Structural determinants of scorpion toxin affinity: the charybdotoxin (alpha-KTX) family of K(+)-channel blocking peptides

Optimization of Pichia pastoris Expression System for High-Level Production of Margatoxin

Margatoxin (MgTx) is a high-affinity blocker of voltage-gated potassium (Kv) channels. It inhibits Kv1.1–Kv1.3 ion channels in picomolar concentrations. This toxin is widely used to study physiological function of Kv ion channels in various cell types, including immune cells. Isolation of native MgTx in large quantities from scorpion venom is not affordable. Chemical synthesis and recombinant production in Escherichia coli need in vitro oxidative refolding for proper disulfide bond formation, resulting in a very low yield of peptide production. The Pichia pastoris expression system offers an economical approach to overcome all these limitations and gives a higher yield of correctly refolded recombinant peptides. In this study, improved heterologous expression of recombinant MgTx (rMgTx) in P. pastoris was obtained by using preferential codons, selecting the hyper-resistant clone against Zeocin, and optimizing the culturing conditions. About 36 ± 4 mg/L of >98% pure His-tagged rMgTx (TrMgTx) was produced, which is a threefold higher yield than has been previously reported. Proteolytic digestion of TrMgTx with factor Xa generated untagged rMgTx (UrMgTx). Both TrMgTx and UrMgTx blocked the Kv1.2 and Kv1.3 currents (patch-clamp) (K_d for Kv1.2 were 64 and 14 pM, and for Kv1.3, 86 and 50 pM, respectively) with comparable potency to the native MgTx. The analysis of the binding kinetics showed that TrMgTx had a lower association rate than UrMgTx for both Kv1.2 and Kv1.3. The dissociation rate of both the analogues was the same for Kv1.3. However, in the case of Kv1.2, TrMgTx showed a much higher dissociation rate with full recovery of the block than UrMgTx. Moreover, in a biological functional assay, both peptides significantly downregulated the expression of early activation markers IL2R and CD40L in activated CD4⁺ T_EM lymphocytes whose activation was Kv1.3 dependent. In conclusion, the authors report that the Pichia expression system is a powerful method to produce disulfide-rich peptides, the overexpression of which could be enhanced noticeably through optimization strategies, making it more cost-effective. Since the presence of the His-tag on rMgTx only mildly altered the block equilibrium and binding kinetics, recombinant toxins could be used in ion channel research without removing the tag and could thus reduce the cost and time demand for toxin production.

Phylloseptin-1 is Leishmanicidal for Amastigotes of Leishmania amazonensis Inside Infected Macrophages

Leishmania protozoans are the causal agents of neglected diseases that represent an important public health issue worldwide. The growing occurrence of drug-resistant strains of Leishmania and severe side effects of available treatments represent an important challenge for the leishmaniases treatment. We have previously reported the leishmanicidal activity of phylloseptin-1 (PSN-1), a peptide found in the skin secretion of Phyllomedusa azurea (=Pithecopus azureus), against Leishmania amazonensis promastigotes. However, its impact on the amastigote form of L. amazonensis and its impact on infected macrophages are unknown. In this work, we evaluated the effects of PSN-1 on amastigotes of L. amazonensis inside macrophages infected in vitro. We assessed the production of hydrogen peroxide and nitric oxide, as well as the levels of inflammatory and immunomodulatory markers (TGF-β, TNF-α and IL-12), in infected and non-infected macrophages treated with PSN-1. Treatment with PSN-1 decreased the number of infected cells and the number of ingested amastigotes per cell when compared with the untreated cells. At 32 µM (64 µg/mL), PSN-1 reduced hydrogen peroxide levels in both infected and uninfected macrophages, whereas it had little effect on NO production or TGF-β release. The effect of PSN-1 on IL-12 and TNF-α secretion depended on its concentration, but, in general, their levels tended to increase as PSN-1 concentration increased. Further in vitro and in vivo studies are needed to clarify the mechanisms of action of PSN-1 and its interaction with the immune system aiming to develop pharmacological applications.

