Quantitative analysis of the fluorescence properties of intrinsically fluorescent proteins in living cells

The main potential of intrinsically fluorescent proteins (IFPs), as noninvasive and site-specific markers, lies in biological applications such as intracellular visualization and molecular genetics. However, photophysical studies of IFPs have been carried out mainly in aqueous solution. Here, we provide a comprehensive analysis of the intracellular environmental effects on the steady-state spectroscopy and excited-state dynamics of green (EGFP) and red (DsRed) fluorescent proteins, using both one- and two-photon excitation. EGFP and DsRed are expressed either in the cytoplasm of rat basophilic leukemia (RBL-2H3) mucosal mast cells or anchored (via LynB protein) to the inner leaflet of the plasma membrane. The fluorescence lifetimes (within ∼10%) and spectra in live cells are basically the same as in aqueous solution, which indicate the absence of both IFP aggregation and cellular environmental effects on the protein folding under our experimental conditions. However, comparative time-resolved anisotropy measurements of EGFP reveal a cytoplasmic viscosity 2. 5 ± 0.3 times larger than that of aqueous solution at room temperature, and also provide some insights into the LynB-EGFP structure and the heterogeneity of the cytoplasmic viscosity. Further, the oligomer configuration and internal depolarization of DsRed, previously observed in solution, persists upon expression in these cells. DsRed also undergoes an instantaneous three-photon induced color change under 740-nm excitation, with efficiently nonradiative green species. These results confirm the implicit assumption that in vitro fluorescence properties of IFPs are essentially valid for in vivo applications, presumably due to the β-barrel protection of the embodied chromophore. We also discuss the relevance of LynB-EGFP anisotropy for specialized domains studies in plasma membranes.