{"gene":"C4BPA","run_date":"2026-04-28T17:12:38","timeline":{"discoveries":[{"year":2001,"finding":"The N-terminal CCP domains 1–3 of the C4BPA alpha-chain are required for C4b binding and complement regulatory activity; CCP2 and CCP3 are most critical, and spatial arrangement (interdomain linkers) between CCP1-4 is required for full function. Polymeric structure confers greater efficiency in degrading cell-surface C4b compared to monomeric variants.","method":"Recombinant deletion and alanine-insertion mutagenesis of 19 C4BP variants; functional assays for C4b binding and factor I cofactor activity","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — extensive mutagenesis with reconstituted functional assays, multiple orthogonal readouts","pmids":["11369776"],"is_preprint":false},{"year":1999,"finding":"A cluster of positively charged amino acids (R39, R64, R66) at the interface between CCP1 and CCP2 of the C4BPA alpha-chain constitutes the C4b-binding site; mutations at these residues reduce C4b binding affinity up to 140-fold and impair factor I cofactor activity. This site is also a specific heparin-binding site.","method":"Site-directed mutagenesis (R39Q, R64Q/R66Q, triple mutant); C4b-binding assays; factor I cofactor degradation assays; heparin binding","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis with in vitro binding and enzymatic assays, multiple mutants tested","pmids":["10383431"],"is_preprint":false},{"year":2003,"finding":"Mutations K126Q/K128Q and F144S/F149S in CCP3 of the C4BPA alpha-chain selectively abolish factor I cofactor activity without affecting C4b/C3b binding affinity or C3-convertase decay acceleration, identifying a region specifically required for the cofactor conformational mechanism distinct from ligand binding.","method":"Site-directed mutagenesis; surface plasmon resonance binding assays; factor I cofactor degradation assays; C3-convertase assembly/decay assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis plus multiple orthogonal functional assays separating binding from catalytic cofactor function","pmids":["12893820"],"is_preprint":false},{"year":2000,"finding":"Positively charged residues R39, K63, R64, and H67 at the CCP1–CCP2 interface of the C4BPA alpha-chain are required for prevention of C3-convertase assembly, acceleration of its decay, and factor I cofactor activity for C4b cleavage in fluid phase.","method":"Recombinant C4BP mutants (Arg/Lys/His to Gln); C3-convertase assembly inhibition and decay acceleration assays; fluid-phase factor I cofactor assays","journal":"Molecular immunology","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with multiple functional complement assays","pmids":["11090879"],"is_preprint":false},{"year":2002,"finding":"SCR2 of the C4BPA alpha-chain is indispensable for C4b binding; SCR2 and SCR3 are required for factor I-mediated C4b cleavage; SCR1–5 participate in C3b cofactor activity with SCR2–4 being absolutely required. Different sets of CCP domains mediate C3b versus C4b cofactor activity.","method":"SCR-deletion mutants of recombinant multimeric C4BP; C3b/C4b-Sepharose binding assays; ELISA; fluid-phase cofactor assays","journal":"Journal of biochemistry","confidence":"High","confidence_rationale":"Tier 1 — systematic deletion mutagenesis with multiple binding and enzymatic readouts","pmids":["12417021"],"is_preprint":false},{"year":2009,"finding":"C4BPA-containing C4BP regulates the lectin pathway C3/C5 convertase with ~7–13-fold greater affinity for C4b deposited via the lectin pathway than via the classical pathway; at high C4b density, all seven alpha-chains simultaneously engage C4b binding (up to ~8 C4b per C4BP heptamer).","method":"Functional complement assays on zymosan and mannan-coated erythrocytes; C4b density titration; IC50 determination for C3 and C5 convertase inhibition","journal":"Molecular immunology","confidence":"Medium","confidence_rationale":"Tier 2 — clean in vitro functional assays with multiple surfaces, single lab","pmids":["19660812"],"is_preprint":false},{"year":2006,"finding":"C4BPA alpha-chain interacts with both the C4c and C4dg subfragments of C4b via adjacent but distinct subsites; filling of the C4dg subsite allosterically increases C4c binding affinity 2–3-fold, revealing synergy between subsites within each CCP1-3 unit.","method":"Surface plasmon resonance binding of C4c and C4dg to wild-type and mutant C4BP; cross-competition experiments; ionic-strength titration","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 1–2 — SPR with mutant panel and cross-competition, single lab","pmids":["16819837"],"is_preprint":false},{"year":2003,"finding":"The C4BPA alpha-chain binds directly to CD40 on human B cells at a site distinct from the CD40L binding site, inducing B cell proliferation, upregulation of CD54 and CD86, and IL-4-dependent IgE isotype switching; this activity is absent in B cells from CD40- or IKKγ/NEMO-deficient patients. C4BP co-localizes with B cells in germinal centers of human tonsils.","method":"Direct binding assays; B cell activation and isotype switching assays; patient-derived B cells (CD40-deficient, IKKγ/NEMO-deficient); immunofluorescence co-localization in tonsil tissue","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 2 — reciprocal functional validation with patient controls, multiple readouts, co-localization","pmids":["12818164"],"is_preprint":false},{"year":1999,"finding":"Protein S binds to the C4BP beta-chain (C4BPB) primarily through SCR-1, with SCR-2 contributing to enhance affinity up to 5-fold; C4BPB SCR-2 specifically augments SCR-1 binding to protein S, and binding to SCR-1-containing constructs reduces protein S cofactor activity for activated protein C.","method":"Chimeric constructs of C4BPB SCRs fused to tPA; protein S binding assays; activated protein C cofactor activity assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — chimeric domain mapping with multiple binding and functional assays, confirmed by follow-up study (PMID 10744423)","pmids":["10329721","10744423"],"is_preprint":false},{"year":1997,"finding":"Residues 447–460 of protein S (within the laminin G1 domain region, around position 450) constitute a binding site for the C4BP beta-chain, as identified by phage-display peptide library selection and confirmed by synthetic peptide inhibition and direct biophysical binding measurements.","method":"Bacteriophage peptide display library selection; synthetic peptide inhibition assays; near-UV circular dichroism and tryptophan fluorescence polarization titrations","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 1–2 — phage display plus biophysical validation, single lab","pmids":["9169428"],"is_preprint":false},{"year":2022,"finding":"Protein S residues Lys255, Glu257, Asp287, Arg410, Lys423, and Glu424 within the LG1 domain are critical for TFPI cofactor function; C4BP beta-chain binding to protein S at this same region almost completely abolishes protein S-mediated enhancement of TFPIα, establishing competitive regulation of the TFPI anticoagulant pathway by C4BP.","method":"N-linked glycosylation scanning mutagenesis; alanine scanning; FXa inhibition assays; C4BP beta-chain expression and binding; plasma TFPI cofactor assays","journal":"Blood advances","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis with multiple functional assays, orthogonal glycosylation and charge-reversal scanning","pmids":["34731882"],"is_preprint":false},{"year":1995,"finding":"C4BPA gene expression is controlled by a hepatic-specific promoter within 369 bp upstream of the transcription start site; the hepatic transcription factor HNF1 binds at −38 bp and is absolutely required for promoter activity, explaining liver-restricted C4BPA expression. The