Cut1 is the separase (separin) of the fission yeast Schizosaccharomyces pombe, a large (1828-residue) CD-clan cysteine endopeptidase (EC 3.4.22.49) and the ortholog of budding yeast Esp1 and human separase (ESPL1). Its catalytic activity resides in a C-terminal peptidase C50 domain with the active-site cysteine at position 1730. Cut1 triggers the metaphase-to-anaphase transition by proteolytically cleaving the kleisin subunit of cohesin (Rad21/Scc1 in mitosis, Rec8 in meiosis), opening the cohesin ring and allowing sister chromatids (or homologues) to separate. Through most of the cell cycle Cut1 is held catalytically inactive by binding its inhibitory chaperone securin Cut2, forming the separase-securin complex; at the metaphase-anaphase transition the APC/C (activated by Slp1/Cdc20) ubiquitinates Cut2 and targets it for degradation, liberating active Cut1. Cut1 is cytoplasmic during interphase and relocates to the spindle pole bodies and mitotic spindle upon entry into prophase, remaining bound to the anaphase spindle midzone, where it also promotes anaphase spindle elongation. Beyond mitosis, Cut1 cleaves Rec8 to drive meiotic chromosome segregation and cleaves cohesin locally during interphase to support DNA repair. Loss of Cut1 produces the "cut" (cell untimely torn) phenotype, in which chromosomes fail to segregate yet cytokinesis proceeds, bisecting the undivided nucleus.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: Cut1 acts in the nucleus, where it cleaves chromatin-bound cohesin to drive chromosome segregation; it also localizes to the nuclear spindle in mitosis. Phylogenetic inference from the separase family is consistent with direct experimental localization in S. pombe. Reason: Nuclear function/localization is supported phylogenetically and confirmed by direct experimental evidence in fission yeast (PMID:9635190 IDA). Supporting Evidence: PMID:12390246 Separase activated by securin destruction cleaves the cohesin subunit Scc1/Rad21 enriched in kinetochores. |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: Cut1 is retained in the cytoplasm during interphase before mobilizing to the spindle pole bodies and spindle in mitosis. The cytoplasmic pool is well documented experimentally and consistent with the phylogenetic inference. Reason: Supported by direct localization data showing an interphase cytoplasmic pool of Cut1. Supporting Evidence: PMID:9635190 Cut1 is retained in the cytoplasm during interphase and moves to the mitotic spindle pole bodies and the spindle upon entry into prophase, when spindles are formed. |
| GO:0044732 mitotic spindle pole body | IBA GO_REF:0000033 | ACCEPT | Summary: Cut1 localizes to the mitotic spindle pole bodies upon entry into prophase. This phylogenetic inference is corroborated by direct experimental evidence in fission yeast. Reason: Consistent with experimental IDA localization (PMID:9635190). Supporting Evidence: PMID:9635190 Cut1 is retained in the cytoplasm during interphase and moves to the mitotic spindle pole bodies and the spindle upon entry into prophase, when spindles are formed. |
| GO:0004197 cysteine-type endopeptidase activity | IBA GO_REF:0000033 | ACCEPT | Summary: This is the core molecular function of Cut1. Separase is a CD-clan cysteine endopeptidase (EC 3.4.22.49) with an active-site cysteine at position 1730 in a C-terminal peptidase C50 domain; it cleaves the cohesin kleisin subunit. The phylogenetic inference is strongly supported across the separase family and by fission yeast experiments showing protease-dependent cohesin cleavage. Reason: Core, well-supported molecular function; UniProt annotates EC 3.4.22.49 with active-site Cys1730, and Rec8 cleavage requires the protease activity. Supporting Evidence: PMID:14532136 we show that separase activation and resultant Rec8 cleavage are required for meiotic chromosome segregation in fission yeast. |
| GO:0072686 mitotic spindle | IBA GO_REF:0000033 | ACCEPT | Summary: Cut1 concentrates along the mitotic spindle, where it is loaded via its interaction with Cut2 and remains bound through anaphase. The phylogenetic inference matches direct experimental localization in S. pombe. Reason: Corroborated by direct experimental evidence (PMID:8978688, PMID:9635190 IDA). Supporting Evidence: PMID:8978688 Cut1 protein concentrates along the short spindle in metaphase as does Cut2. |
| GO:0000280 nuclear division | IEA GO_REF:0000117 | MARK AS OVER ANNOTATED | Summary: Cut1 is required for nuclear division; loss causes the cut phenotype in which nuclear division fails. This generic term is accurate but less informative than the specific process mitotic sister chromatid separation, which captures Cut1's direct mechanistic role. Reason: Accurate but overly general; the specific term mitotic sister chromatid separation (GO:0051306) is the informative annotation already present and better reflects the molecular action. Supporting Evidence: PMID:8065367 They are the cut1+ gene essential for nuclear division |
