Transmembrane domain VI of muscarinic acetylcholine receptors plays an important role in ligand binding and receptor function. A human M(1) (HM(1)) mutant receptor, HM(1)(S388Y, T389P), displayed significantly enhanced agonist potency, binding affinity, and G protein coupling. The mutations are located at the top of transmembrane domain VI and about two helical turns above Tyr381 and Asn382, which are important for ligand binding and receptor function. To determine the functional role of individual mutations of Ser388Tyr and Thr389Pro, we created stable A9 L cell lines expressing HM(1)(S388Y) or HM(1)(T389P) receptors. In phosphatidylinositol hydrolysis assays, muscarinic agonists showed greater potency at the HM(1)(S388Y) and HM(1)(S388Y, T389P) mutants compared with the wild-type and HM(1)(T389P) receptors. Acetylcholine demonstrated 105-fold higher potency at HM(1)(S388Y) receptors than at HM(1)(T389P) receptors. Choline (30 microM, the concentration found in Dulbecco's modified Eagle's medium) exhibited 90% stimulation at HM(1)(S388Y) receptors but was inactive at HM(1)(T389P) receptors. In ligand binding experiments, mutation of Ser388Tyr resulted in significantly increased agonist binding affinity. In contrast, mutation of Thr389Pro did not change agonist binding affinity but rendered multiple agonist binding sites, and the high-affinity binding was sensitive to GTP analogs. These results demonstrate that the Ser388Tyr mutation is responsible for enhanced agonist potency and binding affinity, whereas the Thr389Pro mutation alters G protein interactions. The data suggest that Ser388 and Thr389 are potential targets for modulation of agonist binding and G protein coupling.