FBXL17

FBXL17 is the substrate-recognition F-box subunit of an SCF (Skp1–Cul1–Rbx1) E3 ubiquitin ligase that selects specific substrates for polyubiquitylation and proteasomal degradation across multiple signaling contexts (PMID:27234298, PMID:31560077). Its leucine-rich repeat (LRR) domain mediates both incorporation into the SCF holoenzyme and substrate engagement, and LRR truncation abolishes both holoenzyme association and ubiquitination activity (PMID:31560077). Through this activity FBXL17 promotes Hedgehog signaling by ubiquitylating Sufu in the nucleus, releasing Gli1 from repression; its loss impairs Hh signaling and reduces medulloblastoma growth, and a Gorlin-syndrome Sufu mutation enhances FBXL17-mediated polyubiquitylation to sustain Hh activation (PMID:27234298). Substrate recognition is gated by post-translational modifications: FBXL17 reads an acetylation-dependent IKxxxIK acetyldegron on PRMT1—jointly controlled by p300 acetylation and Sirt1 deacetylation at K200/K205—to drive its degradation (PMID:28883095), and it targets the M1 isoform of spastin (SPAST-M1) by binding its N-terminal BTB domain in the nuclear fraction, with CK2 phosphorylation promoting ubiquitination and a disease-associated Y52C BTB mutation escaping regulation (PMID:35869491). FBXL17 also binds the O-GlcNAcylation enzyme UAP1 and inhibits its phosphorylation, with FBXL17 depletion elevating global O-GlcNAcylation in breast cancer cells (PMID:31560077).

1. 2016 High

   Established FBXL17 as a bona fide SCF substrate-recognition subunit and connected it to a signaling output by showing it ubiquitylates nuclear Sufu to de-repress Gli1 and activate Hedgehog signaling.

   Evidence Reciprocal Co-IP, ubiquitylation assays, siRNA/shRNA knockdown, medulloblastoma tumor models, and Gorlin-syndrome Sufu mutant analysis

   PMID:27234298

   Open questions at the time

   • Does not resolve how FBXL17 is restricted to the nuclear Sufu pool
   • Degron/recognition determinants on Sufu not mapped
2. 2017 High

   Defined a modification-gated recognition logic for FBXL17 by showing it reads an acetylation-dependent IKxxxIK acetyldegron, explaining how substrate PTM state controls degradation timing.

   Evidence In vitro ubiquitylation assays, site-directed mutagenesis of K117/K200/K205, Co-IP, and cycloheximide chase on PRMT1

   PMID:28883095

   Open questions at the time

   • Whether the acetyldegron logic generalizes to other FBXL17 substrates untested
   • Structural basis of acetyl-lysine recognition by the LRR not determined
3. 2019 Medium

   Mapped the LRR domain as the structural element required for both SCF holoenzyme assembly and ligase activity, and broadened the interactome to include UAP1, linking FBXL17 to O-GlcNAcylation control.

   Evidence Yeast two-hybrid screen, Co-IP, LRR truncation constructs, and siRNA knockdown with O-GlcNAcylation measurement in breast cancer cells

   PMID:31560077

   Open questions at the time

   • UAP1 phosphorylation inhibition inferred, not reconstituted
   • Unclear whether UAP1 is a degradation substrate or a non-degradative binding partner
4. 2022 Medium

   Showed FBXL17 recognizes substrates through BTB-domain binding and operates in a fractionation- and phosphorylation-dependent manner, extending its targeting repertoire to SPAST-M1 with disease relevance.

   Evidence Protein chip, Co-IP with BTB domain mapping, fractionation, proteasome inhibitor rescue, CK2 assay, shRNA, and a 3D neuronal differentiation model with disease mutant and chemical inhibitor

   PMID:35869491

   Open questions at the time

   • Single-lab evidence
   • Why degradation is confined to the nuclear fraction not mechanistically explained
5. 2025 Low

   Proposed a unifying role for SCF-FBXL17 as a dimerization quality-control ligase for BTB-domain proteins acting alongside cotranslational folding control.

   Evidence Disome selective profiling, optical tweezers, and ribosome profiling on BTB folding states (preprint)

   PMID:bio_10.1101_2025.08.25.672138

   Open questions at the time

   • Preprint; direct FBXL17 ubiquitylation assay not described
   • FBXL17 role inferred in concert with cotranslational mechanism rather than directly demonstrated

• How FBXL17 substrate selection is partitioned between subcellular compartments and integrated across its diverse PTM-gated degrons remains unresolved.
• No structural model of FBXL17 LRR bound to a substrate degron
• Mechanism restricting activity to the nuclear fraction unknown
• Relationship between BTB-binding and acetyldegron-reading modes not unified