{"gene":"PCSK1","run_date":"2026-06-10T05:19:53","timeline":{"discoveries":[{"year":1991,"finding":"PC1 (PCSK1) cleaves proopiomelanocortin (POMC) preferentially at two pairs of basic residues to produce corticotropin (ACTH) and beta-lipotropin, demonstrating distinct cleavage specificity compared to PC2 (which has broader activity). This was established by co-expressing PC1 and PC2 with POMC in multiple cell lines including BSC-40, PC12, and AtT-20.","method":"Recombinant vaccinia virus co-expression of PC1/PC2 with POMC in multiple cell lines; pulse-chase biosynthetic monitoring","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct enzymatic activity assay with substrate in multiple cell types, replicated across labs","pmids":["2023902"],"is_preprint":false},{"year":1992,"finding":"PC1 can process human prorenin to active renin specifically in cells containing secretory granules (e.g., GH4 somatomammotroph cells) but not in cells lacking granules (CHO or BSC-40 cells), demonstrating that PC1 activity on substrates requires the regulated secretory pathway environment, not merely propeptide removal.","method":"Vaccinia virus expression system; biosynthetic labeling; comparison of processing in granule-containing vs granule-lacking cell lines","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution in multiple cell types, single lab but orthogonal cell contexts","pmids":["1597471"],"is_preprint":false},{"year":1993,"finding":"PC1 and PC2 undergo distinct biosynthetic processing: pro-PC1 (88 kDa) is cleaved to PC1 (83 kDa) in a pre-Golgi compartment (endoplasmic reticulum), with prosegment cleavage occurring early. PC1 is secreted as a glycosylated, sulphated 84 kDa form ~30 min after biosynthesis. N-glycosylation is essential for proper folding and stability of PC1; inhibition with tunicamycin causes ER degradation. Furin cannot cleave pro-PC1.","method":"Pulse-chase analysis; low temperature/brefeldin A/CCCP treatment; tunicamycin inhibition; furin co-expression; vaccinia virus expression in GH4C1 cells","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 / Moderate — multiple orthogonal methods (pharmacological, biochemical, genetic), single lab","pmids":["8397508"],"is_preprint":false},{"year":1993,"finding":"PC1 performs a limited set of POMC cleavages in AtT-20 corticotropes (anterior pituitary pattern: ACTH + beta-LPH), while co-expression of PC2 in these cells enables the full intermediate pituitary cleavage pattern (alpha-MSH, beta-endorphin). PC1-dependent cleavages occur in the earliest biosynthetic steps; PC2-dependent cleavages occur in middle/late steps.","method":"Stable transfection of PC2 cDNA into AtT-20 cells; kinetic analysis of biosynthetic processing products","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct enzymatic substrate processing with temporal kinetics, replicated in multiple studies","pmids":["8380577"],"is_preprint":false},{"year":1993,"finding":"Purified recombinant mouse PC1 is an 87-kDa calcium-dependent serine proteinase with optimal pH 5.5–6.5, preferring substrates with arginine 4 residues N-terminal to the cleavage site. PC1 undergoes autocatalytic prosegment cleavage at the RSKR motif (residues 80–83) early in biosynthesis. PC1 cleaves proenkephalin to yield a peptide B-sized fragment, demonstrating a role in proenkephalin processing.","method":"Purification from CHO cell conditioned medium; fluorogenic substrate specificity studies; pulse-chase biosynthesis; in vitro cleavage of proenkephalin","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — biochemical purification, enzymatic characterization, substrate cleavage, multiple orthogonal methods","pmids":["8449925"],"is_preprint":false},{"year":1993,"finding":"PC1 is concentrated in a trans-Golgi network (TGN)-like compartment in AtT-20 cells and is co-transported with processed peptide (met-enkephalin) to the tips of cell processes (secretory granules). Brefeldin A treatment disrupts PC1 localization to the TGN region, confirming Golgi-dependent localization.","method":"Immunocytochemistry; dual-staining with TGN38 marker; brefeldin A treatment; subcellular fractionation","journal":"Neuroendocrinology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct subcellular localization with pharmacological validation and co-localization, single lab","pmids":["8115023"],"is_preprint":false},{"year":1993,"finding":"PC1 (but not PC2) can cleave prosomatostatin at the dibasic Arg-Lys site to produce somatostatin-14 when co-expressed in COS-7 cells, demonstrating that PC1 mediates dibasic cleavage of prosomatostatin in the regulated secretory pathway.","method":"Co-expression of PC1/PC2 with prosomatostatin in COS-7, AtT-20, and PC12 cells; comparison of processing products by RIA and HPLC","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct substrate processing comparison in multiple cell types, single lab","pmids":["8095501"],"is_preprint":false},{"year":1995,"finding":"PC1/3 and PC2 are purified from bovine adrenal medulla chromaffin granules as 66-kDa Ca2+-dependent serine proteases. PC1/3 shows pH optimum of 6.5, cleaves paired basic and monobasic sites in peptide substrates (Km ~66 µM for Boc-Arg-Val-Arg-Arg-MCA), and is inhibited by EGTA, thiol-blocking reagents, and active-site-directed chloromethyl ketone inhibitors.","method":"Purification from chromaffin granules by chromatography; kinetic studies with fluorogenic peptide substrates; inhibitor profiling; immunoaffinity purification","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — biochemical purification from native tissue with full kinetic and inhibitor characterization, single lab","pmids":["7713926"],"is_preprint":false},{"year":1995,"finding":"The 66-kDa C-terminally truncated form of PC1, generated by cleavage at Arg590-Arg591 in the C-terminal tail, is efficiently routed to secretory granules and retains full processing activity on proneurotensin. C-terminal truncation of PC1 regulates its activity—blocking this cleavage site reduces processing efficiency, suggesting C-terminal processing serves to activate the enzyme.","method":"Site-directed mutagenesis of paired basic sites in C-terminal tail; PC12 cell expression; proneurotensin processing assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis + functional substrate processing assay, single lab","pmids":["7559585"],"is_preprint":false},{"year":1997,"finding":"The RRGDL motif in the P-domain of PC1 is critical for zymogen processing in the ER, C-terminal auto-processing to the 66-kDa form in secretory granules, and proper sorting to the regulated secretory pathway. Mutations in this motif cause increased ER degradation, prevent granule sorting, and route PC1 to the constitutive pathway, reducing POMC processing activity.","method":"Site-directed mutagenesis of RRGDL motif; vaccinia virus expression; pulse-chase analysis; alpha1-PDX inhibitor; immunocytochemistry","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — mutagenesis + multiple functional readouts (sorting, processing, localization), multiple cell types","pmids":["9307023"],"is_preprint":false},{"year":1998,"finding":"The propeptide of PC1/3 (residues 1–98) functions as a potent slow tight-binding inhibitor of active PC1/3 (Ki in low nanomolar range) and also weakly inhibits PC2 (micromolar Ki, competitive). Cleavage within residues 1–71 abrogates inhibitory potency. The propeptide is itself cleaved by mature PC1/3.","method":"Recombinant propeptide isolation from baculovirus-infected insect cells; progress curve kinetic analysis; inhibition assays with murine 71-kDa PC1/3 and human furin","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — purified proteins, kinetic characterization, mutagenesis by proteolytic mapping, single lab","pmids":["9813073"],"is_preprint":false},{"year":1999,"finding":"PC1 propeptide cleavage occurs rapidly in the ER, yet PC1's major substrate processing occurs later (secretory granules). PC1 undergoes a C-terminal processing event that activates the enzyme. ProPC2 requires the chaperone 7B2 for generation of catalytically active PC2, but this is not required for PC1 activation.","method":"Biochemical fractionation; pulse-chase analysis; review integrating multiple experimental systems","journal":"Progress in nucleic acid research and molecular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Strong — synthesizes multiple experimental findings, but this paper is primarily a review; underlying data from multiple labs","pmids":["10506829"],"is_preprint":false},{"year":2000,"finding":"The granin-related protein proSAAS contains a C-terminal hexapeptide (SAAS CT peptide) that is a potent nanomolar inhibitor of PC1, functioning as a tight-binding competitive inhibitor. Recombinant PC1 can cleave the proSAAS CT peptide following the inhibitory hexapeptide, suggesting autoinactivation of the inhibitor.","method":"Recombinant protein inhibition assay; peptide library comparison; proteolytic mapping; structural analogy to 7B2/PC2 system","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct inhibitor kinetics with purified PC1, single lab, two approaches (cleavage + inhibition)","pmids":["10812060"],"is_preprint":false},{"year":2002,"finding":"PC1/3 knockout mice display severe postnatal growth impairment (~60% of normal size) and multiple prohormone processing defects including: failure to cleave GHRH (leading to low GH and IGF-1), defective POMC→ACTH processing in pituitary, hyperproinsulinemia (impaired proinsulin→insulin conversion), and failure of proglucagon→GLP-1/GLP-2 conversion in intestine. This definitively establishes PC1/3 as a key neuroendocrine convertase for these specific substrates in vivo.","method":"Targeted gene disruption (knockout mouse); phenotypic analysis; hormone measurements; mRNA analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean