Biophysics of rhodopsins and optogenetics

Rhodopsins are photoreceptive proteins and 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 respectively possess 11-cis and all-trans retinal, respectively. As cofactors bound with their animal and microbial rhodopsin (seven transmembrane α-helices) environments, 11-cis and all-trans retinal undergo photoisomerization into all-trans and 13-cis retinal forms as part of their functional cycle. While animal rhodopsins are G protein coupled receptors, the function of microbial rhodopsins is highly divergent. Many of the microbial rhodopsins are able to transport ions in a passive or an active manner. These light-gated channels or light-driven pumps represent the main tools for respectively effecting neural excitation and silencing in the emerging field of optogenetics. In this article, the biophysics of rhodopsins and their relationship to optogenetics are reviewed. As history has proven, understanding the molecular mechanism of microbial rhodopsins is a prerequisite for their rational exploitation as the optogenetics tools of the future.