Cooperative exosite-dependent cleavage of synaptobrevin by tetanus toxin light chain

Evolutionary Features in the Structure and Function of Bacterial Toxins

Toxins can function both as a harmful and therapeutic molecule, depending on their concentrations. The diversity in their function allows us to ask some very pertinent questions related to their origin and roles: (a) What makes them such effective molecules? (b) Are there evolutionary features encoded within the structures of the toxins for their function? (c) Is structural hierarchy in the toxins important for maintaining their structure and function? (d) Do protein dynamics play a role in the function of toxins? and (e) Do the evolutionary connections to these unique features and functions provide the fundamental points in driving evolution? In light of the growing evidence in structural biology, it would be appropriate to suggest that protein dynamics and flexibility play a much bigger role in the function of the toxin than the structure itself. Discovery of IDPs (intrinsically disorder proteins), multifunctionality, and the concept of native aggregation are shaking the paradigm of the requirement of a fixed three-dimensional structure for the protein’s function. Growing evidence supporting the above concepts allow us to redesign the structure-function aspects of the protein molecules. An evolutionary model is necessary and needs to be developed to study these important aspects. The criteria for a well-defined model would be: (a) diversity in structure and function, (b) unique functionality, and (c) must belong to a family to define the evolutionary relationships. All these characteristics are largely fulfilled by bacterial toxins. Bacterial toxins are diverse and widely distributed in all three forms of life (Bacteria, Archaea and Eukaryotes). Some of the unique characteristics include structural folding, sequence and functional combination of domains, targeting a cellular process to execute their function, and most importantly their flexibility and dynamics. In this work, we summarize certain unique aspects of bacterial toxins, including role of structure in defining toxin function, uniqueness in their enzymatic function, and interaction with their substrates and other proteins. Finally, we have discussed the evolutionary aspects of toxins in detail, which will help us rethink the current evolutionary theories. A careful study, and appropriate interpretations, will provide answers to several questions related to the structure-function relationship of proteins, in general. Additionally, this will also allow us to refine the current evolution theories.

An unexpected tetanus case

1 million cases of tetanus are estimated to occur worldwide each year, with more than 200 000 deaths. Tetanus is a life-threatening but preventable disease caused by a toxin produced by Clostridium tetani-a Gram-positive bacillus found in high concentrations in soil and animal excrement. Tetanus is almost completely preventable by active immunisation, but very rarely unexpected cases can occur in individuals who have been previously vaccinated. We report a case of generalised tetanus in a 22-year-old woman that arose despite the protective antitoxin antibody in her serum. The patient received all her vaccinations in the USA; her last vaccination was 6 years ago. The case was unusual because the patient had received all standard vaccinations, had no defined port of entry at disease onset, and had symptoms lasting for 6 months. Tetanus can present with unusual clinical forms; therefore, the diagnosis and management of this rare but difficult disease should be updated. In this Grand Round, we review the clinical features, epidemiology, treatment, and prognosis of C tetani infections.

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.

Probing BoNT/A protease exosites: implications for inhibitor design and light chain longevity

Botulinum neurotoxin serotype A (BoNT/A) is one of the most lethal toxins known. Its extreme toxicity is due to its light chain (LC), a zinc protease that cleaves SNAP-25, a synaptosome-associated protein, leading to the inhibition of neuronal activity. Studies on BoNT/A LC have revealed that two regions, termed exosites, can play an important role in BoNT catalytic activity. A clear understanding of how these exosites influence neurotoxin catalytic activity would provide a critical framework for deciphering the mechanism of SNAP-25 cleavage and the design of inhibitors. Herein, based on the crystallographic structure of BoNT/A LC complexed with its substrate, we designed an α-exosite binding probe. Experiments with this unique probe demonstrated that α-exosite binding enhanced both catalytic activity and stability of the LC. These data help delineate why α-exosite binding is needed for SNAP-25 cleavage and also provide new insights into the extended lifetime observed for BoNT/A LC in vivo.

The blockade of the neurotransmitter release apparatus by botulinum neurotoxins

The high toxicity of the seven serotypes of botulinum neurotoxins (BoNT/A to G), together with their specificity and reversibility, includes them in the list A of potential bioterrorism weapons and, at the same time, among the therapeutics of choice for a variety of human syndromes. They invade nerve terminals and cleave specifically the three proteins which form the heterotrimeric SNAP REceptors (SNARE) complex that mediates neurotransmitter release. The BoNT-induced cleavage of the SNARE proteins explains by itself the paralysing activity of the BoNTs because the truncated proteins cannot form the SNARE complex. However, in the case of BoNT/A, the most widely used toxin in therapy, additional factors come into play as it only removes a few residues from the synaptosomal associate protein of 25 kDa C-terminus and this results in a long duration of action. To explain these facts and other experimental data, we present here a model for the assembly of the neuroexocytosis apparatus in which Synaptotagmin and Complexin first assist the zippering of the SNARE complex, and then stabilize and clamp an octameric radial assembly of the SNARE complexes.

Identification of protease exosite-interacting peptides that enhance substrate cleavage kinetics

Many peptidases are thought to require non-active site interaction surfaces, or exosites, to recognize and cleave physiological substrates with high specificity and catalytic efficiency. However, the existence and function of protease exosites remain obscure owing to a lack of effective methods to identify and characterize exosite-interacting substrates. To address this need, we modified the cellular libraries of peptide substrates (CLiPS) methodology to enable the discovery of exosite-interacting peptide ligands. Invariant cleavage motifs recognized by the active sites of thrombin and caspase-7 were displayed on the outer surface of bacteria adjacent to a candidate exosite-interacting peptide. Exosite peptide libraries were then screened for ligands that accelerate cleavage of the active site recognition motif using two-color flow cytometry. Exosite CLiPS (eCLiPS) identified exosite-binding peptides for thrombin that were highly similar to a critical exosite interaction motif in the thrombin substrate, protease-activated receptor 1. Protease activity probes incorporating exosite-binding peptides were cleaved ten-fold faster than substrates without exosite ligands, increasing their sensitivity to thrombin activity in vitro. For comparison, screening with caspase-7 yielded peptides that modestly enhanced (two-fold) substrate cleavage rates. The eCLiPS method provides a new tool to profile the ligand specificity of protease exosites and to develop improved substrates.

