Affinage

C1R

Complement C1r subcomponent · UniProt P00736

Length
705 aa
Mass
80.1 kDa
Annotated
2026-06-09
100 papers in source corpus 35 papers cited in narrative 35 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 6/6 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

C1R encodes a modular serine protease zymogen that serves as the proteolytic engine initiating the classical complement pathway (PMID:814163, PMID:10878362). Each chain is an A (noncatalytic) chain carrying a growth-factor/EGF module and internal repeats, joined by disulfide bonds to a B (catalytic) chain belonging to the trypsin family with a His-Asp-Ser triad and an Asp at the S1 pocket conferring trypsin-like specificity (PMID:6303394, PMID:3021205, PMID:3036070). Through its N-terminal CUB1-EGF-CUB2 region, C1r binds Ca2+ and assembles with C1s into the Ca2+-dependent C1s-C1r-C1r-C1s tetramer, while CUB1/CUB2 modules of C1r provide the high-affinity sites that dock onto the C1q collagen-stem lysines LysB61 and LysC58 to constitute the C1 complex (PMID:8676886, PMID:10408360, PMID:19473974, PMID:23650384). The tetramer holds C1r in a stabilized proenzyme state, and target recognition by the C1q globular heads transmits a mechanical activation signal that disrupts CCP1-SP interfaces to trigger autolytic self-cleavage at Arg446-Ile447 (PMID:8042996, PMID:11823416, PMID:17996945); activated C1r then cleaves and activates C1s with strict specificity for Gln at P2 and Ile at P1', using both active-site and CCP2 exosite contacts (PMID:869924, PMID:23589288). Beyond complement, C1r expression drives pathological tissue responses: it initiates intrarenal complement activation promoting fibrosis (PMID:31509012), and tumor-cell-derived C1r promotes cutaneous squamous cell carcinoma invasion by upregulating MMP production and activating ERK/Akt signaling (PMID:34756877, PMID:31049937). Heterozygous C1R mutations at subunit interfaces and inter-domain hinges cause periodontal Ehlers-Danlos syndrome, acting through intracellular retention and constitutive intracellular activation of C1r/C1s with unregulated extracellular protease activity (PMID:27745832, PMID:31749804).

Mechanistic history

Synthesis pass · year-by-year structured walk · 17 steps
  1. 1976 High

    Established that C1r is a zymogen converted to an active enzyme by proteolytic cleavage and that its sole defined role is to activate C1s within macromolecular C1, defining its position in the cascade.

    Evidence SDS-PAGE, inhibitor studies, and radiolabeled binding/hemolytic reconstitution assays with C1q, C1r, C1s and calcium

    PMID:1249422 PMID:814163

    Open questions at the time
    • Structural basis of activation cleavage not yet defined
    • How C1q transmits the activation signal unknown
  2. 1983 High

    Identified C1r as a trypsin-family serine protease at the sequence level, locating its catalytic His-Asp-Ser triad and the S1-pocket Asp dictating trypsin-like specificity.

    Evidence Protein sequencing of the catalytic B chain via CNBr and protease cleavage

    PMID:6303394

    Open questions at the time
    • Three-dimensional structure not yet solved
    • Determinants of restricted C1s-only substrate specificity unresolved
  3. 1987 High

    Completed the modular architecture by sequencing the A chain and the full cDNA precursor, revealing CUB/EGF-like and internal-repeat modules and homologies that frame later functional dissection.

    Evidence cDNA cloning from liver/HepG2 libraries and protein sequencing of autolytic A-chain fragments with PTM mapping

    PMID:3021205 PMID:3036070

    Open questions at the time
    • Functional contribution of each module untested at this stage
    • Role of beta-hydroxyaspartate and glycosylation in function unknown
  4. 1988 High

    Mapped the C1R and C1S genes as a tail-to-tail pair on chromosome 12p13 expressed primarily in liver, explaining their coordinate biology.

    Evidence Genomic DNA sequencing and RNA blot analysis

    PMID:2459702

    Open questions at the time
    • Regulation of coordinate expression not defined
    • Extrahepatic expression contexts not yet explored
  5. 1996 High

    Defined how C1r assembles into the tetramer, showing the first CUB domain is essential for Ca2+-dependent C1s association and that the N-terminal region controls zymogen stability.

    Evidence Baculovirus expression of deletion mutants with tetramer-formation and stability assays

    PMID:8676886

    Open questions at the time
    • Atomic-level interface with C1s not yet resolved
    • Contribution of individual modules to C1q binding unmapped
  6. 1999 High

    Dissected the activation mechanics, showing autoactivation/C1s cleavage is intrinsic to the SP domain, CCP2 supplies accessory substrate binding, CCP1 mediates dimerization, and one active subunit suffices for full activity.

    Evidence Recombinant domain-deletion mutants, R446Q/S637A and R463Q mutagenesis, sedimentation, calorimetry, and hemolytic reconstitution

    PMID:10408360 PMID:11445589 PMID:11673533 PMID:9916740

    Open questions at the time
    • Trigger linking C1q target recognition to SP-domain cleavage not yet visualized
    • Quantitative role of Ca2+ in restraining activation incomplete
  7. 1994 High

    Showed that tetramer incorporation stabilizes the C1r proenzyme and that the C1q globular head—not its collagen stem or the C1s catalytic region—transmits the activation signal, establishing directional control of activation.

    Evidence Biochemical reconstitution with defined C1q fragments and activation kinetics

    PMID:8042996

    Open questions at the time
    • Physical nature of the transmitted signal undefined
    • Conformational path from globular head to CCP1-SP interface unknown
  8. 2002 High

    Crystal structures of proenzyme and active catalytic regions revealed a head-to-tail homodimer with catalytic and cleavage sites at opposite ends, supporting a mechanical-stress model of activation triggered by target recognition.

