{"gene":"C4BPB","run_date":"2026-04-28T17:12:38","timeline":{"discoveries":[{"year":2003,"finding":"The alpha-chain of human C4BP binds directly to CD40 on human B cells at a site distinct from that used by CD40 ligand, inducing B cell proliferation, upregulation of CD54 and CD86, and IL4-dependent IgE isotype switching. This effect requires CD40 and IKKgamma/NEMO signaling.","method":"Direct binding assay, B cell proliferation assay, isotype switching assay, use of B cells from CD40/IKKgamma-deficient patients, colocalization in germinal centers","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods including direct binding, functional proliferation/switching assays, and genetic deficiency controls","pmids":["12818164"],"is_preprint":false},{"year":2001,"finding":"The N-terminal CCP domains 1–3 of the C4BP alpha-chain are required for C4b binding and complement regulatory activity; CCP2 and CCP3 are most critical. Polymeric C4BP is more efficient than monomeric forms at degrading surface-bound C4b. Spatial arrangements between CCPs are important for full function.","method":"Expression of 19 recombinant C4BP variants (truncated, polymeric with CCP deletions, alanine-insertion mutants); functional assays for C4b binding, factor I cofactor activity, C3-convertase inhibition","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstitution with extensive mutagenesis panel, multiple functional assays","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 C4BP alpha-chain constitutes the C4b-binding site and a specific heparin-binding site. These residues are required for factor I cofactor function and cleavage of specific peptide bonds in C4b.","method":"Site-directed mutagenesis (R39Q, R64Q/R66Q, R39Q/R64Q/R66Q), surface plasmon resonance-based binding assays, factor I cofactor degradation assays, heparin binding assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis with reconstituted in vitro functional assays, multiple mutants","pmids":["10383431"],"is_preprint":false},{"year":2003,"finding":"Mutations K126Q/K128Q and F144S/F149S in CCP3 of the C4BP alpha-chain selectively abolish factor I cofactor activity for C4b and C3b cleavage without affecting C4b/C3b binding affinity or inhibition of C3-convertase assembly/decay, indicating CCP3 contains a surface required for cofactor activity distinct from the binding site.","method":"Site-directed mutagenesis, surface plasmon resonance binding assays, fluid-phase cofactor assays, C3-convertase assembly/decay assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis with multiple orthogonal in vitro functional readouts","pmids":["12893820"],"is_preprint":false},{"year":2000,"finding":"Positively charged residues R39, K63, R64, and H67 at the CCP1-CCP2 interface of C4BP alpha-chain are required for C3-convertase regulation (inhibiting assembly and accelerating decay) and factor I cofactor activity in fluid-phase C4b degradation.","method":"Expression of nine C4BP mutants with positively charged amino acids replaced by glutamine; C3-convertase assembly/decay assays, factor I cofactor assays","journal":"Molecular immunology","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis panel correlated with multiple functional assays","pmids":["11090879"],"is_preprint":false},{"year":2002,"finding":"Deletion mutagenesis of C4BP alpha-chain SCR domains showed SCR2 and SCR3 are indispensable for C4b cleavage cofactor activity (SCR1 contributes additionally), while SCR1-5 participate in C3b cofactor activity with SCR2, 3, 4 being absolutely required. C4b and C3b binding domains partially differ from domains mediating cofactor activity.","method":"SCR-deletion mutants of recombinant multimeric C4BP; C3b/C4b-Sepharose binding, ELISA binding, fluid-phase cofactor assays","journal":"Journal of biochemistry","confidence":"High","confidence_rationale":"Tier 1 — systematic deletion mutagenesis with reconstituted in vitro cofactor assays","pmids":["12417021"],"is_preprint":false},{"year":2009,"finding":"C4BP regulates the lectin pathway C3/C5 convertase; at high C4b density on activating surfaces, all seven alpha-chains engage C4b simultaneously. C4BP has approximately 7–13-fold greater affinity for C4b deposited via the lectin pathway than via the classical pathway, providing stringent regulation of the lectin pathway.","method":"Binding assays on zymosan and mannan-coated erythrocytes; C3/C5 convertase assembly inhibition assays; quantitative C4b per C4BP analysis","journal":"Molecular immunology","confidence":"Medium","confidence_rationale":"Tier 2 — clean in vitro binding and functional assays with quantitative surface analysis, single study","pmids":["19660812"],"is_preprint":false},{"year":2006,"finding":"C4BP interacts with both C4c and C4dg subfragments of C4b via adjacent but distinct subsites within CCP1–3 of the alpha-chain; filling the C4dg subsite enhances C4c binding (and vice versa) by shifting a conformational equilibrium toward a high-affinity state, indicating synergy between C4b subsites.","method":"Surface plasmon resonance with C4c and C4dg subfragments; C4b-binding-defective C4BP mutants; cross-competition experiments","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — SPR with subfragments, multiple mutants, and cross-competition establishing mechanism","pmids":["16819837"],"is_preprint":false},{"year":1999,"finding":"The protein S binding site on C4BP is located in the beta-chain; SCR-1 is the primary binding domain and SCR-2 specifically contributes to the interaction, increasing affinity up to 5-fold over SCR-1 alone. Binding to SCR-1-containing constructs decreases protein S cofactor activity for activated protein C.","method":"Chimeric constructs of C4BP beta-chain SCRs fused to tissue-type plasminogen activator; protein S binding assays; protein S cofactor activity assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — chimeric domain constructs with binding and functional cofactor assays, replicated in follow-up study (PMID 10744423)","pmids":["10329721","10744423"],"is_preprint":false},{"year":1997,"finding":"A region of protein S around residues 447–460 (within the LG-type domain) constitutes a portion of the protein S binding site for C4BP; synthetic peptides spanning this region inhibit protein S–C4BP interaction and directly bind C4BP.","method":"Bacteriophage peptide display library selection against C4BP beta-chain; synthetic peptide inhibition assays; CD spectroscopy and tryptophan fluorescence polarization binding assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — phage display plus direct peptide binding confirmation, single study","pmids":["9169428"],"is_preprint":false},{"year":2022,"finding":"Protein S residues Lys255, Glu257, Asp287, Arg410, Lys423, and Glu424 in the LG1 domain are critical for TFPI cofactor function. C4BP beta-chain binding to protein S LG1 almost completely abolishes this TFPI cofactor function while leaving activated protein C cofactor function intact, demonstrating competitive regulation at a shared LG1 surface.","method":"N-linked glycosylation insertion scanning of protein S LG1; alanine substitution variants; FXa inhibition assays; plasma TFPI cofactor assays; C4BP beta-chain expression and binding","journal":"Blood advances","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with multiple orthogonal functional assays defining residue-level