{"gene":"SPAAR","run_date":"2026-06-10T07:46:38","timeline":{"discoveries":[{"year":2016,"finding":"SPAAR (SPAR polypeptide encoded by LINC00961) localizes to the late endosome/lysosome and interacts with the lysosomal v-ATPase to negatively regulate mTORC1 activation specifically in response to amino acid stimulation, but not growth factor stimulation. In vivo, SPAR downregulation upon skeletal muscle injury enables efficient mTORC1 activation and promotes muscle regeneration.","method":"Subcellular fractionation/localization, co-immunoprecipitation with v-ATPase, CRISPR/Cas9 knockout mouse (SPAR-specific KO maintaining lncRNA expression), in vivo muscle injury model","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP with v-ATPase, specific KO mouse with defined phenotype, multiple orthogonal methods in a single rigorous study","pmids":["28024296"],"is_preprint":false},{"year":2001,"finding":"SPAR (spine-associated RapGAP) is a Rap-specific GTPase-activating protein that interacts with the guanylate kinase-like domain of PSD-95 and forms a complex with PSD-95 and NMDA receptors in brain. In heterologous cells, SPAR reorganizes the actin cytoskeleton and recruits PSD-95 to F-actin. In hippocampal neurons, SPAR localizes to dendritic spines and causes enlargement of spine heads via its RapGAP and actin-interacting domains.","method":"Co-immunoprecipitation from brain lysate, heterologous cell transfection with actin cytoskeleton readout, hippocampal neuron overexpression/dominant-negative constructs, domain deletion analysis","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, domain mutagenesis, neuronal gain/loss-of-function with defined morphological readout, replicated across multiple model systems in one study","pmids":["11502259"],"is_preprint":false},{"year":2008,"finding":"SPAR is targeted for proteasomal degradation by the SCF(β-TrCP) E3 ubiquitin ligase complex. This degradation requires activity-inducible polo-like kinase 2 (Plk2/SNK), which phosphorylates SPAR at a canonical phosphodegron, enabling physical association with β-TrCP. Dominant-negative β-TrCP or Cul1 constructs in hippocampal neurons prevented Plk2-dependent SPAR degradation.","method":"Co-immunoprecipitation of SPAR with SCF(β-TrCP) components, dominant-negative overexpression in hippocampal neurons, phosphodegron identification","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — Co-IP showing physical association with E3 complex, kinase requirement established, dominant-negative rescue in neurons, multiple orthogonal methods","pmids":["18723513"],"is_preprint":false},{"year":2007,"finding":"The EphA4 receptor C-terminus interacts with the PDZ domain of SPAR, mediating EphA4-dependent inactivation of Rap1 and Rap2 GTPases. SPAR-mediated inactivation of Rap1 (but not Rap2) is required for ephrin-A-dependent growth cone collapse in hippocampal neurons and decreased integrin-mediated adhesion in neuronal cells.","method":"Co-immunoprecipitation of EphA4 with SPAR, dominant-negative and constitutively active Rap1/Rap2 constructs, hippocampal neuron growth cone collapse assay, integrin adhesion assay","journal":"The Journal of neuroscience : the official journal of the Society for Neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP, gain/loss-of-function with specific cellular readouts, Rap1 vs Rap2 specificity defined","pmids":["18094260"],"is_preprint":false},{"year":2006,"finding":"ProSAPiP1, a novel PSD protein, binds SPAR via a central coiled-coil/leucine zipper region and recruits SPAR to synapses. ProSAPiP1 also binds ProSAP2/Shank3 via the PDZ domain of Shank3, linking SPAR to the ProSAP/Shank scaffold at excitatory synapses.","method":"Co-immunoprecipitation, yeast two-hybrid, co-localization in hippocampal neurons","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP and localization, single lab, two orthogonal binding methods","pmids":["16522626"],"is_preprint":false},{"year":2009,"finding":"SPAR interacts with PDLIM5/Enigma Homolog (ENH) at the postsynaptic density. PDLIM5 promotes decreased dendritic spine head size and elongated filopodia-like morphology, opposing SPAR's spine head enlargement effect. PKC activation promotes delivery of PDLIM5 into dendritic spines and increases its colocalization with SPAR.","method":"Co-immunoprecipitation, RNAi knockdown of PDLIM5, overexpression in hippocampal neurons, pharmacological PKC activation, fluorescence microscopy","journal":"Molecular and cellular neurosciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP, RNAi, and