{"gene":"PSMG1","run_date":"2026-04-28T19:45:45","timeline":{"discoveries":[{"year":2005,"finding":"PSMG1 (PAC1) and PSMG2 (PAC2) form a heterodimer that acts as a dedicated chaperone for mammalian 20S proteasome assembly. The PAC1–PAC2 complex associates with proteasome precursors, provides a scaffold for alpha-ring formation, keeps alpha-rings competent for subsequent half-proteasome formation, and is degraded upon completion of the mature 20S proteasome. Overexpression of PAC1 or PAC2 accelerates precursor formation, while siRNA knockdown impairs 20S proteasome maturation.","method":"Co-immunoprecipitation, sucrose-gradient sedimentation, siRNA knockdown, overexpression in mammalian cells, native PAGE","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP and functional rescue with multiple orthogonal methods (knockdown, overexpression, fractionation) in a high-impact foundational study","pmids":["16251969"],"is_preprint":false},{"year":2010,"finding":"Conditional knockout of PAC1 (PSMG1) in mice causes early embryonic lethality, demonstrating that PAC1-dependent 20S proteasome assembly is essential for mammalian development. In adult hepatocytes, PAC1 is responsible for producing the majority of 20S proteasomes; PAC1-deficient hepatocytes retain normal 26S proteasome levels but completely lose the free latent 20S proteasome pool, accumulate ubiquitinated proteins, and undergo premature senescence, establishing an essential role for the latent 20S proteasome in cellular homeostasis.","method":"Conditional gene knockout in mice, native PAGE, ubiquitin accumulation assay, cellular senescence assay, proteasome activity assay","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — clean conditional KO with multiple defined cellular phenotypes and mechanistic pathway placement","pmids":["20498273"],"is_preprint":false},{"year":2015,"finding":"iRhom1 physically interacts with the 20S proteasome assembly chaperones PAC1 (PSMG1) and PAC2 (PSMG2), stabilizing their protein levels. Under ER stress, iRhom1 expression is induced and promotes PAC1–PAC2 dimerization; knockdown of iRhom1 impairs PAC1–PAC2 dimerization and reduces proteasome complex assembly and enzymatic activity, particularly in ER-containing microsomes.","method":"Co-immunoprecipitation, native PAGE, proteasome activity assay, siRNA knockdown, fractionation, Drosophila genetic rescue","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2–3 — Co-IP and functional readout in single study; dimerization finding extends prior work","pmids":["26109405"],"is_preprint":false}],"current_model":"PSMG1 (PAC1) forms an obligate heterodimer with PSMG2 (PAC2) that acts as a dedicated scaffold chaperone for mammalian 20S proteasome assembly, promoting alpha-ring formation and half-proteasome maturation, and is degraded upon completion of the mature 20S particle; PAC1 is essential for mammalian development and for maintaining the pool of free latent 20S proteasomes required for cellular homeostasis, with its dimerization and stability further regulated by iRhom1 under ER stress."},"narrative":{"teleology":[{"year":2005,"claim":"Identification of the PAC1–PAC2 heterodimer as a dedicated 20S proteasome assembly chaperone resolved how mammalian alpha-ring formation is initiated and coordinated, establishing that the chaperone is consumed upon proteasome maturation.","evidence":"Reciprocal co-immunoprecipitation, sucrose-gradient sedimentation, siRNA knockdown, and overexpression in mammalian cells","pmids":["16251969"],"confidence":"High","gaps":["In vivo essentiality of PAC1 was not tested","Structural basis of alpha-ring scaffolding by PAC1–PAC2 was not resolved","Regulation of PAC1–PAC2 levels or activity by upstream signals was unknown"]},{"year":2010,"claim":"Genetic ablation of PAC1 in mice demonstrated that the chaperone is essential for embryonic viability and is the primary source of the free latent 20S proteasome pool in adult cells, linking PAC1 function to ubiquitin homeostasis and cellular senescence.","evidence":"Conditional gene knockout in mice with native PAGE, ubiquitin accumulation assay, proteasome activity assay, and senescence assay in hepatocytes","pmids":["20498273"],"confidence":"High","gaps":["Why 26S proteasome levels are maintained despite loss of the free 