fis-2 (F13B9.8) is one of two Caenorhabditis elegans homologs of FIS1, a small (151 aa) tail-anchored protein of the mitochondrial outer membrane. Like other FIS1-family members it has a cytosol-facing tetratricopeptide-repeat (TPR) fold and a single C-terminal transmembrane helix that anchors it in the outer membrane with the TPR body exposed to the cytosol. The worm genome encodes a second FIS1 homolog, fis-1, with which fis-2 acts partly redundantly. In mammals and fungi FIS1 proteins are receptor/adaptor factors implicated in recruiting the dynamin-related GTPase DRP1 to the outer membrane during mitochondrial (and peroxisomal) fission. In C. elegans, however, loss of fis-2 (alone or together with fis-1) does not detectably impair mitochondrial or peroxisomal fission or morphology; the essential fission machinery is instead MFF/DRP-1. The functions demonstrated for fis-2 are downstream of, or parallel to, fission: it has a minor pro-apoptotic role in promoting the elimination of mitochondria in dying cells, acting downstream of the CED-3 caspase and independently of DRP-1 and CED-9, and it contributes (redundantly with fis-1) to the orderly disposal of damaged mitochondria, where loss of both paralogs causes stress-induced accumulation of large autophagic (LGG-1/LC3-positive) aggregates containing mitochondrial remnants. fis-2 is thus a mitochondrial outer membrane FIS1-family protein linked to mitochondrial quality control and cell-death execution rather than to the core fission reaction.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0008289 lipid binding | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Family-level phylogenetic (IBA) inference of a generic "lipid binding" molecular function. There is no C. elegans-specific evidence that FIS-2 binds lipid; its C-terminal transmembrane helix mediates membrane anchoring, which is not the same as a lipid-binding molecular function. This is a weak, uninformative over-propagation for this gene. Reason: No experimental support in worm; the informative molecular function inferred for FIS1-family proteins is adaptor/receptor activity (GO:0060090), not lipid binding. Membrane anchoring via the tail-anchor TM helix does not justify a lipid-binding MF. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: PANTHER:PTN000326738 · FIS1 family node SOURCE WEAK OR INFERRED |
| GO:0060090 molecular adaptor activity | IBA GO_REF:0000033 | ACCEPT | Summary: The most informative molecular-function term available for a FIS1-family protein: an outer-membrane receptor/adaptor that helps bring the fission GTPase DRP-1 into the fission/degradation machinery. Consistent with fis-2 being a tail-anchored mitochondrial outer membrane protein that acts within the DRP-1/MAM disposal machinery. Important caveat: this is entirely family-level inference — no worm FIS-2 protein has been shown to bind DRP-1, an adaptor, or any partner (see knowledge_gaps). Reason: Best-supported and most informative MF term for this gene; consistent with the family function (Drp1 recruitment/adaptor). Retained as the representative molecular function while flagged as an MF-dark gap because it is undemonstrated for fis-2 specifically. Supporting Evidence: PMID:24196833 Recruitment of Drp1 to mitochondria is mediated by proteins that are anchored in the mitochondrial outer membrane PMID:24196833 Fis1 is part of the MAM complex and contributes to a late stage of the removal process but is not required for entry of Drp1 into the MAM or fission |
| GO:0005741 mitochondrial outer membrane | IBA GO_REF:0000033 | ACCEPT | Summary: Mitochondrial outer membrane is the expected site of action for a FIS1-family tail-anchored protein and is consistent with the fis-2 IDA mitochondrion localization (PMID:18722182) and ISS from human FIS1. Accept as the core cellular component, recognizing that OM-resolution localization of worm fis-2 is inferred (homology + tail-anchor topology) rather than directly resolved. Reason: Well-supported localization consistent with the family and with the experimental (IDA) mitochondrial annotation; core to FIS-2 function. |
| GO:0005778 peroxisomal membrane | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Peroxisomal membrane localization is inferred from mammalian/plant FIS1 homologs, which act in peroxisome fission. Plausible for worm fis-2 by homology but not directly demonstrated, and fis-1;fis-2 mutants have normal (punctate) peroxisomes. Retain as a non-core, homology-based localization. Reason: No direct worm evidence for peroxisomal localization or a peroxisomal role; family-level propagation. Not a core function of C. elegans fis-2. Supporting Evidence: PMID:24196833 showing punctate peroxisomes in wild-type and Fis1 mutants |
