Quantification of the intracellular equilibrium dissociation constant of the interaction, K d, is challenging due to the variability of the relative concentrations of the interacting proteins in the cell. Fluorescence lifetime imaging microscopy (FLIM) of the donor provides an accurate measurement of the molecular fraction of donor involved in FRET, but the fraction of bound acceptor is also needed to reliably estimate K d. We present a method that exploits the spectroscopic properties of the widely used eGFP – mCherry FRET pair to rigorously determine the intracellular K d based on imaging the fluorescence lifetime of only the donor (single-channel FLIM). We have assessed the effect of incomplete labelling and determined its range of application for different K d using Monte Carlo simulations. We have demonstrated this method estimating the intracellular K d for the homodimerisaton of the oncogenic protein 3-phosphoinositide-dependent kinase 1 (PDK1) in different cell lines and conditions, revealing a competitive mechanism for its regulation. The measured intracellular K d was validated against in-vitro data. This method provides an accurate and generic tool to quantify protein interactions in situ. (Figure presented.).
- equilibrium dissociation constant
- protein interactions
ASJC Scopus subject areas
- Materials Science(all)
- Biochemistry, Genetics and Molecular Biology(all)
- Physics and Astronomy(all)