Difference between revisions of "Cytoskeleton" - New World Encyclopedia
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| − | [[Image:FluorescentCells.jpg|thumb|right|300px|The eukaryotic cytoskeleton. Actin filaments are shown in red, microtubules in green, and the nuclei are in blue.]]The '''cytoskeleton''' (CSK) is a complex network of protein filaments that extends | + | [[Image:FluorescentCells.jpg|thumb|right|300px|The eukaryotic cytoskeleton. Actin filaments are shown in red, microtubules in green, and the nuclei are in blue.]]The '''cytoskeleton''' (CSK) is a complex network of [[protein]] filaments that extends throughout the [[cytoplasm]] of [[cell (biology)|cells]] acting as a cellular "[[scaffolding]]" or "[[skeleton]]." This internal framework of protein filaments is a dynamic structure that gives cells their various shapes, provides a basis for coordinated and directed movement of cells (using structures such as [[flagellum|flagella]], [[cilium|cilia]] and [[lamellipodia]]), plays an important role in intracellular movement and integration of [[organelle]]s and other sub-cellular structures in the cytoplasm, often protects the cell, and is involved in cell division and [[chromosome]] organization and movement (Alberts et al. 1989). |
| − | + | There are three main types of cytoskeletal filaments: actin filaments, microtubules, and intermediate filaments. In animal cells, the cytoskeleton often is organized from a region near the nucleus where is located the cell's pair of [[centriole]]s (Alberts et al. 1989). | |
The cytoskeleton once was once thought to be unique to [[eukaryote|eukaryotic]] cells, but recent research has identified cytoskeletal structures in [[bacteria]], with [[homology (biology)|homologs]] to all three of the major types of cytoskeletal proteins (actin, tubulin, and intermediate fiber proteins) (Shih and Rothfield 2006). | The cytoskeleton once was once thought to be unique to [[eukaryote|eukaryotic]] cells, but recent research has identified cytoskeletal structures in [[bacteria]], with [[homology (biology)|homologs]] to all three of the major types of cytoskeletal proteins (actin, tubulin, and intermediate fiber proteins) (Shih and Rothfield 2006). | ||
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==The eukaryotic cytoskeleton== | ==The eukaryotic cytoskeleton== | ||
[[Image:MEF_microfilaments.jpg|thumb|right|200px|Actin cytoskeleton of [[mus_musculus|mouse]] [[embryo]] [[fibroblast]]s, stained with [[phalloidin]]]] | [[Image:MEF_microfilaments.jpg|thumb|right|200px|Actin cytoskeleton of [[mus_musculus|mouse]] [[embryo]] [[fibroblast]]s, stained with [[phalloidin]]]] | ||
| − | [[Eukaryotic]] cells contain three main kinds of cytoskeletal filaments, which are microfilaments, intermediate filaments, and microtubules. The cytoskeleton provides the cell's cytoplasm with structure and shape. | + | [[Eukaryotic]] cells contain three main kinds of cytoskeletal filaments, which are microfilaments or actin filaments, intermediate filaments, and microtubules. The cytoskeleton provides the cell's [[cytoplasm]] with structure and shape. |
===Actin filaments / Microfilaments=== | ===Actin filaments / Microfilaments=== | ||
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Around 7 nm or 8 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 [[pseudopod]]ia and [[microvillus|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 [[signal transduction|transduction]]. They are also important for [[cytokinesis]] (specifically, formation of the [[cleavage furrow]]) and, along with [[myosin]], [[Skeletal muscle|muscular contraction]]. [[Actin]]/[[Myosin]] interactions also help produce [[cytoplasmic streaming]] in most cells. | Around 7 nm or 8 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 [[pseudopod]]ia and [[microvillus|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 [[signal transduction|transduction]]. They are also important for [[cytokinesis]] (specifically, formation of the [[cleavage furrow]]) and, along with [[myosin]], [[Skeletal muscle|muscular contraction]]. [[Actin]]/[[Myosin]] interactions also help produce [[cytoplasmic streaming]] in most cells. | ||
===Intermediate filaments=== | ===Intermediate filaments=== | ||
[[Image:KeratinF9.png|thumb|right|200px|Microscopy of keratin filaments inside cells.]] | [[Image:KeratinF9.png|thumb|right|200px|Microscopy of keratin filaments inside cells.]] | ||
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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 [[sarcomere]]s. They also participate in some cell-cell and cell-matrix junctions. | 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 [[sarcomere]]s. They also participate in some cell-cell and cell-matrix junctions. | ||
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===Microtubules=== | ===Microtubules=== | ||
[[Image:Btub.jpg|thumb|right|200px|Microtubules in a gel fixated cell.]] | [[Image:Btub.jpg|thumb|right|200px|Microtubules in a gel fixated cell.]] | ||
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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 [[Guanosine triphosphate|GTP]] for polymerization. They are commonly organized by the [[centrosome]]. | 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 [[Guanosine triphosphate|GTP]] for polymerization. They are commonly organized by the [[centrosome]]. | ||
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==The prokaryotic cytoskeleton== | ==The prokaryotic cytoskeleton== | ||
