Tyrosine 731 of c-Cbl is critical for enhancing PI3 kinase activity. (A) IL-4 activates c-Cbl–associated PI3 kinase in Ba/F3 cells. Lysates from Ba/F3 cells stimulated with or without IL-4 were immunoprecipitated with anti–c-Cbl antibody. The immunoprecipitates were subjected to the PI3 kinase assay. (B) The PI3 kinase assay of mock, HCbl, and YF cells stimulated with or without IL-4. Lysates from mock, HCbl and YF cells stimulated with or without IL-4 were immunoprecipitated with antiphosphotyrosine antibody. The immunoprecipitates were then subjected to the PI-3 kinase assay. (C) The thymidine incorporation assay of HCbl and YF cells in the presence or absence of IL-4 (10 ng/mL). The growth rates were expressed as in Fig 2D. The data for HCbl cells are the same as presented in Fig 2D. (D) Growth of HCbl and YF cells in the presence of IL-4. HCbl and mock cells were cultured in RPMI containing 5% FCS and IL-4 (1 ng/mL), but without IL-3. The data for HCbl cells are the same as presented in Fig 2B.