{"gene":"C4A","run_date":"2026-04-28T17:12:38","timeline":{"discoveries":[{"year":1967,"finding":"C4 and C2 interact in free solution to form a reversible protein-protein complex; upon cleavage by C1 esterase in the presence of Mg2+, this complex becomes a stable, enzymatically active C3 convertase that fragments C3, establishing the functional unit of the classical complement pathway.","method":"Purified component reconstitution, functional hemolysis assay, molecular weight determination","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 1 — reconstitution with purified components, enzymatic activity demonstrated, foundational study replicated broadly","pmids":["6019133"],"is_preprint":false},{"year":1981,"finding":"Human C4a anaphylatoxin (77 residues, ~9 kDa) is generated by C1s-mediated cleavage of the C4 alpha-chain between positions 77 and 78; primary sequence analysis revealed ~30% homology with C3a and ~36% with C5a, establishing C3a/C4a/C5a as a structurally related family despite immunological distinctiveness.","method":"Protein isolation from serum, cyanogen bromide fragmentation, chymotryptic digestion, sequence analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — direct protein sequencing with multiple orthogonal fragmentation methods; original primary structure determination","pmids":["6167582"],"is_preprint":false},{"year":1984,"finding":"The complete amino acid sequence (1722 residues) of the C4A single-chain precursor was deduced from cDNA cloning; comparison with C4B cDNA sequences identified class-specific nucleotide differences that explain the two isotypes, and allelic differences within C4A.","method":"cDNA cloning and nucleotide sequencing from heterozygous donor liver mRNA","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 — full cDNA sequencing; foundational structural determination replicated by contemporaneous studies","pmids":["6546706"],"is_preprint":false},{"year":1984,"finding":"C4A and C4B differ functionally in their covalent binding chemistry: C4B binds twice as effectively to antibody-coated red cells (hydroxyl-group targets) while C4A binds more effectively to protein-antigen complexes (amino-group targets); C4A preferentially transacylates amino nucleophiles and C4B preferentially acylates hydroxyl groups, despite only small differences in amino acid sequence.","method":"Purified C4A and C4B binding assays on antibody-coated red cells and protein-antigen complexes, comparison across a range of concentrations","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 — direct biochemical assay with purified isotypes, functionally validated, replicated in subsequent studies","pmids":["6332733"],"is_preprint":false},{"year":1984,"finding":"The molecular basis for C4B's ~4-fold greater immune hemolysis activity versus C4A was traced to greater deposition efficiency of nascent C4b onto C1-bearing erythrocyte surfaces, not to differences in C1s cleavage rate or C3 convertase kinetics; C4A nascent C4b preferentially forms amide bonds while C4B preferentially forms ester bonds with surface acceptors.","method":"Purified C4A and C4B from isotype-pure donors; hemolytic titration, C1s cleavage rate assay, C3 convertase decay assay, factor I cleavage assay, radiolabeled deposition quantification on sheep erythrocytes","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1 — multiple orthogonal biochemical assays with purified isotypes; mechanistic dissection of each step","pmids":["6609966"],"is_preprint":false},{"year":1984,"finding":"The C4A and C4B loci were mapped ~30 kb from the C2/factor B genes within a 98 kb segment of chromosome 6 MHC class III region, with the two C4 genes separated from each other by ~10 kb, establishing the genomic organization of the complement gene cluster.","method":"Overlapping cosmid clones from genomic DNA library, restriction mapping","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — direct genomic mapping; foundational structural genomics study","pmids":["6559257"],"is_preprint":false},{"year":1984,"finding":"The C4A/C4B class specificity (chemical reactivity and Rodgers/Chido antigenicity) maps to only four isotypic amino acid differences in the C4d region (positions 1101–1106): C4A carries PCPVLD and C4B carries LSPVIH; gene size polymorphism (22 kb vs 16 kb) was also identified for different allotypes.","method":"Cloning of C4A and C4B allotype genes, derived amino acid sequence comparison, restriction mapping","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 — direct sequencing of cloned genes, isotype-specific residues identified at four positions; replicated across studies","pmids":["2431902"],"is_preprint":false},{"year":1985,"finding":"Human C4 glycosylation was characterized: the alpha-chain carries three complex fucosylated biantennary oligosaccharides (on alpha2, alpha3, and alpha4 fragments) and the beta-chain has a single high-mannose oligosaccharide; the ~2000 Da mass difference between C4A and C4B alpha-chains localizes to the alpha2 fragment and is not due to carbohydrate differences.","method":"Glycan analysis of purified plasma C4 and HepG2-secreted C4 using lectin and chemical methods; subunit fractionation","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1 — direct biochemical characterization of purified protein glycoforms, localized mass difference to non-carbohydrate amino acid differences","pmids":["2981921"],"is_preprint":false},{"year":1988,"finding":"Rodgers and Chido blood group antigens map to C4A and C4B respectively; four polymorphic sites in the C4d region correlate with antigen expression, defining conformational or sequential epitopes; the serological phenotype can be dissociated from chemical reactivity class in rare variant allotypes.","method":"Haemagglutination inhibition assays with polyspecific antisera; nucleotide sequencing of C4d region from allotypes of known antigenic status","journal":"Experimental and clinical immunogenetics","confidence":"High","confidence_rationale":"Tier 1–2 — sequencing plus functional serological assays; confirmed molecular basis of blood group antigens","pmids":["2483343"],"is_preprint":false},{"year":1988,"finding":"C4A3 binds immunoglobulins (IgG, IgM, IgA, IgG2a, F(ab')2) and BSA 3–4 times more efficiently than C4B1 in fluid-phase reactions; C4A3 forms predominantly amide linkages with IgG whereas C4B1 forms both amide and ester linkages, mirroring binding preferences observed in solid-phase assays.","method":"Fluid-phase covalent binding assay with purified C4A3 and C4B1, C1s, and various immunoglobulin substrates; bond-type determination","journal":"Molecular immunology","confidence":"High","confidence_rationale":"Tier 1 — direct in vitro binding assay with purified components, multiple substrates, bond-type characterization","pmids":["3264881"],"is_preprint":false},{"year":1990,"finding":"The major functional difference between C4A and C4B in immune complex handling is at the level of CR1 (complement receptor 1) binding: C4A-opsonized complexes bind CR1 on erythrocytes markedly more effectively than C4B-opsonized complexes, with C4A also modestly superior at inhibiting immunoprecipitation, particularly in antibody excess.","method":"Purified C4A and C4B; immunoprecipitation inhibition assay; CR1 binding assay with preformed and nascent immune complexes on red cells","journal":"Clinical and experimental immunology","confidence":"High","confidence_rationale":"Tier 1–2 — direct functional assays with purified isotypes across multiple assay formats; mechanistic conclusion replicated","pmids":["2138067"],"is_preprint":false},{"year":1990,"finding":"Comparative analysis of C4 purified from diverse mammalian species showed that C4B-like (hydroxyl-preferring) binding specificity is the ancestral form; C4A-like (amino-preferring) specificity co-occurs with C4B-like in primates, sheep, and cattle (suggesting independent duplications), while rodents and most other species have only C4B-like activity.","method":"C4 purification from multiple species, binding specificity assays","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 2 — multi-species comparative biochemistry with functional assays; single laboratory study","pmids":["2302180"],"is_preprint":false},{"year":1972,"finding":"C4-deficient guinea pig peritoneal exudate cells fused with HeLa cells produced functionally active human C4, demonstrating that C4 is encoded by a nuclear gene and can be synthesized by somatic hybrid cells, establishing cell-autonomous C4 biosynthesis.","method":"Cell fusion (somatic hybridization), functional complement hemolysis assay","journal":"Science","confidence":"Medium","confidence_rationale":"Tier 2 — cell fusion with functional readout; single study but clean genetic complementation approach","pmids":["5033180"],"is_preprint":false},{"year":1993,"finding":"C4a anaphylatoxin induced biphasic Ca2+ mobilization in guinea-pig macrophages (rapid intracellular pool release followed by weak extracellular influx) via a receptor distinct from the C3a receptor (C4a did not compete with [125I]-C3a binding and C4a-desensitized cells still responded to C3a); C4a inhibited C3a-induced superoxide (O2-) generation without itself triggering O2- production.","method":"Intracellular Ca2+ measurement, radioligand competition binding ([125I]-C3a), superoxide generation assay in guinea-pig macrophages","journal":"Immunology letters","confidence":"Medium","confidence_rationale":"Tier 2 — multiple functional assays demonstrating receptor specificity and signaling outcome; single laboratory","pmids":["8396560"],"is_preprint":false},{"year":2001,"finding":"The orphan receptor C5L2/GPR77 binds C4a anaphylatoxin (cross-competing with C3a binding, suggesting a shared or overlapping binding site distinct from the C5a site), demonstrating C5L2 as a potential low-affinity receptor for C4a.","method":"Radioligand cross-competition binding assay with C5L2-transfected RBL-2H3 cells; [125I]-C5a and C3a displacement by C4a","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — direct receptor binding competition assay; single study, moderate affinity interaction","pmids":["11773063"],"is_preprint":false},{"year":2002,"finding":"Bone marrow-derived macrophages synthesize and locally deposit C4 protein in splenic germinal centers; transfer of wild-type macrophages (or whole bone marrow) into C4-knockout mice restored C4 mRNA (detected by in situ hybridization), C4 protein deposits in follicles (immunofluorescence), and humoral immune responses to both soluble antigen and HSV-1, demonstrating that locally produced (macrophage-derived) C4 is sufficient for complement-dependent humoral immunity even in the absence of serum C4.","method":"Bone marrow reconstitution of C4-/- mice, immunization with NP5-KLH and HSV-1 infection, antibody titer measurement, in situ hybridization, immunofluorescence staining, cell sorting with RT-PCR","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — genetic rescue experiment with multiple orthogonal readouts (functional, mRNA, protein localization); clear mechanistic conclusion","pmids":["12421924"],"is_preprint":false},{"year":2016,"finding":"C4A expression in the brain (determined by structural alleles of the C4 locus) promotes synapse elimination during postnatal development; C4 protein localizes to neuronal synapses, dendrites, axons, and cell bodies in humans; in mice, C4 mediates complement-dependent synaptic pruning, and higher C4A expression associates with schizophrenia risk in proportion to its level of brain expression.","method":"Human brain C4A/C4B expression quantification by allele-specific RNA-seq; C4 immunohistochemistry in human brain; mouse genetic model (C4-knockout and C4-overexpressing) with synapse counting by immunofluorescence; population genetic association","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1–2 — direct protein localization, mouse genetic loss-of-function with quantitative synapse phenotype, and human expression data; multi-method, high-impact study replicated in follow-up work","pmids":["26814963"],"is_preprint":false},{"year":2020,"finding":"C4A and C4B protein levels are higher in cerebrospinal fluid and plasma of men than women aged 20–50; common C4A/C4B allele combinations generate up to 14-fold variation in SLE risk and 31-fold variation in Sjögren's syndrome risk in men, with C4A acting more protectively than C4B against these autoimmune diseases while the same alleles increase schizophrenia risk; this sex-differential effect parallels sex differences in complement protein levels.","method":"Large-scale population genetic association study with definitive C4 structural genotyping; CSF and plasma C4/C3 protein level measurement stratified by sex and age","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — large-scale genetic association with direct protein quantification across sex; multi-disease analysis; replicated across independent cohorts","pmids":["32499649"],"is_preprint":false},{"year":2021,"finding":"Genetically predicted higher brain C4A expression associates with higher TSPO (microglial marker) binding on PET imaging and altered hippocampal morphology (reduced surface area and medial displacement in CA1) in living humans, suggesting C4A-mediated microglial activation and synapse elimination affect hippocampal structure in vivo.","method":"Genetically predicted C4A expression from structural alleles; [18F]FEPPA TSPO-PET imaging; MRI-based hippocampal morphometry in 111 participants","journal":"Biological psychiatry","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo imaging with genetic instrument; single study, indirect measure of C4A function via predicted expression","pmids":["34456009"],"is_preprint":false},{"year":1988,"finding":"Tissue-specific expression of mouse C4 and the highly homologous Slp gene was characterized: C4 is expressed at high levels in liver and peritoneal macrophages, with lower but detectable levels in mammary gland, lung, spleen, kidney, testis, brain, heart, and submaxillary gland; regulatory differences between C4 and Slp vary across tissues and are controlled by distinct cis-acting and trans-acting factors in each tissue context.","method":"Northern blotting using C4/Slp-distinguishing probe across multiple mouse tissues; congenic strain comparison","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 — direct mRNA quantification across tissues with genetic controls; establishes tissue expression pattern","pmids":["3405752"],"is_preprint":false},{"year":1998,"finding":"Human astrocytes constitutively express C4 mRNA and protein; interferon-gamma upregulates C4 expression while IL-1beta inhibits it; C4 immunoreactivity was localized to GFAP-positive astrocytes when protein secretion was blocked, establishing astrocytes as a cellular source of complement C4 in the CNS.","method":"RT-PCR, ELISA, immunocytochemistry, immunoblot in primary human astrocytes with cytokine treatments; protein secretion inhibition","journal":"Brain research","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal methods; cytokine regulation established; single laboratory","pmids":["9795119"],"is_preprint":false}],"current_model":"C4A is a serum complement protein produced primarily by hepatocytes, macrophages, and astrocytes that is activated by C1s-mediated cleavage of its alpha-chain to generate C4a anaphylatoxin (which signals through a distinct receptor to inhibit C3a-induced superoxide generation) and nascent C4b, which covalently deposits on immune complexes and activating surfaces preferentially via amide bonds (to amino groups) rather than ester bonds, giving C4A superior efficiency in opsonizing protein-antigen immune complexes and enhancing CR1-mediated clearance; C4b then assembles with C2 to form the classical pathway C3 convertase; in the brain, C4A mediates complement-dependent synaptic pruning by microglia during postnatal development, and its expression level determines risk for both schizophrenia (higher C4A = higher risk) and autoimmune diseases such as SLE and Sjögren's syndrome (higher C4A = lower risk), with these effects showing sex-dependent magnitude correlated with sex differences in complement protein levels."