{"gene":"PCSK4","run_date":"2026-04-29T11:37:58","timeline":{"discoveries":[{"year":1992,"finding":"PCSK4 (PC4) is a member of the Kex2/subtilisin-like proprotein convertase family, expressed as at least three splice variants (PC4-A, -B, -C) in testicular germ cells (pachytene spermatocytes and round spermatids) but not in Leydig, Sertoli, or peritubular cells, and its expression onset coincides with the first stages of spermiogenesis (postnatal days 19–22), indicating a reproductive developmental function.","method":"Northern analysis, testicular cell separation, in situ hybridization histochemistry, cDNA cloning","journal":"Molecular endocrinology (Baltimore, Md.)","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (Northern, in situ hybridization, cell fractionation) replicated across rat and mouse","pmids":["1448111"],"is_preprint":false},{"year":1997,"finding":"Homozygous disruption of the Pcsk4 locus in mice severely impairs male in vivo and in vitro fertility; spermatogenesis appears morphologically normal, but spermatozoa have reduced fertilizing ability and eggs fertilized by Pcsk4-null sperm fail to develop to the blastocyst stage, establishing PCSK4 as essential for fertilization and early embryonic development.","method":"Homologous recombination knockout in embryonic stem cells, breeding intercrosses, in vitro fertilization assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — clean knockout with specific reproductive phenotype, replicated in vivo and in vitro","pmids":["9192653"],"is_preprint":false},{"year":1998,"finding":"PCSK4 processes the PACAP (pituitary adenylate cyclase-activating polypeptide) precursor into bioactive PACAP38 and PACAP27 in a co-transfection cell assay, demonstrating PACAP precursor as a physiological substrate and suggesting PCSK4 regulates PACAP-mediated germ cell maturation in the testis.","method":"Stable co-transfection of PACAP cDNA with PC4 cDNA in GH4C1 cells, reversed-phase HPLC fractionation, radioimmunoassay, Western blot, cAMP bioactivity assay","journal":"Peptides","confidence":"High","confidence_rationale":"Tier 1 — reconstitution in cell-based system with multiple orthogonal readouts (immunoassay, Western, bioactivity)","pmids":["9493858"],"is_preprint":false},{"year":1999,"finding":"Recombinant PCSK4A (rPC4A) is an enzymatically active serine protease with optimal activity at pH 7.0 and 2 mM Ca²⁺, preferring the substrate Ac-Arg-Lys-Lys-Arg-MCA (KXXR↓ motif); it efficiently cleaves IGF-I and IGF-II pro-peptides and the PACAP precursor, with substrate specificity distinct from furin and PC1. Activity is inhibited by metal chelators, p-chloromercuribenzoic acid, transition-metal ions, and peptide semicarbazone inhibitors.","method":"Vaccinia virus recombinant expression in GH4C1 cells, fluorogenic peptide substrate kinetics (Vmax/Km), synthetic peptide cleavage assays, Western blot, inhibitor profiling","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 — in vitro enzymatic characterization with mutagenesis-informed substrate panel and kinetic analysis","pmids":["10493908"],"is_preprint":false},{"year":2001,"finding":"In addition to spermatids, PCSK4 is expressed in macrophage-like cells of the ovary (theca-interstitium and degenerating corpora lutea); PCSK4-null macrophage-like ovarian cells show elevated TNFα transcripts, and activation of RAW264.7 macrophages with LPS decreases PCSK4 expression while increasing TNFα, suggesting PCSK4 inversely regulates macrophage activation.","method":"β-galactosidase histoenzymatic assay (knockin reporter), RT-PCR, immunohistochemistry with macrophage markers (CD11b, F4/80), LPS activation of RAW264.7 cells","journal":"Journal of reproductive immunology","confidence":"Medium","confidence_rationale":"Tier 2 — clean knockout/reporter model with defined cellular phenotype; single lab","pmids":["11164898"],"is_preprint":false},{"year":2005,"finding":"PCSK4 localizes to acrosomal granules of round spermatids, acrosomal ridges of elongated spermatids, and the plasma membrane overlying the acrosome. PCSK4-null sperm undergo accelerated capacitation, acrosome react at lower zona pellucida concentrations, and exhibit only half the egg-binding ability of wild-type sperm, linking PCSK4's acrosomal localization to its roles in capacitation and zona binding.","method":"Immunohistochemistry, indirect immunofluorescence, immunoelectron microscopy, in vitro capacitation and acrosome reaction assays, zona pellucida egg-binding assays using PCSK4-null vs. wild-type sperm","journal":"Biology of reproduction","confidence":"High","confidence_rationale":"Tier 2 — multiple imaging modalities plus functional assays comparing null vs. wild-type sperm","pmids":["16371590"],"is_preprint":false},{"year":2008,"finding":"Recombinant PCSK4 purified from a Leishmania tarentolae expression system cleaves pro-IGF-2 to produce active IGF-2, confirming a role in IGF-2 maturation. The enzyme is inhibited by the prodomain peptide rPC4(101–116) (IC₅₀ in low µM) and by tetrapeptide chloromethyl ketones (Dec-RVKR/K-cmk, Ki in low nM range).","method":"Leishmania expression and protein purification, fluorogenic substrate assays, proIGF-2 cleavage assay, inhibitor kinetics","journal":"Protein expression and purification","confidence":"High","confidence_rationale":"Tier 1 — purified recombinant enzyme with substrate cleavage validation and inhibitor characterization","pmids":["18485734"],"is_preprint":false},{"year":2009,"finding":"PCSK4-null