promoter lacks a TATA box.","method":"Promoter deletion analysis by transfection; HNF1 binding by footprinting/EMSA; hepatocyte vs. non-hepatocyte cell line transfection comparison","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 2 — promoter deletion series and transcription factor binding assays in relevant cell lines, single lab","pmids":["7772049"],"is_preprint":false},{"year":1995,"finding":"C4BP plasma isoform pattern (α7β1, α7β0, α6β1) is determined by the relative expression levels of the C4BPA and C4BPB genes; genetic factors segregating with the RCA gene cluster control isoform proportions, as demonstrated by HepG2/Hep3B secretion studies and COS cell transfections with different alpha/beta chain ratios.","method":"Hepatocyte cell line (HepG2, Hep3B) secretion analysis; COS cell co-transfection with varying C4BPA/C4BPB ratios; Western blot isoform characterization","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — transfection and cell-secretion experiments with molecular isoform resolution, single lab","pmids":["7561113"],"is_preprint":false},{"year":1995,"finding":"IL-6, IL-1β, and IFN-γ upregulate both C4BPA and C4BPB mRNA in Hep3B cells; TNF-α downregulates both; IFN-γ combined with TNF-α synergistically induces C4BPA mRNA ~10-fold while only marginally increasing C4BPB mRNA, providing a mechanism to maintain C4BPB steady-state levels during acute phase induction.","method":"Cytokine treatment of Hep3B cells; Northern blot analysis of C4BPA and C4BPB mRNA; serial plasma isoform analysis in acute phase patients","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — in vitro cytokine dose-response with molecular readouts correlated with patient samples, single lab","pmids":["7561114"],"is_preprint":false},{"year":2008,"finding":"LPS decreases C4BPα and C4BPβ expression in rat hepatocytes via the NF-κB and MEK/ERK pathways; IL-6 specifically increases C4BPβ expression via the STAT-3 pathway, leading to increased plasma PS–C4BP complex and decreased anticoagulant protein S activity.","method":"LPS and IL-6 treatment of isolated rat hepatocytes in vitro and in vivo; NF-κB, MEK/ERK, and STAT-3 pathway inhibitors; Western blot and RT-PCR; plasma PS activity and PS-C4BP complex measurement","journal":"Journal of thrombosis and haemostasis","confidence":"Medium","confidence_rationale":"Tier 2 — pathway inhibitor experiments with multiple molecular readouts, both in vitro and in vivo, single lab","pmids":["18752574"],"is_preprint":false},{"year":2020,"finding":"C4BPA is expressed intracellularly in cancer cells where it interacts with the NF-κB family member RelA; elevated intracellular C4BPA increases IκBα expression and stabilizes inhibitory IκBα–RelA complexes, sensitizing cells to oxaliplatin-induced apoptosis in vitro and in vivo.","method":"Co-immunoprecipitation of C4BPA with RelA; cancer cell lines with patient-specific C4BPA mutations; IκBα expression and complex stability assays; oxaliplatin apoptosis assays in vitro and xenograft model","journal":"iScience","confidence":"Medium","confidence_rationale":"Tier 2 — reciprocal Co-IP with functional rescue in patient-mutation cell lines and in vivo validation, single lab","pmids":["33205012"],"is_preprint":false},{"year":2019,"finding":"Fetal C4BPA (from fetal cord blood exosomes) binds CD40 on placental villous trophoblast to activate non-canonical NF-κB processing of p100 to p52, inducing pro-labor gene expression; this was supported by computational, crystal structural, and gene functional analyses.","method":"Proteomics of fetal cord blood exosomes; crystal structural analysis; gene functional assays in placental trophoblast cells; p100-to-p52 processing assay","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 — structural and functional gene assays in relevant cell type, single lab","pmids":["30940885"],"is_preprint":false},{"year":1983,"finding":"Chymotryptic cleavage of C4BP yields a 48 kDa N-terminal fragment (from each subunit chain) that retains C4b-regulatory activity; the C4b-binding/active site is located in the N-terminal portion of the subunit chain, and C4BP is composed of 6–8 disulfide-linked 75 kDa subunit chains.","method":"Chymotrypsin cleavage; fragment purification; N-terminal amino acid sequencing; functional activity assay of isolated fragment","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 1–2 — direct biochemical cleavage and N-terminal sequencing with functional validation, foundational study","pmids":["6653778"],"is_preprint":false},{"year":1998,"finding":"A monomeric membrane-anchored form of C4BPA (all 8 SCRs, GPI-anchored) acts as a more efficient factor I cofactor for C3b inactivation than natural multimeric C4BP; blocking studies showed that both the 48 kDa N-terminal fragment and C-terminal domains near the oligomerization region contribute to high C3b-inactivating cofactor activity.","method":"Expression of monomeric GPI-anchored C4BP on swine endothelial cells; fluid-phase factor I cofactor assays with methylamine-treated C3 and C4; monoclonal antibody blocking studies","journal":"Molecular immunology","confidence":"Medium","confidence_rationale":"Tier 2 — functional cell-expression system with antibody blocking, single lab","pmids":["9809581"],"is_preprint":false},{"year":2003,"finding":"Protein S and the C4BP–protein S complex bind to apoptotic (but not healthy) neutrophils; binding is mediated through the Gla domain of protein S, which recognizes negatively charged phospholipids exposed on apoptotic cells, demonstrating that C4BP is recruited to apoptotic surfaces via its protein S partner.","method":"Binding assays with apoptotic vs. non-apoptotic neutrophil populations; anti-Gla domain monoclonal antibody blocking; flow cytometry","journal":"Blood coagulation & fibrinolysis","confidence":"Medium","confidence_rationale":"Tier 2 — antibody blocking with apoptosis-sorted cell populations, single lab","pmids":["12945877"],"is_preprint":false},{"year":2024,"finding":"S-palmitoylation of C4BPA at Cys15 (mouse) / Cys13 and Cys23 (pig, by ZDHHC8) in epididymal epithelial cells is required for C4BPA enrichment into epididymosomes and transfer to sperm surface; palmitoylated C4BPA on sperm protects against complement C4-mediated damage and maintains sperm motility, while the C15S (or C13S/C23S) mutant loses this protective function.","method":"Palmitoylation inhibition studies; C15S and C23S mutagenesis; immunofluorescence localization; sperm motility assays; complement C4 challenge assays; epididymosome fractionation","journal":"International journal of biological macromolecules / Biochimica et biophysica acta","confidence":"Medium","confidence_rationale":"Tier 2 — mutagenesis of palmitoylation site with localization and functional complement assays, two independent studies","pmids":["39370067","41619901"],"is_preprint":false},{"year":2025,"finding":"C4BPA promotes gastric cancer cell proliferation and motility via activation of JAK2/STAT3 phosphorylation in tumor cells and increases C3a and C5a production; C4BPA-derived C5a signals through C5aR1 on macrophages to activate STAT3 and drive M2-like polarization, establishing a C4BPA→C5a→C5aR1→STAT3 axis in tumor-immune crosstalk.","method":"C4BPA knockdown and overexpression in GC cell lines; patient-derived GC organoids; subcutaneous xenograft mouse model; JAK2/STAT3 phosphorylation assays; C3a/C5a measurement; THP-1 macrophage co-culture; recombinant C5a rescue; C5aR1 inhibition","journal":"International immunopharmacology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple genetic perturbation approaches with mechanistic rescue experiments, single lab","pmids":["41237697"],"is_preprint":false},{"year":2022,"finding":"The CCP6 domain of the C4BPA alpha-chain (in oligomeric