Research group led by Associate Professor Keisuke Ikegami reveals how the melatonin receptor MT₁ selectively utilizes multiple intracellular signaling pathways

Novel signaling mechanism of a receptor involved in sleep and circadian rhythms

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Points

• In analyses using cells and mice, we demonstrated that the melatonin receptor MT₁, which is involved in sleep and circadian rhythms, is associated not only with the previously known G_i protein pathway that decreases cAMP levels but also with the G_s protein pathway that increases cAMP levels.
• We elucidated the binding mode between the MT₁ receptor and the G_s protein complex at the structural level, revealing the molecular basis of this novel signaling.
• We found that the MT₁ receptor does not accommodate G_i and G_s proteins in the same manner; instead, it engages the C-terminal α5 helix of the G proteins at distinct positions and angles.
• These findings are expected to contribute to a better understanding of how GPCRs selectively use multiple intracellular signaling pathways and may help develop signal-selective therapeutics.

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Abstract

A research group comprising Kyorin University (Mitaka, Tokyo; President: Taku Watanabe), the Faculty of Medicine; Kyushu University (Fukuoka, Fukuoka; President: Tatsuro Ishibashi), the Faculty of Agriculture; and the University of Tokyo (Bunkyo, Tokyo; President: Teruo Fujii), the Graduate School of Science, has elucidated a mechanism by which the melatonin receptor _{(Note 1)}, MT₁, selectively utilizes multiple intracellular signaling pathways.
G protein-coupled receptors (GPCRs) are an important class of molecules that transmit extracellular signals into the cell and are currently targeted by approximately 30% of all pharmaceuticals. GPCRs convey signals by coupling with intracellular G proteins; however, even the same receptor can elicit distinct cellular responses depending on the type of G protein it binds to.

Melatonin is a hormone involved in regulating sleep-wake cycles and circadian rhythms. Its actions are mediated by the melatonin receptors MT₁ and MT₂, which belong to the GPCR family and are located on the cell membrane. The MT₁ receptor has traditionally been recognized as primarily functioning through the G_i protein pathway, which decreases intracellular cAMP levels. A collaborative research group, including Lecturer Atsuro Oishi from the Faculty of Medicine at Kyorin University, Professor Osamu Nureki from the Graduate School of Science at the University of Tokyo, Associate Professor Keisuke Ikegami from the Faculty of Agriculture at Kyushu University, Dr. Ralf Jockers from INSERM and the Institut Cochin (France), and Dr. Tikhonova from Queen's University Belfast (UK), demonstrated through analyses using cells and mice that the MT₁ receptor is involved not only in the G_i protein pathway but also in the G_s protein pathway, which increases cAMP levels (Figs. 1 and 2). Furthermore, using cryo-electron microscopy _{(Note 2)}, they determined the three-dimensional structure of the complex formed between the MT₁ receptor and the G_s protein and analyzed how the MT₁ receptor accommodates different G proteins (Fig. 2).
As a result, we found that the MT₁ receptor does not accommodate G_s and G_i proteins in the same manner. Instead, it engages the C-terminal α5 helix of the G proteins at distinct positions and angles (Fig. 2). Furthermore, we investigated why MT₂, another melatonin receptor, has a lower propensity to couple with G_s proteins by combining cryo-electron microscopy-based structural analysis, cellular signaling assays, and computational simulations. These analyses revealed that differences in the third intracellular loop between MT₁ and MT₂ are associated with their differential coupling to G_s proteins. These findings deepen our understanding of how GPCRs selectively use multiple intracellular signaling pathways and may contribute to the development of signal-selective therapeutics.
These research findings were published on May 21, 2026, in the UK-based scientific journal Nature Communications.

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Glossary

Note 1: Melatonin receptor
TThe melatonin receptor is classified as a class A GPCR, a major target family in drug discovery that binds low-molecular-weight ligands. Upon binding melatonin, it becomes activated and selectively activates the inhibitory G protein heterotrimer (G_i protein), which suppresses adenylate cyclase activity. There are two subtypes of melatonin receptors, MT₁ and MT₂. Among them, MT₁, predominantly expressed in the suprachiasmatic nucleus of the brain, is known to play a particularly important role in sleep induction.

Note 2: Cryo-electron microscopy
Cryo-electron microscopy is a technique that allows for high-resolution analysis of the three-dimensional structures of molecules by rapidly freezing samples and observing them in a state close to their native condition.

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Publication Information

Title: Structural basis and physiological significance of non-canonical Gs coupling to the melatonin MT₁ receptor
Authors: Atsuro Oishi†,*, Hiroyuki H. Okamoto†, Keisuke Ikegami†, Ronan McHugh, Bernard Masri, Tsukasa Kusakizako, Kazuhiro Kobayashi, Akifumi Takaki, Angeliki Karamitri, Erika Cecon, Julie Dam, Miki Nagase, Irina G. Tikhonova, Osamu Nureki* & Ralf Jockers*
(† co-first authors; * corresponding authors)
Journal: Nature Communications
DOI：10.1038/s41467-026-73555-6

• For more details on this research, click here.

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For Research-related inquiries

Keisuke Ikegami, Associate Professor