Effect of proteasome inhibition on LV transduction of different cell lines. (A) Frequency of GFP-positive cells and (B) MFI of 293T, HeLa, U937, and K562 cells transduced with LV-GFP in the presence of increasing concentrations (0.1 to 2 μM) of MG132 for 12 hours. Missing points at the highest doses in K562 and U937 curves are due to cell death because of MG132 toxicity. Two to 3 experiments are shown for each cell line. (C) CD34⁺ cells exposed (+cyt) or not (–cyt) to cytokines for 20 to 24 hours; HeLa, 293T, U937 and K562 cells were lysed and tested for proteasome chymotrypsin-like activity. Chymotrypsin-like activity was assayed by monitoring the production of 7-amino-4-methylcoumarin (amc) from fluorogenic peptide Suc-LLVY-amc. Values are mean ± SD of 3 measurements from 1 of 2 experiments with similar results.