| GO:0000266 mitochondrial fission | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Mitochondrial fission is the ancestral FIS1-family process, but in C. elegans loss of fis-2 (alone or with fis-1) does NOT impair mitochondrial fission or morphology, and Fis1 homologs are not essential for fission or DRP-1 recruitment (MFF/DRP-1 are the essential factors). Retain as non-core: fis-2 belongs to the fission machinery at the family level but is not required for fission in worm. Reason: Defensible at the family level, but loss-of-function is phenotypically silent for fission in worm; this is not the core, essential fission determinant. Supporting Evidence: PMID:24196833 Our results show that fis-1 and fis-2 single and double mutants have wild-type mitochondrial morphologies, as also shown by others PMID:24196833 Mff and Fis1 are not essential for fission or for Drp1 recruitment to mitochondria in C. elegans |
| GO:0016559 peroxisome fission | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Peroxisome fission is a documented FIS1 function in mammals and plants, but in C. elegans fis-1;fis-2 mutants have normal punctate peroxisomes and only Mff/Drp1 loss produces tubular (fission-defective) peroxisomes. Family-level propagation not supported by direct worm evidence; retain as non-core. Reason: No demonstrated peroxisome-fission role for worm fis-2; the worm data show normal peroxisomes in Fis1 mutants. Keep as homology-based, non-core. Propagation Review Root cause: PROPAGATION BAD Failure modes: FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTN000326738 · FIS1 family node SUPPORTS SOURCE BUT NOT TARGET Supporting Evidence: PMID:24196833 showing punctate peroxisomes in wild-type and Fis1 mutants and tubular peroxisomes in drp-1 mutant and Mff double mutants |
| GO:0000266 mitochondrial fission | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Electronic InterPro2GO mapping of the Fis1 domain (IPR016543) to mitochondrial fission. Same considerations as the IBA mitochondrial-fission annotation: defensible at the family/domain level but not the core essential fission role in worm. Reason: Redundant with the IBA mitochondrial-fission annotation; family/domain-level inference. Non-core given the loss-of-function-silent fission phenotype in worm. |
| GO:0005739 mitochondrion | IEA GO_REF:0000120 | ACCEPT | Summary: Automated electronic annotation to the mitochondrion. Correct and corroborated by the direct experimental (IDA) mitochondrial localization from PMID:18722182 and by ISS from human FIS1. Reason: Well-supported core localization; consistent with the experimental IDA annotation. |
| GO:0005741 mitochondrial outer membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping to mitochondrial outer membrane, consistent with the FIS1 tail-anchor topology and ISS from human FIS1. Reason: Well-supported core localization; redundant with the ISS annotation and consistent with the experimental mitochondrial localization. |
| GO:0005778 peroxisomal membrane | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: UniProt subcellular-location mapping to peroxisomal membrane, transferred from the mammalian FIS1 dual localization. Not directly shown in worm; non-core. Reason: Homology-based peroxisomal localization; no direct C. elegans evidence. Retain as non-core. |
| GO:0005741 mitochondrial outer membrane | ISS GO_REF:0000024 | ACCEPT | Summary: ISS transfer of mitochondrial outer membrane localization from human FIS1 (Q9Y3D6). Consistent with the fis-2 IDA mitochondrial localization and the tail-anchor topology. Reason: Well-supported core localization; consistent with experimental worm data. |
| GO:0005778 peroxisomal membrane | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: ISS transfer of peroxisomal membrane localization from human FIS1. Plausible by homology but not demonstrated in worm; non-core. Reason: Homology-based; no direct worm evidence for peroxisomal localization. |
