Proteasome inhibition activity of EF24 and related curcumin analogs in Xenopus extracts. A) Inhibition of caspase- and chymotrypsin-like proteasome activities and xFANCD2-Ub by MG132, curcumin and curcumin analogs. Proteasome activities and xFANCD2-Ub were monitored using fluorogenic probes and immunoblotting, respectively. IC50 values were determined for each compound using a range of concentrations and represented in histogram graphs. For MG132, numbers above caspase-like and chymotrypsin-like histograms indicate the actual IC50 values. A representative experiment is shown (out of 3 repeats). B) Inhibition of proteasome activities by EF31 and AS153-4 compared to curcumin.