abruptly small bud appeared. Further increase in the beads separation resulted in the
growth of tether. Since the force could be estimated by knowing the trap stiffness and the
shift in the bead center, authors could generate the relation between the inter-bead
distance and the force. The analysis suggested that the axial force monotonously
increased to about 10 pN until a certain inter-bead distance was reached; the force
abruptly dropped to < 5 pN beyond that distance. This drop corresponded to the growth
of a thin tube from the liposome accompanied by the change of the liposome shape from
lemon to a sphere; further growth of the thin tube occurred at the lower forces. Shape
changes of liposome under axial forces have been treated theoretically [eg., 192, 193].
The change in the shape of the body of liposome from the lemon to spherical shape has
been interpreted as a phase transition phenomenon. In an early study, the shape of the
liposome that had been deformed by growing microtubule has been theoretically
analyzed based on a curvature elastic theory of a lipid membrane and the developing
force has been estimated [194]. In that study the shape of the liposome was assumed to
have a rotational symmetry and was regarded as a combination of straight lines and
circular arcs. The resultant relation between the force and the end-to-end distance of
liposome indicated that the force monotonically increased to ~3 pN until the end-to-end
distance of ~15 μm, but the force did not drop upon the extension of the protrusion.
7.20 Mimicking Bacteria Propulsion with Various
Reconstituted Systems
The velocity of Listeria has been shown to be equal to the rate of elongation of actin
filament [195]. As described earlier, Listeria expresses a membrane protein, ActA, to
promote the actin polymerization on the bacterial surface, which pushes the bacterial
body. The validity of this notion has been examined with various reconstituted
systems. In one study [196], the reconstituted system with E-coli artificially
expressing IcsA, an actin-nucleating protein of Shigella, is driven by the actin
polymerization. The genetically engineered E-coli have been shown to move with
a comet tail in the cytoplasm of the egg of Xenopus laevis at a velocity similar to that
of the polymerization-driven bacteria.
The role of factors involved in the propulsion of E-coli expressing IcsA and
Listeria expressing ActA has been analyzed by measuring the velocity of the
polymerization-driven movements. Three factors, Arp2/3 (actin-nucleating protein),
ADF/cofilin (depolymerizing factor) and a capping protein (barbed-end capper),
were necessary [197]. The combination of purified proteins and plastic beads instead
of bacteria has been used to further investigate the role of the bacterial protein:
plastic beads that had been coated with ActA moved at a velocity comparable to that
of Listeria in the Xenopus extract supplemented with exogenous actin [198]. The
bead started moving with the cloud of actin filaments that had been formed around
the bead; apparently, no asymmetric distribution of ActA is necessary for the
movement. In another study, bead was replaced with a liposome of a few microns
7.20 Mimicking Bacteria Propulsion with Various Reconstituted Systems
143
growth of tether. Since the force could be estimated by knowing the trap stiffness and the
shift in the bead center, authors could generate the relation between the inter-bead
distance and the force. The analysis suggested that the axial force monotonously
increased to about 10 pN until a certain inter-bead distance was reached; the force
abruptly dropped to < 5 pN beyond that distance. This drop corresponded to the growth
of a thin tube from the liposome accompanied by the change of the liposome shape from
lemon to a sphere; further growth of the thin tube occurred at the lower forces. Shape
changes of liposome under axial forces have been treated theoretically [eg., 192, 193].
The change in the shape of the body of liposome from the lemon to spherical shape has
been interpreted as a phase transition phenomenon. In an early study, the shape of the
liposome that had been deformed by growing microtubule has been theoretically
analyzed based on a curvature elastic theory of a lipid membrane and the developing
force has been estimated [194]. In that study the shape of the liposome was assumed to
have a rotational symmetry and was regarded as a combination of straight lines and
circular arcs. The resultant relation between the force and the end-to-end distance of
liposome indicated that the force monotonically increased to ~3 pN until the end-to-end
distance of ~15 μm, but the force did not drop upon the extension of the protrusion.
7.20 Mimicking Bacteria Propulsion with Various
Reconstituted Systems
The velocity of Listeria has been shown to be equal to the rate of elongation of actin
filament [195]. As described earlier, Listeria expresses a membrane protein, ActA, to
promote the actin polymerization on the bacterial surface, which pushes the bacterial
body. The validity of this notion has been examined with various reconstituted
systems. In one study [196], the reconstituted system with E-coli artificially
expressing IcsA, an actin-nucleating protein of Shigella, is driven by the actin
polymerization. The genetically engineered E-coli have been shown to move with
a comet tail in the cytoplasm of the egg of Xenopus laevis at a velocity similar to that
of the polymerization-driven bacteria.
The role of factors involved in the propulsion of E-coli expressing IcsA and
Listeria expressing ActA has been analyzed by measuring the velocity of the
polymerization-driven movements. Three factors, Arp2/3 (actin-nucleating protein),
ADF/cofilin (depolymerizing factor) and a capping protein (barbed-end capper),
were necessary [197]. The combination of purified proteins and plastic beads instead
of bacteria has been used to further investigate the role of the bacterial protein:
plastic beads that had been coated with ActA moved at a velocity comparable to that
of Listeria in the Xenopus extract supplemented with exogenous actin [198]. The
bead started moving with the cloud of actin filaments that had been formed around
the bead; apparently, no asymmetric distribution of ActA is necessary for the
movement. In another study, bead was replaced with a liposome of a few microns
7.20 Mimicking Bacteria Propulsion with Various Reconstituted Systems
143