sod-3 encodes a mitochondrial matrix manganese superoxide dismutase (MnSOD) of Caenorhabditis elegans. The nuclear-encoded 218-residue precursor carries an N-terminal mitochondrial transit peptide that directs import into the mitochondrial matrix, where the mature chain adopts the characteristic iron/manganese superoxide dismutase fold and binds one catalytic Mn(2+) ion per subunit. The enzyme dismutates the superoxide anion radical, a by-product of the respiratory electron transport chain, into hydrogen peroxide and molecular oxygen (2 superoxide + 2 H+ -> H2O2 + O2; EC 1.15.1.1), providing antioxidant defense within the organelle. SOD-3 is the paralog of the constitutive, quantitatively dominant mitochondrial MnSOD SOD-2, to which it is ~86% identical; unlike sod-2, sod-3 is expressed at very low basal levels and is strongly induced by the DAF-16/FOXO branch of insulin/IGF-1 signalling, making it a canonical DAF-16 target gene and a widely used transcriptional reporter of insulin/IGF-1 pathway activity and stress. Basal expression is seen in the pharynx and rectum, expanding to vulva, body-wall muscle and hypodermis upon thermal stress. Like SOD-2, SOD-3 physically associates with the mitochondrial respiratory supercomplex I:III:IV, positioning it to scavenge superoxide near its site of production, and it may help stabilize or locally protect the supercomplex, particularly when SOD-2 is absent. Loss of sod-3 alone does not alter lifespan or the activity of the respiratory complexes, but sod-3 shows distinct, non-redundant genetic interactions with electron-transport-chain mutants compared with sod-2.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic (IBA) inference that SOD-3 is active in the mitochondrion. Correct but less specific than the mitochondrial matrix, where this MnSOD acts. Reason: Consistent with the N-terminal mitochondrial transit peptide and with experimental localization, but generic relative to mitochondrial matrix (GO:0005759), which is retained as the core location. Kept as a correct, less-specific companion term. |
| GO:0004784 superoxide dismutase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) inference of superoxide dismutase activity. This is the core molecular function of SOD-3 and is directly confirmed experimentally. Reason: SOD-3 is an experimentally validated manganese superoxide dismutase; the IBA call is fully concordant with the IDA evidence (PMID:9353332, in which the SOD-3 protein was expressed and assayed) and with the Fe/Mn-SOD family assignment. Core function. Supporting Evidence: PMID:9353332 The expressed enzymes, which were not inhibited by hydrogen peroxide or cyanide, are dimeric, show quite different electrophoretic mobilities and isoelectric points, but exhibit comparable specific activities. |
| GO:0030145 manganese ion binding | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) inference of manganese ion binding, the catalytic cofactor of this MnSOD. Concordant with the UniProt Mn(2+) cofactor and Mn-ligand residues (His50, His98, Asp179, His183) and with the enzyme's Mn-type biochemistry. Reason: Correct and specific metal-binding function: this is a Mn-type (not Fe- or Cu/Zn-type) SOD, insensitive to hydrogen peroxide and cyanide, binding one Mn(2+) per subunit. Core cofactor-binding function. Supporting Evidence: PMID:9353332 The expressed enzymes, which were not inhibited by hydrogen peroxide or cyanide, are dimeric, show quite different electrophoretic mobilities and isoelectric points, but exhibit comparable specific activities. |
| GO:0004784 superoxide dismutase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic (IEA) assignment of superoxide dismutase activity from combined automated methods (ARBA/InterPro/EC/RHEA mapping). Redundant with the experimental IDA and phylogenetic IBA calls for the same core function. Reason: Same core molecular function as the IDA/IBA annotations; the EC 1.15.1.1 / RHEA:20696 mapping is correct for this enzyme. Retained as concordant supporting evidence. |
