Cryo-EM structure of the human MT1-Gi signaling complex

Melatonin receptors (MT1 and MT2) transduce inhibitory signaling by melatonin (N-acetyl-5-methoxytryptamine), which is associated with sleep induction and circadian rhythm modulation. Although recently reported crystal structures of ligand-bound MT1 and MT2 elucidated the basis of ligand entry and recognition, the ligand-induced MT1 rearrangement leading to G(i)-coupling remains unclear. Here we report a cryo-EM structure of the human MT1-G(i) signaling complex at 3.3 angstrom resolution, revealing melatonin-induced conformational changes propagated to the G-protein-coupling interface during activation. In contrast to other G(i)-coupled receptors, MT1 exhibits a large outward movement of TM6, which is considered a specific feature of G(s)-coupled receptors. Structural comparison of G(i) and G(s) complexes demonstrated conformational diversity of the C-terminal entry of the G(i) protein, suggesting loose and variable interactions at the end of the alpha 5 helix of G(i) protein. These notions, together with our biochemical and computational analyses, highlight variable binding modes of G alpha(i) and provide the basis for the selectivity of G-protein signaling. A cryo-EM structure of the active human melatonin receptor in complex with G(i) reveals conformational changes upon activation and the molecular basis for G-protein selectivity.