overexpression with morphological readout, single lab","pmids":["19900557"],"is_preprint":false},{"year":2009,"finding":"SPAR interacts with alpha-actinin2, an actin-crosslinking protein. SPAR promotes spine head enlargement while increased alpha-actinin2 favors spine elongation/thinning; together they can generate additive combination spine/filopodia hybrid structures, identifying a molecular pathway bridging the actin cytoskeleton and Rap signaling at synapses.","method":"Co-immunoprecipitation, overexpression of SPAR and alpha-actinin2 in hippocampal neurons, morphological analysis","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — Co-IP and neuronal overexpression with morphological readout, single lab, single study","pmids":["19393616"],"is_preprint":false},{"year":2011,"finding":"LTP induction in hippocampal CA1 neurons triggers ubiquitin-proteasome system (UPS)-dependent degradation of SPAR. This involves two complementary mechanisms: one requiring cyclin-dependent kinase 5 (CDK5) and protein synthesis, and a second requiring UPS and NMDA receptor activation but not CDK5.","method":"Confocal microscopy of eGFP-tagged SPAR in acute hippocampal slices, LTP induction, pharmacological inhibition of CDK5 and proteasome, protein synthesis inhibitor","journal":"Synapse (New York, N.Y.)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — live imaging of tagged protein in acute slices, pharmacological dissection of two mechanistic pathways, single lab","pmids":["21987493"],"is_preprint":false},{"year":2016,"finding":"ProSAPiP1 regulates SPAR levels at the postsynaptic density and the maturation of dendritic spines in hippocampal neurons. Lentiviral overexpression and knockdown of ProSAPiP1 demonstrated that it is dispensable for formation of synaptic specializations per se but controls SPAR abundance at the PSD.","method":"Lentiviral overexpression and knockdown in hippocampal neurons, immunofluorescence quantification of SPAR at PSD","journal":"Frontiers in synaptic neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — bidirectional genetic manipulation with quantitative protein-level readout, single lab","pmids":["27252646"],"is_preprint":false},{"year":2020,"finding":"The SPAAR micropeptide encoded by LINC00961 promotes endothelial cell tubule formation; overexpression of the SPAAR ORF increased tubule formation in primary endothelial cells. Pull-down of SPAAR identified the actin-binding protein SYNE1 as a binding partner of SPAAR in endothelial cells. LINC00961 locus knockout mice showed reduced capillary density in ischemic muscle after hindlimb ischemia.","method":"ORF overexpression in primary ECs, tubule formation assay, pull-down/mass spectrometry identification of SPAAR binding partners, CRISPR KO mouse with hindlimb ischemia model","journal":"Cardiovascular research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pull-down MS identification of binding partner, in vivo KO phenotype, overexpression functional assay, single lab","pmids":["31990292"],"is_preprint":false},{"year":2021,"finding":"SPAAR is conserved across mammals (including marsupials and monotremes) and retains conserved protein structure despite primary sequence divergence. Two independent de novo origination events of 5'-elongated SPAAR isoforms from noncoding sequence occurred in primates, with evidence of adaptive evolution in the extended region.","method":"Syntenic alignments, homology searches, comparative genomics, evolutionary rate analysis","journal":"Genes","confidence":"Low","confidence_rationale":"Tier 4 / Weak — computational/comparative genomic analysis only, no direct biochemical experiment on the protein","pmids":["34946813"],"is_preprint":false}],"current_model":"SPAAR/SPAR encodes two functionally distinct proteins: (1) a lysosome-localized micropeptide (SPAAR) that binds the v-ATPase to specifically suppress amino acid-stimulated mTORC1 activation, enabling tissue-specific regulation of muscle regeneration and angiogenesis; and (2) a postsynaptic RapGAP (SPAR) that forms a complex with PSD-95 and NMDA receptors at dendritic spines, inactivates Rap GTPases downstream of EphA4 to regulate spine morphology and growth cone collapse, and is targeted for proteasomal degradation through activity-induced Plk2/SNK phosphorylation at a phosphodegron recognized by the SCF(β-TrCP) E3 ligase."