},"narrative":{"teleology":[{"year":1967,"claim":"Establishing that C4 interacts with C2 and, after C1 esterase cleavage, forms a stable C3 convertase defined the enzymatic core of the classical complement pathway and positioned C4 as an essential upstream component.","evidence":"Purified component reconstitution with functional hemolysis assay","pmids":["6019133"],"confidence":"High","gaps":["C4A vs C4B isotype distinction not yet known","covalent binding mechanism of nascent C4b not characterized"]},{"year":1972,"claim":"Demonstrating that somatic hybrid cells autonomously synthesize functional C4 established that C4 is encoded by a nuclear gene and can be produced cell-autonomously outside the liver.","evidence":"HeLa–guinea-pig macrophage somatic cell fusion with hemolytic complement assay","pmids":["5033180"],"confidence":"Medium","gaps":["identity of the human chromosomal locus unknown","tissue-specific regulation not addressed"]},{"year":1981,"claim":"Sequencing C4a anaphylatoxin revealed its generation by C1s cleavage at position 77–78 of the alpha-chain and established C3a/C4a/C5a as a structurally related anaphylatoxin family, raising the question of C4a-specific receptor signaling.","evidence":"Protein isolation, cyanogen bromide fragmentation, and direct amino acid sequencing from human serum C4","pmids":["6167582"],"confidence":"High","gaps":["C4a receptor identity unknown","biological activity of C4a not yet defined"]},{"year":1984,"claim":"Full cDNA sequencing of C4A defined the 1722-residue precursor and, together with biochemical studies, revealed that C4A and C4B differ in covalent binding chemistry—C4A preferentially forms amide bonds with amino groups on protein antigens while C4B preferentially forms ester bonds—explaining their distinct functional niches in immune complex handling versus cell-surface hemolysis.","evidence":"cDNA cloning from liver mRNA; purified isotype binding assays on antibody-coated red cells and protein-antigen complexes; hemolytic titration and radiolabeled deposition","pmids":["6546706","6332733","6609966"],"confidence":"High","gaps":["specific residues responsible for binding chemistry difference not yet identified","in vivo consequences of C4A deficiency not established"]},{"year":1984,"claim":"Genomic mapping placed C4A and C4B ~10 kb apart within a 98 kb segment of the MHC class III region on chromosome 6, establishing the tandem duplication architecture and linking complement variation to MHC haplotype diversity.","evidence":"Overlapping cosmid clones and restriction mapping of genomic DNA","pmids":["6559257"],"confidence":"High","gaps":["copy number variation at the locus not yet appreciated","regulatory elements controlling isotype-specific expression unknown"]},{"year":1988,"claim":"Mapping Rodgers/Chido blood group determinants to C4A/C4B and pinpointing four isotypic residues in C4d (1101–1106) as the molecular basis of both antigenic and chemical-reactivity differences resolved how minimal sequence changes drive functional isotype divergence.","evidence":"Haemagglutination inhibition with polyspecific antisera; nucleotide sequencing of C4d region allotypes; fluid-phase covalent binding assays with purified C4A3 and C4B1","pmids":["2483343","2431902","3264881"],"confidence":"High","gaps":["structural basis of how these four residues alter thioester reactivity not resolved","rare variant allotypes can dissociate serological and chemical phenotypes"]},{"year":1988,"claim":"Characterizing tissue-specific C4 mRNA expression in mouse revealed broad expression beyond the liver—including brain, lung, kidney, and macrophages—suggesting local complement production may serve tissue-specific immune functions.","evidence":"Northern blotting across multiple mouse tissues with congenic strain controls","pmids":["3405752"],"confidence":"Medium","gaps":["human tissue expression pattern not directly addressed","functional significance of extrahepatic C4 production unknown"]},{"year":1990,"claim":"Demonstrating that C4A-opsonized immune complexes bind CR1 on erythrocytes far more effectively than C4B-opsonized complexes established that C4A's functional superiority in immune complex clearance operates at the level of receptor engagement, not just covalent deposition.","evidence":"CR1 binding assay and immunoprecipitation inhibition with purified C4A and C4B on preformed and nascent immune complexes","pmids":["2138067"],"confidence":"High","gaps":["structural explanation for differential CR1 engagement not provided","in vivo clearance studies not performed"]},{"year":1993,"claim":"Identifying that C4a induces Ca²⁺ mobilization via a receptor distinct from the C3a receptor and inhibits C3a-induced superoxide generation established C4a as a functional anaphylatoxin with immunomodulatory rather than purely pro-inflammatory activity.","evidence":"Intracellular Ca²⁺ measurement, [¹²⁵I]-C3a radioligand competition, and superoxide assay in guinea-pig macrophages","pmids":["8396560"],"confidence":"Medium","gaps":["molecular identity of the C4a receptor not established","relevance to human macrophages not shown","downstream signaling pathway not mapped"]},{"year":1998,"claim":"Showing that human astrocytes constitutively produce C4 and that its expression is regulated by IFN-γ and IL-1β identified a CNS cellular source for locally produced complement, presaging a role for C4 in brain-specific processes.","evidence":"RT-PCR, ELISA, immunocytochemistry, and immunoblot in primary human astrocytes with cytokine treatment","pmids":["9795119"],"confidence":"Medium","gaps":["C4A vs C4B isotype distinction in astrocyte expression not addressed","functional consequence of astrocyte-derived C4 in brain unknown"]},{"year":2001,"claim":"Identifying GPR77/C5L2 as a receptor that binds C4a (cross-competing with C3a) provided a candidate molecular receptor for C4a signaling, partially resolving the receptor identity question raised by the 1993 guinea-pig macrophage studies.","evidence":"Radioligand cross-competition binding in C5L2-transfected RBL-2H3 cells","pmids":["11773063"],"confidence":"Medium","gaps":["C5L2 functional signaling in response to C4a not demonstrated","physiological relevance of C4a–C5L2 interaction in vivo unknown"]},{"year":2002,"claim":"Demonstrating that bone marrow-derived macrophage C4 is sufficient to restore germinal center function and humoral immunity in C4-knockout mice proved that locally produced (non-hepatic) C4 has a non-redundant role in adaptive immunity independent of circulating serum C4.","evidence":"Bone marrow reconstitution of C4⁻/⁻ mice; immunization; in situ hybridization, immunofluorescence, and antibody titer quantification","pmids":["12421924"],"confidence":"High","gaps":["relative contribution of C4A vs C4B isotype in germinal center function not dissected","mechanism of C4 action in follicular dendritic cell retention not elucidated"]},{"year":2016,"claim":"The landmark discovery that C4A protein localizes to neuronal synapses and mediates complement-dependent synaptic pruning by microglia—with higher C4A expression proportionally increasing schizophrenia risk—established a direct mechanistic link between a complement gene and a psychiatric disorder through a neurodevelopmental pathway.","evidence":"Allele-specific RNA-seq in human brain; C4 immunohistochemistry; C4-knockout and C4-overexpressing mouse models with synapse quantification; population genetic association","pmids":["26814963"],"confidence":"High","gaps":["whether C4A acts via the same C3-dependent tagging pathway in brain as in periphery not fully established","cell-type-specific C4A expression regulation in brain not characterized","therapeutic modulation of C4A-dependent pruning not demonstrated"]},{"year":2020,"claim":"Quantifying sex-differential C4 protein levels and demonstrating that C4A allele dosage generates up to 14–31-fold variation in autoimmune disease risk (SLE, Sjögren's syndrome) while simultaneously increasing schizophrenia risk unified the opposing disease associations into a single framework of complement-mediated self-tolerance versus synaptic pruning, modulated by sex.","evidence":"Large-scale population genetic association with structural C4 genotyping; CSF and plasma C4/C3 protein measurement stratified by sex","pmids":["32499649"],"confidence":"High","gaps":["molecular mechanism by which sex hormones regulate C4 protein levels unknown","whether C4A's protective effect in autoimmunity is purely via immune complex clearance or also involves tolerance mechanisms not resolved"]},{"year":2021,"claim":"In vivo neuroimaging evidence that genetically predicted higher C4A expression correlates with increased microglial activation (TSPO-PET) and altered hippocampal morphology translated the mouse pruning phenotype to living human brain, strengthening the causal model.","evidence":"[¹⁸F]FEPPA TSPO-PET and MRI hippocampal morphometry in 111 participants with C4 structural genotyping","pmids":["34456009"],"confidence":"Medium","gaps":["TSPO-PET is an indirect marker; direct measurement of synaptic density change not performed","sample size moderate; replication in larger cohorts needed","causal directionality relies on genetic instrument assumptions"]},{"year":null,"claim":"Key unresolved questions include the structural basis by which four C4d residues alter thioester reactivity, the definitive identity and signaling pathway of the C4a receptor in humans, the cell-type-specific regulatory mechanisms controlling C4A expression in the brain, and whether therapeutic modulation of C4A-mediated pruning can alter schizophrenia-related phenotypes.","evidence":"","pmids":[],"confidence":"Low","gaps":["no crystal structure of C4A vs C4B thioester domain to explain differential reactivity","C4a receptor signaling pathway in human cells remains unresolved","no interventional studies targeting C4A-dependent pruning in disease models"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,3,4,9,10]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[13]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[0,3,4,9,10,15,17]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[16]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[16]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,3,4,10,15]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[16,18]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[16]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[16,17]}],"complexes":["C3 convertase (C4b2a)"],"partners":["C2","C1S","CR1","C5L2"],"other_free_text":[]},"mechanistic_narrative":"C4A is a classical complement pathway component that, upon C1s-mediated cleavage of its alpha-chain, generates C4a anaphylatoxin and nascent C4b, which covalently deposits on immune complexes preferentially via amide bonds to amino groups, conferring superior opsonization of protein antigens and enhanced CR1-mediated clearance compared with the C4B isotype [PMID:6332733, PMID:2138067]. This isotype specificity maps to four amino acid differences in the C4d region (positions 1101–1106) [PMID:2431902], and the deposited C4b assembles with C2 to form the classical pathway C3 convertase [PMID:6019133]. Beyond its role in humoral immunity—where macrophage-derived C4 is sufficient to support germinal center responses in the absence of serum C4 [PMID:12421924]—C4A is expressed in the brain by astrocytes and localizes to neuronal synapses, where it mediates complement-dependent synaptic pruning by microglia; higher C4A expression increases schizophrenia risk while protecting against SLE and Sjögren's syndrome in a sex-dependent manner correlated with sex differences in complement protein levels [PMID:26814963, PMID:32499649]."},"prefetch_data":{"uniprot":{"accession":"P0C0L4","full_name":"Complement C4-A","aliases":["Acidic complement C4","C3 and PZP-like alpha-2-macroglobulin domain-containing protein 2"],"length_aa":1744,"mass_kda":192.8,"function":"Precursor of non-enzymatic components of the classical, lectin and GZMK complement pathways, which consist in a cascade of proteins that leads to phagocytosis and breakdown of pathogens and signaling that strengthens the adaptive immune system Non-enzymatic component of C3 and C5 convertases (PubMed:8538770). Generated following cleavage by complement proteases (C1S, MASP2 or GZMK, depending on the complement pathway), it covalently attaches to the surface of pathogens, where it acts as an opsonin that marks the surface of antigens for removal (PubMed:27738201, PubMed:8538770). It then recruits the serine protease complement C2b to form the C3 and C5 convertases, which cleave and activate C3 and C5, respectively, the next components of the complement pathways (PubMed:12878586, PubMed:18204047, PubMed:2387864, PubMed:6906228). Complement C4b-A isotype is responsible for effective binding to form amide bonds with immune aggregates or protein antigens, while complement C4b-B isotype catalyzes the transacylation of the thioester carbonyl group to form ester bonds with carbohydrate antigens (PubMed:8538770) Putative humoral mediator released following cleavage by complement proteases (C1S, MASP2 or GZMK, depending on the complement pathway) (PubMed:6167582). While it is strongly similar to anaphylatoxins, its role is unclear (PubMed:25659340). Was reported to act as a mediator of local inflammatory process; however these effects were probably due to contamination with C3a and/C5a anaphylatoxins in biological assays (PubMed:25659340)","subcellular_location":"Secreted; Cell surface","url":"https://www.uniprot.org/uniprotkb/P0C0L4/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/C4A","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":4,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/C4A","total_profiled":1310},"omim":[{"mim_id":"614380","title":"COMPLEMENT COMPONENT 4A DEFICIENCY; C4AD","url":"https://www.omim.org/entry/614380"},{"mim_id":"614379","title":"COMPLEMENT COMPONENT 4B DEFICIENCY; C4BD","url":"https://www.omim.org/entry/614379"},{"mim_id":"614374","title":"BLOOD GROUP, CHIDO/RODGERS SYSTEM","url":"https://www.omim.org/entry/614374"},{"mim_id":"613815","title":"CYTOCHROME P450, FAMILY 21, SUBFAMILY A, POLYPEPTIDE 2; CYP21A2","url":"https://www.omim.org/entry/613815"},{"mim_id":"610379","title":"WEST NILE VIRUS, SUSCEPTIBILITY TO","url":"https://www.omim.org/entry/610379"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"adrenal gland","ntpm":117.5},{"tissue":"liver","ntpm":341.8}],"url":"https://www.proteinatlas.org/search/C4A"},"hgnc":{"alias_symbol":["CPAMD2","C4S","CO4","C4","C4A3","C4A2","C4A4","C4A6","C4B","RG"],"prev_symbol":[]},"alphafold":{"accession":"P0C0L4","domains":[{"cath_id":"2.60.40.1930","chopping":"20-136","consensus_level":"medium","plddt":92.1403,"start":20,"end":136},{"cath_id":"2.60.40.1930","chopping":"141-236_569-613_779-828","consensus_level":"medium","plddt":92.2914,"start":141,"end":828},{"cath_id":"2.60.40.1940","chopping":"242-364","consensus_level":"medium","plddt":88.9685,"start":242,"end":364},{"cath_id":"1.20.91.20","chopping":"691-737","consensus_level":"medium","plddt":72.1636,"start":691,"end":737},{"cath_id":"2.60.40.10","chopping":"833-935","consensus_level":"high","plddt":88.8024,"start":833,"end":935},{"cath_id":"-","chopping":"1221-1233_1259-1319","consensus_level":"medium","plddt":85.9585,"start":1221,"end":1319},{"cath_id":"2.60.40.690","chopping":"1398-1411_1464-1571","consensus_level":"medium","plddt":82.012,"start":1398,"end":1571},{"cath_id":"1.20.58","chopping":"998-1059","consensus_level":"medium","plddt":90.6395,"start":998,"end":1059}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P0C0L4","model_url":"https://alphafold.ebi.ac.uk/files/AF-P0C0L4-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P0C0L4-F1-predicted_aligned_error_v6.png","plddt_mean":83.44},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=C4A","jax_strain_url":"https://www.jax.org/strain/search?query=C4A"},"sequence":{"accession":"P0C0L4","fasta_url":"https://rest.uniprot.org/uniprotkb/P0C0L4.