| GO:0004197 cysteine-type endopeptidase activity | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO assignment of cysteine-type endopeptidase activity based on the separin/peptidase C50 domains. This matches the experimentally established core molecular function of Cut1. Reason: Correct domain-based inference consistent with the curated EC 3.4.22.49 and experimental protease activity. Supporting Evidence: PMID:14532136 We identify two Cut1-dependent cleavage sites within Rec8 sequences and demonstrate that the homologue segregation at meiosis I is completely blocked by the expression of a non-cleavable form of Rec8. |
| GO:0005634 nucleus | IEA GO_REF:0000120 | ACCEPT | Summary: Automated subcellular-location inference of nuclear localization, consistent with the experimentally documented nuclear/spindle pool of Cut1 and its action on chromatin-bound cohesin. Reason: Redundant with, and consistent with, the experimental nucleus IDA annotation. Supporting Evidence: PMID:9635190 Cut1 is retained in the cytoplasm during interphase and moves to the mitotic spindle pole bodies and the spindle upon entry into prophase, when spindles are formed. |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: Automated subcellular-location inference of cytoplasmic localization, matching the documented interphase cytoplasmic pool of Cut1. Reason: Consistent with experimental localization data showing interphase cytoplasmic Cut1. Supporting Evidence: PMID:9635190 Cut1 is retained in the cytoplasm during interphase and moves to the mitotic spindle pole bodies and the spindle upon entry into prophase, when spindles are formed. |
| GO:0006508 proteolysis | IEA GO_REF:0000002 | ACCEPT | Summary: Cut1 is a protease and carries out proteolysis (cleavage of cohesin kleisin). This is the general process parent of its specific function. Accurate but uninformative on its own; the specific endopeptidase activity and sister chromatid separation annotations carry the meaningful content. Reason: Correct general process consistent with Cut1's protease activity. Supporting Evidence: PMID:14532136 we show that separase activation and resultant Rec8 cleavage are required for meiotic chromosome segregation in fission yeast. |
| GO:0098813 nuclear chromosome segregation | IEA GO_REF:0000117 | ACCEPT | Summary: Cut1 mediates nuclear chromosome segregation by cleaving cohesin to release sister chromatids. This is an accurate general process annotation; the more specific mitotic sister chromatid separation term is the core annotation. Reason: Accurate general process consistent with Cut1's role in chromosome segregation. Supporting Evidence: PMID:8978688 Fission yeast Schizosaccharomyces pombe temperature-sensitive (ts) cut1 mutants fail to separate sister chromatids in anaphase but the cells continue to divide, leading to bisection of the undivided nucleus (the cut phenotype). |
| GO:0051306 mitotic sister chromatid separation | NAS PMID:8978688 Fission yeast Cut1 and Cut2 are essential for sister chromat... | ACCEPT | Summary: Core biological process. Cut1 ts mutants fail to separate sister chromatids in anaphase, producing the cut phenotype. This is the central, defining function of Cut1/separase. Reason: Defining function of separase, established by classic cut1 mutant phenotypes and conserved across the family. Supporting Evidence: PMID:8978688 Fission yeast Schizosaccharomyces pombe temperature-sensitive (ts) cut1 mutants fail to separate sister chromatids in anaphase but the cells continue to divide, leading to bisection of the undivided nucleus (the cut phenotype). |
| GO:0051307 meiotic chromosome separation | NAS PMID:14532136 Rec8 cleavage by separase is required for meiotic nuclear di... | KEEP AS NON CORE | Summary: Cut1 cleaves the meiotic kleisin Rec8 to drive meiotic chromosome segregation. This is a genuine but secondary (non-core relative to the mitotic role) function. Reason: Real, experimentally supported meiotic function (Rec8 cleavage), but secondary to the core mitotic sister chromatid separation role. Supporting Evidence: PMID:14532136 we show that separase activation and resultant Rec8 cleavage are required for meiotic chromosome segregation in fission yeast. |
| GO:1990520 separase-securin complex | IPI PMID:8978688 Fission yeast Cut1 and Cut2 are essential for sister chromat... | ACCEPT | Summary: Core cellular component. Cut1 (separase) and Cut2 (securin) physically associate to form the separase-securin complex (ComplexPortal CPX-2573), shown by co-immunoprecipitation and co-sedimentation as large complexes. Reason: Directly demonstrated Cut1-Cut2 physical complex; the inhibitory securin complex is central to separase regulation. Supporting Evidence: PMID:8978688 Cut1 (approximately 200 kDa) and Cut2 (42 kDa) associate, as shown by immunoprecipitation, and co-sediment as large complexes (30 and 40S) in sucrose gradient centrifugation. |
