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

914 Chapter 16: The Cytoskeleton
bacterium
ActA
end
actin
Arp2/3
complex
capping
protein
cofilin
(A) (B) (C)
10 µm 10 µm
end
Figure 16–25 The actin-based movement of Listeria monocytogenes. (A) Fluorescence micrograph of an infected cell
that has been stained to reveal bacteria in red and actin filaments in green. Note the cometlike tail of actin filaments behind
each moving bacterium. Regions of overlap between red and green fluorescence appear yellow. (B) Listeria motility can be
reconstituted in a test tube with ATP and just four purified proteins: actin, Arp 2/3 complex, capping protein, and cofilin. This
micrograph shows the dense actin tails behind bacteria (black). (C) The ActA protein on the bacterial surface activates the
Arp 2/3 complex to nucleate new filament MBoC6 assembly n16.202/16.25 along the sides of existing filaments. Filaments grow at their plus end
until capped by capping protein. Actin is recycled through the action of cofilin, which enhances depolymerization at the minus
ends of the filaments. By this mechanism, polymerization is focused at the rear surface of the bacterium, propelling it forward
(see Movie 23.7). (A, courtesy of Julie Theriot and Tim Mitchison; B, from T.P. Loisel et al., Nature 401:613–616, 1999. With
permission from Macmillan Publishers Ltd.)
cytoplasm at rates of up to 1 μm/sec, leaving behind a long actin “comet tail”
(Figure 16–25; see also Figures 23–28 and 23–29). This motility can be reconstituted
in a test tube by adding the bacteria to a mixture of pure actin, Arp 2/3 complex,
cofilin, and capping protein, illustrating how actin polymerization dynamics
generate movement through spatial regulation of filament assembly and disassembly.
As we shall see, actin-based movement of this sort also underlies membrane
protrusion at the leading edge of motile cells.
Summary
Actin is a highly conserved cytoskeletal protein that is present in high concentrations
in nearly all eukaryotic cells. Nucleation presents a kinetic barrier to actin
polymerization, but once formed, actin filaments undergo dynamic behavior due
to hydrolysis of the bound nucleotide ATP. Actin filaments are polarized and can
undergo treadmilling when a filament assembles at the plus end while simultaneously
depolymerizing at the minus end. In cells, actin filament dynamics are regulated
at every step, and the varied forms and functions of actin depend on a versatile
repertoire of accessory proteins. Approximately half of the actin is kept in a
monomeric form through association with sequestering proteins such as thymosin.
Nucleation factors such as the Arp 2/3 complex and formins promote formation
of branched and parallel filaments, respectively. Interplay between proteins that
bind or cap actin filaments and those that promote filament severing or depolymerization
can slow or accelerate the kinetics of filament assembly and disassembly.
Another class of accessory proteins assembles the filaments into larger ordered structures
by cross-linking them to one another in geometrically defined ways. Connections
between these actin arrays and the plasma membrane of cells give an animal
cell mechanical strength and permit the elaboration of cortical cellular structures
such as lamellipodia, filopodia, and microvilli. By inducing actin filament polymerization
at their surface, intracellular pathogens can hijack the host-cell cytoskeleton
and move around inside the cell.