PSMG2

PSMG2 (PAC2) is a dedicated 20S proteasome assembly chaperone that heterodimerizes with PSMG1 (PAC1) to scaffold alpha-ring formation during proteasome biogenesis; the heterodimer associates with proteasome precursors, maintains alpha-rings competent for half-proteasome dimerization, and is degraded upon completion of 20S assembly (PMID:16251969, PMID:17189198). Loss of PSMG2 impairs proteasome maturation and activity, leading to accumulation of polyubiquitinated proteins, ER stress, compensatory autophagy activation, and apoptosis (PMID:36099919, PMID:40303289). In triple-negative breast cancer, PSMG2 depletion redirects PDPK1 to autophagy-mediated degradation, sensitizing cells to MEK inhibition through disruption of AKT-dependent feedback (PMID:36099919). Germline loss-of-function mutations in PSMG2 cause proteasome-associated autoinflammatory syndrome (PRAAS), characterized by perturbed protein homeostasis and a type I interferon signature (PMID:31827472).

1. 2005 High

   The foundational question of how mammalian 20S proteasome alpha-rings assemble was answered by identifying PSMG2/PSMG1 as a heterodimeric chaperone that scaffolds alpha-ring formation and is degraded upon assembly completion, establishing PSMG2 as a dedicated assembly factor rather than a stoichiometric proteasome subunit.

   Evidence Co-immunoprecipitation, siRNA knockdown, overexpression, and sucrose gradient sedimentation in human cells

   PMID:16251969

   Open questions at the time

   • Structural basis of the PAC1–PAC2 heterodimer interaction with individual alpha subunits was not resolved
   • Whether PAC2 degradation is proteasome-dependent or uses another pathway was not determined
2. 2006 High

   The cooperative relationship between PSMG2 and other assembly chaperones (PAC3, hUmp1) was delineated, showing that triple depletion causes accumulation of disorganized half-proteasomes and establishing that PSMG2 functions within a multi-chaperone system rather than alone.

   Evidence siRNA combinatorial knockdowns with native PAGE and sucrose gradient analysis of proteasome precursors

   PMID:17189198

   Open questions at the time

   • Order of chaperone engagement during assembly was not fully resolved
   • Whether PAC1/PAC2 and PAC3 act sequentially or simultaneously on the same alpha-ring intermediate was unclear
3. 2019 Medium

   The disease relevance of PSMG2 was established when germline loss-of-function mutations were identified as a cause of PRAAS, directly linking the assembly chaperone role to human autoinflammation through impaired proteasome function, ubiquitin accumulation, and a type I interferon signature.

   Evidence Next-generation sequencing of patient cohorts with interferon signature scoring and cytokine profiling

   PMID:31827472

   Open questions at the time

   • Specific PSMG2 mutations and their structural consequences were not biochemically characterized
   • Whether interferon activation is a direct consequence of impaired proteasome assembly or secondary to downstream stress was not distinguished
4. 2022 High

   A cancer-relevant signaling axis downstream of PSMG2 was uncovered: PSMG2 loss impairs proteasome function, activates compensatory autophagy that degrades PDPK1, and thereby sensitizes triple-negative breast cancer cells to MEK inhibition, revealing how proteasome assembly efficiency can rewire survival signaling.

   Evidence CRISPR genome-wide screen, siRNA validation, autophagy inhibitor rescue, and xenograft model in breast cancer cells

   PMID:36099919

   Open questions at the time

   • Whether PDPK1 is a selective autophagy substrate or degraded nonselectively was not determined
   • Generalizability of this axis beyond triple-negative breast cancer was not tested
5. 2025 Medium

   The generality of PSMG2 as a proteasome assembly chaperone controlling ER stress and cellular homeostasis was extended to HNSCC, where knockdown recapitulated ubiquitin accumulation, ER stress, autophagy induction, and additionally suppressed cancer stemness properties.

   Evidence siRNA knockdown with proliferation, apoptosis, stemness, and xenograft assays in HNSCC cell lines

   PMID:40303289

   Open questions at the time

   • Mechanism linking proteasome impairment to loss of stemness was not identified
   • Whether ER stress or autophagy is the primary driver of the growth-inhibitory phenotype was not dissected

• Key unresolved questions include the high-resolution structure of the PSMG1–PSMG2 heterodimer bound to alpha-ring intermediates, the precise degradation mechanism of PSMG2 after 20S assembly, and whether therapeutic targeting of PSMG2 can selectively impair proteasome homeostasis in cancer without triggering autoinflammatory pathology.
• No atomic-resolution structure of human PSMG1–PSMG2 on an alpha-ring intermediate
• Degradation pathway of PSMG2 after assembly completion is unknown
• Therapeutic window between proteasome impairment and autoinflammation is undefined