Jump to content

Cytoskeleton: Difference between revisions

From Wikipedia, the free encyclopedia
Browse history interactively
← Previous editNext edit →
Content deleted Content added
VisualWikitext
No edit summary
Line 32: Line 32:
* the [[mitotic spindle]].
* the [[mitotic spindle]].
* synthesis of the cell wall in plants.
* synthesis of the cell wall in plants.
And p0rn


===Comparison===
===Comparison===

Revision as of 19:16, 16 May 2011

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 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. 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]

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.

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. Although little work has been done on intermediate filaments in plants, there is some evidence that cytosolic intermediate filaments might be present,[5] and plant nuclear filaments have been detected.[6] 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, 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:

And p0rn

Comparison

Cytoskeleton type[7] Diameter (nm)[8] Structure Subunit examples[7]
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.[9] 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.[10] However, some structures in the bacterial cytoskeleton may have yet to be identified.[11]

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 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.[12]

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.[13] 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.[14]

References

  1. ^ Peters RA. "The Harben Lectures, 1929. Reprinted in: Peters, R. A. (1963) Biochemical lesions and lethal synthesis,p. 216. Pergamon Press, Oxford". {{cite journal}}: Cite journal requires |journal= (help)
  2. ^ Frixione E (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. {{cite journal}}: Unknown parameter |month= ignored (help)
  3. ^ Minton AP (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. {{cite journal}}: Unknown parameter |month= ignored (help)
  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.
  5. ^ Shibaoka H, Nagai R (1994). "The plant cytoskeleton". Curr. Opin. Cell Biol. 6 (1): 10–5. doi:10.1016/0955-0674(94)90110-4. PMID 8167014. {{cite journal}}: Unknown parameter |month= ignored (help)
  6. ^ Blumenthal SS, Clark GB, Roux SJ (2004). "Biochemical and immunological characterization of pea nuclear intermediate filament proteins". Planta. 218 (6): 965–75. doi:10.1007/s00425-003-1182-5. PMID 14727112. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  7. ^ a b Unless else specified in boxes, then ref is:Walter F., PhD. Boron (2003). Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. p. 1300. ISBN 1-4160-2328-3. Page 25
  8. ^ Fuchs E, Cleveland DW (1998). "A structural scaffolding of intermediate filaments in health and disease". Science (journal). 279 (5350): 514–9. doi:10.1126/science.279.5350.514. PMID 9438837. {{cite journal}}: Unknown parameter |month= ignored (help)
  9. ^ 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.
  10. ^ Michie KA, Löwe J (2006). "Dynamic filaments of the bacterial cytoskeleton" (PDF). Annu. Rev. Biochem. 75: 467–92. doi:10.1146/annurev.biochem.75.103004.142452. PMID 16756499. [dead link‍]
  11. ^ Briegel A, Dias DP, Li Z, Jensen RB, Frangakis AS, Jensen GJ (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. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  12. ^ Ausmees N, Kuhn JR, Jacobs-Wagner C (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. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  13. ^ Wolosewick JJ, Porter KR (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. {{cite journal}}: Unknown parameter |month= ignored (help)
  14. ^ 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.
Retrieved from "https://wiki.riteme.site/w/index.php?title=Cytoskeleton&oldid=429440078"