Activation of SAPKα by hematopoietic growth factors. MC/9 cells were incubated at 37°C in RPMI 1640 for 1 hour and then left untreated (CON), or treated with saturating doses of IL-4 (IL-4), GM-CSF (GM), SLF (SLF ), or 0.2 mol/L NaCl (NaCl) for the indicated times. Immunoprecipitated SAPKα was analyzed after SDS-PAGE with a separating gel containing GST-c-Jun, using an in-gel kinase assay. The phosphorylation of c-Jun was assessed by autoradiography and the presence of 46- and 55-kD splice variants of SAPKα are indicated.