A tasty discovery about the tongue. (Biomedicine).

Over the past few years, taste researchers have identified various molecules on the tongue that respond to bitter and sweet substances. They've now found another so-called taste receptor, one that responds to almost all of the 20 amino acids that make up proteins.

The new receptor may help explain why people so savor the flavor, called umami, that is typical of cheese, meat, and other glutamate-rich food. When added to a meal, monosodium glutamate, or MSG, elicits this taste, as well.

The amino acid taste-receptor actually uses two previously discovered taste receptor proteins as its subunits, the scientists report in the March 14 Nature. Both the human and mouse versions of this receptor respond to abroad range of amino acids, albeit with different sensitivities to some.

"For a mouse, it's possible that MSG elicits the same perceptual response as other amino acids," notes study coauthor Charles S. Zuker, a Howard Hughes Medical Institute investigator at the University of California, Los Angeles. "The human receptor is significantly more sensitive to glutamate than to any other amino acid."

Two years ago, Nirupa Chaudhari of the University Miami School of Medicine and her colleagues reported a umami-specific taste receptor, one that responds only to glutamate (SN: 1/29/00, p. 68). "Much physiological and behavioral research in rodents and humans suggests that there are at least two and possibly more receptors that underlie umami taste," she says. --J.T.