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

ACTIN AND ACTIN-BINDING PROTEINS
901
state. Addition of a subunit to either end of a filament of n subunits results in a filament
of n + 1 subunits. Thus, the free-energy difference, and therefore the equilibrium
constant (and the critical concentration), must be the same for addition
of subunits at either end of the polymer. In this case, the ratio of the rate constants,
k off /k on , must be identical at the two ends, even though the absolute values
of these rate constants are very different at each end (see Panel 16–2).
The cell takes advantage of actin filament dynamics and polarity to do mechanical
work. Filament elongation proceeds spontaneously when the free-energy
change (∆G) for addition of the soluble subunit is less than zero. This is the case
when the concentration of subunits in solution exceeds the critical concentration.
A cell can couple an energetically unfavorable process to this spontaneous
process; thus, the cell can use free energy released during spontaneous filament
polymerization to move an attached load. For example, by orienting the fast-growing
plus ends of actin filaments toward its leading edge, a motile cell can push its
plasma membrane forward, as we discuss later.
ATP Hydrolysis Within Actin Filaments Leads to Treadmilling at
Steady State
Thus far in our discussion of actin filament dynamics, we have ignored the critical
fact that actin can catalyze the hydrolysis of the nucleoside triphosphate ATP. For
free actin subunits, this hydrolysis proceeds very slowly; however, it is accelerated
when the subunits are incorporated into filaments. Shortly after ATP hydrolysis
occurs, the free phosphate group is released from each subunit, but the ADP
remains trapped in the filament structure. Thus, two different types of filament
structures can exist, one with the “T form” of the nucleotide bound (ATP), and one
with the “D form” bound (ADP).
When the nucleotide is hydrolyzed, much of the free energy released by cleavage
of the phosphate–phosphate bond is stored in the polymer. This makes the
free-energy change for dissociation of a subunit from the D-form polymer more
negative than the free-energy change for dissociation of a subunit from the T-form
polymer. Consequently, the ratio of k off /k on for the D-form polymer, which is
numerically equal to its critical concentration [C c (D)], is larger than the corresponding
ratio for the T-form polymer. Thus, C c (D) is greater than C c (T). At certain
concentrations of free subunits, D-form polymers will therefore shrink while
T-form polymers grow.
In living cells, most soluble actin subunits are in the T form, as the free concentration
of ATP is about tenfold higher than that of ADP. However, the longer the
time that subunits have been in the actin filament, the more likely they are to have
hydrolyzed their ATP. Whether the subunit at each end of a filament is in the T or
the D form depends on the rate of this hydrolysis compared with the rate of subunit
addition. If the concentration of actin monomers is greater than the critical
concentration for both the T-form and D-form polymer, then subunits will add to
the polymer at both ends before the nucleotides in the previously added subunits
are hydrolyzed; as a result, the tips of the actin filament will remain in the T form.
On the other hand, if the subunit concentration is less than the critical concentrations
for both the T-form and D-form polymer, then hydrolysis may occur before
the next subunit is added and both ends of the filament will be in the D form and
will shrink. At intermediate concentrations of actin subunits, it is possible for the
rate of subunit addition to be faster than nucleotide hydrolysis at the plus end,
but slower than nucleotide hydrolysis at the minus end. In this case, the plus end
of the filament remains in the T conformation, while the minus end adopts the D
conformation. The filament then undergoes a net addition of subunits at the plus
end, while simultaneously losing subunits from the minus end. This leads to the
remarkable property of filament treadmilling (Figure 16–14; see Panel 16–2).
At a particular intermediate subunit concentration, the filament growth at the
plus end exactly balances the filament shrinkage at the minus end. Under these
conditions, the subunits cycle rapidly between the free and filamentous states,