Discovery and early investigationActin was first observed ally in 1887 by W.D. Halliburton, who extracted a protein from muscle that 'coagulated' preparations of that he called "myosin-ferment". However, Halliburton was unable to further refine his findings, and the discovery of actin is credited instead to Brunó Ferenc Straub, a young working in 's laboratory at the Institute of Medical Chemistry at the , . Following up on the discovery of Ilona Banga & Szent-Györgyi in 1941 that the coagulation only occurs in some mysosin extractions and was reversed upon the addition of ATP, Straub identified and purified actin from those myosin preparations that did coagulate. Building on Banga's original extraction method, he developed a novel technique for extracting muscle protein that allowed him to isolate substantial amounts of relatively actin, published in 1942. Straub's method is essentially the same as that used in today. Since Straub's protein was necessary to activate the coagulation of myosin, it was dubbed ''actin''. Realizing that Banga's coagulating myosin preparations contained actin as well, Szent-Györgyi called the mixture of both proteins actomyosin. The hostilities of meant Szent-Gyorgyi was unable to publish his lab's work in s. Actin therefore only became well known in the West in 1945, when their paper was published as a supplement to the ''Acta Physiologica Scandinavica''. Straub continued to work on actin, and in 1950 reported that actin contains bound and that, during ization of the protein into s, the is to and inorganic (which remain bound to 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 true only in , and was not supported through experimentation until 2001. The amino acid sequencing of actin was completed by M. Elzinga and co-workers in 1973. The 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 following experiments using co-crystallization with different proteins. The procedure of co-crystallization with different proteins was used repeatedly during the following years, until in 2001 the isolated protein was crystallized along with ADP. However, there is still no high-resolution X-ray structure of F-actin. The crystallization of F-actin was possible due to the use of a conjugate that impedes polymerization by blocking the amino acid .; Christine Oriol-Audit died in the same year that actin was first crystallized but she was the researcher that in 1977 first crystallized actin in the absence of Actin Binding Proteins (ABPs). However, the resulting crystals were too small for the available technology of the time. Although no high-resolution model of actin's filamentous form currently exists, in 2008 Sawaya's team were able to produce a more exact model of its structure based on multiple crystals of actin dimers that bind in different places. This model has subsequently been further refined by Sawaya and Lorenz. Other approaches such as the use of and have recently allowed increasing resolution and better understanding of the nature of the interactions and conformational changes implicated in the formation of actin filaments.
StructureActin's is one of the most highly conserved of the proteins as it has changed little over the course of , differing by no more than 20% in as diverse as and s. It is therefore considered to have an optimised . It has two distinguishing features: it is an that slowly , the "universal energy currency" of biological processes. However, the ATP is required in order to maintain its structural integrity. Its efficient structure is formed by an almost unique process. In addition, it is able to carry out more interactions than any other protein, which allows it to perform a wider variety of functions than other proteins at almost every level of cellular life. is an example of a protein that bonds with actin. Another example is , which can weave actin into bundles or cut the filaments depending on the concentration of cations in the surrounding medium. Actin is one of the most abundant proteins in s, where it is found throughout the cytoplasm. In fact, in it comprises 20% of total cellular protein by weight and between 1% and 5% in other cells. However, there is not only one type of actin; the s that code for actin are defined as a (a family that in plants contains more than 60 elements, including genes and s and in humans more than 30 elements). This means that the genetic information of each individual contains instructions that generate actin variants (called s) that possess slightly different functions. This, in turn, means that eukaryotic organisms different genes that give rise to: α-actin, which is found in contractile structures; β-actin, found at the expanding edge of cells that use the projection of their cellular structures as their means of mobility; and γ-actin, which is found in the filaments of stress fibres. In addition to the similarities that exist between an organism's isoforms there is also an in the structure and function even between organisms contained in different eukaryotic domains. In the actin homologue has been identified, which is a protein that is capable of polymerizing into microfilaments; and in the homologue Ta0583 is even more similar to the eukaryotic actins. Cellular actin has two forms: monomeric globules called G-actin and filaments called F-actin (that is, as filaments made up of many G-actin monomers). F-actin can also be described as a microfilament. Two parallel F-actin strands must rotate 166 degrees to lie correctly on top of each other. This creates the double helix structure of the microfilaments found in the cytoskeleton. Microfilaments measure approximately 7 nm in diameter with the helix repeating every 37 nm. Each molecule of actin is bound to a molecule of (ATP) or (ADP) that is associated with a cation. The most commonly found forms of actin, compared to all the possible combinations, are ATP-G-Actin and ADP-F-actin.
G-Actinimages indicate that G-actin has a globular structure; however, shows that each of these globules consists of two lobes separated by a cleft. This structure represents the “ATPase fold”, which is a centre of enzymatic catalysis that binds ATP and Mg2+ and hydrolyzes the former to ADP plus . This fold is a conserved structural motif that is also found in other proteins that interact with triphosphate s such as (an enzyme used in energy ) or in proteins (a protein family that play an important part in protein folding). G-actin is only functional when it contains either ADP or ATP in its cleft but the form that is bound to ATP predominates in cells when actin is present in its free state. The model of actin that was produced by Kabsch from the of is the most commonly used in structural studies as it was the first to be purified. The G-actin crystallized by Kabsch is approximately 67 x 40 x 37 in size, has a of 41,785 Da and an estimated of 4.8. Its net charge at = 7 is -7. ;Primary structure Elzinga and co-workers first determined the complete for this type of actin in 1973, with later work by the same author adding further detail to the model. It contains 374 residues. Its is highly ic and starts with an ed aspartate in its amino group. While its is and is formed by a preceded by a , which has a degree of functional importance. Both extremes are in close proximity within the I-subdomain. An anomalous is located at position 73. ;Tertiary structure — domains The tertiary structure is formed by two domains known as the large and the small, which are separated by a cleft centred around the location of the bond with - + . Below this there is a deeper notch called a “groove”. In the , despite their names, both have a comparable depth. The normal convention in studies means that a protein is shown with the biggest domain on the left-hand side and the smallest domain on the right-hand side. In this position the smaller domain is in turn divided into two: subdomain I (lower position, residues 1–32, 70–144, and 338–374) and subdomain II (upper position, residues 33–69). The larger domain is also divided in two: subdomain III (lower, residues 145–180 and 270–337) and subdomain IV (higher, residues 181–269). The exposed areas of subdomains I and III are referred to as the “barbed” ends, while the exposed areas of domains II and IV are termed the “pointed" ends. This nomenclature refers to the fact that, due to the small mass of subdomain II actin is polar; the importance of this will be discussed below in the discussion on assembly dynamics. Some authors call the subdomains Ia, Ib, IIa, and IIb, respectively. ;Other important structures The most notable supersecondary structure is a five chain that is composed of a β-meander and a β-α-β clockwise unit. It is present in both domains suggesting that the protein arose from gene duplication. * The binding site is located between two -shaped structures pertaining to the I and III domains. The residues that are involved are Asp11-Lys18 and Asp154-His161 respectively. * The binding site is located just below that for the adenosine nucleotide. ''In vivo'' it is most often formed by or while ''in vitro'' it is formed by a chelating structure made up of and two s from the nucleotide's α-and β- s. This calcium is coordinated with six water molecules that are retained by the amino acids Asp11, Asp154, and . They form a complex with the nucleotide that restricts the movements of the so-called "hinge" region, located between residues 137 and 144. This maintains the native form of the protein until its withdrawal denatures the actin monomer. This region is also important because it determines whether the protein's cleft is in the "open" or "closed" conformation. * It is highly likely that there are at least three other centres with a lesser (intermediate) and still others with a low affinity for divalent cations. It has been suggested that these centres may play a role in the polymerization of actin by acting during the activation stage. * There is a structure in subdomain 2 that is called the “D-loop” because it binds with , it is located between the and residues. It has the appearance of a disorderly element in the majority of crystals, but it looks like a β-sheet when it is complexed with DNase I. It has been proposed that the key event in polymerization is probably the propagation of a conformational change from the centre of the bond with the nucleotide to this domain, which changes from a loop to a spiral. However, this hypothesis has been refuted by other studies.
