Affinage

GM2A

Ganglioside GM2 activator · UniProt P17900

Length
193 aa
Mass
20.8 kDa
Annotated
2026-06-10
14 papers in source corpus 7 papers cited in narrative 7 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 5/5 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

GM2A encodes the GM2 activator protein (GM2-AP), a non-enzymatic lysosomal lipid-binding cofactor that presents membrane-embedded glycosphingolipid substrates to their degradative enzymes (PMID:10942779, PMID:7713516). GM2-AP forms a soluble complex with GM2 ganglioside that enables hydrolysis by beta-hexosaminidase A, and more broadly stimulates degradation of membrane-bound gangliosides such as GM1 by acid beta-galactosidase; this activity requires the endolysosome-specific anionic phospholipids bis(monoacylglycero)phosphate and phosphatidylinositol, which promote binding of both the enzyme and GM2-AP to substrate-carrying membranes (PMID:10942779, PMID:7713516). The substrate-recognition function is modular: an alternatively spliced form spanning amino acids 1–109 selectively stimulates sialidase-mediated removal of NeuAc from GM2 without supporting GalNAc cleavage, localizing the NeuAc-recognition domain to this region (PMID:8631864). Loss-of-function mutations that abolish GM2-AP protein block GM2 catabolism and cause lysosomal ganglioside accumulation, and GM2-AP abundance depends post-translationally on the chaperone GRP94 (PMID:28417072, PMID:31578452). Beyond its catabolic role, GM2A restricts phagosomal escape of Listeria monocytogenes in macrophages downstream of Steap3 (PMID:35569749), and elevated GM2-AP is directly neurotoxic, impairing neurite integrity and neuronal firing (PMID:36131294).

Mechanistic history

Synthesis pass · year-by-year structured walk · 7 steps
  1. 1994 Medium

    Established that GM2-AP works by forming a physical substrate complex with GM2 ganglioside that licenses its enzymatic degradation, framing the protein as a substrate-presenting cofactor rather than an enzyme.

    Evidence cDNA cloning of mouse Gm2a, expression and tissue-distribution analysis

    PMID:7713516

    Open questions at the time
    • Structural basis of GM2 binding not resolved
    • Did not define the lipid/membrane requirements for activity
  2. 1996 High

    Resolved which part of the protein recognizes substrate by mapping the NeuAc-recognition domain to amino acids 1–109 and showing this region stimulates sialidase but not hexosaminidase activity, demonstrating functional modularity.

    Evidence Recombinant expression of a splice form in E. coli with in vitro enzymatic assays against distinct substrates

    PMID:8631864

    Open questions at the time
    • Physiological relevance of the sialidase-stimulating isoform in vivo not established
    • Did not map the GalNAc/hexosaminidase-relevant determinants
  3. 2000 High

    Defined the membrane-lipid context required for activator function, showing that endolysosome-specific anionic phospholipids (BMP, PI) are essential and act by recruiting both GM2-AP and the hydrolase to substrate-bearing membranes.

    Evidence Detergent-free liposomal enzymatic assay with defined lipids plus surface plasmon resonance binding measurements

    PMID:10942779

    Open questions at the time
    • Broadened substrate scope to GM1/beta-galactosidase but did not establish in-cell stoichiometry
    • Molecular geometry of the lipid-protein-enzyme ternary complex not solved
  4. 2017 Medium

    Linked human GM2A mutations causally to disease mechanism by showing that nonsense and missense alleles abolish GM2-AP protein and block GM2 catabolism in patient cells, confirming the enzyme-presentation model is essential in vivo.

    Evidence Western blot, fluorescent ganglioside metabolic labeling, and immunohistochemistry in patient-derived fibroblasts

    PMID:28417072

    Open questions at the time
    • Single patient/lab
    • Did not distinguish failed synthesis from misfolding/degradation for each allele
  5. 2019 Medium

    Identified a post-translational determinant of GM2-AP abundance, showing GRP94 chaperone activity is required to maintain GM2-AP protein and thereby HexA-mediated GM2 turnover, independent of GM2A transcription.

