Distinct primary structures, ligand-binding properties and tissue-specific expression of four human muscarinic acetylcholine receptors.

To investigate the molecular basis for the diversity in muscarinic cholinergic function, we have isolated the genes encoding the human M1 and M2 muscarinic receptors (mAChR) as well as two previously undiscovered mAChR subtypes, designated HM3 and HM4. The amino acid sequence of each subtype reflects a structure consisting of seven, highly conserved transmembrane segments and a large intracellular region unique to each subtype, which may constitute the ligand-binding and effector-coupling domains respectively. Significant differences in affinity for muscarinic ligands were detected in individual mAChR subtypes produced by transfection of mammalian cells. Each subtype exhibited multiple affinity states for agonists; differences among subtypes in the affinities and proportions of such sites suggest the capacity of mAChR subtypes to interact differentially with the cellular effector-coupling apparatus. Subtype-specific mRNA expression was observed in the heart, pancreas and a neuronal cell line, indicating that the regulation of mAChR gene expression contributes to the differentiation of cholinergic activity.