GPCR

GPCR stands for G-protein-coupled receptor. It is the largest superfamily of cell-surface receptors in the human genome, with on the order of 800 members, and it is the receptor family through which most peptide hormones of interest to metabolic and endocrine research exert their effects. A GPCR is a single polypeptide that threads across the plasma membrane seven times, creating seven transmembrane helices and an alternating set of extracellular and intracellular loops. Ligand binding changes the relative orientation of those helices in a way that allows heterotrimeric G proteins - and, separately, arrestins - to dock on the intracellular side and propagate a signal. A comprehensive introduction is Rosenbaum, Rasmussen & Kobilka, Nature 2009 (NLM 19458711).

Class A vs class B peptide-binding GPCRs

GPCRs are grouped into classes on the basis of sequence and structural features. Class A (rhodopsin-like) is the largest and binds a wide variety of small molecules, biogenic amines, and short peptides. Class B (secretin-like) is the family to which most of the large peptide-hormone receptors of metabolic interest belong: GLP-1R, GIPR, GCGR, GHRHR, PTH1R, CRHR, and others. Class B receptors have a large extracellular N-terminal domain that forms the principal binding site for their peptide ligand's C-terminus, while the N-terminus of the peptide engages the transmembrane core in a two-step "two-domain" binding model. This geometry is the reason truncated N-terminal fragments of class B peptide ligands often retain receptor-binding affinity but lose signaling activity. Structural detail is reviewed in Hollenstein et al., Trends Pharmacol. Sci. 2014 (NLM 25407683).

Signaling, bias, and assay readouts

GPCR signaling is not a single event. A given receptor can activate multiple G-protein subtypes (Gs, Gi/o, Gq/11, G12/13) and can recruit arrestins, with the balance among those pathways differing from ligand to ligand - a phenomenon called biased agonism. In vitro research on peptide GPCR ligands therefore typically uses multiple orthogonal readouts: cAMP accumulation (for Gs-coupled receptors), intracellular calcium (for Gq-coupled receptors), BRET- or TR-FRET-based arrestin-recruitment assays, and label-free cellular response measurements. Characterization of a research peptide at a GPCR should describe which readout was used and which cell background was used, because both choices affect the apparent EC50 and efficacy. The pharmacology of biased signaling is summarized in Kenakin, Trends Pharmacol. Sci. 2007 (NLM 17481741).

Open Assay's treatment of GPCR targets

Open Assay indexes peptide ligands by their primary GPCR target and lists, where published, the cell system and readout used in the published characterization. This allows a reader to compare research on two ligands at the same receptor on a methodologically consistent basis rather than on a single number extracted from different assays.