{"gene":"PCSK2","run_date":"2026-04-29T11:37:58","timeline":{"discoveries":[{"year":1991,"finding":"PC2 is a prohormone convertase that cleaves POMC at distinct pairs of basic residues: PC2 cleaves POMC at all five pairs of basic residues analyzed (generating beta-endorphin, N-terminally extended ACTH, and alpha-MSH/desacetyl-alpha-MSH), whereas PC1 preferentially cleaves only two sites, indicating PC2 has broader substrate specificity than PC1.","method":"Recombinant vaccinia virus coexpression of PC1/PC2 with POMC in BSC-40, PC12, and AtT-20 cells; monitoring of cleavage products","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — direct functional assay in multiple cell lines, replicated across labs","pmids":["2023902"],"is_preprint":false},{"year":1991,"finding":"PC2 and PC3 are Kex2-like endoproteases that accurately cleave POMC at paired basic residues in the regulated secretory pathway of mammalian cells; PC2 coexpression with PC3 results in efficient conversion of beta-lipotropin to gamma-lipotropin and beta-endorphin.","method":"Recombinant vaccinia virus expression in processing-deficient BSC-40 cells and bovine adrenomedullary chromaffin cells; analysis of POMC processing products","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — direct reconstitution in mammalian cells with defined cleavage site analysis","pmids":["1647029"],"is_preprint":false},{"year":1992,"finding":"PC2 selectively cleaves proinsulin at the C-peptide-A-chain junction (type II proinsulin processing activity), establishing its role as the endogenous type 2 proinsulin endopeptidase; PC2 immunoreactivity co-elutes with type 2 proinsulin endopeptidase activity and anti-PC2 antisera specifically immunoprecipitates type 2 activity from insulin granule extracts.","method":"Recombinant vaccinia virus coinfection in COS-7 cells; co-elution on chromatography; immunoprecipitation of enzymatic activity; N-terminal sequencing of purified PC2","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1/2 — multiple orthogonal methods (functional assay, co-elution, immunoprecipitation, sequencing) in a single study","pmids":["1528899","1634553"],"is_preprint":false},{"year":1991,"finding":"PC2 expressed in Xenopus oocytes displays calcium-dependent endopeptidase activity with a pH optimum of 5.5, cleaving dibasic (but not monobasic) substrates; activity is inhibited by EDTA and serine protease inhibitors consistent with a calcium-dependent serine protease.","method":"In vitro mRNA synthesis and microinjection into Xenopus oocytes; fluorogenic substrate assay with inhibitor profiling","journal":"FEBS letters","confidence":"High","confidence_rationale":"Tier 1 — in vitro enzymatic characterization with defined substrates and inhibitors","pmids":["2060650"],"is_preprint":false},{"year":1991,"finding":"PC2 biosynthesis involves an 88-kDa prepropolypeptide processed to a 75-kDa glycosylated precursor (proPC2) and then to the 68-kDa mature enzyme; prosegment cleavage of glycosylated proPC2 occurs in the Golgi apparatus, while unglycosylated proPC2 can be cleaved in a pre-Golgi/ER compartment. Furin cannot cleave either proPC1 or proPC2.","method":"Pulse-chase analysis in GH4C1 cells infected with vaccinia recombinants; brefeldin A, low-temperature, and CCCP treatments; coexpression with furin; endoglycosidase H digestion","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1/2 — multiple pharmacological and biochemical approaches in a single study","pmids":["8397508"],"is_preprint":false},{"year":1993,"finding":"ProPC2 undergoes slow autocatalytic maturation; cleavage of the 75-kDa proPC2 to the 68-kDa mature enzyme occurs after the sequence Arg-Lys-Lys-Arg84; deletion of this tetrabasic sequence or mutation of the active site Asp142→Asn blocks cleavage; intermolecular cleavage by wild-type PC2 of catalytically inactive PC2M4 also demonstrated.","method":"Site-directed mutagenesis; cell-free Xenopus egg extract translation/translocation system; pulse-chase analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — active-site mutagenesis and in vitro reconstitution establishing autocatalytic mechanism","pmids":["8276855"],"is_preprint":false},{"year":1994,"finding":"7B2 is a neuroendocrine chaperone that transiently and specifically interacts with proPC2 in the ER early in the secretory pathway; this interaction is required for PC2 activation, as 7B2 prevents premature activation of proPC2. Dissociation coincides with cleavage of 7B2 and proPC2 in later secretory compartments.","method":"In vitro association assay with recombinant 7B2 and pituitary proteins; coimmunoprecipitation with metabolic labeling; pulse-chase analysis","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 — reciprocal coimmunoprecipitation and pulse-chase in multiple cell systems, replicated","pmids":["7913882"],"is_preprint":false},{"year":1994,"finding":"7B2 is a potent endogenous inhibitor of PC2 (but not PC1/PC3); intact 7B2 prevents proPC2 cleavage in vitro; a C-terminal peptide of 7B2 (CT peptide) is specifically inhibitory; the 21-kDa cleavage product of 7B2 is virtually inactive as an inhibitor.","method":"In vitro enzyme inhibition assay with recombinant 7B2; comparison of 7B2 domains","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — direct in vitro enzyme inhibition assay","pmids":["8016065"],"is_preprint":false},{"year":1994,"finding":"PC2 processes POMC in a strict temporal order: PC2 expression in AtT-20 cells (which normally express only PC1) confers all intermediate-pituitary cleavages of POMC but does not affect the earliest processing steps; PC2-dependent cleavages appear only in middle and late biosynthetic steps.","method":"Stable transfection of AtT-20 corticotropes with full-length PC2 cDNA; biosynthetic pulse-chase analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — defined gain-of-function with temporal kinetic analysis","pmids":["8380577"],"is_preprint":false},{"year":1994,"finding":"PC2 is the key endoprotease responsible for proglucagon processing to glucagon in alpha cells; antisense RNA knockdown of PC2 in alpha TC1-6 cells inhibits both PC2 production and proglucagon processing concomitantly.","method":"Antisense RNA expression in alpha TC1-6 cells; continuous and pulse-chase labeling; PC2 immunoblot","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — loss-of-function (antisense) with direct biochemical readout of substrate processing","pmids":["8159732"],"is_preprint":false},{"year":1995,"finding":"The C-terminal region of 7B2 responsible for PC2 inhibition was mapped to a short segment containing Lys171-Lys172; single mutations at this dibasic site strongly diminish and double mutations abolish inhibitory potency of 7B2 toward PC2.","method":"In vitro mutagenesis; prokaryotic expression of mutant 7B2 proteins; in vitro PC1/PC3 and PC2 enzyme inhibition assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstituted inhibition assay with structure-function mutagenesis","pmids":["7782286"],"is_preprint":false},{"year":1995,"finding":"ProPC2 undergoes calcium- and acid pH-dependent aggregation; the pro-form aggregates at pH 7.0 and 6.5 in a calcium-dependent manner and associates with membranes at pH 5.5. The mature PC2 remains soluble under the same conditions. The N-terminal propeptide (amino acids 57-85) can compete pro-PC2 away from membranes.","method":"In vitro Xenopus oocyte translation/translocation; sedimentation assays at varying pH/calcium; peptide competition assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with defined biochemical conditions","pmids":["8034613"],"is_preprint":false},{"year":1995,"finding":"ProPC2 maturation has a pH optimum of 5.5-6.0 and requires high calcium (K0.5 = 2-4 mM), whereas proPC3 matures rapidly at pH 7.0-8.0 without additional calcium; site-directed mutagenesis of active site Asp→Asn suggests proPC2 cleavage is catalyzed by the mature 68-kDa PC2 (intermolecular).","method":"In vitro Xenopus egg extract translation/translocation; pH and calcium titration; active-site mutagenesis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with mutagenesis","pmids":["7836407"],"is_preprint":false},{"year":1995,"finding":"7B2 specifically binds proPC2 (but not furin, PC1, PACE4, or PC5) in the ER; binding is calcium-dependent; the intact hexapeptide RRKRRS of pro-7B2 is critical, and mutations of RR152 abolish while R151 or S156 mutations greatly diminish binding.","method":"Coimmunoprecipitation; site-directed mutagenesis of 7B2; calcium chelation experiments","journal":"Journal of neurochemistry","confidence":"High","confidence_rationale":"Tier 2 — reciprocal coimmunoprecipitation with systematic mutagenesis","pmids":["7722516"],"is_preprint":false},{"year":1995,"finding":"PC2 differentially processes proglucagon compared to PC3: PC2 generates glucagon and leaves the C-terminal half (MPGF) unprocessed (alpha-cell phenotype), whereas PC3 generates GLP-1 and GLP-2 (intestinal L-cell phenotype). In AtT-20/PC2 cells, glicentin is efficiently processed to glucagon by PC2.","method":"Vaccinia virus vector expression; stable transfection of AtT-20/PC2 cells; analysis of proglucagon processing products in multiple cell lines","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1/2 — direct reconstitution in multiple cell systems with defined product analysis","pmids":["7592866"],"is_preprint":false},{"year":1996,"finding":"PC2 cleaves the 7B2 CT peptide at an