Essential Cell Biology 5th edition

606 CHAPTER 17 Cytoskeleton
The contraction of muscle cells represents a highly specialized use of the
basic components of the eukaryotic cytoskeleton. In the following chapter,
we discuss the crucial roles of the cytoskeleton in perhaps the most
fundamental cell movements of all: the segregation of newly replicated
chromosomes and the formation of two daughter cells during the process
of cell division.
ESSENTIAL CONCEPTS
• The cytoplasm of a eukaryotic cell is supported and organized by
a cytoskeleton of intermediate filaments, microtubules, and actin
filaments.
• Intermediate filaments are stable, ropelike polymers—built from
fibrous protein subunits—that give cells mechanical strength. Some
intermediate filaments form the nuclear lamina that supports and
strengthens the nuclear envelope; others are distributed throughout
the cytoplasm.
• Microtubules are stiff, hollow tubes formed by globular tubulin
dimers. They are polarized structures, with a slow-growing minus
end and a fast-growing plus end.
• Microtubules grow out from organizing centers such as the centrosome,
in which the minus ends remain embedded.
• Many microtubules display dynamic instability, alternating rapidly
between growth and shrinkage. Shrinkage is promoted by the
hydrolysis of the GTP that is tightly bound to tubulin dimers, reducing
the affinity of the dimers for their neighbors and thereby promoting
microtubule disassembly.
• Microtubules can be stabilized by localized proteins that capture the
plus ends, thereby helping to position the microtubules and harness
them for specific functions.
• Kinesins and dyneins are microtubule-associated motor proteins
that use the energy of ATP hydrolysis to move unidirectionally along
microtubules. They carry specific organelles, vesicles, and other
types of cargo to particular locations in the cell.
• Eukaryotic cilia and flagella contain a bundle of stable microtubules.
Their rhythmic beating is caused by bending of the microtubules,
driven by the ciliary dynein motor protein.
• Actin filaments are helical polymers of globular actin monomers.
They are more flexible than microtubules and are generally found in
bundles or networks.
• Like microtubules, actin filaments are polarized, with a fast-growing
plus end and a slow-growing minus end. Their assembly and disassembly
are controlled by the hydrolysis of ATP tightly bound to each
actin monomer and by various actin-binding proteins.
• The varied arrangements and functions of actin filaments in cells
stem from the diversity of actin-binding proteins, which can control
actin polymerization, cross-link actin filaments into loose networks
or stiff bundles, attach actin filaments to membranes, or move two
adjacent filaments relative to each other.
• A concentrated network of actin filaments underneath the plasma
membrane forms the bulk of the cell cortex, which is responsible
for the shape and movement of the cell surface, including the movements
involved when a cell crawls along a surface.
• Myosins are motor proteins that use the energy of ATP hydrolysis to
move along actin filaments. In nonmuscle cells, myosin-I can carry
organelles or vesicles along actin-filament tracks, and myosin-II can
cause adjacent actin filaments to slide past each other in contractile
bundles.