Igo1 (also mug134; SPAC10F6.16) is the endosulfine-family phosphoprotein of Schizosaccharomyces pombe, a small (139 residue), largely disordered, non-catalytic protein that is the fission-yeast counterpart of human ENSA/ARPP19 and budding-yeast Igo1/Igo2. Its defining activity is phosphorylation-gated, stoichiometric inhibition of the PP2A-B55 holoenzyme (PP2A-Pab1). In nitrogen-rich medium TORC1 keeps the Greatwall kinase Ppk18 (and its paralog Cek1) inactive, Igo1 is unphosphorylated and PP2A-Pab1 is active. When nitrogen becomes limiting and TORC1 activity falls, Ppk18 phosphorylates Igo1 on the conserved Ser64 of the endosulfine motif and phospho-Igo1 binds and inhibits PP2A-Pab1. Because PP2A-B55 removes Cdk1-dependent phosphorylation, its inhibition lets Cdk1-cyclin B become fully active at a smaller cell size, so cells advance the G2/M transition and divide shorter; cells lacking igo1 fail to accelerate mitosis and to reduce their size on nitrogen shift-down, whereas in rich medium the deletion has no size or morphology phenotype of its own. The same switch relays TORC1 inactivation to TORC2 and to transcription: PP2A-Pab1 dephosphorylates Gad8 Ser546 and the SAGA subunit Taf12, and Igo1-dependent inhibition of the phosphatase allows Gad8 activation and Taf12 phosphorylation, which are needed for timely ste11/mei2 induction, mating, nitrogen-starvation G1 arrest and sporulation; igo1-deleted and igo1-S64A cells are poorly fertile and pab1 deletion suppresses this. During entry into quiescence the Igo1-PP2A-B55 axis, acting through TORC2-Gad8 and Elongator-dependent tRNA anticodon modifications, sustains translation of AAA-lysine-codon-rich transcripts (rap1, sgo2, ctu1 and others), which accounts for the telomere-detachment, subtelomeric-silencing, autophagy and paromomycin-sensitivity phenotypes of igo1 mutants. Igo1 carries additional Cdk1- and PKA-consensus phosphosites of less certain physiological role, and it shows synthetic cell-shape and cell-size defects with loss of the PP2A/PP6 inhibitor Sds23. The transcript is meiotically upregulated (hence the mug134 name). A genome-wide YFP screen places the protein in the cytosol and nucleus.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004864 protein phosphatase inhibitor activity | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT propagation across the endosulfine family node PTN001309504 (seeded by Drosophila Endos, budding-yeast Igo1, human ARPP19 and fission-yeast Igo1 itself); Greatwall-kinase-gated inhibition of PP2A-B55 is the conserved, defining activity of the family and is shown directly for Igo1 in fission yeast. Reason: The node placement at the family root is sound - every characterised endosulfine from yeast to human is a Greatwall/Rim15/MASTL-phosphorylated PP2A-B55 inhibitor - and Igo1 appears in its own WITH/FROM because its PomBase IDA for the child term (Chica et al. 2016) was one of the descendant evidences used to place the IBD, which is expected and not circular. The family-level parent term is the right level for an IBA; the experimental row carries the more specific serine/threonine-phosphatase child. Supporting Evidence: PMID:26776736 Here we show that the greatwall-endosulfine (Ppk18-Igo1 in fission yeast) pathway couples the nutritional environment to the cell-cycle machinery by regulating the activity of PP2A·B55. PMID:26776736 When nutrients are limiting, TORC1 activity falls off, and the activation of greatwall (Ppk18) leads to the phosphorylation of endosulfine (Igo1) and inhibition of PP2A·B55, which in turn allows full activation of Cdk1·CyclinB and entry into mitosis with a smaller cell size. PMID:31553675 Similar to other organisms, the fission yeast endosulfine protein Igo1 inhibits PP2A activity (Chica et al., 2016). file:SCHPO/igo1/igo1-uniprot.txt Belongs to the endosulfine family. |
