Essential Cell Biology 5th edition

574 CHAPTER 17 Cytoskeleton
Figure 17–1 The cytoskeleton gives a cell its shape and allows the
cell to organize its internal components and to move. An animal
cell in culture has been labeled to show two of its major cytoskeletal
systems, the microtubules (green) and the actin filaments (red ). Where
the two filaments overlap, they appear yellow. The DNA in the nucleus
is labeled in blue. (Courtesy of Albert Tousson.)
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Figure 17–2 The three types of protein
filaments that form the cytoskeleton
differ in their composition, mechanical
properties, and roles inside the cell. They
are shown here in epithelial cells, but they
are all found in almost all animal cells.
The cytoskeleton is built on a framework of three types of protein filaments:
intermediate filaments, microtubules, and actin filaments. Each type
of filament has distinct mechanical properties and is formed from a different
protein subunit. A family of fibrous proteins forms the intermediate
filaments; globular tubulin subunits form microtubules; and globular actin
subunits form actin filaments (Figure 17–2). In each case, thousands of
these subunits assemble into fine threads that sometimes extend across
the entire cell.
In this chapter, we consider the structure and function of each of these
protein filament networks. We begin with intermediate filaments, which
provide cells with mechanical strength. We then see how microtubules
organize the cytoplasm of eukaryotic cells and form the hairlike, motile
appendages that enable cells like protozoa and sperm to swim. We next
consider how the actin cytoskeleton supports the cell surface and allows
fibroblasts and other cells to crawl. Finally, we discuss how the actin
cytoskeleton enables our muscles to contract.
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INTERMEDIATE FILAMENTS
MICROTUBULES
ACTIN FILAMENTS
25 nm
Intermediate filaments are ropelike fibers with a
diameter of about 10 nm; they are made of
fibrous intermediate filament proteins. One
type of intermediate filament forms a
meshwork called the nuclear lamina just
beneath the inner nuclear membrane. Other
types extend across the cytoplasm, giving cells
mechanical strength and distributing the
mechanical stresses in an epithelial tissue by
spanning the cytoplasm from one cell–cell
junction to another. Intermediate filaments are
very flexible and have great tensile strength.
They deform under stress but do not rupture.
(L. Norlen et al. Exper. Cell Res. 313:2217–2227,
2007. With permission from Elsevier.)
25 nm
Microtubules are hollow cylinders made of the
protein tubulin. They are long and straight and
typically have one end attached to a single
microtubule-organizing center called a
centrosome. With an outer diameter of 25 nm,
microtubules are more rigid than actin filaments
or intermediate filaments, and they rupture
when stretched. (Micrograph courtesy of
Richard Wade.)
25 nm
Actin filaments (also known as microfilaments)
are helical polymers of the protein actin. They
are flexible structures, with a diameter of about
7 nm, that are organized into a variety of linear
bundles, two-dimensional networks, and
three-dimensional gels. Although actin
filaments are dispersed throughout the cell,
they are most highly concentrated in the cortex,
the layer of cytoplasm just beneath the plasma
membrane. (Micrograph courtesy of Roger
Craig.)