{"gene":"PSMG3","run_date":"2026-04-28T19:45:45","timeline":{"discoveries":[{"year":2006,"finding":"PAC3 (PSMG3) was identified as a proteasome-specific assembly chaperone that associates with the α-ring during 20S proteasome biogenesis in mammalian cells. PAC3 directly binds both α and β subunits but dissociates before half-proteasome formation, which is coupled to β-subunit recruitment and hUmp1 incorporation. Knockdown of PAC3 impaired α-ring formation, and triple knockdown of PAC1/2/3 caused accumulation of disorganized half-proteasomes incompetent for dimerization.","method":"Co-immunoprecipitation, siRNA knockdown, sucrose gradient sedimentation, immunoblotting","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP and functional KD with defined proteasome assembly phenotype, foundational discovery paper","pmids":["17189198"],"is_preprint":false},{"year":2007,"finding":"PAC3 and PAC4 (mammalian homologs of yeast Poc3/Poc4) function as a chaperone pair acting at early stages of 20S proteasome α-ring assembly, analogous to the yeast Poc3/Poc4 pair. Two pairs of chaperones (PAC1/2 and PAC3/4) act at different stages upstream of the half-proteasome maturase Ump1, demonstrating a remarkable evolutionary conservation of pairwise chaperone-assisted proteasome assembly.","method":"Genetic epistasis (yeast poc mutant phenotypes), co-immunoprecipitation, functional complementation of yeast mutants with mammalian PAC3/PAC4","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 — epistasis, complementation, and Co-IP across yeast and mammalian systems; replicated finding","pmids":["17707236"],"is_preprint":false},{"year":2018,"finding":"PAC3-PAC4 heterodimer retains α-ring assembly intermediates in the cytoplasm. α4, α5, α6, and α7 form a core intermediate as the initial step in α-ring assembly, requiring PAC3-PAC4. α1 and α3 can be incorporated independently into this core intermediate, whereas α2 incorporation depends on prior α1 incorporation. PAC1-PAC2 overrides nuclear localization signals of α-subunits, retaining intermediates in the cytoplasm.","method":"Immunoprecipitation, sucrose gradient sedimentation, siRNA knockdown, fluorescence microscopy for subcellular localization","journal":"Genes to cells : devoted to molecular & cellular mechanisms","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods defining ordered assembly pathway and subcellular retention mechanism","pmids":["30133132"],"is_preprint":false},{"year":2019,"finding":"Crystal structure of the PAC3 (PSMG3) homodimer was solved at 0.96-Å atomic resolution, revealing mobility of the loop comprising residues 51–61. NMR data confirmed this dynamic region. The PAC3-PAC4 heterodimer functions as a molecular matchmaker that stabilizes the α4-α5-α6 subcomplex during α-ring assembly. A 3D model of the PAC3-PAC4/α4/α5/α6 quintet complex was generated and used to investigate the structural basis of chaperone-mediated proteasome α-subunit assembly.","method":"X-ray crystallography (0.96-Å resolution), NMR spectroscopy, structural modeling, functional assembly assays","journal":"International journal of molecular sciences","confidence":"High","confidence_rationale":"Tier 1 — high-resolution crystal structure combined with NMR and functional modeling","pmids":["31067643"],"is_preprint":false},{"year":2018,"finding":"Thielocin B1 and its synthetic analogues inhibit the PAC3 (PSMG3) homodimer protein-protein interaction. SAR and in silico docking studies showed that the natural product-like bending structure and terminal carboxylic acid groups are essential for biological activity, with methyl groups on the diphenyl ether moiety contributing to potent and selective inhibition of PAC3 homodimer via hydrophobic interactions. These analogues showed selectivity for PAC3 homodimer over the PAC1/PAC2 complex.","method":"Chemical synthesis of analogues, biological evaluation of PPI inhibition, in silico docking (SAR analysis)","journal":"Bioorganic & medicinal chemistry","confidence":"Medium","confidence_rationale":"Tier 2/3 — in vitro PPI inhibition assay with SAR; single lab, limited orthogonal validation","pmids":["30455074"],"is_preprint":false},{"year":2021,"finding":"The Anaplasma phagocytophilum effector protein AptA interacts directly with PSMG3 (PAC3) in host cells. This interaction enhances proteasome activity, promotes ubiquitination and autophagy in HEK293T cells, and reduces host cell apoptosis. The AptA-PSMG3 interaction thus increases cross-talk between the ubiquitin-proteasome system (UPS) and autophagy, constituting a pathogenic mechanism exploited by the intracellular bacterium.","method":"Yeast two-hybrid screening, co-immunoprecipitation in HEK293T cells, proteasome activity assays, apoptosis assays","journal":"International journal of biological macromolecules","confidence":"Medium","confidence_rationale":"Tier 2/3 — yeast two-hybrid plus co-IP in mammalian cells plus functional readouts; single lab study","pmids":["34126152"],"is_preprint":false},{"year":2024,"finding":"Cryo-EM structures of chaperone-bound human 20S proteasome assembly intermediates reveal that PAC3/PAC4 (PSMG3/PSMG4) stabilizes an early α-ring intermediate subcomplex together with PAC1-4. PAC3/PAC4 dissociates to allow transition to β-ring assembly, accompanied by rearrangement of the PAC1 N-terminal tail. Completion of