Actin is a ubiquitous protein involved in the formation of filaments which are major components of the cytoskeleton. It is the monomeric subunit of microfilaments, one of the three main components of the cytoskeleton, and of the fine filaments that are part of the contractile apparatus of muscle cells.
It is the most abundant protein in the typical eukaryotic cell and accounts for about 15% in some cell types. The protein is highly conserved and forms a wide variety of structures in cells along with a large number of actin-binding proteins. Actin filaments interact with myosin to produce a sliding effect, which is the basis of muscle contraction and many aspects of cell motility, including cytokinesis. The individual actin subunits are known as globular actin (G-actin) which assembles into long filamentous polymers called F-actin. Two parallel filaments of F-actin twist in a helical formation, giving rise to microfilaments of the cytoskeleton. The microfilaments are approximately 7 nm in diameter with a helical ring repeating every 37 nm. Each actin protomer binds to an ATP molecule and has a high affinity site for calcium or magnesium ions, as well as several low affinity sites.
It exists as a monomer at low salt concentrations, but filaments form rapidly as the salt concentration increases, resulting in the hydrolysis of ATP. Actin from many sources forms a close complex with deoxyribonuclease (DNase I), although the significance of this remains unknown. The formation of this complex causes inhibition of DNase I activity and actin loses its ability to polymerize. An ATPase domain of actin has been shown to share similarity with the ATPase domains of the hexokinase and hsp70 proteins. In vertebrates, there are three groups of actin isoforms: alpha, beta, and gamma. Alpha actins are found in muscle tissues and are an important component of the contractile apparatus. Beta and gamma actins coexist in most cell types as components of the cytoskeleton and as mediators of internal cell motility. MreB, an important component of the bacterial cytoskeleton, exhibits high structural homology with its eukaryotic counterpart, actin. Furthermore, it has been suggested that members of the Rho family of small guanosine triphosphatases have emerged as key regulators of the actin cytoskeleton and, through their interaction with multiple target proteins, ensure coordinated control of other cellular activities such as transcription and l membership. of genes.
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