Cytoskeleton

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Cytoskeleton

The cytoskeleton (also CSK) is a cellular "scaffolding

Scaffolding

Scaffolding is a temporary framework used to support people and material in the construction or repair of buildings and other large structures....
" or "skeleton

Skeleton

In biology, a skeleton is a rigid framework that provides protection and structure in many types of animal, particularly those of the phylum Chordata and of the superphylum Ecdysozoa....
" contained within the cytoplasm

Cytoplasm

The cytoplasm is the part of a Cell that is enclosed within the plasma membrane. In eukaryote cells the cytoplasm contains organelles, such as mitochondrion, that are filled with liquid kept separate from the rest of the cytoplasm by biological membranes....
. The cytoskeleton is present in all cells; it was once thought this structure was unique to eukaryote

Eukaryote

Animals, plants, fungus, and protists are eukaryotes , organisms whose Cell are organized into complex structures enclosed within Cell membrane....
s, but recent research has identified the prokaryotic cytoskeleton

Prokaryotic cytoskeleton

The prokaryotic cytoskeleton is the collective name for all structural protein filament in prokaryotes. It was once thought that prokaryotic cells did not possess cytoskeleton , but recent advances in visualization technology and structure determination have shown that filaments indeed exist in these cells....
. It is a dynamic structure that maintains cell shape, protects the cell, enables cellular motion (using structures such as flagella

Flagellum

A flagellum is a tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and it functions in locomotion....
, cilia

Cilium

A cilium is an organelle found in eukaryote cell s. Cilia are tail-like projections extending approximately 5?10 micrometres from the cell body....
and lamellipodia

Lamellipodia

The lamellipodium is a cytoskeleton actin projection on the mobile edge of the cell . It contains a two-dimensional actin mesh; the whole structure pulls the cell across a substrate ....
), and plays important roles in both intracellular transport (the movement of vesicle

Vesicle (biology)

A vesicle is a small bubble of liquid within a cell. More technically, a vesicle is a small, intracellular, membrane-enclosed sac that stores or transports substances within a cell....
s and organelles, for example) and cellular division

Cell division

Cell division is a process by which a cell , called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle....
.

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Encyclopedia

The cytoskeleton (also CSK) is a cellular "scaffolding

Scaffolding

Scaffolding is a temporary framework used to support people and material in the construction or repair of buildings and other large structures....
" or "skeleton

Skeleton

Cytoplasm

Eukaryote

Prokaryotic cytoskeleton

Flagellum

A flagellum is a tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and it functions in locomotion....
, cilia

Cilium

A cilium is an organelle found in eukaryote cell s. Cilia are tail-like projections extending approximately 5?10 micrometres from the cell body....
and lamellipodia

Lamellipodia

Vesicle (biology)

Cell division

Cell division is a process by which a cell , called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle....
. The concept and the term (cytosquelette, in French) was first introduced by French embryologist Paul Wintrebert

Paul Wintrebert

Paul Wintrebert was a French embryologist and a theoretician of developmental biology.He coined the term cytoskeleton in 1931.He held radical epigenetic views....
in 1931.

The eukaryotic cytoskeleton

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

Excluded volume

The concept of excluded volume was introduced by Werner Kuhn in 1934 and applied to polymer molecules shortly thereafter by Paul Flory....
macromolecules from some of the cytosol

Cytosol

The cytosol or intracellular fluid is the liquid found inside cell . In eukaryotes this liquid is separated by cell membranes from the contents of the organelles suspended in the cytosol, such as the mitochondrial matrix inside the mitochondrion....
it adds to the level of macromolecular crowding

Macromolecular crowding

The phenomenon of macromolecular crowding alters the properties of molecules in a solution when high concentrations of macromolecules such as proteins are present....
in this compartment.

Actin filaments / Microfilaments

Around 5-9 nm in diameter, this filament is composed of two intertwined actin chains. Microfilaments are most concentrated just beneath the cell membrane

Cell membrane

The cell membrane is the interface between the cellular machinery inside the cell and the fluid outside.It is a semipermeable lipid bilayer found in all cell ....
, and are responsible for resisting tension and maintaining cellular shape, forming cytoplasmatic protuberances (like pseudopod

Pseudopod

eruses4|eukaryotic cells|the Band|Pseudopod }}Pseudopods or pseudopodia are temporary projections of eukaryotes. Cells having this faculty are generally referred to as amoeboids....
ia and microvilli

