at barbed ends will decrease the number of growth competent filaments but will
increase the length of the growth per filament, because the monomer per filament
available for the polymerization increases. This will reduce the protrusive force, but
if the external force is not very large, the faster protrusion will be possible. This is
called funneling effect [132].
The depolymerization of the actin filaments at the back of lamellipodium occurs
after the filaments are transported by the backward motion of the actin network. This
backward motion is called retrograde flow as shown in Fig. 7.22b; [120, 133]. Myosin
II is assumed to participate in this process. Quite interestingly, it has been suggested
that the retrograde flow contributes to the facilitation of severing of actin filament by
the contractile force [134]. The involvement of myosin may depend on the cell
system because in one type of fibroblast cell, the velocity of the retrograde flow was
little affected by the inhibitors of myosin II activity [120].
Since the intracellular ionic environment (ie., ~0.1 M K
+ and/or 1 mM Mg
2+ ) is
not suitable for depolymerization of actin, actin depolymerizing factor (ADF)/cofilin
plays the central role in the accelerated depolymerization [132, 135–137], as mentioned above. The crystal structure of the actin monomer complexed with a fragment
of an actin depolymerizing factor called twinfilin has been solved [138]. This protein
has an actin interacting domain that is common among ADF, and its binding to actin
filament was found to cause the weakening of the interaction among protomers,
suggesting the facilitation of the depolymerization. As mentioned above, the actin
monomer thus formed is bound by actin-sequestering proteins, thymosin beta-4 and
profilin, which suppress spontaneous polymerization. Actin is then transported from
the rear to the cell edge. The passive diffusion alone may be too slow for the
monomer supply and it has been suggested that some facilitating transport mechanism is involved [139].
⁄
ä
Fig. 7.22 (continued) monomers polymerize to the barbed ends of actin filaments, which are facing
toward the lamellipodial edge. On the other hand, after the network is transported backward, the
filaments in the network are severed by a protein, ADF/cofilin (not shown) and individual monomers coming off the pointed ends are sequestered by proteins, thymosin beata-4 and profilin (not
shown). The actin monomers complexed with thymosin beta-4 or profilin are unpolymerizable and
are transported to the front edge of lamellipodium (dashed arrow; after [127, 128]). This is the
turnover of actin in lamellipodium. The contractile force exerted on actin filaments by myosin II
may facilitate depolymerization as well as the retrograde flow. Panel b, an example of the retrograde
flow of actin bundles in lamellipodium. Actin is fused to red-fluorescent protein and visualized by
epi-fluorescence microscopy. Shown in the upper panel is the lamellipodium; many white streaks
represent actin bundles. The lower panel shows time-lapse images of the area in the box indicated in
the upper panel. Each tile was extracted every 6 s. Black dotted lines connect the positions of the
bright fluorescent clusters probably formed by chance at different locations on the bundles. Because
the actin bundles grow immediately beneath the lamellipodial edge and transported backward along
with the retrograde flow of the actin network in the lamellipodium, those fluorescent spots moved at
the same rate toward the back of lamellipodium. As a result, slopes of the three dotted lines, which
represent the velocities of individual spots, are almost the same. Another remarkable feature is the
actin bundles become obscure at the back of lamellipodium as indicated with the white triangle due
to depolymerizaton of actin. Bar ¼ 10 μm
130
7 Moving Life
increase the length of the growth per filament, because the monomer per filament
available for the polymerization increases. This will reduce the protrusive force, but
if the external force is not very large, the faster protrusion will be possible. This is
called funneling effect [132].
The depolymerization of the actin filaments at the back of lamellipodium occurs
after the filaments are transported by the backward motion of the actin network. This
backward motion is called retrograde flow as shown in Fig. 7.22b; [120, 133]. Myosin
II is assumed to participate in this process. Quite interestingly, it has been suggested
that the retrograde flow contributes to the facilitation of severing of actin filament by
the contractile force [134]. The involvement of myosin may depend on the cell
system because in one type of fibroblast cell, the velocity of the retrograde flow was
little affected by the inhibitors of myosin II activity [120].
Since the intracellular ionic environment (ie., ~0.1 M K
+ and/or 1 mM Mg
2+ ) is
not suitable for depolymerization of actin, actin depolymerizing factor (ADF)/cofilin
plays the central role in the accelerated depolymerization [132, 135–137], as mentioned above. The crystal structure of the actin monomer complexed with a fragment
of an actin depolymerizing factor called twinfilin has been solved [138]. This protein
has an actin interacting domain that is common among ADF, and its binding to actin
filament was found to cause the weakening of the interaction among protomers,
suggesting the facilitation of the depolymerization. As mentioned above, the actin
monomer thus formed is bound by actin-sequestering proteins, thymosin beta-4 and
profilin, which suppress spontaneous polymerization. Actin is then transported from
the rear to the cell edge. The passive diffusion alone may be too slow for the
monomer supply and it has been suggested that some facilitating transport mechanism is involved [139].
⁄
ä
Fig. 7.22 (continued) monomers polymerize to the barbed ends of actin filaments, which are facing
toward the lamellipodial edge. On the other hand, after the network is transported backward, the
filaments in the network are severed by a protein, ADF/cofilin (not shown) and individual monomers coming off the pointed ends are sequestered by proteins, thymosin beata-4 and profilin (not
shown). The actin monomers complexed with thymosin beta-4 or profilin are unpolymerizable and
are transported to the front edge of lamellipodium (dashed arrow; after [127, 128]). This is the
turnover of actin in lamellipodium. The contractile force exerted on actin filaments by myosin II
may facilitate depolymerization as well as the retrograde flow. Panel b, an example of the retrograde
flow of actin bundles in lamellipodium. Actin is fused to red-fluorescent protein and visualized by
epi-fluorescence microscopy. Shown in the upper panel is the lamellipodium; many white streaks
represent actin bundles. The lower panel shows time-lapse images of the area in the box indicated in
the upper panel. Each tile was extracted every 6 s. Black dotted lines connect the positions of the
bright fluorescent clusters probably formed by chance at different locations on the bundles. Because
the actin bundles grow immediately beneath the lamellipodial edge and transported backward along
with the retrograde flow of the actin network in the lamellipodium, those fluorescent spots moved at
the same rate toward the back of lamellipodium. As a result, slopes of the three dotted lines, which
represent the velocities of individual spots, are almost the same. Another remarkable feature is the
actin bundles become obscure at the back of lamellipodium as indicated with the white triangle due
to depolymerizaton of actin. Bar ¼ 10 μm
130
7 Moving Life