A2M

Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0004866 endopeptidase inhibitor activity	IBA GO_REF:0000033	ACCEPT	Summary: Phylogenetic inference of A2M's conserved endopeptidase-inhibitor activity, the defining protease-trapping function of alpha-macroglobulins. Reason: A2M directly inhibits diverse endopeptidases after bait-region cleavage and conformational trapping. The IBA is at the appropriate broad level because A2M acts across protease catalytic classes, and direct work shows dose-dependent inhibition of ADAMTS-7 and ADAMTS-12. Supporting Evidence: PMID:18485748 Furthermore, a(2)M inhibited both ADAMTS-7- and ADAMTS-12-mediated COMP degradation in a concentration (or dose)-dependent manner.
GO:0005576 extracellular region	IBA GO_REF:0000033	ACCEPT	Summary: Phylogenetic inference that A2M is active in the extracellular region, consistent with its role as a circulating plasma protease trap. Reason: Extracellular activity is integral to A2M biology and is independently represented in curated human Reactome reactions involving extracellular metalloproteinases and plasma kallikrein. Supporting Evidence: Reactome:R-HSA-1454781 Alpha 2-macroglobulin (A2M) is a plasma glycoprotein consisting of 4 near-identical subunits (Andersen et al. 1995).
GO:0002020 protease binding	IBA GO_REF:0000033	ACCEPT	Summary: Phylogenetic inference of the conserved ability of A2M to bind and entrap proteases. Reason: Protease binding is a direct and indispensable part of A2M's trapping mechanism, rather than a generic physical association. Curated human pathway evidence describes bait-region cleavage followed by entrapment of the endopeptidase. Supporting Evidence: Reactome:R-HSA-1454781 A2M binding to an endopeptidase is triggered by cleavage of a peptide bond in the 'bait region' of A2M, triggering a conformational change in A2M that in turn entraps the peptidase without blocking the active site (Barrett & Starkey 1973).
GO:0004857 enzyme inhibitor activity	IEA GO_REF:0000117	KEEP AS NON CORE	Summary: A2M inhibits both its core protease targets and, in a secondary context, the non-peptidase enzyme LCAT. Reason: The broad term is not merely redundant with endopeptidase inhibitor activity because A2M-bound lecithin-cholesterol acyltransferase is enzymatically inactive. Retain it as a valid non-core activity while using GO:0004866 for A2M's defining protease-trapping function. Supporting Evidence: PMID:11435418 It is concluded that the binding of LCAT to alpha(2)M inhibits its enzymatic activity.
GO:0004866 endopeptidase inhibitor activity	IEA GO_REF:0000002	ACCEPT	Summary: InterPro-derived electronic annotation of A2M's broad endopeptidase-inhibitor activity. Reason: The term precisely captures the conserved alpha-macroglobulin trapping mechanism and is corroborated by direct inhibition of multiple ADAMTS endopeptidases. Inhibition is steric and substrate-size-dependent: trapped enzymes can remain active on low-molecular-weight substrates while losing access to macromolecular substrates. Supporting Evidence: PMID:18485748 Furthermore, a(2)M inhibited both ADAMTS-7- and ADAMTS-12-mediated COMP degradation in a concentration (or dose)-dependent manner. Reactome:R-HSA-1454781 This blocks enzyme activity against large protein substrates while not preventing activity on low molecular weight substrates.
GO:0005576 extracellular region	IEA GO_REF:0000120	ACCEPT	Summary: Automated localization of the secreted A2M protein to the extracellular region. Reason: This is the correct core localization for a circulating plasma macroglobulin and agrees with the IBA and multiple curated extracellular Reactome reactions. Supporting Evidence: Reactome:R-HSA-1454781 Alpha 2-macroglobulin (A2M) is a plasma glycoprotein consisting of 4 near-identical subunits (Andersen et al. 1995).
GO:0005515 protein binding	IPI PMID:32814053 Interactome Mapping Provides a Network of Neurodegenerative ...	MARK AS OVER ANNOTATED	Summary: Generic protein-binding annotation collapsed from 56 A2M candidate interactions in a large systematic interaction map. Reason: The source reports tens of thousands of candidate interactions from systematic yeast two-hybrid screening. These candidate associations do not establish a mechanistic molecular function for A2M, and the generic protein-binding term is uninformative; many partners are also intracellular and difficult to reconcile with A2M's secreted plasma localization. Supporting Evidence: PMID:32814053 Here, we report on an interactome map that focuses on neurodegenerative disease (ND), connects ∼5,000 human proteins via ∼30,000 candidate interactions and is generated by systematic yeast two-hybrid interaction screening of ∼500 ND-related proteins and integration of literature interactions.
GO:0002020 protease binding	IPI PMID:7679575 Characterization of the antiplasmin activity of human thromb...	ACCEPT	Summary: Direct interaction of A2M with plasmin after release of plasmin from a reversible thrombospondin complex. Reason: The abstract explicitly reports formation of an alpha-2-macroglobulin complex with plasmin. This is a specific protease interaction and directly reflects the core A2M trapping function. Supporting Evidence: PMID:7679575 Similar results were obtained with alpha 2M. Transfer of plasmin from thrombospondin to alpha 2AP or alpha 2M probably required plasmin-thrombospondin-complex dissociation.
GO:0004866 endopeptidase inhibitor activity	IDA PMID:7679575 Characterization of the antiplasmin activity of human thromb...	ACCEPT	Summary: Experimental evidence that A2M captures plasmin, an endopeptidase, from a reversible plasmin-thrombospondin complex. Reason: Although the paper focuses on thrombospondin, its abstract explicitly describes plasmin transfer into an A2M complex and treats A2M as a conventional proteinase inhibitor. This supports the established core endopeptidase-inhibitor function. Supporting Evidence: PMID:7679575 Similar results were obtained with alpha 2M. Transfer of plasmin from thrombospondin to alpha 2AP or alpha 2M probably required plasmin-thrombospondin-complex dissociation.
GO:0048306 calcium-dependent protein binding	IPI PMID:15226301 Identification and characterization of the acidic pH binding...	KEEP AS NON CORE	Summary: Calcium-sensitive binding of activated A2M to cell-surface annexins in an acidic-pH uptake context. Reason: The abstract identifies annexin VI as directly binding activated A2M and states that the relevant binding sites are sensitive to calcium depletion. This supports the annotation, but it is a context-dependent trafficking interaction rather than A2M's core protease-trapping activity; the cached source is abstract-only. Supporting Evidence: PMID:15226301 These sites, like LRP-1, are sensitive to receptor-associated protein and calcium depletion but, unlike LRP-1, are also sensitive to chondroitin sulfate and heparin and capable of directly binding ligands, which do not bind to LRP-1. PMID:15226301 Annexin VI has been identified as a major membrane-associated protein capable of directly binding alpha(2)M(*) at acidic pH.
GO:0031012 extracellular matrix	HDA PMID:28327460 Comprehensive proteomic characterization of stem cell-derive...	KEEP AS NON CORE	Summary: A2M was detected in extracellular matrix produced by cultured human mesenchymal stromal cells in a quantitative proteomic survey. Reason: The source directly reports A2M in bone-marrow-derived cell matrix, making extracellular-matrix association plausible. A2M is a soluble secreted protease inhibitor rather than a structural matrix constituent, so this context-specific localization is non-core. Supporting Evidence: PMID:28327460 Proteinases can be inactivated by protease inhibitors such as TIMP3 and A2M found in Bm ECM.
GO:0031012 extracellular matrix	HDA PMID:28675934 Characterization of the Extracellular Matrix of Normal and D...	KEEP AS NON CORE	Summary: High-throughput proteomic detection of A2M in decellularized normal and diseased human tissue extracellular-matrix preparations. Reason: This HDA record supports matrix association in the sampled tissues and is consistent with independent matrix proteomes. Because abundant soluble A2M can associate with extracellular matrices without being a structural matrix component, retain it as a non-core localization.
GO:0031012 extracellular matrix	HDA PMID:25037231 Extracellular matrix signatures of human primary metastatic ...	KEEP AS NON CORE	Summary: High-throughput proteomic detection of A2M in extracellular-matrix preparations from primary colorectal tumors and liver metastases. Reason: The HDA observation is compatible with A2M's extracellular abundance and with other matrix proteomic detections, but it does not establish A2M as a structural ECM component. Treat the context-specific localization as non-core.
GO:0070062 extracellular exosome	HDA PMID:23533145 In-depth proteomic analyses of exosomes isolated from expres...	KEEP AS NON CORE	Summary: A2M was detected by high-throughput proteomics in exosomes isolated from expressed prostatic secretions in urine. Reason: Retain the experimentally observed vesicle association, but do not treat it as a defining A2M localization. A2M is an abundant secreted protein, and proteomic recovery with extracellular vesicles does not by itself establish a vesicle-specific function.
GO:0005102 signaling receptor binding	IMP PMID:15226301 Identification and characterization of the acidic pH binding...	MODIFY	Summary: Activated A2M binds the LRP1/alpha-2-macroglobulin receptor and participates in receptor-mediated uptake. Reason: Receptor binding is functionally important for clearance of activated A2M complexes, but the generic signaling-receptor term obscures the directly identified partner. GO:0050750 specifically captures binding to LRP1, a low-density-lipoprotein receptor-family member, and the activity must be qualified as activation-dependent because native A2M does not bind LRP1. Proposed replacements: low-density lipoprotein particle receptor binding Supporting Evidence: PMID:15226301 Cell surface annexin VI is also capable of mediating internalization and degradation of cell surface-bound (125)I-TGF-beta(1) and (125)I-alpha(2)M() at pH 6 and of forming ternary complexes with (125)I-alpha(2)M() and LRP-1 at neutral pH as demonstrated by co-immunoprecipitation. PMID:33964423 Native A2M does not interact with LRP1, whereas both A2M-MA and A2M-protease complexes do.
GO:0072562 blood microparticle	HDA PMID:22516433 Proteomic analysis of microvesicles from plasma of healthy d...	KEEP AS NON CORE	Summary: A2M was identified in a proteomic survey of plasma-derived microvesicles from healthy donors. Reason: This is a valid high-throughput compartment association, but A2M is a highly abundant circulating soluble protein and the observation does not establish a microparticle-specific role. Retain it as non-core.
GO:0002020 protease binding	IPI PMID:18485748 Inhibition of ADAMTS-7 and ADAMTS-12 degradation of cartilag...	ACCEPT	Summary: Direct binding and functional inhibition of the extracellular metalloproteases ADAMTS-7 and ADAMTS-12 by A2M. Reason: The study directly tested A2M cleavage by both proteases and dose-dependent blockade of their COMP-degrading activity. This is specific, mechanistically relevant protease binding and a clear example of A2M's core function. Supporting Evidence: PMID:18485748 Both ADAMTS-7 and ADAMTS-12 were able to cleave a(2)M, giving rise to 180- and 105-kDa cleavage products, respectively. Furthermore, a(2)M inhibited both ADAMTS-7- and ADAMTS-12-mediated COMP degradation in a concentration (or dose)-dependent manner.
