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

tension (% of maximum)
960 Chapter 16: The Cytoskeleton
In turn, microtubules influence actin rearrangements and cell adhesion. The
centrosome nucleates a large number of dynamic microtubules, and its repositioning
means that the plus ends of many of these microtubules extend into the
protrusive region of the cell. Direct interactions with microtubules help guide focal
adhesion dynamics in migrating cells. Microtubules might also influence actin filament
formation by delivering Rac-GEFs that bind to the +TIPs traveling on growing
microtubule ends. Microtubules also transport cargoes to and from the focal
adhesions, thereby affecting their signaling and disassembly. Thus, microtubules
reinforce the polarity information that the actin cytoskeleton receives from the
outside world, allowing a sensitive response to weak signals and enabling motility
to persist in the same direction for a prolonged period.
Summary
Whole-cell movements and the large-scale shaping and structuring of cells require
the coordinated activities of all three basic filament systems along with a large variety
of cytoskeletal accessory proteins, including motor proteins. Cell crawling—a
widespread behavior important in embryonic development and also in wound
healing, tissue maintenance, and immune system function in the adult animal—
is a prime example of such complex, coordinated cytoskeletal action. For a cell to
crawl, it must generate and maintain an overall structural polarity, which is influenced
by external cues. In addition, the cell must coordinate protrusion at the leading
edge (by assembly of new actin filaments), adhesion of the newly protruded part
of the cell to the substratum, and forces generated by molecular motors to bring the
cell body forward.
What we don’t know
• How is the cell cortex regulated
locally and globally to coordinate its
activities at different places on the
cell surface? What determines, for
example, where filopodia form?
• How are actin-regulatory proteins
controlled spatially in the cytoplasm to
generate multiple distinct types of actin
arrays in the same cell?
• Are there biologically important
processes occurring inside a
microtubule?
• How can we account for the fact that
there are many different kinesins and
myosins in the cytoplasm but only one
dynein?
• Mutations in the nuclear lamin
proteins cause a large number of
diseases called laminopathies. What do
we not understand about the nuclear
lamina that could account for this fact?
PROBLEMS
Which statements are true? Explain why or why not.
16–1 The role of ATP hydrolysis in actin polymerization
is similar to the role of GTP hydrolysis in tubulin polymerization:
both serve to weaken the bonds in the polymer
and thereby promote depolymerization.
16–2 Motor neurons trigger action potentials in muscle
cell membranes that open voltage-sensitive Ca 2+ channels
in T tubules, allowing extracellular Ca 2+ to enter the cytosol,
bind to troponin C, and initiate rapid muscle contraction.
16–3 In most animal cells, minus-end directed microtubule
motors deliver their cargo to the periphery of the cell,
whereas plus-end directed microtubule motors deliver
their cargo to the interior of the cell.
Discuss the following problems.
16–4 The concentration of actin in cells is 50–100 times
greater than the critical concentration observed for pure
actin in a test tube. How is this possible? What prevents the
actin subunits in cells from polymerizing into filaments?
Why is it advantageous to the cell to maintain such a large
pool of actin subunits?
16–5 Detailed measurements of sarcomere length and
tension during isometric contraction in striated muscle
provided crucial early support for the sliding-filament
100
75
50
25
0
1
1.6
III
2.0 2.2
II
1.3
I
IV 3.6
2
3
sarcomere length (µm)
Figure Q16–1 Tension as a function of sarcomere length during
isometric contraction (Problem 16–5).
model of muscle contraction. Based on your understanding
Problems
of the sliding-filament
EOC pQ16.03/Q16.01
model and the structure of a
sarcomere, propose a molecular explanation for the relationship
of tension to sarcomere length in the portions of
Figure Q16–1 marked I, II, III, and IV. (In this muscle, the
length of the myosin filament is 1.6 μm, and the lengths of
the actin thin filaments that project from the Z discs are 1.0
μm.)
16–6 At 1.4 mg/mL pure tubulin, microtubules grow at
a rate of about 2 μm/min. At this growth rate, how many
αβ-tubulin dimers (8 nm in length) are added to the ends
of a microtubule each second?
4