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one to four. Hence the maximumwork done during one ATPase cycle was measured
as 31 pN/nm (
7.2k
B
T). This result indicates that single myosin heads are able to
convert chemical energy into mechanical work with a maximum ef
ciency of
ΒΌ
36%
(energy liberated by ATP hydrolysis
20k
B
T). This value is similar to the maximum
ef
ciency of contracting muscle
fibers [29, 30]. The similarities between the for-
ce
-
velocity curve and thermodynamic ef
ciency of single actomyosin molecules to
assemble systems such as muscle suggest that the major mechanical and thermody-
namic properties of muscle are essentially the effect of the intrinsic characteristics of
individual actomyosin motors.
2.3.6
Other Types of Molecular Motors
Class-II myosin (muscle myosin) was found to move along actin
filaments by biased
Brownian motion. However, there are various classes of molecule motors. Do they
work by the same mechanism as myosin-II? Here, we investigated class-Vmyosin, a
processive motor that transports organelles in cells.
Myosin-V has two heads, each with a long neck (6IQmotifs) [31], and moves along
an actin
filament in large steps of
36 nm [32]. This step size coincides with an actin
half helical pitch. Electron microscopy [33] and single molecule nano-scale imaging
(FIONA) [10, 34] have shown that each head of myosin-Vmoves alternately along an
actin
filament with a step size (
72 nm) twice as large as that observed in a two-
headed molecule. This result indicates that myosin-V walks along the actin helical
repeats using a hand-over-handmechanism inwhich the rear head detaches from the
actin
filament, diffuses to the forward helical pitch and attaches there while the front
head is still attached. But how is the diffusion of the rear head biased to the forward
target? Also, how does the rear head undergo a Brownian search for the forward
target? The diffusion of the rear head occurs very rapidly (
<
1ms). Therefore, the
process of the diffusion cannot be detected with a suf
cient signal-to-noise ratio
using optical trapping nanometry [32] and single molecule nano-scale imaging
(FIONA) [34]. We again used scanning probe nanometry to resolve the diffusive
process of the head which had been slowed by attaching the head to a relatively large
probe [35]. To do this, we compared single myosin-V heads with short (2IQ motifs)
and long (6IQ motifs) necks to wild-type, two-headed myosin-V. Green
uorescent
protein (GFP) and Myc-tag were fused to the N-terminus and the C-terminus,
respectively. Single myosin-V molecules were captured on the tip of a scanning
probe via c-Myc monoclonal antibody. The number of myosin-Vmolecules on the tip
of a scanning probe was counted by observing the
fluorescence of green
uorescent
protein (GFP) fused to their N-terminus. We were able to observe the stepwise nature
of the rising phases of the displacements of myosin-V 2IQ and 6IQ heads on an
expanded time scale. The size of sub-steps was constant,
5.5 nm, independent of
the length of the neck domain (Figure 2.7a,b). Thus, myosin-V heads move along
actin monomers in a manner similar to myosin-II heads. A two-headed myosin-V
showed processive movement with
36-nm steps as previously observed by optical
trapping nanometry [32]. Furthermore an intermediate step (
18 nm) within each