20S pool was not mechanistically explained","The specific substrates of the latent 20S proteasome pool were not identified","Tissue-specific requirements beyond liver were not fully characterized"]},{"year":2015,"claim":"Discovery that iRhom1 interacts with and stabilizes the PAC1–PAC2 complex under ER stress provided the first upstream regulatory mechanism for proteasome assembly chaperone activity.","evidence":"Co-immunoprecipitation, native PAGE, siRNA knockdown, proteasome activity assay, and subcellular fractionation in mammalian cells with Drosophila genetic rescue","pmids":["26109405"],"confidence":"Medium","gaps":["Finding from a single study; independent confirmation of the iRhom1–PAC1 interaction is lacking","The molecular mechanism by which iRhom1 promotes PAC1–PAC2 dimerization was not resolved","Whether other stress pathways regulate PAC1–PAC2 stability remains unknown"]},{"year":null,"claim":"Structural details of how the PAC1–PAC2 heterodimer scaffolds alpha-ring assembly, the identity of substrates uniquely processed by the latent 20S proteasome pool, and additional upstream regulators of PAC1 remain uncharacterized.","evidence":"","pmids":[],"confidence":"High","gaps":["No high-resolution structure of the PAC1–PAC2 complex bound to proteasome precursors","Specific substrates of the PAC1-dependent latent 20S pool are unknown","Tissue-specific and developmental regulation of PAC1 beyond hepatocytes is largely unexplored"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0044183","term_label":"protein folding chaperone","supporting_discovery_ids":[0,1]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[0]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[2]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,1]}],"complexes":["PAC1–PAC2 heterodimer","20S proteasome precursor"],"partners":["PSMG2","RHBDF1"],"other_free_text":[]},"mechanistic_narrative":"PSMG1 (PAC1) forms an obligate heterodimer with PSMG2 (PAC2) that functions as a dedicated assembly chaperone for the mammalian 20S proteasome, providing a scaffold for alpha-ring formation, maintaining alpha-ring competence for half-proteasome maturation, and undergoing degradation upon completion of the mature 20S particle [PMID:16251969]. Conditional knockout in mice reveals that PAC1 is essential for embryonic development and is responsible for generating the majority of free latent 20S proteasomes in adult cells; loss of PAC1 abolishes the latent 20S pool, causes ubiquitinated protein accumulation, and triggers premature senescence [PMID:20498273]. PAC1–PAC2 dimerization and stability are further regulated by iRhom1, which is induced under ER stress and promotes proteasome assembly, particularly in ER-containing compartments [PMID:26109405]."},"prefetch_data":{"uniprot":{"accession":"O95456","full_name":"Proteasome assembly chaperone 1","aliases":["Chromosome 21 leucine-rich protein","C21-LRP","Down syndrome critical region protein 2","Proteasome chaperone homolog 1","Pba1"],"length_aa":288,"mass_kda":32.9,"function":"Chaperone protein which promotes assembly of the 20S proteasome as part of a heterodimer with PSMG2. The PSMG1-PSMG2 heterodimer binds to the PSMA5 and PSMA7 proteasome subunits, promotes assembly of the proteasome alpha subunits into the heteroheptameric alpha ring and prevents alpha ring dimerization","subcellular_location":"Cytoplasm; Endoplasmic reticulum","url":"https://www.uniprot.org/uniprotkb/O95456/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PSMG1","classification":"Not 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PSMG3","url":"https://www.omim.org/entry/617528"},{"mim_id":"613386","title":"PROTEASOME MATURATION PROTEIN; POMP","url":"https://www.omim.org/entry/613386"},{"mim_id":"612567","title":"INFLAMMATORY BOWEL DISEASE 25, AUTOSOMAL RECESSIVE; IBD25","url":"https://www.omim.org/entry/612567"},{"mim_id":"609702","title":"PROTEASOME ASSEMBLY CHAPERONE 2; PSMG2","url":"https://www.omim.org/entry/609702"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Enhanced","locations":[{"location":"Nucleoplasm","reliability":"Enhanced"},{"location":"Golgi