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.

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

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