Definition and Characterization of the Short αA-Conotoxins:  A Single Residue Determines Dissociation Kinetics from the Fetal Muscle Nicotinic Acetylcholine Receptor^†

We report the definition and characterization of a conotoxin subfamily, designated the short αA-conotoxins (αA_S) and demonstrate that all of these share the unique property of selectively antagonizing the fetal subtype of the mammalian neuromuscular nicotinic acetylcholine receptor (nAChR). We have characterized newly identified αA_S-conotoxins from Conus pergrandis and have conducted a more detailed characterization of αA-conotoxins previously reported from additional Conus species. Among the results, the characterization of the short αA-conotoxins revealed diverse kinetics of a block of the fetal muscle nAChR, particularly in dissociation rates. The structure−function relationships of native αA_S-conotoxins and some analogues revealed a single amino acid locus (alternatively either His or Pro in native peptides) that is a critical determinant of the dissociation kinetics. The unprecedented binding selectivity for the fetal muscle nAChR, coupled with the kinetic diversity, should make αA_S-conotoxins useful ligands for a diverse set of studies. The rapidly reversible peptides may be most suitable for electrophysiological studies, while the relatively irreversible peptides should be most useful for binding and localization studies.