Receptor‐Mediated Endocytosis in the Bloodstream Form of Trypanosoma brucei

The uptake of various host plasma proteins by the bloodstream form of Trypanosoma brucei was studied both biochemically, using radiolabeled proteins, and with the electron microscope, using colloidal gold particles as molecular tracers onto which plasma proteins had been adsorbed. Total plasma proteins and serum albumin were taken up by a mechanism of fluid endocytosis with low clearance (0.1 μ1 [mg cell protein]^‐1 h^‐1), while low‐density lipoprotein (LDL) and transferrin were taken up by a receptor‐mediated process with a clearance of two to three orders of magnitude higher than that of serum albumin. Binding prior to uptake of LDL and transferrin was saturable, depended on the presence of Ca²⁺, and the labeled ligand could be displaced by the homologous but not by heterologous protein. Binding of gold‐labeled proteins was seen only to the membrane of the flagellar pocket and not elsewhere on the plasma membrane. After 1 h of incubation at 30°C with gold‐labeled LDL and transferrin, labeled cellular structures represented respectively half and one‐third of the total volume of all single‐membrane bounded endocytotic and electron‐dense vacuoles within the cell.