    Evidence GRP94 knockdown/knockout cell lines with lipidomics, Western blot, and hexosaminidase activity assays

    PMID:31578452

    Open questions at the time
    • Chaperone-client relationship inferred, not reconstituted in vitro
    • Direct GRP94-GM2AP binding not demonstrated
  6. 2022 Medium

    Extended GM2A function beyond ganglioside catabolism to innate immunity, placing it downstream of Steap3 in a lysosomal pathway that restricts Listeria monocytogenes phagosomal escape.

    Evidence Label-free proteomics of Steap3-KO macrophages plus Gm2a loss-of-function phagosomal escape assays

    PMID:35569749

    Open questions at the time
    • Molecular mechanism connecting GM2A lipid activity to escape restriction unknown
    • Single lab; direct Steap3-GM2A relationship not biochemically defined
  7. 2022 Medium

    Revealed a gain-of-function neurotoxic activity, showing elevated extracellular GM2-AP is sufficient to degrade neurite integrity and suppress neuronal firing.

    Evidence MEA recordings and live-cell neurite imaging in rat cortical and human iPSC-derived neurons treated with cell-derived GM2A and brain extracts

    PMID:36131294

    Open questions at the time
    • Receptor or molecular mechanism downstream of neurotoxicity not identified
    • Relationship between neurotoxic activity and lysosomal cofactor role unresolved

Open questions

Synthesis pass · forward-looking unresolved questions
  • How GM2A's lipid-presentation biochemistry mechanistically connects to its non-lysosomal roles in bacterial restriction and neurotoxicity remains unresolved.
  • No structural model of the GM2AP-lipid-enzyme ternary complex
  • No receptor or effector identified for the neurotoxic and anti-bacterial activities
  • Direct GRP94-GM2AP and Steap3-GM2A physical interactions not biochemically reconstituted

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0008289 lipid binding 3 GO:0098772 molecular function regulator activity 3 GO:0060090 molecular adaptor activity 2
Localization
GO:0005764 lysosome 2
Pathway
R-HSA-1430728 Metabolism 3 R-HSA-168256 Immune System 1
Partners
GRP94

Evidence

Reading pass · 7 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2000 GM2 activator protein (GM2-AP) stimulates the enzymatic degradation of membrane-bound ganglioside GM1 by acid beta-galactosidase in a liposomal, detergent-free assay system. Anionic phospholipids bis(monoacylglycero)phosphate and phosphatidylinositol, which specifically occur in inner membranes of endosomes and lysosomes, are essential for this activator-stimulated hydrolysis. Surface plasmon resonance spectroscopy showed that bis(monoacylglycero)phosphate increases binding of both beta-galactosidase and GM2-AP to substrate-carrying membranes. Liposomal detergent-free enzymatic assay, surface plasmon resonance spectroscopy The Journal of biological chemistry High 10942779
1994 The GM2 activator protein forms a substrate-complex with GM2 ganglioside, which enables degradation of the ganglioside by beta-hexosaminidase A. Mouse Gm2a cDNA encodes an orthologous protein; its transcript (~2.3 kb) is expressed in all tissues, most abundantly in kidney and testis. cDNA cloning, expression analysis, chromosome mapping Genomics Medium 7713516
1996 An alternatively spliced form of GM2 activator protein, GM2A protein (amino acids 1–109 plus a C-terminal VST tripeptide encoded by intron 3), was expressed recombinantly in E. coli. GM2A protein stimulates hydrolysis of NeuAc from GM2 by clostridial sialidase but does NOT stimulate hydrolysis of GalNAc from GM2 by beta-hexosaminidase A, establishing that the NeuAc recognition domain of GM2 activator protein resides within amino acids 1–109. Recombinant protein expression in E. coli, in vitro enzymatic activity assay with purified protein The Journal of biological chemistry High 8631864
2017 Loss-of-function mutations in GM2A (nonsense p.E87X and missense p.P55L) result in absence of GM2-AP protein in patient fibroblasts (by Western blot), indicating both mutations interfere with synthesis and/or folding of the protein. Impaired catabolism of GM2 ganglioside in patient fibroblasts was demonstrated by metabolic labeling with fluorescently labeled GM1 ganglioside and immunohistochemistry with anti-GM2 and anti-GM3 antibodies. Western blot, metabolic labeling with fluorescent ganglioside, immunohistochemistry in patient-derived fibroblasts Molecular genetics and metabolism reports Medium 28417072
2019 GRP94 ablation in brain metastasis cells reduces GM2-AP protein abundance without altering GM2A gene expression, suggesting GM2-AP is a client/substrate of the GRP94 chaperone. Loss of GM2-AP leads to reduced HexA-mediated GM2 hydrolysis, resulting in lysosomal accumulation of GM2 ganglioside. Lipidomic analysis, Western blot, enzymatic activity assay for beta-hexosaminidase, GRP94 knockdown/knockout cell lines Scientific reports Medium 31578452
2022 Elevated GM2A protein from human brain extracts is sufficient to induce loss of neurite integrity and reduction in spontaneous neuronal firing when applied to cultured rat cortical neurons (MEA) and human iPSC-derived neurons, demonstrating a direct neurotoxic activity of GM2A on neuronal structure and function. Multi-electrode array (MEA) recording, live-cell imaging of neurite integrity, treatment with cell-derived GM2A protein and brain extracts Molecular neurodegeneration Medium 36131294
2022 Gm2a inhibits the phagosomal escape of Listeria monocytogenes in macrophages. Proteomic analysis of Steap3-knockout macrophages revealed decreased Gm2a abundance, and functional experiments showed that Gm2a loss (like Steap3 loss) facilitated bacterial escape from the phagosome into the cytoplasm, placing Gm2a downstream of Steap3 in a lysosomal signaling pathway that restricts L. monocytogenes. Label-free quantitative proteomics, Steap3 deletion in macrophages, functional phagosomal escape assay, loss-of-function (Gm2a deletion) Microbes and infection Medium 35569749

