Structural basis of ligand recognition at the human MT₁ melatonin receptor

Melatonin (N-acetyl-5-methoxytryptamine) is a neurohormone that maintains circadian rhythms¹ by synchronization to environmental cues and is involved in diverse physiological processes² such as the regulation of blood pressure and core body temperature, oncogenesis, and immune function³. Melatonin is formed in the pineal gland in a light-regulated manner⁴ by enzymatic conversion from 5-hydroxytryptamine (5-HT or serotonin), and modulates sleep and wakefulness⁵ by activating two high-affinity G-protein-coupled receptors, type 1A (MT₁) and type 1B (MT₂)^3,6. Shift work, travel, and ubiquitous artificial lighting can disrupt natural circadian rhythms; as a result, sleep disorders affect a substantial population in modern society and pose a considerable economic burden⁷. Over-the-counter melatonin is widely used to alleviate jet lag and as a safer alternative to benzodiazepines and other sleeping aids^8,9, and is one of the most popular supplements in the United States¹⁰. Here, we present high-resolution room-temperature X-ray free electron laser (XFEL) structures of MT₁ in complex with four agonists: the insomnia drug ramelteon¹¹, two melatonin analogues, and the mixed melatonin-serotonin antidepressant agomelatine^12,13. The structure of MT₂ is described in an accompanying paper¹⁴. Although the MT₁ and 5-HT receptors have similar endogenous ligands, and agomelatine acts on both receptors, the receptors differ markedly in the structure and composition of their ligand pockets; in MT₁, access to the ligand pocket is tightly sealed from solvent by extracellular loop 2, leaving only a narrow channel between transmembrane helices IV and V that connects it to the lipid bilayer. The binding site is extremely compact, and ligands interact with MT₁ mainly by strong aromatic stacking with Phe179 and auxiliary hydrogen bonds with Asn162 and Gln181. Our structures provide an unexpected example of atypical ligand entry for a non-lipid receptor, lay the molecular foundation of ligand recognition by melatonin receptors, and will facilitate the design of future tool compounds and therapeutic agents, while their comparison to 5-HT receptors yields insights into the evolution and polypharmacology of G-protein-coupled receptors.