Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0003677 DNA binding	IEA GO_REF:0000002	REMOVE	Summary: InterPro domain-based electronic assignment of DNA binding, propagated from a hemimethylated-DNA-binding-like (YccV-like) domain match; FBXO21 is an SCF substrate adaptor with no established DNA-binding function. Reason: This is an over-propagated IEA. FBXO21's UniProt-curated function is purely as the substrate-recognition subunit of an SCF E3 ubiquitin ligase, binding SKP1/CUL1 and substrates (EID1). The DNA binding term derives from an InterPro signature (IPR011722 hemimethylated-DNA-binding/YccV-like, TIGR02097 yccV) detected in the protein's C-terminal region; there is no experimental or curated evidence that FBXO21 binds DNA sequence-specifically or functions in DNA metabolism. The annotation conflates a remote domain-fold match with a molecular function and is biologically misleading for a cytoplasmic/nuclear ubiquitin-ligase adaptor. Supporting Evidence: file:human/FBXO21/FBXO21-uniprot.txt Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
GO:0005515 protein binding	IPI PMID:26085330 FBXO21 mediates the ubiquitylation and proteasomal degradati...	KEEP AS NON CORE	Summary: IPI interaction underpinning the functionally meaningful FBXO21-EID1 substrate relationship (FBXO21 binds the C-terminal region of EID1 and targets it for ubiquitylation). The bare protein binding term is uninformative, but this records a real substrate interaction. Reason: Records the substrate-recognition interaction with EID1, which is biologically central, but bare protein binding (GO:0005515) is uninformative per curation guidelines; the substrate relationship is better captured by the SCF/catabolism process annotations and core functions. Supporting Evidence: PMID:26085330 The central and COOH-terminal portion of FBXO21 was found to interact with the COOH-terminal region of EID1 in transfected cells.
GO:0005515 protein binding	IPI PMID:27705803 A High-Density Map for Navigating the Human Polycomb Complex...	KEEP AS NON CORE	Summary: Interaction captured in a high-throughput affinity-purification map of the human Polycomb complexome. Bare protein binding is uninformative. Reason: High-throughput AP-MS interaction; bare protein binding (GO:0005515) is uninformative and not a core function.
GO:0005515 protein binding	IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling...	KEEP AS NON CORE	Summary: Interaction captured in the BioPlex proteome-scale AP-MS interactome (cell-specific remodeling study). Bare protein binding is uninformative. Reason: High-throughput AP-MS interactome; bare protein binding (GO:0005515) is uninformative and not a core function. Supporting Evidence: PMID:33961781 BioPlex 3.0, results from affinity purification of 10,128 human proteins-half the proteome-in 293T cells and includes 118,162 interactions among 14,586 proteins.
GO:0005515 protein binding	IPI PMID:40205054 Multimodal cell maps as a foundation for structural and func...	KEEP AS NON CORE	Summary: Interaction captured in a multimodal AP-MS/immunofluorescence cell map of U2OS cells. Bare protein binding is uninformative. Reason: High-throughput AP-MS interactome; bare protein binding (GO:0005515) is uninformative and not a core function. Supporting Evidence: PMID:40205054 through joint measurement of biophysical interactions and immunofluorescence images for over 5,100 proteins in U2OS osteosarcoma cells
GO:0019005 SCF ubiquitin ligase complex	NAS PMID:34445249 The SCF Complex Is Essential to Maintain Genome and Chromoso...	ACCEPT	Summary: FBXO21 is the variable F-box substrate-recognition subunit of an SCF (SKP1-CUL1-F-box) E3 ubiquitin ligase complex, binding SKP1 and CUL1. This is a core localization/complex-membership annotation. Reason: Directly supported by UniProt curation (interacts with SKP1 and CUL1) and by a dedicated ComplexPortal entry (CPX-7930, SCF E3 ubiquitin ligase complex, FBXO21 variant). Core to FBXO21's molecular role. Supporting Evidence: file:human/FBXO21/FBXO21-uniprot.txt Directly interacts with SKP1 and CUL1.
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: As the substrate-recognition subunit of an SCF complex, FBXO21 drives SCF-dependent, proteasome-mediated degradation of its substrates (demonstrated for EID1). This is the core biological process. Reason: Core biological process. SCF complexes target substrates with poly-ubiquitin chains for proteasomal degradation (PMID:34445249), and FBXO21 was shown to drive proteasomal degradation of EID1 (PMID:26085330). Specific and well-supported. 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
GO:0005829 cytosol	TAS Reactome:R-HSA-8952618	KEEP AS NON CORE	Summary: Reactome TAS localization to cytosol assigned via a generic CRL1 (cullin-RING ligase) neddylation/cycle reaction. Correct compartment but generic and inherited from pathway context. Reason: Cytosolic localization is consistent with FBXO21 acting in cytoplasmic SCF complexes (and EID1 is degraded in cytoplasm and nucleus), but this annotation is a generic CRL-cycle pathway assignment, redundant across many reactions; non-core.
GO:0005829 cytosol	TAS Reactome:R-HSA-8952620	KEEP AS NON CORE	Summary: Reactome TAS localization to cytosol from a generic CRL1 neddylation/cycle reaction. Reason: Generic CRL-cycle localization; correct but redundant and non-core.
GO:0005829 cytosol	TAS Reactome:R-HSA-8955241	KEEP AS NON CORE	Summary: Reactome TAS localization to cytosol from a generic cytosolic CRL E3 ligase (CAND1-binding) reaction. Reason: Generic CRL-cycle localization; correct but redundant and non-core.
GO:0005829 cytosol	TAS Reactome:R-HSA-8955289	KEEP AS NON CORE	Summary: Reactome TAS localization to cytosol from a generic cytosolic CRL E3 ligase (COMMD/CAND1) reaction. Reason: Generic CRL-cycle localization; correct but redundant and non-core.
GO:0005829 cytosol	TAS Reactome:R-HSA-8956040	KEEP AS NON CORE	Summary: Reactome TAS localization to cytosol from the COP9-signalosome deneddylation reaction on cytosolic CRL complexes. Reason: Generic CRL-cycle localization; correct but redundant and non-core.
GO:0005829 cytosol	TAS Reactome:R-HSA-8956200	KEEP AS NON CORE	Summary: Reactome TAS localization to cytosol from a generic CRL1 (DCUN1D3-binding) reaction. Reason: Generic CRL-cycle localization; correct but redundant and non-core.
GO:0005829 cytosol	TAS Reactome:R-HSA-983140	KEEP AS NON CORE	Summary: Reactome TAS localization to cytosol from a generic ubiquitin-transfer (E2-to-substrate) reaction in the antigen-processing/proteasome pathway. Reason: Generic ubiquitination-cycle localization; correct but redundant and non-core.
GO:0005829 cytosol	TAS Reactome:R-HSA-983147	KEEP AS NON CORE	Summary: Reactome TAS localization to cytosol from a generic reaction (release of E3 from polyubiquitinated substrate). Reason: Generic ubiquitination-cycle localization; correct but redundant and non-core.
GO:0005829 cytosol	TAS Reactome:R-HSA-983156	KEEP AS NON CORE	Summary: Reactome TAS localization to cytosol from a generic polyubiquitination-of-substrate reaction. Reason: Generic ubiquitination-cycle localization; correct but redundant and non-core.
GO:0005829 cytosol	TAS Reactome:R-HSA-983157	KEEP AS NON CORE	Summary: Reactome TAS localization to cytosol from a generic reaction (interaction of E3 with substrate and E2-Ub complex). Reason: Generic ubiquitination-cycle localization; correct but redundant and non-core.
GO:0000151 ubiquitin ligase complex	NAS PMID:10531035 Identification of a family of human F-box proteins.	KEEP AS NON CORE	Summary: FBXO21 is a subunit of a ubiquitin ligase (SCF) complex, the general parent of the more specific SCF ubiquitin ligase complex annotation. Supported by the founding F-box protein family paper. Reason: Correct but generic; the more specific GO:0019005 (SCF ubiquitin ligase complex) better captures FBXO21's complex membership. F-box proteins are one of the four subunits of SCF ubiquitin ligases. Supporting Evidence: PMID:10531035 F-box proteins are one of the four subunits of ubiquitin protein ligases called SCFs.
GO:0004842 ubiquitin-protein transferase activity	NAS PMID:10531035 Identification of a family of human F-box proteins.	MODIFY	Summary: NAS assignment of ubiquitin-protein transferase activity from the founding F-box family paper. However, within the SCF complex the catalytic ubiquitin-transfer activity resides in the RING subunit (RBX1); the F-box protein FBXO21 functions as the substrate-recruiting adaptor, not the catalytic transferase. Reason: F-box proteins do not themselves catalyse ubiquitin transfer; they are the substrate-recognition adaptors that recruit substrates to the SCF, whose catalytic activity is provided by the RING subunit RBX1 (with the E2). The NAS source (PMID:10531035) describes SCF complexes (not FBXO21 itself) as ubiquitin ligases. The more accurate molecular function for an F-box protein is ubiquitin-like ligase-substrate adaptor activity (GO:1990756). Proposed replacements: ubiquitin-like ligase-substrate adaptor activity Supporting Evidence: file:human/FBXO21/FBXO21-uniprot.txt Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
GO:0006511 ubiquitin-dependent protein catabolic process	NAS PMID:10531035 Identification of a family of human F-box proteins.	KEEP AS NON CORE	Summary: FBXO21, as an SCF substrate adaptor, mediates ubiquitin-dependent protein catabolism (directly demonstrated for EID1). General parent of the SCF-dependent catabolism term. Reason: Correct and supported (FBXO21 drives ubiquitin-dependent proteasomal degradation of EID1), but the more specific GO:0031146 (SCF-dependent proteasomal ubiquitin-dependent protein catabolic process) better captures the role; retained as non-core to avoid redundancy. Supporting Evidence: PMID:26085330 FBXO21 mediates the ubiquitylation and proteasomal degradation of EID1.

Core Functions

Substrate-recognition subunit (F-box adaptor) of an SCF (SKP1-CUL1-F-box) E3 ubiquitin ligase complex that binds SKP1/CUL1 via its F-box domain and recruits the substrate EID1, directing SCF(FBXO21)-mediated ubiquitylation and proteasomal degradation of EID1.

Molecular Function:

ubiquitin-like ligase-substrate adaptor activity

Directly Involved In:

SCF-dependent proteasomal ubiquitin-dependent protein catabolic process

In Complex:

SCF ubiquitin ligase complex

Supporting Evidence:

PMID:26085330
An in vitro ubiquitylation assay showed that EID1 is a direct substrate of SCF(FBXO)(21).

As an SCF(FBXO21) substrate receptor, catalyzes non-proteolytic Lys29-linked ubiquitination of the kinase ASK1 (MAP3K5) that promotes ASK1 activation, driving ASK1-JNK/p38 and IRF3-dependent antiviral innate immune signaling and type I interferon induction; this signaling-activating ubiquitination is distinct from the canonical degradative SCF role.

Molecular Function:

ubiquitin-like ligase-substrate adaptor activity

Directly Involved In:

innate immune response

In Complex:

SCF ubiquitin ligase complex

Supporting Evidence:

file:human/FBXO21/FBXO21-deep-research-falcon.md
SCF^FBXO21 mediates **Lys29-linked ubiquitination** of ASK1, demonstrated using ubiquitin mutants (K29-only ubiquitin), and this modification is **non-proteolytic** (ASK1 degradation not affected).

As an SCF(FBXO21) substrate receptor, targets the PI3K regulatory subunit p85alpha (PIK3R1) for ubiquitin-dependent proteasomal degradation, thereby tuning PI3K/AKT versus ERK signaling; in acute myeloid leukemia this activity supports cell survival and blocks differentiation.

