1. A Plethora of Taste Receptors
Sue C Kinnamon 
Neuron 
Volume 25, Issue 3, Pages (March 2000)
DOI: /S (00)

2. Figure 1
Predicted Membrane Topology of G Protein–Linked Taste Receptors
Presumed ligand binding domains are indicated in red. For taste mGluR4 (A) and T1R (B) receptors, the ligand binding is presumed to be in the large extracellular N-terminal domain, based on analogy with synaptic mGluRs. For T2Rs (C), ligand binding is presumed to occur in the extracellular loops and outer portions of transmembrane segments 4–7, which are the most divergent regions among members of this receptor gene family. The presumptive binding regions for T2R receptors differ somewhat from odorant receptors and other short N-terminal receptors, where binding is expected to occur primarily within the transmembrane segments.
Neuron  , DOI: ( /S (00) )

3. Figure 2 Transduction of Umami (A) and Bitter (B) Taste Stimuli
For umami transduction, glutamate binds to taste mGluR4, activating a G protein α subunit that stimulates phosphodiesterase (PDE), causing a reduction in intracellular cAMP. Although the identity of the G protein is not known, likely candidates include α-gustducin and α-transducin. The downstream target of the reduced cAMP has not been identified. Bitter transduction of denatonium and PROP involves binding to mT2R8, which activates the G protein α-gustducin. This stimulates phosphodiesterase (PDE), causing decreases in intracellular cAMP, while in parallel β3/γ13 stimulates phospholipase C-β2 (PLC-β2), resulting in production of inositol trisphosphate (IP3) and diacylglycerol (DAG). The IP3 causes release of Ca2+ from intracellular stores, but the function of decrease in cAMP is not yet clear.
Neuron  , DOI: ( /S (00) )