id: Q4G163 gene_symbol: FBXO43 product_type: PROTEIN status: COMPLETE taxon: id: NCBITaxon:9606 label: Homo sapiens description: >- FBXO43 (Endogenous meiotic inhibitor 2, EMI2/ERP1) is a meiotic cell-cycle regulator and the closest paralog of FBXO5/EMI1. Despite containing an F-box, its established function is as a direct inhibitor of the anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase rather than as a canonical SCF substrate receptor. FBXO43 is the key component of cytostatic factor (CSF) activity that establishes and maintains the arrest of vertebrate oocytes at metaphase of the second meiotic division (MII) until fertilization, acting by binding and inhibiting the APC/C to stabilize cyclin B1 (CCNB1) and sustain CDK1 activity. Mechanistically, EMI2 docks onto the APC/C through a short C-terminal RL tail, and its C-terminal zinc-binding region (ZBR) both impairs association of the coactivator CDC20 with the APC/C core and inhibits APC/C catalytic activity (including UBE2C-dependent ubiquitin chain formation), so that inhibition reflects direct perturbation of APC/C rather than simple pseudosubstrate competition. Upon fertilization, a calcium signal triggers phosphorylation of FBXO43 (at residues including Ser76, Thr234 and Ser334 by kinases such as CaMKII and PLK1), generating a beta-TrCP-recognized phosphodegron that drives its ubiquitination and proteasomal degradation, relieving APC/C inhibition and allowing exit from MII and the metaphase-to-anaphase transition. FBXO43 binds directly to multiple APC/C subunits (ANAPC2, ANAPC4, CDC16, CDC23, ANAPC10, CDC26). It is expressed predominantly in the gonads (testis and oocyte), and loss-of-function variants cause human infertility: oocyte/embryo maturation arrest in females (OZEMA12) and spermatogenic failure/non-obstructive azoospermia with meiotic arrest in males (SPGF64). existing_annotations: - term: id: GO:0005634 label: nucleus evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: is_active_in review: summary: Phylogenetic assignment of nuclear localization, transferred across the EMI1/EMI2 family. action: KEEP_AS_NON_CORE reason: Plausible by family inference, but FBXO43 acts on the APC/C during meiotic divisions where the nuclear envelope has broken down; the localization is not experimentally established for human FBXO43 and is non-core. supported_by: - reference_id: file:human/FBXO43/FBXO43-uniprot.txt supporting_text: Acts by inhibiting the anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase - term: id: GO:0007088 label: regulation of mitotic nuclear division evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: involved_in review: summary: Phylogenetic assignment of regulation of mitotic nuclear division, transferred from the EMI1/EMI2 family. action: MARK_AS_OVER_ANNOTATED reason: FBXO43/EMI2 acts specifically in the meiotic cell cycle (oocyte MII arrest, spermatogenesis); a mitotic-division role is a family-level over-propagation from the mitotic paralog EMI1/FBXO5 and is not supported for FBXO43. supported_by: - reference_id: file:human/FBXO43/FBXO43-uniprot.txt supporting_text: Required to establish and maintain the arrest of oocytes at the second meiotic metaphase until fertilization - term: id: GO:0045835 label: negative regulation of meiotic nuclear division evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: involved_in review: summary: Phylogenetic assignment of negative regulation of meiotic nuclear division, consistent with FBXO43's CSF-mediated MII arrest via APC/C inhibition. Core process. action: ACCEPT reason: Core biological process; FBXO43 inhibits APC/C to arrest oocytes at meiotic metaphase II, directly supported by functional and disease studies. supported_by: - reference_id: file:human/FBXO43/FBXO43-uniprot.txt supporting_text: Required to establish and maintain the arrest of oocytes at the second meiotic metaphase until fertilization - term: id: GO:0007088 label: regulation of mitotic nuclear division evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: involved_in review: summary: InterPro-based electronic assignment of regulation of mitotic nuclear division. action: MARK_AS_OVER_ANNOTATED reason: FBXO43 is a meiotic regulator; the mitotic-division annotation is a family-level over-propagation from the FBX5_43 InterPro signature shared with mitotic EMI1. supported_by: - reference_id: file:human/FBXO43/FBXO43-uniprot.txt supporting_text: Required to establish and maintain the arrest