PSME1

PSME1 (PA28α) is a subunit of the PA28 proteasome activator complex that forms a heptameric ring capping the 20S proteasome core particle, stimulating its peptidase activity and shaping the repertoire of peptides available for MHC class I antigen presentation and broader protein homeostasis. The crystal structure reveals a barrel-shaped heptamer whose binding opens a gated pore in the proteasome α-ring to permit substrate and product passage (PMID:9403698); overexpression at IFN-γ–inducible levels enhances class I–restricted antigen presentation independently of immunoproteasome subunit exchange, while double-knockout mice lose processing of select antigens such as TRP2 (PMID:8610016, PMID:11169410, PMID:11689430). Beyond antigen processing, PSME1-mediated proteasome activation degrades β-catenin to suppress Wnt signaling in bone cells (PMID:34838695), degrades CDK15 to promote breast cancer invasion (PMID:31824858), regulates proteasome load and sensitivity to proteasome inhibitors in multiple myeloma (PMID:33318477), and stabilizes HBV core protein against 26S proteasome degradation to facilitate viral replication (PMID:39281837). PSME1 constitutively localizes to the centrosome together with the 20S proteasome and chaperones, where the complex expands upon proteasome inhibition or accumulation of misfolded proteins, implicating it in centrosome-based protein quality control (PMID:10225950).

1. 1996 High

   Establishing that PA28α has an in vivo function in antigen processing resolved whether this proteasome activator operates only in vitro, showing that physiological-level overexpression directly enhances MHC class I–restricted epitope presentation.

   Evidence Stable transfection of PA28α in murine fibroblasts with CTL recognition assays for two viral antigens

   PMID:8610016

   Open questions at the time

   • Mechanism by which PA28α alters the peptide repertoire was unknown
   • Relative contribution of PA28α versus PA28β was not separated
   • Whether PA28α acts on all antigens or only a subset was untested
2. 1997 High

   Solving the crystal structure of the PA28α heptamer answered how the activator physically engages the proteasome, revealing a barrel that opens a gated pore in the 20S α-ring to allow substrate/product transit.

   Evidence 2.8 Å X-ray crystallography of recombinant human PA28α

   PMID:9403698

   Open questions at the time

   • Structure of the PA28α–20S complex itself was not determined
   • Whether the heteromeric PA28α/β ring adopts the same architecture was unresolved
   • Conformational dynamics during gating remained unknown
3. 1999 High

   Localizing PA28α/β to the centrosome together with the 20S proteasome and chaperones revealed a constitutive proteasome quality-control platform at this organelle, expanding PA28α's role beyond antigen processing.

   Evidence Immunofluorescence colocalization with γ-tubulin, density-gradient fractionation of purified centrosomes, and proteasome inhibitor challenge in HEK293 and HeLa cells

   PMID:10225950

   Open questions at the time

   • The substrates degraded at the centrosome by PA28-activated proteasomes were not identified
   • Whether centrosomal localization is essential for PA28α's antigen-processing role was unclear
4. 2000 High

   Demonstrating that PA28α/β enhances antigen presentation without altering proteasome subunit composition showed that the activator acts directly on 20S peptidase activity rather than by promoting immunoproteasome assembly.

   Evidence Co-transfection of PA28α/β in fibroblasts with proteasome subunit composition analysis and CTL assays

   PMID:11169410

   Open questions at the time

   • The precise cleavage products altered by PA28 binding were not characterized
   • Whether PA28 changes cleavage site preference or only product release rate was unresolved
5. 2001 High

   PA28α/β double-knockout mice showed that the activator is dispensable for most antigen presentation but essential for processing specific epitopes (e.g., TRP2), defining PA28 as a substrate-selective processing factor rather than a global antigen-processing requirement.

   Evidence Genetic double-KO mice with IFN-γ stimulation and antigen-specific CTL assays for multiple antigens

   PMID:11689430

   Open questions at the time

   • Rules governing which epitopes require PA28 were not defined
   • Compensatory roles of PA28γ in vivo were not assessed
   • Whether PA28 dependence maps to primary sequence or proteasome cleavage kinetics was unknown
6. 2016 Medium

   Finding that PA28α/β deletion protects against diabetic microvascular injury extended PSME1's physiological relevance to chronic metabolic disease, linking its proteasome-activating function to inflammatory mediator expression under high glucose.

