FBXO27

FBXO27 is an N-myristoylated F-box protein that assembles into an SCF^FBXO27 (SKP1–CUL1–RBX1–FBXO27) ubiquitin ligase complex and selectively ubiquitinates glycoproteins exposed on damaged lysosomal membranes to promote lysophagy. Its substrate-binding G domain contains a conserved hydrophobic pocket formed by aromatic residues that recognizes high-mannose N-glycans, and proper folding of this domain requires co-expression with SKP1 (PMID:18203720, PMID:21640084). N-myristoylation targets FBXO27 to membranes, enabling rapid recruitment to ruptured lysosomes where it ubiquitinates LAMP1, LAMP2, VAMP3, VAMP7, and other glycoproteins, with LAMP2 ubiquitination enhancing autophagic machinery recruitment (PMID:28743755). A CREG1–FBXO27–LAMP2 regulatory axis controls autophagy in cardiomyocytes and is implicated in diabetic cardiomyopathy (PMID:37658156).

1. 2008 High

   Establishing that FBXO27 is a glycan-binding F-box protein answered how it selects substrates: a conserved G-domain hydrophobic pocket formed by two aromatic residues binds high-mannose N-glycans, and the protein assembles with canonical SCF components (SKP1, CUL1, RBX1).

   Evidence Glycan arrays, site-directed mutagenesis of G-domain aromatic residues, co-immunoprecipitation of SCF components in vitro

   PMID:18203720

   Open questions at the time

   • No physiological substrates identified at this stage
   • Cellular context for glycoprotein ubiquitination unknown
   • No structural model of FBXO27 itself
2. 2011 Medium

   Demonstrating that SKP1 co-expression is required for FBXO27 folding, stability, and glycoprotein-binding competence resolved why isolated FBXO27 aggregates and fails to bind substrates.

   Evidence Co-expression/co-immunoprecipitation, ConA lectin binding assay, aggregate formation analysis

   PMID:21640084

   Open questions at the time

   • Single-lab finding without independent replication
   • Structural basis for SKP1-dependent conformational stabilization not resolved
   • In vivo relevance of this dependency not tested
3. 2015 Medium

   Crystallography of the homologous SKP1–FBG3 complex provided structural context for how the 10-stranded β-sandwich substrate-binding domain accommodates glycans in FBXO27, and mutagenesis explained why the related FBXO44 lacks glycan-binding activity.

   Evidence X-ray crystallography (2.6 Å) of SKP1–FBXO44, structure-based mutational analysis comparing FBXO27 and FBXO44

   PMID:26460611

   Open questions at the time

   • Structure is of the FBXO44 homolog, not FBXO27 directly
   • No co-crystal with glycan substrate
   • Loop conformations in FBXO27 inferred by comparison rather than determined
4. 2017 High

   Identifying FBXO27 as a lysophagy effector resolved its physiological function: N-myristoylation targets it to membranes, enabling rapid accumulation at damaged lysosomes where SCF^FBXO27 ubiquitinates exposed glycoproteins (LAMP1, LAMP2, VAMP3, VAMP7) to recruit autophagic machinery.

   Evidence N-myristoylation characterization, live-cell imaging of lysosome recruitment, Co-IP, ubiquitination screen upon LLOMe-induced damage, substrate identification by mass spectrometry

   PMID:28743755

   Open questions at the time

   • Relative contribution of individual substrates to lysophagy efficiency not dissected
   • Functional redundancy with other glycoprotein-targeting E3 ligases (e.g., FBXO2) during lysophagy unclear
   • In vivo lysophagy phenotype of FBXO27 knockout not reported
5. 2023 Medium

   Placing FBXO27 downstream of CREG1 in cardiomyocytes revealed an upstream regulatory axis: CREG1 represses FBXO27 protein levels, preventing LAMP2 ubiquitination and thereby sustaining autophagy, with relevance to diabetic cardiomyopathy.

   Evidence Genetic overexpression/knockdown in cardiomyocytes, western blotting, LAMP2 rescue experiments

   PMID:37658156

   Open questions at the time

   • Mechanism by which CREG1 inhibits FBXO27 expression (transcriptional vs. post-translational) not defined
   • Single-lab study without independent replication
   • In vivo cardiac phenotype of FBXO27 manipulation not demonstrated

• Key unresolved questions include the direct crystal structure of FBXO27 bound to glycan substrates, in vivo phenotypic consequences of FBXO27 loss in lysophagy and disease models, and the full repertoire of regulatory inputs controlling FBXO27 expression and activity.
• No FBXO27 crystal structure or cryo-EM model available
• No in vivo knockout phenotype reported
• Regulation of FBXO27 beyond CREG1 is uncharacterized