nucleotide-binding domain (NBD) of actin-related protein3 (Arp3) and similar proteins
Arp3, also called actin-like protein 3, is the ATP-binding component of the Arp2/3 complex, a multiprotein complex that mediates actin polymerization upon stimulation by nucleation-promoting factor (NPF). The Arp2/3 complex is comprised of 7 proteins (Arp2, Arp3, and five conserved proteins, ARPC1-5). It generates cytoplasmic branched filaments networks, by promoting nucleation of actin filaments as 70 degrees branches on the side of older filaments. It is activated, by simultaneously binding to a pre-existing filament and a nucleation promoting factor plus an actin monomer. Daughter branches subsequently detach/debranch from the mother filament. Its Arp2 and Arp3 subunits must be loaded with ATP for it to initiate the assembly of branched actin filaments. ATP hydrolysis may be required for branch initiation or debranching. The Arp2/3 complex is also found in the nucleus where it plays a role in promoting de novo actin polymerization and in RNA polymerase II-dependent transcription. This may in part be through regulating nuclear actin polymerization in a way like its function in the cytoplasm. Human Arp3 and Arp3B are encoded by the ACTR3 and ACTR3B genes respectively. Arp3B is also known as actin-related protein Arp4.
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