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

944 Chapter 16: The Cytoskeleton
after assembly and weakens the bonds that hold the microtubule together. Microtubules
are dynamically unstable and liable to catastrophic disassembly, but they can
be stabilized in cells by association with other structures. Microtubule-organizing
centers such as centrosomes protect the minus ends of microtubules and continually
nucleate the formation of new microtubules. Microtubule-associated proteins
(MAPs) stabilize microtubules, and those that localize to the plus end (+TIPs) can
alter the dynamic properties of the microtubule or mediate their interaction with
other structures. Counteracting the stabilizing activity of MAPs are catastrophe
factors, such as kinesin-13 proteins, that act to peel apart microtubule ends. Other
kinesin family members as well as dynein use the energy of ATP hydrolysis to move
unidirectionally along a microtubule. The motor dynein moves toward the minus
end of microtubules, and its sliding of axonemal microtubules underlies the beating
of cilia and flagella. Primary cilia are nonmotile sensory organs found on many
cell types.
INTERMEDIATE FILAMENTS AND SEPTINS
All eukaryotic cells contain actin and tubulin. But the third major type of cytoskeletal
protein, the intermediate filament, forms a cytoplasmic filament only in
some metazoans—including vertebrates, nematodes, and mollusks. Intermediate
filaments are particularly prominent in the cytoplasm of cells that are subject to
mechanical stress and are generally not found in animals that have rigid exoskeletons,
such as arthropods and echinoderms. It seems that intermediate filaments
impart mechanical strength to tissues for the squishier animals.
Cytoplasmic intermediate filaments are closely related to their ancestors, the
much more prevalent nuclear lamins, which are found in many eukaryotes but
missing from unicellular organisms. The nuclear lamins form a meshwork lining
the inner membrane of the nuclear envelope, where they provide anchorage sites
for chromosomes and nuclear pores. Several times during metazoan evolution,
lamin genes have apparently duplicated, and the duplicates have evolved to produce
ropelike, cytoplasmic intermediate filaments. In contrast to the highly conserved
actins and tubulin isoforms that are encoded by a handful of genes, different
families of intermediate filaments are much more diverse and are encoded by
70 different human genes with distinct, cell type-specific functions (Table 16–2).
Table 16–2 Major Types of Intermediate Filament Proteins in Vertebrate Cells
Types of
intermediate
filament
Component polypeptides
Location
Nuclear Lamins A, B, and C Nuclear lamina (inner lining of
nuclear envelope)
Vimentin-like Vimentin Many cells of mesenchymal origin
Desmin
Glial fibrillary acidic protein
Peripherin
Muscle
Glial cells (astrocytes and some
Schwann cells)
Some neurons
Epithelial Type I keratins (acidic) Epithelial cells and their derivatives
Type II keratins (neutral/basic)
(e.g., hair and nails)
Axonal
Neurofilament proteins
(NF-L, NF-M, and NF-H)
Neurons