{"gene":"SPSB3","run_date":"2026-06-10T07:46:41","timeline":{"discoveries":[{"year":2024,"finding":"SPSB3 acts as the substrate receptor for the CRL5 (cullin-RING ubiquitin ligase 5) complex and directly targets nuclear cGAS for ubiquitylation and proteasomal degradation. A cryo-EM structure of nucleosome-bound cGAS complexed with SPSB3 revealed that a conserved Asn-Asn (NN) minimal degron motif at the C-terminus of cGAS is required for SPSB3 recruitment, ubiquitylation, and cGAS protein stability. Interference with SPSB3-regulated nuclear cGAS degradation primes cells for type I interferon signalling.","method":"Cryo-EM structure determination, Co-immunoprecipitation, ubiquitylation assays, CRISPR/loss-of-function experiments, type I interferon signalling readouts","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structure plus functional validation (ubiquitylation assay, mutagenesis of degron, cellular signalling readout) in a single rigorous study","pmids":["38418882"],"is_preprint":false},{"year":2017,"finding":"SPSB3 functions as an E3 ligase component that promotes polyubiquitination and proteasomal degradation of the EMT transcription factor SNAIL in a manner dependent on prior GSK-3β phosphorylation of SNAIL. SPSB3 overexpression inhibits tumor metastasis in vitro and in vivo by reducing SNAIL levels.","method":"Genome-wide siRNA screen (luciferase-based), co-immunoprecipitation, ubiquitination assays, overexpression/knockdown with in vitro and in vivo metastasis assays","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional screen identification plus Co-IP and ubiquitination assay in single lab with in vivo confirmation","pmids":["29059170"],"is_preprint":false},{"year":2005,"finding":"SSB-3 (SPSB3) binds MET (hepatocyte growth factor receptor) similarly to the other SSB family members. Negative finding specific to SPSB3: unlike SSB-1, SSB-2, and SSB-4, SSB-3 does not bind Par-4 (prostate apoptosis response protein-4), indicating its SPRY domain loop regions differ functionally from the other paralogs.","method":"Co-immunoprecipitation (binding to MET); NMR structure of SSB-2 SPRY domain with mutational analysis defining Par-4 binding residues; direct binding assays for Par-4 across SSB family members","journal":"Nature structural & molecular biology / The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP for MET binding (all four SSBs), NMR structure plus mutagenesis for Par-4 non-binding by SSB-3; two independent papers converge","pmids":["16369487","15713673"],"is_preprint":false},{"year":2019,"finding":"SPSB1 and SPSB4, but not SPSB2 and SPSB3, interact with and facilitate ubiquitination and degradation of the circadian clock protein RevErbα, and regulate circadian period. SPSB3 does not participate in this substrate pathway.","method":"Cell-based functional ubiquitin ligase screen, co-immunoprecipitation, ubiquitination assays, circadian period measurements","journal":"Journal of biological rhythms","confidence":"Low","confidence_rationale":"Tier 3 / Weak — negative result for SPSB3 specifically (it does not interact with RevErbα), reported as part of a paralog comparison in a single lab study","pmids":["31607207"],"is_preprint":false},{"year":2025,"finding":"SPSB3 binds FOG-2 independently of the D-L-N-N-N motif recognized by SPSB1/2/4, but SPSB3 fails to trigger FOG-2 ubiquitin-proteasome-dependent degradation, demonstrating that SPSB3 employs a substrate-recognition mechanism distinct from SPSB1/2/4 and is not sufficient for FOG-2 degradation.","method":"Co-immunoprecipitation, western blot, ubiquitination assays, motif mutagenesis, 3T3-L1 adipocyte differentiation model","journal":"Biochemical and biophysical research communications","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, Co-IP and western blot only; SPSB3 binding confirmed but degradation activity is a negative result","pmids":["41418348"],"is_preprint":false},{"year":2026,"finding":"SPSB3 binds the mitochondrial protein TUFM (Tu translation elongation factor, mitochondrial) and promotes its K48- and K63-linked ubiquitination at the K259 residue, leading to TUFM degradation. SPSB3 inhibition prevents TUFM ubiquitination and degradation, attenuating cardiomyocyte apoptosis in myocardial ischemia/reperfusion injury models.","method":"CRISPR/Cas9 high-throughput screening, mass spectrometry, ubiquitin-modified proteomics, co-immunoprecipitation, western blot, amino acid site mutagenesis (K259), MG132/cycloheximide treatment, mouse and cardiomyocyte I/R injury models","journal":"Cell biology and toxicology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP plus site-directed mutagenesis identifying