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

MYOSIN AND ACTIN
915
150 nm
neck or hinge region
C-terminus
2 nm
(A)
100 nm
(B)
light chains
N-terminus
coiled-coil of two α helices
Figure 16–26 Myosin II. (A) The two globular heads and long tail of a myosin II molecule shadowed with platinum can be seen
in this electron micrograph. (B) A myosin II molecule is composed of two heavy chains (each about 2000 amino acids long;
green) and four light chains (blue). The light chains are of two distinct types, and one copy of each type is present on each
myosin head. Dimerization occurs when the two α helices of the heavy chains wrap around each other to form a coiled-coil,
driven by the association of regularly spaced hydrophobic amino acids (see Figure 3–9). The coiled-coil arrangement makes an
extended rod in solution, and this part of the molecule forms the tail. (A, courtesy of David Shotton.)
MYOSIN AND ACTIN
A crucial feature of the actin cytoskeleton is that it can form contractile structures
that cross-link and slide actin filaments relative to one another through the action
of myosin motor proteins. In addition to driving muscle contraction, actin–myosin
assemblies perform important functions in non-muscle cells.
Actin-Based Motor Proteins Are Members of the Myosin
Superfamily
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The first motor protein to be identified was skeletal muscle myosin, which generates
the force for muscle contraction. This protein, now called myosin II, is an
elongated protein formed from two heavy chains and two copies of each of two
light chains. Each heavy chain has a globular head domain at its N-terminus that
contains the force-generating machinery, followed by a very long amino acid
sequence that forms an extended coiled-coil that mediates heavy-chain dimerization
(Figure 16–26). The two light chains bind close to the N-terminal head
domain, while the long coiled-coil tail bundles itself with the tails of other myosin
molecules. These tail–tail interactions form large, bipolar “thick filaments” that
have several hundred myosin heads, oriented in opposite directions at the two
ends of the thick filament (Figure 16–27).
myosin heads
(A)
500 nm
(C)
myosin tail
10 nm
bare zone
myosin heads
(B)
Figure 16–27 The myosin II bipolar thick filament in muscle. (A) Electron micrograph of a myosin II thick filament isolated from frog muscle. Note
the central bare zone, which is free of head domains. (B) Schematic diagram, not drawn to scale. The myosin II molecules aggregate by means of
their tail regions, with their heads projecting to the outside of the filament. The bare zone in the center of the filament consists entirely of myosin II
tails. (C) A small section of a myosin II filament as reconstructed from electron micrographs. An individual myosin molecule is highlighted in green.
The cytoplasmic myosin II filaments in non-muscle cells are much smaller, although similarly organized (see Figure 16–39). (A, courtesy of Murray
Stewart; C, based on R.A. Crowther, R. Padrón and R. Craig, J. Mol. Biol. 184:429–439, 1985. With permission from Academic Press.)
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