| GO:0005634 nucleus | IDA PMID:9635190 Cut1 is loaded onto the spindle by binding to Cut2 and promo... | ACCEPT | Summary: Direct experimental localization of Cut1 to the nuclear spindle/spindle pole bodies in mitosis. Cut1 acts in the nucleus where it cleaves chromatin-bound cohesin. Reason: Direct experimental localization by PomBase curators; consistent with nuclear function. Supporting Evidence: PMID:9635190 Cut1 is retained in the cytoplasm during interphase and moves to the mitotic spindle pole bodies and the spindle upon entry into prophase, when spindles are formed. |
| GO:0140013 meiotic nuclear division | IMP PMID:1934126 Common genes and pathways in the regulation of the mitotic a... | KEEP AS NON CORE | Summary: Genetic evidence places cut1 in the meiotic program: a cut1 cut2 double mutant forms two-spored asci, although a single cut1 mutant alone did not affect sporulation. Cut1 has a genuine meiotic role (Rec8 cleavage), but this particular evidence is indirect; the annotation is best retained as a non-core process. Reason: Meiotic role is real but secondary to the core mitotic function, and the cited genetic evidence is indirect (phenotype seen only in the cut1 cut2 double mutant). Supporting Evidence: PMID:1934126 Single mutants in cut1 or cut2 did not effect sporulation, whereas the double mutant cut1 cut2 formed two-spored asci. |
| GO:0006974 DNA damage response | IMP PMID:15329725 Separase-mediated cleavage of cohesin at interphase is requi... | KEEP AS NON CORE | Summary: Cut1, with securin Cut2, cleaves cohesin locally during interphase to support DNA repair after UV, X-ray and gamma irradiation. Experiments with non-cleavable cohesin and a protease-dead separase show this DNA-repair function acts through cohesin cleavage. This is a genuine but secondary, non-core function. Reason: Experimentally supported interphase DNA-repair role via cohesin cleavage, but distinct from and secondary to the core anaphase chromosome-segregation function. Supporting Evidence: PMID:15329725 Experiments using either mutant cohesin that cannot be cleaved by separase or a protease-dead separase provide evidence that this DNA repair function of securin-separase acts through the cleavage of cohesin. |
| GO:0004175 endopeptidase activity | TAS PMID:12390246 Cut1/separase C-terminus affects spindle pole body positioni... | MODIFY | Summary: Cut1 is an endopeptidase, but the informative molecular function is the more specific cysteine-type endopeptidase activity (GO:0004197): separase is a CD-clan cysteine protease with active-site Cys1730 (EC 3.4.22.49). This generic parent term should be replaced by the cysteine-type term. Reason: The specific cysteine-type endopeptidase activity term is supported and more informative than the generic endopeptidase activity parent. Proposed replacements: cysteine-type endopeptidase activity Supporting Evidence: PMID:12390246 Separase activated by securin destruction cleaves the cohesin subunit Scc1/Rad21 enriched in kinetochores. |
| GO:0044732 mitotic spindle pole body | IDA PMID:9635190 Cut1 is loaded onto the spindle by binding to Cut2 and promo... | ACCEPT | Summary: Direct experimental evidence that Cut1 moves to the mitotic spindle pole bodies upon entry into prophase. Reason: Direct experimental localization to the SPB. Supporting Evidence: PMID:9635190 Cut1 is retained in the cytoplasm during interphase and moves to the mitotic spindle pole bodies and the spindle upon entry into prophase, when spindles are formed. |
| GO:1990023 mitotic spindle midzone | IDA PMID:9635190 Cut1 is loaded onto the spindle by binding to Cut2 and promo... | ACCEPT | Summary: After Cut2 degradation in anaphase, Cut1 remains bound to the elongating anaphase spindle (midzone), where it promotes anaphase spindle movement. Direct experimental localization supports this annotation. Reason: Direct experimental evidence that Cut1 remains associated with the anaphase spindle. Supporting Evidence: PMID:9635190 Cut2 degrades during anaphase, but Cut1 remains bound to the anaphase spindle. |
| GO:0072686 mitotic spindle | IDA PMID:8978688 Fission yeast Cut1 and Cut2 are essential for sister chromat... | ACCEPT | Summary: Direct experimental evidence that Cut1 concentrates along the metaphase spindle, together with Cut2. Reason: Direct experimental spindle localization. Supporting Evidence: PMID:8978688 Cut1 protein concentrates along the short spindle in metaphase as does Cut2. |
