Polypeptide and peptide toxins, magnifying lenses for binding sites in nicotinic acetylcholine receptors

At present the cryo-electron microscopy structure at 4A resolution is known for the Torpedo marmorata nicotinic acetylcholine receptor (nAChR), and high-resolution X-ray structures have been recently determined for bacterial ligand-gated ion channels which have the same type of spatial organization. Together all these structures provide the basis for better understanding functioning of muscle-type and neuronal nAChRs, as well as of other Cys-loop receptors: 5HT3-, glycine-, GABA-A and some other. Detailed information about the ligand-binding sites in nAChRs, necessary both for understanding the receptor functioning and for rational drug design, became available when the X-ray structures were solved for the acetylcholine-binding proteins (AChBP), excellent models for the ligand-binding domains of all Cys-loop receptors. Of special value in this respect are the X-ray structures of AChBP complexes with agonists and antagonists. Among the latter are the complexes with polypeptide and peptide antagonists, that is with protein neurotoxins from snake venoms and peptide neurotoxins (alpha-conotoxins) from poisonous marine snails of Conus genus. The role of a bridge between the AChBP and nAChRs is played by the X-ray structure of the ligand-binding domain of alpha1 subunit of nAChR in the complex with alpha-bungarotoxin. The purpose of this review is to show the role of well-known and new polypeptide and peptide neurotoxins, from the earlier days of nAChRs research until present time, in identification of different nAChR subtypes and mapping their binding sites.