What is actin?
Actin is a developmentally very old protein molecule. As a structural protein it is contained in the cytoplasm of every eukaryotic cell and in the sarcomere of all muscle fibers.
Together with the microtubules and intermediary filaments, it forms the cytoskeleton of each cell in the form of actin filaments. It is partly responsible for the formation of the cell structure and the movement of molecules and cell organelles within the cell. The same applies to the cohesion of cells via tight junctions or adherens junctions. In the muscle fibers actin produces together with the proteins Myosin, Troponin and Tropomyosin the muscle contractions.
Actin can be divided into the three functional units alpha-actin, beta-actin and gamma-actin. Alpha actin is the structural component of the muscle fibers, while beta and gamma actin are mainly found in the cytoplasm of the cells. Actin is a very conserved protein that already occurs with very small differences in the amino acid sequence in unicellular eukaryotic cells. In humans, 10 percent of all protein molecules in the muscle cells are actin. All other cells still contain 1 to 5 percent of this molecule in the cytoplasm.
Function, effect & tasks
Actin fulfills important functions in the cells and muscle fibers. As a component of the cytoskeleton, it forms a dense, three-dimensional network in the cytoplasm of the cell, which holds the cellular structures together.
At certain points of the network, the structures increase to membrane bulges such as microvilli, synapses or pseudopodia. For the cell contacts the Adherens junctions and tight junctions are available. Overall, actin contributes to the stability and shape of cells and tissues. In addition to stability, actin also ensures the transport processes within the cell. It binds important structurally related transmembrane proteins so that they remain in close proximity. With the help of myosines (motor proteins), the actin fibers also take care of transports over short distances.
For example, the vesicles can be transported to the membrane. Longer distances are taken over by the microtubules with the help of the motor proteins kinesin and dynein. Furthermore, actin also ensures cell mobility. Cells need to be able to migrate within the body on many occasions. This applies in particular to immune reactions or wound healing as well as general movements or changes in the shape of cells. The movements can be based on two different processes. On the one hand, the movement can be triggered by a directed polymerization reaction and on the other hand via the actin-myosin interaction.
In the actin-myosin interaction, the actin fibers are built up as fibril bundles that function like pull ropes using myosin. By Actinfilamente cell outgrowths can be formed in the form of pseudopodia (filopodia and lamellipodia). In addition to its multiple functions within the cell, actin is of course responsible for muscle contraction of both skeletal muscle and smooth muscle. These movements are also based on the actin-myosin interaction. To ensure this, many actin filaments are associated with other proteins in a very orderly manner.
Education, occurrence, properties & optimal values
As already mentioned, actin occurs in all eukaryotic organisms and cells. It is an intrinsic component of the cytoplasm and ensures the stability of the cells, the anchoring of structurally related proteins, the short-distance transport of vesicles to the cell membrane and cell motility. Without actin cell survival would not be possible. There are six different actin variants, which are divided into three alpha variants, one beta variant and two gamma variants.
The alpha actins are involved in the development and contraction of the muscles. Beta-actin and gamma-1-actin are very important for the cytoskeleton in the cytoplasm. Gamma-2 actin is again responsible for smooth muscle and intestinal musculature. The synthesis initially forms monomeric globular actin, which is also known as G-actin. The individual monomeric protein molecules are in turn combined to form a filamentary F-actin under polymerization.
In the polymerization process, several spherical monomers combine to form a long filamentary F-actin. Both the construction and the dismantling of the chains are very dynamic. Thus, the Aktingerüst can be adapted quickly to the current requirements. In addition, this process also ensures cell movements. These reactions can be inhibited by so-called cytoskeletal inhibitors. These substances inhibit either polymerizations or depolymerizations. They have medicinal significance as chemotherapy in the context of chemotherapy.
Diseases & Disorders
Since actin is an essential component of all cells, many mutationally induced structural changes lead to the death of the organism. Mutations in genes for alpha actins can cause muscle disease. This is especially true for the Alpha-1 actin.
Due to the fact that alpha-2-actin is responsible for the aortic muscle, a familial thoracic aortic aneurysm may occur in a mutation of the ACTA2 gene. The gene ACTA2 encodes the alpha-2-actin. A mutation of the ACTC1 gene for cardiac alpha actin causes dilated cardiomyopathy. Furthermore, mutation of ACTB as a cytoplasmic beta-actin gene can cause large cell and diffuse B-cell lymphoma. Some autoimmune diseases may have elevated levels of actin antibodies.
In particular, this applies to the autoimmune liver inflammation. It is a chronic hepatitis, which leads in the long term to cirrhosis of the liver. Here an antibody against smooth muscle actin is found. However, in terms of differential diagnosis, autoimmune hepatitis is not so easy to distinguish from chronic viral hepatitis. For chronic viral hepatitis, antibodies against actin can be stimulated to a lesser extent.