knockout with multiple defined substrate-specific phenotypes, replicated across tissues","pmids":["12145326"],"is_preprint":false},{"year":2002,"finding":"Both PC1/3 and PC2 can generate VGF20 from the VGF precursor, while VGF10 production is preferentially mediated by PC1/3. PC1/3 cleaves at the RPR555 site to generate VGF10. The KRKRKK488 motif is the target for VGF20 generation.","method":"Ectopic expression of PC1/3 and PC2 in GH3 cells; site-directed mutagenesis of VGF cleavage sites; mass spectrometry and Edman degradation of products","journal":"Journal of neurochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct substrate cleavage with mutagenesis validation, single lab","pmids":["12065665"],"is_preprint":false},{"year":2003,"finding":"PC2 is more efficient than PC1/3 in generating bioactive CART I (residues 55–102) from pro-CART, while bioactive CART II (residues 62–102) is exclusively generated by PC2. PC1/3 is predominantly responsible for generating intermediate CART fragments (33–102 and 10–89). These results were confirmed in PC2, 7B2, and PC1/3 knockout mouse hypothalamic extracts.","method":"Transient transfection in neuroendocrine cell lines; microsequencing; HPLC/mass spectrometry; knockout mouse hypothalamic extract analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro cell-based assay plus in vivo knockout validation, two orthogonal methods","pmids":["12584191"],"is_preprint":false},{"year":2006,"finding":"A mouse PC1 N222D missense mutation causes obesity, impairs autocatalytic activation of mature PC1, reduces hypothalamic alpha-MSH (through defective POMC processing), and leads to abnormal proinsulin processing and glucose intolerance. This demonstrates that a partial loss of PC1 activity is sufficient to cause obesity in mice.","method":"Mouse mutant allele characterization; enzyme activity assays; hormone measurements; metabolic phenotyping","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo mouse model with mechanistic link to autocatalytic activation and substrate processing","pmids":["16644867"],"is_preprint":false},{"year":2007,"finding":"Expression of PC1/3 (instead of PC2) in alpha-cells redirects proglucagon processing from glucagon production to GLP-1 and GLP-2 production, improving glucose tolerance and promoting beta-cell proliferation. The glucose-lowering effect was attenuated in GLP-1R-/- mice, confirming that PC1/3-mediated GLP-1 production is the key mechanism.","method":"Transplantation of alpha-cells stably expressing PC1/3 vs PC2 in mice; GLP-1R knockout mice; hormone measurements; glucose tolerance tests","journal":"Diabetes","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo cell transplantation with genetic epistasis (GLP-1R KO), multiple readouts","pmids":["17698597"],"is_preprint":false},{"year":2008,"finding":"The PCSK1 nonsynonymous variant N221D (rs6232) significantly impairs PC1/3 catalytic activity in functional assays, linking this common variant to obesity risk.","method":"Functional enzymatic activity assays of N221D mutant PC1/3 compared to wild-type","journal":"Nature genetics","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro enzymatic activity assay, single study; genetic association is large but the functional data is limited in the abstract","pmids":["18604207"],"is_preprint":false},{"year":2008,"finding":"PAX6 transcription factor binds the PC1/3 (PCSK1) promoter and directly upregulates PC1/3 expression. PAX6 deficiency reduces PC1/3 expression, leading to defective proinsulin processing and abnormal glucose metabolism in both mice (Pax6 R266Stop) and humans (PAX6 R240Stop).","method":"Promoter binding assays (ChIP, EMSA); mouse and human genetic models; PC1/3 protein and mRNA measurements; proinsulin processing assays","journal":"Diabetologia","confidence":"High","confidence_rationale":"Tier 2 / Strong — promoter binding demonstrated by ChIP and EMSA, validated in both mouse and human genetic models","pmids":["19034419"],"is_preprint":false},{"year":2009,"finding":"The extreme C-terminal sorting domain of PC1/3 (residues 711–753) contains two alpha-helices (722–728 and 738–750). The second helix is necessary and sufficient to target a constitutively secreted protein to dense core secretory granules (DCSGs). Leucine 745 anchors a hydrophobic patch critical for sorting. Calcium binding by this helix promotes aggregation via the hydrophobic patch, linking calcium to DCSG sorting of PC1/3.","method":"NMR structure determination; functional granule-sorting assays with truncation mutants; calcium binding studies","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR structure plus functional mutagenesis and calcium binding assays in single rigorous study","pmids":["19376969"],"is_preprint":false},{"year":2011,"finding":"PC1/3 exists as multiple ionic forms due to oligomerization and aggregation. The most active form of 87-kDa PC1/3 is a probable homodimer with latent activity revealed by dilution. Preincubation with fluorogenic substrate or peptides containing paired basic residues stabilizes and activates PC1/3, suggesting that substrate binding regulates enzyme activity within secretory granules.","method":"Ion exchange chromatography; 2D gel electrophoresis; gel filtration; cross-linking; enzymatic activity assays with fluorogenic substrates","journal":"Endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple biochemical methods, single lab; mechanistic interpretation of oligomerization and substrate-dependent activation","pmids":["21303942"],"is_preprint":false},{"year":2012,"finding":"PC1/3 is expressed in macrophages, and PC1/3 knockout mice have enlarged, disorganized spleens with depletion of dendritic cells. When challenged with LPS, PC1/3 KO mice exhibit cytokine storm with markedly elevated IL-6, IL-1β, and TNF-α plasma levels and are hypersusceptible to septic shock, demonstrating a role for PC1/3 in regulating innate immune cytokine secretion in macrophages.","method":"PC1/3 knockout mouse; LPS challenge; cytokine ELISA; immunohistochemistry; peritoneal macrophage isolation; electron microscopy","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean knockout with multiple immune phenotypes and cell-autonomous validation in isolated macrophages","pmids":["22396549"],"is_preprint":false},{"year":2012,"finding":"Pax6 directly binds the Pcsk1n (proSAAS) promoter and down-regulates its expression. Elevated proSAAS in Pax6-mutant mice inhibits PC1/3 C-terminal cleavage and activity, impairing proinsulin processing. This inhibition can be rescued by proSAAS knockdown, establishing a Pax6→Pcsk1n→PC1/3 regulatory axis in beta cells.","method":"Luciferase reporter assay; ChIP; EMSA; RNAi knockdown; enzyme activity assays; proinsulin processing measurements","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (ChIP, EMSA, reporter, rescue experiment), single lab","pmids":["23056534"],"is_preprint":false},{"year":2014,"finding":"A heterozygous nonsense mutation PCSK1-p.Arg80* encodes a truncated propeptide that inhibits PC1/3 enzyme activity, likely through a dominant-negative mechanism not requiring strong direct physical interaction. This establishes that truncated PC1/3 propeptide can inhibit wild-type PC1/3 activity in trans.","method":"In vitro functional enzyme activity assays; co-expression studies; family segregation analysis","journal":"International journal of obesity","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct enzyme activity assay with interaction studies, single lab","pmids":["24890885"],"is_preprint":false},{"year":2015,"finding":"PC1/3 inhibition in macrophages (by knockdown or PC1/3 KO) drives macrophages toward an M1 activated phenotype characterized by filopodial extensions, TLR4/MyD88-dependent signaling, increased calcium entry, and secretion of pro-inflammatory factors. This was shown in both NR8383 macrophages and PC1/3 KO mouse macrophages.","method":"Proteomics of secretomes and intracellular proteins; PC1/3 knockdown in NR8383 cells; PC1/3 KO mouse macrophages; calcium imaging","journal":"Molecular & cellular proteomics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — proteomic + KO validation, two cell models, single lab","pmids":["26330543"],"is_preprint":false},{"year":2015,"finding":"ER-retained PC1/3 mutants (G209R and G593R) induce ER stress and exert dominant-negative effects on wild-type PC1/3, blocking prodomain cleavage and decreasing wild-type PC1/3 expression by routing it to a proteasomal degradation pathway. The N221D variant shows ~30% lower enzymatic activity than wild-type in the correct background (wild-type PC1/3, not S357G hypermorphic background).","method":"Site-directed mutagenesis; transfection of WT + mutant constructs; ER stress marker measurement; fluorogenic enzyme activity assays; pulse-chase analysis","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis with direct enzyme activity and dominant-negative mechanistic characterization, single lab","pmids":["26207343"],"is_preprint":false},{"year":2016,"finding":"PC1/3 deficiency in Prader-Willi syndrome (PWS) is due to reduced SNORD116→NHLH2→PCSK1 expression. PC1/3 content and activity were reduced in islets, hypothalamus, and stomach of Snord116 paternal knockout mice, leading to in vivo functional defects in prohormone processing of proinsulin, pro-GHRH, and proghrelin.","method":"iPSC-derived neurons from PWS patients; Snord116 knockout mice; PC1/3 protein measurements; in vivo prohormone processing assays","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 / Strong — human iPSC model plus mouse KO model with multiple in vivo substrate processing measurements","pmids":["27941249"],"is_preprint":false},{"year":2016,"finding":"PC1/3 deficiency in human embryonic stem cell-derived hypothalamic neurons increases unprocessed POMC and decreases ratios of processed POMC-derived