Freeze-fracture studies on the surface membranes of pleomorphic bloodstream and in vitro transformed procyclic Trypanosoma brucei.

The surface membranes of bloodstream long slender, short stumpy and culture procyclic stages of Trypanosoma brucei brucei were compared with respect to freeze-fracture electron microscopy, intramembrane particle (IMP) distribution and beta-hydroxysterol content as shown by the characteristic intramembrane lesions induced by the polyene antibiotic, filipin. Little difference was observed between IMP density of long slender and short stumpy form body membranes: IMP's were more abundant on the protoplasmic face (PF) than on the exoplasmic face (EF). The procyclic culture form body membrane showed an increased density of PF IMPs and a decreased density of EF IMPs over their bloodstream short stumpy form predecessors. Flagellar membrane fracture faces displayed higher IMP densities than body membrane fracture faces of the same trypanosome. The numbers of filipin-induced lesions (FIL) indicated an increased level of beta-hydroxysterols in the short stumpy forms relative to the level in the long slender bloodforms. FIL density was further increased in the body membrane of the procyclic culture form. FIL density was higher in the flagellar membrane than in the corresponding body membrane and FIL were excluded from flagellum to body attachment zones of the flagellar membrane of all stages. The polarity of the FIL in the surface membranes was reversed on transforming from bloodstream to culture procyclic stages. These observations indicate qualitative differences between the surface membranes of the three stages, independent of the presence or absence of the surface coat.