanogen bromide mixtures of the exclusively alkylated and the mercaptolyzed-alkylated neurotoxin. The chromatographically isolated components were identified by N-terminal amino acid sequence analysis. The HPLC patterns showed characteristic differences. The Cys residues predicted in positions 133, 164, 790, 966, and 1059 were found in the sulfhydryl form; Cys 429 and 453 were found disulfide-bridge connecting the light and heavy chains, and Cys 1234 and 1279 were found in an intrachain disulfide-bridge near the C-terminus in the heavy chain.(ABSTRACT TRUNCATED AT 250 WORDS)

ADP-ribosylation of rainbow trout (Oncorhynchus mykiss) actin by botulinum C2 toxin

Intracellular actin of rainbow trout macrophages was ADP-ribosylated by botulinum C2 toxin, which is composed of two nonlinked protein components, component I and trypsinized component II. The actin in the supernatants of various tissue homogenates of the trout was also directly ADP-ribosylated by component I of C2 toxin, indicating that fish actin other than those of land vertebrates is susceptible to enzymatic modification by component I of C2 toxin.

Purification and characterization of the ganglioside-binding fragment of Clostridium botulinum type E neurotoxin

A way of fragmentation of Clostridium botulinum neurotoxin was carried out to elucidate the structure-function relationship of neurotoxin. The hitherto only plausible fragment was isolated from the trypsin-treated heavy chain of botulinum type E neurotoxin. In the presence of 4 M urea, one protein peak emerged from QAE-Sephadex column loaded with the heavy chain mildly treated with trypsin by elution with 0.1 M sodium chloride. Although many protein bands were detected in SDS-PAGE of the treated heavy chain, the eluted protein migrated in a single band to the position of 41,000 Da. The recovery of the 41,000-Da fragment was 28.6%, but with a 2 M urea-containing buffer as eluant, the recovery was less than 12%. The 41,000-Da fragment bound to gangliosides GD1a, GT1b, and GQ1b, to which neurotoxin and the heavy chain bound. The 41,000-Da fragment partially interfered with the binding of 125I-labeled neurotoxin to mouse brain synaptosomes. We have proposed a three-fragment structure (L.H-1.H-2) for botulinum type E neurotoxin. The characters of the 41,000-Da fragment described in this paper seem to substantiated our proposal that type E neurotoxin consists of three fragments, L.H-1.H-2, and that the ganglioside-binding fragment is H-2.

Botulinum neurotoxins are zinc proteins

The available amino acid sequences of 150-kDa botulinum and tetanus neurotoxins show the presence of a closely homologous segment in the middle of the light chain (NH2-terminal 50 kDa), which is the intracellularly active portion of the toxin. This segment contains the zinc binding motif of metalloendopeptidases, HEXXH. Atomic adsorption analysis of botulinum neurotoxins (serotypes A, B, and E) made on the basis of this observation demonstrated the presence of one zinc atom/molecule of 150-kDa neurotoxin. Conditions were found for the removal of the zinc ion with chelating agents and for the restoration of the normal metal content. The conserved segment, which includes the zinc binding motif, was synthesized and shown to bind [65Zn]2+. Chemical modification experiments indicated that two histidines and no cysteines are involved in Zn2+ coordination in agreement with a probable catalytic role for the zinc ion. The present findings suggest the possibility that botulinum neurotoxins are zinc proteases.

Identification of an ion channel-forming motif in the primary structure of tetanus and botulinum neurotoxins

Synthetic peptides with amino acid sequences corresponding to predicted transmembrane segments of tetanus toxin were used as probes to identify a channel-forming motif. A peptide denoted TeTx II, with sequence GVVLLLEYIPEITLPVIAALSIA, forms cation-selective channels when reconstituted in planar lipid bilayers. The single channel conductance in 0.5 M NaCl or KCl is 28 +/- 3 and 24 +/- 2 pS, respectively. In contrast, a peptide with sequence NFIGALETTGVVLLLEYIPEIT, denoted as TeTx I, or a peptide with the same amino acid composition as TeTx II but with a randomized sequence, do not form channels. Conformational energy calculations show that a bundle of four amphipathic alpha-helices is a plausible structural motif underlying observable pore properties. The identified functional module may account for the channel-forming activity of both tetanus toxin and the homologous botulinum toxin A.

Visualizations of binding and internalization of two nonlinked protein components of botulinum C2 toxin in tissue culture cells

Binding and internalization of two nonlinked components of botulinum C2 toxin were visualized in tissue culture cells with components directly labeled with fluorescence. The binding of both untrypsinized and trypsinized component II (UT-II and T-II, respectively) to common specific sites on the cell membrane was evidenced by competitive binding between fluorescence-labeled and unlabeled components. The distribution patterns of fluorescence-labeled T-II and UT-II after binding to cells at 37 degrees C were different; T-II clustered on the cell membrane and entered the cells in endosomes, whereas UT-II entered the cells inefficiently and not in vesicles and was distributed on the nuclear surface. The difference may be due to the multivalent property of T-II, which is not shared with UT-II. Fluorescence-labeled component I, which binds only to cells bound with T-II, entered cells by the same route as T-II did; both colocated on the same clusters on the cell membrane and also in the same vesicles in the cytoplasm. The present results suggest that component I of C2 toxin, which ADP-ribosylates cytoplasmic actin, directly binds to T-II but not to UT-II on the cell membrane and is internalized into cells together with T-II in the same endosomes.