TAS1R3

Taste receptor type 1 member 3 is a is_associated_with::protein that in humans is encoded by the TAS1R3 is_associated_with::gene. The TAS1R3 gene encodes the human homolog of mouse Sac is_associated_with::taste receptor, a major determinant of differences between sweet-sensitive and -insensitive mouse strains in their responsiveness to sucrose, saccharin, and other sweeteners.

Structure
The protein encoded by the TAS1R3 gene is a is_associated_with::G protein-coupled receptor with seven trans-membrane domains and is a component of the heterodimeric is_associated_with::amino acid is_associated_with::taste receptor TAS1R1+3 and is_associated_with::sweet is_associated_with::taste receptor TAS1R2+3. This receptor is formed as a is_associated_with::protein dimer with either is_associated_with::TAS1R1 or is_associated_with::TAS1R2. Experiments have also shown that a homo-dimer of TAS1R3 is also sensitive to natural is_associated_with::sugar substances. This has been hypothesized as the mechanism by which is_associated_with::sugar substitutes do not have the same taste qualities as natural sugars.

Ligands
The is_associated_with::G protein-coupled receptors for sweet and umami taste are formed by dimers of the TAS1R proteins. The TAS1R1+3 taste receptor is sensitive to the 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 many to L-amino acids. The TAS1R2+3 receptor has been shown to respond to natural sugars is_associated_with::sucrose and is_associated_with::fructose, and to artificial sweetners is_associated_with::saccharin, is_associated_with::acesulfame potassium, is_associated_with::dulcin, is_associated_with::guanidinoacetic acid.

Signal transduction
is_associated_with::TAS1R2 and is_associated_with::TAS1R1 receptors have been shown to bind to is_associated_with::G proteins, most often the is_associated_with::gustducin Gα subunit, although a gusducin knock-out has shown small residual activity. is_associated_with::TAS1R2 and is_associated_with::TAS1R1 have also been shown to activate Gαo and Gαi protein subunits. 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. The TAS1R3 protein, however, has been shown in vitro to couple with Gα subunits at a much lower rate than the other TAS1R proteins. While the protein structures of the TAS1R proteins are similar, this experiment shows that the G protein-coupling properties of TAS1R3 may be less important in the transduction of taste signals than the is_associated_with::TAS1R1 and is_associated_with::TAS1R2 proteins.

Location and innervation
TAS1R1+3 expressing cells are found in 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. TAS1R2+3 expressing cells are found in is_associated_with::circumvallate papillae and is_associated_with::foliate papillae near the back 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::glossopharyngeal nerves to send their signals to the brain. TAS1R and TAS2R (bitter) channels are not expressed together in any taste buds.