FNDC-1 is the Caenorhabditis elegans ortholog of mammalian FUNDC1 and a member of the conserved FUN14 family. It is a small (138-residue) multi-pass protein of the mitochondrial outer membrane. FNDC-1 acts as a receptor/adaptor for selective autophagy of mitochondria (mitophagy): loss of fndc-1 impairs mitochondrial clearance in two characterized contexts. First, in the early embryo it contributes to the elimination of sperm-derived (paternal) mitochondria after fertilization, providing a second, ubiquitin-independent route that acts redundantly with, and later than, an initial ubiquitination-dependent mechanism; loss of fndc-1 delays paternal-mitochondria degradation and allows paternal mitochondrial DNA to persist in cross-progeny. Second, in body-wall muscle it mediates mitophagy triggered by hypoxia-reoxygenation, and its loss engages an ATFS-1/UPRmt stress program that is protective against reoxygenation injury. By analogy to mammalian FUNDC1, FNDC-1 is expected to recruit autophagosomal ATG8/LC3 proteins (LGG-1/LGG-2 in the worm) through an LC3-interacting region (LIR) motif, but the LIR and a direct ATG8 interaction have not yet been demonstrated in C. elegans. FNDC-1 thus represents a receptor-mediated, ubiquitin-independent branch of mitochondrial quality control, distinct from the PINK-1/PDR-1 (PINK1/Parkin) pathway.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000422 autophagy of mitochondrion | IBA GO_REF:0000033 | ACCEPT | Summary: The IBA annotation for autophagy of mitochondrion is phylogenetically inferred and well-supported by experimental data in C. elegans. FNDC-1 mediates mitophagy under hypoxia-reoxygenation stress (PMID:33416042) and contributes to paternal mitochondria elimination (PMID:31233739, PMID:31153831). The term accurately captures FNDC-1's role in targeting mitochondria for autophagic degradation. As noted in fndc-1-deep-research-falcon.md, FNDC-1 is the C. elegans ortholog of mammalian FUNDC1. Reason: This annotation is strongly supported by multiple lines of evidence. The Lim et al. 2021 study (PMID:33416042) demonstrated that FNDC-1 mediates mitophagy under hypoxia-reoxygenation conditions, with fndc-1 loss-of-function significantly reducing the percentage of mitochondria undergoing mitophagy as measured by mito-mKeima reporter. The term GO:0000422 is appropriate as it encompasses the broader autophagic process for mitochondria without specifying the pathway (receptor vs. ubiquitin). Supporting Evidence: PMID:31233739 Like FUNDC1, the C. elegans ortholog FNDC-1 is widely expressed in somatic tissues and mediates hypoxic mitophagy. PMID:31153831 If paternal mitochondria are not eliminated via this early process, they are eventually removed from the embryo in a process that depends on the mitophagy adaptor protein, fndc-1. |
| GO:0005741 mitochondrial outer membrane | IBA GO_REF:0000033 | ACCEPT | Summary: The IBA annotation for mitochondrial outer membrane localization is phylogenetically inferred and strongly supported by experimental evidence in C. elegans. Biochemical fractionation and proteinase K accessibility assays confirm FNDC-1 localizes to the OMM (PMID:31233739, PMID:33416042). Reason: This annotation is well-supported by direct experimental evidence. The Lim et al. 2019 study (PMID:31233739) describes FNDC-1 as "expressed on the mitochondrial outer membrane" consistent with its role as a mitophagy receptor. Subsequent studies confirmed this localization through biochemical fractionation and proteinase K sensitivity assays. The FUN14 family proteins are characteristically OMM-localized, and FNDC-1 follows this conserved pattern. Supporting Evidence: PMID:31233739 FUNDC1 (FUN14 domain containing 1) is a mammalian mitophagy receptor expressed on the mitochondrial outer membrane that contributes to mitochondrial quality control following hypoxic stress. Like FUNDC1, the C. elegans ortholog FNDC-1 is widely expressed in somatic tissues and mediates hypoxic mitophagy. |
| GO:0005741 mitochondrial outer membrane | IEA GO_REF:0000044 | ACCEPT | Summary: The IEA annotation based on UniProt subcellular location mapping is redundant with the IBA annotation but remains valid. Both annotations correctly capture OMM localization. Reason: This electronic annotation based on UniProt subcellular location is accurate. UniProt curates the subcellular location based on the primary literature (PMID:31233739), which describes FNDC-1 as a mitochondrial outer membrane protein. The IEA is broader in evidence but still correct. Duplicate annotations with different evidence codes are acceptable in GO. Supporting Evidence: PMID:31233739 FUNDC1 (FUN14 domain containing 1) is a mammalian mitophagy receptor expressed on the mitochondrial outer membrane...Like FUNDC1, the C. elegans ortholog FNDC-1 is widely expressed in somatic tissues and mediates hypoxic mitophagy. |
