Actin is a protein that functions in the contractile system of skeletal muscle, where it is found in the thin filaments. In muscle, fibrous actin (F-actin) is a helical polymer of a globular polypeptide chain, G-actin. Actin is present in all eukaryotes and has a highly conserved protein sequence (Pollard 1990).
Actins of non-muscle cells are encoded by different genes than those of muscle, and have been found to be involved in platelet shape (Lefebvre et al. 1993), endocytosis (Robertson et al. 2009), cell motility (Lazarides and Weber 1974), cell division (Hill et al. 1996), metastasis (Gabbiani et al. 1975), and cell signaling (Liu et al. 1990).
Actin is usually isolated as G-actin, which may be reversibly transformed into a viscous polymerized fibrous form, F-actin, by the addition of neutral salts and at neutral, or slightly alkaline, pH (Chantler and Gratzer 1975). The reaction, which involves bound nucleotide is:
History
While there has been some debate regarding the official discoverer of actin, it is believed that in 1887 Halliburton was the first to make an extract that contained crude actin, and not until 1942 that Straub (Straub 1942) first isolated pure actin. Straub's original method of isolating actin is relatively similar to the procedures used today (Pollard 1990). At that time, however, little was known about muscle structure at the submicroscopic level.
The 1950s brought a greater understanding of the filamentous structure of actin. The sliding filament model was developed, and remains essentially unchanged since then. This theory explains that the thick and thin filaments within the sarcomere slide past one another, shortening the entire length of the sarcomere. In order to slide past one another, the myosin heads will interact with the actin filaments and, using ATP, bend to pull past the actin (Huxley 2004).
During the 1960s and 1970s, two dimensional X-ray patterns of the actin structure were obtained and studied (Huxley 2004). In the 1980s, the first interpretable electron density maps of the actin molecule were acquired (Hirono et al. 1989), and the ATP hydrolysis and phosphate dissociation was characterized (Kono 1988).
The involvement of actin and its associated proteins' involvement in metastasis was investigated in the 1990s, and the crystal structure was solved (Kabsch et al. 1990). Recent research has focused on the functions of actin in endocytosis (Robertson et al 2009), and it has been proposed that actin is a component of chromatin remodeling complexes in RNA biogenesis (Percipalle 2009).
The structure contains four domains. Two of these domains are similar alpha/beta domains, which contain the ATPase catalytic site. The F-actin helix consists of 13 molecules of G-actin in six turns of the helix, which repeat every 360 Angstroms (Branden and Tooze 1999). Each actin molecule consists of five sulfhydryl groups. Actin is associated with tropomyosin and the troponin complex in muscle. Actin also contains a myosin binding site, where it forms temporary complexes with myosin during muscle contraction, and permanently during rigor mortis (Pollard 1990).
Molecular Characteristics
Actin consists of 376 amino acids. The high proportion of proline and glycine (4.9 and 7.5%, respectively) residues contributes to G-actin's globular shape. Six actin genes are expressed in mammals and birds, and these sequences all share a great deal of homology (Vandekerckhove and Weber 1984).
Composition:
The structure contains four domains. Two of these domains are similar alpha/beta domains, which contain the ATPase catalytic site. The F-actin helix consists of 13 molecules of G-actin in six turns of the helix, which repeat every 360 Angstroms (Branden and Tooze 1999). Each actin molecule consists of five sulfhydryl groups. Actin is associated with tropomyosin and the troponin complex in muscle. Actin also contains a myosin binding site, where it forms temporary complexes with myosin during muscle contraction, and permanently during rigor mortis (Pollard 1990).