| GO:0005739 mitochondrion | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Electronic (UniProt SubCell) mitochondrial localization, matching the UniProt subcellular location and the N-terminal mitochondrial transit peptide. Correct but generic relative to the mitochondrial matrix, the specific site of action. Reason: Correct localization but less specific than mitochondrial matrix (GO:0005759), which is retained as the core location for this matrix MnSOD. Kept as a correct, less-specific companion. Supporting Evidence: PMID:9353332 Both deduced protein sequences contain the expected N-terminal mitochondrial transit peptides. |
| GO:0006801 superoxide metabolic process | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Electronic (InterPro2GO) assignment to the general superoxide metabolic process. Correct but less specific than removal of superoxide radicals (GO:0019430), which is the experimentally supported process. Reason: Accurate parent process, but subsumed by the more specific removal of superoxide radicals term retained as core. Kept as a correct, less-informative companion. |
| GO:0046872 metal ion binding | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Electronic (InterPro2GO) generic metal-ion-binding annotation. Subsumed by the specific manganese ion binding (GO:0030145) term. Reason: Correct but uninformative parent of manganese ion binding; the specific Mn(2+) term is retained as core. Kept as a non-core, less-specific companion. |
| GO:0098803 respiratory chain complex | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: Electronic (ARBA) assertion that SOD-3 is part_of the respiratory chain complex. SOD-3 is a soluble matrix MnSOD that physically associates with supercomplex I:III:IV (see the experimental located_in annotation), but it is not a canonical structural subunit of an electron-transport complex, so the part_of qualifier overstates the relationship. Reason: The experimentally supported relationship is association/co-localization with the I:III:IV supercomplex (PMID:23895727, located_in), consistent with local superoxide scavenging and possible supercomplex stabilization; SOD-3 does not carry out or structurally constitute electron transport. The automated part_of qualifier is an over-generalization, so this is retained only as a non-core companion to the experimental located_in annotation rather than as evidence of structural subunit membership. Supporting Evidence: PMID:23895727 In addition, the results show that SOD-3 is also localized to the I:III:IV supercomplex. |
| GO:0098803 respiratory chain complex | IDA PMID:23895727 Novel interactions between mitochondrial superoxide dismutas... | KEEP AS NON CORE | Summary: Direct assay (IDA) showing SOD-3 co-localizes with the mitochondrial supercomplex I:III:IV by blue-native gel Western blotting. A genuine, sod-3-specific localization finding, but a peripheral association rather than the enzyme's core identity; the part_of qualifier overstates it (SOD-3 is not a structural ETC subunit). Reason: Experimentally supported association of SOD-3 with the I:III:IV supercomplex, consistent with local scavenging of superoxide at its site of production and a possible supercomplex-stabilizing role (the authors note SOD-3 may substitute for this SOD-2 function when SOD-2 is absent). Retained as a real but non-core localization; SOD-3's core identity is a matrix MnSOD, not a structural ETC subunit. Supporting Evidence: PMID:23895727 In addition, the results show that SOD-3 is also localized to the I:III:IV supercomplex. |
| GO:0004784 superoxide dismutase activity | IDA PMID:9353332 Cloning, expression, and characterization of two manganese s... | ACCEPT | Summary: Direct assay (IDA) of superoxide dismutase activity: the mature SOD-3 protein was expressed in SOD-deficient E. coli and shown to be an active, Mn-type dismutase. This is the primary experimental evidence for the core function. Reason: Gold-standard experimental support for the defining molecular function. Insensitivity to hydrogen peroxide and cyanide confirms the Mn-type (not Fe- or Cu/Zn-type) mechanism. Both worm MnSODs were expressed and assayed in this study. Core function. Supporting Evidence: PMID:9353332 The expressed enzymes, which were not inhibited by hydrogen peroxide or cyanide, are dimeric, show quite different electrophoretic mobilities and isoelectric points, but exhibit comparable specific activities. |
