Microfilament

Microfilaments are filamentous protein structures in eukaryotic cells.

Together with the microtubules and intermediate filaments form the main body of the cytoskeleton. They consist mainly of the protein actin and are therefore also called actin filaments. The label " microfilaments " is because they significantly thinner with a diameter of only six nanometers are as microtubules and intermediate filaments. Functionally, they play a role in active movement of the cells, wherein the intracellular transport processes and the mechanical stabilization of the cells.

Assembly of the filaments

G- actin ( globular actin, monomer ) binds the nucleotide ATP. This monomer can now connect to other actin molecules - polymerize, with ATP with cleavage (hydrolysis ) of a phosphate residue to ADP. The resulting chain of actin monomers, forming the filamentous form of actin filaments, also called F- actin. The filament consists of two chains of polymerized G -actin monomers umeinanderwinden be helically. The diameter is 7 nm in the cell, both forms are in equilibrium are, in which monomers occur mainly in complex with actin -binding proteins.

Each actin filament has a ( ) end and a ( - ) end. ATP binds to the actin preferably ( ) end, and the filament grows at this end. The ATP is hydrolyzed into ADP in the sequence, whereby the bonding strength to the adjacent actins decreases. The (-) end of the hydrolysis of ATP to ADP is faster than the addition of a new ATP - actin, actin ADP so dissociated and the filament is reduced from this page. But actin monomers bind ATP more strongly than ADP, swap so the nucleotide and can be re- inserted at the ( ) end. This rapid cycle is important for cell movement, and is referred to as treadmilling.

Numerous accompanying proteins control the polymerization and degradation processes. In muscle, the filaments are stabilized for example by tropomyosin, which applies to the entire length of a filament.

Certain proteins also cover the ends and hinder or promote the extension or further degradation. Other proteins prevent or promote the polymerization of G -actin or cause the collapse of the F- actin.

For example, the proteins cofilin and ADF set (actin depolymerizing factor ) to the (-) end and promote the dissociation of actin. The protein profilin other hand, encourages the installation of the ( ) end. The binding of both Cofilin and Profilin is determined by the nucleotide Aktingebundene.

And post-translational modifications are involved in actin polymerization. So every fifth actin monomer is provided in fibroblasts with a arginylation, which has a direct impact on the increased stability of actin filaments. This beta -actin is modified priority.

Adapter and connecting proteins

A large group of concomitant proteins are also referred to as actin -binding proteins, actin filaments crosslinked with each other and with other proteins. Fibrin, villin ( internal skeleton of the microvilli ), and filamin form Espin cross-links and as mechanically rigid bundle. α -actinin also forms bundles (see below) are typically braced with myosin. Filamin the turn forms three-dimensional networks ( gels), as found at the plasma membrane.

Actin filaments radiate into two cell contacts, the adherens contacts and focal contacts. They are anchored via adapter proteins to the protein structures of the contacts. Responsible for this are, among others, again, the α -actinin, vinculin, and talin that. The proteins of the family of ezrin, radixin, moesin (ERM proteins) provide temporary and dynamic binding to the plasma membrane, for example when changing the cell shape and movement of active cell.

Certain protein groups produce a mechanically stable connection between the underlying the plasma membrane and the diaphragm seal Aktinnetz. This because of various genetic diseases and clinically quite significant proteins are dystrophins ( inter alia in the muscle tissue, wherein mutations in the dystrophin complex muscular dystrophy ), and the Spectrine ( inter alia, responsible for the shape of the red blood cells in case of defect, for example, ball -cell anemia ). It is long, thinner proteins that fulfill their tasks in complexes with numerous other proteins.