Affinage

SPART

Spartin · UniProt Q8N0X7

Audit flag: ungrounded claim
Length
666 aa
Mass
72.8 kDa
Annotated
2026-06-10
27 papers in source corpus 14 papers cited in narrative 14 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 8/8 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

Spartin (SPART/SPG20) is a multifunctional adaptor protein that couples membrane and organelle quality control to the ubiquitin system, and its loss causes the autosomal recessive hereditary spastic paraplegia Troyer syndrome (PMID:12134148). Through its N-terminal MIT domain it binds the ESCRT-III protein IST1 with micromolar affinity, an interaction required for spartin's recruitment to the midbody and for completion of cytokinesis; a structure-based MIT mutation that abolishes IST1 binding blocks midbody localization and acts dominant-negatively (PMID:20719964). A central feature of spartin function is its role as a recruitment platform for HECT-family E3 ubiquitin ligases ITCH (AIP4) and WWP1 (AIP5), which it engages via a PPXY motif while itself acting as an adaptor rather than an obligate substrate (PMID:19580544). At lipid droplets, spartin binds TIP47, competes with adipophilin for droplet occupancy, and controls lipid droplet size and number through ITCH/WWP1-dependent ubiquitination (PMID:19307600). On damaged lysosomes, spartin senses lipid-packing defects through amphipathic helices, binds IST1, and recruits and activates ITCH to deposit K63-linked ubiquitin chains that initiate lysophagy (PMID:38503285). The same ITCH-targeting activity underlies antiviral restriction: spartin redistributes ITCH away from endosomes to enable ITCH-mediated ubiquitination of the orthoflavivirus capsid and viral uncoating, establishing spartin as a pan-orthoflavivirus restriction factor (PMID:42161262). Independently, its C-terminal plant-related senescence domain binds cardiolipin and supports mitochondrial membrane potential, calcium uptake, complex I activity, and nuclear-encoded mitochondrial protein import, including the CoQ biosynthetic enzymes COQ7 and COQ9 (PMID:21559443, PMID:31314595, PMID:37433530). In vivo, Spg20-deficient mice recapitulate impaired cytokinesis, increased neuronal axon branching reversed by IST1-binding-competent spartin, and lipid droplet accumulation (PMID:22619377).

Mechanistic history

Synthesis pass · year-by-year structured walk · 13 steps
  1. 2002 Medium

    Established the gene-disease link and first functional context: a frameshift mutation in SPG20 causes Troyer syndrome, with sequence similarity to endosomal trafficking and spastin-like HSP proteins pointing toward membrane biology.

    Evidence Genetic mapping, sequencing, and comparative sequence analysis in Troyer syndrome families

    PMID:12134148

    Open questions at the time
    • Function inferred from homology rather than biochemistry
    • No direct demonstration of trafficking activity
  2. 2005 Medium

    Provided the first direct interaction partner, linking spartin to endocytic machinery and showing it has both cytosolic and membrane-associated pools.

    Evidence Yeast two-hybrid of human brain library with pulldown and redistribution validation

    PMID:16036216

    Open questions at the time
    • No reciprocal Co-IP
    • Functional consequence of Eps15 interaction not established
  3. 2009 High

    Defined spartin as a lipid-droplet regulator and an adaptor for HECT E3 ligases, showing it binds TIP47, competes with adipophilin, and engages ITCH/WWP1 via a PPXY motif while acting as an adaptor rather than obligate substrate.

    Evidence Co-IP, RNAi, LD imaging, ubiquitination assays, and PPXY mutagenesis on endogenous protein across two labs

    PMID:19307600 PMID:19580544

    Open questions at the time
    • Identity of ITCH/WWP1 substrates at LDs not yet defined
    • How spartin selects between the two ligases unclear
  4. 2010 High

    Resolved the mechanism of spartin's role in cell division: the MIT domain binds ESCRT-III IST1 to target spartin to the midbody and complete cytokinesis.

    Evidence SPR binding kinetics, structure-based F24D mutagenesis, siRNA, and co-localization imaging

    PMID:20719964

    Open questions at the time
    • Downstream effector of midbody spartin not defined
    • Relationship between MIT/IST1 axis and the LD/ligase functions unresolved
  5. 2011 Medium

    Identified a distinct mitochondrial function: the PRS domain binds cardiolipin, and spartin loss depolarizes mitochondria and impairs calcium uptake, separating membrane lipid sensing from the MIT/ESCRT activity.