Comparison of Fluorigenic Peptide Substrates PL50, SNAPtide, and BoTest A/E for BoNT/A Detection and Quantification

Detection and quantification of low doses of botulinum toxin serotype A (BoNT/A) in medicinal preparations require precise and sensitive methods. With mounting pressure from governmental authorities to replace the mouse LD50 assay, interest in alternative methods such as the endopeptidase assay, quantifying the toxin active moiety, is growing. Using internal collision-induced fluorescence quenching, Pharmaleads produced peptides encompassing the SNAP-25 cleavage site: a 17-mer (PL63) and a 48-mer (PL50) reaching the previously identified α-exosite, with PL50 showing higher apparent affinity for BoNT/A. Peptide mapping experiments revealed that this increased affinity is mainly due to a connecting peptide sequence between the N-terminus of PL63 and the α-exosite, identifying a new cooperative exosite on BoNT/A. Other endopeptidase substrates available, including SNAPTide and BoTest A/E, are both based on fluorescent resonance energy transfer (FRET) technology. To compare these assays, their limits of detection and quantification were determined using light chain or 150-kDa BoNT/A. Detection limits of PL50 and BoTest were over 100 times better than those using SNAPtide in standard conditions. Although the BoTest possessed a detection limit around 0.2 pM for either BoNT/A form, its quantification limit (9.7 pM) using purified BoNT/A was 12 times inferior to PL50, estimated at 0.8 pM, suitable for medicinal preparation quantification.

Molecular mechanisms of substrate recognition and specificity of botulinum neurotoxin serotype F

BoNTs (botulinum neurotoxins) are both deadly neurotoxins and natural toxins that are widely used in protein therapies to treat numerous neurological disorders of dystonia and spinal spasticity. Understanding the mechanism of action and substrate specificity of BoNTs is a prerequisite to develop antitoxin and novel BoNT-derived protein therapy. To date, there is a lack of detailed information with regard to how BoNTs recognize and hydrolyse the substrate VAMP-2 (vesicle-associated membrane protein 2), even though it is known to be cleaved by four of the seven BoNT serotypes, B, D, F, G and TeNT (tetanus neurotoxin). In the present study we dissected the molecular mechanisms of VAMP-2 recognition by BoNT serotype F for the first time. The initial substrate recognition was mediated through sequential binding of VAMP-2 to the B1, B2 and B3 pockets in LC/F (light chain of BoNT serotype F), which directed VAMP-2 to the active site of LC/F and stabilized the active site substrate recognition, where the P2, P1' and P2' sites of VAMP-2 were specifically recognized by the S2, S1' and S2' pockets of LC/F to promote substrate hydrolysis. The understanding of the molecular mechanisms of LC/F substrate recognition provides insights into the development of antitoxins and engineering novel BoNTs to optimize current therapy and extend therapeutic interventions.

Clostridial neurotoxins: mechanism of SNARE cleavage and outlook on potential substrate specificity reengineering

The clostridial neurotoxin family consists of tetanus neurotoxin and seven distinct botulinum neurotoxins which cause the diseases tetanus and botulism. The extreme potency of these toxins primarily relies not only on their ability to specifically enter motoneurons but also on the activity their catalytic domains display inside presynaptic motoneuronal terminals. Subsequent to neurotoxin binding and endocytosis the catalytic domains become translocated across endosomal membranes and proteolyze unique peptide bonds of one of three soluble N-ethylmaleimide-sensitive fusion protein attachment receptors (SNAREs), vesicle associated membrane protein/synaptobrevin, synaptosome associated protein of 25 kDa, or syntaxin. As these substrate proteins are core components of the vesicular membrane fusion apparatus, cleavage of any of the substrate molecules results in the blockade of neurotransmitter release. This review summarizes the present knowledge about the molecular basis of the specific substrate recognition and cleavage mechanism and assesses the feasibility of reengineering catalytic domains to hydrolyze non-substrate members of the three SNARE families in order to expand the therapeutic application of botulinum neurotoxins.

Cloning and expression of a region of vesicle associated membrane protein2 (VAMP2) gene and its use as a recombinant peptide substrate for assaying clostridial neurotoxins in contaminated biologicals

An assay for the endopeptidase activities of clostridial neurotoxins in contaminated biotherapeutic products has been developed. Based on a synthetic peptide substrate representing amino acid residues 60-94 of the intracellular vesicle associated membrane protein2 (VAMP2), RT-PCR was used to amplify the VAMP2 sequence. The extended insert was digested with EcoRI and SalI and ligated into pGEX4T-1 vector for construction of the pGEX4T-1/VAMP plasmid for expressing in Escherichia coli a fusion protein linked to glutathione S-transferase (GST). The fusion protein was purified by affinity chromatography and used in an ELISA assay for comparison with the commercially available synthetic VAMP peptide and rabbit polyclonal antiserum. The identity of the immunoreactivity of recombinant VAMP2 protein with the chemically synthesized peptide was demonstrated by western blot. Our results indicated that recombinant VAMP2 peptide not only reacted with specific polyclonal antibody in a dose-dependent manner, without any remarkable difference observed between the reactivity of the fusion protein and commercial VAMP2 segment peptide, but also cleaved by botulinum neurotoxin type B (BONT/B) after endopeptidase assay. Thus, recombinant VAMP2 could serve as a replacement for VAMP2 synthetic peptide, potentially useful in endopeptidase assays for replacement of the currently used mouse bioassay for clostridial neurotoxins contaminating biotherapeutic products.

Detection and quantification of botulinum neurotoxin type a by a novel rapid in vitro fluorimetric assay

Botulinum neurotoxin type A (BoNT/A), the most poisonous substance known to humans, is a potential bioterrorism agent. The light-chain protein induces a flaccid paralysis through cleavage of the 25-kDa synaptosome-associated protein (SNAP-25), involved in acetylcholine release at the neuromuscular junction. BoNT/A is widely used as a therapeutic agent and to reduce wrinkles. The toxin is used at very low doses, which have to be accurately quantified. With this aim, internally quenched fluorescent substrates containing the fluorophore/repressor pair pyrenylalanine (Pya)/4-nitrophenylalanine (Nop) were developed. Nop and Pya were, respectively, introduced at positions 197 and 200 of the cleavable fragment (amino acids 187 to 203) of SNAP-25 (with norleucine at position 202 [Nle(202)]), which is acetylated at its N terminus and amidated at its C terminus. Cleavage of this peptide occurred between positions 197 and 198, as in SNAP-25, and was easily quantified by the strong fluorescence emission of the metabolite. To increase the assay sensitivity, the peptide sequence of the previous substrate was lengthened to account for exosite binding to BoNT/A. We synthesized the peptide PL50 (SNAP-25-NH(2) acetylated at positions 156 to 203 [Nop(197), Pya(200), Nle(202)]) and its analogue PL51, in which all methionines were replaced by nonoxidizable Nle. Consistent with a large increase in affinity for BoNT/A, PL50 and PL51 exhibit catalytic efficiencies of 2.6 x 10(6) M(-1) s(-1) and 8.85 x 10(6) M(-1) s(-1), respectively, and behave as the best fluorigenic substrates of BoNT/A reported to date. Under optimized assay conditions, they allow simple quantification of as little as 100 and 60 pg of BoNT/A, respectively, within 2 h with a classical fluorimeter. Calibration of the method against the mouse 50% lethal dose assay unequivocally validates the enzymatic assay.

