Arp2/3 complex subunits are colored as follows: Arp3, orange; Arp2, red; ARPC1, green; ARPC2, cyan; ARPC3 (not shown for clarity); ARPC4, blue; ARPC5, yellow. Actin monomers recruited by WASP are not shown. Purple arrows indicate contact between actin filaments or SPIN90 with ARPC4 that may stimulate a structural change required for activation. Thick gray arrow in left panels indicates potential rigid‐body rotation of Arp2, ARPC1, ARPC5, and the globular portion of ARPC4 hypothesized to move Arp2 into the short‐pitch arrangement (Dalhaimer & Pollard, 2010).