sperm show hyper-tyrosine phosphorylation during capacitation (dependent on PKA, albumin, and calcium) and increased proteolytic processing of ADAM2 from a 46-kDa to a 27-kDa form (dependent on cholesterol efflux), indicating that PCSK4 normally restrains both capacitation-associated signal transduction and ADAM2 processing.","method":"Comparative capacitation time-course experiments with wild-type vs. PCSK4-null sperm, Western blot for tyrosine phosphorylation and ADAM2 processing, PKA inhibitor and calcium chelation experiments","journal":"Fertility and sterility","confidence":"Medium","confidence_rationale":"Tier 2 — controlled null vs. wild-type comparison with pathway-specific inhibitors; single lab","pmids":["19342015"],"is_preprint":false},{"year":2010,"finding":"The proPCSK4 zymogen cannot efficiently self-activate in somatic (HEK293) cells and is retained in the endoplasmic reticulum, where it interacts with the molecular chaperone GRP78/BiP, suggesting that germ cell-specific factors are required for proper PCSK4 zymogen processing and activation.","method":"Transfection of rat or human PCSK4 in HEK293 cells, co-immunoprecipitation with GRP78/BiP, Western blot analysis of intracellular vs. secreted forms","journal":"Molecular and cellular biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 — Co-IP plus cellular fractionation; single lab","pmids":["21080038"],"is_preprint":false},{"year":2012,"finding":"In PCSK4-null spermatozoa, the acrosin-binding protein ACRBP/sp32 is not proteolytically processed from its 58.5 kDa precursor to the mature 27.5 kDa form, identifying ACRBP as a candidate PCSK4 substrate (possibly indirect). Proacrosin also fails to autoactivate in PCSK4-null sperm, and morphological defects in sperm head/acrosome are detected.","method":"2D-DIGE proteomics of PCSK4-null vs. wild-type spermatozoa, Western blot for ACRBP and proacrosin processing, immunolocalization of ACRBP","journal":"Molecular human reproduction","confidence":"Medium","confidence_rationale":"Tier 2 — proteomics plus Western validation in null model; substrate relationship is inferred, not directly reconstituted","pmids":["22357636"],"is_preprint":false},{"year":2012,"finding":"The epididymal serpin CRES inhibits PCSK4 protease activity in vitro in a state-dependent manner: the CRES dimer inhibits with Ki ~8 µM, while the monomer has Ki >100 µM. Both forms block PCSK4-mediated cleavage of pro-IGF-2 in trophoblast cells, suggesting CRES is an endogenous regulator of PCSK4 activity during sperm storage.","method":"Fluorogenic substrate (Boc-RVRR-MCA) inhibition kinetics with recombinant CRES monomer and dimer, pro-IGF-2 processing assay in placental trophoblast cells","journal":"Current molecular medicine","confidence":"Medium","confidence_rationale":"Tier 1 — in vitro enzyme inhibition kinetics with defined inhibitor forms; single lab","pmids":["22827436"],"is_preprint":false},{"year":2018,"finding":"In solitary fibrous tumors causing non-islet cell tumor hypoglycemia (NICTH), an imbalance of high IGF2 expression relative to low PCSK4 expression is associated with accumulation of unprocessed 'big IGF2', directly implicating PCSK4 as the endogenous protease responsible for pro-IGF2 maturation in vivo.","method":"Immunohistochemistry and Western blot quantification of IGF2 and PCSK4 in tumor specimens; serum big IGF2 measurement by western immunoblotting","journal":"The Journal of clinical endocrinology and metabolism","confidence":"Medium","confidence_rationale":"Tier 3 — human tissue correlative study with direct protein quantification linking PCSK4 to IGF2 processing in vivo; supports prior in vitro data","pmids":["29897468"],"is_preprint":false}],"current_model":"PCSK4 is a germ cell-specific subtilisin/kexin-type serine protease expressed predominantly in pachytene spermatocytes and round spermatids, localizing to the sperm acrosome, where it processes substrates including the PACAP precursor, pro-IGF-I/II, and indirectly regulates ACRBP/sp32 and proacrosin maturation; loss of PCSK4 causes male subfertility through accelerated capacitation, impaired zona pellucida binding, and precocious acrosome reaction, while its zymogen activation in the endoplasmic reticulum requires germ cell-specific factors and is regulated by the chaperone GRP78/BiP and by the epididymal serpin CRES."},"narrative":{"teleology":[{"year":1992,"claim":"Identification of PCSK4 as a new proprotein convertase family member with germ cell-restricted expression established the existence of a testis-specific endoprotease likely dedicated to reproductive function.","evidence":"Northern blot, in situ hybridization, and testicular cell fractionation in rat and mouse","pmids":["1448111"],"confidence":"High","gaps":["Enzymatic activity and substrate specificity were unknown","Protein localization within the spermatid was not determined","Function in fertilization was hypothetical"]},{"year":1997,"claim":"Genetic ablation of Pcsk4 demonstrated that it is essential for male fertility and early embryonic viability, transforming PCSK4 from a candidate gene to a functionally validated fertility factor.","evidence":"Homologous recombination knockout in mice with in vivo breeding and in vitro fertilization assays","pmids":["9192653"],"confidence":"High","gaps":["The cellular mechanism of fertilization failure was undefined","Whether the embryonic defect was paternal or oocyte-derived was unclear","Sperm morphology