Receptor and substrate interactions of clostridial neurotoxins

The high potency of clostridial neurotoxins relies predominantly on their neurospecific binding and specific hydrolysis of SNARE proteins. Their multi-step mode of mechanism can be ascribed to their multi-domain three-dimensional structure. The C-terminal H(CC)-domain interacts subsequently with complex polysialo-gangliosides such as GT1b and a synaptic vesicle protein receptor via two neighbouring binding sites, resulting in highly specific uptake of the neurotoxins at synapses of cholinergic motoneurons. After its translocation the enzymatically active light chain specifically hydrolyses specific SNARE proteins, preventing SNARE complex assembly and thereby blocking exocytosis of neurotransmitter.

The familial Mediterranean fever protein, pyrin, is cleaved by caspase-1 and activates NF-kappaB through its N-terminal fragment

Familial Mediterranean fever (FMF) is an autoinflammatory disease caused by mutations in MEFV, which encodes a 781-amino acid protein denoted pyrin. We have previously shown that pyrin regulates caspase-1 activation and IL-1beta production through interaction of its N-terminal PYD motif with the ASC adapter protein, and also modulates IL-1beta production by interaction of its C-terminal B30.2 domain with the catalytic domains of caspase-1. We now asked whether pyrin might itself be a caspase-1 substrate, and found that pyrin is cleaved by caspase-1 at Asp330, a site remote from the B30.2 domain. Pyrin variants harboring FMF-associated B30.2 mutations were cleaved more efficiently than wild-type pyrin. The N-terminal cleaved fragment interacted with the p65 subunit of NF-kappaB and with IkappaB-alpha through its 15-aa bZIP basic domain and adjacent sequences, respectively, and translocated to the nucleus. The interaction of the N-terminal fragment with p65 enhanced entrance of p65 into the nucleus. The interaction of N-terminal pyrin with IkappaB-alpha induced calpain-mediated degradation of IkappaB-alpha, thus potentiating NF-kappaB activation. Absolute and relative quantities of cleaved pyrin and IkappaB-alpha degradation products were substantially increased in leukocytes from FMF patients compared with healthy controls. Our data support a new pyrin/caspase-1 pathway for NF-kappaB activation.

Substrate recognition of VAMP-2 by botulinum neurotoxin B and tetanus neurotoxin

Botulinum neurotoxin (BoNT; serotypes A-G) and tetanus neurotoxin elicit flaccid and spastic paralysis, respectively. These neurotoxins are zinc proteases that cleave SNARE proteins to inhibit synaptic vesicle fusion to the plasma membrane. Although BoNT/B and tetanus neurotoxin (TeNT) cleave VAMP-2 at the same scissile bond, their mechanism(s) of VAMP-2 recognition is not clear. Mapping experiments showed that residues 60-87 of VAMP-2 were sufficient for efficient cleavage by BoNT/B and that residues 40-87 of VAMP-2 were sufficient for efficient TeNT cleavage. Alanine-scanning mutagenesis and kinetic analysis identified three regions within VAMP-2 that were recognized by BoNT/B and TeNT: residues adjacent to the site of scissile bond cleavage (cleavage region) and residues located within N-terminal and C-terminal regions relative to the cleavage region. Analysis of residues within the cleavage region showed that mutations at the P7, P4, P2, and P1' residues of VAMP-2 had the greatest inhibition of LC/B cleavage (> or =32-fold), whereas mutations at P7, P4, P1', and P2' residues of VAMP-2 had the greatest inhibition of LC/TeNT cleavage (> or =64-fold). Residues within the cleavage region influenced catalysis, whereas residues N-terminal and C-terminal to the cleavage region influenced binding affinity. Thus, BoNT/B and TeNT possess similar organization but have unique residues to recognize and cleave VAMP-2. These studies provide new insights into how the clostridial neurotoxins recognize their substrates.

Substrate recognition mechanism of VAMP/synaptobrevin-cleaving clostridial neurotoxins

Botulinum neurotoxins (BoNTs) and tetanus neurotoxin (TeNT) inhibit neurotransmitter release by proteolyzing a single peptide bond in one of the three soluble N-ethylmaleimide-sensitive factor attachment protein receptors SNAP-25, syntaxin, and vesicle-associated membrane protein (VAMP)/synaptobrevin. TeNT and BoNT/B, D, F, and G of the seven known BoNTs cleave the synaptic vesicle protein VAMP/synaptobrevin. Except for BoNT/B and TeNT, they cleave unique peptide bonds, and prior work suggested that different substrate segments are required for the interaction of each toxin. Although the mode of SNAP-25 cleavage by BoNT/A and E has recently been studied in detail, the mechanism of VAMP/synaptobrevin proteolysis is fragmentary. Here, we report the determination of all substrate residues that are involved in the interaction with BoNT/B, D, and F and TeNT by means of systematic mutagenesis of VAMP/synaptobrevin. For each of the toxins, three or more residues clustered at an N-terminal site remote from the respective scissile bond are identified that affect solely substrate binding. These exosites exhibit different sizes and distances to the scissile peptide bonds for each neurotoxin. Substrate segments C-terminal of the cleavage site (P4-P4') do not play a role in the catalytic process. Mutation of residues in the proximity of the scissile bond exclusively affects the turnover number; however, the importance of individual positions at the cleavage sites varied for each toxin. The data show that, similar to the SNAP-25 proteolyzing BoNT/A and E, VAMP/synaptobrevin-specific clostridial neurotoxins also initiate substrate interaction, employing an exosite located N-terminal of the scissile peptide bond.

Structure- and substrate-based inhibitor design for Clostridium botulinum neurotoxin serotype A

The seven antigenically distinct serotypes of Clostridium botulinum neurotoxins cleave specific soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex proteins and block the release of neurotransmitters that cause flaccid paralysis and are considered potential bioweapons. Botulinum neurotoxin type A is the most potent among the clostridial neurotoxins, and to date there is no post-exposure therapeutic intervention available. To develop inhibitors leading to drug design, it is imperative that critical interactions between the enzyme and the substrate near the active site are known. Although enzyme-substrate interactions at exosites away from the active site are mapped in detail for botulinum neurotoxin type A, information about the active site interactions is lacking. Here, we present the crystal structures of botulinum neurotoxin type A catalytic domain in complex with four inhibitory substrate analog tetrapeptides, viz. RRGC, RRGL, RRGI, and RRGM at resolutions of 1.6-1.8 A. These structures show for the first time the interactions between the substrate and enzyme at the active site and delineate residues important for substrate stabilization and catalytic activity. We show that OH of Tyr(366) and NH(2) of Arg(363) are hydrogen-bonded to carbonyl oxygens of P1 and P1' of the substrate analog and position it for catalytic activity. Most importantly, the nucleophilic water is replaced by the amino group of the N-terminal residue of the tetrapeptide. Furthermore, the S1' site is formed by Phe(194), Thr(215), Thr(220), Asp(370), and Arg(363). The K(i) of the best inhibitory tetrapeptide is 157 nm.