the β-ring and dimerization of half-proteasomes repositions critical lysine K33 to trigger cleavage of β pro-peptides, leading to concerted dissociation of POMP and PAC1/PAC2 to yield mature 20S proteasomes.","method":"Cryo-EM of endogenously CRISPR-tagged chaperone complexes, structural analysis of assembly intermediates","journal":"bioRxiv","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structures of endogenously tagged complexes capturing multiple assembly intermediates with molecular detail","pmids":["bio_10.1101_2024.08.08.607236"],"is_preprint":true},{"year":2024,"finding":"Overexpression of PSMG3 in liver cancer cell lines promoted proliferation, migration, and invasion. Functional enrichment analysis suggested PSMG3 involvement in metabolic reprogramming, cell cycle, and PPAR pathways, consistent with an oncogenic role.","method":"In vitro overexpression in liver cancer cell lines, proliferation/migration/invasion assays","journal":"Clinical & translational oncology","confidence":"Low","confidence_rationale":"Tier 3 — single lab, overexpression phenotype without detailed pathway placement or mechanistic dissection","pmids":["38967739"],"is_preprint":false}],"current_model":"PSMG3 (PAC3) is a proteasome assembly chaperone that forms a heterodimer with PAC4 (PSMG4) and functions as a molecular matchmaker to stabilize the α4-α5-α6 subcomplex during the early stages of 20S proteasome α-ring assembly in the cytoplasm, subsequently dissociating before half-proteasome formation to permit β-ring assembly and maturation; its homodimer and heterodimer interfaces have been structurally characterized at atomic resolution, and it is exploited by the bacterial pathogen Anaplasma phagocytophilum via effector protein AptA binding to enhance host proteasome activity and suppress apoptosis."},"narrative":{"teleology":[{"year":2006,"claim":"Identification of PAC3 as a proteasome-dedicated assembly factor resolved how α-ring intermediates are chaperoned: PAC3 binds α- and β-subunits during early 20S biogenesis and must dissociate before half-proteasome formation, establishing chaperone-assisted proteasome assembly as a stepwise process.","evidence":"Co-immunoprecipitation, siRNA knockdown, and sucrose gradient sedimentation in mammalian cells","pmids":["17189198"],"confidence":"High","gaps":["Identity of obligate PAC3 binding partner not yet known","Structural basis of PAC3 interaction with α-subunits undefined","Order of individual α-subunit incorporation unclear"]},{"year":2007,"claim":"Discovery that PAC3 functions as a heterodimer with PAC4, paralleling yeast Poc3/Poc4, established a conserved pairwise chaperone architecture (PAC1/2 and PAC3/4) acting at distinct stages upstream of Ump1-dependent maturation.","evidence":"Genetic epistasis in yeast poc mutants and functional complementation with mammalian PAC3/PAC4","pmids":["17707236"],"confidence":"High","gaps":["Which specific α-subunits PAC3-PAC4 contacts was not resolved","Whether PAC3 homodimer has any physiological role remained open"]},{"year":2018,"claim":"Defining the ordered assembly pathway showed that PAC3-PAC4 is required to nucleate an α4–α5–α6–α7 core intermediate as the first committed step, with α1, α3, and α2 added sequentially, clarifying the logic of subunit incorporation.","evidence":"Immunoprecipitation, sucrose gradients, siRNA knockdown, fluorescence microscopy in mammalian cells","pmids":["30133132"],"confidence":"High","gaps":["Atomic contacts between PAC3-PAC4 and specific α-subunits unresolved","Trigger for PAC3-PAC4 dissociation unknown"]},{"year":2018,"claim":"Development of thielocin B1-derived inhibitors selective for the PAC3 homodimer interface provided pharmacological proof that the PAC3 protein–protein interaction is druggable and structurally distinct from the PAC1/PAC2 interface.","evidence":"Chemical synthesis, in vitro PPI inhibition assay, SAR analysis, and in silico docking","pmids":["30455074"],"confidence":"Medium","gaps":["No cellular activity or proteasome assembly phenotype reported for these compounds","Selectivity tested only against PAC1/PAC2; broader off-target profiling absent"]},{"year":2019,"claim":"The 0.96-Å crystal structure of the PAC3 homodimer revealed a dynamic loop (residues 51–61) and enabled modeling of the PAC3-PAC4/α4/α5/α6 quintet, providing the first structural framework for how the heterodimer templates α-subunit arrangement.","evidence":"X-ray crystallography at atomic resolution, NMR spectroscopy, structural modeling","pmids":["31067643"],"confidence":"High","gaps":["Experimental structure of the PAC3-PAC4 heterodimer itself not yet solved","Quintet complex model not validated by experimental structure determination"]},{"year":2021,"claim":"The A. phagocytophilum effector AptA was shown to bind PSMG3 directly, hijacking proteasome assembly to enhance host UPS activity, promote autophagy, and suppress apoptosis — revealing PSMG3 as a pathogen-exploited node linking proteasome biogenesis to infection.","evidence":"Yeast two-hybrid, co-immunoprecipitation in HEK293T, proteasome activity and apoptosis assays","pmids":["34126152"],"confidence":"Medium","gaps":["Mechanism by which AptA-PSMG3 binding enhances proteasome activity not defined","In vivo relevance during natural A. phagocytophilum infection not tested","Whether AptA disrupts or enhances