Fusion expression and purification of four disulfide-rich peptides reveals enterokinase secondary cleavage sites in animal toxins

Animal toxins are powerful tools for testing the pharmacological, physiological, and structural characteristics of ion channels, proteases, and other receptors. However, most animal toxins are disulfide-rich peptides that are difficult to produce functionally. Here, a glutathione S-transferase (GST) fusion expression strategy was used to produce four recombinant animal toxin peptides, ChTX, StKTx23, BmP01, and ImKTx1, with different isoelectric points from 4.7 to 9.2. GST tags were removed by enterokinase, a widely used and effective commercial protease that cleaves after lysine at the cleavage site DDDDK. Using this strategy, two disulfide-rich animal toxins ChTX and StKTx23 were obtained successfully with a yield of approximately 1-2 mg/l culture. Electrophysiological experiments further showed that these two recombinant toxins showed good bioactivities, indicating that our method was effective in producing large amounts of functional disulfide-rich animal toxins. Interestingly, by analyzing the separated fractions of BmP01, StKTx23, and ImKTx1 using matrix-assisted laser desorption ionization time-of-flight mass spectrometry, four new enterokinase secondary cleavage sites were found, consisting of the sequences "WEYR," "EDK," "QNAR," and "DNDK." To our knowledge, this is the first report of the presence of secondary cleavage sites for commercial enterokinase in animal toxins. These findings will help us use commercial enterokinase appropriately as a cleavage tool in the production of animal toxins.

ImKTx88, a novel selective Kv1.3 channel blocker derived from the scorpion Isometrus maculates

Scorpion toxins are useful in the structure-function research of ion channels and valuable resources for drug design. The Kv1.3 channel is an important pharmacological target for the therapy of T cell-mediated autoimmune diseases, and many toxin peptides targeting Kv1.3 have been identified as good drug candidates in recent years. In this study, a novel toxin gene ImKTx88 was isolated from the venom of the scorpion Isometrus maculates through the construction of the cDNA library method, and the recombinant toxin peptide was purified and characterized physiologically. The mature peptide of ImKTx88 contained 39 amino acid residues including six cysteines and was predicted to be a new member of α-KTx scorpion family by sequence analysis. The electrophysiological experiments further indicated that the rImKTx88 peptide had a novel pharmacological profile: it inhibited Kv1.3 channel current with an IC₅₀ of 91 ± 42 pM, and exhibited very good selectivity for Kv1.3 over Kv1.1 (4200-fold) and Kv1.2 (93000-fold) channels, respectively. All these results suggested that, as a new selective Kv1.3 channel blocker, the ImKTx88 peptide may serve as a potential drug candidate in the therapy of autoimmune diseases.

Scorpion venom and the inflammatory response

Scorpion venoms consist of a complex of several toxins that exhibit a wide range of biological properties and actions, as well as chemical compositions, toxicity, and pharmacokinetic and pharmacodynamic characteristics. These venoms are associated with high morbility and mortality, especially among children. Victims of envenoming by a scorpion suffer a variety of pathologies, involving mainly both sympathetic and parasympathetic stimulation as well as central manifestations such as irritability, hyperthermia, vomiting, profuse salivation, tremor, and convulsion. The clinical signs and symptoms observed in humans and experimental animals are related with an excessive systemic host inflammatory response to stings and stings, respectively. Although the pathophysiology of envenomation is complex and not yet fully understood, venom and immune responses are known to trigger the release of inflammatory mediators that are largely mediated by cytokines. In models of severe systemic inflammation produced by injection of high doses of venom or venoms products, the increase in production of proinflammatory cytokines significantly contributes to immunological imbalance, multiple organ dysfunction and death. The cytokines initiate a cascade of events that lead to illness behaviors such as fever, anorexia, and also physiological events in the host such as activation of vasodilatation, hypotension, and increased of vessel permeability.

Molecular cloning and functional identification of a new K(+) channel blocker, LmKTx10, from the scorpion Lychas mucronatus

Scorpions have a venom gland which is an important determinant in contributing to their successful survival for more than 400 million years. Their venoms contain a diversity of neurotoxins, which represent a tremendous hitherto partially unexplored resource not only for understanding ion channels but also for use in drug design and development. In this investigation, LmKTx10, a new toxin gene was identified from the venom of the scorpion Lychas mucronatus by constructing cDNA library method, and its product was expressed and characterized physiologically. The mature peptide has 38 residues including six conserved cysteines. The electrophysiological experiments further indicated that the recombinant LmKTx10 peptide has an interesting pharmacological profile: it blocks Kv1.3 channel with IC(50)=28nM which is moderate Kv1.3 channel blocking activity compared to the other a-KTxs toxins, and exhibits good selectivity on Kv1.3 over Kv1.1 and Kv1.2, about 60 folds and 450 folds, respectively. These data not only enrich the family of K(+) channel toxins from scorpion venoms but also present a potential drug template for selectively targeting the Kv1.3 channel.