GO:0070062 extracellular exosome	HDA PMID:20458337 MHC class II-associated proteins in B-cell exosomes and pote...	KEEP AS NON CORE	Summary: High-throughput proteomic detection of A2M in B-cell-derived extracellular exosomes. Reason: Retain the measured vesicle association as a context-specific localization. The source is an exosome proteome and the cached record is abstract-only; it does not establish a vesicle-specific A2M mechanism, so this should not be considered core.
GO:0005576 extracellular region	TAS Reactome:R-HSA-1454781	ACCEPT	Summary: Reactome places A2M in the extracellular region while it binds and traps extracellular matrix metalloproteinases. Reason: The reaction and its participants are extracellular, matching A2M's core role as a secreted plasma protease inhibitor. Supporting Evidence: Reactome:R-HSA-1454781 Active metalloproteinases (MMPs) that can be entrapped by A2M include MMP3 (Enghild et al. 1989) MMP1 (Grinnell et al. 1998) and MMP 13 (Beekman et al. 1999).
GO:0005576 extracellular region	TAS Reactome:R-HSA-158340	ACCEPT	Summary: Reactome places A2M in the extracellular region during inactivation of plasma kallikrein. Reason: Formation of a plasma kallikrein-A2M complex occurs in the circulation and is a direct instance of A2M's extracellular protease-control function. Supporting Evidence: Reactome:R-HSA-158340 Activated plasma kallikrein binds to alpha2-macroglobulin (Sottrup-Jensen L et al. 1984), forming a stable and enzymatically inactive complex.
GO:0005576 extracellular region	TAS Reactome:R-HSA-481007	ACCEPT	Summary: Reactome places platelet-stored A2M in the extracellular region following alpha-granule exocytosis. Reason: A2M is a secreted extracellular protein, and release from platelet alpha granules is consistent with that core localization. The parallel alpha-granule-lumen annotation records the pre-exocytosis storage site.
GO:0031093 platelet alpha granule lumen	TAS Reactome:R-HSA-481007	KEEP AS NON CORE	Summary: Reactome identifies A2M as cargo within the platelet alpha-granule lumen before regulated exocytosis. Reason: This curated storage-site localization is plausible for a plasma protease inhibitor and should be retained, but it is cell-type-specific and subordinate to A2M's defining extracellular localization and activity.
GO:0070062 extracellular exosome	HDA PMID:21362503 Protein profile of exosomes from trabecular meshwork cells.	KEEP AS NON CORE	Summary: High-throughput proteomic detection of A2M in exosomes released by trabecular meshwork cells. Reason: The HDA record supports an observed vesicle association in this cell culture context. It does not establish an exosome-specific A2M function, and abundant secreted proteins can accompany vesicle preparations, so the localization is non-core.
GO:0001869 negative regulation of complement activation, lectin pathway	IDA PMID:12538697 Natural substrates and inhibitors of mannan-binding lectin-a...	MARK AS OVER ANNOTATED	Summary: A2M reacts with soluble MASP-1, but later human-serum experiments found that this interaction does not abolish lectin-pathway activation on activator surfaces. Reason: The cited study inferred a possible physiological pathway role from reaction rates measured with recombinant catalytic fragments. A later primary study confirmed fluid-phase A2M-MASP-1 complex formation yet found no suppression of surface lectin-pathway activation. The process-level term therefore overextends a supported protease interaction; the narrower serine-type endopeptidase inhibitor activity remains valid. Supporting Evidence: PMID:12538697 Nevertheless, relative rates of reaction with alpha-2-macroglobulin and C1-inhibitor suggest that alpha-2-macroglobulin may be a significant physiological inhibitor of MASP-1. PMID:23399388 Although α(2)M formed complex with MASP-1 in fluid phase, it could not abolish lectin pathway activation on activator surfaces.
GO:0004867 serine-type endopeptidase inhibitor activity	IDA PMID:12538697 Natural substrates and inhibitors of mannan-binding lectin-a...	ACCEPT	Summary: Direct inhibition of the serine protease MASP-1 by A2M. Reason: MASP-1 is a serine-type endopeptidase, and the source reports its reaction with A2M and identifies A2M as a potentially significant physiological inhibitor. This is a valid specific instance of A2M's core pan-protease inhibitory activity. Supporting Evidence: PMID:12538697 Nevertheless, relative rates of reaction with alpha-2-macroglobulin and C1-inhibitor suggest that alpha-2-macroglobulin may be a significant physiological inhibitor of MASP-1.
GO:0005515 protein binding	IPI PMID:6698368 Mesotrypsin: a new inhibitor-resistant protease from a zymog...	UNDECIDED	Summary: IPI annotation linking A2M to mesotrypsin/PRSS3, an inhibitor-resistant human trypsin-family protease. Reason: The cached source is abstract-only and its abstract does not mention A2M or directly document the interaction represented by the GOA record. Because the full experimental evidence cannot be checked, the annotation should not be rejected or confidently remapped from generic protein binding. Supporting Evidence: PMID:6698368 The most remarkable property of mesotrypsin is its almost total resistance to biological trypsin inhibitors, such as pancreatic trypsin inhibitor, soybean, lima bean, ovomucoid inhibitor, alpha 1-antitrypsin, etc.
GO:0005515 protein binding	IPI PMID:14687906 Complex formation between human kallikrein 13 and serum prot...	MODIFY	Summary: Direct complex formation between A2M and the serine protease kallikrein-13 is represented only as generic protein binding. Reason: The interaction is specifically between A2M and an active protease, so protease binding conveys the biological relationship and is more informative than generic protein binding. Proposed replacements: protease binding Supporting Evidence: PMID:14687906 In vitro analysis indicated that enzymatically active 125I-labeled hK13 forms SDS-stable complexes with alpha2-antiplasmin, alpha2-macroglobulin and alpha1-antichymotrypsin.
GO:0019838 growth factor binding	IDA PMID:9398211 Human alpha 2-macroglobulin is an osteogenic growth peptide-...	KEEP AS NON CORE	Summary: Native and activated A2M directly bind osteogenic growth peptide in human plasma. Reason: Competitive binding and complex electrophoresis support this activity, and the study proposes differential regulation by native versus activated A2M. Growth-peptide carriage is a secondary ligand-binding role rather than the core protease-trapping function. Supporting Evidence: PMID:9398211 We show that OGP binds to both native and activated human plasma alpha 2-macroglobulin (alpha 2M).
GO:0019959 interleukin-8 binding	IPI PMID:10880251 Involvement of alpha-2-macroglobulin receptor in clearance o...	KEEP AS NON CORE	Summary: A2M forms complexes with IL-8 that are recognized and internalized through the A2M receptor on human alveolar macrophages. Reason: The uptake and competition experiments support IL-8 association with A2M and a plausible cytokine-clearance role. This is a secondary cargo-binding function rather than A2M's defining protease-inhibitor activity. Supporting Evidence: PMID:10880251 However,(125)I-rhIL-8-alpha-2-M complexes bound to macrophages, and unlabeled alpha-2-M competed for the binding.
GO:0019966 interleukin-1 binding	IDA PMID:9714181 A modified human alpha 2-macroglobulin derivative that binds...	KEEP AS NON CORE	Summary: A chemically activated A2M derivative binds IL-1 beta with strongly enhanced affinity; native A2M is reported to bind more weakly. Reason: The term remains qualitatively supported because the paper describes binding by A2M and enhancement after conformational stabilization, but the strongest evidence concerns the chemically modified MAC derivative. Retain this as a non-core, state-dependent ligand-binding activity. Supporting Evidence: PMID:9714181 In this study, we chemically modified alpha 2M to stabilize a conformation of the protein (termed MAC, Macroglobulin Activated for Cytokine binding) with greatly increased TNF-alpha- and IL-1 beta-binding activity.
GO:0043120 tumor necrosis factor binding	IDA PMID:9714181 A modified human alpha 2-macroglobulin derivative that binds...	KEEP AS NON CORE	Summary: A chemically activated A2M derivative binds TNF-alpha with much higher affinity than native A2M. Reason: Direct affinity measurements support TNF binding, including lower-affinity binding by native A2M, but the physiological relevance is uncertain because the strongest activity was engineered by chemical modification. Retain as a non-core, conformational-state-dependent activity. Supporting Evidence: PMID:9714181 The equilibrium dissociation constant (KD) for the binding of TNF-alpha to MAC was 80 +/- 20 nM, reflecting a 100-fold increase in affinity compared with native alpha 2M.
GO:0019899 enzyme binding	IPI PMID:11435418 Interaction of lecithin:cholesterol acyltransferase (LCAT).a...	MODIFY	Summary: Direct binding of A2M to lecithin-cholesterol acyltransferase forms a plasma complex and inhibits LCAT activity. Reason: The biochemical interaction is specific and functionally consequential, but generic enzyme binding omits the experimentally shown effect. Replace it with enzyme inhibitor activity because A2M-bound LCAT is inactive; this remains a specialized non-core role. Proposed replacements: enzyme inhibitor activity Supporting Evidence: PMID:11435418 We present evidence for the direct binding of LCAT to alpha(2)-macroglobulin (alpha(2)M) in human plasma to form a complex 18.5 nm in diameter. PMID:11435418 It is concluded that the binding of LCAT to alpha(2)M inhibits its enzymatic activity.
GO:0005576 extracellular region	NAS PMID:14718574 The human plasma proteome: a nonredundant list developed by ...	ACCEPT	Summary: Narrative plasma-proteome evidence places A2M in the extracellular region. Reason: A2M is a canonical abundant plasma protein, and this NAS localization is independently corroborated by phylogenetic, automated, and multiple curated Reactome extracellular annotations. Extracellular localization is central to its physiological protease-trapping role.
GO:0010951 negative regulation of endopeptidase activity	IDA PMID:18485748 Inhibition of ADAMTS-7 and ADAMTS-12 degradation of cartilag...	NEW	Summary: Human A2M directly suppresses endopeptidase activity after bait-region cleavage and conformational trapping. Reason: The molecular-function annotation for endopeptidase inhibitor activity lacks the corresponding biological-process annotation. Direct concentration-dependent inhibition of ADAMTS-7- and ADAMTS-12-mediated substrate degradation supports this specific process term. Supporting Evidence: PMID:18485748 Furthermore, a(2)M inhibited both ADAMTS-7- and ADAMTS-12-mediated COMP degradation in a concentration (or dose)-dependent manner. PMID:34139236 A2M has a unique mechanism of inhibition where it collapses around proteases and irreversibly sequesters them from protein substrates (5).
GO:0140311 protein sequestering activity	IDA PMID:34139236 Development of selective protease inhibitors via engineering...	NEW	Summary: A2M physically entraps proteases and prevents their access to protein substrates. Reason: This term captures the distinctive steric-trapping mechanism that is not conveyed by endopeptidase inhibitor activity alone. Direct bait engineering and structural studies show that cleavage-induced collapse encloses the protease and sequesters it from macromolecular substrates. Supporting Evidence: PMID:34139236 A2M has a unique mechanism of inhibition where it collapses around proteases and irreversibly sequesters them from protein substrates (5). PMID:33964423 These changes collapse the tetramer into a more compact conformation, which encloses an interior protease-trapping cavity.