appeared normal, leaving the functional defect uncharacterized"]},{"year":1998,"claim":"Reconstitution of PACAP precursor processing by PCSK4 co-expression identified the first physiological substrate, linking PCSK4 to neuropeptide maturation in the testis.","evidence":"Co-transfection in GH4C1 cells with HPLC, radioimmunoassay, and cAMP bioactivity readouts","pmids":["9493858"],"confidence":"High","gaps":["In vivo relevance of PACAP processing by PCSK4 was not tested in knockout animals","Whether PACAP processing alone explains the fertility phenotype was unknown"]},{"year":1999,"claim":"Biochemical characterization of purified recombinant PCSK4 defined its catalytic properties (pH 7.0, Ca²⁺-dependent, KXXR↓ preference) and substrate range including pro-IGF-I and pro-IGF-II, distinguishing it from furin and PC1.","evidence":"Vaccinia virus expression, fluorogenic substrate kinetics, synthetic peptide and pro-peptide cleavage assays, inhibitor profiling","pmids":["10493908"],"confidence":"High","gaps":["Substrate cleavage was demonstrated on synthetic and recombinant substrates, not in a physiological germ cell context","Structural basis for substrate selectivity was not determined"]},{"year":2005,"claim":"Subcellular localization to the acrosome and functional assays in null sperm revealed that PCSK4 restrains capacitation rate and is required for normal zona pellucida binding, explaining the mechanism of subfertility.","evidence":"Immunoelectron microscopy, immunofluorescence, capacitation and acrosome reaction assays, zona binding assays comparing null vs. wild-type sperm","pmids":["16371590"],"confidence":"High","gaps":["The molecular targets through which PCSK4 regulates capacitation signaling were not identified","Whether accelerated capacitation is the direct cause or a correlate of reduced zona binding was unclear"]},{"year":2008,"claim":"Independent confirmation of pro-IGF-2 cleavage by highly purified PCSK4 and identification of prodomain-based and chloromethyl ketone inhibitors provided tools and validated IGF-2 maturation as a bona fide PCSK4 function.","evidence":"Leishmania expression system, purified enzyme, pro-IGF-2 cleavage assay, inhibitor kinetics","pmids":["18485734"],"confidence":"High","gaps":["In vivo IGF-2 processing defects in PCSK4-null tissues were not assessed","Whether IGF-2 maturation contributes to the fertility phenotype was untested"]},{"year":2009,"claim":"Discovery that PCSK4-null sperm exhibit hyper-tyrosine phosphorylation and accelerated ADAM2 processing during capacitation identified specific signaling and proteolytic cascades normally held in check by PCSK4.","evidence":"Western blot time-course of capacitation in null vs. wild-type sperm with PKA inhibitors and calcium chelation","pmids":["19342015"],"confidence":"Medium","gaps":["Whether PCSK4 directly cleaves ADAM2 or acts upstream was not resolved","Single-lab observation without independent replication","Identity of the PCSK4-sensitive step in the PKA/tyrosine phosphorylation cascade was not mapped"]},{"year":2010,"claim":"The finding that proPCSK4 is retained in the ER and interacts with GRP78/BiP in somatic cells, but not efficiently self-activated, established that germ cell-specific cofactors are required for zymogen maturation.","evidence":"Transfection in HEK293 cells, co-immunoprecipitation with GRP78/BiP, Western blot of intracellular forms","pmids":["21080038"],"confidence":"Medium","gaps":["The identity of germ cell-specific activation factor(s) was not determined","Co-IP was performed in one direction without reciprocal validation","No structural insight into how the propeptide blocks activation"]},{"year":2012,"claim":"Proteomic and biochemical analysis of PCSK4-null sperm identified ACRBP/sp32 and proacrosin as downstream processing targets, connecting PCSK4 to the acrosin activation cascade critical for zona penetration, while the serpin CRES was identified as an endogenous inhibitor of PCSK4 activity.","evidence":"2D-DIGE proteomics and Western blot of null vs. WT sperm (ACRBP/proacrosin); fluorogenic substrate inhibition kinetics and pro-IGF-2 cleavage assay with recombinant CRES monomer/dimer","pmids":["22357636","22827436"],"confidence":"Medium","gaps":["Direct cleavage of ACRBP by PCSK4 was not reconstituted in vitro","CRES inhibition was shown only in vitro; in vivo relevance in epididymal sperm is inferred","Whether CRES regulation is the primary physiological brake on PCSK4 during sperm storage is untested"]},{"year":2018,"claim":"Correlative analysis of human solitary fibrous tumors showed that low PCSK4 expression coincides with accumulation of unprocessed 'big IGF-2', providing in vivo human evidence that PCSK4 is the endogenous pro-IGF-2 convertase.","evidence":"Immunohistochemistry and Western blot of tumor tissue; serum big IGF-2 measurement","pmids":["29897468"],"confidence":"Medium","gaps":["Correlative human tissue study; causality not demonstrated by PCSK4 overexpression or rescue","Whether other convertases compensate in non-tumor tissues is unknown","No genetic evidence linking PCSK4 mutations to IGF-2-dependent disease in humans"]},{"year":null,"claim":"The identity of germ cell-specific factors required for PCSK4 zymogen activation, the full repertoire of physiological substrates in the acrosome, and whether PCSK4 directly or indirectly regulates capacitation-associated signaling remain unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structural