Proteins that sense light also sense taste, at least in fruit flies

Like many animals, you couldn’t see without proteins called opsins, which dwell in the light-sensitive cells of your eyes. A new study reveals for the first time that fruit flies can also use some of these proteins, nestled at the tip of their nose, to taste noxious molecules in their food. Opsins in our bodies could also serve the same function, researchers speculate.

The results are “paradigm shifting,” says sensory biologist Phyllis Robinson of the University of Maryland, Baltimore County, who wasn’t connected to the research.

The most famous opsin forms the backbone of rhodopsin, the pigment in eye cells known as rods that allow you to see in low light. Your cone cells, which permit vision in bright light, harbor different opsins. Altogether, researchers have uncovered about 1000 other varieties of the proteins in various animals and microbes since rhodopsin was discovered more than 150 years ago.

But the opsin molecular family still offers some surprises, notes neuroscientist Craig Montell of the University of California, Santa Barbara. A handful of studies, including one in 2011 by Montell and his team, have implicated opsins in hearing, touch, and temperature detection.

Montell and colleagues wanted to determine whether any opsins play a role in taste—specifically, whether flies use them to detect a bitter molecule they are known to dislike. The researchers set up a taste test for unmodified Drosophila melanogaster fruit flies and for seven strains that had been genetically altered to each lack a different opsin. All of the flies had the choice between two sugar solutions, one of which was spiked with the bitter compound.

Not surprisingly, the control flies avoided the noxious solution and slurped up the other one. In the genetically modified flies, three of the opsins proved essential. Flies missing any of them didn’t recoil from sugar solutions harboring the bitter molecule, suggesting they were less sensitive to it, the researchers report online today in Current Biology . All three of these opsins can also detect light. Although they are found in the fly’s eyes, the researchers’ analysis suggests the proteins also occur in neurons at the tip of its snorkellike proboscis, which it uses to sample its environment.

Montell and colleagues showed the bitter molecule stimulated these neurons by measuring their electrical impulses, an indicator of how strongly the cells respond. When the researchers dabbed the bitter compound on the flies’ proboscis, neurons with their opsins intact sent off nearly 20 impulses per second. But the same neurons in flies lacking any of the three opsins fired off less than half as many signals, suggesting the bitter molecule had less of an impact on them.

Flies can use another nonopsin protein to sense bitter tastes. However, Montell and colleagues found that only high concentrations of the bitter molecule directly stimulate this protein. The opsins, in contrast, help flies detect lower concentrations, making it easier for the insects to perceive small amounts of substances they should shun. Even though previous research supported nonvisual roles for opsins, Montell says the result is “a bit of a surprise” because nobody had found any evidence of it before.

“It’s a very important new finding” that “expands the role of opsins,” says structural biologist Oliver Ernst of the University of Toronto. He and Robinson agree the study is convincing, using many approaches to confirm that the proteins are involved in taste.

Researchers have long thought that opsins evolved to detect light, but the study suggests another scenario, Montell says. “I would argue that the original role of opsins was not a light sensor, but a chemical sensor.” Only later, he says, did animals co-opt the proteins to sense light. And because humans carry opsins with unknown functions, including some located in the taste buds on our tongue, it’s possible, Montell says, that they have a role in our perception of taste.