form, PRP6-HO7) is sufficient to reprogram pro-inflammatory monocyte-derived dendritic cells to an anti-inflammatory tolerogenic state, downregulating CD83, HLA-DR, CD86, CD80, CD40, IL-12, and TNF-α, and inhibiting T cell alloproliferation; this immunomodulatory activity is independent of complement regulatory activity.","method":"Recombinant CCP6-oligomer (PRP6-HO7) treatment of monocyte-derived DCs; flow cytometry for surface markers; cytokine ELISA; T cell alloproliferation assays; endocytosis and chemotaxis assays; comparison with full-length C4BP isoforms","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 — domain-truncated recombinant protein with multiple orthogonal functional readouts, single lab","pmids":["35547734"],"is_preprint":false}],"current_model":"C4BPA (C4b-binding protein alpha chain) is the primary complement regulatory subunit of the C4BP heptameric complex: its CCP1–3 domains bind C4b (and CCP1–5 for C3b) through a cluster of positively charged residues at the CCP1/CCP2 interface, acting as a cofactor for serine protease factor I to cleave and inactivate C4b and C3b, thereby inhibiting classical, lectin, and to some extent alternative pathway C3/C5-convertases; additionally, C4BPA binds CD40 on B cells to trigger NF-κB-dependent activation, interacts intracellularly with RelA to modulate apoptosis, is palmitoylated at Cys15 for epididymosomal loading onto sperm to protect against complement C4, and the beta-chain partner (C4BPB) sequesters anticoagulant protein S through SCR-1/2, while C4BP(β-) isoform exerts complement-independent immunomodulatory reprogramming of inflammatory myeloid cells via its CCP6 domain."},"narrative":{"teleology":[{"year":1983,"claim":"Initial biochemistry established that the C4b-regulatory activity of C4BP resides in the N-terminal portion of each subunit chain, defining the functional architecture of the multimeric molecule.","evidence":"Chymotryptic cleavage yielding a 48 kDa N-terminal fragment with retained C4b cofactor activity","pmids":["6653778"],"confidence":"Medium","gaps":["Exact domain boundaries not resolved","No identification of individual residues required for binding"]},{"year":1995,"claim":"Transcriptional regulation of C4BPA was defined: hepatic-restricted expression depends on HNF1 binding to a TATA-less promoter, and cytokines (IL-6, IFN-γ, TNF-α) differentially regulate C4BPA versus C4BPB mRNA to control circulating isoform ratios during acute-phase responses.","evidence":"Promoter deletion/EMSA in hepatocyte cell lines; cytokine dose-response Northern blots in Hep3B; COS cell co-transfection isoform reconstitution","pmids":["7772049","7561114","7561113"],"confidence":"Medium","gaps":["Chromatin-level regulatory elements not mapped","In vivo confirmation of isoform ratio control by transcription in human acute-phase limited to correlative data"]},{"year":1999,"claim":"Site-directed mutagenesis pinpointed the C4b-binding site to positively charged residues R39, R64, and R66 at the CCP1–CCP2 interface, simultaneously identifying this region as a heparin-binding site and establishing its role in factor I cofactor function.","evidence":"R39Q, R64Q/R66Q, and triple-mutant recombinant C4BP; C4b binding and factor I cofactor assays","pmids":["10383431"],"confidence":"High","gaps":["No structural data for the C4b–CCP1-2 interface","Heparin-binding significance in vivo unclear"]},{"year":1999,"claim":"The protein S–C4BP interaction was mapped to C4BPB SCR-1 (with SCR-2 augmenting affinity), establishing how C4BP sequesters the anticoagulant cofactor protein S.","evidence":"Chimeric C4BPB SCR–tPA constructs; protein S binding and APC cofactor assays","pmids":["10329721","10744423"],"confidence":"High","gaps":["Atomic-resolution structure of the protein S–C4BPB interface not determined","Regulation of complex dissociation unknown"]},{"year":2001,"claim":"Systematic deletion mutagenesis resolved the domain requirements for C4b versus C3b cofactor activity: CCP1–3 are required for C4b binding while CCP1–5 are needed for C3b, and oligomeric structure enhances cell-surface C4b degradation over monomeric forms.","evidence":"19 recombinant C4BP deletion/insertion variants; C4b and C3b binding and factor I cofactor assays","pmids":["11369776","12417021"],"confidence":"High","gaps":["Mechanism by which multivalency enhances surface-level regulation not structurally resolved","Relative contributions of avidity versus conformational effects undetermined"]},{"year":2003,"claim":"CCP3 residues K126/K128 and F144/F149 were shown to be selectively essential for cofactor catalytic function without affecting C4b/C3b binding or decay acceleration, dissecting the cofactor conformational mechanism from substrate recognition.","evidence":"Site-directed mutagenesis; SPR binding plus factor I cofactor and C3-convertase decay assays","pmids":["12893820"],"confidence":"High","gaps":["No structural basis for how these residues remodel C4b/C3b to expose factor I cleavage sites"]},{"year":2003,"claim":"C4BPA was found to bind CD40 on B cells at a site distinct from CD40L, activating NF-κB-dependent proliferation and IgE class switching — establishing a complement-independent immunomodulatory role.","evidence":"Direct binding assays; B cell activation in wild-type, CD40-deficient, and IKKγ/NEMO-deficient patient B cells; tonsil germinal center co-localization","pmids":["12818164"],"confidence":"High","gaps":["Structural identity of the C4BPA–CD40 interface unknown","Physiological significance in germinal center responses not demonstrated in vivo"]},{"year":2006,"claim":"SPR analysis revealed that C4BPA engages C4b through adjacent but allosterically coupled subsites for C4c and C4dg fragments, explaining cooperative binding.","evidence":"SPR with purified C4c and C4dg subfragments against wild-type and mutant C4BP; ionic-strength titration","pmids":["16819837"],"confidence":"Medium","gaps":["No co-crystal structure of C4BPA–C4b to validate two-subsite model","Only a single lab report"]},{"year":2019,"claim":"Fetal exosomal C4BPA was shown to activate non-canonical NF-κB (p100→p52 processing) via CD40 on placental trophoblast, linking C4BPA to pro-labor signaling.","evidence":"Cord blood exosome proteomics; structural analysis; trophoblast p100-to-p52 processing assay","pmids":["30940885"],"confidence":"Medium","gaps":["In vivo necessity for labor induction not demonstrated","Relative contribution versus CD40L not tested"]},{"year":2020,"claim":"An intracellular role for C4BPA was identified: it interacts with NF-κB subunit RelA, stabilizes IκBα–RelA complexes, and sensitizes cancer cells to oxaliplatin-induced apoptosis.","evidence":"Co-immunoprecipitation of C4BPA–RelA; patient-mutation cell lines; IκBα stabilization assays; xenograft model","pmids":["33205012"],"confidence":"Medium","gaps":["Mechanism of C4BPA nuclear/cytoplasmic entry uncharacterized","Generalizability beyond cancer cell lines uncertain"]},{"year":2022,"claim":"The CCP6 domain alone (in oligomeric form) was shown to reprogram monocyte-derived DCs to a tolerogenic phenotype independently of complement regulation, extending C4BPA immunomodulation beyond CD40-mediated signaling.","evidence":"Recombinant CCP6 oligomer treatment of human monocyte-derived DCs; surface marker flow cytometry; cytokine ELISA; T cell alloproliferation assays","pmids":["35547734"],"confidence":"Medium","gaps":["Receptor on DCs mediating CCP6 recognition unidentified","In vivo tolerogenic function not demonstrated"]},{"year":2022,"claim":"C4BP beta-chain binding to protein S was shown to competitively abolish protein S cofactor enhancement of TFPIα, revealing C4BP as a regulator of the TFPI anticoagulant pathway in addition to the APC pathway.","evidence":"Glycosylation