internal Lys-Lys site in secretory granules, inactivating it; carboxypeptidase E further removes terminal lysines from the cleavage product, providing an efficient mechanism for intracellular inactivation of the CT peptide inhibitor.","method":"Metabolic labeling; immunoprecipitation; in vitro incubation of radiolabeled CT peptide with purified recombinant PC2; RIA; gel filtration","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with purified recombinant PC2 plus cell labeling studies","pmids":["8643504"],"is_preprint":false},{"year":1996,"finding":"The C-terminal domain of PC2 is sufficient to sort it into the regulated secretory pathway (dense-core granules); chimeras of furin with the C-terminal domain of PC2 (F-S-P) were retained intracellularly and released in a regulated manner, whereas truncated furin lacking this domain was not stored.","method":"Chimeric protein construction; stable transfection in AtT-20 cells; immunofluorescence and immuno-electron microscopy","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — domain-swap chimeras with ultrastructural localization","pmids":["8810291"],"is_preprint":false},{"year":1997,"finding":"PC2 (SPC2) is essential in vivo for the processing of proglucagon in alpha cells, prosomatostatin in delta cells, and partially for proinsulin in beta cells; PC2 knockout mice have chronic fasting hypoglycemia, glucagon deficiency, and severely impaired processing of proglucagon, prosomatostatin, and proinsulin in pancreatic islets.","method":"PC2 gene knockout mice (neomycin insertion in exon 3); glucose tolerance tests; pancreatic hormone measurements; islet morphology","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — clean gene knockout with multiple defined biochemical and physiological phenotypes","pmids":["9192619"],"is_preprint":false},{"year":1997,"finding":"PC2 alone, together with carboxypeptidase E, is sufficient for the pancreatic alpha-cell processing pathway of proglucagon to glucagon; purified recombinant PC2 converts oxyntomodulin (a processing intermediate) quantitatively to glucagon in vitro.","method":"Adenovirus-mediated coexpression of proglucagon and PC2 in GH4C1 cells; in vitro incubation of oxyntomodulin with purified recombinant PC2 plus carboxypeptidase E","journal":"FEBS letters","confidence":"High","confidence_rationale":"Tier 1 — purified recombinant enzyme in vitro reconstitution","pmids":["9287128"],"is_preprint":false},{"year":1998,"finding":"PC2 knockout mice show incomplete proinsulin processing: absence of PC2 results in ~60% circulating proinsulin and accumulation of des-31,32 proinsulin (4-5-fold elevation), consistent with PC2's specific role in cleaving proinsulin at the C-peptide/A-chain junction (Lys64-Arg65). PC3 preferentially initiates processing at the B-chain/C-peptide junction.","method":"PC2 knockout mice; pulse-chase experiments in isolated islets; HPLC and RIA analysis of proinsulin processing intermediates","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — clean KO with defined biochemical phenotype and pulse-chase kinetics","pmids":["9452465"],"is_preprint":false},{"year":1998,"finding":"The PC2 proregion is required but not sufficient for 7B2 binding; the P domain is essential for PC2 structural stability; the C-terminal domain is not involved in 7B2 binding; a single substitution Tyr194→Asp in the catalytic domain prevents 7B2 binding and blocks PC2 activation.","method":"Sequential deletions, site-directed mutagenesis, and domain-swapping between PC2 and PC1; expression in AtT-20 cells; coimmunoprecipitation and enzymatic activity assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1/2 — systematic mutagenesis and domain-swap with functional readouts","pmids":["9422782"],"is_preprint":false},{"year":1999,"finding":"ProPC2 maturation is an autocatalytic, intramolecular, pH-driven process: conversion is most extensive between pH 4.0 and 4.9; it is not calcium-dependent; it is not inhibited by the 7B2 CT peptide; and the rate is similar across a 10-fold range of zymogen concentration, indicating intramolecular cleavage.","method":"Purified recombinant proPC2 from CHO cells; pH titration; calcium chelation (EDTA); enzyme concentration-response; N-terminal sequencing of mature enzyme","journal":"Archives of biochemistry and biophysics","confidence":"High","confidence_rationale":"Tier 1 — biochemical reconstitution with purified recombinant enzyme and multiple parameters","pmids":["9989936"],"is_preprint":false},{"year":1999,"finding":"ProPC1 propeptide cleavage occurs in the ER (rapidly), while proPC2 exits the ER without propeptide cleavage, in complex with 7B2; PC2 propeptide removal occurs in maturing secretory granules, likely through autocatalysis, and requires prior association with 7B2 for generation of active enzyme.","method":"Review synthesizing pulse-chase, temperature-block, and coimmunoprecipitation data from multiple studies","journal":"Progress in nucleic acid research and molecular biology","confidence":"Medium","confidence_rationale":"Tier 2 — comprehensive synthesis of prior experimental work, not new primary data","pmids":["10506829"],"is_preprint":false},{"year":2000,"finding":"PC2 is essential for prodynorphin processing in vivo: PC2 knockout mice lack Dyn A-8 completely and show substantial reductions in Dyn A-17 and Dyn B-13, indicating PC2 mediates monobasic cleavage events generating these opioid peptides from prodynorphin.","method":"PC2 gene disruption in mice; enzymological and immunological assays of dynorphin peptides in brain extracts; HPLC; Western blots for other convertases","journal":"Journal of neurochemistry","confidence":"High","confidence_rationale":"Tier 2 — clean KO with specific biochemical phenotype for defined substrates","pmids":["10987860"],"is_preprint":false},{"year":2001,"finding":"PC2 is essential for NH2-terminal processing of proIAPP in vivo; PC2 null mice show accumulation of a ~6-kDa NH2-terminally extended proIAPP intermediate (only COOH-terminally processed), with no detectable mature 4-kDa IAPP, demonstrating that PC3 processes proIAPP only at the COOH-terminal site while PC2 processes the NH2-terminal site.","method":"Western blot and immunofluorescence of islets from PC2 null mice; antisera to NH2- and COOH-terminal flanking regions","journal":"Diabetes","confidence":"High","confidence_rationale":"Tier 2 — clean KO with site-specific cleavage analysis using domain-specific antisera","pmids":["11246872"],"is_preprint":false},{"year":2002,"finding":"PC2 is involved in processing of the VGF precursor: ectopic expression of PC2 in GH3 cells generates VGF20; the KRKRKK488 motif in VGF sequence is the PC2 (and PC1/3) target generating VGF20.","method":"Ectopic expression of PC2 and PC1/3 in GH3 cells; site-directed mutagenesis of VGF sequence; mass spectrometry and Edman degradation of products","journal":"Journal of neurochemistry","confidence":"High","confidence_rationale":"Tier 1/2 — gain-of-function with site-directed mutagenesis and mass spectrometric product identification","pmids":["12065665"],"is_preprint":false},{"year":2002,"finding":"PC2 plays a critical role in processing of pro-neurotensin/neuromedin N in mouse brain; PC2 knockout mice show 15% decrease in neurotensin and 50% decrease in neuromedin N brain levels, with regionally variable loss most pronounced in medial preoptic area and hypothalamic nuclei.","method":"PC2 knockout mice; radioimmunoassay of neurotensin and neuromedin N; immunohistochemistry","journal":"Journal of neurochemistry","confidence":"High","confidence_rationale":"Tier 2 — clean KO with specific substrate measurement","pmids":["12358783"],"is_preprint":false},{"year":2003,"finding":"PC2 is critical for alpha-MSH production in vivo: PC2-deficient mice show essentially complete loss of alpha-MSH in pituitary and brain, with accumulation of ACTH, ACTH-containing intermediates, POMC precursor, and increased beta-endorphin1-31, confirming ACTH as a PC2 substrate for alpha-MSH production.","method":"PC2 knockout mice; RIA for alpha-MSH (non-cross-reacting with POMC, ACTH, beta-endorphin); RP-HPLC; Western blotting with anti-ACTH and anti-beta-endorphin","journal":"Journal of neurochemistry","confidence":"High","confidence_rationale":"Tier 2 — clean KO with multiple substrate/product measurements","pmids":["12859669"],"is_preprint":false},{"year":2003,"finding":"PC2 is exclusively responsible for generating bioactive CART II (residues 62-102) from pro-CART; PC2 is more efficient than PC1/3 in generating CART I (55-102); in vivo studies in PC2 knockout mice confirm PC2 is required for CART II production. PC1/3 predominantly generates intermediate CART fragments.","method":"Transient transfection in cell lines with/without endogenous PC2 or PC1/3; knockout mouse hypothalamic extract analysis; microsequencing; HPLC/MS product identification","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — combined cell-based and KO mouse approaches with MS product identification","pmids":["12584191"],"is_preprint":false},{"year":2004,"finding":"PC2 initiates proglucagon processing at the interdomain site Lys70-Arg71, which is a prerequisite for efficient glucagon production; mutation of this site (K70Q-R71Q or R71A) reduces glucagon production 5-fold by PC2 and increases unprocessed precursor; PC2 can generate glucagon from glicentin but not efficiently from the major proglucagon fragment.","method":"Cotransfection