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

908 Chapter 16: The Cytoskeleton
the filament, the binding of tropomyosin can prevent the actin filament from
interacting with other proteins; this aspect of tropomyosin function is important
in the control of muscle contraction, as we discuss later.
An actin filament that stops growing and is not specifically stabilized in the cell
will depolymerize rapidly, particularly at its plus end, once the actin molecules
N
C
plus end plus end plus end
N
N
C
C
actin
Arp2
Arp3
(A)
activating
factor
other proteins
minus
end
plus
end
+
inactive
Arp2/3 complex
(B)
Arp3
Arp2
active Arp2/3 complex
actin
monomers
nucleated actin filament
100 nm
o
70
(C)
Figure 16–16 Nucleation and actin web formation by the Arp 2/3 complex. (A) The structures of Arp2 and Arp3 compared to the structure
of actin. Although the face of the molecule equivalent to the plus end (top) in both Arp2 and Arp3 is very similar to the plus end of actin itself,
differences on the sides and minus end prevent these actin-related proteins from forming filaments on their own or coassembling into filaments
with actin. (B) A model for actin filament nucleation by the Arp 2/3 complex. In the absence of an activating factor, Arp2 and Arp3 are held by
their accessory proteins in an orientation that prevents them from nucleating a new actin filament. When an activating factor (indicated by the blue
triangle) binds the complex, Arp2 and Arp3 are brought together into a new configuration that resembles the plus end of an actin filament. Actin
subunits can then assemble onto this structure, bypassing the MBoC6 rate-limiting m16.34/16.16
step of filament nucleation. (C) The Arp 2/3 complex nucleates filaments
most efficiently when it is bound to the side of a preexisting actin filament. The result is a filament branch that grows at a 70° angle relative to the
original filament. Repeated rounds of branching nucleation result in a treelike web of actin filaments. (D) Top, electron micrographs of branched actin
filaments formed by mixing purified actin subunits with purified Arp 2/3 complexes. Bottom, reconstructed image of a branch where the crystal
structures of actin (pink) and the Arp 2/3 complex have been fitted to the electron density. The mother filament runs from top to bottom, and the
daughter filament branches off to the right where the Arp 2/3 complex binds to three actin subunits in the mother filament. (D, top, from R.D. Mullins
et al., Proc. Natl Acad. Sci. USA 95:6181–6186, 1998, with permission from National Academy of Sciences; botttom, from N. Volkmann et al.,
Science 293:2456–2459, 2001, with permission from AAAS.)
(D)
10 nm