erg6 (SPBC16E9.05) encodes the sterol 24-C-methyltransferase of Schizosaccharomyces pombe, a SAM-dependent methyltransferase of the Erg6/SMT family. It transfers a methyl group from S-adenosyl-L-methionine to C-24 of zymosterol to form fecosterol (EC 2.1.1.41), adding the C-28 methyl group that distinguishes ergosterol from cholesterol. Sterol analyses of Schizosaccharomyces suggest that C-24 methylation can also occur on lanosterol, giving eburicol, so erg6 may act at two points of the pathway. erg6 deletion mutants are viable but make no ergosterol, accumulate lanosterol, have abnormal morphology, and are sensitive to cycloheximide and staurosporine; erg6 is induced under anaerobiosis via the SREBP Sre1. The enzyme is associated with the ER.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003838 sterol 24-C-methyltransferase activity | IBA GO_REF:0000033 | ACCEPT | Summary: IBA sterol 24-C-methyltransferase from PTN002226525 (fungal and plant SMT clade, incl. S. cerevisiae ERG6). erg6 is the single S. pombe SMT and its deletion abolishes ergosterol; correct core activity. Supporting Evidence: file:SCHPO/erg6/erg6-uniprot.txt Reaction=zymosterol + S-adenosyl-L-methionine = fecosterol + S- PMID:18310029 Disruption mutants of these genes were unable to synthesize ergosterol |
| GO:0003838 sterol 24-C-methyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Combined automated annotation from the UniProt EC 2.1.1.41 / Rhea:21128 assignment (zymosterol + SAM = fecosterol + SAH), which matches the GO term definition exactly. Supporting Evidence: file:SCHPO/erg6/erg6-uniprot.txt Reaction=zymosterol + S-adenosyl-L-methionine = fecosterol + S- |
| GO:0005634 nucleus | HDA PMID:16823372 ORFeome cloning and global analysis of protein localization ... | KEEP AS NON CORE | Summary: ORFeome YFP localisation (Matsuyama et al. 2006) reported a nuclear signal for Erg6-YFP; UniProt records Nucleus from this study and ER only as inferred. The abstract does not give per-protein images. A nuclear-envelope/perinuclear ER signal scored as nucleus is a plausible explanation, but the experimental call cannot be checked here, so it is retained as non-core rather than removed. Reason: High-throughput call not verifiable from the abstract; inconsistent with the ER/lipid-droplet location of SMTs, so not part of the core location. Supporting Evidence: PMID:16823372 Next, we determined the localization of 4,431 proteins, corresponding to file:SCHPO/erg6/erg6-uniprot.txt SUBCELLULAR LOCATION: Nucleus {ECO:0000269|PubMed:16823372}. |
| GO:0005634 nucleus | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: UniProt SubCell mapping of the 'Nucleus' line, which itself derives from the ORFeome HDA call. Same caveat as the HDA row; not a core location for a sterol methyltransferase. Reason: Mirrors the high-throughput nuclear call. Supporting Evidence: file:SCHPO/erg6/erg6-uniprot.txt SUBCELLULAR LOCATION: Nucleus {ECO:0000269|PubMed:16823372}. |
| GO:0005783 endoplasmic reticulum | IBA GO_REF:0000033 | ACCEPT | Summary: IBA ER from PTN002226525; budding-yeast Erg6 is ER- and lipid-droplet-associated and PomBase places the erg6 activity in the ER in its ergosterol GO-CAM. Supporting Evidence: file:SCHPO/erg6/erg6-uniprot.txt Endoplasmic reticulum {ECO:0000305|PubMed:16823372}. |
| GO:0005783 endoplasmic reticulum | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt SubCell mapping of the curator-inferred ER location (ECO:0000305 from the ORFeome study). Consistent with the IBA. Supporting Evidence: file:SCHPO/erg6/erg6-uniprot.txt Endoplasmic reticulum {ECO:0000305|PubMed:16823372}. |