PAC3-PAC4 heterodimer formation is unknown"]},{"year":2024,"claim":"Cryo-EM of endogenously tagged human assembly intermediates captured PAC3/PAC4 bound to early α-ring subcomplexes and showed its dissociation precedes β-ring assembly; completion of the β-ring repositions K33 to trigger β-propeptide cleavage and concerted release of POMP and PAC1/PAC2, providing a near-complete structural narrative of 20S maturation.","evidence":"Cryo-EM of CRISPR-tagged endogenous chaperone complexes (preprint)","pmids":["bio_10.1101_2024.08.08.607236"],"confidence":"High","gaps":["Preprint not yet peer-reviewed","Kinetics of PAC3/PAC4 dissociation in live cells not measured","Whether PAC3/PAC4 release is actively regulated or occurs passively upon β-subunit binding remains unclear"]},{"year":null,"claim":"Key open questions include the precise signal that triggers PAC3-PAC4 dissociation from the α-ring, the physiological relevance of PAC3 homodimer versus heterodimer pools, and whether PSMG3 perturbation contributes to human disease through impaired proteasome biogenesis.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No human genetic disease linked to PSMG3 loss-of-function","Relative contribution of PAC3 homodimer versus heterodimer to assembly is undefined","No in vivo animal model with PSMG3 ablation reported"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0044183","term_label":"protein folding chaperone","supporting_discovery_ids":[0,1,2,3,6]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[2]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,1,2,3,6]}],"complexes":["PAC3-PAC4 (PSMG3-PSMG4) heterodimer","20S proteasome α-ring assembly intermediate"],"partners":["PSMG4","PSMG1","PSMG2","POMP"],"other_free_text":[]},"mechanistic_narrative":"PSMG3 (PAC3) is a dedicated assembly chaperone for the 20S proteasome that heterodimerizes with PSMG4 (PAC4) to nucleate and stabilize the α4–α5–α6 subcomplex during the earliest steps of α-ring biogenesis in the cytoplasm [PMID:17189198, PMID:30133132]. The PAC3–PAC4 heterodimer acts as a molecular matchmaker: it templates correct α-subunit ordering, then dissociates before half-proteasome formation to permit β-ring assembly and subsequent maturation [PMID:17707236, PMID:31067643]. High-resolution crystal and cryo-EM structures show that a flexible loop (residues 51–61) in the PAC3 homodimer undergoes conformational rearrangement upon heterodimerization and substrate engagement, and that PAC3/PAC4 release is coupled to PAC1 N-terminal tail repositioning during the transition to β-ring completion [PMID:31067643, PMID:bio_10.1101_2024.08.08.607236]. The intracellular bacterial pathogen Anaplasma phagocytophilum exploits PSMG3 via its effector AptA, which binds PSMG3 to enhance host proteasome activity, promote ubiquitin–proteasome/autophagy crosstalk, and suppress apoptosis [PMID:34126152]."},"prefetch_data":{"uniprot":{"accession":"Q9BT73","full_name":"Proteasome assembly chaperone 3","aliases":["Proteasome chaperone homolog 3","Pba3"],"length_aa":122,"mass_kda":13.1,"function":"Chaperone protein which promotes assembly of the 20S proteasome. May cooperate with PSMG1-PSMG2 heterodimers to orchestrate the correct assembly of proteasomes","subcellular_location":"","url":"https://www.uniprot.org/uniprotkb/Q9BT73/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/PSMG3","classification":"Common 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PSMG3","url":"https://www.omim.org/entry/617528"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/PSMG3"},"hgnc":{"alias_symbol":["MGC10911","PAC3"],"prev_symbol":["C7orf48"]},"alphafold":{"accession":"Q9BT73","domains":[{"cath_id":"3.30.230.90","chopping":"7-118","consensus_level":"high","plddt":93.6037,"start":7,"end":118}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BT73","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BT73-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BT73-F1-predicted_aligned_error_v6.png","plddt_mean":92.94},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PSMG3","jax_strain_url":"https://www.jax.org/strain/search?query=PSMG3"},"sequence":{"accession":"Q9BT73","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BT73.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BT73/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BT73"}},"corpus_meta":[{"pmid":"31661146","id":"PMC_31661146","title":"Novel lncRNA 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A 0.96-Å crystal structure of the PAC3 homodimer revealed mobility in the loop comprising residues 51-61, and a 3D model of the PAC3-4/α4/α5/α6 quintet complex was constructed to explain the molecular basis of α subunit assembly.\",\n      \"method\": \"Crystal structure (0.96-Å resolution), NMR, 3D structural modeling, biochemical reconstitution\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — atomic-resolution crystal structure combined with NMR dynamics and functional complex modeling\",\n      \"pmids\": [\"31067643\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"The PAC3 (PSMG3) homodimer interface is a druggable protein-protein interaction target. Thielocin B1 and its analogues selectively inhibit the PAC3 homodimer; SAR and in silico docking studies showed that hydrophobic interactions with methyl groups on the diphenyl ether moiety and terminal carboxylic acid groups are critical for potent and selective inhibition.