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

922 Chapter 16: The Cytoskeleton
actin
troponin
complex
tropomyosin – Ca 2+
+ Ca 2+
I C
T
(A)
10 nm
(B)
Figure 16–36 The control of skeletal muscle contraction by troponin. (A) A skeletal-muscle-cell thin filament, showing the
positions of tropomyosin and troponin along the actin filament. Each tropomyosin molecule has seven evenly spaced regions
with similar amino acid sequences, each of which is thought to bind to an actin subunit in the filament. (B) Reconstructed
cryoelectron microscopy image of an actin filament showing the relative position of a superimposed tropomyosin strand in the
presence (dark purple) or absence (light purple) of calcium. (A, adapted from G.N. Phillips, J.P. Fillers and C. Cohen, J. Mol. Biol.
192:111–131, 1986. With permission from Academic Press; B, adapted from C. Xu et al., Biophys. J. 77: 985–992, 1999. With
permission from Elsevier.)
composed of sheets of highly elongated spindle-shaped cells, each with a single
nucleus. Smooth muscle cells do not express the troponins. Instead, elevated
intracellular Ca 2+ levels regulate contraction by a mechanism that depends on
calmodulin (Figure 16–37). Ca 2+ -bound calmodulin activates myosin light-chain
kinase (MLCK), thereby inducing the phosphorylation MBoC6 of smooth m16.78/16.36 muscle myosin
on one of its two light chains. When the light chain is phosphorylated, the myosin
head can interact with actin filaments and cause contraction; when it is dephosphorylated,
the myosin head tends to dissociate from actin and becomes inactive.
The phosphorylation events that regulate contraction in smooth muscle cells
occur relatively slowly, so that maximum contraction often requires nearly a
second (compared with the few milliseconds required for contraction of a skeletal
muscle cell). But rapid activation of contraction is not important in smooth
(A)
(B)
sarcoplasmic
reticulum
outer layer
Ca 2+
active myosin
light-chain kinase
inner layer
+
Ca 2+ /calmodulin
ATP
(C)
calmodulin
myosin lightchain
kinase
PHOSPHORYLATION
OF MYOSIN LIGHT CHAIN
relaxed smooth muscle cell
contracted smooth muscle cell
Figure 16–37 Smooth muscle
contraction. (A) Upon muscle stimulation
by activation of cell-surface receptors,
Ca 2+ released into the cytoplasm from
the sarcoplasmic reticulum (SR) binds to
calmodulin (see Figure 15–29). Ca 2+ -bound
calmodulin then binds myosin light-chain
kinase (MLCK), which phosphorylates
myosin light chain, stimulating myosin
activity. Non-muscle myosin is regulated
by the same mechanism (see Figure
16–39). (B) Smooth muscle cells in a cross
section of cat intestinal wall. The outer
layer of smooth muscle is oriented with
the long axis of its cells extending parallel
along the length of the intestine, and upon
contraction will shorten the intestine. The
inner layer is oriented circularly around
the intestine and when contracted will
cause the intestine to become narrower.
Contraction of both layers squeezes
material through the intestine, much like
squeezing toothpaste out of a tube.
(C) A model for the contractile apparatus
in a smooth muscle cell, with bundles of
contractile filaments containing actin and
myosin (red) oriented obliquely to the long
axis of the cell. Their contraction greatly
shortens the cell. Only a few of the many
bundles are shown. (B, courtesy of
Gwen V. Childs.)