| GO:0006914 autophagy | IEA GO_REF:0000043 | ACCEPT | Summary: The IEA annotation for general autophagy is derived from UniProt keyword mapping. While not incorrect, this term is less specific than the existing annotations for mitophagy (GO:0000423) and autophagy of mitochondrion (GO:0000422). Reason: This electronic annotation is technically correct as FNDC-1 is indeed involved in autophagy (specifically mitophagy). However, it is less informative than the more specific annotations already present. IEA annotations of this type are common and acceptable for broad pathway categorization, even when more specific experimental annotations exist. The term is a parent of the more specific mitophagy terms. Supporting Evidence: PMID:31153831 Elimination of paternal mitochondria after fertilization occurs in many species using the process of selective autophagy. |
| GO:0000423 mitophagy | IMP PMID:31153831 Ubiquitination is required for the initial removal of patern... | ACCEPT | Summary: The IMP annotation for mitophagy based on PMID:31153831 (Molina et al. 2019) is well-supported. This study demonstrated that fndc-1 functions as a mitophagy adaptor for paternal mitochondria elimination, providing a second, ubiquitin-independent mechanism after the initial ubiquitination-dependent pathway. Reason: This annotation is directly supported by the cited publication. Molina et al. 2019 demonstrated that loss of fndc-1 delays paternal mitochondria elimination after fertilization, establishing fndc-1 as a mitophagy adaptor protein. The study shows that fndc-1 provides a temporally distinct mechanism from the ubiquitin-dependent pathway for targeting paternal mitochondria to autophagosomes. Supporting Evidence: PMID:31153831 If paternal mitochondria are not eliminated via this early process, they are eventually removed from the embryo in a process that depends on the mitophagy adaptor protein, fndc-1. Thus, there are two redundant, but temporally distinct mechanisms that target paternal mitochondria for elimination in C. elegans. |
| GO:0000423 mitophagy | IMP PMID:31233739 Fndc-1 contributes to paternal mitochondria elimination in C... | ACCEPT | Summary: The IMP annotation for mitophagy based on PMID:31233739 (Lim et al. 2019) provides independent experimental support. This study specifically characterized FNDC-1's role in paternal mitochondria elimination and hypoxic mitophagy, demonstrating that loss of fndc-1 retards paternal mitochondria degradation. Reason: This annotation is directly supported by the cited publication. Lim et al. 2019 showed that loss of fndc-1 retards the rate of paternal mitochondria degradation and that FNDC-1 functions as a ubiquitin-independent mitophagy receptor. The study also showed that paternal mitochondrial DNA can be detected in cross-progeny of fndc-1 mutant males, providing functional evidence for its role in mitophagy. Supporting Evidence: PMID:31233739 Paternal mitochondria are eliminated following fertilization by selective autophagy... loss of fndc-1 retards the rate of paternal mitochondria degradation, but not that of membranous organelles, a nematode specific membrane compartment whose fusion is required for sperm motility. This is the first example of a ubiquitin-independent mitophagy receptor playing a role in the selective degradation of sperm mitochondria. PMID:31233739 Paternal mitochondrial DNA is normally undetectable in wildtype larva, but can be detected in the cross-progeny of fndc-1 mutant males. |
| GO:0140580 mitochondrion autophagosome adaptor activity | ISS PMID:31233739 Fndc-1 contributes to paternal mitochondria elimination in C... | NEW | Summary: NEW annotation proposed for the molecular function of FNDC-1 as a mitophagy receptor/adaptor. GO:0140580 describes the binding activity that brings together a mitochondrial membrane and an autophagosome during mitophagy, which fits FNDC-1's genetically defined role. It is coded ISS rather than IDA because the adaptor activity is inferred from orthology to mammalian FUNDC1 together with the worm loss-of-function mitophagy phenotype; a direct FNDC-1-ATG8 (LGG-1/LGG-2) binding assay has not been reported in C. elegans. Reason: The existing annotations lack a molecular function (MF) term. FNDC-1 is required for mitophagy in two worm contexts (paternal-mitochondria elimination and hypoxia-reoxygenation) and sits on the mitochondrial outer membrane, consistent with a mitophagy receptor/adaptor that bridges mitochondria to autophagosomes. GO:0140580 (mitochondrion autophagosome adaptor activity) is defined as "The binding activity of a molecule that brings together a mitochondrial membrane and an autophagosome during mitophagy," which precisely names this role. The molecular mechanism itself - an LC3-interacting region (LIR) binding ATG8/LC3 - is established for mammalian FUNDC1 but not yet demonstrated for worm FNDC-1, so ISS (from the FUNDC1 ortholog) is the appropriate evidence basis rather than a direct assay code. The worm ATG8-binding partner (LGG-1/LGG-2) and LIR motif remain to be identified (see knowledge_gaps). Proposed replacements: mitochondrion autophagosome adaptor activity Supporting Evidence: PMID:31233739 This is the first example of a ubiquitin-independent mitophagy receptor playing a role in the selective degradation of sperm mitochondria. PMID:31153831 If paternal mitochondria are not eliminated via this early process, they are eventually removed from the embryo in a process that depends on the mitophagy adaptor protein, fndc-1. file:worm/fndc-1/fndc-1-deep-research-falcon.md encodes the C. elegans ortholog of mammalian FUNDC1, a conserved FUN14-domain protein functioning as a mitophagy receptor |
| GO:0071456 cellular response to hypoxia | IMP PMID:33416042 FNDC-1-mediated mitophagy and ATFS-1 coordinate to protect a... | NEW | Summary: NEW annotation proposed based on FNDC-1's characterized role in hypoxia-reoxygenation response. The Lim et al. 2021 Autophagy study (PMID:33416042) demonstrated that FNDC-1 mediates mitophagy specifically in response to hypoxia-reoxygenation stress in body wall muscle. Reason: In the worm, loss of fndc-1 changes the outcome of hypoxia-reoxygenation (HR): Lim et al. 2021 show FNDC-1 mediates HR-induced mitophagy in body-wall muscle and that its loss protects against HR injury via an ATFS-1/UPRmt program, establishing FNDC-1 as a component of the cellular response to changing oxygen availability (GO:0071456). Note the stimulus studied is hypoxia-reoxygenation specifically; no reoxygenation-specific GO term exists, so GO:0071456 (cellular response to hypoxia) is used as the appropriate parent. IMP is justified by the fndc-1 loss-of-function HR phenotype. Proposed replacements: cellular response to hypoxia Supporting Evidence: PMID:33416042 Here, we provide evidence that FNDC-1 is the C. elegans ortholog of FUNDC1, and that its loss protects against injury in a worm model of HR. PMID:33416042 FUNDC1 (FUN14 domain containing 1) is a mammalian mitophagy receptor that responds to hypoxia-reoxygenation (HR) stress PMID:31233739 FUNDC1 (FUN14 domain containing 1) is a mammalian mitophagy receptor expressed on the mitochondrial outer membrane that contributes to mitochondrial quality control following hypoxic stress. Like FUNDC1, the C. elegans ortholog FNDC-1 is widely expressed in somatic tissues and mediates hypoxic mitophagy. |
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Download this section (compressed HTML)Q: What is the precise LIR motif sequence in C. elegans FNDC-1, and how does phosphorylation regulate its interaction with LGG-1/LGG-2 (C. elegans LC3 homologs)? Mammalian FUNDC1 regulation involves phosphorylation of the LIR motif (Tyr18, Ser13, Ser17) by SRC, CK2, ULK1, and dephosphorylation by PGAM5. The regulatory mechanisms for C. elegans FNDC-1 remain to be fully characterized.
Q: Does FNDC-1 localize to mitochondria-ER contact sites (MAMs) in C. elegans, and what is its relationship with DRP-1-mediated mitochondrial fission during mitophagy? Recent preprints suggest FNDC-1 forms puncta at MAMs and recruits DRP-1 during developmental mitophagy at the oocyte-to-zygote transition. This mechanism needs validation in peer-reviewed studies.
Q: What is the relationship between FNDC-1 and other mitophagy pathways (PINK-1/PDR-1, DCT-1/NIX) in C. elegans across different stress conditions and developmental stages? FNDC-1 operates independently of ubiquitination for paternal mitochondria elimination, but the pathway relationships and possible redundancies in different contexts remain unclear.
Experiment: Co-immunoprecipitation or split-GFP complementation assays to detect direct interaction between FNDC-1 and LGG-1/LGG-2 in vivo. While FNDC-1 is functionally characterized as a mitophagy receptor, direct physical interaction with C. elegans ATG8 homologs has not been demonstrated experimentally.
