FBXO39 (F-box only protein 39) is a 442-amino-acid F-box protein that serves as the substrate-recognition component of an SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex (a Cullin-RING ligase, CRL1). It contains an N-terminal F-box domain (residues ~16-61) through which it docks onto the SKP1-CUL1 scaffold, and a C-terminal leucine-rich-repeat (LRR)/RNI-like region predicted to mediate substrate binding. As an F-box "other" (FBXO) protein, FBXO39 acts as the interchangeable specificity subunit that targets substrate proteins for poly-ubiquitination and subsequent proteasomal degradation; the catalytic RING/E2-recruiting role within the complex is provided by RBX1/CUL1, not by FBXO39 itself. FBXO39 expression is testis-enriched and the gene is a cancer/testis antigen (historically named BCP-20), aberrantly re-expressed in several tumors (colorectal, glioma/glioblastoma, others), where it has been used as a prognostic/immunotherapy-candidate biomarker; knockdown in osteosarcoma cells reduces proliferation and increases apoptosis, indicating functional relevance to tumor-cell viability. The protein remains poorly characterized at the mechanistic level: no validated physiological substrate was established in the curated literature, although an emerging report proposes that FBXO39 promotes degradation of p53 to drive LDHA-mediated aerobic glycolysis in colorectal cancer. Its subcellular localization has not been experimentally determined.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0019005 SCF ubiquitin ligase complex | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (PAN-GO) assignment that FBXO39 is part of an SCF E3 ubiquitin ligase complex, consistent with its F-box domain and the UniProt-documented SKP1/CUL1 interaction. This is the core cellular-component assignment for an F-box protein. Reason: Core localization/complex membership. FBXO39 has an F-box domain and is annotated by UniProt as a substrate-recognition component of the SCF complex that directly interacts with SKP1 and CUL1; ComplexPortal also defines an SCF complex variant containing FBXO39 (CPX-7979). Supporting Evidence: file:human/FBXO39/FBXO39-uniprot.txt Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex. |
| GO:0031146 SCF-dependent proteasomal ubiquitin-dependent protein catabolic process | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (PAN-GO) assignment that FBXO39 participates in SCF-dependent proteasomal protein catabolism, the core biological process for an SCF substrate-recognition subunit. Reason: Core biological process. As an SCF substrate-recognition component, FBXO39's function is to target substrates for SCF/proteasome-mediated degradation, consistent with the F-box domain and the Ubl conjugation pathway keyword in UniProt, and with the functional placement of FBXO39 as an F-box/SCF (CRL1) substrate-recognition receptor. Falcon-sourced literature supports this at the family level but confirms no validated FBXO39 substrate in the curated window (an emerging 2026 p53-degradation mechanism aside), so this remains an inferred core process rather than a substrate-specific one. Supporting Evidence: file:human/FBXO39/FBXO39-uniprot.txt Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex. file:human/FBXO39/FBXO39-deep-research-falcon.md This is strongly supported as a *family-level mechanism* (xuan2024theemergingand pages 1-2, wang2014rolesoffbox pages 1-3), but **FBXO39-specific substrate repertoire remains poorly established** in the retrieved 2023–2024 literature. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | KEEP AS NON CORE | Summary: IntAct binary interaction (HuRI reference interactome) between FBXO39 (Q8N4B4) and MEOX2 (Q6FHY5). The bare protein binding term is uninformative on its own; the interaction is recorded in the UniProt INTERACTION block. Reason: Records a real binary interaction (FBXO39-MEOX2) from a high-throughput interactome, but bare protein binding is uninformative per curation guidelines and is not a core function. MEOX2 is a plausible candidate substrate but this has not been functionally validated. Supporting Evidence: file:human/FBXO39/FBXO39-uniprot.txt Q8N4B4; Q6FHY5: MEOX2; NbExp=3; IntAct=EBI-12383478, EBI-16439278; |
| GO:0019005 SCF ubiquitin ligase complex | NAS PMID:34445249 The SCF Complex Is Essential to Maintain Genome and Chromoso... | ACCEPT | Summary: ComplexPortal NAS assignment (from a review of the SCF complex family) that FBXO39 is part of an SCF E3 ubiquitin ligase complex. Redundant with, and consistent with, the IBA assignment and UniProt FUNCTION. Reason: Core complex membership; corroborated by UniProt (substrate-recognition component; directly interacts with SKP1 and CUL1) and ComplexPortal CPX-7979. The cited review establishes that variable F-box proteins such as FBXO39 form SCF complexes that determine substrate specificity. Supporting Evidence: PMID:34445249 These SCF complexes are distinguishable by variable F-box proteins, which determine substrate specificity. |
| GO:0031146 SCF-dependent proteasomal ubiquitin-dependent protein catabolic process | NAS PMID:34445249 The SCF Complex Is Essential to Maintain Genome and Chromoso... | ACCEPT | Summary: ComplexPortal NAS assignment (from the SCF-family review) that FBXO39 participates in SCF-dependent proteasomal protein catabolism. Redundant with the IBA assignment. Reason: Core biological process; the cited review states that SCF E3 ligase complexes modify substrates with poly-ubiquitin chains to target them for proteasomal degradation, the canonical role of an F-box substrate-recognition subunit. Supporting Evidence: PMID:34445249 encompasses a group of 69 SCF E3 ubiquitin ligase complexes that primarily modify protein substrates with poly-ubiquitin chains to target them for proteasomal degradation |
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Download this section (compressed HTML)Q: What is the physiological substrate (or substrates) recognized by the FBXO39 LRR/C-terminal region, and are the candidate targets (MEOX2 from HuRI; p53 from the emerging colorectal-cancer report) bona fide direct SCF(FBXO39) ubiquitination substrates?
Q: Given the testis-enriched expression of FBXO39, does it have a specific role in spermatogenesis or germ-cell protein turnover, and how does this reconcile with its cancer/testis-antigen (BCP-20) re-expression and pro-tumorigenic phenotypes in cancer?
Q: Where does FBXO39 localize, and does its reported pro-proliferative/anti-apoptotic effect in tumor cells operate through SCF-dependent degradation of a growth-suppressive substrate (e.g. p53)?
Experiment: Reconstitute the SCF(FBXO39) complex (SKP1-CUL1-RBX1-FBXO39) in vitro and perform ubiquitination assays with candidate substrates (MEOX2, p53) to confirm substrate-receptor activity and identify ubiquitin-chain topology.
Experiment: Perform affinity-purification/proximity-labeling mass spectrometry of tagged FBXO39 in testis-derived or germ-cell models and in cancer cells to define the SCF(FBXO39) interactome and candidate substrate repertoire, validated by stabilization upon FBXO39 knockout.
Experiment: Determine FBXO39 subcellular localization (immunofluorescence, fractionation) and test whether its tumor-cell proliferation/apoptosis phenotype requires intact SCF assembly (F-box mutants) and degradation of specific substrates such as p53.
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