studies in GH4C1 cells with wild-type and mutant proglucagon; HPLC analysis of processing products","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 2 — site-directed mutagenesis of substrate with quantitative product analysis","pmids":["15528303"],"is_preprint":false},{"year":2006,"finding":"Impaired NH2-terminal processing of proIAPP by PC2 leads to amyloid formation and cell death; rescue of PC2 expression (via adenoviral PC2) in PC2-null mouse islets expressing human proIAPP restores NH2-terminal processing, decreases amyloid formation, and reduces beta-cell apoptosis.","method":"Adenoviral PC2 overexpression in PC2-null islets expressing human proIAPP; TUNEL assay; immunoblotting for proIAPP processing intermediates","journal":"Diabetes","confidence":"High","confidence_rationale":"Tier 2 — rescue experiment in defined KO background with biochemical and cell death readouts","pmids":["16873681"],"is_preprint":false},{"year":1999,"finding":"The Drosophila PC2 homolog amontillado (amon) is required for hatching behavior; expression is restricted to specific neurons late in embryogenesis, and larvae lacking amontillado chromosomal region fail to hatch normally; ubiquitous amon expression rescues behavior, but a catalytic histidine mutant cannot, establishing that proteolytic activity is required.","method":"Drosophila genetics (deficiency mapping); in situ hybridization; behavioral analysis; ubiquitous rescue with wild-type vs. catalytic mutant amontillado","journal":"The Journal of neuroscience : the official journal of the Society for Neuroscience","confidence":"High","confidence_rationale":"Tier 2 — genetic loss-of-function with catalytic mutant rescue, ortholog with conserved mechanism","pmids":["10436051"],"is_preprint":false},{"year":2003,"finding":"Drosophila amontillado (PC2 homolog) is required during embryogenesis and larval development; amon mutants display partial embryonic lethality and arrest at first-to-second instar larval molt; heat-shock rescue confirms developmental requirement for amon protease activity.","method":"EMS mutagenesis; genetic complementation; heat-shock rescue; developmental analysis","journal":"Genetics","confidence":"High","confidence_rationale":"Tier 2 — clean genetic loss-of-function with rescue; conserved PC2 ortholog","pmids":["12586710"],"is_preprint":false},{"year":2013,"finding":"PC2 (together with its chaperone 7B2) cleaves FGF-23 in osteoblasts; decreased 7B2·PC2 activity in hyp-mice results in reduced FGF-23 cleavage, increased Fgf-23 mRNA (via impaired BMP1 activation from proBMP1 and consequent reduced DMP1 cleavage). Hexa-D-arginine treatment of hyp-mice restores 7B2·PC2 activity, normalizing FGF-23 and rescuing the HYP phenotype.","method":"Transfection of murine osteoblasts with PC2 and 7B2; siRNA knockdown of 7B2; analysis of FGF-23, proBMP1/BMP1, DMP1 in hyp-mouse bone; pharmacological rescue with Hexa-D-arginine","journal":"Journal of bone and mineral research","confidence":"High","confidence_rationale":"Tier 2 — gain-of-function, loss-of-function, and in vivo pharmacological rescue with defined mechanistic pathway","pmids":["22886699"],"is_preprint":false}],"current_model":"PCSK2 (PC2) is a neuroendocrine-specific calcium-dependent subtilisin-like serine endoprotease that resides in dense-core secretory granules, where it undergoes pH-driven autocatalytic zymogen activation facilitated by its obligate chaperone 7B2 (which first stabilizes proPC2 in the ER, then inhibits it via a C-terminal peptide that PC2 itself cleaves to self-disinhibit), and cleaves diverse prohormone precursors—including POMC, proinsulin, proglucagon, prosomatostatin, prodynorphin, proIAPP, pro-neurotensin, pro-CART, VGF, and FGF-23—at dibasic (and some monobasic) sites to generate bioactive peptide hormones and neuropeptides in a tissue-specific manner distinct from the related convertase PC1/PC3."},"narrative":{"teleology":[{"year":1991,"claim":"Establishing that PC2 is a functional prohormone convertase that cleaves POMC at multiple dibasic sites with broader specificity than PC1 answered the fundamental question of which enzymes carry out neuropeptide precursor processing in the regulated secretory pathway.","evidence":"Recombinant vaccinia virus coexpression of PC2 with POMC in BSC-40, PC12, AtT-20, and chromaffin cells with cleavage product analysis","pmids":["2023902","1647029"],"confidence":"High","gaps":["Crystal structure of PC2-substrate complex not determined","Relative contributions of PC1 vs PC2 to each POMC cleavage site in vivo not resolved"]},{"year":1991,"claim":"Enzymatic characterization revealed PC2 is a calcium-dependent serine endoprotease with an acidic pH optimum (~5.5) and selectivity for dibasic substrates, establishing the biochemical framework for understanding where in the secretory pathway PC2 functions.","evidence":"Fluorogenic substrate assays with inhibitor profiling in Xenopus oocyte expression system","pmids":["2060650"],"confidence":"High","gaps":["Kinetic parameters for individual prohormone substrates not yet determined","Structural basis for dibasic selectivity unknown"]},{"year":1992,"claim":"Identifying PC2 as the specific type 2 proinsulin endopeptidase that cleaves at the C-peptide/A-chain junction resolved the long-standing question of which enzyme mediates this specific processing step in insulin biosynthesis.","evidence":"Co-elution chromatography, immunoprecipitation of enzymatic activity, and N-terminal sequencing of purified PC2 from insulin granules","pmids":["1528899","1634553"],"confidence":"High","gaps":["In vivo confirmation in knockout model not yet available at this time"]},{"year":1993,"claim":"Demonstrating that proPC2 maturation is autocatalytic — requiring the tetrabasic propeptide cleavage site and an intact active-site Asp — and can proceed intermolecularly established the zymogen activation mechanism of PC2.","evidence":"Site-directed mutagenesis of cleavage site and active-site residues; cell-free Xenopus egg extract reconstitution; pulse-chase analysis","pmids":["8276855","7836407","8397508"],"confidence":"High","gaps":["Whether intramolecular or intermolecular cleavage dominates in vivo unresolved at this stage"]},{"year":1994,"claim":"Discovery that 7B2 serves as both an obligate ER chaperone for proPC2 folding/transport and a potent endogenous inhibitor of mature PC2 (via its C-terminal peptide) revealed a dual-function regulatory mechanism unique among prohormone convertases.","evidence":"Reciprocal coimmunoprecipitation with metabolic labeling and pulse-chase; in vitro enzyme inhibition assays with recombinant 7B2 and domain fragments","pmids":["7913882","8016065"],"confidence":"High","gaps":["Stoichiometry of the proPC2–7B2 complex not defined","Structural basis of 7B2 chaperone versus inhibitor functions not resolved"]},{"year":1994,"claim":"Showing that PC2 is the enzyme responsible for proglucagon processing to glucagon in pancreatic α-cells — generating the α-cell cleavage pattern distinct from PC1/PC3's intestinal L-cell pattern — established PC2 as the molecular basis for tissue-specific differential proglucagon processing.","evidence":"Antisense PC2 knockdown in αTC1-6 cells; vaccinia expression in AtT-20/PC2 cells with product analysis","pmids":["8159732","7592866"],"confidence":"High","gaps":["In vivo confirmation in whole animal not yet demonstrated"]},{"year":1995,"claim":"Mapping the 7B2 inhibitory determinant to Lys171-Lys172, identifying 7B2's proPC2-binding hexapeptide (RRKRRS), and characterizing pH/calcium-dependent proPC2 aggregation defined the molecular rules governing the 7B2–PC2 interaction and proPC2 sorting into secretory granules.","evidence":"Structure-function mutagenesis of 7B2; coimmunoprecipitation with calcium chelation; in vitro sedimentation assays at varying pH/calcium","pmids":["7782286","7722516","8034613"],"confidence":"High","gaps":["Three-dimensional structure of 7B2–proPC2 complex not determined","Relative contribution of aggregation versus receptor-mediated sorting to granule targeting unknown"]},{"year":1996,"claim":"Discovering that PC2 cleaves the 7B2 CT inhibitory peptide at its internal Lys-Lys site within granules — with carboxypeptidase E trimming the product — established a self-disinhibition mechanism whereby PC2 destroys its own inhibitor to become fully active.","evidence":"In vitro incubation of radiolabeled CT peptide with purified recombinant PC2; metabolic labeling and immunoprecipitation in cells","pmids":["8643504"],"confidence":"High","gaps":["Temporal kinetics of CT peptide destruction relative to substrate processing onset not quantified"]},{"year":1996,"claim":"Demonstrating that the C-terminal domain of PC2 is sufficient for regulated secretory pathway sorting resolved which structural element directs PC2 to dense-core granules rather than the constitutive pathway.","evidence":"Furin–PC2 C-terminal chimeras stably expressed in AtT-20 cells; immunofluorescence and immuno-electron microscopy","pmids":["8810291"],"confidence":"High","gaps":["Identity of the sorting receptor or lipid-binding mechanism not identified"]},{"year":1997,"claim":"PC2 knockout mice revealed that PC2 is essential in vivo for processing proglucagon (to glucagon), prosomatostatin, and proinsulin (at the C-peptide/A-chain junction), causing chronic fasting hypoglycemia and validating decades of in vitro substrate assignments.","evidence":"Gene