Protein That Affects Taste Discovered : Senses: The substance has structure similar to those in eye that capture light. The find supports the theory that a single precursor protein diversified through evolution.

Scientists have discovered a protein that plays a significant role in taste, a finding that they say has broad implications in understanding the evolution of certain biological functions.

The newly discovered protein has an overall structure very similar to those of the proteins in the retina that capture light, New Jersey researchers report today in the British journal Nature.

The similarity “really solidifies” the theory that a broad family of proteins scattered throughout the body are all derived from one precursor that existed hundreds of millions of years ago, said Caltech molecular biologist Melvin Simon.

The close resemblance of the two proteins, he said, is strong evidence supporting the idea that evolution can take a single primitive protein and modify it slightly to provide a variety of functions. That is a “theme that is beginning to be appreciated more and more in molecular biology,” said neuroscientist Charles Glatt of the Johns Hopkins School of Medicine in Baltimore.

Furthermore, Glatt said, the discovery of the taste protein “opens the door” to a complete understanding of the taste system.

Identification of the protein, which apparently interacts with bitter substances, may also have practical implications, said its discoverer, molecular biologist Robert F. Margolskee of the Roche Institute of Molecular Biology in Nutley, N.J. It could lead to the development of scientifically designed food additives, he said, as well as to new ways to restore normal taste in individuals who lose it during cancer chemotherapy.

Taste is one of the more rudimentary human sensory systems. “A lot of what we perceive as taste, such as a delicious dinner or a fine wine, is actually smell or aroma,” Margolskee said in a telephone interview. Humans perceive only four basic tastes: salty, sour, sweet and bitter.

“Before our work, researchers had some ideas about how taste worked, but nothing was known at the molecular level about the proteins involved in taste response,” Margolskee said.

To find the taste proteins, which make up only 1% of the tongue, Margolskee and his colleagues took advantage of recently developed techniques that allow small amounts of DNA (deoxyribonucleic acid, the genetic blueprint of life) to be amplified into much larger amounts that can be more easily manipulated in the laboratory. This allowed them to “fish out” the gene for the taste protein from the complex mixture of genes found in the taste buds of a rat, and then use the gene to produce the protein.

As their “bait,” they used a small segment of DNA that is common in a family of proteins, called G proteins, that act as receptors throughout the body. Many of these receptors sit on the membranes of cells and initiate a chemical reaction within the cells when they bind to a messenger, such as when the tongue receptor binds to a chemical in food. This in turn sends a message to the brain, which is interpreted as a bitter taste, in the case of the newly discovered receptor.

Margolskee found that the new protein, which he named gustducin, is 80% identical to transducin, the receptor protein in the eye.

He said that “probably, several hundred million years ago, there was a G protein that may have done both vision and taste.” Evolutionary pressure forced the G protein gene to duplicate and evolve so that transducin and gustducin were more precisely adapted for their specific roles. At the same time, other G proteins evolved to interact specifically with hormones and other chemical messengers within the body.

Caltech’s Simon and his colleague, molecular biologist Tom Wilkie, have used genetic techniques to demonstrate the relationships between the various G proteins and to trace them back to an ancient ancestor. Interestingly, they had recently predicted, based on those genetic linkages, that the body contains another G protein with a sequence that proved similar to gustducin’s, but whose function they could not predict.

Margolskee’s discovery of the expected protein is a strong confirmation of that evolutionary history, Simon said.