The microtubules are fibers constitutive of the Cytosquelette, as well as the microfilaments of Actine and that the intermediate filaments. They have a diameter of approximately 25 Nm and a variable length because of their dynamics (related to their function within the cell and their lability). Two microtubules are used during the Mitose. The life time of the microtubules is of approximately ten minutes.

Composition

A microtubule is a tube whose wall consists of several protofilaments of tubuline. In general one counts 13 per microtubule of them. Each protofilament itself consists of dimers of Tubuline:
  • tubuline-alpha α
  • and tubuline-beta β.

This polymeric assembly is extremely Labile: the ends of the microtubules polymerize and depolymerize permanently. The two ends of the microtubules have different properties (polarity of the microtubule). There exists a " end; plus" where the dimers are added and a " end; moins" who on the contrary loses his dimers gradually (unless being stabilized) (except for the polarized epithelial cells where the microtubules always do not leave the centrosome). Polymerization brings into play the Hydrolyze GTP (Guanosine TriPhosphate). The microtubules are absent at the procaryotes. Whereas for the eucaryotes, they are constant at Métazoaires, the Protozoa and Protophytes. At Métaphytes (which is also eucaryotes), the microtubules are far from numerous in the somatic cells (absence of centriole) but very present in the sexual cells… Associated proteins: there exist proteins associated with the microtubules or map, the ones have a role in the stabilization of the microtubules, the others are specialized in the movement of the blisters and the organoids along the microtubules; it is of ATPases:

  • the kinésines transport towards the positive end (+), one speaks about anterograde transport.
  • the dynéines transport towards the negative end (-). it is about a retrograde tranport.

It is noted that the proteins allowing the movement make it possible to maintain the place of the organoids of the cells. For example the Appareil of Golgi which is located close to the core (thus close to the Centrosome) will be attached to the dynéines so that those maintain it in this place.

Roles

Cellular division

The microtubules play a very important part in the cellular division (Mitose) and in the cytoplasmic currents thanks to the co-operation between the microfilaments of Actine (the displacement of the chromosomes). Appearance of the microtubules in G2 phase of the interphase.

Cytoplasmic transport

Cellular displacements are done with competition between MY (microfilaments of actine) and the MT (microtubules). The microtubules are very numerous in the Neuron S, in particular in the dendrite S and the axons. They make it possible to forward different components to their ends, serving as true rails on which molecular driving , attached to the components to transport, move. The latter consist, for example, in synaptic blisters and Mitochondrie S.

Transport is possible thanks to P rotéines has ssociés with the M icrotubules (English WFP or MAP):

  • MAP of mobilization
Inter alia the Kinésine (towards the end more),
and the Dynéine (towards the end less).
  • MAP of assembly (proteins Tau).
  • MAP of stabilization.

Cellular mobility

The microtubules mainly compose the axial filament flagellar of the cells Eucaryote S mobiles like the mature Spermatozoïde S and the Protozoaire S.

Others

The microtubules have other roles, among them the maintenance of the three-dimensional form and the Diapédèse and the transport of blisters of Endocytose and Exocytose.

Molecules acting on the MT

There exist short stops like the Colchicine (by reversible reaction), the Vinblastine, the vincristine and the nocadazole. Those modify the balance of the reaction of polymerization and thus support depolymerization. One can use them besides for the establishment of Caryotype S. It should be noted that there exist also activators of polymerization like the Taxol.

Regulation

The microtubules are extremely dynamic structures. They are synthesized and degraded in very short times. Some Médicament S can destabilize them or, on the contrary, stabilize them.

the biogenesis of the microtubules: the MT are polymerized starting from organizing centers (MTOC):

- kinétochores (cellular division).

- basal corpuscle or cinétosome (lashes and whip).
it amorphous matter péricentriolaire (animal cells) or masses amorphous (vegetable cells) for the polymerization of the other MT.

Pathology of the microtubules

Notes and references of the article

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