DIFFERENTIAL TARGETING OF NICOTINIC ACETYLCHOLINE-RECEPTORS BY NOVEL ALPHA-A-CONOTOXINS

We describe the isolation and characterization of two peptide toxins f rom Conus ermimeus venom targeted to nicotinic acetylcholine receptors (nAChRs). The peptide structures have been confirmed by mass spectrom etry and chemical synthesis, In contrast to the 12-18 residue, 4 Cys-c ontaining alpha-conotoxins, the new toxins have 30 residues and 6 Cys residues. The toxins, named alpha A-conotoxins EIVA and EIVB, block bo th Torpedo and mouse alpha 1-containing muscle subtype nAChRs expresse d in Xenopus oocytes at low nanomolar concentrations. In contrast to a lpha-bungarotoxin, alpha A-EIVA is inactive at alpha 7-containing nACh Rs even at micromolar concentrations. In this regard, alpha A-EIVA is similar to the previously described alpha-conotoxins (e.g. alpha-MI an d alpha-GI) which also selectively target alpha 1- versus alpha 7-cont aining nAChRs, However, alpha-MI and alpha-GI discriminate between the alpha/delta versus alpha/gamma subunit interfaces of the mouse muscle nAChR with 10,000-fold selectivity. In contrast, alpha A-conotoxin EI VA blocks both the alpha/gamma site and alpha/delta site with equally high affinity but with distinct kinetics. The alpha A-conotoxins thus represent novel probes for the alpha/gamma as well as the alpha/delta binding sites of the nAChR.