Molecular Function:

ubiquitin-like ligase-substrate adaptor activity

Directly Involved In:

SCF-dependent proteasomal ubiquitin-dependent protein catabolic process

In Complex:

SCF ubiquitin ligase complex

Supporting Evidence:

file:human/FBXO21/FBXO21-deep-research-falcon.md
Dobish et al. used proteomics (TMT MS; K-ε-GG ubiquitin-remnant IP/MS) to identify candidate FBXO21-dependent substrates and nominated **p85α** as a ubiquitination target. They then provided biochemical evidence of p85α ubiquitination dependent on FBXO21 and its F-box domain (ΔF-box mutant inactive), including **in vitro ubiquitination** with immunopurified FBXO21.

References

GO_REF:0000002

Gene Ontology annotation through association of InterPro records with GO terms

PMID:10531035

Identification of a family of human F-box proteins.

F-box proteins are one of the four subunits of SCF ubiquitin protein ligases (with SKP1, a cullin, and ROC1/RBX1); FBXO21 (FBX21) was identified among 26 human F-box proteins using SKP1 as bait.

PMID:26085330

FBXO21 mediates the ubiquitylation and proteasomal degradation of EID1.

FBXO21 is the substrate-recognition subunit of an SCF-type E3; its central/C-terminal region binds the C-terminal region of EID1, and SCF(FBXO21) directly ubiquitylates EID1 to drive its proteasomal degradation in cytoplasm and nucleus.

PMID:27705803

A High-Density Map for Navigating the Human Polycomb Complexome.

PMID:33961781

Dual proteome-scale networks reveal cell-specific remodeling of the human interactome.

PMID:34445249

The SCF Complex Is Essential to Maintain Genome and Chromosome Stability.

The SCF complex is a family of ~69 E3 ubiquitin ligase complexes (SKP1-CUL1-F-box) that poly-ubiquitinate substrates for proteasomal degradation; variable F-box proteins determine substrate specificity.

PMID:40205054

Multimodal cell maps as a foundation for structural and functional genomics.

file:human/FBXO21/FBXO21-deep-research-falcon.md

Falcon deep research report for human FBXO21

FBXO21 is an SCF (SKP1-CUL1-RBX1) substrate receptor with validated substrates EID1, ASK1, p85alpha/PIK3R1, and ABCB1/P-gp, operating in both proteolytic and non-proteolytic ubiquitination modes.

"The research target is **human FBXO21** (UniProt **O94952**; gene **FBXO21**, synonyms **FBX21**, **KIAA0875**), an **F-box only** protein that functions as a substrate-recognition subunit of an **SCF (SKP1–CUL1–RBX1) Cullin-RING E3 ubiquitin ligase** complex. This identity is consistent across primary literature describing **SCF^FBXO21** and its validated substrates (EID1, ASK1, p85α/PIK3R1, and ABCB1/P-gp)."
SCF(FBXO21) mediates non-proteolytic Lys29-linked ubiquitination of ASK1 (MAP3K5) that promotes ASK1 activation rather than degradation, driving antiviral innate signaling and type I interferon responses.

"SCF^FBXO21 mediates **Lys29-linked ubiquitination** of ASK1, demonstrated using ubiquitin mutants (K29-only ubiquitin), and this modification is **non-proteolytic** (ASK1 degradation not affected)."
FBXO21 deficiency impairs virus-induced ASK1-JNK/p38 and IRF3 signaling and reduces IL-6 and IFN-beta induction after viral or nucleic-acid challenge.

"FBXO21 deficiency impaired virus-induced signaling (reduced JNK/p38 and IRF3 activation; reduced nuclear c-Fos/IRF3) and reduced **IL-6 and IFNβ** induction after LPS, nucleic acid agonists, and VSV/HSV-1 infection, linking FBXO21→ASK1 ubiquitination to **ASK1–JNK/p38** antiviral signaling and type I interferon responses."
In acute myeloid leukemia, FBXO21 ubiquitinates the PI3K regulatory subunit p85alpha (PIK3R1) for proteasomal degradation, shaping PI3K/AKT versus ERK signaling; silencing FBXO21 promotes differentiation and chemosensitization.

"Stabilization of p85α (via FBXO21 knockdown) is associated with reduced canonical PI3K signaling (decreased AKT activation) and increased ERK activation in the model, with p85α homodimerization proposed as a mechanistic intermediate; the paper’s model schematic is captured in retrieved figures."
FBXO21 binds and ubiquitinates the multidrug-resistance transporter ABCB1/P-glycoprotein for proteasomal degradation, a turnover that Ser291-phosphorylated CD44 suppresses.

"CD44 physically associates with P-gp at the membrane, and a **Ser291-phosphorylated** form of CD44 inhibits FBXO21-directed degradation of P-gp (Ser291Ala loses protection)."

Reactome:R-HSA-8952618

AcM-UBE2M transfers NEDD8 to CRL1 E3 ubiquitin ligase complex

Reactome:R-HSA-8952620

NEDD8:AcM-UBE2M binds CRL1 E3 ubiquitin ligase complex

Reactome:R-HSA-8955241

CAND1 binds cytosolic CRL E3 ubiquitin ligases

Reactome:R-HSA-8955289

COMMDs displace CAND1 from cytosolic CRL E3 ubiquitin ligase complexes

Reactome:R-HSA-8956040

COP9 signalosome deneddylates cytosolic CRL E3 ubiquitin ligase complexes

Reactome:R-HSA-8956200

MyrG-DCUN1D3 binds CRL1 E3 ubiquitin ligase complex

Reactome:R-HSA-983140

Transfer of Ub from E2 to substrate and release of E2

Reactome:R-HSA-983147

Release of E3 from polyubiquitinated substrate

Reactome:R-HSA-983156

Polyubiquitination of substrate

Reactome:R-HSA-983157

Interaction of E3 with substrate and E2-Ub complex

Suggested Experiments

Experiment: Perform DiPIUS or quantitative ubiquitinome/proteome profiling in FBXO21-knockout versus wild-type cells (with and without proteasome inhibition) to define the endogenous SCF(FBXO21) substrate repertoire.

Experiment: Reconstitute SCF(FBXO21)-mediated ubiquitylation of EID1 in vitro with purified SKP1, CUL1, RBX1, an E2, and an F-box-domain mutant of FBXO21 to confirm that adaptor (SKP1-binding) function is required and to map EID1 ubiquitylation sites and chain linkage.

Experiment: Test whether FBXO21's reported antiviral signalling function requires SCF assembly by comparing wild-type FBXO21 with an F-box-deletion (SKP1-binding-defective) mutant in interferon-pathway reporter and viral-challenge assays.

Deep Research

Falcon

(FBXO21-deep-research-falcon.md)

Research Report: FBXO21 (UniProt O94952) — Functional Annotation (Homo sapiens) Falcon Edison Scientific Literature 15 citations 2 artifacts 2026-06-13T06:34:21.642269

Edison artifact artifact-00 (text/markdown)

## Context ID: pqac-00000016 The requested visual content is found in Figure 6 and Figure 7 of the document. 1. **FBXO21-mediated ubiquitination of p85α**: Figu (image/png)

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Research Report: FBXO21 (UniProt O94952) — Functional Annotation (Homo sapiens)

0) Target verification (gene/protein identity)

The research target is human FBXO21 (UniProt O94952; gene FBXO21, synonyms FBX21, KIAA0875), an F-box only protein that functions as a substrate-recognition subunit of an SCF (SKP1–CUL1–RBX1) Cullin-RING E3 ubiquitin ligase complex. This identity is consistent across primary literature describing SCF^FBXO21 and its validated substrates (EID1, ASK1, p85α/PIK3R1, and ABCB1/P-gp). (watanabe2015fbxo21mediatesthe pages 2-3, watanabe2015fbxo21mediatesthe pages 1-2, yu2016lys29linkageofask1 pages 7-9, zhang2015peptidicdegronin pages 1-2, dobish2023fbxo21mediateddegradation pages 8-9)

1) Key concepts and current understanding

1.1 FBXO21 as an SCF E3 ligase substrate receptor

F-box proteins are best understood as substrate receptors for SCF E3 ligases: they bind SKP1 through the F-box and recruit specific substrates via other domains/regions, enabling the CUL1–RBX1 catalytic module to build ubiquitin chains on the substrate. For FBXO21, co-precipitation of SKP1 and CUL1 with FBXO21 supports its role as an SCF substrate receptor. (watanabe2015fbxo21mediatesthe pages 2-3, yu2016lys29linkageofask1 pages 7-9)

1.2 Two mechanistic “modes” of FBXO21-driven ubiquitination

FBXO21 is notable because its experimentally validated biology spans both:

1) Proteolytic ubiquitination → proteasomal degradation (canonical SCF function), as shown for EID1, p85α (PIK3R1), and ABCB1/P-gp. (watanabe2015fbxo21mediatesthe pages 1-2, dobish2023fbxo21mediateddegradation pages 8-9, ravindranath2015cd44promotesmultidrug pages 7-9)

2) Non-proteolytic ubiquitination → signaling activation, as shown for ASK1, where FBXO21 catalyzes Lys29-linked ubiquitination that promotes ASK1 phosphorylation/activation rather than substrate destruction. (yu2016lys29linkageofask1 pages 14-15, yu2016lys29linkageofask1 pages 7-9)

This duality is important for functional annotation: FBXO21 is not only a “degrader” but can also operate as a signaling regulator through atypical ubiquitin linkages. (yu2016lys29linkageofask1 pages 14-15)

2) Experimentally supported molecular functions, substrates, and pathways

2.1 EID1 is a direct proteolytic substrate of SCF^FBXO21 (2015)

Two independent 2015 studies establish EID1 (EP300-interacting inhibitor of differentiation 1) as a direct FBXO21 substrate.

Substrate identification and validation. Watanabe et al. used differential binding proteomics (DiPIUS) comparing WT FBXO21 to a binding mutant to identify candidate substrates, then validated an FBXO21–EID1 interaction and concluded SCF^FBXO21 ubiquitylates EID1, promoting proteasomal degradation. (Publication date: Aug 2015; https://doi.org/10.1111/gtc.12260) (watanabe2015fbxo21mediatesthe pages 2-3, watanabe2015fbxo21mediatesthe pages 1-2)
Degron concept and biological context. Zhang et al. mapped a peptidic modular degron in EID1 that is necessary and sufficient for SCF^FBXO21-dependent polyubiquitylation and proteasomal degradation in vitro and in vivo. (Publication date: Dec 2015; https://doi.org/10.1073/pnas.1522006112) (zhang2015peptidicdegronin pages 1-2)
Localization context. In Watanabe et al., EID1 degradation by SCF^FBXO21 is reported as occurring predominantly in the cytoplasm, and FBXO21 disruption stabilized EID1 with accumulation in cytoplasm and nucleus (supporting that proteolysis is a major control point for EID1 abundance). (watanabe2015fbxo21mediatesthe pages 1-2)

Functional interpretation. EID1 is a transcriptional regulator (interacting with EP300); thus, FBXO21 likely regulates transcriptional programs indirectly by controlling EID1 protein abundance (rather than through an enzymatic reaction of its own). (watanabe2015fbxo21mediatesthe pages 1-2, zhang2015peptidicdegronin pages 1-2)

2.2 ASK1 is a non-proteolytic substrate: Lys29-linked ubiquitination drives antiviral innate signaling (2016)

Yu et al. show FBXO21 is required for antiviral innate response by catalyzing a non-canonical ubiquitination signal on ASK1 (MAP3K5).

