There is strong evidence that T1R2 and T1R3 proteins function as sweet taste receptors. Although T1R genes are expressed in several different internal organs (reviewed by Bachmanov and Beauchamp[35]), their main sites of expression are taste receptor cells of the taste buds. In mice and rats, T1R2 and T1R3 are co-expressed in the same taste cells, but some taste cells express only T1R3.[36-38] Co-expression of T1R2 and T1R3 in the same taste cells suggested that they may function as heterodimers, which is believed to commonly occur with GPCRs.[39] Consistent with this, cells heterologously expressing both T1R2 and T1R3 respond to sweeteners,[38,40,41] but T1R3 may also function as a low-affinity sugar receptor alone, probably as a homodimer.[42] Finally, genetically engineered mice with targeted mutations of the Tas1r2 or Tas1r3 genes have diminished taste responses to sweeteners.[42,43]