| GO:0019430 removal of superoxide radicals | IMP PMID:9353332 Cloning, expression, and characterization of two manganese s... | ACCEPT | Summary: SOD-3 removes superoxide radicals: heterologous expression of the worm enzyme rescued SOD-deficient E. coli from methyl-viologen (paraquat) oxidative stress. This is the core biological process the enzyme serves. Reason: Functionally correct core process. The evidence is heterologous complementation (protection of SOD-null E. coli against a superoxide generator) rather than a worm sod-3 loss-of-function phenotype, but it directly demonstrates superoxide-radical removal by the SOD-3 protein. Core process. Supporting Evidence: PMID:9353332 Both proteins were shown to be active in E. coli, providing similar protection against methyl viologen-induced oxidative stress. |
| GO:0005739 mitochondrion | IDA PMID:17894411 The MAP kinase JNK-1 of Caenorhabditis elegans: location, ac... | KEEP AS NON CORE | Summary: Direct-assay (IDA) mitochondrial localization of SOD-3 (WormBase, from the full text of Wolf et al. 2008, the paper UniProt cites for the SUBCELLULAR LOCATION). Correct but generic relative to the mitochondrial matrix term. Reason: Mitochondrial localization of this MnSOD is biologically unambiguous and concordant with the transit peptide and matrix localization; retained as a correct, less-specific companion to mitochondrial matrix (GO:0005759). The cached abstract of PMID:17894411 concerns JNK-1/DAF-16 signalling and does not itself state the mitochondrial localization (it is in the full text), so no sod-3-specific verbatim localization quote is available. |
| GO:0005759 mitochondrial matrix | ISS PMID:9353332 Cloning, expression, and characterization of two manganese s... | NEW | Summary: Proposed more-specific localization: as a nuclear-encoded manganese superoxide dismutase bearing an N-terminal mitochondrial transit peptide, SOD-3 is imported into and acts within the mitochondrial matrix, the compartment where Fe/Mn-SOD family MnSODs reside. This refines the generic mitochondrion annotations. Reason: The existing localization annotations (IBA, SubCell IEA, IDA) all use the generic GO:0005739 mitochondrion. The transit peptide and Mn-SOD family assignment place SOD-3 specifically in the mitochondrial matrix, which is retained as the core location. Added as a NEW, more-specific companion inferred from sequence features (ISS) rather than by rewriting the trusted GOA ids. Supporting Evidence: PMID:9353332 Both deduced protein sequences contain the expected N-terminal mitochondrial transit peptides. |
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Download this section (compressed HTML)Q: Does the very low basal level of SOD-3 make a material contribution to superoxide scavenging in the wild-type mitochondrial matrix, or is SOD-3 essentially a stress- and DAF-16-inducible reserve isoform that matters mainly when SOD-2 is limiting?
Q: What is the mechanistic basis of the division of labour between the two nearly identical mitochondrial MnSODs, SOD-2 (constitutive, dominant) and SOD-3 (DAF-16-inducible, low basal), given their distinct and sometimes opposite genetic interactions with electron-transport-chain mutants?
Q: Is the DAF-16-driven induction of sod-3 causally protective (extending lifespan or raising stress resistance), or is it primarily a transcriptional marker of DAF-16 activity, given that eliminating both mitochondrial SODs does not suppress daf-2 longevity?
Experiment: Isoform-resolved, tagged-allele quantification and localization of SOD-3 versus SOD-2 across tissues, developmental stages and stress/DAF-16-active conditions, to define SOD-3's basal contribution and its supercomplex occupancy relative to SOD-2.
Experiment: Catalytically-dead (Mn-ligand mutant) versus wild-type sod-3 rescue in sod-3 and sod-2;sod-3 backgrounds, scoring supercomplex I:III:IV formation, complex I/II activity and ROS damage, to test whether SOD-3 acts by scavenging or as a catalysis-independent supercomplex stabilizer.
