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 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.
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 βand 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,
Figure 16–10 Caulobacter and crescentin. The sickle-shaped bacterium Caulobacter crescentus expresses a protein, crescentin, with a series of coiled-coil 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