{"gene":"SPG21","run_date":"2026-06-10T07:46:39","timeline":{"discoveries":[{"year":2000,"finding":"ACP33/maspardin was identified as a novel intracellular binding partner of CD4 via yeast two-hybrid screen; interaction was confirmed biochemically and mapped to the hydrophobic C-terminal amino acids of CD4. The interaction is mediated by the noncatalytic alpha/beta hydrolase fold domain of ACP33, revealing a peptide-binding function for this domain. ACP33 binding correlates with inhibition of TCR-induced T cell activation.","method":"Yeast two-hybrid screen, co-immunoprecipitation, deletion mutagenesis","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal binding confirmed with mutagenesis mapping in a single lab, two orthogonal methods","pmids":["11113139"],"is_preprint":false},{"year":2003,"finding":"A single base-pair insertion (601insA) in ACP33/SPG21 causes a frameshift and premature truncation (fs201-212X213) of maspardin, establishing loss-of-function mutation as the cause of Mast syndrome (SPG21). Maspardin was noted to localize to intracellular endosomal/trans-Golgi transportation vesicles, suggesting a role in protein transport and sorting.","method":"Sequence analysis of patient DNA, homozygosity mapping, functional annotation from prior localization studies","journal":"American journal of human genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutation identified in all 14 affected cases with frameshift confirmed; localization cited from prior work, not directly demonstrated in this paper","pmids":["14564668"],"is_preprint":false},{"year":2009,"finding":"Maspardin (ACP33/SPG21) localizes prominently to cytoplasm and to membranes, possibly at trans-Golgi network/late endosomal compartments. Immunoprecipitation coupled with mass spectrometry identified the aldehyde dehydrogenase ALDH16A1 as a maspardin-interacting protein; this interaction was confirmed by co-immunoprecipitation of overexpressed proteins and fusion protein pull-down assays, and the two proteins colocalize in cells.","method":"Immunoprecipitation, mass spectrometry, co-IP of overexpressed proteins, fusion protein pull-down, colocalization imaging","journal":"Neurogenetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (IP-MS, co-IP, pulldown, colocalization) in a single lab","pmids":["19184135"],"is_preprint":false},{"year":2010,"finding":"SPG21 knockout mice generated by homologous recombination develop gradually progressive hind limb dysfunction. Cultured cerebral cortical neurons from SPG21-/- mice exhibit significantly more axonal branching than wild-type neurons, indicating a role for maspardin in regulating axon branching.","method":"Homologous recombination knockout mouse, behavioral testing, primary cortical neuron culture, morphological analysis","journal":"Neurogenetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean KO with defined motor phenotype and cellular readout (axon branching), single lab","pmids":["20661613"],"is_preprint":false},{"year":2016,"finding":"Loss of maspardin in SPG21-/- mice attenuates growth, axonal branching, and maturation of primary cortical neurons. SPG21-/- neurons fail to respond to EGF-induced growth, correlating with reduced expression of EGF-EGFR signaling target genes, suggesting maspardin is required for EGF signaling in neurons.","method":"SPG21-/- mouse model, primary cortical neuron culture, beam walk/ledge/hind limb clasp behavioral tests, quantitative RT-PCR","journal":"Neuro-degenerative diseases","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO mouse with cellular phenotype and pathway-level readout (EGF signaling targets), single lab","pmids":["26978163"],"is_preprint":false},{"year":2025,"finding":"Maspardin/SPG21 localizes to endolysosomes via direct interaction with GTP-bound RAB7A. SPG21 depletion does not affect canonical mTORC1 substrates (ULK1, S6K1, 4E-BP1) but specifically reduces phosphorylation of the noncanonical mTORC1 substrate TFEB, leading to enhanced TFEB nuclear translocation and upregulation of a subset of TFEB-target genes. Disease-associated SPG21 variants reduce SPG21 expression and disrupt its endolysosomal localization.","method":"Co-immunoprecipitation, subcellular fractionation/localization, knockdown/knockout biochemical assays, phosphorylation assays, nuclear localization imaging, functional dependency analysis","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal methods (co-IP with RAB7A GTP-specificity, localization, substrate phosphorylation, nuclear translocation, gene expression) establishing mechanistic pathway; independently converges with PMID 41400694","pmids":["40833810"],"is_preprint":false},{"year":2025,"finding":"Maspardin/SPG21 