Ubiquitin chain trimming recycles the substrate binding sites of the 26 S proteasome and promotes degradation of lysine 48‐linked polyubiquitin conjugates

Abstract

The 26 S proteasome possesses two distinct deubiquitinating activities: the ubiquitin (Ub) chain amputation activity removes the entire polyUb chain from the substrates; the Ub chain trimming activity progressively cleaves a polyUb chain from the distal end. The Ub chain amputation activity mediates degradation‐coupled deubiquitination. The Ub chain trimming activity can play a supportive or an inhibitory role in degradation, likely depending on features of the substrates. How Ub chain trimming assists degradation is not clear. We find that inhibition of the chain trimming activity of the 26 S proteasome with Ub aldehyde significantly inhibits degradation of Ub4 (K48)‐UbcH10, and causes accumulation of free Ub4 (generated from chain amputation) that can be retained on the proteasome. Also, a non‐trimmable K48‐mimic Ub4 efficiently targets UbcH10 to the 26 S proteasome, but it can not support efficient degradation of UbcH10 compared to regular K48 Ub4. These results indicate that polyUb chain trimming promotes proteasomal degradation of K48‐linked substrates. Mechanistically, we propose that Ub chain trimming cleaves the proteasome‐bound K48‐linked polyUb chains, which vacates the Ub binding sites of the 26 S proteasome, thus allowing continuous substrate loading.