📄 View Raw YAML

id: P01023
gene_symbol: A2M
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  A2M encodes alpha-2-macroglobulin, an abundant secreted plasma glycoprotein
  assembled as a disulfide-linked homotetramer. It is a broad-spectrum
  endopeptidase inhibitor whose exposed bait region is cleaved by many
  proteases. Bait cleavage triggers a large conformational collapse that
  encloses the protease and restricts its access to protein substrates. An
  internal thioester can additionally form a covalent adduct with the trapped
  enzyme, although tetrameric A2M can entrap proteases without covalent linkage.
  The activated conformation exposes an LRP1-binding site that targets
  A2M-protease complexes for receptor-mediated uptake and degradation. A2M also
  binds several growth factors, cytokines, and plasma enzymes in
  conformation-dependent contexts, but these are secondary to its protease-trap
  function.
existing_annotations:
- term:
    id: GO:0004866
    label: endopeptidase inhibitor activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  supporting_entities:
  - MGI:MGI:87854
  - PANTHER:PTN008320188
  - RGD:2004
  - UniProtKB:P01023
  review:
    summary: Phylogenetic inference of A2M's conserved endopeptidase-inhibitor
      activity, the defining protease-trapping function of alpha-macroglobulins.
    action: ACCEPT
    reason: A2M directly inhibits diverse endopeptidases after bait-region
      cleavage and conformational trapping. The IBA is at the appropriate broad
      level because A2M acts across protease catalytic classes, and direct work
      shows dose-dependent inhibition of ADAMTS-7 and ADAMTS-12.
    additional_reference_ids:
    - PMID:18485748
    supported_by:
    - reference_id: PMID:18485748
      supporting_text: Furthermore, a(2)M inhibited both ADAMTS-7- and
        ADAMTS-12-mediated COMP degradation in a concentration (or dose)-dependent
        manner.
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  supporting_entities:
  - PANTHER:PTN000151154
  - RGD:2004
  review:
    summary: Phylogenetic inference that A2M is active in the extracellular
      region, consistent with its role as a circulating plasma protease trap.
    action: ACCEPT
    reason: Extracellular activity is integral to A2M biology and is independently
      represented in curated human Reactome reactions involving extracellular
      metalloproteinases and plasma kallikrein.
    additional_reference_ids:
    - Reactome:R-HSA-1454781
    - Reactome:R-HSA-158340
    supported_by:
    - reference_id: Reactome:R-HSA-1454781
      supporting_text: Alpha 2-macroglobulin (A2M) is a plasma glycoprotein
        consisting of 4 near-identical subunits (Andersen et al. 1995).
- term:
    id: GO:0002020
    label: protease binding
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  supporting_entities:
  - PANTHER:PTN008320188
  - UniProtKB:P01023
  - UniProtKB:Q7SIH1
  review:
    summary: Phylogenetic inference of the conserved ability of A2M to bind and
      entrap proteases.
    action: ACCEPT
    reason: Protease binding is a direct and indispensable part of A2M's trapping
      mechanism, rather than a generic physical association. Curated human
      pathway evidence describes bait-region cleavage followed by entrapment of
      the endopeptidase.
    additional_reference_ids:
    - Reactome:R-HSA-1454781
    supported_by:
    - reference_id: Reactome:R-HSA-1454781
      supporting_text: A2M binding to an endopeptidase is triggered by cleavage
        of a peptide bond in the 'bait region' of A2M, triggering a conformational
        change in A2M that in turn entraps the peptidase without blocking the
        active site (Barrett & Starkey 1973).
- term:
    id: GO:0004857
    label: enzyme inhibitor activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  qualifier: enables
  supporting_entities:
  - ARBA:ARBA00026999
  review:
    summary: A2M inhibits both its core protease targets and, in a secondary
      context, the non-peptidase enzyme LCAT.
    action: KEEP_AS_NON_CORE
    reason: The broad term is not merely redundant with endopeptidase inhibitor
      activity because A2M-bound lecithin-cholesterol acyltransferase is
      enzymatically inactive. Retain it as a valid non-core activity while using
      GO:0004866 for A2M's defining protease-trapping function.
    additional_reference_ids:
    - PMID:11435418
    supported_by:
    - reference_id: PMID:11435418
      supporting_text: It is concluded that the binding of LCAT to alpha(2)M
        inhibits its enzymatic activity.
- term:
    id: GO:0004866
    label: endopeptidase inhibitor activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  supporting_entities:
  - InterPro:IPR001599
  - InterPro:IPR002890
  review:
    summary: InterPro-derived electronic annotation of A2M's broad
      endopeptidase-inhibitor activity.
    action: ACCEPT
    reason: >-
      The term precisely captures the conserved alpha-macroglobulin
      trapping mechanism and is corroborated by direct inhibition of multiple
      ADAMTS endopeptidases. Inhibition is steric and substrate-size-dependent:
      trapped enzymes can remain active on low-molecular-weight substrates while
      losing access to macromolecular substrates.
    additional_reference_ids:
    - PMID:18485748
    - Reactome:R-HSA-1454781
    supported_by:
    - reference_id: PMID:18485748
      supporting_text: Furthermore, a(2)M inhibited both ADAMTS-7- and
        ADAMTS-12-mediated COMP degradation in a concentration (or dose)-dependent
        manner.
    - reference_id: Reactome:R-HSA-1454781
      supporting_text: This blocks enzyme activity against large protein
        substrates while not preventing activity on low molecular weight
        substrates.
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: located_in
  supporting_entities:
  - InterPro:IPR009048
  - InterPro:IPR011626
  - InterPro:IPR036595
  - UniProtKB-SubCell:SL-0243
  review:
    summary: Automated localization of the secreted A2M protein to the
      extracellular region.
    action: ACCEPT
    reason: This is the correct core localization for a circulating plasma
      macroglobulin and agrees with the IBA and multiple curated extracellular
      Reactome reactions.
    additional_reference_ids:
    - Reactome:R-HSA-1454781
    supported_by:
    - reference_id: Reactome:R-HSA-1454781
      supporting_text: Alpha 2-macroglobulin (A2M) is a plasma glycoprotein
        consisting of 4 near-identical subunits (Andersen et al. 1995).
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:32814053
  qualifier: enables
  supporting_entities:
  - UniProtKB:A4FUJ8
  - UniProtKB:O75558
  - UniProtKB:O75616
  - UniProtKB:O75626-3
  - UniProtKB:O75925
  - UniProtKB:P04141
  - UniProtKB:P04271
  - UniProtKB:P07948
  - UniProtKB:P09017
  - UniProtKB:P0C0S5
  - UniProtKB:P10599
  - UniProtKB:P27986-2
  - UniProtKB:P28347-2
  - UniProtKB:P35222
  - UniProtKB:P41218
  - UniProtKB:P43405-2
  - UniProtKB:P57729
  - UniProtKB:P60409
  - UniProtKB:P63010-2
  - UniProtKB:P68431
  - UniProtKB:Q16637-3
  - UniProtKB:Q6DN90-2
  - UniProtKB:Q6GQQ9-2
  - UniProtKB:Q6PCB6
  - UniProtKB:Q6PIV2
  - UniProtKB:Q7L576
  - UniProtKB:Q7Z6I5
  - UniProtKB:Q86WT6-2
  - UniProtKB:Q8IUC2
  - UniProtKB:Q8N4C7
  - UniProtKB:Q8N895
  - UniProtKB:Q8TBB1
  - UniProtKB:Q8TDB4
  - UniProtKB:Q8WUW1
  - UniProtKB:Q8WVJ9
  - UniProtKB:Q92478
  - UniProtKB:Q92993
  - UniProtKB:Q92993-2
  - UniProtKB:Q96AX1
  - UniProtKB:Q96D59
  - UniProtKB:Q96FW1
  - UniProtKB:Q99497
  - UniProtKB:Q99KR7
  - UniProtKB:Q9BVJ6
  - UniProtKB:Q9H147
  - UniProtKB:Q9NR90-2
  - UniProtKB:Q9UBQ0-2
  - UniProtKB:Q9UHI6
  - UniProtKB:Q9UII2
  - UniProtKB:Q9ULV1
  - UniProtKB:Q9ULX5
  - UniProtKB:Q9UNE7
  - UniProtKB:Q9UNS2
  - UniProtKB:Q9UPM6
  - UniProtKB:Q9Y261-2
  - UniProtKB:Q9Y605
  review:
    summary: Generic protein-binding annotation collapsed from 56 A2M candidate
      interactions in a large systematic interaction map.
    action: MARK_AS_OVER_ANNOTATED
    reason: The source reports tens of thousands of candidate interactions from
      systematic yeast two-hybrid screening. These candidate associations do not
      establish a mechanistic molecular function for A2M, and the generic
      protein-binding term is uninformative; many partners are also intracellular
      and difficult to reconcile with A2M's secreted plasma localization.
    supported_by:
    - reference_id: PMID:32814053
      supporting_text: Here, we report on an interactome map that focuses on
        neurodegenerative disease (ND), connects ∼5,000 human proteins via ∼30,000
        candidate interactions and is generated by systematic yeast two-hybrid
        interaction screening of ∼500 ND-related proteins and integration of
        literature interactions.
- term:
    id: GO:0002020
    label: protease binding
  evidence_type: IPI
  original_reference_id: PMID:7679575
