Ile294Thr and Ser272Pro mutations enhanced the ability of WASp to stimulate actin polymerization in vitro. Sepharose beads coated with WT or mutant GST-WASp constructs or GST-vector alone were incubated with 3 different dilutions of U937 cell lysates for 1 hour at room temperature. (A) SDS-PAGE of bound proteins stained with anti-GST and antiactin antibodies. (B) Actin polymerization activity for each mutant relative to WT.