Light-sensor pulls perplexing double duty

Eye pigment appears to detect temperature too

An eye protein famed for seeing the light also feels the heat.

Rhodopsin, the retinal pigment that makes vision possible in extremely low light, also appears to help developing fruit flies detect minute differences in temperature. Fruit fly larvae that couldn’t make functioning rhodopsin stopped responding to slight differences in air temperature, researchers report in the March 11 Science.

The discovery raises questions about the original role of the ancient, well-studied pigment, versions of which are found in creatures from fungi to mammals, says molecular neuroscientist Craig Montell of the Johns Hopkins University School of Medicine in Baltimore.

“In a way, it’s mind boggling after all this time to discover a new role for it,” says Montell, who led the new research. Maybe the protein plays a role in thermosensation in other animals as well, he speculates.

Researchers worked with larval fruit flies, which crawl rather than fly, making them easier to track. Unlike normal fly larvae, flies that lacked functioning genes for making rhodopsin could not discern their preferred temperature of 18° Celsius from a few degrees higher and lower, Montell and his colleagues discovered. But when the researchers stuck a mouse version of the rhodopsin gene into flies without their own rhodopsin, the larvae sought out their ideal temperature. Further experiments as well as some previous data suggest the temperature-sensing rhodopsin resides not in the flies’ eyes, but rather in cells on the sides of the flies’ bodies.

That rhodopsin may be a multitasker is perplexing as well as surprising, says neurobiologist Baruch Minke of the Hebrew University of Jerusalem.

The pigment is well-known for its stability, extreme sensitivity and singular dedication. By responding to whatever rare photons are available on a moonless night, rhodopsin makes it possible for many animals to see. But like the light-sensitive circuits on a camera, a light-responsive protein should ignore temperature changes that might interfere with its light-capturing capabilities.

“Clearly, it is interesting,” says Minke, coauthor of a commentary on the work in the same issue of Science. “But frankly I don’t know how it can work.”

Rhodopsin has two main parts — an opsin protein that sits within the cell membrane and a light-capturing chromophore, a vitamin A derivative (that’s why vitamin A deficiencies can affect vision). When a photon hits it, the chromophore’s shape changes. That shift signals the opsin protein, which in turn alerts other proteins, kicking of a cascade of cellular events. Perhaps in the developing fruit flies, the rhodopsin that is responding to temperature doesn’t have this light-responsive chromophore, speculates Minke.

Yet previous research on flies suggests that without a chromophore, the pigment can’t survive the trip from inside the cell, where the protein is constructed, to the cell membrane where the protein does its job. Genetic studies — which won’t necessarily be easy to do — might be able to tease out what is happening, says Minke.

“Rhodopsin is very old, and always it functions as a photon sensor,” he says. “But who knows, maybe there’s a version without a chromophore; theoretically, it is possible.”

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