Diverse and elegant mechanisms have evolved to enable organisms to harvest light for a variety of survival functions, including energy generation and the identification of suitable survival environments. A major class of light-sensitive protein consists of 7-transmembrane rhodopsins that can be found across all kingdoms of life and serve a diverse range of functions. Many prokaryotes employ these proteins to control proton gradients and to maintain membrane potential and ionic homeostasis, and many motile microorganisms have evolved opsin-based photoreceptors to modulate flagellar beating and thereby direct phototaxis toward environments with optimal light intensities for photosynthesis.
Owing to their structural simplicity (both light sensation and effector domains are encoded within a single gene) and fast kinetics, microbial opsins can be treated as precise and modular photosensitization components for introduction into non-light sensitive cells to enable rapid optical control of specific cellular processes. In recent years, the development of cellular perturbation tools based on these and other light sensitive proteins has resulted in a technology called optogenetics, referring to the integration of genetic and optical control to achieve gain- or loss-of-function of precisely defined events within specified cells of living tissue.
There is a need in the art for depolarizing and hyperpolarizing optogenetic tools, e.g., for use in controlling neural activity.