Polarized fluorescence resonance energy transfer microscopy.

Current methods for fluorescence resonance energy transfer (FRET) microscopy of living cells involve taking a series of images with alternating excitation colors in separate camera exposures. Here we present a new FRET method based on polarization that requires only one camera exposure and thereby offers the possibility for better time resolution of dynamic associations among subcellular components. Polarized FRET (p-FRET) uses a simultaneous combination of excitation wavelengths from two orthogonally polarized sources, along with an emission channel tri-image splitter outfitted with appropriate polarizers, to concurrently excite and collect fluorescence from free donors, free acceptors, and FRET pairs. Based upon the throughput in each emission channel as premeasured on pure samples of each of the three species, decoupling of an unknown sample's three polarized fluorescence images can be performed to calculate the pixel-by-pixel concentrations of donor, acceptor, and FRET pairs. The theory of this approach is presented here, and its feasibility is experimentally confirmed by measurements on mixtures of cyan fluorescent protein (CFP), citrine ((Cit) a yellow fluorescent protein variant), and linked fusion proteins (CFP-L16-Cit, CFP-L7-Cit, CFP-L54-Cit) in living cells. The effects of shot noise, acceptor polarization, and FRET efficiency on the statistical accuracy of p-FRET experimental results are investigated by a noise-simulation program.