  qualifier: enables
  supporting_entities:
  - UniProtKB:P00747
  review:
    summary: Direct interaction of A2M with plasmin after release of plasmin from
      a reversible thrombospondin complex.
    action: ACCEPT
    reason: The abstract explicitly reports formation of an alpha-2-macroglobulin
      complex with plasmin. This is a specific protease interaction and directly
      reflects the core A2M trapping function.
    supported_by:
    - reference_id: PMID:7679575
      supporting_text: Similar results were obtained with alpha 2M. Transfer of
        plasmin from thrombospondin to alpha 2AP or alpha 2M probably required
        plasmin-thrombospondin-complex dissociation.
- term:
    id: GO:0004866
    label: endopeptidase inhibitor activity
  evidence_type: IDA
  original_reference_id: PMID:7679575
  qualifier: enables
  review:
    summary: Experimental evidence that A2M captures plasmin, an endopeptidase,
      from a reversible plasmin-thrombospondin complex.
    action: ACCEPT
    reason: Although the paper focuses on thrombospondin, its abstract explicitly
      describes plasmin transfer into an A2M complex and treats A2M as a
      conventional proteinase inhibitor. This supports the established core
      endopeptidase-inhibitor function.
    supported_by:
    - reference_id: PMID:7679575
      supporting_text: Similar results were obtained with alpha 2M. Transfer of
        plasmin from thrombospondin to alpha 2AP or alpha 2M probably required
        plasmin-thrombospondin-complex dissociation.
- term:
    id: GO:0048306
    label: calcium-dependent protein binding
  evidence_type: IPI
  original_reference_id: PMID:15226301
  qualifier: enables
  supporting_entities:
  - UniProtKB:E1BGJ0
  - UniProtKB:P04083
  - UniProtKB:P07355
  - UniProtKB:P08133
  - UniProtKB:P09525
  - UniProtKB:P12429
  - UniProtKB:P79134
  review:
    summary: Calcium-sensitive binding of activated A2M to cell-surface annexins
      in an acidic-pH uptake context.
    action: KEEP_AS_NON_CORE
    reason: The abstract identifies annexin VI as directly binding activated A2M
      and states that the relevant binding sites are sensitive to calcium
      depletion. This supports the annotation, but it is a context-dependent
      trafficking interaction rather than A2M's core protease-trapping activity;
      the cached source is abstract-only.
    supported_by:
    - reference_id: PMID:15226301
      supporting_text: These sites, like LRP-1, are sensitive to
        receptor-associated protein and calcium depletion but, unlike LRP-1,
        are also sensitive to chondroitin sulfate and heparin and capable of
        directly binding ligands, which do not bind to LRP-1.
    - reference_id: PMID:15226301
      supporting_text: Annexin VI has been identified as a major membrane-associated
        protein capable of directly binding alpha(2)M(*) at acidic pH.
- term:
    id: GO:0031012
    label: extracellular matrix
  evidence_type: HDA
  original_reference_id: PMID:28327460
  qualifier: colocalizes_with
  review:
    summary: A2M was detected in extracellular matrix produced by cultured human
      mesenchymal stromal cells in a quantitative proteomic survey.
    action: KEEP_AS_NON_CORE
    reason: The source directly reports A2M in bone-marrow-derived cell matrix,
      making extracellular-matrix association plausible. A2M is a soluble
      secreted protease inhibitor rather than a structural matrix constituent,
      so this context-specific localization is non-core.
    supported_by:
    - reference_id: PMID:28327460
      supporting_text: Proteinases can be inactivated by protease inhibitors such
        as TIMP3 and A2M found in Bm ECM.
- term:
    id: GO:0031012
    label: extracellular matrix
  evidence_type: HDA
  original_reference_id: PMID:28675934
  qualifier: located_in
  review:
    summary: High-throughput proteomic detection of A2M in decellularized normal
      and diseased human tissue extracellular-matrix preparations.
    action: KEEP_AS_NON_CORE
    reason: This HDA record supports matrix association in the sampled tissues and
      is consistent with independent matrix proteomes. Because abundant soluble
      A2M can associate with extracellular matrices without being a structural
      matrix component, retain it as a non-core localization.
- term:
    id: GO:0031012
    label: extracellular matrix
  evidence_type: HDA
  original_reference_id: PMID:25037231
  qualifier: located_in
  review:
    summary: High-throughput proteomic detection of A2M in extracellular-matrix
      preparations from primary colorectal tumors and liver metastases.
    action: KEEP_AS_NON_CORE
    reason: The HDA observation is compatible with A2M's extracellular abundance
      and with other matrix proteomic detections, but it does not establish A2M
      as a structural ECM component. Treat the context-specific localization as
      non-core.
- term:
    id: GO:0070062
    label: extracellular exosome
  evidence_type: HDA
  original_reference_id: PMID:23533145
  qualifier: located_in
  review:
    summary: A2M was detected by high-throughput proteomics in exosomes isolated
      from expressed prostatic secretions in urine.
    action: KEEP_AS_NON_CORE
    reason: Retain the experimentally observed vesicle association, but do not
      treat it as a defining A2M localization. A2M is an abundant secreted protein,
      and proteomic recovery with extracellular vesicles does not by itself
      establish a vesicle-specific function.
- term:
    id: GO:0005102
    label: signaling receptor binding
  evidence_type: IMP
  original_reference_id: PMID:15226301
  qualifier: enables
  review:
    summary: Activated A2M binds the LRP1/alpha-2-macroglobulin receptor and
      participates in receptor-mediated uptake.
    action: MODIFY
    reason: Receptor binding is functionally important for clearance of activated
      A2M complexes, but the generic signaling-receptor term obscures the directly
      identified partner. GO:0050750 specifically captures binding to LRP1, a
      low-density-lipoprotein receptor-family member, and the activity must be
      qualified as activation-dependent because native A2M does not bind LRP1.
    proposed_replacement_terms:
    - id: GO:0050750
      label: low-density lipoprotein particle receptor binding
    additional_reference_ids:
    - PMID:1691187
    - PMID:33964423
    supported_by:
    - reference_id: PMID:15226301
      supporting_text: Cell surface annexin VI is also capable of mediating
        internalization and degradation of cell surface-bound (125)I-TGF-beta(1)
        and (125)I-alpha(2)M(*) at pH 6 and of forming ternary complexes with
        (125)I-alpha(2)M(*) and LRP-1 at neutral pH as demonstrated by
        co-immunoprecipitation.
    - reference_id: PMID:33964423
      supporting_text: Native A2M does not interact with LRP1, whereas both A2M-MA
        and A2M-protease complexes do.
- term:
    id: GO:0072562
    label: blood microparticle
  evidence_type: HDA
  original_reference_id: PMID:22516433
  qualifier: located_in
  review:
    summary: A2M was identified in a proteomic survey of plasma-derived
      microvesicles from healthy donors.
    action: KEEP_AS_NON_CORE
    reason: This is a valid high-throughput compartment association, but A2M is a
      highly abundant circulating soluble protein and the observation does not
      establish a microparticle-specific role. Retain it as non-core.
- term:
    id: GO:0002020
    label: protease binding
  evidence_type: IPI
  original_reference_id: PMID:18485748
  qualifier: enables
  supporting_entities:
  - UniProtKB:P58397
  - UniProtKB:Q9UKP4
  review:
    summary: Direct binding and functional inhibition of the extracellular
      metalloproteases ADAMTS-7 and ADAMTS-12 by A2M.
    action: ACCEPT
    reason: The study directly tested A2M cleavage by both proteases and
      dose-dependent blockade of their COMP-degrading activity. This is specific,
      mechanistically relevant protease binding and a clear example of A2M's core
      function.
    supported_by:
    - reference_id: PMID:18485748
      supporting_text: Both ADAMTS-7 and ADAMTS-12 were able to cleave a(2)M,
        giving rise to 180- and 105-kDa cleavage products, respectively. Furthermore,
        a(2)M inhibited both ADAMTS-7- and ADAMTS-12-mediated COMP degradation in
        a concentration (or dose)-dependent manner.
- term:
    id: GO:0070062
    label: extracellular exosome
  evidence_type: HDA
  original_reference_id: PMID:20458337
  qualifier: located_in
  review:
    summary: High-throughput proteomic detection of A2M in B-cell-derived
      extracellular exosomes.
    action: KEEP_AS_NON_CORE
    reason: Retain the measured vesicle association as a context-specific
      localization. The source is an exosome proteome and the cached record is
      abstract-only; it does not establish a vesicle-specific A2M mechanism, so
