Gong Jianke, Yuan Yiyuan, Ward Alex, Kang Lijun, Zhang Bi, Wu Zhiping, Peng Junmin, Feng Zhaoyang, Liu Jianfeng, Xu X Z Shawn
College of Life Science and Technology, Collaborative Innovation Center for Brain Science, and Key Laboratory of Molecular Biophysics of MOE, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China; Life Sciences Institute and Department of Molecular and Integrative Physiology, University of Michigan, Ann Arbor, MI 48109, USA.
Life Sciences Institute and Department of Molecular and Integrative Physiology, University of Michigan, Ann Arbor, MI 48109, USA; Department of Pharmacology, Case Western Reserve University, Cleveland, OH 44106, USA.
Cell. 2016 Nov 17;167(5):1252-1263.e10. doi: 10.1016/j.cell.2016.10.053.
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.
许多动物组织/细胞对光敏感，但在后生动物中仅发现了两种类型的光感受器（即视蛋白和隐花色素）。动物界是否存在未知类型的光感受器这一问题由此产生。LITE-1是一种七跨膜味觉受体（GR）同源物，介导秀丽隐杆线虫的紫外线诱导回避行为。然而，尚不清楚LITE-1是作为化学感受器还是光感受器发挥作用。在此，我们表明LITE-1直接吸收UVA和UVB光，其消光系数是视蛋白和隐花色素的10-100倍，这表明LITE-1在捕获光子方面效率很高。与采用辅基发色团捕获光子的典型光感受器不同，LITE-1严格依赖其蛋白质构象进行光子吸收。我们进一步鉴定出两个对LITE-1功能至关重要的色氨酸残基。有趣的是，与GPCR不同，LITE-1采用反向膜拓扑结构。因此，味觉受体同源物LITE-1代表了动物界一种独特类型的光感受器。