mechanism","pmids":["34731882"],"is_preprint":false},{"year":1995,"finding":"Human C4BP exists as multiple plasma isoforms (alpha7beta1, alpha7beta0, alpha6beta1) whose proportions are genetically determined by the relative expression levels of the C4BPA and C4BPB genes. The beta-chain (encoded by C4BPB) binds protein S, while the alpha-chain mediates complement regulation.","method":"Biochemical characterization of HepG2 and Hep3B cell secretion; COS cell transfection experiments; gel electrophoresis and immunoassay quantification of isoforms","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — transfection and cell secretion experiments establishing genetic basis of isoform composition, single study","pmids":["7561113"],"is_preprint":false},{"year":1995,"finding":"IL-6, IL-1β, and IFN-γ increase both C4BPA and C4BPB mRNA levels, while TNF-α downregulates both. IFN-γ shows a differential effect, and when combined with TNF-α produces synergistic 10-fold induction of C4BPA mRNA but only marginal increase of C4BPB mRNA, providing a mechanism to maintain C4BP beta concentrations during acute phase response.","method":"Hep3B cell treatment with cytokines; Northern blot mRNA quantification of C4BPA and C4BPB; acute phase patient plasma C4BP isoform analysis","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — cell-based mRNA and protein expression experiments with multiple cytokines, single study","pmids":["7561114"],"is_preprint":false},{"year":1996,"finding":"The C4BPB gene promoter region from -126 to +25 drives hepatocyte expression; a critical subfragment (-126 to -90) provides >90% of promoter activity through cooperative binding of HNF-3 and NF-I/CTF transcription factors.","method":"Reporter gene assays in HepG2 cells with promoter deletion constructs; electrophoretic mobility shift assays identifying HNF-3 and NF-I/CTF binding","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — promoter deletion mapping and EMSA in hepatocyte cell line, single study","pmids":["8598458"],"is_preprint":false},{"year":1994,"finding":"The murine C4BPB gene is a single-copy pseudogene containing two in-phase stop codons in its CCP-encoding exons, incompatible with a functional C4BP beta polypeptide. This demonstrates that the mouse lacks a functional beta-chain and thus cannot form the protein S-binding C4BP isoform.","method":"Genomic DNA isolation and sequencing from multiple mouse strains; Southern blotting; chromosomal mapping showing close linkage to C4BPA on mouse chromosome 1","journal":"Genomics","confidence":"High","confidence_rationale":"Tier 1 — direct genomic sequencing across multiple strains establishing pseudogene status","pmids":["7959726"],"is_preprint":false},{"year":2008,"finding":"LPS decreases both C4BPalpha and C4BPbeta expression in rat hepatocytes via NFkappaB and MEK/ERK pathways, while IL-6 specifically increases C4BPbeta expression via STAT-3, leading to increased plasma PS-C4BP complex and decreased protein S anticoagulant activity.","method":"In vivo rat LPS/IL-6 injection; isolated rat hepatocyte treatment; mRNA and protein quantification; pathway inhibitor experiments (NFkappaB, MEK/ERK, STAT-3 inhibitors)","journal":"Journal of thrombosis and haemostasis","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo and in vitro experiments with pharmacological pathway inhibitors, single study","pmids":["18752574"],"is_preprint":false},{"year":1983,"finding":"Chymotryptic cleavage of C4BP yields a 48 kDa N-terminal fragment and a 27 kDa C-terminal fragment joined by disulfide bonds; the 48 kDa fragment released from the N-terminal side retains C4b-binding activity, while the protein S binding site was localized to the core (C-terminal) domain.","method":"Chymotrypsin proteolysis; N-terminal amino acid sequencing; C4b and protein S binding assays of fragments","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 — direct proteolytic mapping combined with binding assays, early structural characterization","pmids":["6653778"],"is_preprint":false},{"year":2003,"finding":"Protein S and the C4BP-protein S complex bind to apoptotic neutrophils (and Jurkat cells) through the Gla domain of protein S interacting with negatively charged phospholipids exposed on apoptotic cells; only the apoptotic neutrophil subpopulation binds these proteins.","method":"Flow cytometry-based binding assays; blocking with anti-Gla domain monoclonal antibody; comparison of apoptotic vs. non-apoptotic neutrophil populations","journal":"Blood coagulation & fibrinolysis","confidence":"Medium","confidence_rationale":"Tier 2 — antibody blocking of defined domain with apoptotic cell model, single study","pmids":["12945877"],"is_preprint":false},{"year":2008,"finding":"Human C4BP injected intraperitoneally inhibits complement-mediated arthritis in mouse models (CAIA and CIA), with C4BP inhibiting the classical but also the alternative pathway when present on activating surfaces, ameliorating disease severity without affecting anti-CII antibody synthesis.","method":"Mouse collagen antibody-induced arthritis (CAIA) and collagen-induced arthritis (CIA) models; intraperitoneal injection of purified human C4BP; classical and alternative pathway activity assays; anti-CII antibody measurement","journal":"Annals of the rheumatic diseases","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo mouse models with complement pathway functional assays, single study","pmids":["18276745"],"is_preprint":false},{"year":2006,"finding":"The hypervariable region (HVR) of streptococcal M proteins folds as a coiled-coil and binds C4BP via a surface spanning four heptad repeats in approximately the N-terminal 27-residue folded nucleus of M4 HVR; the C4BP binding surface of M4 is distinct from the folded coiled-coil core.","method":"NMR spectroscopy of M4 and M22 HVRs free and in complex with C4BP fragment; molecular modeling","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 1 — NMR structure determination with complex, but no mutagenesis validation; single study","pmids":["16584191"],"is_preprint":false},{"year":2024,"finding":"Crystal structures of M68 and M87 HVRs in complex with a C4BP fragment reveal distinct C4BP-binding sequence patterns beyond the previously characterized M2 and M22 patterns; mutagenesis identified critical amino acids for C4BP binding in each pattern type. These patterns are also present in M-like Enn proteins, enabling C4BP recruitment.","method":"X-ray crystallography of HVR-C4BP fragment complexes; site-directed mutagenesis of M proteins and Enn proteins; C4BP binding assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — crystal structures combined with mutagenesis and binding assays","pmids":["38879009"],"is_preprint":false},{"year":1996,"finding":"Streptococcal surface molecules bind C4BP at a site overlapping and indistinguishable from the C4b binding site on CCP1-3 of the alpha-chain, suggesting molecular mimicry of C4b epitopes by bacterial surface proteins.","method":"Competitive inhibition assays with anti-C4BP monoclonal antibodies and C4b; binding assays with C4BP mutants; mapping to SCR 1-3","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — competitive binding with defined antibodies and C4b, single study","pmids":["8943398"],"is_preprint":false},{"year":2001,"finding":"Binding of C4BP to the hypervariable region (HVR) of streptococcal M22 protein contributes to phagocytosis resistance; anti-HVR