Cytoskeleton Structure

Structure of the Cytoskeleton

Role of the Cytoskeleton in Animal Cells

The cytoskeleton gives animal cells structure, strength, and the ability to change shape and move.
In animal cells, the cytoskeleton is a network of filaments that gives the cell its shape and forms the support network for cell functions, such as cell division. In addition to giving cells shape and support, the cytoskeleton creates particular structures and projections essential to the function of specialized cell types. The cytoskeleton organizes the interior of the cell and transports the cytosolic contents, which are cytosol fluid and ions, such as potassium and sodium. It carries out cellular actions such as the process of cell division known as mitosis and allows the cell to respond to stimuli by changing shape and/or moving. There are three types of filaments in the cytoskeleton in eukaryotic cells: actin filaments (also called microfilaments), microtubules, and intermediate filaments. These three filaments are part of a group of proteins known as globular proteins, in that they are made up of spherical, globe-like components. Microtubules are composed of numerous tubulin subunits, called tubulin monomers. There are also three types of motor proteins that are part of the eukaryotic cytoskeleton: myosins, kinesins, and dyneins. Prokaryotes, in contrast, do not contain these proteins. Instead, prokaryotes have cytoskeletons made of proteins homologous to actin, tubulin, and intermediate filament proteins, as well as some proteins unlike any that exist in eukaryotes. Tubulin is a group of protein monomers that polymerize into protofilaments, which in turn form microtubules.

The Three Cytoskeletal Filaments

Three types of cytoskeletal filaments include actin filaments, which are helical polymers of globular G-actin monomers; microtubules, which are hollow tubes formed by tubulin monomers; and intermediate filaments, which are ropelike fibers.

Actin Filaments

Actin is the protein that makes up the actin filament, which plays a central role in cell shape, cell movement, and muscle contraction.

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.

Microtubules and Intermediate Filaments

Microtubules are filaments involved in cellular mitosis and movement, whereas intermediate filaments primarily serve as structural proteins in eukaryotic cells.

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.

Type of Cytoskeletal Protein Present
Cell Type Actin Microtubules Intermediate Filaments
Animal Yes Yes Types I, II, III, IV, V
Plant Yes Yes Type V
Fungi Yes Yes Type V
Protist Yes Yes Type V
Prokaryote No, but homologs MreB, ParM No, but homologs FtsZ, TubZ No, but homolog CreS

Actin filaments, microtubules, and intermediate filaments are the three major types of cytoskeletal proteins. Prokaryotic cells do not contain these types of cytoskeletal proteins, but they do possess homologs of these eukaryotic proteins.

Intermediate Filament Structure and Assembly

Intermediate filaments are strong, fibrous proteins that provide structural support in eukaryotic cells.

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.

Intermediate Filament Proteins in Mammals

Intermediate Filament Protein Type Protein Subunit Tissue Distribution
Type I Acidic keratins Epithelial cells
Type II Basic keratins Epithelial cells
Type III Vimentin, desmin, neuronal proteins Fibroblasts, white blood cells, muscle cells, glial cells, peripheral neurons
Type IV Neurofilament proteins Neurons
Type V Lamins Nuclear lamina
Type VI Nestin Neuronal stem cells

There are six types of intermediate filaments in animal cells.

The structures of all the different intermediate filament proteins share an important similarity. While they vary significantly in size, all types of intermediate filaments have a conserved helical sequence that is used in the formation of filaments. Two protein monomers form dimers by coiling these helical sections together, with the carboxyl and amino ends of the polypeptides aligned. Two dimers then associate, in an antiparallel orientation, to form an antiparallel tetramer that has two equivalent ends and is therefore nonpolar. The tetramers associate end to end to form protofilaments, which then aggregate along their length to form filaments. Many filaments twist together to in a ropelike way to form an intermediate filament.

Intermediate Filament Assembly

Intermediate filaments are bundles of smaller fibers.
While intermediate filaments are generally stable, they may be phosphorylated as a means of regulating their assembly and disassembly. Phosphorylation of vimentin filaments causes them to disassemble into smaller aggregates. The nuclear lamina must disassemble every time mitosis takes place. The various lamins that make up the nuclear lamina are phosphorylated during mitosis and dephosphorylated as the cells enter interphase, which is collectively the G1, S, and G2 phases of mitosis, in which a cell grows, replicates its DNA, and grows again, and the lamina re-forms.

In addition to providing mechanical support to membranes within the cell, intermediate filaments can mediate attachments between neighboring cells. They connect epithelial cells together at cell-cell contacts called desmosomes and to the extracellular matrix at hemidesmosomes. These connections make the tissue more resilient, because forces applied to the tissue are spread over a larger area. In another example of cell-cell connections, desmin intermediate filaments are arranged in parallel arrays around the sarcomeres, which are repeating segments of muscle fibers. Desmin intermediate filaments connect to the plasma membrane and to other desmin filaments surrounding neighboring sarcomeres and provide organization and structural integrity to the muscle tissue. Desmin intermediate filaments also encircle the Z-discs, which are the part of the muscle fibers that mark the ends of the sarcomeres.