EXCHANGE OF W-39 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 th e L-cells exerts an insulinotropic effect at pancreatic beta-cells. Th e GLP-1 receptor belongs to a new subfamily of the superfamily of seve n transmembrane, G-protein-coupled receptors (7 TM receptors). We show that a single point mutation within a nonconserved motif of the N-ter minal, extracellular domain of the GLP-1 receptor results in a dramati c impairment of receptor function. Thus, substitution of W-39 by A or F is followed by a loss of GLP-1 binding. Exchange of K-38 With A (mut ant K) slightly decreased GLP 1 binding affinity. Replacement of the n egatively charged Q(37) by K and K-38 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-t ype receptor and mutant K. Therefore, the presence of an imidazol ring structure in the investigated receptor region is necessary for an int act receptor function. Furthermore, a positive charge at this location is important for the receptor-ligand interaction.