parathyroid hormone 2 receptor, member of the class B family of seven-transmembrane G protein-coupled receptors
The parathyroid hormone 2 receptor (PTH2R), one of the three subtypes of PTH receptor family, is found in mammals and fish, but not in chicken or frog. PTH2R is potently activated by tuberoinfundibular peptide-39 (TIP-39) but not by PTH-related peptide (PTHrP), a paracrine factor that regulates endochondral bone development. PTH, an endocrine hormone that regulates calcium homoeostasis and bone maintenance, strongly activates human PTH2R, but only weakly activates rat and zebrafish PTH2Rs. These results suggest that TIP-39 is a natural ligand for PTH2R. Conversely, PTH1R is activated by PTH and PTHrP, but not by 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.