Expression, purification, and efficacy of the type A botulinum neurotoxin catalytic domain fused to two translocation domain variants

High Yield Preparation of Functionally Active Catalytic-Translocation Domain Module of Botulinum Neurotoxin Type A That Exhibits Uniquely Different Enzyme Kinetics

Botulinum neurotoxins (BoNTs) are the most toxic proteins known to cause flaccid muscle paralysis as a result of inhibition of neurotransmitter release from peripheral cholinergic synapses. BoNT type A (BoNT/A) is a 150 kDa protein consisting of two major subunits: light chain (LC) and heavy chain (HC). The LC is required for the catalytic activity of neurotoxin, whereas the C and N terminal domains of the HC are required for cell binding, and translocation of LC across the endosome membranes, respectively. To better understand the structural and functional aspects of BoNT/A intoxication we report here the development of high yield Escherichia coli expression system (2-20-fold higher yield than the value reported in the literature) for the production of recombinant light chain-translocation domain (rLC-TD/A) module of BoNT/A which is catalytically active and translocation competent. The open reading frame of rLC-TD/A was PCR amplified from deactivated recombinant BoNT/A gene (a non-select agent reagent), and was cloned using pET45b (+) vector to express in E. coli cells. The purification procedure included a sequential order of affinity chromatography, trypsinization, and anion exchange column chromatography. We were able to purify > 95% pure, catalytically active and structurally well-folded protein. Comparison of enzyme kinetics of purified LC-TD/A to full-length toxin and recombinant light chain A suggest that the affinity for the substrate is in between endopeptidase domain and botulinum toxin. The potential application of the purified protein has been discussed in toxicity and translocation assays.

Utilizing Ayurvedic literature for the identification of novel phytochemical inhibitors of botulinum neurotoxin A

ETHNOPHARMACOLOGICAL RELEVANCE

Ayurveda, an ancient holistic system of health care practiced on the Indian subcontinent, utilizes a number of multi-plant formulations and is considered by many as a potential source for novel treatments, as well as the identification of new drugs. Our aim is to identify novel phytochemicals for the inhibition of bacterial exotoxin, botulinum neurotoxin A (BoNT/A) based on Ayurvedic literature. BoNT/A is released by Clostridium species, which when ingested, inhibits the release of acetylcholine by concentrating at the neuromuscular junction and causes flaccid paralysis, resulting in a condition termed as botulism, and may also lead to death due to respiratory arrest.

METHODS

Fifteen plants were selected from the book 'Diagnosis and treatment of diseases in Ayurveda' by Vaidya Bhagwan Dash and Lalitesh Kashyap, based on their frequency of use in the formulations used for the treatment of six diseases with neuromuscular symptoms similar to botulism. Phytochemicals from these plants were screened using in silico, and in vitro methods. Structures of 570 reported phytochemicals from 14 plants were docked inside six reported BoNT/A light chain crystal structures using ensemble docking module in Maestro (Schrödinger, LLE).

RESULTS

From the docking scores and structural diversity, nine compounds including acoric acid 1, three flavonoids, three coumarins derivatives, one kava lactone were selected and screened using an in vitro HPLC-based protease assay. The bioassay results showed that several compounds possess BoNT/A LC inhibition of 50-60% when compared to positive controls NSC 84094 and CB7967495 (80-95%).

CONCLUSION

Further testing of the active compounds identified from Ayurvedic literature and structure-activity studies of acoric acid 1 using more sensitive bioassays is under way. The identification of acoric acid 1, a novel scaffold against BoNT/A, exemplifies the utility of Ayurvedic literature for the discovery of novel drug leads.