antibodies that block C4BP binding cause opsonization, a short HVR deletion eliminates C4BP binding and reduces phagocytosis resistance, and excess purified C4BP blocks opsonizing antibody effects.","method":"Opsonization assays; antibody inhibition of C4BP binding; HVR deletion mutagenesis; competitive addition of purified C4BP to phagocytosis system","journal":"Molecular microbiology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal approaches (antibody competition, mutagenesis, competitive ligand) all pointing to same functional conclusion","pmids":["11703674"],"is_preprint":false},{"year":2019,"finding":"C4BP(β-) isoform (lacking the beta-chain) exerts a novel immunomodulatory activity that 'reprograms' monocyte-derived dendritic cells from a pro-inflammatory to an anti-inflammatory/tolerogenic state; incorporation of the beta-chain into the oligomer interferes with this activity. The CCP6 domain of the C4BP alpha-chain (in PRP6-HO7 construct) is sufficient for immunomodulatory activity independent of complement regulatory function.","method":"In vivo lupus-prone mouse model; histology; anti-dsDNA antibody and complement deposition measurements; transcriptional profiling; cytokine profiling; immunohistochemistry; monocyte-derived DC reprogramming assays","journal":"Kidney international / Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo and in vitro experiments with domain-deletion constructs revealing non-canonical activity, partially replicated across two studies","pmids":["31982108","35547734"],"is_preprint":false}],"current_model":"C4BP (composed of alpha-chains encoded by C4BPA and a unique beta-chain encoded by C4BPB) functions as the principal inhibitor of the classical and lectin complement pathways by binding C4b through a positively charged cluster at the CCP1-CCP2 interface of the alpha-chain (with CCP2 and CCP3 most critical) and serving as a cofactor for factor I-mediated proteolysis of C4b (requiring CCP1-3) and C3b (requiring CCP1-5); the beta-chain's SCR1-2 binds protein S with high affinity, competing with protein S's TFPI cofactor function while the alpha-chain's CCP6 domain mediates a non-canonical immunomodulatory activity that reprograms inflammatory myeloid cells, and the alpha-chain also acts as an activating ligand for CD40 on B cells."},"narrative":{"teleology":[{"year":1983,"claim":"Early proteolytic mapping established that C4BP harbors separable functional domains: an N-terminal region mediating C4b binding and a core/C-terminal domain binding protein S, setting the stage for chain-specific functional dissection.","evidence":"Chymotryptic fragmentation of purified C4BP with binding assays for C4b and protein S","pmids":["6653778"],"confidence":"Medium","gaps":["No chain-level resolution (alpha vs. beta not distinguished)","No domain boundaries defined at the SCR/CCP level"]},{"year":1995,"claim":"Demonstration that C4BP exists as multiple isoforms (alpha7-beta1, alpha7-beta0, alpha6-beta1) whose stoichiometry is genetically set by relative C4BPA/C4BPB expression resolved a long-standing question of why free protein S varies among individuals; it also showed cytokine-driven differential regulation (TNF-α/IFN-γ synergy on C4BPA >> C4BPB) provides a mechanism to shift isoform composition during inflammation.","evidence":"HepG2/Hep3B secretion studies, COS transfection, Northern blots with cytokine treatments in Hep3B cells, acute-phase patient plasma analysis","pmids":["7561113","7561114"],"confidence":"Medium","gaps":["Promoter elements driving differential regulation only partially mapped","In vivo human time-course data lacking"]},{"year":1996,"claim":"Identification of the C4BPB promoter region (-126 to +25) driven by cooperative HNF-3 and NF-I/CTF binding explained hepatocyte-specific expression and provided a framework for understanding how cytokine signaling differentially controls C4BPB transcription.","evidence":"Reporter gene assays with promoter deletions in HepG2; EMSA identifying transcription factor binding sites","pmids":["8598458"],"confidence":"Medium","gaps":["Cytokine-responsive elements not mapped within this promoter","In vivo chromatin accessibility not examined"]},{"year":1994,"claim":"Discovery that the murine C4BPB gene is a pseudogene with in-phase stop codons established that the protein S–C4BP regulatory axis is absent in mice, a critical caveat for interpreting mouse models of complement and coagulation.","evidence":"Genomic sequencing of C4BPB across multiple mouse strains; Southern blotting and chromosomal mapping","pmids":["7959726"],"confidence":"High","gaps":["Functional consequences of absent beta-chain for mouse protein S regulation not fully characterized"]},{"year":1999,"claim":"Mapping the C4b-binding site to a positively charged cluster (R39, R64, R66) at the CCP1–CCP2 junction of the alpha-chain, and identifying beta-chain SCR1–SCR2 as the protein S binding site, achieved residue-level functional assignment for both chains of C4BP.","evidence":"Site-directed mutagenesis with SPR binding and factor I cofactor assays (alpha-chain); chimeric SCR constructs with protein S binding and APC cofactor assays (beta-chain)","pmids":["10383431","10329721","10744423"],"confidence":"High","gaps":["Structural basis of SCR1–SCR2/protein S interface not resolved at atomic level","No crystal structure of beta-chain domains available"]},{"year":2001,"claim":"Systematic CCP-deletion analysis of the alpha-chain defined the minimal domain requirements: CCP1–3 for C4b cofactor activity (CCP2–3 indispensable), CCP1–5 for C3b cofactor activity, with polymerization enhancing surface-bound C4b degradation efficiency.","evidence":"19 recombinant C4BP variants including truncations and deletions tested for C4b/C3b binding, cofactor, and convertase inhibition","pmids":["11369776","12417021"],"confidence":"High","gaps":["Mechanism by which polymerization enhances activity (avidity vs. conformational) not resolved"]},{"year":2003,"claim":"Identification of a cofactor-specific surface on CCP3 (K126/K128, F144/F149) that is essential for factor I–mediated cleavage but dispensable for C4b binding or convertase decay separated binding from catalytic cofactor function, demonstrating that C4BP employs distinct surfaces for substrate capture versus protease activation.","evidence":"Site-directed mutagenesis with SPR, cofactor, and convertase assembly/decay assays","pmids":["12893820"],"confidence":"High","gaps":["Structural mechanism of factor I activation by CCP3 residues unknown","No ternary C4BP–C4b–factor I complex structure"]},{"year":2003,"claim":"Discovery that the C4BP alpha-chain binds CD40 on B cells at a site distinct from CD40L, triggering proliferation, co-stimulatory molecule upregulation, and IL-4-dependent IgE switching, revealed an unexpected immune-regulatory function beyond complement control.","evidence":"Direct binding assays, B cell proliferation and isotype switching assays, CD40-deficient and IKKγ-deficient patient B cells, germinal center colocalization","pmids":["12818164"],"confidence":"High","gaps":["Alpha-chain domain mediating CD40 binding not mapped","Physiological significance in vivo not established"]},{"year":2006,"claim":"SPR analysis of C4c and C4dg subfragments revealed that C4BP binds C4b through synergistic subsites within CCP1–3, where occupancy of one subsite shifts a conformational equilibrium to enhance affinity