Tasting Light: New Type of Photoreceptor Is 50 Times More Efficient Than the Human Eye

Summary: Researchers have discovered a new type of photoreceptor that is about 50 times more efficient at capturing light than the rhodopsin in the human eye.

Source: University of Michigan.

An international team of scientists led by the University of Michigan has discovered a new type of photoreceptor—only the third to be found in animals—that is about 50 times more efficient at capturing light than the rhodopsin in the human eye.

The new receptor protein, LITE-1, was found among a family of taste receptors in invertebrates, and has unusual characteristics that suggest potential future applications ranging from sunscreen to scientific research tools, the team noted in findings published online Nov. 17 in the journal Cell.

“Our experiments also raise the intriguing possibility that it might be possible to genetically engineer other new types of photoreceptors,” said senior study author Shawn Xu, a faculty member of the U-M Life Sciences Institute, where his lab is located.

The LITE-1 receptor was discovered in the eyeless, millimeter-long roundworms known as nematodes, a common model organism in bioscience research.

“LITE-1 actually comes from a family of taste receptor proteins first discovered in insects,” said Xu, who is also a professor in the Department of Molecular and Integrative Physiology at the U-M Medical School. “These, however, are not the same taste receptors as in mammals.”

Xu’s lab previously demonstrated that although they lack eyes, the worms will move away from flashes of light. The new research goes a step further, showing that LITE-1 directly absorbs light, rather than being an intermediary that senses chemicals produced by reactions involving light.

“Photoreceptors convert light into a signal that the body can use,” Xu said. “LITE-1 is unusual in that it is extremely efficient at absorbing both UV-A and UV-B light—10 to 100 times greater than the two other types found in the animal kingdom: opsins and cryptochromes. The next step is to better understand why it has these amazing properties.”

The genetic code of these receptor proteins is also very different from other types of photoreceptors found in plants, animals and microbes, Xu said.

Characterizing the current research as an “entry point,” the researchers said the discovery might prove useful in a variety of ways.

With further study, for example, it might be possible to develop LITE-1 into a sunscreen additive that absorbs harmful rays, or to further scientific research by fostering light sensitivity in new types of cells, the scientists wrote in the paper.

Several characteristics make LITE-1 unusual, Xu said.

Animal photoreceptors typically have two components: a base protein and a light-absorbing chromophore (a role played by retinal, or vitamin A, in human sight). When you break these photoreceptors apart, the chromophore still retains some of its functionality.

This is not the case for LITE-1. Breaking it apart, or “denaturing” it, completely stops its ability to absorb light, rather than just diminishing it—showing that it really is a different model, Xu said.

The new receptor protein, LITE-1, was found among a family of taste receptors in invertebrates, and has unusual characteristics that suggest potential future applications ranging from sunscreen to scientific research tools. NeuroscienceNews.com image is adapted from the University of Michigan press release.
The new receptor protein, LITE-1, was found among a family of taste receptors in invertebrates, and has unusual characteristics that suggest potential future applications ranging from sunscreen to scientific research tools. NeuroscienceNews.com image is adapted from the University of Michigan press release.

The researchers also determined that within the protein, having the amino acid tryptophan in two places was critical to its function.

When a nonlight-sensitive protein in the same family, GUR-3, was modified to add the corresponding tryptophan residues, it reacted strongly to ultraviolet light—with about a third the sensitivity to UV-B as LITE-1.

“This suggests scientists may be able to use similar techniques to genetically engineer other new photoreceptors,” Xu said.

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Additional authors include Jianke Gong and Bi Zhang of U-M and Huazhong University of Science and Technology; Jianfeng Liu of Huazhong University of Science and Technology; Yiyuan Yuan of U-M and Case Western Reserve University; Alex Ward, Lijun Kang and Jianfeng Liu of U-M; Zhiping Wu and Junmin Peng of St. Jude Children’s Research Hospital; and Zhaoyang Feng of Case Western Reserve University.

Funding: The research was supported by the National Eye Institute, National Natural Science Foundation of China, Ministry of Education of China, Program for Changjiang Scholars and Innovative Research Team in University, and the National Institute of General Medical Sciences.

Source: Ian Demsky – University of Michigan
Image Source: NeuroscienceNews.com image is adapted from the University of Michigan press release.
Original Research: Abstract for “The C. elegans Taste Receptor Homolog LITE-1 Is a Photoreceptor” by Jianke Gong, Yiyuan Yuan, Alex Ward, Lijun Kang, Bi Zhang, Zhiping Wu, Junmin Peng, Zhaoyang Feng, Jianfeng Liu, and X.Z. Shawn Xu in Cell. Published online November 17 2016 doi:10.1016/j.cell.2016.10.053

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[cbtabs][cbtab title=”MLA”]University of Michigan “Tasting Light: New Type of Photoreceptor Is 50 Times More Efficient Than the Human Eye.” NeuroscienceNews. NeuroscienceNews, 18 November 2016.
<https://neurosciencenews.com/photoreceptor-light-neuroscience-5556/>.[/cbtab][cbtab title=”APA”]University of Michigan (2016, November 18). Tasting Light: New Type of Photoreceptor Is 50 Times More Efficient Than the Human Eye. NeuroscienceNew. Retrieved November 18, 2016 from https://neurosciencenews.com/photoreceptor-light-neuroscience-5556/[/cbtab][cbtab title=”Chicago”]University of Michigan “Tasting Light: New Type of Photoreceptor Is 50 Times More Efficient Than the Human Eye.” https://neurosciencenews.com/photoreceptor-light-neuroscience-5556/ (accessed November 18, 2016).[/cbtab][/cbtabs]


Abstract

The C. elegans Taste Receptor Homolog LITE-1 Is a Photoreceptor

Highlights
•LITE-1, a taste receptor homolog, is a bona fide photoreceptor that senses UV light
•LITE-1 has a high efficiency of photon capturing
•Photoabsorption by LITE-1 relies on its conformation and requires two Trp residues
•Introducing such a Trp residue into a related protein promotes photosensitivity

Summary
Many animal tissues/cells are photosensitive, yet only two types of photoreceptors (i.e., opsins and cryptochromes) have been discovered in metazoans. The question arises as to whether unknown types of photoreceptors exist in the animal kingdom. LITE-1, a seven-transmembrane gustatory receptor (GR) homolog, mediates UV-light-induced avoidance behavior in C. elegans. However, it is not known whether LITE-1 functions as a chemoreceptor or photoreceptor. Here, we show that LITE-1 directly absorbs both UVA and UVB light with an extinction coefficient 10–100 times that of opsins and cryptochromes, indicating that LITE-1 is highly efficient in capturing photons. Unlike typical photoreceptors employing a prosthetic chromophore to capture photons, LITE-1 strictly depends on its protein conformation for photon absorption. We have further identified two tryptophan residues critical for LITE-1 function. Interestingly, unlike GPCRs, LITE-1 adopts a reversed membrane topology. Thus, LITE-1, a taste receptor homolog, represents a distinct type of photoreceptor in the animal kingdom.

“The C. elegans Taste Receptor Homolog LITE-1 Is a Photoreceptor” by Jianke Gong, Yiyuan Yuan, Alex Ward, Lijun Kang, Bi Zhang, Zhiping Wu, Junmin Peng, Zhaoyang Feng, Jianfeng Liu, and X.Z. Shawn Xu in Cell. Published online November 17 2016 doi:10.1016/j.cell.2016.10.053

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