Identification of Structural Elements of a Scorpion α-Neurotoxin Important for Receptor Site Recognition*

Abstract

α-Neurotoxins from scorpion venoms constitute the most studied group of modifiers of the voltage-sensitive sodium channels, and yet, their toxic site has not been characterized. We used an efficient bacterial expression system for modifying specific amino acid residues of the highly insecticidal α-neurotoxin LqhαIT from the scorpion Leiurus quinquestriatus hebraeus. Toxin variants modified at tight turns, the C-terminal region, and other structurally related regions were subjected to neuropharmacological and structural analyses. This approach highlighted both aromatic (Tyr¹⁰ and Phe¹⁷) and positively charged (Lys⁸, Arg¹⁸, Lys⁶², and Arg⁶⁴) residues that (i) may interact directly with putative recognition points at the receptor site on the sodium channel; (ii) are important for the spatial arrangement of the toxin polypeptide; and (iii) contribute to the formation of an electrostatic potential that may be involved in biorecognition of the receptor site. The latter was supported by a suppressor mutation (E15A) that restored a detrimental effect caused by a K8D substitution. The feasibility of producing anti-insect scorpion neurotoxins with augmented toxicity was demonstrated by the substitution of the C-terminal arginine with histidine. Altogether, the present study provides for the first time an insight into the putative toxic surface of a scorpion neurotoxin affecting sodium channel gating.