Actin is a globular, roughly 42-kDa highly conserved protein found in all eukaryotic cells (the only known exception being nematode sperm) where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans. Actin is the monomeric subunit of two types of filaments in cells: microfilaments, one of the three major components of the cytoskeleton, and thin filaments, part of the contractile apparatus in muscle cells. Thus, actin participates in many important cellular processes including muscle contraction, cell motility, cell division and cytokinesis, vesicle and organelle movement, cell signaling, and the establishment and maintenance of cell junctions and cell shape. Many of these processes are mediated by extensive and intimate interactions of actin with cellular membranes.In vertebrates, three main groups of actin isoforms, alpha, beta, and gamma have been identified. The alpha actins are found in muscle tissues and are a major constituent of the contractile apparatus. The beta and gamma actins co-exist in most cell types as components of the cytoskeleton, and as mediators of internal cell motility.
Actin has four main functions in cells :
1)To form the most dynamic one of the three subclasses of the cytoskeleton, which gives mechanical support to cells, and hardwires the cytoplasm with the surroundings to support signal transduction.
2)To allow cell motility (see Actoclampin molecular motors), including phagocytosis of bacteria by macrophages.
3)In muscle cells to be the scaffold on which myosin proteins generate force to support muscle contraction.
4)In non-muscle cells as a track for cargo transport myosins [non-conventional myosins] such as myosin V and VI. Non-conventional myosins transport cargo, such as vesicles and organelles, in a directed fashion, using ATP hydrolysis, at a rate 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 export of cargos, and myosin VI to be an effective motor for import.
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