Complex formation and substrate binding. FBXO21 forms an SCF complex (co-IP with SKP1/CUL1/RBX1; F-box required). ASK1 was identified as an FBXO21 interactor by endogenous IP + LC-MS and validated by co-IP and GST pull-down. (yu2016lys29linkageofask1 pages 7-9)
Ubiquitin linkage specificity. SCF^FBXO21 mediates Lys29-linked ubiquitination of ASK1, demonstrated using ubiquitin mutants (K29-only ubiquitin), and this modification is non-proteolytic (ASK1 degradation not affected). (yu2016lys29linkageofask1 pages 14-15, yu2016lys29linkageofask1 pages 7-9)
Pathway consequence. FBXO21 deficiency impaired virus-induced signaling (reduced JNK/p38 and IRF3 activation; reduced nuclear c-Fos/IRF3) and reduced IL-6 and IFNβ induction after LPS, nucleic acid agonists, and VSV/HSV-1 infection, linking FBXO21→ASK1 ubiquitination to ASK1–JNK/p38 antiviral signaling and type I interferon responses. (yu2016lys29linkageofask1 pages 14-15, yu2016lys29linkageofask1 pages 2-3)

Functional interpretation. FBXO21 can operate as a signaling E3 adaptor that builds atypical ubiquitin chains (K29) to enhance kinase activation, expanding its role beyond proteasomal turnover. (yu2016lys29linkageofask1 pages 14-15)

2.3 p85α (PIK3R1) is a proteolytic FBXO21 substrate in AML with PI3K/ERK pathway consequences (2023)

A major recent development (high-priority for this report) is a 2023 Leukemia paper linking FBXO21 to AML biology through degradation of p85α (PIK3R1), the regulatory subunit of PI3K.

Substrate nomination and validation. Dobish et al. used proteomics (TMT MS; K-ε-GG ubiquitin-remnant IP/MS) to identify candidate FBXO21-dependent substrates and nominated p85α as a ubiquitination target. They then provided biochemical evidence of p85α ubiquitination dependent on FBXO21 and its F-box domain (ΔF-box mutant inactive), including in vitro ubiquitination with immunopurified FBXO21. (Publication date: Sep 2023; https://doi.org/10.1038/s41375-023-02020-w) (dobish2023fbxo21mediateddegradation pages 8-9, dobish2023fbxo21mediateddegradation pages 3-4, dobish2023fbxo21mediateddegradation pages 7-8)
Proteasomal degradation and ubiquitin chain type. The experiments include proteasome inhibition (MG132) and ubiquitin-IP evidence consistent with proteasomal turnover; an anti-K48 signal is reported in the p85α ubiquitination context, consistent with degradative ubiquitin chains. (dobish2023fbxo21mediateddegradation pages 7-8)
Signaling consequence model. Stabilization of p85α (via FBXO21 knockdown) is associated with reduced canonical PI3K signaling (decreased AKT activation) and increased ERK activation in the model, with p85α homodimerization proposed as a mechanistic intermediate; the paper’s model schematic is captured in retrieved figures. (dobish2023fbxo21mediateddegradation pages 9-10, dobish2023fbxo21mediateddegradation media 6f09b441, dobish2023fbxo21mediateddegradation media e1da3cd9)
Disease and phenotypic impact. Silencing FBXO21 in AML cell lines and primary samples promotes differentiation and inhibits tumor progression; it also sensitizes cells to chemotherapy and perturbs cytokine signaling pathways. (dobish2023fbxo21mediateddegradation pages 1-2, dobish2023fbxo21mediateddegradation pages 3-4)

Evidence from figures (visual support). Figure regions retrieved from Dobish et al. show ubiquitin-IP and in vitro ubiquitination evidence for FBXO21-mediated p85α ubiquitination and the proposed pathway model linking FBXO21 loss to altered PI3K/AKT vs ERK signaling. (dobish2023fbxo21mediateddegradation media 6f09b441, dobish2023fbxo21mediateddegradation media e1da3cd9)

2.4 ABCB1/P-gp is a proteolytic substrate; CD44 phosphorylation protects P-gp from FBXO21 (2015)

Ravindranath et al. demonstrate a mechanistic link between FBXO21 and multidrug resistance by targeting ABCB1/P-glycoprotein (P-gp).

P-gp turnover is ubiquitin–proteasome dependent. In yeast, >50% of P-gp was lost within 2 hours at 37°C, whereas a proteasome-defective mutant accumulated P-gp; MG132 (10 μM) caused time-dependent P-gp accumulation. (Publication date: Jul 2015; https://doi.org/10.18632/oncotarget.4763) (ravindranath2015cd44promotesmultidrug pages 5-7)
FBXO21 binds and ubiquitinates P-gp. Evidence includes reciprocal co-IP, increased ubiquitinated P-gp with FBXO21 overexpression, reduction of P-gp levels, dominant-negative FBXO21 effects, and in vitro reconstitution demonstrating direct ubiquitination by an FBXO21-containing SCF system; a time course shows ubiquitinated complex at 45 min and stronger at 90 min, requiring ubiquitin and FBXO21. (ravindranath2015cd44promotesmultidrug pages 9-10, ravindranath2015cd44promotesmultidrug pages 7-9)
CD44 suppresses FBXO21-mediated P-gp ubiquitination. CD44 physically associates with P-gp at the membrane, and a Ser291-phosphorylated form of CD44 inhibits FBXO21-directed degradation of P-gp (Ser291Ala loses protection). Functionally, CD44 co-expression increased valinomycin resistance ~4-fold relative to P-gp alone in yeast. (ravindranath2015cd44promotesmultidrug pages 1-2, ravindranath2015cd44promotesmultidrug pages 7-9)

Functional interpretation. FBXO21 participates in regulating a clinically important transporter (ABCB1), and CD44 phosphorylation state can rewire this ubiquitin-mediated turnover pathway, linking SCF substrate recognition to drug-resistance phenotypes. (ravindranath2015cd44promotesmultidrug pages 1-2, ravindranath2015cd44promotesmultidrug pages 7-9)

3) Recent developments (2023–2024 prioritized)

3.1 2023: FBXO21–p85α axis in AML (mechanism and quantitative data)

Dobish et al. report that FBXO21 is a critical regulator of AML cell survival/proliferation via p85α degradation with downstream PI3K/ERK consequences. Quantitatively:

Proteomics: 260 proteins upregulated upon FBXO21 silencing (TMT MS). (dobish2023fbxo21mediateddegradation pages 3-4)
Ubiquitinomics: 1297 ubiquitinated peptides identified; 50 peptides more abundant/unique in control, consistent with FBXO21-dependent ubiquitination. (dobish2023fbxo21mediateddegradation pages 3-4)
Chemosensitization: cytarabine IC50 shifted from 42 nM to 23 nM with FBXO21 knockdown. (dobish2023fbxo21mediateddegradation pages 3-4)

These results position FBXO21 as a disease-relevant E3 adaptor and motivate translational targeting. (dobish2023fbxo21mediateddegradation pages 1-2, dobish2023fbxo21mediateddegradation pages 3-4)

3.2 2024: Small-molecule disruption of FBXO21:p85α recognition (preprint)

A 2024 bioRxiv preprint extends the AML work by proposing a drug-like strategy: inhibit the substrate–ligase interface.

The authors report a terphenyl analog (57-057) that blocks FBXO21-mediated p85α ubiquitination in vitro with IC50 2.4 nM and produces dose-dependent accumulation of p85α in AML cells. (Publication date: Dec 2024; https://doi.org/10.1101/2024.12.13.628427) (dobish2024smallmoleculetargeting pages 8-11)
They report an estimated selectivity window in primary cells: ~5-fold selectivity for primary AML versus CD34+ HSPCs (8 nM vs 39 nM). (dobish2024smallmoleculetargeting pages 8-11)

Because this is a preprint, the claims should be interpreted cautiously until peer review and independent replication; nonetheless it is a concrete example of an application/implementation direction built directly on FBXO21 substrate recognition biology. (dobish2024smallmoleculetargeting pages 1-5, dobish2024smallmoleculetargeting pages 8-11)

3.3 2024: Proteome-scale degron/effector discovery corroborates FBXO21 as a degron-binding CRL adaptor

A 2024 Nature study on proteome-scale degradation/stabilization effectors mentions degrons that bind CRL adaptor FBXO21, consistent with the earlier mechanistic picture that FBXO21 recognizes specific degron elements (e.g., in EID1). (Publication date: Mar 2024; https://doi.org/10.1038/s41586-024-07224-3) (context from paper search; direct evidence excerpt not captured here)

4) Current applications and real-world implementations

4.1 Cancer biology and therapeutic concepts

AML (mechanism-driven target concept). FBXO21 is positioned as a regulator of AML proliferation/survival via p85α turnover and PI3K/ERK signaling, with evidence for chemosensitization (cytarabine IC50 shift) upon FBXO21 knockdown. (dobish2023fbxo21mediateddegradation pages 3-4)
Targeting protein–protein interfaces. The 2024 preprint proposes inhibiting FBXO21’s substrate binding to p85α (compound 57-057), an approach analogous in concept to inhibiting E3–substrate contacts rather than inhibiting the proteasome globally. (dobish2024smallmoleculetargeting pages 8-11)
Drug resistance (ABCB1/P-gp). FBXO21-mediated P-gp degradation is a mechanistic lever on multidrug resistance phenotypes, and CD44 phosphorylation-dependent protection provides a plausible intervention node in P-gp-positive tumors. (ravindranath2015cd44promotesmultidrug pages 1-2, ravindranath2015cd44promotesmultidrug pages 7-9)

4.2 Innate immunity modulation

The ASK1 study implies that FBXO21 can be a control point for type I interferon and inflammatory cytokine induction downstream of viral nucleic acid sensing via modulation of ASK1 ubiquitination and MAPK signaling. This suggests potential relevance to antiviral response tuning, although the retrieved excerpts do not provide clinical/therapeutic implementations. (yu2016lys29linkageofask1 pages 14-15, yu2016lys29linkageofask1 pages 2-3)

5) Expert synthesis and evidence-strength assessment

5.1 Strongest supported “primary function” of FBXO21

Across validated targets, the most defensible primary function is:

FBXO21 is an SCF E3 ligase substrate receptor that catalyzes ubiquitination of specific substrates, resulting either in proteasomal degradation (EID1, p85α, ABCB1) or non-proteolytic signaling activation (ASK1 via K29-linked chains). (yu2016lys29linkageofask1 pages 14-15, watanabe2015fbxo21mediatesthe pages 1-2, dobish2023fbxo21mediateddegradation pages 8-9, ravindranath2015cd44promotesmultidrug pages 7-9)

5.2 Subcellular localization: what is known vs unknown

Supported contexts:
EID1 degradation by SCF^FBXO21 is described as occurring predominantly in the cytoplasm. (watanabe2015fbxo21mediatesthe pages 1-2)
The P-gp/CD44 axis is strongly tied to membrane-associated P-gp and CD44 interactions affecting ubiquitination/proteasomal turnover. (ravindranath2015cd44promotesmultidrug pages 1-2, ravindranath2015cd44promotesmultidrug pages 10-11)
Gaps:
The ASK1 and AML p85α studies (in the retrieved excerpts) do not provide definitive FBXO21 compartmental localization, so precise annotation (e.g., cytosol vs nucleus vs organelle) remains limited from these sources alone. (yu2016lys29linkageofask1 pages 7-9, dobish2023fbxo21mediateddegradation pages 8-9)

5.3 Ubiquitin linkage specificity: substrate-dependent

ASK1: explicitly K29-linked, non-proteolytic. (yu2016lys29linkageofask1 pages 14-15, yu2016lys29linkageofask1 pages 7-9)
p85α: evidence includes K48-reactive ubiquitin signal in the ubiquitination context, consistent with proteasomal targeting. (dobish2023fbxo21mediateddegradation pages 7-8)
EID1 and P-gp: polyubiquitylation/proteasomal degradation supported, but chain linkages are not specified in the retrieved excerpts. (watanabe2015fbxo21mediatesthe pages 1-2, ravindranath2015cd44promotesmultidrug pages 7-9)

6) Supplemental disease association signals (database-level)

OpenTargets lists FBXO21 disease associations (e.g., glioblastoma multiforme, hypertension, neurodegenerative disease; with cited literature identifiers), which can be used as hypothesis-generating pointers rather than mechanistic confirmation. (OpenTargets Search: -FBXO21)

Summary table of validated functions

The following table consolidates experimentally supported FBXO21 roles, substrates, outcomes, assays, and key quantitative/statistical notes.

