Tertiary structure of conotoxin GIIIA in aqueous solution

Historical Perspective of the Characterization of Conotoxins Targeting Voltage-Gated Sodium Channels

Marine toxins have potent actions on diverse sodium ion channels regulated by transmembrane voltage (voltage-gated ion channels) or by neurotransmitters (nicotinic acetylcholine receptor channels). Studies of these toxins have focused on varied aspects of venom peptides ranging from evolutionary relationships of predator and prey, biological actions on excitable tissues, potential application as pharmacological intervention in disease therapy, and as part of multiple experimental approaches towards an understanding of the atomistic characterization of ion channel structure. This review examines the historical perspective of the study of conotoxin peptides active on sodium channels gated by transmembrane voltage, which has led to recent advances in ion channel research made possible with the exploitation of the diversity of these marine toxins.

NMR Structure of μ-Conotoxin GIIIC: Leucine 18 Induces Local Repacking of the N-Terminus Resulting in Reduced NaV Channel Potency

μ-Conotoxins are potent and highly specific peptide blockers of voltage-gated sodium channels. In this study, the solution structure of μ-conotoxin GIIIC was determined using 2D NMR spectroscopy and simulated annealing calculations. Despite high sequence similarity, GIIIC adopts a three-dimensional structure that differs from the previously observed conformation of μ-conotoxins GIIIA and GIIIB due to the presence of a bulky, non-polar leucine residue at position 18. The side chain of L18 is oriented towards the core of the molecule and consequently the N-terminus is re-modeled and located closer to L18. The functional characterization of GIIIC defines it as a canonical μ-conotoxin that displays substantial selectivity towards skeletal muscle sodium channels (Na_V), albeit with ~2.5-fold lower potency than GIIIA. GIIIC exhibited a lower potency of inhibition of Na_V1.4 channels, but the same Na_V selectivity profile when compared to GIIIA. These observations suggest that single amino acid differences that significantly affect the structure of the peptide do in fact alter its functional properties. Our work highlights the importance of structural factors, beyond the disulfide pattern and electrostatic interactions, in the understanding of the functional properties of bioactive peptides. The latter thus needs to be considered when designing analogues for further applications.

Mapping of voltage sensor positions in resting and inactivated mammalian sodium channels by LRET

Voltage-gated sodium channels (Navs) play crucial roles in excitable cells. Although vertebrate Nav function has been extensively studied, the detailed structural basis for voltage-dependent gating mechanisms remain obscure. We have assessed the structural changes of the Nav voltage sensor domain using lanthanide-based resonance energy transfer (LRET) between the rat skeletal muscle voltage-gated sodium channel (Nav1.4) and fluorescently labeled Nav1.4-targeting toxins. We generated donor constructs with genetically encoded lanthanide-binding tags (LBTs) inserted at the extracellular end of the S4 segment of each domain (with a single LBT per construct). Three different Bodipy-labeled, Nav1.4-targeting toxins were synthesized as acceptors: β-scorpion toxin (Ts1)-Bodipy, KIIIA-Bodipy, and GIIIA-Bodipy analogs. Functional Nav-LBT channels expressed in Xenopus oocytes were voltage-clamped, and distinct LRET signals were obtained in the resting and slow inactivated states. Intramolecular distances computed from the LRET signals define a geometrical map of Nav1.4 with the bound toxins, and reveal voltage-dependent structural changes related to channel gating.

Human β-defensin 4 with non-native disulfide bridges exhibit antimicrobial activity

Human defensins play multiple roles in innate immunity including direct antimicrobial killing and immunomodulatory activity. They have three disulfide bridges which contribute to the stability of three anti-parallel β-strands. The exact role of disulfide bridges and canonical β-structure in the antimicrobial action is not yet fully understood. In this study, we have explored the antimicrobial activity of human β-defensin 4 (HBD4) analogs that differ in the number and connectivity of disulfide bridges. The cysteine framework was similar to the disulfide bridges present in μ-conotoxins, an unrelated class of peptide toxins. All the analogs possessed enhanced antimicrobial potency as compared to native HBD4. Among the analogs, the single disulfide bridged peptide showed maximum potency. However, there were no marked differences in the secondary structure of the analogs. Subtle variations were observed in the localization and membrane interaction of the analogs with bacteria and Candida albicans, suggesting a role for disulfide bridges in modulating their antimicrobial action. All analogs accumulated in the cytosol where they can bind to anionic molecules such as nucleic acids which would affect several cellular processes leading to cell death. Our study strongly suggests that native disulfide bridges or the canonical β-strands in defensins have not evolved for maximal activity but they play important roles in determining their antimicrobial potency.

Roles of basic amino acid residues in the activity of μ-conotoxin GIIIA and GIIIB, peptide blockers of muscle sodium channels

To study in detail the roles of basic amino acid residues in the activity of μ-conotoxin GIIIA (μ-GIIIA) and GIIIB (μ-GIIIB), specific blockers of muscle sodium channels, seven analogs of μ-GIIIA, and two analogs of μ-GIIIB were synthesized. μ-GIIIA analogs were synthesized by replacing systematically the three Arg residues (Arg1, Arg13, and Arg19) with one, two, and three Lys residues. μ-GIIIB analogs were synthesized by replacing simultaneously all four Lys residues (Lys9, Lys11, Lys16, and Lys19) with Arg residues and further replacement of acidic Asp residues with neutral Ala residues. Circular dichroism spectra of the synthesized analogs suggested that the replacement did not affect the three dimensional structure. The inhibitory effects on the twitch contractions of the rat diaphragm showed that the side chain guanidino group of Arg13 of μ-GIIIA was important for the activity, whereas that of Arg19 had little role for biological activity. Although [Arg9,11,16,19]μ-GIIIB showed higher activity than native μ-GIIIB, highly basic [Ala2,12, Arg9,11,16,19]μ-GIIIB showed lower activity, suggesting that there was an appropriate molecular basicity for the maximum activity.

Molecular dynamics study of binding of µ-conotoxin GIIIA to the voltage-gated sodium channel Na(v)1.4

Homology models of mammalian voltage-gated sodium (NaV) channels based on the crystal structures of the bacterial counterparts are needed to interpret the functional data on sodium channels and understand how they operate. Such models would also be invaluable in structure-based design of therapeutics for diseases involving sodium channels such as chronic pain and heart diseases. Here we construct a homology model for the pore domain of the NaV1.4 channel and use the functional data for the binding of µ-conotoxin GIIIA to NaV1.4 to validate the model. The initial poses for the NaV1.4-GIIIA complex are obtained using the HADDOCK protein docking program, which are then refined in molecular dynamics simulations. The binding mode for the final complex is shown to be in broad agreement with the available mutagenesis data. The standard binding free energy, determined from the potential of mean force calculations, is also in good agreement with the experimental value. Because the pore domains of NaV1 channels are highly homologous, the model constructed for NaV1.4 will provide an excellent template for other NaV1 channels.