    Evidence X-ray crystallography of proenzyme CCP1-CCP2-SP and zymogen/active CCP2-SP fragments, plus NMR of the EGF module

    PMID:11823416 PMID:12429092 PMID:9477945

    Open questions at the time
    • Conformation within the intact C1 complex not directly observed
    • How dimer geometry accommodates self-cleavage geometrically unresolved
  9. 2007 High

    The full active catalytic region structure, capturing an enzyme-product complex with the S1-P1 Asp631-Arg446 salt bridge, yielded a 'split-and-reassembly' model enabling autoactivation without large C1q arm movement.

    Evidence X-ray crystallography with enzyme-product crystal contacts

    PMID:17996945

    Open questions at the time
    • Model not validated in the full assembled C1 complex
    • Kinetics of split-and-reassembly in solution untested
  10. 2010 High

    Defined the Ca2+-driven and C1q-docking determinants: CUB1/CUB2 modules supply the high-affinity C1q sites, Ca2+ binding folds CUB2 with sub-saturating physiological occupancy, and acidic CUB residues coordinate both Ca2+ and C1q lysines.

    Evidence Site-directed mutagenesis with SPR, ITC, lysine-accessibility mass spectrometry, and autoactivation assays

    PMID:19473974 PMID:20178990 PMID:20592021

    Open questions at the time
    • Dynamics linking partial CUB2 saturation to in vivo activation thresholds unclear
    • Architecture of the assembled C1 cone only partly resolved
  11. 2013 High

    Resolved the molecular basis of C1r substrate specificity—Gln at P2 and Ile at P1' with CCP-mediated exosite contacts—and mapped the C1q collagen-stem lysines LysB61/LysC58 that anchor the protease tetramer.

    Evidence Phage-display substrate profiling, substrate mutagenesis, MD simulations, and recombinant C1q mutagenesis with SPR and complement assays

    PMID:23589288 PMID:23650384

    Open questions at the time
    • Specificity determinants for non-complement substrates not surveyed
    • Allosteric coupling between exosite and active site incompletely defined
  12. 2018 High

    The C1r-C1s CUB1-EGF-CUB2 heterodimer crystal structure explained the preference for heterocomplex formation and supported a C1r2C1s2 assembly linked through C1r catalytic domains.

    Evidence X-ray crystallography with SEC-coupled biophysical analysis

    PMID:29311313

    Open questions at the time
    • Full-length tetramer-C1q complex structure still incomplete
    • Conformational changes during activation within the heterodimer untracked
  13. 2004 High

    Revealed a complement-independent proteolytic role for the related C1r-like protein in cleaving prohaptoglobin in the ER, broadening the functional scope of the C1r family.

    Evidence Coexpression in COS-1, active-site and substrate mutagenesis, in vitro cleavage with purified proteins, and RNAi in HepG2

    PMID:15385675

    Open questions at the time
    • Relationship of C1r-LP activity to canonical C1r function unclear
    • Physiological significance of ER prohaptoglobin cleavage not established in vivo
  14. 2016 Medium

    Linked C1R directly to human disease, identifying heterozygous mutations at subunit interfaces and hinges as the cause of periodontal Ehlers-Danlos syndrome via intracellular retention.

    Evidence Whole exome/genome sequencing of 19 families with clinical characterization

    PMID:27745832

    Open questions at the time
    • Functional mechanism not directly tested in this study
    • Connection between protease defect and connective-tissue phenotype unexplained here
  15. 2019 High

    Established the pEDS disease mechanism: mutant C1r is not integrated into normal C1 but causes intracellular activation and constitutive extracellular C1r/C1s protease activity with C4 cleavage.

    Evidence HEK293T overexpression, western blot, SEC, SPR, patient fibroblast analysis, and C4-activation ELISA

    PMID:31749804

    Open questions at the time
    • How constitutive complement activity produces connective-tissue pathology unresolved
    • Tissue-specific consequences in patients not fully mapped
  16. 2021 High

    Extended C1r's roles to host-pathogen and antibody contexts: C1r2s2 stabilizes C1q binding to surface-bound IgG, and the Borrelia lipoprotein BBK32 inhibits C1r by occluding its S1/S1' substrate subsites.

    Evidence Binding/phagocytosis assays with engineered antibodies; X-ray crystallography, SAXS, mutagenesis, SPR and complement assays of the BBK32-C1r complex

    PMID:34155115 PMID:34759015

    Open questions at the time
    • Generality of C1r2s2-mediated C1q stabilization across surfaces not exhausted
    • Other microbial C1r inhibitors not surveyed
  17. 2024 High

    Defined opposing extracellular/cancer roles for C1r: promoting cSCC invasion via MMP induction and ERK/Akt signaling, driving renal fibrosis through complement activation, yet acting as a tumor suppressor in HCC by restraining CRP/NF-kB/HIF-1a glycolysis.

    Evidence CRISPR/shRNA knockout and knockdown, mRNA-seq, xenografts, C1r-null mice, Co-IP, and methylation/reporter assays

    PMID:31049937 PMID:31509012 PMID:34756877 PMID:39150096

    Open questions at the time
    • Whether protease activity versus non-catalytic function drives tumor phenotypes unclear
    • Context-dependence of pro- versus anti-tumor roles mechanistically unreconciled

Open questions

Synthesis pass · forward-looking unresolved questions
  • How C1r reconciles its canonical extracellular complement-initiating protease role with intracellular and complement-independent signaling functions in cancer and fibrosis remains unresolved.
  • Direct catalytic substrates underlying MMP/ERK/Akt and HIF-1a effects unidentified
  • Subcellular site of disease-relevant C1r activity not defined
  • Structure of the fully assembled C1q-tetramer complex incomplete

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140096 catalytic activity, acting on a protein 5 GO:0008092 cytoskeletal protein binding 4 GO:0016787 hydrolase activity 3
Localization
GO:0005576 extracellular region 3 GO:0005783 endoplasmic reticulum 3
Pathway
R-HSA-1643685 Disease 4 R-HSA-392499 Metabolism of proteins 3 R-HSA-168256 Immune System 2
Partners
BBK32C1QAC1QBC1QCC1SCRP
Complex memberships
C1 complexC1s-C1r-C1r-C1s tetramer