| GO:1990520 separase-securin complex | IDA PMID:8978688 Fission yeast Cut1 and Cut2 are essential for sister chromat... | ACCEPT | Summary: Core cellular component: Cut1 forms a physical complex with securin Cut2, demonstrated by co-immunoprecipitation and co-sedimentation. This is the regulatory complex that keeps separase inactive until anaphase. Reason: Directly demonstrated Cut1-Cut2 complex; central to separase regulation. Supporting Evidence: PMID:8978688 Cut1 (approximately 200 kDa) and Cut2 (42 kDa) associate, as shown by immunoprecipitation, and co-sediment as large complexes (30 and 40S) in sucrose gradient centrifugation. |
| GO:0004175 endopeptidase activity | IMP PMID:14532136 Rec8 cleavage by separase is required for meiotic nuclear di... | MODIFY | Summary: Mutant analysis shows Cut1 cleaves Rec8 at two identified sites; a non-cleavable Rec8 blocks meiotic segregation. This demonstrates endopeptidase activity, but the more specific cysteine-type endopeptidase activity (GO:0004197) is the informative molecular function for separase. Reason: The cleavage activity is real, but the specific cysteine-type endopeptidase term is more informative than the generic endopeptidase parent. Proposed replacements: cysteine-type endopeptidase activity Supporting Evidence: PMID:14532136 We identify two Cut1-dependent cleavage sites within Rec8 sequences and demonstrate that the homologue segregation at meiosis I is completely blocked by the expression of a non-cleavable form of Rec8. |
| GO:0051307 meiotic chromosome separation | IMP PMID:14532136 Rec8 cleavage by separase is required for meiotic nuclear di... | KEEP AS NON CORE | Summary: Separase activation and Rec8 cleavage are required for meiotic chromosome segregation; a non-cleavable Rec8 blocks homologue disjunction at meiosis I. A genuine but secondary function relative to the mitotic role. Reason: Experimentally supported meiotic function, secondary to the core mitotic role. Supporting Evidence: PMID:14532136 A non-cleavable form of Rec8 blocks disjunction of homologues at meiosis I. |
| GO:0005829 cytosol | HDA PMID:16823372 ORFeome cloning and global analysis of protein localization ... | ACCEPT | Summary: High-throughput YFP localization assigns Cut1 to the cytosol, consistent with the documented interphase cytoplasmic pool. This is a more granular subcellular term than the cytoplasm annotation and is compatible with the experimental data. Reason: High-throughput localization consistent with the experimentally established interphase cytoplasmic distribution of Cut1. Supporting Evidence: PMID:9635190 Cut1 is retained in the cytoplasm during interphase and moves to the mitotic spindle pole bodies and the spindle upon entry into prophase, when spindles are formed. |
| GO:0051306 mitotic sister chromatid separation | IMP PMID:8065367 Identification of cut8+ and cek1+, a novel protein kinase ge... | ACCEPT | Summary: cut1 was identified as a multicopy suppressor of the anaphase-blocking cut8-563 mutation and described as essential for nuclear division. Together with the classic cut1 ts phenotypes, this supports Cut1's core role in mitotic sister chromatid separation. Although the cited paper foregrounds cut8, the experimental assignment of cut1 to anaphase/sister separation is consistent with the overwhelming evidence for separase function. Reason: Core function of separase; supported here by genetic suppression data and broadly by the cut1 mutant literature. Defer to PomBase curators on the experimental annotation. Supporting Evidence: PMID:8065367 They are the cut1+ gene essential for nuclear division PMID:8978688 Fission yeast Schizosaccharomyces pombe temperature-sensitive (ts) cut1 mutants fail to separate sister chromatids in anaphase but the cells continue to divide, leading to bisection of the undivided nucleus (the cut phenotype). |
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Download this section (compressed HTML)Q: What is the relative biological importance of Cut1's proteolytic cohesin-cleavage function versus its protease-independent contributions to anaphase spindle elongation and SPB positioning?
Q: How is Cut1 catalytic activity regulated beyond securin binding (e.g., phosphorylation, autocleavage) in fission yeast?
Experiment: Quantitative mass spectrometry / N-terminomics on synchronized mitotic and meiotic cells expressing protease-dead Cut1 (Cys1730 mutant) versus wild type to map the full set of in vivo Cut1 cleavage substrates and confirm Rad21/Rec8 cleavage sites.
Experiment: Live-cell imaging of Cut1, Cut2, and Rad21 with degron-based acute depletion to dissect the temporal coupling between securin degradation, Cut1 activation, cohesin cleavage, and anaphase spindle midzone behaviour.
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