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

918 Chapter 16: The Cytoskeleton
(A)
(B)
displacement
bead
X
50 nm
50 nm
Y
0
position
detector
actin
myosin
1 2 3 4 5 6
time (sec)
optical tweezers
development by the fusion of many separate cells. The large muscle cell retains
the many nuclei of the contributing cells. These nuclei lie just beneath the plasma
membrane (Figure 16–31). The bulk of the cytoplasm inside is made up of myofibrils,
which is the name given to MBoC6 the basic n16.303/16.30 contractile elements of the muscle cell.
A myofibril is a cylindrical structure 1–2 μm in diameter that is often as long as
the muscle cell itself. It consists of a long, repeated chain of tiny contractile units—
called sarcomeres, each about 2.2 μm long—which give the vertebrate myofibril its
striated appearance (Figure 16–32).
Each sarcomere is formed from a miniature, precisely ordered array of parallel
and partly overlapping thin and thick filaments. The thin filaments are composed
of actin and associated proteins, and they are attached at their plus ends to a Z
disc at each end of the sarcomere. The capped minus ends of the actin filaments
extend in toward the middle of the sarcomere, where they overlap with thick filaments,
the bipolar assemblies formed from specific muscle isoforms of myosin
II (see Figure 16–27). When this region of overlap is examined in cross section by
electron microscopy, the myosin filaments are arranged in a regular hexagonal
lattice, with the actin filaments evenly spaced between them (Figure 16–33). Cardiac
muscle and smooth muscle also contain sarcomeres, although the organization
is not as regular as that in skeletal muscle.
Figure 16–30 The force of a single
myosin molecule moving along an
actin filament measured using an
optical trap. (A) Schematic of the
experiment, showing an actin filament
with beads attached at both ends and held
in place by focused beams of light called
optical tweezers (Movie 16.4).
The tweezers trap and move the bead,
and can also be used to measure the
force exerted on the bead through the
filament. In this experiment, the filament
was positioned over another bead to
which myosin II motors were attached,
and the optical tweezers were used to
determine the effects of myosin binding
on movement of the actin filament.
(B) These traces show filament movement
in two separate experiments. Initially, when
the actin filament is unattached to myosin,
thermal motion of the filament produces
noisy fluctuations in filament position. When
a single myosin binds to the actin filament,
thermal motion decreases abruptly and a
roughly 10-nm displacement results from
movement of the filament by the motor. The
motor then releases the filament. Because
the ATP concentration is very low in this
experiment, the myosin remains attached
to the actin filament for much longer than
it would in a muscle cell. (Adapted from
C. Rüegg et al., Physiology 17:213–218,
2002. With permission from the American
Physiological Society.)
Figure 16–31 Skeletal muscle cells (also called muscle fibers). (A) These
huge multinucleated cells form by the fusion of many muscle cell precursors,
called myoblasts. Here, a single muscle cell is depicted. In an adult human,
a muscle cell is typically 50 μm in diameter and can be up to several
centimeters long. (B) Fluorescence micrograph of rat muscle, showing the
peripherally located nuclei (blue) in these giant cells. Myofibrils are stained
red. (B, courtesy of Nancy L. Kedersha.)
(A)
nucleus
myofibril
(B)
50 µm