Distribution of intramembranous particles and filipin-sterol complexes in the cell membranes of Toxoplasma gondii.

Toxoplasma gondii possesses a plasma membrane covering the whole cell and, below it, two closely apposed unit membranes interrupted at the anterior and posterior tips of the parasite and at the micropore. Density differences of intramembranous particles (IMP) were observed among the various membranes. The polyene antibiotic filipin was used for the detection of sterols in freeze-fractured membranes of the parasite. Protuberances with a mean diameter of 38 nm, indicative of the formation of filipin-sterol complexes, were seen in the P and E faces of the plasma membrane. The density of filipin-sterol complexes on the P and E faces of the plasma membrane was 130 +/- 45 and 101 +/- 50 protuberances/micron2, respectively. Few or no protuberances were seen on both fracture faces of the intermediate and inner membranes. The results obtained are discussed and compared with those obtained in other parasites enclosed with a complex membrane system.