Essential Cell Biology 5th edition

598 CHAPTER 17 Cytoskeleton
plus end of
newly polymerized
actin filament
leading edge
of cell
plasma
membrane
actin
monomers
(A)
Actin-binding Proteins Influence the Type of Protrusions
Formed at the Leading Edge
Figure 17–35 A web of polymerizing actin
filaments pushes the leading edge of a
lamellipodium forward. (A) A highly motile
keratocyte from frog skin was fixed, dried,
The formation and growth of actin filaments at the leading edge of a
and shadowed with platinum, and examined
ECB5 e17.35-17.35
cell are assisted by various actin-binding proteins. The actin-related proteins—or
ARPs—mentioned earlier promote the formation of a web of
branched actin filaments in lamellipodia. ARPs form complexes that bind
to the sides of existing actin filaments and nucleate the formation of new
filaments, which grow out at an angle to produce side branches. With the
aid of additional actin-binding proteins, this web undergoes continual
assembly at the leading edge and disassembly further back, pushing the
lamellipodium forward (Figure 17–35).
in an electron microscope. Actin filaments
form a dense network, with the fast-growing
plus ends of the filaments terminating at the
leading margin of the lamellipodium (top
of figure; Movie 17.8). (B) Drawing showing
how the nucleation of new actin filaments
(pink ) is mediated by ARP complexes (light
green) attached to the sides of preexisting
actin filaments. The resulting branching
structure pushes the plasma membrane
forward. The plus ends of the actin filaments
become protected from depolymerizing
by capping proteins (dark green), while the
minus ends of actin filaments nearer the
center of the cell continually disassemble
with the help of depolymerizing proteins
(not shown). Because of this directional
growth and disassembly, individual actin
monomers move through this branched
web in a rearward direction, while the web
of actin as a whole undergoes a continual
forward movement. This actin network is
drawn to a different scale than the network
shown in (A). Some pathogenic bacteria
polymerize tails of actin filaments to move
inside the cells they invade (Movie 17.9). (A,
courtesy of Tatyana Svitkina and Gary Borisy.)
0.5 µm
(B)
capping
protein on
plus end
ARP
complex
depolymerizing
actin filaments
The other kind of cell protrusion, the filopodium, depends on formin,
a nucleating protein that attaches to the growing plus ends of actin
filaments and promotes the addition of new monomers to form straight, unbranched
filaments. Formins are also used elsewhere to assemble
unbranched filaments, such as in the contractile ring that pinches a dividing
animal cell in two.
Extracellular Signals Can Alter the Arrangement of
Actin Filaments
Actin-binding proteins control the location, organization, and behavior
of actin filaments. The activities of these proteins are, in turn, controlled
by extracellular signal molecules, allowing the cell to rearrange its actin
cytoskeleton in response to its environment. These extracellular signals
act through a variety of cell-surface receptor proteins, which activate various
intracellular signaling pathways. Many of these pathways converge
on a group of closely related monomeric GTPases that are part of the Rho
protein family. As discussed in Chapter 16, monomeric GTPases behave
as molecular switches that control intracellular processes by cycling
between an active GTP-bound state and an inactive GDP-bound state
(see Figure 16−11B).
In the case of the actin cytoskeleton, different Rho family members alter
the organization of actin filaments in different ways. For example, one