},"narrative":{"mechanistic_narrative":"The SPAAR/SPAR locus (LINC00961) encodes a polypeptide with two functionally distinct, tissue-specific activities [PMID:28024296, PMID:11502259]. In non-neuronal tissues, the SPAAR micropeptide localizes to the late endosome/lysosome and binds the lysosomal v-ATPase to selectively suppress amino acid-stimulated mTORC1 activation, and its downregulation upon skeletal muscle injury licenses mTORC1 signaling to promote muscle regeneration [PMID:28024296]; in endothelial cells SPAAR binds the actin-associated protein SYNE1 and supports tubule formation and capillary density in ischemic muscle [PMID:31990292]. In neurons, SPAR functions as a Rap-specific GTPase-activating protein that binds the guanylate kinase-like domain of PSD-95, forms a complex with NMDA receptors, reorganizes the actin cytoskeleton, and enlarges dendritic spine heads [PMID:11502259]. SPAR acts downstream of the EphA4 receptor, whose C-terminus engages the SPAR PDZ domain to drive Rap1 inactivation required for ephrin-A-dependent growth cone collapse [PMID:18094260], and is integrated into the postsynaptic scaffold and actin machinery through ProSAPiP1/Shank3, PDLIM5, and alpha-actinin2 [PMID:16522626, PMID:19900557, PMID:19393616]. SPAR abundance is tightly controlled by activity-dependent proteasomal degradation: Plk2/SNK phosphorylates a phosphodegron that recruits the SCF(β-TrCP) E3 ligase, and LTP induction triggers UPS-dependent SPAR turnover via CDK5- and NMDA receptor-dependent routes [PMID:18723513, PMID:21987493].","teleology":[{"year":2001,"claim":"Established SPAR as a postsynaptic Rap-GAP that physically couples to the PSD-95/NMDA receptor scaffold and shapes dendritic spine morphology, defining its core synaptic function.","evidence":"Co-IP from brain lysate, heterologous actin readout, and hippocampal neuron gain/loss-of-function with domain deletions","pmids":["11502259"],"confidence":"High","gaps":["Endogenous loss-of-function phenotype not established","Downstream Rap effectors driving spine enlargement not resolved"]},{"year":2006,"claim":"Identified how SPAR is physically recruited to synapses, linking it to the ProSAP/Shank scaffold via ProSAPiP1.","evidence":"Co-IP, yeast two-hybrid, and co-localization in hippocampal neurons","pmids":["16522626"],"confidence":"Medium","gaps":["Single lab","Functional consequence of recruitment for spine signaling not tested here"]},{"year":2007,"claim":"Placed SPAR in a receptor-driven signaling axis by showing EphA4 engages its PDZ domain to inactivate Rap1 for growth cone collapse and adhesion control.","evidence":"Co-IP of EphA4 with SPAR, dominant-negative/constitutively active Rap constructs, growth cone collapse and integrin adhesion assays","pmids":["18094260"],"confidence":"High","gaps":["Spatial dynamics of EphA4-SPAR recruitment not resolved","Rap2-independence of collapse mechanism not fully explained"]},{"year":2008,"claim":"Defined the degradation mechanism controlling SPAR levels: activity-induced Plk2 phosphorylation creates a phosphodegron recognized by SCF(β-TrCP).","evidence":"Co-IP of SPAR with SCF components, phosphodegron mapping, dominant-negative β-TrCP/Cul1 rescue in neurons","pmids":["18723513"],"confidence":"High","gaps":["Precise phosphodegron residues and Plk2 kinetics in vivo","Link to spine remodeling outcome not directly demonstrated"]},{"year":2009,"claim":"Expanded the SPAR interactome to actin/scaffold partners that counterbalance its spine-enlarging activity, revealing combinatorial control of spine morphology.","evidence":"Co-IP, RNAi/overexpression of PDLIM5 and alpha-actinin2 in hippocampal neurons with morphological readouts and PKC pharmacology","pmids":["19900557","19393616"],"confidence":"Medium","gaps":["Single-lab interactions","Direct vs indirect binding to actin machinery not fully dissected"]},{"year":2011,"claim":"Connected SPAR turnover to synaptic plasticity by showing LTP triggers UPS-dependent SPAR degradation through parallel CDK5- and NMDA receptor-dependent routes.","evidence":"Live imaging of eGFP-SPAR in acute hippocampal slices with LTP induction and pharmacological inhibition of CDK5, proteasome, and protein synthesis","pmids":["21987493"],"confidence":"Medium","gaps":["Relationship between CDK5 and Plk2/β-TrCP routes unresolved","Single lab"]},{"year":2016,"claim":"Revealed a second, non-neuronal function of the same locus: the SPAAR micropeptide restrains amino acid-specific mTORC1 activation via the lysosomal v-ATPase to gate muscle regeneration.","evidence":"Subcellular fractionation, reciprocal Co-IP with v-ATPase, SPAR-specific CRISPR KO mouse, in vivo muscle injury model","pmids":["28024296"],"confidence":"High","gaps":["Molecular