the ubiquitination site plus proteasome inhibitor validation, multiple orthogonal methods in single lab","pmids":["42082827"],"is_preprint":false},{"year":2025,"finding":"SPSB3 interacts with ElonginC/B and recruits Cullin5 to form the ECS E3 ligase complex. Knockout of Spsb3 in mice (via CRISPR/Cas9) does not produce defects in sperm quality, fertility, or testis histology, demonstrating that SPSB3 is not essential for spermatogenesis or male fertility under physiological conditions.","method":"CRISPR/Cas9 knockout mice, CASA sperm analysis, histology, immunostaining, TUNEL assay","journal":"American journal of translational research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean KO with defined phenotypic readout (fertility, sperm quality, histology); negative result is robustly established by multiple complementary assays","pmids":["40225996"],"is_preprint":false}],"current_model":"SPSB3 is a substrate-recognition subunit of the CRL5 cullin-RING E3 ubiquitin ligase complex (via ElonginC/B–Cullin5 recruitment) that uses its SPRY domain to recognize specific degron motifs on substrates—most notably an Asn-Asn (NN) motif on nuclear cGAS—targeting them for polyubiquitination and proteasomal degradation; established substrates include nuclear cGAS (suppressing innate immune activation), SNAIL (regulating EMT), and TUFM (modulating cardiomyocyte apoptosis), while SPSB3 binds but does not degrade FOG-2 and does not interact with RevErbα, distinguishing it functionally from its paralogs SPSB1, SPSB2, and SPSB4."},"narrative":{"mechanistic_narrative":"SPSB3 is the substrate-recognition subunit of a CRL5/ECS cullin-RING E3 ubiquitin ligase complex, assembling with ElonginC/B and Cullin5 to direct selected substrates for polyubiquitination and proteasomal degradation [PMID:38418882, PMID:40225996]. Its SPRY domain reads short degron motifs on substrates, most rigorously defined for nuclear cGAS, where a conserved C-terminal Asn-Asn (NN) motif is required for SPSB3 recruitment and cGAS turnover; loss of this degradation primes cells for type I interferon signalling, placing SPSB3 as a brake on innate immune activation [PMID:38418882]. Beyond cGAS, SPSB3 promotes GSK-3β-phosphorylation-dependent ubiquitination and degradation of the EMT transcription factor SNAIL, restraining tumor metastasis [PMID:29059170], and ubiquitinates the mitochondrial elongation factor TUFM at K259 to modulate cardiomyocyte apoptosis in ischemia/reperfusion injury [PMID:42082827]. Substrate selectivity distinguishes SPSB3 from its paralogs: it binds MET and FOG-2 but does not degrade FOG-2, and does not engage RevErbα or Par-4, indicating its SPRY-domain loops recognize substrates through a mechanism distinct from SPSB1/2/4 [PMID:16369487, PMID:15713673, PMID:31607207, PMID:41418348]. SPSB3 is dispensable for spermatogenesis and male fertility in mice [PMID:40225996].","teleology":[{"year":2005,"claim":"Established early that SPSB3 shares some binding partners with its paralogs yet has divergent substrate selectivity, framing the SPRY domain as the determinant of specificity.","evidence":"Co-IP of MET across all SSB family members plus NMR structure and mutagenesis defining Par-4 binding residues, showing SSB-3 binds MET but not Par-4","pmids":["16369487","15713673"],"confidence":"Medium","gaps":["No E3 ligase activity or substrate degradation demonstrated at this stage","Functional consequence of MET binding not established"]},{"year":2017,"claim":"Identified the first degradation substrate, linking SPSB3 to phospho-dependent control of an EMT driver and to metastasis suppression.","evidence":"Genome-wide siRNA luciferase screen, Co-IP, ubiquitination assays, and in vitro/in vivo metastasis assays showing GSK-3β-dependent SNAIL degradation","pmids":["29059170"],"confidence":"Medium","gaps":["Degron motif on SNAIL recognized by SPSB3 not defined","Direct vs indirect ubiquitination not structurally resolved"]},{"year":2019,"claim":"Refined paralog division of labor by showing SPSB3 does not participate in RevErbα turnover or circadian regulation, unlike SPSB1/4.","evidence":"Cell-based ubiquitin ligase screen, Co-IP, ubiquitination assays, and circadian period measurements (negative result for SPSB3)","pmids":["31607207"],"confidence":"Low","gaps":["Negative result reported only within a paralog comparison from a single lab","Does not address what distinguishes SPSB3 substrate loops mechanistically"]},{"year":2024,"claim":"Defined the molecular basis of substrate recognition: SPSB3 is the CRL5 receptor for nuclear cGAS via an NN minimal degron, establishing SPSB3 as a regulator of innate immune activation.","evidence":"Cryo-EM