GPCR family 2, EMR1-like receptor (GPCR_2_EMR1-like_rcpt)
Type: Family

Human epidermal growth factor (EGF)-like module containing mucin-like hormone receptor 1 (EMR1) is a surface receptor of unknown function that belongs to the EGF-seven-transmembrane (EGF-TM7) family of G-protein coupled receptors . Human EMR1 has been reported to be expressed exclusively on eosinophils . It is the the human homologue of F4/80, a monoclonal antibody that recognises a Mus musculus (Mouse) macrophage-restricted cell surface glycoprotein that has been extensively used to characterise macrophage populations in a wide range of immunological studies . Little is known about its possible role in macrophage differentiation and function. The sequence of the F4/80 protein is similar to two protein superfamilies: the N-terminal region contains seven epidermal growth factor (EGF)-like domains, while the C-terminal region contains seven hydrophobic regions whose signature is consistent with membership of the secretin-like superfamily of GPCRs. The EGF and GPCR domains are separated from each other by a serine/threonine-rich domain, a feature reminiscent of mucin-like, single-span, integral membrane glycoproteins with adhesive properties .

This family also comprises EMR3, a marker for mature granulocytes , and EMR4 .

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.