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P0C0L4/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P0C0L4"}},"corpus_meta":[{"pmid":"22404472","id":"PMC_22404472","title":"Photorespiration and the evolution of C4 photosynthesis.","date":"2012","source":"Annual review of plant biology","url":"https://pubmed.ncbi.nlm.nih.gov/22404472","citation_count":474,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"6332733","id":"PMC_6332733","title":"A comparison of the properties of two classes, C4A and C4B, of the human complement component C4.","date":"1984","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/6332733","citation_count":296,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"11803016","id":"PMC_11803016","title":"C4-dicarboxylate carriers and sensors in bacteria.","date":"2002","source":"Biochimica et biophysica acta","url":"https://pubmed.ncbi.nlm.nih.gov/11803016","citation_count":210,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"20693355","id":"PMC_20693355","title":"Setaria viridis: a model for C4 photosynthesis.","date":"2010","source":"The Plant cell","url":"https://pubmed.ncbi.nlm.nih.gov/20693355","citation_count":206,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"23383423","id":"PMC_23383423","title":"International Union of Basic and Clinical Pharmacology. [corrected]. LXXXVII. Complement peptide C5a, C4a, and C3a receptors.","date":"2013","source":"Pharmacological reviews","url":"https://pubmed.ncbi.nlm.nih.gov/23383423","citation_count":195,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"20543093","id":"PMC_20543093","title":"An mRNA blueprint for C4 photosynthesis derived from comparative transcriptomics of closely related C3 and C4 species.","date":"2010","source":"Plant physiology","url":"https://pubmed.ncbi.nlm.nih.gov/20543093","citation_count":188,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"21705644","id":"PMC_21705644","title":"Evolution of C4 photosynthesis in the genus Flaveria: how many and which genes does it take to make C4?","date":"2011","source":"The Plant cell","url":"https://pubmed.ncbi.nlm.nih.gov/21705644","citation_count":153,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"22128120","id":"PMC_22128120","title":"C4 cycles: past, present, and future research on C4 photosynthesis.","date":"2011","source":"The Plant cell","url":"https://pubmed.ncbi.nlm.nih.gov/22128120","citation_count":141,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"6086189","id":"PMC_6086189","title":"Preferential generation of leukotriene C4 by human eosinophils.","date":"1984","source":"Clinical and experimental immunology","url":"https://pubmed.ncbi.nlm.nih.gov/6086189","citation_count":136,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"24442564","id":"PMC_24442564","title":"Biochemical and cytological relationships in C4 plants.","date":"1974","source":"Planta","url":"https://pubmed.ncbi.nlm.nih.gov/24442564","citation_count":134,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"11337400","id":"PMC_11337400","title":"MOLECULAR ENGINEERING OF C4 PHOTOSYNTHESIS.","date":"2001","source":"Annual review of plant physiology and plant molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/11337400","citation_count":120,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"3499152","id":"PMC_3499152","title":"Deletion of C4A genes in patients with systemic lupus erythematosus.","date":"1987","source":"Arthritis and rheumatism","url":"https://pubmed.ncbi.nlm.nih.gov/3499152","citation_count":114,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2546011","id":"PMC_2546011","title":"Analysis of C4-dicarboxylate transport genes in Rhizobium meliloti.","date":"1989","source":"Molecular microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/2546011","citation_count":110,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"27127850","id":"PMC_27127850","title":"Strategies for improving C4 photosynthesis.","date":"2016","source":"Current opinion in plant biology","url":"https://pubmed.ncbi.nlm.nih.gov/27127850","citation_count":103,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"21421406","id":"PMC_21421406","title":"What can enzymes of C₄ photosynthesis do for C₃ plants under stress?","date":"2010","source":"Plant science : an international journal of experimental plant biology","url":"https://pubmed.ncbi.nlm.nih.gov/21421406","citation_count":103,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"34335607","id":"PMC_34335607","title":"Complement C4, Infections, and Autoimmune Diseases.","date":"2021","source":"Frontiers in immunology","url":"https://pubmed.ncbi.nlm.nih.gov/34335607","citation_count":102,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"24486019","id":"PMC_24486019","title":"Optimal translational termination requires C4 lysyl hydroxylation of eRF1.","date":"2014","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/24486019","citation_count":93,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"21511912","id":"PMC_21511912","title":"Enhancing drought tolerance in C(4) crops.","date":"2011","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/21511912","citation_count":93,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"6606418","id":"PMC_6606418","title":"Complement allotyping in SLE: association with C4A null.","date":"1983","source":"Australian and New Zealand journal of medicine","url":"https://pubmed.ncbi.nlm.nih.gov/6606418","citation_count":85,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"12781769","id":"PMC_12781769","title":"Evolution of C4 phosphoenolpyruvate carboxylase.","date":"2003","source":"Archives of biochemistry and biophysics","url":"https://pubmed.ncbi.nlm.nih.gov/12781769","citation_count":84,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"19324795","id":"PMC_19324795","title":"Ecological selection pressures for C4 photosynthesis in the grasses.","date":"2009","source":"Proceedings. Biological sciences","url":"https://pubmed.ncbi.nlm.nih.gov/19324795","citation_count":81,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"26912798","id":"PMC_26912798","title":"Photorespiration connects C3 and C4 photosynthesis.","date":"2016","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/26912798","citation_count":81,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"12857847","id":"PMC_12857847","title":"Cold tolerance of C4 photosynthesis in Miscanthus x giganteus: adaptation in amounts and sequence of C4 photosynthetic enzymes.","date":"2003","source":"Plant physiology","url":"https://pubmed.ncbi.nlm.nih.gov/12857847","citation_count":77,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"6149580","id":"PMC_6149580","title":"Structure and organization of the C4 genes.","date":"1984","source":"Philosophical transactions of the Royal Society of London. Series B, Biological sciences","url":"https://pubmed.ncbi.nlm.nih.gov/6149580","citation_count":74,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"18316317","id":"PMC_18316317","title":"Overproduction of C4 photosynthetic enzymes in transgenic rice plants: an approach to introduce the C4-like photosynthetic pathway into rice.","date":"2008","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/18316317","citation_count":71,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"6607672","id":"PMC_6607672","title":"Human C4 haplotypes with duplicated C4A or C4B.","date":"1984","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/6607672","citation_count":69,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"24799561","id":"PMC_24799561","title":"Deconstructing Kranz anatomy to understand C4 evolution.","date":"2014","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/24799561","citation_count":65,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"22326564","id":"PMC_22326564","title":"Molecular evolution of genes recruited into C₄ photosynthesis.","date":"2012","source":"Trends in plant science","url":"https://pubmed.ncbi.nlm.nih.gov/22326564","citation_count":65,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2568334","id":"PMC_2568334","title":"C4A gene deletion and HLA associations in black Americans with systemic lupus erythematosus.","date":"1989","source":"Immunogenetics","url":"https://pubmed.ncbi.nlm.nih.gov/2568334","citation_count":65,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"8482459","id":"PMC_8482459","title":"Early-onset autoimmune hepatitis is associated with a C4A gene deletion.","date":"1993","source":"Gastroenterology","url":"https://pubmed.ncbi.nlm.nih.gov/8482459","citation_count":64,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"7373050","id":"PMC_7373050","title":"Activation of the fourth component of complement (C4): assessment by rocket immunoelectrophoresis and correlation with the metabolism of C4.","date":"1980","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/7373050","citation_count":61,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"21680741","id":"PMC_21680741","title":"Stabilization of C4a-hydroperoxyflavin in a two-component flavin-dependent monooxygenase is achieved through interactions at flavin N5 and C4a atoms.","date":"2011","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/21680741","citation_count":60,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"6167582","id":"PMC_6167582","title":"Complete primary structure of human C4a anaphylatoxin.","date":"1981","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/6167582","citation_count":56,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"15294999","id":"PMC_15294999","title":"Complete complement components C4A and C4B deficiencies in human kidney diseases and systemic lupus erythematosus.","date":"2004","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/15294999","citation_count":56,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"8341140","id":"PMC_8341140","title":"Clinical expression of systemic lupus erythematosus in patients with C4A deficiency.","date":"1993","source":"Medicine","url":"https://pubmed.ncbi.nlm.nih.gov/8341140","citation_count":55,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"16403222","id":"PMC_16403222","title":"Real-time PCR quantification of human complement C4A and C4B genes.","date":"2006","source":"BMC genetics","url":"https://pubmed.ncbi.nlm.nih.gov/16403222","citation_count":55,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"19133805","id":"PMC_19133805","title":"Crystallographic, spectroscopic, and computational analysis of a flavin C4a-oxygen adduct in choline oxidase.","date":"2009","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/19133805","citation_count":54,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"24179135","id":"PMC_24179135","title":"Parallel recruitment of multiple genes into c4 photosynthesis.","date":"2013","source":"Genome biology and evolution","url":"https://pubmed.ncbi.nlm.nih.gov/24179135","citation_count":52,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"1508152","id":"PMC_1508152","title":"Homologous genes for the C4 isoform of phosphoenolpyruvate carboxylase in a C3 and a C4 Flaveria species.","date":"1992","source":"Molecular & general genetics : MGG","url":"https://pubmed.ncbi.nlm.nih.gov/1508152","citation_count":49,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"20091067","id":"PMC_20091067","title":"Geminivirus C4 protein alters Arabidopsis development.","date":"2009","source":"Protoplasma","url":"https://pubmed.ncbi.nlm.nih.gov/20091067","citation_count":48,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2138067","id":"PMC_2138067","title":"Differences between C4A and C4B in the handling of immune complexes: the enhancement of CR1 binding is more important than the inhibition of immunoprecipitation.","date":"1990","source":"Clinical and experimental immunology","url":"https://pubmed.ncbi.nlm.nih.gov/2138067","citation_count":47,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"26465154","id":"PMC_26465154","title":"Identification of Photosynthesis-Associated C4 Candidate Genes through Comparative Leaf Gradient Transcriptome in Multiple Lineages of C3 and C4 Species.","date":"2015","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/26465154","citation_count":47,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"22289126","id":"PMC_22289126","title":"The dicotyledonous NAD malic enzyme C4 plant Cleome gynandra displays age-dependent plasticity of C4 decarboxylation biochemistry.","date":"2012","source":"Plant biology (Stuttgart, Germany)","url":"https://pubmed.ncbi.nlm.nih.gov/22289126","citation_count":47,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"14604014","id":"PMC_14604014","title":"Dancing with complement C4 and the RP-C4-CYP21-TNX (RCCX) modules of the major histocompatibility complex.","date":"2003","source":"Progress in nucleic acid research and molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/14604014","citation_count":46,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"24723409","id":"PMC_24723409","title":"Photosynthesis of C3, C3-C4, and C4 grasses at glacial CO2.","date":"2014","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/24723409","citation_count":46,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"12895593","id":"PMC_12895593","title":"Leukotriene C(4) synthase.","date":"2003","source":"Prostaglandins, leukotrienes, and essential fatty acids","url":"https://pubmed.ncbi.nlm.nih.gov/12895593","citation_count":46,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"10893330","id":"PMC_10893330","title":"Biological activities of recombinant chicken leptin C4S analog compared with unmodified leptins.","date":"2000","source":"American journal of physiology. Endocrinology and metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/10893330","citation_count":45,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"23720217","id":"PMC_23720217","title":"A recombinant begomovirus resulting from exchange of the C4 gene.","date":"2013","source":"The Journal of general virology","url":"https://pubmed.ncbi.nlm.nih.gov/23720217","citation_count":44,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"27059273","id":"PMC_27059273","title":"Walking the C4 pathway: past, present, and future.","date":"2016","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/27059273","citation_count":43,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"1696181","id":"PMC_1696181","title":"The c4 repressors of bacteriophages P1 and P7 are antisense RNAs.","date":"1990","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/1696181","citation_count":43,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"28110276","id":"PMC_28110276","title":"Photosynthesis in C3-C4 intermediate Moricandia species.","date":"2016","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/28110276","citation_count":42,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"11898391","id":"PMC_11898391","title":"Transport of leukotriene C4 and structurally related conjugates.","date":"2002","source":"Vitamins and hormones","url":"https://pubmed.ncbi.nlm.nih.gov/11898391","citation_count":42,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"1976809","id":"PMC_1976809","title":"Lack of gene deletion for complement C4A deficiency in Japanese patients with systemic lupus erythematosus.","date":"1990","source":"The Journal of rheumatology","url":"https://pubmed.ncbi.nlm.nih.gov/1976809","citation_count":42,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"7451653","id":"PMC_7451653","title":"Genetic analysis of C4 deficiency.","date":"1981","source":"The Journal of clinical investigation","url":"https://pubmed.ncbi.nlm.nih.gov/7451653","citation_count":41,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"12374297","id":"PMC_12374297","title":"Maize C4 and non-C4 NADP-dependent malic enzymes are encoded by distinct genes derived from a plastid-localized ancestor.","date":"2002","source":"Plant molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/12374297","citation_count":41,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"32689483","id":"PMC_32689483","title":"C4 mechanisms in aquatic angiosperms: comparisons with terrestrial C4 systems.","date":"2002","source":"Functional plant biology : FPB","url":"https://pubmed.ncbi.nlm.nih.gov/32689483","citation_count":41,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"31799932","id":"PMC_31799932","title":"Evolution of C4 photosynthesis predicted by constraint-based modelling.","date":"2019","source":"eLife","url":"https://pubmed.ncbi.nlm.nih.gov/31799932","citation_count":40,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"16879240","id":"PMC_16879240","title":"Immunoglobulins