Gq‐coupled Rhodopsin Subfamily Composed of Invertebrate Visual Pigment and Melanopsin †

Rhodopsins (rhodopsins and their related photopigments) are phylogenetically classified into at least seven subfamilies, which are also roughly discriminated by molecular function. The Gq‐coupled rhodopsin subfamily, members of which activate the Gq type G protein upon light absorption, contains pigments which underlie both visual and nonvisual physiologic functions. Gq‐coupled visual pigments have been found in the rhabdomeric photoreceptor cells of varied protostomes, and those of molluskans and arthropods have been extensively investigated. Recently, a novel photopigment, melanopsin, and its homologs have been identified in varied vertebrates. In mammals, melanopsin is localized in retinal ganglion cells and is involved in nonvisual systems, including circadian entrainment and pupillary light responses. More recently, we discovered a melanopsin homolog in amphioxus, the closest living invertebrate to vertebrates. Amphioxus melanopsin is localized in putative nonvisual photoreceptor cells with rhabdomeric morphology and exhibits molecular properties almost identical to those of invertebrate Gq‐coupled visual pigments. The localization and properties of amphioxus melanopsin bridged the functional and evolutionary gap between invertebrate Gq‐coupled visual pigments and vertebrate circadian photopigment melanopsins. Research into the Gq‐coupled rhodopsin subfamily, especially invertebrate melanopsins, will provide an opportunity to investigate the evolution of various physiologic functions, based on orthologous genes, during animal evolution.