# Secretin receptor family

Secretin receptor family (class B GPCR subfamily) consists of secretin receptors regulated by peptide hormones from the glucagon hormone family. The family is different from adhesion G protein-coupled receptors.
The secretin-receptor family of GPCRs include vasoactive intestinal peptide receptors and receptors for secretin, calcitonin and parathyroid hormone/parathyroid hormone-related peptides. These receptors activate adenylyl cyclase and the phosphatidyl-inositol-calcium pathway. The receptors in this family have seven transmembrane helices, like rhodopsin-like GPCRs. However, there is no significant sequence identity between these two GPCR families and the secretin-receptor family has its own characteristic 7TM signature.
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. 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
** PACAPR (ADCYAP1R1)
* Calcitonin receptor InterPro: IPR003287
** CALCR
* Calcitonin receptor-like receptor InterPro: IPR015476
** CALCRL
* Corticotropin-releasing hormone receptor InterPro: IPR003051
** CRHR1; CRHR2
* Glucose-dependent insulinotropic polypeptide receptor/Gastric inhibitory polypeptide receptor InterPro: IPR001749
** GIPR
* Glucagon receptor InterPro: IPR003291
** GCGR
* Glucagon receptor-related InterPro: IPR003290
** GLP1R; GLP2R;
* Growth hormone releasing hormone receptor InterPro: IPR003288
** GHRHR
* Parathyroid hormone receptor InterPro: IPR002170
** PTHR1; PTHR2
* Secretin receptor InterPro: IPR002144
** SCTR
* Vasoactive intestinal peptide receptor InterPro: IPR001571
** VIPR1; VIPR2

## Subfamily B2

Subfamily B2 contains receptors with long extracellular N-termini, such as the leukocyte cell-surface antigen CD97; calcium-independent receptors for latrotoxin and brain-specific angiogenesis inhibitor receptors amongst others. They are otherwise known as Adhesion G protein-coupled receptors.
* Brain-specific angiogenesis inhibitor InterPro: IPR008077
** BAI1; BAI2; BAI3
* CD97 antigen InterPro: IPR003056
** CD97
* EMR hormone receptor InterPro: IPR001740
** CELSR1; CELSR2; CELSR3; EMR1; EMR2; EMR3; EMR4
* GPR56 orphan receptor InterPro: IPR003910
** GPR56; GPR64; GPR97; GPR110; GPR111; GPR112; GPR113; GPR114; GPR115; GPR123; GPR125; GPR126; GPR128; GPR133; GPR144; GPR157
* Latrophilin receptor InterPro: IPR003924
** ELTD1; LPHN1; LPHN2; LPHN3
* Ig-hepta receptor InterPro: IPR008078
** GPR116

## Subfamily B3

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.
* Diuretic hormone receptor InterPro: IPR002001

## Unclassified members

HCTR-5; HCTR-6; KPG 006; KPG 008