Tasting the Bitter and the Sweet

Scientists have found two proteins on the tongue that appear to help us taste the difference between bitter or sweet. The findings, reported in the latest issue of Cell, may lead to potent artificial flavors or to compounds that mask unpleasant tastes.

Biologists have found many of the key genes and proteins that convert light, sound, and other stimuli into sensations that we can experience, but none of the molecules that capture tastes are known, nor do scientists know how they transmit information to the brain, says Charles Zuker, a molecular biologist at the University of California, San Diego. Together with Nicholas Ryba and his colleagues at the National Institute of Dental and Craniofacial Research in Bethesda, Zuker hunted for candidate receptors in a previously created library of genes expressed in taste bud cells.

They found a gene encoding a protein whose amino acid sequence was similar to that of a family of mammalian and fish sensory receptors, suggesting it too might be a receptor. Using antibodies to the protein, which they called TR1, they discovered that it showed up most in taste bud cells near the tongue's tip, the sweet-sensitive region. When the researchers took another spin through the gene library for similar sequences, they found a second gene, whose protein's amino-acid order had a 70% correspondence to TR1's. TR2, as they named it, was expressed mostly on the back of the tongue, an area sensitive to bitter flavor. As further evidence that the genes play a role in taste, the researchers found TR1 and TR2 are expressed only in cells lining the taste bud pores, holes through which flavor molecules must pass before they are recognized. "It was very satisfying to see that these guys are ideally poised to perform this function," Zuker says.

Other researchers find the results compelling. "[The proteins] are showing properties one would expect of an actual taste receptor," says biochemist Joseph Brand of the Monell Chemical Senses Center at the University of Pennsylvania in Philadelphia. Zuker says his group will try to bolster the case by making cultured cells produce the TR1 or TR2 proteins and testing if bitter or sweet compounds activate a reporter gene linked to the receptor. If the findings hold up, he says, scientists may be able to create especially potent flavors by screening for chemicals that fit the receptors exactly. Zuker says that researchers could also search for receptor-blocking molecules that may be used to mask the taste of drugs or other bitter compounds.