id: A0A9L0IQA3 gene_symbol: FBXL20 product_type: PROTEIN status: COMPLETE taxon: id: NCBITaxon:9793 label: Equus asinus description: FBXL20 (F-box/LRR-repeat protein 20; SCRAPPER in mouse) is a substrate-recognition subunit of SCF (SKP1-CUL1-RBX1) E3 ubiquitin ligase complexes. Its N-terminal F-box binds SKP1, linking it to the cullin-RING catalytic core, and its C-terminal leucine-rich repeat domain binds substrates, which are polyubiquitinated and degraded by the proteasome. A C-terminal CAAX-type prenylation motif targets the protein to membranes. In mouse neurons SCRAPPER is enriched at the presynaptic membrane and is the main ubiquitin ligase for the active-zone protein RIM1, tuning synaptic vesicle release; in human cells FBXL20 targets the class III PI3K VPS34/PIK3C3 for degradation after DNA damage, restraining autophagy, and promotes ubiquitination and degradation of the PP2A regulatory subunit PR55alpha/PPP2R2A. The donkey gene product A0A9L0IQA3 (448 aa) is an alternative-first-exon form of FBXL20 that differs from the canonical 436-aa mammalian protein only in its first 26 residues and retains the complete F-box, LRR domain and CAAX motif. existing_annotations: - term: id: GO:0005737 label: cytoplasm evidence_type: IEA original_reference_id: GO_REF:0000044 qualifier: located_in supporting_entities: - UniProtKB-SubCell:SL-0086 review: summary: Cytoplasm, from the UniProt subcellular-location mapping (the donkey SubCell line is itself an ARBA prediction). FBXL20 orthologs are cytoplasmic SCF substrate receptors that associate with membranes through C-terminal prenylation. action: ACCEPT reason: Consistent with the reviewed human ortholog (Cytoplasm, by similarity) and with the mouse ortholog acting as a prenylated, membrane-associated E3 on the cytoplasmic face of the presynaptic membrane. The donkey product keeps the CAAX tail, so the same location applies. Membrane association is an additional, not contradictory, location; no more specific cytoplasmic term is supported for this product without tissue context. supported_by: - reference_id: file:human/FBXL20/FBXL20-uniprot.txt record_section: ["CC:SUBCELLULAR LOCATION"] supporting_text: 'SUBCELLULAR LOCATION: Cytoplasm {ECO:0000250}.' - reference_id: PMID:17803915 supporting_text: The CAAX domain is a carboxyl-terminal membrane-sorting signal induced by prenylation - reference_id: file:EQUAS/FBXL20/FBXL20-bioinformatics/RESULTS-rereview.md supporting_text: Downstream of the motif, A0A9L0IQA3 has the same 422-residue length as the - term: id: GO:0019005 label: SCF ubiquitin ligase complex evidence_type: IEA original_reference_id: GO_REF:0000118 qualifier: part_of supporting_entities: - PANTHER:PTN002541195 review: summary: SCF ubiquitin ligase complex, from TreeGrafter grafting into PANTHER PTHR13318 (subfamily SF47, F-box/LRR-repeat protein 20). The same term reaches mouse Fbxl20 by IBA from the PAINT node PTN002547163, and mouse SCRAPPER was shown to form an SCF complex with Skp1 and Cullin1 in brain. action: ACCEPT reason: The donkey protein is in the FBXL20 subfamily (PTHR13318:SF47) together with the reviewed human, mouse and bovine FBXL20, and it retains the F-box (34-80) that mediates SKP1 binding. SCF membership is experimentally established for the mouse ortholog and is the PAINT-curated ancestral state of the clade. supported_by: - reference_id: PMID:17803915 supporting_text: Similarly, SCRAPPER formed an SCF complex with Skp1 and Cullin1 in mouse brain lysates - reference_id: PMID:34731788 supporting_text: an F-box protein that serves as a substrate recognition component of the SCF (Skp1_Cul1_F-box) E3 ubiquitin ligase complexes that promote proteasomal degradation - reference_id: file:human/FBXL20/FBXL20-uniprot.txt record_section: ["CC:SUBUNIT"] supporting_text: 'SUBUNIT: Interacts with SKP1 and CUL1. {ECO:0000250}.' - term: id: GO:0031146 label: SCF-dependent proteasomal ubiquitin-dependent protein catabolic process evidence_type: IEA original_reference_id: GO_REF:0000118 qualifier: involved_in supporting_entities: - PANTHER:PTN002541195 review: summary: SCF-dependent proteasomal protein catabolism, from TreeGrafter. Mouse SCRAPPER mediates ubiquitination and proteasomal degradation of RIM1, and human FBXL20 that of VPS34 and PR55alpha. action: ACCEPT reason: FBXL20 performs part of this process itself, as the subunit that binds and selects the substrate for SCF-catalysed ubiquitination; knockout of the mouse ortholog lengthens RIM1 half-life and reduces its ubiquitination. Same term and node as the PAINT IBD propagated to mouse Fbxl20. supported_by: - reference_id: PMID:17803915 supporting_text: In neurons from Scrapper -knockout (SCR-KO) mice, RIM1 had a longer half-life with significant reduction in ubiquitination, indicating that SCRAPPER is the predominant ubiquitin ligase that mediates RIM1 degradation. - reference_id: PMID:33234069 supporting_text: Removal of Vps34 by Fbxl20-mediated ubiquitination leads to inhibition of autophagy. - reference_id: PMID:34731788 supporting_text: reduced protein expression of FBXL20 and decreased ubiquitination of PR55α. - term: id: GO:1990756 label: ubiquitin-like ligase-substrate adaptor activity evidence_type: ISS original_reference_id: PMID:34731788 review: summary: FBXL20 binds substrates (RIM1 in mouse, VPS34 and PR55alpha in human) through its LRR domain and links them to the SCF core through its F-box; this adaptor activity