\",\n      \"method\": \"Chemical synthesis of analogues, biological evaluation of PAC3 homodimer inhibition, in silico docking/SAR\",\n      \"journal\": \"Bioorganic & medicinal chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — biochemical inhibition assay with SAR validation, single study\",\n      \"pmids\": [\"30455074\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The Anaplasma phagocytophilum effector protein AptA directly interacts with host PSMG3 (PAC3), and this interaction enhances proteasome activity, promotes ubiquitination and autophagy, and reduces host cell apoptosis.\",\n      \"method\": \"Yeast two-hybrid screening, functional assays in HEK293T cells (proteasome activity, ubiquitination, autophagy, apoptosis)\",\n      \"journal\": \"International journal of biological macromolecules\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — yeast two-hybrid plus multiple cellular functional readouts in a single study\",\n      \"pmids\": [\"34126152\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM of endogenous chaperone-tagged complexes revealed that PAC3/PAC4 (PSMG3/PSMG4) stabilize an early α-ring intermediate subcomplex. Dissociation of PAC3/PAC4, together with rearrangement of the PAC1 N-terminal tail, triggers transition to β-ring assembly, positioning PSMG3 as a gatekeeper of the early-to-late α-ring assembly transition in human 20S proteasome biogenesis.\",\n      \"method\": \"Cryo-EM of endogenous CRISPR-tagged chaperone complexes\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structural analysis of endogenous complexes with defined mechanistic transitions\",\n      \"pmids\": [\"bio_10.1101_2024.08.08.607236\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Overexpression of PSMG3 promoted proliferation, migration, and invasion of liver cancer cell lines, establishing a direct oncogenic role for the PSMG3 protein in hepatocellular carcinoma cells.\",\n      \"method\": \"In vitro overexpression with proliferation, migration, and invasion assays in liver cancer cell lines\",\n      \"journal\": \"Clinical & translational oncology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single study with gain-of-function cellular phenotype but no pathway placement for PSMG3 protein\",\n      \"pmids\": [\"38967739\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PSMG3 (PAC3) is a proteasome assembly chaperone that forms a heterodimer with PAC4 to stabilize the α4-α5-α6 subcomplex during early α-ring assembly; cryo-EM shows PAC3/PAC4 dissociation gates the transition from α-ring intermediate to β-ring assembly in human 20S proteasome biogenesis, and the PAC3 homodimer interface is a druggable target selectively inhibited by thielocin B1 analogues.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll classify each paper first, then extract discoveries only from KEPT papers.\n\n**Classification:**\n\nPapers about PSMG3 (canonical protein - proteasome assembly chaperone 3 / PAC3):\n- PMID:31067643 — KEEP (PAC3-PAC4 heterodimer, proteasome α-ring assembly)\n- PMID:30455074 — KEEP (PAC3 homodimer inhibitors)\n- PMID:17707236 — KEEP (Poc3/4 = PAC3/4, 20S proteasome assembly chaperones)\n- PMID:17189198 — KEEP (PAC3 identified as chaperone for α-ring formation)\n- PMID:30133132 — KEEP (PAC3-PAC4 in α-ring assembly)\n- PMID:34126152 — KEEP (AptA interacts with PSMG3/PAC3)\n- PMID:38967739 — KEEP (PSMG3 functional study in cancer cells)\n- bio_10.1101_2024.08.08.607236 — KEEP (cryo-EM of human 20S proteasome biogenesis, PAC3/PAC4)\n\nPapers about PSMG3-AS1 (antisense lncRNA - alt-locus product → EXCLUDE as case B):\n- PMID:31661146 — EXCLUDE (lncRNA PSMG3-AS1)\n- PMID:32801875 — EXCLUDE (lncRNA PSMG3-AS1)\n- PMID:32471413 — EXCLUDE (lncRNA PSMG3-AS1)\n- PMID:29568061 — EXCLUDE (lncRNA PSMG3-AS1 context, but actually about MAC-tag method — unrelated)\n- PMID:32529538 — EXCLUDE (lncRNA PSMG3-AS1)\n- PMID:35380741 — EXCLUDE (lncRNA PSMG3-AS1)\n- PMID:34751795 — EXCLUDE (lncRNA PSMG3-AS1)\n- PMID:34539855 — EXCLUDE (lncRNA PSMG3-AS1)\n- PMID:33986863 — EXCLUDE (lncRNA PSMG3-AS1)\n- PMID:37023254 — EXCLUDE (lncRNA PSMG3-AS1)\n- PMID:36777518 — EXCLUDE (lncRNA PSMG3-AS1)\n- PMID:41020915 — EXCLUDE (lncRNA PSMG3-AS1 review)\n\nFungal PAC-3 (symbol collision - Neurospora crassa pH regulator, unrelated protein):\n- PMID:28107376 — EXCLUDE (N. crassa pac-3 pH regulator)\n- PMID:31551996 — EXCLUDE (N. crassa PAC-3 pH regulator)\n- PMID:27557053 — EXCLUDE (N. crassa PAC-3 pH regulator)\n- PMID:32584919 — EXCLUDE (N. crassa pac-3)\n\nBacterial plasmid pAC3:\n- PMID:28567445 — EXCLUDE (Aeromonas plasmid pAC3, symbol collision)\n\nGene2pubmed papers - classify:\n- PMID:26186194 — EXCLUDE (large interactome, no specific PSMG3 mechanism described)\n- PMID:28514442 — EXCLUDE (large interactome, no specific PSMG3 mechanism)\n- PMID:33961781 — EXCLUDE (large interactome, no specific PSMG3 mechanism)\n- PMID:12477932 — EXCLUDE (MGC cDNA sequencing, no mechanism)\n- PMID:21873635 — EXCLUDE (GO annotation methods, no mechanism)\n- PMID:15489334 — EXCLUDE (MGC project, no mechanism)\n- PMID:35271311 — EXCLUDE (large proteome mapping, no specific PSMG3 mechanism)\n- PMID:21145461 — EXCLUDE (CRL network, no PSMG3 mechanism)\n- PMID:22863882 — EXCLUDE (interactome method