muscarinic acetylcholine receptor subtype M3, member of the class A family of seven-transmembrane G protein-coupled receptors
Muscarinic acetylcholine receptors (mAChRs) regulate the activity of many fundamental central and peripheral functions. The mAChR family consists of 5 subtypes M1-M5, which can be further divided into two major groups according to their G-protein coupling preference. The M1, M3 and M5 receptors selectively interact with G proteins of the G(q/11) family, whereas the M2 and M4 receptors preferentially link to the G(i/o) types of G proteins. The M3 receptor is mainly located in smooth muscle, exocrine glands and vascular endothelium. It induces vomiting in the central nervous system and is a critical regulator of glucose homeostasis by modulating insulin secretion. Generally, M3 receptor causes contraction of smooth muscle resulting in vasoconstriction and increased glandular secretion. All GPCRs have a common structural architecture comprising of seven-transmembrane (TM) alpha-helices interconnected by three extracellular and three intracellular loops. A general feature of GPCR signaling is agonist-induced conformational changes in the receptors, leading to activation of the heterotrimeric G proteins, which consist of the guanine nucleotide-binding G-alpha subunit and the dimeric G-beta-gamma subunits. The activated G proteins then bind to and activate numerous downstream effector proteins, which generate second messengers that mediate a broad range of cellular and physiological processes.