parathyroid hormone 3 receptor, member of the class B family of seven-transmembrane G protein-coupled receptors
The parathyroid hormone 3 receptor (PTH3R), one of the three subtypes of PTH receptor family, is found in chicken and fish, but it is absent in mammals. On the other hand, the PTH1R is found in all vertebrate species, whereas PTH2R is found in mammals and fish, but not in chicken or frog. PTH1R is is activated by two polypeptide ligands: PTH, an endocrine hormone that regulates calcium homoeostasis and bone maintenance, and PTH-related peptide (PTHrP), a paracrine factor that regulates endochondral bone development. PTH2R is potently activated by tuberoinfundibular peptide-39 (TIP-39), but not by PTHrP. PTH also strongly activates human PTH2R, but only weakly activates rat and zebrafish PTH2Rs, suggesting that TIP-39 is a natural ligand for PTH2R. Conversely, PTH3R binds and responds to both PTH and PTHrP, but not the TIP-39. The PTH family receptors are members of the B1 (or secretin-like) subfamily of class B GPCRs, which include receptors for polypeptide hormones of 27-141 amino-acid residues such as secretin, glucagon, glucagon-like peptide (GLP), and calcitonin gene-related peptide. These receptors contain the large N-terminal extracellular domain (ECD), which plays a critical role in hormone recognition by binding to the C-terminal portion of the peptide. On the other hand, the N-terminal segment of the hormone induces receptor activation by interacting with the receptor transmembrane domains and connecting extracellular loops, triggering intracellular signaling pathways.