Cytoskeleton

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

## Contents

* 1 Overview
* 2 The prokaryotic cytoskeleton
* 2.1 FtsZ
* 2.2 MreB and ParM
* 2.3 Crescentin
* 3 The eukaryotic cytoskeleton
* 3.1 Actin microfilaments
* 3.2 Intermediate filaments
* 3.3 Microtubules
* 3.4 Comparison
* 4 History of cytoskeleton
* 5 References
* 6 Further reading
* 7 External links

## Overview

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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

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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

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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

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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

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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

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File:MEF microfilaments.jpgActin 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

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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

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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:

* 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

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File:Btub.jpgMicrotubules 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:

* 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

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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 |

* vimentin (mesenchyme)
* glial fibrillary acidic protein (glial cells)
* neurofilament proteins (neuronal processes)
* keratins (epithelial cells)
* nuclear lamins

Microtubules |  23 |  protofilaments, in turn consisting of tubulin subunits |  α- and β-tubulin

## History of cytoskeleton

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

## References

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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" (PDF). 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 (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. Unknown parameter `|month=` ignored (help)CS1 maint: Multiple names: authors list (link)
4. ↑ 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. Unknown parameter `|month=` ignored (help)CS1 maint: Multiple names: authors list (link)
5. ↑ Minton AP (1992). "Confinement as a determinant of macromolecular structure and reactivity". Biophys J. 63 (4): 1090–100. doi:10.1016/S0006-3495(92)81663-6. PMC 1262248. PMID 1420928. Unknown parameter `|month=` ignored (help)
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 (1994). "The plant cytoskeleton". Curr Opin Cell Biol. 6 (1): 10–5. PMID 8167014. Unknown parameter `|month=` ignored (help)
8. ↑ 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. Unknown parameter `|month=` ignored (help)CS1 maint: Multiple names: authors list (link)
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. p. 1300\. ISBN 1-4160-2328-3. Page 25
10. ↑ Fuchs E, Cleveland DW (1998). "A structural scaffolding of intermediate filaments in health and disease". Science (journal). 279 (5350): 514–9. PMID 9438837. Unknown parameter `|month=` ignored (help)
11. ↑ 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. Unknown parameter `|month=` ignored (help)

## Further reading

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

## External links

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* 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 great images of actin and microtubule assembly and dynamics.

* v
* t
* e

Proteins of the cytoskeleton

Human|

| Microfilaments
and ABPs|

| Myofilament|

| Actins|

* A1
* A2
* B
* C1
* G1
* G2

|

Myosins|

* 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|

* Wiskott-Aldrich syndrome protein
* Fibrillin
* Filamin
* FLNA
* FLNB
* FLNC
* Espin
* TRIOBP

Intermediate
filaments|

| 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)|

* type I/chromosome 17
* 31
* 32
* 33A
* 33B
* 34
* 35
* 36
* 37
* 38

* type II/chromosome 12
* 81
* 82
* 83
* 84
* 85
* 86

Ungrouped alpha|

* chromosome 17
* 23
* 24
* 25
* 26
* 27
* 28
* 39
* 40

* chromosome 12
* 71
* 72
* 73
* 74
* 75
* 76
* 77
* 78
* 79
* 80

Not alpha|

* Beta-keratin

Type 3|

* Desmin
* GFAP
* Peripherin
* Vimentin

Type 4|

* Internexin
* Nestin
* Neurofilament
* NEFL
* NEFM
* NEFH
* Synemin
* Syncoilin

Type 5|

* Nuclear lamins: A/C
* B1
* B2

Microtubules
and MAPs|

| Tubulins|

* TUBA1A
* TUBA1B
* TUBA1C
* TUBA3C
* TUBA3D
* TUBA3E
* TUBA4A
* TUBA8

|

Kinesins|

* 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

Dyneins|

* 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
* Stathmin
* Tektin
* TEKT1
* TEKT2
* TEKT3
* TEKT4
* TEKT5
* Dynamin
* DNM1
* DNM2
* DNM3

Catenins|

* Alpha catenin
* Beta catenin
* APC
* Plakoglobin (gamma catenin)
* Delta catenin
* GAN

Membrane|

* Dystrophin
* Dystroglycan
* Utrophin
* Ankyrin
* ANK1
* ANK2
* ANK3
* Spectrin
* SPTA1
* SPTAN1
* SPTB
* SPTBN1
* SPTBN2
* SPTBN4
* SPTBN5

Other|

* Plakins
* Corneodesmosin
* Desmoplakin
* Dystonin
* Envoplakin
* MACF1
* Periplakin
* Plectin
* Talin
* TLN1
* Vinculin
* Plakophilin
* PKP1
* PKP2

Nonhuman|

* Major sperm proteins
* Prokaryotic cytoskeleton
* Crescentin
* FtsZ
* MreB

See also: cytoskeletal defects

`{{Phosphate biochemistry}}`

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