Transmembrane Domains 4 and 7 of the M₁ Muscarinic Acetylcholine Receptor are Critical for Ligand Binding and the Receptor Activation Switch

Activation of the muscarinic acetylcholine receptors requires agonist binding followed by a conformational change, but the ligand binding and conformation-switching residues have not been completely identified. Systematic alanine-scanning mutagenesis has been used to assess residues 142-164 in transmembrane helix 4 and 402-421 in transmembrane helix 7 of the M₁ muscarinic acetylcholine receptor. Several inward-facing amino acid side chains in the exofacial parts of transmembrane helices 4 and 7 contribute to acetylcholine binding. Alanine substitution of the aromatic residues in this group reduced signaling efficacy, suggesting that they may form part of a charge-stabilized aromatic cage, which triggers rotation and movement of the transmembrane helices. The mutation of adjacent residues modulated receptor activation, either reducing signaling or causing constitutive activation. In the buried endofacial section of transmembrane helix 7, alanine substitution mutants of the conserved NSXXNPXXY motif displayed strongly reduced signaling efficacy, despite having increased or unchanged acetylcholine affinity. These residues may have dual functions, forming intramolecular contacts that stabilize the receptor in the inactive ground state, but that are broken, allowing them to form new intramolecular bonds in the activated state. This conformational rearrangement is critical to produce a G protein binding site and may represent a key mechanism of receptor activation.