Development of protrusions in the cell is indispensable in the process of cell motility. Membrane protrusion has long been suggested to occur as a result of actin polymerization immediately beneath the cell membrane at the leading edge, but elucidation of the mechanism is insufficient because of the complexity of the cell. To study the mechanism, we prepared giant liposomes containing monomeric actin (100 or 200 M) and introduced KCl into individual liposomes by an electroporation technique. On the electroporation, the giant liposomes deformed. Most importantly, protrusive structure grew from the liposomes containing 200 M actin at rates (ranging from 0.3 to 0.7 m͞s) similar to those obtained in the cell. The deformation occurred in a time range (30 ϳ 100 s) similar to that of actin polymerization monitored in a cuvette (ca. 50 s). Concomitant with deformation, Brownian motion of micron-sized particles entrapped in the liposomes almost ceased. From these observations, we conclude that actin polymerization in the liposomes caused the protrusive formation.Actin is one of the most important components of intracellular structures such as stress fibers or cell cortex. With the aid of a variety of actin-binding proteins and regulatory proteins, it can undergo a cycle of polymerization and depolymerization, thereby participating in protrusive activities that occur at the leading edge of the cell (1-7). Polymerization of actin, hence the elongation of actin filaments immediately beneath the cell membrane, is assumed to be the origin of the protrusive force: the elongating actin filament is supposed to ''push out'' the cell membrane. This possibility has been discussed from the viewpoint of energetics (8, 9). Recently, a mechanistic model has been proposed (10, 11). However, whether cells actually use this mechanism or how this mechanism is realized in the cell remains totally unclear.To elucidate the role of actin polymerization in the protrusive phenomena, several groups have developed model systems, i.e., liposomes containing purified actin monomers (12)(13)(14)(15)(16)(17). In this approach, the actin-containing liposome is regarded as a reconstructed in vitro cell model that provides a unique opportunity to study mechanochemical aspects of the actin filament growth. In these model systems, polymerization of actin encapsulated in liposomes and accompanying shape changes have been achieved by introducing mono-or divalent cations by using ionophores into the liposomes (12-16) or by raising temperatures in the presence of submillimolar Ca 2ϩ ions that had been encapsulated in the liposomes along with actin monomers (17). However, in most cases except for one (13), liposomes did not develop protrusive structure and assumed irregular or elongated shape. In contrast with this, thin and rigid protrusive structure grew from the liposomes containing tubulin (18). The fact that actin filament is far less rigid than a microtubule (19,20) may imply that it is a passive structural unit supporting, rather than generating, ...
