ALPHA-CONOTOXIN EI, A NEW NICOTINIC ACETYLCHOLINE-RECEPTOR ANTAGONISTWITH NOVEL SELECTIVITY

We report the isolation and characterization of a novel nicotinic acet ylcholine receptor (nAChR) ligand. The toxin is an 18 amino acid pepti de and is the first reported alpha-conotoxin from an Atlantic fish-hun ting Conus. The peptide was purified from the venom of Conus ermineus and is called alpha-conotoxin EI. The sequence diverges from that of p reviously isolated alpha-conotoxins. We demonstrate that this structur al divergence has functional consequences. In Torpedo nAChRs, alpha-co notoxin EI selectively binds the agonist site near the alpha/delta sub unit interface in contrast to alpha-conotoxin MI which selectively tar gets the alpha/gamma agonist binding site. In mammalian nAChRs alpha-c onotoxin EI shows high affinity for both the alpha/delta and alpha/gam ma subunit interfaces (with some preference for the alpha/delta site), whereas alpha-conotoxin MI is highly selective for the alpha/gamma li gand binding site. The sequence of the peptide is: s-Pro-Thr-Cys-Asn-M et-Ser-Asn-Pro-Gln-Ile-Cys-NH2, with disulfide bridging between Cys4-C ys10 and Cys5-Cys18, analogous to those of previously described alpha- conotoxins. This sequence has been verified by total chemical synthesi s. Thus, alpha-conotoxin EI is a newly-available tool with unique stru cture and function for characterization of nAChRs.