      this should not be considered core.
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-1454781
  qualifier: located_in
  review:
    summary: Reactome places A2M in the extracellular region while it binds and
      traps extracellular matrix metalloproteinases.
    action: ACCEPT
    reason: The reaction and its participants are extracellular, matching A2M's
      core role as a secreted plasma protease inhibitor.
    supported_by:
    - reference_id: Reactome:R-HSA-1454781
      supporting_text: Active metalloproteinases (MMPs) that can be entrapped by
        A2M include MMP3 (Enghild et al. 1989) MMP1 (Grinnell et al. 1998) and MMP
        13 (Beekman et al. 1999).
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-158340
  qualifier: located_in
  review:
    summary: Reactome places A2M in the extracellular region during inactivation
      of plasma kallikrein.
    action: ACCEPT
    reason: Formation of a plasma kallikrein-A2M complex occurs in the circulation
      and is a direct instance of A2M's extracellular protease-control function.
    supported_by:
    - reference_id: Reactome:R-HSA-158340
      supporting_text: Activated plasma kallikrein binds to alpha2-macroglobulin
        (Sottrup-Jensen L et al. 1984), forming a stable and enzymatically inactive
        complex.
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-481007
  qualifier: located_in
  review:
    summary: Reactome places platelet-stored A2M in the extracellular region
      following alpha-granule exocytosis.
    action: ACCEPT
    reason: A2M is a secreted extracellular protein, and release from platelet
      alpha granules is consistent with that core localization. The parallel
      alpha-granule-lumen annotation records the pre-exocytosis storage site.
- term:
    id: GO:0031093
    label: platelet alpha granule lumen
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-481007
  qualifier: located_in
  review:
    summary: Reactome identifies A2M as cargo within the platelet alpha-granule
      lumen before regulated exocytosis.
    action: KEEP_AS_NON_CORE
    reason: This curated storage-site localization is plausible for a plasma
      protease inhibitor and should be retained, but it is cell-type-specific and
      subordinate to A2M's defining extracellular localization and activity.
- term:
    id: GO:0070062
    label: extracellular exosome
  evidence_type: HDA
  original_reference_id: PMID:21362503
  qualifier: located_in
  review:
    summary: High-throughput proteomic detection of A2M in exosomes released by
      trabecular meshwork cells.
    action: KEEP_AS_NON_CORE
    reason: The HDA record supports an observed vesicle association in this cell
      culture context. It does not establish an exosome-specific A2M function,
      and abundant secreted proteins can accompany vesicle preparations, so the
      localization is non-core.
- term:
    id: GO:0001869
    label: negative regulation of complement activation, lectin pathway
  evidence_type: IDA
  original_reference_id: PMID:12538697
  qualifier: involved_in
  review:
    summary: A2M reacts with soluble MASP-1, but later human-serum experiments
      found that this interaction does not abolish lectin-pathway activation on
      activator surfaces.
    action: MARK_AS_OVER_ANNOTATED
    reason: The cited study inferred a possible physiological pathway role from
      reaction rates measured with recombinant catalytic fragments. A later
      primary study confirmed fluid-phase A2M-MASP-1 complex formation yet found
      no suppression of surface lectin-pathway activation. The process-level term
      therefore overextends a supported protease interaction; the narrower
      serine-type endopeptidase inhibitor activity remains valid.
    additional_reference_ids:
    - PMID:23399388
    supported_by:
    - reference_id: PMID:12538697
      supporting_text: Nevertheless, relative rates of reaction with
        alpha-2-macroglobulin and C1-inhibitor suggest that alpha-2-macroglobulin
        may be a significant physiological inhibitor of MASP-1.
    - reference_id: PMID:23399388
      supporting_text: Although α(2)M formed complex with MASP-1 in fluid phase,
        it could not abolish lectin pathway activation on activator surfaces.
- term:
    id: GO:0004867
    label: serine-type endopeptidase inhibitor activity
  evidence_type: IDA
  original_reference_id: PMID:12538697
  qualifier: enables
  review:
    summary: Direct inhibition of the serine protease MASP-1 by A2M.
    action: ACCEPT
    reason: MASP-1 is a serine-type endopeptidase, and the source reports its
      reaction with A2M and identifies A2M as a potentially significant
      physiological inhibitor. This is a valid specific instance of A2M's core
      pan-protease inhibitory activity.
    supported_by:
    - reference_id: PMID:12538697
      supporting_text: Nevertheless, relative rates of reaction with
        alpha-2-macroglobulin and C1-inhibitor suggest that alpha-2-macroglobulin
        may be a significant physiological inhibitor of MASP-1.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:6698368
  qualifier: enables
  supporting_entities:
  - UniProtKB:P35030
  review:
    summary: IPI annotation linking A2M to mesotrypsin/PRSS3, an inhibitor-resistant
      human trypsin-family protease.
    action: UNDECIDED
    reason: The cached source is abstract-only and its abstract does not mention
      A2M or directly document the interaction represented by the GOA record.
      Because the full experimental evidence cannot be checked, the annotation
      should not be rejected or confidently remapped from generic protein binding.
    supported_by:
    - reference_id: PMID:6698368
      supporting_text: The most remarkable property of mesotrypsin is its almost
        total resistance to biological trypsin inhibitors, such as pancreatic
        trypsin inhibitor, soybean, lima bean, ovomucoid inhibitor, alpha
        1-antitrypsin, etc.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:14687906
  qualifier: enables
  supporting_entities:
  - UniProtKB:Q9UKR3
  review:
    summary: Direct complex formation between A2M and the serine protease
      kallikrein-13 is represented only as generic protein binding.
    action: MODIFY
    reason: The interaction is specifically between A2M and an active protease,
      so protease binding conveys the biological relationship and is more
      informative than generic protein binding.
    proposed_replacement_terms:
    - id: GO:0002020
      label: protease binding
    supported_by:
    - reference_id: PMID:14687906
      supporting_text: In vitro analysis indicated that enzymatically active
        125I-labeled hK13 forms SDS-stable complexes with alpha2-antiplasmin,
        alpha2-macroglobulin and alpha1-antichymotrypsin.
- term:
    id: GO:0019838
    label: growth factor binding
  evidence_type: IDA
  original_reference_id: PMID:9398211
  qualifier: enables
  review:
    summary: Native and activated A2M directly bind osteogenic growth peptide in
      human plasma.
    action: KEEP_AS_NON_CORE
    reason: Competitive binding and complex electrophoresis support this activity,
      and the study proposes differential regulation by native versus activated
      A2M. Growth-peptide carriage is a secondary ligand-binding role rather than
      the core protease-trapping function.
    supported_by:
    - reference_id: PMID:9398211
      supporting_text: We show that OGP binds to both native and activated human
        plasma alpha 2-macroglobulin (alpha 2M).
- term:
    id: GO:0019959
    label: interleukin-8 binding
  evidence_type: IPI
  original_reference_id: PMID:10880251
  qualifier: enables
  supporting_entities:
  - UniProtKB:P10145
  review:
    summary: A2M forms complexes with IL-8 that are recognized and internalized
      through the A2M receptor on human alveolar macrophages.
    action: KEEP_AS_NON_CORE
    reason: The uptake and competition experiments support IL-8 association with
      A2M and a plausible cytokine-clearance role. This is a secondary cargo-binding
      function rather than A2M's defining protease-inhibitor activity.
    supported_by:
    - reference_id: PMID:10880251
      supporting_text: However,(125)I-rhIL-8-alpha-2-M complexes bound to
        macrophages, and unlabeled alpha-2-M competed for the binding.
- term:
    id: GO:0019966
    label: interleukin-1 binding
  evidence_type: IDA
  original_reference_id: PMID:9714181
  qualifier: enables
  review:
    summary: A chemically activated A2M derivative binds IL-1 beta with strongly
      enhanced affinity; native A2M is reported to bind more weakly.
    action: KEEP_AS_NON_CORE
    reason: The term remains qualitatively supported because the paper describes
      binding by A2M and enhancement after conformational stabilization, but the
      strongest evidence concerns the chemically modified MAC derivative. Retain