disruption by neomycin insertion in exon 3; glucose tolerance tests; pancreatic hormone measurements; islet morphology; pulse-chase in isolated islets","pmids":["9192619","9452465"],"confidence":"High","gaps":["CNS neuropeptide processing phenotypes not yet characterized","Compensatory changes in other convertases not fully assessed"]},{"year":1999,"claim":"Purified proPC2 reconstitution demonstrated that zymogen activation is intramolecular, pH-driven (optimum 4.0–4.9), calcium-independent, and not inhibited by the 7B2 CT peptide, settling the debate over the mechanism and compartment of proPC2 maturation.","evidence":"Purified recombinant proPC2 from CHO cells; pH titration; EDTA chelation; concentration-independence kinetics","pmids":["9989936"],"confidence":"High","gaps":["Earlier in vitro data suggested calcium dependence (PMID:7836407); discrepancy with purified system not fully reconciled"]},{"year":2000,"claim":"PC2 knockout mice extended the substrate repertoire to prodynorphin in the brain, showing PC2 mediates monobasic cleavage events required for Dyn A-8 and efficient Dyn A-17/Dyn B-13 production, demonstrating PC2 function beyond classical dibasic processing.","evidence":"PC2 knockout mice; HPLC and immunological quantification of dynorphin peptides in brain extracts","pmids":["10987860"],"confidence":"High","gaps":["Which monobasic sites PC2 cleaves directly versus indirectly not established"]},{"year":2001,"claim":"Establishing that PC2 is specifically required for N-terminal proIAPP processing — and that loss of this processing leads to amyloid-prone intermediate accumulation — linked PC2 activity to islet amyloid pathogenesis.","evidence":"Western blot and immunofluorescence with site-specific antisera in PC2-null mouse islets","pmids":["11246872"],"confidence":"High","gaps":["Whether reduced PC2 activity contributes to human type 2 diabetes amyloidosis not tested"]},{"year":2002,"claim":"Identification of VGF and pro-neurotensin/neuromedin N as additional PC2 substrates broadened the known neuropeptide substrate repertoire and demonstrated region-specific processing deficits in the brain.","evidence":"Ectopic PC2 expression with MS product identification (VGF); PC2-KO mouse brain RIA for neurotensin/neuromedin N","pmids":["12065665","12358783"],"confidence":"High","gaps":["Full CNS peptidomic profiling of PC2-KO not performed","Physiological consequences of VGF processing deficits unknown"]},{"year":2003,"claim":"Demonstrating near-complete loss of α-MSH and exclusive PC2 responsibility for CART II production in knockout mice established PC2 as the essential convertase for melanocortin and CART neuropeptide maturation, with implications for energy homeostasis.","evidence":"PC2-KO mice; RIA, RP-HPLC, and Western blot for α-MSH, ACTH, β-endorphin; cell-based and KO mouse analysis for CART peptides with MS identification","pmids":["12859669","12584191"],"confidence":"High","gaps":["Whether PC2 deficiency-related obesity is solely α-MSH-dependent not resolved"]},{"year":2004,"claim":"Mutagenesis of the proglucagon interdomain site Lys70-Arg71 showed that PC2 initiates processing at this site as a prerequisite for efficient glucagon liberation, defining the ordered cleavage pathway for glucagon biogenesis.","evidence":"Cotransfection of wild-type and mutant proglucagon with PC2 in GH4C1 cells; HPLC product analysis","pmids":["15528303"],"confidence":"High","gaps":["Whether this ordered pathway operates identically in primary α-cells not confirmed"]},{"year":2006,"claim":"Rescue of PC2 expression in PC2-null islets expressing human proIAPP restored N-terminal processing, decreased amyloid deposition, and reduced β-cell apoptosis, establishing a causal link between PC2 processing activity and protection against islet amyloid toxicity.","evidence":"Adenoviral PC2 overexpression in PC2-null islets expressing human proIAPP; TUNEL assay; immunoblotting","pmids":["16873681"],"confidence":"High","gaps":["Relevance to physiological PC2 levels in human β-cells not demonstrated"]},{"year":2013,"claim":"Identifying FGF-23 and proBMP1 as PC2/7B2-dependent substrates in osteoblasts expanded PC2 function beyond classical neuroendocrine processing to bone mineral metabolism, and pharmacological restoration of 7B2·PC2 activity rescued the HYP phenotype.","evidence":"PC2/7B2 transfection and 7B2 siRNA in osteoblasts; analysis of FGF-23, proBMP1, and DMP1 in hyp-mouse bone; Hexa-D-arginine rescue in vivo","pmids":["22886699"],"confidence":"High","gaps":["Whether PC2 processes FGF-23 directly or indirectly via BMP1 activation not fully delineated","Expression level and regulation of PC2 in osteoblasts under physiological conditions unclear"]},{"year":null,"claim":"Key unresolved questions include the structural basis of PC2-7B2 interaction, the complete in vivo peptidomic substrate repertoire, the mechanism by which PC2 achieves monobasic cleavage selectivity, and whether reduced PC2 activity contributes to human metabolic or neurodegenerative disease.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No crystal structure of PC2 alone or in complex with 7B2","Comprehensive peptidomics of PC2-KO tissues not completed","Human genetic studies linking PCSK2 variants to disease phenotypes are sparse"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,1,2,3,5,9,14,17,18,23,24,25,26,27,28,29,33]},{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[0,3,5]}],"localization":[{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[11,16]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[4,6,13]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[4]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,1,2,5,8,9,14,17,18,19,23,24,25,26,27,28,29,30,33]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[17,27,33]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[16]}],"complexes":["proPC2–7B2 complex"],"partners":["SCG5","CPE","POMC","GCG","INS","PDYN","VGF","FGF23"],"other_free_text":[]},"mechanistic_narrative":"PCSK2 (PC2) is a neuroendocrine-specific, calcium-dependent subtilisin-like serine endoprotease that cleaves prohormone and proneuropeptide precursors at dibasic residues within the acidic environment of dense-core secretory granules, generating bioactive peptides including glucagon, α-MSH, β-endorphin, insulin, somatostatin, dynorphins, CART, neuromedin N, and IAPP in a tissue-specific manner distinct from the related convertase PC1/PC3 [PMID:2023902, PMID:9192619, PMID:7592866, PMID:10987860]. PC2 is synthesized as an inactive 75-kDa zymogen (proPC2) that requires the neuroendocrine-specific chaperone 7B2 for ER exit and proper folding; 7B2 binds proPC2 via a calcium-dependent interaction in the ER, accompanies it through the secretory pathway, and its C-terminal peptide potently inhibits mature PC2 until PC2 itself cleaves this inhibitory peptide in secretory granules, providing an elegant self-disinhibition mechanism [PMID:7913882, PMID:8016065, PMID:8643504]. Autocatalytic propeptide removal occurs intramolecularly at acidic pH (optimum ~5.0) within maturing secretory granules, and the C-terminal domain of PC2 is sufficient for sorting into the regulated secretory pathway [PMID:9989936, PMID:8810291]. PC2-null mice exhibit fasting hypoglycemia, near-complete loss of glucagon and α-MSH, severely impaired processing of proinsulin, prosomatostatin, prodynorphin, pro-IAPP, and pro-CART, and accumulation of unprocessed proIAPP that promotes amyloid formation and β-cell apoptosis [PMID:9192619, PMID:12859669, PMID:16873681]."},"prefetch_data":{"uniprot":{"accession":"P16519","full_name":"Neuroendocrine convertase 2","aliases":["KEX2-like endoprotease 2","Prohormone convertase 2","Proprotein convertase 2","PC2"],"length_aa":638,"mass_kda":70.6,"function":"Serine endopeptidase which is involved in the processing of hormone and other protein precursors at sites comprised of pairs of basic amino acid residues. 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SNAP25","url":"https://www.omim.org/entry/600322"},{"mim_id":"300197","title":"ATPase, H+ TRANSPORTING, LYSOSOMAL, ACCESSORY PROTEIN 1; ATP6AP1","url":"https://www.omim.org/entry/300197"},{"mim_id":"188040","title":"THROMBOMODULIN; THBD","url":"https://www.omim.org/entry/188040"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Vesicles","reliability":"Approved"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"adrenal gland","ntpm":26.9},{"tissue":"retina","ntpm":33.2},{"tissue":"thyroid gland","ntpm":54.2}],"url":"https://www.proteinatlas.org/search/PCSK2"},"hgnc":{"alias_symbol":["PC2","SPC2"],"prev_symbol":["NEC2"]},"alphafold":{"accession":"P16519","domains":[{"cath_id":"3.30.70.850","chopping":"30-104","consensus_level":"high","plddt":93.046,"start":30,"end":104},{"cath_id":"3.40.50.200","chopping":"122-401","consensus_level":"high","plddt":96.0583,"start":122,"end":401},{"cath_id":"2.60.120.260","chopping":"466-595","consensus_level":"high","plddt":97.4514,"start":466,"end":595}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P16519","model_url":"https://alphafold.ebi.ac.uk/files/AF-P16519-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P16519-F1-predicted_aligned_error_v6.png","plddt_mean":92.5},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PCSK2","jax_strain_url":"https://www.jax.org/strain/search?query=PCSK2"},"sequence":{"accession":"P16519","fasta_url":"https://rest.uniprot.org/uniprotkb/P16519.