Mcr1 is the Saccharomyces cerevisiae mitochondrial NADH-cytochrome b5 reductase. A single MCR1 translation product is split into p34 and p32 forms: the p34 form arrests in and anchors at the mitochondrial outer membrane, while the p32 form is processed by the inner-membrane protease and released into the mitochondrial intermembrane space. The enzyme reduces cytochrome b5 from NADH and contributes to the D-erythroascorbate antioxidant system, but its in vivo relationship to ER-localized sterol 14-demethylation is unresolved because budding yeast also carries the microsomal paralog Cbr1. Mcr1 can weakly reduce the Dph3 electron carrier in vitro.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003954 NADH dehydrogenase activity | IDA PMID:27694803 Cbr1 is a Dph3 reductase required for the tRNA wobble uridin... | KEEP AS NON CORE | Summary: Mcr1 can use NADH to reduce the Dph3 electron carrier in vitro, but the cited study identifies Cbr1 as the major physiological Dph3 reductase and describes Mcr1 as a slower auxiliary reductase in that assay. Reason: The broad NADH dehydrogenase term is compatible with an NADH-dependent flavoprotein reductase and is directly supported by the recombinant Mcr1 assay in PMID:27694803. It should be kept as a non-core annotation because the assayed Dph3-reductase activity is secondary to Mcr1's established NADH-cytochrome-b5 reductase activity, and the paper itself concluded that Cbr1 is the major physiological reductase for Dph3-dependent tRNA wobble uridine modification. Supporting Evidence: PMID:27694803 both Mcr1 and Ncp1 reduced Dph3 at a slower rate compared to that of Cbr1 under similar reaction conditions PMID:27694803 we identified Cbr1 as the major physiological reductase of Dph3 |
| GO:0003954 NADH dehydrogenase activity | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: ARBA transfers the generic NADH dehydrogenase activity. Mcr1 is unequivocally an NADH-dependent flavoprotein reductase, so the parent activity is true but less informative than the cytochrome-b5 reductase terms. Reason: Mcr1 oxidizes NADH while reducing ferricytochrome b5 and can also reduce Dph3 in vitro, so an NADH dehydrogenase parent activity is not a bad transfer. The term is generic compared with GO:0090524 and GO:0004128, so this ARBA row should be retained only as a non-core, generically defined sibling activity rather than as the specific NADH-cytochrome-b5 reductase chemistry. Propagation Review Root cause: NO FAILURE NON CORE Failure modes: GRANULARITY MISMATCH Sources checked: ARBA:ARBA00088748 SUPPORTS TRANSFER ARBA infers a true but broad NADH-dependent oxidoreductase activity that does not capture Mcr1's cytochrome-b5 acceptor specificity. Supporting Evidence: PMID:8001120 The S. cerevisiae gene MCR1 encodes two mitochondrial isoforms of NADH-cytochrome b5 reductase. PMID:27694803 both Mcr1 and Ncp1 reduced Dph3 at a slower rate compared to that of Cbr1 under similar reaction conditions |
| GO:0004128 cytochrome-b5 reductase activity, acting on NAD(P)H | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT placed cytochrome-b5 reductase activity at PTN000452207, on the mixed eukaryotic MCR1/PGA3 branch of PTHR19370, with budding-yeast MCR1 and PGA3 as experimental descendant seeds. Reason: This is the core activity of Mcr1. The PAINT node assignment is phylogenetically plausible for the NADH-cytochrome-b5 reductase 2 subfamily that contains S. cerevisiae Mcr1, and the target itself is one of the experimental descendants used to ground the node, which is expected and not circular for IBA. Direct experimental work on MCR1 also named the product NADH-cytochrome b5 reductase, so there is no target-specific evidence of loss or divergence. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN000452207 · mixed eukaryotic MCR1/PGA3 node in PTHR19370 SUPPORTS TRANSFER IBD node asserting GO:0004128 on the mixed MCR1/PGA3 branch; the descendant seeds include SGD:S000001633 for MCR1 itself and SGD:S000004594 for yeast PGA3. Supporting Evidence: file:interpro/panther/PTHR19370/PTHR19370-paint.tsv PTHR19370 PTN000452207 GO:0004128 F IBD false PMID:8001120 The S. cerevisiae gene MCR1 encodes two mitochondrial isoforms of NADH-cytochrome b5 reductase. |
