| Receptor | Number of intact genes | Conservation* within each family | Sites of expression | Known ligands (examples) | Structural notes |
---|---|---|---|---|---|---|
Mouse/ human | T1R | 3 (human) 3 (mouse) | 30-40% | Taste cells in the tongue; T1R1 plus T1R3 or T1R2 plus T1R3 are preferentially coexpressed, with few cells expressing all three genes | Sweet chemicals (T1R2 plus T1R3) L-amino acids, umami chemicals (T1R1 plus T1R3) | Similar to V2R (20-30% identical) |
 | T2R | 28 (human); approximately 40 (mouse) | 25% to more than 80% | Taste cells in the tongue; most cells express many, if not all, T2R genes | Bitter compounds, such as cycloheximide (mouse T2R5), salicin (human T2R16), and PTC (human T2R38) | Similar to V1R (15-20% identical) |
 | V1R | 5 (human) 137 (mouse) | 25% to nearly 100% | Basal layer in the vomeronasal organ; probably one V1R gene expressed per cell | 2-heptanone (V1Rb1), 6-hydroxy-6-methyl-3-heptanone, n-pentylacetate, and isobutylamine | Similar to T2R (15-20% identical); important for maternal aggression |
 | V2R | None in human; approximately 140 in mouse | 25% to nearly 100% | Apical layer in the vomeronasal organ; probably one V2R gene expressed per cell | None identified | Similar to T1R (20-30% identical); forms a complex with MHC class Ib and β2-microglobulin; may be required for aggression between males |
 | MHC 1b | None in human; 9 expressed in the mouse | 40% to more than 80% | Apical layer in the vomeronasal organ; one or a few MHC 1b genes expressed per cell; coexpressed with a V2R gene | None (in the VNO) | Forms a complex with β2-microglobulin and V2Rs; may be required for aggression between males |
Fly | GR | 69 | Approximately 15-75% | Spatially defined in one or several taste organs (labellum, labral and cibarial sense organs, legs, wings, and female genitalia) | Sugars and presumably other classes of compounds, including pheromones | Distantly related to olfactory receptor genes (Or83b); gustatory receptor probably recognize the whole spectrum of small soluble ligands; up to 20 subfamilies (with 1-8 genes each) |
 | GR_S†| 8 | 44-75% | Foreleg and labellum (Gr5a), possibly other taste sensory organs | Trehalose (Gr5a) | The knockout fly has lower sensitivity to trehalose; member of a subfamily of 7 genes |