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cytoskeleton

 
American Heritage Dictionary:

cy·to·skel·e·ton

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(sī'tə-skĕl'ĭ-tn) pronunciation
n.
The internal framework of a cell, composed largely of actin filaments and microtubules.


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Britannica Concise Encyclopedia:

cytoskeleton

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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.

For more information on cytoskeleton, visit Britannica.com.

McGraw-Hill Science & Technology Encyclopedia:

Cytoskeleton

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A system of filaments found in the cytoplasm of cells and responsible for the maintenance of and changes in cell shape, cell locomotion, movement of various elements in the cytoplasm, integration of the major cytoplasmic organelles, cell division, chromosome organization and movement, and the adhesion of a cell to a surface or to other cells.

Three major classes of filaments have been resolved on the basis of their diameter and cytoplasmic distribution: actin filaments (or microfilaments) each with an average diameter of 6 nanometers, microtubules with an average diameter of 25 nm, and intermediate filaments whose diameter of 10 nm is intermediate to that of the other two classes. The presence of this system of filaments in all cells, as well as their diversity in structure and cytoplasmic distribution, has been recognized only in the modern period of biology.

A technique that has greatly facilitated the visualization of these filaments, as well as the analysis of their chemical composition, is immunofluorescence applied to cells grown in tissue culture. See also Immunofluorescence.

Actin is the main structural component of actin filaments in all cell types, both muscle and nonmuscle. Actin filaments assume a variety of configurations depending on the type of cell and the state it is in. They extend a considerable distance through the cytoplasm in the form of bundles, also known as stress fibers since they are important in determining the elongated shape of the cell and in enabling the cell to adhere to the substrate and spread out on it. Actin filaments can exist in forms other than straight bundles. In rounded cells that do not adhere strongly to the substrate (such as dividing cells and cancer cells), the filaments form an amorphous meshwork that is quite distinct from the highly organized bundles. The two filamentous states, actin filament bundles and actin filament meshworks, are interconvertible polymeric states of the same molecule. Bundles give the cell its tensile strength, adhesive capability, and structural support, while meshworks provide elastic support and force for cell locomotion.

Microtubules are slender cylindrical structures that exhibit a cytoplasmic distribution distinct from actin filaments. Microtubules originate in structures that are closely associated with the outside surface of the nucleus known as centrioles. The major structural protein of these filaments is known as tubulin. Unlike the other two classes of filaments, microtubules are highly unstable structures and appear to be in a constant state of polymerization-depolymerization. See also Centriole.

Intermediate filaments function as the true cytoskeleton. Unlike microtubules and actin filaments, intermediate filaments are very stable structures. They have a cytoplasmic distribution independent of actin filaments and microtubules. In the intact cell, they anchor the nucleus, positioning it within the cytoplasmic space. During mitosis, they form a filamentous cage around the mitotic spindle which holds the spindle in a fixed place during chromosome movement.

Dictionary of Cultural Literacy: Science:

cytoskeleton

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The inner structural elements, or backbone, of a cell. It consists of microtubules and various filaments that spread out through the cytoplasm, providing both structural support and a means of transport within the cell.

Oxford Dictionary of Biochemistry:

cytoskeleton

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any of the various filamentous elements within the cytoplasm of eukaryotic cells that remain after treatment of the cells with mild detergent to remove membrane constituents and soluble components of the cytoplasm. The term embraces intermediate filaments, microfilaments, microtubules, and the microtrabecular lattice. The various elements of the cytoskeleton not only serve in the maintenance of cellular shape but also have roles in other cellular functions, including cellular movement, cell division, endocytosis, and movement of organelles. (Note: The term was coined to describe hypothetical structural cytoplasmic mosaics envisaged as providing physical support for, and thus spatial coordination of, enzymes effecting sequences of reactions, and as imposing restriction of mobility of the cell. Subsequently the cytoskeleton was regarded also as a possible subcellular site of action of various hormones.)
—cytoskeletal adj.

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Saunders Veterinary Dictionary:

cytoskeleton

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A conspicuous internal reinforcement in the cytoplasm of a cell, consisting of tonofibrils, filaments of the terminal web, and other microfilaments.

Mosby's Dental Dictionary:

cytoskeleton

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n

The intracellular filaments that serve to support or stiffen cells.

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categories related to 'cytoskeleton'

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For a list of words related to cytoskeleton, see:
  • Cell Structure and Function - cytoskeleton: rodlike filaments of various sizes in cytoplasm of most cells, associated with cell shape and movement

Wikipedia on Answers.com:

Cytoskeleton

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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 a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton. It has 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. In 1903 Nikolai K Koltsov proposed that the shape of cells was determined by a network of tubules which he termed the cytoskeleton. The concept of a protein mosaic that dynamically coordinated cytoplasmic biochemistry was proposed by Rudolph Peters in 1929 [1] while the term (cytosquelette, in French) was first introduced by French embryologist Paul Wintrebert in 1931.[2]

Contents

The eukaryotic cytoskeleton

Actin cytoskeleton of mouse embryo fibroblasts, stained with phalloidin.

