Fig. 1.

Transduction of the five basic taste qualities. (A) The sensory membrane of taste cells comes into contact with food. The taste receptor family T1R detects the presence of sweet substances, such as sugars and artificial sweeteners, using the isoforms T1R2 and T1R3 to form a receptor dimer. The alternative dimer (T1R1 + T1R3) detects protein components, in particular monosodium glutamate, and hence mediates the taste quality umami (Japanese for “tasty” or “savory”). The bitter taste arises from cells expressing several members of the T2R family of taste receptors. All taste receptors transmit their signal to the target enzyme phospholipase β2 (PLC) through a GTP-binding signaling protein (G). At the end of this signal transduction cascade is the activation of the cation channel TRPM5 that causes a depolarization of the taste cell. An ion channel that allows Na⁺ ions to enter the cell generates salt taste (ENaC: epithelial sodium channel). Acids may enter the cell in their undissociated form (HA) and cause intracellular acidification. A previously uncharacterized proton channel provides a direct pathway for H⁺ ions into the cell. (B) The method used by Liman et al. (1) to test the effect of acid signals on the sensory membrane of isolated sour-taste cells. The apical pole of the cell is sucked into a glass micropipette so that the sensory apical membrane is exposed to the solution that fills the pipette. This solution contains a compound (“caged H⁺”) that releases H⁺ ions upon illumination with a UV flash. Through the previously uncharacterized proton channels in the sensory membrane, H⁺ ions enter the cell and elicit action potentials in the sour-taste cell.