Essential Cell Biology 5th edition

594 CHAPTER 17 Cytoskeleton
actin with
bound ADP
ADP
ATP
actin with
bound ATP
minus
end
actin filament
3 2 1
3 2
plus
end
1
actin
monomer
minus end
plus end
3 2 1
(A)
P
(B)
TREADMILLING
Figure 17–31 Actin filaments can undergo
treadmilling. (A) Actin monomers in the
cytosol carry ATP, which is hydrolyzed to
ADP soon after assembly into a growing
filament. The ADP molecules remain
trapped within the actin filament, unable to
exchange with ATP until the actin monomer
that carries them dissociates from the
filament. When ATP-actin (dark red ) adds
to the plus end of an actin filament at the
same rate that ADP-actin (light red ) is lost
from the minus end, treadmilling occurs.
(B) When the rates of addition and loss are
equal, the filament stays the same length—
although individual actin monomers (three
of which are numbered here) move through
the filament from the plus to the minus end.
QUESTION 17–5
The formation of actin filaments in
the cytosol is controlled by actinbinding
proteins. Some actin-binding
proteins significantly increase the
rate at which the formation of an
actin filament is initiated. Suggest
a mechanism by which they might
do this.
of GTP to GDP in a microtubule, hydrolysis of ATP to ADP in an actin filament
reduces the strength of binding between the monomers, thereby
decreasing the stability of the polymer. Thus in both cases, nucleotide
hydrolysis ECB5 promotes E17.30/17.31 depolymerization, helping the cell to disassemble its
microtubules and actin filaments after they have formed.
If the concentration of free actin monomers is very high, an actin filament
will grow rapidly, adding monomers at both ends. At intermediate
concentrations of free actin, however, something interesting takes place.
Actin monomers add to the plus end at a rate faster than the bound ATP
can be hydrolyzed, so the plus end grows. At the minus end, by contrast,
ATP is hydrolyzed faster than new monomers can be added; because
ADP-actin destabilizes the structure, the filament loses subunits from
its minus end at the same time as it adds them to the plus end (Figure
17–31A). Individual monomers thus move through the filament from the
plus to the minus end, a behavior called treadmilling. When the rates of
addition and loss are equal, the filament remains the same size (Figure
17–31B).
Actin filament function can be perturbed experimentally by certain toxins
produced by fungi or marine sponges. Some, such as cytochalasin
and latrunculin, prevent actin polymerization; others, such as phalloidin,
stabilize actin filaments against depolymerization (Table 17−2). Addition
of these toxins to cells or tissues, even in low concentrations, instantaneously
freezes cell movements such as cell locomotion. Thus, as with
microtubules, many of the functions of actin filaments depend on the
ability of the filament to assemble and disassemble, the rates of which
depend on the dynamic equilibrium between the actin filaments, the pool
of actin monomers, and various actin-binding proteins.
Many Proteins Bind to Actin and Modify Its Properties
About 5% of the total protein in a typical animal cell is actin; about half
of this actin is assembled into filaments, and the other half remains as
actin monomers in the cytosol. With such a high concentration of actin
monomers—much higher than the concentration required for purified
actin monomers to polymerize spontaneously in a test tube—what, then,
TABLE 17–2 DRUGS THAT AFFECT FILAMENTS
Actin-specific Drugs
Phalloidin
Cytochalasin
Latrunculin
Action
Binds and stabilizes filaments
Caps filament plus ends, preventing polymerization there
Binds actin monomers and prevents their polymerization