A spider toxin that induces a typical effect of scorpion α-toxins but competes with β-toxins on binding to insect sodium channels

δ-Palutoxins from the spider Paracoelotes luctuosus (Araneae: Amaurobiidae) are 36-37 residue long peptides that show preference for insect sodium channels (NaChs) and modulate their function. Although they slow NaCh inactivation in a fashion similar to that of receptor site 3 modifiers, such as scorpion α-toxins, they actually bind with high affinity to the topologically distinct receptor site 4 of scorpion β-toxins. To resolve this riddle, we scanned by Ala mutagenesis the surface of δ-PaluIT2, a δ-palutoxin variant with the highest affinity for insect NaChs, and compared it to the bioactive surface of a scorpion β-toxin. We found three regions on the surface of δ-PaluIT2 important for activity: the first consists of Tyr-22 and Tyr-30 (aromatic), Ser-24 and Met-28 (polar), and Arg-8, Arg-26, Arg-32, and Arg-34 (basic) residues; the second is made of Trp-12; and the third is made of Asp-19, whose substitution by Ala uncoupled the binding from toxicity to lepidopteran larvae. Although spider δ-palutoxins and scorpion β-toxins have developed from different ancestors, they show some commonality in their bioactive surfaces, which may explain their ability to compete for an identical receptor (site 4) on voltage-gated NaChs. Yet, their different mode of channel modulation provides a novel perspective about the structural relatedness of receptor sites 3 and 4, which until now have been considered topologically distinct.