GPCRs represent the largest superfamily of receptors in the human genome. Present on every cell and responding to a plethora of stimuli, GPCRs are involved in a great variety of physiological processes. According to the GRAFS classification, GPCRs can be devided into five classes named Glutamate, Rhodopsin, Adhesion, Frizzled/taste, and Secretin. By far largest and best understood is the Rhodopsin class that includes all classical GPCRs as well as hundreds of olfactory receptors.
The Adhesion class comprises 33 members in humans with a broad distribution in embryonic and larval cells, cells of the reproductive tract, neurons, leukocytes, and a variety of tumors. Most notable is the molecular structure that sets Adhesion-GPCRs apart from other GPCR. Intramolecular processing at a GPCR-proteolytic site (GPS) proximal to the first transmembrane helix gives rise to a membrane-spanning and an extracellular subunit, which subsequently reassociate non-covalently, resulting in expression of a heterodimeric receptor at the cell surface. The extracellular subunits of Adhesion-GPCRs can be exceptionally long and contain a variety of structural domains that are known for the ability to facilitate cell and matrix interactions. Ligand profiles and in vitro studies have indicated a role of Adhesion-GPCRs in cell adhesion and migration. More recent work utilizing genetic models confined this concept by demonstrating that the primary function of Adhesion-GPCRs may relate to the proper positioning of cells in a variety of organ systems. Moreover, growing evidence implies a role of Adhesion-GPCRs in tumor cell metastasis.
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