Molecular basis of the high insecticidal potency of scorpion α-toxins

Scorpion alpha-toxins are similar in their mode of action and three-dimensional structure but differ considerably in affinity for various voltage-gated sodium channels (NaChs). To clarify the molecular basis of the high potency of the alpha-toxin LqhalphaIT ( from Leiurus quinquestriatus hebraeus) for insect NaChs, we identified by mutagenesis the key residues important for activity. We have found that the functional surface is composed of two distinct domains: a conserved Core-domain formed by residues of the loops connecting the secondary structure elements of the molecule core and a variable NC-domain formed by a five-residue turn ( residues 8 - 12) and a C-terminal segment ( residues 56 - 64). We further analyzed the role of these domains in toxin activity on insects by their stepwise construction onto the scaffold of the anti-mammalian alpha-toxin, Aah2 ( from Androctonus australis hector). The chimera harboring both domains, Aah2(LqhalphaIT(face)), was as active to insects as LqhalphaIT. Structure determination of Aah2(LqhalphaIT(face)) by x-ray crystallography revealed that the NC-domain deviates from that of Aah2 and forms an extended protrusion off the molecule core as appears in LqhalphaIT. Notably, such a protrusion is observed in all alpha-toxins active on insects. Altogether, the division of the functional surface into two domains and the unique configuration of the NC-domain illuminate the molecular basis of alpha-toxin specificity for insects and suggest a putative binding mechanism to insect NaChs.