Distinct disulfide isomers of μ-conotoxins KIIIA and KIIIB block voltage-gated sodium channels

In the preparation of synthetic conotoxins containing multiple disulfide bonds, oxidative folding can produce numerous permutations of disulfide bond connectivities. Establishing the native disulfide connectivities thus presents a significant challenge when the venom-derived peptide is not available, as is increasingly the case when conotoxins are identified from cDNA sequences. Here, we investigate the disulfide connectivity of μ-conotoxin KIIIA, which was predicted originally to have a [C1-C9,C2-C15,C4-C16] disulfide pattern based on homology with closely related μ-conotoxins. The two major isomers of synthetic μ-KIIIA formed during oxidative folding were purified and their disulfide connectivities mapped by direct mass spectrometric collision-induced dissociation fragmentation of the disulfide-bonded polypeptides. Our results show that the major oxidative folding product adopts a [C1-C15,C2-C9,C4-C16] disulfide connectivity, while the minor product adopts a [C1-C16,C2-C9,C4-C15] connectivity. Both of these peptides were potent blockers of Na(V)1.2 (K(d) values of 5 and 230 nM, respectively). The solution structure for μ-KIIIA based on nuclear magnetic resonance data was recalculated with the [C1-C15,C2-C9,C4-C16] disulfide pattern; its structure was very similar to the μ-KIIIA structure calculated with the incorrect [C1-C9,C2-C15,C4-C16] disulfide pattern, with an α-helix spanning residues 7-12. In addition, the major folding isomers of μ-KIIIB, an N-terminally extended isoform of μ-KIIIA identified from its cDNA sequence, were isolated. These folding products had the same disulfide connectivities as μ-KIIIA, and both blocked Na(V)1.2 (K(d) values of 470 and 26 nM, respectively). Our results establish that the preferred disulfide pattern of synthetic μ-KIIIA and μ-KIIIB folded in vitro is 1-5/2-4/3-6 but that other disulfide isomers are also potent sodium channel blockers. These findings raise questions about the disulfide pattern(s) of μ-KIIIA in the venom of Conus kinoshitai; indeed, the presence of multiple disulfide isomers in the venom could provide a means of further expanding the snail's repertoire of active peptides.

Identification of Conus peptidylprolyl cis-trans isomerases (PPIases) and assessment of their role in the oxidative folding of conotoxins

Peptidylprolyl cis-trans isomerases (PPIases) are ubiquitous proteins that catalyze the cis-trans isomerization of prolines. A number of proteins, such as Drosophila rhodopsin and the human immunodeficiency viral protein HIV-1 Gag, have been identified as endogenous substrates for PPIases. However, very little is known about the interaction of PPIases with small, disulfide-rich peptides. Marine cone snails synthesize a wide array of cysteine-rich peptides, called conotoxins, many of which contain one or more prolines or hydroxyprolines. To identify whether PPIase-associated cis-trans isomerization of these residues affects the oxidative folding of conotoxins, we identified, sequenced, and expressed three functionally active isoforms of PPIase from the venom gland of Conus novaehollandiae, and we characterized their ability to facilitate oxidative folding of conotoxins in vitro. Three conotoxins, namely mu-GIIIA, mu-SIIIA, and omega-MVIIC, derived from two distinct toxin gene families were assayed. Conus PPIase significantly increased the rate of appearance of the native form of mu-GIIIA, a peptide containing three hydroxyprolines. In contrast, the presence of PPIase had no effect on the folding of mu-SIIIA and omega-MVIIC, peptides containing no or one proline residue, respectively. We further showed that an endoplasmic reticulum-resident PPIase isoform facilitated folding of mu-GIIIA more efficiently than two cytosolic isoforms. This is the first study to demonstrate PPIase-assisted folding of conotoxins, small disulfide-rich peptides with unique structural properties.

Mu-conotoxins as leads in the development of new analgesics

Voltage-gated sodium channels (VGSCs) contain a specific binding site for a family of cone shell toxins known as mu-conotoxins. As some VGSCs are involved in pain perception and mu-conotoxins are able to block these channels, mu-conotoxins show considerable potential as analgesics. Recent studies have advanced our understanding of the three-dimensional structures and structure-function relationships of the mu-conotoxins, including their interaction with VGSCs. Truncated peptide analogues of the native toxins have been created in which secondary structure elements are stabilized by non-native linkers such as lactam bridges. Ultimately, it would be desirable to capture the favourable analgesic properties of the native toxins, in particular their potency and channel sub-type selectivity, in non-peptide mimetics. Such mimetics would constitute lead compounds in the development of new therapeutics for the treatment of pain.

Biochemical characterization of kappaM-RIIIJ, a Kv1.2 channel blocker: evaluation of cardioprotective effects of kappaM-conotoxins

Conus snail (Conus) venoms are a valuable source of pharmacologically active compounds; some of the peptide toxin families from the snail venoms are known to interact with potassium channels. We report the purification, synthesis, and characterization of kappaM-conotoxin RIIIJ from the venom of a fish-hunting species, Conus radiatus. This conopeptide, like a previously characterized peptide in the same family, kappaM-RIIIK, inhibits the homotetrameric human Kv1.2 channels. When tested in Xenopus oocytes, kappaM-RIIIJ has an order of magnitude higher affinity (IC(50) = 33 nm) to Kv1.2 than kappaM-RIIIK (IC(50) = 352 nm). Chimeras of RIIIK and RIIIJ tested on the human Kv1.2 channels revealed that Lys-9 from kappaM-RIIIJ is a determinant of its higher potency against hKv1.2. However, when compared in a model of ischemia/reperfusion, kappaM-RIIIK (100 mug/kg of body weight), administered just before reperfusion, significantly reduces the infarct size in rat hearts in vivo without influencing hemodynamics, providing a potential compound for cardioprotective therapeutics. In contrast, kappaM-RIIIJ does not exert any detectable cardioprotective effect. kappaM-RIIIJ shows more potency for Kv1.2-Kv1.5 and Kv1.2-Kv1.6 heterodimers than kappaM-RIIIK, whereas the affinity of kappaM-RIIIK to Kv1.2-Kv1.7 heterodimeric channels is higher (IC(50) = 680 nm) than that of kappaM-RIIIJ (IC(50) = 3.15 mum). Thus, the cardioprotection seems to correlate to antagonism to heteromultimeric channels, involving the Kv1.2 alpha-subunit rather than antagonism to Kv1.2 homotetramers. Furthermore, kappaM-RIIIK and kappaM-RIIIJ provide a valuable set of probes for understanding the underlying mechanism of cardioprotection.

Conopeptide characterization and classifications: an analysis using ConoServer

Cone snails are carnivorous marine gastropods that have evolved potent venoms to capture their prey. These venoms comprise a rich and diverse cocktail of peptide toxins, or conopeptides, whose high diversity has arisen from an efficient hypermutation mechanism, combined with a high frequency of post-translational modifications. Conopeptides bind with high specificity to distinct membrane receptors, ion channels, and transporters of the central and muscular nervous system. As well as serving their natural function in prey capture, conopeptides have been utilized as versatile tools in neuroscience and have proven valuable as drug leads that target the nervous system in humans. This paper examines current knowledge on conopeptide sequences based on an analysis of gene and peptide sequences in ConoServer (http://www.conoserver.org), a specialized database of conopeptide sequences and three-dimensional structures. We describe updates to the content and organization of ConoServer and discuss correlations between gene superfamilies, cysteine frameworks, pharmacological families targeted by conopeptides, and the phylogeny, habitat, and diet of cone snails. The study identifies gaps in current knowledge of conopeptides and points to potential directions for future research.

Structurally minimized mu-conotoxin analogues as sodium channel blockers: implications for designing conopeptide-based therapeutics

Disulfide bridges that stabilize the native conformation of conotoxins pose a challenge in the synthesis of smaller conotoxin analogues. Herein we describe the synthesis of a minimized analogue of the analgesic mu-conotoxin KIIIA that blocks two sodium channel subtypes, the neuronal Na(V)1.2 and skeletal muscle Na(V)1.4. Three disulfide-deficient analogues of KIIIA were initially synthesized in which the native disulfide bridge formed between either C1-C9, C2-C15, or C4-C16 was removed. Deletion of the first bridge only slightly affected the peptide's bioactivity. To further minimize this analogue, the N-terminal residue was removed and two nonessential serine residues were replaced by a single 5-amino-3-oxapentanoic acid residue. The resulting "polytide" analogue retained the ability to block sodium channels and to produce analgesia. Until now, the peptidomimetic approach applied to conotoxins has progressed only modestly at best; thus, the disulfide-deficient analogues containing backbone spacers provide an alternative advance toward the development of conopeptide-based therapeutics.

