MRX20 (YFR045W) is an uncharacterized inner-membrane protein of Saccharomyces cerevisiae mitochondria that belongs to the mitochondrial carrier family (SLC25/MCF; TC 2.A.29). Its sequence has the canonical mitochondrial-carrier architecture: three tandem Solcar repeats and six predicted transmembrane helices forming the pseudo-threefold-symmetric fold typical of metabolite carriers, and it retains the conserved carrier signature residues found in functionally characterized carriers. Within its family it falls in a fungal-specific subfamily whose members are all uncharacterized, so no specific transported substrate has been established for MRX20; the protein is therefore best described as a putative mitochondrial metabolite carrier of unknown substrate. The gene is non-essential, expressed at low abundance, and its transcription is induced under stress. It was given the "Mrx" name after it co-purified with native mitochondrial-ribosome (MIOREX) complexes in a proteomic study, but a direct role in mitochondrial gene expression has not been demonstrated.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Correct but non-specific mitochondrial localization from phylogenetic propagation. MRX20 is an integral mitochondrial inner-membrane carrier, so "mitochondrion" is accurate but subsumed by the more specific inner-membrane annotation. Reason: Consistent with the mitochondrial-carrier family assignment and the ISS inner-membrane annotation, but less informative than GO:0005743 (mitochondrial inner membrane). |
| GO:0071913 citrate secondary active transmembrane transporter activity | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Over-specific molecular-function propagation. The citrate-transporter activity is driven by the characterized tricarboxylate/citrate carriers in the broad PANTHER family PTHR45788 (CTP1, human SLC25A1, SFC1), but MRX20 falls in the fungal-specific subfamily PTHR45788:SF5 whose members are all uncharacterized mitochondrial carriers with no demonstrated substrate. No citrate-transport activity has been shown for MRX20, and its specific substrate is unknown. Reason: Family-level IBA over-propagation of a specific substrate to a gene in an uncharacterized subfamily; the transported substrate of MRX20 is genuinely unknown. The general carrier/transporter nature is retained in core_functions as transmembrane transporter activity without asserting citrate. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTHR45788 · SUCCINATE/FUMARATE MITOCHONDRIAL TRANSPORTER-RELATED SUPPORTS SOURCE BUT NOT TARGET Broad family node carrying the citrate-carrier signal from characterized members (CTP1, human SLC25A1, SFC1); too general to fix the substrate for MRX20. PANTHER:PTHR45788:SF5 · AFR253WP SOURCE WEAK OR INFERRED MRX20's own subfamily; all members (P43617, SCY_1795, SCRG_05595, AWRI1631_61110) are uncharacterized mitochondrial carriers with no demonstrated citrate transport. Proposed replacements: transmembrane transporter activity |
| GO:0006843 mitochondrial citrate transmembrane transport | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Over-specific biological-process propagation, mirroring the citrate-transporter molecular-function annotation. The citrate-transport process is inferred from the characterized citrate carriers of the family, not from MRX20 or its uncharacterized subfamily. Reason: Same family-level over-propagation as the citrate transporter activity; MRX20's transported substrate is unknown. A general transmembrane-transport process is retained instead. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTHR45788 · SUCCINATE/FUMARATE MITOCHONDRIAL TRANSPORTER-RELATED SUPPORTS SOURCE BUT NOT TARGET Citrate-transport process propagated from characterized citrate carriers in the broad family, not from MRX20 or its uncharacterized subfamily. PANTHER:PTHR45788:SF5 · AFR253WP SOURCE WEAK OR INFERRED MRX20's subfamily is uncharacterized; no member has a demonstrated citrate-transport process. Proposed replacements: mitochondrial transmembrane transport |
| GO:0005743 mitochondrial inner membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Inner-membrane localization from UniProt Subcellular Location mapping, consistent with the multi-pass mitochondrial-carrier assignment and with the ISS annotation from PMID:10930523. This is the best-supported and most specific localization for MRX20. Reason: Agrees with the mitochondrial carrier (SLC25/MCF) family membership, six predicted transmembrane helices, and the independent ISS inner-membrane call. |
| GO:0055085 transmembrane transport | IEA GO_REF:0000002 | ACCEPT | Summary: General transmembrane-transport process from the InterPro mitochondrial-carrier domain signature. Appropriately generic for a carrier whose specific substrate is unknown. Reason: Well supported by the mitochondrial-carrier domain architecture; does not overreach on substrate specificity. |
| GO:0005743 mitochondrial inner membrane | ISS PMID:10930523 The yeast mitochondrial transport proteins: new sequences an... | ACCEPT | Summary: Inner-membrane localization inferred by SGD from sequence/structural similarity, based on the mitochondrial transport protein survey. Concordant with the UniProt SubCell IEA and with the family assignment; this is the core localization. Reason: MRX20 has the canonical mitochondrial-carrier fold (three Solcar repeats, six TM helices) and retains the carrier consensus residues; inner-membrane localization is strongly supported. |
| GO:0003674 molecular_function | ND GO_REF:0000015 | ACCEPT | Summary: Root molecular_function with ND (no biological data). This honestly records that no specific molecular function has been experimentally determined for MRX20. Given that the transported substrate is unknown and the subfamily is uncharacterized, retaining ND is appropriate rather than asserting a specific activity. Reason: Correctly represents the genuine absence of a demonstrated molecular function; a general transmembrane transporter activity is proposed in core_functions on family grounds but no specific catalytic/substrate activity is experimentally established. |
| GO:0006839 mitochondrial transport | ISS PMID:10930523 The yeast mitochondrial transport proteins: new sequences an... | ACCEPT | Summary: General mitochondrial-transport process inferred by SGD from similarity to characterized mitochondrial transport proteins. Appropriately generic and consistent with the carrier-family assignment without committing to a substrate. Reason: Supported by mitochondrial-carrier family membership; correctly avoids substrate-specific over-annotation. Retained as a general (non-core-defining) process for this dark gene. |
| GO:0022857 transmembrane transporter activity | ISS PMID:10930523 The yeast mitochondrial transport proteins: new sequences an... | NEW | Summary: Proposed general molecular-function annotation replacing the over-specific citrate-transporter IBA. MRX20 has the mitochondrial-carrier fold and retained carrier consensus residues, supporting a transmembrane transporter activity without committing to a substrate that has not been experimentally determined. Reason: Captures the defensible, substrate-agnostic transporter activity of this carrier on domain/family grounds, in place of the family-level citrate over-propagation. |
| GO:1990542 mitochondrial transmembrane transport | ISS PMID:10930523 The yeast mitochondrial transport proteins: new sequences an... | NEW | Summary: Proposed mitochondrial transmembrane-transport process replacing the over-specific mitochondrial citrate transport IBA. Reflects the carrier's role in moving a solute across the mitochondrial inner membrane without asserting citrate. Reason: Provides the correctly scoped process term for a mitochondrial carrier of unknown substrate, consistent with the accepted general transmembrane-transport annotations. |
Loading supporting content…
Download this section (compressed HTML)Q: What metabolite(s) does MRX20 transport across the mitochondrial inner membrane, and is the family-level citrate assignment correct for the fungal PTHR45788:SF5 subfamily?