Expression and purification of recombinant TAT-BoNT/A(1-448) under denaturing and native conditions

Botulinum toxin type A can temporarily inhibit muscle contraction. Currently, physicians administer this toxin as a bio-drug in treatment of some muscle contraction disorders. TAT-BoNT/A(1-448) is a functional recombinant protein derived from botulinum toxin light chain. Unlike the full length botulinum toxin, TAT-BoNT/A(1-448) is a self-permeable molecule which can pass through bio-surfaces so can be used as a topical therapeutic agent without injection. To maintain the functionality of TAT-BoNT/A(1-448), it is necessary to restore its normal folding upon expression and purification. In this study, we have investigated and optimized expression conditions for this novel recombinant protein. Under denaturing condition (1 mM IPTG, at 37°C), the chimeric protein was produced as inclusion body and required to be purified using denaturing agents (e.g. urea). Yet, lower incubation temperature (18°C) and less IPTG concentration (0.5 mM) induce a protein under native condition. In such condition, about 60% of the chimeric protein was expressed in soluble form.

TAT-BoNT/A(₁₋₄₄₈), a novel fusion protein as a therapeutic agent: analysis of transcutaneous delivery and enzyme activity

Botulinum neurotoxin type A (BoNT/A) has been used as an injectable therapeutic agent for the treatment of some abnormal muscle contractions. In this study, TAT(47-57) peptide, a cell-penetrating peptide, was fused with the catalytic domain of BoNT/A for therapeutic purposes. HeLa and BE(2)-C cell lines were treated separately with purified TAT-BoNT/A(1-448) recombinant protein, and transduction of protein was analyzed by western blotting. Also, transcutaneous delivery through mouse skin surface was evaluated by immunohistochemistry. The in vitro catalytic activity of TAT-BoNT/A(1-448) was evaluated by HPLC. The presence of recombinant protein was detected in both of the cell lines as well as mouse skin cryosections after 60 and 120 min of incubation. The concentration of intracellular proteins was increased over time. HPLC analysis showed that this fusion protein has a biological activity 1.5 times as much as the full-length BoNT/A(1-448) protein. TAT-BoNT/A(1-448) fusion protein is biologically active and can transmit through living cells in vitro and in vivo successfully and more effectively compared with BoNT/A(1-448) protein as control.

Monoclonal Antibodies Targeting the Alpha-Exosite of Botulinum Neurotoxin Serotype/A Inhibit Catalytic Activity

The paralytic disease botulism is caused by botulinum neurotoxins (BoNT), multi-domain proteins containing a zinc endopeptidase that cleaves the cognate SNARE protein, thereby blocking acetylcholine neurotransmitter release. Antitoxins currently used to treat botulism neutralize circulating BoNT but cannot enter, bind to or neutralize BoNT that has already entered the neuron. The light chain endopeptidase domain (LC) of BoNT serotype A (BoNT/A) was targeted for generation of monoclonal antibodies (mAbs) that could reverse paralysis resulting from intoxication by BoNT/A. Single-chain variable fragment (scFv) libraries from immunized humans and mice were displayed on the surface of yeast, and 19 BoNT/A LC-specific mAbs were isolated by using fluorescence-activated cell sorting (FACS). Affinities of the mAbs for BoNT/A LC ranged from a K_D value of 9.0×10⁻¹¹ M to 3.53×10⁻⁸ M (mean K_D 5.38×10⁻⁹ M and median K_D 1.53×10⁻⁹ M), as determined by flow cytometry analysis. Eleven mAbs inhibited BoNT/A LC catalytic activity with IC₅₀ values ranging from 8.3 ~73×10⁻⁹ M. The fine epitopes of selected mAbs were also mapped by alanine-scanning mutagenesis, revealing that the inhibitory mAbs bound the α-exosite region remote from the BoNT/A LC catalytic center. The results provide mAbs that could prove useful for intracellular reversal of paralysis post-intoxication and further define epitopes that could be targeted by small molecule inhibitors.

Cleavage of SNAP25 and its shorter versions by the protease domain of serotype A botulinum neurotoxin

Various substrates, catalysts, and assay methods are currently used to screen inhibitors for their effect on the proteolytic activity of botulinum neurotoxin. As a result, significant variation exists in the reported results. Recently, we found that one source of variation was the use of various catalysts, and have therefore evaluated its three forms. In this paper, we characterize three substrates under near uniform reaction conditions using the most active catalytic form of the toxin. Bovine serum albumin at varying optimum concentrations stimulated enzymatic activity with all three substrates. Sodium chloride had a stimulating effect on the full length synaptosomal-associated protein of 25 kDa (SNAP25) and its 66-mer substrates but had an inhibitory effect on the 17-mer substrate. We found that under optimum conditions, full length SNAP25 was a better substrate than its shorter 66-mer or 17-mer forms both in terms of kcat, Km, and catalytic efficiency kcat/Km. Assay times greater than 15 min introduced large variations and significantly reduced the catalytic efficiency. In addition to characterizing the three substrates, our results identify potential sources of variations in previous published results, and underscore the importance of using well-defined reaction components and assay conditions.