Source papers

Stage 0 corpus · 14 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2000 Degradation of membrane-bound ganglioside GM1. Stimulation by bis(monoacylglycero)phosphate and the activator proteins SAP-B and GM2-AP. The Journal of biological chemistry 73 10942779
1999 Structure of the GM2A gene: identification of an exon 2 nonsense mutation and a naturally occurring transcript with an in-frame deletion of exon 2. American journal of human genetics 22 10364519
2022 Elevated ganglioside GM2 activator (GM2A) in human brain tissue reduces neurite integrity and spontaneous neuronal activity. Molecular neurodegeneration 17 36131294
2017 Atypical juvenile presentation of GM2 gangliosidosis AB in a patient compound-heterozygote for c.259G > T and c.164C > T mutations in the GM2A gene. Molecular genetics and metabolism reports 17 28417072
2000 The GM2 gangliosidoses databases: allelic variation at the HEXA, HEXB, and GM2A gene loci. Genetics in medicine : official journal of the American College of Medical Genetics 16 11339652
1993 Regional localization of the gene coding for the GM2 activator protein (GM2A) to chromosome 5q32-33 and confirmation of the assignment of GM2AP to chromosome 3. Annals of human genetics 13 8257088
2015 Mutation in GM2A Leads to a Progressive Chorea-dementia Syndrome. Tremor and other hyperkinetic movements (New York, N.Y.) 11 26203402
1996 Characterization of an alternatively spliced GM2 activator protein, GM2A protein. An activator protein which stimulates the enzymatic hydrolysis of N-acetylneuraminic acid, but not N-acetylgalactosamine, from GM2. The Journal of biological chemistry 10 8631864
1993 Refined mapping of the GM2 activator protein (GM2A) locus to 5q31.3-q33.1, distal to the spinal muscular atrophy locus. Genomics 10 8288250
2019 GM2-GM3 gangliosides ratio is dependent on GRP94 through down-regulation of GM2-AP cofactor in brain metastasis cells. Scientific reports 9 31578452
2021 Two patients from Turkey with a novel variant in the GM2A gene and review of the literature. Journal of pediatric endocrinology & metabolism : JPEM 8 33819415
2022 Label-free quantitative proteomics reveals the Steap3-Gm2a axis inhibiting the phagosomal escape of Listeria monocytogenes. Microbes and infection 7 35569749
1994 The mouse gene encoding the GM2 activator protein (Gm2a): cDNA sequence, expression, and chromosome mapping. Genomics 7 7713516
2012 Integrated multiplex ligation dependent probe amplification (MLPA) assays for the detection of alterations in the HEXB, GM2A and SMARCAL1 genes to support the diagnosis of Morbus Sandhoff, M. Tay-Sachs variant AB and Schimke immuno-osseous dysplasia in humans. Molecular and cellular probes 3 23010210

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