associates with the late endosomal/lysosomal membrane and binds RAB7 GTPase. In SPG21 knockout cells, decreased TFEB phosphorylation leads to TFEB nuclear translocation. Mechanistically, SPG21 loss causes redistribution of RAB7 from retromer-positive late endosomes to lysosomes, decreasing its interaction with the GAP TBC1D5, resulting in RAB7 remaining GTP-bound. This recruits more FYCO1 to lysosomes, promoting anterograde lysosome movement along microtubules via FYCO1.","method":"SPG21 knockout cell lines, co-immunoprecipitation (RAB7 binding), TFEB phosphorylation assays, nuclear translocation imaging, lysosome motility assays, proximity/interaction assays for TBC1D5 and FYCO1","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal methods establishing mechanistic pathway from RAB7 positioning to TFEB phosphorylation; independently replicated in parallel with PMID 40833810","pmids":["41400694"],"is_preprint":false}],"current_model":"Maspardin (SPG21/ACP33) is a cytosolic protein that localizes to late endosomes/lysosomes by binding GTP-bound RAB7A, where it acts as a RAB7A effector to promote noncanonical mTORC1-mediated phosphorylation and cytoplasmic retention of the transcription factor TFEB; loss of maspardin redistributes RAB7 to lysosomes, impairs its GAP (TBC1D5) interaction, increases FYCO1 recruitment, drives anterograde lysosome motility, reduces TFEB phosphorylation, and causes TFEB nuclear translocation—while in neurons, maspardin loss results in excessive axon branching, attenuated EGF signaling, and progressive motor dysfunction."},"narrative":{"mechanistic_narrative":"SPG21 (maspardin/ACP33) is a cytosolic protein that governs late endosomal/lysosomal homeostasis by acting as a RAB7A effector, and its loss causes the autosomal recessive neurodegenerative disorder Mast syndrome (SPG21) [PMID:14564668, PMID:40833810]. Maspardin localizes to endolysosomal membranes through direct binding to GTP-bound RAB7A, a step disrupted by disease-associated variants that reduce SPG21 expression [PMID:40833810, PMID:41400694]. At this compartment maspardin selectively restrains the noncanonical mTORC1 substrate TFEB—without affecting canonical substrates ULK1, S6K1, or 4E-BP1—so that SPG21 loss reduces TFEB phosphorylation and drives TFEB nuclear translocation and upregulation of TFEB target genes [PMID:40833810, PMID:41400694]. Mechanistically, maspardin loss redistributes RAB7 from retromer-positive late endosomes to lysosomes, weakening RAB7's interaction with the GAP TBC1D5 so that RAB7 remains GTP-loaded, recruiting FYCO1 and promoting anterograde lysosome motility along microtubules [PMID:41400694]. The alpha/beta hydrolase fold domain mediates peptide binding rather than catalysis, as first defined through its interaction with the cytoplasmic tail of CD4 [PMID:11113139]. In neurons, maspardin is required for normal EGF-EGFR signaling and limits axon branching, and its loss produces excessive axonal branching, impaired neuronal maturation, and progressive motor dysfunction in knockout mice [PMID:20661613, PMID:26978163].","teleology":[{"year":2000,"claim":"Established maspardin as a peptide-binding protein, showing its alpha/beta hydrolase fold acts as a protein-interaction module rather than a catalytic enzyme by binding the cytoplasmic tail of CD4 and dampening T-cell activation.","evidence":"Yeast two-hybrid screen, co-immunoprecipitation, and deletion mutagenesis mapping","pmids":["11113139"],"confidence":"Medium","gaps":["Relevance of CD4 binding to neuronal/endolysosomal function not connected","No catalytic activity demonstrated or excluded for the hydrolase fold"]},{"year":2003,"claim":"Defined SPG21 as a disease gene by linking a loss-of-function frameshift mutation to Mast syndrome, establishing that maspardin deficiency causes neurodegeneration.","evidence":"Homozygosity mapping and patient DNA sequencing identifying 601insA frameshift in all affected cases","pmids":["14564668"],"confidence":"Medium","gaps":["Localization inferred from prior work, not directly demonstrated","Molecular consequence of mutation on protein function untested at this stage"]},{"year":2009,"claim":"Provided the first protein-interaction map beyond CD4 and refined subcellular localization to trans-Golgi/late endosomal compartments, identifying ALDH16A1 as a maspardin partner.","evidence":"IP-mass spectrometry, co-IP of overexpressed proteins, fusion protein pull-down, and colocalization imaging","pmids":["19184135"],"confidence":"Medium","gaps":["Functional significance of the ALDH16A1 interaction unresolved","Interaction validated only with overexpressed proteins in a single lab"]},{"year":2010,"claim":"Connected maspardin loss to a