    Evidence Lipid-binding assay with purified PRS domain plus siRNA functional readouts in neuroblastoma

    PMID:21559443

    Open questions at the time
    • Single lab
    • Mechanistic link between cardiolipin binding and membrane potential not delineated
  6. 2011 Medium

    Showed that epigenetic silencing of SPG20 by promoter hypermethylation phenocopies depletion, causing cytokinesis arrest reversible by demethylation, tying spartin loss to a cancer-relevant proliferation defect.

    Evidence Methylation-specific PCR, siRNA, and cell-cycle analysis with methylation-inhibition rescue

    PMID:21499309

    Open questions at the time
    • Does not establish direct oncogenic mechanism
    • Single lab
  7. 2012 High

    Validated multiple spartin functions in vivo and established that the IST1 interaction is required to restrain neuronal axon branching, integrating cytokinesis, neuronal, BMP-signaling, and lipid-droplet phenotypes in a knockout animal.

    Evidence Spg20-/- mice with primary neuronal/fibroblast culture, imaging, Western blot, and spartin rescue

    PMID:22619377

    Open questions at the time
    • BMP/Smad effect attributed to altered receptor trafficking but mechanism not resolved
    • Adipose LD phenotype not linked to specific ligase activity
  8. 2018 Medium

    Placed spartin loss upstream of EGFR/MAPK signaling, showing knockout activates this pathway to drive proliferation reversible by EGFR or ERK1/2 inhibition.

    Evidence SPG20 knockout cells, proliferation/colony assays, pathway Westerns, pharmacological rescue, and xenografts

    PMID:29673586

    Open questions at the time
    • Direct molecular link between spartin and EGFR not defined
    • Single lab
  9. 2019 Medium

    Deepened the mitochondrial phenotype, showing SPART loss reduces complex I activity and ATP synthesis, increases ROS, and disrupts Ca2+ homeostasis, with rescue restoring calcium handling.

    Evidence SH-SY5Y SPART frameshift model with complex I, ATP, membrane potential, ROS, Ca2+ assays and rescue

    PMID:31314595

    Open questions at the time
    • Whether mitochondrial defects are primary or secondary to import failure not resolved
    • Single lab
  10. 2023 Medium

    Provided a mechanistic basis for the mitochondrial bioenergetic defect: mutant SPART impairs import of nuclear-encoded proteins including COQ7/COQ9, depleting CoQ, with CoQ supplementation restoring ATP comparably to wild-type rescue.

    Evidence Patient fibroblasts, mitochondrial import assay, MS proteomics, CoQ measurement, ATP assay, rescue

    PMID:37433530

    Open questions at the time
    • How spartin supports the import machinery mechanistically unknown
    • Single lab
  11. 2024 High

    Unified spartin's membrane-sensing and ligase-recruitment functions in lysophagy: amphipathic helices detect lipid-packing defects on damaged lysosomes, and spartin binds IST1 and activates ITCH to deposit K63 ubiquitin chains initiating lysosomal autophagy.

    Evidence Cell-based LLOMe damage assays, Co-IP, ubiquitin linkage typing, amphipathic helix mutagenesis, ITCH recruitment assays

    PMID:38503285

    Open questions at the time
    • Identity of the ubiquitinated lysosomal substrate(s) not defined
    • Threshold determining ITCH activation not quantified
  12. 2025 High

    Demonstrated that a viral protease exploits the spartin-ITCH lipid-droplet pathway: HCV NS3/4A cleaves SPG20 at Cys504/Cys562, disrupting TIP47 binding and ITCH recruitment to block ADRP degradation and promote large LD formation.

    Evidence HCV infection/NS3-4A transfection, cleavage-site mutagenesis, Co-IP, ITCH siRNA, immunofluorescence

    PMID:40985721

    Open questions at the time
    • Whether cleavage benefits viral replication directly not established here
    • Single lab
  13. 2026 High

    Established spartin as a pan-orthoflavivirus restriction factor, showing it sequesters ITCH from endosomes to enable ITCH ubiquitination of viral capsid and uncoating, with epistasis confirming an ITCH-dependent mechanism.

