αC-Conotoxin PrXA:: A new family of nicotinic acetylcholine receptor antagonists

We have purified a novel paralytic peptide with 32 AA and a single disulfide bond from the venom of Conus parius, a fish-hunting species. The peptide has the following sequence: TYGIYDAKPOFSCAGLRGGCVLPONLROKFKE-NH2, where O is 4-trans-hydroxyproline. The peptide, designated alpha C-conotoxin PrXA (alpha C-PrXA), is the defining member of a new, structurally distinct family of Conus peptides. The peptide is a competitive nAChR antagonist; all previously characterized conotoxins that competitively antagonize nAChRs are structurally and genetically unrelated. (Most belong to the alpha- and alpha A-conotoxin families.) When administered to mice and fish in vivo, alpha C-PrXA caused paralysis and death. In electrophysiological assays, alpha C-PrXA potently antagonized mouse muscle nicotinic acetylcholine receptors (nAChRs), with IC50 values of 1.8 and 3.0 nM for the adult (alpha 1 beta 1 epsilon delta subunits) and fetal (alpha 1 beta 1 gamma delta subunits) muscle nAChR subtypes, respectively. When tested on a variety of ligand-gated and voltage-gated ion channels, alpha C-PrXA proved to be a highly specific inhibitor of the neuromuscular nAChR. The peptide competes with alpha-bungarotoxin for binding at the alpha/delta and alpha/gamma subunit interfaces of the nAChR, with higher affinity for the alpha/delta subunit interface. alpha C-PrXA is strikingly different from the many conopeptides shown to be nicotinic antagonists; it is most similar in its general biochemical features to the snake toxins known as Waglerins.