Immunological characterization of a non-toxic peptide conferring protection against the toxic fraction (AahG50) of the Androctonus australis hector venom

KAaH1 and KAaH2 are non-toxic peptides, isolated from the venom of the Androctonus australis hector (Aah) scorpion. In a previous study, we showed these peptides to be the most abundant (approximately 10% each) in the toxic fraction (AahG50) of the Aah venom. KAaH1 and KAaH2 showed high sequence identities (approximately 60%) with birtoxin-like peptides, which likewise are the major peptidic components of Parabuthus transvaalicus scorpion venom. Here, we report the immunological characterization of KAaH1 and KAaH2. These peptides were found to be specifically recognized by polyclonal antibodies raised against AahII, the most toxic peptide of Aah venom, and represents the second antigenic group, including toxins from different scorpion species in the world. Moreover, KAaH1 partially inhibits AahII binding to its specific antibody, suggesting some common epitopes between these two peptides. The identification of possible key antigenic residues in KAaH1 was deduced from comparison of its 3-D model with the experimental structure of AahII. Two clusters of putative antigenically important residues were found at the exposed surface; one could be constituted of V3 and D53, the other of D10, T15 and Y16. Polyclonal antibodies raised against KAaH1 in mice were found to cross-react with both AahII and AahG50, and neutralizing 5LD(50)/ml of the toxic fraction. Mice vaccinated with KAaH1 were protected against a challenge of 2LD(50) of AahG50 fraction. All these data suggest that KAaH1 has clear advantages over the use of the whole or part of the venom. KAaH1 is not toxic and could produce sera-neutralizing scorpion toxins, not only from Aah venom, but also toxins of other venoms from Buthus, Leiurus, or Parabuthus scorpion species presenting antigenically related toxins.

Molecular cloning and electrophysiological studies on the first K(+) channel toxin (LmKTx8) derived from scorpion Lychas mucronatus

LmKTx8, the first toxic gene isolated from the venom of scorpion Lychas mucronatus by constructing cDNA library method, was expressed and characterized physiologically. The mature peptide has 40 residues including six conserved cysteines, and is classified as one of alpha-KTx11 subfamily. Using patch-clamp recording, the recombinant LmKTx8 (rLmKTx8) was used to test the effect on voltage-gated K(+) channels (Kv1.3) stably expressed in COS7 cells and large conductance-Ca(2+)-activated K(+) (BK) channels expressed in HEK293. The results of electrophysiological experiments showed that the rLmKTx8 was a potent inhibitor of Kv1.3 channels with an IC(50)=26.40+/-1.62nM, but 100nM rLmKTx8 did not block the BK currents. LmKTx8 or its analogs might serve as a potential candidate for the development of new drugs for autoimmune diseases.

Two toxins from Conus striatus that individually induce tetanic paralysis

We describe structural properties and biological activities of two related O-glycosylated peptide toxins isolated from injected (milked) venom of Conus striatus, a piscivorous snail that captures prey by injecting a venom that induces a violent, spastic paralysis. One 30 amino acid toxin is identified as kappaA-SIVA (termed s4a here), and another 37 amino acid toxin, s4b, corresponds to a putative peptide encoded by a previously reported cDNA. We confirm the amino acid sequences and carry out structural analyses of both mature toxins using multiple mass spectrometric techniques. These include electrospray ionization ion-trap mass spectrometry and nanoelectrospray techniques for small volume samples, as well as matrix-assisted laser desorption/ionization time of flight mass spectrometric analysis as a complementary method to assist in the determination of posttranslational modifications, including O-linked glycosylation. Physiological experiments indicate that both s4a and s4b induce intense repetitive firing of the frog neuromuscular junction, leading to a tetanic contracture in muscle fiber. These effects apparently involve modification of voltage-gated sodium channels in motor axons. Notably, application of either s4a or s4b alone mimics the biological effects of the whole injected venom on fish prey.

