The affinity of scorpion α-toxins for various voltage-gated sodium channels (Navs) differs considerably despite similar structures and activities. It has been proposed that key bioactive residues of the five-residue-turn (residues 8-12) and the C-tail form the NC domain, whose topology is dictated by a cis or trans peptide-bond conformation between residues 9 and 10, which correlates with the potency on insect or mammalian Navs. We examined this hypothesis using Lqh3, an α-like toxin from Leiurus quinquestriatus hebraeus that is highly active in insects and mammalian brain. Lqh3 exhibits slower association kinetics to Navs compared with other α-toxins and its binding to insect Navs is pH-dependent. Mutagenesis of Lqh3 revealed a bi-partite bioactive surface, composed of the Core and NC domains, as found in other α-toxins. Yet, substitutions at the five-residue turn and stabilization of the 9-10 bond in the cis conformation did not affect the activity. However, substitution of hydrogen-bond donors/acceptors at the NC domain reduced the pH-dependency of toxin binding, while retaining its high potency at Drosophila Navs expressed in Xenopus oocytes. Based on these results and the conformational flexibility and rearrangement of intramolecular hydrogen-bonds at the NC domain, evident from the known solution structure, we suggest that acidic pH or specific mutations at the NC domain favor toxin conformations with high affinity for the receptor by stabilizing the bound toxin-receptor complex. Moreover, the C-tail flexibility may account for the slower association rates and suggests a novel mechanism of dynamic conformer selection during toxin binding, enabling α-like toxins to affect a broad range of Navs.
- Scorpion α-like toxin
- Structure-function relationships
- Toxin effect on inactivation
- Toxin receptor site on sodium channel
- pH-dependent toxin binding