High level expression, purification and immunogenicity analysis of a protective recombinant protein against botulinum neurotoxin type E

Botulinum neurotoxin type E heavy chain consists of two domains: N-terminal half as a translocation domain and C-terminal half (Hcc) as a binding domain. In this research a synthetic gene fragment encoding the binding domain of botulinum neurotoxin type E (BoNT/E-Hcc) was highly expressed in Escherichia coli by pGEX4T-1 vector. After purification, the recombinant BoNT/E-Hcc was evaluated by SDS-PAGE and western blot (immunoblot) analysis. Average yields obtained in this research were 3.7 mg recombinant BoNT/E-Hcc per liter of bacterial culture. The recombinant protein was injected in mice for study of its protection ability against botulinum neurotoxin type E challenges. The challenge studies showed that, vaccinated mice were fully protected against 10⁴ × minimum lethal dose of botulinum neurotoxin type E.

What next for botulism vaccine development?

Botulism is a severe neuroparalytic disease caused by the toxins produced from several Clostridium species. Botulinum neurotoxins (BoNTs) cause flaccid paralysis by inducing a blockade at voluntary motor and autonomic cholinergic junctions that, if not treated, can be fatal. Vaccination to elicit protective circulating antibodies that bind, neutralize and clear toxins before they can be internalized and affect cholinergic neurons remains the most effective form of protection against BoNT. A pentavalent BoNT toxoid vaccine administered in the USA under an Investigational New Drug protocol to at-risk workers was discontinued by the CDC in 2011 due to diminished potency and reactogenic effects. Subsequent research efforts have primarily focused on recombinant protein antigens. This review focuses on the development of a recombinant bivalent vaccine (rBV A/B) composed of purified recombinant BoNT/A and BoNT/B receptor-binding domain proteins, as well as presenting a summary of progress and issues associated with alternative vaccines currently being developed against botulism.

The C terminus of the catalytic domain of type A botulinum neurotoxin may facilitate product release from the active site

Botulinum neurotoxins are the most toxic of all compounds. The toxicity is related to a poor zinc endopeptidase activity located in a 50-kDa domain known as light chain (Lc) of the toxin. The C-terminal tail of Lc is not visible in any of the currently available x-ray structures, and it has no known function but undergoes autocatalytic truncations during purification and storage. By synthesizing C-terminal peptides of various lengths, in this study, we have shown that these peptides competitively inhibit the normal catalytic activity of Lc of serotype A (LcA) and have defined the length of the mature LcA to consist of the first 444 residues. Two catalytically inactive mutants also inhibited LcA activity. Our results suggested that the C terminus of LcA might interact at or near its own active site. By using synthetic C-terminal peptides from LcB, LcC1, LcD, LcE, and LcF and their respective substrate peptides, we have shown that the inhibition of activity is specific only for LcA. Although a potent inhibitor with a Ki of 4.5 μm, the largest of our LcA C-terminal peptides stimulated LcA activity when added at near-stoichiometric concentration to three versions of LcA differing in their C-terminal lengths. The result suggested a product removal role of the LcA C terminus. This suggestion is supported by a weak but specific interaction determined by isothermal titration calorimetry between an LcA C-terminal peptide and N-terminal product from a peptide substrate of LcA. Our results also underscore the importance of using a mature LcA as an inhibitor screening target.

