Once activated, the ARP2/3 complex nucleates new actin filaments that extend from the sides of pre-existing mother actin filaments at a 70-degree angle to form Y-branched networks (Firat-Karalar & Welch 2010). These branched actin filaments push the cell membrane forward to form a pseudopod. The ARP2/3 complex is composed of two Arps (actin-related proteins), ARP2 and ARP3, and five unique proteins ARPC1, ARPC2, ARPC3, ARPC4 and ARPC5 (Gournier et al. 2001). Both ARP2 and ARP3 subunits bind ATP. There are two proposed models to explain the process of actin nucleation by ARP2/3 complex: the barbed-end branching model and the dendritic nucleation/side branching model (Le Clainche & Carlier 2008).
In barbed-end branching model, the branching/ternary complex (G-actin-WASP/WAVE-Arp2/3 complex) binds to the barbed end of the mother filament. G-actin bound to VCA domain or one of the Arp subunits incorporates into the mother filament at the barbed end, thus positioning ARP2/3 complex to initiate the daughter branch on the side of the mother filament. ARP2/3 nucleates the formation of new actin filament branches, which elongate at the barbed ends (Le Clainche & Carlier 2008, Pantaloni et al 2000, Le Clainche et al. 2003, Egile et al. 2005). In side branching model, the branching complex binds to the side of the mother actin filament mimicking an actin nucleus and initiates a lateral branch (Le Clainche & Carlier 2008, Amann & Pollard 2001).