basis for amino-acid versus growth-factor selectivity unclear","How v-ATPase binding mechanistically blocks mTORC1 not resolved"]},{"year":2020,"claim":"Extended SPAAR function to angiogenesis, identifying SYNE1 as a binding partner and an in vivo role in capillary formation after ischemia.","evidence":"ORF overexpression and tubule assays in primary endothelial cells, pull-down/MS, LINC00961 KO mouse hindlimb ischemia","pmids":["31990292"],"confidence":"Medium","gaps":["Functional significance of SYNE1 binding not mechanistically defined","Relationship to the v-ATPase/mTORC1 axis in endothelium unknown"]},{"year":2021,"claim":"Provided evolutionary context, showing SPAAR's protein structure is conserved across mammals with independent primate-specific isoform origination.","evidence":"Comparative genomics, syntenic alignment, and evolutionary rate analysis","pmids":["34946813"],"confidence":"Low","gaps":["Computational only, no biochemical test","Functional consequence of primate isoforms unknown"]},{"year":null,"claim":"How a single short ORF executes two unrelated molecular functions — lysosomal mTORC1 suppression versus postsynaptic Rap-GAP/scaffold activity — and whether these reflect distinct isoforms, cell-type-specific partners, or shared structural elements remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structural model reconciling the two activities","Cross-tissue comparison of the same protein product lacking","GAP catalytic versus micropeptide functions not tested in the same system"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,1,3]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[1,4]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[1,6,9]}],"localization":[{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[0]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[0]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[1,3]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,3]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[1,7]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[2]}],"complexes":["PSD-95/NMDA receptor complex","SCF(β-TrCP) E3 ubiquitin ligase","v-ATPase"],"partners":["PSD-95","EPHA4","BTRC","PROSAPIP1","SHANK3","PDLIM5","ACTN2","SYNE1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"A0A1B0GVQ0","full_name":"Small regulatory polypeptide of amino acid response","aliases":[],"length_aa":90,"mass_kda":9.6,"function":"Negative regulator of amino acid sensing and mTORC1, a signaling complex promoting cell growth in response to growth factors, energy levels and amino acids (PubMed:28024296). Negatively regulates mTORC1 activation by inhibiting recruitment of mTORC1 to lysosomes upon stimulation with amino acids: acts by promoting the formation of a tightly bound supercomplex composed of the lysosomal V-ATPase, Ragulator and Rag GTPases, preventing recruitment of mTORC1 (PubMed:28024296). Acts as a regulator of muscle regeneration following injury by regulating mTORC1 activation (By similarity)","subcellular_location":"Late endosome membrane; Lysosome membrane","url":"https://www.uniprot.org/uniprotkb/A0A1B0GVQ0/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SPAAR","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":70,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SPAAR","total_profiled":1310},"omim":[{"mim_id":"617627","title":"SMALL REGULATORY POLYPEPTIDE OF AMINO ACID RESPONSE; SPAAR","url":"https://www.omim.org/entry/617627"},{"mim_id":"613455","title":"MIA SH3 DOMAIN ER EXPORT FACTOR 3; MIA3","url":"https://www.omim.org/entry/613455"},{"mim_id":"313400","title":"SPONDYLOEPIPHYSEAL DYSPLASIA TARDA, X-LINKED; SEDT","url":"https://www.omim.org/entry/313400"},{"mim_id":"300202","title":"TRACKING PROTEIN PARTICLE COMPLEX, SUBUNIT 2; TRAPPC2","url":"https://www.omim.org/entry/300202"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"adipose tissue","ntpm":11.7},{"tissue":"breast","ntpm":10.5},{"tissue":"heart muscle","ntpm":8.7}],"url":"https://www.proteinatlas.org/search/SPAAR"},"hgnc":{"alias_symbol":["SPAR"],"prev_symbol":["LINC00961"]},"alphafold":{"accession":"A0A1B0GVQ0","domains":[{"cath_id":"1.20.5","chopping":"12-48","consensus_level":"medium","plddt":86.1403,"start":12,"end":48}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/A0A1B0GVQ0","model_url":"https://alphafold.ebi.ac.uk/files/AF-A0A1B0GVQ0-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-A0A1B0GVQ0-F1-predicted_aligned_error_v6.png","plddt_mean":67.