Essential Cell Biology 5th edition

Actin Filaments
593
(A) (B) (C) (D) (E)
10 µm
Actin Filaments Are Thin and Flexible
Actin filaments appear in electron micrographs as threads about 7 nm
in diameter. Each filament is a twisted chain ECB5 of E17.28/17.29
identical globular actin
monomers, all of which “point” in the same direction along the axis of
the chain. Like a microtubule, therefore, an actin filament has a structural
polarity, with a plus end and a minus end (Figure 17–30).
Actin filaments are thinner, more flexible, and usually shorter than microtubules.
There are, however, many more of them, so that the total length
of all of the actin filaments in a cell is generally many times greater than
the total length of all of the microtubules. Unlike intermediate filaments
and microtubules, actin filaments rarely occur in isolation in the cell; they
are generally found in cross-linked bundles and networks, which are
much stronger than the individual filaments.
Figure 17–29 Actin filaments allow animal
cells to adopt a variety of shapes and
perform a variety of functions. The actin
filaments in four different structures are
shown here in red: (A) microvilli;
(B) contractile bundles in the cytoplasm;
(C) fingerlike filopodia protruding from the
leading edge of a moving cell; and
(D) contractile ring during cell division.
(E) Micrograph of a cell in which contractile
bundles of actin, like those in (B), are
stained with fluorescently labeled phalloidin,
a molecule that binds specifically to actin
filaments (see Table 17–2, p. 594).
(E, courtesy of Nikon ® MicroscopyU.)
Actin and Tubulin Polymerize by Similar Mechanisms
Like microtubles, actin filaments can grow by the addition of monomers
at either end but their rate of growth is faster at the plus end than at the
minus end. A naked actin filament, like a microtubule without associated
proteins, is inherently unstable, and it can disassemble from both ends. In
living cells, free actin monomers carry a tightly bound nucleoside triphosphate,
in this case ATP. The actin monomer hydrolyzes its bound ATP to
ADP soon after it is incorporated into the filament. As with the hydrolysis
(A)
actin monomer
actin filament
plus end
37 nm
minus end
10 nm
(B) (C) (D)
Figure 17–30 Actin filaments are thin,
flexible protein threads. (A) The subunit
of each actin filament is an actin monomer.
A cleft in the monomer provides a binding
site for ATP or ADP. (B) Arrangement of
actin monomers in an actin filament. Each
filament may be thought of as a twostranded
helix with a twist repeating every
37 nm. Multiple, lateral interactions between
the two strands prevent the strands from
separating. (C) Close-up view showing
the extensive interactions between the
two strands of the actin filament.
(D) Electron micrograph of a negatively
stained actin filament. (D, courtesy of
Roger Craig.)