An intersubunit hydrogen bond in the nicotinic acetylcholine receptor that contributes to channel gating

Abstract

The muscle nicotinic acetylcholine receptor (nAChR) is a large, allosteric, ligand-gated ion channel with the subunit composition α2βγδ. Though 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 α-hydroxyserine (Sah), we find that the backbone N-H of αS191 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 (AChBP). Instead, the hydrogen bonding partner is the extensively researched aspartate γD174/δD180 -— which had originally been identified as a key binding residue for cationic agonists.