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

Abstract

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.

New Tricks of an Old Pattern

“…-KTxs are relatively poor blockers of K ϩ channels, despite presence of the typical functional dyad, Tyr and Lys, important for other activity of the toxin (3). It is still neither clear whether members of the -KTx family might all work through the same mechanism on potassium channels (16,17), nor whether K ϩ channels are indeed their cognate receptors. In part this may be due to the lack of structural and functional characterization of peptides in this family.…”

“…The last and smallest family, the -KTxs, displays purely helical structure stabilized by two disulfide bridges and fold into an ␣-hairpin fold known as CS␣/␣ (cystine-stabilized helix-loop-helix) (11, 14 -17). Peptides belonging to this family have been isolated from only two scorpion genera: Heterometrus (14,15) and Opisthacanthus (16,17). -KTxs are relatively poor blockers of K ϩ channels, despite presence of the typical functional dyad, Tyr and Lys, important for other activity of the toxin (3).…”

Novel Detection Methods Used in Conjunction with Affinity Chromatography for the Identification and Purification of Hydrolytic Enzymes or Enzyme Inhibitors from Insects and Plants

“…Further analysis of data clarified that some forms of plant thionins (e.g. purothionins) and neurotoxins from scorpions (Chagot et al, 2005) and marine snails (Möller et al, 2005) also possess a similar cysteine configuration and two alpha-helices connected by a loop and stabilized by two disulphide bridges. Today it is clear that effective enzyme inhibitors with some additional features (e.g.…”

Structural Similarity between Defense Peptide from Wheat and Scorpion Neurotoxin Permits Rational Functional Design

“…It should be highlighted that this scaffold is shared also by potassium channel blockers -hefutoxin 1 from Heterometrus fulvipes (23) and OmTx1-3 from Opisthacanthus madagascariensis (21).…”

“…This peptide belongs to the ␣-hairpinin family of plant defense peptides (18), which includes both antimicrobial peptides (AMPs) and protease inhibitors (18 -20). Its main structural features are two ␣-helices stabilized by two disulfide bonds and two characteristic cysteine motifs CXXXC where X is any residue; the disulfide pairing is C A number of peptide toxins from venomous animals such as scorpions and sea snails (21)(22)(23) present the same fold. Thus, this kind of peptide structure seems to be a rather universal scaffold for a variety of biological functions.…”