An in vitro assay for detection of tetanus neurotoxin activity: Using antibodies for recognizing the proteolytically generated cleavage product

Tetanus neurotoxin (TeNT(1)) is a bacterial protease which specifically cleaves the vesicle protein synaptobrevin-2 (vesicle associated membrane protein-2, VAMP-2). This proteolytic feature of the toxin has been used to develop a sensitive endopeptidase assay for the detection of TeNT activity as an alternative to the in vivo assay for TeNT toxicity. Recombinant synaptobrevin-2 (rSyb2) is immobilized onto a microtiter plate, and the cleavage of immobilized rSyb2 by TeNT is detected with a polyclonal antibody directed against the newly generated C-terminus of the cleavage product. This antibody is shown to be a highly specific tool for detecting rSyb2 proteolysis by TeNT. The method reaches a detection limit of less than 1pg TeNT/ml. To our knowledge, this is the most sensitive in vitro assay for the detection of TeNT activity, and it is easy to perform. Besides, the assay can also detect the activity of botulinum neurotoxin type B (BoNT/B). The method can be applied to examine the toxicity of TeNT or BoNT/B preparations as well as the influence of chemicals on TeNT and BoNT/B activity. In the future, the assay may also serve as a basis for the replacement of the in vivo safety control of tetanus vaccines.

Botulinum neurotoxin types A, B, and E: fragmentations by autoproteolysis and other mechanisms including by O-phenanthroline-dithiothreitol, and association of the dinucleotides NAD(+)/NADH with the heavy chain of the three neurotoxins

The first evidence of autoproteolytic activity of the approximately 50-kDa light chain of the clostridial neurotoxins (NT) is traceable to the observations that the light chains of botulinum NT serotypes A and E, separated from their approximately 100-kDa heavy chain conjugate, were found cleaved at the amino side of Tyr250 and Arg244, respectively [DasGupta and Foley (1989). Biochimie 71: 1183-1200]. Specific cleavages of the recombinant light chain of NT type A, including at Tyr249-Tyr250, firmly established that the cleavages reported earlier were due to autoproteolysis [Ahmed et al. (2001). J. Protein Chem. 20: 221-231; Ahmed et al. (2003). Biochemistry 42:12539-12549] and not by contaminating proteases or non-enzymatic. We now report many cleavages in the NT types A, B and E and also in their separated light and heavy chains, and identification of several of the peptide bonds cleaved. None of the identified cleaved bonds (-P1-P1' -) in one serotype (except Asp-Pro) was found common in other serotypes or cleaved within itself at a second site. After separation from the heavy chain self-cleavages of the light chains of type A, B and E at Tyr249-Tyr250, Gln258-Ser259 and Ile243-Arg244, respectively indicate an intriguing feature (in the aligned sequences these bonds of type A and B are 2 and type A and E are 4 peptide bonds apart) that may have some role in the NT's structure-function relationship yet to be understood. We point out that autoproteolysis of a single peptide bond (Phe418-Thr419 or Phe422-Glu423) in NT type A reported by Ahmed et al. (2001) can potentially generate proteolytically active light chain freed of the heavy chain; this is an efficient pathway, that by-passes nicking by a trypsin-like protease(s) inside the intrachain disulfide bridge and its reductive cleavage. We offer probable explanations for the observed cleavages such as acid- and metal-mediated (non-catalytic and non-stoichiometric) reactions in addition to autoproteolysis but cannot predict which mechanism(s) of cleavage occur or prevail following NT's entry in the body as poison or therapeutic agent. The metal chelator O-phenanthroline (above critical miceller concentration) in the presence of dithiothreitol cleaved type E NT at limited sites generating discrete 114-, 87-, 49-, 42-, and 31-kDa fragments but degraded NTs type A and B extensively. The limited cleavage of type E NT was dependent on the presence of metal ion(s) bound to the protein and its native (urea sensitive) conformation. The self-cleavage of the NTs at specific sites prompted us to search for specific binding sites on the NTs analogous to SNARE-motifs-the 9-residuelong motifs present on the NT's natural substrates (SNAP-25, syntaxin, VAMP/synaptobrevin); such putative binding motifs (sites) noted on all clostridial NTs are reported here. Their relationship to the observed autoproteolysis remains to be determined experimentally. The dinucleotide NAD(+)/NADH associated with the NTs type A, B and E (2-3 NADH per protein molecule) via their H-chains, and a portion of the H-chain (toward the C-terminus) appears to exhibit limited amino acid sequence homology with lactate dehydrogenase-a representative NAD(+)/NADH binding protein.

Identification of the Amino Acid Residues Rendering TI-VAMP Insensitive toward Botulinum Neurotoxin B

Botulinum neurotoxins types B, D, F, and G, and tetanus neurotoxin inhibit vesicular fusion via proteolytic cleavage of VAMP/Synaptobrevin, a core component of the membrane fusion machinery. Thus, these neurotoxins became widely used tools for investigating vesicular trafficking routes. Except for VAMP-1, VAMP-2, and Cellubrevin, no other member of the VAMP family represents a substrate for these neurotoxins. The molecular basis for this discrepancy is not known. A 34 amino acid residue segment of VAMP-2 was previously suggested to mediate the interaction with botulinum neurotoxin B, but the validity of the data was later questioned. To check whether this segment alone controls the susceptibility toward botulinum neurotoxin B, it was used to replace the corresponding segment in TI-VAMP. The resulting VAMP hybrid and VAMP-2 were hydrolysed at virtually identical rates. Resetting the VAMP-2 portion in the hybrid from either end to TI-VAMP residues gradually reduced the cleavability. A hybrid encompassing merely the VAMP-2 segment 71-80 around the Gln76/Phe77 scissile bond was still hydrolysed, albeit at a approximately tenfold lower cleavage rate. The contribution of each non-conserved amino acid of the whole 34-mer segment to the interaction was investigated employing VAMP-2. We find that the eight non-conserved residues of the 71-80 segment are all necessary for efficient cleavage. Mutation of an additional six residues located upstream and downstream of this segment affects substrate hydrolysis as well. Vice versa, a readily cleavable TI-VAMP molecule requires at the least the replacement of Ile158, Thr161, and the section 165-174 by Asp64, Ala67, and the 71-80 segment of VAMP-2, respectively. However, the insensitivity of TI-VAMP to botulinum neurotoxin B relies on at least 12 amino acid changes versus VAMP-2. These are scattered along an interface of 22 amino acid residues in length.