F-ActinThe classical description of F-actin states that it has a filamentous structure that can be considered to be a single stranded levorotatory with a rotation of 166° around the helical axis and an axial translation of 27.5 , or a single stranded helix with a cross over spacing of 350–380 Å, with each actin surrounded by four more. The symmetry of the actin polymer at 2.17 subunits per turn of a helix is incompatible with the formation of s, which is only possible with a symmetry of exactly 2, 3, 4 or 6 subunits per turn. Therefore, models have to be constructed that explain these anomalies using data from , , crystallization of dimers in different positions and . It should be pointed out that it is not correct to talk of a “structure” for a molecule as dynamic as the actin filament. In reality we talk of distinct structural states, in these the measurement of axial translation remains constant at 27.5 Å while the subunit rotation data shows considerable variability, with displacements of up to 10% from its optimum position commonly seen. Some proteins, such as appear to increase the angle of turn, but again this could be interpreted as the establishment of different structural states. These could be important in the polymerization process. There is less agreement regarding measurements of the turn radius and filament thickness: while the first models assigned a length of 25 Å, current X-ray diffraction data, backed up by cryo-electron microscopy suggests a length of 23.7 Å. These studies have shown the precise contact points between monomers. Some are formed with units of the same chain, between the "barbed" end on one monomer and the "pointed" end of the next one. While the monomers in adjacent chains make lateral contact through projections from subdomain IV, with the most important projections being those formed by the C-terminus and the hydrophobic link formed by three bodies involving residues 39–42, 201–203, and 286. This model suggests that a filament is formed by monomers in a "sheet" formation, in which the subdomains turn about themselves, this form is also found in the bacterial actin homologue . The F-actin polymer is considered to have structural polarity due to the fact that all the microfilament's subunits point towards the same end. This gives rise to a naming convention: the end that possesses an actin subunit that has its ATP binding site exposed is called the "(-) end", while the opposite end where the cleft is directed at a different adjacent monomer is called the "(+) end". The terms "pointed" and "barbed" referring to the two ends of the microfilaments derive from their appearance under when samples are examined following a preparation technique called "decoration". This method consists of the addition of S1 fragments to tissue that has been fixed with . This myosin forms polar bonds with actin monomers, giving rise to a configuration that looks like arrows with feather fletchings along its shaft, where the shaft is the actin and the fletchings are the myosin. Following this logic, the end of the microfilament that does not have any protruding myosin is called the point of the arrow (- end) and the other end is called the barbed end (+ end). A S1 fragment is composed of the head and neck domains of . Under physiological conditions, G-actin (the form) is transformed to F-actin (the form) by ATP, where the role of ATP is essential. The helical F-actin filament found in muscles also contains a molecule, which is a 40 long protein that is wrapped around the F-actin helix. During the resting phase the tropomyosin covers the actin's active sites so that the actin-myosin interaction cannot take place and produce muscular contraction. There are other protein molecules bound to the tropomyosin thread, these are the s that have three polymers: , , and .
FoldingActin can spontaneously acquire a large part of its . However, the way it acquires its from its newly native form is special and almost unique in protein chemistry. The reason for this special route could be the need to avoid the presence of incorrectly folded actin monomers, which could be toxic as they can act as inefficient polymerization terminators. Nevertheless, it is key to establishing the stability of the cytoskeleton, and additionally, it is an essential process for coordinating the . CCT is required in order to ensure that folding takes place correctly. CCT is a group II chaperonin, a large protein complex that assists in the folding of other proteins. CCT is formed of a double ring of eight different subunits (hetero-octameric) and it differs from group I chaperonins like , which is found in Eubacteria and in eukaryotic organelles, as it does not require a co-chaperone to act as a lid over the central cavity. Substrates bind to CCT through specific domains. It was initially thought that it only bound with actin and , although recent studies have shown that it interacts with a large number of s, which possibly function as substrates. It acts through ATP-dependent conformational changes that on occasion require several rounds of liberation and catalysis in order to complete a reaction. In order to successfully complete their folding, both actin and tubulin need to interact with another protein called , which is a heterohexameric complex (formed by six distinct subunits), in an interaction that is so specific that the molecules have coevolved. Actin complexes with prefoldin while it is still being formed, when it is approximately 145 s long, specifically those at the N-terminal. Different recognition sub-units are used for actin or tubulin although there is some overlap. In actin the subunits that bind with prefoldin are probably PFD3 and PFD4, which bind in two places one between residues 60–79 and the other between residues 170–198. The actin is recognized, loaded, and delivered to the cytosolic chaperonin (CCT) in an open conformation by the inner end of prefoldin's "tentacles” (see the image and note). The contact when actin is delivered is so brief that a tertiary complex is not formed, immediately freeing the prefoldin. The CCT then causes actin's sequential folding by forming bonds with its subunits rather than simply enclosing it in its cavity. This is why it possesses specific recognition areas in its apical β-domain. The first stage in the folding consists of the recognition of residues 245–249. Next, other determinants establish contact. Both actin and tubulin bind to CCT in open conformations in the absence of ATP. In actin's case, two subunits are bound during each conformational change, whereas for tubulin binding takes place with four subunits. Actin has specific binding sequences, which interact with the δ and β-CCT subunits or with δ-CCT and ε-CCT. After AMP-PNP is bound to CCT the substrates move within the chaperonin's cavity. It also seems that in the case of actin, the CAP protein is required as a possible cofactor in actin's final folding states. The exact manner by which this process is regulated is still not fully understood, but it is known that the protein PhLP3 (a protein similar to ) inhibits its activity through the formation of a tertiary complex.
ATPase’s catalytic mechanismActin is an , which means that it is an that ATP. This group of enzymes is characterised by their slow reaction rates. It is known that this ATPase is “active”, that is, its speed increases by some 40,000 times when the actin forms part of a filament. A reference value for this rate of hydrolysis under ideal conditions is around 0.3 s−1. Then, the Pi remains bound to the actin next to the ADP for a long time, until it is cooperatively liberated from the interior of the filament. The exact molecular details of the catalytic mechanism are still not fully understood. Although there is much debate on this issue, it seems certain that a "closed" conformation is required for the hydrolysis of ATP, and it is thought that the residues that are involved in the process move to the appropriate distance. The Glu137 is one of the key residues, which is located in subdomain 1. Its function is to bind the water molecule that produces a on the ATP's γ-phosphate , while the nucleotide is strongly bound to subdomains 3 and 4. The slowness of the catalytic process is due to the large distance and skewed position of the water molecule in relation to the reactant. It is highly likely that the conformational change produced by the rotation of the domains between actin's G and F forms moves the Glu137 closer allowing its hydrolysis. This model suggests that the polymerization and ATPase's function would be decoupled straight away. The "open" to "closed" transformation between G and F forms and its implications on the relative motion of several key residues and the formation of water wires have been characterized in and simulations.
GeneticsActin has been one of the most highly conserved proteins throughout evolution because it interacts with a large number of other proteins. It has 80.2% sequence at the level between '' '' and '' '' (a species of yeast), and 95% conservation of the of the protein product. Although most s have only a single actin gene, higher s, in general, several s of actin encoded by a family of related genes. s have at least six actin isoforms coded by separate genes, which are divided into three classes (alpha, , and gamma) according to their s. In general, alpha actins are found in muscle (α-skeletal, α-aortic smooth, α-cardiac), whereas beta and gamma isoforms are prominent in non-muscle cells (β-cytoplasmic, γ1-cytoplasmic, γ2-enteric smooth). Although the amino acid sequences and '' '' properties of the isoforms are highly similar, these isoforms cannot completely substitute for one another '' ''. The typical actin gene has an approximately 100-nucleotide , a 1200-nucleotide region, and a 200-nucleotide . The majority of actin genes are interrupted by s, with up to six 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 s appear to possess genes such as , which encode homologues of actin; these genes are required for the cell's shape to be maintained. The -derived gene ParM encodes an actin-like protein whose polymerized form is , and appears to partition the plasmid into its daughter cells during cell division by a mechanism analogous to that employed by microtubules in eukaryotic . Actin is found in both smooth and rough endoplasmic reticulums.
Nucleation and polymerizationNucleating factors are necessary to stimulate actin polymerization. One such nucleating factor is the , which mimics a G-actin dimer in order to stimulate the nucleation (or formation of the first trimer) of monomeric G-actin. The binds to actin filaments at 70 degrees to form new actin branches off existing actin filaments. Arp2/3-mediated nucleation is necessary for directed cell migration. Also, actin filaments themselves bind ATP, and hydrolysis of this ATP stimulates destabilization of the polymer. The growth of actin filaments can be regulated by thymosin and profilin. Thymosin binds to G-actin to buffer the polymerizing process, while profilin binds to G-actin to exchange Adenosine diphosphate, ADP for , promoting the monomeric addition to the barbed, plus end of F-actin filaments. F-actin is both Strength of materials, strong and dynamic. Unlike other s, such as , whose constituent elements are bound together with covalent bonds, the monomers of actin filaments are assembled by weaker bonds. The lateral bonds with neighbouring monomers resolve this anomaly, which in theory should weaken the structure as they can be broken by thermal agitation. In addition, the weak bonds give the advantage that the filament ends can easily release or incorporate monomers. This means that the filaments can be rapidly remodelled and can change cellular structure in response to an environmental stimulus. Which, along with the biochemical mechanism by which it is brought about is known as the "assembly dynamic". ; ''In vitro'' studies Studies focusing on the accumulation and loss of subunits by microfilaments are carried out '' '' (that is, in the laboratory and not on cellular systems) as the polymerization of the resulting actin gives rise to the same F-actin as produced '' ''. The ''in vivo'' process is controlled by a multitude of proteins in order to make it responsive to cellular demands, this makes it difficult to observe its basic conditions. ''In vitro'' production takes place in a sequential manner: first, there is the "activation phase", when the bonding and exchange of divalent cations occurs in specific places on the G-actin, which is bound to ATP. This produces a conformational change, sometimes called G*-actin or F-actin monomer as it is very similar to the units that are located on the filament. This prepares it for the "nucleation phase", in which the G-actin gives rise to small unstable fragments of F-actin that are able to polymerize. Unstable dimers and trimers are initially formed. The "elongation phase" begins when there are a sufficiently large number of these short polymers. In this phase the filament forms and rapidly grows through the reversible addition of new monomers at both extremes. Finally, a Chemical equilibrium, stationary equilibrium is achieved where the G-actin monomers are exchanged at both ends of the microfilament without any change to its total length. In this last phase the "critical concentration Cc" is defined as the ratio between the assembly constant and the dissociation constant for G-actin, where the dynamic for the addition and elimination of dimers and trimers does not produce a change in the microfilament's length. Under ''in vitro'' conditions Cc is 0.1 μM, which means that at higher values polymerization occurs and at lower values depolymerization occurs. ;Role of ATP hydrolysis As indicated above, although actin hydrolyzes ATP, everything points to the fact that ATP is not required for actin to be assembled, given that, on one hand, the hydrolysis mainly takes place inside the filament, and on the other hand the ADP could also instigate polymerization. This poses the question of understanding which thermodynamics, thermodynamically unfavourable process requires such a prodigious expenditure of energy. The actin cycle, which couples ATP hydrolysis to actin polymerization, consists of the preferential addition of G-actin-ATP monomers to a filament's barbed end, and the simultaneous disassembly of F-actin-ADP monomers at the pointed end where the ADP is subsequently changed into ATP, thereby closing the cycle. This aspect of actin filament formation is known as “treadmilling”. ATP is hydrolysed relatively rapidly just after the addition of a G-actin monomer to the filament. There are two hypotheses regarding how this occurs; the stochastic, which suggests that hydrolysis randomly occurs in a manner that is in some way influenced by the neighbouring molecules; and the vectorial, which suggests that hydrolysis only occurs adjacent to other molecules whose ATP has already been hydrolysed. In either case, the resulting Pi is not released; it remains for some time noncovalent bonding, noncovalently bound to actin's ADP. In this way there are three species of actin in a filament: ATP-Actin, ADP+Pi-Actin and ADP-Actin. The amount of each one of these species present in a filament depends on its length and state: as elongation commences the filament has an approximately equal amount of actin monomers bound with ATP and ADP+Pi and a small amount of ADP-Actin at the (-) end. As the stationary state is reached the situation reverses, with ADP present along the majority of the filament and only the area nearest the (+) end containing ADP+Pi and with ATP only present at the tip. If we compare the filaments that only contain ADP-Actin with those that include ATP, in the former the critical constants are similar at both ends, while Cc for the other two nucleotides is different: At the (+) end Cc+=0.1 μM, while at the (-) end Cc−=0.8 μM, which gives rise to the following situations: * For G-actin-ATP concentrations less than Cc+ no elongation of the filament occurs. * For G-actin-ATP concentrations less than Cc− but greater than Cc+ elongation occurs at the (+) end. * For G-actin-ATP concentrations greater than Cc− the microfilament grows at both ends. It is therefore possible to deduce that the energy produced by hydrolysis is used to create a true “stationary state”, that is a flux, instead of a simple equilibrium, one that is dynamic, polar, and attached to the filament. This justifies the expenditure of energy as it promotes essential biological functions. In addition, the configuration of the different monomer types is detected by actin binding proteins, which also control this dynamism, as will be described in the following section. Microfilament formation by treadmilling has been found to be atypical in stereocilia. In this case the control of the structure's size is totally apical and it is controlled in some way by gene expression, that is, by the total quantity of protein monomer synthesized in any given moment.
