Reduced activation of Akt and Rac in p85α^–/– BMMs. Wild-type and p85α^–/– BMMs were starved overnight and stimulated with M-CSF. Equal amounts of cell lysates were subjected to Akt and Erk MAP kinase activation (A) by Western blot analysis using an anti–phospho Akt or Erk1/2 antibody or to a p21-activated kinase (PAK)–binding pull-down assay (B), which measures active, GTP-bound Rac. Shown are representative blots. Three independent experiments showed similar results. The bars indicate the band intensity of activated Akt and Rac, expressed as arbitrary units.