Retinal proteins: Photochemistry and optogenetics

Abstract

Rhodopsins, which are also called retinal proteins, are photoreceptive proteins. Their photoreactions have attracted many researchers in physics, chemistry and biology. In addition, they are now used as key tools in optogenetics. Although rhodopsin was originally named as a red-colored pigment for vision, the modern meaning of rhodopsin encompasses photoactive proteins containing a retinal chromophore in animals and microbes. Animal and microbial rhodopsins possess 11-cis and all-trans retinal, respectively, to capture light in seven transmembrane α-helices, and photoisomerizations into all-trans and 13-cis forms, respectively, initiate each function. Unlike animal rhodopsins, several kinds of microbial rhodopsins are able to transport ions in a passive or an active manner, and light-gated channels or light-driven pumps, respectively, are the main tools in optogenetics. In this article, historical aspects and recent advances of retinal protein research are reviewed. After general introduction of rhodopsins, the molecular mechanism of bacteriorodopsin, a light-driven H+ pump and the best studied microbial rhodopsin, is described. Then, molecular properties and several variants of channelrhodopsin, the lightgated ion channel, are introduced. As history has proven, understanding the molecular mechanism of microbial rhodopsins is a prerequisite for useful functional design of optogenetics tools in future.