Sra1, Nap1, WAVE1, Abi2 and HSPC300 were overexpressed separately, partially purified, assembled into an Sra1:Nap1 dimer and a WAVE1/Abi2/HSPC300 trimer, respectively, and then assembled into the miniWRC pentamer. Further purification produced homogenous samples. Crystals of miniWRC were obtained by hanging-drop vapor-diffusion at 4 °C. All the data sets were collected at the ID-19 beamline (APS, Chicago) and processed with the HKL3000 suit37 (link) and CCP4 suites38 (link). Experimental phases were determined from Se-MIRAS data collected on samples containing either SeMet-Sra1 or SeMet-Nap1, and analyzed using ShelxD. Phases were improved using MLPHARE and Parrot. The atomic model of the complex was built using Buccaneer and Coot, and refined using Refmac 5. Equilibrium dialysis was done at room temperature and protein concentrations were determined using Deep Purple gel staining (GE Healthcare). Actin polymerization and GST-Rac1 pull-down assays were performed as described previously13 (link). HeLa cells were grown directly on coverslips, fixed in 4% paraformaldehyde, and prepared for immunofluorescence as described25 (link).