| GO:0004128 cytochrome-b5 reductase activity, acting on NAD(P)H | IDA PMID:8001120 Incomplete arrest in the outer membrane sorts NADH-cytochrom... | ACCEPT | Summary: PMID:8001120 identified MCR1 as encoding mitochondrial isoforms of NADH-cytochrome b5 reductase and directly supports the cytochrome-b5 reductase molecular-function annotation. Reason: The term matches the experimentally characterized activity reported in the cited primary MCR1 paper and represents Mcr1's core flavoprotein reductase function. Supporting Evidence: PMID:8001120 The S. cerevisiae gene MCR1 encodes two mitochondrial isoforms of NADH-cytochrome b5 reductase. |
| GO:0004128 cytochrome-b5 reductase activity, acting on NAD(P)H | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA infers cytochrome-b5 reductase activity, a core Mcr1 activity supported independently by direct biochemical work and PAINT. Reason: The ARBA model agrees with both the experimentally assigned Mcr1 NADH-cytochrome-b5 reductase activity and the PTHR19370 PAINT node that places this function on the mixed MCR1/PGA3 branch. No source-side or target-side failure is apparent. Propagation Review Root cause: NO FAILURE CORE Sources checked: ARBA:ARBA00027922 SUPPORTS TRANSFER ARBA infers cytochrome-b5 reductase activity for a protein whose activity is directly established in budding yeast MCR1. Supporting Evidence: PMID:8001120 The S. cerevisiae gene MCR1 encodes two mitochondrial isoforms of NADH-cytochrome b5 reductase. |
| GO:0005515 protein binding | IPI PMID:16606443 Comparative analysis of Saccharomyces cerevisiae WW domains ... | REMOVE | Summary: PMID:16606443 used purified WW-domain probes, including Rsp5 WW domains, on yeast protein microarrays and generated a large in vitro WW-domain interaction map. The cache does not provide evidence for a more specific Mcr1 molecular function downstream of the reported Rsp5 interaction. Reason: Generic protein binding is not informative for Mcr1 function, and the Rsp5 WW-domain hit should not be converted into an enzymatic or adaptor activity without direct evidence of what the interaction does. Removing this annotation does not dispute the reported physical interaction; it only rejects GO:0005515 as a useful molecular-function assertion for Mcr1. Supporting Evidence: PMID:16606443 We used protein microarray technology to generate a protein interaction map for 12 of the 13 WW domains present in proteins of the yeast S. cerevisiae. |
| GO:0005739 mitochondrion | HDA PMID:16823961 Toward the complete yeast mitochondrial proteome: multidimen... | ACCEPT | Summary: The high-throughput PROMITO mitochondrial proteome row is consistent with the direct Mcr1 localization literature: Mcr1 is a mitochondrial protein split between the outer membrane and intermembrane space. Reason: PMID:16823961 is a large-scale mitochondrial proteomics study, and its mitochondrion assignment agrees with the direct PMID:8001120 evidence that MCR1 encodes two mitochondrial NADH-cytochrome-b5 reductase isoforms. The general mitochondrion term is therefore a correct parent component. Supporting Evidence: PMID:16823961 A total of 851 different proteins (PROMITO dataset) were identified by use of multidimensional LC-MS/MS PMID:8001120 The S. cerevisiae gene MCR1 encodes two mitochondrial isoforms of NADH-cytochrome b5 reductase. |
| GO:0005739 mitochondrion | HDA PMID:24769239 Quantitative variations of the mitochondrial proteome and ph... | ACCEPT | Summary: Quantitative proteomics of isolated mitochondria during fermentative and respiratory growth recovered Mcr1 as mitochondrial, matching the direct evidence for Mcr1 outer-membrane and intermembrane-space forms. Reason: The broad mitochondrion assignment from PMID:24769239 is consistent with the established dual submitochondrial localization of Mcr1 and should be retained. Supporting Evidence: PMID:24769239 we performed an overall quantitative proteomic and phosphoproteomic study of isolated mitochondria PMID:8001120 The S. cerevisiae gene MCR1 encodes two mitochondrial isoforms of NADH-cytochrome b5 reductase. |
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | ACCEPT | Summary: The mitochondrion IBA is asserted at the same PTN000452207 PAINT node that carries cytochrome-b5 reductase activity in PTHR19370, with an overlapping but nonidentical MCR1-anchored seed set. Reason: Mcr1 is experimentally localized to mitochondria, with p34 anchored at the mitochondrial outer membrane and p32 released into the intermembrane space. The PAINT node transfer is therefore consistent with the target's own direct evidence and with the mitochondrial localization of this branch. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN000452207 · eukaryotic MCR1-anchored node in PTHR19370 SUPPORTS TRANSFER IBD node asserting mitochondrion from an MCR1 plus trypanosomatid seed set; direct localization evidence for Mcr1 itself confirms that the target inherited this localization. Supporting Evidence: file:interpro/panther/PTHR19370/PTHR19370-paint.tsv PTHR19370 PTN000452207 GO:0005739 C IBD false PMID:8001120 Incomplete translocation arrest in the outer membrane is a novel mechanism by which the product of a single gene is sorted into different compartments of the same organelle. |
