polymerization was driven by K
+ ions introduced by the technique called electroporation. In the electroporation process, electric pulses were applied to actincontaining liposomes to transiently increase the permeability of the membrane to
ions [180], which would allow a inward flow of K
+ ions, which had been included in
the external solution. Indeed, after the application of electric pulse, liposomes
changed their shape over ~1 min; it finally assumed ellipsoidal shape. The polymerization was confirmed by the diminished Brownian motion of 1 μm polystyrene
beads that had been co-encapsulated with actin. Some liposomes even developed
large protrusions on the similar time scale. Protrusions thus developed lasted for at
least 10 min without notable change in their length or thickness suggesting the
structural stability of the protrusion. The observation of fluorescently labeled actin
monomer revealed that actin (probably in the filamentous form) filled the protrusion.
These observations collectively led to a suggestion that the protrusive growth was a
result of actin polymerization. The difference in the shape of liposomes between two
studies might be due to the different time scales on which the polymerization
occurred.
Fluorescently-labeled actin filaments have been shown to execute vigorous
thermal fluctuation indicating the highly flexible nature of then filament
[181]. Lipid membrane is also highly flexible, as mentioned earlier [85, 173]. Therefore, the development of the stable protrusions especially a long protrusion, could be
possible, only if many filaments are oriented in the same direction and grew
simultaneously; the maintenance of the protrusion would be only possible when
the protrusion was filled with actin filaments, as actually observed. In a later study
fascin that bundles actin filaments in the same polarity [182], or alpha-actinin, which
bundles actin filaments in opposite directions both developed thin, but highly rigid
protrusions [183]. This must have been a result of the mechanical strength of the
bundled actin filaments [184, 185].
In the case of the slow polymerization with Ca
2+ ions [36], alignment of actin
filaments could be a result of steric interaction between the lipid membrane and
growing actin filaments. On the other hand, the fast protrusive growth occurred after
the electroporation might be initiated by the localized but large mechanical deformation (budding) of the lipid membrane caused by the electroporation; that the
growing filaments might have turned the initial small protrusions into large, long
protrusions. In the latter case, the lipid membrane exhibited little fluctuation after the
growth of protrusions, and the shape of the liposome with protrusions were stable
over tens of minutes. The tensed membrane seems to have resulted from the force
exerted by the growing filament. Thus, some mechanical work was done by the
growing filaments.
7.19 Reconstituted Systems to Study the Polymerization-Based Phenomena
141
+ ions introduced by the technique called electroporation. In the electroporation process, electric pulses were applied to actincontaining liposomes to transiently increase the permeability of the membrane to
ions [180], which would allow a inward flow of K
+ ions, which had been included in
the external solution. Indeed, after the application of electric pulse, liposomes
changed their shape over ~1 min; it finally assumed ellipsoidal shape. The polymerization was confirmed by the diminished Brownian motion of 1 μm polystyrene
beads that had been co-encapsulated with actin. Some liposomes even developed
large protrusions on the similar time scale. Protrusions thus developed lasted for at
least 10 min without notable change in their length or thickness suggesting the
structural stability of the protrusion. The observation of fluorescently labeled actin
monomer revealed that actin (probably in the filamentous form) filled the protrusion.
These observations collectively led to a suggestion that the protrusive growth was a
result of actin polymerization. The difference in the shape of liposomes between two
studies might be due to the different time scales on which the polymerization
occurred.
Fluorescently-labeled actin filaments have been shown to execute vigorous
thermal fluctuation indicating the highly flexible nature of then filament
[181]. Lipid membrane is also highly flexible, as mentioned earlier [85, 173]. Therefore, the development of the stable protrusions especially a long protrusion, could be
possible, only if many filaments are oriented in the same direction and grew
simultaneously; the maintenance of the protrusion would be only possible when
the protrusion was filled with actin filaments, as actually observed. In a later study
fascin that bundles actin filaments in the same polarity [182], or alpha-actinin, which
bundles actin filaments in opposite directions both developed thin, but highly rigid
protrusions [183]. This must have been a result of the mechanical strength of the
bundled actin filaments [184, 185].
In the case of the slow polymerization with Ca
2+ ions [36], alignment of actin
filaments could be a result of steric interaction between the lipid membrane and
growing actin filaments. On the other hand, the fast protrusive growth occurred after
the electroporation might be initiated by the localized but large mechanical deformation (budding) of the lipid membrane caused by the electroporation; that the
growing filaments might have turned the initial small protrusions into large, long
protrusions. In the latter case, the lipid membrane exhibited little fluctuation after the
growth of protrusions, and the shape of the liposome with protrusions were stable
over tens of minutes. The tensed membrane seems to have resulted from the force
exerted by the growing filament. Thus, some mechanical work was done by the
growing filaments.
7.19 Reconstituted Systems to Study the Polymerization-Based Phenomena
141