Evidence

Reading pass · 35 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
1976 C1r is activated by proteolytic cleavage: each 95,000 Da chain is cleaved into disulfide-linked subunits of 60,000 and 35,000 Da, converting the inactive zymogen into an active enzyme capable of cleaving and activating C1s. Activation is inhibited by calcium, polyanethol sulfonate, C1 inactivator, and DFP. C1 inactivator inhibits C1r by forming a complex via sites on the light chain. Proteolytic cleavage of C1r also occurs upon addition of aggregated IgG to a mixture of C1q, C1r, and C1s in the presence of calcium. Biochemical analysis, SDS-PAGE, inhibitor studies, activation assays Journal of immunology High 1249422
1976 C1r functions as the physical link between C1q and C1s in macromolecular C1: binding of radiolabeled C1r to sensitized erythrocytes required C1q and calcium but not C1s, whereas binding of C1s required C1q, C1r, and calcium. C1r activates C1s and forms C1 in conjunction with C1q and C1s. Activation of C1s by C1r was inhibited by calcium, C1 inactivator, polyanethol sulfonate, and DFP. Radiolabeling binding assays, hemolytic reconstitution assays, inhibitor studies Journal of immunology High 814163
1977 Activated C1r (C1r̄) consists of two peptide chains of approximately 56,000 ('a' chain) and 27,000 ('b' chain) daltons. The N-terminal sequences of both 'b' chains show obvious homology with other serine proteinases. C1r hydrolyzes both lysine and tyrosine ester bonds, but does not hydrolyze any protein substrate except C1s, demonstrating strict substrate specificity. Biochemical characterization, amino acid analysis, N-terminal sequencing, enzymatic assays with synthetic esters The Biochemical journal High 869924
1983 The catalytic B chain of human C1r contains 242 amino acids and belongs to the serine proteinase family. The active site residues His-39, Asp-94, and Ser-191 (equivalent to His-57, Asp-102, Ser-195 in chymotrypsinogen numbering) are identified by sequence analysis. The chain lacks the 'histidine-loop' disulfide bridge. Residue 189 is aspartic acid, consistent with trypsin-like specificity of cleavage. Protein sequencing (CNBr cleavage, tryptic digestion, staphylococcal protease cleavage) Biochemistry High 6303394
1986 C1r is encoded by a cDNA spanning 2493 nucleotides encoding a 705-amino acid polypeptide precursor. The A (noncatalytic) chain contains a potential growth factor domain and two pairs of internal repeats; one pair is closely related to C1s, the other is homologous to beta-2-glycoprotein I, complement factor B, and other proteins. The B (catalytic) chain is homologous to the trypsin family of serine proteases. cDNA cloning and sequencing from human liver and HepG2 cell libraries Biochemistry High 3021205
1987 The A chain of human C1r contains 446 amino acid residues. Position 150 contains a modified amino acid (erythro-beta-hydroxyaspartic acid) in a sequence homologous to vitamin K-dependent coagulation factors. Two carbohydrate moieties are attached to Asn-108 and Asn-204. The A chain undergoes two major autolytic cleavages generating fragments alpha, beta, and gamma. Protein sequencing of autolytic fragments, CNBr cleavage, mass spectrometry The Biochemical journal High 3036070
1988 The human C1R and C1S genes are located in a close tail-to-tail arrangement at a distance of approximately 9.5 kilobases on chromosome 12p13. Both genes are primarily expressed in liver. Multiple mRNA sizes from alternative polyadenylation are observed for both genes. DNA blotting, genomic DNA sequencing, RNA blot analysis, cDNA isolation Proceedings of the National Academy of Sciences High 2459702
1994 C1r autoactivation is regulated by a Ca2+-dependent intramolecular mechanism involving its Ca2+-binding alpha-region. Ca2+ inhibits intramolecular but not intermolecular activation. Incorporation of C1r into the C1s-C1r-C1r-C1s tetramer fully stabilizes C1r in its proenzyme form. C1q causes partial release of Ca2+ inhibitory effect. The collagen-like fragments of C1q bind C1s-C1r-C1r-C1s but fail to trigger C1r activation, demonstrating the globular head domain of C1q transmits the activation signal. The gamma-B catalytic region of C1s plays no role in triggering C1r activation. Biochemical reconstitution, sedimentation analysis, activation kinetics assays, use of C1q fragments The Biochemical journal High 8042996
1996 Domain I (first CUB domain) of C1r is essential for tetramer formation with C1s: deletion mutants lacking domain I abolished Ca2+-dependent tetramer formation with C1s. The N-terminal region contains essential elements controlling activation of the serine protease module, as deletion mutants showed significantly decreased zymogen stability. Baculovirus expression of deletion mutants, functional assays for tetramer formation, stability measurements Molecular immunology High 8676886
1997 The catalytic domain of C1r forms a homodimer via a loose head-to-tail association of two gamma-B monomers. Chemical cross-linking identified one intramonomer cross-link between Lys426 (module V) and C-terminal Asp688 (serine protease domain) and one intermonomer cross-link between N-terminal Gly280 and Glu493 (B domain). The CCP module V interacts with the serine protease on the side opposite to both the active site and the Arg446-Ile447 activation site. Chemical cross-linking, mass spectrometry, N-terminal sequencing, homology modeling Biochemistry Medium 9174342
1998 The EGF-like module of C1r (residues 123-175) was solved by NMR. The C-terminal part (residues Cys144-Ala174) has a well-ordered EGF fold with major and minor beta-sheets, while the N-terminal large loop (between Cys129 and Cys144) is disordered. Addition of calcium at 80 mM did not produce significant structural changes in the isolated EGF module. NMR spectroscopy, simulated annealing/restrained molecular dynamics calculations Biochemistry High 9477945
1999 C1r and C1s associate into a Ca2+-dependent C1s-C1r-C1r-C1s tetramer through their N-terminal CUB1-EGF-CUB2 interaction domains. Ca2+ binding through these domains mediates both C1r-C1s interaction and C1q association. CCP modules IV and V are not involved in tetramer formation but are part of the catalytic gamma-B region. Functional domain mapping, Ca2+-dependent assembly assays, deletion analysis Immunopharmacology Medium 10408360
