Structural Insights into Substrate Recognition and Processing by the 20S Proteasome

Abstract

Four decades of proteasome research have yielded extensive information on ubiquitin-dependent proteolysis. The archetype of proteasomes is a 20S barrel-shaped complex that does not rely on ubiquitin as a degradation signal but can degrade substrates with a considerable unstructured stretch. Since roughly half of all proteasomes in most eukaryotic cells are free 20S complexes, ubiquitin-independent protein degradation may coexist with ubiquitin-dependent degradation by the highly regulated 26S proteasome. This article reviews recent advances in our understanding of the biochemical and structural features that underlie the proteolytic mechanism of 20S proteasomes. The two outer α-rings of 20S proteasomes provide a number of potential docking sites for loosely folded polypeptides. The binding of a substrate can induce asymmetric conformational changes, trigger gate opening, and initiate its own degradation through a protease-driven translocation mechanism. Consequently, the substrate translocates through two additional narrow apertures augmented by the β-catalytic active sites. The overall pulling force through the two annuli results in a protease-like unfolding of the substrate and subsequent proteolysis in the catalytic chamber. Although both proteasomes contain identical β-catalytic active sites, the differential translocation mechanisms yield distinct peptide products. Nonoverlapping substrate repertoires and product outcomes rationalize cohabitation of both proteasome complexes in cells.

Oleuropein Activates Neonatal Neocortical Proteasomes, but Proteasome Gene Targeting by AAV9 Is Variable in a Clinically Relevant Piglet Model of Brain Hypoxia-Ischemia and Hypothermia

“…Proteasomes undergo multiple complex steps to degrade proteins. The t 0 proteasome activity may represent the proteasome's initial substrate binding, docking, and conformational changes that open the gate and pass the substrate through the catalytic core [78]. The P20S proteasome gate opening is controlled by tight allosteric coupling to a tetrahedral transition state regulated by α-ring subunit N-terminal regions that trigger the conformational switch [79].…”

“…These include the free P20S catalytic core, P26S proteasome assembly with the P20S core and 19S regulatory particle, P20S core with proteasome activator subunits, and other hybrids. Several diseases alter proteasome structure and activity [78], including cardiac ischemia [81].…”

Concept and application of circulating proteasomes

“…In addition, oxidatively modified proteins, which largely affect the intracellular signaling pathway and cell viability when they accumulate, also form a class of 20S substrates 17 , 18 . This larger than anticipated physiological and pathological role of the 20S proteasome appears to be mediated by its conformational change upon engagement with substrates (from a resting form to a processing form) 19 . The Glickman group recently demonstrated that the binding of an unstructured model substrate was sufficient to induce conformational changes in α subunits and subsequently 20S gate opening, facilitating substrate hydrolysis without the involvement of the 19S proteasome or other activators 20 .…”

Proteasome in action: substrate degradation by the 26S proteasome

“…Within the 26S proteasome, the 19S regulatory particle (RP) is responsible for recognising the degradation signal and unfolding of the target protein-substrate, whereas the 20S core particle (CP) hydrolyses the unfolded polypeptide into short peptides or amino acids (Figure 1A). Although the 20S CP serves as the catalytic core of the 26S proteasome, it is also quite abundant as a free 20S proteasome complex and may retain basal proteolytic activity for substrates with an unstructured or unfolded stretch [6][7][8]. To broaden its substrate repertoire, various activators attach to the 20S CP aiding substrate processing (SP).…”