metry to study endocytosis and intracellular transport of fluid phase bovine serum albumen gold complexes and membrane bound concanavalin A through endosomal com- partments of bloodstream forms of Trypanosoma brucei

African trypanosomes offer a unique system in which to study the structure, function, regulation, and evolution of the endosomal and lysosomal system. All components of this system are more active in bloodstream forms (BF) that parasitize humans and other mammals than in procyclic forms that develop in tsetse flies (Langreth and Balber, 1975; Pamer et al., 1989; Webster and Fish, 1989; Mbawa et al., 1991a; Brickman and Balber, 1994b). The developmental activation of endocytic and hydrolytic activity in BF reflects the dependence of BF on efficient mechansims that provide macromolecular growth factors (Coppens et al., 1987, 1988; Gillett and Owen, 1992; Grab et al., 1992; Sternberg and McGuigan, 1994; Ligtenberg et al., 1994; Chaudhri et al., 1994). Endocytosis occurs exclusively from the flagellar pocket (FP), a specialized surface domain at the posterior end of the highly polarized BF; in the FP the variant surface glycoprotein (VSG) coat that covers the BF surface is reorganized to permit interaction with growth factors (reviewed by Balber, 1990; Webster and Russell, 1993). At least one lysosomal membrane glycoprotein, CB1-gp, is transported to lysosomes by way of the FP membrane and is transiently exposed on the surface (Brickman and Balber, 1994a). The unique properties of the FP has raised interest in targeting anti-trypanosomal agents to components of the FP and to the internal compartments that communicate with it (Opperdoes et al., 1986). Consequently, elucidating how BF control endocytosis and intracellular transport of material and how these processes differ in trypanosomes and mammalian cells is of considerable practical, as well as evolutionary, interest. The mechanisms that mediate endocytosis and vesicular transport in BF are beginning to be characterized. Several macromolecules enter BF by receptor mediated endocytosis; the receptors bind ligands in the FP (Coppens et al., 1988, 1991, 1992; Webster and Grab, 1988; Grab et al., 1992; Hager et al., 1994; Sternberg and McGuigan, 1994; Ligtenberg et al., 1994; Chaudhri et al., 1994). Other molecules, including bovine serum albumen (BSA) appear to enter BF primarily by fluid phase endocytosis (Coppens et al., 1987; Webster, 1989). Macromolecules are taken up from the FP in coated vesicles and then move anteriorly through a series of morphologically distinct compartments towards the nucleus (Langreth and Balber, 1975; Coppens et al., 1987; Webster, 1989; Webster and Fish, 1989). The rate of uptake is extremely high, and the entire surface of the FP may turn over every 2-3 minutes (Coppens et al., 1987; 3611 Journal of Cell Science 108, 3611-3621 (1995) Printed in Great Britain © The Company of Biologists Limited 1995 JCS1134