Muscarinic acetylcholine receptors contain a region encompassing the second and third transmembrane domains that is rich in conserved aspartic acid residues. To investigate the role of four conserved aspartic acids at positions 71, 99, 105, and 122 in muscarinic receptor function, point mutations in the rat m1 muscarinic receptor gene were made that converted each Asp to Asn, and wild type or mutant genes were stably expressed in Chinese hamster ovary cells that normally lack muscarinic receptors. Substitution of Asp71 or Asp122 with Asn produced mutant receptors that displayed high affinity for carbachol but decreased efficacy and potency, respectively, in agonist-induced activation of phosphoinositide hydrolysis, suggesting that these residues may mediate receptor-GTP binding protein interactions. Substitution of Asp99 or Asp105 with Asn produced marked decreases in ligand binding affinities and/or covalent incorporation of [3H] propylbenzilylcholine mustard, suggesting that these residues may be involved in receptor-ligand interactions.