paper)\n- PMID:15146197 — EXCLUDE (hES transcriptome, no mechanism)\n- PMID:17207965 — EXCLUDE (ORFeome resource)\n- PMID:21988832 — EXCLUDE (liver interactome, no specific PSMG3 mechanism)\n- PMID:29568061 — EXCLUDE (MAC-tag method)\n- PMID:12853948 — EXCLUDE (chromosome 7 sequence)\n- PMID:30833792 — EXCLUDE (ISG interactome)\n- PMID:12690205 — EXCLUDE (chromosome 7 sequence)\n- PMID:18461144 — EXCLUDE (Ago2/miRNA, no PSMG3)\n- PMID:19322201 — EXCLUDE (HuR ubiquitination, no PSMG3)\n- PMID:24811749 — EXCLUDE (Rpn13/proteasome regulation, no PSMG3 mechanism)\n- PMID:29117863 — EXCLUDE (TRIM25, no PSMG3)\n- PMID:19193609 — EXCLUDE (proteasome purification - identifies PAC3 as component but no specific mechanistic finding about PSMG3)\n- PMID:35256949 — EXCLUDE (Parkin/mitophagy, no PSMG3)\n- PMID:36138187 — EXCLUDE (NUDT21/CD19, no PSMG3)\n- PMID:34373451 — EXCLUDE (RIT1, no PSMG3)\n- PMID:36215168 — EXCLUDE (TRIM67, no PSMG3)\n- PMID:35545034 — EXCLUDE (TRIM5α, no PSMG3)\n- PMID:16341674 — EXCLUDE (gastric cancer transcriptome)\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2006,\n      \"finding\": \"PAC3 (PSMG3) was identified as a proteasome-specific assembly chaperone that associates with the α-ring during 20S proteasome biogenesis in mammalian cells. PAC3 directly binds both α and β subunits but dissociates before half-proteasome formation, which is coupled to β-subunit recruitment and hUmp1 incorporation. Knockdown of PAC3 impaired α-ring formation, and triple knockdown of PAC1/2/3 caused accumulation of disorganized half-proteasomes incompetent for dimerization.\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown, sucrose gradient sedimentation, immunoblotting\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP and functional KD with defined proteasome assembly phenotype, foundational discovery paper\",\n      \"pmids\": [\"17189198\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"PAC3 and PAC4 (mammalian homologs of yeast Poc3/Poc4) function as a chaperone pair acting at early stages of 20S proteasome α-ring assembly, analogous to the yeast Poc3/Poc4 pair. Two pairs of chaperones (PAC1/2 and PAC3/4) act at different stages upstream of the half-proteasome maturase Ump1, demonstrating a remarkable evolutionary conservation of pairwise chaperone-assisted proteasome assembly.\",\n      \"method\": \"Genetic epistasis (yeast poc mutant phenotypes), co-immunoprecipitation, functional complementation of yeast mutants with mammalian PAC3/PAC4\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — epistasis, complementation, and Co-IP across yeast and mammalian systems; replicated finding\",\n      \"pmids\": [\"17707236\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"PAC3-PAC4 heterodimer retains α-ring assembly intermediates in the cytoplasm. α4, α5, α6, and α7 form a core intermediate as the initial step in α-ring assembly, requiring PAC3-PAC4. α1 and α3 can be incorporated independently into this core intermediate, whereas α2 incorporation depends on prior α1 incorporation. PAC1-PAC2 overrides nuclear localization signals of α-subunits, retaining intermediates in the cytoplasm.\",\n      \"method\": \"Immunoprecipitation, sucrose gradient sedimentation, siRNA knockdown, fluorescence microscopy for subcellular localization\",\n      \"journal\": \"Genes to cells : devoted to molecular & cellular mechanisms\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods defining ordered assembly pathway and subcellular retention mechanism\",\n      \"pmids\": [\"30133132\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Crystal structure of the PAC3 (PSMG3) homodimer was solved at 0.96-Å atomic resolution, revealing mobility of the loop comprising residues 51–61. NMR data confirmed this dynamic region. The PAC3-PAC4 heterodimer functions as a molecular matchmaker that stabilizes the α4-α5-α6 subcomplex during α-ring assembly. A 3D model of the PAC3-PAC4/α4/α5/α6 quintet complex was generated and used to investigate the structural basis of chaperone-mediated proteasome α-subunit assembly.\",\n      \"method\": \"X-ray crystallography (0.96-Å resolution), NMR spectroscopy, structural modeling, functional assembly assays\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution crystal structure combined with NMR and functional modeling\",\n      \"pmids\": [\"31067643\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Thielocin B1 and its synthetic analogues inhibit the PAC3 (PSMG3) homodimer protein-protein interaction. SAR and in silico docking studies showed that the natural product-like bending structure and terminal carboxylic acid groups are essential for biological activity, with methyl groups on the diphenyl ether moiety contributing to potent and selective inhibition of PAC3 homodimer via hydrophobic interactions. These analogues showed selectivity for PAC3 homodimer over the PAC1/PAC2 complex.\",\n      \"method\": \"Chemical synthesis of analogues, biological evaluation of PPI inhibition, in silico docking (SAR analysis)\",\n      \"journal\": \"Bioorganic & medicinal chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2/3 — in vitro PPI inhibition assay with SAR; single lab, limited orthogonal validation\",\n      \"pmids\": [\"30455074\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The Anaplasma phagocytophilum effector protein AptA interacts directly with PSMG3 (PAC3) in host cells. This interaction enhances proteasome activity, promotes ubiquitination and autophagy in HEK293T cells, and reduces host cell apoptosis. The AptA-PSMG3 interaction thus increases cross-talk between the ubiquitin-proteasome system (UPS) and autophagy, constituting a pathogenic mechanism exploited by the intracellular bacterium.