Hypothesis: FNDC-1 directly binds LGG-1/LGG-2 via its LIR motif to recruit autophagosomes to mitochondria
Experiment: Site-directed mutagenesis of the predicted LIR motif in FNDC-1 followed by functional rescue assays (paternal mitochondria elimination, HR-induced mitophagy). This would confirm that the LIR motif is required for FNDC-1 function in C. elegans and allow comparison with mammalian FUNDC1 regulatory mechanisms.
Hypothesis: The LIR motif is essential for FNDC-1 mitophagy receptor function
Experiment: Phosphoproteomics analysis of FNDC-1 under normoxic vs. hypoxic conditions to identify regulatory phosphorylation sites. This would reveal whether C. elegans FNDC-1 is regulated by phosphorylation similar to mammalian FUNDC1, and identify the relevant kinases/phosphatases in C. elegans.
Hypothesis: FNDC-1 activity is regulated by hypoxia-dependent phosphorylation changes
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The molecular activity of FNDC-1 as a mitophagy receptor is not experimentally established in C. elegans. Its LC3-interacting region (LIR) motif has not been identified or mutated, and a direct physical interaction between FNDC-1 and the worm ATG8 orthologs LGG-1/LGG-2 has not been demonstrated. Whether receptor/adaptor function requires an intact LIR is therefore inferred from mammalian FUNDC1 rather than shown in worm.
OPEN BIOLOGYCURATION MF_DARK
What is known: Genetic loss-of-function establishes that FNDC-1 is required for hypoxia-reoxygenation mitophagy in body-wall muscle and contributes to paternal-mitochondria elimination, and biochemical fractionation/proteinase-K accessibility place it on the mitochondrial outer membrane. The receptor/adaptor role is thus supported genetically and by orthology, but not by any direct binding assay in the worm.
Significance: FNDC-1 is the founding example of a ubiquitin-independent mitophagy receptor for sperm-mitochondria degradation; identifying its ATG8 partner and LIR would define the molecular mechanism and put the GO molecular-function annotation (currently ISS-level, GO:0140580) on direct experimental footing.
What would resolve it: Identify and mutate the FNDC-1 LIR; test FNDC-1-LGG-1/LGG-2 interaction by co-immunoprecipitation, peptide-array/ITC, or split-GFP in vivo, with LIR-mutant rescue of paternal-mitochondria elimination and HR-induced mitophagy.
Provenance (the field's own admissions):
Gap: How FNDC-1 receptor activity is switched on and off in C. elegans is unknown. No phosphorylation or ubiquitination sites have been mapped on the worm protein, and the kinases, phosphatases, and E3 ligases that gate mammalian FUNDC1 (SRC, CK2, ULK1, PGAM5, MARCH5) have no demonstrated counterparts acting on FNDC-1.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: In mammals, reversible LIR phosphorylation and MARCH5-mediated ubiquitination regulate FUNDC1 activity and abundance. In worm, hypoxia-reoxygenation engages FNDC-1-dependent mitophagy and loss of fndc-1 triggers an ATFS-1/UPRmt program, so the downstream stress response is defined, but the upstream regulatory inputs onto FNDC-1 itself are uncharacterized.
Significance: Post-translational control determines when and where FNDC-1 triggers mitophagy; resolving it is needed to place FNDC-1 downstream of hypoxia signaling and to test whether the mammalian regulatory logic is conserved.
What would resolve it: Phosphoproteomic/ubiquitinomic mapping of FNDC-1 under normoxia versus hypoxia-reoxygenation, plus genetic epistasis against candidate worm orthologs of the mammalian regulators.
Provenance (the field's own admissions):
Gap: Whether FNDC-1 is a broadly-acting mitophagy receptor in C. elegans (beyond the two described contexts of paternal-mitochondria elimination and hypoxia-reoxygenation) is unresolved, as is its epistatic relationship to the PINK-1/PDR-1 and DCT-1/NIX pathways. The loss-of-function phenotype is mild and context-restricted, and under some stresses mitochondrial remodeling proceeds without fndc-1.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: Loss of fndc-1 delays but does not abolish paternal-mitochondria elimination because a redundant ubiquitin-dependent pathway acts first and earlier, and it reduces HR-induced mitophagy; no null lethality or gross organismal phenotype is reported.
Significance: Clarifying FNDC-1's physiological scope and its ordering relative to the other worm mitophagy receptors would establish whether it is a general quality-control receptor or a specialist for selective/developmental mitochondrial clearance.
What would resolve it: Systematic mito-mKeima/mito-Keima flux measurements across tissues, ages, and stressors in fndc-1 single and pink-1/pdr-1 or dct-1 double mutants to define genetic-interaction structure.
Provenance (the field's own admissions):
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