peptides (phenocopying PC1/3-null mice), is associated with increased melanocortin receptor expression and increased PRCP (alpha-MSH catabolism), and reduces ACTH secretion.","method":"CRISPR-Cas9 and shRNA-mediated PCSK1 disruption in hESC-derived hypothalamic neurons; POMC peptide measurements; receptor expression analysis","journal":"Stem cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — CRISPR KO and shRNA KD in human neuronal model with multiple substrate and downstream readouts","pmids":["28132887"],"is_preprint":false},{"year":2016,"finding":"A novel PCSK1 variant c.1095+1G>A causes exon 8 skipping, complete loss of PC1/3 enzymatic activity, ER retention of the protein, and ER stress. This mechanistically links splicing mutations to ER quality control in PCSK1 disease.","method":"Exon skipping analysis; enzyme activity assay; cell trafficking experiments; ER stress marker measurement","journal":"Molecular metabolism","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple functional assays (activity, trafficking, ER stress) on a single novel mutant, single lab","pmids":["28271036"],"is_preprint":false},{"year":2016,"finding":"Intestinal bile acids activate TGR5 receptor, which signals through nuclear factor of activated T cells (NFAT) to induce PC1/3 (PCSK1) gene expression in intestinal L-cells, thereby increasing GLP-1 production. This pathway mediates the antidiabetic effect of bile acid binding resins.","method":"TGR5-dependent colestimide treatment in mice; NFAT pathway inhibition; PC1/3 gene expression measurement; GLP-1 secretion assays","journal":"Endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pathway defined with receptor agonist and transcription factor inhibition, in vivo plus cell experiments","pmids":["26789236"],"is_preprint":false},{"year":2020,"finding":"Transcription factor Creb3l1 directly binds a G-box motif in the Pcsk1 promoter and upregulates PC1/3 expression in corticotroph and magnocellular neuroendocrine cells. Viral overexpression of Creb3l1 in supraoptic nuclei increases Pcsk1, and knockdown decreases it. No Creb3l1-Pcsk1 relationship was found in the neurointermediate lobe, indicating cell-type specificity.","method":"RNA-sequencing; viral overexpression and knockdown in vivo; in vitro promoter luciferase assay; ChIP; EMSA","journal":"Journal of neuroendocrinology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (ChIP, EMSA, reporter, in vivo viral manipulation), single lab","pmids":["32319174"],"is_preprint":false},{"year":2021,"finding":"GLP-1 receptor (GLP-1R) agonist liraglutide increases PC1/3 (PCSK1) mRNA expression in a subcluster of pancreatic alpha-cells in a beta-cell GLP-1R-dependent manner. This is associated with increased bihormonal insulin+/glucagon+ cells and expression of other beta-cell-like genes, establishing a GLP-1-mediated paracrine pathway that induces alpha-cell PC1/3 expression.","method":"Mouse model with beta cell-specific GLP-1R; scRNA-seq (DART-Seq); IHC; liraglutide treatment","journal":"JCI insight","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — GLP-1R KO epistasis with single-cell transcriptomics and IHC, single lab","pmids":["33554958"],"is_preprint":false},{"year":1999,"finding":"Leukemia inhibitory factor (LIF) and IL-6 (cytokines signaling through gp130/JAK-STAT) upregulate PC1 protein and mRNA in AtT-20 corticotroph cells. This upregulation increases POMC processing to ACTH. LPS administration in vivo also increases pituitary PC1 and POMC mRNA. PC1 promoter activity is directly increased by LIF/IL-6, demonstrating transcriptional regulation by gp130 cytokines.","method":"AtT-20 cell treatment with LIF/IL-6; PC1 protein and mRNA measurement; PC1 promoter-luciferase transfection assay; in vivo LPS administration","journal":"Molecular and cellular endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reporter assay plus protein/mRNA measurements and in vivo validation, single lab","pmids":["10630414"],"is_preprint":false},{"year":2001,"finding":"Thyroid hormone (T3) negatively regulates PC1 transcription through multiple thyroid hormone response elements (TREs) located at -10 to +19 bp relative to the transcription start site of the hPC1 promoter. TR-alpha1 binds this region as monomer, homodimer, and heterodimer with RXR-beta. Hypothyroidism increases and hyperthyroidism decreases pituitary PC1 mRNA.","method":"Promoter deletion analysis; EMSA with purified TR-alpha1 and RXR-beta; point mutation of TREs; pituitary PC1 mRNA measurement in hypothyroid/hyperthyroid rats","journal":"American journal of physiology. Endocrinology and metabolism","confidence":"High","confidence_rationale":"Tier 2 / Strong — EMSA with purified proteins, mutagenesis of TREs, in vivo hormonal manipulation, multiple methods","pmids":["11120670"],"is_preprint":false},{"year":2004,"finding":"proSAAS expression in AtT-20 cells inhibits both C-terminal processing of PC1 and POMC processing under pulse-chase conditions. The PC1 propeptide expressed in trans also inhibits PC1 C-terminal processing and POMC cleavage, but SAAS CT peptide-propeptide chimeras are less effective in the regulated pathway.","method":"AtT-20 cell transfection with proSAAS and propeptide constructs; pulse-chase analysis; POMC and PC1 processing assays","journal":"The Journal of endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct in-cell inhibition assays, single lab","pmids":["15283695"],"is_preprint":false}],"current_model":"PCSK1-encoded PC1/3 is a calcium-dependent serine endoprotease (subtilisin/kexin family) that undergoes autocatalytic propeptide cleavage in the ER (at RSKR80–83), followed by C-terminal truncation at Arg590-Arg591 in secretory granules to generate the maximally active 66-kDa form; it is sorted to dense core secretory granules via a C-terminal amphipathic helix (residues 738–750) through a calcium-sensitive hydrophobic mechanism; its activity is regulated by endogenous inhibitors (its own propeptide and proSAAS/SAAS CT peptide) and by oligomerization/substrate binding; it cleaves multiple prohormones (POMC→ACTH, proinsulin→insulin, proglucagon→GLP-1/GLP-2, proGHRH, proghrelin, proenkephalin, prosomatostatin, VGF) preferentially at paired basic sites (with preference for Arg at P4), with substrate selectivity and processing efficiency dependent on the regulated secretory pathway; its transcription is regulated by gp130 cytokines (LIF/IL-6), thyroid hormone (negative regulation via TR-alpha1 binding at the proximal promoter), PAX6 (positive), Creb3l1 (positive), and bile acid/TGR5/NFAT signaling; it also plays a role in innate immune regulation in macrophages; loss-of-function in humans and mice causes obesity, hyperproinsulinemia, dwarfism, and multiple endocrinopathies."},"narrative":{"mechanistic_narrative":"PCSK1 encodes PC1/3, a calcium-dependent serine endoprotease of the subtilisin/kexin class that is the principal neuroendocrine convertase of the regulated secretory pathway, cleaving prohormones at paired basic residues with a preference for arginine at the P4 position [PMID:2023902, PMID:8449925, PMID:7713926]. Its activity is built through ordered maturation: autocatalytic removal of the prosegment at the RSKR80–83 motif occurs early in the ER, while major substrate processing and a subsequent C-terminal truncation at Arg590-Arg591 that activates the enzyme occur later in dense core secretory granules [PMID:8449925, PMID:7559585]; N-glycosylation and an intact RRGDL P-domain motif are required for folding, zymogen processing, and granule sorting, with misfolding-prone mutants routed to ER-associated degradation [PMID:8397508, PMID:9307023, PMID:26207343]. Sorting to dense core secretory granules is mediated by a C-terminal amphipathic helix (residues 738–750) anchored by Leu745, whose calcium-promoted aggregation links calcium sensing to granule targeting [PMID:19376969]. Within granules, processing requires the regulated secretory environment rather than propeptide removal alone [PMID:1597471], and PC1/3 activity is restrained by endogenous inhibitors—its own propeptide and the proSAAS-derived SAAS CT peptide—and modulated by oligomerization and substrate-induced activation [PMID:9813073, PMID:10812060, PMID:21303942]. PC1/3 cleaves a defined set of substrates including POMC→ACTH/β-LPH, proinsulin, proGHRH, proglucagon→GLP-1/GLP-2, proenkephalin, prosomatostatin, proneurotensin, VGF, and pro-CART [PMID:2023902, PMID:8449925, PMID:8095501, PMID:7559585, PMID:12145326, PMID:12065665, PMID:12584191], and loss of PC1/3 in mice and humans causes obesity, hyperproinsulinemia, dwarfism, and multiple endocrinopathies [PMID:12145326, PMID:16644867, PMID:24890885]; reduced expression underlies prohormone-processing deficits in Prader-Willi syndrome through a SNORD116→NHLH2→PCSK1 axis [PMID:27941249]. Its transcription is controlled by PAX6, Creb3l1, gp130 cytokines (LIF/IL-6), thyroid hormone (negative, via TR-alpha1), and bile acid/TGR5/NFAT signaling [PMID:19034419, PMID:32319174, PMID:10630414, PMID:11120670, PMID:26789236]. Beyond its endocrine role, PC1/3 restrains innate immune activation, with its loss driving macrophages toward an M1 phenotype and cytokine storm [PMID:22396549, PMID:26330543].","teleology":[{"year":1991,"claim":"Establishing that PC1/3 has a distinct, limited prohormone cleavage specificity answered whether convertases divide labor on a shared substrate, distinguishing it from the broader-acting PC2.","evidence":"Vaccinia co-expression of PC1/PC2 with POMC in multiple cell lines with pulse-chase monitoring","pmids":["2023902"],"confidence":"High","gaps":["Did not define the structural basis of cleavage-site selectivity","Specificity tested only on POMC"]},{"year":1992,"claim":"Showing PC1/3 processes substrates only in granule-containing cells established that its activity depends on the regulated secretory