GTT3 (systematic name YEL017W) encodes an understudied Saccharomyces cerevisiae multi-pass membrane protein of 337 residues. It has a large N-terminal cytoplasmic region containing two disordered, phosphorylation-bearing low-complexity segments, followed by two transmembrane helices that anchor it in the membrane. Global GFP localization and an N-terminal-tag localization library independently place the protein at the nuclear envelope / nuclear periphery. Although named "glutathione transferase 3" by analogy to the yeast Gtt/Gto glutathione transferases, GTT3 carries a distinct, GTT3-specific domain architecture (the InterPro "putative GTT3" signature IPR038872 with Pfam GTT3_N) rather than the canonical soluble glutathione S-transferase fold, and no glutathione-conjugation activity, substrate, or catalytic mechanism has been demonstrated for it. Its molecular function and biological role remain unknown.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016020 membrane | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) inference that Gtt3 is a membrane protein. This is consistent with the UniProt topology model, which annotates two transmembrane helices and classifies Gtt3 as a multi-pass membrane protein. Reason: The membrane localization is well supported: UniProt models transmembrane segments at residues 240-260 and 314-336 and describes Gtt3 as a multi-pass membrane protein, and two independent high-throughput localization datasets place it at the nuclear envelope. The term is broad but not incorrect. A more specific cellular-component term (nuclear membrane) is also present in GOA and better captures the location. Supporting Evidence: UniProt:P39996 TRANSMEM 240..260 UniProt:P39996 TRANSMEM 314..336 |
| GO:0031965 nuclear membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic annotation to nuclear membrane derived from the UniProt Subcellular Location vocabulary. UniProt records "Nucleus membrane; Multi-pass membrane protein", based on the Huh et al. global GFP localization study. Reason: This is the most informative and best-supported cellular-component annotation for Gtt3. It is corroborated by an independent experimental screen (the SWAp-Tag N'-GFP library) that assigns Gtt3 a nuclear-periphery localization (GO:0034399, HDA). The nuclear-envelope location, together with the multi-pass topology, is the core established fact about this protein. Supporting Evidence: UniProt:P39996 SUBCELLULAR LOCATION: Nucleus membrane {ECO:0000269|PubMed:14562095}; PMID:14562095 into 22 distinct subcellular localization categories |
| GO:0034399 nuclear periphery | HDA PMID:26928762 One library to make them all: streamlining the creation of y... | ACCEPT | Summary: High-throughput direct-assay (HDA) localization to the nuclear periphery from the SWAp-Tag (SWAT) N'-terminal GFP library, in which each strain was imaged and manually assigned up to three subcellular localization categories, one of which is "Nuclear periphery". Reason: This experimental localization is consistent with, and complementary to, the UniProt nuclear-membrane annotation and the Huh et al. GFP screen. Nuclear periphery (the nuclear-lumen region proximal to the inner nuclear membrane) is an appropriate descriptor for a nuclear-envelope multi-pass membrane protein. There is no reason to overrule this curator-assigned experimental call. Supporting Evidence: PMID:26928762 Nuclear periphery; Nucleolus; Nucleus |
| GO:0003674 molecular_function | ND GO_REF:0000015 | ACCEPT | Summary: The molecular function of Gtt3 is annotated ND (no data). No enzymatic activity, substrate, catalytic mechanism, transporter activity, or specific binding has been demonstrated for the YEL017W protein. Reason: The ND annotation is the correct and honest representation of the current evidence. Although Gtt3 is named "glutathione transferase 3", this is an annotation-by-name/family assignment (PANTHER PTHR41807; InterPro IPR038872 is literally named "Put_GTT3", i.e. putative). Gtt3 carries a GTT3-specific domain architecture distinct from the canonical soluble glutathione S-transferases Gtt1/Gtt2 and the omega-class Gto1-3, it has a multi-pass membrane topology unlike those soluble enzymes, and no glutathione-conjugation assay has ever been reported for it. Inferring glutathione transferase activity (GO:0004364) would over-annotate the gene, so ND is retained. Supporting Evidence: file:yeast/GTT3/GTT3-deep-research-falcon.md no direct enzymatic assay, substrate specificity, or catalytic mechanism for the specific YEL017W protein was retrieved file:yeast/GTT3/GTT3-deep-research-falcon.md possesses unique domains (Put_GTT3, GTT3_N) that distinguish it from these paralogs |
| GO:0008150 biological_process | ND GO_REF:0000015 | ACCEPT | Summary: The biological process is annotated ND (no data). No specific process has been experimentally assigned to Gtt3. Reason: Retaining ND is appropriate. Unlike its namesake paralogs Gtt1 and Gtt2, which are transcriptionally induced by H2O2 and cumene hydroperoxide and are implicated in oxidative-stress detoxification, GTT3 is not induced under oxidative stress (its transcription is unchanged in a frataxin-deficient oxidative-stress model), which argues against simply transferring an oxidative-stress / glutathione-metabolism process from the paralogs. The only reported deletion phenotype (altered prodeoxyviolacein output in a SCRaMbLE synthetic-chromosome experiment) is mechanistically unresolved and confounded by co-deletions, and cannot support a specific process annotation. Supporting Evidence: file:yeast/GTT3/GTT3-deep-research-falcon.md GTT3 transcription was not detectably altered file:yeast/GTT3/GTT3-deep-research-falcon.md the exact mechanism by which YEL017W deletion increases PDV productivity was not elucidated |
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Download this section (compressed HTML)Q: Is Gtt3 a bona fide glutathione transferase, or is the "glutathione transferase 3" name an unverified family assignment? Does purified Gtt3 catalyze glutathione conjugation of any electrophilic substrate?
