(see Sect. 3.8 for details), and the filament was allowed to interact with myosin
bound to the glass surface. The situation of tug-of-war was established between the
trapped bead and the surface-bound myosin molecules, and hence, the displacement
of the bead by the pulling action of myosin and the accompanying force could be
estimated [7]. Actually, it was difficult to measure the force or movement of a single
myosin molecule with this configuration, because in the presence of ATP, the
dissociation of the myosin head from the actin filament was unavoidable. Lowering
ATP concentration will be required to resolve individual force generation events,
because the bimolecular rate constant for the binding of ATP to myosin head is on
the order of 10
6 M
-1 s
-1
, and hence, at 1 μM ATP, roughly 1 event of the force
generation will be expected [8]. However, the collected data scarcely represented
what would be expected form the single event (the displacement of the bead, ~5 nm,
which was consistent with the estimation utilizing the result of macroscopic force
measurement of skeletal muscle [1] and the value anticipated from the laver arm
action); the displacements often exceeded 20 nm. Thus, the possibility of involvement of several consecutive events driven by several heads could not be denied
[7]. A similar experiment with different configuration has been performed (Fig. 7.6b;
[9]). This configuration allowed the measurement of the single molecule, because the
dissociation of the actin filament from the myosin head could be suppressed. The
result suggested that the distance over which a single myosin molecule exerted a few
piconewton forces on actin filament was about 5 nm.
7.2 Cellular Movements Other Than Muscle Contraction
In the following sections we will describe the movements and dynamics observed in
non-muscle cells. Those include the movement of organelles, change in the cell
shape and crawling motion of the cell body. The transport of organelles depends on
non-muscle type myosin, kinesin or dynein, whereas the contraction of actin filament bundle that occurs in the cell spreading and crawling cell depends on muscletype myosin, but some movements occur independent of those motor proteins (Sect.
7.17).
7.2.1 Three Filament Systems Supporting the Dynamism
in the Cell
The cell has intricate filamentous structures: actin filament, microtubule (Fig. 7.7a
and b, respectively) and intermediate filaments (not shown). Actin filaments (or
F-actin) and microtubules are highly dynamic entities while intermediate filaments
are less dynamic: apart from this difference, they are all assembled and disassembled
through the process of reversible polymerization of monomeric units and
102
7 Moving Life
bound to the glass surface. The situation of tug-of-war was established between the
trapped bead and the surface-bound myosin molecules, and hence, the displacement
of the bead by the pulling action of myosin and the accompanying force could be
estimated [7]. Actually, it was difficult to measure the force or movement of a single
myosin molecule with this configuration, because in the presence of ATP, the
dissociation of the myosin head from the actin filament was unavoidable. Lowering
ATP concentration will be required to resolve individual force generation events,
because the bimolecular rate constant for the binding of ATP to myosin head is on
the order of 10
6 M
-1 s
-1
, and hence, at 1 μM ATP, roughly 1 event of the force
generation will be expected [8]. However, the collected data scarcely represented
what would be expected form the single event (the displacement of the bead, ~5 nm,
which was consistent with the estimation utilizing the result of macroscopic force
measurement of skeletal muscle [1] and the value anticipated from the laver arm
action); the displacements often exceeded 20 nm. Thus, the possibility of involvement of several consecutive events driven by several heads could not be denied
[7]. A similar experiment with different configuration has been performed (Fig. 7.6b;
[9]). This configuration allowed the measurement of the single molecule, because the
dissociation of the actin filament from the myosin head could be suppressed. The
result suggested that the distance over which a single myosin molecule exerted a few
piconewton forces on actin filament was about 5 nm.
7.2 Cellular Movements Other Than Muscle Contraction
In the following sections we will describe the movements and dynamics observed in
non-muscle cells. Those include the movement of organelles, change in the cell
shape and crawling motion of the cell body. The transport of organelles depends on
non-muscle type myosin, kinesin or dynein, whereas the contraction of actin filament bundle that occurs in the cell spreading and crawling cell depends on muscletype myosin, but some movements occur independent of those motor proteins (Sect.
7.17).
7.2.1 Three Filament Systems Supporting the Dynamism
in the Cell
The cell has intricate filamentous structures: actin filament, microtubule (Fig. 7.7a
and b, respectively) and intermediate filaments (not shown). Actin filaments (or
F-actin) and microtubules are highly dynamic entities while intermediate filaments
are less dynamic: apart from this difference, they are all assembled and disassembled
through the process of reversible polymerization of monomeric units and
102
7 Moving Life