Conotoxins: molecular and therapeutic targets

Marine molluscs known as cone snails produce beautiful shells and a complex array of over 50,000 venom peptides evolved for prey capture and defence. Many of these peptides selectively modulate ion channels and transporters, making them a valuable source of new ligands for studying the role these targets play in normal and disease physiology. A number of conopeptides reduce pain in animal models, and several are now in pre-clinical and clinical development for the treatment of severe pain often associated with diseases such as cancer. Less than 1% of cone snail venom peptides are pharmacologically characterised.

Structure, dynamics, and selectivity of the sodium channel blocker mu-conotoxin SIIIA

mu-SIIIA, a novel mu-conotoxin from Conus striatus, appeared to be a selective blocker of tetrodotoxin-resistant sodium channels in frog preparations. It also exhibited potent analgesic activity in mice, although its selectivity profile against mammalian sodium channels remains unknown. We have determined the structure of mu-SIIIA in aqueous solution and characterized its backbone dynamics by NMR and its functional properties electrophysiologically. Consistent with the absence of hydroxyprolines, mu-SIIIA adopts a single conformation with all peptide bonds in the trans conformation. The C-terminal region contains a well-defined helix encompassing residues 11-16, while residues 3-5 in the N-terminal region form a helix-like turn resembling 3 10-helix. The Trp12 and His16 side chains are close together, as in the related conotoxin mu-SmIIIA, but Asn2 is more distant. Dynamics measurements show that the N-terminus and Ser9 have larger-magnitude motions on the subnanosecond time scale, while the C-terminus is more rigid. Cys4, Trp12, and Cys13 undergo significant conformational exchange on microsecond to millisecond time scales. mu-SIIIA is a potent, nearly irreversible blocker of Na V1.2 but also blocks Na V1.4 and Na V1.6 with submicromolar potency. The selectivity profile of mu-SIIIA, including poor activity against the cardiac sodium channel, Na V1.5, is similar to that of the closely related mu-KIIIA, suggesting that the C-terminal regions of both are critical for blocking neuronal Na V1.2. The structural and functional characterization described in this paper of an analgesic mu-conotoxin that targets neuronal subtypes of mammalian sodium channels provides a basis for the design of novel analogues with an improved selectivity profile.

Docking of mu-conotoxin GIIIA in the sodium channel outer vestibule

mu-Conotoxin GIIIA (mu-CTX) is a high-affinity ligand for the outer vestibule of selected isoforms of the voltage-gated Na(+) channel. The detailed bases for the toxin's high affinity binding and isoform selectivity are unclear. The outer vestibule is lined by four pore-forming (P) loops, each with an acidic residue near the mouth of the vestibule. mu-CTX has seven positively charged residues that may interact with these acidic P-loop residues. Using pair-wise alanine replacement of charged toxin and channel residues, in conjunction with double mutant cycle analysis, we determined coupling energies for specific interactions between each P-loop acidic residue and selected toxin residues to systematically establish quantitative restraints on the toxin orientation in the outer vestibule. Xenopus oocytes were injected with the mutant or native Na(+) channel mRNA, and currents measured by two-electrode voltage clamp. Mutant cycle analysis revealed novel, strong, toxin-channel interactions between K9/E403, K11/D1241, K11/D1532, and R19/D1532. Experimentally determined coupling energies for interacting residue pairs provided restraints for molecular dynamics simulations of mu-CTX docking. Our simulations suggest a refined orientation of the toxin in the pore, with toxin basic side-chains playing key roles in high-affinity binding. This modeling also provides a set of testable predictions for toxin-channel interactions, hitherto not described, that may contribute to high-affinity binding and channel isoform selectivity.

Isolation and Structure-Activity of μ-Conotoxin TIIIA, A Potent Inhibitor of Tetrodotoxin-Sensitive Voltage-Gated Sodium Channels

Mu-conotoxins are three-loop peptides produced by cone snails to inhibit voltage-gated sodium channels during prey capture. Using polymerase chain reaction techniques, we identified a gene sequence from the venom duct of Conus tulipa encoding a new mu-conotoxin-TIIIA (TIIIA). A 125I-TIIIA binding assay was established to isolate native TIIIA from the crude venom of Conus striatus. The isolated peptide had three post-translational modifications, including two hydroxyproline residues and C-terminal amidation, and <35% homology to other mu-conotoxins. TIIIA potently displaced [3H]saxitoxin and 125I-TIIIA from rat brain (Nav1.2) and skeletal muscle (Nav1.4) membranes. Alanine and glutamine scans of TIIIA revealed several residues, including Arg14, that were critical for high-affinity binding to tetrodotoxin (TTX)-sensitive Na+ channels. We were surprised to find that [E15A]TIIIA had a 10-fold higher affinity than TIIIA for TTX-sensitive sodium channels (IC50, 15 vs. 148 pM at rat brain membrane). TIIIA was selective for Nav1.2 and -1.4 over Nav1.3, -1.5, -1.7, and -1.8 expressed in Xenopus laevis oocytes and had no effect on rat dorsal root ganglion neuron Na+ current. 1H NMR studies revealed that TIIIA adopted a single conformation in solution that was similar to the major conformation described previously for mu-conotoxin PIIIA. TIIIA and analogs provide new biochemical probes as well as insights into the structure-activity of mu-conotoxins.

Conotoxins down under

In the four decades since toxinologists in Australia and elsewhere started to investigate the active constituents of venomous cone snails, a wealth of information has emerged on the various classes of peptides and proteins that make their venoms such potent bioactive cocktails. This article provides an overview of the current state of knowledge of these venom constituents, several of which are of interest as potential human therapeutics as a consequence of their high potency and exquisite target specificity. With the promise of as many as 50,000 venom components across the entire Conus genus, many more interesting peptides can be anticipated.

Identification of a Novel Pharmacophore for Peptide Toxins Interacting with K+ Channels

KappaM-conotoxin RIIIK blocks TSha1 K+ channels from trout with high affinity by interacting with the ion channel pore. As opposed to many other peptides targeting K+ channels, kappaM-RIIIK does not possess a functional dyad. In this study we combine thermodynamic mutant cycle analysis and docking calculations to derive the binding mode of kappaM-conotoxin RIIIK to the TSha1 channel. The final model reveals a novel pharmacophore, where no positively charged side chain occludes the channel pore. Instead the positive-charged residues of the toxin form a basic ring; kappaM-RIIIK is anchored to the K+ channel via electrostatic interactions of this basic ring with the loop and pore helix residues of the channel. The channel amino acid Glu-354 is likely to be a fundamental determinant of the selectivity of kappaM-RIIIK for the TSha1 channel. The Cgamma-OH of Hyp-15 is in contact with the carbonyls of the selectivity filter, disturbing the charge distribution pattern necessary for the coordination of K+ ions. This novel, experimentally based pharmacophore model proves the existence of diverse binding modes of peptidic toxins to K+ channels and underlines the role of intermolecular electrostatic interactions involving channel loop side chains in determining the selectivity of toxins for specific K+ channel types.

