| Primary Identifier | IPR000466 | Type | Family |
| Short Name | Adeno_A3_rcpt |
| 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 rhodopsin-like GPCRs (GPCRA) represent a widespread protein family that includes hormone, neurotransmitter and light receptors, all of which transduce extracellular signals through interaction with guanine nucleotide-binding (G) proteins. Although their activating ligands vary widely in structure and character, the amino acid sequences of the receptors are very similar and are believed to adopt a common structural framework comprising 7 transmembrane (TM) helices [, , ].In addition to their role in energy metabolism, purines (especiallyadenosine and adenine nucleotides) produce a wide range of pharmacologicaleffects mediated by activation of cell surface receptors. Distinctreceptors exist for adenosine. In the periphery, the main effects ofadenosine include vasodilation, bronchoconstriction, immunosuppresion,inhibition of platelet aggregation, cardiac depression, stimulation ofnociceptive afferents, inhibition of neurotransmitter release andinhibition of the release of other factors, e.g. hormones. In the CNS,adenosine exerts a pre- and post-synaptic depressant action, reducing motoractivity, depressing respiration, inducing sleep and relieving anxiety. Thephysiological role of adenosine is thought to be to adjust energy demandsin line with oxygen supply. Many of the clinical actions of methylxanthinesare thought to be mediated through antagonism of adenosine receptors. Foursubtypes of receptor have been identified, designated A1, A2A, A2B and A3.A3 receptors are found in high levels in the testis, and in lower levels inthe lung, kidney and heart. They are also found in low levels in regionsof the CNS (including the cerebral cortex, striatum and olfactory bulb). Thepresence in high levels in the testis has led to the suggestion that it mayplay a role in reproduction. The A3 receptor inhibits adenylyl cyclasethrough a pertussis-toxin-sensitive G-protein, probably belonging to theGi/Go class. |