Experiment: Redox-biosensor (mitochondrial roGFP/HyPer) measurement of matrix superoxide/H2O2 in wild type, sod-2, sod-3 and sod-2;sod-3 under DAF-16-activating conditions to resolve whether sod-3 induction lowers matrix superoxide and whether that change is required for the associated stress-resistance/longevity phenotypes.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The functional division of labour between the two nearly identical mitochondrial manganese superoxide dismutases, SOD-3 and SOD-2, is undefined. It is unknown why C. elegans maintains a low-basal, DAF-16-inducible MnSOD (sod-3) alongside a constitutive, dominant one (sod-2), what distinguishes their substrates or sub-mitochondrial contexts, and why loss of sod-3 versus sod-2 produces different (sometimes opposite) genetic interactions with electron-transport-chain mutants.
OPEN BIOLOGY BP_DARK
What is known: It is established that sod-2 and sod-3 are ~86% identical mitochondrial MnSODs, that sod-3 is normally expressed at very low levels and induced by DAF-16/insulin signalling whereas sod-2 is constitutive and dominant, that both localize to supercomplex I:III:IV, that loss of sod-3 (unlike sod-2) does not decrease complex I respiration and leaves blue-native supercomplex profiles normal, and that sod-3 has distinct genetic interactions with gas-1, mev-1 and isp-1. What is not established is the mechanistic basis of the non-redundancy at the level of substrate, sub-mitochondrial context, or partner; the authors of the key ETC study explicitly state that the sod-3/supercomplex interaction was still under investigation.
Significance: Two paralogous mitochondrial MnSODs with divergent, non-redundant phenotypes are a clean model for how gene duplication partitions an antioxidant function, and the division of labour determines which isoform is limiting under which stress or signalling state.
Provenance (the field's own admissions):
Gap: Whether the DAF-16-driven induction of sod-3 is causally protective (increasing oxidative-stress resistance or extending lifespan) or is primarily a transcriptional readout of DAF-16 activity is unresolved. It is also unknown whether the very low basal SOD-3 level makes a material contribution to matrix superoxide scavenging in otherwise wild-type animals.
OPEN BIOLOGY BP_DARK
What is known: It is established that sod-3 is a canonical DAF-16 target gene, expressed at very low basal levels and strongly induced upon DAF-16 nuclear translocation (including JNK-1-promoted translocation under thermal stress), and that its induction correlates with stress-resistant, long-lived states. However, simultaneous elimination of both mitochondrial SODs (sod-2 and sod-3) does not suppress the long lifespan of daf-2, arguing that sod-3 induction is not required for insulin/IGF-1-pathway longevity. What is not established is whether sod-3 induction confers a measurable protective benefit on its own, or whether it is chiefly a marker of the DAF-16 program.
Significance: sod-3 is one of the most widely used transcriptional reporters of DAF-16/FOXO activity in aging and stress research; establishing whether its induction is protective or merely a marker directly affects how thousands of studies interpret sod-3 reporter readouts.
Provenance (the field's own admissions):
Gap: Whether SOD-3's association with the respiratory supercomplex I:III:IV reflects only local superoxide scavenging at the site of ROS production, or whether SOD-3 can also act as a catalysis-independent structural stabilizer of the supercomplex (a role proposed for the mitochondrial SODs, and one SOD-3 may assume when SOD-2 is absent), is undetermined.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: It is established that SOD-3 co-localizes with the I:III:IV supercomplex by blue-native gel and that, unlike sod-2 loss, sod-3 loss does not by itself decrease complex I respiration or perturb the supercomplex profile. The open question is causality/mechanism: the mtSODs are proposed to stabilize the supercomplex, and the limited rise in supercomplex ROS damage in sod-2 animals suggests SOD-3 may perform this function when supercomplex levels fall, but a catalysis-independent structural role for SOD-3 remains unproven.
Significance: Distinguishing a scavenging role from a structural role would determine whether SOD-3 is a modular antioxidant or an integral stabilizer of the electron transport chain, with implications for how supercomplex integrity is maintained when the dominant SOD-2 is lost.
Provenance (the field's own admissions):
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