of nucleosome-bound cGAS–SPSB3, Co-IP, ubiquitylation assays, CRISPR loss-of-function, and type I interferon readouts","pmids":["38418882"],"confidence":"High","gaps":["Whether the NN degron generalizes to other SPSB3 substrates not established","Regulation of SPSB3 activity/expression in immune contexts unresolved"]},{"year":2025,"claim":"Confirmed the canonical ECS architecture (ElonginC/B–Cullin5 recruitment) and tested physiological essentiality, showing SPSB3 is dispensable for male fertility.","evidence":"CRISPR/Cas9 knockout mice with CASA sperm analysis, histology, immunostaining, and TUNEL; Co-IP for ElonginC/B/Cullin5 assembly","pmids":["40225996"],"confidence":"Medium","gaps":["Phenotypes in other tissues/contexts not examined","Possible functional redundancy with paralogs masking a phenotype not tested"]},{"year":2025,"claim":"Demonstrated that binding does not equal degradation: SPSB3 engages FOG-2 through a non-canonical motif but fails to drive its turnover, reinforcing a distinct recognition mechanism.","evidence":"Co-IP, western blot, ubiquitination assays, and motif mutagenesis in a 3T3-L1 adipocyte differentiation model","pmids":["41418348"],"confidence":"Low","gaps":["Single lab, Co-IP and western blot only; degradation activity is a negative result","Why binding fails to license ubiquitination not mechanistically explained"]},{"year":2026,"claim":"Extended the substrate repertoire to a mitochondrial target, mapping a specific ubiquitination site and connecting SPSB3 activity to cardiomyocyte survival.","evidence":"CRISPR screen, mass spectrometry/ubiquitin proteomics, Co-IP, K259 site mutagenesis, MG132/CHX, and mouse/cardiomyocyte I/R injury models","pmids":["42082827"],"confidence":"Medium","gaps":["Degron motif on TUFM recognized by SPSB3 not defined","Mixed K48/K63 linkage roles not fully separated"]},{"year":null,"claim":"A unifying degron code for SPSB3 across its diverse substrates and the basis for binding-without-degradation outcomes remain unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No general SPRY-domain recognition rule reconciling cGAS, SNAIL, TUFM, FOG-2, and MET","Regulation of SPSB3 expression/activity across tissues unknown","Determinants converting substrate binding into productive ubiquitination undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,1,5]},{"term_id":"GO:0016874","term_label":"ligase activity","supporting_discovery_ids":[0,6]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,6]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,1,5]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0]}],"complexes":["CRL5/ECS (ElonginB/C–Cullin5) E3 ubiquitin ligase"],"partners":["CUL5","ELOC","ELOB","CGAS","SNAIL","TUFM","MET","FOG-2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q6PJ21","full_name":"SPRY domain-containing SOCS box protein 3","aliases":[],"length_aa":355,"mass_kda":39.4,"function":"Substrate-recognition component of a cullin-5-RING E3 ubiquitin-protein ligase complex (ECS complex, also named CRL5 complex), which mediates the ubiquitination and subsequent proteasomal degradation of target proteins, such as CGAS and SNAI1 (PubMed:29059170, PubMed:38418882). The ECS(SPSB3) complex catalyzes 'Lys-48'-linked ubiquitination of nuclear CGAS in cycling cells, leading to its degradation (PubMed:38418882). Recognizes and binds nucleosome-bound CGAS: ubiquitination and degradation of nuclear CGAS during G1 and G2 phases is required to promote low intranuclear CGAS abundance before the next mitotic cycle (PubMed:38418882). The ECS(SPSB3) complex also mediates ubiquitination and degradation of phosphorylated SNAI1 (PubMed:29059170)","subcellular_location":"Nucleus","url":"https://www.uniprot.org/uniprotkb/Q6PJ21/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SPSB3","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SPSB3","total_profiled":1310},"omim":[{"mim_id":"611659","title":"SPRY DOMAIN- AND SOCS BOX-CONTAINING 3; SPSB3","url":"https://www.omim.org/entry/611659"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Mitochondria","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SPSB3"},"hgnc":{"alias_symbol":["SSB-3"],"prev_symbol":["C16orf31"]},"alphafold":{"accession":"Q6PJ21","domains":[{"cath_id":"2.60.120.920","chopping":"103-275","consensus_level":"high","plddt":97.122,"start":103,"end":275},{"cath_id":"-","chopping":"277-292_305-322","consensus_level":"medium","plddt":89.2588,"start":277,"end":322}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q6PJ21","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q6PJ21-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q6PJ21-F1-predicted_aligned_error_v6.png","plddt_mean":76.69},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SPSB3","jax_strain_url":"https://www.jax.org/strain/search?query=SPSB3"},"sequence":{"accession":"Q6PJ21","fasta_url":"https://rest.uniprot.org/uniprotkb/Q6PJ21.