Morphological changes of liposomes caused by actin assembly : the effect of actin binding proteins and surfactants.

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1.To study the morphogenesis of cells caused by the organization of their internal cytoskeletal network, we characterized the transformation of liposomes encapsulating actin and its crosslinking proteins, fascin, α-actinin, or filamin, using real-time high-intensity dark-field microscopy. The encapsulated G-actin polymerized into actin filaments and formed bundles or gels, depending on the type of actin-crosslinking protein that was co-encapsulated, causing various morphological changes of liposomes. The differences in morphology among transformed liposomes indicate that actin-crosslinking proteins determine liposome shape by organizing their specific actin networks. Morphological analysis reveals that the crosslinking manner, i.e. distance and angular flexibility between adjacent crosslinked actin filaments, is essential for the morphogenesis.2.Gelsolin is one of the best known actin-binding proteins with activities regulated by calcium. We found that plasma gelsolin can be phosphorylated by using the kinase fraction isolated from mitotic HeLa cells. After this phosphorylation, gelsolin no longer requires Ca^<2+> for activity : it severs and subsequently caps actin filaments, and nucleates filament formation in Ca^<2+>-free solutions.3.Dynamic behaviors of liposomes caused by interactions between liposomal membranes and surfactant were studied by direct, real-time observation using high-intensity dark-field microscopy. Solubilization of liposomes by surfactants is thought to be a catastrophic event akin to the explosion of soap bubbles in the air ; however, the actual process has not been clarified. We studied this process experimentally and found that liposomes exposed to various surfactants exhibited novel behavior, namely continuous shrinkage accompanied by intermittent quakes, release of encapsulated liposomes, opening-up, and inside-out topological inversion.