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Model of G-actin ( tertiary structure of the protein with bound ADP , shown in red in the center of the picture)
Model of an F-actin made up of 13 actin subunits (based on the filament model by Ken Holmes)

Actin (English actin ; from ancient Greek ἀκτίς aktis , ray ') is a structural protein that occurs in all eukaryotic cells . It is part of the cytoskeleton and one of the five most common proteins in eukaryotes; in muscle cells every tenth protein molecule is an actin molecule, in other cells the proportion is 1–5%.

Actin occurs in two states: as a globular single molecule or G-actin and strung together as a filament or F-actin . Actin filaments are characterized by their dynamic lengthening and shortening. Actin filaments are counted among the microfilaments of the cell. They serve to stabilize the outer cell shape as well as the formation of cell processes, intracellular shifts and directed cellular movements. In multicellular organisms, they become central components for muscle contraction . Changes in the genes coding for actins can lead to muscle and other diseases.


Actin was first demonstrated experimentally in 1887 by William Dobinson Halliburton . In analogy to the coagulation of blood plasma, he investigated the conditions under which proteins in cell fluids of the muscles change their state ("coagulate"), and highlighted the influence of an extract that he called "myosin ferment". Halliburton was unable to deepen his research, so that the actual discovery of actin is now attributed to Brunó Ferenc Straub , who worked as a research assistant in the Albert Szent-Györgyis laboratory at the Institute for Medicinal Chemistry at the University of Szeged in Hungary on proteins in muscle cells.

Muscle cells ( muscle fibers ) contain myofibrils ( muscle fibrils ); these are made up of proteins - actin forms thin filaments, myosin thicker ones (shown in blue).

In 1942, Straub developed a technique for extracting muscle proteins that allowed him to obtain considerable amounts of relatively pure actin, and which is still used today, essentially unchanged. Szent-Györgyi had previously described the more viscous form of myosin from slow muscle cells as the “activated” form, and since Straub's protein showed the same effect in myosin solutions, it was called “actin”. If you add ATP to the mixture or this composition of the two proteins, called “actomyosin” , its viscosity decreases again.

During the Second World War , Szent-Györgyi and Straub were unable to publish in western science journals. It was not until 1945 that they were able to publish their theses in the Acta Physiologica Scandinavica , with which the term actin also became known in the West. Straub continued his work with actin and reported in 1950 that actin contains bound ATP, which is hydrolyzed to ADP and inorganic phosphate during the polymerization of the microfilaments , whereby the resulting ADP initially remains bound. Straub suggested that the conversion of bound ATP to bound ADP would play a significant role in muscle contraction . In fact, this only applies to smooth muscles , as was only proven experimentally in 2001.

The sequence of the amino acids in the polypeptide chain of an actin was given in full in 1973 by M. Elzinga and co-workers. The crystal structure of G-actin was shown in 1990 by Kabsch and colleagues. In the same year, after carrying out (co) crystallization experiments with different proteins, Holmes and colleagues proposed a model for F-actin. The process of co-crystallization was used repeatedly in the following years, until 2001 the isolated protein could be made crystalline together with ADP. However, there is still no high-resolution X-ray structure analysis for actin. The crystallization of G-actin was made possible by the use of rhodamine-actin (G-actin which is chemically linked to the fluorescent dye rhodamine ), which prevents the polymerization by blocking the amino acid cysteine-374. Christine Oriol-Audit already succeeded in 1977 in crystallizing actin in the absence of actin-binding proteins (ABP). But the resulting crystals were too small for the technology of the time to be able to analyze them further.

Although there is currently no high-resolution model for filamentous actin (F-actin), Sawaya and his team were able to contribute to a more precise picture of the structure in 2008, based on the analysis of various crystals of actin dimers, two of which G-actins are linked via different binding sites. The model based on this was further refined by Sawaya and Lorenz. With methods of cryo- electron microscopy and the use of synchrotron radiation, a higher resolution could recently be achieved with the possibility of better understanding the interactions and conformational changes of G-actin during the transition to F-actin during the formation of actin filaments.