Tyrosine phosphorylation of botulinum neurotoxin protease domains

Botulinum neurotoxins are most potent of all toxins. Their N-terminal light chain domain (Lc) translocates into peripheral cholinergic neurons to exert its endoproteolytic action leading to muscle paralysis. Therapeutic development against these toxins is a major challenge due to their in vitro and in vivo structural differences. Although three-dimensional structures and reaction mechanisms are very similar, the seven serotypes designated A through G vastly vary in their intracellular catalytic stability. To investigate if protein phosphorylation could account for this difference, we employed Src-catalyzed tyrosine phosphorylation of the Lc of six serotypes namely LcA, LcB, LcC1, LcD, LcE, and LcG. Very little phosphorylation was observed with LcD and LcE but LcA, LcB, and LcG were maximally phosphorylated by Src. Phosphorylation of LcA, LcB, and LcG did not affect their secondary and tertiary structures and thermostability significantly. Phosphorylation of Y250 and Y251 made LcA resistant to autocatalysis and drastically reduced its k_cat/K_m for catalysis. A tyrosine residue present near the essential cysteine at the C-terminal tail of LcA, LcB, and LcG was readily phosphorylated in vitro. Inclusion of a competitive inhibitor protected Y426 of LcA from phosphorylation, shedding light on the role of the C-terminus in the enzyme’s substrate or product binding.

Basic tetrapeptides as potent intracellular inhibitors of type A botulinum neurotoxin protease activity

Botulinum neurotoxins (BoNT) are the most potent of all toxins that cause flaccid muscle paralysis leading to death. They are also potential biothreat agents. A systematic investigation of various short peptide inhibitors of the BoNT protease domain with a 17-residue peptide substrate led to arginine-arginine-glycine-cysteine having a basic tetrapeptide structure as the most potent inhibitor. When assayed in the presence of dithiothreitol (DTT), the inhibitory effect was drastically reduced. Replacing the terminal cysteine with one hydrophobic residue eliminated the DTT effect but with two hydrophobic residues made the pentapeptide a poor inhibitor. Replacing the first arginine with cysteine or adding an additional cysteine at the N terminus did not improve inhibition. When assessed using mouse brain lysates, the tetrapeptides also inhibited BoNT/A cleavage of the endogenous SNAP-25. The peptides penetrated the neuronal cell lines, N2A and BE(2)-M17, without adversely affecting metabolic functions as measured by ATP production and P-38 phosphorylation. Biological activity of the peptides persisted within cultured chick motor neurons and rat and mouse cerebellar neurons for more than 40 h and inhibited BoNT/A protease action inside the neurons in a dose- and time-dependent fashion. Our results define a tetrapeptide as the smallest peptide inhibitor in the backdrop of a large substrate protein of 200+ amino acids having multiple interaction regions with its cognate enzyme. The inhibitors should also be valuable candidates for drug development.

Light chain separated from the rest of the type a botulinum neurotoxin molecule is the most catalytically active form

Botulinum neurotoxins (BoNT) are the most potent of all toxins. The 50 kDa N-terminal endopeptidase catalytic light chain (LC) of BoNT is located next to its central, putative translocation domain. After binding to the peripheral neurons, the central domain of BoNT helps the LC translocate into cytosol where its proteolytic action on SNARE (soluble NSF attachment protein receptor) proteins blocks exocytosis of acetyl choline leading to muscle paralysis and eventual death. The translocation domain also contains 105 Å -long stretch of ∼100 residues, known as “belt,” that crosses over and wraps around the LC to shield the active site from solvent. It is not known if the LC gets dissociated from the rest of the molecule in the cytosol before catalysis. To investigate the structural identity of the protease, we prepared four variants of type A BoNT (BoNT/A) LC, and compared their catalytic parameters with those of BoNT/A whole toxin. The four variants were LC + translocation domain, a trypsin-nicked LC + translocation domain, LC + belt, and a free LC. Our results showed that K_m for a 17-residue SNAP-25 (synaptosomal associated protein of 25 kDa) peptide for these constructs was not very different, but the turnover number (k_cat) for the free LC was 6-100-fold higher than those of its four variants. Moreover, none of the four variants of the LC was prone to autocatalysis. Our results clearly demonstrated that in vitro, the LC minus the rest of the molecule is the most catalytically active form. The results may have implication as to the identity of the active, toxic moiety of BoNT/A in vivo.