1999 One active C1r subunit in the C1r homodimer is sufficient for full complement activity. A stable zymogen C1r mutant (R463Q, 'QI') formed an active mixed dimer with wild-type C1r and the resulting C1 complex had full hemolytic activity. The C1r monomer exchange between dimers occurs within 16 hours at pH 7 and 4°C. Site-directed mutagenesis, baculovirus expression, hemolytic reconstitution assay Journal of immunology High 9916740
2000 In serum, C1r and C1s associate exclusively with C1q (not with MBL), while MASP-1, MASP-2, and MAp19 associate exclusively with MBL. Serum deficient in C1r produced no complement activation on IgG-coated surfaces but retained activation on mannan-coated surfaces, demonstrating pathway-specific functional dependence on C1r. Gel-permeation chromatography of serum, functional complement activation assays on coated surfaces, C1r-deficient serum Journal of immunology High 10878362
2001 The ability to autoactivate and cleave C1s is an inherent property of the serine protease (SP) domain of C1r. The CCP2 module significantly increases proteolytic activity on natural substrate C1s (providing accessory binding sites) and greatly stabilizes SP domain structure. CCP1 is not required for autoactivation or C1s cleavage but is essential for dimerization of C1r. Dimerization is not a prerequisite for autoactivation. Recombinant expression in E. coli of domain deletion mutants, esterolytic activity assays, C1s cleavage assays, differential scanning calorimetry Journal of immunology High 11673533
2001 C1r activation is an autolytic (self-cleavage) process: wild-type CCP1/2-SP and CCP2-SP fragments were recovered as two-chain activated proteases, while R446Q and S637A mutants retained single-chain proenzyme structure. CCP1 is essential for dimer assembly (CCP1/2-SP forms 5.5S dimer; CCP2-SP forms 3.2S monomer). CCP2-SP is cleaved more efficiently by thermolysin than CCP1/2-SP. Activated fragments cleave C1s with relative efficiency CCP2-SP > CCP1/2-SP > intact C1r, indicating CCP1 is not involved in C1s recognition. Baculovirus expression, sedimentation velocity, site-directed mutagenesis at cleavage site (R446Q) and active site (S637A), thermolysin activation, C1s cleavage assays The Journal of biological chemistry High 11445589
2002 The crystal structure of the proenzyme catalytic domain of C1r (CCP1-CCP2-SP) was solved at 2.9 Å resolution. The domain associates as a homodimer in an elongated head-to-tail structure with a central opening. The catalytic site of one monomer and the cleavage site of the other are at opposite ends of the dimer. The SP domain shows unusual features and the structure supports that C1r activation in C1 is triggered by a mechanical stress caused by target recognition that disrupts CCP1-SP interfaces. X-ray crystallography of mutated proenzyme form The EMBO journal High 11823416
2002 Crystal structures of both zymogen (S637A mutant) and active CCP2-SP fragments of C1r were determined. Both show restricted CCP2-SP hinge flexibility. The zymogen activation domain exhibits high mobility and the active structure shows restricted access to substrate binding subsites. The active structure shows an unusual alpha-helical conformation of loop E. X-ray crystallography of wild-type and S637A mutant CCP2-SP fragments Structure High 12429092
2007 The crystal structure of the entire active catalytic region of human C1r (CCP1-CCP2-SP) was determined. An enzyme-product complex in the crystal lattice shows a crucial S1-P1 salt bridge between Asp631 and Arg446 residues, and intermolecular interaction between the CCP2 module and the SP domain. A 'split-and-reassembly' model for C1r autoactivation is proposed, allowing activation without large-scale directed movement of C1q arms. X-ray crystallography, structural analysis of enzyme-product crystal contacts Molecular immunology High 17996945
2009 High-affinity C1q-binding sites of the C1s-C1r-C1r-C1s tetramer are contributed by the C1r CUB1 and CUB2 modules (with lower affinity sites from C1s CUB1). Mutations targeting the Glu137-Glu-Asp139 stretch in the C1r EGF module had no effect on C1 assembly, ruling out a previous interaction model. All binding sites implicate acidic residues also contributing Ca2+ ligands. Six C1q binding sites total (one per C1q stem) are contributed by the tetramer. Site-directed mutagenesis, surface plasmon resonance binding assays The Journal of biological chemistry High 19473974
2010 Mass spectrometry-based lysine accessibility mapping of the C1s-C1r-C1r-C1s tetramer in free and C1-bound forms revealed that both C1s CUB1-EGF-CUB2 interaction domains, which are distant in the free tetramer, associate with each other in the C1 complex. The C1s serine protease domain is partly positioned inside the C1q cone in the proenzyme C1 complex. Chemical modification (lysine labeling), label-free semi-quantitative mass spectrometry The Journal of biological chemistry Medium 20592021
2010 Ca2+ binding to the CUB2 domain of C1r induces folding from a flexible, disordered conformation to a compact, folded structure. CUB2 binds Ca2+ with a relatively high Kd (~430 μM), meaning only ~74% saturation occurs at physiological blood calcium levels. The disordered Ca2+-free CUB2 form may provide the flexibility required for C1 activation. Ca2+ inhibits autoactivation of native isolated C1r zymogen. Recombinant domain production, isothermal titration calorimetry, C1r autoactivation assays with varying Ca2+ concentrations, structural analysis The Journal of biological chemistry High 20178990
2013 C1r specificity is for Gln at P2 and Ile at P1' positions in substrates, matching its physiological substrates. Removing Gln at P2 or Ile at P1' in the C1s substrate reduced C1r activation rate. Substituting Gln at P2 of MASP-3 (not normally cleaved by C1r) enabled its efficient activation. CCP domains of C1r make exosite contributions to C1s activation beyond active site interactions. Phage library substrate specificity profiling, mutagenesis of substrate cleavage sites, molecular dynamics simulations, structural modeling The Journal of biological chemistry High 23589288
2013 LysB61 and LysC58 of C1q collagen-like stems each play a key role in interaction with C1s-C1r-C1r-C1s; LysA59 is involved to a lesser degree. These lysines form salt bridges with outer acidic Ca2+ ligands of the C1r and C1s CUB domains. Recombinant C1q expression with site-directed mutagenesis, surface plasmon resonance, functional complement activation assays Proceedings of the National Academy of Sciences High 23650384