pathway environment, not merely propeptide removal.","evidence":"Vaccinia expression with biosynthetic labeling comparing granule-containing vs granule-lacking cells using prorenin","pmids":["1597471"],"confidence":"High","gaps":["Molecular features of the granule environment required were not defined","Did not identify cofactors supplied by granules"]},{"year":1993,"claim":"Biochemical purification and biosynthetic mapping defined PC1/3 as a calcium-dependent acidic-pH serine protease with P4 arginine preference that undergoes early autocatalytic prosegment removal at RSKR80–83, and localized it to the TGN/granule pathway.","evidence":"Recombinant purification, fluorogenic substrate kinetics, pulse-chase, tunicamycin/BFA treatments, immunocytochemistry across cell lines","pmids":["8397508","8380577","8449925","8095501","8115023"],"confidence":"High","gaps":["C-terminal processing event not yet mechanistically defined","Full in vivo substrate repertoire unknown"]},{"year":1995,"claim":"Identifying C-terminal truncation at Arg590-Arg591 as an activating, granule-routing event answered how PC1/3 reaches maximal activity after ER prosegment removal.","evidence":"Site-directed mutagenesis of C-terminal paired basic sites with proneurotensin processing in PC12 cells; purification from chromaffin granules with kinetics","pmids":["7559585","7713926"],"confidence":"High","gaps":["Identity of the protease performing C-terminal cleavage in granules not resolved","Quantitative contribution of truncation vs oligomerization to activation unclear"]},{"year":1997,"claim":"Defining the RRGDL P-domain motif as essential for zymogen processing, C-terminal autoprocessing, and granule sorting linked a single structural element to the full maturation program.","evidence":"Site-directed mutagenesis, vaccinia expression, pulse-chase, alpha1-PDX inhibition, immunocytochemistry","pmids":["9307023"],"confidence":"High","gaps":["Atomic-level mechanism of how RRGDL coordinates folding and sorting not resolved"]},{"year":2000,"claim":"Characterizing the PC1/3 propeptide and the proSAAS-derived SAAS CT peptide as nanomolar inhibitors answered how the enzyme is held latent and activated in a controlled manner.","evidence":"Recombinant propeptide and proSAAS peptide inhibition kinetics, progress-curve analysis, proteolytic mapping","pmids":["9813073","10812060"],"confidence":"High","gaps":["In vivo stoichiometry of inhibitor-to-enzyme not established","Timing of inhibitor inactivation within granules not directly observed"]},{"year":2002,"claim":"Knockout of PC1/3 in mice answered which prohormones depend on it in vivo, establishing it as the key convertase for GHRH, POMC, proinsulin, and proglucagon and explaining the growth/endocrine phenotypes.","evidence":"Targeted gene disruption with hormone measurements and tissue-specific processing analysis; complementary VGF cleavage-site mapping","pmids":["12145326","12065665"],"confidence":"High","gaps":["Did not separate developmental from acute requirements","Tissue-specific contributions of redundant convertases not fully dissected"]},{"year":2003,"claim":"Comparing PC1/3 and PC2 on pro-CART refined the division of labor between convertases on a shared substrate, assigning intermediate-fragment generation to PC1/3.","evidence":"Transfection in neuroendocrine lines with microsequencing and knockout mouse hypothalamic extract validation","pmids":["12584191"],"confidence":"High","gaps":["Physiological role of PC1/3-generated CART intermediates not defined"]},{"year":2008,"claim":"Functional characterization of the N221D/N222D variants linked partial PC1/3 activity loss to obesity, establishing that hypomorphic alleles are pathogenic.","evidence":"In vitro enzymatic activity assays of variant PC1/3; mouse N222D mutant metabolic phenotyping","pmids":["18604207","16644867"],"confidence":"Medium","gaps":["Quantitative threshold of activity loss causing obesity not defined","Background-dependence of the assayed activity (noted later in #26) complicates interpretation"]},{"year":2009,"claim":"NMR structure and sorting assays of the C-terminal domain answered how PC1/3 is targeted to dense core granules, defining a calcium-responsive amphipathic helix anchored by Leu745.","evidence":"NMR structure determination, truncation/granule-sorting assays, calcium binding studies","pmids":["19376969"],"confidence":"High","gaps":["Granule receptor or membrane partner engaging the helix not identified","Link between aggregation and physiological sorting efficiency not quantified in vivo"]},{"year":2011,"claim":"Demonstrating oligomerization and substrate-induced activation provided a model for how PC1/3 activity is tuned by its own substrates within granules.","evidence":"Ion exchange, gel filtration, cross-linking, and fluorogenic activity assays with peptide preincubation","pmids":["21303942"],"confidence":"Medium","gaps":["Single-lab biochemical model not validated in intact granules","Physiological relevance of homodimer latency unconfirmed in vivo"]},{"year":2012,"claim":"Discovering immune phenotypes in PC1/3 knockout mice extended its function beyond endocrine prohormone processing to restraint of innate immune cytokine secretion.","evidence":"Knockout mouse LPS challenge, cytokine ELISA, macrophage isolation, electron microscopy; later proteomic M1-polarization analysis","pmids":["22396549","26330543"],"confidence":"High","gaps":["Macrophage substrate(s) processed by PC1/3 not identified","Mechanism linking PC1/3 to TLR4/MyD88 signaling not resolved"]},{"year":2012,"claim":"Mapping PAX6→Pcsk1n→PC1/3 and direct PAX6 activation of PCSK1 answered how transcriptional control couples beta-cell identity to prohormone-processing capacity.","evidence":"ChIP, EMSA, luciferase reporters, RNAi rescue, and proinsulin processing assays in mouse and human models","pmids":["19034419","23056534"],"confidence":"High","gaps":["Quantitative contribution of each arm (direct activation vs proSAAS repression) to net activity not partitioned"]},{"year":2016,"claim":"Defining ER-retention dominant-negative mutants and a splice variant established how diverse PCSK1 mutations converge on ER quality control and proteasomal degradation to reduce wild-type activity.","evidence":"Mutagenesis, co-expression of WT+mutant, ER stress markers, pulse-chase, and exon-skipping/trafficking analysis","pmids":["26207343","28271036","24890885"],"confidence":"High","gaps":["In vivo contribution of dominant-negative trans-inhibition to human disease severity not quantified"]},{"year":2017,"claim":"Human stem-cell and Prader-Willi models answered whether PC1/3 deficiency phenocopies in human neurons and connected PCSK1 to a disease gene network via SNORD116→NHLH2→PCSK1.","evidence":"CRISPR/shRNA disruption in hESC-derived hypothalamic neurons; iPSC PWS neurons and Snord116 KO mice with in vivo prohormone processing","pmids":["28132887","27941249"],"confidence":"High","gaps":["Therapeutic correction of PC1/3 deficiency in PWS not demonstrated","Relative contribution to PWS phenotype vs other affected genes unclear"]},{"year":2020,"claim":"Identifying Creb3l1, gp130 cytokines, thyroid hormone, and bile acid/TGR5/NFAT inputs answered how PC1/3 expression is matched to physiological state across cell types.","evidence":"ChIP/EMSA/reporter and in vivo viral manipulation (Creb3l1); promoter-reporter and in vivo (LIF/IL-6, T3); TGR5 agonist with NFAT inhibition; GLP-1R-dependent alpha-cell scRNA-seq","pmids":["32319174","10630414","11120670","26789236","33554958"],"confidence":"High","gaps":["Integration of competing transcriptional inputs in single cells not modeled","Whether expression changes translate to proportional active-enzyme changes not always measured"]},{"year":null,"claim":"The molecular substrate(s) and signaling mechanism by which PC1/3 restrains macrophage M1 activation and innate cytokine release remain undefined.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No immune substrate identified","Connection between PC1/3 catalytic activity and TLR4/MyD88/calcium signaling not mechanistically established"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,4,7,8,13]},{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[4,7,10]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[10,12,24]}],"localization":[{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[1,5,8,20]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[5]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[2,26,29]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,4,13,8]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[9,20]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[22,25]}],"complexes":[],"partners":["PCSK1N","PCSK2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P29120","full_name":"Neuroendocrine convertase 1","aliases":["Prohormone convertase 1","Proprotein convertase 1","PC1"],"length_aa":753,"mass_kda":84.2,"function":"Involved in the processing of hormone and other protein precursors at sites comprised of pairs of basic amino acid residues. 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This was established by co-expressing PC1 and PC2 with POMC in multiple cell lines including BSC-40, PC12, and AtT-20.\",\n      \"method\": \"Recombinant vaccinia virus co-expression of PC1/PC2 with POMC in multiple cell lines; pulse-chase biosynthetic monitoring\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct enzymatic activity assay with substrate in multiple cell types, replicated across labs\",\n      \"pmids\": [\"2023902\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"PC1 can process human prorenin to active renin specifically in cells containing secretory granules (e.g., GH4 somatomammotroph cells) but not in cells lacking granules (CHO or BSC-40 cells), demonstrating that PC1 activity on substrates requires the regulated secretory pathway environment, not merely propeptide removal.