diameter [199]. Like bacteria, the comet tail grew from the surface and extended to
the back of the liposome. The actin filaments were bound to the liposome through
ActA. An advantage of using liposome was that the forces exerted on the moved
object could be visualized. Thus, the liposome coated by ActA, moving with a comet
tail, was pulled back at the central part of the rear and was pushed forward along its
side. These observations suggested that some actin filaments were bound to the
liposome by ActA around the rearmost region, whereas other filaments obliquely
pushed the side of the liposome. Similar force distribution has been demonstrated
using oil droplet [200]. The spontaneous breakage of the symmetric distribution of
actin filaments around the bead is postulated to be the key to the directed motion
[201, 202]. An elastic energy of the gel made of actin filaments around the bead has
been assumed to be released through fracture of the gel [198]: this might propel the
objects. In another study, the force exerted by the actin polymerization on the surface
of the cantilever of atomic force microscope has been measured. The cantilever was
coated with ActA and actin filaments were allowed to grow at the lower surface of
the cantilever in a Xenopus egg. The cantilever was pushed up demonstrating >
100 nN force was produced [177]. Another study utilizing an N-WASP-coated
plastic bead has shown to generate a few nN force when actin was allowed to
polymerized from the bead surface in the presence of Arp2/3, gelsolin (a barbedend capping protein), ADF/cofilin, profilin and actin [203].
The reconstituted system has been also used to study the motility of the Ascaris
sperm, which, as mentioned above, does not depend on actin-myosin system.
Addition of ATP to the sperm extract showed a formation of a fibrous structure
that bound a plasma membrane vesicle and had an arrangement of the filaments
similar to that made of MSP (major sperm protein) in vivo. The growth of the fibrous
structure, which occurred at the interface of the fiber and the plasma membrane of
the vesicle, seemed to propel the membrane vesicle [204]. The movement realized in
this intriguing system has been treated theoretically [205].
7.21 Polymerization Force: Theoretical Studies
7.21.1 Hill’s Theory
As mentioned in Sect. 7.16, Hill [147] developed the theory based on irreversible
thermodynamics to explain the polymerization-based force development. There are
several assumptions: (1) The monomer and polymer coexist in the solution; (2) One
end of the polymer is fixed and the other end is touching an obstacle, which is pushed
against the polymer end by an external force F; (3) The polymer was assumed to be a
linear chain of monomer and infinitely rigid. As before, we call the monomer
incorporated in the polymer “protomer” to distinguish it from the monomer in
solution. The chemical potential of a monomer in solution is designated as μ and
that of protomer, μ pro . Hill assumed that the value of μ pro does not depend on the
position of the protomer (monomer in the polymer); this is a simplification, because
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7 Moving Life
the back of the liposome. The actin filaments were bound to the liposome through
ActA. An advantage of using liposome was that the forces exerted on the moved
object could be visualized. Thus, the liposome coated by ActA, moving with a comet
tail, was pulled back at the central part of the rear and was pushed forward along its
side. These observations suggested that some actin filaments were bound to the
liposome by ActA around the rearmost region, whereas other filaments obliquely
pushed the side of the liposome. Similar force distribution has been demonstrated
using oil droplet [200]. The spontaneous breakage of the symmetric distribution of
actin filaments around the bead is postulated to be the key to the directed motion
[201, 202]. An elastic energy of the gel made of actin filaments around the bead has
been assumed to be released through fracture of the gel [198]: this might propel the
objects. In another study, the force exerted by the actin polymerization on the surface
of the cantilever of atomic force microscope has been measured. The cantilever was
coated with ActA and actin filaments were allowed to grow at the lower surface of
the cantilever in a Xenopus egg. The cantilever was pushed up demonstrating >
100 nN force was produced [177]. Another study utilizing an N-WASP-coated
plastic bead has shown to generate a few nN force when actin was allowed to
polymerized from the bead surface in the presence of Arp2/3, gelsolin (a barbedend capping protein), ADF/cofilin, profilin and actin [203].
The reconstituted system has been also used to study the motility of the Ascaris
sperm, which, as mentioned above, does not depend on actin-myosin system.
Addition of ATP to the sperm extract showed a formation of a fibrous structure
that bound a plasma membrane vesicle and had an arrangement of the filaments
similar to that made of MSP (major sperm protein) in vivo. The growth of the fibrous
structure, which occurred at the interface of the fiber and the plasma membrane of
the vesicle, seemed to propel the membrane vesicle [204]. The movement realized in
this intriguing system has been treated theoretically [205].
7.21 Polymerization Force: Theoretical Studies
7.21.1 Hill’s Theory
As mentioned in Sect. 7.16, Hill [147] developed the theory based on irreversible
thermodynamics to explain the polymerization-based force development. There are
several assumptions: (1) The monomer and polymer coexist in the solution; (2) One
end of the polymer is fixed and the other end is touching an obstacle, which is pushed
against the polymer end by an external force F; (3) The polymer was assumed to be a
linear chain of monomer and infinitely rigid. As before, we call the monomer
incorporated in the polymer “protomer” to distinguish it from the monomer in
solution. The chemical potential of a monomer in solution is designated as μ and
that of protomer, μ pro . Hill assumed that the value of μ pro does not depend on the
position of the protomer (monomer in the polymer); this is a simplification, because
144
7 Moving Life