Molecular analysis of the sea anemone toxin Av3 reveals selectivity to insects and demonstrates the heterogeneity of receptor site-3 on voltage-gated Na⁺ channels

Av3 is a short peptide toxin from the sea anemone Ammonia viridis shown to be active on crustaceans and inactive on mammals. It inhibits inactivation of Na_vs (voltage-gated Na⁺ channels) like the structurally dissimilar scorpion α-toxins and type I sea anemone toxins that bind to receptor site-3. To examine the potency and mode of interaction of Av3 with insect Na_vs, we established a system for its expression, mutagenized it throughout, and analysed it in toxicity, binding and electrophysiological assays. The recombinant Av3 was found to be highly toxic to blowfly larvae (ED₅₀ = 2.65 ± 0.46 pmol/100 mg), to compete well with the site-3 toxin LqhαIT (from the scorpion Leiurus quinquestriatus) on binding to cockroach neuronal membranes (K_i = 21.4 ± 7.1 nM), and to inhibit the inactivation of Drosophila melanogaster channel, DmNa_v1, but not that of mammalian Na_vs expressed in Xenopus oocytes. Moreover, like other site-3 toxins, the activity of Av3 was synergically enhanced by ligands of receptor site-4 (e.g. scorpion β-toxins). The bioactive surface of Av3 was found to consist mainly of aromatic residues and did not resemble any of the bioactive surfaces of other site-3 toxins. These analyses have portrayed a toxin that might interact with receptor site-3 in a different fashion compared with other ligands of this site. This assumption was corroborated by a D1701R mutation in DmNa_v1, which has been shown to abolish the activity of all other site-3 ligands, except Av3. All in all, the present study provides further evidence for the heterogeneity of receptor site-3, and raises Av3 as a unique model for design of selective anti-insect compounds.