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.[3] Cytoskeletal elements interact extensively and intimately with cellular membranes.[4]

Microfilaments (actin filaments)

These are the thinnest filaments of the cytoskeleton. They are composed of linear polymers of actin subunits, and generate force by elongation at one end of the filament coupled with shrinkage at the other, causing net movement of the intervening strand. They also act as tracks for the movement of myosin molecules that attach to the microfilament and "walk" along them. Actin structures are controlled by the Rho family of small GTP-binding proteins such as Rho itself for contractile acto-myosin filaments ("stress fibers"), Rac for lamellipodia and Cdc42 for filopodia.

Intermediate filaments

Microscopy of keratin filaments inside cells.
Main article: intermediate filament

These filaments, around 10 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:

Microtubules

Microtubules in a gel fixated cell.
Main article: microtubule

Microtubules are hollow cylinders about 23 nm in diameter (lumen = approximately 15 nm in diameter), most commonly comprising 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, where in 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:

Comparison

Cytoskeleton type[5] Diameter (nm)[6] Structure Subunit examples[5]
Microfilaments     6  double helix  actin
Intermediate filaments    10  two anti-parallel helices/dimers, forming tetramers
Microtubules    23  protofilaments, in turn consisting of tubulin subunits  α- and β-tubulin

The prokaryotic cytoskeleton

Main article: 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.[7] 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.[8] However, some structures in the bacterial cytoskeleton may have yet to be identified.[9]

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 3rd 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.[10]

History

Microtrabeculae

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.[11] 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 than an artifact of certain types of fixation treatment, although we have yet to fully understand the complexity of the cell's cytoskeleton.[12]

References

  1. ^ Peters RA. The Harben Lectures, 1929. Reprinted in: Peters, R. A. (1963) Biochemical lesions and lethal synthesis,p. 216. Pergamon Press, Oxford.. 
  2. ^ Frixione E (June 2000). "Recurring views on the structure and function of the cytoskeleton: a 300-year epic". Cell motility and the cytoskeleton 46 (2): 73–94. doi:10.1002/1097-0169(200006)46:2<73::AID-CM1>3.0.CO;2-0. PMID 10891854. 
  3. ^ Minton AP (October 1992). "Confinement as a determinant of macromolecular structure and reactivity". Biophys. J. 63 (4): 1090–100. Bibcode 1992BpJ....63.1090M. doi:10.1016/S0006-3495(92)81663-6. PMC 1262248. PMID 1420928. http://www.biophysj.org/cgi/reprint/63/4/1090. 
  4. ^ Doherty GJ and McMahon HT (2008). "Mediation, Modulation and Consequences of Membrane-Cytoskeleton Interactions". Annual Review of Biophysics 37: 65–95. doi:10.1146/annurev.biophys.37.032807.125912. PMID 18573073. http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.biophys.37.032807.125912. 
  5. ^ a b Unless else specified in boxes, then ref is:Walter F., PhD. Boron (2003). Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. pp. 1300. ISBN 1-4160-2328-3.  Page 25
  6. ^ Fuchs E, Cleveland DW (January 1998). "A structural scaffolding of intermediate filaments in health and disease". Science 279 (5350): 514–9. doi:10.1126/science.279.5350.514. PMID 9438837. http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=9438837. 
  7. ^ Shih YL, Rothfield L (2006). "The bacterial cytoskeleton". Microbiol. Mol. Biol. Rev. 70 (3): 729–54. doi:10.1128/MMBR.00017-06. PMC 1594594. PMID 16959967. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1594594. 
  8. ^ Michie KA, Löwe J (2006). "Dynamic filaments of the bacterial cytoskeleton". Annu. Rev. Biochem. 75: 467–92. doi:10.1146/annurev.biochem.75.103004.142452. PMID 16756499. http://www2.mrc-lmb.cam.ac.uk/SS/Lowe_J/group/PDF/annrev2006.pdf. [dead link]
  9. ^ Briegel A, Dias DP, Li Z, Jensen RB, Frangakis AS, Jensen GJ (October 2006). "Multiple large filament bundles observed in Caulobacter crescentus by electron cryotomography". Mol. Microbiol. 62 (1): 5–14. doi:10.1111/j.1365-2958.2006.05355.x. PMID 16987173. 
  10. ^ Ausmees N, Kuhn JR, Jacobs-Wagner C (December 2003). "The bacterial cytoskeleton: an intermediate filament-like function in cell shape". Cell 115 (6): 705–13. doi:10.1016/S0092-8674(03)00935-8. PMID 14675535. 
  11. ^ Wolosewick JJ, Porter KR (July 1979). "Microtrabecular lattice of the cytoplasmic ground substance. Artifact or reality". J. Cell Biol. 82 (1): 114–39. doi:10.1083/jcb.82.1.114. PMC 2110423. PMID 479294. http://www.jcb.org/cgi/pmidlookup?view=long&pmid=479294. 
  12. ^ Heuser J (2002). "Whatever happened to the 'microtrabecular concept'?". Biol Cell 94 (9): 561–96. doi:10.1016/S0248-4900(02)00013-8. PMID 12732437. 