2018 The crystal structure of the C1r-C1s heterodimer (CUB1-EGF-CUB2 fragments) was determined. Both fragments are L-shaped and interlock to form a compact antiparallel heterodimer with Ca2+ from each subcomponent at the interface. Contacts involving all three domains of each protease are more extensive than C1r or C1s homodimers, explaining preferential heterocomplex formation. The data support a C1r2C1s2 model where two C1r-C1s dimers are linked via C1r catalytic domains. X-ray crystallography, size-exclusion chromatography-coupled biophysical analyses Proceedings of the National Academy of Sciences High 29311313
2021 C1r2s2 proteases stabilize C1q binding to surface-bound IgG: the presence of C1r2s2 increases the avidity of C1q for surface-bound IgG on various surfaces including S. aureus. The extent of C1r2s2 contribution to C1q-IgG stability differs between human IgG subclasses. Hexamer-enhancing mutations improve C1q-IgG stability both in the absence and presence of C1r2s2. Binding assays on various coated surfaces, antibody engineering with hexamer-enhancing mutations, complement-dependent phagocytosis by neutrophils Proceedings of the National Academy of Sciences Medium 34155115
2021 Crystal structure of BBK32 (Borrelia burgdorferi lipoprotein) in complex with activated human C1r was determined. BBK32 inhibits C1r by occluding substrate interaction subsites S1 and S1', and B loop-interacting residues of BBK32 contribute to full inhibitory activity. Structure-guided mutagenesis validated the predicted molecular interface by SPR and complement function assays. X-ray crystallography, SAXS, size-exclusion chromatography, site-directed mutagenesis, surface plasmon resonance, complement functional assays Journal of immunology High 34759015
2016 Heterozygous missense or in-frame insertion/deletion mutations in C1R cause periodontal Ehlers-Danlos syndrome. Pathogenic variants involve subunit interfaces or inter-domain hinges of C1r and are associated with intracellular retention and mild endoplasmic reticulum enlargement, connecting the C1r protein to connective tissue homeostasis via a dominant gain-of-function or dominant-negative mechanism. Whole exome/genome sequencing of 19 families, clinical characterization American journal of human genetics Medium 27745832
2019 pEDS-associated C1R variants are not integrated into the C1 complex but cause extracellular presence of catalytic C1r/C1s activities. Variants show domain-specific abnormalities of intracellular processing and secretion with preservation of serine protease function in supernatants. Patient fibroblasts exhibit activated C1s and activation of externally added C4, while control cells secrete proenzyme C1s without C4 activation. The pathogenesis involves intracellular activation of C1r and/or C1s leading to constitutive complement cascade activation. In vitro overexpression in HEK293T cells, western blot, size exclusion chromatography, SPR, patient-derived fibroblast analysis, ELISA for C4 activation Frontiers in immunology High 31749804
2021 C1r promotes invasion of cutaneous squamous cell carcinoma (cSCC) cells by upregulating matrix metalloproteinase production. CRISPR/Cas9 knockout of C1r in cSCC cells decreased proliferation, migration, and invasion through collagen type I, suppressed xenograft growth, and promoted apoptosis. C1r knockout decreased production of MMP-1, MMP-13, MMP-10, and MMP-12, and inhibited MMP-13 expression and collagen degradation in vivo. CRISPR/Cas9 knockout, mRNA sequencing, western blot, xenograft tumor model, immunohistochemistry The Journal of investigative dermatology High 34756877
2019 C1r promotes cSCC tumor cell growth and vascularization. Knockdown of C1r in cSCC cells inhibited activation of ERK1/2 and Akt signaling pathways, promoted apoptosis, and significantly suppressed growth and vascularization of human cSCC xenograft tumors in vivo. shRNA knockdown, western blot for ERK1/2 and Akt phosphorylation, xenograft tumor model The British journal of dermatology Medium 31049937
2019 C1r expression in kidney tubular epithelial cells initiates complement cascade activation leading to fibrosis. C1r-null mice had reduced acute tubular injury, reduced C1s and C3 fragment formation, and reduced organ fibrosis after folic acid administration. In vitro, C1s expression was shown to be dependent on increased C1r expression; interferon-γ induces expression of both proteases. C1r global knockout mice, folic acid kidney fibrosis model, immunohistochemistry, in situ hybridization, in vitro siRNA knockdown in renal epithelial cells American journal of physiology. Renal physiology High 31509012
2004 C1r-like protein (C1r-LP) mediates proteolytic cleavage of prohaptoglobin (proHp) in the endoplasmic reticulum. Coexpression of proHp and C1r-LP in COS-1 cells effected ER cleavage; mutation of the active-site serine abolished the reaction. Incubation of affinity-purified C1r-LP with proHp in vitro led to cleavage. Substitution of Gly for Arg-161 in proHp blocked cleavage. C1r-LP showed specificity for proHp over pro-C1s. RNAi suppression of C1r-LP in HepG2 cells reduced proHp cleavage by up to 45%. Coexpression in COS-1 cells, active-site mutagenesis, in vitro cleavage assay with purified proteins, substrate mutagenesis, RNAi knockdown Proceedings of the National Academy of Sciences High 15385675
2024 In hepatocellular carcinoma (HCC), C1r inhibits HIF-1α-mediated glycolysis. C1r promoter is hypermethylated by DNMT1 and DNMT3a, reducing C1r expression. CRP binds to C1r; free CRP activates NF-κB signaling, which boosts HIF-1α expression and glycolysis. Downregulation of C1r promoted xenograft tumor formation. C1r acts as a tumor suppressor in HCC. RNA-seq, methylation-specific PCR, co-immunoprecipitation, dual-luciferase reporter assays, in vitro and in vivo HCC experiments Molecular carcinogenesis Medium 39150096
1987 C1 activation by cardiolipin vesicles proceeds through a C1r-independent mechanism: C1s binds to C1q on cardiolipin vesicles and is specifically cleaved into its active chains (58 kDa and 28 kDa) in the absence of C1r. This is in contrast to immune complex-induced C1 activation, which requires C1r for C1s activation. C1r-depleted reconstitution experiments, SDS-PAGE analysis of C1s cleavage products, inhibitor studies Journal of immunology Medium 3029222