\",\n      \"method\": \"Vaccinia virus expression system; biosynthetic labeling; comparison of processing in granule-containing vs granule-lacking cell lines\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution in multiple cell types, single lab but orthogonal cell contexts\",\n      \"pmids\": [\"1597471\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"PC1 and PC2 undergo distinct biosynthetic processing: pro-PC1 (88 kDa) is cleaved to PC1 (83 kDa) in a pre-Golgi compartment (endoplasmic reticulum), with prosegment cleavage occurring early. PC1 is secreted as a glycosylated, sulphated 84 kDa form ~30 min after biosynthesis. N-glycosylation is essential for proper folding and stability of PC1; inhibition with tunicamycin causes ER degradation. Furin cannot cleave pro-PC1.\",\n      \"method\": \"Pulse-chase analysis; low temperature/brefeldin A/CCCP treatment; tunicamycin inhibition; furin co-expression; vaccinia virus expression in GH4C1 cells\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — multiple orthogonal methods (pharmacological, biochemical, genetic), single lab\",\n      \"pmids\": [\"8397508\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"PC1 performs a limited set of POMC cleavages in AtT-20 corticotropes (anterior pituitary pattern: ACTH + beta-LPH), while co-expression of PC2 in these cells enables the full intermediate pituitary cleavage pattern (alpha-MSH, beta-endorphin). PC1-dependent cleavages occur in the earliest biosynthetic steps; PC2-dependent cleavages occur in middle/late steps.\",\n      \"method\": \"Stable transfection of PC2 cDNA into AtT-20 cells; kinetic analysis of biosynthetic processing products\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct enzymatic substrate processing with temporal kinetics, replicated in multiple studies\",\n      \"pmids\": [\"8380577\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"Purified recombinant mouse PC1 is an 87-kDa calcium-dependent serine proteinase with optimal pH 5.5–6.5, preferring substrates with arginine 4 residues N-terminal to the cleavage site. PC1 undergoes autocatalytic prosegment cleavage at the RSKR motif (residues 80–83) early in biosynthesis. PC1 cleaves proenkephalin to yield a peptide B-sized fragment, demonstrating a role in proenkephalin processing.\",\n      \"method\": \"Purification from CHO cell conditioned medium; fluorogenic substrate specificity studies; pulse-chase biosynthesis; in vitro cleavage of proenkephalin\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — biochemical purification, enzymatic characterization, substrate cleavage, multiple orthogonal methods\",\n      \"pmids\": [\"8449925\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"PC1 is concentrated in a trans-Golgi network (TGN)-like compartment in AtT-20 cells and is co-transported with processed peptide (met-enkephalin) to the tips of cell processes (secretory granules). Brefeldin A treatment disrupts PC1 localization to the TGN region, confirming Golgi-dependent localization.\",\n      \"method\": \"Immunocytochemistry; dual-staining with TGN38 marker; brefeldin A treatment; subcellular fractionation\",\n      \"journal\": \"Neuroendocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct subcellular localization with pharmacological validation and co-localization, single lab\",\n      \"pmids\": [\"8115023\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"PC1 (but not PC2) can cleave prosomatostatin at the dibasic Arg-Lys site to produce somatostatin-14 when co-expressed in COS-7 cells, demonstrating that PC1 mediates dibasic cleavage of prosomatostatin in the regulated secretory pathway.\",\n      \"method\": \"Co-expression of PC1/PC2 with prosomatostatin in COS-7, AtT-20, and PC12 cells; comparison of processing products by RIA and HPLC\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct substrate processing comparison in multiple cell types, single lab\",\n      \"pmids\": [\"8095501\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"PC1/3 and PC2 are purified from bovine adrenal medulla chromaffin granules as 66-kDa Ca2+-dependent serine proteases. PC1/3 shows pH optimum of 6.5, cleaves paired basic and monobasic sites in peptide substrates (Km ~66 µM for Boc-Arg-Val-Arg-Arg-MCA), and is inhibited by EGTA, thiol-blocking reagents, and active-site-directed chloromethyl ketone inhibitors.\",\n      \"method\": \"Purification from chromaffin granules by chromatography; kinetic studies with fluorogenic peptide substrates; inhibitor profiling; immunoaffinity purification\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — biochemical purification from native tissue with full kinetic and inhibitor characterization, single lab\",\n      \"pmids\": [\"7713926\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"The 66-kDa C-terminally truncated form of PC1, generated by cleavage at Arg590-Arg591 in the C-terminal tail, is efficiently routed to secretory granules and retains full processing activity on proneurotensin. C-terminal truncation of PC1 regulates its activity—blocking this cleavage site reduces processing efficiency, suggesting C-terminal processing serves to activate the enzyme.\",\n      \"method\": \"Site-directed mutagenesis of paired basic sites in C-terminal tail; PC12 cell expression; proneurotensin processing assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis + functional substrate processing assay, single lab\",\n      \"pmids\": [\"7559585\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"The RRGDL motif in the P-domain of PC1 is critical for zymogen processing in the ER, C-terminal auto-processing to the 66-kDa form in secretory granules, and proper sorting to the regulated secretory pathway. Mutations in this motif cause increased ER degradation, prevent granule sorting, and route PC1 to the constitutive pathway, reducing POMC processing activity.\",\n      \"method\": \"Site-directed mutagenesis of RRGDL motif; vaccinia virus expression; pulse-chase analysis; alpha1-PDX inhibitor; immunocytochemistry\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — mutagenesis + multiple functional readouts (sorting, processing, localization), multiple cell types\",\n      \"pmids\": [\"9307023\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The propeptide of PC1/3 (residues 1–98) functions as a potent slow tight-binding inhibitor of active PC1/3 (Ki in low nanomolar range) and also weakly inhibits PC2 (micromolar Ki, competitive). Cleavage within residues 1–71 abrogates inhibitory potency. The propeptide is itself cleaved by mature PC1/3.\",\n      \"method\": \"Recombinant propeptide isolation from baculovirus-infected insect cells; progress curve kinetic analysis; inhibition assays with murine 71-kDa PC1/3 and human furin\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — purified proteins, kinetic characterization, mutagenesis by proteolytic mapping, single lab\",\n      \"pmids\": [\"9813073\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"PC1 propeptide cleavage occurs rapidly in the ER, yet PC1's major substrate processing occurs later (secretory granules). PC1 undergoes a C-terminal processing event that activates the enzyme. ProPC2 requires the chaperone 7B2 for generation of catalytically active PC2, but this is not required for PC1 activation.\",\n      \"method\": \"Biochemical fractionation; pulse-chase analysis; review integrating multiple experimental systems\",\n      \"journal\": \"Progress in nucleic acid research and molecular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Strong — synthesizes multiple experimental findings, but this paper is primarily a review; underlying data from multiple labs\",\n      \"pmids\": [\"10506829\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The granin-related protein proSAAS contains a C-terminal hexapeptide (SAAS CT peptide) that is a potent nanomolar inhibitor of PC1, functioning as a tight-binding competitive inhibitor. Recombinant PC1 can cleave the proSAAS CT peptide following the inhibitory hexapeptide, suggesting autoinactivation of the inhibitor.\",\n      \"method\": \"Recombinant protein inhibition assay; peptide library comparison; proteolytic mapping; structural analogy to 7B2/PC2 system\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct inhibitor kinetics with purified PC1, single lab, two approaches (cleavage + inhibition)\",\n      \"pmids\": [\"10812060\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"PC1/3 knockout mice display severe postnatal growth impairment (~60% of normal size) and multiple prohormone processing defects including: failure to cleave GHRH (leading to low GH and IGF-1), defective POMC→ACTH processing in pituitary, hyperproinsulinemia (impaired proinsulin→insulin conversion), and failure of proglucagon→GLP-1/GLP-2 conversion in intestine. This definitively establishes PC1/3 as a key neuroendocrine convertase for these specific substrates in vivo.\",\n      \"method\": \"Targeted gene disruption (knockout mouse); phenotypic analysis; hormone measurements; mRNA analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean knockout with multiple defined substrate-specific phenotypes, replicated across tissues\",\n      \"pmids\": [\"12145326\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Both PC1/3 and PC2 can generate VGF20 from the VGF precursor, while VGF10 production is preferentially mediated by PC1/3. PC1/3 cleaves at the RPR555 site to generate VGF10. The KRKRKK488 motif is the target for VGF20 generation.