      this as a non-core, state-dependent ligand-binding activity.
    supported_by:
    - reference_id: PMID:9714181
      supporting_text: In this study, we chemically modified alpha 2M to stabilize
        a conformation of the protein (termed MAC, Macroglobulin Activated for
        Cytokine binding) with greatly increased TNF-alpha- and IL-1 beta-binding
        activity.
- term:
    id: GO:0043120
    label: tumor necrosis factor binding
  evidence_type: IDA
  original_reference_id: PMID:9714181
  qualifier: enables
  review:
    summary: A chemically activated A2M derivative binds TNF-alpha with much
      higher affinity than native A2M.
    action: KEEP_AS_NON_CORE
    reason: Direct affinity measurements support TNF binding, including lower-affinity
      binding by native A2M, but the physiological relevance is uncertain because
      the strongest activity was engineered by chemical modification. Retain as
      a non-core, conformational-state-dependent activity.
    supported_by:
    - reference_id: PMID:9714181
      supporting_text: The equilibrium dissociation constant (KD) for the binding
        of TNF-alpha to MAC was 80 +/- 20 nM, reflecting a 100-fold increase in
        affinity compared with native alpha 2M.
- term:
    id: GO:0019899
    label: enzyme binding
  evidence_type: IPI
  original_reference_id: PMID:11435418
  qualifier: enables
  supporting_entities:
  - UniProtKB:P04180
  review:
    summary: Direct binding of A2M to lecithin-cholesterol acyltransferase forms
      a plasma complex and inhibits LCAT activity.
    action: MODIFY
    reason: The biochemical interaction is specific and functionally
      consequential, but generic enzyme binding omits the experimentally shown
      effect. Replace it with enzyme inhibitor activity because A2M-bound LCAT is
      inactive; this remains a specialized non-core role.
    proposed_replacement_terms:
    - id: GO:0004857
      label: enzyme inhibitor activity
    supported_by:
    - reference_id: PMID:11435418
      supporting_text: We present evidence for the direct binding of LCAT to
        alpha(2)-macroglobulin (alpha(2)M) in human plasma to form a complex 18.5
        nm in diameter.
    - reference_id: PMID:11435418
      supporting_text: It is concluded that the binding of LCAT to alpha(2)M
        inhibits its enzymatic activity.
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: NAS
  original_reference_id: PMID:14718574
  qualifier: located_in
  review:
    summary: Narrative plasma-proteome evidence places A2M in the extracellular
      region.
    action: ACCEPT
    reason: A2M is a canonical abundant plasma protein, and this NAS localization
      is independently corroborated by phylogenetic, automated, and multiple
      curated Reactome extracellular annotations. Extracellular localization is
      central to its physiological protease-trapping role.
    additional_reference_ids:
    - Reactome:R-HSA-1454781
    - Reactome:R-HSA-158340
- term:
    id: GO:0010951
    label: negative regulation of endopeptidase activity
  evidence_type: IDA
  original_reference_id: PMID:18485748
  qualifier: involved_in
  review:
    summary: Human A2M directly suppresses endopeptidase activity after
      bait-region cleavage and conformational trapping.
    action: NEW
    reason: The molecular-function annotation for endopeptidase inhibitor
      activity lacks the corresponding biological-process annotation. Direct
      concentration-dependent inhibition of ADAMTS-7- and ADAMTS-12-mediated
      substrate degradation supports this specific process term.
    additional_reference_ids:
    - PMID:34139236
    supported_by:
    - reference_id: PMID:18485748
      supporting_text: Furthermore, a(2)M inhibited both ADAMTS-7- and
        ADAMTS-12-mediated COMP degradation in a concentration (or dose)-dependent
        manner.
    - reference_id: PMID:34139236
      supporting_text: A2M has a unique mechanism of inhibition where it collapses
        around proteases and irreversibly sequesters them from protein substrates
        (5).
- term:
    id: GO:0140311
    label: protein sequestering activity
  evidence_type: IDA
  original_reference_id: PMID:34139236
  qualifier: enables
  review:
    summary: A2M physically entraps proteases and prevents their access to
      protein substrates.
    action: NEW
    reason: This term captures the distinctive steric-trapping mechanism that
      is not conveyed by endopeptidase inhibitor activity alone. Direct bait
      engineering and structural studies show that cleavage-induced collapse
      encloses the protease and sequesters it from macromolecular substrates.
    additional_reference_ids:
    - PMID:33964423
    supported_by:
    - reference_id: PMID:34139236
      supporting_text: A2M has a unique mechanism of inhibition where it collapses
        around proteases and irreversibly sequesters them from protein substrates
        (5).
    - reference_id: PMID:33964423
      supporting_text: These changes collapse the tetramer into a more compact
        conformation, which encloses an interior protease-trapping cavity.
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000117
  title: Electronic Gene Ontology annotations created by ARBA machine learning models
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:1691187
  title: 'The human alpha 2-macroglobulin receptor: identification of a 420-kD cell
    surface glycoprotein specific for the activated conformation of alpha 2-macroglobulin.'
  findings:
  - statement: The alpha-2-macroglobulin receptor binds conformationally
      activated A2M but not native A2M.
    supporting_text: The purified 420-kD protein binds to the conformationally
      altered forms of alpha 2M that are known to specifically interact with
      alpha 2M receptors and does not bind to native alpha 2M.
    reference_section_type: ABSTRACT
  full_text_unavailable: true
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: The PubMed-cached abstract directly establishes the activated
      conformation specificity of receptor binding; the paper predates naming
      this receptor as LRP1.
- id: PMID:6203908
  title: Primary structure of human alpha 2-macroglobulin. V. The complete structure.
  findings:
  - statement: Human A2M is a tetrameric plasma glycoprotein.
    supporting_text: The primary structure of the tetrameric plasma glycoprotein
      human alpha 2-macroglobulin has been determined.
    reference_section_type: ABSTRACT
  - statement: An internal Cys-Gln thioester provides an activatable site capable
      of covalent nucleophile capture.
    supporting_text: The beta-SH group of Cys-949 is thiol esterified to the
      gamma-carbonyl group of Glx-952, thus forming an activatable reactive site
      which can mediate covalent binding of nucleophiles.
    reference_section_type: ABSTRACT
  full_text_unavailable: true
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: The PubMed-cached abstract directly supports the tetrameric
      architecture, bait-cleavage region, and internal thioester; residue
      numbers follow the mature-chain convention used by the paper.
- id: PMID:10880251
  title: Involvement of alpha-2-macroglobulin receptor in clearance of interleukin
    8-alpha-2-macroglobulin complexes by human alveolar macrophages.
  findings:
  - statement: IL-8-A2M complexes bind human alveolar macrophages and undergo an
      A2M-receptor-dependent clearance route.
    supporting_text: However,(125)I-rhIL-8-alpha-2-M complexes bound to
      macrophages, and unlabeled alpha-2-M competed for the binding.
    reference_section_type: ABSTRACT
  full_text_unavailable: true
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: The abstract directly supports a context-specific cytokine
      cargo interaction and uptake mechanism, not a defining A2M function.
- id: PMID:11435418
  title: Interaction of lecithin:cholesterol acyltransferase (LCAT).alpha 2-macroglobulin
    complex with low density lipoprotein receptor-related protein (LRP). Evidence
    for an alpha 2-macroglobulin/LRP receptor-mediated system participating in LCAT
    clearance.
  findings:
  - statement: A2M binds LCAT in human plasma, inhibits its catalytic activity,
      and permits LRP-dependent clearance of activated A2M-LCAT complexes.
    supporting_text: It is concluded that the binding of LCAT to alpha(2)M
      inhibits its enzymatic activity.
    reference_section_type: ABSTRACT
  full_text_unavailable: true
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: The abstract directly supports non-peptidase enzyme inhibition
      and a specialized A2M cargo-clearance role.
- id: PMID:12538697
  title: 'Natural substrates and inhibitors of mannan-binding lectin-associated serine
    protease-1 and -2: a study on recombinant catalytic fragments.'
  findings:
  - statement: Recombinant soluble MASP fragments react with A2M, leading the