at the other, explaining the high cooperativity of C4b recognition.","evidence":"SPR with C4b subfragments, C4BP mutants, and cross-competition experiments","pmids":["16819837"],"confidence":"High","gaps":["Atomic-level structural basis of conformational shift not determined"]},{"year":2008,"claim":"In vivo demonstration that IL-6 upregulates C4BPB specifically through STAT3, while LPS downregulates both chains via NF-κB/MEK-ERK, provided a signaling-level mechanism for how inflammation shifts C4BP isoforms to increase protein S sequestration and reduce anticoagulant activity.","evidence":"Rat LPS/IL-6 injection, isolated hepatocyte treatment, pathway inhibitor experiments","pmids":["18752574"],"confidence":"Medium","gaps":["Direct STAT3 binding to C4BPB promoter not shown","Translation to human hepatocytes not confirmed"]},{"year":2019,"claim":"The beta-chain-lacking C4BP(β−) isoform was shown to reprogram inflammatory myeloid cells to an anti-inflammatory/tolerogenic state via the alpha-chain CCP6 domain, while incorporation of the beta-chain suppresses this activity, establishing the beta-chain as a negative modulator of C4BP's non-canonical immunomodulatory function.","evidence":"Lupus-prone mouse model, monocyte-derived DC reprogramming, domain-deletion constructs, cytokine and transcriptional profiling","pmids":["31982108","35547734"],"confidence":"Medium","gaps":["Receptor for CCP6-mediated myeloid reprogramming unidentified","Mechanism by which beta-chain interferes with CCP6 activity unclear","Human in vivo relevance not demonstrated"]},{"year":2022,"claim":"Residue-level mapping showed that C4BP beta-chain binding to protein S LG1 domain abolishes TFPI cofactor function while preserving APC cofactor function, resolving a longstanding question of how protein S simultaneously participates in anticoagulant and complement pathways and establishing competitive regulation at a shared surface.","evidence":"Glycosylation insertion scanning, alanine mutagenesis of protein S LG1, FXa inhibition assays, plasma TFPI cofactor assays","pmids":["34731882"],"confidence":"High","gaps":["Structural model of beta-chain SCR1–2/protein S LG1 complex lacking","In vivo consequences of selective TFPI pathway inhibition not tested"]},{"year":2024,"claim":"Crystal structures of streptococcal M-protein HVRs in complex with C4BP alpha-chain fragments revealed multiple distinct C4BP-binding sequence patterns beyond previously known types, showing that pathogen exploitation of the C4b-binding site has diversified through convergent evolution including in M-like Enn proteins.","evidence":"X-ray crystallography, site-directed mutagenesis of M proteins and Enn proteins, C4BP binding assays","pmids":["38879009"],"confidence":"High","gaps":["Full-length C4BP–M protein complex structure not available","Functional impact on complement evasion in animal infection models not tested with these specific patterns"]},{"year":null,"claim":"Key unresolved questions include: the atomic structure of the beta-chain SCR1–2/protein S complex, the receptor through which the C4BP(β−) CCP6 domain reprograms myeloid cells, the structural mechanism by which beta-chain incorporation suppresses immunomodulatory activity, and whether isoform-specific C4BP functions are druggable targets in thromboinflammatory disease.","evidence":"","pmids":[],"confidence":"Low","gaps":["No crystal/cryo-EM structure of beta-chain or its complex with protein S","CCP6 myeloid receptor unidentified","No therapeutic modulation studies targeting isoform balance"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[1,2,3,4,5,6,7,8,10]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[8,10,11]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[11,16,17]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[1,2,3,4,5,6,7,18,23]},{"term_id":"R-HSA-109582","term_label":"Hemostasis","supporting_discovery_ids":[8,10,15]}],"complexes":["C4b-binding protein (C4BP)"],"partners":["C4BPA","PROS1","C4B","C3","CFI","CD40"],"other_free_text":[]},"mechanistic_narrative":"C4BPB encodes the beta-chain of C4b-binding protein (C4BP), a major soluble inhibitor of the classical and lectin complement pathways whose alpha-chains bind C4b through a positively charged cluster at the CCP1–CCP2 interface and serve as cofactor for factor I–mediated cleavage of C4b and C3b [PMID:10383431, PMID:11369776, PMID:12893820]. The beta-chain's SCR1–SCR2 domains constitute the high-affinity protein S binding site; this interaction competitively abolishes protein S cofactor function for TFPI without affecting its activated protein C cofactor activity, thereby linking complement regulation to coagulation control [PMID:10329721, PMID:34731882]. Plasma C4BP isoform composition (alpha7-beta1, alpha7-beta0, alpha6-beta1) is genetically determined by relative C4BPA and C4BPB expression levels, and differential cytokine regulation—IL-6 selectively upregulates C4BPB via STAT3 while TNF-α/IFN-γ combinations preferentially induce C4BPA—shifts the balance toward beta-chain-lacking isoforms during acute-phase responses, augmenting a non-canonical immunomodulatory activity of the alpha-chain CCP6 domain that reprograms inflammatory myeloid cells [PMID:7561114, PMID:18752574, PMID:31982108]. Multiple streptococcal M-protein hypervariable regions recruit C4BP to the bacterial surface via the alpha-chain CCP1–3 C4b-binding site, enabling phagocytosis resistance through molecular mimicry [PMID:8943398, PMID:11703674, PMID:38879009]."},"prefetch_data":{"uniprot":{"accession":"P20851","full_name":"C4b-binding protein beta chain","aliases":[],"length_aa":252,"mass_kda":28.4,"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. It also interacts with anticoagulant protein S and with serum amyloid P component. The beta chain binds protein S","subcellular_location":"Secreted","url":"https://www.uniprot.org/uniprotkb/P20851/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/C4BPB","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/C4BPB","total_profiled":1310},"omim":[{"mim_id":"176880","title":"PROTEIN S; PROS1","url":"https://www.omim.org/entry/176880"},{"mim_id":"171835","title":"6-@PHOSPHOFRUCTO-2-KINASE/FRUCTOSE-2,6-BISPHOSPHATASE 2; PFKFB2","url":"https://www.omim.org/entry/171835"},{"mim_id":"134370","title":"COMPLEMENT FACTOR H; CFH","url":"https://www.omim.org/entry/134370"},{"mim_id":"120831","title":"COMPLEMENT COMPONENT 4-BINDING PROTEIN, BETA CHAIN; C4BPB","url":"https://www.omim.org/entry/120831"},{"mim_id":"120830","title":"COMPLEMENT COMPONENT 4-BINDING PROTEIN, ALPHA; C4BPA","url":"https://www.omim.org/entry/120830"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Uncertain","locations":[{"location":"Golgi apparatus","reliability":"Uncertain"}],"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":"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":"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":"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_2024.11.29.24317861","title":"Identification of disease-specific extracellular vesicle-associated plasma protein biomarkers for Duchenne Muscular Dystrophy and Facioscapulohumeral Muscular Dystrophy","date":"2024-11-30","source":"bioRxiv","url":"https://doi.org/10.1101/2024.11.29.24317861","citation_count":0,"is_preprint":true},{"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":38263,"output_tokens":5686,"usd":0.10004},"stage2":{"model":"claude-opus-4-6","input_tokens":9287,"output_tokens":3413,"usd":0.19764},"total_usd":0.29768,"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\": 2003,\n      \"finding\": \"The alpha-chain of human C4BP binds directly to CD40 on human B cells at a site distinct from that used by CD40 ligand, inducing B cell proliferation, upregulation of CD54 and CD86, and IL4-dependent IgE isotype switching. This effect requires CD40 and IKKgamma/NEMO signaling.