Mapping of Single Amino Acid Residues Required for Selective Activation of G by the m3 Muscarinic Acetylcholine Receptor (*)

Each G protein-coupled receptor can interact only with a limited number of the many structurally similar G proteins expressed within a cell. This study was undertaken to identify single amino acids required for selectively coupling the m3 muscarinic acetylcholine receptor to G proteins of the G family. To this goal, distinct intracellular segments/amino acids of the m3 receptor were systematically substituted into the structurally closely related m2 muscarinic receptor, which couples to G proteins, not G proteins. The resultant mutant receptors were expressed in COS-7 cells and studied for their ability to induce agonist-dependent stimulation of phosphatidylinositol hydrolysis, a response known to be mediated by G proteins of the G class. Using this approach, we were able to identify four amino acids in the second intracellular loop and four amino acids at the C terminus of the third intracellular loop of the m3 muscarinic receptor that are essential for efficient G activation. We could demonstrate that these amino acids, together with a short segment at the N terminus of the third intracellular loop, fully account for the G protein coupling preference of the m3 muscarinic receptor. Taken together, our data strongly suggest that only a limited number of amino acids, located on different intracellular regions, are required to determine the functional profile of a given G protein-coupled receptor.