of oocytes at the second meiotic metaphase until fertilization - term: id: GO:0045835 label: negative regulation of meiotic nuclear division evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: involved_in review: summary: InterPro-based electronic assignment of negative regulation of meiotic nuclear division. Core process. action: ACCEPT reason: Core biological process; redundant with the IBA and experimental support for meiotic arrest via APC/C inhibition. supported_by: - reference_id: file:human/FBXO43/FBXO43-uniprot.txt supporting_text: Required to establish and maintain the arrest of oocytes at the second meiotic metaphase until fertilization - term: id: GO:0008270 label: zinc ion binding evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: enables review: summary: InterPro/keyword-based assignment of zinc ion binding via the ZBR-type zinc-finger domain (residues 636-684) that coordinates two zinc ions. action: KEEP_AS_NON_CORE reason: Accurate structural feature of the ZBR domain (important for APC/C inhibition), but a structural attribute subsidiary to the core APC/C-inhibitor function rather than a standalone core function. supported_by: - reference_id: file:human/FBXO43/FBXO43-uniprot.txt supporting_text: ZN_FING 636..684 - term: id: GO:0010948 label: negative regulation of cell cycle process evidence_type: IEA original_reference_id: GO_REF:0000117 qualifier: involved_in review: summary: ARBA machine-learning assignment of negative regulation of a cell-cycle process, consistent with FBXO43's APC/C inhibition arresting the meiotic cell cycle. action: ACCEPT reason: Correct (generic parent of the specific meiotic-arrest process); consistent with the IMP-supported negative regulation of the meiotic cell cycle. supported_by: - reference_id: file:human/FBXO43/FBXO43-uniprot.txt supporting_text: Acts by inhibiting the anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase - term: id: GO:0045835 label: negative regulation of meiotic nuclear division evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: involved_in review: summary: InterPro-based electronic assignment of negative regulation of meiotic nuclear division. Core process. (Second InterPro record.) action: ACCEPT reason: Core biological process; redundant with the IBA and experimental support for meiotic arrest via APC/C inhibition. supported_by: - reference_id: file:human/FBXO43/FBXO43-uniprot.txt supporting_text: Required to establish and maintain the arrest of oocytes at the second meiotic metaphase until fertilization - term: id: GO:0005515 label: protein binding evidence_type: IPI original_reference_id: PMID:32296183 qualifier: enables review: summary: Binary interactome reference map interaction (e.g. PPP2R1A/SKP1). Bare protein binding is uninformative. action: KEEP_AS_NON_CORE reason: High-throughput interactome; bare protein binding is uninformative per curation guidelines. supported_by: - reference_id: file:human/FBXO43/FBXO43-uniprot.txt supporting_text: 'Q4G163; P30153: PPP2R1A; NbExp=3; IntAct=EBI-12053217, EBI-302388;' - term: id: GO:1990090 label: cellular response to nerve growth factor stimulus evidence_type: IEA original_reference_id: GO_REF:0000107 qualifier: involved_in review: summary: Ortholog-based electronic transfer of a cellular response to nerve growth factor. There is no biological support for FBXO43 acting in NGF signaling; FBXO43 is a gonad-restricted meiotic APC/C inhibitor. action: REMOVE reason: Biologically implausible electronic over-propagation. FBXO43 is expressed in testis/oocyte and functions in meiotic APC/C inhibition; an NGF-response role contradicts its tissue specificity and documented function, and rests only on Ensembl Compara ortholog transfer. supported_by: - reference_id: file:human/FBXO43/FBXO43-uniprot.txt supporting_text: Expressed in the testis - term: id: GO:0016567 label: protein ubiquitination evidence_type: IEA original_reference_id: GO_REF:0000041 qualifier: involved_in review: summary: UniPathway-derived generic protein ubiquitination annotation, propagated from the F-box/UPA00143 mapping. action: MARK_AS_OVER_ANNOTATED reason: FBXO43's established role is inhibition of the APC/C ubiquitin ligase, and it is itself a phosphodegron substrate; UniProt notes that SKP1 interaction does not occur (PMID:34595750). A generic productive-ubiquitination annotation overstates a catalytic role; the UniProt FUNCTION line about promoting ubiquitination is hedged ("probably") and