Relaxin receptor (IPR008112)
Short name: Relaxin_rcpt
Overlapping homologous superfamilies
- G protein-coupled receptor, rhodopsin-like (IPR000276)
- Relaxin receptor (IPR008112)
The insulin family of proteins groups together several evolutionarily related active peptides [PMID: 6107857]: these include insulin [PMID: 6243748, PMID: 503234], relaxin [PMID: 10601981, PMID: 8735594], insect prothoracicotropic hormone (bombyxin) [PMID: 8683595], insulin-like growth factors (IGF1 and IGF2) [PMID: 2036417, PMID: 1319992], mammalian Leydig cell-specific insulin-like peptide (gene INSL3), early placenta insulin-like peptide (ELIP) (gene INSL4), locust insulin-related peptide (LIRP), molluscan insulin-related peptides (MIP), and Caenorhabditis elegans insulin-like peptides. The 3D structures of a number of family members have been determined [PMID: 2036417, PMID: 1319992, PMID: 9141131]. The fold comprises two polypeptide chains (A and B) linked by two disulphide bonds: all share a conserved arrangement of 4 cysteines in their A chain, the first of which is linked by a disulphide bond to the third, while the second and fourth are linked by interchain disulphide bonds to cysteines in the B chain.
Insulin is found in many animals, and is involved in the regulation of normal glucose homeostasis. It also has other specific physiological effects, such as increasing the permeability of cells to monosaccharides, amino acids and fatty acids, and accelerating glycolysis and glycogen synthesis in the liver [PMID: 6243748]. Insulin exerts its effects by interaction with a cell-surface receptor, which may also result in the promotion of cell growth [PMID: 6243748].
Insulin is synthesised as a prepropeptide from which an endoplasmic reticulum-targeting sequence is cleaved to yield proinsulin. The sequence of prosinsulin contains 2 well-conserved regions (designated A and B), separated by an intervening connecting region (C), which is variable between species [PMID: 503234]. The connecting region is cleaved, liberating the active protein, which contains the A and B chains, held together by 2 disulphide bonds [PMID: 503234].
Relaxin has diverse actions in the reproductive tract and in other tissues during pregnancy [PMID: 12217491]. Although binding sites for relaxin have been found in reproductive tissue, the nature of the receptor was previously unknown. Recently, two orphan GPCRs, LGR7 and LGR8, have been identified as receptors for the hormone. These two receptors contain large extracellular N-termini with leucine-rich repeat regions, and are structurally similar to the gonadotropin and thyrotropin receptors. LGR7 is expressed in the brain, kidney, testis, placenta, uterus, ovary, adrenal gland, prostate, skin and heart, while LGR8 is expressed mainly in the brain, kidney, muscle, testis, thyroid, uterus, peripheral blood cells and bone marrow. Upon binding to LGR7 or 8, relaxin stimulates a dose-dependent increase in cyclic AMP production, indicating coupling of the receptors to G proteins.
- PR01739 (RELAXINR)