Analysis of the molecular mechanism of the novel topological transformation of membrane vesicle and the construction of the cell-model

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(1) Liposome manipulation. The morphologies of living cells are based on the mechanical characteristics of cytoskeleton and membrane. To investigate the role of lipid membrane in the morphogenesis, we manipulated the giant liposomes that encapsulated two polystyrene beads by using double beam laser tweezers. As mechanical force was applied onto the beads to push a liposome membrane from inside, a spherical liposome transformed into a lemon shape with increasing the tension, and subsequently a tubular membrane projection was generated at the end of the lemon-shaped liposome. In an elongation stage of the lemon shape, the force required for the transformation became larger. Just before a membrane tube was generated, the force reached the maximum strength. However, after the membrane tube was developed, the force suddenly decreased, and kept constant strength independently from the tube length. These results indicate that lipid membrane can form the membrane tube by simple applying of mechanical force.(2) Cell model developed with giant liposome. It is well established that many kinds of actin binding proteins play regulatory roles in the organization of actin networks in living cells. To study such organization roles, we characterized the transformation of liposomes encapsulating actin with its crosslinking proteins, fascin, a-actinin or filamin. As the actin polymerized, which caused various morphological changes of liposomes. The differences in morphology indicate that the actin-crosslinking proteins actually determine liposome shape by organizing their specific actin networks.