scanning and alanine scanning mutagenesis of protein S LG1 domain; FXa inhibition assays with C4BP beta-chain","pmids":["34731882"],"confidence":"High","gaps":["In vivo impact on TFPI-dependent anticoagulation not tested in animal models"]},{"year":2024,"claim":"S-palmitoylation of C4BPA at N-terminal cysteines by ZDHHC8 in epididymal epithelium was identified as the mechanism for C4BPA loading onto epididymosomes and transfer to sperm, where it provides complement C4 protection and maintains motility.","evidence":"Palmitoylation site mutagenesis (C15S, C13S/C23S); epididymosome fractionation; sperm complement challenge and motility assays in mouse and pig","pmids":["39370067","41619901"],"confidence":"Medium","gaps":["Male fertility consequence of C4BPA palmitoylation loss not assessed in knockout animals","Contribution relative to other complement regulators on sperm not quantified"]},{"year":2025,"claim":"In gastric cancer, C4BPA was found to promote tumor growth via JAK2/STAT3 activation and to generate C5a that polarizes macrophages to an M2-like phenotype through C5aR1/STAT3, defining a tumor-immune crosstalk axis.","evidence":"C4BPA knockdown/overexpression in GC cell lines and organoids; xenograft model; THP-1 macrophage co-culture with C5a rescue and C5aR1 inhibition","pmids":["41237697"],"confidence":"Medium","gaps":["Mechanism by which C4BPA increases C5a generation not resolved","Single-lab finding not yet replicated"]},{"year":null,"claim":"No high-resolution co-crystal structure of C4BPA bound to C4b or CD40 exists; the DC receptor for CCP6-mediated tolerogenic reprogramming is unidentified; and the physiological significance of intracellular C4BPA–RelA interaction outside cancer contexts is unknown.","evidence":"","pmids":[],"confidence":"Low","gaps":["Structural basis of C4b and CD40 recognition at atomic resolution","Identity of DC receptor for CCP6","In vivo relevance of intracellular C4BPA functions"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,1,2,3,4,5,18]},{"term_id":"GO:0048018","term_label":"receptor ligand activity","supporting_discovery_ids":[7,16]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[0,5,6,17,19]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[20]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[15]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,1,2,3,4,5,7,22]},{"term_id":"R-HSA-109582","term_label":"Hemostasis","supporting_discovery_ids":[8,10]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[7,15,16,21]}],"complexes":["C4b-binding protein (C4BP)"],"partners":["C4B","C3","CFI","CD40","RELA","PROS1","C4BPB","ZDHHC8"],"other_free_text":[]},"mechanistic_narrative":"C4BPA is the principal complement-regulatory subunit of the multimeric C4b-binding protein (C4BP) complex, functioning as an obligate cofactor for serine protease factor I in the proteolytic inactivation of C4b and C3b, thereby suppressing classical, lectin, and alternative complement pathway convertases [PMID:11369776, PMID:10383431, PMID:12893820, PMID:19660812]. The C4b-binding site maps to a cluster of positively charged residues (R39, R64, R66) at the CCP1–CCP2 interface, while CCP3 residues K126/K128 and F144/F149 are selectively required for cofactor catalytic function distinct from substrate binding; CCP1–3 mediate C4b regulation and CCP1–5 are needed for C3b cofactor activity [PMID:10383431, PMID:12893820, PMID:12417021]. Beyond complement regulation, C4BPA binds CD40 on B cells and trophoblasts to activate NF-κB signaling, promoting B cell proliferation, IgE class switching, and pro-labor gene expression, and its CCP6 domain independently reprograms dendritic cells toward a tolerogenic phenotype [PMID:12818164, PMID:30940885, PMID:35547734]. C4BPA is S-palmitoylated at its N-terminal cysteine for epididymosomal loading onto sperm to confer complement protection, and can interact intracellularly with RelA to stabilize IκBα and modulate apoptotic sensitivity [PMID:39370067, PMID:33205012]."},"prefetch_data":{"uniprot":{"accession":"P04003","full_name":"C4b-binding protein alpha chain","aliases":["Proline-rich protein","PRP"],"length_aa":597,"mass_kda":67.0,"function":"Controls the classical pathway of complement activation. It binds as a cofactor to C3b/C4b inactivator (C3bINA), which then hydrolyzes the complement fragment C4b. It also accelerates the degradation of the C4bC2a complex (C3 convertase) by dissociating the complement fragment C2a. Alpha chain binds C4b. It also interacts with anticoagulant protein S and with serum amyloid P component","subcellular_location":"Secreted","url":"https://www.uniprot.org/uniprotkb/P04003/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/C4BPA","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/C4BPA","total_profiled":1310},"omim":[{"mim_id":"614514","title":"THROMBOPHILIA DUE TO PROTEIN S DEFICIENCY, AUTOSOMAL RECESSIVE; THPH6","url":"https://www.omim.org/entry/614514"},{"mim_id":"612922","title":"HEMOLYTIC UREMIC SYNDROME, ATYPICAL, SUSCEPTIBILITY TO, 2; AHUS2","url":"https://www.omim.org/entry/612922"},{"mim_id":"612336","title":"THROMBOPHILIA DUE TO PROTEIN S DEFICIENCY, AUTOSOMAL DOMINANT; THPH5","url":"https://www.omim.org/entry/612336"},{"mim_id":"235400","title":"HEMOLYTIC UREMIC SYNDROME, ATYPICAL, SUSCEPTIBILITY TO, 1; AHUS1","url":"https://www.omim.org/entry/235400"},{"mim_id":"176880","title":"PROTEIN S; PROS1","url":"https://www.omim.org/entry/176880"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in 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international journal of clinical chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/7758211","citation_count":2,"is_preprint":false},{"pmid":"40237358","id":"PMC_40237358","title":"Identification of serum C4BPA as a potential diagnostic marker of right ventricular remodelling via proteomic analysis.","date":"2025","source":"ESC heart failure","url":"https://pubmed.ncbi.nlm.nih.gov/40237358","citation_count":1,"is_preprint":false},{"pmid":"39272751","id":"PMC_39272751","title":"Altered Expression of C4BPA and CXCL1 Genes in the Endometrium of Patients with Recurrent Implantation Failure after In Vitro Fertilization and Thin Endometrium.","date":"2024","source":"Diagnostics (Basel, Switzerland)","url":"https://pubmed.ncbi.nlm.nih.gov/39272751","citation_count":1,"is_preprint":false},{"pmid":"41053817","id":"PMC_41053817","title":"Identification of C4BPA as a genetically informed drug target in NSCLC: an integrative single-cell and multi-omics study based on the druggable genes.","date":"2025","source":"Human genomics","url":"https://pubmed.ncbi.nlm.nih.gov/41053817","citation_count":0,"is_preprint":false},{"pmid":"41237697","id":"PMC_41237697","title":"Tumor-derived C4BPA promotes macrophage M2-like polarization through C5a -C5aR1-STAT3 axis and drives gastric cancer progression.","date":"2025","source":"International immunopharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/41237697","citation_count":0,"is_preprint":false},{"pmid":"38712057","id":"PMC_38712057","title":"Conservation of C4BP-binding Sequence Patterns in Streptococcus pyogenes M and Enn Proteins.","date":"2024","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/38712057","citation_count":0,"is_preprint":false},{"pmid":"41851078","id":"PMC_41851078","title":"Impact of C4BPA on Muscle progenitor cell differentiation: insights for Duchenne muscular dystrophy treatment.","date":"2026","source":"Cell death & disease","url":"https://pubmed.ncbi.nlm.nih.gov/41851078","citation_count":0,"is_preprint":false},{"pmid":"41349955","id":"PMC_41349955","title":"Culture-attenuated pathogenic Leptospira lose the ability to survive complement lytic activity due to decreased C4BP uptake.","date":"2025","source":"Microbes