MEK1/2 is critical for Bim phosphorylation in mitogenically stimulated B cells and proteasomal inhibitors inhibit the reduction in Bim levels. (A) Western blot analysis of lysates from purified (wt) B cells that were left untreated (lane 1) or had been stimulated with Fab2 anti-IgM antibody fragments plus IL-2, IL-4, IL-5 for 1 h (lanes 2–4) or 3 h (lanes 5–7), plus (lanes 3, 6) or minus (lanes 2, 5) the addition of the MEK1/2 inhibitor UO126 (10 µM) or the carrier, DMSO (lanes 4, 7). (B) Western blot analysis of lysates from purified (wt) B cells that were left untreated (lanes 1, 7) or had been stimulated for 3 h with Fab2 anti-IgM antibody fragments plus IL-2, IL-4, IL-5 (lanes 2–6 and 8–12) plus (lanes 3–5 and 9–11) or minus (lanes 2 and 8) the addition of PS341 (lanes 3–5) or MG132 (lanes 9–11) or the carrier, DMSO (lanes, 6 and 12).