model of PCSK4 exists","Direct cleavage of ACRBP and ADAM2 has not been reconstituted","No human genetic studies have linked PCSK4 mutations to male infertility"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[2,3,6,9,10]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[8]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[5]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[2,3,6,9]},{"term_id":"R-HSA-1474165","term_label":"Reproduction","supporting_discovery_ids":[1,5,7]}],"complexes":[],"partners":["GRP78","CRES","ACRBP","ADAM2"],"other_free_text":[]},"mechanistic_narrative":"PCSK4 is a germ cell-specific subtilisin/kexin-type serine proprotein convertase that functions in sperm maturation, capacitation control, and fertilization competence. It is expressed in pachytene spermatocytes and round spermatids, localizes to the sperm acrosome, and cleaves substrates including the PACAP precursor, pro-IGF-I, and pro-IGF-II at basic amino acid motifs with an optimum at pH 7.0 and 2 mM Ca²⁺ [PMID:1448111, PMID:10493908, PMID:16371590]. PCSK4-null male mice are severely subfertile, exhibiting accelerated capacitation with hyper-tyrosine phosphorylation, precocious acrosome reaction, impaired zona pellucida binding, defective processing of ACRBP/sp32 and proacrosin, and embryo developmental failure [PMID:9192653, PMID:16371590, PMID:19342015, PMID:22357636]. Zymogen activation requires germ cell-specific factors and involves ER retention with the chaperone GRP78/BiP, while enzyme activity is negatively regulated by the epididymal serpin CRES [PMID:21080038, PMID:22827436]."},"prefetch_data":{"uniprot":{"accession":"Q6UW60","full_name":"Proprotein convertase subtilisin/kexin type 4","aliases":["Proprotein convertase 4","PC4"],"length_aa":755,"mass_kda":82.8,"function":"Proprotein convertase involved in the processing of hormone and other protein precursors at sites comprised of pairs of basic amino acid residues (By similarity). In males, important for ADAM2 processing as well as other acrosomal proteins with roles in fertilization and critical for normal fertilization events such as sperm capacitation, acrosome reaction and binding of sperm to zona pellucida (By similarity). Also plays a role in female fertility, involved in the regulation of trophoblast migration and placental development, may be through the proteolytical processing and activation of proteins such as IGF2 (PubMed:16040806). May also participate in folliculogenesis in the ovaries (By similarity)","subcellular_location":"Membrane; Cytoplasmic vesicle, secretory vesicle, acrosome membrane","url":"https://www.uniprot.org/uniprotkb/Q6UW60/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PCSK4","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/PCSK4","total_profiled":1310},"omim":[{"mim_id":"600488","title":"PROPROTEIN CONVERTASE, SUBTILISIN/KEXIN-TYPE, 5; PCSK5","url":"https://www.omim.org/entry/600488"},{"mim_id":"600487","title":"PROPROTEIN CONVERTASE, SUBTILISIN/KEXIN-TYPE, 4; PCSK4","url":"https://www.omim.org/entry/600487"},{"mim_id":"147470","title":"INSULIN-LIKE GROWTH FACTOR II; IGF2","url":"https://www.omim.org/entry/147470"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"testis","ntpm":62.5}],"url":"https://www.proteinatlas.org/search/PCSK4"},"hgnc":{"alias_symbol":["PC4","SPC5","DKFZp434B217","MGC34749"],"prev_symbol":[]},"alphafold":{"accession":"Q6UW60","domains":[{"cath_id":"3.30.70.850","chopping":"33-105","consensus_level":"high","plddt":90.0752,"start":33,"end":105},{"cath_id":"3.40.50.200","chopping":"137-444","consensus_level":"high","plddt":95.833,"start":137,"end":444},{"cath_id":"2.60.120.260","chopping":"452-577","consensus_level":"high","plddt":96.5883,"start":452,"end":577},{"cath_id":"2.10.220.10","chopping":"590-635_643-683","consensus_level":"medium","plddt":84.2723,"start":590,"end":683}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q6UW60","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q6UW60-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q6UW60-F1-predicted_aligned_error_v6.png","plddt_mean":84.31},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PCSK4","jax_strain_url":"https://www.jax.org/strain/search?query=PCSK4"},"sequence":{"accession":"Q6UW60","fasta_url":"https://rest.uniprot.org/uniprotkb/Q6UW60.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q6UW60/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q6UW60"}},"corpus_meta":[{"pmid":"8062391","id":"PMC_8062391","title":"Purification, 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cells by encapsulation of Pc4 in silica nanoparticles.","date":"2009","source":"Toxicology and applied pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/19695274","citation_count":115,"is_preprint":false},{"pmid":"9482861","id":"PMC_9482861","title":"A dynamic model for PC4 coactivator function in RNA polymerase II transcription.","date":"1998","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/9482861","citation_count":109,"is_preprint":false},{"pmid":"8617240","id":"PMC_8617240","title":"Yeast SUB1 is a suppressor of TFIIB mutations and has homology to the human co-activator PC4.","date":"1996","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/8617240","citation_count":101,"is_preprint":false},{"pmid":"7628453","id":"PMC_7628453","title":"The coactivator p15 (PC4) initiates transcriptional activation during TFIIA-TFIID-promoter complex formation.","date":"1995","source":"The EMBO 