NMR solution structure of Cn12, a novel peptide from the Mexican scorpion Centruroides noxius with a typical β-toxin sequence but with α-like physiological activity

Abstract

Cn12 isolated from the venom of the scorpion Centruroides noxius has 67 amino-acid residues, closely packed with four disulfide bridges. Its primary structure and disulfide bridges were determined. Cn12 is not lethal to mammals and arthropods in vivo at doses up to 100 μg per animal. Its 3D structure was determined by proton NMR using 850 distance constraints, 36 φ angles derived from 36 coupling constants obtained by two different methods, and 22 hydrogen bonds. The overall structure has a two and half turn α-helix (residues 24-32), three strands of antiparallel β-sheet (residues 2-4, 37-40 and 45-48), and a type II turn (residues 41-44). The amino-acid sequence of Cn12 resembles the β scorpion toxin class, although patch-clamp experiments showed the induction of supplementary slow inactivation of Na + channels in F-11 cells (mouse neuroblastoma N18TG-2 x rat DRG2), which means that it behaves more like an α scorpion toxin. This behaviour prompted us to analyse Na + channel binding sites using information from 112 Na + channel gene clones available in the literature, focusing on the extracytoplasmic loops of the S5-S6 transmembrane segments of domain I and the S3-S4 segments of domain IV, sites considered to be responsible for binding α scorpion toxins.