not experimentally established. supported_by: - reference_id: file:human/FBXO43/FBXO43-uniprot.txt supporting_text: According to PubMed:34595750 interaction with SKP1 does not occur - term: id: GO:0019005 label: SCF ubiquitin ligase complex evidence_type: NAS original_reference_id: PMID:34445249 qualifier: part_of review: summary: ComplexPortal author-statement assigning FBXO43 to an SCF ubiquitin ligase complex based on its F-box. However, UniProt records that FBXO43 does not interact with SKP1 (PMID:34595750), and FBXO43 instead binds directly to APC/C subunits. action: MARK_AS_OVER_ANNOTATED reason: The SCF assignment is a default F-box-family inference. The experimental record indicates FBXO43 does not bind SKP1 and acts as an APC/C inhibitor, not an SCF substrate receptor. Retaining it as part_of SCF over-states a complex membership not supported for FBXO43. supported_by: - reference_id: file:human/FBXO43/FBXO43-uniprot.txt supporting_text: According to PubMed:34595750 interaction with SKP1 does not occur - 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: ComplexPortal author-statement assigning FBXO43 to SCF-dependent proteasomal protein catabolism. FBXO43 is itself degraded by the ubiquitin-proteasome system after calcium-triggered phosphorylation; no productive SCF substrate-receptor role is established and SKP1 interaction is reported not to occur. action: MARK_AS_OVER_ANNOTATED reason: Over-extension from the F-box-family default. FBXO43's documented ubiquitin/proteasome link is as a substrate, and its core activity is APC/C inhibition; an SCF-dependent catabolic involved_in annotation is not supported. supported_by: - reference_id: file:human/FBXO43/FBXO43-uniprot.txt supporting_text: Ubiquitinated in response to calcium, which promotes proteasomal degradation - term: id: GO:0005515 label: protein binding evidence_type: IPI original_reference_id: PMID:34595750 qualifier: enables review: summary: Interactions demonstrated with APC/C subunits (ANAPC2, ANAPC4, CDC16, CDC23, ANAPC10, CDC26) during spermatogenesis. These direct contacts, together with the C-terminal RL-tail docking module and ZBR, underlie EMI2's inhibition of the APC/C. Bare protein binding is uninformative. action: KEEP_AS_NON_CORE reason: Records the functionally central direct interactions with APC/C subunits, but bare protein binding is uninformative; the inhibitory APC/C relationship is captured by the meiotic-division annotations. EMI2 docks via an RL tail and uses its ZBR to disrupt CDC20 association and UBE2C-dependent ubiquitylation. additional_reference_ids: - file:human/FBXO43/FBXO43-deep-research-falcon.md supported_by: - reference_id: file:human/FBXO43/FBXO43-uniprot.txt supporting_text: Interacts with ANAPC2; the interaction is direct, ANAPC4, CDC16, CDC23; the interaction is direct, ANAPC10; the interaction is direct and CDC26, during spermatogenesis - reference_id: file:human/FBXO43/FBXO43-deep-research-falcon.md supporting_text: 'EMI2 requires a short **C-terminal RL tail** for physical docking to the APC/C; this docking is required for inhibitory activity, facilitating the functional engagement of other inhibitory regions (including ZBR).' - term: id: GO:0010948 label: negative regulation of cell cycle process evidence_type: IMP original_reference_id: PMID:34052850 qualifier: involved_in review: summary: Mutant-phenotype evidence (FBXO43 infertility variants; failure to upregulate CCNB1; failure to rescue mouse oocyte phenotype) that FBXO43 negatively regulates the meiotic cell-cycle by inhibiting APC/C. Core process. action: ACCEPT reason: Core biological process with direct mutant-phenotype support; FBXO43 restrains the meiotic cell cycle (MII arrest) via APC/C inhibition and CCNB1 stabilization. supported_by: - reference_id: PMID:34052850 supporting_text: FBXO43, an inhibitor of the anaphase-promoting complex/cyclosome, mediates Metaphase II arrest as a component of the cytostatic factor in oocytes - term: id: GO:1990948 label: ubiquitin ligase inhibitor activity evidence_type: IMP original_reference_id: PMID:34595750 qualifier: enables review: summary: Proposed molecular function annotation. FBXO43 acts as a direct inhibitor of the APC/C ubiquitin ligase; loss-of-function abolishes this inhibition, causing meiotic arrest. This captures the core molecular function not represented in the seeded GOA. action: NEW reason: FBXO43's core molecular function is inhibition of the APC/C E3 ligase (a ubiquitin