The secretin-receptor family GPCRs exist in many animal species. Data mining with the Pfam signature has identified members in fungi, although due to their presumed non-hormonal function they are more commonly referred to as Adhesion G protein-coupled receptors, making the Adhesion subfamily the more basal group.[6] Three distinct sub-families (B1-B3) are recognized.
Subfamily B1
Subfamily B1 contains classical hormone receptors, such as receptors for secretin and glucagon, that are all involved in cAMP-mediated signalling pathways.
Pituitary adenylate cyclase-activating polypeptide type 1 receptor InterPro: IPR002285
Subfamily B3 includes Methuselah and other Drosophila proteins. Other than the typical seven-transmembrane region, characteristic structural features include an amino-terminal extracellular domain involved in ligand binding, and an intracellular loop (IC3) required for specific G-protein coupling.