Metal substitution of tetanus neurotoxin

Solvent water interactions within the active site of the membrane type I matrix metalloproteinase

Matrix metalloproteinases (MMP) are an important family of proteases which catalyze the degradation of extracellular matrix components. While the mechanism of peptide cleavage is well established, the process of enzyme regeneration, which represents the rate limiting step of the catalytic cycle, remains unresolved. This step involves the loss of the newly formed N-terminus (amine) and C-terminus (carboxylate) protein fragments from the site of catalysis coupled with the inclusion of one or more solvent waters. Here we report a novel crystal structure of membrane type I MMP (MT1-MMP or MMP-14), which includes a small peptide bound at the catalytic Zn site via its C-terminus. This structure models the initial product state formed immediately after peptide cleavage but before the final proton transfer to the bound amine; the amine is not present in our system and as such proton transfer cannot occur. Modeling of the protein, including earlier structural data of Bertini and coworkers [I. Bertini, et al., Angew. Chem., Int. Ed., 2006, 45, 7952-7955], suggests that the C-terminus of the peptide is positioned to form an H-bond network to the amine site, which is mediated by a single oxygen of the functionally important Glu240 residue, facilitating efficient proton transfer. Additional quantum chemical calculations complemented with magneto-optical and magnetic resonance spectroscopies clarify the role of two additional, non-catalytic first coordination sphere waters identified in the crystal structure. One of these auxiliary waters acts to stabilize key intermediates of the reaction, while the second is proposed to facilitate C-fragment release, triggered by protonation of the amine. Together these results complete the enzymatic cycle of MMPs and provide new design criteria for inhibitors with improved efficacy.

An investigation of the pH dependence of copper-substituted anthrax lethal factor and its mechanistic implications

Anthrax lethal factor (LF) is a zinc-dependent endopeptidase involved in the cleavage of proteins critical to the maintenance of host signaling pathways during anthrax infections. Although zinc is typically regarded as the native metal ion in vivo, LF is highly tolerant to metal substitution, with its replacement by copper yielding an enzyme (CuLF) 4.5-fold more active than the native zinc protein (at pH 7). The current study demonstrates that by careful choice of the buffer, ionic strength, pH and substrate, the activity ratio of CuLF and native LF can be increased to >40-fold. Using a fluorogenic LF substrate, such optimized assay conditions can be exploited to detect LF concentrations as low as 2 pM. In contrast to the zinc form, CuLF was found to be inhibited by bromide and iodide ions, to be resistant to metal loss under acidic conditions, and to display a sharp pH dependence with significantly shifted alkaline limb towards more acidic conditions. The alkaline limb in the enzyme's pH profile is suggested to originate from changes in the protonation state of the metal-bound water molecule which serves as the nucleophile in the catalytic mechanism. Based on these observations and studies on other zinc proteases, a minimal mechanism for LF is proposed.

The Molecular Basis of Toxins' Interactions with Intracellular Signaling via Discrete Portals

An understanding of the molecular mechanisms by which microbial, plant or animal-secreted toxins exert their action provides the most important element for assessment of human health risks and opens new insights into therapies addressing a plethora of pathologies, ranging from neurological disorders to cancer, using toxinomimetic agents. Recently, molecular and cellular biology dissecting tools have provided a wealth of information on the action of these diverse toxins, yet, an integrated framework to explain their selective toxicity is still lacking. In this review, specific examples of different toxins are emphasized to illustrate the fundamental mechanisms of toxicity at different biochemical, molecular and cellular- levels with particular consideration for the nervous system. The target of primary action has been highlighted and operationally classified into 13 sub-categories. Selected examples of toxins were assigned to each target category, denominated as portal, and the modulation of the different portal’s signaling was featured. The first portal encompasses the plasma membrane lipid domains, which give rise to pores when challenged for example with pardaxin, a fish toxin, or is subject to degradation when enzymes of lipid metabolism such as phospholipases A₂ (PLA₂) or phospholipase C (PLC) act upon it. Several major portals consist of ion channels, pumps, transporters and ligand gated ionotropic receptors which many toxins act on, disturbing the intracellular ion homeostasis. Another group of portals consists of G-protein-coupled and tyrosine kinase receptors that, upon interaction with discrete toxins, alter second messengers towards pathological levels. Lastly, subcellular organelles such as mitochondria, nucleus, protein- and RNA-synthesis machineries, cytoskeletal networks and exocytic vesicles are also portals targeted and deregulated by other diverse group of toxins. A fundamental concept can be drawn from these seemingly different toxins with respect to the site of action and the secondary messengers and signaling cascades they trigger in the host. While the interaction with the initial portal is largely determined by the chemical nature of the toxin, once inside the cell, several ubiquitous second messengers and protein kinases/ phosphatases pathways are impaired, to attain toxicity. Therefore, toxins represent one of the most promising natural molecules for developing novel therapeutics that selectively target the major cellular portals involved in human physiology and diseases.

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.

