Summary: Lucifer yellow (LY), an impermeable fluorescent dye used as a marker for fluid phase endocytosis, was internalized by As observed by fluorescence microscopy, incubation of with LY in potassium phosphate buffer (pH 6.0) and glucose (2%, w/v) resulted in localization of the dye inside vacuoles. Sodium azide and carbonyl cyanide -chlorophenylhydrazone, which are inhibitors of energy metabolism, decreased the uptake of the dye. The optimum temperature for uptake was 30.C; no internalization was observed at 0.C. Quantification of cell-associated LY by fluorescence spectrometry showed an uptake linear with time and not saturable over a 400-fold range of concentration. Thus, internalized LY into vacuoles by a nonsaturable and time-, temperature- and energy-dependent process consistent with fluid phase endocytosis. Both the yeast and mould phase of this dimorphic fungus endocytosed LY. Growth in complex medium appeared to be required to enable the cells to internalize LY. However, addition of peptone or yeast extract to the phosphate buffer/glucose assay medium interfered with LY uptake by causing an apparent increase of exocytosis. These studies provide the first evidence of fluid phase endocytosis in and may explain how some large molecules, such as toxins and cationic proteins, enter


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