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P16519/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P16519"}},"corpus_meta":[{"pmid":"2023902","id":"PMC_2023902","title":"PC1 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monitoring of cleavage products\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct functional assay in multiple cell lines, replicated across labs\",\n      \"pmids\": [\"2023902\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"PC2 and PC3 are Kex2-like endoproteases that accurately cleave POMC at paired basic residues in the regulated secretory pathway of mammalian cells; PC2 coexpression with PC3 results in efficient conversion of beta-lipotropin to gamma-lipotropin and beta-endorphin.\",\n      \"method\": \"Recombinant vaccinia virus expression in processing-deficient BSC-40 cells and bovine adrenomedullary chromaffin cells; analysis of POMC processing products\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct reconstitution in mammalian cells with defined cleavage site analysis\",\n      \"pmids\": [\"1647029\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"PC2 selectively cleaves proinsulin at the C-peptide-A-chain junction (type II proinsulin processing activity), establishing its role as the endogenous type 2 proinsulin endopeptidase; PC2 immunoreactivity co-elutes with type 2 proinsulin endopeptidase activity and anti-PC2 antisera specifically immunoprecipitates type 2 activity from insulin granule extracts.\",\n      \"method\": \"Recombinant vaccinia virus coinfection in COS-7 cells; co-elution on chromatography; immunoprecipitation of enzymatic activity; N-terminal sequencing of purified PC2\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — multiple orthogonal methods (functional assay, co-elution, immunoprecipitation, sequencing) in a single study\",\n      \"pmids\": [\"1528899\", \"1634553\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"PC2 expressed in Xenopus oocytes displays calcium-dependent endopeptidase activity with a pH optimum of 5.5, cleaving dibasic (but not monobasic) substrates; activity is inhibited by EDTA and serine protease inhibitors consistent with a calcium-dependent serine protease.\",\n      \"method\": \"In vitro mRNA synthesis and microinjection into Xenopus oocytes; fluorogenic substrate assay with inhibitor profiling\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro enzymatic characterization with defined substrates and inhibitors\",\n      \"pmids\": [\"2060650\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"PC2 biosynthesis involves an 88-kDa prepropolypeptide processed to a 75-kDa glycosylated precursor (proPC2) and then to the 68-kDa mature enzyme; prosegment cleavage of glycosylated proPC2 occurs in the Golgi apparatus, while unglycosylated proPC2 can be cleaved in a pre-Golgi/ER compartment. Furin cannot cleave either proPC1 or proPC2.\",\n      \"method\": \"Pulse-chase analysis in GH4C1 cells infected with vaccinia recombinants; brefeldin A, low-temperature, and CCCP treatments; coexpression with furin; endoglycosidase H digestion\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — multiple pharmacological and biochemical approaches in a single study\",\n      \"pmids\": [\"8397508\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"ProPC2 undergoes slow autocatalytic maturation; cleavage of the 75-kDa proPC2 to the 68-kDa mature enzyme occurs after the sequence Arg-Lys-Lys-Arg84; deletion of this tetrabasic sequence or mutation of the active site Asp142→Asn blocks cleavage; intermolecular cleavage by wild-type PC2 of catalytically inactive PC2M4 also demonstrated.\",\n      \"method\": \"Site-directed mutagenesis; cell-free Xenopus egg extract translation/translocation system; pulse-chase analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — active-site mutagenesis and in vitro reconstitution establishing autocatalytic mechanism\",\n      \"pmids\": [\"8276855\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"7B2 is a neuroendocrine chaperone that transiently and specifically interacts with proPC2 in the ER early in the secretory pathway; this interaction is required for PC2 activation, as 7B2 prevents premature activation of proPC2. Dissociation coincides with cleavage of 7B2 and proPC2 in later secretory compartments.\",\n      \"method\": \"In vitro association assay with recombinant 7B2 and pituitary proteins; coimmunoprecipitation with metabolic labeling; pulse-chase analysis\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal coimmunoprecipitation and pulse-chase in multiple cell systems, replicated\",\n      \"pmids\": [\"7913882\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"7B2 is a potent endogenous inhibitor of PC2 (but not PC1/PC3); intact 7B2 prevents proPC2 cleavage in vitro; a C-terminal peptide of 7B2 (CT peptide) is specifically inhibitory; the 21-kDa cleavage product of 7B2 is virtually inactive as an inhibitor.\",\n      \"method\": \"In vitro enzyme inhibition assay with recombinant 7B2; comparison of 7B2 domains\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct in vitro enzyme inhibition assay\",\n      \"pmids\": [\"8016065\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"PC2 processes POMC in a strict temporal order: PC2 expression in AtT-20 cells (which normally express only PC1) confers all intermediate-pituitary cleavages of POMC but does not affect the earliest processing steps; PC2-dependent cleavages appear only in middle and late biosynthetic steps.\",\n      \"method\": \"Stable transfection of AtT-20 corticotropes with full-length PC2 cDNA; biosynthetic pulse-chase analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — defined gain-of-function with temporal kinetic analysis\",\n      \"pmids\": [\"8380577\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"PC2 is the key endoprotease responsible for proglucagon processing to glucagon in alpha cells; antisense RNA knockdown of PC2 in alpha TC1-6 cells inhibits both PC2 production and proglucagon processing concomitantly.\",\n      \"method\": \"Antisense RNA expression in alpha TC1-6 cells; continuous and pulse-chase labeling; PC2 immunoblot\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function (antisense) with direct biochemical readout of substrate processing\",\n      \"pmids\": [\"8159732\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"The C-terminal region of 7B2 responsible for PC2 inhibition was mapped to a short segment containing Lys171-Lys172; single mutations at this dibasic site strongly diminish and double mutations abolish inhibitory potency of 7B2 toward PC2.\",\n      \"method\": \"In vitro mutagenesis; prokaryotic expression of mutant 7B2 proteins; in vitro PC1/PC3 and PC2 enzyme inhibition assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted inhibition assay with structure-function mutagenesis\",\n      \"pmids\": [\"7782286\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"ProPC2 undergoes calcium- and acid pH-dependent aggregation; the pro-form aggregates at pH 7.0 and 6.5 in a calcium-dependent manner and associates with membranes at pH 5.5. The mature PC2 remains soluble under the same conditions. The N-terminal propeptide (amino acids 57-85) can compete pro-PC2 away from membranes.\",\n      \"method\": \"In vitro Xenopus oocyte translation/translocation; sedimentation assays at varying pH/calcium; peptide competition assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with defined biochemical conditions\",\n      \"pmids\": [\"8034613\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"ProPC2 maturation has a pH optimum of 5.5-6.0 and requires high calcium (K0.5 = 2-4 mM), whereas proPC3 matures rapidly at pH 7.0-8.0 without additional calcium; site-directed mutagenesis of active site Asp→Asn suggests proPC2 cleavage is catalyzed by the mature 68-kDa PC2 (intermolecular).\",\n      \"method\": \"In vitro Xenopus egg extract translation/translocation; pH and calcium titration; active-site mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with mutagenesis\",\n      \"pmids\": [\"7836407\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"7B2 specifically binds proPC2 (but not furin, PC1, PACE4, or PC5) in the ER; binding is calcium-dependent; the intact hexapeptide RRKRRS of pro-7B2 is critical, and mutations of RR152 abolish while R151 or S156 mutations greatly diminish binding.\",\n      \"method\": \"Coimmunoprecipitation; site-directed mutagenesis of 7B2; calcium chelation experiments\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal coimmunoprecipitation with systematic mutagenesis\",\n      \"pmids\": [\"7722516\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"PC2 differentially processes proglucagon compared to PC3: PC2 generates glucagon and leaves the C-terminal half (MPGF) unprocessed (alpha-cell phenotype), whereas PC3 generates GLP-1 and GLP-2 (intestinal L-cell phenotype). In AtT-20/PC2 cells, glicentin is efficiently processed to glucagon by PC2.\",\n      \"method\": \"Vaccinia virus vector expression; stable transfection of AtT-20/PC2 cells; analysis of proglucagon processing products in multiple cell lines\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — direct reconstitution in multiple cell systems with defined product analysis\",\n      \"pmids\": [\"7592866\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"PC2 cleaves the 7B2 CT peptide at an internal Lys-Lys site in secretory granules, inactivating it; carboxypeptidase E further removes terminal lysines from the cleavage product, providing an efficient mechanism for intracellular inactivation of the CT peptide inhibitor.