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SPAAR","jax_strain_url":"https://www.jax.org/strain/search?query=SPAAR"},"sequence":{"accession":"A0A1B0GVQ0","fasta_url":"https://rest.uniprot.org/uniprotkb/A0A1B0GVQ0.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/A0A1B0GVQ0/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/A0A1B0GVQ0"}},"corpus_meta":[{"pmid":"28024296","id":"PMC_28024296","title":"mTORC1 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to the scaffolding protein ProSAP2/Shank3.","date":"2006","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/16522626","citation_count":49,"is_preprint":false},{"pmid":"27219485","id":"PMC_27219485","title":"Spiral artery remodeling and maternal cardiovascular risk: the spiral artery remodeling (SPAR) study.","date":"2016","source":"Journal of hypertension","url":"https://pubmed.ncbi.nlm.nih.gov/27219485","citation_count":40,"is_preprint":false},{"pmid":"19900557","id":"PMC_19900557","title":"Postsynaptic PDLIM5/Enigma Homolog binds SPAR and causes dendritic spine shrinkage.","date":"2009","source":"Molecular and cellular neurosciences","url":"https://pubmed.ncbi.nlm.nih.gov/19900557","citation_count":38,"is_preprint":false},{"pmid":"29156520","id":"PMC_29156520","title":"Long noncoding RNA LINC00961 inhibits cell invasion and metastasis in human non-small cell lung cancer.","date":"2017","source":"Biomedicine & pharmacotherapy = Biomedecine & 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In vivo, SPAR downregulation upon skeletal muscle injury enables efficient mTORC1 activation and promotes muscle regeneration.\",\n      \"method\": \"Subcellular fractionation/localization, co-immunoprecipitation with v-ATPase, CRISPR/Cas9 knockout mouse (SPAR-specific KO maintaining lncRNA expression), in vivo muscle injury model\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP with v-ATPase, specific KO mouse with defined phenotype, multiple orthogonal methods in a single rigorous study\",\n      \"pmids\": [\"28024296\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"SPAR (spine-associated RapGAP) is a Rap-specific GTPase-activating protein that interacts with the guanylate kinase-like domain of PSD-95 and forms a complex with PSD-95 and NMDA receptors in brain. In heterologous cells, SPAR reorganizes the actin cytoskeleton and recruits PSD-95 to F-actin. In hippocampal neurons, SPAR localizes to dendritic spines and causes enlargement of spine heads via its RapGAP and actin-interacting domains.\",\n      \"method\": \"Co-immunoprecipitation from brain lysate, heterologous cell transfection with actin cytoskeleton readout, hippocampal neuron overexpression/dominant-negative constructs, domain deletion analysis\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, domain mutagenesis, neuronal gain/loss-of-function with defined morphological readout, replicated across multiple model systems in one study\",\n      \"pmids\": [\"11502259\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"SPAR is targeted for proteasomal degradation by the SCF(β-TrCP) E3 ubiquitin ligase complex. This degradation requires activity-inducible polo-like kinase 2 (Plk2/SNK), which phosphorylates SPAR at a canonical phosphodegron, enabling physical association with β-TrCP. Dominant-negative β-TrCP or Cul1 constructs in hippocampal neurons prevented Plk2-dependent SPAR degradation.\",\n      \"method\": \"Co-immunoprecipitation of SPAR with SCF(β-TrCP) components, dominant-negative overexpression in hippocampal neurons, phosphodegron identification\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — Co-IP showing physical association with E3 complex, kinase requirement established, dominant-negative rescue in neurons, multiple orthogonal methods\",\n      \"pmids\": [\"18723513\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The EphA4 receptor C-terminus interacts with the PDZ domain of SPAR, mediating EphA4-dependent inactivation of Rap1 and Rap2 GTPases. SPAR-mediated inactivation of Rap1 (but not Rap2) is required for ephrin-A-dependent growth cone collapse in hippocampal neurons and decreased integrin-mediated adhesion in neuronal cells.