Liselotte Bjerre Knudsen

Different domains of the glucagon and glucagon-like peptide-1 receptors provide the critical determinants of ligand selectivity.

Glucagon and glucagon‐like peptide‐1 (GLP‐1) are homologous peptide hormones with important functions in glucose metabolism. The receptors for glucagon and GLP‐1 are homologous family B G‐protein coupled receptors. The GLP‐1 receptor amino‐terminal extracellular domain is a major determinant of glucagon/GLP‐1 selectivity of the GLP‐1 receptor. However, the divergent residues in glucagon and GLP‐1 that determine specificity for the GLP‐1 receptor amino‐terminal extracellular domain are not known. Less is known about how the glucagon receptor distinguishes between glucagon and GLP‐1. We analysed chimeric glucagon/GLP‐1 peptides for their ability to bind and activate the glucagon receptor, the GLP‐1 receptor and chimeric glucagon/GLP‐1 receptors. The chimeric peptide GLP‐1(7–20)/glucagon(15–29) was unable to bind and activate the glucagon receptor. Substituting the glucagon receptor core domain with the GLP‐1 receptor core domain (chimera A) completely rescued the affinity and potency of GLP‐1(7–20)/glucagon(15–29) without compromising the affinity and potency of glucagon. Substituting transmembrane segment 1 (TM1), TM6, TM7, the third extracellular loop and the intracellular carboxy‐terminus of chimera A with the corresponding glucagon receptor segments re‐established the ability to distinguish GLP‐1(7–20)/glucagon(15–29) from glucagon. Corroborant results were obtained with the opposite chimeric peptide glucagon(1–14)/GLP‐1(21–37). The results suggest that the glucagon and GLP‐1 receptor amino‐terminal extracellular domains determine specificity for the divergent residues in the glucagon and GLP‐1 carboxy‐terminals respectively. The GLP‐1 receptor core domain is not a critical determinant of glucagon/GLP‐1 selectivity. Conversely, the glucagon receptor core domain contains two or more sub‐segments which strongly determine specificity for divergent residues in the glucagon amino‐terminus.