| GO:0005741 mitochondrial outer membrane | HDA PMID:16407407 Proteomic analysis of the yeast mitochondrial outer membrane... | ACCEPT | Summary: Highly purified mitochondrial outer-membrane proteomics detected Mcr1 in the same submitochondrial compartment that PMID:8001120 had directly established for the p34 form. Reason: This high-throughput outer-membrane row is consistent with direct biochemical localization of the p34 NADH-cytochrome-b5 reductase form in the mitochondrial outer membrane. The paper's abstract is general and does not name Mcr1, but the SGD experimental annotation should not be second-guessed from an abstract-only cache when the localization is independently well established. Supporting Evidence: PMID:16407407 We used a eukaryotic model organism, the yeast Saccharomyces cerevisiae, to determine the proteome of highly purified mitochondrial outer membranes. PMID:8001120 The primary translation product has an amino-terminal matrix-targeting signal, followed by a stretch of 21 uncharged amino acids. This precursor protein is inserted into the outer membrane |
| GO:0005741 mitochondrial outer membrane | IDA PMID:16689936 Integral membrane proteins in the mitochondrial outer membra... | ACCEPT | Summary: The FEBS J study of yeast mitochondrial outer-membrane integral proteins supports Mcr1 outer-membrane localization, which agrees with the direct p34-form localization reported in PMID:8001120. Reason: The cached PMID:16689936 abstract summarizes an outer-membrane proteomics study without listing every assayed protein. Because SGD assigned this experimental row from the full text and direct work has already shown that the p34 Mcr1 form anchors in the mitochondrial outer membrane, the annotation is retained. Supporting Evidence: PMID:16689936 Integral membrane proteins in the mitochondrial outer membrane of Saccharomyces cerevisiae. PMID:8001120 This precursor protein is inserted into the outer membrane, but only about one-third of the molecules become firmly anchored to the outer face of that membrane. |
| GO:0005741 mitochondrial outer membrane | IDA PMID:8001120 Incomplete arrest in the outer membrane sorts NADH-cytochrom... | ACCEPT | Summary: PMID:8001120 directly showed that Mcr1 is sorted into two mitochondrial forms, one of which becomes firmly anchored to the outer mitochondrial membrane. Reason: The mitochondrial outer-membrane annotation captures the p34 form produced by incomplete translocation arrest and is directly supported by the cited primary MCR1 paper. Supporting Evidence: PMID:8001120 This precursor protein is inserted into the outer membrane, but only about one-third of the molecules become firmly anchored to the outer face of that membrane. |
| GO:0005741 mitochondrial outer membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt's subcellular-location vocabulary mapping to mitochondrial outer membrane reflects the experimentally supported p34 Mcr1 form. Reason: The subcellular-location mapping is a correct electronic restatement of a UniProt location that is itself backed by direct Mcr1 sorting experiments: one fraction of the Mcr1 precursor arrests and anchors in the mitochondrial outer membrane. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB-SubCell:SL-0172 SUPPORTS TRANSFER UniProt subcellular-location mapping correctly maps Mcr1's annotated p34 outer-membrane location to GO:0005741. Supporting Evidence: PMID:8001120 This precursor protein is inserted into the outer membrane, but only about one-third of the molecules become firmly anchored to the outer face of that membrane. |
