Editors' ChoiceReceptors

A taste for the light

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Sci. Signal.  29 Nov 2016:
Vol. 9, Issue 456, pp. ec279
DOI: 10.1126/scisignal.aal4600

Although Caenorhabditis elegans lack eyes, they can detect light and avoid excessive UV exposure. This light-avoidance behavior requires LITE-1, a seven-transmembrane receptor in the gustatory receptor family. Gong et al. sought to determine whether LITE-1 directly detects UV light or end products from light-induced chemical reactions, such as reactive oxygen species. The authors generated worms that overexpressed tagged LITE-1 in muscle cells. Analysis of the orientation of LITE-1 indicated that it had an intracellular N-terminus and an extracellular C-terminus, a topology that is reversed compared with that of seven-transmembrane photoreceptors in the opsin family. UV exposure triggered the generation of calcium transients in LITE-1–overexpressing muscles and induced muscle contractions and body paralysis in the transgenic worms. LITE-1 purified from the muscle tissue of transgenic worms captured both UVB and UVA light, a property that was abolished by denaturation. This result indicated that photosensitivity was a property that was intrinsic to LITE-1, unlike cryptochromes or opsins, which require a chromophore for photoabsorption. UVA did not induce calcium transients in muscle or body paralysis in worms with muscle-specific expression of LITE-1 variants with mutations in the transmembrane domain. However, these mutant worms retained sensitivity to UVB light. Photobleaching reduced the ability of LITE-1 to capture UVA light, but not UVB light. Mutational analysis indicated that Trp77 and Trp328 were required for the absorption of both UVA and UVB light by LITE-1. The gustatory receptor GUR-3, which is structurally related to LITE-1 but does not have tryptophan residues at the positions corresponding to Trp77 and Trp328, was not photosensitive. However, a mutant form of GUR-3 with a tryptophan residue at the position corresponding to Trp77 absorbed UVB light, which induced body paralysis in worms expressing this mutant GUR-3 in muscle. Thus, LITE-1 is a photoreceptor with properties distinct from those of previously characterized photoreceptors.

J. Gong, Y. Yuan, A. Ward, L. Kang, B. Zhang, Z. Wu, J. Peng, Z. Feng, J. Liu, X. Z. S. Xu, The C. elegans taste receptor homolog LITE-1 is a photoreceptor. Cell 167, 1252–1263 (2016). [PubMed]