and infection","url":"https://pubmed.ncbi.nlm.nih.gov/41349955","citation_count":0,"is_preprint":false},{"pmid":"39599529","id":"PMC_39599529","title":"Survival of Borrelia burgdorferi Strain B31 in Human Serum Is Not Dependent on C4BP Binding to the Bacterial Surface.","date":"2024","source":"Pathogens (Basel, Switzerland)","url":"https://pubmed.ncbi.nlm.nih.gov/39599529","citation_count":0,"is_preprint":false},{"pmid":"41619901","id":"PMC_41619901","title":"Palmitoylation of C4BPA in porcine epididymosomes mitigates complement C4-mediated damage to sperm.","date":"2026","source":"Biochimica et biophysica acta. Molecular and cell biology of lipids","url":"https://pubmed.ncbi.nlm.nih.gov/41619901","citation_count":0,"is_preprint":false},{"pmid":"41745945","id":"PMC_41745945","title":"Knockout of the C4BPA Gene Promotes Mitophagy via Activation of the Pink1/Parkin Pathway and Alleviates the Inflammatory Response by Inhibiting the NF-κB Signalling Pathway in Bovine Mammary Epithelial Cells.","date":"2026","source":"Veterinary sciences","url":"https://pubmed.ncbi.nlm.nih.gov/41745945","citation_count":0,"is_preprint":false},{"pmid":"41648161","id":"PMC_41648161","title":"C4BP occludes the non-opsonic interaction of Neisseria gonorrhoeae with human neutrophil CEACAMs.","date":"2026","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/41648161","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2024.10.21.619360","title":"A surface lipoprotein on<i>Pasteurella multocida</i>binds complement factor I to promote immune evasion","date":"2024-10-21","source":"bioRxiv","url":"https://doi.org/10.1101/2024.10.21.619360","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":44450,"output_tokens":5812,"usd":0.110265},"stage2":{"model":"claude-opus-4-6","input_tokens":9509,"output_tokens":3502,"usd":0.202643},"total_usd":0.312908,"stage1_batch_id":"msgbatch_01TqNM6p6FsoMBT2jzQTwKBr","stage2_batch_id":"msgbatch_01Ya473eco8sTHbvRymQaU8o","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2001,\n      \"finding\": \"The N-terminal CCP domains 1–3 of the C4BPA alpha-chain are required for C4b binding and complement regulatory activity; CCP2 and CCP3 are most critical, and spatial arrangement (interdomain linkers) between CCP1-4 is required for full function. Polymeric structure confers greater efficiency in degrading cell-surface C4b compared to monomeric variants.\",\n      \"method\": \"Recombinant deletion and alanine-insertion mutagenesis of 19 C4BP variants; functional assays for C4b binding and factor I cofactor activity\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — extensive mutagenesis with reconstituted functional assays, multiple orthogonal readouts\",\n      \"pmids\": [\"11369776\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"A cluster of positively charged amino acids (R39, R64, R66) at the interface between CCP1 and CCP2 of the C4BPA alpha-chain constitutes the C4b-binding site; mutations at these residues reduce C4b binding affinity up to 140-fold and impair factor I cofactor activity. This site is also a specific heparin-binding site.\",\n      \"method\": \"Site-directed mutagenesis (R39Q, R64Q/R66Q, triple mutant); C4b-binding assays; factor I cofactor degradation assays; heparin binding\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with in vitro binding and enzymatic assays, multiple mutants tested\",\n      \"pmids\": [\"10383431\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Mutations K126Q/K128Q and F144S/F149S in CCP3 of the C4BPA alpha-chain selectively abolish factor I cofactor activity without affecting C4b/C3b binding affinity or C3-convertase decay acceleration, identifying a region specifically required for the cofactor conformational mechanism distinct from ligand binding.\",\n      \"method\": \"Site-directed mutagenesis; surface plasmon resonance binding assays; factor I cofactor degradation assays; C3-convertase assembly/decay assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis plus multiple orthogonal functional assays separating binding from catalytic cofactor function\",\n      \"pmids\": [\"12893820\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Positively charged residues R39, K63, R64, and H67 at the CCP1–CCP2 interface of the C4BPA alpha-chain are required for prevention of C3-convertase assembly, acceleration of its decay, and factor I cofactor activity for C4b cleavage in fluid phase.\",\n      \"method\": \"Recombinant C4BP mutants (Arg/Lys/His to Gln); C3-convertase assembly inhibition and decay acceleration assays; fluid-phase factor I cofactor assays\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with multiple functional complement assays\",\n      \"pmids\": [\"11090879\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"SCR2 of the C4BPA alpha-chain is indispensable for C4b binding; SCR2 and SCR3 are required for factor I-mediated C4b cleavage; SCR1–5 participate in C3b cofactor activity with SCR2–4 being absolutely required. Different sets of CCP domains mediate C3b versus C4b cofactor activity.\",\n      \"method\": \"SCR-deletion mutants of recombinant multimeric C4BP; C3b/C4b-Sepharose binding assays; ELISA; fluid-phase cofactor assays\",\n      \"journal\": \"Journal of biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic deletion mutagenesis with multiple binding and enzymatic readouts\",\n      \"pmids\": [\"12417021\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"C4BPA-containing C4BP regulates the lectin pathway C3/C5 convertase with ~7–13-fold greater affinity for C4b deposited via the lectin pathway than via the classical pathway; at high C4b density, all seven alpha-chains simultaneously engage C4b binding (up to ~8 C4b per C4BP heptamer).\",\n      \"method\": \"Functional complement assays on zymosan and mannan-coated erythrocytes; C4b density titration; IC50 determination for C3 and C5 convertase inhibition\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean in vitro functional assays with multiple surfaces, single lab\",\n      \"pmids\": [\"19660812\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"C4BPA alpha-chain interacts with both the C4c and C4dg subfragments of C4b via adjacent but distinct subsites; filling of the C4dg subsite allosterically increases C4c binding affinity 2–3-fold, revealing synergy between subsites within each CCP1-3 unit.\",\n      \"method\": \"Surface plasmon resonance binding of C4c and C4dg to wild-type and mutant C4BP; cross-competition experiments; ionic-strength titration\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 — SPR with mutant panel and cross-competition, single lab\",\n      \"pmids\": [\"16819837\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"The C4BPA alpha-chain binds directly to CD40 on human B cells at a site distinct from the CD40L binding site, inducing B cell proliferation, upregulation of CD54 and CD86, and IL-4-dependent IgE isotype switching; this activity is absent in B cells from CD40- or IKKγ/NEMO-deficient patients. C4BP co-localizes with B cells in germinal centers of human tonsils.\",\n      \"method\": \"Direct binding assays; B cell activation and isotype switching assays; patient-derived B cells (CD40-deficient, IKKγ/NEMO-deficient); immunofluorescence co-localization in tonsil tissue\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal functional validation with patient controls, multiple readouts, co-localization\",\n      \"pmids\": [\"12818164\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Protein S binds to the C4BP beta-chain (C4BPB) primarily through SCR-1, with SCR-2 contributing to enhance affinity up to 5-fold; C4BPB SCR-2 specifically augments SCR-1 binding to protein S, and binding to SCR-1-containing constructs reduces protein S cofactor activity for activated protein C.