Finding/role	Direct substrate/partner	Ubiquitin outcome (degradation vs non-proteolytic; linkage if known)	Key assays/evidence	Biological context/pathway	Publication (first author year)	URL/DOI
SCF^FBXO21 recognizes and degrades EID1	EID1 (EP300-interacting inhibitor of differentiation 1)	Proteasomal degradation; polyubiquitylation shown, linkage not specified in the cited evidence	Differential proteomics (DiPIUS), FLAG-IP/LC-MS/MS, co-IP with SKP1/CUL1, interaction mapping, FBXO21 overexpression causing EID1 downregulation, CRISPR/Cas9 FBXO21 disruption stabilizing EID1, in vitro ubiquitylation; degradation reported predominantly in cytoplasm (watanabe2015fbxo21mediatesthe pages 2-3, watanabe2015fbxo21mediatesthe pages 1-2, zhang2015peptidicdegronin pages 1-2)	Protein turnover of a transcriptional repressor; SCF E3 ligase substrate recognition/degron biology	Watanabe 2015; Zhang 2015	https://doi.org/10.1111/gtc.12260 ; https://doi.org/10.1073/pnas.1522006112
SCF^FBXO21 activates antiviral signaling through ASK1 modification	ASK1 (MAP3K5)	Non-proteolytic polyubiquitylation; Lys29-linked chains required; no degradation effect detected (yu2016lys29linkageofask1 pages 14-15, yu2016lys29linkageofask1 pages 7-9, yu2016lys29linkageofask1 pages 2-3)	Endogenous IP + LC-MS identification, co-IP, GST pull-down, domain mapping, Fbxo21 knockout/reconstitution, linkage-specific ubiquitination assays using K29-only ubiquitin mutants, viral infection models (VSV, HSV-1), cytokine and phospho-signaling readouts (yu2016lys29linkageofask1 pages 14-15, yu2016lys29linkageofask1 pages 7-9, yu2016lys29linkageofask1 pages 2-3)	Antiviral innate immunity; ASK1-JNK/p38 signaling promotes type I IFN and IL-6 responses after viral nucleic acid sensing/infection (yu2016lys29linkageofask1 pages 14-15, yu2016lys29linkageofask1 pages 2-3)	Yu 2016	URL/DOI not available in retrieved metadata
FBXO21 promotes p85α turnover in AML	p85α / PIK3R1	Proteasomal degradation; ubiquitination supported with K48-reactive signal in cited evidence (dobish2023fbxo21mediateddegradation pages 7-8)	TMT proteomics and K-ε-GG ubiquitin-remnant IP/MS substrate nomination; co-expression/co-IP; ubiquitin IP in HEK293T and MOLM-13; MG132 rescue; ΔF-box mutant loses activity; in vitro ubiquitination with immunopurified FBXO21; stabilization upon FBXO21 knockdown (dobish2023fbxo21mediateddegradation pages 8-9, dobish2023fbxo21mediateddegradation pages 3-4, dobish2023fbxo21mediateddegradation pages 7-8)	AML proliferation/survival; loss of FBXO21 stabilizes p85α, decreases canonical PI3K/AKT signaling, promotes p85α homodimerization and ERK activation, increases CXCL10, and sensitizes to chemotherapy (dobish2023fbxo21mediateddegradation pages 9-10, dobish2023fbxo21mediateddegradation pages 8-9, dobish2023fbxo21mediateddegradation pages 1-2, dobish2023fbxo21mediateddegradation media 6f09b441)	Dobish 2023	https://doi.org/10.1038/s41375-023-02020-w
Clinical association of FBXO21 in AML	FBXO21 expression (not a direct substrate row, but disease association from same study)	Not applicable	AML datasets and experimental models showed higher FBXO21 expression associated with poorer outcomes; FBXO21 knockdown sensitized cells to cytarabine with IC50 shift from 42 nM to 23 nM; overexpression accelerated disease onset in NSG mice (dobish2023fbxo21mediateddegradation pages 1-2, dobish2023fbxo21mediateddegradation pages 3-4)	Prognostic association and therapy-response modulation in AML	Dobish 2023	https://doi.org/10.1038/s41375-023-02020-w
FBXO21 targets multidrug transporter P-gp/ABCB1; CD44 protects it	ABCB1 / P-glycoprotein; CD44 (protective antagonist of degradation)	Proteasomal degradation of P-gp by FBXO21; CD44 Ser291-phosphorylated form inhibits FBXO21-directed degradation (abstract-level evidence in retrieved context)	Reported binding of FBXO21 to P-gp, in vivo ubiquitination of P-gp, in vitro ubiquitination assay, and functional protection by CD44 from FBXO21-mediated ubiquitination/degradation (retrieved abstract/snippet) (OpenTargets Search: -FBXO21)	Multidrug resistance; CD44 increases P-gp-mediated drug resistance by blocking FBXO21-driven turnover (OpenTargets Search: -FBXO21)	Ravindranath 2015	https://doi.org/10.18632/oncotarget.4763
Small-molecule disruption of FBXO21:p85α interaction (preprint)	FBXO21:p85α interface; compound 57-057	Blocks FBXO21-mediated p85α ubiquitylation, thereby stabilizing p85α; reported K692 site and YccV-domain-dependent binding in cited excerpt (dobish2024smallmoleculetargeting pages 11-14, dobish2024smallmoleculetargeting pages 1-5)	In vitro ubiquitination assays; computational modeling of p85α degron; mutation analyses (K692R, Y467A); dose-dependent p85α accumulation in cells; pathway/dimerization assays; AML viability/colony assays; in vivo antileukemia activity claimed in excerpt (dobish2024smallmoleculetargeting pages 11-14, dobish2024smallmoleculetargeting pages 1-5, dobish2024smallmoleculetargeting pages 8-11)	Translational targeting concept in AML: reduced AKT/canonical PI3K signaling, altered p85/p110 vs p85/p85 dimerization, induction of AML cell death (dobish2024smallmoleculetargeting pages 11-14, dobish2024smallmoleculetargeting pages 1-5, dobish2024smallmoleculetargeting pages 8-11)	Dobish 2024 preprint	https://doi.org/10.1101/2024.12.13.628427
Reported selectivity statistics for 57-057 (preprint)	Primary AML cells vs healthy CD34+ HSPCs	Not applicable	FBXO21:p85α disruptor 57-057 inhibited FBXO21-mediated p85α ubiquitination with IC50 2.4 nM; showed ~5-fold selectivity for primary AML versus CD34+ HSPCs, with reported activity 8 nM vs 39 nM (dobish2024smallmoleculetargeting pages 8-11)	Early preclinical therapeutic window estimate for targeting FBXO21 biology in AML	Dobish 2024 preprint	https://doi.org/10.1101/2024.12.13.628427

Table: This table summarizes experimentally supported human FBXO21 functions, substrates, ubiquitin outcomes, and disease contexts. It highlights both foundational mechanistic studies and the recent AML-focused therapeutic preprint on the FBXO21:p85α interaction.

Key references (publication dates and URLs)

Watanabe K, Yumimoto K, Nakayama KI. “FBXO21 mediates the ubiquitylation and proteasomal degradation of EID1.” Genes to Cells (Aug 2015). https://doi.org/10.1111/gtc.12260 (watanabe2015fbxo21mediatesthe pages 2-3, watanabe2015fbxo21mediatesthe pages 1-2)
Zhang C et al. “Peptidic degron in EID1 is recognized by an SCF E3 ligase complex containing the orphan F-box protein FBXO21.” PNAS (Dec 2015). https://doi.org/10.1073/pnas.1522006112 (zhang2015peptidicdegronin pages 1-2)
Yu Z et al. “Lys29-linkage of ASK1 by Skp1–Cullin1–Fbxo21 ubiquitin ligase complex is required for antiviral innate response.” (2016; journal/DOI not available in retrieved metadata). (yu2016lys29linkageofask1 pages 14-15, yu2016lys29linkageofask1 pages 7-9, yu2016lys29linkageofask1 pages 2-3)
Ravindranath AK et al. “CD44 promotes multi-drug resistance by protecting P-glycoprotein from FBXO21-mediated ubiquitination.” Oncotarget (Jul 2015). https://doi.org/10.18632/oncotarget.4763 (ravindranath2015cd44promotesmultidrug pages 1-2, ravindranath2015cd44promotesmultidrug pages 7-9, ravindranath2015cd44promotesmultidrug pages 5-7)
Dobish KK et al. “FBXO21 mediated degradation of p85α regulates proliferation and survival of acute myeloid leukemia.” Leukemia (Sep 2023). https://doi.org/10.1038/s41375-023-02020-w (dobish2023fbxo21mediateddegradation pages 8-9, dobish2023fbxo21mediateddegradation pages 3-4, dobish2023fbxo21mediateddegradation media 6f09b441)
Dobish KK et al. “Small molecule targeting of FBXO21 mediated p85α ubiquitylation in acute myeloid leukemia.” bioRxiv (Dec 2024). https://doi.org/10.1101/2024.12.13.628427 (dobish2024smallmoleculetargeting pages 8-11)

Figures (visual evidence retrieved)

Ubiquitin-IP and in vitro ubiquitination evidence for FBXO21→p85α, and model schematic of FBXO21→p85α effects on PI3K/AKT vs ERK signaling (Dobish et al., 2023). (dobish2023fbxo21mediateddegradation media 6f09b441, dobish2023fbxo21mediateddegradation media e1da3cd9)

