Since the discovery of lysosomes, it was believed that this organelle degrades cellular proteins. However, several independent researchers showed strong evidence for non-lysosomal intracellular protein degradation, but this process's mechanisms were unclear until proteasome discovery. The ubiquitin-proteasome pathway is a major avenue for the degradation of proteins in the cytosol and nucleus of eukaryotic cells. Proteins are marked for degradation via the attachment of a ubiquitin group whereby the proteasome recognizes, unfold, and digests these proteins. The 26S proteasome is a cylindrical shaped proteolytic complex (Figure 1 (left)) that incorporates the ATP-dependent 19S caps, which recognize the proteins marked for degradation and unfold substrates in turn fed to the 20S proteasome, where proteolysis occurs. 20S core particle is a four stacked heptameric ring composed of a and B units (a 7, B 7, B 7, a 7). subunits guard the entrance to the active site by allowing access to only unfolded proteins.
The catalytic activities are confined to the B subunits responsible for mediating the enzymatic activity of the proteasome. Three of the B-subunits in the B-ring are catalytically active, and they have an N-terminal threonine (side-chain hydroxyl group activated by the N-terminal amine moiety) that acts as the proteolytic nucleophile (Figure 1 (right)). The proteasome has at least three distinct peptidase activities and, by comparison with substrate specificities of known proteases, they are designated as chymotrypsin-like (B5-subunit), trypsin-like (B2-subunit), and caspase-like activities (B1- subunit).