rrg8 (SPAC31G5.06) is a small (234-residue) fungal protein of unknown molecular function whose mitochondrial role is supported by orthology. Its budding-yeast ortholog RRG8/YPR116W (alias MTA1) is one of several accessory factors required for efficient 5'-end processing of mitochondrial tRNAs: it associates with the mitochondrial inner membrane, sediments as a high-molecular-weight complex, and co-purifies with Rpm2p, the protein subunit of mitochondrial RNase P (the endonuclease that cleaves the 5' leader of pre-tRNAs). Loss of the ortholog reduces mitochondrial tRNA levels and causes respiratory deficiency and defects in mitochondrial genome maintenance and mitochondrial protein synthesis. In fission yeast, where loss of mitochondrial DNA is lethal, deletion of rrg8 is inviable. The fission-yeast protein carries no recognizable domain, catalytic motif, or match to the RRG8 family signature PF17068/IPR031415 found on the budding-yeast ortholog. No direct biochemical activity has been demonstrated for either protein; the S. pombe role in mitochondrial tRNA processing and gene expression is inferred from ortholog genetics rather than measured directly.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005634 nucleus | IEA GO_REF:0000044 | UNDECIDED | Summary: Electronic (UniProt Subcellular Location keyword-mapping) nuclear annotation, ultimately derived from a high-throughput C-terminal YFP-tagging screen. Reason: The nucleus call traces to the genome-scale YFP-tagging ORFeome screen (PMID:16823372) via UniProt Subcellular Location and GO_REF:0000044, so it ultimately reflects a direct S. pombe localization experiment despite the IEA evidence code. It conflicts with the mitochondrial ISO transfer from the characterized S. cerevisiae ortholog. Because the cached paper is abstract-only and accessible searches did not expose the rrg8-specific image/table, there is not enough evidence to decide whether the nuclear signal is genuine, condition-dependent, or an artifact. UniProt preserves the protein-specific experimental attribution, whereas the current PomBase GOA curates only mitochondrion as a specific organellar location. Retain as UNDECIDED pending inspection or replication of the image. Supporting Evidence: file:SCHPO/rrg8/rrg8-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm {ECO:0000269|PubMed:16823372}. Nucleus {ECO:0000269|PubMed:16823372}. |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | MARK AS OVER ANNOTATED | Summary: Electronic (UniProt Subcellular Location keyword-mapping) cytoplasmic annotation from the same high-throughput YFP-tagging screen. Reason: Cytoplasm derives from the direct genome-scale S. pombe YFP screen (PMID:16823372), as recorded protein-specifically by UniProt. It does not conflict with mitochondrion: GO:0005739 is part of GO:0005737, so mitochondrial localization entails the broad cytoplasmic parent. Mark this true but uninformative parent term as over-annotated; the manually curated mitochondrion term is the informative core location. Supporting Evidence: file:SCHPO/rrg8/rrg8-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm {ECO:0000269|PubMed:16823372}. Nucleus {ECO:0000269|PubMed:16823372}. |
| GO:0003674 molecular_function | ND GO_REF:0000015 | ACCEPT | Summary: Root molecular_function term with the No-Data (ND) evidence code, correctly recording that no molecular function has been determined for rrg8. Reason: The molecular function of rrg8 is genuinely unknown. O14106 has no recognizable domain, catalytic motif, or family signature (no InterPro/Pfam/PANTHER assignment); the budding-yeast ortholog carries PF17068/IPR031415 but is still described as "of unknown function". The ND annotation to the root term is the honest representation of this gap. |