\",\n      \"method\": \"Co-immunoprecipitation of EphA4 with SPAR, dominant-negative and constitutively active Rap1/Rap2 constructs, hippocampal neuron growth cone collapse assay, integrin adhesion assay\",\n      \"journal\": \"The Journal of neuroscience : the official journal of the Society for Neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP, gain/loss-of-function with specific cellular readouts, Rap1 vs Rap2 specificity defined\",\n      \"pmids\": [\"18094260\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"ProSAPiP1, a novel PSD protein, binds SPAR via a central coiled-coil/leucine zipper region and recruits SPAR to synapses. ProSAPiP1 also binds ProSAP2/Shank3 via the PDZ domain of Shank3, linking SPAR to the ProSAP/Shank scaffold at excitatory synapses.\",\n      \"method\": \"Co-immunoprecipitation, yeast two-hybrid, co-localization in hippocampal neurons\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP and localization, single lab, two orthogonal binding methods\",\n      \"pmids\": [\"16522626\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"SPAR interacts with PDLIM5/Enigma Homolog (ENH) at the postsynaptic density. PDLIM5 promotes decreased dendritic spine head size and elongated filopodia-like morphology, opposing SPAR's spine head enlargement effect. PKC activation promotes delivery of PDLIM5 into dendritic spines and increases its colocalization with SPAR.\",\n      \"method\": \"Co-immunoprecipitation, RNAi knockdown of PDLIM5, overexpression in hippocampal neurons, pharmacological PKC activation, fluorescence microscopy\",\n      \"journal\": \"Molecular and cellular neurosciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP, RNAi, and overexpression with morphological readout, single lab\",\n      \"pmids\": [\"19900557\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"SPAR interacts with alpha-actinin2, an actin-crosslinking protein. SPAR promotes spine head enlargement while increased alpha-actinin2 favors spine elongation/thinning; together they can generate additive combination spine/filopodia hybrid structures, identifying a molecular pathway bridging the actin cytoskeleton and Rap signaling at synapses.\",\n      \"method\": \"Co-immunoprecipitation, overexpression of SPAR and alpha-actinin2 in hippocampal neurons, morphological analysis\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — Co-IP and neuronal overexpression with morphological readout, single lab, single study\",\n      \"pmids\": [\"19393616\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"LTP induction in hippocampal CA1 neurons triggers ubiquitin-proteasome system (UPS)-dependent degradation of SPAR. This involves two complementary mechanisms: one requiring cyclin-dependent kinase 5 (CDK5) and protein synthesis, and a second requiring UPS and NMDA receptor activation but not CDK5.\",\n      \"method\": \"Confocal microscopy of eGFP-tagged SPAR in acute hippocampal slices, LTP induction, pharmacological inhibition of CDK5 and proteasome, protein synthesis inhibitor\",\n      \"journal\": \"Synapse (New York, N.Y.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — live imaging of tagged protein in acute slices, pharmacological dissection of two mechanistic pathways, single lab\",\n      \"pmids\": [\"21987493\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"ProSAPiP1 regulates SPAR levels at the postsynaptic density and the maturation of dendritic spines in hippocampal neurons. Lentiviral overexpression and knockdown of ProSAPiP1 demonstrated that it is dispensable for formation of synaptic specializations per se but controls SPAR abundance at the PSD.\",\n      \"method\": \"Lentiviral overexpression and knockdown in hippocampal neurons, immunofluorescence quantification of SPAR at PSD\",\n      \"journal\": \"Frontiers in synaptic neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — bidirectional genetic manipulation with quantitative protein-level readout, single lab\",\n      \"pmids\": [\"27252646\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"The SPAAR micropeptide encoded by LINC00961 promotes endothelial cell tubule formation; overexpression of the SPAAR ORF increased tubule formation in primary endothelial cells. Pull-down of SPAAR identified the actin-binding protein SYNE1 as a binding partner of SPAAR in endothelial cells. LINC00961 locus knockout mice showed reduced capillary density in ischemic muscle after hindlimb ischemia.\",\n      \"method\": \"ORF overexpression in primary ECs, tubule formation assay, pull-down/mass spectrometry identification of SPAAR binding partners, CRISPR KO mouse with hindlimb ischemia model\",\n      \"journal\": \"Cardiovascular research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pull-down MS identification of binding partner, in vivo KO phenotype, overexpression functional assay, single lab\",\n      \"pmids\": [\"31990292\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"SPAAR is conserved across mammals (including marsupials and monotremes) and retains conserved protein structure despite primary sequence divergence. Two independent de novo origination events of 5'-elongated SPAAR isoforms from noncoding sequence occurred in primates, with evidence of adaptive evolution in the extended region.