    Evidence Genome-wide CRISPRa screen, SPART/ITCH single and double knockouts, Spg20-/- and Itch-/- mice, viral assays, capsid ubiquitination

    PMID:42161262

    Open questions at the time
    • How spartin spatially redistributes ITCH at the molecular level not detailed
    • Breadth across other virus families not tested

Open questions

Synthesis pass · forward-looking unresolved questions
  • It remains unresolved how spartin's distinct domain activities — MIT/IST1 targeting, PPXY-mediated E3 ligase recruitment, amphipathic membrane sensing, and PRS/cardiolipin-dependent mitochondrial support — are coordinated and which contributes to Troyer syndrome neuropathology.
  • No structural model integrating the multiple domains
  • Tissue-specific determinant of motor-neuron vulnerability unknown
  • Direct ubiquitination substrates at each membrane site incompletely defined

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0060090 molecular adaptor activity 2 GO:0098772 molecular function regulator activity 2 GO:0008289 lipid binding 1 GO:0140299 molecular sensor activity 1
Localization
GO:0005739 mitochondrion 2 GO:0005811 lipid droplet 2 GO:0005829 cytosol 2 GO:0005764 lysosome 1 GO:0005768 endosome 1
Pathway
R-HSA-1430728 Metabolism 3 R-HSA-1640170 Cell Cycle 3 R-HSA-392499 Metabolism of proteins 2 R-HSA-168256 Immune System 1 R-HSA-9612973 Autophagy 1
Partners
EPS15IST1ITCHTIP47WWP1