\",\n      \"method\": \"Chimeric constructs of C4BPB SCRs fused to tPA; protein S binding assays; activated protein C cofactor activity assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — chimeric domain mapping with multiple binding and functional assays, confirmed by follow-up study (PMID 10744423)\",\n      \"pmids\": [\"10329721\", \"10744423\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Residues 447–460 of protein S (within the laminin G1 domain region, around position 450) constitute a binding site for the C4BP beta-chain, as identified by phage-display peptide library selection and confirmed by synthetic peptide inhibition and direct biophysical binding measurements.\",\n      \"method\": \"Bacteriophage peptide display library selection; synthetic peptide inhibition assays; near-UV circular dichroism and tryptophan fluorescence polarization titrations\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 — phage display plus biophysical validation, single lab\",\n      \"pmids\": [\"9169428\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Protein S residues Lys255, Glu257, Asp287, Arg410, Lys423, and Glu424 within the LG1 domain are critical for TFPI cofactor function; C4BP beta-chain binding to protein S at this same region almost completely abolishes protein S-mediated enhancement of TFPIα, establishing competitive regulation of the TFPI anticoagulant pathway by C4BP.\",\n      \"method\": \"N-linked glycosylation scanning mutagenesis; alanine scanning; FXa inhibition assays; C4BP beta-chain expression and binding; plasma TFPI cofactor assays\",\n      \"journal\": \"Blood advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with multiple functional assays, orthogonal glycosylation and charge-reversal scanning\",\n      \"pmids\": [\"34731882\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"C4BPA gene expression is controlled by a hepatic-specific promoter within 369 bp upstream of the transcription start site; the hepatic transcription factor HNF1 binds at −38 bp and is absolutely required for promoter activity, explaining liver-restricted C4BPA expression. The promoter lacks a TATA box.\",\n      \"method\": \"Promoter deletion analysis by transfection; HNF1 binding by footprinting/EMSA; hepatocyte vs. non-hepatocyte cell line transfection comparison\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — promoter deletion series and transcription factor binding assays in relevant cell lines, single lab\",\n      \"pmids\": [\"7772049\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"C4BP plasma isoform pattern (α7β1, α7β0, α6β1) is determined by the relative expression levels of the C4BPA and C4BPB genes; genetic factors segregating with the RCA gene cluster control isoform proportions, as demonstrated by HepG2/Hep3B secretion studies and COS cell transfections with different alpha/beta chain ratios.\",\n      \"method\": \"Hepatocyte cell line (HepG2, Hep3B) secretion analysis; COS cell co-transfection with varying C4BPA/C4BPB ratios; Western blot isoform characterization\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — transfection and cell-secretion experiments with molecular isoform resolution, single lab\",\n      \"pmids\": [\"7561113\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"IL-6, IL-1β, and IFN-γ upregulate both C4BPA and C4BPB mRNA in Hep3B cells; TNF-α downregulates both; IFN-γ combined with TNF-α synergistically induces C4BPA mRNA ~10-fold while only marginally increasing C4BPB mRNA, providing a mechanism to maintain C4BPB steady-state levels during acute phase induction.\",\n      \"method\": \"Cytokine treatment of Hep3B cells; Northern blot analysis of C4BPA and C4BPB mRNA; serial plasma isoform analysis in acute phase patients\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro cytokine dose-response with molecular readouts correlated with patient samples, single lab\",\n      \"pmids\": [\"7561114\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"LPS decreases C4BPα and C4BPβ expression in rat hepatocytes via the NF-κB and MEK/ERK pathways; IL-6 specifically increases C4BPβ expression via the STAT-3 pathway, leading to increased plasma PS–C4BP complex and decreased anticoagulant protein S activity.\",\n      \"method\": \"LPS and IL-6 treatment of isolated rat hepatocytes in vitro and in vivo; NF-κB, MEK/ERK, and STAT-3 pathway inhibitors; Western blot and RT-PCR; plasma PS activity and PS-C4BP complex measurement\",\n      \"journal\": \"Journal of thrombosis and haemostasis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pathway inhibitor experiments with multiple molecular readouts, both in vitro and in vivo, single lab\",\n      \"pmids\": [\"18752574\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"C4BPA is expressed intracellularly in cancer cells where it interacts with the NF-κB family member RelA; elevated intracellular C4BPA increases IκBα expression and stabilizes inhibitory IκBα–RelA complexes, sensitizing cells to oxaliplatin-induced apoptosis in vitro and in vivo.\",\n      \"method\": \"Co-immunoprecipitation of C4BPA with RelA; cancer cell lines with patient-specific C4BPA mutations; IκBα expression and complex stability assays; oxaliplatin apoptosis assays in vitro and xenograft model\",\n      \"journal\": \"iScience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP with functional rescue in patient-mutation cell lines and in vivo validation, single lab\",\n      \"pmids\": [\"33205012\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Fetal C4BPA (from fetal cord blood exosomes) binds CD40 on placental villous trophoblast to activate non-canonical NF-κB processing of p100 to p52, inducing pro-labor gene expression; this was supported by computational, crystal structural, and gene functional analyses.\",\n      \"method\": \"Proteomics of fetal cord blood exosomes; crystal structural analysis; gene functional assays in placental trophoblast cells; p100-to-p52 processing assay\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — structural and functional gene assays in relevant cell type, single lab\",\n      \"pmids\": [\"30940885\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1983,\n      \"finding\": \"Chymotryptic cleavage of C4BP yields a 48 kDa N-terminal fragment (from each subunit chain) that retains C4b-regulatory activity; the C4b-binding/active site is located in the N-terminal portion of the subunit chain, and C4BP is composed of 6–8 disulfide-linked 75 kDa subunit chains.\",\n      \"method\": \"Chymotrypsin cleavage; fragment purification; N-terminal amino acid sequencing; functional activity assay of isolated fragment\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 — direct biochemical cleavage and N-terminal sequencing with functional validation, foundational study\",\n      \"pmids\": [\"6653778\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"A monomeric membrane-anchored form of C4BPA (all 8 SCRs, GPI-anchored) acts as a more efficient factor I cofactor for C3b inactivation than natural multimeric C4BP; blocking studies showed that both the 48 kDa N-terminal fragment and C-terminal domains near the oligomerization region contribute to high C3b-inactivating cofactor activity.\",\n      \"method\": \"Expression of monomeric GPI-anchored C4BP on swine endothelial cells; fluid-phase factor I cofactor assays with methylamine-treated C3 and C4; monoclonal antibody blocking studies\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional cell-expression system with antibody blocking, single lab\",\n      \"pmids\": [\"9809581\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Protein S and the C4BP–protein S complex bind to apoptotic (but not healthy) neutrophils; binding is mediated through the Gla domain of protein S, which recognizes negatively charged phospholipids exposed on apoptotic cells, demonstrating that C4BP is recruited to apoptotic surfaces via its protein S partner.