Synthesis of a biotin derivative of iberiotoxin: binding interactions with streptavidin and the BK Ca2+-activated K+ channel expressed in a human cell line

Iberiotoxin (IbTx) is a scorpion venom peptide that inhibits BK Ca2+-activated K+ channels with high affinity and specificity. Automated solid-phase synthesis was used to prepare a biotin-labeled derivative (IbTx-LC-biotin) of IbTx by substitution of Asp19 of the native 37-residue peptide with N--(D-biotin-6-amidocaproate)-L-lysine. Both IbTx-LC-biotin and its complex with streptavidin (StrAv) block single BK channels from rat skeletal muscle with nanomolar affinity, indicating that the biotin-labeled residue, either alone or in complex with StrAv, does not obstruct the toxin binding interaction with the BK channel. IbTx-LC-biotin exhibits high affinity (KD = 26 nM) and a slow dissociation rate (koff = 5.4 x 10(-4) s(-1)) in a macroscopic blocking assay of whole-cell current of the cloned human BK channel. Titration of IbTx-LC-biotin with StrAv monitored by high performance size exclusion chromatography is consistent with a stoichiometry of two binding sites for IbTx-LC-biotin per StrAv tetramer, indicating that steric interference hinders simultaneous binding of two toxin molecules on each of the two biotin-binding faces of StrAv. In combination with fluorescent conjugates of StrAv or anti-biotin antibody, IbTx-LC-biotin was used to image the surface distribution of BK channels on a transfected cell line. Fluorescence microscopy revealed a patch-like surface distribution of BK channel protein. The results support the feasibility of using IbTx-LC-biotin and similar biotin-tagged K+ channel toxins for diverse applications in cellular neurobiology. .

Novel α-KTx peptides from the venom of the scorpion Centruroides elegans selectively blockade Kv1.3 over IKCa1 K+ channels of T cells

From the venom of the Mexican scorpion Centruroides elegans Thorell five peptides were isolated to homogeneity by chromatographic procedures and their full amino acid sequence was determined by automatic Edman degradation. They all belong to the Noxiustoxin subfamily of scorpion toxins and were given the systematic names alpha-KTx 2.8 to 2.12, with trivial names Ce1 to Ce5, respectively. They have 39 amino acid residues, except for Ce3 which has only 38, but all of them have three disulfide bridges, and have molecular weights of 4255, 4267, 4249, 4295 and 4255 atomic mass units, respectively for Ce1 to Ce5. The C-terminal residues of Ce2, Ce4 and Ce5 were found to be amidated. The electrophysiological assay (whole-cell patch-clamp) showed that out of the five peptides, Ce1 (alpha-KTx 2.8), Ce2 (alpha-KTX2.9) and Ce4 (alpha-KTx 2.11) were effective blockers of Kv1.3 channels of human T lymphocytes, whereas these peptides did not inhibit the Ca2+-activated K+ channels (IKCa1) of the same cells. The equilibrium dissociation constants of these peptides for Kv1.3 were 0.70, 0.25 and 0.98nM for Ce1, Ce2 and Ce4, respectively. Furthermore, toxins Ce1, Ce2 and Ce4 practically did not inhibit the related voltage gated Shaker K+ channels, and rKv2.1 channels of the Shab family. The high affinity blockage of Kv1.3 channels by these peptides and their selectivity for Kv1.3 over IKCa1 may have significance in the development of novel tools for suppressing the function of those T cell subsets whose proliferation critically depends on the activity of Kv1.3 channels.

Probing the pH-dependent structural features of alpha-KTx12.1, a potassium channel blocker from the scorpion Tityus serrulatus

Potassium channels are widespread in living cells and are involved in many diseases. The scorpion toxin alpha-KTx(12.1) interacts with various K(+) channels, suggesting its capacity to match diverse channel pores. It is recognized that tissue injuries may affect the pH at toxins site of action, thereby modulating both protein conformation and activity. To better understand its molecular mechanism of action, we studied alpha-KTx(12.1) using pH as a tool to explore its plasticity and NMR in combination with MD calculations to detect it. The toxin solution structure consists of an alpha-helix and a triple-stranded beta-sheet stabilized by four disulfide bridges. The NMR results show, in addition, that His28 possesses an unusually low pK(a) of 5.2. The best set of protein conformers is obtained at pH 4.5, while at pH 7.0, the reduced number of NOEs resulting from a faster hydrogen exchange does not allow to reach a good structural convergence. Nonetheless, MD calculations show that the toxin structure does not vary significantly in that pH range, while conformational changes and modifications of the surface charge distribution occur when His28 is fully protonated. Moreover, essential dynamics analysis reveals variations in the toxin's coherent motions. In conclusion, His28, with its low pK(a) value, provides alpha-KTx(12.1) with the ability to preserve its active conformation over a wide pH interval, thus expanding the range of cellular conditions where the toxin can fully exhibit its activity. Overall, the results further underline the role of histidine as a natural controller of proteins' functionality.