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q6PJ21/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q6PJ21"}},"corpus_meta":[{"pmid":"10782989","id":"PMC_10782989","title":"Interaction of E. coli single-stranded DNA binding protein (SSB) with exonuclease I. The carboxy-terminus of SSB is the recognition site for the nuclease.","date":"2000","source":"Biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/10782989","citation_count":105,"is_preprint":false},{"pmid":"16369487","id":"PMC_16369487","title":"The SPRY domain of SSB-2 adopts a novel fold that presents conserved Par-4-binding residues.","date":"2005","source":"Nature structural & molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/16369487","citation_count":64,"is_preprint":false},{"pmid":"27881980","id":"PMC_27881980","title":"Comparative Genomics of DNA Recombination and Repair in Cyanobacteria: Biotechnological Implications.","date":"2016","source":"Frontiers in microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/27881980","citation_count":59,"is_preprint":false},{"pmid":"38418882","id":"PMC_38418882","title":"The CRL5-SPSB3 ubiquitin ligase targets nuclear cGAS for 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A cryo-EM structure of nucleosome-bound cGAS complexed with SPSB3 revealed that a conserved Asn-Asn (NN) minimal degron motif at the C-terminus of cGAS is required for SPSB3 recruitment, ubiquitylation, and cGAS protein stability. Interference with SPSB3-regulated nuclear cGAS degradation primes cells for type I interferon signalling.\",\n      \"method\": \"Cryo-EM structure determination, Co-immunoprecipitation, ubiquitylation assays, CRISPR/loss-of-function experiments, type I interferon signalling readouts\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structure plus functional validation (ubiquitylation assay, mutagenesis of degron, cellular signalling readout) in a single rigorous study\",\n      \"pmids\": [\"38418882\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"SPSB3 functions as an E3 ligase component that promotes polyubiquitination and proteasomal degradation of the EMT transcription factor SNAIL in a manner dependent on prior GSK-3β phosphorylation of SNAIL. SPSB3 overexpression inhibits tumor metastasis in vitro and in vivo by reducing SNAIL levels.\",\n      \"method\": \"Genome-wide siRNA screen (luciferase-based), co-immunoprecipitation, ubiquitination assays, overexpression/knockdown with in vitro and in vivo metastasis assays\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional screen identification plus Co-IP and ubiquitination assay in single lab with in vivo confirmation\",\n      \"pmids\": [\"29059170\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"SSB-3 (SPSB3) binds MET (hepatocyte growth factor receptor) similarly to the other SSB family members. Negative finding specific to SPSB3: unlike SSB-1, SSB-2, and SSB-4, SSB-3 does not bind Par-4 (prostate apoptosis response protein-4), indicating its SPRY domain loop regions differ functionally from the other paralogs.\",\n      \"method\": \"Co-immunoprecipitation (binding to MET); NMR structure of SSB-2 SPRY domain with mutational analysis defining Par-4 binding residues; direct binding assays for Par-4 across SSB family members\",\n      \"journal\": \"Nature structural & molecular biology / The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP for MET binding (all four SSBs), NMR structure plus mutagenesis for Par-4 non-binding by SSB-3; two independent papers converge\",\n      \"pmids\": [\"16369487\", \"15713673\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"SPSB1 and SPSB4, but not SPSB2 and SPSB3, interact with and facilitate ubiquitination and degradation of the circadian clock protein RevErbα, and regulate circadian period. SPSB3 does not participate in this substrate pathway.\",\n      \"method\": \"Cell-based functional ubiquitin ligase screen, co-immunoprecipitation, ubiquitination assays, circadian period measurements\",\n      \"journal\": \"Journal of biological rhythms\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — negative result for SPSB3 specifically (it does not interact with RevErbα), reported as part of a paralog comparison in a single lab study\",\n      \"pmids\": [\"31607207\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SPSB3 binds FOG-2 independently of the D-L-N-N-N motif recognized by SPSB1/2/4, but SPSB3 fails to trigger FOG-2 ubiquitin-proteasome-dependent degradation, demonstrating that SPSB3 employs a substrate-recognition mechanism distinct from SPSB1/2/4 and is not sufficient for FOG-2 degradation.