Essential Cell Biology 5th edition

Actin Filaments
597
CONTRACTION
actin cortex
cortex under tension
myosin motor
proteins slide along
actin filaments
lamellipodium
substratum
ACTIN POLYMERIZATION AT
PLUS END PROTRUDES
LAMELLIPODIUM
movement of unpolymerized actin
ATTACHMENT
Figure 17–33 Forces generated in the
actin-filament-rich cortex help move a
cell forward. Actin polymerization at the
leading edge of the cell pushes the plasma
membrane forward (protrusion) and forms
new regions of actin cortex, shown here in
red. New points of anchorage are made
between the bottom of the cell and the
surface (substratum) on which the cell is
crawling (attachment). Contraction at the
rear of the cell—mediated by myosin motor
proteins moving along actin filaments—then
draws the body of the cell forward. New
anchorage points are established at the
front, and old ones are released at the back,
as the cell crawls forward. The same cycle is
repeated over and over again, moving the
cell forward in a stepwise fashion.
focal contacts
(contain integrins)
FURTHER PROTRUSION
plasma membrane and elongate by the addition of actin monomers at
their plus ends. In this way, the filaments push out the membrane without
tearing it.
When the lamellipodia and filopodia touch down on a favorable surface,
they stick: transmembrane ECB5 proteins e17.33/17.33 in their plasma membrane, known
as integrins (discussed in Chapter 20), adhere to molecules either in the
extracellular matrix or on the surface of a neighboring cell over which
the moving cell is slithering. Meanwhile, on the intracellular face of the
crawling cell’s plasma membrane, integrins capture actin filaments in the
cortex, thereby creating a robust anchorage for the crawling cell (see
Figures 17–33 and 20–14C). To use this anchorage to drag its body forward,
the cell calls on the help of myosin motor proteins, which slide
along actin filaments, as we discuss shortly.
QUESTION 17–6
Suppose that the actin molecules
in a cultured skin cell have been
randomly labeled in such a way
that 1 in 10,000 molecules carries
a fluorescent marker. What would
you expect to see if you examined
the lamellipodium (leading edge)
of this cell through a fluorescence
microscope? Assume that your
microscope is sensitive enough to
detect single fluorescent molecules.
lamellipodium
filopodium
lamellipodium
(A)
(B)
5 µm
filopodium
Figure 17–34 Actin filaments allow animal cells to migrate. (A) Schematic drawing of a fibroblast, showing flattened lamellipodia
and fine filopodia projecting from its surface, especially in the regions of the leading edge. (B) Scanning electron micrograph showing
lamellipodia and filopodia at the leading edge of a human fibroblast migrating in culture; the arrow shows the direction of cell
movement. As the cell moves forward, the lamellipodia that fail to attach to the substratum are swept backward over the upper surface
of the cell— a movement referred to as ruffling. (B, courtesy of Julian Heath.)
ECB5 e17.34/17.34