An unusual fold for potassium channel blockers: NMR structure of three toxins from the scorpion Opisthacanthus madagascariensis

Architecture et Fonction des Macromolécules Biologiques, UMR 6098, CNRS et Universités d'Aix-Marseille I et II, 31 Chemin Joseph Aiguier, 13402 Marseille Cedex 20, France.
Biochemical Journal (Impact Factor: 4.4). 06/2005; 388(Pt 1):263-71. DOI: 10.1042/BJ20041705
Source: PubMed


The Om-toxins are short peptides (23-27 amino acids) purified from the venom of the scorpion Opisthacanthus madagascariensis. Their pharmacological targets are thought to be potassium channels. Like Csalpha/beta (cystine-stabilized alpha/beta) toxins, the Om-toxins alter the electrophysiological properties of these channels; however, they do not share any sequence similarity with other scorpion toxins. We herein demonstrate by electrophysiological experiments that Om-toxins decrease the amplitude of the K+ current of the rat channels Kv1.1 and Kv1.2, as well as human Kv1.3. We also determine the solution structure of three of the toxins by use of two-dimensional proton NMR techniques followed by distance geometry and molecular dynamics. The structures of these three peptides display an uncommon fold for ion-channel blockers, Csalpha/alpha (cystine-stabilized alpha-helix-loop-helix), i.e. two alpha-helices connected by a loop and stabilized by two disulphide bridges. We compare the structures obtained and the dipole moments resulting from the electrostatic anisotropy of these peptides with those of the only other toxin known to share the same fold, namely kappa-hefutoxin1.

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Available from: Hervé Darbon, Mar 25, 2015
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    • "Kv1.3 channels at 500 mM (Chagot et al., 2005). OcyC8 (k- KTx2.5) inhibits the K þ currents of the Kv1.1 and Kv1.4 channels showing IC 50 values of 217 and 71, respectively (Camargos et al., 2011). "
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    ABSTRACT: The number and types of venom components that affect ion-channel function are reviewed. These are the most important venom components responsible for human intoxication, deserving medical attention, often requiring the use of specific anti-venoms. Special emphasis is given to peptides that recognize Na(+)-, K(+)- and Ca(++)-channels of excitable cells. Knowledge generated by direct isolation of peptides from venom and components deduced from cloned genes, whose amino acid sequences are deposited into databanks are now adays in the order of 1.5 thousands, out of an estimate biodiversity closed to 300,000. Here the diversity of components is briefly reviewed with mention to specific references. Structural characteristic are discussed with examples taken from published work. The principal mechanisms of action of the three different types of peptides are also reviewed. Na(+)-channel specific venom components usually are modifier of the open and closing kinetic mechanisms of the ion-channels, whereas peptides affecting K(+)-channels are normally pore blocking agents. The Ryanodine Ca(++)-channel specific peptides are known for causing sub-conducting stages of the channels conductance and some were shown to be able to internalize penetrating inside the muscle cells.
    Toxicon 07/2013; 76. DOI:10.1016/j.toxicon.2013.07.012 · 2.49 Impact Factor
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    • "Scorpion KTxs were originally classified into three families named ␣, ␤ and ␥ [29], all of them have the above mentioned, highly conserved, ␣/␤ structural arrangement. Latter, scorpion KTxs presenting a different structural arrangement, with only two ␣-helices stabilized by two disulfide bonds, CS␣/␣, were described, and they were named ␬-KTxs [6] [28] [4]. Among the almost 200 scorpion KTxs described until now (for a complete list see, "
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    ABSTRACT: Opisthacanthus cayaporum belongs to the Liochelidae family, and the scorpions from this genus occur in southern Africa, Central America and South America and, therefore, can be considered a true Gondwana heritage. In this communication, the isolation, primary structure characterization, and K(+)-channel blocking activity of new peptide from this scorpion venom are reported. OcyKTx2 is a 34 amino acid long peptide with four disulfide bridges and molecular mass of 3807Da. Electrophysiological assays conducted with pure OcyKTx2 showed that this toxin reversibly blocks Shaker B K(+)-channels with a Kd of 82nM, and presents an even better affinity towards hKv1.3, blocking it with a Kd of ∼ 18nM. OcyKTx2 shares high sequence identity with peptides belonging to subfamily 6 of alpha-KTxs that clustered very closely in the phylogenetic tree included here. Sequence comparison, chain length and number of disulfide bridges analysis classify OcyKTx2 into subfamily 6 of the α-KTx scorpion toxins (systematic name, α-KTx6.17).
    Peptides 05/2013; 46. DOI:10.1016/j.peptides.2013.04.021 · 2.62 Impact Factor
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    • "All toxins, except HeTx203 and HeTx204, contain six cysteines with alignment patterns similar to the known acidic toxin α-KTxs BmP01 [25], [40], indicating that they may adopt the CS-α/β fold scaffold motif. The HeTx203 and HeTx204 peptides may adopt the CS-α/α fold scaffold using four cysteines, a pattern similar to that of the known acidic κ-KTx OmTx3 [12]. All new KTxs are acidic peptides with pI values 4.66 to 5.48. "
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    ABSTRACT: Although the basic scorpion K(+) channel toxins (KTxs) are well-known pharmacological tools and potential drug candidates, characterization the acidic KTxs still has the great significance for their potential selectivity towards different K(+) channel subtypes. Unfortunately, research on the acidic KTxs has been ignored for several years and progressed slowly. Here, we describe the identification of nine new acidic KTxs by cDNA cloning and bioinformatic analyses. Seven of these toxins belong to three new α-KTx subfamilies (α-KTx28, α-KTx29, and α-KTx30), and two are new members of the known κ-KTx2 subfamily. ImKTx104 containing three disulfide bridges, the first member of the α-KTx28 subfamily, has a low sequence homology with other known KTxs, and its NMR structure suggests ImKTx104 adopts a modified cystine-stabilized α-helix-loop-β-sheet (CS-α/β) fold motif that has no apparent α-helixs and β-sheets, but still stabilized by three disulfide bridges. These newly described acidic KTxs exhibit differential pharmacological effects on potassium channels. Acidic scorpion toxin ImKTx104 was the first peptide inhibitor found to affect KCNQ1 channel, which is insensitive to the basic KTxs and is strongly associated with human cardiac abnormalities. ImKTx104 selectively inhibited KCNQ1 channel with a K(d) of 11.69 µM, but was less effective against the basic KTxs-sensitive potassium channels. In addition to the ImKTx104 toxin, HeTx204 peptide, containing a cystine-stabilized α-helix-loop-helix (CS-α/α) fold scaffold motif, blocked both Kv1.3 and KCNQ1 channels. StKTx23 toxin, with a cystine-stabilized α-helix-loop-β-sheet (CS-α/β) fold motif, could inhibit Kv1.3 channel, but not the KCNQ1 channel. These findings characterize the structural and functional diversity of acidic KTxs, and could accelerate the development and clinical use of acidic KTxs as pharmacological tools and potential drugs.
    PLoS ONE 04/2012; 7(4):e35154. DOI:10.1371/journal.pone.0035154 · 3.23 Impact Factor
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