Fbxo42 (CG6758) is a 667-residue F-box protein with an N-terminal F-box (residues 21-70) followed by a kelch-type beta-propeller (Pfam Beta-prop_FBX42). It is the substrate-recognition subunit of an SCF (SkpA-Cul1-Roc1a) cullin-RING ubiquitin ligase; its F-box is required for binding to SkpA. Two substrates or targets are characterised in the fly. In S2 cells and ER-stressed tissues, SCF-Fbxo42 binds the RNA-binding protein Ataxin-2, promotes its ubiquitylation (Ataxin-2 Cys244 is the candidate acceptor site), and drives its proteasomal degradation. Fbxo42 is recruited to Ataxin-2 granules under ER or arsenite stress, and this releases sequestered Xbp1 mRNA for translation late in the unfolded protein response; loss of Fbxo42 suppresses Xbp1s-induced retinal cell death. In the female germline, SCF-Fbxo42 lowers the level of the PP2A-B56 regulatory subunit Widerborst (Wrd) and is required for assembly and maintenance of the synaptonemal complex and for karyosome formation. Fbxo42 is expressed in the larval brain, ring gland and eye disc and in the adult brain, testis and ovary.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0019005 SCF ubiquitin ligase complex | IBA GO_REF:0000033 | ACCEPT | Summary: SCF membership is inherited by the FBXO42 clade and is directly shown for the fly protein. Reason: PAINT placed this term at PTN002924600, the FBXO42 node (PANTHER PTHR46432, subfamily SF1, which also contains human FBXO42 Q6P3S6 and mouse Fbxo42 Q6PDJ6). The fly gene's own IDA is the descendant evidence; its appearance in WITH/FROM marks experimental grounding on the target, not circularity. Two independent fly studies show F-box-dependent binding to SkpA and Cul1 (PMID:33382409, PMID:40804044). Supporting Evidence: PMID:33382409 Altogether, we found that two SCF complexes that consist of SkpAβCul1βSlimb/Ξ²Trcp (SCF-Slimb/Ξ²Trcp) and SkpAβCul1βFbxo42 (SCF-Fbxo42) are important for proper formation and maintenance of the synaptonemal complex PMID:40804044 SkpA co-immunoprecipitates with FLAG-HA-Fbxo42, but not with a deletion of the F-box domain (FLAG-HA-ΞFbxo42) file:DROME/Fbxo42/Fbxo42-uniprot.txt DR InterPro; IPR001810; F-box_dom. |
| GO:0019005 SCF ubiquitin ligase complex | IDA PMID:33382409 SCF-Fbxo42 promotes synaptonemal complex assembly by downreg... | ACCEPT | Summary: Ovary immunoprecipitation-mass spectrometry places Fbxo42 in an SkpA-Cul1 SCF complex. Reason: GFP-Fbxo42 immunoprecipitates from ovaries were enriched for SCF subunits, and Fbxo42 was among the F-box proteins recovered with GFP-SkpA. PMID:40804044 independently confirms SkpA binding that depends on the F-box. Supporting Evidence: PMID:33382409 SCF subunits, PP2A core subunits and 14β3-3 proteins were significantly enriched in GFP-Fbxo42 immunoprecipitates PMID:40804044 SkpA co-immunoprecipitates with FLAG-HA-Fbxo42, but not with a deletion of the F-box domain (FLAG-HA-ΞFbxo42) |
| GO:0019005 SCF ubiquitin ligase complex | ISM GO_REF:0000050 | ACCEPT | Summary: The F-box sequence model is concordant with the direct complex evidence. Reason: The F-box (PROSITE PS50181, residues 21-70) is the SkpA-binding module of SCF substrate receptors. Here the prediction is confirmed experimentally: deleting the F-box abolishes SkpA binding (PMID:40804044). Supporting Evidence: file:DROME/Fbxo42/Fbxo42-uniprot.txt FT DOMAIN 21..70 FT /note="F-box" FT /evidence="ECO:0000259|PROSITE:PS50181" PMID:40804044 SkpA co-immunoprecipitates with FLAG-HA-Fbxo42, but not with a deletion of the F-box domain (FLAG-HA-ΞFbxo42) |
| GO:0031146 SCF-dependent proteasomal ubiquitin-dependent protein catabolic process | ISM GO_REF:0000050 | ACCEPT | Summary: Fbxo42 directs SCF-dependent ubiquitylation and proteasomal turnover of Ataxin-2. Reason: This is the core process for the gene, and it is now supported directly, not only by sequence model. In S2 cells and fly tissues, Fbxo42 is needed for Ataxin-2-ubiquitin conjugates. Overexpressing Fbxo42 accelerates Ataxin-2 turnover in cycloheximide chase, depleting it slows turnover, and MG132 stabilises Ataxin-2 (PMID:40804044). In the ovary, Fbxo42 depletion raises PP2A-B56 (Wrd) levels (PMID:33382409), but Wrd ubiquitylation was not tested there. Fbxo42 is the substrate-recognition subunit; Roc1a-Cul1 and the E2 carry out ubiquitin transfer. Supporting Evidence: PMID:40804044 We conclude that Fbxo42 promotes the degradation of Ataxin-2 by mediating the ubiquitylation of the C244 residue. PMID:40804044 Treating cells with CHX and MG132, an inhibitor of the proteosome, led to a significant stabilization of Ataxin-2-GFP PMID:40804044 while Fbxo42 RNAi depletion protects Ataxin-2-GFP from degradation PMID:33382409 In the Fbxo42 depletion, GFP-Wrd showed significant increases in region 2 of the germarium |
