We have monitored the mixing of both aqueous intracellular and membrane-bound fluorescent dyes during the fusion of human red blood cells to influenza hemagglutinin-expressing fibroblasts using fluorescence spectroscopy and low light, image-enhanced video microscopy. The water-soluble fluorescent dye, N-(7-nitrobenzofurazan-4-yl)taurine, was incorporated into intact human red blood cells. The fluorescence of the dye in the intact red blood cell was partially quenched by hemoglobin. The lipid fluorophore, octadecylrhodamine, was incorporated into the membrane of the same red blood cell at self-quenching concentrations (Morris, S. J., D. P. Sarkar, J. M. White, and R. Blumenthal. 1989. J. Biol. Chem. 264: 3972-3978). Fusion, which allowed movement of the water-soluble dye from the cytoplasm of the red blood cell into the hemagglutinin-expressing fibroblasts, and movement of octadecylrhodamine from membranes of red blood cell to the plasma membrane of the fibroblasts, was observed by fluorescence microscopy as a spatial relocation of dyes, and monitored by spectrofluorometry as an increase in fluorescence. Upon lowering the pH below 5.4, fluorescence increased after a delay of about 30 s at 37 degrees C, reaching a maximum within 3 min. The kinetics, pH profile, and temperature dependence were similar for both fluorescent events measured simultaneously, indicating that influenza hemagglutinin-induced fusion rapidly establishes bilayer continuity and exchange of cytoplasmic contents.

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