Elastase-like Activity Is Dominant to Chymotrypsin-like Activity in 20S Proteasome's beta 5 Catalytic Subunit

摘要

The ubiquitin/proteasome system is the major protein degradation pathway in eukaryotes with several key catalytic cores. Targeting the beta 5 subunit with small-molecule inhibitors is an established therapeutic strategy for hematologic cancers. Herein, we report a mouse-trap-like conformational change that influences molecular recognition depending on the substitution pattern of a bound ligand. Variation of the size of P1 residues from the highly beta 5-selective proteasome inhibitor BSc2118 allows for discrimination between inhibitory strength and substrate conversion. We found that increasing molecular size strengthens inhibition, whereas decreasing P1 size accelerates substrate conversion. Evaluation of substrate hydrolysis after silencing of beta 5 activity reveals significant residual activity for large residues exclusively. Thus, classification of the beta 5 subunit as chymotrypsin-like and the use of the standard tyrosine-containing substrate should be reconsidered.