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biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/21080038","citation_count":5,"is_preprint":false},{"pmid":"24560843","id":"PMC_24560843","title":"A transmembrane domain determines the localization of rice stripe virus pc4 to plasmodesmata and is essential for its function as a movement protein.","date":"2014","source":"Virus research","url":"https://pubmed.ncbi.nlm.nih.gov/24560843","citation_count":5,"is_preprint":false},{"pmid":"30561148","id":"PMC_30561148","title":"The effect of phosphate ion on the ssDNA binding mode of MoSub1, a Sub1/PC4 homolog from rice blast fungus.","date":"2018","source":"Proteins","url":"https://pubmed.ncbi.nlm.nih.gov/30561148","citation_count":5,"is_preprint":false},{"pmid":"39772744","id":"PMC_39772744","title":"The single-stranded DNA-binding factor SUB1/PC4 alleviates replication stress at telomeres and is a vulnerability of ALT cancer cells.","date":"2025","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/39772744","citation_count":4,"is_preprint":false},{"pmid":"39468351","id":"PMC_39468351","title":"Human PC4 supports telomere stability and viability in cells utilizing the alternative lengthening of telomeres mechanism.","date":"2024","source":"EMBO reports","url":"https://pubmed.ncbi.nlm.nih.gov/39468351","citation_count":4,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":43762,"output_tokens":3607,"usd":0.092696},"stage2":{"model":"claude-opus-4-6","input_tokens":6687,"output_tokens":2669,"usd":0.15024},"total_usd":0.242936,"stage1_batch_id":"msgbatch_012ZiiCUBfB62NCTQd4Bk1Jo","stage2_batch_id":"msgbatch_01Xuqq9aUHoQouS7UVGZNStZ","note":"batch pricing = 50% of standard"},"stage1_raw":"Looking at the corpus, I need to distinguish between two completely different genes that share the \"PC4\" symbol:\n\n1. **PCSK4** (Proprotein Convertase Subtilisin/Kexin Type 4) — a testis-specific serine protease involved in sperm maturation and fertilization (PMIDs: 1448111, 9192653, 10493908, 30561148, etc.)\n\n2. **PC4/SUB1** (Positive Cofactor 4, also called TFIP1 or coactivator PC4) — a transcriptional coactivator/ssDNA-binding protein (PMIDs: 8062391, 9482861, etc.)\n\n3. **IFRD1/TIS7/PC4** — an interferon-related developmental regulator (PMIDs: 7756174, 21127072, etc.)\n\nThe query gene is **PCSK4**. I will focus only on discoveries about the proprotein convertase PCSK4 (the serine protease expressed in testicular germ cells). Papers about the transcriptional coactivator PC4 (SUB1/TFIP1) are symbol collisions with a different protein. Papers about IFRD1/TIS7/PC4 are also a different protein.\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1992,\n      \"finding\": \"PCSK4 (PC4) is a member of the Kex2/subtilisin-like proprotein convertase family, expressed as at least three splice variants (PC4-A, -B, -C) in testicular germ cells (pachytene spermatocytes and round spermatids) but not in Leydig, Sertoli, or peritubular cells, and its expression onset coincides with the first stages of spermiogenesis (postnatal days 19–22), indicating a reproductive developmental function.\",\n      \"method\": \"Northern analysis, testicular cell separation, in situ hybridization histochemistry, cDNA cloning\",\n      \"journal\": \"Molecular endocrinology (Baltimore, Md.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (Northern, in situ hybridization, cell fractionation) replicated across rat and mouse\",\n      \"pmids\": [\"1448111\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Homozygous disruption of the Pcsk4 locus in mice severely impairs male in vivo and in vitro fertility; spermatogenesis appears morphologically normal, but spermatozoa have reduced fertilizing ability and eggs fertilized by Pcsk4-null sperm fail to develop to the blastocyst stage, establishing PCSK4 as essential for fertilization and early embryonic development.\",\n      \"method\": \"Homologous recombination knockout in embryonic stem cells, breeding intercrosses, in vitro fertilization assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean knockout with specific reproductive phenotype, replicated in vivo and in vitro\",\n      \"pmids\": [\"9192653\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"PCSK4 processes the PACAP (pituitary adenylate cyclase-activating polypeptide) precursor into bioactive PACAP38 and PACAP27 in a co-transfection cell assay, demonstrating PACAP precursor as a physiological substrate and suggesting PCSK4 regulates PACAP-mediated germ cell maturation in the testis.\",\n      \"method\": \"Stable co-transfection of PACAP cDNA with PC4 cDNA in GH4C1 cells, reversed-phase HPLC fractionation, radioimmunoassay, Western blot, cAMP bioactivity assay\",\n      \"journal\": \"Peptides\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution in cell-based system with multiple orthogonal readouts (immunoassay, Western, bioactivity)\",\n      \"pmids\": [\"9493858\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Recombinant PCSK4A (rPC4A) is an enzymatically active serine protease with optimal activity at pH 7.0 and 2 mM Ca²⁺, preferring the substrate Ac-Arg-Lys-Lys-Arg-MCA (KXXR↓ motif); it efficiently cleaves IGF-I and IGF-II pro-peptides and the PACAP precursor, with substrate specificity distinct from furin and PC1. Activity is inhibited by metal chelators, p-chloromercuribenzoic acid, transition-metal ions, and peptide semicarbazone inhibitors.