Structure-guided rational design of red fluorescent proteins: towards designer genetically-encoded fluorophores

•Red fluorescent proteins (RFPs) are useful for whole-animal imaging.•RFPs tend to be dimmer and mature more slowly than green or yellow FPs.•Structure-guided rational design enables the improvement of key RFP properties.•Key properties include brightness, monomerization, maturation, and photostability.•Future rational design will pave the way to designer far-red fluorophores. Red fluorescent proteins (RFPs) have become an integral part of modern biological research due to their longer excitation and emission wavelengths. Protein engineering efforts have improved many key properties of RFPs for their practical use in imaging. Even so, continued engineering is required to overcome the shortcomings of the red chromophore and create RFPs with photophysical properties rivalling those of their optimized green and yellow counterparts. Here, we highlight recent examples of structure-guided rational design of RFPs to improve brightness, monomerization, maturation, and photostability, and discuss possible pathways for the future engineering of designer RFPs tailored to specific applications.