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The cytoskeleton was previously thought to be a feature only of [[eukaryote|eukaryotic]] cells, but [[homology (biology)|homologues]] to all the major proteins of the eukaryotic cytoskeleton have recently been found in [[prokaryotes]].<ref name=Shih>{{cite journal |author=Shih YL, Rothfield L |title=The bacterial cytoskeleton |journal=Microbiol. Mol. Biol. Rev. |volume=70 |issue=3 |pages=729–54 |year=2006 |pmid=16959967 |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=16959967 |doi=10.1128/MMBR.00017-06}}</ref> Although the evolutionary relationships are so distant that they are not obvious from protein sequence comparisons alone, the similarity of their three-dimensional [[protein structure|structures]] and similar functions in maintaining cell shape and polarity provides strong evidence that the eukaryotic and prokaryotic cytoskeletons are truly homologous.<ref>{{cite journal |author=Michie KA, Löwe J |title=Dynamic filaments of the bacterial cytoskeleton |journal=Annu. Rev. Biochem. |volume=75 |issue= |pages=467–92 |year=2006 |pmid=16756499 |url=http://www2.mrc-lmb.cam.ac.uk/SS/Lowe_J/group/PDF/annrev2006.pdf |doi=10.1146/annurev.biochem.75.103004.142452}}</ref> | The cytoskeleton was previously thought to be a feature only of [[eukaryote|eukaryotic]] cells, but [[homology (biology)|homologues]] to all the major proteins of the eukaryotic cytoskeleton have recently been found in [[prokaryotes]].<ref name=Shih>{{cite journal |author=Shih YL, Rothfield L |title=The bacterial cytoskeleton |journal=Microbiol. Mol. Biol. Rev. |volume=70 |issue=3 |pages=729–54 |year=2006 |pmid=16959967 |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=16959967 |doi=10.1128/MMBR.00017-06}}</ref> Although the evolutionary relationships are so distant that they are not obvious from protein sequence comparisons alone, the similarity of their three-dimensional [[protein structure|structures]] and similar functions in maintaining cell shape and polarity provides strong evidence that the eukaryotic and prokaryotic cytoskeletons are truly homologous.<ref>{{cite journal |author=Michie KA, Löwe J |title=Dynamic filaments of the bacterial cytoskeleton |journal=Annu. Rev. Biochem. |volume=75 |issue= |pages=467–92 |year=2006 |pmid=16756499 |url=http://www2.mrc-lmb.cam.ac.uk/SS/Lowe_J/group/PDF/annrev2006.pdf |doi=10.1146/annurev.biochem.75.103004.142452}}</ref> | ||
Revision as of 13:02, 9 July 2008
The cytoskeleton (CSK) is a complex network of protein filaments that extends throughout the cytoplasm of cells acting as a cellular "scaffolding" or "skeleton." This internal framework of protein filaments is a dynamic structure that gives cells their various shapes, provides a basis for coordinated and directed movement of cells (using structures such as flagella, cilia and lamellipodia), plays an important role in intracellular movement and integration of organelles and other sub-cellular structures in the cytoplasm, often protects the cell, and is involved in cell division and chromosome organization and movement (Alberts et al. 1989).
There are three main types of cytoskeletal filaments: actin filaments, microtubules, and intermediate filaments. In animal cells, the cytoskeleton often is organized from a region near the nucleus where is located the cell's pair of centrioles (Alberts et al. 1989).
The cytoskeleton once was once thought to be unique to eukaryotic cells, but recent research has identified cytoskeletal structures in bacteria, with homologs to all three of the major types of cytoskeletal proteins (actin, tubulin, and intermediate fiber proteins) (Shih and Rothfield 2006).
The eukaryotic cytoskeleton
Eukaryotic cells contain three main kinds of cytoskeletal filaments, which are microfilaments or actin filaments, intermediate filaments, and microtubules. The cytoskeleton provides the cell's cytoplasm with structure and shape.
Actin filaments / Microfilaments
Around 7 nm or 8 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) [1] | Structure | Subunit examples[1] |
|---|---|---|---|
| Microfilaments | 8-10 | double helix | actin |
| Intermediate filaments | 8-10 | two parallel helices/dimers, forming tetramers |
|
| 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[2].
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.[3] 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.[4]
- "In recent years it has been shown that bacteria contain a number of cytoskeletal structures. The bacterial cytoplasmic elements include homologs of the three major types of eukaryotic cytoskeletal proteins (actin, tubulin, and intermediate filament proteins) and a fourth group, the MinD-ParA group, that appears to be unique to bacteria. The cytoskeletal structures play important roles in cell division, cell polarity, cell shape regulation, plasmid partition, and other functions." Shuh and Rothfield 2006
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, but the mechanism by which it does this is currently unclear.
ReferencesISBN links support NWE through referral fees
- ↑ 1.0 1.1 Unless else specified in boxes, then ref is:Walter F., PhD. Boron (2003). Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders, 1300. ISBN 1-4160-2328-3. Page 25
- ↑ Heuser J (2002). Whatever happened to the 'microtrabecular concept'?. Biol Cell 94 (9): 561–96.
- ↑ Shih YL, Rothfield L (2006). The bacterial cytoskeleton. Microbiol. Mol. Biol. Rev. 70 (3): 729–54.
- ↑ Michie KA, Löwe J (2006). Dynamic filaments of the bacterial cytoskeleton. Annu. Rev. Biochem. 75: 467–92.
- Alberts, B., D. Bray, J. Lewis, M. Raff, K. Roberts, and J. D. Watson. Molecular Biology of the Cell, 2nd edition. New York: Garland Publishing, 1989. ISBN 0824036956.
homologues to all the major proteins of the eukaryotic cytoskeleton have recently been found in prokaryotes.[1]
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.)
| Proteins of the Cytoskeleton |
|---|
| Microfilaments - Actins | Myosins | Actin-binding proteins |
| Prokaryotic cytoskeleton - FtsZ | MreB | Crescentin |
| Intermediate filaments - Keratins | Type III IF proteins | Neurofilaments | Lamins | Intermediate filament-associated proteins |
| Microtubules - Tubulins | Dyneins | Kinesins | Microtubule-associated proteins |
| Other - Major Sperm Proteins |
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- ↑ Shih YL, Rothfield L (2006). The bacterial cytoskeleton. Microbiol. Mol. Biol. Rev. 70 (3): 729–54.