Source papers

Stage 0 corpus · 100 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
1992 The HLA-A,B "negative" mutant cell line C1R expresses a novel HLA-B35 allele, which also has a point mutation in the translation initiation codon. Journal of immunology (Baltimore, Md. : 1950) 270 1541831
1977 The structure and enzymic activities of the C1r and C1s subcomponents of C1, the first component of human serum complement. The Biochemical journal 141 869924
1986 Nucleotide sequence of the cDNA coding for human complement C1r. Biochemistry 129 3021205
1976 Physicochemical and functional characterization of the C1r subunit of the first complement component. Journal of immunology (Baltimore, Md. : 1950) 123 814163
1972 C1r deficiency: an inborn error associated with cutaneous and renal disease. The Journal of clinical investigation 112 4623164
1976 Activation of C1r by proteolytic cleavage. Journal of immunology (Baltimore, Md. : 1950) 108 1249422
2002 The crystal structure of the zymogen catalytic domain of complement protease C1r reveals that a disruptive mechanical stress is required to trigger activation of the C1 complex. The EMBO journal 90 11823416
2000 Interaction of C1q and mannan-binding lectin (MBL) with C1r, C1s, MBL-associated serine proteases 1 and 2, and the MBL-associated protein MAp19. Journal of immunology (Baltimore, Md. : 1950) 89 10878362
1998 2-amino-4H-3,1-benzoxazin-4-ones as inhibitors of C1r serine protease. Journal of medicinal chemistry 86 9544206
2003 Complete sequencing and expression of three complement components, C1r, C4 and C1 inhibitor, of the classical activation pathway of the complement system in rainbow trout Oncorhynchus mykiss. Immunogenetics 85 14628103
2016 Periodontal Ehlers-Danlos Syndrome Is Caused by Mutations in C1R and C1S, which Encode Subcomponents C1r and C1s of Complement. American journal of human genetics 79 27745832
1970 Deficiency of C1r in human serum. Effects on the structure and function of macromolecular C1. The Journal of experimental medicine 78 4988128
2019 Tumour-cell-derived complement components C1r and C1s promote growth of cutaneous squamous cell carcinoma. The British journal of dermatology 73 31049937
2009 Identification of the C1q-binding Sites of Human C1r and C1s: a refined three-dimensional model of the C1 complex of complement. The Journal of biological chemistry 66 19473974
1988 Human genes for complement components C1r and C1s in a close tail-to-tail arrangement. Proceedings of the National Academy of Sciences of the United States of America 60 2459702
2018 Structure of the C1r-C1s interaction of the C1 complex of complement activation. Proceedings of the National Academy of Sciences of the United States of America 58 29311313
2009 Early complement proteases: C1r, C1s and MASPs. A structural insight into activation and functions. Molecular immunology 56 19477526
2004 Prohaptoglobin is proteolytically cleaved in the endoplasmic reticulum by the complement C1r-like protein. Proceedings of the National Academy of Sciences of the United States of America 53 15385675
2013 Expression of recombinant human complement C1q allows identification of the C1r/C1s-binding sites. Proceedings of the National Academy of Sciences of the United States of America 51 23650384
1987 Complete amino acid sequence of the A chain of human complement-classical-pathway enzyme C1r. The Biochemical journal 51 3036070
2002 Monomeric structures of the zymogen and active catalytic domain of complement protease c1r: further insights into the c1 activation mechanism. Structure (London, England : 1993) 47 12429092
2021 C1q binding to surface-bound IgG is stabilized by C1r2s2 proteases. Proceedings of the National Academy of Sciences of the United States of America 46 34155115
1991 Complement components C1r/C1s, bone morphogenic protein 1 and Xenopus laevis developmentally regulated protein UVS.2 share common repeats. FEBS letters 45 2026272
1983 Complete amino acid sequence of the catalytic chain of human complement subcomponent C1-r. Biochemistry 45 6303394
1997 A neoepitope-based enzyme immunoassay for quantification of C1-inhibitor in complex with C1r and C1s. Scandinavian journal of immunology 44 9420617
2021 C1r Upregulates Production of Matrix Metalloproteinase-13 and Promotes Invasion of Cutaneous Squamous Cell Carcinoma. The Journal of investigative dermatology 41 34756877
1989 Complement genes C1r and C1s feature an intronless serine protease domain closely related to haptoglobin. Journal of molecular biology 41 2553984
1998 Solution structure of the epidermal growth factor (EGF)-like module of human complement protease C1r, an atypical member of the EGF family. Biochemistry 40 9477945
1994 A 100-kDa protein in the C4-activating component of Ra-reactive factor is a new serine protease having module organization similar to C1r and C1s. Journal of immunology (Baltimore, Md. : 1950) 39 8133044
2009 Analysis of human C1q by combined bottom-up and top-down mass spectrometry: detailed mapping of post-translational modifications and insights into the C1r/C1s binding sites. Molecular & cellular proteomics : MCP 37 20008834
1997 Structure and assembly of the catalytic region of human complement protease C1r: a three-dimensional model based on chemical cross-linking and homology modeling. Biochemistry 36 9174342
2007 Revisiting the mechanism of the autoactivation of the complement protease C1r in the C1 complex: structure of the active catalytic region of C1r. Molecular immunology 34 17996945
2001 The role of the individual domains in the structure and function of the catalytic region of a modular serine protease, C1r. Journal of immunology (Baltimore, Md. : 1950) 34 11673533