The anthrax lethal factor and its MAPK kinase-specific metalloprotease activity

The anthrax lethal factor is a multi-domain protein toxin released by Bacillus anthracis which enters cells in a process mediated by the protective antigen and specific cell receptors. In the cytosol, the lethal factor cleaves the N-terminal tail of many MAPK kinases, thus deranging a major cell signaling pathway. The structural features at the basis of these activities of LF are reviewed here with particular attention to the proteolytic activity and to the identification of specific inhibitors. A significant similarity between the metalloprotease domain of the lethal factor and of that of the clostridial neurotoxins has been noted and is discussed.

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.

Transition Metal−Peptide Binding Studied by Metal-Catalyzed Oxidation Reactions and Mass Spectrometry

We have identified conditions that allow metal-catalyzed oxidation (MCO) reactions and mass spectrometry (MS) to correctly identify binding sites of first-row transition metal ions to model peptides. This work extends the applicability of the MCO/MS method to metals other than Cu(II). When the appropriate reducing agent (ascorbate, 10 mM) and oxidizing agent concentrations (1 mM persulfate, atmospheric O2, or both) are used, metal-bound amino acids can be sufficiently and specifically oxidized for clear identification by MS. The MCO reactions with Mn(II), Fe(II), Co(II), and Ni(II) occur to lesser extents than with Cu(II), but oxidation is still extensive enough to allow easy identification of the metal-bound residues. With the exception of aspartic acid, the known metal-binding amino acids of angiotensin I and bacitracin A are oxidized, while no oxidation is observed at nonbinding residues. Failure to oxidize aspartic acid is likely due to the relatively slow reactivity of its carboxylic acid side chain with reactive oxygen species, suggesting that the current MCO/MS protocol is transparent to such acidic residues. Overall, this study indicates that, just as is possible for Cu(II), the MCO/MS method should be suitable for determining the Mn(II)-, Fe(II)-, Co(II)-, and Ni(II)-binding sites of metalloproteins.

His1205 and His1223 are essential for the activity of the mitogenic Pasteurella multocida toxin

Pasteurella multocida produces a 146-kDa protein toxin (PMT), which activates multiple cellular signal-transduction pathways, resulting in the activation of PLCbeta, Rho, JNK, and ERK. In addition to an essential cysteine residue at position 1165, PMT contains several histidine residues in the catalytically important C-terminal part of the protein. To elucidate the role of the histidine residues, we treated PMT with the histidine-modifying substance diethyl pyrocarbonate (DEPC). DEPC inhibited PMT in a time- and concentration-dependent manner, suggesting that one or several histidine residues are essential for the biological activity of PMT. In experiments in which PMT was directly delivered into the cytosol of EBL cells by electroporation, we show that DEPC treatment inhibits the catalytically important histidine residues. Leucine substitutions of eight individual histidine residues in the C-terminal catalytic domain of PMT were constructed, and the effect on the biological activity of PMT was analyzed by determining PLCbeta, Rho, and ERK activation. Substitution of two histidine residues, H1205 and H1223, led to inactivation of the resulting PMT proteins, indicating that H1205 and H1223 play an important role in biological activity of the toxin. In addition, we show that the mutant toxins appear to be correctly folded, as judged by protease digestion. The precise function of H1205 and H1223 is not yet known. However, treatment of PMT with the cation chelating substance 1,10-phenantroline led to inactivation of the toxin, indicating that the essential histidine residues and cysteine 1165 might be involved in metal ion binding.

Botulinum and tetanus neurotoxins: structure, function and therapeutic utility

The toxic products of the anaerobic bacteria Clostridium botulinum, Clostridium butyricum, Clostridium barati and Clostridium tetani are the causative agents of botulism and tetanus. The ability of botulinum neurotoxins to disrupt neurotransmission, often for prolonged periods, has been exploited for use in several medical applications and the toxins, as licensed pharmaceutical products, now represent the therapeutics of choice for the treatment for several neuromuscular conditions. Research into the structures and activities of botulinum and tetanus toxins has revealed features of these proteins that might be useful in the design of improved vaccines, effective inhibitors and novel biopharmaceuticals. Here, we discuss the relationships between structure, mechanism of action and therapeutic use.

Active-site mutagenesis of tetanus neurotoxin implicates TYR-375 and GLU-271 in metalloproteolytic activity

Tetanus neurotoxin (TeNT) blocks neurotransmitter release by cleaving VAMP/synaptobrevin, a membrane associated protein involved in synaptic vesicle fusion. Such activity is exerted by the N-terminal 50kDa domain of TeNT which is a zinc-dependent endopeptidase (TeNT-L-chain). Based on the three-dimensional structure of botulinum neurotoxin serotype A (BoNT/A) and serotype B (BoNT/B), two proteins closely related to TeNT, and on X-ray scattering studies of TeNT, we have designed mutations at two active site residues to probe their involvement in activity. The active site of metalloproteases is composed of a primary sphere of residues co-ordinating the zinc atom, and a secondary sphere of residues that determines proteolytic specificity and activity. Glu-261 and Glu-267 directly co-ordinates the zinc atom in BoNT/A and BoNT/B respectively and the corresponding residue of TeNT was replaced by Asp or by the non conservative residue Ala. Tyr-365 is 4.3A away from zinc in BoNT/A, and the corresponding residue of TeNT was replaced by Phe or by Ala. The purified mutants had CD, fluorescence and UV spectra closely similar to those of the wild-type molecule. The proteolytic activity of TeNT-Asp-271 (E271D) is similar to that of the native molecule, whereas that of TeNT-Phe-375 (Y375F) is lower than the control. Interestingly, the two Ala mutants are completely devoid of enzymatic activity. These results demonstrate that both Glu-271 and Tyr-375 are essential for the proteolytic activity of TeNT.