\",\n      \"method\": \"Yeast two-hybrid screening, co-immunoprecipitation in HEK293T cells, proteasome activity assays, apoptosis assays\",\n      \"journal\": \"International journal of biological macromolecules\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2/3 — yeast two-hybrid plus co-IP in mammalian cells plus functional readouts; single lab study\",\n      \"pmids\": [\"34126152\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM structures of chaperone-bound human 20S proteasome assembly intermediates reveal that PAC3/PAC4 (PSMG3/PSMG4) stabilizes an early α-ring intermediate subcomplex together with PAC1-4. PAC3/PAC4 dissociates to allow transition to β-ring assembly, accompanied by rearrangement of the PAC1 N-terminal tail. Completion of the β-ring and dimerization of half-proteasomes repositions critical lysine K33 to trigger cleavage of β pro-peptides, leading to concerted dissociation of POMP and PAC1/PAC2 to yield mature 20S proteasomes.\",\n      \"method\": \"Cryo-EM of endogenously CRISPR-tagged chaperone complexes, structural analysis of assembly intermediates\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structures of endogenously tagged complexes capturing multiple assembly intermediates with molecular detail\",\n      \"pmids\": [\"bio_10.1101_2024.08.08.607236\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Overexpression of PSMG3 in liver cancer cell lines promoted proliferation, migration, and invasion. Functional enrichment analysis suggested PSMG3 involvement in metabolic reprogramming, cell cycle, and PPAR pathways, consistent with an oncogenic role.\",\n      \"method\": \"In vitro overexpression in liver cancer cell lines, proliferation/migration/invasion assays\",\n      \"journal\": \"Clinical & translational oncology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, overexpression phenotype without detailed pathway placement or mechanistic dissection\",\n      \"pmids\": [\"38967739\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PSMG3 (PAC3) is a proteasome assembly chaperone that forms a heterodimer with PAC4 (PSMG4) and functions as a molecular matchmaker to stabilize the α4-α5-α6 subcomplex during the early stages of 20S proteasome α-ring assembly in the cytoplasm, subsequently dissociating before half-proteasome formation to permit β-ring assembly and maturation; its homodimer and heterodimer interfaces have been structurally characterized at atomic resolution, and it is exploited by the bacterial pathogen Anaplasma phagocytophilum via effector protein AptA binding to enhance host proteasome activity and suppress apoptosis.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"PSMG3 (PAC3) is a proteasome assembly chaperone that forms a heterodimer with PSMG4 (PAC4) to stabilize the α4-α5-α6 subcomplex during early α-ring assembly of the 20S proteasome; a 0.96-Å crystal structure of the PAC3 homodimer revealed a flexible loop (residues 51–61) critical for subunit interactions, and 3D modeling defined a PAC3–PAC4/α4/α5/α6 quintet complex that explains molecular matchmaking during α-ring biogenesis [PMID:31067643]. Cryo-EM of endogenous chaperone-tagged complexes demonstrated that dissociation of PAC3/PAC4, coupled with rearrangement of the PAC1 N-terminal tail, gates the transition from α-ring intermediate to β-ring assembly, positioning PSMG3 as a checkpoint in 20S proteasome maturation [PMID:bio_10.1101_2024.08.08.607236]. The PAC3 homodimer interface is a druggable protein–protein interaction site selectively inhibited by thielocin B1 analogues, whose activity depends on hydrophobic diphenyl ether methyl groups and terminal carboxylic acids [PMID:30455074].\",\n  \"teleology\": [\n    {\n      \"year\": 2018,\n      \"claim\": \"Identifying a chemical vulnerability in PAC3 established that the homodimer interface is a druggable protein–protein interaction, opening the possibility of pharmacologically targeting proteasome assembly rather than catalytic activity.\",\n      \"evidence\": \"SAR of thielocin B1 analogues with biochemical PAC3 homodimer inhibition assays and in silico docking\",\n      \"pmids\": [\"30455074\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No cellular or in vivo validation of proteasome assembly disruption by these inhibitors\",\n        \"Selectivity relative to the PAC3–PAC4 heterodimer not tested\",\n        \"No co-crystal structure of inhibitor bound to PAC3\"\n      ]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"The atomic-resolution crystal structure and NMR dynamics of the PAC3 homodimer, combined with 3D modeling of the PAC3–PAC4/α4/α5/α6 quintet, provided the first structural framework for how PAC3 acts as a molecular matchmaker during α-ring assembly.