2001 Assembly and enzymatic properties of the catalytic domain of human complement protease C1r. The Journal of biological chemistry 33 11445589
2011 Clinical presentations and molecular basis of complement C1r deficiency in a male African-American patient with systemic lupus erythematosus. Lupus 30 21784777
1988 Assignment of the complement serine protease genes C1r and C1s to chromosome 12 region 12p13. Human genetics 30 2834284
1985 Primary structure of the A chain of human complement-classical-pathway enzyme C1r. N-terminal sequences and alignment of autolytic fragments and CNBr-cleavage peptides. The Biochemical journal 30 2983658
1971 C1r, subunit of the first complement component: purification, properties, and assay based on its linking role. The Journal of clinical investigation 30 4100685
1992 Recombinant human complement subcomponent C1s lacking beta-hydroxyasparagine, sialic acid, and one of its two carbohydrate chains still reassembles with C1q and C1r to form a functional C1 complex. Biochemistry 29 1533159
1995 Complement components C1q, C1r/C1s, and C1INH in rheumatoid arthritis. Correlation of in situ hybridization and northern blot results with function and protein concentration in synovium and primary cell cultures. Arthritis and rheumatism 28 7718002
1987 Antibody-independent activation of C1. I. Differences in the mechanism of C1 activation by nonimmune activators and by immune complexes: C1r-independent activation of C1s by cardiolipin vesicles. Journal of immunology (Baltimore, Md. : 1950) 28 3029222
2001 Molecular cloning of the complement (C1r/C1s/MASP2-like serine proteases from the common carp (Cyprinus carpio). Immunogenetics 25 11220628
1996 Exon structure of the gene encoding the human mannose-binding protein-associated serine protease light chain: comparison with complement C1r and C1s genes. International immunology 25 8921412
1994 Activation of human complement serine-proteinase C1r is down-regulated by a Ca(2+)-dependent intramolecular control that is released in the C1 complex through a signal transmitted by C1q. The Biochemical journal 25 8042996
2019 C1R Mutations Trigger Constitutive Complement 1 Activation in Periodontal Ehlers-Danlos Syndrome. Frontiers in immunology 24 31749804
1989 Expression of hemolytically active human complement component C1r proenzyme in insect cells using a baculovirus vector. Complement and inflammation 24 2557186
1987 Calcium-sensitive thermal transitions and domain structure of human complement subcomponent C1r. Biochemistry 23 2823884
1999 Structure and functions of the interaction domains of C1r and C1s: keystones of the architecture of the C1 complex. Immunopharmacology 21 10408360
1984 Structure and activity of C1r and C1s. Philosophical transactions of the Royal Society of London. Series B, Biological sciences 21 6149574
1979 The enzymatic nature of human c1r: a subcomponent of the first component of complement. Journal of immunology (Baltimore, Md. : 1950) 21 469256
2021 A Structural Basis for Inhibition of the Complement Initiator Protease C1r by Lyme Disease Spirochetes. Journal of immunology (Baltimore, Md. : 1950) 20 34759015
2018 Phenotype-specific recombinant haptoglobin polymers co-expressed with C1r-like protein as optimized hemoglobin-binding therapeutics. BMC biotechnology 20 29544494
2017 C1r and C1s from Nile tilapia (Oreochromis niloticus): Molecular characterization, transcriptional profiling upon bacterial and IFN-γ inductions and potential role in response to bacterial infection. Fish & shellfish immunology 20 28882800
2019 Complement C1r serine protease contributes to kidney fibrosis. American journal of physiology. Renal physiology 19 31509012
2012 Zebrafish scube1 (signal peptide-CUB (complement protein C1r/C1s, Uegf, and Bmp1)-EGF (epidermal growth factor) domain-containing protein 1) is involved in primitive hematopoiesis. The Journal of biological chemistry 19 23271740
2010 Mapping surface accessibility of the C1r/C1s tetramer by chemical modification and mass spectrometry provides new insights into assembly of the human C1 complex. The Journal of biological chemistry 19 20592021
2014 Classical complement pathway components C1r and C1s: purification from human serum and in recombinant form and functional characterization. Methods in molecular biology (Clifton, N.J.) 18 24218249
2003 Complement C1r and C1s genes are duplicated in the mouse: differential expression generates alternative isomorphs in the liver and in the male reproductive system. The Biochemical journal 18 12513694
1994 Unique C1 inhibitor dysfunction in a kindred without angioedema. I. A mutant C1 INH that inhibits C1-s but not C1-r. Journal of immunology (Baltimore, Md. : 1950) 17 8144914
2013 Molecular determinants of the substrate specificity of the complement-initiating protease, C1r. The Journal of biological chemistry 16 23589288
2010 Calcium-dependent conformational flexibility of a CUB domain controls activation of the complement serine protease C1r. The Journal of biological chemistry 16 20178990
2015 Characterization of rock bream (Oplegnathus fasciatus) complement components C1r and C1s in terms of molecular aspects, genomic modulation, and immune responsive transcriptional profiles following bacterial and viral pathogen exposure. Fish & shellfish immunology 15 26241508
1986 Biosynthesis of the subcomponents C1q, C1r and C1s of the first component of complement (C1) by guinea pig hepatocyte primary cultures. European journal of immunology 15 3019707
1993 Mapping of MYF5, C1R, MYHL, TPI1, IAPP, A2MR and RNR onto sheep chromosome 3q. Animal genetics 14 8273915
2005 A novel human complement-related protein, C1r-like protease (C1r-LP), specifically cleaves pro-C1s. The Biochemical journal 13 15527420
