Refined structure of BeM9 reveals arginine hand, an overlooked structural motif in scorpion toxins affecting sodium channels.

Sodium channel alpha-toxins from scorpion venom (α-NaTx) inhibit the inactivation of voltage-gated sodium channels. We used solution NMR to investigate the structure of BeM9 toxin from Mesobuthus eupeus scorpion, a prototype α-NaTx classified as an "α-like" toxin due to its wide spectrum of activity on insect and mammalian channels. We identified a new motif that we named "arginine hand," whereby arginine side chain forms several hydrogen bonds with main chain atoms. The arginine hand was found in the "specificity module," a part of the molecule that dictates toxin selectivity; and just single arginine-to-lysine point mutation drastically changed BeM9 selectivity profile.