Actin Nucleation by Arp-2/3 Complex

Arp2/3 complex is a seven-subunit protein complex that plays a major role in the regulation of the actin cytoskeleton. It is a major component of the actin cytoskeleton and is found in most actin cytoskeleton-containing eukaryotic cells.Two of its subunits, the Actin-Related Proteins ARP2 and ARP3, closely resemble the structure of monomeric actin and serve as nucleation sites for new actin filaments. The complex binds to the sides of existing filaments and initiates growth of a new filament at a distinctive 70 degree angle from the mother. Branched actin networks are created as a result of this nucleation of new filaments. The regulation of rearrangements of the actin cytoskeleton is important for processes like cell locomotion, phagocytosis, and intracellular motility of lipid vesicles.Actin is a family of globular multi-functional proteins that form microfilaments. An actin protein 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. It can be present as either a free monomer called G-actin (globular) or as part of a linear polymer microfilament called F-actin (filamentous), both of which are essential for such important cellular functions as the mobility and contraction of cells during cell division. Nucleating factors are necessary to stimulate actin polymerization. One such nucleating factor is the Arp2/3 complex, which mimics a G-actin dimer in order to stimulate the nucleation (or formation of the first trimer) of monomeric G-actin. The Arp2/3 complex 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 ADP for ATP, promoting the monomeric addition to the barbed, plus end of F-actin filaments.

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