History and Perspectives of Light-Sensing Proteins

Abstract

Light-sensing proteins used in optogenetics are rhodopsins. The word ‘rhodopsin’ originates from the Greek words ‘rhodo’ and ‘opsis’, indicating rose and sight, respectively. Although the classical meaning of rhodopsin is the red-colored pigment in our eyes, 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; photoisomerizations into all-trans and 13-cis forms, respectively, initiate each function. We can find ion-transporting proteins in microbial rhodopsins, such as light-gated channels and light-driven pumps, which are the main tools in optogenetics. In this chapter, historical aspects and molecular properties of rhodopsins are introduced. The first part, What is Rhodopsin?, gives a general introduction to rhodopsin. The molecular mechanism of bacteriorhodopsin, a light-driven proton pump and the best studied microbial rhodopsin, is then described. In the section Channelrhodopsin, the Light-Gated Ion Channel by Light, molecular properties and several variants are introduced. History has proven that an understanding of the molecular mechanisms of microbial rhodopsins is a prerequisite for useful functional design of optogenetics tools in future.