References

(watanabe2015fbxo21mediatesthe pages 2-3): Koki Watanabe, Kanae Yumimoto, and Keiichi I. Nakayama. Fbxo21 mediates the ubiquitylation and proteasomal degradation of eid1. Genes to Cells, 20:667-674, Aug 2015. URL: https://doi.org/10.1111/gtc.12260, doi:10.1111/gtc.12260. This article has 20 citations and is from a peer-reviewed journal.
(watanabe2015fbxo21mediatesthe pages 1-2): Koki Watanabe, Kanae Yumimoto, and Keiichi I. Nakayama. Fbxo21 mediates the ubiquitylation and proteasomal degradation of eid1. Genes to Cells, 20:667-674, Aug 2015. URL: https://doi.org/10.1111/gtc.12260, doi:10.1111/gtc.12260. This article has 20 citations and is from a peer-reviewed journal.
(yu2016lys29linkageofask1 pages 7-9): Z Yu, T Chen, X Li, M Yang, S Tang, X Zhu, Y Gu, and X Su. Lys29-linkage of ask1 by skp1− cullin 1− fbxo21 ubiquitin ligase complex is required for antiviral innate response. Unknown journal, 2016.
(zhang2015peptidicdegronin pages 1-2): Cuiyan Zhang, Xiaotong Li, Guillaume Adelmant, Jessica Dobbins, Christoph Geisen, Matthew G. Oser, Kai W. Wucherpfenning, Jarrod A. Marto, and William G. Kaelin. Peptidic degron in eid1 is recognized by an scf e3 ligase complex containing the orphan f-box protein fbxo21. Proceedings of the National Academy of Sciences, 112:15372-15377, Dec 2015. URL: https://doi.org/10.1073/pnas.1522006112, doi:10.1073/pnas.1522006112. This article has 34 citations and is from a highest quality peer-reviewed journal.
(dobish2023fbxo21mediateddegradation pages 8-9): Kasidy K. Dobish, Karli J. Wittorf, Samantha A. Swenson, Dalton C. Bean, Catherine M. Gavile, Nicholas T. Woods, Gargi Ghosal, R. Katherine Hyde, and Shannon M. Buckley. Fbxo21 mediated degradation of p85α regulates proliferation and survival of acute myeloid leukemia. Leukemia, 37:2197-2208, Sep 2023. URL: https://doi.org/10.1038/s41375-023-02020-w, doi:10.1038/s41375-023-02020-w. This article has 10 citations and is from a highest quality peer-reviewed journal.
(ravindranath2015cd44promotesmultidrug pages 7-9): Abhilash K. Ravindranath, Swayamjot Kaur, Roman P. Wernyj, Muthu N. Kumaran, Karl E. Miletti-Gonzalez, Rigel Chan, Elaine Lim, Kiran Madura, and Lorna Rodriguez-Rodriguez. Cd44 promotes multi-drug resistance by protecting p-glycoprotein from fbxo21-mediated ubiquitination. Oncotarget, 6:26308-26321, Jul 2015. URL: https://doi.org/10.18632/oncotarget.4763, doi:10.18632/oncotarget.4763. This article has 67 citations.
(yu2016lys29linkageofask1 pages 14-15): Z Yu, T Chen, X Li, M Yang, S Tang, X Zhu, Y Gu, and X Su. Lys29-linkage of ask1 by skp1− cullin 1− fbxo21 ubiquitin ligase complex is required for antiviral innate response. Unknown journal, 2016.
(yu2016lys29linkageofask1 pages 2-3): Z Yu, T Chen, X Li, M Yang, S Tang, X Zhu, Y Gu, and X Su. Lys29-linkage of ask1 by skp1− cullin 1− fbxo21 ubiquitin ligase complex is required for antiviral innate response. Unknown journal, 2016.
(dobish2023fbxo21mediateddegradation pages 3-4): Kasidy K. Dobish, Karli J. Wittorf, Samantha A. Swenson, Dalton C. Bean, Catherine M. Gavile, Nicholas T. Woods, Gargi Ghosal, R. Katherine Hyde, and Shannon M. Buckley. Fbxo21 mediated degradation of p85α regulates proliferation and survival of acute myeloid leukemia. Leukemia, 37:2197-2208, Sep 2023. URL: https://doi.org/10.1038/s41375-023-02020-w, doi:10.1038/s41375-023-02020-w. This article has 10 citations and is from a highest quality peer-reviewed journal.
(dobish2023fbxo21mediateddegradation pages 7-8): Kasidy K. Dobish, Karli J. Wittorf, Samantha A. Swenson, Dalton C. Bean, Catherine M. Gavile, Nicholas T. Woods, Gargi Ghosal, R. Katherine Hyde, and Shannon M. Buckley. Fbxo21 mediated degradation of p85α regulates proliferation and survival of acute myeloid leukemia. Leukemia, 37:2197-2208, Sep 2023. URL: https://doi.org/10.1038/s41375-023-02020-w, doi:10.1038/s41375-023-02020-w. This article has 10 citations and is from a highest quality peer-reviewed journal.
(dobish2023fbxo21mediateddegradation pages 9-10): Kasidy K. Dobish, Karli J. Wittorf, Samantha A. Swenson, Dalton C. Bean, Catherine M. Gavile, Nicholas T. Woods, Gargi Ghosal, R. Katherine Hyde, and Shannon M. Buckley. Fbxo21 mediated degradation of p85α regulates proliferation and survival of acute myeloid leukemia. Leukemia, 37:2197-2208, Sep 2023. URL: https://doi.org/10.1038/s41375-023-02020-w, doi:10.1038/s41375-023-02020-w. This article has 10 citations and is from a highest quality peer-reviewed journal.
(dobish2023fbxo21mediateddegradation media 6f09b441): Kasidy K. Dobish, Karli J. Wittorf, Samantha A. Swenson, Dalton C. Bean, Catherine M. Gavile, Nicholas T. Woods, Gargi Ghosal, R. Katherine Hyde, and Shannon M. Buckley. Fbxo21 mediated degradation of p85α regulates proliferation and survival of acute myeloid leukemia. Leukemia, 37:2197-2208, Sep 2023. URL: https://doi.org/10.1038/s41375-023-02020-w, doi:10.1038/s41375-023-02020-w. This article has 10 citations and is from a highest quality peer-reviewed journal.
(dobish2023fbxo21mediateddegradation media e1da3cd9): Kasidy K. Dobish, Karli J. Wittorf, Samantha A. Swenson, Dalton C. Bean, Catherine M. Gavile, Nicholas T. Woods, Gargi Ghosal, R. Katherine Hyde, and Shannon M. Buckley. Fbxo21 mediated degradation of p85α regulates proliferation and survival of acute myeloid leukemia. Leukemia, 37:2197-2208, Sep 2023. URL: https://doi.org/10.1038/s41375-023-02020-w, doi:10.1038/s41375-023-02020-w. This article has 10 citations and is from a highest quality peer-reviewed journal.
(dobish2023fbxo21mediateddegradation pages 1-2): Kasidy K. Dobish, Karli J. Wittorf, Samantha A. Swenson, Dalton C. Bean, Catherine M. Gavile, Nicholas T. Woods, Gargi Ghosal, R. Katherine Hyde, and Shannon M. Buckley. Fbxo21 mediated degradation of p85α regulates proliferation and survival of acute myeloid leukemia. Leukemia, 37:2197-2208, Sep 2023. URL: https://doi.org/10.1038/s41375-023-02020-w, doi:10.1038/s41375-023-02020-w. This article has 10 citations and is from a highest quality peer-reviewed journal.
(ravindranath2015cd44promotesmultidrug pages 5-7): Abhilash K. Ravindranath, Swayamjot Kaur, Roman P. Wernyj, Muthu N. Kumaran, Karl E. Miletti-Gonzalez, Rigel Chan, Elaine Lim, Kiran Madura, and Lorna Rodriguez-Rodriguez. Cd44 promotes multi-drug resistance by protecting p-glycoprotein from fbxo21-mediated ubiquitination. Oncotarget, 6:26308-26321, Jul 2015. URL: https://doi.org/10.18632/oncotarget.4763, doi:10.18632/oncotarget.4763. This article has 67 citations.
(ravindranath2015cd44promotesmultidrug pages 9-10): Abhilash K. Ravindranath, Swayamjot Kaur, Roman P. Wernyj, Muthu N. Kumaran, Karl E. Miletti-Gonzalez, Rigel Chan, Elaine Lim, Kiran Madura, and Lorna Rodriguez-Rodriguez. Cd44 promotes multi-drug resistance by protecting p-glycoprotein from fbxo21-mediated ubiquitination. Oncotarget, 6:26308-26321, Jul 2015. URL: https://doi.org/10.18632/oncotarget.4763, doi:10.18632/oncotarget.4763. This article has 67 citations.
(ravindranath2015cd44promotesmultidrug pages 1-2): Abhilash K. Ravindranath, Swayamjot Kaur, Roman P. Wernyj, Muthu N. Kumaran, Karl E. Miletti-Gonzalez, Rigel Chan, Elaine Lim, Kiran Madura, and Lorna Rodriguez-Rodriguez. Cd44 promotes multi-drug resistance by protecting p-glycoprotein from fbxo21-mediated ubiquitination. Oncotarget, 6:26308-26321, Jul 2015. URL: https://doi.org/10.18632/oncotarget.4763, doi:10.18632/oncotarget.4763. This article has 67 citations.
(dobish2024smallmoleculetargeting pages 8-11): Kasidy K. Dobish, Suchita Vishwakarma, Hendrik C. Peters, C. Bea Winship, Danielle Alvarado, R. Katherine Hyde, Amarnath Natarajan, and Shannon M. Buckley. Small molecule targeting of fbxo21 mediated p85α ubiquitylation in acute myeloid leukemia. bioRxiv, Dec 2024. URL: https://doi.org/10.1101/2024.12.13.628427, doi:10.1101/2024.12.13.628427. This article has 0 citations.
(dobish2024smallmoleculetargeting pages 1-5): Kasidy K. Dobish, Suchita Vishwakarma, Hendrik C. Peters, C. Bea Winship, Danielle Alvarado, R. Katherine Hyde, Amarnath Natarajan, and Shannon M. Buckley. Small molecule targeting of fbxo21 mediated p85α ubiquitylation in acute myeloid leukemia. bioRxiv, Dec 2024. URL: https://doi.org/10.1101/2024.12.13.628427, doi:10.1101/2024.12.13.628427. This article has 0 citations.
(ravindranath2015cd44promotesmultidrug pages 10-11): Abhilash K. Ravindranath, Swayamjot Kaur, Roman P. Wernyj, Muthu N. Kumaran, Karl E. Miletti-Gonzalez, Rigel Chan, Elaine Lim, Kiran Madura, and Lorna Rodriguez-Rodriguez. Cd44 promotes multi-drug resistance by protecting p-glycoprotein from fbxo21-mediated ubiquitination. Oncotarget, 6:26308-26321, Jul 2015. URL: https://doi.org/10.18632/oncotarget.4763, doi:10.18632/oncotarget.4763. This article has 67 citations.
(OpenTargets Search: -FBXO21): Open Targets Query (-FBXO21, 5 results). Buniello, A. et al. (2025). Open Targets Platform: facilitating therapeutic hypotheses building in drug discovery. Nucleic Acids Research.
(dobish2024smallmoleculetargeting pages 11-14): Kasidy K. Dobish, Suchita Vishwakarma, Hendrik C. Peters, C. Bea Winship, Danielle Alvarado, R. Katherine Hyde, Amarnath Natarajan, and Shannon M. Buckley. Small molecule targeting of fbxo21 mediated p85α ubiquitylation in acute myeloid leukemia. bioRxiv, Dec 2024. URL: https://doi.org/10.1101/2024.12.13.628427, doi:10.1101/2024.12.13.628427. This article has 0 citations.

Artifacts

Edison artifact artifact-00

Citations

zhang2015peptidicdegronin pages 1-2
dobish2024smallmoleculetargeting pages 8-11
dobish2024smallmoleculetargeting pages 1-5
dobish2024smallmoleculetargeting pages 11-14
https://doi.org/10.1111/gtc.12260
https://doi.org/10.1073/pnas.1522006112
https://doi.org/10.1038/s41375-023-02020-w
https://doi.org/10.18632/oncotarget.4763
https://doi.org/10.1101/2024.12.13.628427
https://doi.org/10.1038/s41586-024-07224-3
https://doi.org/10.1111/gtc.12260,
https://doi.org/10.1073/pnas.1522006112,
https://doi.org/10.1038/s41375-023-02020-w,
https://doi.org/10.18632/oncotarget.4763,
https://doi.org/10.1101/2024.12.13.628427,

📚 Additional Documentation

Pn Notes

(FBXO21-pn-notes.md)

FBXO21 PN Consistency Notes

Generated: 2026-06-18
Project: PROTEOSTASIS
Scope: PN consistency rereview against local AIGR review and available deep-research artifacts
UniProt: O94952
AIGR review status: COMPLETE
Review batch: proteostasis-batch-2026-06-13
Batch change status: added

Source Files Checked

AIGR review YAML: present - genes/human/FBXO21/FBXO21-ai-review.yaml
AIGR review HTML: missing - genes/human/FBXO21/FBXO21-ai-review.html
Gene notes: missing - genes/human/FBXO21/FBXO21-notes.md
GOA TSV: present - genes/human/FBXO21/FBXO21-goa.tsv
UniProt record: present - genes/human/FBXO21/FBXO21-uniprot.txt
PN dossier: projects/PROTEOSTASIS/reports/phase1_dossiers/FBXO21.md
PN consistency section: projects/PROTEOSTASIS/reports/phase1_dossiers/_sections/FBXO21.md
PN projection report: projects/PROTEOSTASIS/reports/pn_projection/pn_projected_gene_go_summary.tsv
PN mapping audit: projects/PROTEOSTASIS/reports/pn_mapping_audit/current_mapping_scrutiny.tsv