| GO:0004865 protein serine/threonine phosphatase inhibitor activity | IDA PMID:26776736 Nutritional Control of Cell Size by the Greatwall-Endosulfin... | ACCEPT | Summary: Chica et al. 2016 (abstract-only in the cache) established that Ppk18-phosphorylated Igo1 inhibits the PP2A-B55 (PP2A-Pab1) serine/threonine phosphatase, that igo1 deletion leaves PP2A-B55 inappropriately active under nitrogen limitation, and that reducing PP2A-B55 suppresses igo1 phenotypes. Reason: This is the core molecular function of the gene. PP2A is a serine/threonine phosphatase, so the child term is the right level of specificity. Only the abstract is cached, but the abstract itself states the phosphorylation-dependent inhibition of PP2A·B55, the PomBase curator read the full text, and the deep-research synthesis records the Ser64 dependence and the pab1 epistasis. The activity is the same Greatwall-site-phosphorylation-gated inhibition demonstrated biochemically for the budding-yeast and human family members. Supporting Evidence: PMID:26776736 When nutrients are limiting, TORC1 activity falls off, and the activation of greatwall (Ppk18) leads to the phosphorylation of endosulfine (Igo1) and inhibition of PP2A·B55, which in turn allows full activation of Cdk1·CyclinB and entry into mitosis with a smaller cell size. file:SCHPO/igo1/igo1-deep-research-falcon.md Phosphorylated Igo1 then strongly suppresses PP2A-B55 activity file:SCHPO/igo1/igo1-deep-research-falcon.md **Ppk18 phosphorylation of Igo1 at Ser64** is the central activating event supported by the mechanistic literature. Loss of *igo1* causes abnormally high PP2A-B55 activity under nitrogen-poor or nitrogen-free conditions, while genetically reducing PP2A-B55 can rescue multiple *igo1Δ* phenotypes. PMID:31553675 Similar to other organisms, the fission yeast endosulfine protein Igo1 inhibits PP2A activity (Chica et al., 2016). |
| GO:0005634 nucleus | HDA PMID:16823372 ORFeome cloning and global analysis of protein localization ... | ACCEPT | Summary: Genome-wide ORFeome YFP-tagging screen (Matsuyama et al. 2006) scored Igo1-YFP in the nucleus as well as the cytosol; this is the only direct localisation datum for the protein. Reason: High-throughput but consistent with the biology - Igo1 is a small soluble protein without targeting signals, and its effector PP2A-Pab1 has nuclear substrates (the SAGA subunit Taf12, Cdk1 substrates at mitotic entry) as well as cytosolic ones (Gad8). The screen was done in rich medium, so a nutrient-dependent redistribution is not excluded. UniProt derives its subcellular-location statement from the same screen. The deep-research report states that it found no direct Igo1 microscopy, but that is a gap in its retrieval (the ORFeome screen is direct microscopy), not a contradiction. Supporting Evidence: PMID:16823372 Next, we determined the localization of 4,431 proteins, corresponding to approximately 90% of the fission yeast proteome, by tagging each ORF with the yellow fluorescent protein. file:SCHPO/igo1/igo1-uniprot.txt SUBCELLULAR LOCATION: Nucleus {ECO:0000269|PubMed:16823372}. Cytoplasm file:SCHPO/igo1/igo1-deep-research-falcon.md The retrieved studies report nuclear and telomeric localization changes for downstream proteins, not direct Igo1 microscopy or fractionation. |
| GO:0005634 nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location keyword mapping (SL-0191 Nucleus); the UniProt statement is itself derived from the ORFeome YFP screen (PubMed 16823372). Reason: Correct, if redundant with the HDA row from the same primary observation. A nuclear pool is functionally plausible given the nuclear PP2A-Pab1 substrates (Taf12, mitotic Cdk1 targets). Supporting Evidence: PMID:16823372 Next, we determined the localization of 4,431 proteins, corresponding to approximately 90% of the fission yeast proteome, by tagging each ORF with the yellow fluorescent protein. file:SCHPO/igo1/igo1-uniprot.txt SUBCELLULAR LOCATION: Nucleus {ECO:0000269|PubMed:16823372}. Cytoplasm file:SCHPO/igo1/igo1-deep-research-falcon.md The retrieved studies report nuclear and telomeric localization changes for downstream proteins, not direct Igo1 microscopy or fractionation. |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT propagation of cytoplasmic localisation across the endosulfine family, seeded by rat Ensa/Arpp19 and budding-yeast Igo1. Reason: Family-level placement is sound (endosulfines are small soluble cytoplasmic/nuclear proteins) and it is confirmed on the target by the ORFeome screen, which scored Igo1-YFP in the cytosol. Igo1 must act where PP2A-Pab1 and its substrate Gad8 are, which is largely cytoplasmic. Supporting Evidence: PMID:16823372 Next, we determined the localization of 4,431 proteins, corresponding to approximately 90% of the fission yeast proteome, by tagging each ORF with the yellow fluorescent protein. file:SCHPO/igo1/igo1-uniprot.txt SUBCELLULAR