Minimal conformation of the alpha-conotoxin ImI for the alpha 7 neuronal nicotinic acetylcholine receptor recognition: correlated CD, NMR and bindingstudies

The alpha-ImI conotoxin, a selective potent inhibitor of the mammalian neur onal alpha 7 nicotinic acetylcholine receptor (n-AchR), was shown by point mutation or by L-alanine scanning to display two regions essential for bioa ctivity: the active site Asp(5)-Pro(6)-Arg(7) in the first loop and Trp(10) in the second loop. The deletion of the Cys(3),Cys(12) disulfide bond in t he a-ImI scaffold, e.g. peptide II, had no effect on its binding affinity, CD spectra, NMR studies and structure calculations were carried out on the mild type alpha-ImI, the weakest analog (R7A) and peptide II (equipotent to alpha-ImI) in order to point out the conformational differences between th ese compounds, Then, an attempt to correlate the conformational data and th e affinity results was proposed. CD and NMR data were identical for the R7A analog and alpha-ImI, revealing the crucial functional role of the Arg(7) Side chain. On the other hand, the scaffold of the first loop in peptide II was shown by NMR to represent the minimal conformation for the optimal int eraction of the toxin with the neuronal alpha 7 n-AchR, Last, the beta-turn forming property of the 6th residue (Pro) in the active site of the alpha- ImI can he correlated with its affinity. (C) 1999 Federation of European Bi ochemical Societies.