Proteasome activator 28γ (PA28γ) allosterically activates trypsin-like proteolysis by binding to the α-ring of the 20S proteasome

Mapping the Human Proteome with Physical Access to DNA

“…Overall, these were functions known to occur within chromatin, however, our DNA interactome highlighted specific proteins that came in direct contact with DNA when executing them. For example, we found proteins related to the ubiquitin-proteasome system such as PSM3, which forms an 11S regulatory complex (PA28γ) that promotes the degradation of TP53 and many other proteins involved in cell cycle progression, apoptosis and DNA repair (Thomas and Smith, 2022; Zhang and Zhang, 2008). Our data implied that PSM3 came in physical contact with DNA when targeting the proteasome within chromatin.…”

Structure, Function, and Allosteric Regulation of the 20S Proteasome by the 11S/PA28 Family of Proteasome Activators

“…It has also been hypothesized to regulate the 20S CP through mechanisms distinct from those of PA28αβ. As opposed to controlling substrate entry, PA28γ has been suggested to enhance the proteolytic active site of the 20S CP, which is responsible for cleavage after basic amino acids [ 39 ], a function recently demonstrated conclusively by Thomas and Smith in 2022 [ 45 ]. Enigmatically, alterations such as a single-point mutation and changes in purification strategies have been found to shift PA28γ’s function from a trypsin-like activator to a gate-opener [ 44 , 46 , 47 ].…”

“…PA28γ’s IDR comprises a cluster of positive residues followed by a cluster of negative residues ( Figure 4 C). Interestingly, the presence of the IDR does not influence the PA28γ complex assembly nor its functional interactions with the proteasome [ 45 , 68 ]. The structural characterization of these IDRs, either individually through NMR or within the context of the protein complex, remains elusive, leaving the question of their physiological roles open-ended.…”

“…This open-gate conformation is quantifiable in a proteasome activity assay using fluorogenic peptide substrates that are specific for each of the three proteolytic subunits. When compared against a wild-type proteasome, PA28αβ–20S CP complexes can enhance 20S peptide degradation rates by 200-fold [ 39 , 45 ], which is the result of inducing 20S gate-opening. What is also important about this gate-opening function is the ability of the PA28αβ–20S CP complex to form a hybrid complex with the 19S RP.…”

“…Recently, Thomas and Smith established PA28γ’s function as a T-L proteolytic-site activator, using allosteric regulation to activate the 20S CP in the presence of peptide substrates without requiring any supplementary mechanisms to fulfill this role [ 45 ]. The study used a constitutively open 20S CP to deconvolute gateing contributions to the quantified enzymatic activities.…”