LOCATION: Nucleus {ECO:0000269|PubMed:16823372}. Cytoplasm file:SCHPO/igo1/igo1-deep-research-falcon.md The retrieved studies report nuclear and telomeric localization changes for downstream proteins, not direct Igo1 microscopy or fractionation. |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location keyword mapping (SL-0086 Cytoplasm), derived from the ORFeome YFP screen. Reason: Correct and consistent with the HDA cytosol row and the family IBA; a generic cytoplasm term from a keyword mapping is acceptable. Supporting Evidence: PMID:16823372 Next, we determined the localization of 4,431 proteins, corresponding to approximately 90% of the fission yeast proteome, by tagging each ORF with the yellow fluorescent protein. file:SCHPO/igo1/igo1-uniprot.txt SUBCELLULAR LOCATION: Nucleus {ECO:0000269|PubMed:16823372}. Cytoplasm file:SCHPO/igo1/igo1-deep-research-falcon.md The retrieved studies report nuclear and telomeric localization changes for downstream proteins, not direct Igo1 microscopy or fractionation. |
| GO:0005829 cytosol | HDA PMID:16823372 ORFeome cloning and global analysis of protein localization ... | ACCEPT | Summary: Genome-wide ORFeome YFP-tagging screen scored Igo1-YFP in the cytosol (and nucleus). Reason: The only direct localisation datum; a soluble regulatory phosphoprotein acting on a soluble PP2A holoenzyme is expected to be cytosolic. Consistent with the cytosolic localisation of its kinase Ppk18 in the same screen. Supporting Evidence: PMID:16823372 Next, we determined the localization of 4,431 proteins, corresponding to approximately 90% of the fission yeast proteome, by tagging each ORF with the yellow fluorescent protein. file:SCHPO/igo1/igo1-uniprot.txt SUBCELLULAR LOCATION: Nucleus {ECO:0000269|PubMed:16823372}. Cytoplasm file:SCHPO/igo1/igo1-deep-research-falcon.md The retrieved studies report nuclear and telomeric localization changes for downstream proteins, not direct Igo1 microscopy or fractionation. |
| GO:0031139 positive regulation of conjugation with cellular fusion | IGI PMID:29079657 TORC1 and TORC2 converge to regulate the SAGA co-activator i... | ACCEPT | Summary: Laboucarie et al. 2017 (abstract-only in the cache; full text consulted at PMC) show that igo1 deletion and the igo1-S64A knock-in weakly induce ste11+/mei2+ on starvation and are sterile, that these mutants fail to phosphorylate the SAGA subunit Taf12, and that deleting pab1 (PP2A-B55; the IGI partner SPAC227.07c) suppresses the sterility and restores Taf12 phosphorylation. Reason: Igo1 does the work here - it is the phosphoprotein that inhibits PP2A-Pab1 once starvation is sensed, which is what allows Taf12 phosphorylation and TORC2-Gad8 activation and hence commitment to mating - and the pab1 epistasis pins the effector. The cached abstract states only the PP2A/TORC1 and Gad8/TORC2 arms; the igo1 mutant data (Fig 5, Appendix Fig S8) are in the full text, which the PomBase curator read. Independent support comes from Martin et al. 2017 (igo1 deletion blocks Gad8 Ser546 phosphorylation and mei2 induction on TORC1 inactivation) and from the deep-research synthesis. Promotion of sexual differentiation is a genuine, if pleiotropic, output of the same nutrient-signalling switch and is retained as a second core function, consistent with the Ppk18 review. Supporting Evidence: PMID:29079657 Taf12 phosphorylation increases early upon starvation and is controlled by the opposing activities of the PP2A phosphatase, which is activated by TORC1, and the TORC2-activated Gad8AKT kinase. PMID:29079657 Mutational analyses suggest that Taf12 phosphorylation prevents cells from committing to differentiation until starvation reaches a critical level. file:SCHPO/igo1/igo1-deep-research-falcon.md PP2A/Pab1 associates with Gad8 and opposes phosphorylation of Gad8 at **Ser546**. Inhibiting PP2A-B55 through Igo1 permits accumulation of Gad8-S546 phosphorylation and greater TORC2 output. file:SCHPO/igo1/igo1-deep-research-falcon.md Cells lacking Igo1 fail to arrest normally in G1, show impaired starvation-induced cell-size reduction, and have reduced sporulation. |
