Effects of lipid-phase separation on the filipin action on membranes of ergosterol-replaced Tetrahymena cells, as studied by freeze-fracture electron microscopy.

The effects of lipid-phase separation on the filipin action on pellicle membranes of ergosterol-replaced Tetrahymena pyriformis cells were studied by freeze-fracture electron microscopy. The pellicle membranes with phase separations induced by chilling from 34 degrees C (growth temperature) to lower temperatures (30, 22 and 15 degrees C) were treated with filipin. This produced filipin-induced lesions ("pits") only in the particulated (liquid) regions along the margin between solid and liquid domains, while they were produced in the particle-free (solid) areas when membranes were chilled to 15 degrees C. The pellicle membranes with lesions induced by filipin at 34 degrees C were chilled to 22 degrees C. This chilling raised larger particle-free areas and more condensed particle-aggregations on the membranes than on the membranes without the filipin treatment. These results suggest that the membrane fluidity affects induction and development of the ergosterol-filipin complex in the membrane.