Functions as 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. The Arp2/3 complex is involved in organizing the actin system in cell motility and chemotaxis, in phagocytosis and macropinocytosis, at late steps of endosome processing, and in mitosis. In concert with a group of other proteins, the Arp2/3 complex plays a general role in the rapid activation and adaptation of the actin system to its multiple functions. Component of the Arp2/3 complex composed of arpB/Arp2, arpC/Arp3, arcA/p41-arc, arcB/p34-arc, arcC/p21-arc, arcD/p20-arc and arcE/p16-arc. Interacts with carmil (via the region between the LRR domain and COOH-terminal proline-rich domain); carmil is required for Arp2/3-dependent actin nucleation. Arp2/3 complex, MyoB, MyoC, and the alpha and beta subunits of capping protein all form a larger complex with carmil. Belongs to the WD repeat ARPC1 family.