RFFL

RFFL (Rififylin/CARP-2) is a RING-finger E3 ubiquitin ligase that organizes endosomal protein quality control and membrane trafficking through an N-terminal FYVE-like domain that targets it to recycling endocytic membranes and a C-terminal RING domain that confers ligase activity (PMID:15229288, PMID:18450452). At the endocytic recycling compartment it restrains cargo recycling to the plasma membrane: it directly ubiquitylates Rab11 effectors including EHD1, MICALL1, and Rab11-FIPs, and its prolonged engagement of these effectors clusters recycling tubulovesicular membranes and delays transferrin exit (PMID:15229288, PMID:30659120). RFFL provides a chaperone-independent peripheral quality-control pathway by directly recognizing the disordered regions of unfolded ΔF508-CFTR at the plasma membrane and appending K63-linked polyubiquitin to route it for lysosomal degradation; loss of RFFL stabilizes surface CFTR and potentiates pharmacological rescue by folding correctors and translational readthrough drugs (PMID:29503157, PMID:41723327). Through RING-dependent ubiquitination RFFL also degrades the internalized TNF receptor effector RIPK1 to dampen TNF-induced NF-κB signaling (PMID:18450452), promotes ERAD-coupled proteasomal turnover of the hERG channel in association with VCP/p97 (PMID:30401747), and exerts opposing RING-dependent control over cardiac Kv4.3 and Kv1.4 channel levels, shaping repolarization (PMID:38367666). At damaged mitochondria RFFL acts upstream of mitophagy, interacting with PRKN/parkin to promote its stable recruitment and directly ubiquitylating mitofusin 2 to control mitochondrial morphology, with overexpression rescuing the hyperfused phenotype of Charcot-Marie-Tooth type 2A MFN2 mutants (PMID:35373701, PMID:40444323). Additional proteasomal substrates identified by proteomics and biochemistry include DHX9, JMJD6, and DNAJB11, and RFFL influences lipid homeostasis (PMID:37550275, PMID:40444323, PMID:40568870).

1. 2004 Medium

   Established that RFFL is a recycling-compartment protein whose FYVE-like domain, not its RING domain, governs its membrane localization and its inhibitory effect on endocytic recycling.

   Evidence Overexpression and deletion-mutant analysis with transferrin recycling assays in HeLa cells

   PMID:15229288

   Open questions at the time

   • Did not identify ubiquitination substrates at the ERC
   • RING-domain catalytic role left undefined
   • Mechanism of recycling inhibition not resolved to specific effectors
2. 2008 High

   Defined a signaling function for endosomal RFFL by showing it ubiquitinates and degrades RIPK1 within the internalized TNF-receptor complex to negatively regulate NF-κB.

   Evidence siRNA knockdown, reciprocal Co-IP, ubiquitination and NF-κB reporter assays with endosomal imaging

   PMID:18450452

   Open questions at the time

   • Ubiquitin linkage type on RIPK1 not specified
   • How receptor internalization recruits RFFL not detailed
3. 2011 Medium

   Connected RFFL-driven recycling delay to organ physiology, linking cardiomyocyte Rffl overexpression to altered beat frequency, QT interval, and hypertension.

   Evidence Congenic rat model with transferrin recycling assays and ECG telemetry

   PMID:21357277

   Open questions at the time

   • No direct ubiquitination substrate identified for the cardiac phenotype
   • Correlative physiology without molecular dissection
4. 2012 Medium

   Extended RFFL recycling control to renal proximal tubules, where increased expression delays endosomal recycling and contributes to proteinuria.

   Evidence Congenic rat model with primary proximal tubule recycling and polyubiquitination assays

   PMID:22891072

   Open questions at the time

   • Specific surface cargo affected not pinned down
   • Direct substrate-ligase relationship not established
5. 2018 High

   Identified RFFL as the E3 ligase mediating chaperone-independent peripheral quality control of ΔF508-CFTR via direct recognition of disordered regions and K63-linked polyubiquitination.

   Evidence Genome-wide siRNA screen, linkage-specific ubiquitin analysis, direct binding and functional CFTR assays

   PMID:29503157

   Open questions at the time

   • Structure of the RFFL substrate-binding region not solved here
   • Whether the mechanism generalizes to other misfolded surface proteins untested
6. 2018 High

   Showed RFFL couples to ERAD by degrading the ER-retained hERG channel in a RING- and VCP/p97-dependent manner, implicating it in cardiac repolarization.

