It used to be the case that, if you wanted to know anything about taste, the best person to ask was probably a chef. We still know relatively little about the molecular basis of taste perception compared with, say, vision or touch. But a report by Charles Zuker, Nick Ryba and colleagues (Cell 96, 541-551; 1999) now adds to the picture. Using standard molecular biological recipes, these authors have discovered two potential mammalian taste receptors.

Mammals are thought to have five basic taste modalities — sweet, bitter, salty, sour and umami (the taste of monosodium glutamate). Different regions of the tongue prefer these various modalities. So, for example, the circumvallate papillae at the back of the tongue are particularly sensitive to bitter substances, whereas the fungiform papillae at the front of the tongue prefer sweet compounds.

Zuker, Ryba and co-workers identified the new proteins — known as TR1 and TR2 — starting from complementary DNA libraries of sequences expressed only in taste cells. Both proteins are guanine-nucleotide-binding (G) protein-coupled receptors with sequence homologies to other candidate chemosensory receptors.

When they studied the topographic expression of TR1 and TR2, the authors found that TR1 is expressed in all fungiform taste buds, and that TR2 localizes to the circumvallate taste buds. What's more, as the image on the left shows, these proteins (green) do not colocalize with gustducin (red).

Studies using knockout mice indicate that gustducin is involved in bitter and sweet transduction. But this image shows that it is probably not responsible for signalling from TR1 and TR2. Finding the agents that are could well be the next chapter in the taste story.