Intrinsic Förster Resonance Energy Transfer Imaging Technique for Detection of Native Protein in Live Cells: Label-free Protein Imaging

To detect a native target protein and its interaction with drugs in live cells, we developed an intrinsic fluorescence resonance energy transfer (iFRET) imaging technique (iFIT). The iFIT employs a target-specific fluorescent probe in combination with a deep UV biological microscope. The iFRET utilizes tryptophan residues of the target protein and a cell-permeable target-specific probe, which is selectively excited by the intrinsic tryptophan fluorescence (λem = 350 nm), as FRET donors and acceptors, respectively. As the Forster distance between the tryptophan residues and the iFIT probe is about 2 nm, an iFRET signal is generated by specific interaction of the target protein and the selective iFIT probe. A deep UV biological microscope equipped with a quartz objective and two optical filters (288 and 365 nm) on a shutter was constructed to facilitate the detection of the iFRET signal upon binding of the probe to its target, in live cells. We successfully achieved imaging of native streptavidin in live cells with the thus developed iFIT, after simple treatment of live cells with the corresponding iFIT probes.