   Evidence Reciprocal Co-IP, RING-domain mutant, dominant-negative VCP, and patch-clamp electrophysiology in cardiomyocytes

   PMID:30401747

   Open questions at the time

   • Direct ubiquitin transfer to hERG in vitro not reconstituted
   • ERAD versus peripheral pathway contributions not fully separated
7. 2019 High

   Resolved the molecular basis of RFFL's recycling phenotype by showing it directly ubiquitylates Rab11 effectors (EHD1, MICALL1, Rab11-FIPs), with redundancy among E3 ligases revealed by KO.

   Evidence BioID interactome, in vitro ubiquitylation reconstitution, RFFL KO, and dominant-negative mutant with recycling assays

   PMID:30659120

   Open questions at the time

   • Ubiquitin linkage and fate of effectors not fully defined
   • Identity of redundant E3 ligases unknown
8. 2022 Medium

   Placed RFFL upstream of mitophagy by showing it associates with damaged mitochondria and interacts with PRKN to promote stable parkin recruitment.

   Evidence RFFL KO cells, Co-IP, live confocal imaging, and CCCP mitochondrial damage assays

   PMID:35373701

   Open questions at the time

   • Substrate ubiquitinated to recruit PRKN not identified
   • Whether endosomal localization is required for mitochondrial function unclear
9. 2023 High

   Demonstrated that RFFL's substrate-binding region is pharmacologically targetable, as α-tocopherol succinate binds it and blocks ΔF508-CFTR ubiquitination without affecting catalysis.

   Evidence Chemical array screening with NMR binding, ubiquitination assays, and functional CFTR rescue

   PMID:37543348

   Open questions at the time

   • High-resolution structure of the binding pocket not determined
   • Selectivity across RFFL substrates not mapped
10. 2023 Medium

    Expanded the RFFL substrate repertoire to DHX9, whose RFFL-mediated proteasomal degradation is antagonized by LINC01016 to control PI3K/AKT signaling in breast cancer.

    Evidence RNA pull-down, mass spectrometry, Co-IP, and ubiquitination/proteasome inhibitor assays

    PMID:37550275

    Open questions at the time

    • Direct in vitro ubiquitylation of DHX9 not shown
    • Ubiquitin linkage type unspecified
11. 2024 Medium

    Showed RFFL exerts divergent RING-dependent control over cardiac potassium channels, lowering Kv4.3 while raising Kv1.4, thereby tuning repolarization reserve.

    Evidence Adenoviral overexpression in rabbit cardiomyocytes, patch clamp, RING-domain mutant, and computational modeling

    PMID:38367666

    Open questions at the time

    • Direct ubiquitination of Kv4.3/Kv1.4 not demonstrated
    • Mechanism producing opposite effects on the two channels unexplained
12. 2025 High

    Established RFFL as a direct E3 ligase for MFN2 controlling mitochondrial morphology, with disease relevance shown by rescue of CMT2A MFN2 mutant hyperfusion.

    Evidence In vitro ubiquitylation reconstitution, CRISPR KO with EM, Co-IP, RING-domain mutant, and disease-mutant rescue

    PMID:40444323

    Open questions at the time

    • Whether RFFL acts on MFN2 at endosome-mitochondria contacts unresolved
    • Link between MFN turnover and observed lipid homeostasis defects not mechanistically detailed
13. 2025 Medium

    Used unbiased proteomics to identify JMJD6 and DNAJB11 as endogenous proteasomal RFFL substrates and reinforced a role in lipid metabolism.

    Evidence CRISPR KO with label-free quantitative proteomics, in vivo ubiquitination, and proteasome inhibitor experiments across three cell lines

    PMID:40568870

    Open questions at the time

    • Direct binding/ubiquitylation not reconstituted for each substrate
    • Biological consequences of JMJD6/DNAJB11 degradation not defined
14. 2026 Medium

    Showed RFFL controls peripheral quality control of full-length CFTR restored by readthrough drugs, with knockdown enhancing functional rescue.

    Evidence siRNA knockdown with ubiquitination and Ussing chamber/patch clamp CFTR assays in epithelial cells

    PMID:41723327

    Open questions at the time

    • Whether RFFL inhibition is safe/selective in vivo untested
    • Extent of overlap with the ΔF508 mechanism not fully delineated

• How RFFL's single FYVE-like/RING architecture coordinates its diverse roles across recycling endosomes, the plasma membrane, and damaged mitochondria, and what determines its substrate selectivity and ubiquitin-linkage choice, remains unresolved.
• No structural model of substrate recognition
• Spatial regulation of RFFL recruitment to distinct organelles unexplained
• Linkage-type determinants across substrates undefined