| GO:0035556 intracellular signal transduction | IMP PMID:26776736 Nutritional Control of Cell Size by the Greatwall-Endosulfin... | ACCEPT | Summary: Igo1 is the relay component of the TORC1 -| Ppk18 -> Igo1 -| PP2A-B55 -> Cdk1 signalling cascade that transduces nitrogen limitation into mitotic entry; igo1 deletion abolishes the nutrient-dependent cell-size response. Reason: Correct - Igo1 sits in the middle of an intracellular signalling relay and the phenotype is on the single igo1 deletion. The term is generic; the pathway-specific information is carried by the sibling GO:1905287 annotation from the same paper, and PomBase applies the same generic term to Ppk18 for this paper. A MODIFY to TORC1 signaling (GO:0038202) was considered and rejected, because Igo1 is not a direct TORC1 target (TORC1 acts on Ppk18 upstream) and the paper does not claim it is. Supporting Evidence: PMID:26776736 Here we show that the greatwall-endosulfine (Ppk18-Igo1 in fission yeast) pathway couples the nutritional environment to the cell-cycle machinery by regulating the activity of PP2A·B55. PMID:26776736 In the presence of nutrients, greatwall (Ppk18) protein kinase is inhibited by TORC1 and PP2A·B55 is active. PMID:26776736 When nutrients are limiting, TORC1 activity falls off, and the activation of greatwall (Ppk18) leads to the phosphorylation of endosulfine (Igo1) and inhibition of PP2A·B55, which in turn allows full activation of Cdk1·CyclinB and entry into mitosis with a smaller cell size. file:SCHPO/igo1/igo1-deep-research-falcon.md This is an indirect signaling role: Igo1 does not phosphorylate Gad8. Rather, it prevents PP2A-B55 from reversing the phosphorylation state needed for TORC2–Gad8 signaling. |
| GO:1902472 regulation of mitotic cytokinesis, division site positioning | IGI PMID:31553675 The phosphatase inhibitor Sds23 regulates cell division symm... | MARK AS OVER ANNOTATED | Summary: Schutt and Moseley 2019 (full text cached) combined igo1 deletion with deletion of sds23 (the PP2A/PP6 inhibitor, SPBC646.13). The double mutant had significantly more bent/curved cells and divided at a larger size than either single mutant, but the authors state that the division-symmetry (cell half ratio) trend was not significantly different from sds23 deletion alone, and igo1 deletion alone has no division-site defect. Reason: The IGI is real and the curator's reading is defensible, but the specific claim, that Igo1 regulates division-site positioning, rests on a trend the authors themselves flag as not significant; the significant synthetic phenotypes in the same figure are cell shape and cell size at division, which the paper attributes to both inhibitors converging on PP2A to control mitotic entry (already captured by GO:1905287). The annotation is not wrong in spirit - excess PP2A activity in the double mutant may well perturb contractile-ring placement, and PP2A itself is implicated in cytokinesis - but it overstates the evidence for a division-plane role of Igo1. Graded identically for Ppk18, whose IGI row rests on the same panel. Supporting Evidence: PMID:31553675 Both igo1∆ and ppk18∆ mutants alone did not display defects in cell shape, cell length at division, or division symmetry under normal growing conditions, consistent with previous reports (Figure 5, B–E; Chica et al., 2016). PMID:31553675 Lastly, the cell half ratio was even further reduced for double mutant cells, but we note that the double mutants were not significantly different from sds23∆ alone despite this trend (Figure 5E). PMID:31553675 Second, the double mutants divided at a larger cell size than the single mutants (Figure 5D), consistent with both pathways acting on PP2A to control mitotic entry. PMID:31553675 First, the percentage of cells with aberrant morphology, specifically bent and curved cells, was increased in the sds23∆ igo1∆ and sds23∆ ppk18∆ double mutant cells compared with single mutants (Figure 5C). PMID:31553675 Our observations likely reflect a broader role for regulation of PP2A in cell polarity and cytokinesis because sds23∆ phenotypes were exacerbated when combined with mutations in the fission yeast Ensa homologue, Igo1. |