The role of zinc binding in the biological activity of botulinum toxin

Botulinum toxin is a zinc-dependent endoprotease that acts on vulnerable cells to cleave polypeptides that are essential for exocytosis. To exert this poisoning effect, the toxin must proceed through a complex sequence of events that involves binding, productive internalization, and intracellular expression of catalytic activity. Results presented in this study show that soluble chelators rapidly strip Zn(2+) from its binding site in botulinum toxin, and this stripping of cation results in the loss of catalytic activity in cell-free or broken cell preparations. Stripped toxin is still active against intact neuromuscular junctions, presumably because internalized toxin binds cytosolic Zn(2+). In contrast to soluble chelators, immobilized chelators have no effect on bound Zn(2+), nor do they alter toxin activity. The latter finding is because of the fact that the spontaneous loss of Zn(2+) from its coordination site in botulinum toxin is relatively slow. When exogenous Zn(2+) is added to toxin that has been stripped by soluble chelators, the molecule rebinds cation and regains catalytic and neuromuscular blocking activity. Exogenous Zn(2+) can restore toxin activity either when the toxin is free in solution on the cell exterior or when it has been internalized and is in the cytosol. The fact that stripped toxin can reach the cytosol means that the loss of bound Zn(2+) does not produce conformational changes that block internalization. Similarly, the fact that stripped toxin in the cytosol can be reactivated by ambient Zn(2+) or exogenous Zn(2+) means that productive internalization does not produce conformational changes that block rebinding of cation.

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.

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.

Recombinant and truncated tetanus neurotoxin light chain: cloning, expression, purification, and proteolytic activity

Tetanus neurotoxin (TeNT) consists of two disulfide-linked polypeptide chains, heavy (H) and light (L). The L chain is a zinc endopeptidase protein highly specific for vesicle-associated membrane protein (VAMP), which is an essential component of the exocytosis apparatus. Here we describe the cloning of the L chain of TeNT from Clostridium tetani strain Y-IV-3 (WS 15) and its expression in Escherichia coli as a glutathione S-transferase fusion protein. The full-length recombinant L chain, corresponding to residues 1-457, was obtained as a mixture of proteins of slightly different mass with identical N-terminal ends. To obtain a product useful for structural analysis and crystallization, a COOH-terminally truncated L chain (residues 1-427) was cloned, expressed, and purified with high yield. This truncated L chain is more active than the full-length and wild-type proteins in the hydrolysis of VAMP. Preliminary experiments of crystallization of the truncated recombinant L chain gave encouraging results.

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.

Inhibition by tetanus toxin of sodium-dependent, high-affinity [3H]5-hydroxytryptamine uptake in rat synaptosomes

Tetanus toxin (TeTx) is a powerful clostridial neurotoxin that inhibits Ca2+-dependent neurotransmitter secretion as do the botulinum neurotoxins (BoNTs). We found that TeTx (but not BoNT/A) produced a specific time- and dose-dependent inhibition of Na+-dependent [3H]5-hydroxytryptamine (serotonin, 5-HT) uptake in rat CNS synaptosomes. This effect was found in all CNS tryptaminergic areas, being maximal in the hippocampus and occipital cortex. TeTx produced the maximum reduction in [3H]5-HT uptake after 30 min of preincubation, being significant also at lower doses (10(-12) M) or shorter incubation times (10 min). Serotonin transport inhibitors such as fenfluramine (IC50, 11.0 +/- 0.9 microM), paroxetine (IC50, 33.5 +/- 0.1 microM), and imipramine (IC50, 89.9 +/- 5.7 microM) were 3 or 4 orders of magnitude less potent than TeTx (IC50, 8.7 +/- 1.0 nM). Of the two fragments of TeTx, (the C-terminal portion of the neurotoxin heavy chain, which is responsible for the binding to the nerve tissue) was consistently more effective than the L-H(N) fragment (the light neurotoxin chain disulfide linked to the N-terminal portion of the heavy chain, which is responsible for the toxic metalloprotease action) as inhibitor of [3H]5-HT uptake in synaptosomal preparations (56 +/- 5% and 95 +/- 3% with respect to control, respectively). Antagonism of the toxin-induced [3H]5-HT uptake blockade could not be reversed by zinc chelators but did have the ability to antagonize the TeTx inhibition of basal and K+-evoked [3H]5-HT release in rat synaptosomes. The reduction in serotonin accumulation induced by TeTx could be responsible for some tetanic symptoms that have been related to the serotonergic system.