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).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005634 nucleus | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic assignment of nuclear localization, transferred across the EMI1/EMI2 family. 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. Supporting Evidence: file:human/FBXO43/FBXO43-uniprot.txt Acts by inhibiting the anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase |
| GO:0007088 regulation of mitotic nuclear division | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Phylogenetic assignment of regulation of mitotic nuclear division, transferred from the EMI1/EMI2 family. 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. Supporting Evidence: file:human/FBXO43/FBXO43-uniprot.txt Required to establish and maintain the arrest of oocytes at the second meiotic metaphase until fertilization |
| GO:0045835 negative regulation of meiotic nuclear division | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic assignment of negative regulation of meiotic nuclear division, consistent with FBXO43's CSF-mediated MII arrest via APC/C inhibition. Core process. Reason: Core biological process; FBXO43 inhibits APC/C to arrest oocytes at meiotic metaphase II, directly supported by functional and disease studies. Supporting Evidence: file:human/FBXO43/FBXO43-uniprot.txt Required to establish and maintain the arrest of oocytes at the second meiotic metaphase until fertilization |
| GO:0007088 regulation of mitotic nuclear division | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: InterPro-based electronic assignment of regulation of mitotic nuclear division. 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. Supporting Evidence: file:human/FBXO43/FBXO43-uniprot.txt Required to establish and maintain the arrest of oocytes at the second meiotic metaphase until fertilization |
| GO:0045835 negative regulation of meiotic nuclear division | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based electronic assignment of negative regulation of meiotic nuclear division. Core process. Reason: Core biological process; redundant with the IBA and experimental support for meiotic arrest via APC/C inhibition. Supporting Evidence: file:human/FBXO43/FBXO43-uniprot.txt Required to establish and maintain the arrest of oocytes at the second meiotic metaphase until fertilization |
| GO:0008270 zinc ion binding | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: InterPro/keyword-based assignment of zinc ion binding via the ZBR-type zinc-finger domain (residues 636-684) that coordinates two zinc ions. 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. Supporting Evidence: file:human/FBXO43/FBXO43-uniprot.txt ZN_FING 636..684 |
| GO:0010948 negative regulation of cell cycle process | IEA GO_REF:0000117 | ACCEPT | 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. Reason: Correct (generic parent of the specific meiotic-arrest process); consistent with the IMP-supported negative regulation of the meiotic cell cycle. Supporting Evidence: file:human/FBXO43/FBXO43-uniprot.txt Acts by inhibiting the anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase |
| GO:0045835 negative regulation of meiotic nuclear division | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based electronic assignment of negative regulation of meiotic nuclear division. Core process. (Second InterPro record.) Reason: Core biological process; redundant with the IBA and experimental support for meiotic arrest via APC/C inhibition. Supporting Evidence: file:human/FBXO43/FBXO43-uniprot.txt Required to establish and maintain the arrest of oocytes at the second meiotic metaphase until fertilization |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | KEEP AS NON CORE | Summary: Binary interactome reference map interaction (e.g. PPP2R1A/SKP1). Bare protein binding is uninformative. Reason: High-throughput interactome; bare protein binding is uninformative per curation guidelines. Supporting Evidence: file:human/FBXO43/FBXO43-uniprot.txt Q4G163; P30153: PPP2R1A; NbExp=3; IntAct=EBI-12053217, EBI-302388; |
| GO:1990090 cellular response to nerve growth factor stimulus | IEA GO_REF:0000107 | REMOVE | 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. 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. Supporting Evidence: file:human/FBXO43/FBXO43-uniprot.txt Expressed in the testis |
| GO:0016567 protein ubiquitination | IEA GO_REF:0000041 | MARK AS OVER ANNOTATED | Summary: UniPathway-derived generic protein ubiquitination annotation, propagated from the F-box/UPA00143 mapping. 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. Supporting Evidence: file:human/FBXO43/FBXO43-uniprot.txt According to PubMed:34595750 interaction with SKP1 does not occur |
| GO:0019005 SCF ubiquitin ligase complex | NAS PMID:34445249 The SCF Complex Is Essential to Maintain Genome and Chromoso... | MARK AS OVER ANNOTATED | 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. 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. Supporting Evidence: file:human/FBXO43/FBXO43-uniprot.txt According to PubMed:34595750 interaction with SKP1 does not occur |
| GO:0031146 SCF-dependent proteasomal ubiquitin-dependent protein catabolic process | NAS PMID:34445249 The SCF Complex Is Essential to Maintain Genome and Chromoso... | MARK AS OVER ANNOTATED | 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. 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. Supporting Evidence: file:human/FBXO43/FBXO43-uniprot.txt Ubiquitinated in response to calcium, which promotes proteasomal degradation |
| GO:0005515 protein binding | IPI PMID:34595750 A homozygous loss-of-function mutation in FBXO43 causes huma... | KEEP AS NON CORE | 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. 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. Supporting Evidence: file:human/FBXO43/FBXO43-uniprot.txt Interacts with ANAPC2; the interaction is direct, ANAPC4, CDC16, CDC23; the interaction is direct, ANAPC10; the interaction is direct and CDC26, during spermatogenesis file:human/FBXO43/FBXO43-deep-research-falcon.md 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). |
| GO:0010948 negative regulation of cell cycle process | IMP PMID:34052850 FBXO43 variants in patients with female infertility characte... | ACCEPT | 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. Reason: Core biological process with direct mutant-phenotype support; FBXO43 restrains the meiotic cell cycle (MII arrest) via APC/C inhibition and CCNB1 stabilization. Supporting Evidence: PMID:34052850 FBXO43, an inhibitor of the anaphase-promoting complex/cyclosome, mediates Metaphase II arrest as a component of the cytostatic factor in oocytes |
| GO:1990948 ubiquitin ligase inhibitor activity | IMP PMID:34595750 A homozygous loss-of-function mutation in FBXO43 causes huma... | NEW | 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. 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. Supporting Evidence: PMID:34595750 FBXO43 is a direct inhibitor of the anaphase-promoting complex/cyclosome (APC/C) E3 ligase file:human/FBXO43/FBXO43-deep-research-falcon.md 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. |
Loading supporting content…
Download this section (compressed HTML)Q: 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?
Q: 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?
Q: 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?
Experiment: 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.
Experiment: 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.
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)