FBXO43

FBXO43 (Emi2/XErp1) is a direct inhibitor of the anaphase-promoting complex/cyclosome (APC/C) that enforces meiotic metaphase arrest by blocking the destruction of APC/C substrates such as Cyclin B1 (PMID:15753281, PMID:34052850). Its zinc-binding region (ZBR) domain achieves inhibition by two distinct mechanisms: displacing the coactivator Cdc20 from the APC/C core and blocking UBE2C-driven ubiquitin chain elongation by the ANAPC2–ANAPC11 catalytic module, while an adjacent post-ZBR region docks onto the cullin subunit ANAPC2 (PMID:25161877). Consistent with a direct-inhibitor rather than SCF role, FBXO43 contacts APC/C subunits ANAPC2, ANAPC8, and ANAPC10 but not SKP1 (PMID:34595750). This APC/C-restraining activity maintains meiotic arrest, and loss-of-function variants that destabilize FBXO43 and lower Cyclin B1 cause meiotic arrest and parthenogenetic activation, linking FBXO43 to human meiotic/reproductive failure (PMID:34595750, PMID:34052850). Beyond classical meiosis, transient FBXO43-mediated inhibition of APC/C-Cdh1 is required to activate Plk1 and drive centriole amplification in multiciliated cell progenitors (PMID:35363516). In somatic and cancer contexts FBXO43 instead stabilizes pro-proliferative factors—Cyclin D1, AKT1-phosphorylated SKP2, and PCNA—to promote cell cycle progression, proliferation, and invasion (PMID:36937431, PMID:38604312, PMID:34645483).

1. 2005 High

   Established FBXO43/Emi2 as the stable APC/C inhibitor responsible for cytostatic-factor metaphase II arrest and showed how fertilization releases the brake, answering what holds the egg arrested and how it is reversed.

   Evidence Antibody depletion, exogenous protein add-back, and Plk1-inhibitor experiments in Xenopus egg extracts

   PMID:15753281

   Open questions at the time

   • Molecular mechanism of APC/C inhibition not yet resolved
   • Did not define the domain or residues required for inhibition
2. 2014 High

   Resolved the dual molecular mechanism of inhibition—Cdc20 displacement plus blockade of UBE2C-mediated chain elongation by the ZBR, with cullin docking by the post-ZBR region—explaining how a single domain suppresses APC/C catalysis.

   Evidence In vitro ubiquitylation reconstitution, ZBR mutagenesis and transplantation into the Emi1/FBXO5 ZBR, structural characterization, and co-IP in HEK293T

   PMID:25161877

   Open questions at the time

   • No full structure of the FBXO43–APC/C complex
   • How calcium/Plk1 signaling relieves this inhibition mechanistically not addressed
3. 2021 Medium

   Confirmed in mammals that FBXO43 acts as a direct APC/C inhibitor rather than an SCF substrate-receptor by mapping subunit contacts and tying loss-of-function to human meiotic arrest, extending the Xenopus model to human male and female meiosis.

   Evidence Co-IP from mouse testis showing ANAPC2/ANAPC8/ANAPC10 but not SKP1 binding; whole-exome sequencing of patients; oocyte rescue and HEK293T variant Western blots establishing Cyclin B1 as the effector

   PMID:34052850 PMID:34595750

   Open questions at the time

   • Single-lab Co-IP without reciprocal in vivo validation
   • Stoichiometry and dynamics of FBXO43–APC/C engagement during meiotic progression not defined
4. 2022 High

   Revealed a non-meiotic role in which transient FBXO43 inhibition of APC/C-Cdh1 activates Plk1 to drive centriole amplification in multiciliated cell progenitors, showing the emi2–APC/C axis is repurposed after cell-cycle exit.

   Evidence Genetic epistasis (loss-of-function and rescue), phosphorylation assays, and centriole/basal-body phenotypic readouts in the Xenopus MCC differentiation system

   PMID:35363516

   Open questions at the time

   • Direct biochemical demonstration of FBXO43-Cdh1 inhibition not shown in this system
   • Mechanism timing the switch from inhibition to down-regulation unclear
5. 2021 Medium

   Began characterizing somatic/oncogenic FBXO43 by identifying PCNA as an interactor whose abundance is required for FBXO43-driven proliferation and invasion, opening a cancer-context function distinct from APC/C inhibition.

   Evidence Mass spectrometry, co-IP, PCNA overexpression rescue, and xenograft model in breast cancer cells

   PMID:34645483

   Open questions at the time

   • Whether FBXO43 directly ubiquitinates/stabilizes PCNA not established
   • Single-lab finding without independent replication
6. 2023 Medium

   Extended the oncogenic role by showing FBXO43 promotes polyubiquitination and stabilization of Cyclin D1 and influences UBE2C/p53 turnover, with m6A regulation (METTL3/IGF2BP2) controlling FBXO43 mRNA, framing how FBXO43 is regulated and how it drives proliferation in HCC.

   Evidence Co-IP, in vivo ubiquitination assays, CCND1/UBE2C knockdown rescue, and m6A writer/reader functional assays in HCC cells

   PMID:36765911 PMID:36937431

   Open questions at the time

   • Apparent stabilizing ubiquitination contrasts with the APC/C-inhibitory role and is not mechanistically reconciled
   • Direct enzymatic relationship between FBXO43 and these substrates not reconstituted
7. 2024 Medium

   Identified AKT1-phosphorylated SKP2 as an FBXO43 partner whose auto-ubiquitylation FBXO43 suppresses, linking phospho-dependent substrate recognition to cell-cycle progression across multiple cancer types.

   Evidence Co-IP, AKT1 phosphorylation experiments, ubiquitylation assays, and loss-of-function cell-cycle readouts in NSCLC, HCC, and sarcoma lines

   PMID:38604312

   Open questions at the time

   • Single-lab study
   • How FBXO43 reduces SKP2 auto-ubiquitylation mechanistically not defined

• How FBXO43 reconciles its APC/C-inhibitory function in meiosis with substrate-stabilizing roles in somatic cancer cells, and what governs its context-specific partner selection, remains unresolved.
• No unified biochemical model linking APC/C inhibition and Cyclin D1/SKP2/PCNA stabilization
• No structure of FBXO43 bound to its somatic partners
• Tissue-specific regulation of which activity dominates is unknown