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

898 Chapter 16: The Cytoskeleton
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Summary
The cytoplasm of eukaryotic cells is spatially organized by a network of protein filaments
known as the cytoskeleton. This network contains three principal types of
filaments: actin filaments, microtubules, and intermediate filaments. All three types
of filaments form as helical assemblies of subunits that self-associate using a combination
of end-to-end and side-to-side MBoC6 m16.28/16.10 protein contacts. Differences in the structure
of the subunits and the manner of their self-assembly give the filaments different
mechanical properties. Subunit assembly and disassembly constantly remodel all
three types of cytoskeletal filaments. Actin and tubulin (the subunits of actin filaments
and microtubules, respectively) bind and hydrolyze nucleoside triphosphates
(ATP and GTP, respectively), and assemble head-to-tail to generate polarized filaments
capable of generating force. In living cells, accessory proteins modulate the
dynamics and organization of cytoskeletal filaments, resulting in complex events
such as cell division or migration, and generating elaborate cellular architecture
to form polarized tissues such as epithelia. Bacterial cells also contain homologs of
actin, tubulin, and intermediate filaments that form dynamic structures that help
control cell shape and division.
Figure 16–10 Caulobacter and
crescentin. The sickle-shaped bacterium
Caulobacter crescentus expresses a
protein, crescentin, with a series of coiledcoil
domains similar in size and organization
to the domains of eukaryotic intermediate
filaments. (A) The crescentin protein forms
a fiber (labeled in red) that runs down
the inner side of the curving bacterial cell
wall. (B) When the gene is disrupted, the
bacteria grow as straight rods (bottom).
(From N. Ausmees, J.R. Kuhn and
C. Jacobs-Wagner, Cell 115:705–713,
2003. With permission from Elsevier.)
ACTIN AND ACTIN-BINDING PROTEINS
The actin cytoskeleton performs a wide range of functions in diverse cell types.
Each actin subunit, sometimes called globular or G-actin, is a 375-amino-acid
polypeptide carrying a tightly associated molecule of ATP or ADP (Figure
16–11A). Actin is extraordinarily well conserved among eukaryotes. The amino
acid sequences of actins from different eukaryotic species are usually about 90%
identical. Small variations in actin amino acid sequence can cause significant
functional differences: In vertebrates, for example, there are three isoforms of
actin, termed α, β, and γ, that differ slightly in their amino acid sequences and
have distinct functions. α-Actin is expressed only in muscle cells, while β- and
γ-actins are found together in almost all non-muscle cells.
Actin Subunits Assemble Head-to-Tail to Create Flexible,
Polar Filaments
Actin subunits assemble head-to-tail to form a tight, right-handed helix, forming
a structure about 8 nm wide called filamentous or F-actin (Figure 16–11B and C).
Because the asymmetrical actin subunits of a filament all point in the same direction,
filaments are polar and have structurally different ends: a slower-growing
minus end and a faster-growing plus end. The minus end is also referred to as the
“pointed end” and the plus end as the “barbed end,” because of the “arrowhead”
appearance of the complex formed between actin filaments and the motor protein
myosin (Figure 16–12). Within the filament, the subunits are positioned with
their nucleotide-binding cleft directed toward the minus end.
Individual actin filaments are quite flexible. The stiffness of a filament can be
characterized by its persistence length, the minimum filament length at which random
thermal fluctuations are likely to cause it to bend. The persistence length
of an actin filament is only a few tens of micrometers. In a living cell, however,