Actin is encoded by a family of genes. Humans have six paralogous variants which differ only in a few amino acids and which are expressed in different tissue types ; Functionally, these isoforms can be differentiated as alpha-, beta- or gamma-actins.

gene protein Gene locus Length ( AA ) OMIM Localization pathology
ACTA1 alpha-1 1q42.13 375 102610 Skeletal muscles Nemaline myopathy type 3 (NM3); Congenital Myopathies (CM / CFTD)
ACTA2 alpha-2 10q22-24 375 102620 Smooth musculature; aorta Familial thoracic aortic aneurysm type 6 (AAT6)
ACTB cytoplasmic-1; beta 7p15-p12 374 102630 Cytoplasm; Cytoskeleton juvenile-onset dystonia ; Diffuse large B-cell lymphoma
ACTC1 cardiac alpha 15q11-14 375 102540 Heart muscle Dilated cardiomyopathy type 1R (DCM1R); Hypertrophic cardiomyopathy type 11 (HCM11)
ACTG1 cytoplasmic-2; gamma-1 17q25 374 102560 Cytoplasm; Cytoskeleton non-syndromic sensorineural deafness autosomal dominant type 20 (DFNA20)
ACTG2 gamma-2 2p13.1 374 102545 Smooth musculature; Intestines

If actin is present as a single molecule (monomer), it is referred to as globular actin or G-actin . Its polypeptide chain , which is unfolded into a protein of spherical shape, has a length of 375 amino acids and a weight of approx. 42  kDa . Actin is an evolutionarily highly conserved protein; For example, the actins of algae and those of humans only differ from each other in a seventh of their amino acids, or the coding gene segments match over 85% in their base sequence . A bacterial homolog of actin is FtsA .

Actin filaments

Actin filaments, colors represent different layers.
Endothelial cells of the pulmonary artery of a bovine under the microscope after marking with different fluorescent dyes. The actin filaments appear red (due to TRITC ), while the microtubules appear green (due to FITC ). The DNA of the six cell nuclei appears blue (through DAPI ).

Monomeric globular or G-actin can attach to one another to form dimeric , oligomeric and polymeric structures. Polymeric filamentary or F-actin is the main component of microfilaments . The so-called polymerization process from G-actin to F-actin is a non-covalent stringing together of G-actin units to form a double-chain, helical actin filament. This process is dynamic - as is the reverse process, the degradation of the filament to G-actin. The actin network of a cell adapts quickly to current requirements and thus contributes to cell movement. The build-up and breakdown of actin filaments can be inhibited by cytoskeleton inhibitors .



As part of the cytoskeleton, actin forms a dense, stiff, three-dimensional cortical network below the plasma membrane, which is linked by the above-mentioned connecting proteins. This network is more pronounced at certain specific points in the cell, e.g. B. in membrane bulges ( microvilli , pseudopodia , synapses ) and at certain cell contacts ( adherens junctions , tight junctions ) and thus contribute to the shape and stability of cells and tissues.

Anchoring and transport route

Many transmembrane proteins (channels, pumps, receptors, cell adhesion proteins) are held directly or indirectly in place on this cortical actin network. Functionally related proteins are kept in close proximity. The short-distance transport of vesicles to the membrane by myosins, a class of motor proteins, takes place along the actin network (while the long-distance transport is carried out by microtubules with their motor proteins dynein and kinesin ). The myosins partly take over the charges brought by Dynein / Kinesin.

Cell motility

Many eukaryotic cells have a high degree of mobility, called cell motility. It allows the cells to change their shape in a targeted manner up to and including wandering movements, or cell migration ; for example in the development of nerve cells or in unicellular organisms such as amoebas . Cells of the immune system use their motility to render cells recognized as foreign in the body harmless; skin cells need them, for example, so that wounds can heal. This mobility is made possible by various processes.

In order to "feel" the cell environment and initiate a new direction of movement, the formation of cell outgrowths such as filopodia and lamellipodia play an important role. These are formed and stabilized by actin filaments. With actin polymerisation directed in the direction of movement, a cell can react to signals from the cell periphery and develop cell processes . Through actin-myosin interaction in fibril bundles (stress fibers), these become contractile pull cords that run through the cell and brace them with elements of the base or matrix .

