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

FUNCTION AND ORIGIN OF THE CYTOSKELETON
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BREAKAGE IN
MIDDLE BREAKS
ONE BOND
REMOVAL FROM
ONE END BREAKS
ONE BOND
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SINGLE PROTOFILAMENT: THERMALLY UNSTABLE
BREAKING IN TWO REQUIRES
BREAKING MULTIPLE BONDS
REMOVAL FROM ONE END BREAKS
A SMALL NUMBER OF BONDS
Figure 16–5 The thermal stability of
cytoskeletal filaments with dynamic
ends. A protofilament consisting of a single
strand of subunits is thermally unstable,
since breakage of a single bond between
subunits is sufficient to break the filament.
In contrast, formation of a cytoskeletal
filament from more than one protofilament
allows the ends to be dynamic, while
enabling the filaments themselves to
be resistant to thermal breakage. In a
microtubule, for example, removing a single
subunit dimer from the end of the filament
requires breaking noncovalent bonds with a
maximum of three other subunits, whereas
fracturing the filament in the middle requires
breaking noncovalent bonds in all thirteen
protofilaments.
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MULTIPLE PROTOFILAMENTS: THERMALLY STABLE
distribution and the dynamic behavior of the filaments, converting information
received through signaling pathways into cytoskeletal action. These accessory
proteins bind to the filaments or their subunits to determine the sites of assembly
of new filaments, to regulate the partitioning of polymer proteins between filament
and subunit forms, to change the kinetics of filament assembly and disassembly,
to harness energy to generate force, and to link filaments to one another
or to other cell structures such as organelles and the plasma membrane. In these
processes, the accessory proteins bring cytoskeletal structure under the control
of extracellular and intracellular signals, including those that trigger the dramatic
transformations of the cytoskeleton that occur during each cell cycle. Acting
together, the accessory proteins enable a eukaryotic cell to maintain a highly organized
but flexible internal structure and, in many cases, to move.
lateral pulling
intermediate
filament
MBoC6 m16.08/16.05
200 nm
Figure 16–6 Flexibility and stretch in
an intermediate filament. Intermediate
filaments are formed from elongated fibrous
subunits with strong lateral contacts,
resulting in resistance to stretching
forces. When a tiny mechanical probe is
dragged across an intermediate filament,
the filament is stretched over three times
its length before it breaks, as illustrated
by the fluorescently labeled filaments in
the photomicrographs. This technique
is termed atomic force microscopy (see
Figure 9–33). (Adapted from L. Kreplak et
al., J. Mol. Biol. 354:569–577, 2005. With
permission from Elsevier.)