Structure of the Cytoskeleton

An actin filament is a polymer made of actin monomers and has a diameter of about 7 nm. Actin filaments, which are also known as microfilaments or F-actin, play a major role in muscle contraction, cell movement, and cell shape. Actin, also called G-actin, is a protein found in all eukaryotic cells. Its monomeric form is the subunit of actin filaments. Actin is a globular protein that, in the actin filament, is arranged in a helix that completes a turn every 13 subunits. Actin filaments, along with filaments of the motor protein myosin, are the main contractile components of muscle cells. A motor protein is a protein that uses the energy released by the hydrolysis of adenosine triphosphate (ATP), which is the biological unit of energy, to move along a cytoskeletal filament. Muscle-like assemblies of actin and myosin are also observed in nonmuscle cells. These include the contractile ring that separates cells at the end of mitosis, stress fibers in fibroblasts—cells that makes the extracellular matrix, including collagen—and adhesion belts in epithelial cells, which are cells that cover the surface of the body as well as the outside and inside of many internal organs. Actin and myosin also produce movement of the cellular contents in plant cells, a phenomenon known as cytoplasmic streaming.

Actin filaments also serve structural purposes in eukaryotic cells. In animal cells, the cell cortex is a network of actin filaments beneath the plasma membrane. The filaments of the cortex are cross-linked by a variety of actin-binding proteins into a stiff mesh that gives the cell its shape. More specialized structures, such as the microvilli of the cells lining the intestine and the hair cells of the inner ear, are formed by bundles of parallel actin filaments, which are also cross-linked for stability.

Actin is constructed of globular proteins that join together to create long strands, called filaments. Actin filaments are considered polar filaments because subunits are more easily added to one end (the "plus" end) than the other (the "minus" end). Each actin subunit faces the same direction, with different filament ends termed either barbed (with a hook) or pointed (spear-shaped). It is the ability of the subunits to join together or break apart in preferred directions that allows cellular structures to form in response to specific cell signals.

A microtubule is a long tubulin filament that plays a role in cell structure, organization, mitosis, and movement. Like actin filaments, microtubules are polar and often originate from a microtubule organizing center (MTOC), which is the general term for a place in the cell from which microtubules grow. The fast-growing end of the microtubule extends away from the MTOC. In contrast to thin actin filaments, which create dense networks or bundles, microtubules are not cross-linked with each other. Instead, they perform their functions individually or in an array. The motor proteins kinesin and dynein both move along microtubules and are used in cellular transport. Microtubules and actin filaments can work together, for example, when carrying out mitosis.

An intermediate filament is a ropelike cytoskeletal filament about 8–12 nm in diameter that functions primarily as a structural protein in eukaryotic cells. Unlike actin filaments and microtubules, intermediate filaments may be composed of a number of different proteins and are arranged in a less regular fashion. Types I, II, III, and IV intermediate filaments are only found in animal cells, while Type V are found in nearly all eukaryotes. The intermediate filaments are more stable than either actin filaments or microtubules, and they play several different structural roles but are not involved in cell movement.

Prokaryotes do not contain cytoskeletal proteins as such. They do contain homologs, or similar structures, to structures formed from actin, microtubules, or intermediate filaments. Instead of actin, prokaryotes have MreB, which is a bacterial protein that determines the shape of nonspherical bacteria. ParM is also similar to actin, but it behaves more like tubulin. It joins together in two directions. In place of microtubules, prokaryotes have FtsZ, which is a filament-like ring at the center of the cell that tightens during cell division. FtsZ is a protein that is essential for cell division. The homolog CreS, or crescentin, is the prokaryotic version of intermediate filaments, and it forms a long filament that goes from pole to pole in bacterium.

Intermediate filaments are the most stable filaments of the cytoskeleton. Actin filaments and microtubules are dynamic and constantly polymerize and depolymerize; that is, they join together to form larger molecules (polymerization) or revert back to monomers. The filaments can form transient structures suited to changing conditions or that facilitate cell movement. Intermediate filaments, in contrast, are tough, durable, and insoluble fibers that support areas of cells that are subject to various stresses that would break or damage neurons or epithelial cells.

There are six types of intermediate filaments, five of which are present only in animal cells and a sixth that is observed in nearly all eukaryotic cells. Intermediate filament diameters range from 8 to 12 nm. Each type of intermediate filament is formed from different protein monomers. Types I and II intermediate filaments are the keratins, which are found mainly in epithelial cells. There are many members of the keratin subfamilies, and the mixtures of different keratins present vary by the location of the tissue. Type III intermediate filaments include vimentin, found in fibroblasts and white blood cells; desmin, found in muscle cells; and proteins specific to glial and peripheral nerve cells. Type IV intermediate filaments are neurofilament proteins, where they support the long axons and dendrites of mature nerve cells, and alpha-internexin, which is present in developing nerve cells. The one Type VI protein, nestin, is also associated with the developing central nervous system and is found in stem cells there. Type V intermediate filaments, the lamins, are observed in most eukaryotic cells, where they form the nuclear lamina, part of the nuclear envelope, which is a barrier that separates the nucleus of a cell from the cytoplasm. The structure of lamins differs from the other intermediate filaments. Rather than a ropelike structure, it is a square mesh of filaments that surrounds the inner nuclear membrane.