Evidence

Reading pass · 14 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2002 SPG20 encodes spartin, which by comparative sequence analysis shares similarity with molecules involved in endosomal trafficking and with spastin (a microtubule-interacting HSP protein); a frameshift mutation in SPG20 causes Troyer syndrome. Genetic mapping, sequencing, and comparative sequence analysis Nature genetics Medium 12134148
2005 Spartin (SPG20) interacts with Eps15, a protein involved in endocytosis and cell proliferation; spartin is both cytosolic and membrane-associated. Yeast two-hybrid screen of adult human brain library, fusion protein pulldown, cellular redistribution assay, anti-spartin antibody fractionation Biochemical and biophysical research communications Medium 16036216
2009 Spartin associates with lipid droplet surfaces and regulates lipid droplet size and number; it binds TIP47 and competes with adipophilin for LD occupancy; WWP1 (AIP5) binds spartin, transfers ubiquitin to it, removes it from LDs, and reduces its levels. The Troyer syndrome truncation mutant lacks these activities. Co-IP, RNA interference knockdown, overexpression, lipid droplet imaging, ubiquitination assays The Journal of cell biology High 19307600
2009 Endogenous spartin localizes to a cytosolic pool that is recruited to endosomes and lipid droplets; cytosolic spartin is mono-ubiquitinated; spartin interacts via a PPXY motif with ubiquitin E3 ligases AIP4 (ITCH) and AIP5 (WWP1); the PPXY motif and these ligases are not required for spartin's own ubiquitination, suggesting spartin acts as an adaptor. Subcellular fractionation, immunofluorescence, endogenous protein immunoprecipitation, PPXY motif mutagenesis The Biochemical journal High 19580544
2010 The spartin MIT domain binds ESCRT-III protein IST1 with micromolar affinity (but not CHMP1-7); spartin co-localizes with IST1 at the midbody; IST1 depletion reduces spartin at midbodies; spartin depletion impairs cytokinesis; a structure-based F24D mutation in the MIT domain blocks the spartin–IST1 interaction, prevents midbody localization, and acts dominant-negatively to impair cytokinesis. Yeast two-hybrid, in vitro surface plasmon resonance, siRNA knockdown, immunofluorescence co-localization, dominant-negative mutation (F24D) Molecular biology of the cell High 20719964
2011 The plant-related senescence (PRS) domain of spartin interacts specifically with cardiolipin but not phosphatidylcholine or phosphatidylethanolamine; spartin knockdown depolarizes mitochondrial membrane potential and significantly decreases mitochondrial calcium uptake. Lipid-binding assay with purified PRS domain, siRNA knockdown in neuroblastoma cells, mitochondrial membrane potential measurements, mitochondrial calcium uptake assay PloS one Medium 21559443
2011 SPG20 promoter hypermethylation silences spartin expression; spartin depletion in cancer cells causes cytokinesis arrest, reversed by inhibiting SPG20 methylation. Methylation-specific PCR, qRT-PCR, siRNA knockdown, cell cycle analysis Oncogene Medium 21499309
2012 Spg20-/- mice show: (1) impaired cytokinesis in embryonic fibroblasts and binucleated chondrocytes in bone growth plates; (2) increased axon branching in cortical neurons reversed by reintroduction of spartin requiring its IST1 interaction; (3) elevated BMP/Smad1/5 phosphorylation in fibroblasts suggesting altered BMP receptor trafficking; (4) increased lipid droplet numbers and altered perilipin levels in adipose tissue. Spg20 knockout mice, primary neuronal culture, primary fibroblast culture, immunofluorescence, Western blot, rescue with wild-type spartin Human molecular genetics High 22619377
2019 SPART loss-of-function leads to decreased mitochondrial complex I (NADH dehydrogenase) activity, decreased ATP synthesis, defective mitochondrial membrane potential, increased ROS, and altered intracellular Ca2+ homeostasis; re-expression of wild-type spartin restores Ca2+ homeostasis. SH-SY5Y cell model with SPART frameshift mutation, enzymatic complex I activity assay, ATP synthesis assay, mitochondrial membrane potential measurement, ROS detection, Ca2+ imaging, wild-type rescue FASEB journal Medium 31314595
2018 Knockout of SPG20/spartin activates the EGFR/MAPK signaling pathway to promote cell proliferation; EGFR kinase or ERK1/2 inhibitors rescue the proliferation phenotype caused by spartin loss. SPG20 knockout cells, in vitro proliferation assays, colony formation, Western blot for pathway activation, pharmacological inhibition with EGFR and ERK1/2 inhibitors, in vivo xenografts Biochemical and biophysical research communications Medium 29673586
2023 Mutant SPART causes impaired mitochondrial import of nuclear-encoded proteins, leading to significant decrease of CoQ10 biosynthesis enzymes COQ7 and COQ9 and severe reduction in CoQ content; CoQ supplementation restores cellular ATP levels to the same extent as re-expression of wild-type SPART. Patient-derived fibroblasts, mitochondrial protein import assay, mass spectrometry proteomics, CoQ content measurement, ATP assay, wild-type SPART rescue Open biology Medium 37433530
2024 SPG20 detects lipid-packing defects in damaged lysosomal membranes via sensory amphipathic helices before membrane rupture; SPG20 binds IST1 on damaged lysosomes and, when lipid-packing defects are extensive (e.g., during lipid peroxidation), recruits and activates ITCH to mark the lysosome with K63-linked ubiquitin chains, initiating lysophagy. Human cell-based assays, lysosomal damage assays (LLOMe), co-immunoprecipitation, ubiquitin linkage-specific antibodies, SPG20 amphipathic helix mutagenesis, IST1 binding assays, ITCH recruitment assays Molecular cell High 38503285
2025 HCV NS3/4A protease cleaves SPG20 at Cys504 and Cys562; cleavage disrupts SPG20–TIP47 interaction and prevents ITCH recruitment to LD-associated ADRP/adipophilin, thereby blocking ubiquitin-dependent ADRP degradation and promoting large lipid droplet formation. Immunoblot of HCV-infected cells, NS3/4A transfection (active vs. inactive mutant), site-directed mutagenesis of cleavage sites, co-immunoprecipitation of SPG20 with TIP47, siRNA knockdown of ITCH, immunofluorescence Journal of virology High 40985721
2026 SPART interacts with and disrupts the endosomal localization of ITCH E3-ubiquitin ligase; ITCH ubiquitinates the ZIKV capsid to trigger viral uncoating; loss of SPART enhances ZIKV replication through an ITCH-dependent mechanism (effect absent in SPART-ITCH double knockouts), establishing SPART as a pan-orthoflavivirus restriction factor acting via ITCH-mediated capsid ubiquitination. Genome-wide CRISPR activation screen, SPART/ITCH single and double knockout cell lines, Spg20-/- and Itch-/- mice infected with ZIKV, viral replication assays, ITCH localization imaging, ubiquitination assays on viral capsid Cell host & microbe High 42161262

