Endocytosis and intracellular transport has been studied in the bloodstream forms of Trypanosoma brucei by light and electron microscopy, using colloidal gold coupled to bovine transferrin (transferrin-gold). The endocytosed transferrin-gold, visualized by silver intensification for light microscopy, was present in vesicular structures between the cell nucleus and flagellar pocket of the organism. At the ultrastructural level, transferrin-gold was present after a 10-min incubation in the flagellar pocket, coated vesicles, cisternal networks, and lysosomelike structures. Endocytosis and intracellular processing of T. brucei variable surface glycoprotein (VSG) was studied using two preparations of affinity-purified rabbit IgG directed against different parts of the VSG. One preparation of IgG was directed against the cross-reacting determinant (CRD): a complex glycolipid side chain covalently linked to the COOH-terminus of the VSG molecule. The other was directed against determinants on the rest of the VSG molecule. When the two IgG preparations were used on thawed, thin cryosections of trypanosomes that had been incubated in transferrin-gold before fixation, the organelles involved with transferrin-gold endocytosis labeled with both antibodies, as well as many vesicular, tubular, and vacuolar structures that did not contain endocytosed transferrin-gold. Both antibodies also labeled the cell surface. In double-labeling experiments both antibodies were closely associated except that IgG directed against the VSG molecule labeled all the cisternae of the Golgi apparatus, whereas anti-CRD IgG was shown to label only half of the Golgi apparatus. Evidence for sorting of VSG molecules from endocytosed transferrin-gold was found. Double-labeling experiments also showed some tubular profiles which labeled on one side with anti-CRD IgG and on the other side with anti-VSG IgG, suggesting a possible segregation of parts of the VSG molecule.

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