Evolutionary Diversification of Mesobuthus α-Scorpion Toxins Affecting Sodium Channels

Abstract

␣-Scorpion toxins constitute a family of peptide modulators that induce a prolongation of the action potential of excitable cells by inhibiting voltage-gated sodium channel inactivation. Although they all adopt a conserved structural scaffold, the potency and phylogentic preference of these toxins largely vary, which render them an intriguing model for studying evolutionary diversification among family members. Here, we report molecular characterization of a new multigene family of ␣-toxins comprising 13 members (named MeuNaTx␣-1 to MeuNaTx␣-13) from the scorpion Mesobuthus eupeus. Of them, five native toxins (MeuNaTx␣-1 to -5) were purified to homogeneity from the venom and the solution structure of MeuNaTx␣ Venomous animals, such as sea anemones, scorpions, spiders, cone snails, jellyfish, and snakes, represent a distinct group of organisms that evolve toxins to help them capture prey and defend themselves from predators (1). These animals occupy diverse ecological niches and have different evolutionary histories and food sources, but they convergently select voltage-gated sodium channels (VGSCs) 1 as molecular targets for their toxins that can strongly modify VGSCs' functions to alter electrical excitability of an animal's neuromuscular system and thus cause rapid immobilization of both prey and competitors (2, 3). VGSCs play vital roles in the normal functioning of excitable tissues (neurons, muscles, and heart). These large, complex membrane proteins are composed of one principal ␣-subunit of 220 -260 kDa associated with one or more auxiliary ␤-subunits of ϳ33-36 kDa in some tissues of mammals and the tipE subunit in insects (4 -6). The ␣-subunit of VGSCs includes four homologous domains (DI-DIV) connected by intracellular and extracellular loops, each domain containing six helical transmembrane segments (S1-S6), and a hairpin-like P loop between S5 and S6 that forms the ion selectivity filter. Thus far, at least seven distinct receptor sites have been identified on the ␣-subunits for various animal-derived toxins that either affects ion permeation or voltage-dependent gating properties (7) 1 The abbreviations used are: VGSC, voltage-gated sodium channel; 3Ј-UTR, 3Ј-untranslational region; CS␣␤, cysteine-stabilized ␣-helical and ␤-sheet; MALDI-TOF MS, matrix-assisted laser desorption ionization time-of-flight mass spectrometry; MeuNaTx␣, Mesobuthus eupeus ␣-scorpion toxin; PSS, positively selected site; RACE, rapid amplification of cDNA end; RP-HPLC, reversed-phase high-performance liquid chromatography; TTX, tetrodotoxin. Research

Modular Organization of α-Toxins from Scorpion Venom Mirrors Domain Structure of Their Targets, Sodium Channels

“…Similarly to typical ␣-like toxins, BeM9 was found to affect both insect and mammalian Na v s, with the exception of Na v 1.2, the major isoform found in the central nervous system. Both MeuNaTx␣-1 and -2 were characterized as ␣-like toxins by Zhu et al (36), supporting our assumptions.…”

“…Electrophysiology-All stages were performed as reported previously (36). For the expression of Na v channels (rNa v 1.2, rNa v 1.4, hNa v 1.5, mNa v 1.6, the insect channel DmNa v 1 (Para), and the auxiliary subunits r␤1, h␤1, and TipE) in Xenopus laevis oocytes, linearized plasmids were transcribed using the T7 or SP6 mMESSAGE-mMACHINE transcription kit (Ambion, Carlsbad, CA).…”

The Fungal Defensin Family Enlarged

“…The extension of a C-terminal sequence via convergent evolution was also recently observed in interleukin 6 (IL-6), a class-I helical cytokine, of two leporids ( Oryctolagus and Pentalagus ) [16]. The presence of C-terminal Gly-Arg or Gly-Arg-Arg in some dermatophyte-derived fDLPs suggest that they may be amidated, as previously observed in some animal toxins, e.g., the Mesobuthus α-toxins [17]. Interestingly, the mature peptide of micaDLP is larger in size than that of other members in this family, as identified by an N-terminal extension of 38 amino acids (Figure 2).…”

The pharmacology of voltage-gated sodium channel activators

“…Of these, the pharmacological activity of MeuNaTxα-1, and has been comprehensibly characterized. MeuNaTxα-1 modulates Na V 1.2, Na V 1.3, Na V 1.6 and Na V 1.7, MeuNaTxα-2 affects Na V 1.4 only, whereas MeuNaTxα-5 is the least selective modulating Na V 1.3-1.7 (Zhu et al, 2012). These peptides also potently slow inactivation in the insect channel counterpart DmNaV1.…”