New insights into clostridial neurotoxin-SNARE interactions

Botulinum neurotoxin serotype A (BoNT/A) has achieved a dichotomous status in modern medicine; it is both a versatile treatment for several neurological disorders and a lethal poison responsible for causing the neuroparalytic syndrome botulism. The extent of paralysis largely depends on the dosage of toxin received. The toxins block neurotransmitter release by delivering their Zn(2+)-dependent protease components to the presynaptic side of chemical synapses. These highly specialized enzymes exclusively hydrolyze peptide bonds within SNARE (soluble N-ethylmaleiamide-sensitive factor attachment protein receptor) proteins. Recently, the structural basis for the highly specific interaction between BoNT/A and its target SNARE, SNAP-25 (synaptosomal-associated protein of 25kDa), was elucidated. New details regarding the nature of the toxin-SNARE interactions could be exploited for novel inhibitor design.

2.3 A crystal structure of tetanus neurotoxin light chain

TeNT is the causative agent of the neuroparalytic disease tetanus. A key component of TeNT is its light chain, a Zn(2+) endopeptidase that targets SNAREs. Recent structural studies of closely related BoNT endopeptidases indicate that substrate-binding exosites remote from a conserved active site are the primary determinants of substrate specificity. Here we report the 2.3 A X-ray crystal structure of TeNT-LC, determined by combined molecular replacement and MAD phasing. As expected, the overall structure of TeNT-LC is similar to the other known CNT light chain structures, including a conserved thermolysin-like core inserted between structurally distinct amino- and carboxy-terminal regions. Differences between TeNT-LC and the other CNT light chains are mainly limited to surface features such as unique electrostatic potential profiles. An analysis of surface residue conservation reveals a pattern of relatively high variability matching the path of substrate binding around BoNT/A, possibly serving to accommodate the variations in different SNARE targets of the CNT group.

Partial Protection against Botulinum B Neurotoxin-Induced Blocking of Exocytosis by a Potent Inhibitor of Its Metallopeptidase Activity

Clostridium botulinum neurotoxins (BoNTs) cause botulism, which is characterized by a flaccid paralysis, through inhibition of acetylcholine release by peripheral cholinergic nerve terminals. This is due to the zinc metallopeptidase activity of the neurotoxin, cleaving one component (synaptobrevin for BoNT/B) of the exocytosis machinery. Yet, there are no specific agents able to control the peptidase-related effects of BoNT/B. We recently developed the first compounds to inhibit this enzymatic activity in the nanomolar range. Here we report that two of our best inhibitors prevent the BoNT/B-induced cleavage of native synaptobrevin on synaptic vesicles, and partially inhibit the suppression of [3H]noradrenaline release from synaptosomes that is caused by BoNT/B. These results were obtained at micromolar concentrations, consistent with the measured inhibitory potency of these inhibitors on the native toxin. These compounds provide a new way to possibly prevent and/or to control the neurotoxin effects of botulinum.

Structural analysis of the catalytic domain of tetanus neurotoxin

Clostridium neurotoxins, comprising the tetanus neurotoxin and the seven antigenically distinct botulinum neurotoxins (BoNT/A-G), are among the known most potent bacterial protein toxins to humans. Although they have similar function, sequences and three-dimensional structures, the substrate specificity and the selectivity of peptide bond cleavage are different and unique. Tetanus and botulinum type B neurotoxins enzymatically cleave the same substrate, vesicle-associated membrane protein, at the same peptide bond though the optimum length of substrate peptide required for cleavage by them is different. Here, we present the first experimentally determined three-dimensional structure of the catalytic domain of tetanus neurotoxin and analyze its active site. The structure provides insight into the active site of tetanus toxin's proteolytic activity and the importance of the nucleophilic water and the role of the zinc ion. The probable reason for different modes of binding of vesicle-associated membrane protein to botulinum neurotoxin type B and the tetanus toxin is discussed. The structure provides a basis for designing a novel recombinant vaccine or structure-based drugs for tetanus.

Exosite-driven substrate specificity and function in coagulation

Macromolecular substrate recognition and serine proteinase specificity lie at the heart of the tightly regulated hemostatic response. Mechanisms established for the less specific serine proteinases of digestion have played a dominant role in guiding investigations of the basis for the narrow specificities exhibited by the coagulation enzymes. These concepts have also dominated the development of specific inhibitors of coagulation for therapeutic purposes. Studies of the enzymology and physical biochemistry of prothrombinase challenge these prevailing ideas by establishing a principal role for exosites within the enzyme in determining substrate recognition and directing the action of the enzyme on its biological substrate. Mechanisms by which narrow protein substrate specificity is achieved by prothrombinase also apply to several other reactions of coagulation. These strategies are increasingly evident in the action of other families of enzymes that act with high specificity on protein substrates. Exosite-driven enzymic function probably represents a widely employed biological strategy for the achievement of high macromolecular substrate specificity.

Substrate recognition strategy for botulinum neurotoxin serotype A

Clostridal neurotoxins (CNTs) are the causative agents of the neuroparalytic diseases botulism and tetanus. CNTs impair neuronal exocytosis through specific proteolysis of essential proteins called SNAREs. SNARE assembly into a low-energy ternary complex is believed to catalyse membrane fusion, precipitating neurotransmitter release; this process is attenuated in response to SNARE proteolysis. Site-specific SNARE hydrolysis is catalysed by the CNT light chains, a unique group of zinc-dependent endopeptidases. The means by which a CNT properly identifies and cleaves its target SNARE has been a subject of much speculation; it is thought to use one or more regions of enzyme-substrate interaction remote from the active site (exosites). Here we report the first structure of a CNT endopeptidase in complex with its target SNARE at a resolution of 2.1 A: botulinum neurotoxin serotype A (BoNT/A) protease bound to human SNAP-25. The structure, together with enzyme kinetic data, reveals an array of exosites that determine substrate specificity. Substrate orientation is similar to that of the general zinc-dependent metalloprotease thermolysin. We observe significant structural changes near the toxin's catalytic pocket upon substrate binding, probably serving to render the protease competent for catalysis. The novel structures of the substrate-recognition exosites could be used for designing inhibitors specific to BoNT/A.

Synthesis of substrates and inhibitors of botulinum neurotoxin type A metalloprotease

Botulinum neurotoxin (BoNT) metalloproteases and related proteases are the most selective proteases known. X-ray crystal structures suggest that the active sites of the native enzymes exist in catalytically incompetent forms that must be activated by substrate binding. In order to characterize the postulated substrate-induced conformational changes for enzyme activation, we synthesized a series of transition-state analog inhibitors in which the dipeptide cleavage site is replaced by tetrahedral intermediate analogs within the minimal substrate peptide sequence. In this paper, we report our efforts to design inhibitors of BoNT/A metalloprotease. We confirm that an effective substrate sequence for BoNT/A metalloprotease is a 17-mer peptide corresponding to residues 187-203 of SNAP-25. A more stable substrate, Nle202SNAP-25 [187-203] was synthesized in order to develop an assay for proteolytic activity of BoNT/A metalloprotease that can be used to monitor time-dependent inhibition. Alpha-thiol amide analogs of Gln-197 were incorporated via solid-phase peptide synthesis into both 17-mer minimal peptide substrate sequences. The synthesis, characterization and inhibition kinetics for the alpha-thiol amide analogs of holotoxin A substrate are described. These substrate-derived inhibitors were shown to be submicromolar inhibitors of BoNT/A catalytic activity.