      authors to propose a possible physiological MASP-1 inhibitor role.
    supporting_text: Nevertheless, relative rates of reaction with
      alpha-2-macroglobulin and C1-inhibitor suggest that alpha-2-macroglobulin
      may be a significant physiological inhibitor of MASP-1.
    reference_section_type: ABSTRACT
  full_text_unavailable: true
  reference_review:
    relevance: MEDIUM
    correctness: DISPUTED
    review_notes: The cited experiment supports soluble MASP-1 inhibition, but
      its pathway-level physiological extrapolation is contradicted by
      PMID:23399388, which found no abolition of surface lectin-pathway
      activation in human serum.
- id: PMID:14687906
  title: Complex formation between human kallikrein 13 and serum protease inhibitors.
  findings: []
- id: PMID:14718574
  title: 'The human plasma proteome: a nonredundant list developed by combination
    of four separate sources.'
  findings: []
- id: PMID:15226301
  title: Identification and characterization of the acidic pH binding sites for growth
    regulatory ligands of low density lipoprotein receptor-related protein-1.
  findings:
  - statement: Activated A2M forms complexes with LRP1 and undergoes cell-surface
      internalization and degradation in an annexin-VI- and pH-dependent context.
    supporting_text: Cell surface annexin VI is also capable of mediating
      internalization and degradation of cell surface-bound (125)I-TGF-beta(1)
      and (125)I-alpha(2)M(*) at pH 6 and of forming ternary complexes with
      (125)I-alpha(2)M(*) and LRP-1 at neutral pH as demonstrated by
      co-immunoprecipitation.
    reference_section_type: ABSTRACT
  full_text_unavailable: true
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: The abstract directly supports activated-A2M uptake and LRP1
      complex formation, but the annexin-VI and acidic-pH context makes it
      supporting rather than defining evidence.
- id: PMID:18485748
  title: Inhibition of ADAMTS-7 and ADAMTS-12 degradation of cartilage oligomeric
    matrix protein by alpha-2-macroglobulin.
  findings:
  - statement: A2M is cleaved by ADAMTS-7 and ADAMTS-12 and dose-dependently
      inhibits their degradation of cartilage oligomeric matrix protein.
    supporting_text: Furthermore, a(2)M inhibited both ADAMTS-7- and
      ADAMTS-12-mediated COMP degradation in a concentration (or dose)-dependent
      manner.
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Full text is cached, and the direct inhibition assay strongly
      supports the core molecular function and the new negative-regulation
      process annotation.
- id: PMID:20458337
  title: MHC class II-associated proteins in B-cell exosomes and potential functional
    implications for exosome biogenesis.
  findings: []
- id: PMID:21362503
  title: Protein profile of exosomes from trabecular meshwork cells.
  findings: []
- id: PMID:22516433
  title: Proteomic analysis of microvesicles from plasma of healthy donors reveals
    high individual variability.
  findings: []
- id: PMID:23399388
  title: 'The control of the complement lectin pathway activation revisited: both
    C1-inhibitor and antithrombin are likely physiological inhibitors, while
    α2-macroglobulin is not.'
  findings:
  - statement: A2M forms a fluid-phase complex with MASP-1 but does not abolish
      lectin-pathway activation on activator surfaces.
    supporting_text: Although α(2)M formed complex with MASP-1 in fluid phase,
      it could not abolish lectin pathway activation on activator surfaces.
    reference_section_type: ABSTRACT
  full_text_unavailable: true
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: The PubMed-cached abstract directly tests the process-level
      inference from PMID:12538697 in human serum and supports narrowing it to a
      soluble protease interaction rather than lectin-pathway regulation.
- id: PMID:23533145
  title: In-depth proteomic analyses of exosomes isolated from expressed prostatic
    secretions in urine.
  findings: []
- id: PMID:25037231
  title: Extracellular matrix signatures of human primary metastatic colon cancers
    and their metastases to liver.
  findings: []
- id: PMID:28327460
  title: Comprehensive proteomic characterization of stem cell-derived extracellular
    matrices.
  findings: []
- id: PMID:28675934
  title: Characterization of the Extracellular Matrix of Normal and Diseased Tissues
    Using Proteomics.
  findings: []
- id: PMID:32814053
  title: Interactome Mapping Provides a Network of Neurodegenerative Disease Proteins
    and Uncovers Widespread Protein Aggregation in Affected Brains.
  findings:
  - statement: The study reports a large systematic yeast two-hybrid candidate
      interaction network rather than an A2M-specific mechanism.
    supporting_text: Here, we report on an interactome map that focuses on
      neurodegenerative disease (ND), connects ∼5,000 human proteins via ∼30,000
      candidate interactions and is generated by systematic yeast two-hybrid
      interaction screening of ∼500 ND-related proteins and integration of
      literature interactions.
    reference_section_type: ABSTRACT
  full_text_unavailable: true
  reference_review:
    relevance: LOW
    correctness: UNVERIFIED
    review_notes: The accessible abstract verifies the screen scale, but does
      not expose the A2M-specific binary records. The generic binding tuple is
      therefore not treated as mechanistic evidence.
- id: PMID:33964423
  title: Structural Investigations of Human A2M Identify a Hollow Native Conformation
    That Underlies Its Distinctive Protease-Trapping Mechanism.
  findings:
  - statement: Bait-region cleavage rearranges A2M's disulfide-linked dimers and
      collapses the tetramer around an internal protease-trapping cavity.
    supporting_text: These changes collapse the tetramer into a more compact
      conformation, which encloses an interior protease-trapping cavity.
    reference_section_type: ABSTRACT
  - statement: LRP1 recognizes activated A2M conformations but not native A2M.
    supporting_text: Native A2M does not interact with LRP1, whereas both A2M-MA
      and A2M-protease complexes do.
    reference_section_type: INTRODUCTION
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Full text is cached. The combined structural study directly
      supports the native-to-collapsed transition and independently summarizes
      the established activation dependence of LRP1 binding.
- id: PMID:34139236
  title: Development of selective protease inhibitors via engineering of the bait
    region of human α(2)-macroglobulin.
  findings:
  - statement: A2M collapses around proteases and sequesters them from protein
      substrates rather than blocking the catalytic site directly.
    supporting_text: A2M has a unique mechanism of inhibition where it collapses
      around proteases and irreversibly sequesters them from protein substrates
      (5).
    reference_section_type: INTRODUCTION
  - statement: The permissive wild-type bait region makes A2M a broad-spectrum
      inhibitor, while engineered substrate sequences redirect its protease
      selectivity.
    supporting_text: As the wild-type bait region is permissive to cleavage by
      most human proteases, A2M is accordingly a broad-spectrum protease
      inhibitor.
    reference_section_type: ABSTRACT
  - statement: Proteolytic activation exposes an LRP1-binding site that targets
      A2M-protease complexes for internalization and degradation.
    supporting_text: As a consequence of its conformational change, A2M reveals
      a binding site that interacts with a scavenger receptor, low-density
      lipoprotein receptor-related protein 1 (LRP1), and this interaction quickly
      removes A2M-protease complexes from circulation through internalization and
      endosomal degradation, primarily in the liver (12, 13).
    reference_section_type: INTRODUCTION
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Full text is cached. Bait specificity is established
      experimentally in this study; the LRP1-clearance statement summarizes
      earlier work and is corroborated by PMID:1691187 and PMID:15226301.
- id: PMID:6698368
  title: 'Mesotrypsin: a new inhibitor-resistant protease from a zymogen in human
    pancreatic tissue and fluid.'
  findings:
  - statement: The accessible abstract characterizes mesotrypsin inhibitor
      resistance but does not expose an A2M-specific experiment.
    supporting_text: The enzyme was poorly inhibited by soybean trypsin
      inhibitor, chicken ovomucoid, human serum alpha 1-proteinase inhibitor and
      human pancreatic secretory trypsin inhibitor.
    reference_section_type: ABSTRACT
  full_text_unavailable: true
  reference_review:
    relevance: LOW
    correctness: UNVERIFIED
    review_notes: The cached record is abstract-only and never mentions A2M, so
      the curator's full-text evidence cannot be assessed and the annotation is
