mammalian taste receptor 2, subtypes 7 and 9, member of the seven-transmembrane G protein-coupled receptor superfamily
This group includes the mammalian taste receptor 2 (TAS2R) subtypes 7 and 9, which function as bitter taste receptors. The human TAS2R family contains about 25 functional members, which are glycoproteins and have the ability to form both homomeric and heteromeric receptor complexes. Five basic tastes are perceived by animals: bitter, sweet, sour, salty, and umami (the taste of glutamate, MSG). Among these, sour and salty are mediated by ion channels, while the perception of umami and sweet tastes is mediated by the TAS1R taste receptors, which belong to the class C GPCR family. The TAS2Rs in humans have a short extracellular N-terminus and the ligand binds within the transmembrane domain, whereas the TAS1Rs have a large N-terminal extracellular domain composed of the Venus flytrap module that forms the orthosteric (primary) ligand binding site. Signal transduction of bitter taste involves binding of bitter compounds to TAS2Rs linked to the alpha-subunit of gustducin, a heterotrimeric G protein expressed in taste receptor cells. This G-alpha subunit stimulates phosphodiesterase and decreases cAMP and cGMP levels. Further steps in the signaling cascade is still unknown. The beta-gamma-subunit of gustducin also mediates bitter taste transduction by activating phospholipase C, which leads to an increased formation of IP3 (inositol triphosphate) and DAG (diacylglycerol), thereby causing release of Ca2+ from intracellular stores and enhanced neurotransmitter release.
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