Immunology of scorpion toxins and perspectives for generation of anti-venom vaccines

Scorpions and other venomous animals contain concentrates of biologically active substances developed to block vital physiological and biochemical functions of the victims. These have contrasting human health concerns, provide important pharmacological raw material and pose a serious threat to human life and health in tropical and subtropical regions. Because only occasional and minor quantities of venom are introduced into the human organism with a scorpion sting and their mortal effect is an acute phenomenon these substances are unknown to the immune defense system and thus no immunity has appeared against them during evolution. Antidotes prepared from animal anti-sera are effective against some species of scorpions but depend on the manufacturer and the availability of product to the medical community. Although significant progress has been made in immunological studies of certain groups of toxins, few centers are dedicated to this research. Information is still insufficient to generate a comprehensive picture of the subject and to propose vaccines against venoms. A novel approach based on mimotopes selected from phage-displayed random peptide libraries show potential to impel further progress of toxin immunological studies and to provide putative vaccine resources. In this report we revise the "state of the art" in the field.

Discrepin, a new peptide of the sub-family α-ktx15, isolated from the scorpion Tityus discrepans irreversibly blocks K+-channels (IA currents) of cerebellum granular cells

A new peptide was purified from the venom of the Venezuelan scorpion Tityus discrepans, by high-performance liquid chromatography and its amino acid sequence was completed by Edman degradation and mass spectrometry analysis. It contains 38 amino acid residues with a molecular weight of 4177.7 atomic mass units, tightly folded by three disulfide bridges, and has a pyroglutamic acid at the N-terminal region. This peptide, named Discrepin, was shown to block preferentially the IA currents of the voltage-dependent K+ -channel of rat cerebellum granular cells in culture. The K+ -currents are inhibited in an apparently irreversible manner, whose 50% inhibitory effect is reached with a 190 nM toxin concentration. The systematic nomenclature proposed for this toxin is alpha-KTx15.6.

NMR solution structure of Cn12, a novel peptide from the Mexican scorpion Centruroides noxius with a typical beta-toxin sequence but with alpha-like physiological activity

Cn12 isolated from the venom of the scorpion Centruroides noxius has 67 amino-acid residues, closely packed with four disulfide bridges. Its primary structure and disulfide bridges were determined. Cn12 is not lethal to mammals and arthropods in vivo at doses up to 100 microg per animal. Its 3D structure was determined by proton NMR using 850 distance constraints, 36 phi angles derived from 36 coupling constants obtained by two different methods, and 22 hydrogen bonds. The overall structure has a two and half turn alpha-helix (residues 24-32), three strands of antiparallel beta-sheet (residues 2-4, 37-40 and 45-48), and a type II turn (residues 41-44). The amino-acid sequence of Cn12 resembles the beta scorpion toxin class, although patch-clamp experiments showed the induction of supplementary slow inactivation of Na(+) channels in F-11 cells (mouse neuroblastoma N18TG-2 x rat DRG2), which means that it behaves more like an alpha scorpion toxin. This behaviour prompted us to analyse Na(+) channel binding sites using information from 112 Na(+) channel gene clones available in the literature, focusing on the extracytoplasmic loops of the S5-S6 transmembrane segments of domain I and the S3-S4 segments of domain IV, sites considered to be responsible for binding alpha scorpion toxins.

Novel interactions between K+ channels and scorpion toxins

K(+) channels are macromolecules embedded in biological membranes, where they play a key role in cellular excitability and signal transduction pathways. Knowledge of their structure should help improve our understanding of their function and lead to the design of therapeutic compounds. Most pharmacological and structural characteristics of these channels have been elucidated by using high-affinity channel blockers isolated from scorpion venoms. Recent data on the three-dimensional structures of K(+) channels and novel scorpion toxins suggest a variety of novel interacting modes of these channels and toxins, which should help increase our understanding of the K(+) channel structure-function relationship.

Solution structure of CnErg1 (Ergtoxin), a HERG specific scorpion toxin

The three-dimensional structure of chemically synthesized CnErg1 (Ergtoxin), which specifically blocks HERG (human ether-a-go-go-related gene) K+ channels, was determined by nuclear magnetic resonance spectroscopy. CnErg1 consists of a triple-stranded beta-sheet and an alpha-helix, as is typical of K+ channel scorpion toxins. The peptide structure differs from the canonical structures in that the first beta-strand is shorter and is nearer to the second beta-strand rather than to the third beta-strand on the C-terminus. There is also a large hydrophobic patch on the surface of the toxin, surrounding a central lysine residue, Lys13. We postulate that this hydrophobic patch is likely to form part of the binding surface of the toxin.

