Cytosquelette

The cytosquelette (to be reserved for the cytoplasm, but cf cytomotosquelette) of a cell is the organized whole of the biological polymers which confer to him the essence of its mechanical properties. The terminological reference to the " Skeleton " the vertebrate ones is however misleading since

all the components of the cytosquelette are renewed by polymerization permanently
the cytosquelette is at the origin of the majority of the forces exerted by the cell to move and nourish themselves, it in what it is connected rather with a whole of " Muscles ".
finally, the mechanical properties of the cytosquelette are very variable according to the components and the situations considered.

The cytosquelettes of all the Eucaryotes are rather similar (although important differences exist between the animal and vegetable cells), while those recently discovered at the Procaryotes seem organized in a completely different way.

The cytosquelette of the eucaryotes

Composition and total structure

The cytosquelette consists of biological protein polymers, which one sometimes describes as fibers being given their important size on a cellular scale. One classifies them in three categories:

the filaments of actine, made of actine (of which there exist various types). One finds also these same filaments in great quantity in the muscle fibers. Their diameter borders 7 Nm, and them Length of persistence of approximately 17 µm. They are thus flexible filaments enough . They are also directed because of asymmetry of the monomers of actine and their helical assembly: one of the ends (+) can polymerize much more quickly than the other (-). One speaks about polarized filaments .
the intermediate filaments. They are the least dynamic elements of the cytosquelette, but their study develops quickly. They are very important for the structure of the core .ils allow the inking of the organoids.
the Microtubule S are the most rigid components of the cytosquelette. Their length of persistence is indeed of several millimetres, which largely exceeds the scale of the cell, for a ray varying of 15 with 25nm according to the types of microtubules. This rigidity their is conferred by a structure out of tube due to the particular assembly of the monomers which compose them. The microtubules are in the same way polarized that filaments of actine, but biochemistry of polymerization is different. In particular, there exists a dynamic instability which can lead to a very brutal shortening of a microtubule, which can be at the origin of an important force.

The polymers are organized in networks, beams or cables according to the roles which they fill. This high level of organization is made possible by the presence of auxiliary hundreds of proteins:

proteins of bridging (usually indicated by the English terms " crosslinkers" or " crosslinking proteins"). This term refers to the physics of polymers, where the addition of certain components (as suffers it for rubber) led to the formation of bridgings between the chains and completely modify the physical properties of material. They is indeed what occurs, in a way even more spectacular, in the case of the cytosquelette. Indeed, the majority of the crosslinkers are controlled by the cell by means of other proteins of regulation, which allows reorganizations sometimes very fast cytosquelette.
proteins of connection (sometimes regarded as a particular case of the preceding ones, but important primarily in the case of filaments of actine)
the depolymerizing proteins cap and proteins (capping, severing proteins) which control the rates of polymerization of the filaments at their ends
the proteins of anchoring

The cytosquelette of the plants

The shape of the cell being mainly defined by the osmotic interaction pressure/resistance of the wall, one notes a less importance of the cytosquelette: absence of microfilament of actine and intermediate filaments (except on the level of the core. only the network of microtubules is developed and is organized in a helicoid structure plating itself against the membrane (one suspects their importance in the direction of synthesis of cellulose fibers of the wall)

Localized structures

Principal functions

The cytosquelette contributes to many functions within the cell:

regulation of the shape of the cell.
anchoring with the membranes of the close cells.
formation of protrusions or membrane involutions (important for the Phagocytosis and the cellular migration: Pseudopode S)
the maintenance of the internal structure, and in particular of the cellular compartments.
the transport of proteins or ARNm in a particular place of surface.
the separation of the chromosomes at the time of the mitosis
formation and mitotic contraction of the ring allowing the physical separation of two cells girls (cytodiérèse).
contraction of the muscular cells.

This list could not be exhaustive, of many unknown roles being highlighted regularly by the research which is very active in this field.

Mechanical properties

The cytosquelette of the procaryotes

One recently highlighted the presence of a cytosquelette at the procaryotes, in particular thanks to work of Rut Carballido-López and his team. Those Ci discovered the Mreb protein, counterpart with protein of actine, and similar structure, located under the membrane and pretense to play a big role in the structure and the cellular form. The FtsZ protein, would also play a part in the cytodiérèse of the bacteria.

Random links: