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== Introduction == | == Introduction == | ||
Actin is a spherical 43 kilo-Dalton protein that is found in many cell types. It belongs to the class of proteins known as [http://en.wikipedia.org/wiki/Motor_proteins motor proteins]. Motor proteins function in the movement of substances across a substrate by utilizing [http://en.wikipedia.org/wiki/Adenosine_triphosphate ATP] as an energy source. It is one of the most highly conserved proteins differing by no more than 20% in species ranging from yeast to humans.<ref>PMID: 18573073</ref> In an alignment of the 81 unique actin protein sequences, scientists found that 17.4% of positions in the consensus sequence are invariant in all actins, 35.6 % of positions are invariant or have a difference in only one of the 81 actins, and 40.4% of positions are invariant or have what are considered conservative changes | Actin is a spherical 43 kilo-Dalton protein that is found in many cell types. It belongs to the class of proteins known as [http://en.wikipedia.org/wiki/Motor_proteins motor proteins]. Motor proteins function in the movement of substances across a substrate by utilizing [http://en.wikipedia.org/wiki/Adenosine_triphosphate ATP] as an energy source. It is one of the most highly conserved proteins differing by no more than 20% in species ranging from yeast to humans.<ref>PMID: 18573073</ref> In an alignment of the 81 unique actin protein sequences, scientists found that 17.4% of positions in the consensus sequence are invariant in all actins, 35.6 % of positions are invariant or have a difference in only one of the 81 actins, and 40.4% of positions are invariant or have what are considered conservative changes. <ref>PMID: 8489492</ref> Actin is the monomeric subunit of microfilaments, one of the three key components of the [http://en.wikipedia.org/wiki/Cytoskeleton cytoskeleton] and thin filaments, which is part of the contractile unit in muscle cells. The cytoskeleton is a three-dimensional complex of filamentous protein that creates the shape and structure of cells, as well as fills in the gaps between organelles. This is what provides the cells with motility. There are three main proteins that make up the cytoskeleton. They are microfilaments, intermediate filaments and microtubules.<ref>Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular biology of the cell. (3 ed.). New York: Garland Science.</ref> Actin exists in two forms, the monomeric, globular form (G-actin) and the polymeric, filamentous form (F-actin).<ref>PMID: 20672362</ref> In vertebrates there are three main groups of actin isoforms which include alpha, beta, and gamma. These isotypes (α, β and γ), show >90% amino-acid homology between isotypes and >98% homology within members of a particular isotypic group.<ref>Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular biology of the cell. (3 ed.). New York: Garland Science.</ref> The alpha actins, found in muscle tissues, are a major constituent of the contractile apparatus. The beta and gamma actins coexist in most cell types as components of the cytoskeleton, and as mediators of internal cell motility. | ||
Actin is the monomeric subunit of microfilaments, one of the three key components of the [http://en.wikipedia.org/wiki/Cytoskeleton cytoskeleton] and thin filaments, which is part of the contractile unit in muscle cells. The cytoskeleton is a three-dimensional complex of filamentous protein that creates the shape and structure of cells, as well as fills in the gaps between organelles. This is what provides the cells with motility. There are three main proteins that make up the cytoskeleton. They are microfilaments, intermediate filaments and microtubules. | |||
=History= | =History= | ||
In 1942, [http://en.wikipedia.org/wiki/Brun%C3%B3_Ferenc_Straub Bruno Ferenc Straub] isolated actin as a water soluble component of muscle acetone powder. He found that at increased ionic strength the G-actin molecules would come together to form the F-actin filament. Actin was later found in non-muscle cells and its structure was seen through the use of [http://en.wikipedia.org/wiki/Immunofluorescence_microscopy immunofluorescence microscopy]. The next major accomplishment came with the discovery of the amino acid sequence. This allowed researchers to locate chemical and enzymatic alterations and to study how they affected the properties of the molecule. The high conservation of actin’s structure throughout various species was discovered through [http://en.wikipedia.org/wiki/Immunofluorescence_microscopy comparative sequence analysis]. In 1950, Straub reported that actin contained bound ATP and that during the polymerization of the monomers into microfilaments, the ATP was hydrolyzed to ADP and inorganic phosphate. | In 1942, [http://en.wikipedia.org/wiki/Brun%C3%B3_Ferenc_Straub Bruno Ferenc Straub] isolated actin as a water soluble component of muscle acetone powder. He found that at increased ionic strength the G-actin molecules would come together to form the F-actin filament. Actin was later found in non-muscle cells and its structure was seen through the use of [http://en.wikipedia.org/wiki/Immunofluorescence_microscopy immunofluorescence microscopy]. The next major accomplishment came with the discovery of the amino acid sequence. This allowed researchers to locate chemical and enzymatic alterations and to study how they affected the properties of the molecule. The high conservation of actin’s structure throughout various species was discovered through [http://en.wikipedia.org/wiki/Immunofluorescence_microscopy comparative sequence analysis]. In 1950, Straub reported that actin contained bound ATP and that during the polymerization of the monomers into microfilaments, the ATP was hydrolyzed to ADP and inorganic phosphate.<ref>PMID: 2673365</ref> His discovery led to his suggestion that ATP-bound actin transforming into ADP-bound actin was what created muscle contractions. In the early 1980’s the principles of actin polymerization were exposed and the first actin-binding proteins (ABPs) were characterized according to their in vivo and in vitro functions. In 1990, the crystal structure of G-actin was solved by Kabsch.<ref>PMID: 2395459</ref>In this same year a model of F-actin was suggested by Holmes et al. The model was created by fitting a helix of G-actin structures according to low-resolution fiber diffraction data from the F-actin.<ref>PMID: 2395461</ref> Further advances in cryoelectron microscopy allowed scientists to find the binding sites for myosin and tropomyosin. A combination of [http://en.wikipedia.org/wiki/X-ray_crystallography X-ray crystallography] with fiber diffraction led to an atomic model of F-actin.<ref>PMID: 1388079</ref> After some debate actin also became generally accepted as an important structural and functional component of the cell nucleus <ref>PMID: 20672362</ref> Since this pioneering work by Straub, interest in actin has increased tremendously and so has the realization of how complex its structure is. | ||
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[[Image:Actin Image.gif]] | [[Image:Actin Image.gif]] | ||
Actin exists in both its monomeric (globular, G-actin) and polymeric (filamentous or F-actin) form. The actin monomer is approximately pear shaped. Actin is composed of 376 residues that are folded into two large domains, each comprised of two subdomains. Its secondary structure is 41% helical (22 helices; 153 residues) and 20% beta sheet (20 strands; 77 residues). | Actin exists in both its monomeric (globular, G-actin) and polymeric (filamentous or F-actin) form. The actin monomer is approximately pear shaped. Actin is composed of 376 residues that are folded into two large domains, each comprised of two subdomains. Its secondary structure is 41% helical (22 helices; 153 residues) and 20% beta sheet (20 strands; 77 residues). The actin filament is a double-stranded, right-handed helix with a half-pitch of 37 nm and a one-start left-handed genetic helix with a rise of 2.75 nm per monomer. The width of the filament is within the range of 7–10 nm. <ref>PMID: 20672362</ref> The large domains are organized to form a hinged molecule with a deep cleft. Subdomains 1 and 3 are structurally related, whereas subdomains 2 and 4 can be viewed as large insertions into subdomains 1 and 3, respectively. There is moderately little contact between the two major domains of actin; the polypeptide chain passes twice between these domains at the loop centered at residue Lys336 and at the linker helix Gln137-Ser145, which functions as the hinge axis between the domains. As a result, two clefts are formed between the domains. Within the upper cleft are the cofactors, an adenine nucleotide and a divalent metal ion, usually magnesium. It is thought that the cofactors interact with the domains on either side, increasing their connectivity. The lower cleft between domains 1 and 3 is lined by residues Tyr143, Ala144, Gly146, Thr148, Gly168, Ile341, Ile345, Leu346, Leu349, Thr351, and Met355, which are predominantly hydrophobic. This cleft constitutes the major binding site for most ABPs, and is thus called the targetbinding or hydrophobic cleft.Most of the interaction that occur in the subunit are electrostatic in character but there are also hydrophobic interactions.<ref>PMID: 2395459</ref> | ||
Under physiologic conditions, G-actin is transformed to F-actin by ATP. After G-actin is bound to an ATP molecule it can then bind to another ATP bound monomer to form an unstable dimer. It can then add a third ATP bound monomer to create a stable trimer that serves as a basis for building fibrous actin (F-actin). Once the F-actin is formed the ATP can be hydrolyzed, keeping the bound ADP and releasing an inorganic phosphate. The polymer can undergo a process known as treadmilling. This is where the polymer continuously grows at its positive end but disassembles at its negative end. This process requires a permanent source of energy such as ATP | Under physiologic conditions, G-actin is transformed to F-actin by ATP. After G-actin is bound to an ATP molecule it can then bind to another ATP bound monomer to form an unstable dimer. It can then add a third ATP bound monomer to create a stable trimer that serves as a basis for building fibrous actin (F-actin). Once the F-actin is formed the ATP can be hydrolyzed, keeping the bound ADP and releasing an inorganic phosphate. The polymer can undergo a process known as treadmilling. This is where the polymer continuously grows at its positive end but disassembles at its negative end. This process requires a permanent source of energy such as ATP <ref>Boron, W., & Boulpaep, E. (2008). Medical physiology. (2 ed., pp. 14-20). Saunders Elsevier. Retrieved from http://books.google.com/books/about/Medical_Physiology_E_Book.html?id=HlMJRw08ihgC</ref> | ||
Unlike G-actin, F-actin does not form crystalline arrays that can be analyzed by X-ray crystallography. Instead it can be found by using fiber diffraction, electron-microscopy data and mathematical models. The structure can then be refined and remodeled as necessary. (ACTIN: General Principles from Studies in Yeast) The first high-resolution structural model of the actin filament was at a resolution of 8 A ˚ and was proposed by Holmes et al., 1990. Recently, an improved high-resolution F-actin model was created by Oda et al., 2009 with a resolution of 3.3 A ˚ in the radial and 5.6 A ˚ in the equatorial directions. | Unlike G-actin, F-actin does not form crystalline arrays that can be analyzed by X-ray crystallography. Instead it can be found by using fiber diffraction, electron-microscopy data and mathematical models. The structure can then be refined and remodeled as necessary. (ACTIN: General Principles from Studies in Yeast) The first high-resolution structural model of the actin filament was at a resolution of 8 A ˚ and was proposed by Holmes et al., 1990. Recently, an improved high-resolution F-actin model was created by Oda et al., 2009 with a resolution of 3.3 A ˚ in the radial and 5.6 A ˚ in the equatorial directions. | ||
Vertebrates express three main actin isoforms, including three α-isoforms of skeletal, cardiac, and smooth muscles and the β- and γ- isoforms expressed in nonmuscle and muscle cells. Actin isoforms are different in a few of its amino acids, with most variations occurring toward the N terminus. Actin also undergoes various forms of posttranslational modifications. For instance, His73 of skeletal muscle α-actin is methylated, the N-terminal methionine and cysteine residues are acetylated and cleaved, and the resulting N-terminal aspartic acid is then re-acetylated. Since the original determination of the crystal structure of G-actin, over 80 structures of actin have been reported. The majority of these structures have been obtained as complexes with actin-binding proteins (ABPs). (Actin Structure and Function) | Vertebrates express three main actin isoforms, including three α-isoforms of skeletal, cardiac, and smooth muscles and the β- and γ- isoforms expressed in nonmuscle and muscle cells. Actin isoforms are different in a few of its amino acids, with most variations occurring toward the N terminus. Actin also undergoes various forms of posttranslational modifications. For instance, His73 of skeletal muscle α-actin is methylated, the N-terminal methionine and cysteine residues are acetylated and cleaved, and the resulting N-terminal aspartic acid is then re-acetylated. Since the original determination of the crystal structure of G-actin, over 80 structures of actin have been reported. The majority of these structures have been obtained as complexes with actin-binding proteins (ABPs). (Actin Structure and Function) | ||
