Characterization and three-dimensional structure determination of psi-conotoxin Piiif, a novel noncompetitive antagonist of nicotinic acetylcholine receptors.

A novel inhibitor of nicotinic acetylcholine receptors (nAChRs), psi-conotoxin Piiif, was isolated from the venom of Conus purpurascens and found to have the sequence GOOCCLYGSCROFOGCYNALCCRK-NH2. The sequence is highly homologous to that of psi-conotoxin Piiie, a previously identified noncompetitive inhibitor of Torpedo electroplax nAChR, also isolated from C. purpurascens. Both psi-conotoxins block Torpedo and mouse nicotinic acetylcholine receptors (nAChRs), but psi-Piiif is less potent by a factor of 10(1)-10(2). A high-resolution structure of psi-Piiif was determined by NMR and molecular modeling calculations. Psi-Piiif analogues containing [(13)C]-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 A for the main body of the peptide between residues 4 and 22 and 0.47 A for all residues. The overall backbone conformation is closely similar to psi-Piiie, the main difference being in the degree of conformational disorder at the two termini. Psi-Piiie and psi-Piiif have similar locations of positive charge density, although psi-Piiif has a lower overall charge. One disulfide bridge of psi-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 psi-Piiie and psi-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 psi-conotoxins.