Of the five basic tastes — bitter, sweet, umami, sour and salty — umami is the least well known and certainly the least romantic. Yet most of us have experienced this savoury flavour, and commonly associate it with oriental cuisine. Xiaodong Li and colleagues now reveal the molecular mechanism behind perception of the umami taste (F. Zhang et al. Proc. Natl Acad. Sci. USA doi:10.1073/pnas.0810174106; 2008).

The umami taste comes from the amino acid glutamate, although ribonucleotides such as IMP and GMP greatly enhance its intensity. Like the sweet taste, perception of this taste depends on G-protein-coupled receptors of class C (C-GPCRs), which are found in cell membranes and also include many receptors of physiological importance. In C-GPCRs, a 'Venus flytrap' (VFT) domain, which binds to ligands on the cell surface, is connected to a transmembrane domain.

Although the receptors for the umami taste (consisting of T1R1–T1R3) and the sweet taste (T1R2–T1R3) share the same transmembrane subunit (T1R3), Li and colleagues show — using different combinations of these three subunits — that, in each case, the unique subunit mediates taste recognition. Moreover, four amino-acid residues deep inside the VFT domain seem to be essential for recognition of glutamate.

Credit: PAQUIN/PHOTOCUISINE/CORBIS

Enhancement of the umami taste by IMP/GMP, however, seems to depend on a different set of four amino acids in the opening to the VFT domain. So the authors propose that IMP binds near glutamate, supporting the ligand-bound, closed conformation of the VFT domain.

Regulatory molecules such as IMP that modulate the activity of GPCRs are of great interest to the drug industry — for example, they are more selective than agonist molecules, which might affect the activity of related receptors. As there are few known naturally occurring examples of IMP-like regulatory molecules, the significance of Li and colleagues' results goes beyond unravelling the mysteries of a curious taste.