Scorpion Neurotoxin BeM9 Derivative Uncovers Unique Interaction Mode with Nav1.5 Sodium Channel Isoform

Scorpion α-neurotoxins are classical ligands of voltage-gated sodium channels that inhibit their inactivation. The strength of this effect depends on the organism and channel isoform, and the precise mechanisms explaining the differences in activity are still unknown. Previously, we have shown that scorpion α-toxins are characterized by a modular structure. They consist of a conserved and structurally stable core module and a variable and mobile specificity module, which determines the selectivity for different channels. We noted a higher mobility of the specificity module in toxins active against mammals compared to insect-active toxins. We then hypothesized that the enhanced mobility in mammal toxins was provided by two conserved glycine residues that enclose the N-terminal loop of the specificity module. To test this assumption, we obtained a derivative of the neurotoxin BeM9 from the venom of the scorpion Mesobuthus eupeus with two replacements of amino acid residues in the corresponding positions with glycine (A4G and Y17G). Unexpectedly, it turned out that BeM9GG lost its activity against Nav1.5 channel isoform, characteristic of mammalian cardiac muscle. A comparison of two known structures of voltage-gated sodium channel complexes with scorpion toxins made it possible to explain the observed effect. We hypothesize an essential role of the membrane in the interaction of toxins with the Nav1.5 isoform.