Arp2/3 complex

Many actin-related molecules create a free barbed end for polymerization by uncapping or severing pre-existing filaments and using these as actin nucleation cores. However, the Arp2/3 complex stimulates actin polymerization by creating a new nucleation core. Actin nucleation is an initial step in the formation of an actin filament. The nucleation core activity of Arp2/3 is activated by Nucleation Promoting Factors (NPFs) including members of the Wiskott-Aldrich syndrome family protein (WASP, N-WASP, WAVE, and WASH proteins). The V domain of a WASP protein interacts with actin monomers while the CA region associates with the Arp2/3 complex to create a nucleation core. However, de novo nucleation followed by polymerization is not sufficient to form integrated actin networks, since these newly synthesized polymers would not be associated with pre-existing filaments. Thus, the Arp2/3 complex binds to pre-existing filaments so that the new filaments can grow on the old ones and form a functional actin cytoskeleton. Large conformational changes occur on nucleotide and WASP binding. ==Cellular uses of Arp2/3==

The Arp2/3 complex appears to be important in a variety of specialized cell functions that involve the actin cytoskeleton. The complex is found in cellular regions characterized by dynamic actin filament activity: in macropinocytic cups, in the leading edge of motile cells (lamellipodia), and in motile actin patches in yeast. == Subunits ==

The Arp2/3 complex is composed of seven subunits: Arp2/ACTR2, Arp3/ACTR3, p41/ARPC1A&B/Arc40/Sop2/p40, p34/ARPC2/ARC35/p35, p21/ARPC3/ARC18/p19, p20/ARPC4/ARC19/p18, p16/ARPC5/ARC15/p14. The subunits Arp2 and Arp3 closely resemble monomeric actin allowing for a thermodynamically stable actin-like dimer. p41 has been proposed to interact with nucleation promoting factors (NPFs) because it is only known to have minor contacts with the mother filament and there is a major loss of nucleation efficiency in the absence of p41. p34 and p20 dimerize to form a structural backbone that mediates the interaction with the mother filament. p21 forms a bridge between Arp3 and the mother filament, increasing nucleation efficiency. p16 tethers Arp2 to the rest of the complex. Similarly, ARPC5 has a homolog ARPC5L (ARPC5-like). Compared to ARPC5, ARPC5L supports more rapid and dynamic reorganization of filaments in regions such as lamellipodia involved in cell motility. == Structural insights ==

The Arp2/3 complex is composed of seven subunits: two actin-related proteins (Arp2 and Arp3) and five additional subunits (ARPC1–ARPC5). In its inactive state, the complex adopts a compact conformation where Arp2 and Arp3 are spatially separated and not aligned in a way that mimics an actin dimer. This conformation prevents spontaneous actin nucleation. Upon activation by nucleation-promoting factors (NPFs)—such as WASP—the Arp2/3 complex undergoes a dramatic conformational rearrangement. Cryo-electron microscopy and molecular dynamics simulations have revealed that the binding of NPFs and ATP induces repositioning of Arp2 and Arp3 into a short-pitch actin-like dimer, the geometry required for nucleating a new actin filament branch. This transition forms a structural "template" that resembles a barbed-end actin nucleus, which then elongates by recruiting actin monomers. The core subunits, particularly ARPC2 and ARPC4, form a structural scaffold that binds the side of an existing ("mother") actin filament at a 70-degree angle. This binding provides the platform from which a new ("daughter") filament can branch. Structural data show that ATP binding is essential for this transition. == Cellular and physiological roles ==

The Arp2/3 complex is essential for the spatial and temporal regulation of actin cytoskeleton dynamics. One of the most well-characterized roles of the Arp2/3 complex is in cell motility, particularly through the formation of lamellipodia. This is vital in processes such as cell movement and vesicle trafficking. The complex, activated by nucleation-promoting factors like WASP and WAVE, catalyzes the branching of actin filaments to push the plasma membrane forward. In neuronal development, the Arp2/3 complex plays a pivotal role in axon guidance, organization of radial glial cells, and dendritic spine formation. Disruption of Arp2/3 function in mouse models leads to severe abnormalities in the cortical layering of neurons, loss of neuronal polarity, and impaired synaptic plasticity, indicating that actin nucleation is critical for establishing and maintaining neuronal networks. In the immune system, Arp2/3-mediated actin remodeling is necessary for phagocytosis, immune synapse formation, and cell migration. == References ==