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

CELL POLARIZATION AND MIGRATION
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(A)
Figure 16–77 Migratory keratocytes from
a fish epidermis. (A) Light micrographs of
a keratocyte in culture, taken about
15 seconds apart. This cell is moving
at about 15 μm/min (Movie 16.13 and
see Movie 1.1). (B) Keratocyte seen by
scanning electron microscopy, showing
its broad, flat lamellipodium and small
cell body, including the nucleus, carried
up above the substratum at the rear.
(C) Distribution of cytoskeletal filaments in
this cell. Actin filaments (red) fill the large
lamellipodium and are responsible for
the cell’s rapid movement. Microtubules
(green) and intermediate filaments (blue)
are restricted to the regions close to the
nucleus. (A and B, courtesy of Juliet Lee.)
(B)
10 µm
(C)
actin cortex. Recruitment of myosin II and contraction of actin and myosin can
then power retraction of membrane blebs. Alternatively, extension of new blebs
from old ones can drive cell migration.
Lamellipodia Contain All of the Machinery Required for Cell Motility
Lamellipodia have been particularly well studied in the epithelial cells of the epidermis
of fish and frogs; these epithelial cells are known as keratocytes because
of their abundant keratin filaments. These cells normally cover the animal by
forming an epithelial sheet, and they are specialized to close wounds very rapidly,
moving at rates of up to 30 μm/min. MBoC6 When m16.87/16.79
cultured as individual cells, keratocytes
assume a distinctive shape with a very large lamellipodium and a small, trailing
cell body that is not attached to the substratum (Figure 16–77). Fragments of this
lamellipodium can be sliced off with a micropipette. Although the fragments generally
lack microtubules and membrane-enclosed organelles, they continue to
crawl normally, looking like tiny keratocytes.
The dynamic behavior of actin filaments in keratocyte lamellipodia can be
studied by labeling a small patch of actin and examining its fate. This reveals that,
while the lamellipodia crawl forward, the actin filaments remain stationary with
respect to the substratum. The actin filaments in the meshwork are mostly oriented
with their plus ends facing forward. The minus ends are frequently attached
to the sides of other actin filaments by Arp 2/3 complexes (see Figure 16–16),
helping to form the two-dimensional web (Figure 16–78). The web as a whole is
undergoing treadmilling, assembling at the front and disassembling at the back,
reminiscent of the treadmilling that occurs in individual actin filaments discussed
previously (see Figure 16–14).
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10 µm
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Figure 16–78 Actin filament nucleation and web formation by the
Arp 2/3 complex in lamellipodia. (A) A keratocyte with actin filaments
labeled in red by fluorescent phalloidin and the Arp 2/3 complex labeled in
green with an antibody against one of its subunits. The Arp 2/3 complex
is highly concentrated near the front of the lamellipodium, where actin
nucleation is most active. (B) Electron micrograph of a platinum-shadowed
replica of the leading edge of a keratocyte, showing the dense actin filament
meshwork. The labels denote areas enlarged in (C). (C) Close-up views of the
marked regions of the actin web at the leading edge shown in (B). Numerous
branched filaments can be seen, with the characteristic 70° angle formed
when the Arp 2/3 complex nucleates a new actin filament off the side of a
preexisting filament (see Figure 16–16). (From T. Svitkina and G. Borisy,
J. Cell Biol. 145:1009–1026, 1999. With permission from the authors.)
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