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

The alpha-like toxin from the venom of the scorpion Leiurus quinquestriatus hebraeus (Lqh iii) binds with high affinity to receptor site 3 on insect s odium channels but does not bind to rat brain synaptosomes, The binding aff inity 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 histidi nes. Radioiodination of Tyr(14) of Lqh III abolished its binding to locust but not cockroach sodium channels, whereas the noniodinated toxin bound equ ally well to both neuronal preparations. Radioiodination of Tyr(10) or Tyr( 21) of th, structurally similar cu-toxin from L. quinquestriatus hebraeus ( Lqh alpha IT), as well as their substitution by phenylalanine, had only min or effects on binding to cockroach neuronal membranes. However, substitutio n of Tyr(21), but not Tyr(14), by leucine decreased the binding affinity of Lqh alpha IT similar to 87-fold. Thus, Tyr(14) is involved in the bioactiv ity of Lqh III to locust receptor site 3 and is not crucial for the binding of Lqh alpha IT to this site. In turn, the aromatic ring of Tyr21 takes pa rt in the bioactivity of Lqh alpha IT to insects. These results highlight s ubtle architectural variations between locust and cockroach receptor site 3 , in addition to previous results demonstrating the competence of Lqh III t o differentiate between insect and mammalian sodium channel subtypes.