Herwig, L. et al. Cell Chem. Biol. 24, 415–425 (2017).

Near-infrared fluorescent proteins (NIFPs) have long excitation and emission wavelengths compatible with imaging deeply into tissues. However, despite substantial progress in discovering and engineering such tools, the photophysical properties of NIFPs still stand to be improved. Herwig et al. developed brighter, red-shifted variants of Archaerhodopsin-3 (Arch-3). Arch-3 is a microbial rhodopsin that emits red fluorescence when bound to its natural chromophore. The team substituted the natural chromophore for a derivative, which led to an ~200-nm red shift in fluorescence emission. They also introduced mutations to increase fluorescence brightness. The final tools excite and emit around 760 and 775 nm, respectively, which make them the reddest available NIFPs. They were shown to be brightly fluorescent in Escherichia coli.