Complete stabilization of tyrosine-phosphorylated Stat5A in the nucleus by MG132. 32D (EpoR wt) cells were starved overnight and pretreated with or without MG132, followed by stimulation with IL-3. After removal of the cytokine, cytoplasmic and nuclear extracts were prepared at the indicated time points and subjected to Western blot analysis with anti–phospho-Stat5 (α-pStat5), anti-Stat5A (α-Stat5A), or anti–nuclear protein YY1 (α-YY1) antibodies.