\",\n      \"method\": \"Co-immunoprecipitation, western blot, ubiquitination assays, motif mutagenesis, 3T3-L1 adipocyte differentiation model\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, Co-IP and western blot only; SPSB3 binding confirmed but degradation activity is a negative result\",\n      \"pmids\": [\"41418348\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"SPSB3 binds the mitochondrial protein TUFM (Tu translation elongation factor, mitochondrial) and promotes its K48- and K63-linked ubiquitination at the K259 residue, leading to TUFM degradation. SPSB3 inhibition prevents TUFM ubiquitination and degradation, attenuating cardiomyocyte apoptosis in myocardial ischemia/reperfusion injury models.\",\n      \"method\": \"CRISPR/Cas9 high-throughput screening, mass spectrometry, ubiquitin-modified proteomics, co-immunoprecipitation, western blot, amino acid site mutagenesis (K259), MG132/cycloheximide treatment, mouse and cardiomyocyte I/R injury models\",\n      \"journal\": \"Cell biology and toxicology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP plus site-directed mutagenesis identifying the ubiquitination site plus proteasome inhibitor validation, multiple orthogonal methods in single lab\",\n      \"pmids\": [\"42082827\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SPSB3 interacts with ElonginC/B and recruits Cullin5 to form the ECS E3 ligase complex. Knockout of Spsb3 in mice (via CRISPR/Cas9) does not produce defects in sperm quality, fertility, or testis histology, demonstrating that SPSB3 is not essential for spermatogenesis or male fertility under physiological conditions.\",\n      \"method\": \"CRISPR/Cas9 knockout mice, CASA sperm analysis, histology, immunostaining, TUNEL assay\",\n      \"journal\": \"American journal of translational research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean KO with defined phenotypic readout (fertility, sperm quality, histology); negative result is robustly established by multiple complementary assays\",\n      \"pmids\": [\"40225996\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SPSB3 is a substrate-recognition subunit of the CRL5 cullin-RING E3 ubiquitin ligase complex (via ElonginC/B–Cullin5 recruitment) that uses its SPRY domain to recognize specific degron motifs on substrates—most notably an Asn-Asn (NN) motif on nuclear cGAS—targeting them for polyubiquitination and proteasomal degradation; established substrates include nuclear cGAS (suppressing innate immune activation), SNAIL (regulating EMT), and TUFM (modulating cardiomyocyte apoptosis), while SPSB3 binds but does not degrade FOG-2 and does not interact with RevErbα, distinguishing it functionally from its paralogs SPSB1, SPSB2, and SPSB4.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SPSB3 is the substrate-recognition subunit of a CRL5/ECS cullin-RING E3 ubiquitin ligase complex, assembling with ElonginC/B and Cullin5 to direct selected substrates for polyubiquitination and proteasomal degradation [#0, #6]. Its SPRY domain reads short degron motifs on substrates, most rigorously defined for nuclear cGAS, where a conserved C-terminal Asn-Asn (NN) motif is required for SPSB3 recruitment and cGAS turnover; loss of this degradation primes cells for type I interferon signalling, placing SPSB3 as a brake on innate immune activation [#0]. Beyond cGAS, SPSB3 promotes GSK-3\\u03b2-phosphorylation-dependent ubiquitination and degradation of the EMT transcription factor SNAIL, restraining tumor metastasis [#1], and ubiquitinates the mitochondrial elongation factor TUFM at K259 to modulate cardiomyocyte apoptosis in ischemia/reperfusion injury [#5]. Substrate selectivity distinguishes SPSB3 from its paralogs: it binds MET and FOG-2 but does not degrade FOG-2, and does not engage RevErb\\u03b1 or Par-4, indicating its SPRY-domain loops recognize substrates through a mechanism distinct from SPSB1/2/4 [#2, #3, #4]. SPSB3 is dispensable for spermatogenesis and male fertility in mice [#6].\",\n  \"teleology\": [\n    {\n      \"year\": 2005,\n      \"claim\": \"Established early that SPSB3 shares some binding partners with its paralogs yet has divergent substrate selectivity, framing the SPRY domain as the determinant of specificity.