Cytoskeleton

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The eukaryotic cytoskeleton. Actin filaments are shown in red, microtubules in green, and the nuclei are in blue.

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Editor In-Chief: Henry A. Hoff

Overview

The cytoskeleton is a cellular "scaffolding" or "skeleton" contained, as all other organelles, within the cytoplasm. It is contained in all eukaryotic cells and recent research has shown it can be present in prokaryotic cells too.[1] It is a dynamic structure that maintains cell shape, and also has been known to protect the cell, enables some cell motion (using structures such as flagella and cilia), and plays important roles in both intra-cellular transport (the movement of vesicles and organelles, for example) and cellular division. It is a bone-like structure floating around within the cytoplasm.

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.[1] Although the evolutionary relationships are so distant that they are not obvious from protein sequence comparisons alone, the similarity of their three-dimensional structures provides strong evidence that the eukaryotic and prokaryotic cytoskeletons are truly homologous.[2] However, some structures in the bacterial cytoskeleton may have yet to be identified.[3]

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.[4]

The eukaryotic cytoskeleton

File:MEF microfilaments.jpg
Actin cytoskeleton of mouse embryo fibroblasts, stained with phalloidin

The cytoskeleton provides the cell with structure and shape, and by excluded volume macromolecules from some of the cytosol add to the level of macromolecular crowding in this compartment.[5] Cytoskeletal elements (the membrane skeleton) interact extensively and intimately with cellular membranes.[6]

Intimately connected to the cytoskeleton across the nuclear envelope (NE) is the nucleoskeleton (NS), which provides shape and support for the NE and the important activities of the nucleus.

Eukaryotic cells contain three main kinds of cytoskeletal filaments, which are microfilaments, intermediate filaments, and microtubules.

Actin microfilaments

Main article: Microfilament

Around 6 nm in diameter, this filament type 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.

Further information: Actin

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,[7] and plant nuclear filaments have been detected.[8] Like actin filaments, they function in the maintenance of cell-shape by bearing tension (microtubules, by contrast, resist compression. 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

File:Btub.jpg
Microtubules in a gel fixated cell.
Main article: Microtubules

Microtubules are hollow cylinders about 23 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:

Comparison

Cytoskeleton type[9] Diameter (nm)[10] Structure Subunit examples[9]
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

History of cytoskeleton

The concept and the term (cytosquelette, in French) was first introduced by French embryologist Paul Wintrebert in 1931.[11]

References

  1. 1.0 1.1 Shih Y L, Rothfield L (2006). "The Bacterial Cytoskeleton". Microbiol Mol Biol Rev 70 (3): 729-754. PMID 16959967.
  2. 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.
  3. Briegel A, Dias DP, Li Z, Jensen RB, Frangakis AS, Jensen GJ (Oct 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.
  4. Ausmees N, Kuhn JR, Jacobs-Wagner C (Dec 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.
  5. Minton AP (Oct 1992). "Confinement as a determinant of macromolecular structure and reactivity". Biophys J. 63 (4): 1090–100. doi:10.1016/S0006-3495(92)81663-6. PMID 1420928.
  6. Doherty GJ, 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.
  7. Shibaoka H, Nagai R (Feb 1994). "The plant cytoskeleton". Curr Opin Cell Biol. 6 (1): 10–5. PMID 8167014.
  8. Blumenthal SS, Clark GB, Roux SJ (Apr 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.
  9. 9.0 9.1 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
  10. Fuchs E, Cleveland DW (January 1998). "A structural scaffolding of intermediate filaments in health and disease". Science (journal) 279 (5350): 514–9. PMID 9438837.
  11. 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:<73::AID-CM1>3.0.CO;2-0 10.1002/1097-0169(200006)46:2<73::AID-CM1>3.0.CO;2-0. PMID 10891854.

Further reading

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

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