Figure 1.
Opsins as GEVIs. (A) Principle of opsin-based sensors. The retinal chromophore within the protein becomes fluorescent when protonated as a result of membrane potential depolarization. (B) Detailed mechanism of PROPS voltage sensing (concept from Kralj et al., 2011). When membrane potential is negative, protons move away from the Schiff base, causing the chromophore to become less fluorescent. As the membrane depolarizes, protons move toward the Schiff base and increase the quantum yield of the chromophore. The ratio of protonated to deprontonated species depends upon the voltage drop V between the Schiff base and the cytoplasm; in general V < Vmem. (C) A new concept for opsin-based GEVIs fuses fluorescent proteins to the opsin and uses differences in FRET efficiency between the fluorescent protein and the retinal for sensing.