This family is known as Family B, the secretin-receptor family or family 2 of the G-protein-coupled receptors (GCPRs). They have been described in many animal species, but not in plants, fungi or prokaryotes. Three distinct sub-families are recognised. Subfamily B1 contains classical hormone receptors, such as receptors for secretin and glucagon, that are all involved in cAMP-mediated signalling pathways. Subfamily B2 contains receptors with long extracellular N-termini, such as the leukocyte cell-surface antigen CD97 (Swiss:P48960); calcium-independent receptors for latrotoxin (such as Swiss:O94910), and brain-specific angiogenesis inhibitors (such as Swiss:O14514) amongst others. Subfamily B3 includes Methuselah and other Drosophila proteins (e.g. Swiss:P83119). 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 [1].

1: Family-B G-protein-coupled receptors. Harmar AJ; Genome Biol 2001;2:REVIEWS3013. PMID:11790261