\",\n      \"method\": \"Metabolic labeling; immunoprecipitation; in vitro incubation of radiolabeled CT peptide with purified recombinant PC2; RIA; gel filtration\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with purified recombinant PC2 plus cell labeling studies\",\n      \"pmids\": [\"8643504\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"The C-terminal domain of PC2 is sufficient to sort it into the regulated secretory pathway (dense-core granules); chimeras of furin with the C-terminal domain of PC2 (F-S-P) were retained intracellularly and released in a regulated manner, whereas truncated furin lacking this domain was not stored.\",\n      \"method\": \"Chimeric protein construction; stable transfection in AtT-20 cells; immunofluorescence and immuno-electron microscopy\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — domain-swap chimeras with ultrastructural localization\",\n      \"pmids\": [\"8810291\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"PC2 (SPC2) is essential in vivo for the processing of proglucagon in alpha cells, prosomatostatin in delta cells, and partially for proinsulin in beta cells; PC2 knockout mice have chronic fasting hypoglycemia, glucagon deficiency, and severely impaired processing of proglucagon, prosomatostatin, and proinsulin in pancreatic islets.\",\n      \"method\": \"PC2 gene knockout mice (neomycin insertion in exon 3); glucose tolerance tests; pancreatic hormone measurements; islet morphology\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean gene knockout with multiple defined biochemical and physiological phenotypes\",\n      \"pmids\": [\"9192619\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"PC2 alone, together with carboxypeptidase E, is sufficient for the pancreatic alpha-cell processing pathway of proglucagon to glucagon; purified recombinant PC2 converts oxyntomodulin (a processing intermediate) quantitatively to glucagon in vitro.\",\n      \"method\": \"Adenovirus-mediated coexpression of proglucagon and PC2 in GH4C1 cells; in vitro incubation of oxyntomodulin with purified recombinant PC2 plus carboxypeptidase E\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — purified recombinant enzyme in vitro reconstitution\",\n      \"pmids\": [\"9287128\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"PC2 knockout mice show incomplete proinsulin processing: absence of PC2 results in ~60% circulating proinsulin and accumulation of des-31,32 proinsulin (4-5-fold elevation), consistent with PC2's specific role in cleaving proinsulin at the C-peptide/A-chain junction (Lys64-Arg65). PC3 preferentially initiates processing at the B-chain/C-peptide junction.\",\n      \"method\": \"PC2 knockout mice; pulse-chase experiments in isolated islets; HPLC and RIA analysis of proinsulin processing intermediates\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined biochemical phenotype and pulse-chase kinetics\",\n      \"pmids\": [\"9452465\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The PC2 proregion is required but not sufficient for 7B2 binding; the P domain is essential for PC2 structural stability; the C-terminal domain is not involved in 7B2 binding; a single substitution Tyr194→Asp in the catalytic domain prevents 7B2 binding and blocks PC2 activation.\",\n      \"method\": \"Sequential deletions, site-directed mutagenesis, and domain-swapping between PC2 and PC1; expression in AtT-20 cells; coimmunoprecipitation and enzymatic activity assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — systematic mutagenesis and domain-swap with functional readouts\",\n      \"pmids\": [\"9422782\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"ProPC2 maturation is an autocatalytic, intramolecular, pH-driven process: conversion is most extensive between pH 4.0 and 4.9; it is not calcium-dependent; it is not inhibited by the 7B2 CT peptide; and the rate is similar across a 10-fold range of zymogen concentration, indicating intramolecular cleavage.\",\n      \"method\": \"Purified recombinant proPC2 from CHO cells; pH titration; calcium chelation (EDTA); enzyme concentration-response; N-terminal sequencing of mature enzyme\",\n      \"journal\": \"Archives of biochemistry and biophysics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — biochemical reconstitution with purified recombinant enzyme and multiple parameters\",\n      \"pmids\": [\"9989936\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"ProPC1 propeptide cleavage occurs in the ER (rapidly), while proPC2 exits the ER without propeptide cleavage, in complex with 7B2; PC2 propeptide removal occurs in maturing secretory granules, likely through autocatalysis, and requires prior association with 7B2 for generation of active enzyme.\",\n      \"method\": \"Review synthesizing pulse-chase, temperature-block, and coimmunoprecipitation data from multiple studies\",\n      \"journal\": \"Progress in nucleic acid research and molecular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — comprehensive synthesis of prior experimental work, not new primary data\",\n      \"pmids\": [\"10506829\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"PC2 is essential for prodynorphin processing in vivo: PC2 knockout mice lack Dyn A-8 completely and show substantial reductions in Dyn A-17 and Dyn B-13, indicating PC2 mediates monobasic cleavage events generating these opioid peptides from prodynorphin.\",\n      \"method\": \"PC2 gene disruption in mice; enzymological and immunological assays of dynorphin peptides in brain extracts; HPLC; Western blots for other convertases\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with specific biochemical phenotype for defined substrates\",\n      \"pmids\": [\"10987860\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"PC2 is essential for NH2-terminal processing of proIAPP in vivo; PC2 null mice show accumulation of a ~6-kDa NH2-terminally extended proIAPP intermediate (only COOH-terminally processed), with no detectable mature 4-kDa IAPP, demonstrating that PC3 processes proIAPP only at the COOH-terminal site while PC2 processes the NH2-terminal site.\",\n      \"method\": \"Western blot and immunofluorescence of islets from PC2 null mice; antisera to NH2- and COOH-terminal flanking regions\",\n      \"journal\": \"Diabetes\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with site-specific cleavage analysis using domain-specific antisera\",\n      \"pmids\": [\"11246872\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"PC2 is involved in processing of the VGF precursor: ectopic expression of PC2 in GH3 cells generates VGF20; the KRKRKK488 motif in VGF sequence is the PC2 (and PC1/3) target generating VGF20.\",\n      \"method\": \"Ectopic expression of PC2 and PC1/3 in GH3 cells; site-directed mutagenesis of VGF sequence; mass spectrometry and Edman degradation of products\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — gain-of-function with site-directed mutagenesis and mass spectrometric product identification\",\n      \"pmids\": [\"12065665\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"PC2 plays a critical role in processing of pro-neurotensin/neuromedin N in mouse brain; PC2 knockout mice show 15% decrease in neurotensin and 50% decrease in neuromedin N brain levels, with regionally variable loss most pronounced in medial preoptic area and hypothalamic nuclei.\",\n      \"method\": \"PC2 knockout mice; radioimmunoassay of neurotensin and neuromedin N; immunohistochemistry\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with specific substrate measurement\",\n      \"pmids\": [\"12358783\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"PC2 is critical for alpha-MSH production in vivo: PC2-deficient mice show essentially complete loss of alpha-MSH in pituitary and brain, with accumulation of ACTH, ACTH-containing intermediates, POMC precursor, and increased beta-endorphin1-31, confirming ACTH as a PC2 substrate for alpha-MSH production.\",\n      \"method\": \"PC2 knockout mice; RIA for alpha-MSH (non-cross-reacting with POMC, ACTH, beta-endorphin); RP-HPLC; Western blotting with anti-ACTH and anti-beta-endorphin\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with multiple substrate/product measurements\",\n      \"pmids\": [\"12859669\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"PC2 is exclusively responsible for generating bioactive CART II (residues 62-102) from pro-CART; PC2 is more efficient than PC1/3 in generating CART I (55-102); in vivo studies in PC2 knockout mice confirm PC2 is required for CART II production. PC1/3 predominantly generates intermediate CART fragments.\",\n      \"method\": \"Transient transfection in cell lines with/without endogenous PC2 or PC1/3; knockout mouse hypothalamic extract analysis; microsequencing; HPLC/MS product identification\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — combined cell-based and KO mouse approaches with MS product identification\",\n      \"pmids\": [\"12584191\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"PC2 initiates proglucagon processing at the interdomain site Lys70-Arg71, which is a prerequisite for efficient glucagon production; mutation of this site (K70Q-R71Q or R71A) reduces glucagon production 5-fold by PC2 and increases unprocessed precursor; PC2 can generate glucagon from glicentin but not efficiently from the major proglucagon fragment.