| GO:0006694 steroid biosynthetic process | IEA GO_REF:0000002 | TOO GENERAL | Summary: InterPro2GO steroid biosynthetic process from the SMT C-terminal domain signature; correct broad parent of ergosterol biosynthesis. Supporting Evidence: file:SCHPO/erg6/erg6-uniprot.txt PATHWAY: Steroid metabolism; ergosterol biosynthesis. |
| GO:0006696 ergosterol biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: IBA ergosterol biosynthetic process from the SMT clade; confirmed for S. pombe by the deletion mutant. Supporting Evidence: PMID:18310029 Disruption mutants of these genes were unable to synthesize ergosterol |
| GO:0006696 ergosterol biosynthetic process | IMP PMID:18310029 Multiple functions of ergosterol in the fission yeast Schizo... | ACCEPT | Summary: Iwaki et al. 2008: erg6 disruptants cannot synthesise ergosterol. Direct experimental evidence for the process. Supporting Evidence: PMID:18310029 Disruption mutants of these genes were unable to synthesize ergosterol PMID:18310029 These results indicate that these proteins are responsible for ergosterol biosynthesis in fission yeast |
| GO:0006696 ergosterol biosynthetic process | IMP PMID:23145048 A genetic and pharmacological analysis of isoprenoid pathway... | ACCEPT | Summary: Nakamura et al. 2012 (LC-MS/MS): ergosterol was not detected in erg6-deleted cells, whereas lanosterol rose about 136-fold relative to erg31 erg32 double mutants. Direct support. Supporting Evidence: PMID:23145048 On the other hand, in Ξerg6 cells, Ξerg31Ξerg32 cells, and Ξerg5 cells, the ergosterol levels were extremely lower than that in wild-type cells PMID:23145048 Intriguingly, lanosterol levels in Ξerg6 cells were about 136 folds higher than that in Ξerg31Ξerg32 |
| GO:0008168 methyltransferase activity | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO methyltransferase activity from the class I SAM-MTase signature; correct but a distant parent of the specific sterol 24-C-methyltransferase activity. Reason: Too general; GO:0003838 is the specific activity. Proposed replacements: sterol 24-C-methyltransferase activity Supporting Evidence: file:SCHPO/erg6/erg6-uniprot.txt Reaction=zymosterol + S-adenosyl-L-methionine = fecosterol + S- |
| GO:0008757 S-adenosylmethionine-dependent methyltransferase activity | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO S-adenosylmethionine-dependent methyltransferase activity; correct parent of GO:0003838 but less informative. Reason: Too general; GO:0003838 is the specific activity. Proposed replacements: sterol 24-C-methyltransferase activity Supporting Evidence: file:SCHPO/erg6/erg6-uniprot.txt Reaction=zymosterol + S-adenosyl-L-methionine = fecosterol + S- |
| GO:0016126 sterol biosynthetic process | IEA GO_REF:0000041 | TOO GENERAL | Summary: UniPathway ergosterol biosynthesis mapping to sterol biosynthetic process; correct general parent. Supporting Evidence: file:SCHPO/erg6/erg6-uniprot.txt PATHWAY: Steroid metabolism; ergosterol biosynthesis. |
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Download this section (compressed HTML)Q: Does S. pombe Erg6 methylate lanosterol to eburicol in vitro, as suggested by eburicol accumulation under azole treatment in Schizosaccharomyces? GO has no lanosterol 24-C-methyltransferase term (GO:0003838 is defined on zymosterol only).
Q: Is the nuclear YFP signal reported for Erg6 in the ORFeome screen a nuclear-envelope/ER signal, and does Erg6 also reside on lipid droplets in fission yeast as in budding yeast?
Experiment: Express S. pombe Erg6 and measure SAM-dependent methylation of zymosterol and lanosterol (kinetics, product identification by GC-MS).
Hypothesis: S. pombe Erg6 accepts both zymosterol and lanosterol as methyl acceptors.
Type: enzyme kinetics
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