      left UNDECIDED rather than removed.
- id: PMID:7679575
  title: Characterization of the antiplasmin activity of human thrombospondin-1 in
    solution.
  findings: []
- id: PMID:9398211
  title: Human alpha 2-macroglobulin is an osteogenic growth peptide-binding protein.
  findings:
  - statement: Osteogenic growth peptide binds both native and activated A2M in
      human plasma.
    supporting_text: We show that OGP binds to both native and activated human
      plasma alpha 2-macroglobulin (alpha 2M).
    reference_section_type: ABSTRACT
  full_text_unavailable: true
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Direct binding supports a secondary ligand-carriage activity;
      the native and activated conformations have different downstream effects.
- id: PMID:9714181
  title: A modified human alpha 2-macroglobulin derivative that binds tumor necrosis
    factor-alpha and interleukin-1 beta with high affinity in vitro and reverses lipopolysaccharide
    toxicity in vivo in mice.
  findings:
  - statement: Chemical activation produces an A2M conformation with greatly
      enhanced TNF-alpha and IL-1-beta binding relative to native A2M.
    supporting_text: The equilibrium dissociation constant (KD) for the binding
      of TNF-alpha to MAC was 80 +/- 20 nM, reflecting a 100-fold increase in
      affinity compared with native alpha 2M.
    reference_section_type: ABSTRACT
  full_text_unavailable: true
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: The paper directly supports state-dependent cytokine binding,
      but its strongest affinity and in-vivo rescue results concern a chemically
      modified A2M derivative rather than native A2M.
- id: Reactome:R-HSA-1454781
  title: MMP1,3,13 (2, 7-12, 19) binding by Alpha-2 macroglubulin
  findings: []
- id: Reactome:R-HSA-158340
  title: kallikrein + alpha2-macroglobulin -> kallikrein:alpha2-macrogloulin
  findings: []
- id: Reactome:R-HSA-481007
  title: Exocytosis of platelet alpha granule contents
  findings: []
core_functions:
- description: >-
    Broad-spectrum extracellular endopeptidase inhibition by conformational
    trapping. Protease cleavage of A2M's exposed bait region collapses the
    disulfide-linked tetramer around the enzyme, sequestering it from protein
    substrates; the internal thioester can additionally capture the protease
    covalently. Because susceptibility is determined largely by the bait
    sequence, A2M inhibits endopeptidases from multiple catalytic classes.
  molecular_function:
    id: GO:0004866
    label: endopeptidase inhibitor activity
  directly_involved_in:
  - id: GO:0010951
    label: negative regulation of endopeptidase activity
  locations:
  - id: GO:0005576
    label: extracellular region
  supported_by:
  - reference_id: PMID:33964423
    supporting_text: These changes collapse the tetramer into a more compact
      conformation, which encloses an interior protease-trapping cavity.
  - reference_id: PMID:34139236
    supporting_text: As the wild-type bait region is permissive to cleavage by
      most human proteases, A2M is accordingly a broad-spectrum protease
      inhibitor.
  - reference_id: PMID:18485748
    supporting_text: Furthermore, a(2)M inhibited both ADAMTS-7- and
      ADAMTS-12-mediated COMP degradation in a concentration (or dose)-dependent
      manner.
- description: >-
    Sequesters bait-cleaving proteases inside the collapsed A2M tetramer so they
    cannot reach protein substrates. This steric trap is mechanistically
    distinct from active-site blockade; noncovalent trapping has been reported
    for tetrameric A2M even though covalent capture often accompanies activation.
  molecular_function:
    id: GO:0140311
    label: protein sequestering activity
  locations:
  - id: GO:0005576
    label: extracellular region
  supported_by:
  - reference_id: PMID:34139236
    supporting_text: A2M has a unique mechanism of inhibition where it collapses
      around proteases and irreversibly sequesters them from protein substrates
      (5).
  - reference_id: PMID:34139236
    supporting_text: Protease trapping is associated with covalent conjugation
      of the protease by a thiol ester bond that is initially buried within A2M
      and exposed upon its proteolysis-induced conformational change; although
      covalent conjugation is required for protease trapping by the monomeric
      αMs, tetramers such as A2M can trap proteases noncovalently (8, 9, 10, 11).
  - reference_id: PMID:33964423
    supporting_text: Protease inhibition by A2M and other αMs is enacted by a
      unique trapping mechanism whereby proteases are sequestered inside the αM
      protease inhibitor, restricting their access to protein substrates (2).
- description: >-
    Binds LRP1 only after bait cleavage or chemical activation exposes the
    receptor-binding determinant. This conformation-dependent interaction
    targets activated A2M and A2M-protease complexes for receptor-mediated
    internalization and degradation.
  molecular_function:
    id: GO:0050750
    label: low-density lipoprotein particle receptor binding
  locations:
  - id: GO:0005576
    label: extracellular region
  supported_by:
  - reference_id: PMID:1691187
    supporting_text: The purified 420-kD protein binds to the conformationally
      altered forms of alpha 2M that are known to specifically interact with
      alpha 2M receptors and does not bind to native alpha 2M.
  - reference_id: PMID:34139236
    supporting_text: As a consequence of its conformational change, A2M reveals
      a binding site that interacts with a scavenger receptor, low-density
      lipoprotein receptor-related protein 1 (LRP1), and this interaction quickly
      removes A2M-protease complexes from circulation through internalization and
      endosomal degradation, primarily in the liver (12, 13).
proposed_new_terms: []
suggested_questions:
- question: >-
    What fraction of physiological A2M inhibition requires thioester-mediated
    covalent attachment rather than noncovalent entrapment by the tetramer for
    each major class of circulating or extracellular protease?
- question: >-
    In which fluid-phase or tissue contexts, if any, does A2M materially restrain
    MASP1 without suppressing surface lectin-pathway complement activation?
- question: >-
    Which protease-bound A2M complexes dominate LRP1-dependent hepatic clearance
    under normal, inflammatory, and tissue-injury conditions?
- question: >-
    Which reported cytokine, growth-factor, and plasma-enzyme interactions occur
    with native A2M at physiological concentrations, and which require an
    activated or chemically modified conformation?
suggested_experiments:
- hypothesis: >-
    Native tetrameric A2M inhibits many proteases without thioester-mediated
    covalent attachment, whereas bait cleavage and conformational collapse are
    universally required.
  description: >-
    Compare wild-type A2M with bait-resistant, thioester-deficient, and
    collapse-defective recombinant variants across representative serine,
    cysteine, metallo, and aspartic endopeptidases. Measure bait cleavage,
    conformational change, covalent adduct formation, protease capture, and
    residual cleavage of protein substrates under plasma-like conditions.
- hypothesis: >-
    Only activated A2M and A2M-protease complexes undergo efficient
    LRP1-dependent hepatic uptake and degradation.
  description: >-
    Quantify uptake, endosomal delivery, and degradation of labeled native A2M,
    methylamine-activated A2M, and defined A2M-protease complexes in primary
    human hepatocytes or liver organoids with LRP1 knockout and receptor-rescue
    controls.
- hypothesis: >-
    A2M can trap soluble MASP1 without measurably suppressing activator-surface
    lectin-pathway signaling in human serum.
  description: >-
    Compare fluid-phase MASP1 complex formation, protease activity, and C3/C4
    deposition on defined activator surfaces in A2M-depleted human serum,
    matched untreated serum, and depleted serum reconstituted with purified
    wild-type or bait-resistant A2M.

Gene Description

Existing Annotations Review

Core Functions

Sequesters bait-cleaving proteases inside the collapsed A2M tetramer so they cannot reach protein substrates. This steric trap is mechanistically distinct from active-site blockade; noncovalent trapping has been reported for tetrameric A2M even though covalent capture often accompanies activation.

Binds LRP1 only after bait cleavage or chemical activation exposes the receptor-binding determinant. This conformation-dependent interaction targets activated A2M and A2M-protease complexes for receptor-mediated internalization and degradation.

References

Suggested Questions for Experts

Suggested Experiments

📚 Additional Documentation

Notes

A2M review notes

2026-08-08 — initial review

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