Of the five tastes — sweet, salty, sour, bitter and umami — sour is one of the most mysterious. Bite into a piece of lemon and — bing! — your brain gets a message that something sour has arrived. But unlike sweet and bitter, for example, for which biologists have identified proteins on the tongue’s taste cells that detect the molecules involved, the sourness of acids like lemon juice and vinegar has remained enigmatic, with the exact details of how we pick up on it little understood. Now, however, in a paper published last month in Science, researchers report that they have found a protein in mouse taste cells that is likely a key player in the detection of sour flavors.
There’s just one strange thing, though: Biologists have known about this protein for years. It was previously identified in the inner ear, or vestibular system, of mice, humans and many other creatures, where it is required for developing a sense of balance.
The results suggest a fascinating truth about evolution: The first place something is discovered may not be the last place it turns up. If it has proved advantageous over the eons, a protein whose purpose we thought we understood may have a rich private life of its own elsewhere in the body, just waiting to be found.
Similar discoveries have cropped up more and more in the last decade as researchers look more closely at which genes cells are using. This approach has led to the revelations that smell receptors are alive and well in the kidneys, bitter taste receptors dot the sinuses and testes, and sweet taste receptors are found in the bladder.
It seems these proteins are acting as sensors in these other tissues as well. But rather than sending a message about the strawberry you’re biting into, they are rigged to send messages about invading bacteria, for instance, or to help adjust blood pressure after a meal. They may have been evolution’s version of an especially useful Tinkertoy piece: versatile, easy to assemble in new configurations, already in the tool kit.
It may have been a similar situation for this protein implicated in both taste and balance.
Emily Liman of the University of Southern California, who led the study, and colleagues were on the hunt for a sour sensor when they made the discovery chronicled in their latest paper. They had made a list of all the genes expressed in sour taste cells and identified those that might make cells sensitive to acids. They had tested about 40 before they came to one called Otop1.
“Because this gene was already known to be involved in the formation of the vestibular system, we thought it was very unlikely to play a role in taste,” Dr. Liman said. “It was really just a shot in the dark. We had all but given up, and we thought, ‘Let’s test this last one.’”
Otop1 worked like a charm. “It was a huge, huge surprise for us,” she said.
It isn’t yet clear why Otop1 would be useful in both the ear and the tongue. But the overlap makes a kind of sense. The ability to balance depends on the formation of tiny crystals of calcium carbonate in the inner ear. They rest on top of sensitive hair cells and shift gently as we move, giving feedback that lets us stay upright.
Those crystals will dissolve, however, if they are placed in an acid. Perhaps Otop1’s ability to make cells sensitive to acids is important in the process of creating these crucial structures. Exactly which role the protein plays there remains to be seen, Dr. Liman said. But it’s fascinating to see that in places as distinct as the mouth and the ear, evolution has given the same protein very different, but similarly important, jobs to do.
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