Extreme sensitivity of botulinum neurotoxin domains towards mild agitation

Botulinum neurotoxins (BoNTs) and their fragments are targets of therapeutic developments and are increasingly used as therapeutic, prophylactic, and research reagents. However, published data on their properties vary widely. In order to gain a better understanding of these variations, we initiated a systematic investigation of the stability parameters of catalytic light chains (Lc) as well as of cell surface binding domains (Hc) of the neurotoxin. When followed by CD spectroscopy, we noticed that the recombinant light chains of serotypes A (LcA), B, D, E, and G rapidly lost their secondary structures by mild stirring. Denaturation of LcA increased with stirring speed and temperature resulting in a catalytically inactive precipitate. Reducing agents or an anaerobic environment were ineffective in the denaturation. Under identical conditions, bovine serum albumin, ovalbumin, carboxypeptidase B, and of thermolysin, a structural and functional analogue of LcA, remained unchanged. Hc domains of serotype A, B, C, E, and F were also denatured by mild stirring. Adding the nonionic detergent Tween-20 to LcA completely prevented the denaturation. We speculate that the BoNT domains undergo surface denaturation due to rapid exposure of hydrophobic residues by mechanical agitation. This study has important implications for handling BoNT proteins used in therapeutic development.

Identification and biochemical characterization of small-molecule inhibitors of Clostridium botulinum neurotoxin serotype A

An integrated strategy that combined in silico screening and tiered biochemical assays (enzymatic, in vitro, and ex vivo) was used to identify and characterize effective small-molecule inhibitors of Clostridium botulinum neurotoxin serotype A (BoNT/A). Virtual screening was initially performed by computationally docking compounds of the National Cancer Institute (NCI) database into the active site of BoNT/A light chain (LC). A total of 100 high-scoring compounds were evaluated in a high-performance liquid chromatography (HPLC)-based protease assay using recombinant full-length BoNT/A LC. Seven compounds that significantly inhibited the BoNT/A protease activity were selected. Database search queries of the best candidate hit [7-((4-nitro-anilino)(phenyl)methyl)-8-quinolinol (NSC 1010)] were performed to mine its nontoxic analogs. Fifty-five analogs of NSC 1010 were synthesized and examined by the HPLC-based assay. Of these, five quinolinol derivatives that potently inhibited both full-length BoNT/A LC and truncated BoNT/A LC (residues 1 to 425) were selected for further inhibition studies in neuroblastoma (N2a) cell-based and tissue-based mouse phrenic nerve hemidiaphragm assays. Consistent with enzymatic assays, in vitro and ex vivo studies revealed that these five quinolinol-based analogs effectively neutralized BoNT/A toxicity, with CB 7969312 exhibiting ex vivo protection at 0.5 microM. To date, this is the most potent BoNT/A small-molecule inhibitor that showed activity in an ex vivo assay. The reduced toxicity and high potency demonstrated by these five compounds at the biochemical, cellular, and tissue levels are distinctive among the BoNT/A small-molecule inhibitors reported thus far. This study demonstrates the utility of a multidisciplinary approach (in silico screening coupled with biochemical testing) for identifying promising small-molecule BoNT/A inhibitors.

Production of catalytically inactive BoNT/A1 holoprotein and comparison with BoNT/A1 subunit vaccines against toxin subtypes A1, A2, and A3

A recombinant, catalytically inactive Clostridium botulinum neurotoxin A1 holoprotein (ciBoNT/A1 HP) was constructed by introducing amino acid substitutions H223A, E224A, and H227A in the active site to ablate proteolytic activity. ciBoNT/A1 HP was produced in the yeast Pichia pastoris and the purified product was evaluated as a vaccine candidate by comparison against recombinant BoNT/A1 LC, LC-belt, LC-H(n), and H(c) antigens and a LC-H(n)+H(c) combination in mouse potency and efficacy bioassays when challenged with BoNT/A subtypes /A1, /A2, and /A3. A single dose of ciBoNT/A1 HP provided equivalent or greater protective immunity, not only against the homologous toxin, but also against two distinct toxin subtypes with significant amino acid divergence. Only the LC-H(n)+H(c) combination provided comparable protection against /A1; however, it was less effective against subtypes /A2 and /A3. Differences in protective immunity diminished after multiple vaccinations with either ciBoNT/A1 HP or BoNT/A1 H(c), and the survival rates were more comparable at the toxin levels used to challenge.

