Structure of the agonist-activated M2 muscarinic receptor (M2R) The first high-resolution X-ray structures of two muscarinic receptor subtypes (M2R and M3R) bound to orthosteric antagonists (QNB and tiotropium, respectively) were published in 2012. Recently, my section was involved in a multi-lab collaborative effort that resulted in the determination of the structure of the active confirmation of the M2R in complex with an orthosteric agonist (iperoxo). The active M2R confirmation is characterized by a significant outward displacement of the cytoplasmic end of TM6, together with a smaller outward movement of the C-terminal portion of TM5 and a rearrangement of the highly conserved NPXXY (TM7) and DRY (cytoplasmic end of TM3) motifs. Interestingly, similar conformational changes have been described for the active-state conformations of other class A GPCRs (GPCRs of the rhodopsin family). Notably, agonist (iperoxo) binding to the M2R causes a significant contraction of the orthosteric binding site, which completely occludes the bound agonist from solvent. Following agonist binding, TM5, TM6, and TM7 move inward toward the ligand, and TM3 undergoes a slight rotation about its axis. The M2R-iperoxo structure clearly shows that these conformational changes result in the closure of a lid of tyrosine residues above the agonist ligand. It is also worth noting that virtually all M2R amino acid side chains that are in contact with the agonist are also involved in the binding of QNB, a muscarinic antagonist, in the inactive M2R conformation. Structure of the M2R in complex with a positive allosteric modulator During the past decades, the M2R has served as an excellent model system for studying the regulation of GPCR function by small allosteric modulators. Recently, my section was involved in a multi-lab collaboration that led to the publication of a high resolution X-ray structure of the agonist (iperoxo)-occupied M2R in complex with a positive allosteric modulator (LY2119620) that selectively enhances the affinity of orthosteric agonists for the M2R. This new structure revealed that the allosteric modulator binds directly above the orthosteric agonist in the M2R-iperoxo-LY2119620 complex and that the modulator engages in extensive interactions with the extracellular vestibule. Notably, the structure of the M2R in complex with the allosteric modulator is remarkably similar to that of the M2R structure in complex with the orthosteric agonist (iperoxo) alone. This observation strongly suggests that the binding site for the allosteric modulator is largely pre-formed after binding of the orthosteric agonist. Since binding of the orthosteric agonist (iperoxo) causes a contraction of the extracellular vestibule, the positive allosteric modulator is able to engage in extensive interactions with this 'outer' cavity of the M2R. These observations strongly suggest that muscarinic positive allosteric modulators enhance the receptor affinity of orthosteric agonists by stabilizing the active receptor conformation by interfering with the dissociation of the orthosteric agonist from its binding pocket. These new high-resolution structures should prove useful for the development of novel muscarinic ligands that selectively act on distinct muscarinic receptor subtypes. Many studies suggest that such agents may provide considerable therapeutic benefits. Recent reviews: Kruse AC, Hu J, Kobilka BK, Wess J. Muscarinic acetylcholine receptor X-ray structures: potential implications for drug development. Curr Opin Pharmacol 16, 24-30, 2014. Kruse AC, Kobilka BK, Gautam D, Sexton PM, Christopoulos A, Wess J. Muscarinic acetylcholine receptors: novel opportunities for drug development. Nat Rev Drug Discov 13, 549-60, 2014.