ligase inhibitor activity, GO:1990948, consistent with its EMI1/FBXO5 paralog), directly supported by the disease loss-of-function study; this term is absent from the existing GOA and is added here. Mechanistically the ZBR impairs CDC20 coactivator association and inhibits UBE2C-dependent ubiquitin chain formation, so inhibition is a direct perturbation of APC/C activity. additional_reference_ids: - file:human/FBXO43/FBXO43-deep-research-falcon.md supported_by: - reference_id: PMID:34595750 supporting_text: FBXO43 is a direct inhibitor of the anaphase-promoting complex/cyclosome (APC/C) E3 ligase - reference_id: file:human/FBXO43/FBXO43-deep-research-falcon.md supporting_text: 'the ZBR can (i) **impair association of the APC/C coactivator CDC20 with the APC/C core**, and (ii) **inhibit APC/C catalytic activity**, including UBE2C-dependent ubiquitin chain formation/elongation steps—implying EMI2 is not merely a stoichiometric blocker but directly perturbs APC/C function.' references: - id: GO_REF:0000002 title: Gene Ontology annotation through association of InterPro records with GO terms findings: [] - id: GO_REF:0000033 title: Annotation inferences using phylogenetic trees findings: [] - id: GO_REF:0000041 title: Gene Ontology annotation based on UniPathway vocabulary mapping findings: [] - id: GO_REF:0000107 title: Automatic transfer of experimentally verified manual GO annotation data to orthologs using Ensembl Compara findings: [] - id: GO_REF:0000117 title: Electronic Gene Ontology annotations created by ARBA machine learning models findings: [] - id: PMID:32296183 title: A reference map of the human binary protein interactome. findings: [] reference_review: relevance: LOW correctness: VERIFIED review_notes: High-throughput binary interactome; source of a bare protein binding annotation. - id: PMID:34052850 title: FBXO43 variants in patients with female infertility characterized by early embryonic arrest. findings: - statement: Homozygous FBXO43 variants cause female infertility with early embryonic arrest; FBXO43 inhibits the APC/C and mediates metaphase II arrest as a cytostatic-factor component, with the truncating variant failing to upregulate CCNB1 and failing to rescue the mouse oocyte phenotype. reference_section_type: ABSTRACT reference_review: relevance: HIGH correctness: VERIFIED review_notes: Establishes the OZEMA12 disease link and the meiotic APC/C-inhibitor (CSF) function; abstract-only in cache but explicit on function. - id: PMID:34445249 title: The SCF Complex Is Essential to Maintain Genome and Chromosome Stability. findings: - statement: Review of the 69 SCF E3 ubiquitin ligase complexes in which variable F-box proteins determine substrate specificity. reference_section_type: ABSTRACT reference_review: relevance: LOW correctness: VERIFIED review_notes: Family-level review used as NAS basis for SCF-complex and SCF-catabolism annotations; does not establish a productive SCF role for FBXO43, which is reported not to bind SKP1. - id: PMID:34595750 title: A homozygous loss-of-function mutation in FBXO43 causes human non-obstructive azoospermia. findings: - statement: FBXO43 is a direct inhibitor of the APC/C E3 ligase; a homozygous nonsense mutation causes non-obstructive azoospermia with meiotic arrest at early diplotene; FBXO43 interacts directly with multiple APC/C subunits and does not interact with SKP1. reference_section_type: ABSTRACT reference_review: relevance: HIGH correctness: VERIFIED review_notes: Establishes the SPGF64 disease link, direct APC/C-subunit interactions, the absence of SKP1 binding, and the direct APC/C-inhibitor function; abstract-only in cache. - id: file:human/FBXO43/FBXO43-deep-research-falcon.md title: Falcon deep research report for human FBXO43 findings: - statement: FBXO43/EMI2 is the meiotic APC/C inhibitor underlying cytostatic-factor (CSF) metaphase-II arrest in vertebrate oocytes, suppressing APC/C-mediated cyclin B destruction to sustain MPF/CDK1 until fertilization. supporting_text: FBXO43/EMI2 is best understood functionally as a **meiotic inhibitor of the anaphase-promoting complex/cyclosome (APC/C)**—a multi-subunit E3 ubiquitin ligase controlling anaphase onset and exit from M phase. In vertebrate oocytes, APC/C inhibition is the core biochemical requirement for maintaining **metaphase II arrest** (cytostatic factor/CSF arrest) until fertilization. - statement: The EMI2 C-terminal zinc-binding region (ZBR) inhibits APC/C by both impairing