The peptide hormone GLP-1 (glucagon-like peptide-1) is secreted by the duodenum, which subsequently triggers the release of insulin from pancreatic cells. The ability of GLP-1 to induce insulin secretion in dependence on high glucose levels, renders this component potentially useful in the treatment of non-insulin dependent diabetus mellitus (NIDDM). However, the peptide nature of GLP-1 prevents its direct application due to its short half-life in vivo. A rational design of new agonists will therefore greatly benefit from structural knowledge of the interaction between GLP-1 and its receptor. The receptor protein for GLP-1 belongs to class B of G-protein coupled receptors. The protein has a large N-terminal extracellular domain that was shown to be involved in the ligand binding. As for other peptide-hormone receptors such as receptors for parathyroid hormone (PTH), vasoactive intestinal peptide (VIP), calcitonin or pituitary adenylate cyclase activating peptide (PACAP), the N-terminal domain contains six conserved cysteine residues, which are assumed to form three disulfide bonds. As the extracellular domain of the GLP-1 receptor can specifically bind its peptide, we set out to generate large amounts of that portion of the molecule for functional and structural analysis The N-terminal, extracellular domain of the receptor for glucagon-like peptide 1 (GLP-1 receptor) was expressed at a high level in E. coli and isolated as inclusion bodies. Renaturation with concomitant disulfide bond formation was achieved from guanidinium-solubilized material. A soluble and active fraction of the protein was isolated by ion exchange chromatography and gel filtration. Complex formation with GLP-1 was shown by cross-linking experiments, surface plasmon resonance measurements, and isothermal titration calorimetry. The existence of disulfide bridges in the N-terminal receptor fragment was proven after digestion of the protein with chymotrypsin. Further analysis revealed a disulfide-binding pattern with links between cysteines 46 and 71, 62 and 104, and between 85 and 126.