Associated proteinsThe actin cytoskeleton '' '' is not exclusively composed of actin, other proteins are required for its formation, continuance, and function. These proteins are called ''actin-binding proteins'' (ABP) and they are involved in actin's polymerization, depolymerization, stability, organisation in bundles or networks, fragmentation, and destruction. The diversity of these proteins is such that actin is thought to be the protein that takes part in the greatest number of protein-protein interactions. For example, G-actin sequestering elements exist that impede its incorporation into microfilaments. There are also proteins that stimulate its polymerization or that give complexity to the synthesizing networks. * Thymosins, Thymosin β-4 is a 5 kDa protein that can bind with G-actin-ATP in a 1:1 stoichiometry; which means that one unit of thymosin β-4 binds to one unit of G-actin. Its role is to impede the incorporation of the monomers into the growing polymer. * Profilin, is a cytosolic protein with a molecular weight of 15 kDa, which also binds with G-actin-ATP or -ADP with a stoichiometry of 1:1, but it has a different function as it facilitates the replacement of ADP nucleotides by ATP. It is also implicated in other cellular functions, such as the binding of proline repetitions in other proteins or of lipids that act as second messenger system, secondary messengers. Other proteins that bind to actin regulate the length of the microfilaments by cutting them, which gives rise to new active ends for polymerization. For example, if a microfilament with two ends is cut twice, there will be three new microfilaments with six ends. This new situation favors the dynamics of assembly and disassembly. The most notable of these proteins are gelsolin and . These proteins first achieve a cut by binding to an actin monomer located in the polymer they then change the actin monomer's Protein conformation, conformation while remaining bound to the newly generated (+) end. This has the effect of impeding the addition or exchange of new G-actin subunits. Depolymerization is encouraged as the (-) ends are not linked to any other molecule. Other proteins that bind with actin cover the ends of F-actin in order to stabilize them, but they are unable to break them. Examples of this type of protein are CapZ, which binds the (+) ends depending on a cell's levels of calcium, Ca2+/calmodulin. These levels depend on the cell's internal and external signals and are involved in the regulation of its biological functions). Another example is tropomodulin (that binds to the (-) end). Tropomodulin basically acts to stabilize the F-actin present in the myofibrils present in sarcomeres, which are structures characterized by their great stability. The is widely found in all Eukaryote, eukaryotic organisms. It is composed of seven subunits, some of which possess a topology that is clearly related to their biological function: two of the subunits, ARP2 and ARP3, have a structure similar to that of actin monomers. This homology allows both units to act as nucleation, nucleation agents in the polymerization of G-actin and F-actin. This complex is also required in more complicated processes such as in establishing dendrite, dendritic structures and also in anastomosis (the reconnection of two branching structures that had previously been joined, such as in blood vessels).
Chemical inhibitorsThere are a number of toxins that interfere with actin's dynamics, either by preventing it from polymerizing (latrunculin and cytochalasin D) or by stabilizing it (phalloidin): * Latrunculin is a toxin produced by sponges. It binds to G-actin preventing it from binding with microfilaments. * Cytocalasin D, is an alkaloid produced by fungi, that binds to the (+) end of F-actin preventing the addition of new monomers. Cytocalasin D has been found to disrupt actin's dynamics, activating protein p53 in animals. * Phalloidin, is a toxin that has been isolated from the death cap mushroom ''Amanita phalloides''. It binds to the interface between adjacent actin monomers in the F-actin polymer, preventing its depolymerization.
Functions and locationActin forms filaments ('F-actin' or s) are essential elements of the eukaryotic , able to undergo very fast polymerization and depolymerization dynamics. In most cells actin filaments form larger-scale networks which are essential for many key functions in cells: * Various types of actin networks (made of actin filaments) give mechanical support to cells, and provide trafficking routes through the cytoplasm to aid signal transduction. * Rapid assembly and disassembly of actin network enables cells to migrate (Cell migration). * In metazoan cells, to be the scaffold on which proteins generate force to support muscle contraction. * In non-muscle cells, to be a track for cargo transport myosins (nonconventional myosins) such as myosin V and VI. Nonconventional myosins use ATP hydrolysis to transport cargo, such as Vesicle (biology), vesicles and organelles, in a directed fashion much faster than diffusion. Myosin V walks towards the barbed end of actin filaments, while myosin VI walks toward the pointed end. Most actin filaments are arranged with the barbed end toward the cellular membrane and the pointed end toward the cellular interior. This arrangement allows myosin V to be an effective motor for the export of cargos, and myosin VI to be an effective motor for import. The actin protein is found in both the cytoplasm and the cell nucleus. Its location is regulated by cell membrane signal transduction pathways that integrate the stimuli that a cell receives stimulating the restructuring of the actin networks in response. In ''Dictyostelium'', phospholipase D has been found to intervene in inositol phosphate pathways. Actin filaments are particularly stable and abundant in myocyte, muscle fibres. Within the sarcomere (the basic morphological and physiological unit of muscle fibres) actin is present in both the I and A bands; myosin is also present in the latter.
CytoskeletonMicrofilaments are involved in the movement of all mobile cells, including non-muscular types, and drugs that disrupt F-actin organization (such as the cytochalasins) affect the activity of these cells. Actin comprises 2% of the total amount of proteins in hepatocytes, 10% in fibroblasts, 15% in amoebas and up to 50–80% in activated platelets. There are a number of different types of actin with slightly different structures and functions. This means that α-actin is found exclusively in muscle fibres, while types β and γ are found in other cells. In addition, as the latter types have a high turnover rate the majority of them are found outside permanent structures. This means that the microfilaments found in cells other than muscle cells are present in three forms: * Microfilament networks - Animal cells commonly have a cell cortex under the that contains a large number of microfilaments, which precludes the presence of s. This network is connected with numerous Receptor (biochemistry), receptor cells that signal transduction, relay signals to the outside of a cell. * Microfilament bundles - These extremely long microfilaments are located in networks and, in association with contractile proteins such as non-muscular , they are involved in the movement of substances at an intracellular level. * Periodic actin rings - A periodic structure constructed of evenly spaced actin rings is recently found to specifically exist in axons (not dendrites). In this structure, the actin rings, together with spectrin tetramers that link the neighboring actin rings, form a cohesive that supports the axon membrane. The structure periodicity may also regulate the sodium ion channels in axons.
YeastsActin's cytoskeleton is key to the processes of , , determination of cell polarity and morphogenesis in s. In addition to relying on actin these processes involve 20 or 30 associated proteins, which all have a high degree of evolutionary conservation, along with many signalling molecules. Together these elements allow a spatially and temporally modulated assembly that defines a cell's response to both internal and external stimuli. Yeasts contain three main elements that are associated with actin: patches, cables, and rings that, despite not being present for long, are subject to a dynamic equilibrium due to continual polymerization and depolymerization. They possess a number of accessory proteins including ADF/cofilin, which has a molecular weight of 16kDa and is coded for by a single gene, called ''COF1''; Aip1, a cofilin cofactor that promotes the disassembly of microfilaments; Srv2/CAP, a process regulator related to adenylate cyclase proteins; a profilin with a molecular weight of approximately 14 kDa that is related/associated with actin monomers; and twinfilin, a 40 kDa protein involved in the organization of patches.