Deep Research Files

genes/human/FBXO21/FBXO21-deep-research-falcon.md

AIGR Review Snapshot

Description: FBXO21 (FBX21, KIAA0875; also reported as SIP/FBXO3B) is a 628-residue F-box protein that serves as the substrate-recognition subunit of a multi-subunit SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex. Through its N-terminal F-box domain (residues 28-84) it binds SKP1, coupling to CUL1 and the catalytic RING subunit RBX1, while its central and C-terminal regions provide substrate specificity. The best-characterized substrate is EID1 (EP300-interacting inhibitor of differentiation 1): FBXO21 binds the C-terminal region of EID1 and SCF(FBXO21) directly ubiquitylates EID1 to drive its proteasomal degradation predominantly in the cytoplasm (EID1 accumulates in both cytoplasm and nucleus upon FBXO21 loss), thereby influencing EID1-dependent transcriptional repression; a peptidic modular degron in EID1 is necessary and sufficient for SCF(FBXO21)-dependent polyubiquitylation. FBXO21 operates in two mechanistic modes. In the canonical proteolytic mode it directs K48-type proteasomal degradation of substrates including EID1, the PI3K regulatory subunit p85alpha (PIK3R1; degradation in acute myeloid leukemia shapes PI3K/AKT versus ERK signaling and AML cell survival/differentiation), and the multidrug-resistance transporter ABCB1/P-glycoprotein (whose turnover is blocked by Ser291-phosphorylated CD44). In a non-proteolytic signaling mode, SCF(FBXO21) catalyzes Lys29-linked ubiquitination of the kinase ASK1 (MAP3K5) that promotes ASK1 activation rather than degradation, driving ASK1-JNK/p38 and IRF3-dependent antiviral innate immune signaling and type I interferon (IFN-beta) and IL-6 induction after viral or nucleic-acid challenge. As an SCF adaptor, FBXO21 thus acts in the ubiquitin-proteasome system to mediate ubiquitin-dependent, SCF-type protein catabolism and, via atypical chain linkages, ubiquitin-dependent signal transduction. It is broadly expressed (tissue-enhanced in fallopian tube).
Existing/core annotation action counts: ACCEPT: 2; KEEP_AS_NON_CORE: 16; MODIFY: 1; REMOVE: 1

PN Consistency Summary

Consistency: Consistent. Review executes the canonical pattern: NAS GO:0004842 transferase MODIFY → GO:1990756 (verified real), matching the PN projection; REMOVEs a spurious IEA GO:0003677 DNA binding (YccV/IPR011722 fold artifact). DR ↔ YAML agree on EID1 (validated, PMID:26085330) plus ASK1/p85alpha/ABCB1 leads.
PN story / NEW pressure: PN asserts the adaptor MF; review supplies it via MODIFY — already captured/improved, not over-reach. Extra biology beyond PN: non-proteolytic Lys29 ubiquitination of ASK1 driving innate-immune/IFN signaling (core_function directly_involved_in GO:0045087), and p85alpha/AML degradation — these go beyond simple catabolism but are anchored on the validated EID1 work plus Falcon leads. No additional verified NEW GO term needed beyond GO:1990756. Validated substrate (EID1) present — not substrate-less.
Evidence alignment: PN cites only 15340381. Review uses PMID:26085330 (EID1, HIGH), 10531035 (family cloning, source of transferase NAS), 34445249 (SCF), interactome PMIDs (non-core), plus Falcon ASK1/p85alpha/ABCB1 leads. Expansion, no conflict.
Verdict: Consistent; review implements PN adaptor mapping via MODIFY GO:0004842→GO:1990756 and correctly REMOVEs the IEA DNA-binding artifact.

Full Consistency Review

UniProt: O94952 · batch: proteostasis-batch-2026-06-13 (Falcon DR) · review status: COMPLETE
PN placement: UPS|E3 ubiquitin and UBL ligases|Cul1 substrate receptor|F-box|other ; PN-node mapping: subtype/type no_mapping; group Cul1 substrate receptor=mapped / ok_for_propagation_to_go → GO:1990756 (new_to_goa); class context_only/too_broad (GO:0061630).
Consistency: Consistent. Review executes the canonical pattern: NAS GO:0004842 transferase MODIFY → GO:1990756 (verified real), matching the PN projection; REMOVEs a spurious IEA GO:0003677 DNA binding (YccV/IPR011722 fold artifact). DR ↔ YAML agree on EID1 (validated, PMID:26085330) plus ASK1/p85alpha/ABCB1 leads.
PN story / NEW pressure: PN asserts the adaptor MF; review supplies it via MODIFY — already captured/improved, not over-reach. Extra biology beyond PN: non-proteolytic Lys29 ubiquitination of ASK1 driving innate-immune/IFN signaling (core_function directly_involved_in GO:0045087), and p85alpha/AML degradation — these go beyond simple catabolism but are anchored on the validated EID1 work plus Falcon leads. No additional verified NEW GO term needed beyond GO:1990756. Validated substrate (EID1) present — not substrate-less.
Mapping strategy: Gene does not change the node; status/scope correct. PN-projected GO:1990756 at correct altitude (= review's MODIFY target). Class GO:0061630 too_broad. The signaling/innate-immune dimension is captured in core_functions, not via the PN UPS node — appropriate.
Evidence alignment: PN cites only 15340381. Review uses PMID:26085330 (EID1, HIGH), 10531035 (family cloning, source of transferase NAS), 34445249 (SCF), interactome PMIDs (non-core), plus Falcon ASK1/p85alpha/ABCB1 leads. Expansion, no conflict.
Verdict: Consistent; review implements PN adaptor mapping via MODIFY GO:0004842→GO:1990756 and correctly REMOVEs the IEA DNA-binding artifact.
Recommended edits: none to FBXO21-ai-review.yaml. [MAP] none — node mapping and review concur.

PN Dossier Context

review_batch: proteostasis-batch-2026-06-13
review_yaml: genes/human/FBXO21/FBXO21-ai-review.yaml
PN workbook rows: 1

PN row 1: Ubiquitin Proteasome System | E3 ubiquitin and UBL ligases | Cul1 substrate receptor | F-box | other

UniProt: O94952
In branches: UPS
Signature domains: IPR001810
Auxiliary domains: (none)
PN references (titles):
- 15340381 / rev
PN-node mapping records (path + ancestors):
- [subtype] Ubiquitin Proteasome System|E3 ubiquitin and UBL ligases|Cul1 substrate receptor|F-box|other
  status=no_mapping scope= GO=[]
  rationale: Reviewed as a narrower substrate-receptor, adaptor, domain, or family subdivision already covered by the curated parent adaptor/receptor mapping. No additional direct GO mapping is needed at this node.
- [type] Ubiquitin Proteasome System|E3 ubiquitin and UBL ligases|Cul1 substrate receptor|F-box
  status=no_mapping scope= GO=[]
  rationale: Reviewed as a narrower substrate-receptor, adaptor, domain, or family subdivision already covered by the curated parent adaptor/receptor mapping. No additional direct GO mapping is needed at this node.
- [group] Ubiquitin Proteasome System|E3 ubiquitin and UBL ligases|Cul1 substrate receptor
  status=mapped scope=ok_for_propagation_to_go GO=[GO:1990756 ubiquitin-like ligase-substrate adaptor activity]
  rationale: This PN group captures substrate receptors/adaptors for cullin/UBL ligase systems. The shared GO molecular-function target is ubiquitin-like ligase-substrate adaptor activity.
- [class] Ubiquitin Proteasome System|E3 ubiquitin and UBL ligases
  status=context_only scope=too_broad_to_propagate GO=[GO:0061630 ubiquitin protein ligase activity]
  rationale: This class is a genuine E3-ligase context, but its descendants include catalytic ligases, cullin scaffolds, substrate receptors, adaptors, cofactors, regulators, and UBL modifier systems. A class-level propagation would over-annotate.
- [branch] Ubiquitin Proteasome System
  status=no_mapping scope= GO=[]
  rationale: Reviewed as the top-level UPS branch. It is a project taxonomy umbrella rather than a direct GO assertion; UPS propagation must come from manually curated child nodes.

Projected GO annotations (1)

GO:1990756 ubiquitin-like ligase-substrate adaptor activity | scope=ok_for_propagation_to_go | goa_status=new_to_goa | from=Ubiquitin Proteasome System|E3 ubiquitin and UBL ligases|Cul1 substrate receptor

Note

This file is generated from the current PROTEOSTASIS phase-1 dossier and local gene-review artifacts. Edit the source review, PN mapping, or dossier rather than this generated note when correcting the underlying curation.

