Anionic Lipid Domains: Correlation with Functional Topography in a Mammalian Cell Membrane

Polymyxin B was used to explore distribution of anionic phospholipids in sperm plasma membranes by electron microscopy of freeze-fracture replicas. After exposure to Hepes/Tris-buffered polymyxin at 4 mM, phosphatidylcholine liposomes showed no perturbations nor did they fluoresce with dansylated incubation. When phosphatidylethanolamine was included in the liposomes, they became perturbed and fluoresced. Plasma membranes of Drosophila larval cells, containing or lacking cholesterol, were also disrupted by polymyxin. The cell membranes of guinea pig sperm were likewise disrupted but in specific functional areas. Fusional membrane domains showed protrusions; the stable membrane of the flagellum revealed diffuse bubbling. Regions of well-defined particle arrays and the postacrosomal segment maintained smooth contours. By fluorescence microscopy, we detected the same heterogeneous binding of the polymyxin dansyl derivative.