apparatus","reliability":"Enhanced"},{"location":"Cytosol","reliability":"Enhanced"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"testis","ntpm":37.6}],"url":"https://www.proteinatlas.org/search/PSMG1"},"hgnc":{"alias_symbol":["c21-LRP","LRPC21","PAC1"],"prev_symbol":["DSCR2"]},"alphafold":{"accession":"O95456","domains":[{"cath_id":"3.40.50.10900","chopping":"53-102_116-283","consensus_level":"medium","plddt":88.1453,"start":53,"end":283}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O95456","model_url":"https://alphafold.ebi.ac.uk/files/AF-O95456-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O95456-F1-predicted_aligned_error_v6.png","plddt_mean":81.75},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PSMG1","jax_strain_url":"https://www.jax.org/strain/search?query=PSMG1"},"sequence":{"accession":"O95456","fasta_url":"https://rest.uniprot.org/uniprotkb/O95456.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O95456/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O95456"}},"corpus_meta":[{"pmid":"21350482","id":"PMC_21350482","title":"Post-traumatic 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standard","round2_stage1":{"model":"claude-sonnet-4-6","input_tokens":63345,"output_tokens":1646,"usd":0.107362},"round2_rules_fired":"R3","round2_stage2":{"model":"claude-opus-4-6","input_tokens":4414,"output_tokens":1320,"usd":0.082605}},"stage1_raw":"Looking at the corpus, I need to identify which papers are about the human gene PSMG1 (Proteasome Assembly Chaperone 1, also known as PAC1 in the context of proteasome biology). The vast majority of papers in this corpus are about completely different genes that share the \"PAC1\" symbol:\n\n1. DUSP2/PAC1 - a MAP kinase phosphatase (papers 2, 3, 5, 8, 14, etc.)\n2. ADCYAP1R1/PAC1 receptor - a GPCR for PACAP neuropeptide (papers 1, 7, 10, etc.)\n3. Yeast/fungal pac1 genes (papers 4, 6, 9, 23, 24, etc.)\n\nThe human gene PSMG1 encodes a 20S proteasome assembly chaperone. I need to find papers specifically about this protein. Paper [38] is clearly about PSMG1 as a proteasome assembly chaperone, and paper [53] mentions PAC1/2 in the context of proteasome assembly chaperones. These are the relevant papers.\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2010,\n      \"finding\": \"PAC1 (PSMG1) functions as a proteasome assembly chaperone essential for the initial assembly of the 20S proteasome catalytic core. PAC1 conditional knockout mice exhibit early embryonic lethality, demonstrating it is essential for mammalian development. In quiescent adult hepatocytes, PAC1 is responsible for producing the majority of the 20S proteasome. PAC1-deficient hepatocytes retain normal amounts of 26S proteasome but completely lose free latent 20S proteasomes, accumulate ubiquitinated proteins, and exhibit premature senescence, establishing that both the PAC1-dependent assembly pathway and latent 20S proteasomes are required for cellular homeostasis.\",\n      \"method\": \"Conditional knockout mouse generation, proteasome fractionation, ubiquitinated protein accumulation assay, cellular senescence assay\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean conditional KO with multiple defined cellular phenotypes and biochemical readouts, replicated across multiple tissue types\",\n      \"pmids\": [\"20498273\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"iRhom1 interacts physically with the 20S proteasome assembly chaperones PAC1 (PSMG1) and PAC2, affecting their protein stability. Knockdown of iRhom1 impairs dimerization of PAC1 and PAC2 under ER stress, and reduces proteasome complex assembly and enzymatic activity. Overexpression of iRhom1 enhances proteasome activity, identifying iRhom1 as a regulator of proteasome assembly that functions via the PAC1/PAC2 chaperone system under ER stress.\",\n      \"method\": \"Co-immunoprecipitation, native-gel analysis, fractionation, siRNA knockdown, cDNA overexpression screen, in vivo Drosophila model\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single lab with Co-IP and functional knockdown data, multiple orthogonal methods but no structural validation\",\n      \"pmids\": [\"26109405\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PSMG1 (PAC1) is an essential mammalian proteasome assembly chaperone that mediates the initial assembly of the 20S proteasome catalytic core; its loss eliminates free latent 20S proteasomes, causes accumulation of ubiquitinated proteins and premature cellular senescence, and results in embryonic lethality, while its activity is regulated by interaction with iRhom1 which stabilizes PAC1/PAC2 dimerization particularly under ER stress conditions.