PlantsPlant genome studies have revealed the existence of protein isovariants within the actin family of genes. Within ''Arabidopsis thaliana'', a dicotyledon used as a model organism, there are ten types of actin, nine types of α-tubulins, six β-tubulins, six profilins, and dozens of myosins. This diversity is explained by the evolutionary necessity of possessing variants that slightly differ in their temporal and spatial expression. The majority of these proteins were jointly expressed in the Tissue (biology), tissue analysed. Actin networks are distributed throughout the cytoplasm of cells that have been cultivated '' ''. There is a concentration of the network around the nucleus that is connected via spokes to the cellular cortex, this network is highly dynamic, with a continuous polymerization and depolymerization. Even though the majority of plant cells have a Plant cell wall, cell wall that defines their morphology and impedes their movement, their microfilaments can generate sufficient force to achieve a number of cellular activities, such as, the cytoplasmic currents generated by the microfilaments and myosin. Actin is also involved in the movement of organelles and in cellular morphogenesis, which involve as well as the elongation and differentiation of the cell. The most notable proteins associated with the actin cytoskeleton in plants include: , which belongs to the same family as gelsolin/severin and is able to cut microfilaments and bind actin monomers in the presence of calcium cations; fimbrin, which is able to recognize and unite actin monomers and which is involved in the formation of networks (by a different regulation process from that of animals and yeasts); formins, which are able to act as an F-actin polymerization nucleating agent; , a typical molecular motor that is specific to eukaryotes and which in ''Arabidopsis thaliana'' is coded for by 17 genes in two distinct classes; CHUP1, which can bind actin and is implicated in the spatial distribution of chloroplasts in the cell; KAM1/MUR3 that define the morphology of the Golgi apparatus as well as the composition of xyloglucans in the cell wall; NtWLIM1, which facilitates the emergence of actin cell structures; and ERD10, which is involved in the association of organelles within cell membrane, membranes and microfilaments and which seems to play a role that is involved in an organism's reaction to Stress (biology), stress.
Nuclear actinNuclear actin was first noticed and described in 1977 by Clark and Merriam. Authors describe a protein present in the nuclear fraction, obtained from ''Xenopus laevis'' oocytes, which shows the same features as skeletal muscle actin. Since that time there have been many scientific reports about the structure and functions of actin in the nucleus (for review see: Hofmann 2009.) The controlled level of actin in the nucleus, its interaction with actin-binding proteins (ABP) and the presence of different isoforms allows actin to play an important role in many important nuclear processes.
Transport of actin through the nuclear membraneThe actin sequence does not contain a nuclear localization signal. The small size of actin (about 43 kDa) allows it to enter the nucleus by passive diffusion. Actin however shuttles between cytoplasm and nucleus quite quickly, which indicates the existence of active transport. The import of actin into the nucleus (probably in a complex with cofilin) is facilitated by the import protein importin 9. Low level of actin in the nucleus seems to be very important, because actin has two nuclear export signals (NES) into its sequence. Microinjected actin is quickly removed from the nucleus to the cytoplasm. Actin is exported at least in two ways, through XPO1, exportin 1 (EXP1) and XPO6, exportin 6 (Exp6). Specific modifications, such as SUMOylation, allows for nuclear actin retention. It was demonstrated that a mutation preventing SUMOylation causes rapid export of beta actin from the nucleus. Based on the experimental results a general mechanism of nuclear actin transport can be proposed: * In the cytoplasm cofilin bind ADP-actin monomers. This complex is actively imported into the nucleus. * Higher concentration of ATP in the nucleus (compared to the cytoplasm) promote ADP to ATP exchange in the actin-cofilin complex. This weakens the strength of binding of these two proteins. * Cofilin-actin complex finally dissociate after cofilin phosphorylation by nuclear LIM kinase. * Actin is SUMOylated and in this form is retained inside the nucleus. * Actin can form complexes with profilin and leave the nucleus via exportin 6.
The organization of nuclear actinNuclear actin exists mainly as a monomer, but can also form dynamic oligomers and short polymers. Nuclear actin organization varies in different cell types. For example, in ''Xenopus'' oocytes (with higher nuclear actin level in comparison to somatic cells) actin forms filaments, which stabilize nucleus architecture. These filaments can be observed under the microscope thanks to fluorophore-conjugated phalloidin staining. In somatic cell nuclei, however, actin filaments cannot be observed using this technique. The DNase I inhibition assay, so far the only test which allows the quantification of the polymerized actin directly in biological samples, has revealed that endogenous nuclear actin indeed occurs mainly in a monomeric form. Precisely controlled level of actin in the cell nucleus, lower than in the cytoplasm, prevents the formation of filaments. The polymerization is also reduced by the limited access to actin monomers, which are bound in complexes with ABPs, mainly cofilin.
Actin isoforms in the cell nucleusLittle attention is paid to actin isoforms; however, it has been shown that different isoforms of actin are present in the cell nucleus. Actin isoforms, despite of their high sequence similarity, have different biochemical properties such as polymerization and depolymerization kinetic. They also show different localization and functions. The level of actin isoforms, both in the cytoplasm and the nucleus, may change for example in response to stimulation of cell growth or arrest of proliferation and transcriptional activity. Research concerns on nuclear actin are usually focused on isoform beta. However the use of antibodies directed against different actin isoforms allows identifying not only the cytoplasmic beta in the cell nucleus, but also: * gamma actin in the cell nuclei of human melanoma, * alpha skeletal muscle actin in the nuclei of mouse myoblasts, * cytoplasmic gamma actin and also alpha smooth muscle actin in the nucleus of the foetal mouse fibroblast The presence of different isoforms of actin may have a significant effect on its function in nuclear processes, especially because the level of individual isoforms can be controlled independently.
Nuclear actin functionsFunctions of actin in the nucleus are associated with its ability to polymerize and interaction with variety of ABPs and with structural elements of the nucleus. Nuclear actin is involved in: * Architecture of the nucleus - Interaction of actin with alpha II-spectrin and other proteins are important for maintaining proper shape of the nucleus. * Transcription – Actin is involved in chromatin reorganization, transcription initiation and interaction with the transcription complex. Actin takes part in the regulation of chromatin structure, interacting with RNA polymerase I, II and III. In Pol I transcription, actin and myosin (MYO1C, which binds DNA) act as a molecular motor. For Pol II transcription, β-actin is needed for the formation of the preinitiation complex. Pol III contains β-actin as a subunit. Actin can also be a component of chromatin remodelling complexes as well as pre-mRNP particles (that is, precursor messenger RNA bundled in proteins), and is involved in nuclear pore, nuclear export of RNAs and proteins. * Regulation of gene activity – Actin binds to the regulatory regions of different kinds of genes. Actin's ability to regulate gene activity is used in the molecular reprogramming method, which allows differentiated cells return to their embryonic state. * Translocation of the activated chromosome fragment from under membrane region to euchromatin where transcription starts. This movement requires the interaction of actin and myosin. * Integration of different cellular compartments. Actin is a molecule that integrates cytoplasmic and nuclear signal transduction pathways. An example is the activation of transcription in response to serum stimulation of cells ''in vitro''. * Immune response - Nuclear actin polymerizes upon T-cell receptor stimulation and is required for cytokine expression and antibody production ''in vivo''. Due to its ability to undergo conformational changes and interaction with many proteins, actin acts as a regulator of formation and activity of protein complexes such as transcriptional complex.
Outline of a muscle contractionIn muscle cells, actomyosin myofibrils make up much of the cytoplasmic material. These myofibrils are made of ''thin filaments'' of actin (typically around 7 nm in diameter), and ''thick filaments'' of the motor-protein (typically around 15 nm in diameter). These myofibrils use energy derived from to create movements of cells, such as muscle contraction. Using the hydrolysis of ATP for energy, myosin heads undergo a cycle during which they attach to thin filaments, exert a tension, and then, depending on the load, perform a power stroke that causes the thin filaments to slide past, shortening the muscle. In contractile bundles, the actin-bundling protein alpha-actinin separates each thin filament by ≈35 nm. This increase in distance allows thick filaments to fit in between and interact, enabling deformation or contraction. In deformation, one end of myosin is bound to the plasma membrane, while the other end "walks" toward 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" toward their filament's plus end, sliding the actin filaments closer to 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 , the division of one cell into two.