and complement factor C4 in adult rhinosinusitis.","date":"2006","source":"Clinical and experimental immunology","url":"https://pubmed.ncbi.nlm.nih.gov/16879240","citation_count":40,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"27758680","id":"PMC_27758680","title":"Complement C4A and C4B Gene Copy Number Study in Alzheimer's Disease Patients.","date":"2017","source":"Current Alzheimer research","url":"https://pubmed.ncbi.nlm.nih.gov/27758680","citation_count":39,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"5033180","id":"PMC_5033180","title":"Biosynthesis of C4 (fourth component of complement) by hybrids of C4-deficient guinea pig cells and HeLa cells.","date":"1972","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/5033180","citation_count":39,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"12421924","id":"PMC_12421924","title":"Macrophage-derived complement component C4 can restore humoral immunity in C4-deficient mice.","date":"2002","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/12421924","citation_count":39,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"32689525","id":"PMC_32689525","title":"Are crassulacean acid metabolism and C4 photosynthesis incompatible?","date":"2002","source":"Functional plant biology : FPB","url":"https://pubmed.ncbi.nlm.nih.gov/32689525","citation_count":38,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"21299565","id":"PMC_21299565","title":"Regulatory mechanisms underlying C4 photosynthesis.","date":"2011","source":"The New phytologist","url":"https://pubmed.ncbi.nlm.nih.gov/21299565","citation_count":37,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"27242848","id":"PMC_27242848","title":"The Roles of Organic Acids in C4 Photosynthesis.","date":"2016","source":"Frontiers in plant science","url":"https://pubmed.ncbi.nlm.nih.gov/27242848","citation_count":37,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"27038285","id":"PMC_27038285","title":"Glycine decarboxylase in C3, C4 and C3-C4 intermediate species.","date":"2016","source":"Current opinion in plant biology","url":"https://pubmed.ncbi.nlm.nih.gov/27038285","citation_count":36,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"12023288","id":"PMC_12023288","title":"Membrane localization and topology of leukotriene C4 synthase.","date":"2002","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12023288","citation_count":36,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2981921","id":"PMC_2981921","title":"Oligosaccharide structure of human C4.","date":"1985","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/2981921","citation_count":36,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"9795119","id":"PMC_9795119","title":"Expression of complement C4 and C9 genes by human astrocytes.","date":"1998","source":"Brain research","url":"https://pubmed.ncbi.nlm.nih.gov/9795119","citation_count":35,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2302180","id":"PMC_2302180","title":"The complement component C4 of mammals.","date":"1990","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/2302180","citation_count":35,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"6308844","id":"PMC_6308844","title":"Effects of leukotrienes B4 and C4 on coronary circulation and myocardial contractility.","date":"1983","source":"Surgery","url":"https://pubmed.ncbi.nlm.nih.gov/6308844","citation_count":35,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"12968987","id":"PMC_12968987","title":"Leukotriene C4 synthase polymorphisms and responsiveness to leukotriene antagonists in asthma.","date":"2003","source":"British journal of clinical pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/12968987","citation_count":34,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"10849339","id":"PMC_10849339","title":"The evolution of C4 plants: acquisition of cis-regulatory sequences in the promoter of C4-type pyruvate, orthophosphate dikinase gene.","date":"2000","source":"The Plant journal : for cell and molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/10849339","citation_count":34,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"21631531","id":"PMC_21631531","title":"Carbonic anhydrase and the molecular evolution of C4 photosynthesis.","date":"2011","source":"Plant, cell & environment","url":"https://pubmed.ncbi.nlm.nih.gov/21631531","citation_count":32,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"23494362","id":"PMC_23494362","title":"One decade after the discovery of single-cell C4 species in terrestrial plants: what did we learn about the minimal requirements of C4 photosynthesis?","date":"2013","source":"Photosynthesis research","url":"https://pubmed.ncbi.nlm.nih.gov/23494362","citation_count":32,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"27055266","id":"PMC_27055266","title":"Finding the genes to build C4 rice.","date":"2016","source":"Current opinion in plant biology","url":"https://pubmed.ncbi.nlm.nih.gov/27055266","citation_count":31,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"18979631","id":"PMC_18979631","title":"Metastasis-associated C4.4A, a GPI-anchored protein cleaved by ADAM10 and ADAM17.","date":"2008","source":"Biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/18979631","citation_count":31,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"23703454","id":"PMC_23703454","title":"The recurrent assembly of C4 photosynthesis, an evolutionary tale.","date":"2013","source":"Photosynthesis research","url":"https://pubmed.ncbi.nlm.nih.gov/23703454","citation_count":30,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2044237","id":"PMC_2044237","title":"DR3 and nonDR3 associated complement component C4A deficiency in systemic lupus erythematosus.","date":"1991","source":"Clinical immunology and immunopathology","url":"https://pubmed.ncbi.nlm.nih.gov/2044237","citation_count":29,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"3264881","id":"PMC_3264881","title":"The fluid-phase binding of human C4 and its genetic variants, C4A3 and C4B1, to immunoglobulins.","date":"1988","source":"Molecular immunology","url":"https://pubmed.ncbi.nlm.nih.gov/3264881","citation_count":29,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"25643361","id":"PMC_25643361","title":"Developmental genetic mechanisms of C4 syndrome based on transcriptome analysis of C3 cotyledons and C4 assimilating shoots in Haloxylon ammodendron.","date":"2015","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/25643361","citation_count":28,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"18432942","id":"PMC_18432942","title":"Human complement components C4A and C4B genetic diversities: complex genotypes and phenotypes.","date":"2005","source":"Current protocols in immunology","url":"https://pubmed.ncbi.nlm.nih.gov/18432942","citation_count":27,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"32095676","id":"PMC_32095676","title":"Erythritol: Another C4 Platform Chemical in Biomass Refinery.","date":"2020","source":"ACS omega","url":"https://pubmed.ncbi.nlm.nih.gov/32095676","citation_count":27,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"22404718","id":"PMC_22404718","title":"C4.4A is associated with tumor budding and epithelial-mesenchymal transition of colorectal cancer.","date":"2012","source":"Cancer science","url":"https://pubmed.ncbi.nlm.nih.gov/22404718","citation_count":25,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"3405752","id":"PMC_3405752","title":"Tissue-specific variation in C4 and Slp gene regulation.","date":"1988","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/3405752","citation_count":25,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"33178234","id":"PMC_33178234","title":"A Partial C4 Photosynthetic Biochemical Pathway in Rice.","date":"2020","source":"Frontiers in plant science","url":"https://pubmed.ncbi.nlm.nih.gov/33178234","citation_count":25,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"8396560","id":"PMC_8396560","title":"Cellular responses of guinea-pig macrophages to C4a; inhibition of C3a-induced O2- generation by C4a.","date":"1993","source":"Immunology letters","url":"https://pubmed.ncbi.nlm.nih.gov/8396560","citation_count":23,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"10591082","id":"PMC_10591082","title":"The biochemical, molecular, and genomic aspects of leukotriene C4 synthase.","date":"1999","source":"Proceedings of the Association of American Physicians","url":"https://pubmed.ncbi.nlm.nih.gov/10591082","citation_count":23,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2483343","id":"PMC_2483343","title":"Antigenic determinants of human C4, Rodgers and Chido.","date":"1988","source":"Experimental and clinical immunogenetics","url":"https://pubmed.ncbi.nlm.nih.gov/2483343","citation_count":23,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"34142128","id":"PMC_34142128","title":"Finding the C4 sweet spot: cellular compartmentation of carbohydrate metabolism in C4 photosynthesis.","date":"2021","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/34142128","citation_count":22,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"18263632","id":"PMC_18263632","title":"Consequences of C4 photosynthesis for the partitioning of growth: a test using C3 and C4 subspecies of Alloteropsis semialata under nitrogen-limitation.","date":"2008","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/18263632","citation_count":22,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"29274820","id":"PMC_29274820","title":"Characterization and Stability of Trypanosoma cruzi 24-C4 (Tc24-C4), a Candidate Antigen for a Therapeutic Vaccine Against Chagas Disease.","date":"2017","source":"Journal of pharmaceutical sciences","url":"https://pubmed.ncbi.nlm.nih.gov/29274820","citation_count":22,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"27535993","id":"PMC_27535993","title":"A generalized stoichiometric model of C3, C2, C2+C4, and C4 photosynthetic metabolism.","date":"2016","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/27535993","citation_count":21,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"1545698","id":"PMC_1545698","title":"Neonatal lupus erythematosus syndrome: analysis of C4 allotypes and C4 genes in 18 families.","date":"1992","source":"Medicine","url":"https://pubmed.ncbi.nlm.nih.gov/1545698","citation_count":21,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"27986845","id":"PMC_27986845","title":"Unique photosynthetic phenotypes in Portulaca (Portulacaceae): C3-C4 intermediates and NAD-ME C4 species with Pilosoid-type Kranz anatomy.","date":"2016","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/27986845","citation_count":20,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"3260957","id":"PMC_3260957","title":"C4 null phenotypes among lupus erythematosus patients are predominantly the result of deletions covering C4 and closely linked 21-hydroxylase A genes.","date":"1988","source":"Journal of medical genetics","url":"https://pubmed.ncbi.nlm.nih.gov/3260957","citation_count":20,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"34456009","id":"PMC_34456009","title":"Genetically Predicted Brain C4A Expression Is Associated With TSPO and Hippocampal Morphology.","date":"2021","source":"Biological psychiatry","url":"https://pubmed.ncbi.nlm.nih.gov/34456009","citation_count":19,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2890280","id":"PMC_2890280","title":"Leukotriene C4 metabolism by hepatoma cells and liver.","date":"1987","source":"Advances in enzyme regulation","url":"https://pubmed.ncbi.nlm.nih.gov/2890280","citation_count":19,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"26492168","id":"PMC_26492168","title":"Toward understanding the molecular mechanism of a geminivirus C4 protein.","date":"2015","source":"Plant signaling & behavior","url":"https://pubmed.ncbi.nlm.nih.gov/26492168","citation_count":19,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"28062590","id":"PMC_28062590","title":"Shared characteristics underpinning C4 leaf maturation derived from analysis of multiple C3 and C4 species of Flaveria.","date":"2017","source":"Journal of experimental botany","url":"https://pubmed.ncbi.nlm.nih.gov/28062590","citation_count":19,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"2029208","id":"PMC_2029208","title":"Influence of C4 null alleles on C4 activation in systemic lupus erythematosus.","date":"1991","source":"Annals of the rheumatic diseases","url":"https://pubmed.ncbi.nlm.nih.gov/2029208","citation_count":18,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"26814963","id":"PMC_26814963","title":"Schizophrenia risk from complex variation of complement component 4.","date":"2016","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/26814963","citation_count":1806,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"12477932","id":"PMC_12477932","title":"Generation and initial analysis of more than 15,000 full-length human and mouse cDNA sequences.","date":"2002","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/12477932","citation_count":1479,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"12754519","id":"PMC_12754519","title":"Identification and quantification of N-linked glycoproteins using hydrazide chemistry, stable isotope labeling and mass spectrometry.","date":"2003","source":"Nature biotechnology","url":"https://pubmed.ncbi.nlm.nih.gov/12754519","citation_count":1176,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"26186194","id":"PMC_26186194","title":"The BioPlex Network: A Systematic Exploration of the Human Interactome.","date":"2015","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/26186194","citation_count":1118,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"28514442","id":"PMC_28514442","title":"Architecture of the human interactome defines protein communities and disease networks.","date":"2017","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/28514442","citation_count":1085,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"26496610","id":"PMC_26496610","title":"A human interactome in three quantitative dimensions organized by stoichiometries and abundances.","date":"2015","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/26496610","citation_count":1015,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"29507755","id":"PMC_29507755","title":"VIRMA mediates preferential m6A mRNA methylation in 3'UTR and near stop codon and associates with alternative polyadenylation.","date":"2018","source":"Cell discovery","url":"https://pubmed.ncbi.nlm.nih.gov/29507755","citation_count":829,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"33961781","id":"PMC_33961781","title":"Dual proteome-scale networks reveal cell-specific remodeling of the human interactome.","date":"2021","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/33961781","citation_count":705,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"21873635","id":"PMC_21873635","title":"Phylogenetic-based propagation of functional annotations within the Gene Ontology consortium.","date":"2011","source":"Briefings in bioinformatics","url":"https://pubmed.ncbi.nlm.nih.gov/21873635","citation_count":656,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"22074402","id":"PMC_22074402","title":"Antimicrobial peptides: key components of the innate immune system.","date":"2011","source":"Critical reviews in biotechnology","url":"https://pubmed.ncbi.nlm.nih.gov/22074402","citation_count":565,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"23535732","id":"PMC_23535732","title":"Identification of 23 new prostate cancer susceptibility loci using the iCOGS custom genotyping array.","date":"2013","source":"Nature genetics","url":"https://pubmed.ncbi.nlm.nih.gov/23535732","citation_count":463,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"15199963","id":"PMC_15199963","title":"The lectin-complement pathway--its role in innate immunity and evolution.","date":"2004","source":"Immunological reviews","url":"https://pubmed.ncbi.nlm.nih.gov/15199963","citation_count":443,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"6546707","id":"PMC_6546707","title":"The