Quantification of protein interaction in living cells by two-photon spectral imaging with fluorescent protein fluorescence resonance energy transfer pair devoid of acceptor bleed-through.

Fluorescence resonance energy transfer (FRET) between fluorescent proteins (FPs) is a powerful method to visualize and quantify protein-protein interaction in living cells. Unfortunately, the emission bleed-through of FPs limits the usage of this complex technique. To circumvent undesirable excitation of the acceptor fluorophore, using two-photon excitation, we searched for FRET pairs that show selective excitation of the donor but not of the acceptor fluorescent molecule. We found this property in the fluorescent cyan fluorescent protein (CFP)/yellow fluorescent protein (YFP) and YFP/mCherry FRET pairs and performed two-photon excited FRET spectral imaging to quantify protein interactions on the later pair that shows better spectral discrimination. Applying non-negative matrix factorization to unmix two-photon excited spectral imaging data, we were able to eliminate the donor bleed-through as well as the autofluorescence. As a result, we achieved FRET quantification by means of a single spectral acquisition, making the FRET approach not only easy and straightforward but also less prone to calculation artifacts. As an application of our approach, the intermolecular interaction of amyloid precursor protein and the adaptor protein Fe65 associated with Alzheimer's disease was quantified. We believe that the FRET approach using two-photon and fluorescent YFP/mCherry pair is a promising method to monitor protein interaction in living cells.