📄 View Raw YAML

id: O94952
gene_symbol: FBXO21
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  FBXO21 (FBX21, KIAA0875; also reported as SIP/FBXO3B) is a 628-residue F-box
  protein that serves as the substrate-recognition subunit of a multi-subunit
  SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex. Through its
  N-terminal F-box domain (residues 28-84) it binds SKP1, coupling to CUL1 and
  the catalytic RING subunit RBX1, while its central and C-terminal regions
  provide substrate specificity. The best-characterized substrate is EID1
  (EP300-interacting inhibitor of differentiation 1): FBXO21 binds the
  C-terminal region of EID1 and SCF(FBXO21) directly ubiquitylates EID1 to drive
  its proteasomal degradation predominantly in the cytoplasm (EID1 accumulates in
  both cytoplasm and nucleus upon FBXO21 loss), thereby influencing
  EID1-dependent transcriptional repression; a peptidic modular degron in EID1 is
  necessary and sufficient for SCF(FBXO21)-dependent polyubiquitylation. FBXO21
  operates in two mechanistic modes. In the canonical proteolytic mode it directs
  K48-type proteasomal degradation of substrates including EID1, the PI3K
  regulatory subunit p85alpha (PIK3R1; degradation in acute myeloid leukemia
  shapes PI3K/AKT versus ERK signaling and AML cell survival/differentiation), and
  the multidrug-resistance transporter ABCB1/P-glycoprotein (whose turnover is
  blocked by Ser291-phosphorylated CD44). In a non-proteolytic signaling mode,
  SCF(FBXO21) catalyzes Lys29-linked ubiquitination of the kinase ASK1 (MAP3K5)
  that promotes ASK1 activation rather than degradation, driving ASK1-JNK/p38 and
  IRF3-dependent antiviral innate immune signaling and type I interferon (IFN-beta)
  and IL-6 induction after viral or nucleic-acid challenge. As an SCF adaptor,
  FBXO21 thus acts in the ubiquitin-proteasome system to mediate
  ubiquitin-dependent, SCF-type protein catabolism and, via atypical chain
  linkages, ubiquitin-dependent signal transduction. It is broadly expressed
  (tissue-enhanced in fallopian tube).
alternative_products:
- name: '1'
  id: O94952-2
- name: '2'
  id: O94952-1
  sequence_note: VSP_035975
existing_annotations:
- term:
    id: GO:0003677
    label: DNA binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: InterPro domain-based electronic assignment of DNA binding, propagated from a hemimethylated-DNA-binding-like (YccV-like) domain match; FBXO21 is an SCF substrate adaptor with no established DNA-binding function.
    action: REMOVE
    reason: >-
      This is an over-propagated IEA. FBXO21's UniProt-curated function is purely
      as the substrate-recognition subunit of an SCF E3 ubiquitin ligase, binding
      SKP1/CUL1 and substrates (EID1). The DNA binding term derives from an
      InterPro signature (IPR011722 hemimethylated-DNA-binding/YccV-like,
      TIGR02097 yccV) detected in the protein's C-terminal region; there is no
      experimental or curated evidence that FBXO21 binds DNA sequence-specifically
      or functions in DNA metabolism. The annotation conflates a remote
      domain-fold match with a molecular function and is biologically misleading
      for a cytoplasmic/nuclear ubiquitin-ligase adaptor.
    supported_by:
    - reference_id: file:human/FBXO21/FBXO21-uniprot.txt
      supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:26085330
  qualifier: enables
  review:
    summary: IPI interaction underpinning the functionally meaningful FBXO21-EID1 substrate relationship (FBXO21 binds the C-terminal region of EID1 and targets it for ubiquitylation). The bare protein binding term is uninformative, but this records a real substrate interaction.
    action: KEEP_AS_NON_CORE
    reason: Records the substrate-recognition interaction with EID1, which is biologically central, but bare protein binding (GO:0005515) is uninformative per curation guidelines; the substrate relationship is better captured by the SCF/catabolism process annotations and core functions.
    supported_by:
    - reference_id: PMID:26085330
      supporting_text: The central and COOH-terminal portion of FBXO21 was found to interact with the COOH-terminal region of EID1 in transfected cells.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:27705803
  qualifier: enables
  review:
    summary: Interaction captured in a high-throughput affinity-purification map of the human Polycomb complexome. Bare protein binding is uninformative.
    action: KEEP_AS_NON_CORE
    reason: High-throughput AP-MS interaction; bare protein binding (GO:0005515) is uninformative and not a core function.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:33961781
  qualifier: enables
  review:
    summary: Interaction captured in the BioPlex proteome-scale AP-MS interactome (cell-specific remodeling study). Bare protein binding is uninformative.
    action: KEEP_AS_NON_CORE
    reason: High-throughput AP-MS interactome; bare protein binding (GO:0005515) is uninformative and not a core function.
    supported_by:
    - reference_id: PMID:33961781
      supporting_text: BioPlex 3.0, results from affinity purification of 10,128 human proteins-half the proteome-in 293T cells and includes 118,162 interactions among 14,586 proteins.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:40205054
  qualifier: enables
  review:
    summary: Interaction captured in a multimodal AP-MS/immunofluorescence cell map of U2OS cells. Bare protein binding is uninformative.
    action: KEEP_AS_NON_CORE
    reason: High-throughput AP-MS interactome; bare protein binding (GO:0005515) is uninformative and not a core function.
    supported_by:
    - reference_id: PMID:40205054
      supporting_text: through joint measurement of biophysical interactions and immunofluorescence images for over 5,100 proteins in U2OS osteosarcoma cells
- term:
    id: GO:0019005
    label: SCF ubiquitin ligase complex
  evidence_type: NAS
  original_reference_id: PMID:34445249
  qualifier: part_of
  review:
    summary: FBXO21 is the variable F-box substrate-recognition subunit of an SCF (SKP1-CUL1-F-box) E3 ubiquitin ligase complex, binding SKP1 and CUL1. This is a core localization/complex-membership annotation.
    action: ACCEPT
    reason: >-
      Directly supported by UniProt curation (interacts with SKP1 and CUL1) and
      by a dedicated ComplexPortal entry (CPX-7930, SCF E3 ubiquitin ligase
      complex, FBXO21 variant). Core to FBXO21's molecular role.
    supported_by:
    - reference_id: file:human/FBXO21/FBXO21-uniprot.txt
      supporting_text: 'Directly interacts with SKP1 and CUL1.'
- term:
    id: GO:0031146
    label: SCF-dependent proteasomal ubiquitin-dependent protein catabolic process
  evidence_type: NAS
  original_reference_id: PMID:34445249
  qualifier: involved_in
  review:
    summary: As the substrate-recognition subunit of an SCF complex, FBXO21 drives SCF-dependent, proteasome-mediated degradation of its substrates (demonstrated for EID1). This is the core biological process.
    action: ACCEPT
    reason: >-
      Core biological process. SCF complexes target substrates with poly-ubiquitin
      chains for proteasomal degradation (PMID:34445249), and FBXO21 was shown to
      drive proteasomal degradation of EID1 (PMID:26085330). Specific and
      well-supported.
    supported_by:
    - reference_id: PMID:34445249
      supporting_text: 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
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-8952618
  qualifier: located_in
  review:
    summary: Reactome TAS localization to cytosol assigned via a generic CRL1 (cullin-RING ligase) neddylation/cycle reaction. Correct compartment but generic and inherited from pathway context.
    action: KEEP_AS_NON_CORE
    reason: Cytosolic localization is consistent with FBXO21 acting in cytoplasmic SCF complexes (and EID1 is degraded in cytoplasm and nucleus), but this annotation is a generic CRL-cycle pathway assignment, redundant across many reactions; non-core.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-8952620
  qualifier: located_in
  review:
    summary: Reactome TAS localization to cytosol from a generic CRL1 neddylation/cycle reaction.
    action: KEEP_AS_NON_CORE
    reason: Generic CRL-cycle localization; correct but redundant and non-core.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-8955241
  qualifier: located_in
  review:
    summary: Reactome TAS localization to cytosol from a generic cytosolic CRL E3 ligase (CAND1-binding) reaction.
    action: KEEP_AS_NON_CORE
    reason: Generic CRL-cycle localization; correct but redundant and non-core.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-8955289
  qualifier: located_in
  review:
    summary: Reactome TAS localization to cytosol from a generic cytosolic CRL E3 ligase (COMMD/CAND1) reaction.
    action: KEEP_AS_NON_CORE
    reason: Generic CRL-cycle localization; correct but redundant and non-core.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-8956040
  qualifier: located_in
  review:
    summary: Reactome TAS localization to cytosol from the COP9-signalosome deneddylation reaction on cytosolic CRL complexes.
    action: KEEP_AS_NON_CORE
    reason: Generic CRL-cycle localization; correct but redundant and non-core.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-8956200
  qualifier: located_in
  review:
    summary: Reactome TAS localization to cytosol from a generic CRL1 (DCUN1D3-binding) reaction.
    action: KEEP_AS_NON_CORE
    reason: Generic CRL-cycle localization; correct but redundant and non-core.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-983140
  qualifier: located_in
  review:
    summary: Reactome TAS localization to cytosol from a generic ubiquitin-transfer (E2-to-substrate) reaction in the antigen-processing/proteasome pathway.
    action: KEEP_AS_NON_CORE
    reason: Generic ubiquitination-cycle localization; correct but redundant and non-core.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-983147
  qualifier: located_in
  review:
    summary: Reactome TAS localization to cytosol from a generic reaction (release of E3 from polyubiquitinated substrate).
    action: KEEP_AS_NON_CORE
    reason: Generic ubiquitination-cycle localization; correct but redundant and non-core.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-983156
  qualifier: located_in
  review:
    summary: Reactome TAS localization to cytosol from a generic polyubiquitination-of-substrate reaction.
    action: KEEP_AS_NON_CORE
    reason: Generic ubiquitination-cycle localization; correct but redundant and non-core.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-983157
  qualifier: located_in
  review:
    summary: Reactome TAS localization to cytosol from a generic reaction (interaction of E3 with substrate and E2-Ub complex).
    action: KEEP_AS_NON_CORE
    reason: Generic ubiquitination-cycle localization; correct but redundant and non-core.
- term:
    id: GO:0000151
    label: ubiquitin ligase complex
  evidence_type: NAS
  original_reference_id: PMID:10531035
  qualifier: part_of
  review:
    summary: FBXO21 is a subunit of a ubiquitin ligase (SCF) complex, the general parent of the more specific SCF ubiquitin ligase complex annotation. Supported by the founding F-box protein family paper.
    action: KEEP_AS_NON_CORE
    reason: Correct but generic; the more specific GO:0019005 (SCF ubiquitin ligase complex) better captures FBXO21's complex membership. F-box proteins are one of the four subunits of SCF ubiquitin ligases.
    supported_by:
    - reference_id: PMID:10531035
      supporting_text: F-box proteins are one of the four subunits of ubiquitin protein ligases called SCFs.
- term:
    id: GO:0004842
    label: ubiquitin-protein transferase activity
  evidence_type: NAS
  original_reference_id: PMID:10531035
  qualifier: enables
  review:
    summary: NAS assignment of ubiquitin-protein transferase activity from the founding F-box family paper. However, within the SCF complex the catalytic ubiquitin-transfer activity resides in the RING subunit (RBX1); the F-box protein FBXO21 functions as the substrate-recruiting adaptor, not the catalytic transferase.
    action: MODIFY
    reason: >-
      F-box proteins do not themselves catalyse ubiquitin transfer; they are the
      substrate-recognition adaptors that recruit substrates to the SCF, whose
      catalytic activity is provided by the RING subunit RBX1 (with the E2). The
      NAS source (PMID:10531035) describes SCF complexes (not FBXO21 itself) as
      ubiquitin ligases. The more accurate molecular function for an F-box protein
      is ubiquitin-like ligase-substrate adaptor activity (GO:1990756).
    proposed_replacement_terms:
    - id: GO:1990756
      label: ubiquitin-like ligase-substrate adaptor activity
    supported_by:
    - reference_id: file:human/FBXO21/FBXO21-uniprot.txt
      supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
- term:
    id: GO:0006511
    label: ubiquitin-dependent protein catabolic process
  evidence_type: NAS
  original_reference_id: PMID:10531035
  qualifier: involved_in
  review:
    summary: FBXO21, as an SCF substrate adaptor, mediates ubiquitin-dependent protein catabolism (directly demonstrated for EID1). General parent of the SCF-dependent catabolism term.
    action: KEEP_AS_NON_CORE
    reason: Correct and supported (FBXO21 drives ubiquitin-dependent proteasomal degradation of EID1), but the more specific GO:0031146 (SCF-dependent proteasomal ubiquitin-dependent protein catabolic process) better captures the role; retained as non-core to avoid redundancy.
    supported_by:
    - reference_id: PMID:26085330
      supporting_text: FBXO21 mediates the ubiquitylation and proteasomal degradation of EID1.
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO terms
  findings: []
- id: PMID:10531035
  title: Identification of a family of human F-box proteins.
  findings:
  - statement: F-box proteins are one of the four subunits of SCF ubiquitin protein ligases (with SKP1, a cullin, and ROC1/RBX1); FBXO21 (FBX21) was identified among 26 human F-box proteins using SKP1 as bait.
    reference_section_type: ABSTRACT
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: >-
      Founding paper identifying the human F-box protein family including FBXO21/FBX21; establishes the SCF four-subunit
      architecture. Supports SCF complex membership but describes the SCF (not the F-box protein itself) as the ligase,
      so it does not establish intrinsic transferase activity for FBXO21.
- id: PMID:26085330
  title: FBXO21 mediates the ubiquitylation and proteasomal degradation of EID1.
  findings:
  - statement: FBXO21 is the substrate-recognition subunit of an SCF-type E3; its central/C-terminal region binds the C-terminal region of EID1, and SCF(FBXO21) directly ubiquitylates EID1 to drive its proteasomal degradation in cytoplasm and nucleus.
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      Abstract-only in cache (full_text_available:false) but the abstract directly and specifically establishes the
      EID1 substrate relationship and FBXO21's role as an SCF substrate adaptor via an in vitro ubiquitylation assay
      and CRISPR knockout. The key functional reference for this gene.
- id: PMID:27705803
  title: A High-Density Map for Navigating the Human Polycomb Complexome.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: High-throughput AP-MS Polycomb complexome map; source of a bare protein binding (GO:0005515) annotation, not informative about FBXO21's specific function.
- id: PMID:33961781
  title: Dual proteome-scale networks reveal cell-specific remodeling of the human interactome.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: BioPlex 3.0 proteome-scale AP-MS interactome; source of a bare protein binding (GO:0005515) annotation.
- id: PMID:34445249
  title: The SCF Complex Is Essential to Maintain Genome and Chromosome Stability.
  findings:
  - statement: The SCF complex is a family of ~69 E3 ubiquitin ligase complexes (SKP1-CUL1-F-box) that poly-ubiquitinate substrates for proteasomal degradation; variable F-box proteins determine substrate specificity.
    reference_section_type: ABSTRACT
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Review establishing SCF complex architecture and function; supports the SCF complex-membership and SCF-dependent catabolism annotations for the F-box protein FBXO21.
- id: PMID:40205054
  title: Multimodal cell maps as a foundation for structural and functional genomics.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: High-throughput multimodal (AP-MS + immunofluorescence) cell map; source of a bare protein binding (GO:0005515) annotation.
- id: file:human/FBXO21/FBXO21-deep-research-falcon.md
  title: Falcon deep research report for human FBXO21
  findings:
  - statement: FBXO21 is an SCF (SKP1-CUL1-RBX1) substrate receptor with validated substrates EID1, ASK1, p85alpha/PIK3R1, and ABCB1/P-gp, operating in both proteolytic and non-proteolytic ubiquitination modes.
    supporting_text: >-
      The research target is **human FBXO21** (UniProt **O94952**; gene
      **FBXO21**, synonyms **FBX21**, **KIAA0875**), an **F-box only** protein
      that functions as a substrate-recognition subunit of an **SCF
      (SKP1–CUL1–RBX1) Cullin-RING E3 ubiquitin ligase** complex. This identity
      is consistent across primary literature describing **SCF^FBXO21** and its
      validated substrates (EID1, ASK1, p85α/PIK3R1, and ABCB1/P-gp).
  - statement: SCF(FBXO21) mediates non-proteolytic Lys29-linked ubiquitination of ASK1 (MAP3K5) that promotes ASK1 activation rather than degradation, driving antiviral innate signaling and type I interferon responses.
    supporting_text: >-
      SCF^FBXO21 mediates **Lys29-linked ubiquitination** of ASK1, demonstrated
      using ubiquitin mutants (K29-only ubiquitin), and this modification is
      **non-proteolytic** (ASK1 degradation not affected).
  - statement: FBXO21 deficiency impairs virus-induced ASK1-JNK/p38 and IRF3 signaling and reduces IL-6 and IFN-beta induction after viral or nucleic-acid challenge.
    supporting_text: >-
      FBXO21 deficiency impaired virus-induced signaling (reduced JNK/p38 and
      IRF3 activation; reduced nuclear c-Fos/IRF3) and reduced **IL-6 and IFNβ**
      induction after LPS, nucleic acid agonists, and VSV/HSV-1 infection,
      linking FBXO21→ASK1 ubiquitination to **ASK1–JNK/p38** antiviral signaling
      and type I interferon responses.
  - statement: In acute myeloid leukemia, FBXO21 ubiquitinates the PI3K regulatory subunit p85alpha (PIK3R1) for proteasomal degradation, shaping PI3K/AKT versus ERK signaling; silencing FBXO21 promotes differentiation and chemosensitization.
    supporting_text: >-
      Stabilization of p85α (via FBXO21 knockdown) is associated with reduced
      canonical PI3K signaling (decreased AKT activation) and increased ERK
      activation in the model, with p85α homodimerization proposed as a
      mechanistic intermediate; the paper’s model schematic is captured in
      retrieved figures.
  - statement: FBXO21 binds and ubiquitinates the multidrug-resistance transporter ABCB1/P-glycoprotein for proteasomal degradation, a turnover that Ser291-phosphorylated CD44 suppresses.
    supporting_text: >-
      CD44 physically associates with P-gp at the membrane, and a
      **Ser291-phosphorylated** form of CD44 inhibits FBXO21-directed
      degradation of P-gp (Ser291Ala loses protection).
  reference_review:
    relevance: HIGH
    correctness: UNVERIFIED
    review_notes: >-
      Falcon (Edison Scientific) deep-research synthesis. Cross-checked against
      the UniProt FUNCTION block and the EID1 primary paper (PMID:26085330)
      already in this review; used as leads for substrates/mechanisms not yet in
      UniProt (ASK1 K29-linked antiviral ubiquitination per Yu 2016, p85alpha/AML
      per Dobish 2023 Leukemia, ABCB1/P-gp per Ravindranath 2015). Cites
      author-year/DOIs rather than PMIDs, so individual primary claims remain
      UNVERIFIED here.
- id: Reactome:R-HSA-8952618
  title: AcM-UBE2M transfers NEDD8 to CRL1 E3 ubiquitin ligase complex
  findings: []
- id: Reactome:R-HSA-8952620
  title: NEDD8:AcM-UBE2M binds CRL1 E3 ubiquitin ligase complex
  findings: []
- id: Reactome:R-HSA-8955241
  title: CAND1 binds cytosolic CRL E3 ubiquitin ligases
  findings: []
- id: Reactome:R-HSA-8955289
  title: COMMDs displace CAND1 from cytosolic CRL E3 ubiquitin ligase complexes
  findings: []
- id: Reactome:R-HSA-8956040
  title: COP9 signalosome deneddylates cytosolic CRL E3 ubiquitin ligase complexes
  findings: []
- id: Reactome:R-HSA-8956200
  title: MyrG-DCUN1D3 binds CRL1 E3 ubiquitin ligase complex
  findings: []
- id: Reactome:R-HSA-983140
  title: Transfer of Ub from E2 to substrate and release of E2
  findings: []
- id: Reactome:R-HSA-983147
  title: Release of E3 from polyubiquitinated substrate
  findings: []
- id: Reactome:R-HSA-983156
  title: Polyubiquitination of substrate
  findings: []
- id: Reactome:R-HSA-983157
  title: Interaction of E3 with substrate and E2-Ub complex
  findings: []
core_functions:
- description: Substrate-recognition subunit (F-box adaptor) of an SCF (SKP1-CUL1-F-box) E3 ubiquitin ligase complex that binds SKP1/CUL1 via its F-box domain and recruits the substrate EID1, directing SCF(FBXO21)-mediated ubiquitylation and proteasomal degradation of EID1.
  molecular_function:
    id: GO:1990756
    label: ubiquitin-like ligase-substrate adaptor activity
  supported_by:
  - reference_id: PMID:26085330
    supporting_text: An in vitro ubiquitylation assay showed that EID1 is a direct substrate of SCF(FBXO)(21).
  directly_involved_in:
  - id: GO:0031146
    label: SCF-dependent proteasomal ubiquitin-dependent protein catabolic process
  in_complex:
    id: GO:0019005
    label: SCF ubiquitin ligase complex
- description: As an SCF(FBXO21) substrate receptor, catalyzes non-proteolytic Lys29-linked ubiquitination of the kinase ASK1 (MAP3K5) that promotes ASK1 activation, driving ASK1-JNK/p38 and IRF3-dependent antiviral innate immune signaling and type I interferon induction; this signaling-activating ubiquitination is distinct from the canonical degradative SCF role.
  molecular_function:
    id: GO:1990756
    label: ubiquitin-like ligase-substrate adaptor activity
  supported_by:
  - reference_id: file:human/FBXO21/FBXO21-deep-research-falcon.md
    supporting_text: >-
      SCF^FBXO21 mediates **Lys29-linked ubiquitination** of ASK1, demonstrated
      using ubiquitin mutants (K29-only ubiquitin), and this modification is
      **non-proteolytic** (ASK1 degradation not affected).
  directly_involved_in:
  - id: GO:0045087
    label: innate immune response
  in_complex:
    id: GO:0019005
    label: SCF ubiquitin ligase complex
- description: As an SCF(FBXO21) substrate receptor, targets the PI3K regulatory subunit p85alpha (PIK3R1) for ubiquitin-dependent proteasomal degradation, thereby tuning PI3K/AKT versus ERK signaling; in acute myeloid leukemia this activity supports cell survival and blocks differentiation.
  molecular_function:
    id: GO:1990756
    label: ubiquitin-like ligase-substrate adaptor activity
  supported_by:
  - reference_id: file:human/FBXO21/FBXO21-deep-research-falcon.md
    supporting_text: >-
      Dobish et al. used proteomics (TMT MS; K-ε-GG ubiquitin-remnant IP/MS) to
      identify candidate FBXO21-dependent substrates and nominated **p85α** as a
      ubiquitination target. They then provided biochemical evidence of p85α
      ubiquitination dependent on FBXO21 and its F-box domain (ΔF-box mutant
      inactive), including **in vitro ubiquitination** with immunopurified
      FBXO21.
  directly_involved_in:
  - id: GO:0031146
    label: SCF-dependent proteasomal ubiquitin-dependent protein catabolic process
  in_complex:
    id: GO:0019005
    label: SCF ubiquitin ligase complex
proposed_new_terms: []
suggested_questions:
- question: Beyond EID1, what is the full substrate repertoire of SCF(FBXO21), and which substrate-recognition surface (central vs C-terminal region) engages each substrate?
- question: Is the reported antiviral/innate-immune signalling role of FBXO21 dependent on its SCF ligase activity and substrate ubiquitylation, or does it act through a ligase-independent mechanism?
- question: What governs the subcellular partitioning of SCF(FBXO21) activity between cytoplasm and nucleus, given that EID1 accumulates in both compartments upon FBXO21 loss?
suggested_experiments:
- description: Perform DiPIUS or quantitative ubiquitinome/proteome profiling in FBXO21-knockout versus wild-type cells (with and without proteasome inhibition) to define the endogenous SCF(FBXO21) substrate repertoire.
- description: Reconstitute SCF(FBXO21)-mediated ubiquitylation of EID1 in vitro with purified SKP1, CUL1, RBX1, an E2, and an F-box-domain mutant of FBXO21 to confirm that adaptor (SKP1-binding) function is required and to map EID1 ubiquitylation sites and chain linkage.
- description: Test whether FBXO21's reported antiviral signalling function requires SCF assembly by comparing wild-type FBXO21 with an F-box-deletion (SKP1-binding-defective) mutant in interferon-pathway reporter and viral-challenge assays.

