Tritrichomonas foetus: Localization of filipin-sterol complexes in cell membranes

The polyene antibiotic, filipin, was used as a probe for the detection of sterols in the freeze-fractured plasma membrane and the flagellar membranes of the pathogenic protozoa, Tritrichomonas foetus. A homogeneous distribution of filipin-sterol complexes was seen throughout the plasma membrane, and the membrane of the three anterior and the one recurrent flagella. No or very few filipin-sterol complexes were observed in some specialized regions such as the base of the flagella (necklace), the portion of the recurrent flagellum, and that part of the cell body to which the flagellum was attached. The density of filipin-sterol complexes varied from one cell to the other. In some cells, about 205 complexes/μm² were seen. A larger number of filipin-sterol complexes were observed on both faces of the membrane of cytoplasmic structures, probably corresponding to vacuoles. No complexes were seen in the nuclear membrane and in the membrane of the endoplasmic reticulum. Very few or no complexes were observed in the membrane of the hydrogenosomes. Treatment of living cells with filipin induced aggregation of filipin-sterol complexes at some points of the plasma membrane.