Hydrophobic pairwise interactions stabilize α-conotoxin MI in the muscle acetylcholine receptor binding site

The present work delineates pairwise interactions underlying the nanomolar affinity of alpha-conotoxin MI (CTx MI) for the alpha-delta site of the muscle acetylcholine receptor (AChR). We mutated all non-cysteine residues in CTx MI, expressed the alpha(2)beta delta(2) pentameric form of the AChR in 293 human embryonic kidney cells, and measured binding of the mutant toxins by competition against the initial rate of I-125-alpha-bungarotoxin binding. The CTx MI mutations P6G, A7V, G9S, and Y12T all decrease affinity for alpha(2)beta delta(2) pentamers by 10,000-fold. Side chains at these four positions localize to a restricted region of the known three-dimensional structure of CTx MI. Mutations of the AChR reveal major contributions to CTx MI affinity by Tyr-198 in the alpha subunit and by the selectivity determinants Ser-36, Tyr-113, and Ile-178 in the delta subunit. By using double mutant cycles analysis, we find that Tyr-12 of CTx MI interacts strongly with all three selectivity determinants in the delta subunit and that delta Ser-36 and delta Ile-178 are interdependent in stabilizing Tyr-12, We find additional strong interactions between Gly-9 and Pro-6 in CTx MI and selectivity determinants in the delta subunit, and between Ala-7 and Pro-6 and Tyr-198 in the alpha subunit. The overall results reveal the orientation of CTx MI when bound to the alpha-delta interface and show that primarily hydrophobic interactions stabilize the complex.