\",\n      \"method\": \"Direct binding assay, B cell proliferation assay, isotype switching assay, use of B cells from CD40/IKKgamma-deficient patients, colocalization in germinal centers\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods including direct binding, functional proliferation/switching assays, and genetic deficiency controls\",\n      \"pmids\": [\"12818164\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"The N-terminal CCP domains 1–3 of the C4BP alpha-chain are required for C4b binding and complement regulatory activity; CCP2 and CCP3 are most critical. Polymeric C4BP is more efficient than monomeric forms at degrading surface-bound C4b. Spatial arrangements between CCPs are important for full function.\",\n      \"method\": \"Expression of 19 recombinant C4BP variants (truncated, polymeric with CCP deletions, alanine-insertion mutants); functional assays for C4b binding, factor I cofactor activity, C3-convertase inhibition\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution with extensive mutagenesis panel, multiple functional assays\",\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 C4BP alpha-chain constitutes the C4b-binding site and a specific heparin-binding site. These residues are required for factor I cofactor function and cleavage of specific peptide bonds in C4b.\",\n      \"method\": \"Site-directed mutagenesis (R39Q, R64Q/R66Q, R39Q/R64Q/R66Q), surface plasmon resonance-based binding assays, factor I cofactor degradation assays, heparin binding assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with reconstituted in vitro functional assays, multiple mutants\",\n      \"pmids\": [\"10383431\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Mutations K126Q/K128Q and F144S/F149S in CCP3 of the C4BP alpha-chain selectively abolish factor I cofactor activity for C4b and C3b cleavage without affecting C4b/C3b binding affinity or inhibition of C3-convertase assembly/decay, indicating CCP3 contains a surface required for cofactor activity distinct from the binding site.\",\n      \"method\": \"Site-directed mutagenesis, surface plasmon resonance binding assays, fluid-phase cofactor assays, C3-convertase assembly/decay assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with multiple orthogonal in vitro functional readouts\",\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 C4BP alpha-chain are required for C3-convertase regulation (inhibiting assembly and accelerating decay) and factor I cofactor activity in fluid-phase C4b degradation.\",\n      \"method\": \"Expression of nine C4BP mutants with positively charged amino acids replaced by glutamine; C3-convertase assembly/decay assays, factor I cofactor assays\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis panel correlated with multiple functional assays\",\n      \"pmids\": [\"11090879\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Deletion mutagenesis of C4BP alpha-chain SCR domains showed SCR2 and SCR3 are indispensable for C4b cleavage cofactor activity (SCR1 contributes additionally), while SCR1-5 participate in C3b cofactor activity with SCR2, 3, 4 being absolutely required. C4b and C3b binding domains partially differ from domains mediating cofactor activity.\",\n      \"method\": \"SCR-deletion mutants of recombinant multimeric C4BP; C3b/C4b-Sepharose binding, ELISA binding, fluid-phase cofactor assays\",\n      \"journal\": \"Journal of biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic deletion mutagenesis with reconstituted in vitro cofactor assays\",\n      \"pmids\": [\"12417021\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"C4BP regulates the lectin pathway C3/C5 convertase; at high C4b density on activating surfaces, all seven alpha-chains engage C4b simultaneously. C4BP has approximately 7–13-fold greater affinity for C4b deposited via the lectin pathway than via the classical pathway, providing stringent regulation of the lectin pathway.\",\n      \"method\": \"Binding assays on zymosan and mannan-coated erythrocytes; C3/C5 convertase assembly inhibition assays; quantitative C4b per C4BP analysis\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean in vitro binding and functional assays with quantitative surface analysis, single study\",\n      \"pmids\": [\"19660812\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"C4BP interacts with both C4c and C4dg subfragments of C4b via adjacent but distinct subsites within CCP1–3 of the alpha-chain; filling the C4dg subsite enhances C4c binding (and vice versa) by shifting a conformational equilibrium toward a high-affinity state, indicating synergy between C4b subsites.\",\n      \"method\": \"Surface plasmon resonance with C4c and C4dg subfragments; C4b-binding-defective C4BP mutants; cross-competition experiments\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — SPR with subfragments, multiple mutants, and cross-competition establishing mechanism\",\n      \"pmids\": [\"16819837\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"The protein S binding site on C4BP is located in the beta-chain; SCR-1 is the primary binding domain and SCR-2 specifically contributes to the interaction, increasing affinity up to 5-fold over SCR-1 alone. Binding to SCR-1-containing constructs decreases protein S cofactor activity for activated protein C.\",\n      \"method\": \"Chimeric constructs of C4BP beta-chain SCRs fused to tissue-type plasminogen activator; protein S binding assays; protein S cofactor activity assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — chimeric domain constructs with binding and functional cofactor assays, replicated in follow-up study (PMID 10744423)\",\n      \"pmids\": [\"10329721\", \"10744423\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"A region of protein S around residues 447–460 (within the LG-type domain) constitutes a portion of the protein S binding site for C4BP; synthetic peptides spanning this region inhibit protein S–C4BP interaction and directly bind C4BP.\",\n      \"method\": \"Bacteriophage peptide display library selection against C4BP beta-chain; synthetic peptide inhibition assays; CD spectroscopy and tryptophan fluorescence polarization binding assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — phage display plus direct peptide binding confirmation, single study\",\n      \"pmids\": [\"9169428\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Protein S residues Lys255, Glu257, Asp287, Arg410, Lys423, and Glu424 in the LG1 domain are critical for TFPI cofactor function. C4BP beta-chain binding to protein S LG1 almost completely abolishes this TFPI cofactor function while leaving activated protein C cofactor function intact, demonstrating competitive regulation at a shared LG1 surface.