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.