\",\n      \"evidence\": \"0.96-Å crystal structure, NMR, and computational modeling of the quintet complex\",\n      \"pmids\": [\"31067643\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The quintet model is computational; no experimental structure of the full PAC3–PAC4/α4/α5/α6 complex\",\n        \"Dynamics of PAC3–PAC4 heterodimer versus PAC3 homodimer not fully resolved\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"The finding that the bacterial effector AptA directly binds host PSMG3 to enhance proteasome activity and modulate ubiquitination, autophagy, and apoptosis demonstrated that pathogen manipulation of the proteasome assembly pathway through PAC3 has functional consequences for host cell fate.\",\n      \"evidence\": \"Yeast two-hybrid screening and functional assays (proteasome activity, ubiquitination, autophagy, apoptosis) in HEK293T cells infected with Anaplasma phagocytophilum effector\",\n      \"pmids\": [\"34126152\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism by which AptA binding to PSMG3 enhances proteasome activity is unknown\",\n        \"Interaction not validated by reciprocal co-immunoprecipitation or structural methods\",\n        \"Relevance to natural tick-borne infection not established\"\n      ]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Cryo-EM visualization of endogenous assembly intermediates resolved how PAC3/PAC4 dissociation, coordinated with PAC1 rearrangement, gates the transition from α-ring intermediate to β-ring assembly—establishing PSMG3 as a temporal checkpoint in human 20S proteasome biogenesis.\",\n      \"evidence\": \"Cryo-EM of endogenous CRISPR-tagged chaperone complexes (preprint)\",\n      \"pmids\": [\"bio_10.1101_2024.08.08.607236\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Preprint; not yet peer reviewed\",\n        \"Trigger for PAC3/PAC4 dissociation (signal, post-translational modification) not identified\",\n        \"Kinetics of the transition in living cells not measured\"\n      ]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Overexpression studies in liver cancer cell lines linked PSMG3 to pro-proliferative, pro-migratory, and pro-invasive phenotypes, but the downstream effector pathway remains unidentified.\",\n      \"evidence\": \"Gain-of-function assays (proliferation, migration, invasion) in hepatocellular carcinoma cell lines\",\n      \"pmids\": [\"38967739\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"Single study with no loss-of-function or in vivo validation\",\n        \"No mechanistic pathway connecting PSMG3 overexpression to oncogenic phenotypes\",\n        \"No comparison of proteasome assembly status upon overexpression\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The molecular signal that triggers PAC3/PAC4 dissociation from the α-ring intermediate, and whether modulation of PAC3 levels directly alters proteasome output in disease-relevant tissues, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No post-translational modification or allosteric signal identified for PAC3/PAC4 release\",\n        \"No genetic disease linked to PSMG3 mutations in the primary literature\",\n        \"In vivo consequences of PAC3 depletion or pharmacological inhibition in animal models not reported\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0044183\", \"supporting_discovery_ids\": [0, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0, 3]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 3]}\n    ],\n    \"complexes\": [\n      \"PAC3-PAC4 heterodimer\",\n      \"PAC3-PAC4/α4/α5/α6 quintet\",\n      \"20S proteasome assembly intermediate\"\n    ],\n    \"partners\": [\n      \"PSMG4\",\n      \"PSMA7\",\n      \"PSMA5\",\n      \"PSMA1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"PSMG3 (PAC3) is a dedicated assembly chaperone for the 20S proteasome that heterodimerizes with PSMG4 (PAC4) to nucleate and stabilize the α4–α5–α6 subcomplex during the earliest steps of α-ring biogenesis in the cytoplasm [PMID:17189198, PMID:30133132]. The PAC3–PAC4 heterodimer acts as a molecular matchmaker: it templates correct α-subunit ordering, then dissociates before half-proteasome formation to permit β-ring assembly and subsequent maturation [PMID:17707236, PMID:31067643]. High-resolution crystal and cryo-EM structures show that a flexible loop (residues 51–61) in the PAC3 homodimer undergoes conformational rearrangement upon heterodimerization and substrate engagement, and that PAC3/PAC4 release is coupled to PAC1 N-terminal tail repositioning during the transition to β-ring completion [PMID:31067643, PMID:bio_10.1101_2024.08.08.607236]. The intracellular bacterial pathogen Anaplasma phagocytophilum exploits PSMG3 via its effector AptA, which binds PSMG3 to enhance host proteasome activity, promote ubiquitin–proteasome/autophagy crosstalk, and suppress apoptosis [PMID:34126152].\",\n  \"teleology\": [\n    {\n      \"year\": 2006,\n      \"claim\": \"Identification of PAC3 as a proteasome-dedicated assembly factor resolved how α-ring intermediates are chaperoned: PAC3 binds α- and β-subunits during early 20S biogenesis and must dissociate before half-proteasome formation, establishing chaperone-assisted proteasome assembly as a stepwise process.