FBXO28

FBXO28 is an F-box protein that serves as the substrate-recognition subunit of SCF (SKP1/CUL1/F-box) E3 ubiquitin ligase complexes, directing ubiquitination of diverse substrates for proteasomal degradation across processes spanning cell division, oncogenesis, and metabolism (PMID:31678254, PMID:37596321). As an SCF component it directly binds and ubiquitinates multiple targets, including the EMT regulator SNAI2 in cooperation with PKA (PMID:37596321), the chromatin-remodeling factor SMARCC2 in pancreatic cancer (PMID:37348029), and the small GTPase RAB27A via K48-linked chains (PMID:38696123); it also assembles a complex with TRAF6 that promotes K63-linked ubiquitination and activation of ERK5 (PMID:31184423), and supports PFKFB4-dependent ubiquitylation of HIF-1α (PMID:36115843). SCFᶠᴮˣᵒ²⁸ activity additionally drives FBXO28 auto-ubiquitination and self-degradation, a neddylation-dependent autoregulatory loop (PMID:31678254). Beyond its ligase function, FBXO28 interacts with topoisomerase IIα to restrain its decatenation activity and ensure proper mitotic chromosome segregation, with loss of function producing lagging chromosomes, multipolar spindles, and multinucleation (PMID:27754753), and it localizes to chromosomes and acentriolar MTOCs in oocytes where it governs actin-based spindle migration during asymmetric meiotic division (PMID:38960234).

1. 2016 High

   Established the first defined cellular role for FBXO28 by showing it physically restrains topoisomerase IIα decatenation activity to safeguard mitotic fidelity, rather than acting purely as a degradation adaptor.

   Evidence Reciprocal co-IP, co-localization, siRNA knockdown, decatenation assay, and live-cell imaging in human cells

   PMID:27754753

   Open questions at the time

   • Whether topoisomerase IIα regulation requires SCF ligase activity or is a non-degradative interaction is not resolved
   • No structural basis for the FBXO28–TOP2A interface
2. 2018 Medium

   Linked FBXO28 as an SCF substrate-recruiting subunit to pancreatic β-cell survival, extending its relevance to metabolic/endocrine biology.

   Evidence siRNA knockdown, overexpression, and apoptosis/insulin-secretion assays in human islets

   PMID:29587369

   Open questions at the time

   • The degradation substrate mediating the survival effect was not identified
   • Mechanism downstream of FBXO28 in β-cells unknown
3. 2019 High

   Defined FBXO28's autoregulation, showing SCFᶠᴮˣᵒ²⁸ self-ubiquitinates in a neddylation- and F-box-dependent manner, establishing its own turnover control.

   Evidence Ubiquitination assay, cycloheximide chase, MLN4924 inhibition, F-box deletion mutant, siRNA in human cells

   PMID:31678254

   Open questions at the time

   • Physiological signals controlling auto-degradation not defined
   • Lysine residues mediating self-ubiquitination not mapped
4. 2019 Medium

   Showed FBXO28 also participates in non-degradative K63-linked ubiquitination by forming a complex with TRAF6 to activate ERK5 signaling, broadening its ubiquitin chain repertoire.

   Evidence Co-IP, K63-ubiquitination assay, in vitro and in vivo (rat Thy-1 nephritis) knockdown, promoter analysis

   PMID:31184423

   Open questions at the time

   • Whether FBXO28 functions here within canonical SCF is unclear
   • Direct E3 versus scaffolding role with TRAF6 not dissected
5. 2022 Medium

   Implicated FBXO28 in hypoxia signaling, showing PFKFB4 directs FBXO28 ligase activity toward HIF-1α ubiquitylation and degradation in glioblastoma.

   Evidence Mass spectrometry of immunoprecipitated PFKFB4, ubiquitylation assay, siRNA, Western blot

   PMID:36115843

   Open questions at the time

   • Direct binding of FBXO28 to HIF-1α not demonstrated
   • How PFKFB4 redirects substrate selection unknown
6. 2023 Medium

   Identified SNAI2 and SMARCC2 as direct degradation substrates, framing FBXO28 as a tumor suppressor restraining EMT and tumor progression.

   Evidence Co-IP/IP-MS, ubiquitination assays, rescue and in vitro/in vivo metastasis assays in liver and pancreatic cancer models

   PMID:37348029 PMID:37596321

   Open questions at the time

   • Degron motifs in substrates not mapped
   • Context determining substrate choice across tissues unresolved
7. 2024 Medium

   Established RAB27A as a K48-linked degradation substrate and defined a meiotic role, showing FBXO28 controls actin assembly and asymmetric spindle migration in oocytes.

   Evidence Co-IP, ubiquitination, CHX chase, in vivo HFD model and rescue (RAB27A); morpholino knockdown with mRNA rescue, live imaging, IF in mouse oocytes

   PMID:38696123 PMID:38960234

   Open questions at the time

   • Whether oocyte actin defects depend on RAB27A degradation is untested
   • Link between FBXO28 and ARPC2/ARP3 expression is indirect
8. 2026 Medium

   Placed FBXO28-mediated RAB27A degradation downstream of receptor signaling, showing APLNR recruits FBXO28 upon ligand stimulation to drive K48-linked Rab27a turnover.

   Evidence Co-IP, K48-ubiquitination assay, APLNR knockdown, MG132 inhibition, in vivo murine allergy model

   PMID:42068044

   Open questions at the time

   • Mechanism of APLNR-dependent FBXO28 recruitment not defined
   • Whether this represents the same SCF complex active elsewhere is unclear

• How FBXO28 selects among its many substrates in a tissue- and signal-specific manner, and whether its degradative and non-degradative (K63, topoisomerase-regulatory) activities share a common SCF assembly, remain unresolved.
• No structural model of FBXO28 substrate-binding interface
• Substrate degrons and recruitment cofactors largely unmapped
• Relationship between mitotic, meiotic, and ligase functions undefined