Exchange of W39 by A within the N-terminal extracellular domain of the GLP-1 receptor results in a loss of receptor function.

The proglucagon-derived glucagon-like peptide-1 (GLP-1) secreted by the L-cells exerts an insulinotropic effect at pancreatic beta-cells. The GLP-1 receptor belongs to a new subfamily of the superfamily of seven transmembrane, G-protein-coupled receptors (7 TM receptors). We show that a single point mutation within a nonconserved motif of the N-terminal, extracellular domain of the GLP-1 receptor results in a dramatic impairment of receptor function. Thus, substitution of W39 by A or F is followed by a loss of GLP-1 binding. Exchange of K38 with A (mutant K) slightly decreased GLP-1 binding affinity. Replacement of the negatively charged Q37 by K and K38 by A, which is identical with a shift of the positively charged K one position upstream, resulted in a receptor mutant able to bind GLP-1 with higher affinity as the wild-type receptor and mutant K. Therefore, the presence of an imidazol ring structure in the investigated receptor region is necessary for an intact receptor function Furthermore, a positive charge at this location is important for the receptor-ligand interaction.