Q: Is the co-purification of MRX20 with MIOREX/mitochondrial-ribosome complexes a direct, functionally meaningful interaction, or an abundance-driven artifact of a membrane protein?
Experiment: Express and purify recombinant MRX20, reconstitute into proteoliposomes, and assay uptake/exchange of candidate solutes (citrate and other di-/tricarboxylates, amino acids, nucleotides, cofactors) to identify the transported substrate.
Hypothesis: MRX20 is a functional mitochondrial metabolite carrier with a substrate distinct from citrate.
Type: in vitro transport reconstitution
Experiment: Phenotype the MRX20 deletion across respiratory/non-fermentable carbon sources and stress conditions, with mitochondrial metabolomics and synthetic genetic array analysis to place MRX20 in a pathway.
Hypothesis: Loss of MRX20 perturbs a specific mitochondrial process under particular growth or stress conditions.
Type: conditional phenotyping and genetic interaction mapping
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The physiological substrate transported by MRX20 is unknown. It has the mitochondrial-carrier fold and the conserved carrier signature residues, but its specific solute (whether a tricarboxylate/citrate as in related family members, another metabolite, or none) has never been measured, and no transport assay for MRX20 exists.
OPEN BIOLOGY MF_DARK
What is known: It is firmly established that MRX20 is an integral mitochondrial inner-membrane protein of the SLC25/mitochondrial-carrier family with the canonical three-repeat, six-transmembrane-helix architecture and retained carrier consensus residues. What is unknown is the identity of the transported substrate; the GOA citrate-transporter annotations derive from characterized citrate carriers elsewhere in the family, not from MRX20 or its uncharacterized fungal subfamily (PTHR45788:SF5).
Significance: Assigning MRX20's substrate would convert a "putative carrier of unknown substrate" into a defined metabolite-transport function and would test whether the family-level citrate propagation is correct or an over-annotation for the fungal subfamily.
What would resolve it: Reconstitute recombinant MRX20 into liposomes and screen candidate solutes (di-/tricarboxylates, amino acids, nucleotides, cofactors) for transport, and/or perform targeted mitochondrial metabolomics in an MRX20 deletion strain.
Provenance (the field's own admissions):
Gap: Whether MRX20 has any direct role in mitochondrial gene expression is unknown. The "Mrx" name reflects only proteomic co-purification with mitochondrial-ribosome (MIOREX) complexes, and no experiment has shown that MRX20 participates in transcription, RNA processing, translation, or ribosome assembly.
OPEN BIOLOGY BP_DARK
What is known: MRX20 was recovered in the interactome of native mitochondrial-ribosome (MIOREX) complexes and named accordingly. It is otherwise characterized only as a membrane-embedded carrier-family protein; a functional (as opposed to co-purification) link to gene-expression machinery has not been demonstrated.
Significance: A carrier that genuinely couples metabolite transport to the organization of mitochondrial gene expression would be mechanistically novel; conversely, confirming it is a bystander of the pulldown would clarify that the "Mrx" name is associative only.
What would resolve it: Test MRX20 deletion/depletion for mitochondrial transcription, mt-mRNA stability, and translation phenotypes, and determine whether the MIOREX association is direct and stoichiometric or an abundance-driven co-purification.
Provenance (the field's own admissions):
Gap: The biological process MRX20 serves and the basis of its loss-of-function phenotypes are unknown. The deletion is viable with only weak, pleiotropic high-throughput phenotypes, none of which points to a specific pathway.
OPEN BIOLOGY BP_DARK
What is known: MRX20 is non-essential, expressed at low abundance, and stress-inducible. Reported deletion phenotypes are mild and non-specific, so they do not define the pathway in which MRX20 acts.
Significance: Linking MRX20 to a defined metabolic or mitochondrial-maintenance pathway would turn a stress-inducible dark carrier into an interpretable node of mitochondrial physiology.
What would resolve it: Condition-specific phenotyping (respiratory/non-fermentable carbon sources, metabolic and oxidative stresses) combined with synthetic-genetic-array and metabolomic profiling of the deletion strain.
Provenance (the field's own admissions):
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)