Exocytosis has been proposed to contain four sequential steps, namely docking, priming, fusion, and recycling, and to be regulated by various proteins-protein interactions. Synaptosomal-associated protein of 25 kDa (SNAP25) has recently been found to bind rabphilin, the Rab3A specific binding protein, in vitro. However, it is still unclear whether SNAP25 and rabphilin interact during exocytosis within cells in vivo. This problem was addressed by the integration of fluorescence resonance energy transfer (FRET) with high sensitivity fluorescence lifetime imaging microscopy (FLIM) to observe this protein-protein interaction. Enhanced green fluorescence protein-labeled SNAP25 (donor) and red fluorescence protein-labeled rabphilin (acceptor) were expressed in neuroendocrine PC12 cells as a FRET pair and ATP stimulation was carried out for various durations. With 10 s stimulation, a 0.17-ns left shift of the lifetime peak was found when compared with donor only. Analysis of the lifetime image further suggested that the lifetime recovered to a similar level as the donor only in a time dependent manner. Four-dimensional (4D) images by FLIM provided useful information indicating that the interaction of SNAP25 and rabphilin occurred particularly within optical sections near cell membrane. Together the results suggest that SNAP25 bound rabphilin loosely at docking step before exocytosis and the binding became tighter at the very start of exocytosis. Finally, these two proteins dissociated after stimulation. To our knowledge, this is the first report to demonstrate the interaction of SNAP25 and rabphilin in situ using the FLIM-FRET technique within neuroendocrine cells.
2007 Wiley-Liss, Inc