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 (By similarity). Seems to contact the pointed end of the daughter actin filament (By similarity). Required during embryogenesis for the developmental migration of tail hemocytes anteriorly, along the ventral midline (PubMed:25739458). Component of the Arp2/3 complex. Belongs to the actin family. ARP3 subfamily.