Small tripeptide surrogates with low nanomolar affinity as potent inhibitors of the botulinum neurotoxin B metallo-proteolytic activity

Botulinum neurotoxin type B is a high-weight (150 kDa) protein produced by the anaerobic bacillus Clostridium botulinum. This metallo-protease neurotoxin cleaves synaptobrevin, a protein, which is crucial to neurotransmission, resulting in the muscle paralysis, which characterizes botulism. Inhibition of the metallo-peptidase activity is a possible approach to obtain specific therapeutics to treat botulism. We have previously reported a successful attempt to block the proteolytic activity of this neurotoxin with new, selective amino-thiol inhibitors endowed with Ki values in the 15-20 nanomolar range. With the aim of increasing the affinity and bioavailability of this first series of inhibitors we have optimized the residue that fits the P(1) subsite of the enzyme by comparing a series of ligands that contain subtle but significant variants of the parent structure. In addition, this strategy provided a simplification of the synthesis of BoNT/B inhibitors by reducing the possible number of stereoisomers. As such we were able to enhance the inhibitory potency whilst reducing the size as compared to the initial privileged structure yielding the first pseudo-tripeptide inhibitors with Ki values in the low nanomolar range.

Crystal structure of Clostridium botulinum neurotoxin protease in a product-bound state: Evidence for noncanonical zinc protease activity

Clostridium botulinum neurotoxins (BoNTs), the most potent toxins known, disrupt neurotransmission through proteolysis of proteins involved in neuroexocytosis. The light chains of BoNTs are unique zinc proteases that have stringent substrate specificity and require exceptionally long substrates. We have determined the crystal structure of the protease domain from BoNT serotype A (BoNT/A). The structure reveals a homodimer in a product-bound state, with loop F242-V257 from each monomer deeply buried in its partner's catalytic site. The loop, which acts as a substrate, is oriented in reverse of the canonical direction for other zinc proteases. The Y249-Y250 peptide bond of the substrate loop is hydrolyzed, leaving the Y249 product carboxylate coordinated to the catalytic zinc. From the crystal structure of the BoNT/A protease, detailed models of noncanonical binding and proteolysis can be derived which we propose are also consistent with BoNT/A binding and proteolysis of natural substrate synaptosome-associated protein of 25 kDa (SNAP-25). The proposed BoNT/A substrate-binding mode and catalytic mechanism are markedly different from those previously proposed for the BoNT serotype B.

Implication of pH in the catalytic properties of anthrax lethal factor

The anthrax lethal factor (LF) is a Zn(2+)-endopeptidase specific for mitogen-activated protein kinase kinases (MAPKKs), which are cleaved within their N-terminal region. Much line of effort was carried out to elucidate the catalytic activity of LF for designing the inhibitor and to understand the cellular mechanism of its cytotoxicity. Current assay methods to analyze the LF activity have been based on a synthetic peptide, consisting of 15-20 residues around being cleaved. However, there are accumulating reports that the region distal to cleavage site is required for the LF-mediated proteolysis of substrate. In this study, we demonstrate the catalytic properties of LF, using the full-length native substrate, MEK. We described the catalytic properties of LF focused on the effects of the pH alteration, which was encountered during the endocytosis of lethal toxin, and of the requirement for metal ions. We present the first evidence that additional metal ions are required for the LF catalyzed hydrolysis of native substrate, and that the pH alteration causes a significant change of catalytic properties of LF.

Design and synthesis of substrate-based inhibitors of botulinum neurotoxin type B metalloprotease

Botulinum toxin (BoNT) metalloproteases and related proteases are the most selective proteases known. X-ray crystal structures suggest that the native enzymes exist in catalytically incompetent forms that must be activated by substrate binding. In order to characterize the postulated substrate-induced conformational changes, we synthesized a series of transition state analog inhibitors (TSI) in which the dipeptide cleavage site has been replaced by tetrahedral intermediate analogs within the minimal substrate peptide sequence. Reduced amide, alpha-hydroxyamide, alpha-thio-amide, and hydroxyethylamine analogs of -Gln-Phe- were incorporated via solid phase peptide synthesis into 35-mer analogs of the minimal peptide substrate sequence. The synthesis, characterization, and inhibition kinetics for four series of compounds against holotoxin BoNT/B is described. The alpha-thiol amide derivatives of the 35-mer substrate were found to inhibit BONT/B in the low micromolar range.

Free-energy contributions to complex formation between botulinum neurotoxin type B and synaptobrevin fragment

Free-energy terms that contribute to complex formation between the catalytic domain of botulinum neurotoxin type B (BoNT/B-L(C)) and a 36-residue synaptobrevin fragment were estimated by using a combination of microscopic simulations and continuum methods. The complex for a non-hydrolyzed substrate was calculated by optimizing an energy function applied to the X-ray co-crystal structure of BoNT/B-L(C) bound with reaction products from a cleaved synaptobrevin peptide, refined to high crystallographic thermal factors. The estimated absolute binding affinity of the simulation structure is in good qualitative agreement with the experimental free energy of Michaelis complex formation, given the approximations of the model calculations. The simulation structure revealed significant complex stabilization from the hydrophobic effect, while the electrostatic cost of releasing water molecules from the interface determined to be highly unfavorable. By partitioning the total electrostatic and hydrophobic terms into residue free-energy contributions, a binding-affinity 'signature' for synaptobrevin was developed from the optimized conformation. The results demonstrate the effect of substrate length on complex formation and identify a peripheral high-affinity binding site near the N-terminal region that might initiate cooperative activation responsible for the large minimal substrate length requirement. The so-called SNARE motif is observed to contribute negligible free energy of binding.

[Botulism: the agent, mode of action of the botulinum neurotoxins, forms of acquisition, treatment and prevention]

The botulinum neurotoxins are produced by anaerobic, spore-forming bacteria belonging to the Clostridium genus. They are synthesised as a single chain protein (150 kDa), which is not or weakly active. The active form results from a proteolysis cleaving the precursor in a light chain (about 50 kDa) and a heavy chain (about 100 kDa), which are linked by a disulfide bridge. The heavy chain is involved in the recognition of a specific neuronal surface receptor and mediates the internalization of the light chain into the cytosol. The light chain is responsible for the intracellular activity. It catalyses the proteolysis of SNARE proteins, which are involved in the exocytosis of synaptic vesicles containing acetylcholine. Hence, the release of acetylcholine at the neuromuscular junction is blocked, leading to a flaccid paralysis. Human botulism, usually type A, B or E, is associated with intoxination, ingestion of preformed toxin in food, with digestive toxi-infection, mainly in newborns (infant botulism), or with wound contamination (wound botulism). The treatment of botulism is usually symptomatic. The specific treatment is based on the serotherapy or on the use of purified specific antibodies. The vaccination against botulism is efficient. However, since the botulinum neurotoxins are widely used for the treatment of numerous dystonias, a generalised vaccination is not conceivable.