\",\n      \"method\": \"Binding assays with apoptotic vs. non-apoptotic neutrophil populations; anti-Gla domain monoclonal antibody blocking; flow cytometry\",\n      \"journal\": \"Blood coagulation & fibrinolysis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — antibody blocking with apoptosis-sorted cell populations, single lab\",\n      \"pmids\": [\"12945877\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"S-palmitoylation of C4BPA at Cys15 (mouse) / Cys13 and Cys23 (pig, by ZDHHC8) in epididymal epithelial cells is required for C4BPA enrichment into epididymosomes and transfer to sperm surface; palmitoylated C4BPA on sperm protects against complement C4-mediated damage and maintains sperm motility, while the C15S (or C13S/C23S) mutant loses this protective function.\",\n      \"method\": \"Palmitoylation inhibition studies; C15S and C23S mutagenesis; immunofluorescence localization; sperm motility assays; complement C4 challenge assays; epididymosome fractionation\",\n      \"journal\": \"International journal of biological macromolecules / Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — mutagenesis of palmitoylation site with localization and functional complement assays, two independent studies\",\n      \"pmids\": [\"39370067\", \"41619901\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"C4BPA promotes gastric cancer cell proliferation and motility via activation of JAK2/STAT3 phosphorylation in tumor cells and increases C3a and C5a production; C4BPA-derived C5a signals through C5aR1 on macrophages to activate STAT3 and drive M2-like polarization, establishing a C4BPA→C5a→C5aR1→STAT3 axis in tumor-immune crosstalk.\",\n      \"method\": \"C4BPA knockdown and overexpression in GC cell lines; patient-derived GC organoids; subcutaneous xenograft mouse model; JAK2/STAT3 phosphorylation assays; C3a/C5a measurement; THP-1 macrophage co-culture; recombinant C5a rescue; C5aR1 inhibition\",\n      \"journal\": \"International immunopharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic perturbation approaches with mechanistic rescue experiments, single lab\",\n      \"pmids\": [\"41237697\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"The CCP6 domain of the C4BPA alpha-chain (in oligomeric form, PRP6-HO7) is sufficient to reprogram pro-inflammatory monocyte-derived dendritic cells to an anti-inflammatory tolerogenic state, downregulating CD83, HLA-DR, CD86, CD80, CD40, IL-12, and TNF-α, and inhibiting T cell alloproliferation; this immunomodulatory activity is independent of complement regulatory activity.\",\n      \"method\": \"Recombinant CCP6-oligomer (PRP6-HO7) treatment of monocyte-derived DCs; flow cytometry for surface markers; cytokine ELISA; T cell alloproliferation assays; endocytosis and chemotaxis assays; comparison with full-length C4BP isoforms\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — domain-truncated recombinant protein with multiple orthogonal functional readouts, single lab\",\n      \"pmids\": [\"35547734\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"C4BPA (C4b-binding protein alpha chain) is the primary complement regulatory subunit of the C4BP heptameric complex: its CCP1–3 domains bind C4b (and CCP1–5 for C3b) through a cluster of positively charged residues at the CCP1/CCP2 interface, acting as a cofactor for serine protease factor I to cleave and inactivate C4b and C3b, thereby inhibiting classical, lectin, and to some extent alternative pathway C3/C5-convertases; additionally, C4BPA binds CD40 on B cells to trigger NF-κB-dependent activation, interacts intracellularly with RelA to modulate apoptosis, is palmitoylated at Cys15 for epididymosomal loading onto sperm to protect against complement C4, and the beta-chain partner (C4BPB) sequesters anticoagulant protein S through SCR-1/2, while C4BP(β-) isoform exerts complement-independent immunomodulatory reprogramming of inflammatory myeloid cells via its CCP6 domain.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"C4BPA is the principal complement-regulatory subunit of the multimeric C4b-binding protein (C4BP) complex, functioning as an obligate cofactor for serine protease factor I in the proteolytic inactivation of C4b and C3b, thereby suppressing classical, lectin, and alternative complement pathway convertases [PMID:11369776, PMID:10383431, PMID:12893820, PMID:19660812]. The C4b-binding site maps to a cluster of positively charged residues (R39, R64, R66) at the CCP1–CCP2 interface, while CCP3 residues K126/K128 and F144/F149 are selectively required for cofactor catalytic function distinct from substrate binding; CCP1–3 mediate C4b regulation and CCP1–5 are needed for C3b cofactor activity [PMID:10383431, PMID:12893820, PMID:12417021]. Beyond complement regulation, C4BPA binds CD40 on B cells and trophoblasts to activate NF-κB signaling, promoting B cell proliferation, IgE class switching, and pro-labor gene expression, and its CCP6 domain independently reprograms dendritic cells toward a tolerogenic phenotype [PMID:12818164, PMID:30940885, PMID:35547734]. C4BPA is S-palmitoylated at its N-terminal cysteine for epididymosomal loading onto sperm to confer complement protection, and can interact intracellularly with RelA to stabilize IκBα and modulate apoptotic sensitivity [PMID:39370067, PMID:33205012].\",\n  \"teleology\": [\n    {\n      \"year\": 1983,\n      \"claim\": \"Initial biochemistry established that the C4b-regulatory activity of C4BP resides in the N-terminal portion of each subunit chain, defining the functional architecture of the multimeric molecule.\",\n      \"evidence\": \"Chymotryptic cleavage yielding a 48 kDa N-terminal fragment with retained C4b cofactor activity\",\n      \"pmids\": [\"6653778\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Exact domain boundaries not resolved\", \"No identification of individual residues required for binding\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Transcriptional regulation of C4BPA was defined: hepatic-restricted expression depends on HNF1 binding to a TATA-less promoter, and cytokines (IL-6, IFN-γ, TNF-α) differentially regulate C4BPA versus C4BPB mRNA to control circulating isoform ratios during acute-phase responses.\",\n      \"evidence\": \"Promoter deletion/EMSA in hepatocyte cell lines; cytokine dose-response Northern blots in Hep3B; COS cell co-transfection isoform reconstitution\",\n      \"pmids\": [\"7772049\", \"7561114\", \"7561113\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Chromatin-level regulatory elements not mapped\", \"In vivo confirmation of isoform ratio control by transcription in human acute-phase limited to correlative data\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Site-directed mutagenesis pinpointed the C4b-binding site to positively charged residues R39, R64, and R66 at the CCP1–CCP2 interface, simultaneously identifying this region as a heparin-binding site and establishing its role in factor I cofactor function.\",\n      \"evidence\": \"R39Q, R64Q/R66Q, and triple-mutant recombinant C4BP; C4b binding and factor I cofactor assays\",\n      \"pmids\": [\"10383431\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structural data for the C4b–CCP1-2 interface\", \"Heparin-binding significance in vivo unclear\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"The protein S–C4BP interaction was mapped to C4BPB SCR-1 (with SCR-2 augmenting affinity), establishing how C4BP sequesters the anticoagulant cofactor protein S.