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

946 Chapter 16: The Cytoskeleton
to several millimeters for microtubules and about ten micrometers for actin), but
they are extremely difficult to break and can be stretched to over three times their
length (see Figure 16–6).
Less is understood about the mechanism of assembly and disassembly of
intermediate filaments than of actin filaments and microtubules. In pure protein
solutions, intermediate filaments are extremely stable due to tight association
of subunits, but some types of intermediate filaments, including vimentin, form
highly dynamic structures in cells such as fibroblasts. Protein phosphorylation
probably regulates their disassembly, in much the same way that phosphorylation
regulates the disassembly of nuclear lamins in mitosis (see Figure 12–18). As evidence
for rapid turnover, labeled subunits microinjected into tissue-culture cells
incorporate into intermediate filaments within a few minutes. Remodeling of the
intermediate filament network accompanies events requiring dynamic cellular
reorganization, such as division, migration, and differentiation.
Intermediate Filaments Impart Mechanical Stability
to Animal Cells
Keratins are the most diverse intermediate filament family: there are about 20
found in different types of human epithelial cells and about 10 more that are specific
to hair and nails; analysis of the human genome sequence has revealed that
there are 54 distinct keratins. Every keratin filament is made up of an equal mixture
of type I (acidic) and type II (neutral/basic) keratin proteins; these form a
heterodimer filament subunit (see Figure 16–67). Cross-linked keratin networks
held together by disulfide bonds can survive even the death of their cells, forming
tough coverings for animals, as in the outer layer of skin and in hair, nails, claws,
and scales. The diversity in keratins is clinically useful in the diagnosis of epithelial
cancers (carcinomas), as the particular set of keratins expressed gives an indication
of the epithelial tissue in which the cancer originated and thus can help to
guide the choice of treatment.
A single epithelial cell may produce multiple types of keratins, and these
copolymerize into a single network (Figure 16–68). Keratin filaments impart
mechanical strength to epithelial tissues in part by anchoring the intermediate
filaments at sites of cell–cell contact, called desmosomes, or cell–matrix contact,
called hemidesmosomes (see Figure 16–4). We discuss these important adhesive
structures in Chapter 19. Accessory proteins, such as filaggrin, bundle keratin filaments
in differentiating cells of the epidermis to give the outermost layers of the
10 µm
Figure 16–68 Keratin filaments in
epithelial cells. Immunofluorescence
micrograph of the network of keratin
filaments (blue) in a sheet of epithelial
cells in culture. The filaments in each cell
are indirectly connected to those of its
neighbors by desmosomes (discussed in
Chapter 19). A second protein (red) has
been stained to reveal the location of the
cell boundaries. (Courtesy of Kathleen
Green and Evangeline Amargo.)