Scorpion Neuropeptides Affecting the Insect Voltage Gated Sodium Channels - Diversity and Cooperativity
Long-chain neurotoxins derived from the venom of the Buthidae scorpions, which affect voltage-gated sodium channels (VGSCs) are divided to the α category (which affects sodium inactivation) and ß category (affecting the sodium activation stage). The excitatory and depressant insect-selective toxins from scorpion venom belong to the ß category. We provide here two pieces of information concerning the chemistry and agrotechnical significance of the insect-selective toxins.
The efficacy of the recombinant baculovirus (expressing an anti-insect toxin) is attributed mainly to its ability (a) to continuously provide and translocate the gene of the expressed toxin to the insect central nervous system; (b) to provide a mechanism for insecticide resistance management. The latter is based on the pharmacological flexibility of the voltage-gated sodium channel. Thus, channel mutations conferring resistance to a given class of insecticidal agents (such as the KDR phenomenon) may greatly increase susceptibility to the toxin-expressing baculoviral bioinsecticides.
The study reveals an insect-selective toxin derived from the α category of scorpion neuropeptides. By the aid of reverse-phase HPLC column chromatography, RT-PCR, cloning and various toxicity assays, a new insect selective toxin designated as BjαIT was isolated from the venom of the Judean Black Scorpion (Buthotus judaicus), and its full primary sequence was determined: MNYLVVICFALLLMTVVESGRDAYIADNLNCAY-TCGSNSYCNTEWCTKNGAVSGYCQWLGKYGNACWCINLPDKVPIRIPGACR (leader sequence is underlined). Despite its lack of toxicity to mammals and potent toxicity to insects, BjαIT reveals an amino acid sequence and an inferred spatial arrangement that is characteristic of the well-known scorpion α-toxins highly toxic to mammals. BjαITs sharp distinction between insects and mammals was also revealed by its effect on sodium conductance of two cloned neuronal VGSCs heterologously expressed in Xenopus laevis oocytes and assayed with the two-electrode voltage-clamp technique. BjαIT completely inhibits the inactivation process of the insect para/tipE VGSC at a concentration of 100 nM, in contrast to the rat brain Nav1.2/ß1 which is resistant to the toxin.