The eukaryotic cytoskeleton. Actin filaments are shown in red, microtubules in green, and the nuclei are in blue.
Actin is a globular structural protein that polymerizes in a helically fashion to form actin filaments (or microfilaments). These form the cytoskeleton - a three-dimensional network inside an eukaryotic cell. Actin filaments provide mechanical support for the cell, determine the cell shape, enable cell movements (through lamellipodia, filopodia, or pseudopodia); and participate in certain cell junctions, in cytoplasmic streaming and in contraction of the cell during cytokinesis. In muscle cells they play an essential role, along with myosin, in muscle contraction. In the cytosol, actin is predominantly bound to ATP, but can also bind to ADP. An ATP-actin complex polymerizes faster and dissociates slower than an ADP-actin complex. Actin is one of the most abundant proteins in many eukaryotic cells, with concentrations of over 100 μM. It is also one of the most highly conserved proteins, differing by no more than 5% in species as diverse as algae and humans.
F-Actin; surface representation of 13 subunit repeat based on Ken Holmes' actin filament model
The individual subunits of actin are known as globular actin (G-actin), while the filamentous polymer composed of G-actin subunits (a microfilament), is called F-actin. The microfilaments are the thinnest component of the cytoskeleton, measuring only 7 nm in diameter. Much like the microtubules, actin filaments are polar, with a fast growing plus (+) or barbed end and a slow growing minus (-) or pointed end. The terms barbed and
pointed endcome from the arrow-like appearance of microfilaments decorated with the motor domain of myosin as seen in electronmicrographs. Filaments elongate approximately 10 times faster at the plus (+) end than the minus (-) end. This phenomenon is known as the treadmill effect. The process of actin polymerization, nucleation, starts with the association of three G-actin monomers into a trimer. ATP-actin then binds the plus (+) end, and the ATP is subsequently hydrolyzed, which reduces the binding strength between neighboring units and generally destabilizes the filament. ADP-actin dissociates from the minus end and the increase in ADP-actin stimulates the exchange of bound ADP for ATP, leading to more ATP-actin units. This rapid turnover is important for the cell’s movement. End-capping proteins such as CapZ prevent the addition or loss of monomers at the filament end where actin turnover is unfavourable like in the muscle apparatus.
The protein cofilin binds to ADP-actin units and promotes their dissociation from the minus end and prevents their reassembly. The protein profilin reverses this effect by stimulating the exchange of bound ADP for ATP. In addition, ATP-actin units bound to profilin will dissociate from cofilin and are then free to polymerize. Another important component in filament production is the Arp2/3 complex, which nucleates new actin filaments while bound to existing filaments, thus creating a branched network. All of these three proteins are regulated by cell signaling mechanism.
Actin filaments are assembled in two general types of structures: bundles and networks. Actin-binding proteins dictate the formation of either structure since they cross-link actin filaments. Actin filaments have the appearance of a double-stranded helix.
In non-muscle actin bundles, the filaments are held together such that they are parallel to each other by actin-bundling proteins and/or cationic species. Bundles play a role in many cellular processes such as cell division (cytokinesis) and cell movement. For example, in vertebrates, the actin-bundling protein villin is almost entirely responsible for causing bundle formations in the microvilli of intestinal cells.
Actin, together with myosin filaments, form actomyosin, which provides the mechanism for muscle contraction. Muscular contraction uses ATP for energy. The ATP allows, through hydrolysis, the myosin head to extend up and bind with the actin filament. The myosin head then releases after moving the actin filament in a relaxing or contracting movement by usage of ADP.
In contractile bundles, the actin-bundling protein actinin separates each filament by 40 nm. This increase in distance allows the motor protein myosin to interact with the filament, enabling deformation or contraction. In the first case, one end of myosin is bound to the plasma membrane while the other end walks towards the plus end of the actin filament. This pulls the membrane into a different shape relative to the cell cortex. For contraction, the myosin molecule is usually bound to two separate filaments and both ends simultaneously walk towards their filament's plus end, sliding the actin filaments over each other. This results in the shortening, or contraction, of the actin bundle (but not the filament). This mechanism is responsible for muscle contraction and cytokinesis, the division of one cell into two.
Actin networks, along with many actin-binding proteins (such as the Arp2/3 complex and filamin) form a complex network at the cortical regions of the cell. Recent studies have also suggested that actin network on the cell cortex serve as barriers for molecular diffusion within the plasmic membrane.
Actin is one of the most highly conserved proteins, with 80.2% sequence conservation at the gene level between Homo sapiens and Saccharomyces cerevisiae, and 95% conservation of the primary structure of the protein product.
Although most yeasts have only a single actin gene, higher eukaryotes generally express several isoforms of actin encoded by a family of related genes. Mammals have at least six actins, which are divided into three classes (alpha, beta and gamma) according to their isoelectric point. Alpha actins are generally found in muscle, whereas beta and gamma isoforms are prominent in non-muscle cells. Although there are small differences in sequence and properties between the isoforms, all actins assemble into microfilaments and are essentially identical in the majority of tests performed in vitro.
The typical actin gene has an approximately 100 nucleotide 5' UTR, a 1200 nucleotide translated region, and a 200 nucleotide 3' UTR. The majority of actin genes are interrupted by introns, with up to 6 introns in any of 19 well-characterised locations. The high conservation of the family makes actin the favoured model for studies comparing the introns-early and introns-late models of intron evolution.
All non-spherical prokaryotes appear to possess genes such as MreB which encode homologues of actin; these genes are required for the cell's shape to be maintained. The plasmid-derived gene ParM encodes an actin-like protein whose polymerised form is dynamically unstable, and appears to partition the plasmid DNA into the daughter cells during cell division by a mechanism analogous to that employed by microtubules in eukaryotic mitosis.
Actin was first observed experimentally in 1887 by W.D. Halliburton, who extracted a protein from muscle which 'coagulated' preparations of myosin, and which he dubbed "myosin-ferment". However, Halliburton was unable to further characterise his findings and the discovery of actin is generally credited instead to Brúnó F. Straub, a young biochemist working in Albert Szent-Gyorgyi's laboratory at the Institute of Medical Chemistry at the University of Szeged, Hungary.
In 1942 Straub developed a novel technique for extracting muscle protein that allowed him to isolate substantial amounts of relatively pure actin. Straub's method is essentially the same as that used in laboratories today. Szent-Gyorgyi had previously described the more viscous form of myosin produced by slow muscle extractions as 'activated' myosin, and since Straub's protein produced the activating effect, it was dubbed 'actin'. The hostilities of World War II meant that Szent-Gyorgyi and Straub were unable to publish the work in Western scientific journals; it became well-known in the West only in 1945, when it was published as a supplement to the Acta Physiologica Scandinavica.
Straub continued to work on actin and in 1950 reported that actin contains bound ATP and that, during polymerisation of the protein into microfilaments, the nucleotide is hydrolysed to ADP and inorganic phosphate (which remain bound in the microfilament). Straub suggested that the transformation of ATP-bound actin to ADP-bound actin played a role in muscular contraction. In fact this is only true in smooth muscle, and was not experimentally supported until 2001.
The crystal structure of G-actin was solved in 1990 by Kabsch and colleagues. In the same year a model for F-actin was proposed by Holmes and colleagues. The model was derived by fitting a helix of G-actin structures according to low-resolution fibre diffraction data from the filament. Several models of the filament have been proposed since. However there is still no x-ray structure of F-actin.
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