Biomedical Engineering Reference
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proposed more than 10 years ago (Boyer 1993 ) and directly observed (Noji et al.
1997 ; Mehta et al. 1999 ) by attaching a fl uorescent actin fi lament to the g subunit as
a marker. Further data on motor rotation were obtained from other experiments car-
ried out on single molecules of ATP synthase (Yasuda et al. 1998, 2001 ; Montemagno
and Bachand 1999 ; Sambongi et al. 1999 ; Itoh et al. 2004 ; Rondelez et al. 2005 ) .
2.2
Myosin
The term myosin refers to at least 14 classes of proteins, each containing actin-based
motors ( 2003 ; Schliwa and Woehlke 2003 ). Myosin is composed of two large heads,
containing a catalytic unit for ATP hydrolysis, connected to a more or less long tail.
Myosin-II provides the power for all our voluntary motions (running, walking, lifting,
etc.) as well as for involuntary muscles (e.g., beating heart). In muscle cells, many
myosin-II molecules combine by aligning their tails, staggered one relative to the
next. Muscle cells are also fi lled with cables of actin, which are used as a ladder on
which myosin climbs. The head groups of myosin extend from the surface of the
resulting fi lament like bristles in a bottle brush. The bristling head groups act indepen-
dently and provide the power to contract muscles. They reach from the myosin fi la-
ment to a neighboring actin fi lament and attach to it. Breakage of an ATP molecule,
then, forces the myosin head into a radically different shape. It bends near the center
and drags the myosin fi lament along the actin fi lament. This is the power stroke of
muscle contraction. In a rapidly contracting muscle, each myosin head may stroke fi ve
times a second, each stroke moving the fi lament about 10 nm. The working stroke of
myosin-II was observed (Finer et al. 1994 ; Reconditi et al. 2004 ) by optical methods.
These experiments, however, could not resolve a number of issues, such as the motor
mechanism, which are still the subject of extensive investigations.
Unlike myosin-II, myosin-V is a processive motor, i.e., it moves progressively
along its actin track, transporting organelles. In order to take consecutive steps along
the actin fi lament without dissociating, the two heads must operate in a coordinated
manner, but the mechanism has been controversial. Two models have been postu-
lated: the “hand-over-hand” model, in which the two heads alternate in the lead
(akin to human walking), and the “inchworm” model, in which the heads shuffl e
along one behind the other, with one of the two always leading. Recent detailed
studies (Forkey et al. 2003 ; Molloy and Veigel 2003 ; Yildiz et al. 2003 ) strongly
support a hand-over-hand mechanism of motility, and not an inchworm model.
2.3
Kinesin
Kinesin is a processive two-headed motor protein which moves along microtubule
tracks. It transports a variety of cargo, including membranous organelles, mRNA,
intermediate fi laments, and signaling molecules. Models for the motility cycles of
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