Extraction and inhibition of enzymatic activity of botulinum neurotoxins/A1, /A2, and /A3 by a panel of monoclonal anti-BoNT/A antibodies

Botulinum neurotoxins (BoNTs) are extremely potent toxins that are capable of causing death or respiratory failure leading to long-term intensive care. Treatment includes serotype-specific antitoxins, which must be administered early in the course of the intoxication. Rapidly determining human exposure to BoNT is an important public health goal. In previous work, our laboratory focused on developing Endopep-MS, a mass spectrometry-based endopeptidase method for detecting and differentiating BoNT/A–G serotypes in buffer and BoNT/A, /B, /E, and /F in clinical samples. We have previously reported the effectiveness of antibody-capture to purify and concentrate BoNTs from complex matrices, such as clinical samples. Because some antibodies inhibit or neutralize the activity of BoNT, the choice of antibody with which to extract the toxin is critical. In this work, we evaluated a panel of 16 anti-BoNT/A monoclonal antibodies (mAbs) for their ability to inhibit the in vitro activity of BoNT/A1, /A2, and /A3 complex as well as the recombinant LC of A1. We also evaluated the same antibody panel for the ability to extract BoNT/A1, /A2, and /A3. Among the mAbs, there were significant differences in extraction efficiency, ability to extract BoNT/A subtypes, and inhibitory effect on BoNT catalytic activity. The mAbs binding the C-terminal portion of the BoNT/A heavy chain had optimal properties for use in the Endopep-MS assay.

Bivalent recombinant vaccine for botulinum neurotoxin types A and B based on a polypeptide comprising their effector and translocation domains that is protective against the predominant A and B subtypes

The botulinum neurotoxins (BoNTs) are a large family of extremely potent, neuroparalytic, dichain proteins which act at the peripheral nervous system. The wide genetic diversity observed with this neurotoxin family poses a significant challenge for the development of an effective botulinum vaccine. The present study describes a vaccine development platform based on protein fragments representing the N-terminal two-thirds of each toxin molecule. These fragments, designated LH(N), comprise the light chain and translocation domains of each neurotoxin and are devoid of any neuron-binding activity. Using codon-optimized genes, LH(N) fragments derived from BoNT serotypes A and B were expressed in Escherichia coli in high yield with >1 g of purified, soluble fragment recoverable from 4.5 liter-scale fermentations. The protective efficacy of LH(N)/A was significantly enhanced by treatment with formaldehyde, which induced intramolecular cross-linking but virtually no aggregation of the fragment. A single immunization of the modified fragment protected mice from challenge with a 10(3) 50% lethal dose (LD(50)) of BoNT/A(1) with an 50% effective dose (ED(50)) of 50 ng of the vaccine. In similar experiments, the LH(N)/A vaccine was shown to protect mice against challenge with BoNT/A subtypes A(1), A(2), and A(3), which is the first demonstration of single-dose protection by a vaccine against the principal toxin subtypes of BoNT/A. The LH(N)/B vaccine was also highly efficacious, giving an ED(50) of approximately 140 ng to a challenge of 10(3) LD(50) of BoNT/B(1). In addition, LH(N)/B provided single-dose protection in mice against BoNT/B(4) (nonproteolytic toxin subtype).

