Characterization and Three-Dimensional Structure Determination of ψ-Conotoxin Piiif, a Novel Noncompetitive Antagonist of Nicotinic Acetylcholine Receptors

A novel inhibitor of nicotinic acetylcholine receptors (nAChRs), ψ-conotoxin Piiif, was isolated from the venom of Conus purpurascens and found to have the sequence GOOCCLYGSCROFOGCYNALCCRK-nh 2. The sequence is highly homologous to that of ψ-conotoxin Piiie, a previously identified noncompetitive inhibitor of Torpedo electroplax nAChR, also isolated from C. purpurascens. Both ψ-conotoxins block Torpedo and mouse nicotinic acetylcholine receptors (nAChRs), but ψ-Piiif is less potent by a factor of 101−102. A high-resolution structure of ψ-Piiif was determined by NMR and molecular modeling calculations. ψ-Piiif analogues containing [13C]-labeled cysteine at selected positions were synthesized to resolve spectral overlap of Cys side chain proton signals. The structures are well-converged, with backbone atom position RMSDs of 0.21 Å for the main body of the peptide between residues 4 and 22 and 0.47 Å for all residues. The overall backbone conformation is closely similar to ψ-Piiie, the main difference being in the degree of conformational disorder at the two termini. ψ-Piiie and ψ-Piiif have similar locations of positive charge density, although ψ-Piiif has a lower overall charge. One disulfide bridge of ψ-Piiif appears to undergo dynamic conformational fluctuations based on both the model and on experimental observation. Chimeras in which the three intercysteine loops were swapped between ψ-Piiie and ψ-Piiif were tested for inhibitory activity against Torpedo nAChRs. The third loop, which contains no charged residues in either peptide, is the prime determinant of potency in these ψ-conotoxins.