G protein-coupled receptor 135, member of the class A family of seven-transmembrane G protein-coupled receptors
GPR135, also known as the somastostatin- and angiotensin-like peptide receptor (SALPR), is found in various tissues including eye, brain, cervix, stomach, and testis. Pharmacological studies have shown that relaxin-3 (R3) is a high-affinity endogenous ligand for GPR135. R3 has recently been identified as a new member of the insulin/relaxin family of peptide hormones and is exclusively expressed in the brain neurons. In addition to GPR135, R3 also acts as an agonist for GPR142, a pseudogene in the rat, and can activate LGR7 (leucine repeat-containing G-protein receptor-7), which is the main receptor for relaxin-1 (R1) and relaxin-2 (R2). While R1 and R2 are hormones primarily associated with reproduction and pregnancy, R3 is involved in neuroendocrine and sensory processing. All GPCRs have a common structural architecture comprising of seven-transmembrane (TM) alpha-helices interconnected by three extracellular and three intracellular loops. A general feature of GPCR signaling is agonist-induced conformational changes in the receptors, leading to activation of the heterotrimeric G proteins, which consist of the guanine nucleotide-binding G-alpha subunit and the dimeric G-beta-gamma subunits. The activated G proteins then bind to and activate numerous downstream effector proteins, which generate second messengers that mediate a broad range of cellular and physiological processes.