Structure and Dynamics of the M ₃ Muscarinic Acetylcholine Receptor

Abstract

Acetylcholine (ACh), the first neurotransmitter to be identified ¹ , exerts many of its physiological actions via activation of a family of G protein-coupled receptors (GPCRs) known as muscarinic ACh receptors (mAChRs). Although the five mAChR subtypes (M ₁-M ₅) share a high degree of sequence homology, they show pronounced differences in G protein coupling preference and the physiological responses they mediate. ^{2– 4} Unfortunately, despite decades of effort, no therapeutic agents endowed with clear mAChR subtype selectivity have been developed to exploit these differences. ^{5– 6} We describe here the structure of the G _q/11-coupled M ₃ mAChR bound to the bronchodilator drug tiotropium and identify the binding mode for this clinically important drug. This structure, together with that of the G _i/o-coupled M ₂ receptor, offers new possibilities for the design of mAChR subtype-selective ligands. Importantly, the M ₃ receptor structure allows the first structural comparison between two members of a mammalian GPCR subfamily displaying different G-protein coupling selectivities. Furthermore, molecular dynamics simulations suggest that tiotropium binds transiently to an allosteric site en route to the binding pocket of both receptors. These simulations offer a structural view of an allosteric binding mode for an orthosteric GPCR ligand and raise additional opportunities for the design of ligands with different affinities or binding kinetics for different mAChR subtypes. Our findings not only offer new insights into the structure and function of one of the most important GPCR families, but may also facilitate the design of improved therapeutics targeting these critical receptors.