Oxidative folding of conotoxins sharing an identical disulfide bridging framework

Conotoxins are short, disulfide-rich peptide neurotoxins produced in the venom of predatory marine cone snails. It is generally accepted that an estimated 100,000 unique conotoxins fall into only a handful of structural groups, based on their disulfide bridging frameworks. This unique molecular diversity poses a protein folding problem of relationships between hypervariability of amino acid sequences and mechanism(s) of oxidative folding. In this study, we present a comparative analysis of the folding properties of four conotoxins sharing an identical pattern of cysteine residues forming three disulfide bridges, but otherwise differing significantly in their primary amino acid sequence. Oxidative folding properties of M-superfamily conotoxins GIIIA, PIIIA, SmIIIA and RIIIK varied with respect to kinetics and thermodynamics. Based on rates for establishing the steady-state distribution of the folding species, two distinct folding mechanisms could be distinguished: first, rapid-collapse folding characterized by very fast, but low-yield accumulation of the correctly folded form; and second, slow-rearrangement folding resulting in higher accumulation of the properly folded form via the reshuffling of disulfide bonds within folding intermediates. Effects of changing the folding conditions indicated that the rapid-collapse and the slow-rearrangement mechanisms were mainly determined by either repulsive electrostatic or productive noncovalent interactions, respectively. The differences in folding kinetics for these two mechanisms were minimized in the presence of protein disulfide isomerase. Taken together, folding properties of conotoxins from the M-superfamily presented in this work and from the O-superfamily published previously suggest that conotoxin sequence diversity is also reflected in their folding properties, and that sequence information rather than a cysteine pattern determines the in vitro folding mechanisms of conotoxins.

Structure-Activity Relationships of Hainantoxin-IV and Structure Determination of Active and Inactive Sodium Channel Blockers

Hainantoxin-IV (HNTX-IV) can specifically inhibit the neuronal tetrodotoxin-sensitive sodium channels and defines a new class of depressant spider toxin. The sequence of native HNTX-IV is ECLGFGKGCNPSNDQCCKSSNLVCSRKHRWCKYEI-NH(2). In the present study, to obtain further insight into the primary and tertiary structural requirements of neuronal sodium channel blockers, we determined the solution structure of HNTX-IV as a typical inhibitor cystine knot motif and synthesized four mutants designed based on the predicted sites followed by structural elucidation of two inactive mutants. Pharmacological studies indicated that the S12A and R26A mutants had activities near that of native HNTX-IV, while K27A and R29A demonstrated activities reduced by 2 orders of magnitude. (1)H MR analysis showed the similar molecular conformations for native HNTX-IV and four synthetic mutants. Furthermore, in the determined structures of K27A and R29A, the side chains of residues 27 and 29 were located in the identical spatial position to those of native HNTX-IV. These results suggested that residues Ser(12), Arg(26), Lys(27), and Arg(29) were not responsible for stabilizing the distinct conformation of HNTX-IV, but Lys(27) and Arg(29) were critical for the bioactivities. The potency reductions produced by Ala substitutions were primarily due to the direct interaction of the essential residues Lys(27) and Arg(29) with sodium channels rather than to a conformational change. After comparison of these structures and activities with correlated toxins, we hypothesized that residues Lys(27), Arg(29), His(28), Lys(32), Phe(5), and Trp(30) clustered on one face of HNTX-IV were responsible for ligand binding.

Novel interactions identified between micro -Conotoxin and the Na+ channel domain I P-loop: implications for toxin-pore binding geometry

micro -Conotoxins ( micro -CTX) are peptides that inhibit Na(+) flux by blocking the Na(+) channel pore. Toxin residue arginine 13 is critical for both high affinity binding and for complete block of the single channel current, prompting the simple conventional view that residue 13 (R13) leads toxin docking by entering the channel along the pore axis. To date, the strongest interactions identified are between micro -CTX and domain II (DII) or DIII pore residues of the rat skeletal muscle (Na(v)1.4) Na(+) channels, but little data is available for the role of the DI P-loop in micro -CTX binding due to the lack of critical determinants identified in this domain. Despite being an essential determinant of isoform-specific tetrodotoxin sensitivity, the DI-Y401C variant had little effect on micro -CTX block. Here we report that the charge-changing substitution Y401K dramatically reduced the micro -CTX affinity ( approximately 300-fold). Using mutant cycle analysis, we demonstrate that K401 couples strongly to R13 (DeltaDeltaG > 3.0 kcal/mol) but not R1, K11, or R14 (<<1 kcal/mol). Unlike K401, however, a significant coupling was detected between toxin residue 14 and DI-E403K (DeltaDeltaG = 1.4 kcal/mol for the E403K-Q14D pair). This appears to underlie the ability of DI-E403K channels to discriminate between the GIIIA and GIIIB isoforms of micro -CTX (p < 0.05), whereas Y401K, DII-E758Q, and DIII-D1241K do not. We also identify five additional, novel toxin-channel interactions (>0.75 kcal/mol) in DII (E758-K16, D762-R13, D762-K16, E765-R13, E765-K16). Considered together, these new interactions suggest that the R13 side chain and the bulk of the bound toxin micro -CTX molecule may be significantly tilted with respect to pore axis.

Structural basis for tetrodotoxin-resistant sodium channel binding by mu-conotoxin SmIIIA

SmIIIA is a new micro-conotoxin isolated recently from Conus stercusmuscarum. Although it shares several biochemical characteristics with other micro-conotoxins (the arrangement of cysteine residues and a conserved arginine believed to interact with residues near the channel pore), it has several distinctive features, including the absence of hydroxyproline, and is the first specific antagonist of tetrodotoxin-resistant voltage-gated sodium channels to be characterized. It therefore represents a potentially useful tool to investigate the functional roles of these channels. We have determined the three-dimensional structure of SmIIIA in aqueous solution. Consistent with the absence of hydroxyprolines, SmIIIA adopts a single conformation with all peptide bonds in the trans configuration. The spatial orientations of several conserved Arg and Lys side chains, including Arg14 (using a consensus numbering system), which plays a key role in sodium channel binding, are similar to those in other micro-conotoxins but the N-terminal regions differ, reflecting the trans conformation for the peptide bond preceding residue 8 in SmIIIA, as opposed to the cis conformation in micro-conotoxins GIIIA and GIIIB. Comparison of the surfaces of SmIIIA with other micro-conotoxins suggests that the affinity of SmIIIA for TTX-resistant channels is influenced by the Trp15 side chain, which is unique to SmIIIA. Arg17, which replaces Lys in the other micro-conotoxins, may also be important. Consistent with these inferences from the structure, assays of two chimeras of SmIIIA and PIIIA in which their N- and C-terminal halves were recombined, indicated that residues in the C-terminal half of SmIIIA confer affinity for tetrodotoxin-resistant sodium channels in the cell bodies of frog sympathetic neurons. SmIIIA and the chimera possessing the C-terminal half of SmIIIA also inhibit tetrodotoxin-resistant sodium channels in the postganglionic axons of sympathetic neurons, as indicated by their inhibition of C-neuron compound action potentials that persist in the presence of tetrodotoxin.