Actin’s role in muscle contractionThe helical F-actin filament found in muscles also contains a molecule, a 40- protein that is wrapped around the F-actin helix. During the resting phase the tropomyosin covers the actin's active sites so that the actin-myosin interaction cannot take place and produce muscular contraction (the interaction gives rise to a movement between the two proteins that, because it is repeated many times, produces a contraction). There are other protein molecules bound to the tropomyosin thread, these include the s that have three polymers: , , and . Tropomyosin's regulatory function depends on its interaction with troponin in the presence of Ca2+ ions. Both actin and are involved in contraction and relaxation and they make up 90% of muscle protein. The overall process is initiated by an external signal, typically through an action potential stimulating the muscle, which contains specialized cells whose interiors are rich in actin and myosin filaments. The contraction-relaxation cycle comprises the following steps: # Depolarization of the sarcolemma and transmission of an action potential through the T-tubules. # Opening of the sarcoplasmic reticulum’s calcium, Ca2+ channels. # Increase in cytosolic Ca2+ concentrations and the interaction of these cations with troponin causing a conformational change in its . This in turn alters the structure of tropomyosin, which covers actin's active site, allowing the formation of myosin-actin cross-links (the latter being present as thin filaments). # Movement of myosin heads over the thin filaments, this can either involve ATP or be independent of ATP. The former mechanism, mediated by activity in the myosin heads, causes the movement of the actin filaments towards the Sarcomere, Z-disc. # Ca2+ capture by the sarcoplasmic reticulum, causing a new conformational change in tropomyosin that inhibits the actin-myosin interaction.
Other biological processesThe traditional image of actin's function relates it to the maintenance of the cytoskeleton and, therefore, the organization and movement of organelles, as well as the determination of a cell's shape. However, actin has a wider role in eukaryotic cell physiology, in addition to similar functions in s. * Cytokinesis. Cell division in animal cells and yeasts normally involves the separation of the parent cell into two daughter cells through the constriction of the central circumference. This process involves a constricting ring composed of actin, myosin, and actinin, α-actinin. In the fission yeast ''Schizosaccharomyces pombe'', actin is actively formed in the constricting ring with the participation of Arp2/3, Arp3, the formin Cdc12, profilin, and WASp, along with preformed microfilaments. Once the ring has been constructed the structure is maintained by a continual assembly and disassembly that, aided by the Arp2/3 complex and formins, is key to one of the central processes of cytokinesis. The totality of the contractile ring, the spindle apparatus, microtubules, and the dense peripheral material is called the "Fleming body" or "intermediate body". * Apoptosis. During programmed cell death the ICE/ced-3 family of proteases (one of the interleukin-1β-converter proteases) degrade actin into two fragments ''in vivo''; one of the fragments is 15 kDa and the other 31 kDa. This represents one of the mechanisms involved in destroying cell viability that form the basis of apoptosis. The protease calpain has also been shown to be involved in this type of cell destruction; just as the use of calpain inhibitors has been shown to decrease actin proteolysis and the degradation of (another of the characteristic elements of apoptosis). On the other hand, the Stress (biology), stress-induced triggering of apoptosis causes the reorganization of the actin cytoskeleton (which also involves its polymerization), giving rise to structures called stress fibers; this is activated by the MAPK/ERK pathway, MAP kinase pathway. * Cellular adhesion and Developmental biology, development. The adhesion between cells is a characteristic of multicellular organisms that enables Tissue (biology), tissue specialization and therefore increases cell complexity. Adhesion of cell epithelium, epithelia involves the actin cytoskeleton in each of the joined cells as well as cadherins acting as extracellular elements with the connection between the two mediated by catenins. Interfering in actin dynamics has repercussions for an organism's development, in fact actin is such a crucial element that systems of redundant s are available. For example, if the α-actinin or gelation factor gene has been removed in ''Dictyostelium'' individuals do not show an anomalous phenotype possibly due to the fact that each of the proteins can perform the function of the other. However, the development of Mutation, double mutations that lack both gene types is affected. * Gene expression modulation. Actin's state of polymerization affects the pattern of . In 1997, it was discovered that cytocalasin D-mediated depolymerization in Schwann cells causes a specific pattern of expression for the genes involved in the myelinization of this type of Neuron, nerve cell. F-actin has been shown to modify the transcriptome in some of the life stages of unicellular organisms, such as the fungus ''Candida albicans''. In addition, proteins that are similar to actin play a regulatory role during spermatogenesis in Muridae, mice and, in yeasts, actin-like proteins are thought to play a role in the regulation of Epigenetics, gene expression. In fact, actin is capable of acting as a transcription initiator when it reacts with a type of nuclear myosin that interacts with RNA polymerases and other enzymes involved in the transcription process. * Stereocilia dynamics. Some cells develop fine filliform outgrowths on their surface that have a Somatosensory system, mechanosensory function. For example, this type of organelle is present in the Organ of Corti, which is located in the ear. The main characteristic of these structures is that their length can be modified. The molecular architecture of the stereocilia includes a paracrystalline actin core in dynamic equilibrium with the monomers present in the adjacent cytosol. Type VI and VIIa myosins are present throughout this core, while myosin XVa is present in its extremities in quantities that are proportional to the length of the stereocilia. * Intrinsic chirality. Actomyosin networks have been implicated in generating an intrinsic chirality in individual cells. Cells grown out on chiral surfaces can show a directional left/right bias that is actomyosin dependent.
Molecular pathologyThe majority of mammals possess six different actin s. Of these, two code for the (''ACTB'' and ''ACTG1'') while the other four are involved in skeletal striated muscle (''ACTA1''), smooth muscle tissue (''ACTA2''), Intestine, intestinal muscles (''ACTG2'') and cardiac muscle (''ACTC1''). The actin in the cytoskeleton is involved in the Pathogenesis, pathogenic mechanisms of many Pathogen, infectious agents, including HIV. The vast majority of the s that affect actin are point mutations that have a Dominance (genetics), dominant effect, with the exception of six mutations involved in nemaline myopathy. This is because in many cases the mutant of the actin monomer acts as a “cap” by preventing the elongation of F-actin.