\",\n      \"evidence\": \"Chimeric C4BPB SCR–tPA constructs; protein S binding and APC cofactor assays\",\n      \"pmids\": [\"10329721\", \"10744423\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-resolution structure of the protein S–C4BPB interface not determined\", \"Regulation of complex dissociation unknown\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Systematic deletion mutagenesis resolved the domain requirements for C4b versus C3b cofactor activity: CCP1–3 are required for C4b binding while CCP1–5 are needed for C3b, and oligomeric structure enhances cell-surface C4b degradation over monomeric forms.\",\n      \"evidence\": \"19 recombinant C4BP deletion/insertion variants; C4b and C3b binding and factor I cofactor assays\",\n      \"pmids\": [\"11369776\", \"12417021\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which multivalency enhances surface-level regulation not structurally resolved\", \"Relative contributions of avidity versus conformational effects undetermined\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"CCP3 residues K126/K128 and F144/F149 were shown to be selectively essential for cofactor catalytic function without affecting C4b/C3b binding or decay acceleration, dissecting the cofactor conformational mechanism from substrate recognition.\",\n      \"evidence\": \"Site-directed mutagenesis; SPR binding plus factor I cofactor and C3-convertase decay assays\",\n      \"pmids\": [\"12893820\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structural basis for how these residues remodel C4b/C3b to expose factor I cleavage sites\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"C4BPA was found to bind CD40 on B cells at a site distinct from CD40L, activating NF-κB-dependent proliferation and IgE class switching — establishing a complement-independent immunomodulatory role.\",\n      \"evidence\": \"Direct binding assays; B cell activation in wild-type, CD40-deficient, and IKKγ/NEMO-deficient patient B cells; tonsil germinal center co-localization\",\n      \"pmids\": [\"12818164\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural identity of the C4BPA–CD40 interface unknown\", \"Physiological significance in germinal center responses not demonstrated in vivo\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"SPR analysis revealed that C4BPA engages C4b through adjacent but allosterically coupled subsites for C4c and C4dg fragments, explaining cooperative binding.\",\n      \"evidence\": \"SPR with purified C4c and C4dg subfragments against wild-type and mutant C4BP; ionic-strength titration\",\n      \"pmids\": [\"16819837\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No co-crystal structure of C4BPA–C4b to validate two-subsite model\", \"Only a single lab report\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Fetal exosomal C4BPA was shown to activate non-canonical NF-κB (p100→p52 processing) via CD40 on placental trophoblast, linking C4BPA to pro-labor signaling.\",\n      \"evidence\": \"Cord blood exosome proteomics; structural analysis; trophoblast p100-to-p52 processing assay\",\n      \"pmids\": [\"30940885\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo necessity for labor induction not demonstrated\", \"Relative contribution versus CD40L not tested\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"An intracellular role for C4BPA was identified: it interacts with NF-κB subunit RelA, stabilizes IκBα–RelA complexes, and sensitizes cancer cells to oxaliplatin-induced apoptosis.\",\n      \"evidence\": \"Co-immunoprecipitation of C4BPA–RelA; patient-mutation cell lines; IκBα stabilization assays; xenograft model\",\n      \"pmids\": [\"33205012\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of C4BPA nuclear/cytoplasmic entry uncharacterized\", \"Generalizability beyond cancer cell lines uncertain\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"The CCP6 domain alone (in oligomeric form) was shown to reprogram monocyte-derived DCs to a tolerogenic phenotype independently of complement regulation, extending C4BPA immunomodulation beyond CD40-mediated signaling.\",\n      \"evidence\": \"Recombinant CCP6 oligomer treatment of human monocyte-derived DCs; surface marker flow cytometry; cytokine ELISA; T cell alloproliferation assays\",\n      \"pmids\": [\"35547734\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Receptor on DCs mediating CCP6 recognition unidentified\", \"In vivo tolerogenic function not demonstrated\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"C4BP beta-chain binding to protein S was shown to competitively abolish protein S cofactor enhancement of TFPIα, revealing C4BP as a regulator of the TFPI anticoagulant pathway in addition to the APC pathway.\",\n      \"evidence\": \"Glycosylation scanning and alanine scanning mutagenesis of protein S LG1 domain; FXa inhibition assays with C4BP beta-chain\",\n      \"pmids\": [\"34731882\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo impact on TFPI-dependent anticoagulation not tested in animal models\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"S-palmitoylation of C4BPA at N-terminal cysteines by ZDHHC8 in epididymal epithelium was identified as the mechanism for C4BPA loading onto epididymosomes and transfer to sperm, where it provides complement C4 protection and maintains motility.\",\n      \"evidence\": \"Palmitoylation site mutagenesis (C15S, C13S/C23S); epididymosome fractionation; sperm complement challenge and motility assays in mouse and pig\",\n      \"pmids\": [\"39370067\", \"41619901\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Male fertility consequence of C4BPA palmitoylation loss not assessed in knockout animals\", \"Contribution relative to other complement regulators on sperm not quantified\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"In gastric cancer, C4BPA was found to promote tumor growth via JAK2/STAT3 activation and to generate C5a that polarizes macrophages to an M2-like phenotype through C5aR1/STAT3, defining a tumor-immune crosstalk axis.\",\n      \"evidence\": \"C4BPA knockdown/overexpression in GC cell lines and organoids; xenograft model; THP-1 macrophage co-culture with C5a rescue and C5aR1 inhibition\",\n      \"pmids\": [\"41237697\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which C4BPA increases C5a generation not resolved\", \"Single-lab finding not yet replicated\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"No high-resolution co-crystal structure of C4BPA bound to C4b or CD40 exists; the DC receptor for CCP6-mediated tolerogenic reprogramming is unidentified; and the physiological significance of intracellular C4BPA–RelA interaction outside cancer contexts is unknown.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Structural basis of C4b and CD40 recognition at atomic resolution\", \"Identity of DC receptor for CCP6\", \"In vivo relevance of intracellular C4BPA functions\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1, 2, 3, 4, 5, 18]},\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [7, 16]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 5, 6, 17, 19]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [20]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [15]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 1, 2, 3, 4, 5, 7, 22]},\n      {\"term_id\": \"R-HSA-109582\", \"supporting_discovery_ids\": [8, 10]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [7, 15, 16, 21]}\n    ],\n    \"complexes\": [\n      \"C4b-binding protein (C4BP)\"\n    ],\n    \"partners\": [\n      \"C4B\",\n      \"C3\",\n      \"CFI\",\n      \"CD40\",\n      \"RELA\",\n      \"PROS1\",\n      \"C4BPB\",\n      \"ZDHHC8\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}