BUDDING AND FISSION OF VESICLES

We report on budding and fission of protein-free vesicles swollen from a natural lipid mixture of bovine brain sphingomyelins. Budding was i nduced by increasing the area-to-volume ratio through heating. Morphol ogical changes were monitored by phase contrast microscopy and correla ted with the thermal behavior of the bilayer by differential scanning calorimetry. Freeze fracture electron microscopy revealed that budding and fission are not restricted to giant vesicles but also occur on le ngth scales relevant for cellular processes. We also observed osmotica lly induced budding and fission in mixtures of dimyristoyl phosphatidy lcholine with cholesterol. We find that these shape transitions are dr iven by liquid/gel domain formation and/or coupling of the spontaneous curvature of the membrane to the local lipid composition. Our results provide evidence that coat proteins are not necessary for budding and fission of vesicles. The physics of the lipid bilayer is rich enough to explain the observed behavior.