CycT1-U7 promotes the degradation of Tat. A. Western blotting depicts the steady-state expression of Tat proteins co-expressed with wt or mutant CycT1. HA-epitope tagged Tat (lanes 1–3) and HA-tagged CycT1 1–280 (wt: lane 2) or HA-CycT1-U7 (lane 3) were co-expressed in 293T cells. Twenty-four hours after transfection, cells were lysed with RIPA buffer (25 mM Hepes-KOH, 150 mM KCl, 1 mM EDTA, 1% Triton X100, 0.1% NP-40, pH 7.4), and soluble proteins were separated by 12% SDS-PAGE. The ectopically expressed CycT1 and Tat proteins were detected by anti-HA antibody. The endogenous proteins (CycT1, Cdk9 and Tubulin) were also detected by Western blotting. B. The expression of CycT1-U7 and Tat was restored by proteasome inhibitors. 293T cells were transfected with HA-tagged wt CycT1 (1–280) (lanes 8 to 10) or HA-CycT1-U7 (lanes 1 to 7) and HA-Tat as described above. Twenty-four hours after transfection, cells were treated with DMSO (lanes 1, 4 and 8), MG-132 (50 μM: lanes 2, 3 and 9) or Epoxomicin (50 μM: lanes 4 to 7 and 10) for 1 (lanes 2 and 5), 3 (lanes 3, 6, 9 and 10) and 5 hours (lane 7). Cells were then lysed in RIPA buffer and subjected to SDS-PAGE. The ectopically expressed CycT1 and Tat proteins were detected by anti-HA antibodies.