Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

916 Chapter 16: The Cytoskeleton
(A)
myosin head
actin filament
glass slide
5 µm
Figure 16–28 Direct evidence for the
motor activity of the myosin head. In this
experiment, purified myosin heads were
attached to a glass slide, and then actin
filaments labeled with fluorescent phalloidin
were added and allowed to bind to the
myosin heads. (A) When ATP was added,
the actin filaments began to glide along
the surface, owing to the many individual
steps taken by each of the dozens of
myosin heads bound to each filament. The
video frames shown in this sequence were
recorded about 0.6 second apart; the two
actin filaments shown (one red and one
green) were moving in opposite directions
at a rate of about 4 μm/sec. (B) Diagram
of the experiment. The large red arrows
indicate the direction of actin filament
movement (Movie 16.2). (A, courtesy of
James Spudich.)
(B)
Each myosin head binds and hydrolyzes ATP, using the energy of ATP hydrolysis
to walk toward the plus end of an actin filament (Figure 16–28). The opposing
orientation of the heads in the thick filament makes the filament efficient at sliding
pairs of oppositely oriented actin filaments toward each other, shortening the
muscle. In skeletal muscle, in which carefully arranged actin filaments are aligned
MBoC6 m16.56/16.28
in “thin filament” arrays surrounding the myosin thick filaments, the ATP-driven
sliding of actin filaments results in a powerful contraction. Cardiac and smooth
muscle contain myosin II molecules that are similarly arranged, although different
genes encode them.
Myosin Generates Force by Coupling ATP Hydrolysis to
Conformational Changes
Motor proteins use structural changes in their ATP-binding sites to produce cyclic
interactions with a cytoskeletal filament. Each cycle of ATP binding, hydrolysis,
and release propels them forward in a single direction to a new binding site along
the filament. For myosin II, each step of the movement along actin is generated by
the swinging of an 8.5-nm-long α helix, or lever arm, which is structurally stabilized
by the binding of light chains. At the base of this lever arm next to the head,
there is a pistonlike helix that connects movements at the ATP-binding cleft in the
head to small rotations of the so-called converter domain. A small change at this
point can swing the helix like a long lever, causing the far end of the helix to move
by about 5.0 nm.
These changes in the conformation of the myosin are coupled to changes in
its binding affinity for actin, allowing the myosin head to release its grip on the
actin filament at one point and snatch hold of it again at another. The full mechanochemical
cycle of nucleotide binding, nucleotide hydrolysis, and phosphate
release (which causes the “power stroke”) produces a single step of movement
(Figure 16–29). At low ATP concentrations, the interval between the force-producing
step and the binding of the next ATP is long enough that single steps can
be observed (Figure 16–30).
Sliding of Myosin II Along Actin Filaments Causes Muscles to
Contract
Muscle contraction is the most familiar and best-understood form of movement in
animals. In vertebrates, running, walking, swimming, and flying all depend on the
rapid contraction of skeletal muscle on its scaffolding of bone, while involuntary