Localization of the sites and characterization of the mechanisms by which anti-light chain antibodies neutralize the actions of the botulinum holotoxin

The recombinant, catalytically active light chain of botulinum toxin type A was evaluated as a potential vaccine candidate. Previous studies have shown that the light chain can elicit protective immunity in vivo. [Kiyatkin N, Maksymowych AB, Simpson LL. Induction of immune response by oral administration of recombinant botulinum toxin. Infect Immun 1997;65(11):4586-91], but the underlying basis for this observation was not determined. In the present study, antibodies directed against the light chain were shown to act at three different sites in the body to produce neutralization. Firstly, these antibodies acted to block toxin absorption into the body. This was demonstrated in vitro, in studies on binding and transport of toxin across epithelial monolayers, and in vivo, in studies on inhalation poisoning. Secondly, anti-light chain antibodies acted to promote clearance of toxin from the general circulation. This was demonstrated in vivo in studies on toxin levels in blood and in parallel studies on toxin accumulation in liver and spleen. Finally, anti-light chain antibodies acted to protect cholinergic nerves from botulinum toxin action. This was demonstrated in two types of in vitro assays: rate of paralysis of murine phrenic nerve-hemidiaphragm preparations and extent of binding to Neuro-2a cells. When taken together, these data show that anti-light chain antibodies can evoke three layers of protection against botulinum toxin.

Identification of residues surrounding the active site of type A botulinum neurotoxin important for substrate recognition and catalytic activity

Type A botulinum neurotoxin is one of the most lethal of the seven serotypes and is increasingly used as a therapeutic agent in neuromuscular dysfunctions. Its toxic function is related to zinc-endopeptidase activity of the N-terminal light chain (LC) on synaptosome-associated protein-25 kDa (SNAP-25) of the SNARE complex. To understand the determinants of substrate specificity and assist the development of strategies for effective inhibitors, we used site-directed mutagenesis to investigate the effects of 13 polar residues of the LC on substrate binding and catalysis. Selection of the residues for mutation was based on a computational analysis of the three-dimensional structure of the LC modeled with a 17-residue substrate fragment of SNAP-25. Steady-state kinetic parameters for proteolysis of the substrate fragment were determined for a set of 16 single mutants. Of the mutated residues non-conserved among the serotypes, replacement of Arg-230 and Asp-369 by polar or apolar residues resulted in drastic lowering of the catalytic rate constant (k(ca)), but had less effect on substrate affinity (K(m)). Substitution of Arg-230 with Lys decreased the catalytic efficiency (k(cat)/K(m)) by 50-fold, whereas replacement by Leu yielded an inactive protein. Removal of the electrostatic charge at Asp-369 by mutation to Asn resulted in 140-fold decrease in k(cat)/K(m). Replacement of other variable residues surrounding the catalytic cleft (Glu-54, Glu-63, Asn-66, Asp-130, Asn-161, Glu-163, Glu-170, Glu-256), had only marginal effect on decreasing the catalytic efficiency, but unexpectedly the substitution of Lys-165 with Leu resulted in fourfold increase in k(cat)/K(m). For comparison purposes, two conserved residues Arg-362 and Tyr-365 were investigated with substitutions of Leu and Phe, respectively, and their catalytic efficiency decreased 140- and 10-fold, respectively, whereas substitution of the tyrosine ring with Asn abolished activity. The altered catalytic efficiencies of the mutants were not due to any significant changes in secondary or tertiary structures, or in zinc content and thermal stability. We suggest that, despite the large minimal substrate size for catalysis, only a few non-conserved residues surrounding the active site are important to render the LC competent for catalysis or provide conformational selection of the substrate.

Fine and domain-level epitope mapping of botulinum neurotoxin type A neutralizing antibodies by yeast surface display

Botulinum neurotoxin (BoNT), the most poisonous substance known, causes naturally occurring human disease (botulism) and is one of the top six biothreat agents. Botulism is treated with polyclonal antibodies produced in horses that are associated with a high incidence of systemic reactions. Human monoclonal antibodies (mAbs) are under development as a safer therapy. Identifying neutralizing epitopes on BoNTs is an important step in generating neutralizing mAbs, and has implications for vaccine development. Here, we show that the three domains of BoNT serotype A (BoNT/A) can be displayed on the surface of yeast and used to epitope map six mAbs to the toxin domains they bind. The use of yeast obviates the need to express and purify each domain, and it should prove possible to display domains of other BoNT subtypes and serotypes for epitope mapping. Using a library of yeast-displayed BoNT/A binding domain (H(C)) mutants and selecting for loss of binding, the fine epitopes of three neutralizing BoNT/A mAbs were identified. Two mAbs bind the C-terminal subdomain of H(C), with one binding near the toxin sialoganglioside binding site. The most potently neutralizing mAb binds the N-terminal subdomain of H(C), in an area not previously thought to be functionally important. Modeling the epitopes shows how all three mAbs could bind BoNT/A simultaneously and may explain, in part, the dramatic synergy observed on in vivo toxin neutralization when these antibodies are combined. The results demonstrate how yeast display can be used for domain-level and fine mapping of conformational BoNT antibody epitopes and the mapping results identify three neutralizing BoNT/A epitopes.

