A homology model of the M1 muscarinic acetylcholine receptor, based on the X-ray structure of bovine rhodopsin, has been used to interpret the results of scanning and point mutagenesis studies on the receptor's transmembrane (TM) domain. Potential intramolecular interactions that are important for the stability of the protein fold have been identified. The residues contributing to the binding site for the antagonist, N-methyl scopolamine, and the agonist, acetylcholine, have been mapped. The positively charged headgroups of these ligands probably bind in a charge-stabilized aromatic cage formed by amino acid side chains in TM helices TM3, TM6 and TM7, while residues in TM4 may participate as part of a peripheral docking site. Closure of the cage around the headgroup of acetylcholine may be part of the mechanism for transducing binding energy into receptor activation, probably by disrupting a set of Van der Waals interactions between residues lying beneath the binding site that help to constrain the receptor to the inactive state, in the absence of agonist. This may trigger the reorganization of a hydrogen-bonding network between highly conserved residues in the core of the receptor, whose integrity is crucial for achievement of the activated state.