oca8 (SPCC16A11.10c) encodes one of two microsomal-type cytochrome b5 proteins in fission yeast (the other is cyb502). It has an N-terminal heme-binding cytochrome b5 domain and a C-terminal tail anchor for insertion into the endoplasmic reticulum membrane. By homology with S. cerevisiae Cyb5 it is a one-electron carrier that accepts electrons from NADH-cytochrome b5 reductase (Cbr1) and passes them to ER oxygenases of ergosterol synthesis, such as the C-5 sterol desaturases (Erg31/Erg32) and the C-4 methyl sterol oxidase. A genome-wide C-terminal YFP-tag screen reported a mitochondrial signal. The protein has not been characterised biochemically, and its relation to Cyb502 is unknown. Deletion is viable.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005739 mitochondrion | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Keyword mapping of the UniProt "Mitochondrion" location, which UniProt attributes to the S. pombe ORFeome YFP screen. That screen tagged ORFs at the C terminus, which for a tail-anchored protein such as Oca8 can mistarget the fusion. PomBase did not make a matching HDA annotation. Some cytochromes b5 do reside in the mitochondrial outer membrane, so a mitochondrial pool is possible but unconfirmed. Reason: Single high-throughput observation with a known tagging caveat; not the location of the inferred core activity. Supporting Evidence: file:SCHPO/oca8/oca8-uniprot.txt Cytoplasmic side {ECO:0000250}. Mitochondrion PMID:16823372 First, we evaluated ORF prediction in the fission yeast genome project by expressing each ORF tagged at the 3' terminus. |
| GO:0005789 endoplasmic reticulum membrane | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT node PTN001064421 seeded by S. cerevisiae Cyb5 (ER membrane). Oca8 is a tail-anchored b5 within this clade. Reason: Core location by sound phylogenetic inference, consistent with the C-terminal TM anchor. Supporting Evidence: file:SCHPO/oca8/oca8-uniprot.txt CC -!- SUBCELLULAR LOCATION: Endoplasmic reticulum membrane {ECO:0000250}; |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Keyword mapping of the by-similarity ER membrane location; consistent with the tail anchor. Reason: Core location. Supporting Evidence: file:SCHPO/oca8/oca8-uniprot.txt CC -!- SUBCELLULAR LOCATION: Endoplasmic reticulum membrane {ECO:0000250}; |
| GO:0006696 ergosterol biosynthetic process | ISO GO_REF:0000024 | ACCEPT | Summary: PomBase transfer from S. cerevisiae Cyb5, which supplies electrons to the sterol C-4 methyl oxidase and can support CYP51. Oca8 is a direct homolog; cytochrome b5 does part of the work of these oxidation steps by reducing the enzymes. Reason: Sound ortholog transfer; matches the PomBase GO-CAM (Oca8 feeding Erg31/Erg32). Supporting Evidence: PMID:6163470 the 14CO2 formation from the 14C-labeled lanosterol was inhibited by antibodies to yeast cytochrome b5 |
| GO:0009055 electron transfer activity | ISO GO_REF:0000024 | ACCEPT | Summary: PomBase transfer of electron transfer activity from S. cerevisiae Cyb5. Oca8 retains the heme-binding b5 fold and the heme-binding-site signature. Reason: Core MF of a cytochrome b5. Supporting Evidence: PMID:10622712 can be wholly and efficiently supported by the cytochrome b5/NADH cytochrome b5 reductase electron transport system file:SCHPO/oca8/oca8-uniprot.txt CC -!- FUNCTION: Membrane bound hemoprotein which function as an electron |
| GO:0020037 heme binding | IBA GO_REF:0000033 | ACCEPT | Summary: Heme binding assigned at an ancestral cytochrome b5 node; Oca8 keeps both heme-ligand positions (38 and 62). Reason: Correct; heme is the redox centre. Supporting Evidence: file:SCHPO/oca8/oca8-uniprot.txt DR InterPro; IPR018506; Cyt_B5_heme-BS. |
| GO:0020037 heme binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the cytochrome b5 heme-binding-site signature (IPR018506), which Oca8 matches. Reason: Correct. Supporting Evidence: file:SCHPO/oca8/oca8-uniprot.txt DR InterPro; IPR018506; Cyt_B5_heme-BS. |
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Download this section (compressed HTML)Q: Is Oca8 partly mitochondrial, as the C-terminally tagged ORFeome fusion suggested, and does it serve acceptors different from those of Cyb502?
Experiment: Tag Oca8 at the N terminus (preserving the tail anchor) and co-localise it with ER and mitochondrial markers; combine with cyb502 oca8 double deletion and sterol GC-MS profiling.
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