defined neuronal phenotype, showing knockout mice recapitulate progressive motor dysfunction and that maspardin restrains axon branching.","evidence":"Homologous recombination knockout mouse with behavioral testing and primary cortical neuron morphology","pmids":["20661613"],"confidence":"Medium","gaps":["Molecular pathway linking maspardin to axon branching unidentified","Cell-autonomy of the motor phenotype not dissected"]},{"year":2016,"claim":"Placed maspardin within a growth-factor signaling axis, showing it is required for EGF-induced neuronal growth and EGF-EGFR target gene expression.","evidence":"SPG21-/- primary cortical neuron culture, behavioral tests, and quantitative RT-PCR of EGF signaling targets","pmids":["26978163"],"confidence":"Medium","gaps":["Direct biochemical link between maspardin and EGFR signaling not established","How attenuated EGF signaling relates to endolysosomal function unknown"]},{"year":2025,"claim":"Resolved the core molecular mechanism: maspardin is a RAB7A effector that controls RAB7 nucleotide state and lysosome positioning, selectively gating noncanonical mTORC1 phosphorylation of TFEB.","evidence":"Two independent studies using SPG21 knockout/knockdown cells with GTP-specific RAB7 co-IP, TBC1D5/FYCO1 interaction assays, TFEB phosphorylation and nuclear translocation imaging, and lysosome motility assays","pmids":["40833810","41400694"],"confidence":"High","gaps":["Direct mechanism by which maspardin promotes TFEB phosphorylation at the lysosome not fully defined","How disease variants mechanistically impair RAB7 binding beyond reduced expression unresolved","Whether the neuronal axon-branching/EGF phenotypes derive from the RAB7-TFEB axis untested"]},{"year":null,"claim":"It remains unknown how the endolysosomal RAB7-TFEB regulatory role of maspardin mechanistically produces the neuronal axon-branching, EGF-signaling, and progressive motor phenotypes of Mast syndrome.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No in vivo demonstration that TFEB dysregulation drives the SPG21 motor phenotype","Catalytic versus scaffolding role of the hydrolase fold in RAB7 effector function unresolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[5,6]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[5,6]}],"localization":[{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[5,6]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[2,6]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[2]}],"pathway":[],"complexes":[],"partners":["RAB7A","TBC1D5","FYCO1","TFEB","CD4","ALDH16A1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9NZD8","full_name":"Maspardin","aliases":["Acid cluster protein 33","Spastic paraplegia 21 autosomal recessive Mast syndrome protein","Spastic paraplegia 21 protein"],"length_aa":308,"mass_kda":35.0,"function":"May play a role as a negative regulatory factor in CD4-dependent T-cell activation","subcellular_location":"Cytoplasm, cytosol; Membrane; Endosome membrane; Golgi apparatus, trans-Golgi network membrane","url":"https://www.uniprot.org/uniprotkb/Q9NZD8/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SPG21","classification":"Not Classified","n_dependent_lines":69,"n_total_lines":1208,"dependency_fraction":0.057119205298013245},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000090487","cell_line_id":"CID001864","localizations":[{"compartment":"vesicles","grade":3}],"interactors":[{"gene":"FAM50A","stoichiometry":10.0},{"gene":"RAB7A","stoichiometry":0.2},{"gene":"VAMP7","stoichiometry":0.2},{"gene":"ARL8B","stoichiometry":0.2},{"gene":"VPS29","stoichiometry":0.2},{"gene":"VPS35","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID001864","total_profiled":1310},"omim":[{"mim_id":"613358","title":"ALDEHYDE DEHYDROGENASE 16 FAMILY, MEMBER A1; ALDH16A1","url":"https://www.omim.org/entry/613358"},{"mim_id":"608181","title":"ACIDIC CLUSTER PROTEIN, 33-KD; ACP33","url":"https://www.omim.org/entry/608181"},{"mim_id":"604360","title":"SPASTIC PARAPLEGIA 11, AUTOSOMAL RECESSIVE; SPG11","url":"https://www.omim.org/entry/604360"},{"mim_id":"270800","title":"SPASTIC PARAPLEGIA 5A, AUTOSOMAL RECESSIVE; SPG5A","url":"https://www.omim.org/entry/270800"},{"mim_id":"248900","title":"MAST SYNDROME; MASTS","url":"https://www.omim.org/entry/248900"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Vesicles","reliability":"Supported"},{"location":"Cytosol","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SPG21"},"hgnc":{"alias_symbol":["ACP33","GL010","BM-019","MAST","ABHD21"],"prev_symbol":[]},"alphafold":{"accession":"Q9NZD8","domains":[{"cath_id":"3.40.50.1820","chopping":"3-306","consensus_level":"high","plddt":93.3592,"start":3,"end":306}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NZD8","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NZD8-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NZD8-F1-predicted_aligned_error_v6.png","plddt_mean":92.81},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SPG21","jax_strain_url":"https://www.jax.org/strain/search?query=SPG21"},"sequence":{"accession":"Q9NZD8","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9NZD8.