Fluorescent
probes are valuable tools for visualizing the spatiotemporal dynamics
of molecules in living cells. Here we developed a genetically encoded
antibody probe with antigen-dependent fluorescence intensity called
“Flashbody”. We first created a fusion of EGFP to the
single chain variable region fragment (scFv) of antibody against seven
amino acids of the bone Gla protein C-terminus (BGPC7) called BGP
Fluobody, which successfully showed the intracellular localization
of BGPC7-tagged protein. To generate BGP Flashbody, circularly permuted
GFP was inserted in between two variable region fragments, and the
linkers were optimized, resulting in fluorescence intensity increase
of 300% upon binding with BGPC7 in a dose-dependent manner. Live-cell
imaging using BGP Flashbody showed that BGPC7 fused with cell penetrating
peptide was able to enter through the plasma membrane by forming a
nucleation zone, while it penetrated the nuclear membrane with different
mechanism. The construction of Flashbody will be possible for a range
of antibody fragments and opens up new possibilities for visualizing
a myriad of molecules of interest.