Protrusive growth from giant liposomes driven by actin polymerization

Development of protrusions in the cell is indispensable in the process of c ell motility, Membrane protrusion has long been suggested to occur as a res ult of actin polymerization immediately beneath the cell membrane at the le ading edge, but elucidation of the mechanism is insufficient because of the complexity of the cell. To study the mechanism, we prepared giant liposome s containing monomeric actin (100 or 200 mu M) and introduced KCl into indi vidual liposomes by an electroporation technique. On the electroporation, t he giant liposomes deformed, Most importantly, protrusive structure grew fr om the liposomes containing 200 mu M actin at rates (ranging from 0.3 to 0. 7 mu m/s) similar to those obtained in the cell. The deformation occurred i n a time range (30 similar to 100 s) similar to that of actin polymerizatio n 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 lipo somes caused the protrusive formation.