Insights into peptide and protein function: a convergent approach

From viruses to multicellular organisms, life is inseparable from the genetic instructions aimed at regulating its maintenance, division, multiplication, differentiation and death (apoptosis). Over the past 15 years, structural studies have begun to resolve the complex reactions involved in these fundamental processes in biology. The three-dimensional representations of the complexes formed with peptides and/or proteins have allowed interpretation of the biochemical data and formulation of novel hypotheses about the control and execution of these processes. Moreover, they have opened the way to rational approaches for designing compounds able to interfere with these crucial events in normal or pathological conditions. Various results obtained in our laboratory in these fields are briefly summarized in this review.

Tetanus and botulinum neurotoxins: turning bad guys into good by research

The neuroparalytic syndromes of tetanus and botulism are caused by neurotoxins produced by bacteria of the genus Clostridium. They are 150 kDa proteins consisting of three-domains, endowed with different functions: neurospecific binding, membrane translocation and specific proteolysis of three key components of the neuroexocytosis apparatus. After binding to the presynaptic membrane of motoneurons, tetanus neurotoxin (TeNT) is internalized and transported retroaxonally to the spinal cord, where it blocks neurotransmitter release from spinal inhibitory interneurons. In contrast, the seven botulinum neurotoxins (BoNT) act at the periphery and inhibit acetylcholine release from peripheral cholinergic nerve terminals. TeNT and BoNT-B, -D, -F and -G cleave specifically at single but different peptide bonds, VAMP/synaptobrevin, a membrane protein of small synaptic vesicles. BoNT types -A, -C and -E cleave SNAP-25 at different sites within the COOH-terminus, whereas BoNT-C also cleaves syntaxin. BoNTs are increasingly used in medicine for the treatment of human diseases characterized by hyperfunction of cholinergic terminals.

Evidence for a vesicle-mediated maintenance of store-operated calcium channels in a human embryonic kidney cell line

Direct microinjection of the clostridial neurotoxins botulinum neurotoxin A light chain or tetanus neurotoxin into cells of a human embryonic kidney cell line significantly reduced calcium entry after depletion of internal calcium stores by cyclopiazonic acid, a reversible inhibitor of the sarcoplasmic-endoplasmic reticular calcium-ATPases. Botulinum neurotoxin A light chain specifically hydrolyzes a synaptosomal-associated protein of 25 kilodaltons (SNAP-25), and tetanus neurotoxin specifically hydrolyzes synaptobrevin-2 (vesicle-associated membrane protein 2, VAMP-2) and cellubrevin (vesicle-associated membrane protein 3, VAMP-3). Since these substrate proteins are required for vesicle docking and fusion, inhibition of store-operated calcium entry by botulinum neurotoxin A light chain and tetanus neurotoxin supports a model in which vesicle fusion is a prerequisite for activation of store-operated calcium entry. Brefeldin A, a fungal metabolite that interferes with vesicle traffic, partially reduced calcium entry following store depletion. The size of the reserve pool of vesicles or parallel vesicle recycling pathways employing brefeldin A-sensitive and brefeldin A-insensitive ADP-ribosylation factors may explain the failure of brefeldin A to completely inhibit store-operated calcium entry.

Proteinase inhibitors and activators strategic targets for therapeutic intervention

The contributions in this meeting represent the state of the art in study of proteinases and their biological and therapeutic regulation. The near future of proteinase research will be fashioned by the new horizons of proteomics research [61], adding substance to the genomic data. Even new approaches in drug discovery, such as combinatorial chemistry, impact upon the understanding of the proteinase function, as with the discovery of a novel allosteric exosite in FVIIa by probing with libraries of ligands [62]. What is without question is that proteinases will remain at the forefront of understanding and intervention in human biochemistry and human disease pathology.

How botulinum and tetanus neurotoxins block neurotransmitter release

Botulinum neurotoxins (BoNT, serotypes A-G) and tetanus neurotoxin (TeNT) are bacterial proteins that comprise a light chain (M(r) approximately 50) disulfide linked to a heavy chain (M(r) approximately 100). By inhibiting neurotransmitter release at distinct synapses, these toxins cause two severe neuroparalytic diseases, tetanus and botulism. The cellular and molecular modes of action of these toxins have almost been deciphered. After binding to specific membrane acceptors, BoNTs and TeNT are internalized via endocytosis into nerve terminals. Subsequently, their light chain (a zinc-dependent endopeptidase) is translocated into the cytosolic compartment where it cleaves one of three essential proteins involved in the exocytotic machinery: vesicle associated membrane protein (also termed synaptobrevin), syntaxin, and synaptosomal associated protein of 25 kDa. The aim of this review is to explain how the proteolytic attack at specific sites of the targets for BoNTs and TeNT induces perturbations of the fusogenic SNARE complex dynamics and how these alterations can account for the inhibition of spontaneous and evoked quantal neurotransmitter release by the neurotoxins.

Neurotoxins affecting neuroexocytosis

Nerve terminals are specific sites of action of a very large number of toxins produced by many different organisms. The mechanism of action of three groups of presynaptic neurotoxins that interfere directly with the process of neurotransmitter release is reviewed, whereas presynaptic neurotoxins acting on ion channels are not dealt with here. These neurotoxins can be grouped in three large families: 1) the clostridial neurotoxins that act inside nerves and block neurotransmitter release via their metalloproteolytic activity directed specifically on SNARE proteins; 2) the snake presynaptic neurotoxins with phospholipase A(2) activity, whose site of action is still undefined and which induce the release of acethylcholine followed by impairment of synaptic functions; and 3) the excitatory latrotoxin-like neurotoxins that induce a massive release of neurotransmitter at peripheral and central synapses. Their modes of binding, sites of action, and biochemical activities are discussed in relation to the symptoms of the diseases they cause. The use of these toxins in cell biology and neuroscience is considered as well as the therapeutic utilization of the botulinum neurotoxins in human diseases characterized by hyperfunction of cholinergic terminals.

Activity-dependent changes in partial VAMP complexes during neurotransmitter release

The temporal sequence of SNARE protein interactions that cause exocytosis is unknown. Blockade of synaptic neurotransmitter release through cleavage of VAMP/synaptobrevin by tetanus toxin light chain (TeNT-LC) was accelerated by nerve stimulation. Botulinum/B neurotoxin light chain (BoNT/B-LC), which cleaves VAMP at the same site as TeNT-LC, did not require stimulation. Because TeNT-LC requires the N-terminal coil domain of VAMP for binding but BoNT/B-LC requires the C-terminal coil domain, it seems that, before nerve activity, the N-terminal domain is shielded in a protein complex, but the C-terminal domain is exposed. This N-terminal complex lasts until nerve activity occurs and may serve to cock synaptic vesicles for immediate exocytosis upon Ca2+ entry.

