Muscarinic acetylcholine receptors: typical G-coupled receptors

The muscarinic acetylcholine receptors (mAChRs) are members of the class of G-protein coupled receptors. 5 mAChR genes have been cloned and sequenced. The receptor subtypes have a characteristic tissue distribution which correlates with their pharmacological and functional properties. The mAChRs appear to be monomeric in their active state. They conform to a common generic structure. The receptor polypeptide chain is believed to traverse the cell-surface membrane 7 times, forming a helical cluster which contains the ligand binding site. Amino-acid residues important in ligand binding have been identified both by affinity-labelling and peptide sequencing, and by site-directed mutagenesis and expression studies. The cytoplasmic domain of the receptor is composed of 3 cytoplasmic loops, and the C-terminal tail of the receptor polypeptide chain. The cytoplasmic domain binds, recognises and activates specific hetero-trimeric GTP-binding proteins (G-proteins) in an agonist-dependent manner. The activated, GTP-liganded G-proteins then mediate a wide variety of events, which include, in the case of mAChRs, the inhibition of adenylyl cyclase, the breakdown of polyphosphoinositides, the opening and closing of potassium channels, and the production of arachidonic acid. Mutational analysis shows that the N-terminal portion of the third intracellular loop of the mAChR sequences is important in conveying specificity for G-protein recognition. The receptor sequences contain a number of additional features which are functionally important. These include potential substrate sites for receptor-specific kinases, and a cysteine residue which is likely to be palmitoylated.