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

906 Chapter 16: The Cytoskeleton
in vitro, when the monomer concentration is 0.2 μM, filament half-life, a measure
of how long an individual actin monomer spends in a filament as it treadmills, is
approximately 30 minutes. In a non-muscle vertebrate cell, actin half-life in filaments
is only 30 seconds, demonstrating that cellular factors modify the dynamic
behavior of actin filaments. Actin-binding proteins dramatically alter actin filament
dynamics and organization through spatial and temporal control of monomer
availability, filament nucleation, elongation, and depolymerization. In the
following sections, we describe the ways in which these accessory proteins modify
actin function in the cell.
Monomer Availability Controls Actin Filament Assembly
In most non-muscle vertebrate cells, approximately 50% of the actin is in
filaments and 50% is soluble—and yet the soluble monomer concentration is
50–200 μM, well above the critical concentration. Why does so little of the actin
polymerize into filaments? The reason is that the cell contains proteins that bind
to the actin monomers and make polymerization much less favorable (an action
similar to that of the drug latrunculin). A small protein called thymosin is the most
abundant of these proteins. Actin monomers bound to thymosin are in a locked
state, where they cannot associate with either the plus or minus ends of actin filaments
and can neither hydrolyze nor exchange their bound nucleotide.
How do cells recruit actin monomers from this buffered storage pool and use
them for polymerization? The answer depends on another monomer-binding protein
called profilin. Profilin binds to the face of the actin monomer opposite the
ATP-binding cleft, blocking the side of the monomer that would normally associate
with the filament minus end, while leaving exposed the site on the monomer
that binds to the plus end (Figure 16–15). When the profilin–actin complex binds
a free plus end, a conformational change in actin reduces its affinity for profilin
and the profilin falls off, leaving the actin filament one subunit longer. Profilin
competes with thymosin for binding to individual actin monomers. Thus, by regulating
the local activity of profilin, cells can control the movement of actin subunits
from the sequestered thymosin-bound pool onto filament plus ends.
Several mechanisms regulate profilin activity, including profilin phosphorylation
and profilin binding to inositol phospholipids. These mechanisms can define
the sites where profilin acts. For example, profilin is required for filament assembly
at the plasma membrane, where it is recruited by an interaction with acidic
membrane phospholipids. At this location, extracellular signals can activate profilin
to produce local actin polymerization and the extension of actin-rich motile
structures such as filopodia and lamellipodia.
Actin-Nucleating Factors Accelerate Polymerization and Generate
Branched or Straight Filaments
In addition to the availability of active actin subunits, a second prerequisite for
cellular actin polymerization is filament nucleation. Proteins that contain actin
monomer binding motifs linked in tandem mediate the simplest mechanism of
filament nucleation. These actin-nucleating proteins bring several actin subunits
together to form a seed. In most cases, actin nucleation is catalyzed by one of two
different types of factors: the Arp 2/3 complex or the formins. The first of these is
a complex of proteins that includes two actin-related proteins, or ARPs, each of
which is about 45% identical to actin. The Arp 2/3 complex nucleates actin filament
growth from the minus end, allowing rapid elongation at the plus end (Figure
16–16A and B). The complex can attach to the side of another actin filament
while remaining bound to the minus end of the filament that it has nucleated,
thereby building individual filaments into a treelike web (Figure 16–16C and D).
Formins are dimeric proteins that nucleate the growth of straight, unbranched
filaments that can be cross-linked by other proteins to form parallel bundles. Each
formin subunit has a binding site for monomeric actin, and the formin dimer
appears to nucleate actin filament polymerization by capturing two monomers.