filament was facing to the bacterial surface [158]. Since the complex is enriched in
the rear region of the bacterial body, and neither myosin II nor non-muscle myosin
has been found in that region, polymerization of actin was likely to push the
bacterium forward.
Listeria possesses a membrane protein ActA on its surface to utilize the host cell
actin. It has been demonstrated [159] that Act A allows the polymerization of actin at
or near the bacterial surface. This protein contains domains that bind arp2/3, actin
and VASP. With ActA, Listeria utilizes the cellular machinery to nucleate the actin
polymerization in the vicinity of bacterial surface and convert the polymerization
into a mechanical work to penetrate the crowded cytoplasm. In case of another
bacterium, Shigella, IcsA, like cdc42, promotes actin polymerization by activating
N-WASP (Neural Wiscott-Aldrich Syndrome Protein), the activator of Arp2/3
complex.
The actin polymerization-based, comet-type motility seems to be quite general,
because it is not limited to the motility of micro-organisms. The actin polymerization
has been observed in the transport of a membrane vesicle called endosome that are
formed in the process of endocytosis (invagination and subsequent closure of the cell
membrane [24]). The vesicle binds N-WASP, forming a comet tail at its back
[160]. Thus, the bacteria and endosomes seem to utilize the regulatory machinery
that induces actin polymerization on the cell membranes. As described earlier, the
membrane vesicles are transported on actin filament or microtubule by appropriate
motor proteins (myosin I, V, VI, kinesin, and cytoplasmic dynein [161, 162]). Thus,
the intracellular transport of endosomes seems to be supported by the two very different mechanisms.
Another remarkable example is that of the sperm of a nematode, Ascaris. Like
animal cells, Ascaris sperm crawls on the surface by extending lamellipodium at the
front, followed by the retraction of the tail [163]. In the lamellipodium, neither actin
nor myosin is present, but a dimer of major sperm protein (MSP) is structured into
the filamentous network in the front edge; the MSP dissociates at the back of the
lamellipodium.
7.18 Polymerization Force Developed by Lamellipodium
7.18.1 Keratocyte: Measurement with Atomic Force
Microscope
Protrusive force developed by the lamellipodia of keratocyte has been measured by
atomic force microscopy [164]. In this study the force was measured by the
deflection of an AFM cantilever, which had been placed vertically with its back
pushed against the lamellipodial edge. Forces on the order of nanonewton were
developed by the lamellipodial edge; this was apparent from a considerable indentation of the cell edge around the cantilever. The result demonstrates that a large
7.18 Polymerization Force Developed by Lamellipodium
135
the rear region of the bacterial body, and neither myosin II nor non-muscle myosin
has been found in that region, polymerization of actin was likely to push the
bacterium forward.
Listeria possesses a membrane protein ActA on its surface to utilize the host cell
actin. It has been demonstrated [159] that Act A allows the polymerization of actin at
or near the bacterial surface. This protein contains domains that bind arp2/3, actin
and VASP. With ActA, Listeria utilizes the cellular machinery to nucleate the actin
polymerization in the vicinity of bacterial surface and convert the polymerization
into a mechanical work to penetrate the crowded cytoplasm. In case of another
bacterium, Shigella, IcsA, like cdc42, promotes actin polymerization by activating
N-WASP (Neural Wiscott-Aldrich Syndrome Protein), the activator of Arp2/3
complex.
The actin polymerization-based, comet-type motility seems to be quite general,
because it is not limited to the motility of micro-organisms. The actin polymerization
has been observed in the transport of a membrane vesicle called endosome that are
formed in the process of endocytosis (invagination and subsequent closure of the cell
membrane [24]). The vesicle binds N-WASP, forming a comet tail at its back
[160]. Thus, the bacteria and endosomes seem to utilize the regulatory machinery
that induces actin polymerization on the cell membranes. As described earlier, the
membrane vesicles are transported on actin filament or microtubule by appropriate
motor proteins (myosin I, V, VI, kinesin, and cytoplasmic dynein [161, 162]). Thus,
the intracellular transport of endosomes seems to be supported by the two very different mechanisms.
Another remarkable example is that of the sperm of a nematode, Ascaris. Like
animal cells, Ascaris sperm crawls on the surface by extending lamellipodium at the
front, followed by the retraction of the tail [163]. In the lamellipodium, neither actin
nor myosin is present, but a dimer of major sperm protein (MSP) is structured into
the filamentous network in the front edge; the MSP dissociates at the back of the
lamellipodium.
7.18 Polymerization Force Developed by Lamellipodium
7.18.1 Keratocyte: Measurement with Atomic Force
Microscope
Protrusive force developed by the lamellipodia of keratocyte has been measured by
atomic force microscopy [164]. In this study the force was measured by the
deflection of an AFM cantilever, which had been placed vertically with its back
pushed against the lamellipodial edge. Forces on the order of nanonewton were
developed by the lamellipodial edge; this was apparent from a considerable indentation of the cell edge around the cantilever. The result demonstrates that a large
7.18 Polymerization Force Developed by Lamellipodium
135