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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remain strikingly dynamic and are continuously remodeled and replaced every 48
hours, even within stable cell-surface structures that persist for decades.
Besides forming stable, specialized cell-surface protrusions, the cytoskeleton
is also responsible for large-scale cellular polarity, enabling cells to tell the
difference between top and bottom, or front and back. The large-scale polarity
information conveyed by cytoskeletal organization is often maintained over the
lifetime of the cell. Polarized epithelial cells use organized arrays of microtubules,
actin filaments, and intermediate filaments to maintain the critical differences
between the apical surface and the basolateral surface. They also must maintain
strong adhesive contacts with one another to enable this single layer of cells to
serve as an effective physical barrier (Figure 16–4).
Filaments Assemble from Protein Subunits That Impart Specific
Physical and Dynamic Properties
Cytoskeletal filaments can reach from one end of the cell to the other, spanning
tens or even hundreds of micrometers. Yet the individual protein molecules that
form the filaments are only a few nanometers in size. The cell builds the filaments
by assembling large numbers of the small subunits, like building a skyscraper out
of bricks. Because these subunits are small, they can diffuse rapidly in the cytosol,
whereas the assembled filaments cannot. In this way, cells can undergo rapid
structural reorganizations, disassembling filaments at one site and reassembling
them at another site far away.
Actin filaments and microtubules are built from subunits that are compact
and globular—actin subunits for actin filaments and tubulin subunits for microtubules—whereas
intermediate filaments are made up of smaller subunits that are
themselves elongated and fibrous. All three major types of cytoskeletal filaments
form as helical assemblies of subunits (see Figure 3–22) that self-associate, using
a combination of end-to-end and side-to-side protein contacts. Differences in
the structures of the subunits and the strengths of the attractive forces between
APICAL
microvillus
–
+
+
–
–
– –
–
nucleus
+
+
+
+
intermediate filaments
microtubules
actin microfilaments
–
–
+
+
BASAL
terminal web
of actin
adherens junction
desmosome
hemidesmosome
basal lamina
Figure 16–4 Organization of the
cytoskeleton in polarized epithelial cells.
All the components of the cytoskeleton
cooperate to produce the characteristic
shapes of specialized cells, including the
epithelial cells that line the small intestine,
diagrammed here. At the apical (upper)
surface, facing the intestinal lumen,
bundled actin filaments (red) form microvilli
that increase the cell surface area available
for absorbing nutrients from food. Below
the microvilli, a circumferential band of
actin filaments is connected to cell–cell
adherens junctions that anchor the cells to
each other. Intermediate filaments (blue)
are anchored to other kinds of adhesive
structures, including desmosomes and
hemidesmosomes, that connect the
epithelial cells into a sturdy sheet and
attach them to the underlying extracellular
matrix; these structures are discussed
in Chapter 19. Microtubules (green) run
vertically from the top of the cell to the
bottom and provide a global coordinate
system that enables the cell to direct newly
synthesized components to their proper
locations.