Background information
The sense of taste provides animals with valuable information about the quality and nutritional value of food. There are four widely accepted categories of taste perception, sweet, bitter, salty, and sour. A controversial fifth taste, known as umami or monosodium glutamate (MSG), has also been described. A family of G protein coupled receptors are involved in taste perception, and includes T1R, which is involved in sweet and umami taste perception, and T2R, which is involved in bitter taste perception. The T1R family consists of three members, T1R1, T1R2, and T1R3 (1-4). These proteins form heterodimers, which alters the selectivity of the subunits (1-4). The T1R2 and T1R3 heterodimer functions as a receptor for sweet taste, and recognizes several sweet-tasting molecules, such as sucrose, saccharin, dulcin, and acesulfame-K (1–4). The T1R1 and T1R3 heterodimer recognizes L-amino-acids to perceive umami taste. Sweet taste transduction is carried out by two pathways (2). First, sucrose and other sugars activate Gas via the T1Rs, which activates adenylyl cyclase to generate cAMP. Artificial sweeteners bind to either Gbg or Gaq coupled T1Rs to activate PLCb2 and generate IP3 and DAG. Both pathways ultimately lead to neurotransmitter release. The mouse T1R3 gene maps to chromosome 4 near the Sac locus, a primary determinant of sweet preference in mice, and it is expressed in a subset of taste cells in circumvallate, foliate, and fungiform taste papillae.