Microvillus

Microvilli are microscopic cellular membrane protrusions that increase the surface area of cells, and are involved in a wide variety of functions, including absorption , secretion, cellular adhesion, and mechanotransduction....
- 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

Signal transduction

In biology, 'signal transduction' refers to any process by which a cell converts one kind of signal or stimulus into another. Most processes of signal transduction involve ordered sequences of biochemistry chemical reaction inside the cell, which are carried out by enzymes, activated by Second messenger systems, resulting in a signal tran...
. They are also important for cytokinesis

Cytokinesis

Cytokinesis is the process where the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation to the next....
(specifically, formation of the cleavage furrow

Cleavage furrow

In cell biology, the cleavage furrow is the indentation that begins the process of cytokinesis, by which animal and some algae cell s undergo cytokinesis....
) and, along with myosin

Myosin

Myosins are a large family of motor proteins found in eukaryotic Biological tissue. They are responsible for actin-based motility.Following the discovery, by Pollard and Korn, of enzymes with myosin-like function in Acanthamoeba, a large number of divergent myosin genes have been discovered throughout eukaryotes....
, muscular contraction

Skeletal muscle

They generally contract voluntarily , although they can contract involuntarily through Reflex action. The whole muscle is wrapped in a special type of connective tissue, epimysium....
. Actin

Actin

Actin is a Globular_protein, roughly 42-kDa protein found in all Eukaryote where it may be present at concentrations of over 100 ?M. It is also one of the most highly-Conservation proteins, differing by no more than 20% in species as diverse as algae and humans....
/Myosin

Myosin

Cytoplasmic streaming

Cytoplasmic streaming is the flowing of cytoplasm in eukaryotic cells. This occurs in both plant and animal cells. It creates cytoplasmic reorganization during cell reproduction....
in most cells.

Intermediate filaments

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). Intermediate filaments organize the internal tridimensional structure of the cell, anchoring organelles and serving as structural components of the nuclear lamina

Nuclear lamina

The nuclear lamina is a dense fibrillar network inside the Cell nucleus of an eukaryotic cell. It is composed of intermediate filaments and membrane associated proteins....
and sarcomere

Sarcomere

"A-band" redirects here. For other uses of the term see A band.A sarcomere is the basic unit of a muscle's cross-striated myofibril. Sarcomeres are multi-protein complexes composed of three different filament systems....
s. They also participate in some cell-cell and cell-matrix junctions.

Different intermediate filaments are:

made of vimentin
Vimentin

Vimentin is a member of the intermediate filament family of proteins. Intermediate filaments are an important structural feature of eukaryotic cells....
s, being the common structural support of many cells.
made of keratin
Keratin

Keratins are a family of fibrous protein; tough and insoluble, they form the hard but mineral structures found in reptiles, birds, amphibians and mammals....
, found in skin
Skin

The skin is the outer covering of the body, also known as the epidermis. It is the largest organ of the integumentary system made up of multiple layers of epithelial biological tissue, and guards the underlying muscles, bones, ligaments and organ s....
cells, hair
Hair

Hair is a protein filament that epidermal growth from hair follicle deep within the dermis. The fine, soft hair found on many nonhuman mammals is typically called fur; wool is the characteristically curly hair found on sheep and goats....
and nails
Nail (anatomy)

A nail is a horn -like structure at the end of an animal's finger or toe. See also claw....
.
neurofilament
Neurofilament

Neurofilaments are the 10 nanometer or intermediate filaments found specifically in neurons. The subunits of neurofilaments are related structurally to the 10nm or intermediate filaments of other tissues such as the keratin subunits, which make 10nm filaments expressed specifically in epithelia....
s of neural cells.
made of lamin
Lamin

Nuclear Lamins, also known as Class V intermediate filaments, are fibrous proteins providing structural function and transcriptional regulation in the cell nucleus....
, giving structural support to the nuclear envelope.

Microtubules

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

Tubulin

Tubulin is one of several members of a small family of globular proteins. The most common members of the tubulin family are a-tubulin and ?-tubulin, the proteins that make up microtubules....
. They have a very dynamic behaviour, binding GTP

Guanosine triphosphate

Guanosine-5'-triphosphate is a purine nucleotide. One role is as substrate for the synthesis of RNA during transcription . Its structure is similar to that of the guanine nucleoside, the only difference being that there are three phosphate groups attached to the 5' carbon....
for polymerization. They are commonly organized by the centrosome

Centrosome

In cell biology, the centrosome is an organelle that serves as the main microtubule organizing center of the animal cell as well as a regulator of cell-cycle progression....
.

