Phylogenetic specificity of cholinergic ligands: alpha-conotoxin SI.

The alpha-conotoxins are small peptide neurotoxins from the venom of fish-hunting cone snails which block nicotinic acetylcholine receptors (nAChRs). We describe the purification, characterization, and chemical synthesis of a new alpha-conotoxin from Conus striatus, alpha-conotoxin SI. In contrast to other AChR ligands, alpha-SI discriminates between different vertebrate nAChRs. The sequence of alpha-conotoxin SI is Ile-Cys-Cys-Asn-Pro5-Ala-Cys-Gly-Pro-Lys10-Tyr-Ser-Cys-NH2. This sequence was confirmed by chemical synthesis. A des-Ile-alpha-SI derivative was also synthesized and is biologically active. Although alpha-conotoxin SI is highly homologous to previously described alpha-conotoxins, it has one noteworthy sequence feature which may account for its novel biological specificity. In all other alpha-conotoxins, there is a positively charged amino acid at residue 9; in alpha-conotoxin SI, this is replaced by proline. The discovery that different alpha-conotoxins can vary by orders of magnitude in their apparent affinity for different vertebrate receptors demonstrates that alpha-conotoxins will be useful probes for investigating phylogenetic differences between vertebrate nAChRs.