Cross bridges between actin filaments and the heads of myosin molecules are the basis of the actin-myosin interaction

Contractile structures

The contraction apparatus of all types of muscles , i.e. all macroscopic movements of the body and its internal organs (e.g. intestinal peristalsis ), are based on the actin-myosin interaction . Numerous actin filaments, myosin II and other proteins are arranged in large numbers in a highly ordered manner. See the articles on muscle tissue for details .

Individual evidence

  1. Uncomplexed Actin , Protein Data Bank
  2. KC Holmes, D. Popp, W. Gebhard, W. Kabsch: Atomic model of the actin filament. In: Nature. 347, 1990, pp. 21-22. PMID 2395461 .
  3. Renate Wahrig-Burfeind (Ed.): True. Illustrated dictionary of the German language . ADAC-Verlag, Munich 2004, ISBN 3-577-10051-6 , pp. 39 .
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  5. ^ WD Halliburton : On Muscle Plasma . In: J. Physiol. (Lond.) . tape 8 , no. 3-4 , August 1887, pp. 133-202 , PMID 16991477 , PMC 1485127 (free full text). See in particular p. 147.
  6. A. Szent-Gyorgyi: Studies on muscle . In: Acta Physiol Scand . tape 9 , Suppl, 1945, pp. 25 .
  7. a b F. B. Straub, G. Feuer: Adenosine triphosphate. The functional group of actin. 1950 . In: Biochim. Biophys. Acta . tape 1000 , 1989, pp. 180-195 , doi : 10.1016 / 0006-3002 (50) 90052-7 , PMID 2673365 .
  8. M. Bárány, JT Barron, L. Gu, K. Bárány: Exchange of the actin-bound nucleotide in intact arterial smooth muscle . In: J. Biol. Chem. Volume 276 , no. 51 , December 2001, p. 48398-48403 , PMID 11602582 . doi : 10.1074 / jbc.M106227200 (currently not available)
  9. M. Elzinga, JH Collins, WM Kuehl, RS Adelstein: Complete amino-acid sequence of actin of rabbit skeletal muscle . In: Proc. Natl. Acad. Sci. USA band 70 , no. 9 September 1973, p. 2687–2691 , doi : 10.1073 / pnas.70.9.2687 , PMID 4517681 , PMC 427084 (free full text).
  10. W. Kabsch, HG man heart, D. Suck, EF Pai, KC Holmes: Atomic structure of the actin: DNase I complex . In: Nature . tape 347 , no. 6288 , September 1990, pp. 37-44 , doi : 10.1038 / 347037a0 , PMID 2395459 .
  11. PDB  1J6Z ; LR Otterbein, P. Graceffa, R. Dominguez: The crystal structure of uncomplexed actin in the ADP state . In: Science . tape 293 , no. 5530 , July 2001, p. 708-711 , doi : 10.1126 / science.1059700 , PMID 11474115 .
  12. C. Oriol, C. Dubord, F. Landon: Crystallization of native striated-muscle actin . In: FEBS Lett. tape 73 , no. 1 , January 1977, p. 89-91 , doi : 10.1016 / 0014-5793 (77) 80022-7 , PMID 320040 .
  13. MR Sawaya, DS Kudryashov, I. Pashkov, H. Adisetiyo, E. Reisler, TO Yeates: multiple crystal structures of actin dimers and Their implications for interactions in the actin filament . In: Acta Crystallogr. D Biol. Crystallogr. tape 64 , Pt 4, April 2008, p. 454-465 , doi : 10.1107 / S0907444908003351 , PMID 18391412 , PMC 2631129 (free full text).
  14. A. Narita, S. Takeda, A. Yamashita, Y. Maéda: Structural basis of actin filament capping at the barbed-end: a cryo-electron microscopy study . In: EMBO J. Band 25 , no. 23 November 2006, pp. 5626-5633 , doi : 10.1038 / sj.emboj.7601395 , PMID 17110933 , PMC 1679762 (free full text).
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