OTA directly activates 26S pure proteasome but not 20S pure proteasome. (a) 26S and (b) 20S (0.3 ng) pure proteasome complexes were treated with 5 or 10 µM OTA or vehicle (VHC) (0.2% v/v Et-OH) in 10 mM HEPES buffer (pH 7.6) for 1 h at 37 °C. OTA-treated 26S and 20S proteasome complexes were mixed with specific luminogenic substrates (Suc-LLVY-Glo, Z-nLPnLD-Glo, and Z-LRR-Glo for chymotrypsin-, caspase-, and trypsin-like activities, respectively) in the presence of 250 μM ATP for 26S proteasome and luminogenic signals were detected. Results were shown relative to the vehicle (VHC) control (ns: Nonsignificant, ** p < 0.01, *** p < 0.001, **** p < 0.0001).