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll systematically classify each paper before extracting discoveries.\n\n**Classification:**\n\nThe gene query is PSMG1 (Proteasome Assembly Chaperone 1, also known as PAC1 in the proteasome assembly context).\n\nFrom the retrieved papers [1-100]: These are overwhelmingly about:\n- PAC1 = DUSP2 (dual specificity phosphatase 2, phosphatase of activated cells 1) — alias collision\n- PAC1 = ADCYAP1R1 (pituitary adenylate cyclase-activating polypeptide type I receptor) — alias collision\n- PAC1 = yeast/fungal pH-regulatory transcription factors — symbol collision\n- PAC1 = fission yeast RNase III homolog — symbol collision\n- PAC1 = yeast LIS1 homolog (nuclear migration) — symbol collision\n- PAC-1 = procaspase-activating compound — completely unrelated\n- PAC1 = herpesvirus packaging motif — unrelated\n- PAC1 = bacterial β-lactamase — unrelated\n\nOnly paper [38] from the retrieved corpus describes PSMG1 (proteasome assembly chaperone PAC1/PSMG1).\n\nFrom the gene2pubmed-curated papers: Paper [22] clearly describes PSMG1 (PAC1/PAC2 heterodimer, proteasome assembly chaperone). Paper [53] (iRhom1/PAC1/PAC2) also describes PSMG1. Papers [38] from retrieved corpus and [22], [53] from curated are KEEP.\n\nOther curated papers are large-scale interactome/genomic studies that may list PSMG1 as an interactor but don't provide mechanistic findings about PSMG1 specifically — I'll check for mechanistic content about PSMG1.\n\n- [2] BioPlex: large-scale AP-MS, no specific PSMG1 mechanism described\n- [3] BioPlex 2.0: same\n- [4] Human interactome: same\n- [5] HuRI: same\n- [8] BioPlex 3.0: same\n- [9] Census of complexes: same\n- [16] Metazoan complexes: same\n- [19] Size-exclusion: same\n- [20] IBD GWAS: GWAS association near PSMG1 — expression/association, not mechanistic\n- [29] Rpn13 autoubiquitination: mentions proteasome but not PSMG1 specifically\n\nPaper [22] is the foundational mechanism paper for PSMG1. Paper [38] is the PAC1 knockout paper. Paper [53] describes iRhom1 interaction with PAC1/PAC2.\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2005,\n      \"finding\": \"PSMG1 (PAC1) and PSMG2 (PAC2) form a heterodimer that acts as a dedicated chaperone for mammalian 20S proteasome assembly. The PAC1–PAC2 complex associates with proteasome precursors, provides a scaffold for alpha-ring formation, keeps alpha-rings competent for subsequent half-proteasome formation, and is degraded upon completion of the mature 20S proteasome. Overexpression of PAC1 or PAC2 accelerates precursor formation, while siRNA knockdown impairs 20S proteasome maturation.\",\n      \"method\": \"Co-immunoprecipitation, sucrose-gradient sedimentation, siRNA knockdown, overexpression in mammalian cells, native PAGE\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP and functional rescue with multiple orthogonal methods (knockdown, overexpression, fractionation) in a high-impact foundational study\",\n      \"pmids\": [\"16251969\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Conditional knockout of PAC1 (PSMG1) in mice causes early embryonic lethality, demonstrating that PAC1-dependent 20S proteasome assembly is essential for mammalian development. In adult hepatocytes, PAC1 is responsible for producing the majority of 20S proteasomes; PAC1-deficient hepatocytes retain normal 26S proteasome levels but completely lose the free latent 20S proteasome pool, accumulate ubiquitinated proteins, and undergo premature senescence, establishing an essential role for the latent 20S proteasome in cellular homeostasis.\",\n      \"method\": \"Conditional gene knockout in mice, native PAGE, ubiquitin accumulation assay, cellular senescence assay, proteasome activity assay\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean conditional KO with multiple defined cellular phenotypes and mechanistic pathway placement\",\n      \"pmids\": [\"20498273\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"iRhom1 physically interacts with the 20S proteasome assembly chaperones PAC1 (PSMG1) and PAC2 (PSMG2), stabilizing their protein levels. Under ER stress, iRhom1 expression is induced and promotes PAC1–PAC2 dimerization; knockdown of iRhom1 impairs PAC1–PAC2 dimerization and reduces proteasome complex assembly and enzymatic activity, particularly in ER-containing microsomes.