| GO:0005758 mitochondrial intermembrane space | EXP PMID:22984289 Intermembrane space proteome of yeast mitochondria. | ACCEPT | Summary: The yeast IMS proteome study used soluble Mcr1 as a known Bax-released intermembrane-space marker, directly confirming Mcr1 localization in the IMS. Reason: This row is consistent with both the 2012 IMS proteome work and the original discovery that a processed Mcr1 form is released into the intermembrane space. Mcr1's dual outer-membrane/IMS localization is a well-supported consequence of incomplete translocation arrest. Supporting Evidence: PMID:22984289 IMS proteins like cytochrome c or the soluble isoform of Mcr1 were largely released into the SN PMID:8001120 The remaining molecules pass through the outer membrane into the inner membrane, are cleaved by inner membrane protease 1, and are released into the intermembrane space. |
| GO:0005758 mitochondrial intermembrane space | IDA PMID:8001120 Incomplete arrest in the outer membrane sorts NADH-cytochrom... | ACCEPT | Summary: PMID:8001120 directly established that a processed Mcr1 isoform is released into the mitochondrial intermembrane space. Reason: The intermembrane-space annotation captures the p32 Mcr1 form generated after inner membrane protease cleavage and is directly supported by the cited primary paper. Supporting Evidence: PMID:8001120 The remaining molecules pass through the outer membrane into the inner membrane, are cleaved by inner membrane protease 1, and are released into the intermembrane space. |
| GO:0005758 mitochondrial intermembrane space | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt's intermembrane-space subcellular-location mapping reflects the processed soluble Mcr1 form confirmed by the original sorting paper and by the later IMS proteome study. Reason: The GO_REF:0000044 row is an appropriate electronic mapping of UniProt's experimentally backed Mcr1 intermembrane-space location. Direct evidence shows that a processed Mcr1 isoform reaches the IMS. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB-SubCell:SL-0169 SUPPORTS TRANSFER UniProt subcellular-location mapping correctly maps the processed soluble Mcr1 form to GO:0005758. Supporting Evidence: PMID:8001120 The remaining molecules pass through the outer membrane into the inner membrane, are cleaved by inner membrane protease 1, and are released into the intermembrane space. PMID:22984289 IMS proteins like cytochrome c or the soluble isoform of Mcr1 were largely released into the SN |
| GO:0006696 ergosterol biosynthetic process | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: PAINT placed ergosterol biosynthesis at PTN000452208 in PTHR19370, seeded only by budding-yeast MCR1. The inherited assertion should be retained, but the available CYP51 reconstitution evidence used the microsomal cytochrome-b5 reductase system and does not make this a core Mcr1 role. Reason: The PTN000452208 IBD node is seeded by MCR1 itself, so this IBA is a valid self-seeded PAINT assertion rather than a circular transfer. However, the only locally available CYP51 reconstitution abstract describes the microsomal cytochrome-b5/NADH-cytochrome-b5-reductase electron-transfer system that more naturally corresponds to the ER CBR1 branch. The inherited ergosterol row is therefore kept as a supported non-core process while the in vivo Cbr1-versus-Mcr1 donor relationship remains unsettled. Propagation Review Root cause: NO FAILURE NON CORE Sources checked: PANTHER:PTN000452208 · ergosterol-biosynthesis node in PTHR19370 SUPPORTS TRANSFER IBD node asserting GO:0006696 from the self-seeded budding-yeast MCR1 row at SGD:S000001633. Supporting Evidence: file:interpro/panther/PTHR19370/PTHR19370-paint.tsv PTHR19370 PTN000452208 GO:0006696 P IBD false PMID:10622712 fungal CYP51 mediated sterol 14alpha-demethylation can be wholly and efficiently supported by the cytochrome b5/NADH cytochrome b5 reductase electron transport system |
| GO:0006696 ergosterol biosynthetic process | IDA PMID:10622712 Biodiversity of the P450 catalytic cycle: yeast cytochrome b... | KEEP AS NON CORE | Summary: The cytochrome-b5/NADH cytochrome-b5 reductase electron-transfer system can support the fungal CYP51 sterol 14-demethylation reaction in vitro. This supports retention of the SGD row, but the experiment does not resolve whether mitochondrial Mcr1 or ER Cbr1 supplies CYP51 electrons in vivo. Reason: CYP51-mediated sterol 14-demethylation is a catalytic step in ergosterol biosynthesis, and PMID:10622712 showed that the fungal reaction can be driven by cytochrome b5 plus NADH cytochrome-b5 reductase. SGD's experimental MCR1 row is retained, but the cached abstract frames this as a microsomal electron-transfer system and does not establish that Mcr1, rather than the ER-localized paralog Cbr1, is the physiological CYP51 reductase in vivo. Supporting Evidence: PMID:10622712 fungal CYP51 mediated sterol 14alpha-demethylation can be wholly and efficiently supported by the cytochrome b5/NADH cytochrome b5 reductase electron transport system |