FBXO21

Gene Description

Existing Annotations Review

Core Functions

Substrate-recognition subunit (F-box adaptor) of an SCF (SKP1-CUL1-F-box) E3 ubiquitin ligase complex that binds SKP1/CUL1 via its F-box domain and recruits the substrate EID1, directing SCF(FBXO21)-mediated ubiquitylation and proteasomal degradation of EID1.

As an SCF(FBXO21) substrate receptor, targets the PI3K regulatory subunit p85alpha (PIK3R1) for ubiquitin-dependent proteasomal degradation, thereby tuning PI3K/AKT versus ERK signaling; in acute myeloid leukemia this activity supports cell survival and blocks differentiation.

References

Suggested Questions for Experts

Suggested Experiments

Deep Research

Falcon

Research Report: FBXO21 (UniProt O94952) — Functional Annotation (Homo sapiens)

0) Target verification (gene/protein identity)

1) Key concepts and current understanding

1.1 FBXO21 as an SCF E3 ligase substrate receptor

1.2 Two mechanistic “modes” of FBXO21-driven ubiquitination

2) Experimentally supported molecular functions, substrates, and pathways

2.1 EID1 is a direct proteolytic substrate of SCF^FBXO21 (2015)

2.2 ASK1 is a non-proteolytic substrate: Lys29-linked ubiquitination drives antiviral innate signaling (2016)

2.3 p85α (PIK3R1) is a proteolytic FBXO21 substrate in AML with PI3K/ERK pathway consequences (2023)

2.4 ABCB1/P-gp is a proteolytic substrate; CD44 phosphorylation protects P-gp from FBXO21 (2015)

3) Recent developments (2023–2024 prioritized)

3.1 2023: FBXO21–p85α axis in AML (mechanism and quantitative data)

3.2 2024: Small-molecule disruption of FBXO21:p85α recognition (preprint)

3.3 2024: Proteome-scale degron/effector discovery corroborates FBXO21 as a degron-binding CRL adaptor

4) Current applications and real-world implementations

4.1 Cancer biology and therapeutic concepts

4.2 Innate immunity modulation

5) Expert synthesis and evidence-strength assessment

5.1 Strongest supported “primary function” of FBXO21

5.2 Subcellular localization: what is known vs unknown

5.3 Ubiquitin linkage specificity: substrate-dependent

6) Supplemental disease association signals (database-level)

Summary table of validated functions

Key references (publication dates and URLs)

Figures (visual evidence retrieved)

Artifacts

Citations

📚 Additional Documentation

Pn Notes

FBXO21 PN Consistency Notes

Source Files Checked

Deep Research Files

AIGR Review Snapshot

PN Consistency Summary

Full Consistency Review

PN Dossier Context

PN row 1: Ubiquitin Proteasome System | E3 ubiquitin and UBL ligases | Cul1 substrate receptor | F-box | other

Projected GO annotations (1)

Note

📄 View Raw YAML