PSMG4

PSMG4 (PAC4) is a dedicated chaperone for 20S proteasome biogenesis that heterodimerizes with PAC3 to nucleate and stabilize the earliest α-ring assembly intermediate. The PAC3–PAC4 complex acts as a molecular matchmaker that promotes formation of an α4–α5–α6 subcomplex (and the broader α4–α7 intermediate), presenting a hydrophobic surface with charge complementarity to α4–α5 subunits; dissociation of PAC3–PAC4 from the completed α-ring is required for subsequent β-ring incorporation and maturation of the 20S core particle (PMID:17707236, PMID:28263418, PMID:31067643, PMID:39294158). Cryo-EM of endogenous and reconstituted assembly intermediates confirms that PAC3–PAC4 occupies the α-ring at the earliest captured stage and must vacate before POMP-dependent β-subunit processing can proceed (PMID:39294158). The Psmg4 transcript is additionally subject to regulated alternative splicing (exon 2 skipping) controlled by a CHAtRF/SRSF5 axis, and reduction of the full-length PSMG4 isoform promotes pathological cardiac hypertrophy in mice and human iPSC-derived cardiomyocytes (PMID:41907183).

1. 2007 High

   Identification of PAC3–PAC4 as a heterodimeric chaperone pair acting early in α-ring assembly established that 20S proteasome biogenesis uses multiple dedicated, stage-specific assembly factors beyond the previously known PAC1–PAC2 and UMP1.

   Evidence Genetic screen, co-IP, and functional complementation in yeast and mammalian cells

   PMID:17707236

   Open questions at the time

   • Structural basis of the PAC3–PAC4 interaction with α-subunits was unknown
   • Precise α-subunit intermediate stabilized by PAC3–PAC4 was not defined
   • Mechanism of PAC3–PAC4 dissociation not addressed
2. 2008 High

   Biochemical reconstitution confirmed that PAC3–PAC4 acts before β-subunit incorporation and before PAC1–PAC2, establishing the temporal order of chaperone action during α-ring heptamer assembly.

   Evidence Biochemical reconstitution and co-IP of assembly intermediates

   PMID:18786393

   Open questions at the time

   • No atomic structure of PAC4 or the PAC3–PAC4 complex
   • Which specific α-subunit subcomplex is chaperoned was not resolved
3. 2017 High

   The 1.90-Å crystal structure of PAC4 revealed a hydrophobic binding surface complementary to PAC3 and charge-complementary to α4–α5 subunits, providing the first structural explanation for how PAC3–PAC4 recognizes its substrate within the assembling α-ring.

   Evidence X-ray crystallography of human PAC4

   PMID:28263418

   Open questions at the time

   • No co-crystal of the PAC3–PAC4–α-subunit complex
   • Dynamic conformational changes during assembly not captured
4. 2018 High

   Knockdown studies showed PAC3–PAC4 is specifically required for the α4–α7 intermediate — the earliest step of α-ring formation — while PAC1–PAC2 retains intermediates in the cytoplasm by masking α-subunit NLS sequences, delineating non-overlapping chaperone functions.

   Evidence siRNA knockdown, co-IP, subcellular fractionation, fluorescence microscopy

   PMID:30133132

   Open questions at the time

   • Structural visualization of endogenous intermediates lacking
   • Mechanism triggering PAC3–PAC4 release still unknown
5. 2019 High

   Ultra-high-resolution structure and NMR dynamics of PAC3 identified a flexible loop (residues 51–61) critical for forming the five-component PAC3–PAC4–α4–α5–α6 quintet, defining PAC3–PAC4 as a molecular matchmaker that templates the α4–α5–α6 subcomplex.

   Evidence 0.96-Å crystal structure, NMR spectroscopy, 3D modeling, biochemical assays

   PMID:31067643

   Open questions at the time

   • Full reconstitution of the quintet complex at atomic resolution not achieved
   • Whether PAC3–PAC4 catalytically accelerates or only thermodynamically stabilizes assembly unclear
6. 2024 High

   Cryo-EM of endogenous chaperone-bound complexes demonstrated that PAC3–PAC4 dissociation from the α-ring is a prerequisite for β-ring assembly, and that mature 20S formation requires concerted release of POMP and PAC1–PAC2 triggered by β-subunit K33 repositioning, completing the structural map of the assembly pathway.

   Evidence Cryo-EM of CRISPR/Cas9 endogenously tagged chaperone complexes (peer-reviewed); complemented by cryo-EM of recombinant subcomplexes (preprint)

   PMID:38328185 PMID:39294158

   Open questions at the time

   • Signal that triggers PAC3–PAC4 release specifically (versus passive displacement) not resolved
   • Kinetics of chaperone handoff in live cells not measured
7. 2026 Medium

   Discovery that the CHAtRF/SRSF5 axis controls Psmg4 exon 2 alternative splicing revealed a non-canonical regulatory layer: reduced full-length PSMG4 from exon skipping drives pathological cardiac hypertrophy, linking proteasome assembly chaperone expression to heart disease.

   Evidence RNA immunoprecipitation, splicing assays, overexpression/loss-of-function in mice and hiPSC-CMs, AngII-induced hypertrophy model

   PMID:41907183

   Open questions at the time

   • Mechanism connecting reduced PSMG4 to hypertrophic signaling (proteasome insufficiency vs. other) not established
   • Whether exon 2–skipped isoform has any residual function is unknown
   • Single-lab study; independent replication pending

• The signal that specifically triggers PAC3–PAC4 release from the completed α-ring, the kinetics of chaperone handoff in living cells, and the precise mechanism by which reduced PSMG4 causes cardiac hypertrophy remain unresolved.
• No real-time imaging of PAC3–PAC4 dynamics during assembly in vivo
• Downstream effectors linking PSMG4 deficiency to hypertrophic gene program unknown
• Whether PAC3–PAC4 participates in immunoproteasome-specific assembly is unresolved