This family is known as Family B, the secretin-receptor family or family 2 of the G-protein-coupled receptors.They have been described in many animal species, but not in plants, fungi or prokaryotes. Three distinct sub-families (B1-B3) are recognized. Many secretin receptors are regulated by peptide hormones from the glucagon hormone family.
The secretin-like GPCRs include secretin, calcitonin, parathyroid hormone/parathyroid hormone-related peptides and vasoactive intestinal peptide receptors, all of which activate adenylyl cyclase and the phosphatidyl-inositol-calcium pathway. These receptors have 7 transmembrane helices, like rhodopsin-like GPCRs. However,there is no significant sequence identity between these families: the secretin-like receptors thus bear their own unique '7TM' signature.
Subfamily B1 contains classical hormone receptors, such as receptors for secretin and glucagon, that are all involved in cAMP-mediated signalling pathways.
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.
