Cytoskeleton
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/* Intermediate filaments */
[[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''' (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 [[eukaryote]]s, 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 [[flagellum|flagella]], [[cilium|cilia]] and [[lamellipodia]]), and plays important roles in both intracellular transport (the movement of [[vesicle (biology)|vesicle]]s and organelles, for example) and [[Cell division|cellular division]].
==The eukaryotic cytoskeleton==
[[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.
===Actin filaments / Microfilaments===
{{Main article|Microfilament}}
{{See|Actin}}
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 [[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===
[[Image:KeratinF9.png|thumb|right|200px|Microscopy of keratin filaments inside cells.]]
{{main article|intermediate filament}}
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<!-- microfilaments, not microtubules= "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.
Different intermediate filaments are:
* made of [[vimentin]]s, being the common structural support of many cells.
* made of [[keratin]], found in [[skin]] cells, [[hair]] and [[nail (anatomy)|nails]].
* [[neurofilament]]s of neural cells.
* made of [[lamin]], giving structural support to the nuclear envelope.
===Microtubules===
[[Image:Btub.jpg|thumb|right|200px|Microtubules in a gel fixated cell.]]
{{main article|microtubule}}
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]].
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 [[dynein]]s and [[kinesin]]s, they transport [[organelles]] like [[mitochondria]] or [[vesicle (biology)|vesicle]]s).
* the [[axoneme]] of [[cilium|cilia]] and [[flagellum|flagella]].
* the [[mitotic spindle]].
* synthesis of the cell wall in plants.
===Comparison===
{|class=wikitable
|-
! Cytoskeleton type !! Diameter ([[nanometre|nm]]) <ref name=boron25Unless> Unless else specified in boxes, then ref is:{{cite book |author=Walter F., PhD. Boron |title=Medical Physiology: A Cellular And Molecular Approaoch |publisher=Elsevier/Saunders |location= |year=2003 |pages=1300 |isbn=1-4160-2328-3 |oclc= |doi=}} Page 25 </ref> !! Structure !! Subunit examples<ref name=boron25Unless/>
|-
! [[Microfilaments]]
| 8-10 || [[double helix]] || [[actin]]
|-
! [[Intermediate filament]]s
| 8-10 || two parallel [[helix|helices]]/dimers, forming tetramers ||
*[[vimentin]] ([[mesenchyme]])
*[[glial fibrillary acidic protein]] ([[glial cell]]s)
*[[neurofilament]] proteins ([[neuronal process]]es)
*[[keratin]]s ([[epithelial cell]]s)
*[[nuclear lamins]]
|-
![[Microtubule]]s
| 25 || [[protofilament]]s, in turn consisting of tubulin subunits || [[α-bubulin|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<ref>{{cite journal | author=Heuser J | title = Whatever happened to the 'microtrabecular concept'? | journal = Biol Cell | year = 2002 | volume = 94| issue = 9| pages = 561–96 | doi=10.1016/S0248-4900(02)00013-8}}</ref>.
==The prokaryotic cytoskeleton==
{{main|prokaryotic cytoskeleton}}
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>
===FtsZ===
[[FtsZ]] was the first protein of the prokaryotic cytoskeleton to be identified. Like tubulin, FtsZ forms filaments in the presence of [[Guanosine triphosphate|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 [[gene]]s 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 [[plasmid]]s 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 [[Analogy (biology)|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.
==References==
{{Refimprove|date=December 2007}}
<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==
* [http://www.biochemweb.org/cytoskeleton.shtml Cytoskeleton, Cell Motility and Motors - The Virtual Library of Biochemistry and Cell Biology]
* [http://www.cytoskeletons.com Cytoskeleton database, clinical trials, recent literature, lab registry ...]
* [http://aimediaserver.com/studiodaily/videoplayer/?src=harvard/harvard.swf&width=640&height=520 Animation of leukocyte adhesion] (Animation with some images of actin and microtubule assembly and dynamics.)
{{Cytoskeletal Proteins}}
[[Category:Cell anatomy]]
[[Category:Cytoskeleton]]
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