\",\n      \"evidence\": \"Co-IP of MET across all SSB family members plus NMR structure and mutagenesis defining Par-4 binding residues, showing SSB-3 binds MET but not Par-4\",\n      \"pmids\": [\"16369487\", \"15713673\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No E3 ligase activity or substrate degradation demonstrated at this stage\", \"Functional consequence of MET binding not established\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Identified the first degradation substrate, linking SPSB3 to phospho-dependent control of an EMT driver and to metastasis suppression.\",\n      \"evidence\": \"Genome-wide siRNA luciferase screen, Co-IP, ubiquitination assays, and in vitro/in vivo metastasis assays showing GSK-3\\u03b2-dependent SNAIL degradation\",\n      \"pmids\": [\"29059170\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Degron motif on SNAIL recognized by SPSB3 not defined\", \"Direct vs indirect ubiquitination not structurally resolved\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Refined paralog division of labor by showing SPSB3 does not participate in RevErb\\u03b1 turnover or circadian regulation, unlike SPSB1/4.\",\n      \"evidence\": \"Cell-based ubiquitin ligase screen, Co-IP, ubiquitination assays, and circadian period measurements (negative result for SPSB3)\",\n      \"pmids\": [\"31607207\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Negative result reported only within a paralog comparison from a single lab\", \"Does not address what distinguishes SPSB3 substrate loops mechanistically\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Defined the molecular basis of substrate recognition: SPSB3 is the CRL5 receptor for nuclear cGAS via an NN minimal degron, establishing SPSB3 as a regulator of innate immune activation.\",\n      \"evidence\": \"Cryo-EM of nucleosome-bound cGAS\\u2013SPSB3, Co-IP, ubiquitylation assays, CRISPR loss-of-function, and type I interferon readouts\",\n      \"pmids\": [\"38418882\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether the NN degron generalizes to other SPSB3 substrates not established\", \"Regulation of SPSB3 activity/expression in immune contexts unresolved\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Confirmed the canonical ECS architecture (ElonginC/B\\u2013Cullin5 recruitment) and tested physiological essentiality, showing SPSB3 is dispensable for male fertility.\",\n      \"evidence\": \"CRISPR/Cas9 knockout mice with CASA sperm analysis, histology, immunostaining, and TUNEL; Co-IP for ElonginC/B/Cullin5 assembly\",\n      \"pmids\": [\"40225996\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Phenotypes in other tissues/contexts not examined\", \"Possible functional redundancy with paralogs masking a phenotype not tested\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Demonstrated that binding does not equal degradation: SPSB3 engages FOG-2 through a non-canonical motif but fails to drive its turnover, reinforcing a distinct recognition mechanism.\",\n      \"evidence\": \"Co-IP, western blot, ubiquitination assays, and motif mutagenesis in a 3T3-L1 adipocyte differentiation model\",\n      \"pmids\": [\"41418348\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Single lab, Co-IP and western blot only; degradation activity is a negative result\", \"Why binding fails to license ubiquitination not mechanistically explained\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Extended the substrate repertoire to a mitochondrial target, mapping a specific ubiquitination site and connecting SPSB3 activity to cardiomyocyte survival.\",\n      \"evidence\": \"CRISPR screen, mass spectrometry/ubiquitin proteomics, Co-IP, K259 site mutagenesis, MG132/CHX, and mouse/cardiomyocyte I/R injury models\",\n      \"pmids\": [\"42082827\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Degron motif on TUFM recognized by SPSB3 not defined\", \"Mixed K48/K63 linkage roles not fully separated\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"A unifying degron code for SPSB3 across its diverse substrates and the basis for binding-without-degradation outcomes remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No general SPRY-domain recognition rule reconciling cGAS, SNAIL, TUFM, FOG-2, and MET\", \"Regulation of SPSB3 expression/activity across tissues unknown\", \"Determinants converting substrate binding into productive ubiquitination undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 1, 5]},\n      {\"term_id\": \"GO:0016874\", \"supporting_discovery_ids\": [0, 6]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 1, 5]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"complexes\": [\"CRL5/ECS (ElonginB/C–Cullin5) E3 ubiquitin ligase\"],\n    \"partners\": [\"CUL5\", \"ELOC\", \"ELOB\", \"cGAS\", \"SNAIL\", \"TUFM\", \"MET\", \"FOG-2\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}