\",\n      \"method\": \"Cotransfection studies in GH4C1 cells with wild-type and mutant proglucagon; HPLC analysis of processing products\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — site-directed mutagenesis of substrate with quantitative product analysis\",\n      \"pmids\": [\"15528303\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Impaired NH2-terminal processing of proIAPP by PC2 leads to amyloid formation and cell death; rescue of PC2 expression (via adenoviral PC2) in PC2-null mouse islets expressing human proIAPP restores NH2-terminal processing, decreases amyloid formation, and reduces beta-cell apoptosis.\",\n      \"method\": \"Adenoviral PC2 overexpression in PC2-null islets expressing human proIAPP; TUNEL assay; immunoblotting for proIAPP processing intermediates\",\n      \"journal\": \"Diabetes\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — rescue experiment in defined KO background with biochemical and cell death readouts\",\n      \"pmids\": [\"16873681\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"The Drosophila PC2 homolog amontillado (amon) is required for hatching behavior; expression is restricted to specific neurons late in embryogenesis, and larvae lacking amontillado chromosomal region fail to hatch normally; ubiquitous amon expression rescues behavior, but a catalytic histidine mutant cannot, establishing that proteolytic activity is required.\",\n      \"method\": \"Drosophila genetics (deficiency mapping); in situ hybridization; behavioral analysis; ubiquitous rescue with wild-type vs. catalytic mutant amontillado\",\n      \"journal\": \"The Journal of neuroscience : the official journal of the Society for Neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic loss-of-function with catalytic mutant rescue, ortholog with conserved mechanism\",\n      \"pmids\": [\"10436051\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Drosophila amontillado (PC2 homolog) is required during embryogenesis and larval development; amon mutants display partial embryonic lethality and arrest at first-to-second instar larval molt; heat-shock rescue confirms developmental requirement for amon protease activity.\",\n      \"method\": \"EMS mutagenesis; genetic complementation; heat-shock rescue; developmental analysis\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean genetic loss-of-function with rescue; conserved PC2 ortholog\",\n      \"pmids\": [\"12586710\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"PC2 (together with its chaperone 7B2) cleaves FGF-23 in osteoblasts; decreased 7B2·PC2 activity in hyp-mice results in reduced FGF-23 cleavage, increased Fgf-23 mRNA (via impaired BMP1 activation from proBMP1 and consequent reduced DMP1 cleavage). Hexa-D-arginine treatment of hyp-mice restores 7B2·PC2 activity, normalizing FGF-23 and rescuing the HYP phenotype.\",\n      \"method\": \"Transfection of murine osteoblasts with PC2 and 7B2; siRNA knockdown of 7B2; analysis of FGF-23, proBMP1/BMP1, DMP1 in hyp-mouse bone; pharmacological rescue with Hexa-D-arginine\",\n      \"journal\": \"Journal of bone and mineral research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — gain-of-function, loss-of-function, and in vivo pharmacological rescue with defined mechanistic pathway\",\n      \"pmids\": [\"22886699\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PCSK2 (PC2) is a neuroendocrine-specific calcium-dependent subtilisin-like serine endoprotease that resides in dense-core secretory granules, where it undergoes pH-driven autocatalytic zymogen activation facilitated by its obligate chaperone 7B2 (which first stabilizes proPC2 in the ER, then inhibits it via a C-terminal peptide that PC2 itself cleaves to self-disinhibit), and cleaves diverse prohormone precursors—including POMC, proinsulin, proglucagon, prosomatostatin, prodynorphin, proIAPP, pro-neurotensin, pro-CART, VGF, and FGF-23—at dibasic (and some monobasic) sites to generate bioactive peptide hormones and neuropeptides in a tissue-specific manner distinct from the related convertase PC1/PC3.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"PCSK2 (PC2) is a neuroendocrine-specific, calcium-dependent subtilisin-like serine endoprotease that cleaves prohormone and proneuropeptide precursors at dibasic residues within the acidic environment of dense-core secretory granules, generating bioactive peptides including glucagon, α-MSH, β-endorphin, insulin, somatostatin, dynorphins, CART, neuromedin N, and IAPP in a tissue-specific manner distinct from the related convertase PC1/PC3 [PMID:2023902, PMID:9192619, PMID:7592866, PMID:10987860]. PC2 is synthesized as an inactive 75-kDa zymogen (proPC2) that requires the neuroendocrine-specific chaperone 7B2 for ER exit and proper folding; 7B2 binds proPC2 via a calcium-dependent interaction in the ER, accompanies it through the secretory pathway, and its C-terminal peptide potently inhibits mature PC2 until PC2 itself cleaves this inhibitory peptide in secretory granules, providing an elegant self-disinhibition mechanism [PMID:7913882, PMID:8016065, PMID:8643504]. Autocatalytic propeptide removal occurs intramolecularly at acidic pH (optimum ~5.0) within maturing secretory granules, and the C-terminal domain of PC2 is sufficient for sorting into the regulated secretory pathway [PMID:9989936, PMID:8810291]. PC2-null mice exhibit fasting hypoglycemia, near-complete loss of glucagon and α-MSH, severely impaired processing of proinsulin, prosomatostatin, prodynorphin, pro-IAPP, and pro-CART, and accumulation of unprocessed proIAPP that promotes amyloid formation and β-cell apoptosis [PMID:9192619, PMID:12859669, PMID:16873681].\",\n  \"teleology\": [\n    {\n      \"year\": 1991,\n      \"claim\": \"Establishing that PC2 is a functional prohormone convertase that cleaves POMC at multiple dibasic sites with broader specificity than PC1 answered the fundamental question of which enzymes carry out neuropeptide precursor processing in the regulated secretory pathway.\",\n      \"evidence\": \"Recombinant vaccinia virus coexpression of PC2 with POMC in BSC-40, PC12, AtT-20, and chromaffin cells with cleavage product analysis\",\n      \"pmids\": [\"2023902\", \"1647029\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Crystal structure of PC2-substrate complex not determined\", \"Relative contributions of PC1 vs PC2 to each POMC cleavage site in vivo not resolved\"]\n    },\n    {\n      \"year\": 1991,\n      \"claim\": \"Enzymatic characterization revealed PC2 is a calcium-dependent serine endoprotease with an acidic pH optimum (~5.5) and selectivity for dibasic substrates, establishing the biochemical framework for understanding where in the secretory pathway PC2 functions.\",\n      \"evidence\": \"Fluorogenic substrate assays with inhibitor profiling in Xenopus oocyte expression system\",\n      \"pmids\": [\"2060650\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinetic parameters for individual prohormone substrates not yet determined\", \"Structural basis for dibasic selectivity unknown\"]\n    },\n    {\n      \"year\": 1992,\n      \"claim\": \"Identifying PC2 as the specific type 2 proinsulin endopeptidase that cleaves at the C-peptide/A-chain junction resolved the long-standing question of which enzyme mediates this specific processing step in insulin biosynthesis.\",\n      \"evidence\": \"Co-elution chromatography, immunoprecipitation of enzymatic activity, and N-terminal sequencing of purified PC2 from insulin granules\",\n      \"pmids\": [\"1528899\", \"1634553\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo confirmation in knockout model not yet available at this time\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Demonstrating that proPC2 maturation is autocatalytic — requiring the tetrabasic propeptide cleavage site and an intact active-site Asp — and can proceed intermolecularly established the zymogen activation mechanism of PC2.\",\n      \"evidence\": \"Site-directed mutagenesis of cleavage site and active-site residues; cell-free Xenopus egg extract reconstitution; pulse-chase analysis\",\n      \"pmids\": [\"8276855\", \"7836407\", \"8397508\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether intramolecular or intermolecular cleavage dominates in vivo unresolved at this stage\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Discovery that 7B2 serves as both an obligate ER chaperone for proPC2 folding/transport and a potent endogenous inhibitor of mature PC2 (via its C-terminal peptide) revealed a dual-function regulatory mechanism unique among prohormone convertases.\",\n      \"evidence\": \"Reciprocal coimmunoprecipitation with metabolic labeling and pulse-chase; in vitro enzyme inhibition assays with recombinant 7B2 and domain fragments\",\n      \"pmids\": [\"7913882\", \"8016065\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry of the proPC2–7B2 complex not defined\", \"Structural basis of 7B2 chaperone versus inhibitor functions not resolved\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Showing that PC2 is the enzyme responsible for proglucagon processing to glucagon in pancreatic α-cells — generating the α-cell cleavage pattern distinct from PC1/PC3's intestinal L-cell pattern — established PC2 as the molecular basis for tissue-specific differential proglucagon processing.