retinoic acid-inducible orphan G-protein-coupled receptor 3; class C family of seven-transmembrane G protein-coupled receptors, group 5, member C
Retinoic acid-inducible G-protein-coupled receptors (RAIGs), also referred to as GPCR class C group 5, are a group consisting of four orphan receptors RAIG1 (GPRC5A), RAIG2 (GPRC5B), RAIG3 (GPRC5C), and RAIG4 (GPRC5D). Unlike other members of the class C GPCRs which contain a large N-terminal extracellular domain, RAIGs have a shorter N-terminus. Thus, it is unlikely that RAIGs bind an agonist at its N-terminus domain. Instead, the agonists may bind to the seven-transmembrane domain of these receptors. In addition, RAIG2 and RAIG3 contain a cleavable signal peptide whereas RAIG1 and RAIG4 do not. Although their expression is induced by retinoic acid (vitamin A analog), their biological function is not clearly understood. To date, no ligand is known for the members of RAIG family. Three receptor types (RAIG1-3) are found in vertebrates, while RAIG4 is only present in mammals. They show distinct tissue distribution with RAIG1 being primarily expressed in the lung, RAIG2 in the brain and placenta, RAIG3 in the brain, kidney and liver, and RAIG4 in the skin. The specific function of RAIG3 is unknown; however, this protein may play a role in mediating the effects of retinoic acid on embryogenesis, differentiation, and tumorigenesis through interaction with a G-protein signaling cascade.