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

Dynein

Dynein is a motor protein in biological cells which converts the chemical energy contained in Adenosine triphosphate into the mechanical energy of movement....
. 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
Dynein

Dynein is a motor protein in biological cells which converts the chemical energy contained in Adenosine triphosphate into the mechanical energy of movement....
s and kinesin
Kinesin

Kinesins are a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule cables powered by the dephosphorylation of Adenosine triphosphate ....
s, they transport organelles like mitochondria or vesicle
Vesicle (biology)

A vesicle is a small bubble of liquid within a cell. More technically, a vesicle is a small, intracellular, membrane-enclosed sac that stores or transports substances within a cell....
s).
the axoneme
Axoneme

Numerous eukaryotic Cell carry whip-like appendages whose inner core consists of a cytoskeleton structure called the axoneme.The axoneme serves as the "skeleton" of these organelles, both giving support to the structure and, in some cases, causing it to bend....
of cilia
Cilium

A cilium is an organelle found in eukaryote cell s. Cilia are tail-like projections extending approximately 5?10 micrometres from the cell body....
and flagella
Flagellum

A flagellum is a tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and it functions in locomotion....
.
the mitotic spindle
Mitotic spindle

In cell biology, the spindle apparatus is the structure that separates the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryote....
.
synthesis of the cell wall in plants.

Comparison

Cytoskeleton type	Diameter (nm Nanometre A nanometre is a Units of measurement of length in the metric system, equal to one billionth of a metre .It is one of the more often used units for very small lengths, and equals ten ?ngstr?m, an internationally recognized non-International System of Units of length.... )	Structure	Subunit examples
Microfilaments	8-10	double helix Double helix In geometry a double helix typically consists of two congruence helix with the same axis, differing by a translation along the axis, which may or may not be half-way....	actin Actin Actin is a Globular_protein, roughly 42-kDa protein found in all Eukaryote where it may be present at concentrations of over 100 ?M. It is also one of the most highly-Conservation proteins, differing by no more than 20% in species as diverse as algae and humans....
Intermediate filament Intermediate filament Intermediate filaments are a family of related proteins that share common structural and sequence features. Intermediate filaments have an average diameter of 10 nanometers, which is between that of actin and microtubules, although they were initially designated 'intermediate' because their average diameter was between those of narrower mi... s	8-10	two anti-parallel helices Helix A helix is a special kind of space curve, i.e. a Differentiable manifold curve in three-space. As a mental image of a helix one may take the spring .... /dimers, forming tetramers	vimentin Vimentin Vimentin is a member of the intermediate filament family of proteins. Intermediate filaments are an important structural feature of eukaryotic cells.... (mesenchyme Mesenchyme Mesenchyme is a type of loose connective tissue, of mesodermal origin and located within the embryo mesoderm, consisting of a ground substance Matrix containing a loose aggregate of unspecialized cell which are capable of developing into connective tissue, bone, cartilage, the lymphatic system, and the circulatory system.... ) glial fibrillary acidic protein Glial fibrillary acidic protein Glial fibrillary acidic protein is an intermediate filament protein that is thought to be specific for astrocytes in CNS. Later it was shown that GFAP also is expressed by other cell types in CNS - ependymal cells.... (glial cell Glial cell Glial cells, commonly called neuroglia or simply glia , are non-neuronal cell that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system.... s) neurofilament Neurofilament Neurofilaments are the 10 nanometer or intermediate filaments found specifically in neurons. The subunits of neurofilaments are related structurally to the 10nm or intermediate filaments of other tissues such as the keratin subunits, which make 10nm filaments expressed specifically in epithelia.... proteins (neuronal process Neuronal process Neuronal process may refer to:dendriteaxon... es) keratin Keratin Keratins are a family of fibrous protein; tough and insoluble, they form the hard but mineral structures found in reptiles, birds, amphibians and mammals.... s (epithelial cells) nuclear lamins
Microtubule Microtubule Microtubules are one of the components of the cytoskeleton. They have a diameter of 25 Nanometre and length varying from 200 nanometers to 25 micrometers.... s	25	protofilaments, in turn consisting of tubulin subunits	a- and ß-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.

