Recombinant Phototagging Enables Fluorescent Labeling of Biomolecules

Fluorescent labeling of biomolecules relies on small-molecule conjugation or fusion with fluorescent proteins, each with its own limitations, including (1) the need for additional downstream processing steps or (2) affecting the functionality of the labeled biomolecule. In light of this, there is a need for a minimally perturbative, genetically encodable labeling strategy. We previously observed that the C-terminal peptide fragment (CTPF) released upon photocleavage of the photoconvertible protein PhoCl1 exhibits red fluorescence. We leverage this phenomenon to achieve postexpression fluorescent labeling by genetically fusing PhoCl1 to the target peptide or protein to be labeled. This strategy was validated for diverse biomolecules, including peptides, proteins, and enzymes. Furthermore, we demonstrate its application in visualizing liquid-liquid phase separation. We anticipate that this strategy for inducing red fluorescence in nonfluorescent biomolecules via photocleavable proteins will open new avenues for minimally perturbative fluorescent labeling.