\",\n      \"method\": \"Ectopic expression of PC1/3 and PC2 in GH3 cells; site-directed mutagenesis of VGF cleavage sites; mass spectrometry and Edman degradation of products\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct substrate cleavage with mutagenesis validation, single lab\",\n      \"pmids\": [\"12065665\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"PC2 is more efficient than PC1/3 in generating bioactive CART I (residues 55–102) from pro-CART, while bioactive CART II (residues 62–102) is exclusively generated by PC2. PC1/3 is predominantly responsible for generating intermediate CART fragments (33–102 and 10–89). These results were confirmed in PC2, 7B2, and PC1/3 knockout mouse hypothalamic extracts.\",\n      \"method\": \"Transient transfection in neuroendocrine cell lines; microsequencing; HPLC/mass spectrometry; knockout mouse hypothalamic extract analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro cell-based assay plus in vivo knockout validation, two orthogonal methods\",\n      \"pmids\": [\"12584191\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"A mouse PC1 N222D missense mutation causes obesity, impairs autocatalytic activation of mature PC1, reduces hypothalamic alpha-MSH (through defective POMC processing), and leads to abnormal proinsulin processing and glucose intolerance. This demonstrates that a partial loss of PC1 activity is sufficient to cause obesity in mice.\",\n      \"method\": \"Mouse mutant allele characterization; enzyme activity assays; hormone measurements; metabolic phenotyping\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo mouse model with mechanistic link to autocatalytic activation and substrate processing\",\n      \"pmids\": [\"16644867\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Expression of PC1/3 (instead of PC2) in alpha-cells redirects proglucagon processing from glucagon production to GLP-1 and GLP-2 production, improving glucose tolerance and promoting beta-cell proliferation. The glucose-lowering effect was attenuated in GLP-1R-/- mice, confirming that PC1/3-mediated GLP-1 production is the key mechanism.\",\n      \"method\": \"Transplantation of alpha-cells stably expressing PC1/3 vs PC2 in mice; GLP-1R knockout mice; hormone measurements; glucose tolerance tests\",\n      \"journal\": \"Diabetes\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo cell transplantation with genetic epistasis (GLP-1R KO), multiple readouts\",\n      \"pmids\": [\"17698597\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"The PCSK1 nonsynonymous variant N221D (rs6232) significantly impairs PC1/3 catalytic activity in functional assays, linking this common variant to obesity risk.\",\n      \"method\": \"Functional enzymatic activity assays of N221D mutant PC1/3 compared to wild-type\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro enzymatic activity assay, single study; genetic association is large but the functional data is limited in the abstract\",\n      \"pmids\": [\"18604207\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"PAX6 transcription factor binds the PC1/3 (PCSK1) promoter and directly upregulates PC1/3 expression. PAX6 deficiency reduces PC1/3 expression, leading to defective proinsulin processing and abnormal glucose metabolism in both mice (Pax6 R266Stop) and humans (PAX6 R240Stop).\",\n      \"method\": \"Promoter binding assays (ChIP, EMSA); mouse and human genetic models; PC1/3 protein and mRNA measurements; proinsulin processing assays\",\n      \"journal\": \"Diabetologia\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — promoter binding demonstrated by ChIP and EMSA, validated in both mouse and human genetic models\",\n      \"pmids\": [\"19034419\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The extreme C-terminal sorting domain of PC1/3 (residues 711–753) contains two alpha-helices (722–728 and 738–750). The second helix is necessary and sufficient to target a constitutively secreted protein to dense core secretory granules (DCSGs). Leucine 745 anchors a hydrophobic patch critical for sorting. Calcium binding by this helix promotes aggregation via the hydrophobic patch, linking calcium to DCSG sorting of PC1/3.\",\n      \"method\": \"NMR structure determination; functional granule-sorting assays with truncation mutants; calcium binding studies\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR structure plus functional mutagenesis and calcium binding assays in single rigorous study\",\n      \"pmids\": [\"19376969\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"PC1/3 exists as multiple ionic forms due to oligomerization and aggregation. The most active form of 87-kDa PC1/3 is a probable homodimer with latent activity revealed by dilution. Preincubation with fluorogenic substrate or peptides containing paired basic residues stabilizes and activates PC1/3, suggesting that substrate binding regulates enzyme activity within secretory granules.\",\n      \"method\": \"Ion exchange chromatography; 2D gel electrophoresis; gel filtration; cross-linking; enzymatic activity assays with fluorogenic substrates\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple biochemical methods, single lab; mechanistic interpretation of oligomerization and substrate-dependent activation\",\n      \"pmids\": [\"21303942\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"PC1/3 is expressed in macrophages, and PC1/3 knockout mice have enlarged, disorganized spleens with depletion of dendritic cells. When challenged with LPS, PC1/3 KO mice exhibit cytokine storm with markedly elevated IL-6, IL-1β, and TNF-α plasma levels and are hypersusceptible to septic shock, demonstrating a role for PC1/3 in regulating innate immune cytokine secretion in macrophages.\",\n      \"method\": \"PC1/3 knockout mouse; LPS challenge; cytokine ELISA; immunohistochemistry; peritoneal macrophage isolation; electron microscopy\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean knockout with multiple immune phenotypes and cell-autonomous validation in isolated macrophages\",\n      \"pmids\": [\"22396549\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Pax6 directly binds the Pcsk1n (proSAAS) promoter and down-regulates its expression. Elevated proSAAS in Pax6-mutant mice inhibits PC1/3 C-terminal cleavage and activity, impairing proinsulin processing. This inhibition can be rescued by proSAAS knockdown, establishing a Pax6→Pcsk1n→PC1/3 regulatory axis in beta cells.\",\n      \"method\": \"Luciferase reporter assay; ChIP; EMSA; RNAi knockdown; enzyme activity assays; proinsulin processing measurements\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (ChIP, EMSA, reporter, rescue experiment), single lab\",\n      \"pmids\": [\"23056534\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"A heterozygous nonsense mutation PCSK1-p.Arg80* encodes a truncated propeptide that inhibits PC1/3 enzyme activity, likely through a dominant-negative mechanism not requiring strong direct physical interaction. This establishes that truncated PC1/3 propeptide can inhibit wild-type PC1/3 activity in trans.\",\n      \"method\": \"In vitro functional enzyme activity assays; co-expression studies; family segregation analysis\",\n      \"journal\": \"International journal of obesity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct enzyme activity assay with interaction studies, single lab\",\n      \"pmids\": [\"24890885\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"PC1/3 inhibition in macrophages (by knockdown or PC1/3 KO) drives macrophages toward an M1 activated phenotype characterized by filopodial extensions, TLR4/MyD88-dependent signaling, increased calcium entry, and secretion of pro-inflammatory factors. This was shown in both NR8383 macrophages and PC1/3 KO mouse macrophages.\",\n      \"method\": \"Proteomics of secretomes and intracellular proteins; PC1/3 knockdown in NR8383 cells; PC1/3 KO mouse macrophages; calcium imaging\",\n      \"journal\": \"Molecular & cellular proteomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — proteomic + KO validation, two cell models, single lab\",\n      \"pmids\": [\"26330543\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"ER-retained PC1/3 mutants (G209R and G593R) induce ER stress and exert dominant-negative effects on wild-type PC1/3, blocking prodomain cleavage and decreasing wild-type PC1/3 expression by routing it to a proteasomal degradation pathway. The N221D variant shows ~30% lower enzymatic activity than wild-type in the correct background (wild-type PC1/3, not S357G hypermorphic background).\",\n      \"method\": \"Site-directed mutagenesis; transfection of WT + mutant constructs; ER stress marker measurement; fluorogenic enzyme activity assays; pulse-chase analysis\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis with direct enzyme activity and dominant-negative mechanistic characterization, single lab\",\n      \"pmids\": [\"26207343\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"PC1/3 deficiency in Prader-Willi syndrome (PWS) is due to reduced SNORD116→NHLH2→PCSK1 expression. PC1/3 content and activity were reduced in islets, hypothalamus, and stomach of Snord116 paternal knockout mice, leading to in vivo functional defects in prohormone processing of proinsulin, pro-GHRH, and proghrelin.\",\n      \"method\": \"iPSC-derived neurons from PWS patients; Snord116 knockout mice; PC1/3 protein measurements; in vivo prohormone processing assays\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — human iPSC model plus mouse KO model with multiple in vivo substrate processing measurements\",\n      \"pmids\": [\"27941249\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"PC1/3 deficiency in human embryonic stem cell-derived hypothalamic neurons increases unprocessed POMC and decreases ratios of processed POMC-derived peptides (phenocopying PC1/3-null mice), is associated with increased melanocortin receptor expression and increased PRCP (alpha-MSH catabolism), and reduces ACTH secretion.