| GO:1905088 positive regulation of synaptonemal complex assembly | IMP PMID:33382409 SCF-Fbxo42 promotes synaptonemal complex assembly by downreg... | ACCEPT | Summary: Germline Fbxo42 depletion blocks assembly of the filamentous synaptonemal complex and causes its premature disassembly. Reason: Several independent shRNAs give the same defect. The proposed mechanism is regulatory: by lowering PP2A-B56 (Wrd), Fbxo42 shifts the balance toward phosphorylation and so toward synaptonemal-complex assembly, and Wrd overexpression phenocopies the knockdown. A regulation term, rather than participation in assembly, therefore describes Fbxo42's role at the right level. This is a physiological role specific to the germline, alongside the Ataxin-2 role in somatic stress responses. Supporting Evidence: PMID:33382409 Depletion of CG6758 (which we call Fbxo42) prevented assembly of the fully filamentous synaptonemal complex and prematurely disassembled the partially formed structure PMID:33382409 Our biochemical and genetic evidence suggests that Fbxo42 promotes synaptonemal complex assembly by negatively controlling the level of the protein phosphatase PP2A-B56. |
| GO:1990756 ubiquitin-like ligase-substrate adaptor activity | IBA GO_REF:0000033 | ACCEPT | Summary: The substrate-adaptor activity of an F-box protein is inherited across the FBXO42 clade. Reason: The PAINT node PTN002924600 covers PANTHER PTHR46432:SF1, which includes fly Fbxo42 and the vertebrate FBXO42 orthologs. The fly protein links SkpA, through its F-box, to a bound substrate (Ataxin-2). The term correctly names an adaptor activity and not intrinsic E3 catalysis. Supporting Evidence: PMID:40804044 SkpA co-immunoprecipitates with FLAG-HA-Fbxo42, but not with a deletion of the F-box domain (FLAG-HA-ΞFbxo42) PMID:40804044 Ataxin-2-GFP can co-IP Fbxo42 in S2 cells file:DROME/Fbxo42/Fbxo42-uniprot.txt DR PANTHER; PTHR46432:SF1; F-BOX ONLY PROTEIN 42; 1. |
| GO:1990756 ubiquitin-like ligase-substrate adaptor activity | IPI PMID:33382409 SCF-Fbxo42 promotes synaptonemal complex assembly by downreg... | ACCEPT | Summary: Fbxo42 binds SkpA (FBgn0025637) and the PP2A-B56 pathway (Wrd, FBgn0042693) in ovaries. Reason: The cached text shows that the GFP-Fbxo42 immunoprecipitates contained SCF subunits and the PP2A core A and C subunits. Fbxo42 depletion raised Wrd levels. The curators, who had the full figures and supplements, recorded Wrd as the IPI partner. Even if PP2A-B56 is bound through the PP2A core, the function is right: Fbxo42 bridges SCF to a regulated target. Direct substrate binding is independently shown for Ataxin-2 (PMID:40804044). Supporting Evidence: PMID:33382409 SCF subunits, PP2A core subunits and 14β3-3 proteins were significantly enriched in GFP-Fbxo42 immunoprecipitates PMID:33382409 Our biochemical and genetic evidence suggests that Fbxo42 promotes synaptonemal complex assembly by negatively controlling the level of the protein phosphatase PP2A-B56. |
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Download this section (compressed HTML)Q: Should Fbxo42 receive a cellular-component annotation to cytoplasmic stress granules? Fbxo42 foci decorate Ataxin-2 granules after DTT or arsenite treatment (PMID:40804044), but it is recruited to the granule surface and is not shown to be a stable constituent.
Q: Should a process term capture the Ataxin-2/Xbp1 role, for example a regulation term under the IRE1-mediated unfolded protein response? Fbxo42 acts by degrading a translational repressor of Xbp1 mRNA, and the organismal readout (suppression of Xbp1s-induced retinal degeneration) is a downstream phenotype.
Q: Is PP2A-B56 (Wrd) a direct SCF-Fbxo42 ubiquitylation substrate, or does Fbxo42 lower its level indirectly through the PP2A A/C core subunits it co-purifies with?
Experiment: In vitro reconstitution of SCF-Fbxo42 with Ataxin-2 and Wrd, combined with mass-spectrometric mapping of ubiquitylation sites, to separate direct from indirect substrates.
Experiment: Endogenous tagging of Fbxo42 with live imaging, with and without ER stress, to define its basal subcellular localization and the kinetics of its recruitment to Ataxin-2 granules.
Experiment: Test whether human FBXO42 (same PANTHER subfamily PTHR46432:SF1) ubiquitylates ATXN2 or ATXN2L under ER stress, to establish whether the Ataxin-2 substrate relationship is conserved.
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