Pathology associated with ''ACTA1''''ACTA1'' is the gene that codes for the α- of actin that is predominant in human skeletal striated muscles, although it is also expressed in heart muscle and in the Thyroid, thyroid gland. Its DNA sequence consists of seven exons that produce five known Transcription (genetics), transcripts. The majority of these consist of point mutations causing substitution of s. The mutations are in many cases associated with a phenotype that determines the severity and the course of the affliction. The mutation alters the structure and function of skeletal muscles producing one of three forms of myopathy: type 3 nemaline myopathy, Congenital myopathy, congenital myopathy with an excess of thin myofilaments (CM) and Congenital myopathy#Congenital fiber type disproportion, congenital myopathy with fibre type disproportion (CMFTD). Mutations have also been found that produce Central core disease, core myopathies. Although their phenotypes are similar, in addition to typical nemaline myopathy some specialists distinguish another type of myopathy called actinic nemaline myopathy. In the former, clumps of actin form instead of the typical rods. It is important to state that a patient can show more than one of these phenotypes in a biopsy. The most common symptoms consist of a typical facial morphology (myopathic Facies (medical), facies), muscular weakness, a delay in motor development and respiratory difficulties. The course of the illness, its gravity, and the age at which it appears are all variable and overlapping forms of myopathy are also found. A symptom of nemaline myopathy is that "nemaline rods" appear in differing places in type 1 muscle fibres. These rods are non-pathognomonic structures that have a similar composition to the Z disks found in the sarcomere. The pathogenesis of this myopathy is very varied. Many mutations occur in the region of actin's indentation near to its binding sites, while others occur in Domain 2, or in the areas where interaction occurs with associated proteins. This goes some way to explain the great variety of clumps that form in these cases, such as Nemaline or Intranuclear Bodies or Zebra Bodies. Changes in actin's Protein folding, folding occur in nemaline myopathy as well as changes in its aggregation and there are also changes in the Gene expression, expression of other associated proteins. In some variants where intranuclear bodies are found the changes in the folding masks the Nuclear pore#Export of proteins, nucleus's protein exportation signal so that the accumulation of actin's mutated form occurs in the cell nucleus. On the other hand, it appears that mutations to ''ACTA1'' that give rise to a CFTDM have a greater effect on sarcomeric function than on its structure. Recent investigations have tried to understand this apparent paradox, which suggests there is no clear correlation between the number of rods and muscular weakness. It appears that some mutations are able to induce a greater apoptosis rate in type II muscular fibres.
In smooth muscleThere are two isoforms that code for actins in the smooth muscle tissue: ''ACTG2'' codes for the largest actin isoform, which has nine exons, one of which, the one located at the 5' end, is not Translation (biology), translated. It is a γ-actin that is expressed in the enteric smooth muscle. No mutations to this gene have been found that correspond to pathologies, although DNA microarray, microarrays have shown that this protein is more often expressed in cases that are resistant to chemotherapy using cisplatin. ''ACTA2'' codes for an α-actin located in the smooth muscle, and also in vascular smooth muscle. It has been noted that the MYH11 mutation could be responsible for at least 14% of hereditary Aortic aneurism, thoracic aortic aneurisms particularly Type 6. This is because the mutated variant produces an incorrect filamentary assembly and a reduced capacity for vascular smooth muscle contraction. Degradation of the Aorta, aortic media has been recorded in these individuals, with areas of disorganization and hyperplasia as well as stenosis of the aorta's vasa vasorum. The number of afflictions that the gene is implicated in is increasing. It has been related to Moyamoya disease and it seems likely that certain mutations in heterozygosis could confer a predisposition to many vascular pathologies, such as thoracic aortic aneurysm and ischaemic heart disease. The α-actin found in smooth muscles is also an interesting marker for evaluating the progress of liver cirrhosis.
In heart muscleThe ''ACTC1'' gene codes for the α-actin isoform present in heart muscle. It was first sequenced by Hamada and co-workers in 1982, when it was found that it is interrupted by five introns. It was the first of the six genes where alleles were found that were implicated in pathological processes. A number of structural disorders associated with point mutations of this gene have been described that cause malfunctioning of the heart, such as Type 1R dilated cardiomyopathy and Type 11 hypertrophic cardiomyopathy. Certain defects of the Atrium (heart), atrial septum have been described recently that could also be related to these mutations. Two cases of dilated cardiomyopathy have been studied involving a substitution of highly conserved s belonging to the protein domains that bind and intersperse with the sarcomere, Z discs. This has led to the theory that the dilation is produced by a defect in the transmission of muscle contraction, contractile force in the myocytes. The mutations in ACTC1 are responsible for at least 5% of hypertrophic cardiomyopathies. The existence of a number of point mutations have also been found: * Mutation E101K: changes of net charge and formation of a weak electrostatic link in the actomyosin-binding site. * P166A: interaction zone between actin monomers. * A333P: actin-myosin interaction zone. Pathogenesis appears to involve a compensatory mechanism: the mutated proteins act like toxins with a dominant effect, decreasing the heart's ability to Heart#Functioning, contract causing abnormal mechanical behaviour such that the hypertrophy, that is usually delayed, is a consequence of the cardiac muscle's normal response to stress (physiology), stress. Recent studies have discovered ACTC1 mutations that are implicated in two other pathological processes: Infantile idiopathic restrictive cardiomyopathy, and Noncompaction cardiomyopathy, noncompaction of the left ventricular myocardium.
In cytoplasmatic actins''ACTB'' is a highly complex Locus (genetics), locus. A number of s exist that are distributed throughout the genome, and its sequence contains six exons that can give rise to up to 21 different transcriptions by alternative splicing, which are known as the β-actins. Consistent with this complexity, its products are also found in a number of locations and they form part of a wide variety of processes ( , NuA4 histone-acyltransferase complex, cell nucleus) and in addition they are associated with the mechanisms of a great number of pathological processes (Cancer, carcinomas, juvenile dystonia, infection mechanisms, nervous system malformations and tumour invasion, among others). A new form of actin has been discovered, kappa actin, which appears to substitute for β-actin in processes relating to tumours. Three pathological processes have so far been discovered that are caused by a direct alteration in gene sequence: * Hemangiopericytoma with t(7;12)(p22;q13)-translocations is a rare affliction, in which a Mutation#By effect on structure, translocational mutation causes the fusion of the ''ACTB'' gene over GLI1 in Chromosome 12 (human), Chromosome 12. * Juvenile onset dystonia is a rare degenerative disease that affects the central nervous system; in particular, it affects areas of the neocortex and thalamus, where rod-like eosinophilic inclusions are formed. The affected individuals represent a phenotype with deformities on the median line, sensory Deafness, hearing loss and dystonia. It is caused by a point mutation in which the amino acid tryptophan replaces arginine in position 183. This alters actin's interaction with the ADF/ system, which regulates the dynamics of Neuron, nerve cell cytoskeleton formation. * A dominant point mutation has also been discovered that causes neutrophil granulocyte dysfunction and recurring s. It appears that the mutation modifies the domain responsible for binding between profilin and other regulatory proteins. Actin's affinity for profilin is greatly reduced in this allele. The ''ACTG1'' locus codes for the cytosolic γ-actin protein that is responsible for the formation of cytoskeletal s. It contains six exons, giving rise to 22 different Messenger RNA, mRNAs, which produce four complete s whose form of expression is probably dependent on the type of Tissue (biology), tissue they are found in. It also has two different Promoter (genetics), DNA promoters. It has been noted that the sequences translated from this locus and from that of β-actin are very similar to the predicted ones, suggesting a common ancestral sequence that suffered duplication and genetic conversion. In terms of pathology, it has been associated with processes such as amyloidosis, retinitis pigmentosa, infection mechanisms, kidney diseases, and various types of congenital hearing loss. Six autosomal-dominant point mutations in the sequence have been found to cause various types of hearing loss, particularly sensorineural hearing loss linked to the DFNA 20/26 locus. It seems that they affect the stereocilia of the ciliated cells present in the inner ear's Organ of Corti. β-actin is the most abundant protein found in human tissue, but it is not very abundant in ciliated cells, which explains the location of the pathology. On the other hand, it appears that the majority of these mutations affect the areas involved in linking with other proteins, particularly actomyosin. Some experiments have suggested that the pathological mechanism for this type of hearing loss relates to the F-actin in the mutations being more sensitive to cofilin than normal. However, although there is no record of any case, it is known that γ-actin is also expressed in skeletal muscles, and although it is present in small quantities, model organisms have shown that its absence can give rise to myopathies.