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: InterPro signatures for the NADH-cytochrome-b5 reductase/FAD-dependent oxidoreductase fold support the very broad oxidoreductase parent term for Mcr1. Reason: Mcr1 is a FAD-containing NADH-dependent oxidoreductase, so InterPro2GO's generic GO:0016491 mapping is not wrong. It is much less informative than the cytochrome-b5 reductase activity terms and should be treated as a non-core ancestor rather than as the curated functional endpoint. Propagation Review Root cause: NO FAILURE NON CORE Failure modes: GRANULARITY MISMATCH Sources checked: InterPro:IPR001433 SUPPORTS TRANSFER InterPro signature supports Mcr1's flavoprotein oxidoreductase architecture but not a specific electron acceptor. InterPro:IPR001709 SUPPORTS TRANSFER InterPro signature supports Mcr1's NADH-dependent oxidoreductase architecture but not a specific electron acceptor. InterPro:IPR001834 SUPPORTS TRANSFER Family-level signature for NADH-cytochrome-b5 reductases; the InterPro2GO mapping only asserts the oxidoreductase ancestor. InterPro:IPR017927 SUPPORTS TRANSFER InterPro signature supports the conserved FAD/NAD(P)H reductase fold but not the specific cytochrome-b5 acceptor. Supporting Evidence: PMID:8001120 The S. cerevisiae gene MCR1 encodes two mitochondrial isoforms of NADH-cytochrome b5 reductase. |
| GO:0034599 cellular response to oxidative stress | IMP PMID:11420140 Mitochondrial NADH-cytochrome b(5) reductase plays a crucial... | KEEP AS NON CORE | Summary: Mcr1 reduces the D-erythroascorbyl free radical and mcr1 disruption makes cells hypersensitive to hydrogen peroxide and menadione, supporting a direct but secondary role in the cellular oxidative-stress response. Reason: PMID:11420140 provides perturbation evidence that Mcr1 helps maintain the yeast D-erythroascorbate antioxidant system and contributes to resistance against oxidative damage. This process row is therefore supported, but it is a condition-level consequence of the reductase chemistry rather than the most informative core molecular function of Mcr1. Supporting Evidence: PMID:11420140 the MCR1 product, mitochondrial NADH-cytochrome b(5) reductase, plays a key role in the NADH-dependent reduction of D-erythroascorbyl free radical in S. cerevisiae PMID:11420140 the mcr1 disruptant cells were hypersensitive to hydrogen peroxide and menadione |
| GO:0090524 cytochrome-b5 reductase activity, acting on NADH | IEA GO_REF:0000120 | ACCEPT | Summary: RHEA:46680 and EC:1.6.2.2 map exactly to NADH:ferricytochrome-b5 oxidoreductase activity, the specific reaction catalyzed by Mcr1. Reason: This is the best existing molecular-function term for the Mcr1 reaction: it specifies NADH rather than NADPH as the electron donor for cytochrome b5 reduction and matches the enzymatic activity described in the primary MCR1 literature. Propagation Review Root cause: NO FAILURE CORE Sources checked: RHEA:46680 SUPPORTS TRANSFER Rhea reaction for NADH-dependent reduction of ferricytochrome b5, matching Mcr1's curated EC 1.6.2.2 activity. EC:1.6.2.2 SUPPORTS TRANSFER Enzyme Commission mapping for NADH:ferricytochrome-b5 oxidoreductase, the activity experimentally assigned to budding-yeast Mcr1. Supporting Evidence: PMID:8001120 The S. cerevisiae gene MCR1 encodes two mitochondrial isoforms of NADH-cytochrome b5 reductase. |
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Download this section (compressed HTML)Q: Does Mcr1 supply electrons to CYP51 in vivo, or is the ER-localized paralog Cbr1 the physiological reductase for sterol 14-demethylation in budding yeast?
Q: Which physiological cytochrome-b5-dependent reactions are fed by the outer-membrane p34 form versus the soluble intermembrane-space p32 form of Mcr1?
Q: Does Mcr1 reduce Dph3 in vivo only under high electron demand or Cbr1 limitation, or does the in vitro Dph3 activity remain physiologically marginal in all normal contexts?
Experiment: Build separation-of-localization Mcr1 variants that force the reductase to the mitochondrial outer membrane or intermembrane space, then measure ergosterol intermediates, D-erythroascorbyl-radical reduction, and Dph3-dependent mcm5s2U/diphthamide readouts.
Mitochondrial NADH-cytochrome b5 reductase; involved in ergosterol biosynthesis
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