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9NZD8/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NZD8"}},"corpus_meta":[{"pmid":"22554690","id":"PMC_22554690","title":"Hereditary spastic paraplegias with autosomal dominant, recessive, X-linked, or maternal trait of inheritance.","date":"2012","source":"Journal of the neurological sciences","url":"https://pubmed.ncbi.nlm.nih.gov/22554690","citation_count":229,"is_preprint":false},{"pmid":"24833714","id":"PMC_24833714","title":"Overlapping phenotypes in complex spastic paraplegias SPG11, SPG15, SPG35 and SPG48.","date":"2014","source":"Brain : a journal of neurology","url":"https://pubmed.ncbi.nlm.nih.gov/24833714","citation_count":132,"is_preprint":false},{"pmid":"14564668","id":"PMC_14564668","title":"Maspardin is mutated in mast syndrome, a complicated form of hereditary spastic paraplegia associated with dementia.","date":"2003","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/14564668","citation_count":128,"is_preprint":false},{"pmid":"26556829","id":"PMC_26556829","title":"ALS5/SPG11/KIAA1840 mutations cause autosomal recessive axonal Charcot-Marie-Tooth 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intracellular binding partner of CD4 via yeast two-hybrid screen; interaction was confirmed biochemically and mapped to the hydrophobic C-terminal amino acids of CD4. The interaction is mediated by the noncatalytic alpha/beta hydrolase fold domain of ACP33, revealing a peptide-binding function for this domain. ACP33 binding correlates with inhibition of TCR-induced T cell activation.\",\n      \"method\": \"Yeast two-hybrid screen, co-immunoprecipitation, deletion mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal binding confirmed with mutagenesis mapping in a single lab, two orthogonal methods\",\n      \"pmids\": [\"11113139\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"A single base-pair insertion (601insA) in ACP33/SPG21 causes a frameshift and premature truncation (fs201-212X213) of maspardin, establishing loss-of-function mutation as the cause of Mast syndrome (SPG21). Maspardin was noted to localize to intracellular endosomal/trans-Golgi transportation vesicles, suggesting a role in protein transport and sorting.\",\n      \"method\": \"Sequence analysis of patient DNA, homozygosity mapping, functional annotation from prior localization studies\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutation identified in all 14 affected cases with frameshift confirmed; localization cited from prior work, not directly demonstrated in this paper\",\n      \"pmids\": [\"14564668\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Maspardin (ACP33/SPG21) localizes prominently to cytoplasm and to membranes, possibly at trans-Golgi network/late endosomal compartments. Immunoprecipitation coupled with mass spectrometry identified the aldehyde dehydrogenase ALDH16A1 as a maspardin-interacting protein; this interaction was confirmed by co-immunoprecipitation of overexpressed proteins and fusion protein pull-down assays, and the two proteins colocalize in cells.\",\n      \"method\": \"Immunoprecipitation, mass spectrometry, co-IP of overexpressed proteins, fusion protein pull-down, colocalization imaging\",\n      \"journal\": \"Neurogenetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (IP-MS, co-IP, pulldown, colocalization) in a single lab\",\n      \"pmids\": [\"19184135\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"SPG21 knockout mice generated by homologous recombination develop gradually progressive hind limb dysfunction. Cultured cerebral cortical neurons from SPG21-/- mice exhibit significantly more axonal branching than wild-type neurons, indicating a role for maspardin in regulating axon branching.\",\n      \"method\": \"Homologous recombination knockout mouse, behavioral testing, primary cortical neuron culture, morphological analysis\",\n      \"journal\": \"Neurogenetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean KO with defined motor phenotype and cellular readout (axon branching), single lab\",\n      \"pmids\": [\"20661613\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Loss of maspardin in SPG21-/- mice attenuates growth, axonal branching, and maturation of primary cortical neurons. SPG21-/- neurons fail to respond to EGF-induced growth, correlating with reduced expression of EGF-EGFR signaling target genes, suggesting maspardin is required for EGF signaling in neurons.