Sensory perception of sweet, bitter, and umami (glutamate) taste

TAS1R-containing sweet and umami receptors and TAS2R bitter receptors are each physically associated with a particular heterotrimeric G protein complex, the gustducin complex, containing GNAT3 (gustducin), GNB1 or GNB3, and GNG13. Upon binding an agonist ligand, the receptor activates the alpha subunit, GNAT3, to exchange GDP for GTP, which results in a conformational change in GNAT3 that causes the receptor-gustducin complex to dissociate, yielding GNAT3:GTP, GNB1,3:GNG13, and the receptor:ligand. The GNB1,3:GNG13 complex binds and activates Phospholipase C beta-2 (PLCB2), which then hydrolyzes phosphoinositol 4,5-bisphosphate (PI(4,5)P2) to yield diacylglycerol and inositol 1,4,5-trisphosphate (I(1,4,5)P3). I(1,4,5)P3 binds and activates the calcium channel IP3-gated Ca-channel type 3 (ITPR3) and ITPR3 then releases calcium ions from the endoplasmic reticulum into the cytosol. The increased cytosolic calcium activates the TRPM5 cation channels, which then transport sodium ions along the concentration gradient from the extracellular region to the cytosol (reviewed in Aroke et al. 2020). The depolarization activates SCN2A, SCN3A, and SCN9A channels, which transport further sodium ions from the extracellular region to the cytosol. The depolarization of the plasma membrane opens CALHM1:CALHM3 channels, which transport ATP, a neurotransmitter in the olfactory system, from the cytosol to the extracellular region.

Taste receptors were initially discovered in taste buds of the tongue and have now been found in several other tissues including nasal epithelium (Barnham et al. 2015, inferred from rodent homologs in Tizzano et al. 2011), the respiratory system, pancreatic islet cells, sperm, leukocytes, and enteroendocrine cells of the gut (inferred from rat and mouse homologs in Wu et al. 2002).