SCF-induced activation of Rac1 and Rac2 GTPases and p38 MAPK are coordinated through Fyn kinase. (A) fyn^+/+ and fyn^−/− BMMCs were activated with SCF (50 ng/mL) for 0, 5, 15, or 30 minutes. CRIB pull-down assays were used to detect activated Rac GTPases (Rac^GTP), as described in “Materials and methods.” SCLs and pull-down samples were immunoblotted (IB) with anti-Rac1 and anti-Rac2 antibodies. In all experiments, 10% of input was used for loading of GTPases with excess GDP or GTPγS to serve as controls. (B) fyn^−/− BMMCs transduced with MSCV or MSCV-Fyn were analyzed as described. (C) fyn^+/+ and fyn^−/− BMMCs were plated on fibronectin-coated wells and treated with SCF (50 ng/mL) for 0, 5, or 15 minutes. SCLs were prepared and IB with: antiphosphotyrosine (pY), antiphospho-Akt (pAkt), anti-Akt, antiphospho-p38 (pp38), or anti-p38. Arrow at right indicates a phosphoprotein that appears to be elevated in fyn^−/− BMMCs.