Q: What is the functional significance of Gtt3's nuclear-envelope multi-pass membrane topology, and which nuclear-membrane proteins or complexes does it interact with?
Q: Do the N-terminal Cdk1/phosphorylation sites regulate Gtt3, and does Gtt3 have a cell-cycle- or nuclear-envelope-related role distinct from the oxidative-stress roles of its paralogs Gtt1/Gtt2?
Experiment: Purify recombinant Gtt3 and assay glutathione S-transferase activity against canonical (CDNB) and alternative electrophilic substrates, alongside Gtt1/Gtt2 as positive controls, to test the GST hypothesis directly.
Type: biochemistry
Experiment: Determine the Gtt3 protein fold (experimental structure or AlphaFold analysis of the GTT3_N domain) and compare it with the canonical cytosolic GST fold to assess whether a catalytic GSH-binding site is present.
Type: structural_biology
Experiment: Identify Gtt3 physical interactors by affinity-purification mass spectrometry of endogenously tagged, functional Gtt3, focusing on nuclear-envelope and membrane-protein partners.
Type: proteomics
Experiment: Phenotype a gtt3-null strain (and gtt3 combined with gtt1/gtt2/gto deletions) across stress conditions and cell-cycle assays to assign a biological process and test for redundancy with the other glutathione transferases.
Type: genetics
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The molecular function of Gtt3 (YEL017W) is unknown. Despite the name "glutathione transferase 3", no glutathione-conjugation (or any other) enzymatic activity, substrate, catalytic mechanism, transporter activity, or specific binding partner has ever been demonstrated for the protein; it is not even established that Gtt3 is a bona fide glutathione transferase.
OPEN BIOLOGY MF_DARK
What is known: What is established is the protein's location and topology: Gtt3 is a multi-pass membrane protein (two transmembrane helices) with a large cytoplasmic N-terminal region, localized to the nuclear envelope / nuclear periphery by two independent high-throughput screens (Huh et al. global GFP; the SWAp-Tag N'-GFP library), and it is expressed at the protein level and phosphorylated on several N-terminal serines. Its GTT3-specific domain architecture (InterPro "putative GTT3" IPR038872 / Pfam GTT3_N) differs from the soluble Gtt1/Gtt2 and omega-class Gto1-3 glutathione transferases, and unlike them it is not induced by oxidative stress.
Significance: Gtt3 belongs to a conserved family (PANTHER PTHR41807 spans more than 1200 proteins across more than 2500 taxa) that has no experimentally established molecular function at any level, so resolving Gtt3 would illuminate a widespread but wholly dark protein family. The mismatch between its "glutathione transferase" name and its atypical membrane topology and non-canonical domains makes it a candidate mis-annotation whose correct function is unknown.
What would resolve it: Biochemical activity screens on purified Gtt3 (including glutathione-conjugation assays with CDNB and other electrophiles to test the GST hypothesis directly); structure determination / AlphaFold-based fold assignment to test whether the GTT3_N domain adopts a GST fold; affinity-purification mass spectrometry to identify nuclear-envelope partners; and phenotyping of a gtt3-null (and its combination with gtt1/gtt2/gto deletions) to assign a biological process.
Provenance (the field's own admissions):
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