CDC20 coactivator association with the APC/C core and inhibiting UBE2C-dependent ubiquitin chain formation, i.e. it directly perturbs APC/C catalytic activity rather than acting only as a stoichiometric/pseudosubstrate blocker. supporting_text: 'the ZBR can (i) **impair association of the APC/C coactivator CDC20 with the APC/C core**, and (ii) **inhibit APC/C catalytic activity**, including UBE2C-dependent ubiquitin chain formation/elongation steps—implying EMI2 is not merely a stoichiometric blocker but directly perturbs APC/C function.' - statement: EMI2 requires a short C-terminal RL tail for physical docking to the APC/C, and this docking is required for its inhibitory activity, enabling engagement of the D-box and ZBR inhibitory modules. supporting_text: 'EMI2 requires a short **C-terminal RL tail** for physical docking to the APC/C; this docking is required for inhibitory activity, facilitating the functional engagement of other inhibitory regions (including ZBR).' - statement: At fertilization, Ca2+/CaMKII and Plx1/PLK1 phosphorylation generate a beta-TrCP-recognized phosphodegron driving EMI2 ubiquitination and degradation, releasing APC/C inhibition for meiotic exit. supporting_text: 'Ca2+-activated signaling (via **CaMKII**) and **Plx1/Plk1** phosphorylation leads to **β-TrCP recognition**, ubiquitination, and degradation of EMI2, thereby releasing APC/C inhibition and allowing meiotic exit.' reference_review: relevance: HIGH correctness: UNVERIFIED review_notes: Falcon deep-research synthesis; anchors FBXO43/EMI2 as the meiotic CSF APC/C inhibitor and adds the RL-tail docking requirement, ZBR-mediated disruption of CDC20 association and UBE2C ubiquitylation, and the beta-TrCP/CaMKII/PLK1 degradation switch. Treated as leads cross-checked against UniProt and the cached disease PMIDs; mechanistic detail rests largely on Xenopus/mouse oocyte systems. core_functions: - description: Direct inhibitor of the anaphase-promoting complex/cyclosome (APC/C) E3 ubiquitin ligase; binds directly to multiple APC/C subunits (ANAPC2, ANAPC4, CDC16, CDC23, ANAPC10, CDC26) to suppress its activity. molecular_function: id: GO:1990948 label: ubiquitin ligase inhibitor activity supported_by: - reference_id: PMID:34595750 supporting_text: FBXO43 is a direct inhibitor of the anaphase-promoting complex/cyclosome (APC/C) E3 ligase directly_involved_in: - id: GO:0045835 label: negative regulation of meiotic nuclear division - description: Cytostatic-factor (CSF) component that establishes and maintains arrest of oocytes at metaphase of the second meiotic division until fertilization, by inhibiting APC/C to stabilize cyclin B1 and sustain CDK1 activity; calcium-triggered degradation at fertilization relieves the arrest. molecular_function: id: GO:1990948 label: ubiquitin ligase inhibitor activity supported_by: - reference_id: PMID:34052850 supporting_text: FBXO43, an inhibitor of the anaphase-promoting complex/cyclosome, mediates Metaphase II arrest as a component of the cytostatic factor in oocytes directly_involved_in: - id: GO:0045835 label: negative regulation of meiotic nuclear division proposed_new_terms: [] suggested_questions: - question: Does human FBXO43/EMI2 ever assemble a functional SCF complex and target any substrate for ubiquitination, given that SKP1 interaction is reported not to occur and its dominant role is direct APC/C inhibition? - question: How do the calcium-triggered phosphorylation events (CaMKII, PLK1) on FBXO43 quantitatively convert it from an APC/C inhibitor into a degradation substrate to time the metaphase-II-to-anaphase transition at fertilization? - question: Does human FBXO43/EMI2 have a non-meiotic, post-mitotic role analogous to the reported Xenopus function in multiciliated-cell progenitors (transient APC/C^CDH1 inhibition enabling PLK1 activation and centriole amplification), and if so in which human tissues? suggested_experiments: - description: Reconstitute APC/C inhibition in vitro with purified human FBXO43 and APC/C(CDC20) and measure cyclin B1 ubiquitination, mapping which APC/C-subunit contacts (ANAPC2/ANAPC10/CDC23) are required for inhibition versus its proposed pseudosubstrate mode. - description: In human or mouse oocytes, perform degron-tagged FBXO43 depletion/add-back with phosphosite mutants (Ser76, Thr234, Ser334) and live imaging of MII arrest and exit to test the requirement of each phosphorylation event for calcium-triggered destruction and anaphase onset.