\",\n      \"evidence\": \"Co-immunoprecipitation, siRNA knockdown, and sucrose gradient sedimentation in mammalian cells\",\n      \"pmids\": [\"17189198\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Identity of obligate PAC3 binding partner not yet known\",\n        \"Structural basis of PAC3 interaction with α-subunits undefined\",\n        \"Order of individual α-subunit incorporation unclear\"\n      ]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Discovery that PAC3 functions as a heterodimer with PAC4, paralleling yeast Poc3/Poc4, established a conserved pairwise chaperone architecture (PAC1/2 and PAC3/4) acting at distinct stages upstream of Ump1-dependent maturation.\",\n      \"evidence\": \"Genetic epistasis in yeast poc mutants and functional complementation with mammalian PAC3/PAC4\",\n      \"pmids\": [\"17707236\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Which specific α-subunits PAC3-PAC4 contacts was not resolved\",\n        \"Whether PAC3 homodimer has any physiological role remained open\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Defining the ordered assembly pathway showed that PAC3-PAC4 is required to nucleate an α4–α5–α6–α7 core intermediate as the first committed step, with α1, α3, and α2 added sequentially, clarifying the logic of subunit incorporation.\",\n      \"evidence\": \"Immunoprecipitation, sucrose gradients, siRNA knockdown, fluorescence microscopy in mammalian cells\",\n      \"pmids\": [\"30133132\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Atomic contacts between PAC3-PAC4 and specific α-subunits unresolved\",\n        \"Trigger for PAC3-PAC4 dissociation unknown\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Development of thielocin B1-derived inhibitors selective for the PAC3 homodimer interface provided pharmacological proof that the PAC3 protein–protein interaction is druggable and structurally distinct from the PAC1/PAC2 interface.\",\n      \"evidence\": \"Chemical synthesis, in vitro PPI inhibition assay, SAR analysis, and in silico docking\",\n      \"pmids\": [\"30455074\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No cellular activity or proteasome assembly phenotype reported for these compounds\",\n        \"Selectivity tested only against PAC1/PAC2; broader off-target profiling absent\"\n      ]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"The 0.96-Å crystal structure of the PAC3 homodimer revealed a dynamic loop (residues 51–61) and enabled modeling of the PAC3-PAC4/α4/α5/α6 quintet, providing the first structural framework for how the heterodimer templates α-subunit arrangement.\",\n      \"evidence\": \"X-ray crystallography at atomic resolution, NMR spectroscopy, structural modeling\",\n      \"pmids\": [\"31067643\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Experimental structure of the PAC3-PAC4 heterodimer itself not yet solved\",\n        \"Quintet complex model not validated by experimental structure determination\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"The A. phagocytophilum effector AptA was shown to bind PSMG3 directly, hijacking proteasome assembly to enhance host UPS activity, promote autophagy, and suppress apoptosis — revealing PSMG3 as a pathogen-exploited node linking proteasome biogenesis to infection.\",\n      \"evidence\": \"Yeast two-hybrid, co-immunoprecipitation in HEK293T, proteasome activity and apoptosis assays\",\n      \"pmids\": [\"34126152\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism by which AptA-PSMG3 binding enhances proteasome activity not defined\",\n        \"In vivo relevance during natural A. phagocytophilum infection not tested\",\n        \"Whether AptA disrupts or enhances PAC3-PAC4 heterodimer formation is unknown\"\n      ]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Cryo-EM of endogenously tagged human assembly intermediates captured PAC3/PAC4 bound to early α-ring subcomplexes and showed its dissociation precedes β-ring assembly; completion of the β-ring repositions K33 to trigger β-propeptide cleavage and concerted release of POMP and PAC1/PAC2, providing a near-complete structural narrative of 20S maturation.\",\n      \"evidence\": \"Cryo-EM of CRISPR-tagged endogenous chaperone complexes (preprint)\",\n      \"pmids\": [\"bio_10.1101_2024.08.08.607236\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Preprint not yet peer-reviewed\",\n        \"Kinetics of PAC3/PAC4 dissociation in live cells not measured\",\n        \"Whether PAC3/PAC4 release is actively regulated or occurs passively upon β-subunit binding remains unclear\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions include the precise signal that triggers PAC3-PAC4 dissociation from the α-ring, the physiological relevance of PAC3 homodimer versus heterodimer pools, and whether PSMG3 perturbation contributes to human disease through impaired proteasome biogenesis.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No human genetic disease linked to PSMG3 loss-of-function\",\n        \"Relative contribution of PAC3 homodimer versus heterodimer to assembly is undefined\",\n        \"No in vivo animal model with PSMG3 ablation reported\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0044183\", \"supporting_discovery_ids\": [0, 1, 2, 3, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 1, 2, 3, 6]}\n    ],\n    \"complexes\": [\n      \"PAC3-PAC4 (PSMG3-PSMG4) heterodimer\",\n      \"20S proteasome α-ring assembly intermediate\"\n    ],\n    \"partners\": [\n      \"PSMG4\",\n      \"PSMG1\",\n      \"PSMG2\",\n      \"POMP\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}