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

myofibril
MYOSIN AND ACTIN
919
Figure 16–32 Skeletal muscle myofibrils. (A) Low-magnification electron
micrograph of a longitudinal section through a skeletal muscle cell of a
rabbit, showing the regular pattern of cross-striations. The cell contains
many myofibrils aligned in parallel (see Figure 16–31). (B) Detail of the
skeletal muscle shown in (A), showing portions of two adjacent myofibrils
and the definition of a sarcomere (black arrow). (C) Schematic diagram of
a single sarcomere, showing the origin of the dark and light bands seen in
the electron micrographs. The Z discs, at each end of the sarcomere, are
attachment sites for the plus ends of actin filaments (thin filaments); the
M line, or midline, is the location of proteins that link adjacent myosin II
filaments (thick filaments) to one another. (D) When the sarcomere contracts,
the actin and myosin filaments slide past one another without shortening.
(A and B, courtesy of Roger Craig.)
(A)
2 µm
Z disc dark band light band
Sarcomere shortening is caused by the myosin filaments sliding past the actin
thin filaments, with no change in the length of either type of filament (see Figure
16–32C and D). Bipolar thick filaments walk toward the plus ends of two sets of
thin filaments of opposite orientations, driven by dozens of independent myosin
heads that are positioned to interact with each thin filament. Because there is no
coordination among the movements of the myosin heads, it is critical that they
remain tightly bound to the actin filament for only a small fraction of each ATPase
cycle so that they do not hold one another back. Each myosin thick filament has
about 300 heads (294 in frog muscle), and each head cycles about five times per
second in the course of a rapid contraction—sliding the myosin and actin filaments
past one another at rates of up to 15 μm/sec and enabling the sarcomere
to shorten by 10% of its length in less than one-fiftieth of a second. The rapid synchronized
shortening of the thousands of sarcomeres lying end-to-end in each
myofibril enables skeletal muscle to contract rapidly enough for running and flying,
or for playing the piano.
Accessory proteins produce the remarkable uniformity in filament organization,
length, and spacing in the sarcomere (Figure 16–34). The actin filament plus
ends are anchored in the Z disc, which is built from CapZ and α-actinin; the Z disc
caps the filaments (preventing depolymerization), while holding them together in
a regularly spaced bundle. The precise length of each thin filament is influenced
by a protein of enormous size, called nebulin, which consists almost entirely of a
repeating 35-amino-acid actin-binding motif. Nebulin stretches from the Z disc
toward the minus end of each thin filament, which is capped and stabilized by tropomodulin.
Although there is some slow exchange of actin subunits at both ends
of the muscle thin filament, such that the components of the thin filament turn
over with a half-life of several days, the actin filaments in sarcomeres are remarkably
stable compared with those found in most other cell types, whose dynamic
actin filaments turn over with half-lives of a few minutes or less.
(B)
(C)
thick filament (myosin)
thin filament (actin)
light band
Z disc
(D)
one sarcomere
dark band
M line
light band
Z disc
MBoC6 m16.74/16.32
1 µm
Figure 16–33 Electron micrographs
of an insect flight muscle viewed in
cross section. The myosin and actin
filaments are packed together with almost
crystalline regularity. Unlike their vertebrate
counterparts, these myosin filaments
have a hollow center, as seen in the
enlargement on the right. The geometry
of the hexagonal lattice is slightly different
in vertebrate muscle. (From J. Auber,
J. de Microsc. 8:197–232, 1969. With
permission from Societé Française de
Microscopie Électronique.)