\",\n      \"method\": \"Vaccinia virus recombinant expression in GH4C1 cells, fluorogenic peptide substrate kinetics (Vmax/Km), synthetic peptide cleavage assays, Western blot, inhibitor profiling\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro enzymatic characterization with mutagenesis-informed substrate panel and kinetic analysis\",\n      \"pmids\": [\"10493908\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"In addition to spermatids, PCSK4 is expressed in macrophage-like cells of the ovary (theca-interstitium and degenerating corpora lutea); PCSK4-null macrophage-like ovarian cells show elevated TNFα transcripts, and activation of RAW264.7 macrophages with LPS decreases PCSK4 expression while increasing TNFα, suggesting PCSK4 inversely regulates macrophage activation.\",\n      \"method\": \"β-galactosidase histoenzymatic assay (knockin reporter), RT-PCR, immunohistochemistry with macrophage markers (CD11b, F4/80), LPS activation of RAW264.7 cells\",\n      \"journal\": \"Journal of reproductive immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean knockout/reporter model with defined cellular phenotype; single lab\",\n      \"pmids\": [\"11164898\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"PCSK4 localizes to acrosomal granules of round spermatids, acrosomal ridges of elongated spermatids, and the plasma membrane overlying the acrosome. PCSK4-null sperm undergo accelerated capacitation, acrosome react at lower zona pellucida concentrations, and exhibit only half the egg-binding ability of wild-type sperm, linking PCSK4's acrosomal localization to its roles in capacitation and zona binding.\",\n      \"method\": \"Immunohistochemistry, indirect immunofluorescence, immunoelectron microscopy, in vitro capacitation and acrosome reaction assays, zona pellucida egg-binding assays using PCSK4-null vs. wild-type sperm\",\n      \"journal\": \"Biology of reproduction\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple imaging modalities plus functional assays comparing null vs. wild-type sperm\",\n      \"pmids\": [\"16371590\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Recombinant PCSK4 purified from a Leishmania tarentolae expression system cleaves pro-IGF-2 to produce active IGF-2, confirming a role in IGF-2 maturation. The enzyme is inhibited by the prodomain peptide rPC4(101–116) (IC₅₀ in low µM) and by tetrapeptide chloromethyl ketones (Dec-RVKR/K-cmk, Ki in low nM range).\",\n      \"method\": \"Leishmania expression and protein purification, fluorogenic substrate assays, proIGF-2 cleavage assay, inhibitor kinetics\",\n      \"journal\": \"Protein expression and purification\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — purified recombinant enzyme with substrate cleavage validation and inhibitor characterization\",\n      \"pmids\": [\"18485734\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"PCSK4-null sperm show hyper-tyrosine phosphorylation during capacitation (dependent on PKA, albumin, and calcium) and increased proteolytic processing of ADAM2 from a 46-kDa to a 27-kDa form (dependent on cholesterol efflux), indicating that PCSK4 normally restrains both capacitation-associated signal transduction and ADAM2 processing.\",\n      \"method\": \"Comparative capacitation time-course experiments with wild-type vs. PCSK4-null sperm, Western blot for tyrosine phosphorylation and ADAM2 processing, PKA inhibitor and calcium chelation experiments\",\n      \"journal\": \"Fertility and sterility\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — controlled null vs. wild-type comparison with pathway-specific inhibitors; single lab\",\n      \"pmids\": [\"19342015\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The proPCSK4 zymogen cannot efficiently self-activate in somatic (HEK293) cells and is retained in the endoplasmic reticulum, where it interacts with the molecular chaperone GRP78/BiP, suggesting that germ cell-specific factors are required for proper PCSK4 zymogen processing and activation.\",\n      \"method\": \"Transfection of rat or human PCSK4 in HEK293 cells, co-immunoprecipitation with GRP78/BiP, Western blot analysis of intracellular vs. secreted forms\",\n      \"journal\": \"Molecular and cellular biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP plus cellular fractionation; single lab\",\n      \"pmids\": [\"21080038\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"In PCSK4-null spermatozoa, the acrosin-binding protein ACRBP/sp32 is not proteolytically processed from its 58.5 kDa precursor to the mature 27.5 kDa form, identifying ACRBP as a candidate PCSK4 substrate (possibly indirect). Proacrosin also fails to autoactivate in PCSK4-null sperm, and morphological defects in sperm head/acrosome are detected.\",\n      \"method\": \"2D-DIGE proteomics of PCSK4-null vs. wild-type spermatozoa, Western blot for ACRBP and proacrosin processing, immunolocalization of ACRBP\",\n      \"journal\": \"Molecular human reproduction\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — proteomics plus Western validation in null model; substrate relationship is inferred, not directly reconstituted\",\n      \"pmids\": [\"22357636\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"The epididymal serpin CRES inhibits PCSK4 protease activity in vitro in a state-dependent manner: the CRES dimer inhibits with Ki ~8 µM, while the monomer has Ki >100 µM. Both forms block PCSK4-mediated cleavage of pro-IGF-2 in trophoblast cells, suggesting CRES is an endogenous regulator of PCSK4 activity during sperm storage.