| GO:0005739 mitochondrion | ISO GO_REF:0000024 | ACCEPT | Summary: Mitochondrial localization transferred by orthology (ISO) from the S. cerevisiae ortholog RRG8/MTA1 (SGD:S000006320), which localizes to the matrix side of the mitochondrial inner membrane and is GFP-tagged in mitochondria. Reason: The ISO transfer is well supported: the ortholog is experimentally mitochondrial (inner-membrane-associated; GFP in mitochondria, PMID:19751518) and functions there in mitochondrial tRNA processing (PMID:30759361). Mitochondrion is the single cellular component curated by PomBase for rrg8 and represents its core inferred location. Retain the curator's manual ISO call, while noting that low sequence identity and the direct S. pombe cyto/nuclear screen make native localization an important confirmation gap. Supporting Evidence: PMID:19751518 Proteins Rrg1, Rrg2, and Rrg5 through Rrg10 have been localized to mitochondria by high-throughput green fluorescent protein (GFP) fusion protein localization [35] and/or mitochondrial proteome analysis [6,34]. PMID:30759361 Mta1p, Mta2p, Pet130p, and Gep5p are associated with the mitochondrial inner membrane |
| GO:0097745 mitochondrial tRNA 5'-end processing | ISO GO_REF:0000024 | ACCEPT | Summary: Involvement in mitochondrial tRNA 5'-end processing transferred by orthology (ISO) from the S. cerevisiae ortholog RRG8/MTA1, which is required for efficient 5' processing of mitochondrial tRNAs and associates with mitochondrial RNase P (Rpm2p). Reason: The process assignment is well supported by ortholog genetics and biochemistry: mta1 null mutants have severely reduced mitochondrial tRNA levels, ts mutants accumulate pre-tRNA transcripts, and Mta1p co-immunopurifies with Rpm2p, the protein subunit of mitochondrial RNase P (PMID:30759361). This is the best-supported biological process for rrg8 and is consistent with its essentiality in fission yeast, where loss of mtDNA/mitochondrial gene expression is lethal. Note this is a process (BP) annotation, not a claim that rrg8 is the catalytic nuclease. Retain the manual ISO call, but do not upgrade it without direct S. pombe tRNA-processing evidence. Supporting Evidence: PMID:30759361 Here, we identify four novel genes MTA1, MTA2, GEP5 and PET130 of the Saccharomycetaceae family that are necessary for an efficient processing of mitochondrial tRNAs. PMID:30759361 co-immunopurification of Rpm2 with Mta1p |
Loading supporting contentβ¦
Download this section (compressed HTML)Q: Is Rrg8 a stoichiometric subunit of fission-yeast mitochondrial RNase P, or a transient assembly/maturation factor for the enzyme?
Q: Does the essential phenotype of rrg8 deletion in S. pombe reflect specifically defective mitochondrial tRNA processing (and consequent loss of mitochondrial translation and mtDNA), or an additional/independent function?
Experiment: Endogenously epitope-tag rrg8, purify from isolated mitochondria under native conditions, and test by mass spectrometry / co-immunoprecipitation for association with candidate RNase P protein partners and other Mta1/Mta2/Gep5/Pet130-equivalent factors, and test association with the rnpB RNA. In parallel, use a conditional rrg8 allele and northern blotting / primer extension to test for accumulation of unprocessed mitochondrial pre-tRNA 5' leaders.
Hypothesis: Rrg8 associates with mitochondrial RNase P and is required for 5' processing of mitochondrial pre-tRNAs in S. pombe.
Type: affinity purification-mass spectrometry and RNA processing assay
Experiment: Determine or predict the structure of Rrg8 (cryo-EM within the mitochondrial RNase P complex, or high-confidence structure prediction plus DALI/Foldseek search) to identify structural homologs, a possible RNA-binding surface, or a degenerate catalytic site, and use conservation mapping across fungal orthologs to nominate functionally important residues for mutagenesis.
Hypothesis: Rrg8 possesses an assignable fold and candidate functional surface despite lacking recognized sequence signatures.
Type: structural biology / structure-based homology detection
What is not known β curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The molecular function of rrg8 is unknown. No biochemical activity, catalytic residue, or recognizable domain/family match has been identified for the fission-yeast protein; the budding-yeast ortholog carries PF17068/IPR031415 but also lacks an assigned molecular activity. Whether Rrg8 is an enzyme, an RNA-binding protein, or a structural scaffold is undetermined.