\",\n      \"method\": \"CRISPR-Cas9 and shRNA-mediated PCSK1 disruption in hESC-derived hypothalamic neurons; POMC peptide measurements; receptor expression analysis\",\n      \"journal\": \"Stem cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — CRISPR KO and shRNA KD in human neuronal model with multiple substrate and downstream readouts\",\n      \"pmids\": [\"28132887\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"A novel PCSK1 variant c.1095+1G>A causes exon 8 skipping, complete loss of PC1/3 enzymatic activity, ER retention of the protein, and ER stress. This mechanistically links splicing mutations to ER quality control in PCSK1 disease.\",\n      \"method\": \"Exon skipping analysis; enzyme activity assay; cell trafficking experiments; ER stress marker measurement\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple functional assays (activity, trafficking, ER stress) on a single novel mutant, single lab\",\n      \"pmids\": [\"28271036\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Intestinal bile acids activate TGR5 receptor, which signals through nuclear factor of activated T cells (NFAT) to induce PC1/3 (PCSK1) gene expression in intestinal L-cells, thereby increasing GLP-1 production. This pathway mediates the antidiabetic effect of bile acid binding resins.\",\n      \"method\": \"TGR5-dependent colestimide treatment in mice; NFAT pathway inhibition; PC1/3 gene expression measurement; GLP-1 secretion assays\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pathway defined with receptor agonist and transcription factor inhibition, in vivo plus cell experiments\",\n      \"pmids\": [\"26789236\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Transcription factor Creb3l1 directly binds a G-box motif in the Pcsk1 promoter and upregulates PC1/3 expression in corticotroph and magnocellular neuroendocrine cells. Viral overexpression of Creb3l1 in supraoptic nuclei increases Pcsk1, and knockdown decreases it. No Creb3l1-Pcsk1 relationship was found in the neurointermediate lobe, indicating cell-type specificity.\",\n      \"method\": \"RNA-sequencing; viral overexpression and knockdown in vivo; in vitro promoter luciferase assay; ChIP; EMSA\",\n      \"journal\": \"Journal of neuroendocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (ChIP, EMSA, reporter, in vivo viral manipulation), single lab\",\n      \"pmids\": [\"32319174\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"GLP-1 receptor (GLP-1R) agonist liraglutide increases PC1/3 (PCSK1) mRNA expression in a subcluster of pancreatic alpha-cells in a beta-cell GLP-1R-dependent manner. This is associated with increased bihormonal insulin+/glucagon+ cells and expression of other beta-cell-like genes, establishing a GLP-1-mediated paracrine pathway that induces alpha-cell PC1/3 expression.\",\n      \"method\": \"Mouse model with beta cell-specific GLP-1R; scRNA-seq (DART-Seq); IHC; liraglutide treatment\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — GLP-1R KO epistasis with single-cell transcriptomics and IHC, single lab\",\n      \"pmids\": [\"33554958\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Leukemia inhibitory factor (LIF) and IL-6 (cytokines signaling through gp130/JAK-STAT) upregulate PC1 protein and mRNA in AtT-20 corticotroph cells. This upregulation increases POMC processing to ACTH. LPS administration in vivo also increases pituitary PC1 and POMC mRNA. PC1 promoter activity is directly increased by LIF/IL-6, demonstrating transcriptional regulation by gp130 cytokines.\",\n      \"method\": \"AtT-20 cell treatment with LIF/IL-6; PC1 protein and mRNA measurement; PC1 promoter-luciferase transfection assay; in vivo LPS administration\",\n      \"journal\": \"Molecular and cellular endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reporter assay plus protein/mRNA measurements and in vivo validation, single lab\",\n      \"pmids\": [\"10630414\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Thyroid hormone (T3) negatively regulates PC1 transcription through multiple thyroid hormone response elements (TREs) located at -10 to +19 bp relative to the transcription start site of the hPC1 promoter. TR-alpha1 binds this region as monomer, homodimer, and heterodimer with RXR-beta. Hypothyroidism increases and hyperthyroidism decreases pituitary PC1 mRNA.\",\n      \"method\": \"Promoter deletion analysis; EMSA with purified TR-alpha1 and RXR-beta; point mutation of TREs; pituitary PC1 mRNA measurement in hypothyroid/hyperthyroid rats\",\n      \"journal\": \"American journal of physiology. Endocrinology and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — EMSA with purified proteins, mutagenesis of TREs, in vivo hormonal manipulation, multiple methods\",\n      \"pmids\": [\"11120670\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"proSAAS expression in AtT-20 cells inhibits both C-terminal processing of PC1 and POMC processing under pulse-chase conditions. The PC1 propeptide expressed in trans also inhibits PC1 C-terminal processing and POMC cleavage, but SAAS CT peptide-propeptide chimeras are less effective in the regulated pathway.\",\n      \"method\": \"AtT-20 cell transfection with proSAAS and propeptide constructs; pulse-chase analysis; POMC and PC1 processing assays\",\n      \"journal\": \"The Journal of endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct in-cell inhibition assays, single lab\",\n      \"pmids\": [\"15283695\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PCSK1-encoded PC1/3 is a calcium-dependent serine endoprotease (subtilisin/kexin family) that undergoes autocatalytic propeptide cleavage in the ER (at RSKR80–83), followed by C-terminal truncation at Arg590-Arg591 in secretory granules to generate the maximally active 66-kDa form; it is sorted to dense core secretory granules via a C-terminal amphipathic helix (residues 738–750) through a calcium-sensitive hydrophobic mechanism; its activity is regulated by endogenous inhibitors (its own propeptide and proSAAS/SAAS CT peptide) and by oligomerization/substrate binding; it cleaves multiple prohormones (POMC→ACTH, proinsulin→insulin, proglucagon→GLP-1/GLP-2, proGHRH, proghrelin, proenkephalin, prosomatostatin, VGF) preferentially at paired basic sites (with preference for Arg at P4), with substrate selectivity and processing efficiency dependent on the regulated secretory pathway; its transcription is regulated by gp130 cytokines (LIF/IL-6), thyroid hormone (negative regulation via TR-alpha1 binding at the proximal promoter), PAX6 (positive), Creb3l1 (positive), and bile acid/TGR5/NFAT signaling; it also plays a role in innate immune regulation in macrophages; loss-of-function in humans and mice causes obesity, hyperproinsulinemia, dwarfism, and multiple endocrinopathies.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"PCSK1 encodes PC1/3, a calcium-dependent serine endoprotease of the subtilisin/kexin class that is the principal neuroendocrine convertase of the regulated secretory pathway, cleaving prohormones at paired basic residues with a preference for arginine at the P4 position [#0, #4, #7]. Its activity is built through ordered maturation: autocatalytic removal of the prosegment at the RSKR80\\u201383 motif occurs early in the ER, while major substrate processing and a subsequent C-terminal truncation at Arg590-Arg591 that activates the enzyme occur later in dense core secretory granules [#4, #8]; N-glycosylation and an intact RRGDL P-domain motif are required for folding, zymogen processing, and granule sorting, with misfolding-prone mutants routed to ER-associated degradation [#2, #9, #26]. Sorting to dense core secretory granules is mediated by a C-terminal amphipathic helix (residues 738\\u2013750) anchored by Leu745, whose calcium-promoted aggregation links calcium sensing to granule targeting [#20]. Within granules, processing requires the regulated secretory environment rather than propeptide removal alone [#1], and PC1/3 activity is restrained by endogenous inhibitors\\u2014its own propeptide and the proSAAS-derived SAAS CT peptide\\u2014and modulated by oligomerization and substrate-induced activation [#10, #12, #21]. PC1/3 cleaves a defined set of substrates including POMC\\u2192ACTH/\\u03b2-LPH, proinsulin, proGHRH, proglucagon\\u2192GLP-1/GLP-2, proenkephalin, prosomatostatin, proneurotensin, VGF, and pro-CART [#0, #4, #6, #8, #13, #14, #15], and loss of PC1/3 in mice and humans causes obesity, hyperproinsulinemia, dwarfism, and multiple endocrinopathies [#13, #16, #24]; reduced expression underlies prohormone-processing deficits in Prader-Willi syndrome through a SNORD116\\u2192NHLH2\\u2192PCSK1 axis [#27]. Its transcription is controlled by PAX6, Creb3l1, gp130 cytokines (LIF/IL-6), thyroid hormone (negative, via TR-alpha1), and bile acid/TGR5/NFAT signaling [#19, #31, #33, #34, #30]. Beyond its endocrine role, PC1/3 restrains innate immune activation, with its loss driving macrophages toward an M1 phenotype and cytokine storm [#22, #25].\",\n  \"teleology\": [\n    {\n      \"year\": 1991,\n      \"claim\": \"Establishing that PC1/3 has a distinct, limited prohormone cleavage specificity answered whether convertases divide labor on a shared substrate, distinguishing it from the broader-acting PC2.\",\n      \"evidence\": \"Vaccinia co-expression of PC1/PC2 with POMC in multiple cell lines with pulse-chase monitoring\",\n      \"pmids\": [\"2023902\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not define the structural basis of cleavage-site selectivity\", \"Specificity tested only on POMC\"]\n    },\n    {\n      \"year\": 1992,\n      \"claim\": \"Showing PC1/3 processes substrates only in granule-containing cells established that its activity depends on the regulated secretory pathway environment, not merely propeptide removal.