An Intersubunit Hydrogen Bond in the Nicotinic Acetylcholine Receptor That Contributes to Channel Gating

The muscle nicotinic acetylcholine receptor is a large, allosteric, ligand-gated ion channel with the subunit composition alpha(2)beta gamma delta. Although much is now known about the structure of the binding site, relatively little is understood about how the binding event is communicated to the channel gate, causing the pore to open. Here we identify a key hydrogen bond near the binding site that is involved in the gating pathway. Using mutant cycle analysis with the novel unnatural residue alpha-hydroxyserine, we find that the backbone N-H of alpha Ser-191 in loop C makes a hydrogen bond to an anionic side chain of the complementary subunit upon agonist binding. However, the anionic partner is not the glutamate predicted by the crystal structures of the homologous acetylcholine-binding protein. Instead, the hydrogen-bonding partner is the extensively researched aspartate gamma Asp-174/delta Asp-180, which had originally been identified as a key binding residue for cationic agonists.