Endocytosis and intracellular occurrence of the variant surface glycoprotein in Trypanosoma congolense.

Trypanosoma congolense bloodstream forms were examined for binding sites of polyclonal anti-variant surface glycoprotein (VSG) antibodies using immunoelectron microscopy. Besides the surface, the antibodies labeled intracellular vesicles, the tubular membrane system, secondary lysosomes, and the digestive vacuole. Protein A gold (PAG), peroxidase gold (POG), anti-VSG antibodies preincubated with PAG, ferritin, concanavalin A-ferritin, and microperoxidase were examined for their suitability as endocytosis tracers in combination with immunoelectron microscopy. Endocytosis of PAG and POG was most effective and was mediated by vesicles transporting the tracer to secondary lysosomes. Gold particles eventually accumulated in the digestive vacuole. Apparantly only low amounts of VSG were internalized during endocytosis. VSG export from the cell interior to the flagellar pocket was not observed during excessive endocytosis of PAG, whereas after incubation with substances causing the formation of filopodia by binding to the surface coat, VSG-labeled vesicles were present near the flagellar pocket.