The Actine is a important Protéine Bi-globular for the cellular architecture and movements. It is present in all the cells of the body and especially in the muscular cells. It can represent up to 10% of the proteinic total mass of the cells. This protein was highly preserved during the evolution of the eucaryotes, since the identity between isotypic of actine human and the yeast actine (S. cerevisiae) are higher than 90%. The majority of the cells eucaryotes have many genes of actine coding slightly different proteins.

Structure

In the mammals, there exist 7 Isotype S of actine:

• 4 isoformes of actines alpha (present in the striated muscles skeletal and cardiac and the smooth muscles),
• 2 isoformes of actines gamma (present in the enteric smooth muscle and nonmuscular fabrics),
• 1 isoforme of actine beta (nonmuscular).

The actine is a protein whose diameter is of 5.5 Nm, which contains a rare amino-acid: the 3-methyl histidine. It is consisted a polypeptide of 375 amino-acids. In the cell, one finds it in two forms:

• Actine G (Globular), soluble monomeric form in aqueous solution.

The monomer of actine G is associated with a divalent cation such as the ion calcium or magnesium (magnesium in vivo) and a nucleotide of the type ATP or ADP depending on the phosphorylation state of nucleotide. In the absence of these two cofacteurs, the actine is denatured easily. The thymosine blocks polymerization by joining the monomers actine G related to the ATP.

• Actine F (Filamentous) or microfilament, of 7 Nm diameter and which is a polymer of actine G.

This filament is a dextral helicoid arrangement, with a turn of propeller comprising 13 monomers and a length of 37nm.

Polymerization

See also: Filament of actine

It starts with a phase known as of nucleation during which dimers (or trimères) of actine (called cores) are assembled. This stage, dévaforable thermodynamically, are a slow stage. If the concentration in monomers of actine (actine known as G) is higher than a critical concentration, the actine G is assembled in filaments starting from the preformed cores. It is the stage of elongation of the filaments. This fast stage is often called phase of polymerization, although the filamentous actine (known as actine-F) is not a genuine polymer (the monomers are not dependant between them by a covalent bond within a filament). Once formed, the filaments of actine are with balance between dissociation of the filaments at the ends and association of monomers at the ends. In the cells, the spontaneous formation of cores of actine is very unfavourable. Also, the cells have recourse to proteins known as nucléateurs. This days, three nucléateurs were identified: the Arp2/3 complex (complex composed of 7 pennies proteinic units), formines, and the protein Whorl.

Localization and role

In the muscular contraction, the actine polymerized binds to another protein, the Myosine. The latter clings to polymer actine and makes it slide compared to it; with the other end of the filament of actine, another filament of myosine makes the same thing in a symmetrical way; the two filaments of myosine thus approach one the other, it is the muscular contraction.

Other roles:

• contractile Ring of the cells in division at the time of the cytodiérèse allowing to separate the cells resulting from the mitosis (or of the meiosis).
• Belt of Desmosomes to the apical pole of the epithelial cells.
• Maintenance of the Microvillosité S of the intestinal epithelial cells.
• Emission of Lamellipode S which makes it possible the cell to lengthen in a given direction what allows the cellular migration or the catch of preys (at the Amibe S and the phagocytic cells of the immune system like the Macrophage S). In the case of the cellular migration, the microfilaments of actine bind to focal points of adhesion which are used as fulcrums on the extracellular Matrice, necessary to advance. They are the proteins associated with the Intégrine S which bind to the actine.

See too

Filament of actine

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