Binding mode of alpha-conotoxins to an acetylcholine binding protein determined by saturation transfer difference NMR

The saturation transfer difference (STD) NMR technique was employed to study the complex of the alpha-conotoxins Vc1.1 and MII bound to the acetylcholine binding protein (AChBP) from Lymnea stagnalis, a model system of the 7 subunit of the nicotinic acetylcholine receptor. MII was found to be the more potent ligand for AChBP, consistent with data from electrophysiology measurements for the nicotinic acetylcholine receptor. Both peptides displayed strong interactions on aromatic residues in the alpha-helical part of their sequences, i. e., Tyr10 in Vc1.1 and His9 in MII respectively. From the STD NMR spectra it was determined that the peptides are buried in the nicotinic binding site of ACBP as has been previously shown for the conotoxins PnIA[A10L, D14K], ImI and TxIA[A10L] by X-ray crystallography. This study demonstrates the value of STD NMR in the study of conotoxin binding to receptor proteins.