structural basis of the multiple forms of human complement component C4.","date":"1984","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/6546707","citation_count":435,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"16344560","id":"PMC_16344560","title":"Diversification of transcriptional modulation: large-scale identification and characterization of putative alternative promoters of human genes.","date":"2005","source":"Genome research","url":"https://pubmed.ncbi.nlm.nih.gov/16344560","citation_count":409,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"6559257","id":"PMC_6559257","title":"A molecular map of the human major histocompatibility complex class III region linking complement genes C4, C2 and factor B.","date":"1984","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/6559257","citation_count":406,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"17503323","id":"PMC_17503323","title":"Gene copy-number variation and associated polymorphisms of complement component C4 in human systemic lupus erythematosus (SLE): low copy number is a risk factor for and high copy number is a protective factor against SLE susceptibility in European Americans.","date":"2007","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/17503323","citation_count":370,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"16335952","id":"PMC_16335952","title":"Human plasma N-glycoproteome analysis by immunoaffinity subtraction, hydrazide chemistry, and mass spectrometry.","date":"2005","source":"Journal of proteome research","url":"https://pubmed.ncbi.nlm.nih.gov/16335952","citation_count":350,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"30110629","id":"PMC_30110629","title":"Enhancer Activity Requires CBP/P300 Bromodomain-Dependent Histone H3K27 Acetylation.","date":"2018","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/30110629","citation_count":274,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"35031058","id":"PMC_35031058","title":"EWSR1-induced circNEIL3 promotes glioma progression and exosome-mediated macrophage immunosuppressive polarization via stabilizing IGF2BP3.","date":"2022","source":"Molecular cancer","url":"https://pubmed.ncbi.nlm.nih.gov/35031058","citation_count":257,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"15174051","id":"PMC_15174051","title":"An investigation into the human serum \"interactome\".","date":"2004","source":"Electrophoresis","url":"https://pubmed.ncbi.nlm.nih.gov/15174051","citation_count":247,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"6019133","id":"PMC_6019133","title":"Formation and functional significance of a molecular complex derived from the second and the fourth component of human complement.","date":"1967","source":"The Journal of experimental medicine","url":"https://pubmed.ncbi.nlm.nih.gov/6019133","citation_count":245,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"14574404","id":"PMC_14574404","title":"The DNA sequence and analysis of human chromosome 6.","date":"2003","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/14574404","citation_count":242,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"6609966","id":"PMC_6609966","title":"The molecular basis for the difference in immune hemolysis activity of the Chido and Rodgers isotypes of human complement component C4.","date":"1984","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/6609966","citation_count":227,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"2431902","id":"PMC_2431902","title":"Structural basis of the polymorphism of human complement components C4A and C4B: gene size, reactivity and antigenicity.","date":"1986","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/2431902","citation_count":197,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"11773063","id":"PMC_11773063","title":"The orphan receptor C5L2 has high affinity binding sites for complement fragments C5a and C5a des-Arg(74).","date":"2001","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/11773063","citation_count":189,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"32499649","id":"PMC_32499649","title":"Complement genes contribute sex-biased vulnerability in diverse disorders.","date":"2020","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/32499649","citation_count":189,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"12894949","id":"PMC_12894949","title":"Paternal, maternal, and biparental molecular markers provide unique windows onto the evolutionary history of macaque monkeys.","date":"2003","source":"Evolution; international journal of organic evolution","url":"https://pubmed.ncbi.nlm.nih.gov/12894949","citation_count":175,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"25544563","id":"PMC_25544563","title":"Global mapping of herpesvirus-host protein complexes reveals a transcription strategy for late genes.","date":"2014","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/25544563","citation_count":173,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"32807901","id":"PMC_32807901","title":"UFMylation maintains tumour suppressor p53 stability by antagonizing its ubiquitination.","date":"2020","source":"Nature cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/32807901","citation_count":168,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"20936779","id":"PMC_20936779","title":"A human MAP kinase interactome.","date":"2010","source":"Nature methods","url":"https://pubmed.ncbi.nlm.nih.gov/20936779","citation_count":165,"is_preprint":false,"source_track":"gene2pubmed"}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":45267,"output_tokens":3645,"usd":0.095238},"stage2":{"model":"claude-opus-4-6","input_tokens":7062,"output_tokens":3235,"usd":0.174278},"total_usd":0.613811,"stage1_batch_id":"msgbatch_01TqNM6p6FsoMBT2jzQTwKBr","stage2_batch_id":"msgbatch_01Ya473eco8sTHbvRymQaU8o","note":"batch pricing = 50% of standard","round2_stage1":{"model":"claude-sonnet-4-6","input_tokens":58290,"output_tokens":7230,"usd":0.14166},"round2_rules_fired":"R2,R3","round2_stage2":{"model":"claude-opus-4-6","input_tokens":8813,"output_tokens":3641,"usd":0.202635}},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1984,\n      \"finding\": \"C4A and C4B differ in their covalent binding specificity: C4B reacts more effectively with hydroxyl groups (binding twice as effectively to antibody-coated red cells), while C4A reacts more effectively with amino groups (binding better to protein-antigen complexes), despite very small differences in amino acid sequence.\",\n      \"method\": \"In vitro binding assays with purified C4A and C4B proteins tested against antibody-coated red cells and protein-antigen complexes\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct biochemical assay with purified proteins, foundational paper with 296 citations replicated by subsequent studies\",\n      \"pmids\": [\"6332733\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1981,\n      \"finding\": \"C4a anaphylatoxin is generated by C1s cleavage of the C4 alpha-chain between positions 77 and 78, producing a 77-residue cationic polypeptide (Mr ~9000) devoid of histidine, tryptophan, and carbohydrate, with ~30% homology to C3a and ~36% to C5a.\",\n      \"method\": \"Protein purification from serum, sequence analysis of cyanogen bromide fragments and chymotryptic peptides\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct protein sequencing and biochemical characterization of purified anaphylatoxin\",\n      \"pmids\": [\"6167582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1984,\n      \"finding\": \"C4A and C4B are encoded by at least two separate loci (each ~16 kb) mapping in the HLA Class III region on chromosome 6, with class-specific differences in nucleotide sequence in the C4d region that distinguish C4A from C4B.\",\n      \"method\": \"cDNA cloning, nucleotide sequence analysis, restriction mapping of cosmid genomic library, hybridization with C4A/C4B-specific synthetic oligonucleotide probes\",\n      \"journal\": \"Philosophical transactions of the Royal Society of London. Series B, Biological sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct genomic sequencing and molecular characterization\",\n      \"pmids\": [\"6149580\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"C4A is markedly more effective than C4B in enhancing binding of immune complexes to the complement receptor CR1 on red cells (both preformed and nascent complexes), identifying CR1-mediated immune complex handling as the major functional difference between C4A and C4B isotypes.\",\n      \"method\": \"Purified C4A and C4B compared in immunoprecipitation inhibition assays and CR1 binding assays with red cells\",\n      \"journal\": \"Clinical and experimental immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro assay with purified proteins, multiple readouts (immunoprecipitation and CR1 binding), consistent with EMBO 1984 findings\",\n      \"pmids\": [\"2138067\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"C4A3 binds 3-4 times more IgG than C4B1 in fluid-phase systems, predominantly via amide linkage, whereas C4B1 forms both amide and acyl ester bonds; C4A3 also shows higher binding efficiency for IgM, IgA, IgG2a, F(ab')2, and BSA.\",\n      \"method\": \"Fluid-phase covalent binding assay with purified C4A3 and C4B1 variants, antibody, and purified C1s; linkage characterized biochemically\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with purified components, multiple substrates tested, linkage chemistry determined\",\n      \"pmids\": [\"3264881\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1985,\n      \"finding\": \"The alpha- and beta-chains of human C4 are glycosylated (the gamma-chain is not); the alpha-chain carries three complex fucosylated biantennary oligosaccharides and sulfation on the alpha-4 fragment; the ~2000 Da molecular weight difference between C4A and C4B alpha-chains is not due to carbohydrate and resides in the alpha-2 fragment.\",\n      \"method\": \"Biochemical analysis of oligosaccharide structures on purified plasma C4 and HepG2-secreted C4 using enzymatic treatments and chain fractionation\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct biochemical characterization localizing structural differences between C4A and C4B\",\n      \"pmids\": [\"2981921\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1980,\n      \"finding\": \"Activation of C4 by the classical complement pathway generates the fragment C4d, detectable in plasma by rocket immunoelectrophoresis; the C4d/C4 ratio correlates with in vivo radiolabeled C4 metabolism, providing a measure of C4 activation in diseases such as SLE and rheumatoid arthritis.\",\n      \"method\": \"Rocket immunoelectrophoresis with anti-C4d antibodies, correlated with radiolabeled C4 metabolic studies in patients\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo metabolic labeling correlated with immunoelectrophoresis, establishes activation product identity\",\n      \"pmids\": [\"7373050\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1972,\n      \"finding\": \"Human C4 is synthesized by cells of hepatic/non-hepatic origin: hybrid cells formed by fusion of C4-deficient guinea pig peritoneal exudate cells with HeLa cells produced functionally active human C4, demonstrating that C4 biosynthesis can be restored by complementation.\",\n      \"method\": \"Cell fusion (somatic cell hybridization), functional assay for C4 activity in hybrid cell supernatants\",\n      \"journal\": \"Science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — cell biology reconstitution experiment establishing cellular biosynthesis capacity\",\n      \"pmids\": [\"5033180\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Human astrocytes constitutively express complement C4 mRNA and protein; C4 expression is upregulated by interferon-gamma and inhibited by IL-1beta; C4 immunoreactivity is localized to GFAP-positive astrocytes when protein secretion is blocked.\",\n      \"method\": \"RT-PCR, ELISA, immunocytochemistry, immunoblot of primary human astrocytes with cytokine treatments\",\n      \"journal\": \"Brain research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (RT-PCR, ELISA, immunocytochemistry) in primary human cells with functional cytokine manipulation\",\n      \"pmids\": [\"9795119\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Bone marrow-derived macrophages produce sufficient C4 locally (within splenic germinal centers) to restore humoral immunity in C4-deficient mice; splenic macrophages (CD11b+, CD11c-) are identified as a cellular source of C4 synthesis within the spleen.\",\n      \"method\": \"Bone marrow reconstitution of C4-/- mice, immunization with NP-KLH and HSV-1, in situ hybridization for C4 mRNA, immunofluorescence for C4 protein deposits, cell sorting with C4-specific RT-PCR\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (bone marrow chimeras, in situ hybridization, immunofluorescence, cell sorting + RT-PCR) with functional immune readout\",\n      \"pmids\": [\"12421924\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"C4a induces a biphasic Ca2+ mobilization in guinea-pig macrophages (intracellular pool release followed by extracellular Ca2+ influx) via a receptor distinct from the C3a receptor; C4a-induced signaling does not trigger O2- generation but inhibits C3a-induced O2- generation.\",\n      \"method\": \"Ca2+ mobilization assays, [125I]-C3a competitive binding assay, O2- generation assay in guinea-pig macrophages\",\n      \"journal\": \"Immunology letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple functional assays (Ca2+ mobilization, binding competition, O2- generation) in primary macrophages establishing receptor pharmacology of C4a\",\n      \"pmids\": [\"8396560\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"C4 null alleles, particularly C4A Q0, are associated with reduced C4d (activation product) concentrations independent of total C4 concentration, suggesting that silent C4A genes condition poor C4 activation—a critical step in clearance of immune complexes via the classical pathway.\",\n      \"method\": \"C4 phenotyping by protein electrophoresis, C4 and C4d quantification in SLE patient plasma\",\n      \"journal\": \"Annals of the rheumatic diseases\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — clinical association study with protein quantification; mechanistic inference rather than direct experimental manipulation\",\n      \"pmids\": [\"2029208\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1984,\n      \"finding\": \"Human C4 haplotypes can carry duplicated C4A or C4B loci, with diploid genomes carrying 2-4 expressed C4 structural genes; mean serum C4 levels roughly reflect the number of expressed structural genes.\",\n      \"method\": \"Family genetic studies with HLA typing, complement allotyping, and segregation analysis across multiple generations\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic segregation analysis in families establishing copy number variation and gene dosage effect on protein levels\",\n      \"pmids\": [\"6607672\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"Among mammalian species, chimpanzee and rhesus monkey express two C4 types with C4A-like and C4B-like binding specificities similar to humans; rat, guinea pig, whale, rabbit, dog and pig each express a single C4B-like type; sheep and cattle express both types, suggesting the C4 gene duplication either predates mammalian divergence or occurred multiple independent times.\",\n      \"method\": \"Purification of C4 from multiple mammalian species and binding specificity testing\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 — biochemical binding assays with purified proteins across multiple species, establishing evolutionary conservation of functional diversification\",\n      \"pmids\": [\"2302180\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Complete C4 deficiency can result from a 2-bp GT deletion in exon 13 of C4A causing protein truncation, or from intronic mutations (including splice site mutation at intron 28 donor site) in C4B genes; mutation hotspots cluster in exon 13 and a 2.6-kb region spanning exons 20-29.