Essential Cell Biology 5th edition

600 CHAPTER 17 Cytoskeleton
Figure 17–37 Myosin-I is the simplest
myosin. (A) Myosin-I has a single globular
head that attaches to an actin filament and
a tail that attaches to another molecule or
organelle in the cell. (B) This arrangement
allows the head domain to move a vesicle
relative to an actin filament, which in this
case is anchored to the plasma membrane.
(C) Myosin-I can also bind to an actin
filament in the cell cortex, ultimately pulling
the plasma membrane into a new shape.
Note that the head group always walks
toward the plus end of the actin filament.
(A)
(B)
myosin-I
plus end
70 nm
head
domain
plasma
membrane
tail
myosin-l
vesicle
minus end
(C)
minus end
plus end
myosin-l
plasma membrane
MUSCLE CONTRACTION
Muscle contraction ECB5 is the E17.36/17.37
best understood of animal cell movements. In
vertebrates, running, walking, swimming, and flying all depend on the
ability of skeletal muscle to contract strongly and move various bones.
Involuntary movements such as heart pumping and gut peristalsis
depend on cardiac muscle and smooth muscle, respectively, which are
formed from muscle cells that differ in structure from skeletal muscle but
use actin and myosin in a similar way to contract. Much of our understanding
of the mechanisms of cell movement originated from studies of
muscle cell contraction. In this section, we discuss how actin and myosin
interact to produce this contraction.
Muscle Contraction Depends on Interacting Filaments
of Actin and Myosin
Muscle myosin belongs to the myosin-II subfamily of myosins. These
proteins are dimers, with two globular ATPase heads at one end and a
single coiled-coil tail at the other (Figure 17–38A). Clusters of myosin-II
molecules bind to each other through their coiled-coil tails, forming a
bipolar myosin filament from which the heads project (Figure 17–38B).
Figure 17–38 Myosin-II molecules
can associate with one another
to form myosin filaments. (A) A
molecule of myosin-II contains two
identical heavy chains, each with
a globular head and an extended
tail. (It also contains two light chains
bound to each head, but these are
not shown.) The tails of the two heavy
chains form a single coiled-coil tail.
(B) The coiled-coil tails of myosin-II
molecules associate with one another
to form a bipolar myosin filament in
which the heads project outward from
the middle in opposite directions.
The bare region in the middle of the
filament consists of tails only.
(A)
(B)
myosin-II molecule
head
150 nm
myosin-II filament
tail
bare region
(myosin tails only)
1 µm
myosin heads