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
907
free actin
monomer
plus-end growth
actin filament
free actin
monomer
plus-end growth
+
_
profilin
+
_
actin–
thymosin
complex
no binding
no plus-end growth
actin–
profilin
complex
rapid plus-end growth
actin–profilin complex
free actin
monomer
thymosin
profilin
+
actin filament
_
actin–thymosin complex
PROFILIN COMPETES WITH THYMOSIN FOR BINDING TO ACTIN MONOMERS
AND PROMOTES ASSEMBLY
Figure 16–15 Effects of thymosin and profilin on actin polymerization. An actin monomer bound to thymosin is sterically
prevented from binding to and elongating the plus end of an actin filament (left). An actin monomer bound to profilin, on the
other hand, is capable of elongating a filament (right). Thymosin and profilin cannot both bind to a single actin monomer at the
same time. In a cell in which most of the
MBoC6
actin
m16.37/16.15
monomer is bound to thymosin, the activation of a small amount of profilin can
produce rapid filament assembly. As indicated (bottom), profilin binds to actin monomers that are transiently released from
the thymosin-bound monomer pool, shuttles them onto the plus ends of actin filaments, and is then released and recycled for
further rounds of filament elongation.
As the newly nucleated filament grows, the formin dimer remains associated with
the rapidly growing plus end while still allowing the addition of new subunits at
that end (Figure 16–17). This mechanism of filament assembly is clearly different
from that used by the Arp 2/3 complex, which remains stably bound to the
filament minus end, preventing subunit addition or loss at that end. Formin-dependent
actin filament growth is strongly enhanced by the association of actin
monomers with profilin (Figure 16–18).
Like profilin activation, actin filament nucleation by Arp 2/3 complexes and
formins occurs primarily at the plasma membrane, and the highest density of
actin filaments in most cells is at the cell periphery. The layer just beneath the
plasma membrane is called the cell cortex, and the actin filaments in this region
determine the shape and movement of the cell surface, allowing the cell to change
its shape and stiffness rapidly in response to changes in its external environment.
Actin-Filament-Binding Proteins Alter Filament Dynamics
Actin filament behavior is regulated by two major classes of binding proteins:
those that bind along the side of a filament and those that bind to the ends (see
Panel 16–3). Side-binding proteins include tropomyosin, an elongated protein
that binds simultaneously to six or seven adjacent actin subunits along each of the
two grooves of the helical actin filament. In addition to stabilizing and stiffening