Sweet finding: researchers propose candidate sour sensor.

Two teams of scientists have identified a protein on the surfaces of select tongue cells that may be the long-sought detector of sour taste.

People and some other animals, including mice, distinguish five recognized tastes: sweet, bitter, sour, salty, and umami, the flavor of monosodium glutamate. Over the past 6 years, researchers including Charles S. Zuker of the University of California, San Diego have ferreted out proteins on tongue-cell surfaces responsible for receiving sweet, bitter, and umami sensations.

To locate the sour-taste receptor, Zuker's team started with a few assumptions based on previous findings. For example, a sour-detecting protein would weave in and out of the membranes of tongue cells, as the taste receptors already identified do. Previous studies also suggested that each tongue cell produces no more than one type of taste receptor.

Zuker's team scanned the mouse genome, looking first for genes that encode membrane-spanning proteins that resemble the known taste receptors. To narrow down the thousands of candidates, the researchers focused on genes that make rare proteins in the body, rather than genes responsible for proteins that carry out general functions in all cells.

Zuker and his colleagues searched in tongue tissue for each protein that met those criteria. They eventually located a single protein, called PKD2L1, that's in some taste bud cells but not in those that detect sweet, bitter, and umami flavors.

To test whether PKD2L1 is important for sensing sour, the researchers engineered mice so that any cells bearing the protein died before the animals were born. Zuker's team reports in the Aug. 24 Nature that nerves in these rodents' tongues responded normally to other tastes but didn't respond when the researchers gave the animals solutions of sour chemicals such as citric acid or vinegar.

The mice "were completely insensitive, just like we were dabbing their tongues with water," Zuker says. His team's findings "show that sour taste is mediated by cells bearing this unique receptor protein," he adds.

Those results are "really exciting," says Hiroaki Matsunami of Duke University Medical Center in Durham, N.C., whose team also proposes PKD2L1 as a sourtaste receptor. That work appears in the Aug. 15 Proceedings of the National Academy of Sciences.

Matsunami speculates that researchers may someday exploit this research to change the taste of foods--for example, by increasing a soda's sourness without upping its tooth-degrading acidity.