\",\n      \"method\": \"N-linked glycosylation insertion scanning of protein S LG1; alanine substitution variants; FXa inhibition assays; plasma TFPI cofactor assays; C4BP beta-chain expression and binding\",\n      \"journal\": \"Blood advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with multiple orthogonal functional assays defining residue-level mechanism\",\n      \"pmids\": [\"34731882\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Human C4BP exists as multiple plasma isoforms (alpha7beta1, alpha7beta0, alpha6beta1) whose proportions are genetically determined by the relative expression levels of the C4BPA and C4BPB genes. The beta-chain (encoded by C4BPB) binds protein S, while the alpha-chain mediates complement regulation.\",\n      \"method\": \"Biochemical characterization of HepG2 and Hep3B cell secretion; COS cell transfection experiments; gel electrophoresis and immunoassay quantification of isoforms\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — transfection and cell secretion experiments establishing genetic basis of isoform composition, single study\",\n      \"pmids\": [\"7561113\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"IL-6, IL-1β, and IFN-γ increase both C4BPA and C4BPB mRNA levels, while TNF-α downregulates both. IFN-γ shows a differential effect, and when combined with TNF-α produces synergistic 10-fold induction of C4BPA mRNA but only marginal increase of C4BPB mRNA, providing a mechanism to maintain C4BP beta concentrations during acute phase response.\",\n      \"method\": \"Hep3B cell treatment with cytokines; Northern blot mRNA quantification of C4BPA and C4BPB; acute phase patient plasma C4BP isoform analysis\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — cell-based mRNA and protein expression experiments with multiple cytokines, single study\",\n      \"pmids\": [\"7561114\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"The C4BPB gene promoter region from -126 to +25 drives hepatocyte expression; a critical subfragment (-126 to -90) provides >90% of promoter activity through cooperative binding of HNF-3 and NF-I/CTF transcription factors.\",\n      \"method\": \"Reporter gene assays in HepG2 cells with promoter deletion constructs; electrophoretic mobility shift assays identifying HNF-3 and NF-I/CTF binding\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — promoter deletion mapping and EMSA in hepatocyte cell line, single study\",\n      \"pmids\": [\"8598458\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"The murine C4BPB gene is a single-copy pseudogene containing two in-phase stop codons in its CCP-encoding exons, incompatible with a functional C4BP beta polypeptide. This demonstrates that the mouse lacks a functional beta-chain and thus cannot form the protein S-binding C4BP isoform.\",\n      \"method\": \"Genomic DNA isolation and sequencing from multiple mouse strains; Southern blotting; chromosomal mapping showing close linkage to C4BPA on mouse chromosome 1\",\n      \"journal\": \"Genomics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct genomic sequencing across multiple strains establishing pseudogene status\",\n      \"pmids\": [\"7959726\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"LPS decreases both C4BPalpha and C4BPbeta expression in rat hepatocytes via NFkappaB and MEK/ERK pathways, while IL-6 specifically increases C4BPbeta expression via STAT-3, leading to increased plasma PS-C4BP complex and decreased protein S anticoagulant activity.\",\n      \"method\": \"In vivo rat LPS/IL-6 injection; isolated rat hepatocyte treatment; mRNA and protein quantification; pathway inhibitor experiments (NFkappaB, MEK/ERK, STAT-3 inhibitors)\",\n      \"journal\": \"Journal of thrombosis and haemostasis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo and in vitro experiments with pharmacological pathway inhibitors, single study\",\n      \"pmids\": [\"18752574\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1983,\n      \"finding\": \"Chymotryptic cleavage of C4BP yields a 48 kDa N-terminal fragment and a 27 kDa C-terminal fragment joined by disulfide bonds; the 48 kDa fragment released from the N-terminal side retains C4b-binding activity, while the protein S binding site was localized to the core (C-terminal) domain.\",\n      \"method\": \"Chymotrypsin proteolysis; N-terminal amino acid sequencing; C4b and protein S binding assays of fragments\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct proteolytic mapping combined with binding assays, early structural characterization\",\n      \"pmids\": [\"6653778\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Protein S and the C4BP-protein S complex bind to apoptotic neutrophils (and Jurkat cells) through the Gla domain of protein S interacting with negatively charged phospholipids exposed on apoptotic cells; only the apoptotic neutrophil subpopulation binds these proteins.\",\n      \"method\": \"Flow cytometry-based binding assays; blocking with anti-Gla domain monoclonal antibody; comparison of apoptotic vs. non-apoptotic neutrophil populations\",\n      \"journal\": \"Blood coagulation & fibrinolysis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — antibody blocking of defined domain with apoptotic cell model, single study\",\n      \"pmids\": [\"12945877\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Human C4BP injected intraperitoneally inhibits complement-mediated arthritis in mouse models (CAIA and CIA), with C4BP inhibiting the classical but also the alternative pathway when present on activating surfaces, ameliorating disease severity without affecting anti-CII antibody synthesis.\",\n      \"method\": \"Mouse collagen antibody-induced arthritis (CAIA) and collagen-induced arthritis (CIA) models; intraperitoneal injection of purified human C4BP; classical and alternative pathway activity assays; anti-CII antibody measurement\",\n      \"journal\": \"Annals of the rheumatic diseases\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo mouse models with complement pathway functional assays, single study\",\n      \"pmids\": [\"18276745\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The hypervariable region (HVR) of streptococcal M proteins folds as a coiled-coil and binds C4BP via a surface spanning four heptad repeats in approximately the N-terminal 27-residue folded nucleus of M4 HVR; the C4BP binding surface of M4 is distinct from the folded coiled-coil core.\",\n      \"method\": \"NMR spectroscopy of M4 and M22 HVRs free and in complex with C4BP fragment; molecular modeling\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — NMR structure determination with complex, but no mutagenesis validation; single study\",\n      \"pmids\": [\"16584191\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Crystal structures of M68 and M87 HVRs in complex with a C4BP fragment reveal distinct C4BP-binding sequence patterns beyond the previously characterized M2 and M22 patterns; mutagenesis identified critical amino acids for C4BP binding in each pattern type. These patterns are also present in M-like Enn proteins, enabling C4BP recruitment.\",\n      \"method\": \"X-ray crystallography of HVR-C4BP fragment complexes; site-directed mutagenesis of M proteins and Enn proteins; C4BP binding assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structures combined with mutagenesis and binding assays\",\n      \"pmids\": [\"38879009\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Streptococcal surface molecules bind C4BP at a site overlapping and indistinguishable from the C4b binding site on CCP1-3 of the alpha-chain, suggesting molecular mimicry of C4b epitopes by bacterial surface proteins.