Structural insights into the galanin receptors signaling

Galanin is a biologically active neuropeptide, and functions through three distinct G protein-coupled receptors (GPCRs), namely GALR1, GALR2 and GLAR3. GALR signaling plays important roles in regulating various physiological processes such as energy metabolism, neuropathic pain, epileptic activity, and sleep homeostasis. GALR1 and GALR3 signal through the Gi/o pathway, whereas GALR2 signals mainly through the Gq/11 pathway. However, the molecular basis for galanin recognition and G protein selectivity of GALRs remains poorly understood. Here, we report the cryoelectron microscopy structures of the GALR1-Go and the GALR2-Gq complexes bound to the endogenous ligand galanin or spexin. The galanin peptide mainly adopts an alpha helical structure, which binds at the extracellular vestibule of the receptors, nearly parallel to the membrane plane without penetrating deeply into the receptor core. Structural analysis combined with functional studies reveals important structural determinants for the G protein selectivity of GALRs as well as other class A GPCRs. In addition, we show that the zinc ion is a negative allosteric regulator of GALR1 but not GALR2. Our studies provide insight into the mechanisms of G protein selectivity of GPCRs and highlight potential novel function of the neuromodulator zinc ion as a modulator of GPCR signaling in the central nervous system. Significance StatementGalanin exerts various physiological functions through galanin receptors, including antinociceptive activity, depression and sleep. Here, we reveal a distinct binding site and binding pose of galanin peptide in galanin receptors from that of the published structures of peptide-bound GPCRs. Moreover, our work show that the neuromodulator zinc ion negatively modulates galanin signaling in the central nervous system, and further advances our understanding of mechanisms of G protein selectivity of GPCRs. These unique features of galanin receptors can be exploited for rational design of subtype selective ligands for treatments of neurological disorders.