TAS1R1

Taste receptor type 1 member 1 is a is_associated_with::protein that in humans is encoded by the TAS1R1 is_associated_with::gene.

Structure
The protein encoded by the TAS1R1 gene is a is_associated_with::G protein-coupled receptor with seven trans-membrane domains and is a component of the heterodimeric amino acid taste receptor T1R1+3. This receptor is formed as a dimer of the TAS1R1 and is_associated_with::TAS1R3 proteins. Moreover, the TAS1R1 protein is not functional outside of formation of the 1+3 heterodimer. The TAS1R1+3 receptor has been shown to respond to L-is_associated_with::amino acids but not to their D-enantiomers or other compounds. This ability to bind L-is_associated_with::amino acids, specifically L-is_associated_with::glutamine, enables the body to sense the is_associated_with::umami, or savory, taste. Multiple transcript variants encoding several different isoforms have been found for this gene, which may account for differing taste thresholds among individuals for the is_associated_with::umami taste. Another interesting quality of the TAS1R1 and is_associated_with::TAS1R2 proteins is their spontaneous activity in the absence of the extracellular domains and binding ligands. This may mean that the extracellular domain regulates function of the receptor by preventing spontaneous action as well as binding to activating ligands such as L-is_associated_with::glutamine.

Ligands
The is_associated_with::umami taste is distinctly related to the compound is_associated_with::monosodium glutamate(MSG). Synthesized in 1908 by Japanese chemist is_associated_with::Kikunae Ikeda, this flavor-enhancing compound led to the naming of a new flavor quality that was named “is_associated_with::umami”, the Japanese word for “tasty”. The TAS1R1+3 taste receptor is sensitive to the is_associated_with::glutamate in MSG as well as the synergistic taste-enhancer molecules is_associated_with::inosine monophosphate (IMP) and is_associated_with::guanosine monophosphate (GMP). These taste-enhancer molecules are unable to activate the receptor alone, but are rather used to enhance receptor responses to many L-amino acids.

Signal transduction
TAS1R1 and is_associated_with::TAS1R2 receptors have been shown to bind to is_associated_with::G proteins, most often the is_associated_with::gustducin Gα subunit, although a gustducin knock-out has shown small residual activity. TAS1R1 and is_associated_with::TAS1R2 have also been shown to activate Gαo and Gαi. This suggests that TAS1R1 and is_associated_with::TAS1R2 are is_associated_with::G protein-coupled receptors that inhibit is_associated_with::adenylyl cyclases to decrease is_associated_with::cyclic guanosine monophosphate (cGMP) levels in is_associated_with::taste receptors.

Research done by creating knock-outs of common channels activated by sensory G-protein is_associated_with::second messenger systems has also shown a connection between is_associated_with::umami taste perception and the is_associated_with::phosphatidylinositol (PIP2) pathway. The nonselecive cation is_associated_with::Transient Receptor Potential channel TRPM5 has been shown to correlate with both umami and sweet taste. Also, the is_associated_with::phospholipase PLCβ2 was shown to similarly correlate with umami and sweet taste. This suggests that activation of the G-protein pathway and subsequent activation of PLC β2 and the TRPM5 channel in these taste cells functions to activate the cell.

Location and innervation
TAS1R1+3 expressing cells are found mostly in the is_associated_with::fungiform papillae at the tip and edges of the tongue and palate taste receptor cells in the roof of the mouth. These cells are shown to synapse upon the is_associated_with::chorda tympani nerves to send their signals to the brain, although some activation of the is_associated_with::glossopharyngeal nerve has been found. TAS1R and TAS2R (bitter) channels are not expressed together in taste buds.