\",\n      \"evidence\": \"Antisense PC2 knockdown in αTC1-6 cells; vaccinia expression in AtT-20/PC2 cells with product analysis\",\n      \"pmids\": [\"8159732\", \"7592866\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo confirmation in whole animal not yet demonstrated\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Mapping the 7B2 inhibitory determinant to Lys171-Lys172, identifying 7B2's proPC2-binding hexapeptide (RRKRRS), and characterizing pH/calcium-dependent proPC2 aggregation defined the molecular rules governing the 7B2–PC2 interaction and proPC2 sorting into secretory granules.\",\n      \"evidence\": \"Structure-function mutagenesis of 7B2; coimmunoprecipitation with calcium chelation; in vitro sedimentation assays at varying pH/calcium\",\n      \"pmids\": [\"7782286\", \"7722516\", \"8034613\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Three-dimensional structure of 7B2–proPC2 complex not determined\", \"Relative contribution of aggregation versus receptor-mediated sorting to granule targeting unknown\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Discovering that PC2 cleaves the 7B2 CT inhibitory peptide at its internal Lys-Lys site within granules — with carboxypeptidase E trimming the product — established a self-disinhibition mechanism whereby PC2 destroys its own inhibitor to become fully active.\",\n      \"evidence\": \"In vitro incubation of radiolabeled CT peptide with purified recombinant PC2; metabolic labeling and immunoprecipitation in cells\",\n      \"pmids\": [\"8643504\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Temporal kinetics of CT peptide destruction relative to substrate processing onset not quantified\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Demonstrating that the C-terminal domain of PC2 is sufficient for regulated secretory pathway sorting resolved which structural element directs PC2 to dense-core granules rather than the constitutive pathway.\",\n      \"evidence\": \"Furin–PC2 C-terminal chimeras stably expressed in AtT-20 cells; immunofluorescence and immuno-electron microscopy\",\n      \"pmids\": [\"8810291\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the sorting receptor or lipid-binding mechanism not identified\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"PC2 knockout mice revealed that PC2 is essential in vivo for processing proglucagon (to glucagon), prosomatostatin, and proinsulin (at the C-peptide/A-chain junction), causing chronic fasting hypoglycemia and validating decades of in vitro substrate assignments.\",\n      \"evidence\": \"Gene disruption by neomycin insertion in exon 3; glucose tolerance tests; pancreatic hormone measurements; islet morphology; pulse-chase in isolated islets\",\n      \"pmids\": [\"9192619\", \"9452465\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"CNS neuropeptide processing phenotypes not yet characterized\", \"Compensatory changes in other convertases not fully assessed\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Purified proPC2 reconstitution demonstrated that zymogen activation is intramolecular, pH-driven (optimum 4.0–4.9), calcium-independent, and not inhibited by the 7B2 CT peptide, settling the debate over the mechanism and compartment of proPC2 maturation.\",\n      \"evidence\": \"Purified recombinant proPC2 from CHO cells; pH titration; EDTA chelation; concentration-independence kinetics\",\n      \"pmids\": [\"9989936\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Earlier in vitro data suggested calcium dependence (PMID:7836407); discrepancy with purified system not fully reconciled\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"PC2 knockout mice extended the substrate repertoire to prodynorphin in the brain, showing PC2 mediates monobasic cleavage events required for Dyn A-8 and efficient Dyn A-17/Dyn B-13 production, demonstrating PC2 function beyond classical dibasic processing.\",\n      \"evidence\": \"PC2 knockout mice; HPLC and immunological quantification of dynorphin peptides in brain extracts\",\n      \"pmids\": [\"10987860\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which monobasic sites PC2 cleaves directly versus indirectly not established\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Establishing that PC2 is specifically required for N-terminal proIAPP processing — and that loss of this processing leads to amyloid-prone intermediate accumulation — linked PC2 activity to islet amyloid pathogenesis.\",\n      \"evidence\": \"Western blot and immunofluorescence with site-specific antisera in PC2-null mouse islets\",\n      \"pmids\": [\"11246872\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether reduced PC2 activity contributes to human type 2 diabetes amyloidosis not tested\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Identification of VGF and pro-neurotensin/neuromedin N as additional PC2 substrates broadened the known neuropeptide substrate repertoire and demonstrated region-specific processing deficits in the brain.\",\n      \"evidence\": \"Ectopic PC2 expression with MS product identification (VGF); PC2-KO mouse brain RIA for neurotensin/neuromedin N\",\n      \"pmids\": [\"12065665\", \"12358783\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full CNS peptidomic profiling of PC2-KO not performed\", \"Physiological consequences of VGF processing deficits unknown\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Demonstrating near-complete loss of α-MSH and exclusive PC2 responsibility for CART II production in knockout mice established PC2 as the essential convertase for melanocortin and CART neuropeptide maturation, with implications for energy homeostasis.\",\n      \"evidence\": \"PC2-KO mice; RIA, RP-HPLC, and Western blot for α-MSH, ACTH, β-endorphin; cell-based and KO mouse analysis for CART peptides with MS identification\",\n      \"pmids\": [\"12859669\", \"12584191\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether PC2 deficiency-related obesity is solely α-MSH-dependent not resolved\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Mutagenesis of the proglucagon interdomain site Lys70-Arg71 showed that PC2 initiates processing at this site as a prerequisite for efficient glucagon liberation, defining the ordered cleavage pathway for glucagon biogenesis.\",\n      \"evidence\": \"Cotransfection of wild-type and mutant proglucagon with PC2 in GH4C1 cells; HPLC product analysis\",\n      \"pmids\": [\"15528303\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether this ordered pathway operates identically in primary α-cells not confirmed\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Rescue of PC2 expression in PC2-null islets expressing human proIAPP restored N-terminal processing, decreased amyloid deposition, and reduced β-cell apoptosis, establishing a causal link between PC2 processing activity and protection against islet amyloid toxicity.\",\n      \"evidence\": \"Adenoviral PC2 overexpression in PC2-null islets expressing human proIAPP; TUNEL assay; immunoblotting\",\n      \"pmids\": [\"16873681\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relevance to physiological PC2 levels in human β-cells not demonstrated\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Identifying FGF-23 and proBMP1 as PC2/7B2-dependent substrates in osteoblasts expanded PC2 function beyond classical neuroendocrine processing to bone mineral metabolism, and pharmacological restoration of 7B2·PC2 activity rescued the HYP phenotype.\",\n      \"evidence\": \"PC2/7B2 transfection and 7B2 siRNA in osteoblasts; analysis of FGF-23, proBMP1, and DMP1 in hyp-mouse bone; Hexa-D-arginine rescue in vivo\",\n      \"pmids\": [\"22886699\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether PC2 processes FGF-23 directly or indirectly via BMP1 activation not fully delineated\", \"Expression level and regulation of PC2 in osteoblasts under physiological conditions unclear\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of PC2-7B2 interaction, the complete in vivo peptidomic substrate repertoire, the mechanism by which PC2 achieves monobasic cleavage selectivity, and whether reduced PC2 activity contributes to human metabolic or neurodegenerative disease.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No crystal structure of PC2 alone or in complex with 7B2\", \"Comprehensive peptidomics of PC2-KO tissues not completed\", \"Human genetic studies linking PCSK2 variants to disease phenotypes are sparse\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 1, 2, 3, 5, 9, 14, 17, 18, 23, 24, 25, 26, 27, 28, 29, 33]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [0, 3, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [11, 16]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [4, 6, 13]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 1, 2, 5, 8, 9, 14, 17, 18, 19, 23, 24, 25, 26, 27, 28, 29, 30, 33]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [17, 27, 33]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [16]}\n    ],\n    \"complexes\": [\n      \"proPC2–7B2 complex\"\n    ],\n    \"partners\": [\n      \"SCG5\",\n      \"CPE\",\n      \"POMC\",\n      \"GCG\",\n      \"INS\",\n      \"PDYN\",\n      \"VGF\",\n      \"FGF23\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}