\",\n      \"method\": \"Immunofixation, genomic restriction fragment analysis, pulsed field gel electrophoresis, gene sequencing of mutant C4A and C4B genes from complete C4-deficient patients\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct sequencing identifying causative mutations with multiple complementary genomic methods\",\n      \"pmids\": [\"15294999\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Higher genetically predicted brain C4A expression is associated with higher TSPO (microglial marker) signal on PET imaging and altered hippocampal morphology (reduced surface area and medial displacement in CA1 area), linking C4A to microglial activity and hippocampal structure in living humans.\",\n      \"method\": \"Genetic prediction of brain C4A expression from RCCX structural elements combined with [18F]FEPPA TSPO-PET imaging and MRI morphometry in 111 participants\",\n      \"journal\": \"Biological psychiatry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo multimodal imaging with genetic prediction approach; single study but two independent imaging modalities\",\n      \"pmids\": [\"34456009\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Human complement C4A is encoded within the HLA class III region alongside the paralogous C4B gene; after activation by C1s cleavage of the alpha-chain, C4A preferentially forms amide bonds with amino groups on immune complexes (particularly immunoglobulins), making it markedly more effective than C4B at enhancing CR1-mediated immune complex binding and clearance, while the small released fragment C4a signals through a distinct receptor on macrophages to mobilize Ca2+ and suppress C3a-induced superoxide generation; C4A is synthesized by hepatocytes, macrophages, and astrocytes, and its gene dosage determines plasma protein levels, with copy number variation and null alleles linked to impaired classical pathway activation and susceptibility to autoimmune diseases including SLE.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll systematically classify each paper before extracting discoveries.\n\n**Classification:**\n\nThe target gene is **C4A** — human complement component 4A, a serum protein involved in the classical complement pathway.\n\n**KEEP papers** (about complement C4A):\n- PMID:6332733 — C4A vs C4B properties (complement)\n- PMID:23383423 — complement peptide receptors including C4a anaphylatoxin\n- PMID:3499152 — C4A deletion in SLE\n- PMID:6149580 — C4 gene structure\n- PMID:26814963 — C4A in schizophrenia/synapse elimination\n- PMID:6606418 — C4A null in SLE\n- PMID:6167582 — C4a anaphylatoxin primary structure\n- PMID:15294999 — complete C4A/C4B deficiency\n- PMID:2568334 — C4A deletion in black Americans with SLE\n- PMID:8482459 — autoimmune hepatitis and C4A deletion\n- PMID:7373050 — C4 activation, C4d\n- PMID:16403222 — real-time PCR quantification C4A/C4B\n- PMID:2138067 — C4A vs C4B immune complex handling\n- PMID:14604014 — C4 genetics, RCCX modules\n- PMID:2302180 — C4 complement mammalian biochemistry\n- PMID:27758680 — C4A/C4B CNV in Alzheimer's\n- PMID:5033180 — C4 biosynthesis by hybrid cells\n- PMID:12421924 — macrophage-derived C4 restores humoral immunity\n- PMID:16879240 — C4A null in rhinosinusitis\n- PMID:7451653 — genetic analysis of C4 deficiency\n- PMID:2981921 — oligosaccharide structure of human C4\n- PMID:8396560 — C4a cellular responses in macrophages\n- PMID:3264881 — fluid-phase binding C4A3 and C4B1 to immunoglobulins\n- PMID:3405752 — tissue-specific C4 and Slp gene regulation (mouse)\n- PMID:2044237 — DR3 and nonDR3 C4A deficiency in SLE\n- PMID:2029208 — C4 null alleles and C4 activation in SLE\n- PMID:1545698 — NLE and C4 allotypes\n- PMID:18432942 — C4A/C4B genotypes/phenotypes methods\n- PMID:6546706 — structural basis C4A and C4B (cDNA sequences) [gene2pubmed]\n- PMID:6559257 — molecular map MHC class III, C4 genes\n- PMID:6019133 — C4/C2 complex = C3 convertase\n- PMID:14574404 — chromosome 6 sequence (mentions C4)\n- PMID:6609966 — molecular basis C4A vs C4B hemolysis (isotype binding)\n- PMID:2431902 — structural basis C4A/C4B polymorphism\n- PMID:11773063 — C5L2 binds C5a/C3a/C4a\n- PMID:32499649 — complement C4 sex-biased vulnerability\n- PMID:17503323 — C4 gene CNV and SLE\n- PMID:34456009 — brain C4A expression, TSPO, hippocampal morphology\n- PMID:9795119 — astrocytes express C4\n- PMID:1976809 — C4A gene deletion in Japanese SLE\n- PMID:3260957 — C4 null phenotypes in LE\n- PMID:2483343 — antigenic determinants C4A/C4B (Rodgers/Chido)\n\n**EXCLUDE papers:**\n- All C4 photosynthesis papers (PMID:22404472, 20693355, 20543093, 21705644, 22128120, etc.) — plant biology symbol collision\n- PMID:11803016 — C4-dicarboxylate bacterial carriers\n- PMID:2546011 — C4-dicarboxylate transport in Rhizobium\n- PMID:6086189 — leukotriene C4 (different molecule)\n- PMID:21680741 — C4a-hydroperoxyflavin in monooxygenase (flavin C4a atom, not complement C4A)\n- PMID:19133805 — flavin C4a-oxygen adduct (not complement C4A)\n- PMID:24486019 — C4 lysyl hydroxylation of eRF1 (Jmjd4/eRF1, not complement C4A)\n- PMID:20091067 — geminivirus C4 protein (plant virus)\n- PMID:23720217 — begomovirus C4 gene (plant virus)\n- PMID:26492168 — geminivirus C4 protein mechanism (plant virus)\n- PMID:12895593 — leukotriene C4 synthase\n- PMID:12023288 — leukotriene C4 synthase membrane topology\n- PMID:11898391 — leukotriene C4 transport\n- PMID:12968987 — leukotriene C4 synthase polymorphisms\n- PMID:10591082 — leukotriene C4 synthase biochemistry\n- PMID:6308844 — leukotriene C4 coronary effects\n- PMID:2890280 — leukotriene C4 metabolism\n- PMID:10893330 — chicken leptin C4S analog (cysteine mutation, not complement)\n- PMID:22404718 — C4.4A in colorectal cancer (different protein, LYPD3)\n- PMID:18979631 — C4.4A substrate for ADAM10/17 (LYPD3, not C4A complement)\n- PMID:29274820 — Tc24-C4 Trypanosoma antigen (different protein entirely)\n- PMID:1696181 — bacteriophage c4 repressor (antisense RNA, bacteria)\n- PMID:12477932 — MGC cDNA sequences (general, not mechanistic for C4A)\n- PMID:12754519 — N-linked glycoproteins MS (general proteomics)\n- PMID:26186194 — BioPlex interactome (general)\n- PMID:28514442 — BioPlex 2.0 (general)\n- PMID:26496610 — human interactome (general)\n- PMID:29507755 — VIRMA m6A methylation (not C4A)\n- PMID:33961781 — BioPlex 3.0 (general)\n- PMID:21873635 — Gene Ontology propagation (general)\n- PMID:22074402 — antimicrobial peptides (general)\n- PMID:23535732 — prostate cancer GWAS (general)\n- PMID:15199963 — lectin-complement pathway (general complement, not C4A specific mechanism)\n- PMID:16344560 — alternative promoters (general)\n- PMID:16335952 — plasma N-glycoproteome (general)\n- PMID:15174051 — serum interactome (general)\n- PMID:6019133 — C3 convertase formation (KEEP — directly mechanistic for C4)\n- PMID:30110629 — CBP/P300 bromodomain H3K27Ac (not C4A)\n- PMID:35031058 — circNEIL3 glioma (not C4A)\n- PMID:12894949 — macaque phylogenetics with C4 intron as marker (not mechanistic for C4A)\n- PMID:25544563 — herpesvirus-host complexes (not C4A)\n- PMID:32807901 — UFMylation/p53 (not C4A)\n- PMID:20936779 — MAPK interactome (not C4A)\n- PMID:24179135 — parallel recruitment C4 photosynthesis (plant)\n- PMID:32095676 — erythritol C4 platform chemical (not C4A)\n\nNow extracting discoveries from KEPT papers:\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1967,\n      \"finding\": \"C4 and C2 interact in free solution to form a reversible protein-protein complex; upon cleavage by C1 esterase in the presence of Mg2+, this complex becomes a stable, enzymatically active C3 convertase that fragments C3, establishing the functional unit of the classical complement pathway.\",\n      \"method\": \"Purified component reconstitution, functional hemolysis assay, molecular weight determination\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution with purified components, enzymatic activity demonstrated, foundational study replicated broadly\",\n      \"pmids\": [\"6019133\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1981,\n      \"finding\": \"Human C4a anaphylatoxin (77 residues, ~9 kDa) is generated by C1s-mediated cleavage of the C4 alpha-chain between positions 77 and 78; primary sequence analysis revealed ~30% homology with C3a and ~36% with C5a, establishing C3a/C4a/C5a as a structurally related family despite immunological distinctiveness.\",\n      \"method\": \"Protein isolation from serum, cyanogen bromide fragmentation, chymotryptic digestion, sequence analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct protein sequencing with multiple orthogonal fragmentation methods; original primary structure determination\",\n      \"pmids\": [\"6167582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1984,\n      \"finding\": \"The complete amino acid sequence (1722 residues) of the C4A single-chain precursor was deduced from cDNA cloning; comparison with C4B cDNA sequences identified class-specific nucleotide differences that explain the two isotypes, and allelic differences within C4A.\",\n      \"method\": \"cDNA cloning and nucleotide sequencing from heterozygous donor liver mRNA\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — full cDNA sequencing; foundational structural determination replicated by contemporaneous studies\",\n      \"pmids\": [\"6546706\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1984,\n      \"finding\": \"C4A and C4B differ functionally in their covalent binding chemistry: C4B binds twice as effectively to antibody-coated red cells (hydroxyl-group targets) while C4A binds more effectively to protein-antigen complexes (amino-group targets); C4A preferentially transacylates amino nucleophiles and C4B preferentially acylates hydroxyl groups, despite only small differences in amino acid sequence.\",\n      \"method\": \"Purified C4A and C4B binding assays on antibody-coated red cells and protein-antigen complexes, comparison across a range of concentrations\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct biochemical assay with purified isotypes, functionally validated, replicated in subsequent studies\",\n      \"pmids\": [\"6332733\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1984,\n      \"finding\": \"The molecular basis for C4B's ~4-fold greater immune hemolysis activity versus C4A was traced to greater deposition efficiency of nascent C4b onto C1-bearing erythrocyte surfaces, not to differences in C1s cleavage rate or C3 convertase kinetics; C4A nascent C4b preferentially forms amide bonds while C4B preferentially forms ester bonds with surface acceptors.\",\n      \"method\": \"Purified C4A and C4B from isotype-pure donors; hemolytic titration, C1s cleavage rate assay, C3 convertase decay assay, factor I cleavage assay, radiolabeled deposition quantification on sheep erythrocytes\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple orthogonal biochemical assays with purified isotypes; mechanistic dissection of each step\",\n      \"pmids\": [\"6609966\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1984,\n      \"finding\": \"The C4A and C4B loci were mapped ~30 kb from the C2/factor B genes within a 98 kb segment of chromosome 6 MHC class III region, with the two C4 genes separated from each other by ~10 kb, establishing the genomic organization of the complement gene cluster.\",\n      \"method\": \"Overlapping cosmid clones from genomic DNA library, restriction mapping\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct genomic mapping; foundational structural genomics study\",\n      \"pmids\": [\"6559257\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1984,\n      \"finding\": \"The C4A/C4B class specificity (chemical reactivity and Rodgers/Chido antigenicity) maps to only four isotypic amino acid differences in the C4d region (positions 1101–1106): C4A carries PCPVLD and C4B carries LSPVIH; gene size polymorphism (22 kb vs 16 kb) was also identified for different allotypes.\",\n      \"method\": \"Cloning of C4A and C4B allotype genes, derived amino acid sequence comparison, restriction mapping\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct sequencing of cloned genes, isotype-specific residues identified at four positions; replicated across studies\",\n      \"pmids\": [\"2431902\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1985,\n      \"finding\": \"Human C4 glycosylation was characterized: the alpha-chain carries three complex fucosylated biantennary oligosaccharides (on alpha2, alpha3, and alpha4 fragments) and the beta-chain has a single high-mannose oligosaccharide; the ~2000 Da mass difference between C4A and C4B alpha-chains localizes to the alpha2 fragment and is not due to carbohydrate differences.\",\n      \"method\": \"Glycan analysis of purified plasma C4 and HepG2-secreted C4 using lectin and chemical methods; subunit fractionation\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct biochemical characterization of purified protein glycoforms, localized mass difference to non-carbohydrate amino acid differences\",\n      \"pmids\": [\"2981921\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"Rodgers and Chido blood group antigens map to C4A and C4B respectively; four polymorphic sites in the C4d region correlate with antigen expression, defining conformational or sequential epitopes; the serological phenotype can be dissociated from chemical reactivity class in rare variant allotypes.\",\n      \"method\": \"Haemagglutination inhibition assays with polyspecific antisera; nucleotide sequencing of C4d region from allotypes of known antigenic status\",\n      \"journal\": \"Experimental and clinical immunogenetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — sequencing plus functional serological assays; confirmed molecular basis of blood group antigens\",\n      \"pmids\": [\"2483343\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"C4A3 binds immunoglobulins (IgG, IgM, IgA, IgG2a, F(ab')2) and BSA 3–4 times more efficiently than C4B1 in fluid-phase reactions; C4A3 forms predominantly amide linkages with IgG whereas C4B1 forms both amide and ester linkages, mirroring binding preferences observed in solid-phase assays.\",\n      \"method\": \"Fluid-phase covalent binding assay with purified C4A3 and C4B1, C1s, and various immunoglobulin substrates; bond-type determination\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct in vitro binding assay with purified components, multiple substrates, bond-type characterization\",\n      \"pmids\": [\"3264881\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"The major functional difference between C4A and C4B in immune complex handling is at the level of CR1 (complement receptor 1) binding: C4A-opsonized complexes bind CR1 on erythrocytes markedly more effectively than C4B-opsonized complexes, with C4A also modestly superior at inhibiting immunoprecipitation, particularly in antibody excess.\",\n      \"method\": \"Purified C4A and C4B; immunoprecipitation inhibition assay; CR1 binding assay with preformed and nascent immune complexes on red cells\",\n      \"journal\": \"Clinical and experimental immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — direct functional assays with purified isotypes across multiple assay formats; mechanistic conclusion replicated\",\n      \"pmids\": [\"2138067\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"Comparative analysis of C4 purified from diverse mammalian species showed that C4B-like (hydroxyl-preferring) binding specificity is the ancestral form; C4A-like (amino-preferring) specificity co-occurs with C4B-like in primates, sheep, and cattle (suggesting independent duplications), while rodents and most other species have only C4B-like activity.\",\n      \"method\": \"C4 purification from multiple species, binding specificity assays\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multi-species comparative biochemistry with functional assays; single laboratory study\",\n      \"pmids\": [\"2302180\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1972,\n      \"finding\": \"C4-deficient guinea pig peritoneal exudate cells fused with HeLa cells produced functionally active human C4, demonstrating that C4 is encoded by a nuclear gene and can be synthesized by somatic hybrid cells, establishing cell-autonomous C4 biosynthesis.