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.