The subunits of actin filaments and microtubules are asymmetrical and bind to one another head-to-tail so that they all point in one direction. This subunit polarity gives the filaments structural polarity along their length, and makes the two ends of each polymer behave differently. In addition, actin and tubulin subunits are both enzymes that catalyze the hydrolysis of a nucleoside triphosphate— ATP and GTP, respectively. As we discuss later, the energy derived from nucleotide hydrolysis enables the filaments to remodel rapidly. By controlling when and where actin and microtubules assemble, the cell harnesses the polar and dynamic properties of these filaments to generate force in a specific direction, to move the leading edge of a migrating cell forward, for example, or to pull chromosomes apart during cell division. In contrast, the subunits of intermediate filaments are symmetrical, and thus do not form polarized filaments with two different ends. Intermediate filament subunits also do not catalyze the hydrolysis of nucleotides. Nevertheless, intermediate filaments can be disassembled rapidly when required. In mitosis , for example, kinases phosphorylate the subunits, leading to their dissociation.
Cytoskeletal filaments in living cells are not built by simply stringing subunits together in single file. A thousand tubulin subunits lined up end-to-end, for example, would span the diameter of a small eukaryotic cell, but a filament formed in this way would lack the strength to avoid breakage by ambient thermal energy, unless each subunit in the filament was bound extremely tightly to its two neighbors. Such tight binding would limit the rate at which the filaments could disassemble, making the cytoskeleton a static and less useful structure. To provide both strength and adaptability, microtubules are built of 13 protofilaments—linear strings of subunits joined end-to-end—that associate with one another laterally to form a hollow cylinder. The addition or loss of a subunit at the end of one protofilament makes or breaks a small number of bonds. In contrast, loss of a subunit from the middle of the filament requires breaking many more bonds, while breaking it in two requires breaking bonds in multiple protofilaments all at the same time (Figure 16–5). The greater energy required to break multiple noncovalent bonds simultaneously allows microtubules to resist thermal breakage, while allowing rapid subunit addition and loss at the filament ends. Helical actin filaments are much thinner and therefore require much less energy to break. However, multiple actin filaments are often bundled together inside cells, providing mechanical strength, while allowing dynamic behavior of filament ends.
As with other specific protein–protein interactions, many hydrophobic interactions and noncovalent bonds hold the subunits in a cytoskeletal filament together (see Figure 3–4). The locations and types of subunit–subunit contacts differ for the different filaments. Intermediate filaments, for example, assemble by forming strong lateral contacts between α-helical coiled-coils, which extend over most of the length of each elongated fibrous subunit. Because the individual subunits are staggered in the filament, intermediate filaments form strong, ropelike structures that tolerate stretching and bending to a greater extent than do either actin filaments or microtubules (Figure 16–6).
The cell regulates the length and stability of its cytoskeletal filaments, as well as their number and geometry. It does so largely by regulating their attachments to one another and to other components of the cell, so that the filaments can form a wide variety of higher-order structures. Direct covalent modification of the filament subunits regulates some filament properties, but most of the regulation is performed by hundreds of accessory proteins that determine the spatial
SINGLE PROTOFILAMENT: THERMALLY UNSTABLE