Schematic representations of the precursors and assembled components of the branch junction. (A) Drawing of the inactive Arp2/3 complex and a standard actin filament. (B) Model of the branch junction. The following conformational changes are proposed to occur: (1) opening the nucleotide-binding clefts of mother filament subunits M2 and M4; (2) converting subunit M4 from a filament to a monomeric conformation; (3) converting Arp3 into filament conformation; (4) moving Arp2 tethered by ARPC5 next to Arp3 to form the first two subunits of the daughter filament; and (5) converting Arp2 into filament conformation.