FURTHER-STUDIES ON THE ENDOCYTIC ACTIVITY OF TRITRICHOMONAS-FETUS

The endocytic activity of Tritrichomonas foetus was studied at the ult rastructural level using gold-labeled macromolecules (bovine lactoferr in, human and bovine transferrin, bovine albumin, human low-density li poprotein, horseradish peroxidase, and protein A). All macromolecules were ingested by the protozoan. Binding experiments showed that only b ovine lactoferrin bound to the parasite surface in a process that coul d be inhibited by the unlabeled protein, suggesting that it binds and is internalized via receptors. Label-fracture experiments showed that the receptors were distributed in clusters that did not colocalize wit h intramembranous particles. Kinetics analysis of the internalization of bovine lactoferrin and horseradish peroxidase, associated with the cytochemical detection of acid phosphatase, revealed that proteins wer e rapidly ingested through small uncoated vesicles and delivered to ac id phosphatase-containing compartments. The colocalization of gold-lab eled proteins and reaction product indicative of enzyme activity was c onfirmed by electron spectroscopic imaging. Simultaneous incubation of cells in the presence of two proteins labeled with gold particles of different diameters showed that they were ingested through the same pa thway and were concentrated into cytoplasmic vacuoles corresponding to lysosome-like organelles. These data suggest that the endocytic proce ss in T. foetus is very rapid and that the intracellular pathway for r eceptor-mediated and fluid-phase endocytosis seems to be the same.