System of microscopic filaments or fibres, present in the cytoplasm of eukaryotic cells (see eukaryote), that organizes other cell components, maintains cell shape, and is responsible for cell locomotion and for movement of the organelles within it. Three major types of filaments make up the cytoskeleton: actin filaments, microtubules, and intermediate filaments. Actin filaments occur as constantly changing bundles of parallel fibres; they help determine cell shape, help the cell adhere to surfaces, help the cell move, and assist in cell division during mitosis. Intermediate filaments are very stable structures that form the cell's true skeleton; they anchor the nucleus within the cell and give the cell its elastic properties.

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The cytoskeleton (also CSK) is a cellular "scaffolding" or "skeleton" contained within the cytoplasm. The cytoskeleton is present in all cells; it was once thought this structure was unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton. It is a dynamic structure that maintains cell shape, often protects the cell, enables cellular motion (using structures such as flagella, cilia and lamellipodia), and plays important roles in both intracellular transport (the movement of vesicles and organelles, for example) and cellular division.

The eukaryotic cytoskeleton

Eukaryotic cells contain three main kinds of cytoskeletal filaments, which are microfilaments, intermediate filaments, and microtubules. The cytoskeleton provides the cell with structure and shape, and by excluding macromolecules from some of the cytosol it adds to the level of macromolecular crowding in this compartment.

Actin filaments / Microfilaments

Around 7 nm in diameter, this filament is composed of two intertwined actin chains. Microfilaments are most concentrated just beneath the cell membrane, and are responsible for resisting tension and maintaining cellular shape, forming cytoplasmatic protuberances (like pseudopodia and microvilli- although these by different mechanisms), and participation in some cell-to-cell or cell-to-matrix junctions. In association with these latter roles, microfilaments are essential to transduction. They are also important for cytokinesis (specifically, formation of the cleavage furrow) and, along with myosin, muscular contraction. Actin/Myosin interactions also help produce cytoplasmic streaming in most cells.

Intermediate filaments

These filaments, 8 to 12 nanometers in diameter, are more stable (strongly bound) than actin filaments, and heterogeneous constituents of the cytoskeleton. Like actin filaments, they function in the maintenance of cell-shape by bearing tension (microtubules, by contrast, resist compression. It may be useful to think of micro- and intermediate filaments as cables, and of microtubules as cellular support beams). Intermediate filaments organize the internal tridimensional structure of the cell, anchoring organelles and serving as structural components of the nuclear lamina and sarcomeres. They also participate in some cell-cell and cell-matrix junctions.

Different intermediate filaments are:

• made of vimentins, being the common structural support of many cells.
• made of keratin, found in skin cells, hair and nails.
• neurofilaments of neural cells.
• made of lamin, giving structural support to the nuclear envelope.

Microtubules

Microtubules are hollow cylinders about 25 nm in diameter (lumen = approximately 15nm in diameter), most commonly comprised of 13 protofilaments which, in turn, are polymers of alpha and beta tubulin. They have a very dynamic behaviour, binding GTP for polymerization. They are commonly organized by the centrosome.

In nine triplet sets (star-shaped), they form the centrioles, and in nine doublets oriented about two additional microtubules (wheel-shaped) they form cilia and flagella. The latter formation is commonly referred to as a "9+2" arrangement, wherein each doublet is connected to another by the protein dynein. As both flagella and cilia are structural components of the cell, and are maintained by microtubules, they can be considered part of the cytoskeleton.

They play key roles in:

• intracellular transport (associated with dyneins and kinesins, they transport organelles like mitochondria or vesicles).
• the axoneme of cilia and flagella.
• the mitotic spindle.
• synthesis of the cell wall in plants.

Comparison

Cytoskeleton type	Diameter (nm)	Structure	Subunit examples
Microfilaments	8-10	double helix	actin
Intermediate filaments	8-10	two parallel helices/dimers, forming tetramers	vimentin (mesenchyme) glial fibrillary acidic protein (glial cells) neurofilament proteins (neuronal processes) keratins (epithelial cells) nuclear lamins
Microtubules	25	protofilaments, in turn consisting of tubulin subunits	α- and β-tubulin

Microtrabeculae - a further structural network?

A fourth eukaryotic cytoskeletal element, microtrabeculae, was proposed by Keith Porter based on images obtained from high-voltage electron microscopy of whole cells in the 1970s. The images showed short, filamentous structures of unknown molecular composition associated with known cytoplasmic structures. Porter proposed that this microtrabecular structure represented a novel filamentous network distinct from microtubules, filamentous actin, or intermediate filaments. It is now generally accepted that microtrabeculae are nothing more that an artefact of certain types of fixation treatment though we have yet to fully understand the complexity of the cell's cytoskeleton.

The prokaryotic cytoskeleton

The cytoskeleton was previously thought to be a feature only of eukaryotic cells, but homologues to all the major proteins of the eukaryotic cytoskeleton have recently been found in prokaryotes. Although the evolutionary relationships are so distant that they are not obvious from protein sequence comparisons alone, the similarity of their three-dimensional structures and similar functions in maintaining cell shape and polarity provides strong evidence that the eukaryotic and prokaryotic cytoskeletons are truly homologous. However, some structures in the bacterial cytoskeleton may have yet to be identified.

FtsZ

FtsZ was the first protein of the prokaryotic cytoskeleton to be identified. Like tubulin, FtsZ forms filaments in the presence of GTP, but these filaments do not group into tubules. During cell division, FtsZ is the first protein to move to the division site, and is essential for recruiting other proteins that synthesize the new cell wall between the dividing cells.

MreB and ParM

Prokaryotic actin-like proteins, such as MreB, are involved in the maintenance of cell shape. All non-spherical bacteria have genes encoding actin-like proteins, and these proteins form a helical network beneath the cell membrane that guides the proteins involved in cell wall biosynthesis.

Some plasmids encode a partitioning system that involves an actin-like protein ParM. Filaments of ParM exhibit dynamic instability, and may partition plasmid DNA into the dividing daughter cells by a mechanism analogous to that used by microtubules during eukaryotic mitosis.

Crescentin

The bacterium Caulobacter crescentus contains a third protein, crescentin, that is related to the intermediate filaments of eukaryotic cells. Crescentin is also involved in maintaining cell shape, such as helical and vibrioid forms of bacteria, but the mechanism by which it does this is currently unclear.

References

Further reading

• Linda A. Amos and W. Gradshaw Amos, Molecules of the Cytoskeletion, Guilford, ISBN 0-89862-404-5, LoC QP552.C96A46 1991

External links

• Cytoskeleton, Cell Motility and Motors - The Virtual Library of Biochemistry and Cell Biology
• Cytoskeleton database, clinical trials, recent literature, lab registry ...
• Animation of leukocyte adhesion (Animation with some images of actin and microtubule assembly and dynamics.)