Fluorescence-Activating and Absorption-Shifting Tags for Advanced Imaging and Biosensing.

Fluorescent labels and biosensors play central roles in biological and medical research. Targeted to specific biomolecules or cells, they allow noninvasive imaging of the machinery that govern cells and organisms in real time. Recently, chemogenetic reporters made of organic dyes specifically anchored to genetic tags have challenged the paradigm of fully genetically encoded fluorescent proteins. Combining the advantage of synthetic fluorophores with the targeting selectivity of genetically encoded tags, these chemogenetic reporters open new exciting prospects for studying cell biochemistry and biology. In this Account, we present the growing toolbox of fluorescence-activating and absorption-shifting tags (FASTs), small monomeric proteins of 14 kDa (125 amino acids residues) that can be used as markers to monitor gene expression and protein localization in live cells and organisms. Engineered by directed protein evolution from the photoactive yellow protein (PYP) from the bacterium