Dependence of mu-conotoxin block of sodium channels on ionic strength but not on the permeating [Na+]: implications for the distinctive mechanistic interactions between Na+ and K+ channel pore-blocking toxins and their molecular targets

Mu-conotoxins (mu-CTXs) are Na+ channel-blocking, 22-amino acid peptides produced by the sea snail Conus geographus. Although K+ channel pore-blocking toxins show specific interactions with permeant ions and strong dependence on the ionic strength (mu), no such dependence has been reported for mu-CTX and Na+ channels. Such properties would offer insight into the binding and blocking mechanism of mu-CTX as well as functional and structural properties of the Na+ channel pore. Here we studied the effects of mu and permeant ion concentration ([Na+]) on mu-CTX block of rat skeletal muscle (mu1, Nav1.4) Na+ channels. Mu-CTX sensitivity of wild-type and E758Q channels increased significantly (by approximately 20-fold) when mu was lowered by substituting external Na+ with equimolar sucrose (from 140 to 35 mm Na+); however, toxin block was unaltered (p > 0.05) when mu was maintained by replacement of [Na+] with N-methyl-d-glucamine (NMG+), suggesting that the enhanced sensitivity at low mu was not due to reduction in [Na+]. Single-channel recordings identified the association rate constant, k(on), as the primary determinant of the changes in affinity (k(on) increased 40- and 333-fold for mu-CTX D2N/R13Q and D12N/R13Q, respectively, when symmetric 200 mm Na+ was reduced to 50 mm). In contrast, dissociation rates changed <2-fold for the same derivatives under the same conditions. Experiments with additional mu-CTX derivatives identified toxin residues Arg-1, Arg-13, and Lys-16 as important contributors to the sensitivity to external mu. Taken together, our findings indicate that mu-CTX block of Na+ channels depends critically on mu but not specifically on [Na+], contrasting with the known behavior of pore-blocking K+ channel toxins. These findings suggest that different degrees of ion interaction, underlying the fundamental conduction mechanisms of Na+ and K+ channels, are mirrored in ion interactions with pore-blocking toxins.

Conotoxins as sensors of local pH and electrostatic potential in the outer vestibule of the sodium channel

We examined the block of voltage-dependent rat skeletal muscle sodium channels by derivatives of mu-conotoxin GIIIA (muCTX) having either histidine, glutamate, or alanine residues substituted for arginine-13. Toxin binding and dissociation were observed as current fluctuations from single, batrachotoxin-treated sodium channels in planar lipid bilayers. R13X derivatives of muCTX only partially block the single-channel current, enabling us to directly monitor properties of both muCTX-bound and -unbound states under different conditions. The fractional residual current through the bound channel changes with pH according to a single-site titration curve for toxin derivatives R13E and R13H, reflecting the effect of changing the charge on residue 13, in the bound state. Experiments with R13A provided a control reflecting the effects of titration of all residues on toxin and channel other than toxin residue 13. The apparent pKs for the titration of residual conductance are shifted 2-3 pH units positive from the nominal pK values for histidine and glutamate, respectively, and from the values for these specific residues, determined in the toxin molecule in free solution by NMR measurements. Toxin affinity also changes dramatically as a function of pH, almost entirely due to changes in the association rate constant, kon. Interpreted electrostatically, our results suggest that, even in the presence of the bound cationic toxin, the channel vestibule strongly favors cation entry with an equivalent local electrostatic potential more negative than -100 mV at the level of the "outer charged ring" formed by channel residues E403, E758, D1241, and D1532. Association rates are apparently limited at a transition state where the pK of toxin residue 13 is closer to the solution value than in the bound state. The action of these unique peptides can thus be used to sense the local environment in the ligand--receptor complex during individual molecular transitions and defined conformational states.

Molecular basis of isoform-specific micro-conotoxin block of cardiac, skeletal muscle, and brain Na+ channels

mu-Conotoxins (mu-CTXs) block skeletal muscle Na(+) channels with an affinity 1-2 orders of magnitude higher than cardiac and brain Na(+) channels. Although a number of conserved pore residues are recognized as critical determinants of mu-CTX block, the molecular basis of isoform-specific toxin sensitivity remains unresolved. Sequence comparison of the domain II (DII) S5-S6 loops of rat skeletal muscle (mu1, Na(v)1.4), human heart (hh1, Na(v)1.5), and rat brain (rb1, Na(v)1.1) Na(+) channels reveals substantial divergence in their N-terminal S5-P linkers even though the P-S6 and C-terminal P segments are almost identical. We used Na(v)1.4 as the backbone and systematically converted these DII S5-P isoform variants to the corresponding residues in Na(v)1.1 and Na(v)1.5. The Na(v)1.4-->Na(v)1.5 variant substitutions V724R, C725S, A728S, D730S, and C731S (Na(v)1.4 numbering) reduced block of Na(v)1.4 by 4-, 86-, 12-, 185-, and 55-fold respectively, rendering the skeletal muscle isoform more "cardiac-like." Conversely, an Na(v)1.5--> Na(v)1.4 chimeric construct in which the Na(v)1.4 DII S5-P linker replaces the analogous segment in Na(v)1.5 showed enhanced mu-CTX block. However, these variant determinants are conserved between Na(v)1.1 and Na(v)1.4 and thus cannot explain their different sensitivities to mu-CTX. Comparison of their sequences reveals two variants at Na(v)1.4 positions 729 and 732: Ser and Asn in Na(v)1.4 compared with Thr and Lys in Na(v)1.1, respectively. The double mutation S729T/N732K rendered Na(v)1.4 more "brain-like" (30-fold downward arrow in block), and the converse mutation T925S/K928N in Na(v)1.1 reproduced the high affinity blocking phenotype of Na(v)1.4. We conclude that the DII S5-P linker, although lying outside the conventional ion-conducting pore, plays a prominent role in mu-CTX binding, thus shaping isoform-specific toxin sensitivity.

Solution structure of mu-conotoxin PIIIA, a preferential inhibitor of persistent tetrodotoxin-sensitive sodium channels

Mu-conotoxins are peptide inhibitors of voltage-sensitive sodium channels (VSSCs). Synthetic forms of mu-conotoxins PIIIA and PIIIA-(2-22) were found to inhibit tetrodotoxin (TTX)-sensitive VSSC current but had little effect on TTX-resistant VSSC current in sensory ganglion neurons. In rat brain neurons, these peptides preferentially inhibited the persistent over the transient VSSC current. Radioligand binding assays revealed that PIIIA, PIIIA-(2-22), and mu-conotoxins GIIIB discriminated among TTX-sensitive VSSCs in rat brain, that these and GIIIC discriminated among the corresponding VSSCs in human brain, and GIIIA had low affinity for neuronal VSSCs. (1)H NMR studies found that PIIIA adopts two conformations in solution due to cis/trans isomerization at hydroxyproline 8. The major trans conformation results in a three-dimensional structure that is significantly different from the previously identified conformation of mu-conotoxins GIIIA and GIIIB that selectively target TTX-sensitive muscle VSSCs. Comparison of the structures and activity of PIIIA to muscle-selective mu-conotoxins provides an insight into the structural requirements for inhibition of different TTX-sensitive sodium channels by mu-conotoxins.

Critical molecular determinants of voltage-gated sodium channel sensitivity to mu-conotoxins GIIIA/B

GIIIA/B mu-conotoxins block the rat skeletal muscle sodium channel (rNa(v)1.4) with high affinity by binding to specific residues in the pore. However, human Na(v)1.4 (hNa(v)1.4) channels, which are resistant to block by GIIIA/B, have these same pore residues. We used chimera constructs, site-directed mutagenesis, and electrophysiological techniques to investigate which residues determine GIIIA/B selectivity. Exchange of serine 729 in the D2/S5-S6 linker of rat Na(v)1.4 with leucine (S729L), the corresponding residue in hNa(v)1.4, reduces the sensitivity of rNa(v)1.4 by approximately 20-fold and largely accounts for the differential sensitivity of rNa(v)1.4 and hNa(v)1.4 to both GIIIA and GIIIB. To determine whether D2/S5-S6 linker residues might contribute to the resistance of neuronal channels to GIIIA/B, we exchanged residues in this linker that differed between rNa(v)1.4 and neuronal channels. Substitution of aspargine 732 with lysine (N732K), the corresponding residue in rNa(v)1.1a and rNa(v)1.7, reduced the GIIIB sensitivity of rNa(v)1.4 by approximately 20-fold. The N732K substitution, however, only reduced GIIIA sensitivity of rNa(v)1.4 by approximately 4-fold, demonstrating that GIIIA and GIIIB have distinct interactions with the D2/S5-S6 linker. Our data indicate that naturally occurring variants in the extra-pore region of the D2/S5-S6 linker contribute to the isoform-specific sensitivity of sodium channels to GIIIA/B. Because S729 and N732 are not part of the high-affinity binding site for mu-conotoxins, these extra-pore residues probably influence the accessibility of the toxin to the binding site within the pore and/or the stability of the toxin-channel complex. Our results should aid the development of toxins that block specific neuronal sodium channel isoforms.