is its molecular function. action: NEW reason: None of the donkey GOA rows captures the molecular function. Transferred from the FBXL20 orthologs of the same PANTHER subfamily (SF47), where mouse SCRAPPER binds RIM1 and is required for its ubiquitination and human FBXL20 binds PR55alpha and is required for its ubiquitination. The PAINT tree has IBDs for this term at other PTHR13318 nodes (PTN004419270, PTN009029653) but not on the FBXL20 lineage, whose donors carry no experimental MF annotation beyond protein binding; the absence reflects missing descendant MF evidence, not a curated exclusion. The donkey product retains every domain needed (F-box, LRR, CAAX). supported_by: - reference_id: PMID:34731788 supporting_text: FBXL20 interacts with PR55α in vitro and in vivo - reference_id: PMID:34731788 supporting_text: an F-box protein that serves as a substrate recognition component of the SCF (Skp1_Cul1_F-box) E3 ubiquitin ligase complexes that promote proteasomal degradation - reference_id: PMID:17803915 supporting_text: Consistent with a function as a ubiquitin ligase, ubiquitinated proteins coimmunoprecipitated (co-IPed) with FLAG-tagged SCRAPPER (FLAG-SCR) from lysates of MG132-treated (a proteasome inhibitor) cells - reference_id: file:EQUAS/FBXL20/FBXL20-bioinformatics/RESULTS-rereview.md supporting_text: Downstream of the motif, A0A9L0IQA3 has the same 422-residue length as the references: - id: GO_REF:0000044 title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt findings: [] - id: GO_REF:0000118 title: TreeGrafter-generated GO annotations findings: [] - id: file:human/FBXL20/FBXL20-uniprot.txt title: UniProt record for human FBXL20 (Q96IG2) findings: [] - id: file:EQUAS/FBXL20/FBXL20-bioinformatics/RESULTS.md title: 'FBXL20: donkey-human sequence comparison' findings: [] reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: Local reproducible global alignment; confirms full-length coverage of human FBXL20 and an identical C-terminus. Its reading of the N-terminal difference as a 12-residue insertion is superseded by RESULTS-rereview.md (alternative first exon). No longer cited as support in this re-review; kept for provenance. - id: file:EQUAS/FBXL20/FBXL20-bioinformatics/RESULTS-rereview.md title: 'FBXL20 (donkey A0A9L0IQA3): N-terminal exon comparison (re-review, 2026-10-21)' findings: [] reference_review: relevance: HIGH correctness: VERIFIED review_notes: Reproducible script (nterm_exon_rereview.py) over UniProt sequences; shows the donkey product is an alternative-first-exon form with the canonical 422-residue F-box/LRR/CAAX body. - id: PMID:17803915 title: SCRAPPER-dependent ubiquitination of active zone protein RIM1 regulates synaptic vesicle release. findings: [] reference_review: relevance: HIGH correctness: VERIFIED review_notes: 'Mouse ortholog (Fbxl20/SCRAPPER) study, full text cached: SCF complex with Skp1 and Cullin1, CAAX-dependent membrane targeting, RIM1 ubiquitination and degradation, knockout synaptic phenotype. Synaptic terms are neural-context roles and are not proposed for this donkey product.' - id: PMID:34731788 title: p53/FBXL20 axis negatively regulates the protein stability of PR55α, a regulatory subunit of PP2A Ser/Thr phosphatase. findings: [] reference_review: relevance: HIGH correctness: VERIFIED review_notes: 'Human cell study, full text cached: FBXL20 binds PR55alpha and is required for its ubiquitination.' - id: PMID:33234069 title: 'The FBXL family of F-box proteins: variations on a theme.' findings: [] reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: Family review summarizing FBXL20-VPS34 autophagy regulation and FBXL2 paralogy. core_functions: - description: Substrate-recognition subunit of SCF E3 ubiquitin ligase complexes; binds SKP1 through its F-box and substrates (such as RIM1, VPS34 and PR55alpha in other mammals) through its LRR domain, directing them to ubiquitination and proteasomal degradation. molecular_function: id: GO:1990756 label: ubiquitin-like ligase-substrate adaptor activity directly_involved_in: - id: GO:0031146 label: SCF-dependent proteasomal ubiquitin-dependent protein catabolic process locations: - id: GO:0005737 label: cytoplasm in_complex: id: GO:0019005 label: SCF ubiquitin ligase complex supported_by: - reference_id: PMID:17803915 supporting_text: Similarly, SCRAPPER formed an SCF complex with Skp1 and Cullin1 in mouse brain lysates - reference_id: PMID:34731788 supporting_text: an F-box protein that serves as a substrate recognition component of the SCF (Skp1_Cul1_F-box) E3 ubiquitin ligase complexes that promote proteasomal degradation - reference_id: PMID:33234069 supporting_text: Removal of Vps34 by Fbxl20-mediated ubiquitination leads to inhibition of autophagy. suggested_questions: - question: Which FBXL20 first exons are used in donkey tissues, and does the alternative N-terminal segment of A0A9L0IQA3 (also found in horse F6YIL4) change localization, stability or substrate choice relative to the canonical MRRDVNGVTKSRFE start? - question: Does donkey FBXL20 regulate VPS34 levels and autophagy after DNA damage, and RIM1 turnover at presynaptic terminals, as its human and mouse orthologs do? suggested_experiments: - description: RT-PCR or long-read transcript sequencing of donkey brain and other tissues to determine which FBXL20 first exons are expressed, followed by comparison of the encoded N-terminal variants for SKP1 binding and membrane association.