Structural implications on the interaction of scorpion α-like toxins with the sodium channel receptor site inferred from toxin iodination and pH-dependent binding

The α-like toxin from the venom of the scorpion Leiurus quinquestriatus hebraeus (Lqh III) binds with high affinity to receptor site 3 on insect sodium channels but does not bind to rat brain synaptosomes. The binding affinity of Lqh III to cockroach neuronal membranes was fivefold higher at pH 6.5 than at pH 7.5. This correlated with an increase in the electropositive charge on the toxin surface resulting from protonation of its four histidines. Radioiodination of Tyr¹⁴ of Lqh III abolished its binding to locust but not cockroach sodium channels, whereas the noniodinated toxin bound equally well to both neuronal preparations. Radioiodination of Tyr¹⁰ or Tyr²¹ of the structurally similar α-toxin from L. quinquestriatus hebraeus (LqhαlT), as well as their substitution by phenylalanine, had only minor effects on binding to cockroach neuronal membranes. However, substitution of Tyr²¹, but not Tyr¹⁴, by leucine decreased the binding affinity of LqhαlT ~87-fold. Thus, Tyr¹⁴ is involved in the bioactivity of Lqh III to locust receptor site 3 and is not crucial for the binding of LqhαlT to this site. In turn, the aromatic ring of Tyr²¹ takes part in the bioactivity of LqhαlT to insects. These results highlight subtle architectural variations between locust and cockroach receptor site 3, in addition to previous results demonstrating the competence of Lqh III to differentiate between insect and mammalian sodium channel subtypes.