Charge conversion enables quantification of the proximity between a normally-neutral mu-conotoxin (GIIIA) site and the Na+ channel pore

mu-Conotoxin (mu-CTX) inhibits Na+ flux by obstructing the Na+ channel pore. Previous studies of mu-CTX have focused only on charged toxin residues, ignoring the neutral sites. Here we investigated the proximity between the C-terminal neutral alanine (A22) of mu-CTX and the Na+ channel pore by replacing it with the negatively charged glutamate. The analog A22E and wild-type (WT) mu-CTX exhibited identical nuclear magnetic resonance spectra except at the site of replacement, verifying that they have identical backbone structures. A22E significantly reduced mu-CTX affinity for WT mu1 Na+ channels (90-fold), as if the inserted glutamate repels the anionic pore receptor. We then looked for the interacting partner(s) of residue 22 by determining the potency of block of Y401K, Y401A, E758Q, D762K, D762A, E765K, E765A and D1241K channels by WT mu-CTX and A22E, followed by mutant cycle analysis to assess their individual couplings. Our results show that A22E interacts strongly with E765K from domain II (DII) (deltadeltaG=2.2 +/- 0.1 vs. <1 kcal/mol for others). We conclude that mu-CTX residue 22 closely associates with the DII pore in the toxin-bound channel complex. The approach taken may be further exploited to study the proximity of other neutral toxin residues with the Na+ channel pore.

Generation of polyclonal antibody against mu-conotoxin GIIIA using an immunogen of [Cys(5)]mu-conotoxin GIIIA site-specifically conjugated with bovine serum albumin

mu-Conotoxin GIIIA, one of the strong peptide toxins in the cone shell, preferentially blocks the skeletal muscle-type sodium channels in vertebrates. The toxicity of mu-conotoxin GIIIA is nearly equal to that of tetrodotoxin. The generation of an antibody for the native toxins is analytically useful, but practically difficult due to its high toxicity to animals. In this study, we generated the polyclonal antibody for mu-conotoxin GIIIA using a specific conjugation method in which the immunogen was detoxified while retaining the active-site structure for the sodium channels. ELISA analysis showed that the generated antibody recognized the native toxin folded with three disulfide bridges, but not the linear one. Furthermore, the physiologically active mutants of GIIIA were recognized while the inactive mutants were not, suggesting that the newly generated antibody can selectively recognize the physiologically active toxins. These methods for generating an antibody against peptide toxins will be applicable to other peptide toxins.

Electrostatic and steric contributions to block of the skeletal muscle sodium channel by mu-conotoxin

Pore-blocking toxins are valuable probes of ion channels that underlie electrical signaling. To be effective inhibitors, they must show high affinity and specificity and prevent ion conduction. The 22-residue sea snail peptide, mu-conotoxin GIIIA, blocks the skeletal muscle sodium channel completely. Partially blocking peptides, derived by making single or paired amino acid substitutions in mu-conotoxin GIIIA, allow a novel analysis of blocking mechanisms. Replacement of one critical residue (Arg-13) yielded peptides that only partially blocked single-channel current. These derivatives, and others with simultaneous substitution of a second residue, were used to elucidate the structural basis of the toxin's blocking action. The charge at residue-13 was the most striking determinant. A positive charge was necessary, though not sufficient, for complete block. Blocking efficacy increased with increasing residue-13 side chain size, regardless of charge, suggesting a steric contribution to inhibition. Charges grouped on one side of the toxin molecule at positions 2, 12, and 14 had a weaker influence, whereas residue-16, on the opposite face of the toxin, was more influential. Most directly interpreted, the data suggest that one side of the toxin is masked by close apposition to a binding surface on the pore, whereas the other side, bearing Lys-16, is exposed to an aqueous cavity accessible to entering ions. Strong charge-dependent effects emanate from this toxin surface. In the native toxin, Arg-13 probably presents a strategically placed electrostatic barrier rather than effecting a complete steric occlusion of the pore. This differs from other well-described channel inhibitors such as the charybdotoxin family of potassium channel blockers and the sodium channel-blocking guanidinium toxins (tetrodotoxin and saxitoxin), which appear to occlude the narrow part of the pore.

Clockwise domain arrangement of the sodium channel revealed by (mu)-conotoxin (GIIIA) docking orientation

mu-Conotoxins (mu-CTXs) specifically inhibit Na(+) flux by occluding the pore of voltage-gated Na(+) channels. Although the three-dimensional structures of mu-CTXs are well defined, the molecular configuration of the channel receptor is much less certain; even the fundamental question of whether the four homologous Na(+) channel domains are arranged in a clockwise or counter-clockwise configuration remains unanswered. Residues Asp(762) and Glu(765) from domain II and Asp(1241) from domain III of rat skeletal muscle Na(+) channels are known to be critical for mu-CTX binding. We probed toxin-channel interactions by determining the potency of block of wild-type, D762K, E765K, and D1241C channels by wild-type and point-mutated mu-CTXs (R1A, Q14D, K11A, K16A, and R19A). Individual interaction energies for different toxin-channel pairs were quantified from the half-blocking concentrations using mutant cycle analysis. We find that Asp(762) and Glu(765) interact strongly with Gln(14) and Arg(19) but not Arg(1) and that Asp(1241) is tightly coupled to Lys(16) but not Arg(1) or Lys(11). These newly identified toxin-channel interactions within adjacent domains, interpreted in light of the known asymmetric toxin structure, fix the orientation of the toxin with respect to the channel and reveal that the four internal domains of Na(+) channels are arranged in a clockwise configuration as viewed from the extracellular surface.

mu-conotoxin GIIIA interactions with the voltage-gated Na(+) channel predict a clockwise arrangement of the domains

Voltage-gated Na(+) channels underlie the electrical activity of most excitable cells, and these channels are the targets of many antiarrhythmic, anticonvulsant, and local anesthetic drugs. The channel pore is formed by a single polypeptide chain, containing four different, but homologous domains that are thought to arrange themselves circumferentially to form the ion permeation pathway. Although several structural models have been proposed, there has been no agreement concerning whether the four domains are arranged in a clockwise or a counterclockwise pattern around the pore, which is a fundamental question about the tertiary structure of the channel. We have probed the local architecture of the rat adult skeletal muscle Na(+) channel (mu1) outer vestibule and selectivity filter using mu-conotoxin GIIIA (mu-CTX), a neurotoxin of known structure that binds in this region. Interactions between the pore-forming loops from three different domains and four toxin residues were distinguished by mutant cycle analysis. Three of these residues, Gln-14, Hydroxyproline-17 (Hyp-17), and Lys-16 are arranged approximately at right angles to each other in a plane above the critical Arg-13 that binds directly in the ion permeation pathway. Interaction points were identified between Hyp-17 and channel residue Met-1240 of domain III and between Lys-16 and Glu-403 of domain I and Asp-1532 of domain IV. These interactions were estimated to contribute -1.0+/-0.1, -0.9+/-0.3, and -1.4+/-0.1 kcal/mol of coupling energy to the native toxin-channel complex, respectively. mu-CTX residues Gln-14 and Arg-1, both on the same side of the toxin molecule, interacted with Thr-759 of domain II. Three analytical approaches to the pattern of interactions predict that the channel domains most probably are arranged in a clockwise configuration around the pore as viewed from the extracellular surface.

