C HEK293T cells were transfected as indicated, at 18 h after transfection, the cells were treated with the proteasome inhibitor MG132 (20μM) for 6 hours and then harvested. Cell lysates were immunoprecipitated by Flag-antibody (M2 beads) and Ubiquitin-conjugated GPX1 was detected by Western blotting with anti-Myc antibody. 