PSMG3

PSMG3 (PAC3) is a proteasome assembly chaperone that acts as a molecular matchmaker during biogenesis of the 20S proteasome α-ring (PMID:31067643). Functioning as a PAC3-PAC4 heterodimer, it stabilizes the α4-α5-α6 subcomplex during α-ring formation, a role supported by an atomic-resolution crystal structure of the PAC3 homodimer and an NMR-validated PAC3-4/α4/α5/α6 quintet complex model (PMID:31067643). Cryo-EM of endogenous CRISPR-tagged chaperone complexes places PAC3 within the PAC1-4 chaperone set that stabilizes an early α-ring intermediate, from which PAC3/PAC4 dissociate upon the transition to β-ring assembly, ahead of β pro-peptide cleavage and the concerted release of POMP and PAC1/PAC2 that yields the mature 20S proteasome [PMID:bio_10.1101_2024.08.08.607236]. The PAC3 homodimer interface is a druggable target: thielocin B1 analogues selectively inhibit this protein-protein interaction through hydrophobic contacts and terminal carboxylic acid groups (PMID:30455074). PSMG3 is also exploited by the Anaplasma phagocytophilum effector AptA, which binds PSMG3 directly to enhance host proteasome activity and autophagy and to suppress apoptosis (PMID:34126152).

1. 2018 Medium

   Before this work the PAC3 homodimer interface was not known to be a tractable small-molecule target; demonstrating selective chemical inhibition established it as a druggable protein-protein interaction distinct from PAC1/PAC2.

   Evidence Synthesis and biological evaluation of thielocin B1 analogues against PAC3 homodimer vs PAC1/PAC2, with SAR and in silico docking

   PMID:30455074

   Open questions at the time

   • Cellular consequences of PAC3 dimer inhibition on proteasome assembly not shown
   • Selectivity validated by docking but without co-crystal structure of inhibitor bound
   • No demonstration of effect on proteasome maturation in cells
2. 2019 High

   Resolved how PAC3 contributes to α-ring assembly by defining it as a matchmaker that, with PAC4, stabilizes the α4-α5-α6 subcomplex, providing atomic-resolution structural grounding for its mechanism.

   Evidence 0.96-Å crystal structure of the PAC3 homodimer, NMR, and 3D modeling of a PAC3-4/α4/α5/α6 quintet complex with biochemical validation

   PMID:31067643

   Open questions at the time

   • Timing and trigger of PAC3 release during assembly not resolved by static structure
   • Function of the mobile 51–61 loop not assigned
   • Model of the quintet complex not validated by full-complex structure
3. 2021 Medium

   Identified a pathogen route to hijack PSMG3, showing the Anaplasma effector AptA binds PSMG3 to reprogram host proteasome activity, autophagy, and apoptosis — extending PSMG3 relevance to host-pathogen interaction.

   Evidence Yeast two-hybrid identification of AptA-PSMG3 interaction plus functional assays in HEK293T cells (proteasome activity, ubiquitination, autophagy, apoptosis)

   PMID:34126152

   Open questions at the time

   • Interaction not confirmed by orthogonal pulldown or in infection context
   • Whether AptA acts via PSMG3's assembly-chaperone function or an independent activity is unresolved
   • Mechanism linking PSMG3 binding to UPS-autophagy crosstalk not defined
4. 2024 High

   Placed PAC3 within the ordered choreography of 20S assembly, capturing endogenous intermediates and defining the precise transition at which PAC3/PAC4 dissociate during β-ring formation.

   Evidence Cryo-EM of endogenous CRISPR-tagged chaperone-bound assembly intermediates (preprint)

   PMID:bio_10.1101_2024.08.08.607236

   Open questions at the time

   • Preprint not peer-reviewed
   • Molecular signal triggering PAC3/PAC4 release not mechanistically dissected
   • Fate of dissociated PAC3 not tracked
5. 2024 Low

   Raised a candidate disease role by linking PSMG3 overexpression to oncogenic phenotypes in hepatocellular carcinoma cells.

   Evidence Overexpression in liver cancer cell lines with proliferation, migration, and invasion assays

   PMID:38967739

   Open questions at the time

   • Single-lab overexpression phenotype without mechanistic or pathway placement
   • No connection drawn to PSMG3's proteasome-assembly function
   • No in vivo or patient-level validation

• How PSMG3's assembly-chaperone activity mechanistically connects to its reported roles in host-pathogen UPS-autophagy crosstalk and cancer cell phenotypes remains unresolved.
• No mechanistic link between assembly chaperone function and oncogenic phenotype
• AptA-PSMG3 functional axis not validated beyond a single study