Expression, purification, and characterization of Clostridium botulinum type B light chain

A full-length synthetic gene encoding the light chain of botulinum neurotoxin serotype B, approximately 50 kDa (BoNT/B LC), has been cloned into a bacterial expression vector pET24a+. BoNT/B LC was expressed in Escherichia coli BL21.DE3.pLysS and isolated from the soluble fraction. The resultant protein was purified to homogeneity by cation chromatography and was determined to be >98% pure as assessed by SDS-polyacrylamide gel stained with SilverXpress and analyzed by densitometry. Mass spectroscopic analysis indicated the protein to be 50.8 kDa, which equaled the theoretically expected mass. N-terminal sequencing of the purified protein showed the sequence corresponded to the known reported sequence. The recombinant BoNT/B light chain was found to be highly stable, catalytically active, and has been used to prepare antisera that neutralizes against BoNT/B challenge. Characterization of the protein including pH, temperature, and the stability of the protein in the presence or absence of zinc is described within. The influence of pH differences, buffer, and added zinc on secondary and tertiary structure of BoNT/B light chain was analyzed by circular dichroism and tryptophan fluorescence measurements. Optimal conditions for obtaining maximum metalloprotease activity and stabilizing the protein for long term storage were determined. We further analyzed the thermal denaturation of BoNT/B LC as a function of temperature to probe the pH and added zinc effects on light chain stability. The synthetic BoNT/B LC has been found to be highly active on its substrate (vesicle associated membrane protein-2) and, therefore, can serve as a useful reagent for BoNT/B research.

Preparation of specifically activatable endopeptidase derivatives of Clostridium botulinum toxins type A, B, and C and their applications

Clostridium botulinum neurotoxins are potently toxic proteins of 150 kDa with specific endopeptidase activity for SNARE proteins involved in vesicle docking and release. Following treatment with trypsin, a fragment of botulinum neurotoxin serotype A that lacks the C-terminal domain responsible for neuronal cell binding, but retains full catalytic activity, can be obtained. Known as the LH(N) fragment, we report the development of a recombinant expression and purification scheme for the isolation of comparable fragments of neurotoxin serotypes B and C. Expressed as maltose-binding protein fusions, both have specific proteolytic sites present between the fusion tag and the light chain to facilitate removal of the fusion, and between the light chain endopeptidase and the H(N) translocation domains to facilitate activation of the single polypeptide. We have also used this approach to prepare a new variant of LH(N)/A with a specific activation site that avoids the need to use trypsin. All three LH(N)s are enzymatically active and are of low toxicity. The production of specifically activatable LH(N)/A, LH(N)/B, and LH(N)/C extends the opportunities for exploitation of neurotoxin fragments. The potential utility of these fragments is discussed.

Inhibition of the Protease Activity of the Light Chain of Type A Botulinum Neurotoxin by Aqueous Extract from Stinging Nettle (Urtica dioica) Leaf

We investigated the inhibitory effect of stinging nettle leaf extract on the protease activity of botulinum neurotoxin type A and B light chains. The nettle leaf infusion was fractionated and HPLC-based enzymatic assays were performed to determine the capacity of each fraction to inhibit the protease activity of botulinum neurotoxin type A and B light chains. Assay results demonstrated that a water-soluble fraction obtained from the nettle leaf infusion inhibited type A, but did not inhibit type B light chain protease activity. The inhibition mode of water soluble fraction against protease activity of type A light chain was analyzed and found to be a non-competitive.