   Evidence PA28α/β double-KO mice in STZ-induced diabetes, mesangial and retinal pericyte cultures, PA28-blocking peptides, OPN/MCP-1 readouts

   PMID:27830089

   Open questions at the time

   • The specific proteasome substrates mediating OPN/MCP-1 upregulation were not identified
   • Whether PA28α or PA28β is the rate-limiting subunit in this context was not determined
   • Blocking peptide specificity in vivo was not fully validated
7. 2019 Medium

   Identification of CDK15 as a target degraded via PA28α/β-activated immunoproteasome placed PSME1 in a pro-invasive axis in breast cancer, establishing a proteasome-dependent mechanism for metastasis promotion.

   Evidence siRNA knockdown of PA28α/β and β5i in breast cancer cells with migration/invasion assays and CDK15 protein level measurement

   PMID:31824858

   Open questions at the time

   • Direct ubiquitination or proteasomal degradation of CDK15 was not reconstituted
   • In vivo metastasis data were limited
   • Whether CDK15 degradation is immunoproteasome-specific was not conclusively shown
8. 2020 Medium

   Showing that PA28α knockdown reduces proteasome load and confers resistance to proteasome inhibitors in myeloma revealed PSME1 as a determinant of proteasome dependence in cancer, with therapeutic implications.

   Evidence shRNA/siRNA knockdown in MM cell lines with proteasome activity and bortezomib/carfilzomib sensitivity assays

   PMID:33318477

   Open questions at the time

   • The mechanism linking reduced PA28α to decreased proteasome load was not delineated
   • Clinical correlation with patient drug response was not established
   • Whether PA28α overexpression is sufficient to resensitize resistant cells was not tested
9. 2021 Medium

   Two studies in 2021 expanded PSME1's non-immune roles: PA28α overexpression in vivo reduced hippocampal protein aggregation and improved cognition independently of boosted proteasome peptidase activity, while recombinant PSME1/2 suppressed β-catenin to inhibit Wnt signaling and osteogenesis, establishing PA28α as a modulator of proteostasis and developmental signaling beyond antigen processing.

   Evidence PA28α transgenic mice with behavioral testing and aggregation assays; recombinant PSME1/2 in osteoblast/osteoclast cultures with β-catenin western blot and OVX mouse model

   PMID:33720528 PMID:34838695

   Open questions at the time

   • The anti-aggregation mechanism independent of proteasome activation was not identified
   • Whether β-catenin is a direct proteasomal substrate of PA28-activated proteasomes was not shown
   • Sex-specific effects (female-only cognitive benefit) were unexplained mechanistically
10. 2023 Medium

    Discovery that PSME1 directly binds HBV core protein and shields it from 26S proteasome degradation revealed a virus-co-opted stabilization function, demonstrating that PA28α can also inhibit specific substrate degradation rather than only promote it.

    Evidence APEX2 proximity labeling, co-IP, siRNA knockdown with HBc stability and HBV transcription assays

    PMID:39281837

    Open questions at the time

    • Structural basis for PA28α–HBc interaction was not determined
    • Whether stabilization requires the assembled heptamer or monomeric PA28α was not tested
    • Relevance to HBV pathogenesis in vivo was not demonstrated
11. 2024 Medium

    Identification of a CCDC92–PA28α–ABCA1 axis in podocytes showed that upstream regulators can channel PSME1 activity toward degradation of specific membrane transporters, broadening the list of physiologically relevant PA28α substrates.

    Evidence Podocyte-specific Ccdc92 KO mice, co-IP, cholesterol efflux and ABCA1 stability assays under high glucose

    PMID:38228909

    Open questions at the time

    • Whether PA28α activates 20S proteasomes or an alternative pathway for ABCA1 degradation was not distinguished
    • Direct ubiquitination status of ABCA1 in this axis was not characterized

• Key open questions include the structural basis of heteromeric PA28α/β ring assembly on the 20S proteasome, the rules determining which substrates require PA28-activated versus 26S-mediated degradation, and the mechanism underlying PA28α's proteasome-activity-independent anti-aggregation function.
• No high-resolution structure of the PA28α/β–20S holocomplex
• No systematic identification of PA28-dependent proteasomal substrates
• Anti-aggregation mechanism independent of peptidase activation is uncharacterized