Epo rapidly and transiently activates p38 and MAPKAP kinase-2 during erythroid differentiation. (A) SKT6 cells were stimulated with Epo for 0, 3, 5, 15, 30, 60, and 180 minutes (lanes 1 through 7) and for 1, 2, 3 and 4.5 days (lanes 8 through 11). The p38 activity was measured in the immunoprecipitates with anti-p38–specific antibody and GST-ATF-2 as a substrate. Arrow indicates the phosphorylated GST-ATF-2 (molecular weight, 40 kD). The lower panel shows the immunoblots of the immunoprecipitated p38 in each lane. (B) The MAPKAP kinase-2 assay was performed in the immunoprecipitates of SKT6 cells in the presence of MAPKAP kinase-2 substrate peptide as a substrate at various time points after Epo stimulation, as indicated.