2022 Autoantibodies against Complement Classical Pathway Components C1q, C1r, C1s and C1-Inh in Patients with Lupus Nephritis. International journal of molecular sciences 12 36012546
2002 Structure, function and molecular genetics of human and murine C1r. Immunobiology 12 12396000
1998 Structural and functional studies on C1r and C1s: new insights into the mechanisms involved in C1 activity and assembly. Immunobiology 12 9777414
1996 Functional effects of domain deletions in a multidomain serine protease, C1r. Molecular immunology 12 8676886
2014 The diagnostic role of signal peptide-C1r/C1s, Uegf, and Bmp1-epidermal growth factor domain-containing protein 1 in non-ST-elevation acute coronary syndrome. The American journal of emergency medicine 11 25445868
1999 One active C1r subunit is sufficient for the activity of the complement C1 complex: stabilization of C1r in the zymogen form by point mutations. Journal of immunology (Baltimore, Md. : 1950) 11 9916740
1989 Biosynthesis of C1r and C1s subcomponents. Behring Institute Mitteilungen 11 2552984
1976 The role of C1 esterase inhibitor in the activation of C1r, a subcomponent of the first component of complement from human plasma. Journal of biochemistry 11 1270407
2024 Downregulation of C1R promotes hepatocellular carcinoma development by activating HIF-1α-regulated glycolysis. Molecular carcinogenesis 10 39150096
2016 The diagnostic significance of signal peptide-complement C1r/C1s, Uegf, and Bmp1-epidermal growth factor domain-containing protein-1 levels in pulmonary embolism. Annals of thoracic medicine 10 27803754
1989 FN-C1q and C1 INH C1r-C1s complexes as indicators of complement activation in patients with chronic lymphocytic leukaemia. Immunology letters 10 2550362
2007 Conformationally altered hyaluronan restricts complement classical pathway activation by binding to C1q, C1r, C1s, C2, C5 and C9, and suppresses WOX1 expression in prostate DU145 cells. International journal of molecular medicine 9 17143562
2020 Complement C1q Interacts With LRP1 Clusters II and IV Through a Site Close but Different From the Binding Site of Its C1r and C1s-Associated Proteases. Frontiers in immunology 8 33193398
2003 A novel murine complement-related gene encoding a C1r-like serum protein. Molecular immunology 8 12686506
2002 C1r-C1s-C1inhibitor (C1rs-C1inh) complex measurements in tears of patients before and after penetrating keratoplasty. Current eye research 8 12187480
1994 Non-coordinated biosynthesis of early complement components in a deficiency of complement proteins C1r and C1s. Scandinavian journal of immunology 8 7939409
2023 Conformational dynamics of complement protease C1r inhibitor proteins from Lyme disease- and relapsing fever-causing spirochetes. The Journal of biological chemistry 7 37380082
2018 A Chinese family with periodontal Ehlers-Danlos syndrome associated with missense mutation in the C1R gene. Journal of clinical periodontology 7 30025171
2004 A novel human dendritic cell-derived C1r-like serine protease analog inhibits complement-mediated cytotoxicity. Biochemical and biophysical research communications 7 15358180
2003 The human complement component C1R gene: the exon-intron structure and the molecular basis of allelic diversity. Annals of human genetics 7 12914573
2001 Identification of cDNA encoding a serine protease homologous to human complement C1r precursor from grafted mouse skin. The Journal of investigative dermatology 7 11231310
1996 Inefficient assembly limits transport and cell surface expression of HLA-Cw4 molecules in C1R. Tissue antigens 7 9008307
1990 Trimer and tetramer complexes containing C1 esterase inhibitor, C1r and C1s, in serum and synovial fluid of patients with rheumatic disease. Journal of immunological methods 7 2338498
1985 C1r serine proteinase of human complement: a case of intramolecular autolytic activation. Bioscience reports 7 3006820
2003 A polymorphism in the complement component C1r is not associated with sporadic Alzheimer's disease. Neuroscience letters 6 12499050
1989 The structures of human C1r and C1s and their relationship to other serine proteases. Behring Institute Mitteilungen 6 2552983
1989 Human complement C1r and C1s proteins and genes: studies with molecular probes. Behring Institute Mitteilungen 6 2572213
2023 Therapeutic Intervention of Neuroinflammatory Alzheimer Disease Model by Inhibition of Classical Complement Pathway with the Use of Anti-C1r Loaded Exosomes. Research square 5 37886595
1993 Protein engineering studies on C1r and C1s. Behring Institute Mitteilungen 5 8172556
1989 Structure and function of C1r and C1s: current concepts. Behring Institute Mitteilungen 5 2552982
2018 Diagnostic value of signal peptide-Complement C1r/C1s, Uegf, and Bmp1-epidermal growth factor domain-containing protein 1 on serum and tissue samples in non-small cell lung cancer. Turk gogus kalp damar cerrahisi dergisi 4 32082741
2017 Molecular features and the transcriptional and functional delineation of complement system activators C1r and C1s from Sebastes schlegelii. Developmental and comparative immunology 4 29247723
1988 Genetic polymorphism of human C1R subcomponent of the first complement component in the Japanese population. Forensic science international 4 2850274
1982 Characterization of the activation of the human C1r complement molecule. Molecular immunology 4 6285181
1988 A rapid and efficient method for the purification of the complement subcomponents C1r and C1s in zymogen form using fast protein chromatography. Journal of immunological methods 3 2832479

Missed literature

Know a paper Affinage missed for C1R? Flag it for the maintainers and the community.

No submissions yet.