\",\n      \"method\": \"Syntenic alignments, homology searches, comparative genomics, evolutionary rate analysis\",\n      \"journal\": \"Genes\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 / Weak — computational/comparative genomic analysis only, no direct biochemical experiment on the protein\",\n      \"pmids\": [\"34946813\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SPAAR/SPAR encodes two functionally distinct proteins: (1) a lysosome-localized micropeptide (SPAAR) that binds the v-ATPase to specifically suppress amino acid-stimulated mTORC1 activation, enabling tissue-specific regulation of muscle regeneration and angiogenesis; and (2) a postsynaptic RapGAP (SPAR) that forms a complex with PSD-95 and NMDA receptors at dendritic spines, inactivates Rap GTPases downstream of EphA4 to regulate spine morphology and growth cone collapse, and is targeted for proteasomal degradation through activity-induced Plk2/SNK phosphorylation at a phosphodegron recognized by the SCF(β-TrCP) E3 ligase.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"The SPAAR/SPAR locus (LINC00961) encodes a polypeptide with two functionally distinct, tissue-specific activities [#0, #1]. In non-neuronal tissues, the SPAAR micropeptide localizes to the late endosome/lysosome and binds the lysosomal v-ATPase to selectively suppress amino acid-stimulated mTORC1 activation, and its downregulation upon skeletal muscle injury licenses mTORC1 signaling to promote muscle regeneration [#0]; in endothelial cells SPAAR binds the actin-associated protein SYNE1 and supports tubule formation and capillary density in ischemic muscle [#9]. In neurons, SPAR functions as a Rap-specific GTPase-activating protein that binds the guanylate kinase-like domain of PSD-95, forms a complex with NMDA receptors, reorganizes the actin cytoskeleton, and enlarges dendritic spine heads [#1]. SPAR acts downstream of the EphA4 receptor, whose C-terminus engages the SPAR PDZ domain to drive Rap1 inactivation required for ephrin-A-dependent growth cone collapse [#3], and is integrated into the postsynaptic scaffold and actin machinery through ProSAPiP1/Shank3, PDLIM5, and alpha-actinin2 [#4, #5, #6]. SPAR abundance is tightly controlled by activity-dependent proteasomal degradation: Plk2/SNK phosphorylates a phosphodegron that recruits the SCF(β-TrCP) E3 ligase, and LTP induction triggers UPS-dependent SPAR turnover via CDK5- and NMDA receptor-dependent routes [#2, #7].\",\n  \"teleology\": [\n    {\n      \"year\": 2001,\n      \"claim\": \"Established SPAR as a postsynaptic Rap-GAP that physically couples to the PSD-95/NMDA receptor scaffold and shapes dendritic spine morphology, defining its core synaptic function.\",\n      \"evidence\": \"Co-IP from brain lysate, heterologous actin readout, and hippocampal neuron gain/loss-of-function with domain deletions\",\n      \"pmids\": [\"11502259\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endogenous loss-of-function phenotype not established\", \"Downstream Rap effectors driving spine enlargement not resolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Identified how SPAR is physically recruited to synapses, linking it to the ProSAP/Shank scaffold via ProSAPiP1.\",\n      \"evidence\": \"Co-IP, yeast two-hybrid, and co-localization in hippocampal neurons\",\n      \"pmids\": [\"16522626\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Functional consequence of recruitment for spine signaling not tested here\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Placed SPAR in a receptor-driven signaling axis by showing EphA4 engages its PDZ domain to inactivate Rap1 for growth cone collapse and adhesion control.\",\n      \"evidence\": \"Co-IP of EphA4 with SPAR, dominant-negative/constitutively active Rap constructs, growth cone collapse and integrin adhesion assays\",\n      \"pmids\": [\"18094260\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Spatial dynamics of EphA4-SPAR recruitment not resolved\", \"Rap2-independence of collapse mechanism not fully explained\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Defined the degradation mechanism controlling SPAR levels: activity-induced Plk2 phosphorylation creates a phosphodegron recognized by SCF(β-TrCP).