Sequence homology and structural analysis of the clostridial neurotoxins

The clostridial neurotoxins (CNTs), comprised of tetanus neurotoxin (TeNT) and the seven serotypes of botulinum neurotoxin (BoNT A-G), specifically bind to neuronal cells and disrupt neurotransmitter release by cleaving proteins involved in synaptic vesicle membrane fusion. In this study, multiple CNT sequences were analyzed within the context of the 1277 residue BoNT/A crystal structure to gain insight into the events of binding, pore formation, translocation, and catalysis that are required for toxicity. A comparison of the TeNT-binding domain structure to that of BoNT/A reveals striking differences in their surface properties. Further, the solvent accessibility of a key tryptophan in the C terminus of the BoNT/A-binding domain refines the location of the ganglioside-binding site. Data collected from a single frozen crystal of BoNT/A are included in this study, revealing slight differences in the binding domain orientation as well as density for a previously unobserved translocation domain loop. This loop and the conservation of charged residues with structural proximity to putative pore-forming sequences lend insight into the CNT mechanism of pore formation and translocation. The sequence analysis of the catalytic domain revealed an area near the active-site likely to account for specificity differences between the CNTs. It revealed also a tertiary structure, highly conserved in primary sequence, which seems critical to catalysis but is 30 A from the active-site zinc ion. This observation, along with an analysis of the 54 residue "belt" from the translocation domain are discussed with respect to the mechanism of catalysis.

Tetanus and botulinum neurotoxins: mechanism of action and therapeutic uses

The clostridial neurotoxins responsible for tetanus and botulism are proteins consisting of three domains endowed with different functions: neurospecific binding, membrane translocation and proteolysis for specific components of the neuroexocytosis apparatus. Tetanus neurotoxin (TeNT) binds to the presynaptic membrane of the neuromuscular junction, is internalized and transported retroaxonally to the spinal cord. The spastic paralysis induced by the toxin is due to the blockade of neurotransmitter release from spinal inhibitory interneurons. In contrast, the seven serotypes of botulinum neurotoxins (BoNTs) act at the periphery by inducing a flaccid paralysis due to the inhibition of acetylcholine release at the neuromuscular junction. TeNT and BoNT serotypes B, D, F and G cleave specifically at single but different peptide bonds, of the vesicle associated membrane protein (VAMP) synaptobrevin, a membrane protein of small synaptic vesicles (SSVs). BoNT types A, C and E cleave SNAP-25 at different sites located within the carboxyl-terminus, while BoNT type C additionally cleaves syntaxin. The remarkable specificity of BoNTs is exploited in the treatment of human diseases characterized by a hyperfunction of cholinergic terminals.

Metallopeptidase inhibitors of tetanus toxin: A combinatorial approach

The bacterial protein tetanus toxin (TeNt), which belongs to the family of zinc endopeptidases, cleaves synaptobrevin, an essential synaptic protein component of the neurotransmitter exocytosis apparatus, at a single peptide bond (Gln76-Phe77). This protease activity is a particularly attractive target for designing potent and selective synthetic inhibitors as a possible drug therapy for tetanus. beta-Aminothiols mimicking Gln76 of synaptobrevin have been previously shown to inhibit the tetanus neurotoxin enzymatic activity in the 35-250 microM range. These compounds have now been modified to interact with S' subsites of the TeNt active site, with the aim of increasing their inhibitory potencies. Combinatorial libraries of pseudotripeptides, containing an ethylene sulfonamide or an m-sulfonamidophenyl moiety as the P1 side chain and natural amino acids in P1' and P2' positions, were synthesized. The best inhibitory activity was observed with Tyr and His as P1' and P2' components, respectively. This led to new inhibitors of TeNt with Ki values in the 3-4 microM range. These molecules are the most potent inhibitors of TeNt described so far.

Proteolysis of SNAP-25 isoforms by botulinum neurotoxin types A, C, and E: domains and amino acid residues controlling the formation of enzyme-substrate complexes and cleavage

Tetanus toxin and the seven serologically distinct botulinal neurotoxins (BoNT/A to BoNT/G) abrogate synaptic transmission at nerve endings through the action of their light chains (L chains), which proteolytically cleave VAMP (vesicle-associated membrane protein)/synaptobrevin, SNAP-25 (synaptosome-associated protein of 25 kDa), or syntaxin. BoNT/C was reported to proteolyze both syntaxin and SNAP-25. Here, we demonstrate that cleavage of SNAP-25 occurs between Arg198 and Ala199, depends on the presence of regions Asn93 to Glu145 and Ile156 to Met202, and requires about 1,000-fold higher L chain concentrations in comparison with BoNT/A and BoNT/E. Analyses of the BoNT/A and BoNT/E cleavage sites revealed that changes in the carboxyl-terminal residues, in contrast with changes in the amino-terminal residues, drastically impair proteolysis. A proteolytically inactive BoNT/A L chain mutant failed to bind to VAMP/synaptobrevin and syntaxin, but formed a stable complex (KD = 1.9 x 10(-7) M) with SNAP-25. The minimal essential domain of SNAP-25 required for cleavage by BoNT/A involves the segment Met146-Gln197, and binding was optimal only with full-length SNAP-25. Proteolysis by BoNT/E required the presence of the domain Ile156-Asp186. Murine SNAP-23 was cleaved by BoNT/E and, to a reduced extent, by BoNT/A, whereas human SNAP-23 was resistant to all clostridial L chains. Lys185Asp or Pro182Arg mutations of human SNAP-23 induced susceptibility toward BoNT/E or toward both BoNT/A and BoNT/E, respectively.

Beta-amino-thiols inhibit the zinc metallopeptidase activity of tetanus toxin light chain

Tetanus neurotoxin is a 150-kDa protein produced by Clostridium tetani, which causes the lethal spastic paralytic syndromes of tetanus by blocking inhibitory neurotransmitter release at central synapses. The toxin light chain (50 kDa) has a zinc endopeptidase activity specific for synaptobrevin, an essential component of the neuroexocytosis apparatus. Previous unsuccessful attempts to block the proteolytic activity of this neurotoxin with well-known inhibitors of other zinc proteases led us to study the design of specific inhibitors as a possible drug therapy to prevent the progressive evolution of tetanus following infection. Starting from the synaptobrevin sequence at the level of the cleavage site by tetanus neurotoxin (Gln76-Phe77), a thiol analogue of glutamine demonstrated inhibitory activities in the millimolar range. A structure-activity relationship performed with this compound led us to determine the requirement for the correct positioning of the thiol group, the primary amino group, and a carboxamide or sulfonamide group on the side chain. This resulted in the design of a beta-amino-(4-sulfamoylphenyl)glycine-thiol, the first significantly efficient inhibitor of tetanus neurotoxin with a Ki value of 35 +/- 5 microM.