Other pathological mechanismsSome infectious agents use actin, especially cytoplasmic actin, in their Alternation of generations, life cycle. Two basic forms are present in : * ''Listeria monocytogenes'', some species of ''Rickettsia'', ''Shigella flexneri'' and other intracellular germs escape from phagocytosis, phagocytic vacuoles by coating themselves with a capsule of actin filaments. ''L. monocytogenes'' and ''S. flexneri'' both generate a tail in the form of a "comet tail" that gives them mobility. Each species exhibits small differences in the molecular polymerization mechanism of their "comet tails". Different displacement velocities have been observed, for example, with ''Listeria'' and ''Shigella'' found to be the fastest. Many experiments have demonstrated this mechanism ''in vitro''. This indicates that the bacteria are not using a myosin-like protein motor, and it appears that their propulsion is acquired from the pressure exerted by the polymerization that takes place near to the microorganism's cell wall. The bacteria have previously been surrounded by ABPs from the host, and as a minimum the covering contains , Ena/Vasp homology proteins, Ena/VASP proteins, cofilin, a buffering protein and nucleation promoters, such as vinculin complex. Through these movements they form protrusions that reach the neighbouring cells, infecting them as well so that the can only fight the infection through cell immunity. The movement could be caused by the modification of the curve and debranching of the filaments. Other species, such as ''Mycobacterium marinum'' and ''Burkholderia pseudomallei'', are also capable of localized polymerization of cellular actin to aid their movement through a mechanism that is centered on the Arp2/3 complex. In addition the vaccine ''Vaccinia'' also uses elements of the actin cytoskeleton for its dissemination. * ''Pseudomonas aeruginosa'' is able to form a protective biofilm in order to escape a host (biology), host organism’s defences, especially Neutrophil granulocyte, white blood cells and Antibacterial, antibiotics. The biofilm is constructed using and actin filaments from the host organism. In addition to the previously cited example, actin polymerization is stimulated in the initial steps of the internalization of some viruses, notably HIV, by, for example, inactivating the cofilin complex. The role that actin plays in the invasion process of cancer cells has still not been determined.
EvolutionThe eukaryotic cytoskeleton of organisms among all Phylogenetic, taxonomic groups have similar components to actin and tubulin. For example, the protein that is coded by the ''ACTG2'' gene in humans is completely equivalent to the Homology (biology), homologues present in rats and mice, even though at a level the similarity decreases to 92%. However, there are major differences with the equivalents in prokaryotes (FtsZ and ), where the similarity between nucleotide sequences is between 40−50 % among different and species. Some authors suggest that the ancestral protein that gave rise to the model eukaryotic actin resembles the proteins present in modern bacterial cytoskeletons. Some authors point out that the behaviour of actin, tubulin, and histone, a protein involved in the stabilization and regulation of DNA, are similar in their ability to bind nucleotides and in their ability of take advantage of Brownian motion. It has also been suggested that they all have a common ancestor. Therefore, ary processes resulted in the diversification of ancestral proteins into the varieties present today, conserving, among others, actins as efficient molecules that were able to tackle essential ancestral biological processes, such as .
Equivalents in bacteriaThe Cytoskeleton#Prokaryotic cytoskeleton, bacterial cytoskeleton may not be as complex as that found in s; however, it contains proteins that are highly similar to actin monomers and polymers. The bacterial protein polymerizes into thin non-helical filaments and occasionally into helical structures similar to F-actin. Furthermore, its crystalline structure is very similar to that of G-actin (in terms of its three-dimensional conformation), there are even similarities between the MreB protofilaments and F-actin. The bacterial cytoskeleton also contains the FtsZ proteins, which are similar to . Bacteria therefore possess a cytoskeleton with homologous elements to actin (for example, MreB, AlfA, ParM, FtsA, and MamK), even though the amino acid sequence of these proteins diverges from that present in animal cells. However, such proteins have a high degree of protein structure, structural similarity to eukaryotic actin. The highly dynamic microfilaments formed by the aggregation of MreB and ParM are essential to cell viability and they are involved in cell morphogenesis, chromosome segregation, and cell polarity. ParM is an actin homologue that is coded in a and it is involved in the regulation of plasmid DNA. ParMs from different bacterial plasmids can form astonishingly diverse helical structures comprising two or four strands to maintain faithful plasmid inheritance.
ApplicationsActin is used in scientific and technological laboratories as a track for molecular motors such as myosin (either in muscle tissue or outside it) and as a necessary component for cellular functioning. It can also be used as a diagnostic tool, as several of its anomalous variants are related to the appearance of specific pathologies. *Nanotechnology. Actin-myosin systems act as molecular motors that permit the transport of vesicles and organelles throughout the cytoplasm. It is possible that actin could be applied to nanotechnology as its dynamic ability has been harnessed in a number of experiments including those carried out in acellular systems. The underlying idea is to use the microfilaments as tracks to guide molecular motors that can transport a given load. That is actin could be used to define a circuit along which a load can be transported in a more or less controlled and directed manner. In terms of general applications, it could be used for the directed transport of molecules for deposit in determined locations, which would permit the controlled assembly of nanostructures. These attributes could be applied to laboratory processes such as on ''lab-on-a-chip'', in nanocomponent mechanics and in nanotransformers that convert mechanical energy into electrical energy. *Actin is used as an internal control in western blots to ascertain that equal amounts of protein have been loaded on each lane of the gel. In the blot example shown on the left side, 75 µg of total protein was loaded in each well. The blot was reacted with anti-β-actin antibody (for other details of the blot see the reference ) The use of actin as an internal control is based on the assumption that its expression is practically constant and independent of experimental conditions. By comparing the expression of the gene of interest to that of the actin, it is possible to obtain a relative quantity that can be compared between different experiments, whenever the expression of the latter is constant. It is worth pointing out that actin does not always have the desired stability in its . *Health. Some s of actin cause diseases; for this reason techniques for their detection have been developed. In addition, actin can be used as an indirect marker in surgical pathology: it is possible to use variations in the pattern of its distribution in tissue as a marker of invasion in neoplasm, neoplasia, vasculitis, and other conditions. Further, due to actin's close association with the apparatus of muscular contraction its levels in skeletal muscle diminishes when these tissues atrophy, it can therefore be used as a marker of this physiological process. *Food technology. It is possible to determine the quality of certain processed foods, such as embutido, sausages, by quantifying the amount of actin present in the constituent meat. Traditionally, a method has been used that is based on the detection of histidine, 3-methylhistidine in samples of these products, as this compound is present in actin and F-myosin's heavy chain (both are major components of muscle). The generation of this compound in flesh derives from the methylation of histidine residues present in both proteins.
GenesHuman genes encoding actin proteins include: * ACTA1 — alpha actin 1, skeletal muscle * ACTA2 — alpha actin 2, smooth muscle, aorta * ACTB — beta actin * ACTC1 — actin, alpha, cardiac muscle 1 * ACTG1 — gamma actin 1 * ACTG2 — gamma actin 2, smooth muscle, enteric
See also* Actin remodeling — effect on cell structure and shape * Actin-binding protein * Active matter * Arp2/3 * Filopodia * FtsZ * Intermediate filament * Lamellipodium * Motor protein — converts chemical energy into mechanical work * — one of the actin homologues in bacteria *Neuron * Phallotoxin