is halted when the growth of actin filaments is inhibited by cytochalasin, a mold
toxin that binds to the barbed end of actin filament and blocks the turnover of actin
there [125, 126]. Based on this observation and the absence of the myosin II in
lamellipodium, it has been postulated that the growing actin filaments push out the
membrane. The flexible nature of individual actin filaments seems to be impossible
to be reconciled with this notion, but the mechanical weakness is probably compensated by the existence of the large number of the cross-linked filaments immediately
beneath the lamellipodial edge as schematically shown in Fig. 7.22a [124, 127].
In the case of filopodium, the extension occurs at the structure called tip complex
that resides at the tip of filopodium. In the tip complex, nucleation of actin polymerization at the barbed end is promoted by the actin-nucleating protein, formin that
enables the pre-existing filament incorporating actin monomer albeit the presence of
an acitn-sequestering protein. The activity of formin is under the control of a small
GTPase called cdc42 [128].
7.14 Actin Turnover in Lamellipodium
In fast-moving cell like keratocyte, the protrusive growth of lamellipodia should be
fast, if the protrusion is intimately coupled to the growth of the actin filament. For
example, if a keratocyte moves at ~20 μm/min [129], the rate of protrusion is the
same, because in keratocyte moving without changing its shape, the velocity of
protrusion is the same as that of the tail retraction. Assuming that the protrusion is
driven by actin polymerization, the rate of the elongation of actin filaments along the
direction of the extension of the lamellipodial edge is also the same as the moving
velocity. Then, the rate of the addition of monomer to individual filament should be
~100 monomer/s (individual filaments are impinging on the edge at angles around
50 degrees; modal value measured in fibroblast; [130], but this will not quantitatively
change the conclusion here).
In this situation, the actin monomer concentration should be high in the vicinity of
the cell edge. Synthesis of actin is probably too slow to achieve the high concentration [131]. Thus, the supply of actin monomer must come from the actin network. It
is known that the actin filaments in the network depolymerize at the rear of the
network. Therefore, the actin monomer generated by depolymerization is likely to
contribute to the supply of actin monomer. Still, the intrinsic rate of depolymerization in the intracellular ionic condition is too slow (7.6 monomer per second; [42]).
Cell utilizes two strategies to circumvent this problem. One is to accelerate the
depolymerization at the pointed end. This is achieved by a combination of the
severing of actin filaments by weakening the inter-protomer binding in the filament
(see below) at the back of the network and the capping of some barbed ends by a
capping protein in the vicinity of lamellipodial edge. The severing will increase the
number of pointed ends, and hence, increase the number of the dissociating monomer. The dissociated monomer is sequestered by monomer sequestering proteins
such as thymosin beta-4 and profilin to prevent the re-polymerization. The capping
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7 Moving Life
toxin that binds to the barbed end of actin filament and blocks the turnover of actin
there [125, 126]. Based on this observation and the absence of the myosin II in
lamellipodium, it has been postulated that the growing actin filaments push out the
membrane. The flexible nature of individual actin filaments seems to be impossible
to be reconciled with this notion, but the mechanical weakness is probably compensated by the existence of the large number of the cross-linked filaments immediately
beneath the lamellipodial edge as schematically shown in Fig. 7.22a [124, 127].
In the case of filopodium, the extension occurs at the structure called tip complex
that resides at the tip of filopodium. In the tip complex, nucleation of actin polymerization at the barbed end is promoted by the actin-nucleating protein, formin that
enables the pre-existing filament incorporating actin monomer albeit the presence of
an acitn-sequestering protein. The activity of formin is under the control of a small
GTPase called cdc42 [128].
7.14 Actin Turnover in Lamellipodium
In fast-moving cell like keratocyte, the protrusive growth of lamellipodia should be
fast, if the protrusion is intimately coupled to the growth of the actin filament. For
example, if a keratocyte moves at ~20 μm/min [129], the rate of protrusion is the
same, because in keratocyte moving without changing its shape, the velocity of
protrusion is the same as that of the tail retraction. Assuming that the protrusion is
driven by actin polymerization, the rate of the elongation of actin filaments along the
direction of the extension of the lamellipodial edge is also the same as the moving
velocity. Then, the rate of the addition of monomer to individual filament should be
~100 monomer/s (individual filaments are impinging on the edge at angles around
50 degrees; modal value measured in fibroblast; [130], but this will not quantitatively
change the conclusion here).
In this situation, the actin monomer concentration should be high in the vicinity of
the cell edge. Synthesis of actin is probably too slow to achieve the high concentration [131]. Thus, the supply of actin monomer must come from the actin network. It
is known that the actin filaments in the network depolymerize at the rear of the
network. Therefore, the actin monomer generated by depolymerization is likely to
contribute to the supply of actin monomer. Still, the intrinsic rate of depolymerization in the intracellular ionic condition is too slow (7.6 monomer per second; [42]).
Cell utilizes two strategies to circumvent this problem. One is to accelerate the
depolymerization at the pointed end. This is achieved by a combination of the
severing of actin filaments by weakening the inter-protomer binding in the filament
(see below) at the back of the network and the capping of some barbed ends by a
capping protein in the vicinity of lamellipodial edge. The severing will increase the
number of pointed ends, and hence, increase the number of the dissociating monomer. The dissociated monomer is sequestered by monomer sequestering proteins
such as thymosin beta-4 and profilin to prevent the re-polymerization. The capping
128
7 Moving Life