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Candidates for Taste Receptors Identified
In a world of lime-laced tortilla chips, hoppy beers and countless flavors of ice cream, our sense of taste might seem to exist purely for pleasure. But the ability to distinguish bitter from sweet does far more than help us choose between light and dark chocolates. Sweetness is a signal that what we're eating will give us energy, while bitterness can warn that we may be about to swallow something toxic. In our primordial past—long before nutritional labeling—heeding such cues was essential to survival, as it still is for animals living in the wild.
Important as taste may be, its molecular underpinnings have remained a mystery. A team led by Howard Hughes Medical Institute investigator Charles Zuker, however, has taken a step toward solving part of that mystery. In collaboration with Mark Hoon and Nicholas Ryba of the National Institute of Dental and Craniofacial Research, Zuker and colleagues Elliot Adler and Jurgen Lindemeier of the University of California, San Diego have isolated genes encoding two proteins that may function as taste receptors. They published their findings in the February 19, 1999, issue of the journal Cell.
The researchers used the latest DNA cloning and screening techniques to generate and sift through libraries of rat genes. From these libraries, they identified a large number of genes that were expressed solely or primarily in taste buds. Two such genes, dubbed TR1 and TR2, were especially promising candidates, says Zuker, because they resembled genes that encode other types of sensory receptors.
To be good receptor candidates, though, they needed to do more than simply resemble other receptors—they also had to be expressed in the right places. In mammals, taste receptor cells are organized into the onion-shaped clusters called taste buds. Taste buds reside in papillae, tiny protuberances that cover the upper surface and sides of the tongue. Different types of papillae are found on different parts of the tongue. At the very back of the tongue are circumvallate papillae, which seem particularly sensitive to bitterness. Along the sides of the back of the tongue are foliate papillae, sensitive to both sourness and bitterness. Fungiform papillae, located at the front of the tongue, are most sensitive to salty, sour and sweet compounds.
Reprinted from the June 1999 HHMI Bulletin,Vol. 12,
No.2, pp 10-13. To
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