BTK-2, a new inhibitor of the Kv1.1 potassium channel purified from Indian scorpion Buthus tamulus

A novel inhibitor of voltage-gated potassium channel was isolated and purified to homogeneity from the venom of the red scorpion Buthus tamulus. The primary sequence of this toxin, named BTK-2, as determined by peptide sequencing shows that it has 32 amino acid residues with six conserved cysteines. The molecular weight of the toxin was found to be 3452 Da. It was found to block the human potassium channel hKv1.1 (IC(50)=4.6 microM). BTK-2 shows 40-70% sequence similarity to the family of the short-chain toxins that specifically block potassium channels. Multiple sequence alignment helps to categorize the toxin in the ninth subfamily of the K+ channel blockers. The modeled structure of BTK-2 shows an alpha/beta scaffold similar to those of the other short scorpion toxins. Comparative analysis of the structure with those of the other toxins helps to identify the possible structure-function relationship that leads to the difference in the specificity of BTK-2 from that of the other scorpion toxins. The toxin can also be used to study the assembly of the hKv1.1 channel.

New binding site on common molecular scaffold provides HERG channel specificity of scorpion toxin BeKm-1

The scorpion toxin BeKm-1 is unique among a variety of known short scorpion toxins affecting potassium channels in its selective action on ether-a-go-go-related gene (ERG)-type channels. BeKm-1 shares the common molecular scaffold with other short scorpion toxins. The toxin spatial structure resolved by NMR consists of a short alpha-helix and a triple-stranded antiparallel beta-sheet. By toxin mutagenesis study we identified the residues that are important for the binding of BeKm-1 to the human ERG K+ (HERG) channel. The most critical residues (Tyr-11, Lys-18, Arg-20, Lys-23) are located in the alpha-helix and following loop whereas the "traditional" functional site of other short scorpion toxins is formed by residues from the beta-sheet. Thus the unique location of the binding site of BeKm-1 provides its specificity toward the HERG channel.

A new class of scorpion toxin binding sites related to an A-type K+ channel: pharmacological characterization and localization in rat brain

A new scorpion toxin (3751.8 Da) was isolated from the Buthus martensi venom, sequenced and chemically synthesized (sBmTX3). The A-type current of striatum neurons in culture completely disappeared when 1 microM sBmTX3 was applied (Kd=54 nM), whereas the sustained K+ current was unaffected. 125I-sBmTX3 specifically bound to rat brain synaptosomes (maximum binding=14 fmol x mg(-1) of protein, Kd=0.21 nM). A panel of toxins yet described as specific ligands for K+ channels were unable to compete with 125I-sBmTX3. A high density of 125I-sBmTX3 binding sites was found in the striatum, hippocampus, superior colliculus, and cerebellum in the adult rat brain.

Residue Gly1326 of the N-type calcium channel alpha 1B subunit controls reversibility of omega-conotoxin GVIA and MVIIA block

We recently reported that amino acid residues contained within a putative EF hand motif in the domain III S5-H5 region of the alpha(1B) subunit affected the relative barium:calcium permeability of N-type calcium channels (Feng, Z. P., Hamid, J., Doering, C., Jarvis, S. E., Bosey, G. M., Bourinet, E., Snutch, T. P., and Zamponi, G. W. (2001) J. Biol. Chem. 276, 5726-5730). Since this region partially overlaps with residues previously implicated in block of the channel by omega-conotoxin GVIA, we assessed the effects of mutations in the putative EF hand domain on channel block by omega-conotoxin GVIA and the structurally related omega-conotoxin MVIIA. Both of the toxins irreversibly block the activity of wild type alpha(1B) N-type channels. We find that in addition to previously identified amino acid residues, residues in positions 1326 and 1332 are important determinants of omega-conotoxin GVIA blockade. Substitution of residue Glu(1332) to arginine slows the time course of development of block. Point mutations in position Gly(1326) to either arginine, glutamic acid, or proline dramatically decrease the time constant for development of the block. Additionally, in the G1326P mutant channel activity was almost completely recovered following washout. A qualitatively similar result was obtained with omega-conotoxin MVIIA, suggesting that common molecular determinants underlie block by these two toxins. Taken together the data suggest that residue Gly(1326) may form a barrier, which controls the access of peptide toxins to their blocking site within the outer vestibule of the channel pore and also stabilizes the toxin-channel interaction.