\",\n      \"method\": \"Co-immunoprecipitation, native PAGE, proteasome activity assay, siRNA knockdown, fractionation, Drosophila genetic rescue\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — Co-IP and functional readout in single study; dimerization finding extends prior work\",\n      \"pmids\": [\"26109405\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PSMG1 (PAC1) forms an obligate heterodimer with PSMG2 (PAC2) that acts as a dedicated scaffold chaperone for mammalian 20S proteasome assembly, promoting alpha-ring formation and half-proteasome maturation, and is degraded upon completion of the mature 20S particle; PAC1 is essential for mammalian development and for maintaining the pool of free latent 20S proteasomes required for cellular homeostasis, with its dimerization and stability further regulated by iRhom1 under ER stress.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"PSMG1 (PAC1) is a proteasome assembly chaperone essential for the initial assembly of the 20S proteasome catalytic core. Conditional knockout in mice causes early embryonic lethality, and PAC1-deficient hepatocytes completely lose free latent 20S proteasomes, accumulate ubiquitinated proteins, and undergo premature senescence, demonstrating that PAC1-dependent assembly is required for cellular homeostasis [PMID:20498273]. iRhom1 physically interacts with PAC1 and PAC2, stabilizing their dimerization particularly under ER stress to promote proteasome assembly and activity [PMID:26109405].\",\n  \"teleology\": [\n    {\n      \"year\": 2010,\n      \"claim\": \"Whether PAC1 is essential in vivo and what cellular consequences follow its loss were unknown; conditional knockout revealed that PAC1 is required for embryonic viability and is the primary source of free latent 20S proteasomes in quiescent cells, whose absence triggers ubiquitin accumulation and premature senescence.\",\n      \"evidence\": \"Conditional knockout mouse with proteasome fractionation, ubiquitinated protein accumulation, and senescence assays in hepatocytes\",\n      \"pmids\": [\"20498273\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Mechanism by which loss of latent 20S proteasomes specifically causes senescence is not defined\",\n        \"Whether PAC1 contributes to 26S proteasome assembly in proliferating cells remains unclear\",\n        \"No structural detail on PAC1/PAC2 chaperone interaction with assembling α-ring intermediates\"\n      ]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"How PAC1/PAC2 chaperone activity is regulated was unknown; identification of iRhom1 as a physical interactor that stabilizes PAC1–PAC2 dimerization under ER stress established a regulatory layer linking ER stress signaling to proteasome biogenesis.\",\n      \"evidence\": \"Co-immunoprecipitation, native-gel analysis, siRNA knockdown, and overexpression in mammalian cells and Drosophila\",\n      \"pmids\": [\"26109405\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Interaction between iRhom1 and PAC1/PAC2 lacks structural validation or mapping of binding interfaces\",\n        \"Whether iRhom1-mediated regulation operates outside ER stress conditions is untested\",\n        \"No independent replication of the iRhom1–PAC1 interaction by another group\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural basis for PAC1/PAC2 chaperone recognition of α-ring assembly intermediates, the signaling pathways connecting PAC1 loss to senescence, and additional regulators of PAC1 activity remain uncharacterized.