External links

Structures of the cell / organelles (TH H1.00.01.2-3)
Endomembrane system
Cytoskeleton
Endosymbionts
Other internal
External
B strc: edmb (perx), skel (ctrs), epit, cili, mito, nucl (chro)
Proteins of the cytoskeleton
Human
A1, A2, B, C1, G1, G2
I (MYO1A, MYO1B, MYO1C, MYO1D, MYO1E, MYO1F, MYO1G, MYO1H) · II (MYH1, MYH2, MYH3, MYH4, MYH6, MYH7, MYH7B, MYH8, MYH9, MYH10, MYH11, MYH13, MYH14, MYH15, MYH16) · III (MYO3A, MYO3B) · V (MYO5A, MYO5B, MYO5C) · VI (MYO6) · VII (MYO7A, MYO7B) · IX (MYO9A, MYO9B) · X (MYO10) · XV (MYO15A) · XVIII (MYO18A, MYO18B) · LC (MYL1, MYL2, MYL3, MYL4, MYL5, MYL6, MYL6B, MYL7, MYL9, MYLIP, MYLK, MYLK2, MYLL1)
Other

Tropomodulin (1, 2, 3, 4) · Troponin (T 1 2 3, C 1 2, I 1 2 3) · Tropomyosin (1, 2, 3, 4)

Actinin (1, 2, 3, 4) · Arp2/3 complex · actin depolymerizing factors (Cofilin (1, 2) · Destrin) · Gelsolin · Profilin (1, 2) · Titin
Other
Type 1/2
(Keratin,
Cytokeratin)
Epithelial keratins
(soft alpha-keratins)

type I/chromosome 17 (10, 12, 13, 14, 15, 16, 17, 19, 20) · chromosome 12 (18) · none (21)

type II/chromosome 12 (1, 2A, 3, 4, 5, 6A, 6B, 7, 8, 9)
Hair keratins
(hard alpha-keratins)
Ungrouped alpha
Not alpha
Type 3
Type 4
Type 5
KIF1A, KIF1B, KIF2A, KIF2C, KIF3B, KIF3C, KIF4A, KIF4B, KIF5A, KIF5B, KIF5C, KIF6, KIF7, KIF9, KIF11, KIF12, KIF13A, KIF13B, KIF14, KIF15, KIF16B, KIF17, KIF18A, KIF18B, KIF19, KIF20A, KIF20B, KIF21A, KIF21B, KIF22, KIF23, KIF24, KIF25, KIF26A, KIF26B, KIF27, KIFC1, KIFC2, KIFC3

axonemal: DNAH1, DNAH2, DNAH3, DNAH5, DNAH6, DNAH7, DNAH8, DNAH9, DNAH10, DNAH11, DNAH12, DNAH13, DNAH14, DNAH17, DNAI1, DNAI2, DNALI1, DNAL1, DNAL4

cytoplasmic: DYNC1H1, DYNC2H1, DYNC1I1, DYNC1I2, DYNC1LI1, DYNC1LI2, DYNC2LI1, DYNLL1, DYNLL2, DYNLRB1, DYNLRB2, DYNLT1, DYNLT3
Other
Tau protein · Dynactin (DCTN1) · Tubulins (TUBA1A, TUBA1B, TUBA1C, TUBA3C, TUBA3D, TUBA3E, TUBA4A, TUBA8) · Stathmin · Tektin (TEKT1, TEKT2, TEKT3, TEKT4, TEKT5) · Dynamin (DNM1, DNM2, DNM3)
Other
Nonhuman

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American Heritage Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.  Read more
Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 1994-2012 Encyclopædia Britannica, Inc. All rights reserved.  Read more
McGraw-Hill Science & Technology Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.  Read more
Dictionary of Cultural Literacy: Science. The New Dictionary of Cultural Literacy, Third Edition Edited by E.D. Hirsch, Jr., Joseph F. Kett, and James Trefil. Copyright © 2002 by Houghton Mifflin Company. Published by Houghton Mifflin. All rights reserved.  Read more
 Oxford Dictionary of Biochemistry. Oxford University Press. Oxford Dictionary of Biochemistry and Molecular Biology © 1997, 2000, 2006 All rights reserved.  Read more
Saunders Veterinary Dictionary. Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved.  Read more
Mosby's Dental Dictionary. Mosby's Dental Dictionary. Copyright © 2004 by Elsevier, Inc. All rights reserved.  Read more
Random House Word Menu. © 2010 Write Brothers Inc. Word Menu is a registered trademark of the Estate of Stephen Glazier. Write Brothers Inc. All rights reserved.  Read more
Wikipedia on Answers.com. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article Cytoskeleton.  Read more

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