\",\n      \"evidence\": \"Vaccinia expression with biosynthetic labeling comparing granule-containing vs granule-lacking cells using prorenin\",\n      \"pmids\": [\"1597471\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular features of the granule environment required were not defined\", \"Did not identify cofactors supplied by granules\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Biochemical purification and biosynthetic mapping defined PC1/3 as a calcium-dependent acidic-pH serine protease with P4 arginine preference that undergoes early autocatalytic prosegment removal at RSKR80\\u201383, and localized it to the TGN/granule pathway.\",\n      \"evidence\": \"Recombinant purification, fluorogenic substrate kinetics, pulse-chase, tunicamycin/BFA treatments, immunocytochemistry across cell lines\",\n      \"pmids\": [\"8397508\", \"8380577\", \"8449925\", \"8095501\", \"8115023\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"C-terminal processing event not yet mechanistically defined\", \"Full in vivo substrate repertoire unknown\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Identifying C-terminal truncation at Arg590-Arg591 as an activating, granule-routing event answered how PC1/3 reaches maximal activity after ER prosegment removal.\",\n      \"evidence\": \"Site-directed mutagenesis of C-terminal paired basic sites with proneurotensin processing in PC12 cells; purification from chromaffin granules with kinetics\",\n      \"pmids\": [\"7559585\", \"7713926\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the protease performing C-terminal cleavage in granules not resolved\", \"Quantitative contribution of truncation vs oligomerization to activation unclear\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Defining the RRGDL P-domain motif as essential for zymogen processing, C-terminal autoprocessing, and granule sorting linked a single structural element to the full maturation program.\",\n      \"evidence\": \"Site-directed mutagenesis, vaccinia expression, pulse-chase, alpha1-PDX inhibition, immunocytochemistry\",\n      \"pmids\": [\"9307023\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-level mechanism of how RRGDL coordinates folding and sorting not resolved\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Characterizing the PC1/3 propeptide and the proSAAS-derived SAAS CT peptide as nanomolar inhibitors answered how the enzyme is held latent and activated in a controlled manner.\",\n      \"evidence\": \"Recombinant propeptide and proSAAS peptide inhibition kinetics, progress-curve analysis, proteolytic mapping\",\n      \"pmids\": [\"9813073\", \"10812060\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo stoichiometry of inhibitor-to-enzyme not established\", \"Timing of inhibitor inactivation within granules not directly observed\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Knockout of PC1/3 in mice answered which prohormones depend on it in vivo, establishing it as the key convertase for GHRH, POMC, proinsulin, and proglucagon and explaining the growth/endocrine phenotypes.\",\n      \"evidence\": \"Targeted gene disruption with hormone measurements and tissue-specific processing analysis; complementary VGF cleavage-site mapping\",\n      \"pmids\": [\"12145326\", \"12065665\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not separate developmental from acute requirements\", \"Tissue-specific contributions of redundant convertases not fully dissected\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Comparing PC1/3 and PC2 on pro-CART refined the division of labor between convertases on a shared substrate, assigning intermediate-fragment generation to PC1/3.\",\n      \"evidence\": \"Transfection in neuroendocrine lines with microsequencing and knockout mouse hypothalamic extract validation\",\n      \"pmids\": [\"12584191\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological role of PC1/3-generated CART intermediates not defined\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Functional characterization of the N221D/N222D variants linked partial PC1/3 activity loss to obesity, establishing that hypomorphic alleles are pathogenic.\",\n      \"evidence\": \"In vitro enzymatic activity assays of variant PC1/3; mouse N222D mutant metabolic phenotyping\",\n      \"pmids\": [\"18604207\", \"16644867\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Quantitative threshold of activity loss causing obesity not defined\", \"Background-dependence of the assayed activity (noted later in #26) complicates interpretation\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"NMR structure and sorting assays of the C-terminal domain answered how PC1/3 is targeted to dense core granules, defining a calcium-responsive amphipathic helix anchored by Leu745.\",\n      \"evidence\": \"NMR structure determination, truncation/granule-sorting assays, calcium binding studies\",\n      \"pmids\": [\"19376969\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Granule receptor or membrane partner engaging the helix not identified\", \"Link between aggregation and physiological sorting efficiency not quantified in vivo\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Demonstrating oligomerization and substrate-induced activation provided a model for how PC1/3 activity is tuned by its own substrates within granules.\",\n      \"evidence\": \"Ion exchange, gel filtration, cross-linking, and fluorogenic activity assays with peptide preincubation\",\n      \"pmids\": [\"21303942\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab biochemical model not validated in intact granules\", \"Physiological relevance of homodimer latency unconfirmed in vivo\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Discovering immune phenotypes in PC1/3 knockout mice extended its function beyond endocrine prohormone processing to restraint of innate immune cytokine secretion.\",\n      \"evidence\": \"Knockout mouse LPS challenge, cytokine ELISA, macrophage isolation, electron microscopy; later proteomic M1-polarization analysis\",\n      \"pmids\": [\"22396549\", \"26330543\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Macrophage substrate(s) processed by PC1/3 not identified\", \"Mechanism linking PC1/3 to TLR4/MyD88 signaling not resolved\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Mapping PAX6\\u2192Pcsk1n\\u2192PC1/3 and direct PAX6 activation of PCSK1 answered how transcriptional control couples beta-cell identity to prohormone-processing capacity.\",\n      \"evidence\": \"ChIP, EMSA, luciferase reporters, RNAi rescue, and proinsulin processing assays in mouse and human models\",\n      \"pmids\": [\"19034419\", \"23056534\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Quantitative contribution of each arm (direct activation vs proSAAS repression) to net activity not partitioned\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Defining ER-retention dominant-negative mutants and a splice variant established how diverse PCSK1 mutations converge on ER quality control and proteasomal degradation to reduce wild-type activity.\",\n      \"evidence\": \"Mutagenesis, co-expression of WT+mutant, ER stress markers, pulse-chase, and exon-skipping/trafficking analysis\",\n      \"pmids\": [\"26207343\", \"28271036\", \"24890885\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo contribution of dominant-negative trans-inhibition to human disease severity not quantified\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Human stem-cell and Prader-Willi models answered whether PC1/3 deficiency phenocopies in human neurons and connected PCSK1 to a disease gene network via SNORD116\\u2192NHLH2\\u2192PCSK1.\",\n      \"evidence\": \"CRISPR/shRNA disruption in hESC-derived hypothalamic neurons; iPSC PWS neurons and Snord116 KO mice with in vivo prohormone processing\",\n      \"pmids\": [\"28132887\", \"27941249\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Therapeutic correction of PC1/3 deficiency in PWS not demonstrated\", \"Relative contribution to PWS phenotype vs other affected genes unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Identifying Creb3l1, gp130 cytokines, thyroid hormone, and bile acid/TGR5/NFAT inputs answered how PC1/3 expression is matched to physiological state across cell types.\",\n      \"evidence\": \"ChIP/EMSA/reporter and in vivo viral manipulation (Creb3l1); promoter-reporter and in vivo (LIF/IL-6, T3); TGR5 agonist with NFAT inhibition; GLP-1R-dependent alpha-cell scRNA-seq\",\n      \"pmids\": [\"32319174\", \"10630414\", \"11120670\", \"26789236\", \"33554958\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Integration of competing transcriptional inputs in single cells not modeled\", \"Whether expression changes translate to proportional active-enzyme changes not always measured\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The molecular substrate(s) and signaling mechanism by which PC1/3 restrains macrophage M1 activation and innate cytokine release remain undefined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No immune substrate identified\", \"Connection between PC1/3 catalytic activity and TLR4/MyD88/calcium signaling not mechanistically established\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 4, 7, 8, 13]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [4, 7, 10]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [10, 12, 24]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [1, 5, 8, 20]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [5]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [2, 26, 29]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 4, 13, 8]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [9, 20]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [22, 25]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"PCSK1N\", \"PCSK2\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}