because this part is no longer needed for the movement. The key is the different
critical concentration of the barbed and pointed ends. ATPase activity of actin is
essential to maintain the kinetic difference between the two ends. A detailed
theoretical analysis [209] was performed including relevant biochemical processes.
Those are the sequestration of actin monomer by thymosin beta-4, the exchange of
ADP bound to actin monomer with solution ATP by profilin, binding of profilinATP-bound actin monomer complex to the barbed and pointed ends and severing of
actin filament. The study has considered a steady state of the amount of actin
filament in the network. It was found that if the part of the barbed ends is incompetent of polymerization due to the capping, the uncapped filaments grow faster than
the shrinkage of the pointed ends to maintain the steady state of the network. This is
because actin monomers polymerize at the limited number of the barbed ends that
are uncapped (funneling effect; [132]). To compensate for the depletion of
polymerization-competent monomer, actin filaments in the network are severed at
the back of the network by the severing protein (ADF/cofilin); depolymerization of
actin occurs predominantly at the pointed end due to the higher critical concentration
at this end than the barbed end, since the actin protomer near the filament end is in
the ADP-bound form. The newly exposed barbed end is thought to be capped to
block futile polymerization (not producing mechanical work). The actin monomer
generated by the depolymerization binds ADP and will be sequestered by thymosinbeta 4. Profilin also binds the ADP-bound actin monomer, but the bound ADP is
rapidly exchanged to ATP. The ATPase activity of actin thus maintains the directionality in an individual actin filament. The slow release of Pi might enhance the
vectorial nature; the distinction of the kinetic properties between the ADP-Pi bound
protomer and ADP-bound protomer will only occur some distance from the barbed
end due to the delayed release of Pi form the protomer.
7.24 The Experimental Evidence of the Polymerization
Force by a Single Actin Filament
The theoretical treatment of the force accompanying the polymerization should be
applicable to general cases, but the experiment with a single actin filament to confirm
the force-velocity relation is difficult, because of an intrinsic flexibility of a single
actin filament. An attempt has been made to measure the force by tethering an actin
filament to the glass surface with chemically modified myosin that are unable to
hydrolyze ATP, but binds actin strongly; the barbed end is bound by formin that had
been also bound to the surface [210]. With the growth of an actin filament by the
nucleating activity of formin at the barbed end, the filament started buckling. The
shape of the buckled filament was analyzed by the elastic rod theory. The buckling
force depended on the conditions for the attachment of the filaments to two proteins
was between 1-10 pN, depending on the attachment condition (freely pivoting
around the tethered point or direction of the filament fixed). It also strongly depended
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7 Moving Life
critical concentration of the barbed and pointed ends. ATPase activity of actin is
essential to maintain the kinetic difference between the two ends. A detailed
theoretical analysis [209] was performed including relevant biochemical processes.
Those are the sequestration of actin monomer by thymosin beta-4, the exchange of
ADP bound to actin monomer with solution ATP by profilin, binding of profilinATP-bound actin monomer complex to the barbed and pointed ends and severing of
actin filament. The study has considered a steady state of the amount of actin
filament in the network. It was found that if the part of the barbed ends is incompetent of polymerization due to the capping, the uncapped filaments grow faster than
the shrinkage of the pointed ends to maintain the steady state of the network. This is
because actin monomers polymerize at the limited number of the barbed ends that
are uncapped (funneling effect; [132]). To compensate for the depletion of
polymerization-competent monomer, actin filaments in the network are severed at
the back of the network by the severing protein (ADF/cofilin); depolymerization of
actin occurs predominantly at the pointed end due to the higher critical concentration
at this end than the barbed end, since the actin protomer near the filament end is in
the ADP-bound form. The newly exposed barbed end is thought to be capped to
block futile polymerization (not producing mechanical work). The actin monomer
generated by the depolymerization binds ADP and will be sequestered by thymosinbeta 4. Profilin also binds the ADP-bound actin monomer, but the bound ADP is
rapidly exchanged to ATP. The ATPase activity of actin thus maintains the directionality in an individual actin filament. The slow release of Pi might enhance the
vectorial nature; the distinction of the kinetic properties between the ADP-Pi bound
protomer and ADP-bound protomer will only occur some distance from the barbed
end due to the delayed release of Pi form the protomer.
7.24 The Experimental Evidence of the Polymerization
Force by a Single Actin Filament
The theoretical treatment of the force accompanying the polymerization should be
applicable to general cases, but the experiment with a single actin filament to confirm
the force-velocity relation is difficult, because of an intrinsic flexibility of a single
actin filament. An attempt has been made to measure the force by tethering an actin
filament to the glass surface with chemically modified myosin that are unable to
hydrolyze ATP, but binds actin strongly; the barbed end is bound by formin that had
been also bound to the surface [210]. With the growth of an actin filament by the
nucleating activity of formin at the barbed end, the filament started buckling. The
shape of the buckled filament was analyzed by the elastic rod theory. The buckling
force depended on the conditions for the attachment of the filaments to two proteins
was between 1-10 pN, depending on the attachment condition (freely pivoting
around the tethered point or direction of the filament fixed). It also strongly depended
150
7 Moving Life