| GO:1905287 positive regulation of G2/M transition of mitotic cell cycle involved in cellular response to nitrogen starvation | IMP PMID:26776736 Nutritional Control of Cell Size by the Greatwall-Endosulfin... | ACCEPT | Summary: Central function: under nitrogen limitation, TORC1 inhibition activates Ppk18, which phosphorylates Igo1 on Ser64; phospho-Igo1 inhibits PP2A-B55, permitting full Cdk1-cyclin B activation and entry into mitosis at a smaller cell size. igo1 deletion cells fail to advance mitosis and to reduce their size on nitrogen shift-down. Reason: The best-supported and most specific process annotation for the gene, from single-gene loss-of-function experiments in the primary fission-yeast study. Igo1 performs a step of the process - it is the entity that binds and inhibits PP2A-B55 - so involvement is participation, not mere necessity. The compound term is contorted but it is exactly what the paper shows, and it is the same term PomBase applies to Ppk18 for the same paper. Supporting Evidence: PMID:26776736 When nutrients are limiting, TORC1 activity falls off, and the activation of greatwall (Ppk18) leads to the phosphorylation of endosulfine (Igo1) and inhibition of PP2A·B55, which in turn allows full activation of Cdk1·CyclinB and entry into mitosis with a smaller cell size. PMID:26776736 High levels of PP2A·B55 prevent the activation of mitotic Cdk1·Cyclin B, and cells increase in size in G2 before they undergo mitosis. file:SCHPO/igo1/igo1-deep-research-falcon.md *igo1Δ* cells do not execute the normal acceleration of G2/M or starvation-associated reduction in division size, placing Igo1 between nutrient sensing and the core cell-cycle machinery PMID:31553675 Second, the double mutants divided at a larger cell size than the single mutants (Figure 5D), consistent with both pathways acting on PP2A to control mitotic entry. |
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Download this section (compressed HTML)Q: Is inhibition of PP2A-Pab1 by phospho-Igo1 direct and stoichiometric, as for ENSA/ARPP19 and budding-yeast Igo1 (which bind the B55/catalytic-subunit interface and are slowly dephosphorylated), and what is the affinity? No purified fission-yeast Igo1-PP2A-Pab1 binding or inhibition constant has been reported, and the PomBase IDA rests on in vivo phosphatase activity and epistasis.
Suggested experts: Sergio Moreno, Anna Castro
Q: What do the Cdk1-consensus (Ser31, Ser89, Ser102) and PKA-consensus (Ser76, Ser115) phosphosites of Igo1 do? The Igo1-4A mutant has nutrient-dependent size phenotypes, but whether these sites modulate Ser64-dependent PP2A-B55 inhibition, Igo1 turnover, or PP2A-B55 substrate-competition (as the PKA site does for ENSA/ARPP19) is unresolved.
Suggested experts: Sergio Moreno, Livia Perez-Hidalgo
Q: Should the biological-process counterpart of the inhibitor activity (GO:0010923 negative regulation of phosphatase activity) be added? Human ENSA/ARPP19 carry it, but the fission-yeast PP2A/PP6 inhibitor Sds23 carries only the molecular-function term, so PomBase appears to capture this through the MF annotation alone and no NEW term is proposed here. The human ENSA review in this repository does propose GO:0010923 as NEW, because human ARPP19 carries it by IDA; the divergence is a per-MOD convention judgment, not an oversight.
Suggested experts: Valerie Wood
Experiment: Phosphorylate recombinant Igo1 (wild-type and S64A) with purified Ppk18 or a hyperactive Greatwall, then measure binding to and inhibition of PP2A-Pab1 affinity-purified from S. pombe (and, as a specificity control, PP2A-Par1/Par2 B56 holoenzymes) on a phospho-Ser/Thr substrate; complement with co-immunoprecipitation of Igo1-MYC with Pab1 in rich versus nitrogen-free medium.
Hypothesis: Ser64-phosphorylated Igo1 is a direct, high-affinity inhibitor of the fission-yeast PP2A-Pab1 holoenzyme.
Type: biochemistry
Experiment: Compare igo1 deletion with igo1 deletion carrying a phosphomimetic gad8-S546E allele or reduced pab1 expression for ste11/mei2 induction, mating efficiency, polysome profiles, mcm5s2U34 tRNA modification and Rap1 protein levels after nitrogen removal; a full rescue by gad8-S546E would place every downstream phenotype on the single TORC2 branch.
Hypothesis: The differentiation, quiescence-translation and telomere phenotypes of igo1 mutants are all explained by excess PP2A-Pab1 activity acting on Gad8.
Type: genetics
Experiment: Image endogenously tagged Igo1 (C-terminal mNeonGreen, verified functional by size-at-division on nitrogen shift-down) in rich medium, after nitrogen removal and after TORC1 inactivation (tor2-51), quantifying nuclear/cytosolic ratio alongside Ser64 phosphorylation.
Hypothesis: Igo1 redistributes between cytosol and nucleus when the switch is activated.
Type: cell biology
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