\",\n      \"method\": \"SPG21-/- mouse model, primary cortical neuron culture, beam walk/ledge/hind limb clasp behavioral tests, quantitative RT-PCR\",\n      \"journal\": \"Neuro-degenerative diseases\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO mouse with cellular phenotype and pathway-level readout (EGF signaling targets), single lab\",\n      \"pmids\": [\"26978163\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Maspardin/SPG21 localizes to endolysosomes via direct interaction with GTP-bound RAB7A. SPG21 depletion does not affect canonical mTORC1 substrates (ULK1, S6K1, 4E-BP1) but specifically reduces phosphorylation of the noncanonical mTORC1 substrate TFEB, leading to enhanced TFEB nuclear translocation and upregulation of a subset of TFEB-target genes. Disease-associated SPG21 variants reduce SPG21 expression and disrupt its endolysosomal localization.\",\n      \"method\": \"Co-immunoprecipitation, subcellular fractionation/localization, knockdown/knockout biochemical assays, phosphorylation assays, nuclear localization imaging, functional dependency analysis\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal methods (co-IP with RAB7A GTP-specificity, localization, substrate phosphorylation, nuclear translocation, gene expression) establishing mechanistic pathway; independently converges with PMID 41400694\",\n      \"pmids\": [\"40833810\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Maspardin/SPG21 associates with the late endosomal/lysosomal membrane and binds RAB7 GTPase. In SPG21 knockout cells, decreased TFEB phosphorylation leads to TFEB nuclear translocation. Mechanistically, SPG21 loss causes redistribution of RAB7 from retromer-positive late endosomes to lysosomes, decreasing its interaction with the GAP TBC1D5, resulting in RAB7 remaining GTP-bound. This recruits more FYCO1 to lysosomes, promoting anterograde lysosome movement along microtubules via FYCO1.\",\n      \"method\": \"SPG21 knockout cell lines, co-immunoprecipitation (RAB7 binding), TFEB phosphorylation assays, nuclear translocation imaging, lysosome motility assays, proximity/interaction assays for TBC1D5 and FYCO1\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal methods establishing mechanistic pathway from RAB7 positioning to TFEB phosphorylation; independently replicated in parallel with PMID 40833810\",\n      \"pmids\": [\"41400694\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Maspardin (SPG21/ACP33) is a cytosolic protein that localizes to late endosomes/lysosomes by binding GTP-bound RAB7A, where it acts as a RAB7A effector to promote noncanonical mTORC1-mediated phosphorylation and cytoplasmic retention of the transcription factor TFEB; loss of maspardin redistributes RAB7 to lysosomes, impairs its GAP (TBC1D5) interaction, increases FYCO1 recruitment, drives anterograde lysosome motility, reduces TFEB phosphorylation, and causes TFEB nuclear translocation—while in neurons, maspardin loss results in excessive axon branching, attenuated EGF signaling, and progressive motor dysfunction.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SPG21 (maspardin/ACP33) is a cytosolic protein that governs late endosomal/lysosomal homeostasis by acting as a RAB7A effector, and its loss causes the autosomal recessive neurodegenerative disorder Mast syndrome (SPG21) [#1, #5]. Maspardin localizes to endolysosomal membranes through direct binding to GTP-bound RAB7A, a step disrupted by disease-associated variants that reduce SPG21 expression [#5, #6]. At this compartment maspardin selectively restrains the noncanonical mTORC1 substrate TFEB—without affecting canonical substrates ULK1, S6K1, or 4E-BP1—so that SPG21 loss reduces TFEB phosphorylation and drives TFEB nuclear translocation and upregulation of TFEB target genes [#5, #6]. Mechanistically, maspardin loss redistributes RAB7 from retromer-positive late endosomes to lysosomes, weakening RAB7's interaction with the GAP TBC1D5 so that RAB7 remains GTP-loaded, recruiting FYCO1 and promoting anterograde lysosome motility along microtubules [#6]. The alpha/beta hydrolase fold domain mediates peptide binding rather than catalysis, as first defined through its interaction with the cytoplasmic tail of CD4 [#0]. In neurons, maspardin is required for normal EGF-EGFR signaling and limits axon branching, and its loss produces excessive axonal branching, impaired neuronal maturation, and progressive motor dysfunction in knockout mice [#3, #4].