\",\n      \"method\": \"Fluorogenic substrate (Boc-RVRR-MCA) inhibition kinetics with recombinant CRES monomer and dimer, pro-IGF-2 processing assay in placental trophoblast cells\",\n      \"journal\": \"Current molecular medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro enzyme inhibition kinetics with defined inhibitor forms; single lab\",\n      \"pmids\": [\"22827436\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"In solitary fibrous tumors causing non-islet cell tumor hypoglycemia (NICTH), an imbalance of high IGF2 expression relative to low PCSK4 expression is associated with accumulation of unprocessed 'big IGF2', directly implicating PCSK4 as the endogenous protease responsible for pro-IGF2 maturation in vivo.\",\n      \"method\": \"Immunohistochemistry and Western blot quantification of IGF2 and PCSK4 in tumor specimens; serum big IGF2 measurement by western immunoblotting\",\n      \"journal\": \"The Journal of clinical endocrinology and metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — human tissue correlative study with direct protein quantification linking PCSK4 to IGF2 processing in vivo; supports prior in vitro data\",\n      \"pmids\": [\"29897468\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PCSK4 is a germ cell-specific subtilisin/kexin-type serine protease expressed predominantly in pachytene spermatocytes and round spermatids, localizing to the sperm acrosome, where it processes substrates including the PACAP precursor, pro-IGF-I/II, and indirectly regulates ACRBP/sp32 and proacrosin maturation; loss of PCSK4 causes male subfertility through accelerated capacitation, impaired zona pellucida binding, and precocious acrosome reaction, while its zymogen activation in the endoplasmic reticulum requires germ cell-specific factors and is regulated by the chaperone GRP78/BiP and by the epididymal serpin CRES.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"PCSK4 is a germ cell-specific subtilisin/kexin-type serine proprotein convertase that functions in sperm maturation, capacitation control, and fertilization competence. It is expressed in pachytene spermatocytes and round spermatids, localizes to the sperm acrosome, and cleaves substrates including the PACAP precursor, pro-IGF-I, and pro-IGF-II at basic amino acid motifs with an optimum at pH 7.0 and 2 mM Ca²⁺ [PMID:1448111, PMID:10493908, PMID:16371590]. PCSK4-null male mice are severely subfertile, exhibiting accelerated capacitation with hyper-tyrosine phosphorylation, precocious acrosome reaction, impaired zona pellucida binding, defective processing of ACRBP/sp32 and proacrosin, and embryo developmental failure [PMID:9192653, PMID:16371590, PMID:19342015, PMID:22357636]. Zymogen activation requires germ cell-specific factors and involves ER retention with the chaperone GRP78/BiP, while enzyme activity is negatively regulated by the epididymal serpin CRES [PMID:21080038, PMID:22827436].\",\n  \"teleology\": [\n    {\n      \"year\": 1992,\n      \"claim\": \"Identification of PCSK4 as a new proprotein convertase family member with germ cell-restricted expression established the existence of a testis-specific endoprotease likely dedicated to reproductive function.\",\n      \"evidence\": \"Northern blot, in situ hybridization, and testicular cell fractionation in rat and mouse\",\n      \"pmids\": [\"1448111\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Enzymatic activity and substrate specificity were unknown\", \"Protein localization within the spermatid was not determined\", \"Function in fertilization was hypothetical\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Genetic ablation of Pcsk4 demonstrated that it is essential for male fertility and early embryonic viability, transforming PCSK4 from a candidate gene to a functionally validated fertility factor.\",\n      \"evidence\": \"Homologous recombination knockout in mice with in vivo breeding and in vitro fertilization assays\",\n      \"pmids\": [\"9192653\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The cellular mechanism of fertilization failure was undefined\", \"Whether the embryonic defect was paternal or oocyte-derived was unclear\", \"Sperm morphology appeared normal, leaving the functional defect uncharacterized\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Reconstitution of PACAP precursor processing by PCSK4 co-expression identified the first physiological substrate, linking PCSK4 to neuropeptide maturation in the testis.\",\n      \"evidence\": \"Co-transfection in GH4C1 cells with HPLC, radioimmunoassay, and cAMP bioactivity readouts\",\n      \"pmids\": [\"9493858\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of PACAP processing by PCSK4 was not tested in knockout animals\", \"Whether PACAP processing alone explains the fertility phenotype was unknown\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Biochemical characterization of purified recombinant PCSK4 defined its catalytic properties (pH 7.0, Ca²⁺-dependent, KXXR↓ preference) and substrate range including pro-IGF-I and pro-IGF-II, distinguishing it from furin and PC1.