| Description: |
G protein-coupled receptors (GPCRs) constitute a vast protein family that encompasses a wide range of functions, including various autocrine, paracrine and endocrine processes. They show considerable diversity at the sequence level, on the basis of which they can be separated into distinct groups []. The term clan can be used to describe the GPCRs, as they embrace a group of families for which there are indications of evolutionary relationship, but between which there is no statistically significant similarity in sequence []. The currently known clan members include rhodopsin-like GPCRs (Class A, GPCRA), secretin-like GPCRs (Class B, GPCRB), metabotropic glutamate receptor family (Class C, GPCRC), fungal mating pheromone receptors (Class D, GPCRD), cAMP receptors (Class E, GPCRE) and frizzled/smoothened (Class F, GPCRF) [, , , , ]. GPCRs are major drug targets, and are consequently the subject of considerable research interest. It has been reported that the repertoire of GPCRs for endogenous ligands consists of approximately 400 receptors in humans and mice []. Most GPCRs are identified on the basis of their DNA sequences, rather than the ligand they bind, those that are unmatched to known natural ligands are designated by as orphan GPCRs, or unclassified GPCRs [].The secretin-like GPCRs include secretin [], calcitonin [], parathyroid hormone/parathyroid hormone-related peptides []and vasoactive intestinal peptide [], all of which activate adenylyl cyclase and the phosphatidyl-inositol-calcium pathway. These receptors contain seven transmembrane regions, in a manner reminiscent of the rhodopsins and other receptors believed to interact with G-proteins (however there is no significant sequence identity between these families, the secretin-like receptors thus bear their own unique '7TM' signature). Their N-terminal is probably located on the extracellular side of the membrane and potentially glycosylated. This N-terminal region contains a long conserved region which allows the binding of large peptidic ligand such as glucagon, secretin, VIP and PACAP; this region contains five conserved cysteines residues which could be involved in disulphide bond. The C-terminal region of these receptor is probably cytoplasmic. Every receptor gene in this family is encoded on multiple exons, and several of these genes are alternatively spliced to yield functionally distinct products. The Adhesion G Protein-Coupled Receptors (aGPCRs) constitute an evolutionary ancient membrane protein family. The receptors contain a 7-TM domain with phylogeny suggesting ancestry to the Family B/2 (secretin receptor family, Class B/2) G-Protein-Coupled Receptors. aGPCRs are distinguished by their large amino-terminal regions that typically contain multiple modular motifs such as EGF (Epidermal Growth Factor-like), cadherin and immunoglobulin domains as well as novel lineage-specific structures. A defining feature of aGPCRs is the GPCR Autoproteoolysis-Inducing (GAIN) domain linking the N-terminal structure to the 7-TM region. Most aGPCRs undergo autocatalytic cleavage here, at the GPCR proteolysis site (GPS) into N-terminal and C-terminal fragments [].Adhesion G protein-coupled receptor E2 (ADGRE2) protein is a member of the EGF-7TM subclass of aGPCRs and has an N-terminal extracellular region that consists of 5 tandem EGF-like adhesion domains, an internal mucin-like stalk domain containing a short G-protein proteolytic site and a C-terminal seven-pass transmembrane domain. ADGRE2 undergoes autocatalytic cleavage within its G-protein proteolytic site motif. It is expressed predominantly in myeloid leukocytes but also on the surface of lung mast cells and the HMC1 human mast-cell line. The endogenous ligand is dermatan sulfate. The most closely related paralogue of ADGRE2 is ADGRE5 (also called CD97). Ligand binding of ADGRE5 mediates cell-cell adhesion of leukocytes and mediates an essential role in leukocyte migration []. |