Novel structural determinants of mu-conotoxin (GIIIB) block in rat skeletal muscle (mu1) Na+ channels

mu-Conotoxin (mu-CTX) specifically occludes the pore of voltage-dependent Na(+) channels. In the rat skeletal muscle Na(+) channel (mu1), we examined the contribution of charged residues between the P loops and S6 in all four domains to mu-CTX block. Conversion of the negatively charged domain II (DII) residues Asp-762 and Glu-765 to cysteine increased the IC(50) for mu-CTX block by approximately 100-fold (wild-type = 22.3 +/- 7.0 nm; D762C = 2558 +/- 250 nm; E765C = 2020 +/- 379 nm). Restoration or reversal of charge by external modification of the cysteine-substituted channels with methanethiosulfonate reagents (methanethiosulfonate ethylsulfonate (MTSES) and methanethiosulfonate ethylammonium (MTSEA)) did not affect mu-CTX block (D762C: IC(50, MTSEA+) = 2165.1 +/- 250 nm; IC(50, MTSES-) = 2753.5 +/- 456.9 nm; E765C: IC(50, MTSEA+) = 2200.1 +/- 550.3 nm; IC(50, MTSES-) = 3248.1 +/- 2011.9 nm) compared with their unmodified counterparts. In contrast, the charge-conserving mutations D762E (IC(50) = 21.9 +/- 4.3 nm) and E765D (IC(50) = 22.0 +/- 7.0 nm) preserved wild-type blocking behavior, whereas the charge reversal mutants D762K (IC(50) = 4139.9 +/- 687.9 nm) and E765K (IC(50) = 4202.7 +/- 1088.0 nm) destabilized mu-CTX block even further, suggesting a prominent electrostatic component of the interactions between these DII residues and mu-CTX. Kinetic analysis of mu-CTX block reveals that the changes in toxin sensitivity are largely due to accelerated toxin dissociation (k(off)) rates with little changes in association (k(on)) rates. We conclude that the acidic residues at positions 762 and 765 are key determinants of mu-CTX block, primarily by virtue of their negative charge. The inability of the bulky MTSES or MTSEA side chain to modify mu-CTX sensitivity places steric constraints on the sites of toxin interaction.

Conus peptides targeted to specific nicotinic acetylcholine receptor subtypes

The venoms of predatory cone snails represent a rich combinatorial-like library of evolutionarily selected, neuropharmacologically active peptides. A major fraction of the venom components are conotoxins--small, disulfide-rich peptides that potently and specifically target components of the neuromuscular system, particularly ligand- and voltage-gated ion channels. This review focuses on Conus peptides, which act at nicotinic acetylcholine receptors. These nicotinic antagonist peptides from Conus are broadly divided into two groups: those that act at the neuromuscular junction and those that act at subtypes of neuronal nicotinic acetylcholine receptors. The latter include peptides specific for the alpha 7, alpha 3 beta 2, and alpha 3 beta 4 nicotinic receptor subtypes. The degree of specificity exhibited by these peptides is remarkable, particularly given their relatively small size. As a group the nicotinic acetylcholine receptor-targeted Conus peptides represent an increasingly well-defined set of tools for probing the structure, function, and physiological role of nicotinic acetylcholine receptors.

Charged residues between the selectivity filter and S6 segments contribute to the permeation phenotype of the sodium channel

The deep regions of the Na(+) channel pore around the selectivity filter have been studied extensively; however, little is known about the adjacent linkers between the P loops and S6. The presence of conserved charged residues, including five in a row in domain III (D-III), hints that these linkers may play a role in permeation. To characterize the structural topology and function of these linkers, we neutralized the charged residues (from position 411 in D-I and its homologues in D-II, -III, and -IV to the putative start sites of S6) individually by cysteine substitution. Several cysteine mutants displayed enhanced sensitivities to Cd(2+) block relative to wild-type and/or were modifiable by external sulfhydryl-specific methanethiosulfonate reagents when expressed in TSA-201 cells, indicating that these amino acids reside in the permeation pathway. While neutralization of positive charges did not alter single-channel conductance, negative charge neutralizations generally reduced conductance, suggesting that such charges facilitate ion permeation. The electrical distances for Cd(2+) binding to these residues reveal a secondary "dip" into the membrane field of the linkers in domains II and IV. Our findings demonstrate significant functional roles and surprising structural features of these previously unexplored external charged residues.

Functional and structural features of gamma-zeathionins, a new class of sodium channel blockers

Gamma1- and gamma2-zeathionins (gamma1-Z and gamma2-Z) are members of a family of small and basic peptides involved in plant protection. These plant defensins exhibit remarkable structural similarity to scorpion neurotoxins and insect defensins. In the present report, we used the whole-cell patch clamp technique to investigate the inhibition of the sodium current (I(Na)) by gamma1-Z and gamma2-Z in the GH3 cell line. Both gamma1-Z and gamma2-Z rapidly and reversibly inhibited I(Na) without changing the kinetics or voltage dependence of activation or inactivation. To our knowledge, this is the first example of a plant protein that inhibits the sodium channel. From structural comparisons with the mu-conotoxins, a family of peptides that block the sodium channel, we detected some similar features that could provide the basis of inhibition of sodium channels by gamma-zeathionins.

Extrapore residues of the S5-S6 loop of domain 2 of the voltage-gated skeletal muscle sodium channel (rSkM1) contribute to the mu-conotoxin GIIIA binding site

The tetradomain voltage-gated sodium channels from rat skeletal muscle (rSkM1) and from human heart (hH1) possess different sensitivities to the 22-amino-acid peptide toxin, mu-conotoxin GIIIA (mu-CTX). rSkM1 is sensitive (IC50 = 51.4 nM) whereas hH1 is relatively resistant (IC50 = 5700 nM) to the action of the toxin, a difference in sensitivity of >100-fold. The affinity of the mu-CTX for a chimera formed from domain 1 (D1), D2, and D3 from rSkM1and D4 from hH1 (SSSH; S indicates origin of domain is skeletal muscle and H indicates origin of domain is heart) was paradoxically increased approximately fourfold relative to that of rSkM1. The source of D3 is unimportant regarding the difference in the relative affinity of rSkM1 and hH1 for mu-CTX. Binding of mu-CTX to HSSS was substantially decreased (IC50 = 1145 nM). Another chimera with a major portion of D2 deriving form hH1 showed no detectable binding of mu-CTX (IC50 > 10 microM). These data indicate that D1 and, especially, D2 play crucial roles in forming the mu-CTX receptor. Charge-neutralizing mutations in D1 and D2 (Asp384, Asp762, and Glu765) had no effect on toxin binding. However, mutations at a neutral and an anionic site (residues 728 and 730) in S5-S6/D2 of rSkM1, which are not in the putative pore region, were found to decrease significantly the mu-CTX affinity with little effect on tetrodotoxin binding (</=1.3-fold increase in affinity). Furthermore, substitution at Asp730 with cysteine and exposure to Cd2+ or methanethiosulfonate reagents had no significant effect on sodium currents, consistent with this residue not contributing to the pore.