The prokaryotic cytoskeleton

The cytoskeleton was previously thought to be a feature only of eukaryotic

Eukaryote

Animals, plants, fungus, and protists are eukaryotes , organisms whose Cell are organized into complex structures enclosed within Cell membrane....
cells, but homologues

Homology (biology)

In evolutionary biology, homology refers to any similarity between characteristics that is due to their common descent. The word homologous derives from the ancient Greek ??????e??, 'to agree'....
to all the major proteins of the eukaryotic cytoskeleton have recently been found in prokaryotes. Although the evolutionary relationships are so distant that they are not obvious from protein sequence comparisons alone, the similarity of their three-dimensional structures

Protein structure

Proteins are an important class of biological macromolecules present in all biological organisms, made up of such chemical element as carbon,hydrogen, nitrogen, oxygen, and sulphur....
and similar functions in maintaining cell shape and polarity provides strong evidence that the eukaryotic and prokaryotic cytoskeletons are truly homologous. However, some structures in the bacterial cytoskeleton may have yet to be identified.

FtsZ

FtsZ is a protein encoded by the ftsZ gene that assembles into a ring at the future site of the septum of bacterial cell division. FtsZ, named after filamenting temperature-sensitive...
was the first protein of the prokaryotic cytoskeleton to be identified. Like tubulin, FtsZ forms filaments in the presence of GTP

Guanosine triphosphate

Guanosine-5'-triphosphate is a purine nucleotide. One role is as substrate for the synthesis of RNA during transcription . Its structure is similar to that of the guanine nucleoside, the only difference being that there are three phosphate groups attached to the 5' carbon....
, but these filaments do not group into tubules. During cell division

Cell division

Cell division is a process by which a cell , called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle....
, FtsZ is the first protein to move to the division site, and is essential for recruiting other proteins that synthesize the new cell wall

Cell wall

A cell wall is a tough, flexible and sometimes fairly rigid layer that surrounds some types of cell . It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism....
between the dividing cells.

MreB and ParM

Prokaryotic actin-like proteins, such as MreB

MreB

MreB is a protein found in bacteria that has been identified as a homology of actin, as indicated by similarities in tertiary structure and conservation of active site amino acid sequence....
, are involved in the maintenance of cell shape. All non-spherical bacteria have gene

Gene

A gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cell and pass genetic trait to offspring....
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

Biosynthesis

Biosynthesis is a phenomenon wherein chemical compounds are produced from simpler reagents. Biosynthesis, unlike chemosynthesis, takes place within living organisms and is generally catalyst by enzymes....
.

Some plasmid

Plasmid

File:plasmid .svgA plasmid is an extra-chromosomal DNA molecule separate from the chromosome which is capable of replicating independently of the chromosomal DNA....
s encode a partitioning system that involves an actin-like protein ParM

PARM

PARM can refer to:*PARM 1 mine, Panzer-Abwehr Richtmine * Authentic Party of the Mexican Revolution, Partido Aut?ntico de la Revoluci?n Mexicana ...
. Filaments of ParM exhibit dynamic instability, and may partition plasmid DNA into the dividing daughter cells by a mechanism analogous

Analogy (biology)

Two structures in biology are said to be analogous if they perform the same or similar function by a similar mechanism but evolved separately....
to that used by microtubules during eukaryotic mitosis

Mitosis

Mitosis is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus, into two identical sets in two daughter nuclei....
.

Crescentin

The bacterium Caulobacter crescentus
Caulobacter crescentus

Caulobacter crescentus is a Gram-negative, oligotrophic bacterium widely distributed in fresh water lakes and streams. It plays an important role in the carbon cycle....
contains a third protein, crescentin

Crescentin

Crescentin is a protein which is a bacterial relative of the intermediate filaments found in eukaryotic cells. Just as tubulins and actins, the other major Cytoskeleton, have prokaryotic Homology in, respectively, the FtsZ and MreB proteins, intermediate filaments are linked to the crescentin protein....
, that is related to the intermediate filaments of eukaryotic cells. Crescentin is also involved in maintaining cell shape, such as helical and vibrioid

Vibrio

Vibrio is a genus of Gram-negative bacteria possessing a curved rod shape. Typically found in Seawater, Vibrio are Facultative anaerobic organism that test positive for oxidase and do not form spores....
forms of bacteria, but the mechanism by which it does this is currently unclear.

External links

(Animation with some images of actin and microtubule assembly and dynamics.)

Cytoskeleton

The eukaryotic cytoskeleton

Actin filaments / Microfilaments

Intermediate filaments

Microtubules

Comparison

Microtrabeculae - a further structural network?

The prokaryotic cytoskeleton

FtsZ

MreB and ParM

Crescentin

Further reading

External links