\",\n      \"evidence\": \"Vaccinia virus expression, fluorogenic substrate kinetics, synthetic peptide and pro-peptide cleavage assays, inhibitor profiling\",\n      \"pmids\": [\"10493908\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Substrate cleavage was demonstrated on synthetic and recombinant substrates, not in a physiological germ cell context\", \"Structural basis for substrate selectivity was not determined\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Subcellular localization to the acrosome and functional assays in null sperm revealed that PCSK4 restrains capacitation rate and is required for normal zona pellucida binding, explaining the mechanism of subfertility.\",\n      \"evidence\": \"Immunoelectron microscopy, immunofluorescence, capacitation and acrosome reaction assays, zona binding assays comparing null vs. wild-type sperm\",\n      \"pmids\": [\"16371590\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The molecular targets through which PCSK4 regulates capacitation signaling were not identified\", \"Whether accelerated capacitation is the direct cause or a correlate of reduced zona binding was unclear\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Independent confirmation of pro-IGF-2 cleavage by highly purified PCSK4 and identification of prodomain-based and chloromethyl ketone inhibitors provided tools and validated IGF-2 maturation as a bona fide PCSK4 function.\",\n      \"evidence\": \"Leishmania expression system, purified enzyme, pro-IGF-2 cleavage assay, inhibitor kinetics\",\n      \"pmids\": [\"18485734\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo IGF-2 processing defects in PCSK4-null tissues were not assessed\", \"Whether IGF-2 maturation contributes to the fertility phenotype was untested\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Discovery that PCSK4-null sperm exhibit hyper-tyrosine phosphorylation and accelerated ADAM2 processing during capacitation identified specific signaling and proteolytic cascades normally held in check by PCSK4.\",\n      \"evidence\": \"Western blot time-course of capacitation in null vs. wild-type sperm with PKA inhibitors and calcium chelation\",\n      \"pmids\": [\"19342015\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether PCSK4 directly cleaves ADAM2 or acts upstream was not resolved\", \"Single-lab observation without independent replication\", \"Identity of the PCSK4-sensitive step in the PKA/tyrosine phosphorylation cascade was not mapped\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"The finding that proPCSK4 is retained in the ER and interacts with GRP78/BiP in somatic cells, but not efficiently self-activated, established that germ cell-specific cofactors are required for zymogen maturation.\",\n      \"evidence\": \"Transfection in HEK293 cells, co-immunoprecipitation with GRP78/BiP, Western blot of intracellular forms\",\n      \"pmids\": [\"21080038\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"The identity of germ cell-specific activation factor(s) was not determined\", \"Co-IP was performed in one direction without reciprocal validation\", \"No structural insight into how the propeptide blocks activation\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Proteomic and biochemical analysis of PCSK4-null sperm identified ACRBP/sp32 and proacrosin as downstream processing targets, connecting PCSK4 to the acrosin activation cascade critical for zona penetration, while the serpin CRES was identified as an endogenous inhibitor of PCSK4 activity.\",\n      \"evidence\": \"2D-DIGE proteomics and Western blot of null vs. WT sperm (ACRBP/proacrosin); fluorogenic substrate inhibition kinetics and pro-IGF-2 cleavage assay with recombinant CRES monomer/dimer\",\n      \"pmids\": [\"22357636\", \"22827436\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct cleavage of ACRBP by PCSK4 was not reconstituted in vitro\", \"CRES inhibition was shown only in vitro; in vivo relevance in epididymal sperm is inferred\", \"Whether CRES regulation is the primary physiological brake on PCSK4 during sperm storage is untested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Correlative analysis of human solitary fibrous tumors showed that low PCSK4 expression coincides with accumulation of unprocessed 'big IGF-2', providing in vivo human evidence that PCSK4 is the endogenous pro-IGF-2 convertase.\",\n      \"evidence\": \"Immunohistochemistry and Western blot of tumor tissue; serum big IGF-2 measurement\",\n      \"pmids\": [\"29897468\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Correlative human tissue study; causality not demonstrated by PCSK4 overexpression or rescue\", \"Whether other convertases compensate in non-tumor tissues is unknown\", \"No genetic evidence linking PCSK4 mutations to IGF-2-dependent disease in humans\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The identity of germ cell-specific factors required for PCSK4 zymogen activation, the full repertoire of physiological substrates in the acrosome, and whether PCSK4 directly or indirectly regulates capacitation-associated signaling remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structural model of PCSK4 exists\", \"Direct cleavage of ACRBP and ADAM2 has not been reconstituted\", \"No human genetic studies have linked PCSK4 mutations to male infertility\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [2, 3, 6, 9, 10]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [8]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [2, 3, 6, 9]},\n      {\"term_id\": \"R-HSA-1474165\", \"supporting_discovery_ids\": [1, 5, 7]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"GRP78\", \"CRES\", \"ACRBP\", \"ADAM2\"],\n    \"other_free_text\": []\n  }\n}\n```"}