\",\n      \"evidence\": \"Co-IP of SPAR with SCF components, phosphodegron mapping, dominant-negative β-TrCP/Cul1 rescue in neurons\",\n      \"pmids\": [\"18723513\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise phosphodegron residues and Plk2 kinetics in vivo\", \"Link to spine remodeling outcome not directly demonstrated\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Expanded the SPAR interactome to actin/scaffold partners that counterbalance its spine-enlarging activity, revealing combinatorial control of spine morphology.\",\n      \"evidence\": \"Co-IP, RNAi/overexpression of PDLIM5 and alpha-actinin2 in hippocampal neurons with morphological readouts and PKC pharmacology\",\n      \"pmids\": [\"19900557\", \"19393616\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab interactions\", \"Direct vs indirect binding to actin machinery not fully dissected\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Connected SPAR turnover to synaptic plasticity by showing LTP triggers UPS-dependent SPAR degradation through parallel CDK5- and NMDA receptor-dependent routes.\",\n      \"evidence\": \"Live imaging of eGFP-SPAR in acute hippocampal slices with LTP induction and pharmacological inhibition of CDK5, proteasome, and protein synthesis\",\n      \"pmids\": [\"21987493\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Relationship between CDK5 and Plk2/β-TrCP routes unresolved\", \"Single lab\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Revealed a second, non-neuronal function of the same locus: the SPAAR micropeptide restrains amino acid-specific mTORC1 activation via the lysosomal v-ATPase to gate muscle regeneration.\",\n      \"evidence\": \"Subcellular fractionation, reciprocal Co-IP with v-ATPase, SPAR-specific CRISPR KO mouse, in vivo muscle injury model\",\n      \"pmids\": [\"28024296\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis for amino-acid versus growth-factor selectivity unclear\", \"How v-ATPase binding mechanistically blocks mTORC1 not resolved\"],\n      \"also_pmids\": [\"27252646\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Extended SPAAR function to angiogenesis, identifying SYNE1 as a binding partner and an in vivo role in capillary formation after ischemia.\",\n      \"evidence\": \"ORF overexpression and tubule assays in primary endothelial cells, pull-down/MS, LINC00961 KO mouse hindlimb ischemia\",\n      \"pmids\": [\"31990292\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional significance of SYNE1 binding not mechanistically defined\", \"Relationship to the v-ATPase/mTORC1 axis in endothelium unknown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Provided evolutionary context, showing SPAAR's protein structure is conserved across mammals with independent primate-specific isoform origination.\",\n      \"evidence\": \"Comparative genomics, syntenic alignment, and evolutionary rate analysis\",\n      \"pmids\": [\"34946813\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Computational only, no biochemical test\", \"Functional consequence of primate isoforms unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How a single short ORF executes two unrelated molecular functions — lysosomal mTORC1 suppression versus postsynaptic Rap-GAP/scaffold activity — and whether these reflect distinct isoforms, cell-type-specific partners, or shared structural elements remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structural model reconciling the two activities\", \"Cross-tissue comparison of the same protein product lacking\", \"GAP catalytic versus micropeptide functions not tested in the same system\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1, 3]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [1, 4]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [1, 6, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 3]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 3]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [1, 7]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"complexes\": [\n      \"PSD-95/NMDA receptor complex\",\n      \"SCF(β-TrCP) E3 ubiquitin ligase\",\n      \"v-ATPase\"\n    ],\n    \"partners\": [\n      \"PSD-95\",\n      \"EphA4\",\n      \"BTRC\",\n      \"ProSAPiP1\",\n      \"SHANK3\",\n      \"PDLIM5\",\n      \"ACTN2\",\n      \"SYNE1\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"tie","faith_supported":5,"faith_total":5,"faith_pct":100.0}}