\",\n      \"method\": \"Cell fusion (somatic hybridization), functional complement hemolysis assay\",\n      \"journal\": \"Science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — cell fusion with functional readout; single study but clean genetic complementation approach\",\n      \"pmids\": [\"5033180\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"C4a anaphylatoxin induced biphasic Ca2+ mobilization in guinea-pig macrophages (rapid intracellular pool release followed by weak extracellular influx) via a receptor distinct from the C3a receptor (C4a did not compete with [125I]-C3a binding and C4a-desensitized cells still responded to C3a); C4a inhibited C3a-induced superoxide (O2-) generation without itself triggering O2- production.\",\n      \"method\": \"Intracellular Ca2+ measurement, radioligand competition binding ([125I]-C3a), superoxide generation assay in guinea-pig macrophages\",\n      \"journal\": \"Immunology letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple functional assays demonstrating receptor specificity and signaling outcome; single laboratory\",\n      \"pmids\": [\"8396560\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"The orphan receptor C5L2/GPR77 binds C4a anaphylatoxin (cross-competing with C3a binding, suggesting a shared or overlapping binding site distinct from the C5a site), demonstrating C5L2 as a potential low-affinity receptor for C4a.\",\n      \"method\": \"Radioligand cross-competition binding assay with C5L2-transfected RBL-2H3 cells; [125I]-C5a and C3a displacement by C4a\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct receptor binding competition assay; single study, moderate affinity interaction\",\n      \"pmids\": [\"11773063\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Bone marrow-derived macrophages synthesize and locally deposit C4 protein in splenic germinal centers; transfer of wild-type macrophages (or whole bone marrow) into C4-knockout mice restored C4 mRNA (detected by in situ hybridization), C4 protein deposits in follicles (immunofluorescence), and humoral immune responses to both soluble antigen and HSV-1, demonstrating that locally produced (macrophage-derived) C4 is sufficient for complement-dependent humoral immunity even in the absence of serum C4.\",\n      \"method\": \"Bone marrow reconstitution of C4-/- mice, immunization with NP5-KLH and HSV-1 infection, antibody titer measurement, in situ hybridization, immunofluorescence staining, cell sorting with RT-PCR\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic rescue experiment with multiple orthogonal readouts (functional, mRNA, protein localization); clear mechanistic conclusion\",\n      \"pmids\": [\"12421924\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"C4A expression in the brain (determined by structural alleles of the C4 locus) promotes synapse elimination during postnatal development; C4 protein localizes to neuronal synapses, dendrites, axons, and cell bodies in humans; in mice, C4 mediates complement-dependent synaptic pruning, and higher C4A expression associates with schizophrenia risk in proportion to its level of brain expression.\",\n      \"method\": \"Human brain C4A/C4B expression quantification by allele-specific RNA-seq; C4 immunohistochemistry in human brain; mouse genetic model (C4-knockout and C4-overexpressing) with synapse counting by immunofluorescence; population genetic association\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — direct protein localization, mouse genetic loss-of-function with quantitative synapse phenotype, and human expression data; multi-method, high-impact study replicated in follow-up work\",\n      \"pmids\": [\"26814963\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"C4A and C4B protein levels are higher in cerebrospinal fluid and plasma of men than women aged 20–50; common C4A/C4B allele combinations generate up to 14-fold variation in SLE risk and 31-fold variation in Sjögren's syndrome risk in men, with C4A acting more protectively than C4B against these autoimmune diseases while the same alleles increase schizophrenia risk; this sex-differential effect parallels sex differences in complement protein levels.\",\n      \"method\": \"Large-scale population genetic association study with definitive C4 structural genotyping; CSF and plasma C4/C3 protein level measurement stratified by sex and age\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — large-scale genetic association with direct protein quantification across sex; multi-disease analysis; replicated across independent cohorts\",\n      \"pmids\": [\"32499649\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Genetically predicted higher brain C4A expression associates with higher TSPO (microglial marker) binding on PET imaging and altered hippocampal morphology (reduced surface area and medial displacement in CA1) in living humans, suggesting C4A-mediated microglial activation and synapse elimination affect hippocampal structure in vivo.\",\n      \"method\": \"Genetically predicted C4A expression from structural alleles; [18F]FEPPA TSPO-PET imaging; MRI-based hippocampal morphometry in 111 participants\",\n      \"journal\": \"Biological psychiatry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo imaging with genetic instrument; single study, indirect measure of C4A function via predicted expression\",\n      \"pmids\": [\"34456009\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"Tissue-specific expression of mouse C4 and the highly homologous Slp gene was characterized: C4 is expressed at high levels in liver and peritoneal macrophages, with lower but detectable levels in mammary gland, lung, spleen, kidney, testis, brain, heart, and submaxillary gland; regulatory differences between C4 and Slp vary across tissues and are controlled by distinct cis-acting and trans-acting factors in each tissue context.\",\n      \"method\": \"Northern blotting using C4/Slp-distinguishing probe across multiple mouse tissues; congenic strain comparison\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct mRNA quantification across tissues with genetic controls; establishes tissue expression pattern\",\n      \"pmids\": [\"3405752\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Human astrocytes constitutively express C4 mRNA and protein; interferon-gamma upregulates C4 expression while IL-1beta inhibits it; C4 immunoreactivity was localized to GFAP-positive astrocytes when protein secretion was blocked, establishing astrocytes as a cellular source of complement C4 in the CNS.\",\n      \"method\": \"RT-PCR, ELISA, immunocytochemistry, immunoblot in primary human astrocytes with cytokine treatments; protein secretion inhibition\",\n      \"journal\": \"Brain research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods; cytokine regulation established; single laboratory\",\n      \"pmids\": [\"9795119\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"C4A is a serum complement protein produced primarily by hepatocytes, macrophages, and astrocytes that is activated by C1s-mediated cleavage of its alpha-chain to generate C4a anaphylatoxin (which signals through a distinct receptor to inhibit C3a-induced superoxide generation) and nascent C4b, which covalently deposits on immune complexes and activating surfaces preferentially via amide bonds (to amino groups) rather than ester bonds, giving C4A superior efficiency in opsonizing protein-antigen immune complexes and enhancing CR1-mediated clearance; C4b then assembles with C2 to form the classical pathway C3 convertase; in the brain, C4A mediates complement-dependent synaptic pruning by microglia during postnatal development, and its expression level determines risk for both schizophrenia (higher C4A = higher risk) and autoimmune diseases such as SLE and Sjögren's syndrome (higher C4A = lower risk), with these effects showing sex-dependent magnitude correlated with sex differences in complement protein levels.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"C4A is a complement component encoded within the HLA class III region that, upon activation by C1s cleavage of its alpha-chain, preferentially forms amide bonds with amino groups on immune complexes—particularly immunoglobulins—making it substantially more effective than its paralog C4B at promoting CR1-mediated immune complex binding and clearance [PMID:6332733, PMID:2138067, PMID:3264881]. C4A is synthesized by hepatocytes, bone marrow-derived macrophages within splenic germinal centers, and astrocytes in the CNS, with expression modulated by cytokines such as IFN-γ [PMID:12421924, PMID:9795119]. The released anaphylatoxin fragment C4a signals through a receptor distinct from C3aR on macrophages, triggering biphasic Ca²⁺ mobilization and suppressing C3a-induced superoxide generation [PMID:8396560]. C4A gene copy number directly determines plasma C4 protein levels, and null alleles—arising from frameshift or splice-site mutations—impair classical pathway activation, with complete C4 deficiency linked to susceptibility to systemic lupus erythematosus [PMID:6607672, PMID:15294999, PMID:2029208].\",\n  \"teleology\": [\n    {\n      \"year\": 1972,\n      \"claim\": \"Establishing that C4 could be synthesized by non-hepatic cells resolved whether complement production was restricted to the liver and opened the question of local tissue-level complement function.\",\n      \"evidence\": \"Somatic cell hybridization of C4-deficient guinea pig cells with HeLa cells restored functional C4 secretion\",\n      \"pmids\": [\"5033180\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Cell fusion system does not identify which physiological cell types produce C4 in vivo\",\n        \"No distinction between C4A and C4B isotype production\"\n      ]\n    },\n    {\n      \"year\": 1980,\n      \"claim\": \"Detection of the C4d activation fragment in plasma provided the first means to quantify in vivo C4 activation and link it to disease states such as SLE.\",\n      \"evidence\": \"Rocket immunoelectrophoresis with anti-C4d correlated with radiolabeled C4 metabolic turnover in patients\",\n      \"pmids\": [\"7373050\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Could not distinguish C4A-derived from C4B-derived C4d\",\n        \"Correlation with disease does not establish causation\"\n      ]\n    },\n    {\n      \"year\": 1981,\n      \"claim\": \"Sequencing of the C4a anaphylatoxin defined the C1s cleavage site and revealed structural homology to C3a and C5a, establishing C4a as a member of the anaphylatoxin family.\",\n      \"evidence\": \"Protein purification from serum followed by sequencing of cyanogen bromide and chymotryptic peptides\",\n      \"pmids\": [\"6167582\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No receptor or signaling function for C4a had been identified\",\n        \"Functional consequences of C4a release remained unknown\"\n      ]\n    },\n    {\n      \"year\": 1984,\n      \"claim\": \"Discovery that C4A and C4B are encoded by separate loci in the MHC class III region, and that their functional divergence maps to the C4d region, resolved the genetic basis of isotypic diversity and established the RCCX modular framework.\",\n      \"evidence\": \"cDNA cloning, nucleotide sequencing, cosmid restriction mapping, and hybridization with isotype-specific oligonucleotides\",\n      \"pmids\": [\"6149580\", \"6607672\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Exact amino acid residues responsible for binding specificity not yet pinpointed\",\n        \"Mechanisms generating copy number variation not defined\"\n      ]\n    },\n    {\n      \"year\": 1984,\n      \"claim\": \"Biochemical demonstration that C4A preferentially forms amide bonds with amino groups while C4B prefers hydroxyl groups provided the molecular explanation for isotype-specific immune complex handling.\",\n      \"evidence\": \"In vitro binding assays with purified C4A and C4B against antibody-coated red cells and protein-antigen complexes\",\n      \"pmids\": [\"6332733\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis for differential thioester reactivity not resolved\",\n        \"Relative physiological importance of each isotype in vivo not tested\"\n      ]\n    },\n    {\n      \"year\": 1988,\n      \"claim\": \"Quantitative binding studies confirmed C4A's 3–4-fold superiority over C4B for IgG binding via amide linkage, extending this to multiple immunoglobulin classes and establishing amide-bond formation as the dominant mechanism.\",\n      \"evidence\": \"Fluid-phase covalent binding assays with purified C4A3 and C4B1 and multiple substrates including IgG, IgM, IgA, and BSA\",\n      \"pmids\": [\"3264881\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"In vivo confirmation of preferential C4A–immunoglobulin interaction not yet available\",\n        \"Contribution of non-covalent interactions not assessed\"\n      ]\n    },\n    {\n      \"year\": 1990,\n      \"claim\": \"Demonstrating that C4A is markedly more effective than C4B at promoting CR1-mediated immune complex binding on erythrocytes identified the downstream functional consequence of C4A's amino-group specificity for immune complex clearance.\",\n      \"evidence\": \"Immunoprecipitation inhibition and CR1 binding assays with purified isotypes and red cells\",\n      \"pmids\": [\"2138067\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether C4A deficiency alone suffices to impair immune complex clearance in vivo not directly tested\",\n        \"Contribution of C4A to CR1-independent clearance pathways unknown\"\n      ]\n    },\n    {\n      \"year\": 1990,\n      \"claim\": \"Cross-species comparison showed C4A-like and C4B-like binding specificities exist in primates, ruminants, and other mammals, indicating that functional diversification of C4 is evolutionarily ancient.\",\n      \"evidence\": \"Purification and binding specificity testing of C4 from chimpanzee, rhesus monkey, sheep, cattle, and other mammals\",\n      \"pmids\": [\"2302180\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether functional diversification arose by a single ancient duplication or convergent events remains unresolved\",\n        \"Sequence determinants of isotype specificity not compared across species\"\n      ]\n    },\n    {\n      \"year\": 1991,\n      \"claim\": \"Linking C4A null alleles to reduced C4d activation products in SLE patients provided genetic evidence that C4A gene dosage conditions classical pathway activation capacity.\",\n      \"evidence\": \"C4 phenotyping by electrophoresis and C4/C4d quantification in SLE patient plasma\",\n      \"pmids\": [\"2029208\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"Observational association in a patient cohort without experimental manipulation; causality not established\",\n        \"Confounding by linkage disequilibrium with other MHC genes not excluded\",\n        \"Small sample size and single-center study\"\n      ]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Identification of a distinct C4a receptor on macrophages that triggers Ca²⁺ mobilization and cross-inhibits C3a-induced superoxide generation established C4a as a signaling molecule with immunomodulatory function independent of complement opsonization.\",\n      \"evidence\": \"Ca²⁺ mobilization, ¹²⁵I-C3a competitive binding, and superoxide generation assays in guinea pig macrophages\",\n      \"pmids\": [\"8396560\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Molecular identity of the C4a receptor not determined\",\n        \"Physiological relevance of C4a-mediated C3a antagonism in vivo not tested\",\n        \"Only guinea pig macrophages examined; human relevance assumed\"\n      ]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Demonstration that human astrocytes constitutively produce C4 and upregulate it in response to IFN-γ established a CNS-local source of complement C4 with potential relevance to neuroinflammation.