\",\n      \"method\": \"Competitive inhibition assays with anti-C4BP monoclonal antibodies and C4b; binding assays with C4BP mutants; mapping to SCR 1-3\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — competitive binding with defined antibodies and C4b, single study\",\n      \"pmids\": [\"8943398\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Binding of C4BP to the hypervariable region (HVR) of streptococcal M22 protein contributes to phagocytosis resistance; anti-HVR antibodies that block C4BP binding cause opsonization, a short HVR deletion eliminates C4BP binding and reduces phagocytosis resistance, and excess purified C4BP blocks opsonizing antibody effects.\",\n      \"method\": \"Opsonization assays; antibody inhibition of C4BP binding; HVR deletion mutagenesis; competitive addition of purified C4BP to phagocytosis system\",\n      \"journal\": \"Molecular microbiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal approaches (antibody competition, mutagenesis, competitive ligand) all pointing to same functional conclusion\",\n      \"pmids\": [\"11703674\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"C4BP(β-) isoform (lacking the beta-chain) exerts a novel immunomodulatory activity that 'reprograms' monocyte-derived dendritic cells from a pro-inflammatory to an anti-inflammatory/tolerogenic state; incorporation of the beta-chain into the oligomer interferes with this activity. The CCP6 domain of the C4BP alpha-chain (in PRP6-HO7 construct) is sufficient for immunomodulatory activity independent of complement regulatory function.\",\n      \"method\": \"In vivo lupus-prone mouse model; histology; anti-dsDNA antibody and complement deposition measurements; transcriptional profiling; cytokine profiling; immunohistochemistry; monocyte-derived DC reprogramming assays\",\n      \"journal\": \"Kidney international / Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo and in vitro experiments with domain-deletion constructs revealing non-canonical activity, partially replicated across two studies\",\n      \"pmids\": [\"31982108\", \"35547734\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"C4BP (composed of alpha-chains encoded by C4BPA and a unique beta-chain encoded by C4BPB) functions as the principal inhibitor of the classical and lectin complement pathways by binding C4b through a positively charged cluster at the CCP1-CCP2 interface of the alpha-chain (with CCP2 and CCP3 most critical) and serving as a cofactor for factor I-mediated proteolysis of C4b (requiring CCP1-3) and C3b (requiring CCP1-5); the beta-chain's SCR1-2 binds protein S with high affinity, competing with protein S's TFPI cofactor function while the alpha-chain's CCP6 domain mediates a non-canonical immunomodulatory activity that reprograms inflammatory myeloid cells, and the alpha-chain also acts as an activating ligand for CD40 on B cells.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"C4BPB encodes the beta-chain of C4b-binding protein (C4BP), a major soluble inhibitor of the classical and lectin complement pathways whose alpha-chains bind C4b through a positively charged cluster at the CCP1–CCP2 interface and serve as cofactor for factor I–mediated cleavage of C4b and C3b [PMID:10383431, PMID:11369776, PMID:12893820]. The beta-chain's SCR1–SCR2 domains constitute the high-affinity protein S binding site; this interaction competitively abolishes protein S cofactor function for TFPI without affecting its activated protein C cofactor activity, thereby linking complement regulation to coagulation control [PMID:10329721, PMID:34731882]. Plasma C4BP isoform composition (alpha7-beta1, alpha7-beta0, alpha6-beta1) is genetically determined by relative C4BPA and C4BPB expression levels, and differential cytokine regulation—IL-6 selectively upregulates C4BPB via STAT3 while TNF-α/IFN-γ combinations preferentially induce C4BPA—shifts the balance toward beta-chain-lacking isoforms during acute-phase responses, augmenting a non-canonical immunomodulatory activity of the alpha-chain CCP6 domain that reprograms inflammatory myeloid cells [PMID:7561114, PMID:18752574, PMID:31982108]. Multiple streptococcal M-protein hypervariable regions recruit C4BP to the bacterial surface via the alpha-chain CCP1–3 C4b-binding site, enabling phagocytosis resistance through molecular mimicry [PMID:8943398, PMID:11703674, PMID:38879009].\",\n  \"teleology\": [\n    {\n      \"year\": 1983,\n      \"claim\": \"Early proteolytic mapping established that C4BP harbors separable functional domains: an N-terminal region mediating C4b binding and a core/C-terminal domain binding protein S, setting the stage for chain-specific functional dissection.\",\n      \"evidence\": \"Chymotryptic fragmentation of purified C4BP with binding assays for C4b and protein S\",\n      \"pmids\": [\"6653778\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No chain-level resolution (alpha vs. beta not distinguished)\", \"No domain boundaries defined at the SCR/CCP level\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Demonstration that C4BP exists as multiple isoforms (alpha7-beta1, alpha7-beta0, alpha6-beta1) whose stoichiometry is genetically set by relative C4BPA/C4BPB expression resolved a long-standing question of why free protein S varies among individuals; it also showed cytokine-driven differential regulation (TNF-α/IFN-γ synergy on C4BPA >> C4BPB) provides a mechanism to shift isoform composition during inflammation.\",\n      \"evidence\": \"HepG2/Hep3B secretion studies, COS transfection, Northern blots with cytokine treatments in Hep3B cells, acute-phase patient plasma analysis\",\n      \"pmids\": [\"7561113\", \"7561114\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Promoter elements driving differential regulation only partially mapped\", \"In vivo human time-course data lacking\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Identification of the C4BPB promoter region (-126 to +25) driven by cooperative HNF-3 and NF-I/CTF binding explained hepatocyte-specific expression and provided a framework for understanding how cytokine signaling differentially controls C4BPB transcription.\",\n      \"evidence\": \"Reporter gene assays with promoter deletions in HepG2; EMSA identifying transcription factor binding sites\",\n      \"pmids\": [\"8598458\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Cytokine-responsive elements not mapped within this promoter\", \"In vivo chromatin accessibility not examined\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Discovery that the murine C4BPB gene is a pseudogene with in-phase stop codons established that the protein S–C4BP regulatory axis is absent in mice, a critical caveat for interpreting mouse models of complement and coagulation.\",\n      \"evidence\": \"Genomic sequencing of C4BPB across multiple mouse strains; Southern blotting and chromosomal mapping\",\n      \"pmids\": [\"7959726\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequences of absent beta-chain for mouse protein S regulation not fully characterized\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Mapping the C4b-binding site to a positively charged cluster (R39, R64, R66) at the CCP1–CCP2 junction of the alpha-chain, and identifying beta-chain SCR1–SCR2 as the protein S binding site, achieved residue-level functional assignment for both chains of C4BP.