Three-dimensional solution structure of conotoxin psi-PIIIE, an acetylcholine gated ion channel antagonist

The three-dimensional structure of conotoxin psi-PIIIE, a 24-amino acid peptide from Conus purpurascens, has been solved using two-dimensional (2D) 1H NMR spectroscopy. Conotoxin psi-PIIIE contains the same disulfide bonding pattern as the mu-conotoxins, which target skeletal muscle sodium channels, but has been shown to antagonize the acetylcholine gated cation channel through a noncompetitive mechanism. Structural information was obtained by the analysis of a series of 2D NOESY spectra as well as measurement of coupling constants from 1D 1H and PE-COSY NMR experiments. Molecular modeling calculations included the use of the distance geometry (DG) algorithm, simulated annealing techniques, and the restrained molecular dynamics method. The resulting structures are considerably similar to the previously published structures for the mu-conotoxins GIIIA and GIIIB, despite the lack of sequence conservation between conotoxin psi-PIIIE and the mu-conotoxins. The structure consists of a series of tight turns, each turn occurring in the position analogous to those of turns described in mu-GIIIA and mu-GIIIB. This suggests the disulfide bonding pattern is able to largely direct the structure of the peptides, creating a stable structural motif which allows extensive sequence substitution of non-cystine residues.

On the structural basis for size-selective permeation of organic cations through the voltage-gated sodium channel. Effect of alanine mutations at the DEKA locus on selectivity, inhibition by Ca2+ and H+, and molecular sieving

Recent evidence indicates that ionic selectivity in voltage-gated Na+ channels is mediated by a small number of residues in P-region segments that link transmembrane elements S5 and S6 in each of four homologous domains denoted I, II, III, and IV. Important determinants for this function appear to be a set of conserved charged residues in the first three homologous domains, Asp(I), Glu(II), and Lys(III), located in a region of the pore called the DEKA locus. In this study, we examined several Ala-substitution mutations of these residues for alterations in ionic selectivity, inhibition of macroscopic current by external Ca2+ and H+, and molecular sieving behavior using a series of organic cations ranging in size from ammonium to tetraethylammonium. Whole-cell recording of wild-type and mutant channels of the rat muscle micro1 Na+ channel stably expressed in HEK293 cells was used to compare macroscopic current-voltage behavior in the presence of various external cations and an intracellular reference solution containing Cs+ and very low Ca2+. In particular, we tested the hypothesis that the Lys residue in domain III of the DEKA locus is responsible for restricting the permeation of large organic cations. Mutation of Lys(III) to Ala largely eliminated selectivity among the group IA monovalent alkali cations (Li+, Na+, K+, Rb+, Cs+) and permitted inward current of group IIA divalent cations (Mg2+, Ca2+, Sr2+, Ba2+). This same mutation also resulted in the acquisition of permeability to many large organic cations such as methylammonium, tetramethylammonium, and tetraethylammonium, all of which are impermeant in the native channel. The results lead to the conclusion that charged residues of the DEKA locus play an important role in molecular sieving behavior of the Na+ channel pore, a function that has been previously attributed to a hypothetical region of the channel called the "selectivity filter." A detailed examination of individual contributions of the Asp(I), Glu(II), and Lys(III) residues and the dependence on molecular size suggests that relative permeability of organic cations is a complex function of the size, charge, and polarity of these residues and cation substrates. As judged by effects on macroscopic conductance, charged residues of the DEKA locus also appear to play a role in the mechanisms of block by external Ca2+ and H+, but are not essential for the positive shift in activation voltage that is produced by these ions.

Pore residues critical for mu-CTX binding to rat skeletal muscle Na+ channels revealed by cysteine mutagenesis

We have studied mu-conotoxin (mu-CTX) block of rat skeletal muscle sodium channel (rSkM1) currents in which single amino acids within the pore (P-loop) were substituted with cysteine. Among 17 cysteine mutants expressed in Xenopus oocytes, 7 showed significant alterations in sensitivity to mu-CTX compared to wild-type rSkM1 channel (IC50 = 17.5 +/- 2.8 nM). E758C and D1241C were less sensitive to mu-CTX block (IC50 = 220 +/- 39 nM and 112 +/- 24 nM, respectively), whereas the tryptophan mutants W402C, W1239C, and W1531C showed enhanced mu-CTX sensitivity (IC50 = 1.9 +/- 0.1, 4.9 +/- 0.9, and 5.5 +/- 0.4 nM, respectively). D400C and Y401C also showed statistically significant yet modest (approximately twofold) changes in sensitivity to mu-CTX block compared to WT (p < 0.05). Application of the negatively charged, sulfhydryl-reactive compound methanethiosulfonate-ethylsulfonate (MTSES) enhanced the toxin sensitivity of D1241C (IC50 = 46.3 +/- 12 nM) while having little effect on E758C mutant channels (IC50 = 199.8 +/- 21.8 nM). On the other hand, the positively charged methanethiosulfonate-ethylammonium (MTSEA) completely abolished the mu-CTX sensitivity of E758C (IC50 > 1 microM) and increased the IC50 of D1241C by about threefold. Applications of MTSEA, MTSES, and the neutral MTSBN (benzyl methanethiosulfonate) to the tryptophan-to-cysteine mutants partially or fully restored the wild-type mu-CTX sensitivity, suggesting that the bulkiness of the tryptophan's indole group is a determinant of toxin binding. In support of this suggestion, the blocking IC50 of W1531A (7.5 +/- 1.3 nM) was similar to W1531C, whereas W1531Y showed reduced toxin sensitivity (14.6 +/- 3.5 nM) similar to that of the wild-type channel. Our results demonstrate that charge at positions 758 and 1241 are important for mu-CTX toxin binding and further suggest that the tryptophan residues within the pore in domains I, III, and IV negatively influence toxin-channel interaction.

Solution structure of the sodium channel antagonist conotoxin GS: a new molecular caliper for probing sodium channel geometry

BACKGROUND

The venoms of Conus snails contain small, disulfide-rich inhibitors of voltage-dependent sodium channels. Conotoxin GS is a 34-residue polypeptide isolated from Conus geographus that interacts with the extracellular entrance of skeletal muscle sodium channels to prevent sodium ion conduction. Although conotoxin GS binds competitively with mu conotoxin GIIIA to the sodium channel surface, the two toxin types have little sequence identity with one another, and conotoxin GS has a four-loop structural framework rather than the characteristic three-loop mu-conotoxin framework. The structural study of conotoxin GS will form the basis for establishing a structure-activity relationship and understanding its interaction with the pore region of sodium channels.

RESULTS

The three-dimensional structure of conotoxin GS was determined using two-dimensional NMR spectroscopy. The protein exhibits a compact fold incorporating a beta hairpin and several turns. An unusual feature of conotoxin GS is the exceptionally high proportion (100%) of cis-imide bond geometry for the three proline or hydroxyproline residues. The structure of conotoxin GS bears little resemblance to the three-loop mu conotoxins, consistent with the low sequence identity between the two toxin types and their different structural framework. However, the tertiary structure and cystine-knot motif formed by the three disulfide bonds is similar to that present in several other polypeptide ion channel inhibitors.

CONCLUSIONS

This is the first three-dimensional structure of a 'four-loop' sodium channel inhibitor, and it represents a valuable new structural probe for the pore region of voltage-dependent sodium channels. The distribution of amino acid sidechains in the structure creates several polar and charged patches, and comparison with the mu conotoxins provides a basis for determining the binding surface of the conotoxin GS polypeptide.

Molecular basis of charge movement in voltage-gated sodium channels

Voltage-dependent movement of a sodium channel S4 segment was examined by cysteine scanning mutagenesis and testing accessibility of the residues to hydrophilic cysteine-modifying reagents. These experiments indicate that 2 charged S4 residues move completely from an internally accessible to an externally accessible location in response to depolarization by passage through a short "channel" in the protein. The energetic problems of S4 movement have thus been solved in the same way that may ion channels achieve highly selective and rapid ion permeation through an open pore, by restricting the contact region between the permion and its channel.