Essential Cell Biology 5th edition

Actin Filaments
599
(A)
UNSTIMULATED CELLS
(B)
Rho ACTIVATION
Figure 17–36 Activation of Rho-family
GTPases can have a dramatic effect on
the organization of actin filaments in
fibroblasts. In these micrographs, actin is
stained with fluorescently labeled phalloidin
(see Table 17−2, p. 594). (A) Unstimulated
fibroblasts have actin filaments primarily in
the cortex. (B) Microinjection of an activated
form of Rho promotes the rapid assembly
of bundles of long, unbranched actin
filaments; because myosin is associated
with these bundles, they are contractile.
(C) Microinjection of an activated form
of Rac, a GTP-binding protein similar to
Rho, causes the formation of an enormous
lamellipodium that extends from the entire
circumference of the cell. (D) Microinjection
of an activated form of Cdc42, another Rho
family member, stimulates the protrusion
of a forest of filopodia at the cell periphery.
(Courtesy of Catherine Nobes.)
(C)
Rac ACTIVATION
(D)
Cdc42 ACTIVATION
20 µm
Rho family member triggers the bundling of actin filaments and activation
of the formin proteins that promote the formation of filopodia. Another
Rho GTPase might stimulate ARP complexes at the cell’s leading edge,
promoting the formation of lamellipodia and membrane ruffling. Finally,
activation of the founding member of the Rho family drives the bundling
of actin filaments with myosin motor proteins and the clustering of cellsurface
integrins, actions that promote cell crawling (see Figure 17−33).
ECB5 e17.37/17.36
Examples of these dramatic, Rho-driven cytoskeletal rearrangements are
shown in Figure 17–36.
Actin Associates with Myosin to Form Contractile
Structures
Perhaps the most familiar of all the actin-binding proteins is myosin.
Myosins belong to a family of motor proteins that bind to and hydrolyze
ATP, which provides the energy for their movement along actin filaments
toward the plus end. Myosin, like actin, was first discovered in skeletal
muscle, and much of what we know about the interaction of these two
proteins was learned from studies of muscle. There are numerous types of
myosins in cells, of which the myosin-I and myosin-II subfamilies are the
most abundant.
Myosin-I molecules, which are present in all cell types, have a head
domain and a tail (Figure 17–37A). The head domain binds to an actin
filament and has the ATP-hydrolyzing motor activity that enables it to
move along the filament in a repetitive cycle of binding, detachment,
and rebinding (Movie 17.10). The tail varies among the different types
of myosin-I and determines what type of cargo the myosin will carry.
For example, the tail may bind to a particular type of vesicle and propel
it through the cell along actin filament tracks (Figure 17–37B), or it may
bind to the plasma membrane and pull it into a different shape (Figure
17–36C).
Myosin-II is structurally and mechanistically more complex than myosin-I.
Muscle cells make use of a specialized form of myosin-II to drive
muscle contraction, as we discuss next.
QUESTION 17–7
At the leading edge of a crawling
cell, the plus ends of actin filaments
are located close to the plasma
membrane, and actin monomers
are added at these ends, pushing
the membrane outward to form
lamellipodia or filopodia. What do
you suppose holds the filaments at
their other ends to prevent them
from just being pushed into the
cell’s interior?