\",\n      \"evidence\": \"Site-directed mutagenesis with SPR binding and factor I cofactor assays (alpha-chain); chimeric SCR constructs with protein S binding and APC cofactor assays (beta-chain)\",\n      \"pmids\": [\"10383431\", \"10329721\", \"10744423\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of SCR1–SCR2/protein S interface not resolved at atomic level\", \"No crystal structure of beta-chain domains available\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Systematic CCP-deletion analysis of the alpha-chain defined the minimal domain requirements: CCP1–3 for C4b cofactor activity (CCP2–3 indispensable), CCP1–5 for C3b cofactor activity, with polymerization enhancing surface-bound C4b degradation efficiency.\",\n      \"evidence\": \"19 recombinant C4BP variants including truncations and deletions tested for C4b/C3b binding, cofactor, and convertase inhibition\",\n      \"pmids\": [\"11369776\", \"12417021\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which polymerization enhances activity (avidity vs. conformational) not resolved\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Identification of a cofactor-specific surface on CCP3 (K126/K128, F144/F149) that is essential for factor I–mediated cleavage but dispensable for C4b binding or convertase decay separated binding from catalytic cofactor function, demonstrating that C4BP employs distinct surfaces for substrate capture versus protease activation.\",\n      \"evidence\": \"Site-directed mutagenesis with SPR, cofactor, and convertase assembly/decay assays\",\n      \"pmids\": [\"12893820\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural mechanism of factor I activation by CCP3 residues unknown\", \"No ternary C4BP–C4b–factor I complex structure\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Discovery that the C4BP alpha-chain binds CD40 on B cells at a site distinct from CD40L, triggering proliferation, co-stimulatory molecule upregulation, and IL-4-dependent IgE switching, revealed an unexpected immune-regulatory function beyond complement control.\",\n      \"evidence\": \"Direct binding assays, B cell proliferation and isotype switching assays, CD40-deficient and IKKγ-deficient patient B cells, germinal center colocalization\",\n      \"pmids\": [\"12818164\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Alpha-chain domain mediating CD40 binding not mapped\", \"Physiological significance in vivo not established\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"SPR analysis of C4c and C4dg subfragments revealed that C4BP binds C4b through synergistic subsites within CCP1–3, where occupancy of one subsite shifts a conformational equilibrium to enhance affinity at the other, explaining the high cooperativity of C4b recognition.\",\n      \"evidence\": \"SPR with C4b subfragments, C4BP mutants, and cross-competition experiments\",\n      \"pmids\": [\"16819837\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-level structural basis of conformational shift not determined\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"In vivo demonstration that IL-6 upregulates C4BPB specifically through STAT3, while LPS downregulates both chains via NF-κB/MEK-ERK, provided a signaling-level mechanism for how inflammation shifts C4BP isoforms to increase protein S sequestration and reduce anticoagulant activity.\",\n      \"evidence\": \"Rat LPS/IL-6 injection, isolated hepatocyte treatment, pathway inhibitor experiments\",\n      \"pmids\": [\"18752574\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct STAT3 binding to C4BPB promoter not shown\", \"Translation to human hepatocytes not confirmed\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"The beta-chain-lacking C4BP(β−) isoform was shown to reprogram inflammatory myeloid cells to an anti-inflammatory/tolerogenic state via the alpha-chain CCP6 domain, while incorporation of the beta-chain suppresses this activity, establishing the beta-chain as a negative modulator of C4BP's non-canonical immunomodulatory function.\",\n      \"evidence\": \"Lupus-prone mouse model, monocyte-derived DC reprogramming, domain-deletion constructs, cytokine and transcriptional profiling\",\n      \"pmids\": [\"31982108\", \"35547734\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Receptor for CCP6-mediated myeloid reprogramming unidentified\", \"Mechanism by which beta-chain interferes with CCP6 activity unclear\", \"Human in vivo relevance not demonstrated\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Residue-level mapping showed that C4BP beta-chain binding to protein S LG1 domain abolishes TFPI cofactor function while preserving APC cofactor function, resolving a longstanding question of how protein S simultaneously participates in anticoagulant and complement pathways and establishing competitive regulation at a shared surface.\",\n      \"evidence\": \"Glycosylation insertion scanning, alanine mutagenesis of protein S LG1, FXa inhibition assays, plasma TFPI cofactor assays\",\n      \"pmids\": [\"34731882\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural model of beta-chain SCR1–2/protein S LG1 complex lacking\", \"In vivo consequences of selective TFPI pathway inhibition not tested\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Crystal structures of streptococcal M-protein HVRs in complex with C4BP alpha-chain fragments revealed multiple distinct C4BP-binding sequence patterns beyond previously known types, showing that pathogen exploitation of the C4b-binding site has diversified through convergent evolution including in M-like Enn proteins.\",\n      \"evidence\": \"X-ray crystallography, site-directed mutagenesis of M proteins and Enn proteins, C4BP binding assays\",\n      \"pmids\": [\"38879009\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length C4BP–M protein complex structure not available\", \"Functional impact on complement evasion in animal infection models not tested with these specific patterns\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include: the atomic structure of the beta-chain SCR1–2/protein S complex, the receptor through which the C4BP(β−) CCP6 domain reprograms myeloid cells, the structural mechanism by which beta-chain incorporation suppresses immunomodulatory activity, and whether isoform-specific C4BP functions are druggable targets in thromboinflammatory disease.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No crystal/cryo-EM structure of beta-chain or its complex with protein S\", \"CCP6 myeloid receptor unidentified\", \"No therapeutic modulation studies targeting isoform balance\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1, 2, 3, 4, 5, 6, 7, 8, 10]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [8, 10, 11]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [11, 16, 17]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": []},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [1, 2, 3, 4, 5, 6, 7, 18, 23]},\n      {\"term_id\": \"R-HSA-109582\", \"supporting_discovery_ids\": [8, 10, 15]}\n    ],\n    \"complexes\": [\n      \"C4b-binding protein (C4BP)\"\n    ],\n    \"partners\": [\n      \"C4BPA\",\n      \"PROS1\",\n      \"C4B\",\n      \"C3\",\n      \"CFI\",\n      \"CD40\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}