Class B G protein-coupled receptors (GPCRs) exert essential action in hormonal homeostasis and are important therapeutic targets for a variety of diseases including metabolic disorders such as type 2 diabetes. These receptors consist of an extracellular domain (ECD) and a transmembrane domain (TMD), both of which are required to interact with their cognate peptide ligands and to regulate downstream signal transduction. Due to difficulties in high-quality protein preparation, determination of the structure of full-length class B GPCRs remains a challenge, thus limiting the understanding of molecular mechanisms of receptor action.