VCA/CA ligands increase the FRET efficiency between labels on Arp2 and Arp3 more than between labels on ArpC1 and ArpC3. Distributions of diffusion-based smFRET events for (A) Arp2cys-Arp3cys construct and (B) ArpC1cys-ArpC3cys construct. Conditions: 75 pM labeled Arp2/3 complex in KMET buffer (50 mM KCl, 1 mM MgCl2, 1 mM EGTA, 0.1 mM ATP, 1 mM DTT, and 10 mM Tris⋅HCl, pH 7.0) with top row, 40 nM unlabeled Arp2/3 complex; second row, 20 μM WASP-VCA from fission yeast Wsp1p; third row, 20 μM WASP-CA from fission yeast Wsp1p; and fourth row, 2 μM actin-VCA from fission yeast Wsp1p. (Insets) Representative stoichiometry-filtered ALEX histograms. Gray: representative histograms of smFRET events collected over 60 min. Orange: fitted curves for Arp2cys-Arp3cys construct. Blue: fitted curves for ArpC1cys-ArpC3cys construct. Vertical dashed reference lines indicate the peak center for Arp2/3 complex alone.