type 1 taste receptors, member of the class C of seven-transmembrane G protein-coupled receptors
This subfamily represents the type I taste receptors (TAS1Rs) that belongs to the class C family of G protein-coupled receptors. The functional TAS1Rs are obligatory heterodimers built from three known members, TAS1R1-3. TAS1R1 combines with TAS1R3 to form an umami taste receptor, which is responsible for the perception of savory taste, such as the food additive monosodium glutamate (MSG); whereas the combination of TAS1R2-TAS1R3 forms a sweet-taste receptor for sugars and D-amino acids. On the other hand, the type II taste receptors (TAS2Rs), which belong to the class A family of GPCRs, recognize bitter tasting compounds. In the case of sweet, for example, the TAS1R2-TAS1R3 heterodimer activates phospholipase C (PLC) via alpha-gustducin, a heterodimeric G protein that is involved in perception of sweet and bitter tastes. This activation leads to generation of inositol (1, 4, 5)-trisphosphate (IP3) and diacylglycerol (DAG), and consequently increases intracellular Ca2+ mobilization and activates a cation channel, TRPM5. In contrast to the TAS1R2-TAS1R3 heterodimer, TAS1R3 alone could activate adenylate cyclase leading to cAMP formation in the absence of alpha-gustducin. Each TAS1R contains a large extracellular Venus flytrap-like domain in the N-terminus, cysteine-rich domain (CRD) and seven-transmembrane (7TM) domain, which are characteristics of the class C GPCRs. The Venus flytrap-like domain shares strong sequence homology to bacterial periplasmic binding proteins and possess the orthosteric amino acid and calcium binding sites for members of the class C, including CaSR, GABA-B1, GPRC6A, mGlu, and TAS1R receptors.