Functions as ATP-binding component of the Arp2/3 complex which is involved in regulation of actin polymerization and together with an activating nucleation-promoting factor (NPF) mediates the formation of branched actin networks. Seems to contact the pointed end of the daughter actin filament. Regulates the directionality of cell expansion by regulating the actin organization, and thus the microtubules distribution and the fusion of small vacuoles (By similarity). Component of the Arp2/3 complex. Belongs to the actin family. ARP2 subfamily.