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No high-resolution structure of PAC1/PAC2 bound to α-ring intermediates\",\n        \"Mechanism linking latent 20S proteasome depletion to senescence induction is undefined\",\n        \"Tissue-specific roles of PAC1 beyond liver have not been systematically explored\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0044183\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"complexes\": [\"PAC1–PAC2 heterodimer\"],\n    \"partners\": [\"PSMG2\", \"RHBDF1\"],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"PSMG1 (PAC1) forms an obligate heterodimer with PSMG2 (PAC2) that functions as a dedicated assembly chaperone for the mammalian 20S proteasome, providing a scaffold for alpha-ring formation, maintaining alpha-ring competence for half-proteasome maturation, and undergoing degradation upon completion of the mature 20S particle [PMID:16251969]. Conditional knockout in mice reveals that PAC1 is essential for embryonic development and is responsible for generating the majority of free latent 20S proteasomes in adult cells; loss of PAC1 abolishes the latent 20S pool, causes ubiquitinated protein accumulation, and triggers premature senescence [PMID:20498273]. PAC1–PAC2 dimerization and stability are further regulated by iRhom1, which is induced under ER stress and promotes proteasome assembly, particularly in ER-containing compartments [PMID:26109405].\",\n  \"teleology\": [\n    {\n      \"year\": 2005,\n      \"claim\": \"Identification of the PAC1–PAC2 heterodimer as a dedicated 20S proteasome assembly chaperone resolved how mammalian alpha-ring formation is initiated and coordinated, establishing that the chaperone is consumed upon proteasome maturation.\",\n      \"evidence\": \"Reciprocal co-immunoprecipitation, sucrose-gradient sedimentation, siRNA knockdown, and overexpression in mammalian cells\",\n      \"pmids\": [\"16251969\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"In vivo essentiality of PAC1 was not tested\",\n        \"Structural basis of alpha-ring scaffolding by PAC1–PAC2 was not resolved\",\n        \"Regulation of PAC1–PAC2 levels or activity by upstream signals was unknown\"\n      ]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Genetic ablation of PAC1 in mice demonstrated that the chaperone is essential for embryonic viability and is the primary source of the free latent 20S proteasome pool in adult cells, linking PAC1 function to ubiquitin homeostasis and cellular senescence.\",\n      \"evidence\": \"Conditional gene knockout in mice with native PAGE, ubiquitin accumulation assay, proteasome activity assay, and senescence assay in hepatocytes\",\n      \"pmids\": [\"20498273\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Why 26S proteasome levels are maintained despite loss of the free 20S pool was not mechanistically explained\",\n        \"The specific substrates of the latent 20S proteasome pool were not identified\",\n        \"Tissue-specific requirements beyond liver were not fully characterized\"\n      ]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Discovery that iRhom1 interacts with and stabilizes the PAC1–PAC2 complex under ER stress provided the first upstream regulatory mechanism for proteasome assembly chaperone activity.\",\n      \"evidence\": \"Co-immunoprecipitation, native PAGE, siRNA knockdown, proteasome activity assay, and subcellular fractionation in mammalian cells with Drosophila genetic rescue\",\n      \"pmids\": [\"26109405\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Finding from a single study; independent confirmation of the iRhom1–PAC1 interaction is lacking\",\n        \"The molecular mechanism by which iRhom1 promotes PAC1–PAC2 dimerization was not resolved\",\n        \"Whether other stress pathways regulate PAC1–PAC2 stability remains unknown\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Structural details of how the PAC1–PAC2 heterodimer scaffolds alpha-ring assembly, the identity of substrates uniquely processed by the latent 20S proteasome pool, and additional upstream regulators of PAC1 remain uncharacterized.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No high-resolution structure of the PAC1–PAC2 complex bound to proteasome precursors\",\n        \"Specific substrates of the PAC1-dependent latent 20S pool are unknown\",\n        \"Tissue-specific and developmental regulation of PAC1 beyond hepatocytes is largely unexplored\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0044183\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"complexes\": [\n      \"PAC1–PAC2 heterodimer\",\n      \"20S proteasome precursor\"\n    ],\n    \"partners\": [\n      \"PSMG2\",\n      \"RHBDF1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}