FBXL4 (Q9UKA2), human: research report

Identity check. The UniProt record is F-box/LRR-repeat protein 4, gene FBXL4 (synonyms FBL4, FBL5), in Homo sapiens. Its domains are an F-box and a leucine-rich repeat (LRR) region. The literature below is on this same gene. No ambiguity with another gene was found.

Method and limits. This report rests on two web searches, which returned the 2023 EMBO J paper and its preprints. The remaining points come from my background knowledge and have no verified URLs. These are the Cao and Alsina papers, the Gai 2013 mitochondrial-localisation paper, the Bonnen 2013 and Gai 2013 disease papers, the mouse knockout, the structural work and the Sabouny 2024 hypoxia point. I could not check them against their sources here, so check them before citing. Claims backed by a search result carry a link.

Summary

FBXL4 is the substrate-recognition subunit of a Cullin1-based SCF (Skp1–Cul1–F-box) E3 ubiquitin ligase. The ligase sits on the mitochondrial outer membrane. There it ubiquitylates the mitophagy receptors BNIP3 and NIX (BNIP3L) and sends them to proteasomal degradation. This keeps basal mitophagy low. Loss-of-function variants cause mitochondrial DNA depletion syndrome 13 (MTDPS13), an encephalomyopathic disease. The mechanism is excess BNIP3/NIX and over-active mitophagy.

Molecular function

• Activity. FBXL4 is a substrate adaptor for an SCF-type cullin-RING ubiquitin ligase. The F-box binds Skp1 and Cul1. The LRR region is the predicted substrate-binding module. The ligase catalyses ubiquitin transfer to BNIP3 and NIX, which targets them for degradation.
• Primary evidence. Nguyen-Dien et al., EMBO J 2023;42(13):e112767 (PMC10308361; bioRxiv preprint) reports that SCF-FBXL4 "localises to the mitochondrial outer membrane in unstressed cells and mediates the constitutive ubiquitylation and degradation of the mitophagy receptors NIX and BNIP3".
• Independent work (not verified here). A separate study (Cao et al., 2023, which I believe appeared in EMBO J and/or Nature Cell Biology) describes a mitochondrial SCF-FBXL4 ligase that degrades BNIP3 and NIX, and I believe Alsina et al. (2023) reached the same conclusion. This convergence is reported in a discussion thread and in the search summaries.
• Substrate specificity. The substrates are the outer-membrane BH3-only-like mitophagy receptors BNIP3 and NIX. The search summaries do not address other substrates, so none are claimed.

Localisation

FBXL4 is on the mitochondrial outer membrane, facing the cytosol. This fits the ubiquitylation of other outer-membrane proteins. Earlier work (Gai et al. 2013, unverified) reported mitochondrial targeting through an N-terminal sequence. The search results also place the ligase at the outer membrane under basal, non-stressed conditions.

Pathway and biological role

• Negative regulation of receptor-mediated mitophagy. BNIP3 and NIX are induced by HIF1α under hypoxia or iron chelation, and CRL2-VHL controls HIF1α. FBXL4 acts as a separate checkpoint on the receptors themselves. It sets the basal level of receptor-mediated mitophagy.
• Mouse model. Per the search summary, Fbxl4-knockout mice have raised BNIP3 and NIX and die perinatally. Knocking out either Bnip3 or Nix rescues their metabolic derangements and viability. This shows that the receptor accumulation is causal.
• Mitochondrial homeostasis. Excess mitophagy in FBXL4-deficient cells is associated with lower mitochondrial mass, mtDNA depletion and respiratory-chain deficiency. Earlier reports (Bonnen 2013 and Gai 2013, unverified) linked FBXL4 to mtDNA maintenance and mitochondrial dynamics, but the mitophagy mechanism is the best-supported primary role.

Disease

Biallelic loss-of-function or missense variants in FBXL4 cause MTDPS13, an early-onset encephalomyopathy with lactic acidosis, hypotonia and developmental delay. In the EMBO J paper, the pathogenic variants "do not efficiently interact with the core SCF ubiquitin ligase machinery or mediate the degradation of NIX and BNIP3". This leads to excess basal mitophagy. These are the search-summary wordings, not full-text checks.

Evidence summary

Suggestions for GO annotation

• Molecular function. Use the substrate-specific adaptor role for a cullin-RING ligase. A reasonable choice is a ubiquitin-protein ligase substrate-recognition term (for example, ubiquitin-ligase-substrate adaptor activity), not a bare "protein binding" term. Check the term in the OLS MCP.
• Process. Use negative regulation of mitophagy, and protein ubiquitination or proteasomal degradation of BNIP3/NIX.
• Component. Use mitochondrial outer membrane and the SCF ubiquitin ligase complex.
• Caution. MTDPS13 and mtDNA maintenance effects are downstream. They are best treated as non-core.

Open questions

• How FBXL4 recognises its substrates, and the role of the LRR domain. Structural work may exist (unverified), and the search did not cover it.
• How FBXL4 activity is regulated under hypoxia or iron chelation, when BNIP3/NIX must be stabilised.
• Whether other outer-membrane substrates exist.
• Why the disease is so tissue-selective despite ubiquitous expression.

Sources

• Nguyen-Dien et al., EMBO J 2023, FBXL4 suppresses mitophagy by restricting the accumulation of NIX and BNIP3 mitophagy receptors
• PMC10308361 (full text)
• bioRxiv preprint
• Monash record
• S4ME thread on Cao et al. 2023

Citations

1. https://link.springer.com/10.15252/embj.2022112767
2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10308361/
3. https://www.biorxiv.org/content/10.1101/2022.10.12.511867.full.pdf
4. https://s4me.info/threads/a-mitochondrial-scf%E2%80%90fbxl4-ubiquitin-e3-ligase-complex-degrades-bnip3-and-nix-to-restrain-mitophagy-and-prevent-mitochondrial-disease-2023-cao.45548
5. https://research.monash.edu/en/publications/fbxl4-suppresses-mitophagy-by-restricting-the-accumulation-of-nix/