Scientists Identify Sour Taste Receptor in Fruit Flies

Sour taste allows for the detection of hydrogen ions and organic acids. It is one of the five basic tastes and, along with other chemical and textural features, allows animals from flies to humans to discriminate between foods that are safe and appealing from other options that are dangerous. Now, a team of scientists from Kookmin University in Korea and the University of California, Santa Barbara, has discovered the first sour taste receptor used by an animal to choose between different foods options.

The receptors required for the gustatory responses to sour compounds have been elusive. Rimal et al discover a Drosophila taste receptor, IR7a, which is required for rejecting a sour chemical. Surprisingly, IR7a is narrowly tuned to acetic acid, suggesting that carboxylic acids are detected by a repertoire of receptors. Image credit: Rimal et al, doi: 10.1016/j.celrep.2019.01.042.

The receptors required for the gustatory responses to sour compounds have been elusive. Rimal et al discover a Drosophila taste receptor, IR7a, which is required for rejecting a sour chemical. Surprisingly, IR7a is narrowly tuned to acetic acid, suggesting that carboxylic acids are detected by a repertoire of receptors. Image credit: Rimal et al, doi: 10.1016/j.celrep.2019.01.042.

Until now, researchers didn’t know what receptors allow animals to evaluate food on the basis of sourness.

“In mammals, a protein called Otopetrin1 is a candidate. But, in fact, the precise receptors are not known in any animal,” said University of California, Santa Barbara’s Professor Craig Montell, co-lead author of the study.

“It’s really a major question in the taste field, and I think we all care about taste. I mean, we all think about it. And taste is extremely important, not only for enjoyment, but for survival.”

In their search for a sour receptor, Professor Montell and colleagues provided fruit flies (Drosophila melanogaster) with choices between sugar alone versus sugar combined with various concentrations of different acids.

The scientists then established low concentrations of various acids that attracted the animals and high levels that repulsed them.

Many of the flies had known mutations affecting their receptor proteins, and the team tracked to what extent the behavior of these flies differed from their more conventional counterparts.

The researchers discovered that a protein, ionotropic receptor 7a (IR7a), was required for the flies to detect acetic acid, the active ingredient in vinegar, and to make decisions based on acetic acid concentration.

Those without the protein couldn’t recognize the acid, however a mutated IR7a didn’t affect the animals’ ability to sense any of the other acids the team tested.

“It was surprising just how narrowly tuned this receptor was,” Professor Montell said.

“Since we tested a limited number of acids, we don’t believe the protein responds only to acetic acid. But even so, it’s still narrowly tuned. We expected that one receptor would account for many sour compounds.”

Once the study authors discovered that flies needed IR7a to detect acetic acid, they were curious whether this protein was sufficient to produce the ability on its own.

They introduced it into fly neurons that reacted to sugars to see if they became acid sensitive. Remarkably, they did.

However, when the team introduced IR7a in mammalian cells grown in a Petri dish, the cells did not become sensitive to acetic acid. These results suggest that IR7a works in concert with other proteins in taste receptor cells to detect acetic acid, something they plan to continue investigating in future research.

“The discovery here is finding an enigmatic type of receptor that’s important in something very basic: a sour taste,” Professor Montell said.

The discovery is reported in a paper in the journal Cell Reports.

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Suman Rimal et al. 2019. Mechanism of Acetic Acid Gustatory Repulsion in Drosophila. Cell Reports 26 (6): 1432-1442; doi: 10.1016/j.celrep.2019.01.042

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