\",\n      \"evidence\": \"RT-PCR, ELISA, immunocytochemistry, and immunoblot of primary human astrocytes with cytokine treatments\",\n      \"pmids\": [\"9795119\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether astrocyte-derived C4 is predominantly C4A or C4B not determined\",\n        \"Functional consequences of local C4 production in the CNS not tested\"\n      ]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Bone marrow chimera experiments proved that macrophage-derived C4 produced locally in splenic germinal centers is sufficient to restore humoral immunity, establishing that extrahepatic C4 production has non-redundant immune function.\",\n      \"evidence\": \"Bone marrow reconstitution of C4⁻/⁻ mice, immunization, in situ hybridization, and cell sorting with RT-PCR\",\n      \"pmids\": [\"12421924\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Mouse model does not distinguish C4A from C4B (mice have single C4 gene)\",\n        \"Quantitative contribution of local vs. circulating C4 not determined\"\n      ]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Identification of specific causative mutations (frameshift in exon 13, splice-site mutation in intron 28) in C4A and C4B genes defined the molecular basis of complete C4 deficiency.\",\n      \"evidence\": \"Genomic sequencing, immunofixation, restriction fragment analysis, and pulsed field gel electrophoresis of C4-deficient patients\",\n      \"pmids\": [\"15294999\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Genotype-phenotype correlation for partial deficiency states not fully characterized\",\n        \"Rescue experiments to confirm causality of individual mutations not performed\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Genetically predicted higher brain C4A expression was associated with increased microglial activation (TSPO-PET) and altered hippocampal morphology, providing in vivo human evidence linking C4A to synaptic pruning-related brain phenotypes.\",\n      \"evidence\": \"Genetic prediction of brain C4A expression from RCCX structure combined with [18F]FEPPA TSPO-PET and MRI morphometry in 111 participants\",\n      \"pmids\": [\"34456009\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Association study; causal mechanism linking C4A to microglial activation not demonstrated\",\n        \"Single cohort without independent replication\",\n        \"Whether C4A acts directly on microglia or indirectly via complement-tagged synapses is unresolved\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The molecular identity of the C4a receptor, the structural basis for the thioester reactivity difference between C4A and C4B at atomic resolution, and the precise mechanism by which C4A influences synaptic pruning and microglial activation in the human brain remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"C4a receptor has not been molecularly cloned or characterized\",\n        \"No crystal or cryo-EM structure comparing C4A and C4B thioester domains\",\n        \"Causal pathway from C4A gene dosage to hippocampal remodeling not established experimentally\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [10]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 3, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 3, 4, 6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 1, 3, 4, 6, 9, 10]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"C1S\",\n      \"CR1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"C4A is a classical complement pathway component that, upon C1s-mediated cleavage of its alpha-chain, generates C4a anaphylatoxin and nascent C4b, which covalently deposits on immune complexes preferentially via amide bonds to amino groups, conferring superior opsonization of protein antigens and enhanced CR1-mediated clearance compared with the C4B isotype [PMID:6332733, PMID:2138067]. This isotype specificity maps to four amino acid differences in the C4d region (positions 1101–1106) [PMID:2431902], and the deposited C4b assembles with C2 to form the classical pathway C3 convertase [PMID:6019133]. Beyond its role in humoral immunity—where macrophage-derived C4 is sufficient to support germinal center responses in the absence of serum C4 [PMID:12421924]—C4A is expressed in the brain by astrocytes and localizes to neuronal synapses, where it mediates complement-dependent synaptic pruning by microglia; higher C4A expression increases schizophrenia risk while protecting against SLE and Sjögren's syndrome in a sex-dependent manner correlated with sex differences in complement protein levels [PMID:26814963, PMID:32499649].\",\n  \"teleology\": [\n    {\n      \"year\": 1967,\n      \"claim\": \"Establishing that C4 interacts with C2 and, after C1 esterase cleavage, forms a stable C3 convertase defined the enzymatic core of the classical complement pathway and positioned C4 as an essential upstream component.\",\n      \"evidence\": \"Purified component reconstitution with functional hemolysis assay\",\n      \"pmids\": [\"6019133\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"C4A vs C4B isotype distinction not yet known\", \"covalent binding mechanism of nascent C4b not characterized\"]\n    },\n    {\n      \"year\": 1972,\n      \"claim\": \"Demonstrating that somatic hybrid cells autonomously synthesize functional C4 established that C4 is encoded by a nuclear gene and can be produced cell-autonomously outside the liver.\",\n      \"evidence\": \"HeLa–guinea-pig macrophage somatic cell fusion with hemolytic complement assay\",\n      \"pmids\": [\"5033180\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"identity of the human chromosomal locus unknown\", \"tissue-specific regulation not addressed\"]\n    },\n    {\n      \"year\": 1981,\n      \"claim\": \"Sequencing C4a anaphylatoxin revealed its generation by C1s cleavage at position 77–78 of the alpha-chain and established C3a/C4a/C5a as a structurally related anaphylatoxin family, raising the question of C4a-specific receptor signaling.\",\n      \"evidence\": \"Protein isolation, cyanogen bromide fragmentation, and direct amino acid sequencing from human serum C4\",\n      \"pmids\": [\"6167582\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"C4a receptor identity unknown\", \"biological activity of C4a not yet defined\"]\n    },\n    {\n      \"year\": 1984,\n      \"claim\": \"Full cDNA sequencing of C4A defined the 1722-residue precursor and, together with biochemical studies, revealed that C4A and C4B differ in covalent binding chemistry—C4A preferentially forms amide bonds with amino groups on protein antigens while C4B preferentially forms ester bonds—explaining their distinct functional niches in immune complex handling versus cell-surface hemolysis.\",\n      \"evidence\": \"cDNA cloning from liver mRNA; purified isotype binding assays on antibody-coated red cells and protein-antigen complexes; hemolytic titration and radiolabeled deposition\",\n      \"pmids\": [\"6546706\", \"6332733\", \"6609966\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"specific residues responsible for binding chemistry difference not yet identified\", \"in vivo consequences of C4A deficiency not established\"]\n    },\n    {\n      \"year\": 1984,\n      \"claim\": \"Genomic mapping placed C4A and C4B ~10 kb apart within a 98 kb segment of the MHC class III region on chromosome 6, establishing the tandem duplication architecture and linking complement variation to MHC haplotype diversity.\",\n      \"evidence\": \"Overlapping cosmid clones and restriction mapping of genomic DNA\",\n      \"pmids\": [\"6559257\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"copy number variation at the locus not yet appreciated\", \"regulatory elements controlling isotype-specific expression unknown\"]\n    },\n    {\n      \"year\": 1988,\n      \"claim\": \"Mapping Rodgers/Chido blood group determinants to C4A/C4B and pinpointing four isotypic residues in C4d (1101–1106) as the molecular basis of both antigenic and chemical-reactivity differences resolved how minimal sequence changes drive functional isotype divergence.\",\n      \"evidence\": \"Haemagglutination inhibition with polyspecific antisera; nucleotide sequencing of C4d region allotypes; fluid-phase covalent binding assays with purified C4A3 and C4B1\",\n      \"pmids\": [\"2483343\", \"2431902\", \"3264881\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"structural basis of how these four residues alter thioester reactivity not resolved\", \"rare variant allotypes can dissociate serological and chemical phenotypes\"]\n    },\n    {\n      \"year\": 1988,\n      \"claim\": \"Characterizing tissue-specific C4 mRNA expression in mouse revealed broad expression beyond the liver—including brain, lung, kidney, and macrophages—suggesting local complement production may serve tissue-specific immune functions.\",\n      \"evidence\": \"Northern blotting across multiple mouse tissues with congenic strain controls\",\n      \"pmids\": [\"3405752\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"human tissue expression pattern not directly addressed\", \"functional significance of extrahepatic C4 production unknown\"]\n    },\n    {\n      \"year\": 1990,\n      \"claim\": \"Demonstrating that C4A-opsonized immune complexes bind CR1 on erythrocytes far more effectively than C4B-opsonized complexes established that C4A's functional superiority in immune complex clearance operates at the level of receptor engagement, not just covalent deposition.\",\n      \"evidence\": \"CR1 binding assay and immunoprecipitation inhibition with purified C4A and C4B on preformed and nascent immune complexes\",\n      \"pmids\": [\"2138067\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"structural explanation for differential CR1 engagement not provided\", \"in vivo clearance studies not performed\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Identifying that C4a induces Ca²⁺ mobilization via a receptor distinct from the C3a receptor and inhibits C3a-induced superoxide generation established C4a as a functional anaphylatoxin with immunomodulatory rather than purely pro-inflammatory activity.\",\n      \"evidence\": \"Intracellular Ca²⁺ measurement, [¹²⁵I]-C3a radioligand competition, and superoxide assay in guinea-pig macrophages\",\n      \"pmids\": [\"8396560\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"molecular identity of the C4a receptor not established\", \"relevance to human macrophages not shown\", \"downstream signaling pathway not mapped\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Showing that human astrocytes constitutively produce C4 and that its expression is regulated by IFN-γ and IL-1β identified a CNS cellular source for locally produced complement, presaging a role for C4 in brain-specific processes.\",\n      \"evidence\": \"RT-PCR, ELISA, immunocytochemistry, and immunoblot in primary human astrocytes with cytokine treatment\",\n      \"pmids\": [\"9795119\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"C4A vs C4B isotype distinction in astrocyte expression not addressed\", \"functional consequence of astrocyte-derived C4 in brain unknown\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Identifying GPR77/C5L2 as a receptor that binds C4a (cross-competing with C3a) provided a candidate molecular receptor for C4a signaling, partially resolving the receptor identity question raised by the 1993 guinea-pig macrophage studies.\",\n      \"evidence\": \"Radioligand cross-competition binding in C5L2-transfected RBL-2H3 cells\",\n      \"pmids\": [\"11773063\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"C5L2 functional signaling in response to C4a not demonstrated\", \"physiological relevance of C4a–C5L2 interaction in vivo unknown\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Demonstrating that bone marrow-derived macrophage C4 is sufficient to restore germinal center function and humoral immunity in C4-knockout mice proved that locally produced (non-hepatic) C4 has a non-redundant role in adaptive immunity independent of circulating serum C4.\",\n      \"evidence\": \"Bone marrow reconstitution of C4⁻/⁻ mice; immunization; in situ hybridization, immunofluorescence, and antibody titer quantification\",\n      \"pmids\": [\"12421924\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"relative contribution of C4A vs C4B isotype in germinal center function not dissected\", \"mechanism of C4 action in follicular dendritic cell retention not elucidated\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"The landmark discovery that C4A protein localizes to neuronal synapses and mediates complement-dependent synaptic pruning by microglia—with higher C4A expression proportionally increasing schizophrenia risk—established a direct mechanistic link between a complement gene and a psychiatric disorder through a neurodevelopmental pathway.\",\n      \"evidence\": \"Allele-specific RNA-seq in human brain; C4 immunohistochemistry; C4-knockout and C4-overexpressing mouse models with synapse quantification; population genetic association\",\n      \"pmids\": [\"26814963\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"whether C4A acts via the same C3-dependent tagging pathway in brain as in periphery not fully established\", \"cell-type-specific C4A expression regulation in brain not characterized\", \"therapeutic modulation of C4A-dependent pruning not demonstrated\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Quantifying sex-differential C4 protein levels and demonstrating that C4A allele dosage generates up to 14–31-fold variation in autoimmune disease risk (SLE, Sjögren's syndrome) while simultaneously increasing schizophrenia risk unified the opposing disease associations into a single framework of complement-mediated self-tolerance versus synaptic pruning, modulated by sex.\",\n      \"evidence\": \"Large-scale population genetic association with structural C4 genotyping; CSF and plasma C4/C3 protein measurement stratified by sex\",\n      \"pmids\": [\"32499649\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"molecular mechanism by which sex hormones regulate C4 protein levels unknown\", \"whether C4A's protective effect in autoimmunity is purely via immune complex clearance or also involves tolerance mechanisms not resolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"In vivo neuroimaging evidence that genetically predicted higher C4A expression correlates with increased microglial activation (TSPO-PET) and altered hippocampal morphology translated the mouse pruning phenotype to living human brain, strengthening the causal model.\",\n      \"evidence\": \"[¹⁸F]FEPPA TSPO-PET and MRI hippocampal morphometry in 111 participants with C4 structural genotyping\",\n      \"pmids\": [\"34456009\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"TSPO-PET is an indirect marker; direct measurement of synaptic density change not performed\", \"sample size moderate; replication in larger cohorts needed\", \"causal directionality relies on genetic instrument assumptions\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis by which four C4d residues alter thioester reactivity, the definitive identity and signaling pathway of the C4a receptor in humans, the cell-type-specific regulatory mechanisms controlling C4A expression in the brain, and whether therapeutic modulation of C4A-mediated pruning can alter schizophrenia-related phenotypes.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"no crystal structure of C4A vs C4B thioester domain to explain differential reactivity\", \"C4a receptor signaling pathway in human cells remains unresolved\", \"no interventional studies targeting C4A-dependent pruning in disease models\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 3, 4, 9, 10]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 3, 4, 9, 10, 15, 17]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [16]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [16]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 3, 4, 10, 15]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [16, 18]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [16]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [16, 17]}\n    ],\n    \"complexes\": [\n      \"C3 convertase (C4b2a)\"\n    ],\n    \"partners\": [\n      \"C2\",\n      \"C1S\",\n      \"CR1\",\n      \"C5L2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}