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Established maspardin as a peptide-binding protein, showing its alpha/beta hydrolase fold acts as a protein-interaction module rather than a catalytic enzyme by binding the cytoplasmic tail of CD4 and dampening T-cell activation.\",\n      \"evidence\": \"Yeast two-hybrid screen, co-immunoprecipitation, and deletion mutagenesis mapping\",\n      \"pmids\": [\"11113139\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Relevance of CD4 binding to neuronal/endolysosomal function not connected\", \"No catalytic activity demonstrated or excluded for the hydrolase fold\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Defined SPG21 as a disease gene by linking a loss-of-function frameshift mutation to Mast syndrome, establishing that maspardin deficiency causes neurodegeneration.\",\n      \"evidence\": \"Homozygosity mapping and patient DNA sequencing identifying 601insA frameshift in all affected cases\",\n      \"pmids\": [\"14564668\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Localization inferred from prior work, not directly demonstrated\", \"Molecular consequence of mutation on protein function untested at this stage\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Provided the first protein-interaction map beyond CD4 and refined subcellular localization to trans-Golgi/late endosomal compartments, identifying ALDH16A1 as a maspardin partner.\",\n      \"evidence\": \"IP-mass spectrometry, co-IP of overexpressed proteins, fusion protein pull-down, and colocalization imaging\",\n      \"pmids\": [\"19184135\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Functional significance of the ALDH16A1 interaction unresolved\", \"Interaction validated only with overexpressed proteins in a single lab\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Connected maspardin loss to a defined neuronal phenotype, showing knockout mice recapitulate progressive motor dysfunction and that maspardin restrains axon branching.\",\n      \"evidence\": \"Homologous recombination knockout mouse with behavioral testing and primary cortical neuron morphology\",\n      \"pmids\": [\"20661613\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Molecular pathway linking maspardin to axon branching unidentified\", \"Cell-autonomy of the motor phenotype not dissected\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Placed maspardin within a growth-factor signaling axis, showing it is required for EGF-induced neuronal growth and EGF-EGFR target gene expression.\",\n      \"evidence\": \"SPG21-/- primary cortical neuron culture, behavioral tests, and quantitative RT-PCR of EGF signaling targets\",\n      \"pmids\": [\"26978163\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Direct biochemical link between maspardin and EGFR signaling not established\", \"How attenuated EGF signaling relates to endolysosomal function unknown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Resolved the core molecular mechanism: maspardin is a RAB7A effector that controls RAB7 nucleotide state and lysosome positioning, selectively gating noncanonical mTORC1 phosphorylation of TFEB.\",\n      \"evidence\": \"Two independent studies using SPG21 knockout/knockdown cells with GTP-specific RAB7 co-IP, TBC1D5/FYCO1 interaction assays, TFEB phosphorylation and nuclear translocation imaging, and lysosome motility assays\",\n      \"pmids\": [\"40833810\", \"41400694\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Direct mechanism by which maspardin promotes TFEB phosphorylation at the lysosome not fully defined\", \"How disease variants mechanistically impair RAB7 binding beyond reduced expression unresolved\", \"Whether the neuronal axon-branching/EGF phenotypes derive from the RAB7-TFEB axis untested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unknown how the endolysosomal RAB7-TFEB regulatory role of maspardin mechanistically produces the neuronal axon-branching, EGF-signaling, and progressive motor phenotypes of Mast syndrome.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No in vivo demonstration that TFEB dysregulation drives the SPG21 motor phenotype\", \"Catalytic versus scaffolding role of the hydrolase fold in RAB7 effector function unresolved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [5, 6]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [5, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [5, 6]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [2, 6]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": []}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"RAB7A\", \"TBC1D5\", \"FYCO1\", \"TFEB\", \"CD4\", \"ALDH16A1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}