Identification of an atropine-agonized mutant M3 muscarinic acetylcholine receptor (M3R). To generate an M3R that can be activated by atropine, we applied error-prone random mutagenesis and screened approximately 1 x 107 mutant M3R clones for atropine sensitivity in a yeast expression system. In one clone we identified the mutations R179K and A489P (A) that enable the M3R to be activated by atropine (B). Both mutated positions are fully conserved within Gq-coupled muscarinic acetylcholine receptors.