| GO:0036498 IRE1-mediated unfolded protein response | HEP PMID:16184190 Genetic interactions due to constitutive and inducible gene ... | KEEP AS NON CORE | Summary: High-throughput expression-profiling (HEP) annotation: fis-2 (F13B9.8) was identified as a UPR-regulated gene in a genome-wide microarray study of the C. elegans unfolded protein response. This reflects transcriptional co-regulation of fis-2 with the IRE1/XBP-1 UPR, not a demonstrated functional role of fis-2 within the UPR pathway. fis-2 is not named in the paper's text (the evidence is in supplementary tables), so no verbatim supporting quote is available. Reason: Expression-based (HEP) association only; being a UPR target gene does not establish that fis-2 acts in the UPR. Retain as a peripheral, non-core annotation reflecting transcriptional responsiveness, not core function. |
| GO:0005739 mitochondrion | IDA PMID:18722182 Caenorhabditis elegans drp-1 and fis-2 regulate distinct cel... | ACCEPT | Summary: Direct experimental (IDA) mitochondrial localization of FIS-2 curated by WormBase from Breckenridge et al. 2008. This is the strongest, fis-2-specific evidence for FIS-2 localization and anchors the core cellular component. The cached record is abstract-only, so the localization figure is not quotable here; the annotation is retained on the WormBase curator's IDA assignment. Reason: Direct experimental assay in C. elegans (WormBase IDA); the experimental basis for FIS-2 mitochondrial localization. |
| GO:0006915 apoptotic process | IGI PMID:18722182 Caenorhabditis elegans drp-1 and fis-2 regulate distinct cel... | ACCEPT | Summary: The only individually demonstrated fis-2 function. Genetic-interaction (IGI) evidence: fis-2 has a minor pro-apoptotic role, revealed in sensitized genetic backgrounds, acting downstream of the CED-3 caspase and independently of DRP-1 and CED-9 to promote elimination of mitochondria in dying cells and facilitate cell-death execution. The effect is minor (fis-2 single mutants show only small changes in cell-corpse numbers), so this is a genuine but non-essential role. Reason: Experimentally supported, fis-2-specific role in cell-death execution (IGI with ced-3). This is the best-characterized function of fis-2 and should be retained. Supporting Evidence: PMID:18722182 minor proapoptotic roles for drp-1 and fis-2, a homolog of human Fis1, are revealed in sensitized genetic backgrounds PMID:18722182 reveal distinct roles for drp-1 and fis-2 as mediators of cell-death execution downstream of caspase activation |
| GO:0000423 mitophagy | IGI PMID:24196833 Mutations in Fis1 disrupt orderly disposal of defective mito... | NEW | Summary: Proposed new annotation (not currently in GOA). Genetic loss of both FIS1 paralogs (the fis-1;fis-2 double mutant) disrupts the orderly disposal of defective mitochondria: mitochondrial stress induces accumulation of large LGG-1/LC3-positive autophagic aggregates containing mitochondrial remnants and DRP-1, and these aggregates/mitophagosome spots are suppressed by drp-1, mff, and pink-1 mutations. fis-2 is genetically part of this pathway (drp-1 fis-1 fis-2 and pink-1 fis-1 fis-2 triple mutants). IMPORTANT: the disposal phenotype was only demonstrated in the fis-1;fis-2 double, so fis-2's contribution is redundant with fis-1, not shown to be fis-2-autonomous. Reason: Captures the experimentally supported biological process in which fis-2 participates (coupling stress-induced fission to mitophagic disposal), currently absent from GOA. Annotated as IGI (redundancy with fis-1) to reflect that the phenotype requires loss of both paralogs. Supporting Evidence: PMID:24196833 pink-1 single and pink-1 fis-1 fis-2 triple mutant animals had no colocalizing spots, as expected for a complete block of mitophagy PMID:24196833 a drp-1 fis-1 fis-2 triple mutant strain also had reduced number and size of aggregates when treated with Paraquat |
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Download this section (compressed HTML)Q: Does a fis-2 single loss-of-function (independent of fis-1) measurably alter mitophagic flux or mitochondrial quality control, or is fis-2 fully redundant with fis-1?
Q: Does C. elegans FIS-2 physically interact with DRP-1 or MAM/ER-mitochondria components, and does it use an adaptor intermediate, as mammalian FIS1 does under stress?
Q: Do the two worm FIS1 paralogs partition functionally (fis-2 toward cell-death-execution disposal, fis-1 toward UVC/mtDNA-damage removal), or are they interchangeable?
Experiment: Compare stage-resolved mitophagy flux (mito-Rosella / mtKeima or LGG-1 puncta maturation) after Paraquat or antimycin A across wild-type, fis-2 single, fis-1 single, and fis-1;fis-2 double mutants, with allele-specific rescue, to isolate any fis-2-autonomous step.