OPEN BIOLOGY MF_DARK
What is known: The protein is 234 residues, carries a single low-complexity region and no recognizable InterPro/Pfam/PANTHER domain, and UniProt lists no FUNCTION and only "Predicted" protein existence. The S. cerevisiae ortholog RRG8/MTA1 matches the narrow RRG8 signature PF17068/IPR031415 but is still described as a protein "of unknown function", so a family match does not resolve the activity gap.
Significance: rrg8 is essential in fission yeast and its ortholog is required for respiratory growth, yet the molecular basis of these phenotypes is completely unexplained. It is a conserved, single-copy, essential "dark" gene of exactly the type the unknome program targets.
What would resolve it: Biochemical characterization of purified rrg8 / Rrg8-family protein: test for nuclease, RNA-binding, or protein-scaffolding activity in mitochondrial RNase P reconstitution; structural determination (X-ray/cryo-EM or high-confidence structure prediction) to assign a fold and candidate active site.
Provenance (the field's own admissions):
Gap: Whether rrg8 is a stable subunit of mitochondrial RNase P, a transient assembly/maturation factor for the enzyme, or acts on tRNA processing indirectly is unresolved. The mechanism by which it promotes 5' pre-tRNA cleavage is not defined.
OPEN BIOLOGYONTOLOGY RESIDUAL_SUBGAP
What is known: In the ortholog, Mta1p (together with Mta2p, Gep5p, Pet130p) is inner-membrane-associated, sediments as a high-molecular-weight complex, and co-immunopurifies with the RNase P protein subunit Rpm2p (PMID:30759361); null mutants have reduced mitochondrial tRNA levels and ts mutants accumulate pre-tRNA transcripts. This establishes a functional link to RNase P but not the mechanistic role (subunit vs assembly factor vs indirect). The corresponding protein composition of S. pombe mitochondrial RNase P, including whether it has an Rpm2-equivalent partner, is itself unresolved in the evidence reviewed here.
Significance: Distinguishing a bona fide RNase P subunit from an accessory assembly factor would clarify the architecture of fungal mitochondrial RNase P and determine whether an appropriate GO complex/subunit term (currently lacking) is needed to express rrg8's contribution.
What would resolve it: Native purification / cryo-EM of fungal mitochondrial RNase P to test for stoichiometric inclusion of Rrg8; in-organello pre-tRNA processing assays in rrg8 conditional mutants; define whether the RNase P activity or its assembly is lost.
Provenance (the field's own admissions):
Gap: The direct role of the S. pombe protein itself has not been measured. All functional annotations (mitochondrion, mitochondrial tRNA 5'-end processing) are transferred by orthology from S. cerevisiae; there is no fission-yeast biochemistry, localization by native methods, or in-organello assay for rrg8.
OPEN BIOLOGYCURATION MF_DARK
What is known: rrg8 deletion is inviable in fission yeast (PomBase; microscopy evidence PMID:23697806), and the protein is expressed (detected at RNA and protein level, increased in meiosis, PMID:39367033). The essential phenotype is consistent with a mitochondrial gene-expression role because loss of mtDNA is lethal in fission yeast (PMID:20473289), but the specific molecular defect in rrg8-null cells has not been characterized. A focused OpenScientist comparison found only about 23% pairwise identity to budding-yeast Mta1/Rrg8, no O14106 match to the donor's Pfam family, and a low-confidence O14106 AlphaFold model; these do not refute the curated 1:1 orthology but increase the transfer risk.
Significance: Direct fission-yeast evidence would upgrade the ISO annotations to experimental support and test whether the mitochondrial tRNA-processing role is conserved in S. pombe or whether the essential phenotype reflects a different or additional function.
What would resolve it: Localize an endogenously tagged Rrg8 by fluorescence and subcellular fractionation; analyze mitochondrial tRNA processing and mtDNA maintenance in a conditional (e.g. tetO/thiamine- repressible) rrg8 allele.
Provenance (the field's own admissions):
Loading supporting contentβ¦
Download this section (compressed HTML)Loading supporting contentβ¦
Download this section (compressed HTML)Loading supporting contentβ¦
Download this section (compressed HTML)