Source papers

Stage 0 corpus · 27 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2002 SPG20 is mutated in Troyer syndrome, an hereditary spastic paraplegia. Nature genetics 200 12134148
2009 A role for ubiquitin ligases and Spartin/SPG20 in lipid droplet turnover. The Journal of cell biology 117 19307600
2010 SPG20 protein spartin is recruited to midbodies by ESCRT-III protein Ist1 and participates in cytokinesis. Molecular biology of the cell 67 20719964
2011 SPG20, a novel biomarker for early detection of colorectal cancer, encodes a regulator of cytokinesis. Oncogene 63 21499309
2009 Endogenous spartin (SPG20) is recruited to endosomes and lipid droplets and interacts with the ubiquitin E3 ligases AIP4 and AIP5. The Biochemical journal 61 19580544
2024 Lysosomal damage sensing and lysophagy initiation by SPG20-ITCH. Molecular cell 58 38503285
2012 Spg20-/- mice reveal multimodal functions for Troyer syndrome protein spartin in lipid droplet maintenance, cytokinesis and BMP signaling. Human molecular genetics 48 22619377
2011 SPG20 protein spartin associates with cardiolipin via its plant-related senescence domain and regulates mitochondrial Ca2+ homeostasis. PloS one 34 21559443
2005 The Troyer syndrome (SPG20) protein spartin interacts with Eps15. Biochemical and biophysical research communications 29 16036216
2017 Methylation changes of SIRT1, KLF4, DAPK1 and SPG20 in B-lymphocytes derived from follicular and diffuse large B-cell lymphoma. Leukemia research 25 28324774
2017 Detection of SPG20 gene promoter-methylated DNA, as a novel epigenetic biomarker, in plasma for colorectal cancer diagnosis using the MethyLight method. Oncology letters 24 28521434
2018 Methylation-induced silencing of SPG20 facilitates gastric cancer cell proliferation by activating the EGFR/MAPK pathway. Biochemical and biophysical research communications 18 29673586
2019 A novel mutation in SPART gene causes a severe neurodevelopmental delay due to mitochondrial dysfunction with complex I impairments and altered pyruvate metabolism. FASEB journal : official publication of the Federation of American Societies for Experimental Biology 15 31314595
2017 SPG20 mutation in three siblings with familial hereditary spastic paraplegia. Cold Spring Harbor molecular case studies 15 28679690
2016 Novel Homozygous Missense Mutation in SPG20 Gene Results in Troyer Syndrome Associated with Mitochondrial Cytochrome c Oxidase Deficiency. JIMD reports 15 27539578
2019 Methylomics analysis identifies a putative STAT3 target, SPG20, as a noninvasive epigenetic biomarker for early detection of gastric cancer. PloS one 14 31194837
2015 Recurrent null mutation in SPG20 leads to Troyer syndrome. Molecular and cellular probes 12 26003402
2022 Methylation of FBN1, SPG20, ITF2, RUNX3, SNCA, MLH1, and SEPT9 genes in circulating cell-free DNA as biomarkers of colorectal cancer. Cancer biomarkers : section A of Disease markers 11 34957998
2021 KLF4, DAPK1 and SPG20 promoter methylation is not affected by DNMT1 silencing and hypomethylating drugs in lymphoma cells. Oncology reports 11 34751409
2017 Novel SPG20 mutation in an extended family with Troyer syndrome. Metabolic brain disease 10 28875386
2019 Dwarfism in Troyer syndrome: a family with SPG20 compound heterozygous mutations and a literature review. Annals of the New York Academy of Sciences 8 31535723
2023 Mutant SPART causes defects in mitochondrial protein import and bioenergetics reversed by Coenzyme Q. Open biology 7 37433330
2019 Aberrant methylation status of SPG20 promoter in hepatocellular carcinoma: A potential tumor metastasis biomarker. Cancer genetics 4 31109594
2023 SPART links autophagy machinery and lipid droplets in motor neurons. Autophagy 3 37573517
2025 Hepatitis C virus NS3/4A protease cleaves SPG20, a key regulator of lipid droplet turnover, to promote lipid droplet formation. Journal of virology 2 40985721
2026 A CRISPR activation screen identifies SPART as a pan-orthoflavivirus restriction factor. Cell host & microbe 0 42161262
2020 A novel missense mutation (c.1006C>T) of SPG20 gene associated with Troyer syndrome. Journal of genetics 0 32661208

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