Molecular basis for allosteric agonism and G protein subtype selectivity of galanin receptors

Peptide hormones and neuropeptides are complex signaling molecules that predominately function through G protein-coupled receptors (GPCRs). Two unanswered questions remaining in the field of peptide-GPCR signaling systems pertain to the basis for the diverse binding modes of peptide ligands and the specificity of G protein coupling. Here, we report the structures of a neuropeptide, galanin, bound to its receptors, GAL1R and GAL2R, in complex with their primary G protein subtypes G(i) and G(q), respectively. The structures reveal a unique binding pose of galanin, which almost ‘lays flat’ on the top of the receptor transmembrane domain pocket in an α-helical conformation, and acts as an ‘allosteric-like’ agonist via a distinct signal transduction cascade. The structures also uncover the important features of intracellular loop 2 (ICL2) that mediate specific interactions with G(q), thus determining the selective coupling of G(q) to GAL2R. ICL2 replacement in G(i)-coupled GAL1R, μOR, 5-HT(1A)R, and G(s)-coupled β(2)AR and D1R with that of GAL2R promotes G(q) coupling of these receptors, highlighting the dominant roles of ICL2 in G(q) selectivity. Together our results provide insights into peptide ligand recognition and allosteric activation of galanin receptors and uncover a general structural element for G(q) coupling selectivity.