The unicellular parasite Trypanosoma brucei rapidly removes host-derived immunoglobulin (Ig) from its cell surface, which is dominated by a single type of glycosylphosphatidylinositol-anchored variant surface glycoprotein (VSG). We have determined the mechanism of antibody clearance and found that Ig-VSG immune complexes are passively sorted to the posterior cell pole, where they are endocytosed. The backward movement of immune complexes requires forward cellular motility but is independent of endocytosis and of actin function. We suggest that the hydrodynamic flow acting on swimming trypanosomes causes directional movement of Ig-VSG immune complexes in the plane of the plasma membrane, that is, immunoglobulins attached to VSG function as molecular sails. Protein sorting by hydrodynamic forces helps to protect trypanosomes against complement-mediated immune destruction in culture and possibly in infected mammals but likewise may be of functional significance at the surface of other cell types such as epithelial cells lining blood vessels

Equations 1 and 2 were published incorrectly in both the print and online versions of the paper. The corrected equations are shown below. We apologise for any inconvenience caused. Erratum ne*(t)/ns =

1. The properties of a transport system for the uptake of fatty acids into Escherichia coli cells are described. The system is induced under the same conditions as the enzymes involved in fatty acid degradation. I n accordance with the chain-length specificity of the acyl-CoA synthetase, only fatty acids with more than eight carbon atoms are taken up. The reduced rate of uptake of fatty acids in mutants with various lesions in fatty acid degradation indicates that transport and metabolism are tightly coupled processes. The absence of an efflux reaction and the inability of a mutant lacking the acyl-CoA synthetase to take up fatty acids support the hypothesis that this enzyme is involved in a vectorial acylation during the permeation process.2. Mutants lacking one or all of the enzymes enoyl-CoA hydrase, b-hydroxyacyl-CoA dehydrogenase and thiolase, when assayed with substrates having four carbon atoms, can partially degrade long-chain length and medium-chain length fatty acids. This suggests that there are at least two types of enzyme activity in the cell which differ in their specificities for acyl-CoA substrates of various chain-lengths.3. A new old locus situated between the genes proA and proB on the E . coli linkage group is described. Lesions in this gene make the cell unable to degrade fatty acids of any carbon number, and must therefore lack an element common to the degradation of all fatty acids. Fatty a,cids taken up by these mutants are exclusively incorporated into membrane phospholipids.An inducible system for the synthesis of the enzymes of fatty acid degradation has been described in Escherichia coli [l-41. Since the enzymes are formed in response to the presence of fatty acids in the growth medium the cells must be able to take up fatty acids. The properties of such a transport system are described in this communication. Moreover, a further physiological analysis of mutants unable to degrade fatty acids (old mutants) is presented. Previously described old mutants with lesions in the metE-rha region, which lack one or several enzymes of the @-oxidation cycle, are still able to partially degrade oleate. For three out of the four reactions of the cycle which have been studied, the results suggest the presence of enzymes with different chainlength specificities for their respective substrates This paper is dedicated to Prof. Dr. Feodor Lynen on the occasion of his 60th birthday.Enzymes.

Abstract. In Trypanosoma brucei, the products of two genes, ESAG 6 and ESAG 7, located upstream of the variant surface glycoprotein gene in a polycistronic expression site form a glycosylphosphatidylinositolanchored transferrin-binding protein (TFBP) complex. It is shown by gel filtration and membrane-binding experiments that the TFBP complex is heterodimeric and binds one molecule of transferrin with high affinity (2,300 binding sites per cell; K D = 2.1 nM for the dominant expression site from T. brucei strain 427 and KD = 131 nM for ES1.3A of the EATRO 1125 stock). The ternary transferrin-TFBP complexes with iron-loaded or iron-flee ligand are stable between pH 5 and 8. Cellular transferrin uptake can be inhibited by 90% with Fab fragments from anti-TFBP antibodies. After uptake, the TFBP complex and its ligand are routed to lysosomes where transferrin is proteolytically degraded. While the degradation products are released from the cells, iron remains cell associated and the TFBP complex is probably recycled to the membrane of the flagellar pocket, the only site for exo-and endocytosis in this organism. It is concluded that the TFBP complex serves as the receptor for the uptake of transferrin in T. brucei by a mechanism distinct from that in mammalian cells.T RANSFERRIN (TF) 1 is the major serum glycoprotein that transports iron to most tissues in mammals. Diferric-TF (holo-TF) binds to a specific cell-surface receptor, the ligand-receptor complex is endocytosed and delivered to endosomes (for reviews see Crichton and Charloteaux-Wauters, 1987;Huebers and Finch, 1987). The low pH of endosomes triggers the release of iron from holo-TF but the resulting iron-free TF (apo-TF) remains bound to the receptor and is recycled back to the cell surface where, at neutral pH, apo-TF dissociates from the receptor. The human TF receptor is a transmembrane glycoprotein composed of two identical disulphide-linked subunits of 90 kD; each monomer can bind one molecule of TF. The intracellular domain of the receptor subunits contains specific signals for uptake in clathrin-coated vesicles (Trowbridge et al., 1993).African trypanosomes, the causative agents of sleeping sickness in humans and Nagana in cattle, are unicellular, flagellated protozoa that live extracellularly in the blood Address all correspondence to P. Overath,

Recently, proteins linked to glycosylphosphatidylinositol (GPI) residues have received considerable attention both for their association with lipid microdomains and for their specific transport between cellular membranes. Basic features of trafficking of GPI-anchored proteins or glycolipids may be explored in flagellated protozoan parasites, which offer the advantage that their surface is dominated by these components. In Trypanosoma brucei, the GPI-anchored variant surface glycoprotein (VSG) is efficiently sorted at multiple intracellular levels, leading to a 50-fold higher membrane concentration at the cell surface compared with the endoplasmic reticulum. We have studied the membrane and VSG flow at an invagination of the plasma membrane, the flagellar pocket, the sole region for endoand exocytosis in this organism. VSG enters trypanosomes in large clathrin-coated vesicles (135 nm in diameter), which deliver their cargo to endosomes. In the lumen of cisternal endosomes, VSG is concentrated by default, because a distinct class of small clathrin-coated vesicles (50 -60 nm in diameter) budding from the cisternae is depleted in VSG. TbRAB11-positive cisternal endosomes, containing VSG, fragment by an unknown process giving rise to intensely TbRAB11-as well as VSG-positive, disk-like carriers (154 nm in diameter, 34 nm in thickness), which are shown to fuse with the flagellar pocket membrane, thereby recycling VSG back to the cell surface.