G protein-coupled receptor 45, member of the class A family of seven-transmembrane G protein-coupled receptors
This subgroup includes the human orphan receptor GPR45 and closely related proteins found in vertebrates. GPR45 is also called PSP24 in Xenopus and PSP24-alpha (or PSP24-1) in mammals. GPR45 shows the highest sequence homology with GPR63 (PSP24-beta, or PSP24-2). PSP24 was originally identified as a novel, high-affinity lysophosphatidic acid (LPA) receptor in Xenopus laevis oocytes; however, PSP24 receptors (GPR45 and GPR63) have not been shown to be activated by LPA. Mammalian PSP24 receptors are highly expressed in neuronal cells of cerebellum and their expression level remains constant from the early embryonic stages to adulthood, suggesting the important role of PSP24s in brain neuronal functions. Members of this subgroup contain the highly conserved Asp-Arg-Tyr/Phe (DRY/F) motif found in the third transmembrane helix (TM3) of the rhodopsin-like class A receptors which is important for efficient G protein-coupled signal transduction. 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.