Hypothesis: FIS-2 makes a distinct, non-redundant contribution to stress-induced mitophagy that is masked by the paralog fis-1 in double-mutant analyses.
Experiment: Use proximity labeling (TurboID) or co-immunoprecipitation of endogenously tagged FIS-2 from C. elegans under basal and mitochondrial-stress conditions to identify DRP-1, MAM, and autophagy/apoptosis interactors and to test for adaptor intermediates, comparing the FIS-2 and FIS-1 interactomes.
Hypothesis: Worm FIS-2 has direct or adaptor-mediated partners at the mitochondrial outer membrane / ER-mitochondrial interface.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The molecular activity of FIS-2 is entirely undetermined experimentally. Its only molecular-function annotation ("molecular adaptor activity") is a phylogenetic (IBA) propagation from the FIS1 family; no C. elegans FIS-2 protein has been shown to bind DRP-1, a fission adaptor, lipid, or any other partner, and no biochemical or structural study of worm FIS-2 exists.
OPEN BIOLOGY MF_DARK
What is known: Established: FIS-2 is a FIS1-family tail-anchored protein of the mitochondrial outer membrane (IDA mitochondrion; ISS/tail-anchor topology) that participates genetically in mitochondrial disposal. Unknown: what FIS-2 actually does at the molecular level - its direct binding partner(s) and the biochemical activity, if any, beyond membrane anchoring.
Significance: FIS1 has long been presumed a fission receptor; worm (and mammalian) data reassign the family to mitochondrial quality-control roles whose mechanism is unresolved. For the fis-2 paralog specifically, even the assumed adaptor activity is untested, making it a genuinely dark molecular function relevant to PINK1/Parkin quality-control biology.
Provenance (the field's own admissions):
Gap: Whether FIS-2 has any non-redundant, fis-2-autonomous role in mitochondrial disposal/mitophagy is unresolved. Every mitophagy/LGG-1-aggregate phenotype was scored in the fis-1(tm1867); fis-2(gk414) double mutant (the "Fis1 mutant"), and both single mutants have wild-type mitochondrial morphology, so the individual contribution of fis-2 to orderly disposal (versus complete redundancy with, or dispensability relative to, fis-1) has not been measured.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: Established: the fis-1;fis-2 double mutant accumulates stress-induced LGG-1 aggregates and lies genetically downstream of MFF/DRP-1 and PINK-1. Unknown: what a fis-2 single loss-of-function does to mitophagic flux, and whether fis-2 contributes anything the paralog fis-1 does not.
Significance: Distinguishing redundant from specialized paralog functions is required before fis-2 can be assigned a specific mitophagy role rather than a family-level one, and determines whether fis-2 is a distinct quality-control node or a fis-1 backup.
Provenance (the field's own admissions):
Gap: The direct binding partners and interactome of C. elegans FIS-2 are unmapped. It is unknown whether FIS-2 binds DRP-1 (directly or via an adaptor intermediate analogous to yeast Mdv1/Caf4), whether it joins an ER-mitochondria (MAM) fission/disposal complex as mammalian FIS1 does under stress, and which downstream autophagy or apoptotic-machinery factors it engages.
OPEN BIOLOGYCURATION MF_DARK
What is known: Established (in mammals): stress induces a DRP-1-FIS1 co-immunoprecipitable complex that also contains ER/MAM proteins. Unknown (for worm FIS-2): its physical interactors, and whether the mammalian complex applies to this paralog.
Provenance (the field's own admissions):
Gap: The mechanism and biological significance of the fis-2-specific pro-apoptotic role are unknown. fis-2 promotes elimination of mitochondria in dying cells downstream of CED-3 and independently of DRP-1/CED-9, but whether this uses the same membrane machinery as its (redundant) mitophagy role, why the effect is only detectable in sensitized backgrounds, and why this activity is foregrounded for fis-2 but not fis-1, are all unresolved.
OPEN BIOLOGY MF_DARK
What is known: Established: in sensitized backgrounds fis-2 has a minor, CED-3-dependent, DRP-1/CED-9-independent pro-apoptotic effect on mitochondrial elimination. Unknown: the molecular basis of this activity and its relationship to the paralog-redundant mitophagy pathway.
Provenance (the field's own admissions):
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