A filament of actine , or microfilament , is a Homopolymère of Actine, protein of 42 kDa (Unit of atomic mass). It is a constituent essence of the Cytosquelette of the cells eucaryotes, as well as muscle fibers. The actine in the form of filaments is sometimes called actine F (Filamentous), while the monomeric form is called actine G (Globular).
Structure of a filament
A filament of actine has a structure in double helix with a diameter of approximately 7 Nm. Its length of persistence is approximately 17 µm, that is to say the order of magnitude of the diameter of the cells.
Dynamics of polymerization
the actine G globular is polymerized in actine F (filament of actine). Polymerization starts by a phase of nucleation, where are formed mainly of the trimères. The monomers are assembled then according to a double helix, which thus does not have a center of symmetry. With the one of the ends, noted (+), the constant kinetics are in order of magnitude 10 times higher than those of the other end, noted (-). Moreover, the monomers associated with ATP (ATP-actine), present in majority in the alive cells, tend more to polymerize than those associated with ADP (ADP-actine).The actine associated with a filament tends to hydrolize its ATP. This property is, with the polarity of the filament, at the origin of the phenomenon known as of " carpet roulant" (English: threadmilling). Indeed, the end (+) will tend to collect in very large majority of ATP-actine, supporting consequently polymerization at this end. On the other hand, the end (-) being less active, the actine filament which is close from there spent more time in filament form, and is mainly in the form of ADP-actine. Consequently, at the end (-) balance is moved towards depolymerization.
These two concomitant displacements of balance make that the chain grows permanently side (+) and decrease side (-). If one maintains a monomer central fixed, the whole of the chain thus seems to move. The contribution of energy necessary to maintain this state except balance is done in the surrounding liquid medium, where ADP-actine is regenerated in ATP-actine. The " term; carpet roulant" suggest that the monomers leaving the end (-) return to be fixed at the end (+) after a passage in solution. However, no mass is transported macroscopically. This process allows the realization of a molecular Moteur which makes it possible certain cells to move by means of a Lamellipode. It is also at the origin of the movement of the bacterium Listeria. However, several other proteins are necessary and the actine only cannot convert the chemical energy of hydrolysis of the ATP into work.
Associated proteins
The proteins associated with the actine (in English, Actin associated proteins, AAP) are the key of control by the cell of its stock of actine. They make it possible to control polymerization and to organize the filaments spatially. They in their turn are controlled by regulating proteins which form part of the complex network interragissant with all the cell.
Here some examples:
- Gelsoline : allows to pass from the actine FREEZING to the actine GROUND.
- ARP2/3: protein close to the actine generating its polymerization, signal starting.
- Fimbrine: maintains the filaments tight of actine fasciculée.
- Cofiline: impose an additional torsion on the actine involving its depolymerization.
- Villine: maintains the filaments tight of actine fasciculée, specific of the microvillosities.
- Filamine: block the réticulée actine to prevent it from passing in actine fasciculée.
- Profiline: at the monomeric actine helping with the rehabilitation of the actine in polymer at the end +
- Thymosine is fixed: is fixed at the end - globular actine and prevents passages ATP-ADP.
- Myosine of the type 1: maintains the actine broad fasciculée with the assistance of alpha-actinine.
- Myosine of the type 2: muscular contraction.
- Myosine of the type 5: protein engine able to move towards the end +.
- Tropomoduline (stabilization).
- Gelosine (disintegration).
- formine: nucleator
- protein of cap (capping protein): binds at the end + and prevents its polymerization.
Certain toxins act on the filaments of actine:
- Cytochalasine: block polymerization
- Phalloïdine: be opposed to depolymerization
- Latrunculline: fixes themselves on the sub-units and prevents polymerization
Superstructures
The filaments of actine organize in very diverse ways according to their biological functions. Here some examples of structures observed and their principal properties: beams, cables of tension, networks, comets.
Roles
- Maintenance of the structural buildings for example of villosities.
- intracellular Movements: contraction of the muscular cells by bringing together of the filaments of actine around the Sarcomère causing the concentration of the muscles.
- cellular movements: displacement of the ameba, the leucocytes (pseudopodes).
- the Cytodiérèse (at the cellular end of the division).
- formation of the cellular junctions.
| Random links: | Binary X | Atomic Artist | Xantho (city) | Francisca Sadornil Ruiz | Francis Dhomont |