Identification of compound 5929407 as a proteasome inhibitor. (A) In vivo proteasome activity assay. Flow cytometry diagrams showing proteasome inhibition-dependent expression of GFP in GFPu-1 cells treated with the indicated drugs (gray area), compared with non-treated cells (white area). In the first panel, three independent non-treated samples are plotted to show variability. (B) Compounds with proteasome inhibitor activity preferentially inhibit proteasome chymotrypsin- and caspase-like activities. Proteasome activities were monitored using fluorogenic probes processed by cellular extracts obtained from HeLa cells exposed to the various proteasome inhibitors (n = 3, mean ± SEM).