Clb2 is the principal mitotic B-type cyclin (cyclin A/B subfamily) of the budding yeast Saccharomyces cerevisiae and the regulatory subunit of the cyclin-dependent kinase Cdc28 (Cdk1). Clb2 has no catalytic activity of its own: by binding Cdc28 it forms the Clb2-Cdc28-Cks1 holoenzyme, induces the active kinase conformation and confers substrate preference and subcellular targeting on the kinase. CLB2 is transcribed from late S phase as part of the Mcm1-Fkh2-Ndd1-dependent CLB2 cluster (with positive feedback through Clb2-Cdc28 phosphorylation of Ndd1 and Fkh2), so Clb2-Cdc28 activity rises through G2 and supplies the bulk of the M-phase CDK activity that drives the G2/M transition. Its outputs include separation of the duplicated spindle pole bodies and bipolar spindle assembly (via phosphorylation of the kinesin-5 motors Kip1 and Cin8), promotion of anaphase spindle elongation through Net1 phosphorylation and Cdc14 release (FEAR pathway), the switch from polarized to isotropic bud growth, inhibition of the G1 transcriptional program, retention of phosphorylated Cdc6 in a licensing-incompetent complex to prevent re-replication, and, at DNA double-strand breaks, local Cdk1 activity toward Fun30 and checkpoint proteins. Clb2-Cdc28 is held in check by Swe1-mediated inhibitory phosphorylation of Cdc28 Tyr19 (reversed by Mih1), by the stoichiometric inhibitor Sic1 and by the GSK-3 kinase Mck1; Clb2 is destroyed by APC/C-Cdc20 from anaphase and by APC/C-Cdh1 (Hct1) for mitotic exit and through G1, and its levels are minimal in G1 and undetectable during meiosis. The protein is primarily nuclear (bipartite NLS, importin-dependent) but shuttles via a leucine-rich NES and is also found on the mitotic spindle, at spindle pole bodies and, uniquely among the Clbs, at the mother-bud neck through Bud3-dependent targeting. CLB1 is a close paralog with partially redundant function; clb1 clb2 double mutants are inviable and arrest in G2/M.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000082 G1/S transition of mitotic cell cycle | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: IBA propagation of G1/S transition from the deep cyclin node PTN000019791, which spans G1/S cyclins (cyclin E/A/D, Cln/Clb5-type) and mitotic B-type cyclins. Clb2 is the late-S/G2/M cyclin of budding yeast: its transcription (CLB2 cluster, Mcm1-Fkh2-Ndd1) starts in late S phase, it is absent in G1, and G1/S commitment is executed by Cln1-3 and Clb5/6-Cdc28. Reason: Clb2-Cdc28 acts at G2/M, not G1/S. Grandin & Reed show Clb1/Clb2 are expressed and activate Cdc28 only from late S phase up to mitosis, and Clb2-Cdc28 actually antagonises the G1 program (SBF inhibition) and holds Cdc6 in a licensing-incompetent complex. The G1/S function belongs to other members of the ancestral cyclin node and has functionally diverged from the B-type mitotic cyclins, so the propagated term overstates Clb2's role (same call as the human CCNB1 and fission-yeast cdc13 reviews). Propagation Review Root cause: PROPAGATION BAD Failure modes: FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTN000019791 SUPPORTS SOURCE BUT NOT TARGET Ancestral cyclin node grouping G1/S cyclins with mitotic A/B cyclins; the G1/S-transition function is inherited by the G1/S cyclins (e.g. Clb5, cyclin E/A), not by the mitotic B-type cyclin Clb2. Supporting Evidence: PMID:8455600 Clb1 and Clb2 were expressed and activated p34cdc28 later in the mitotic cell cycle, starting in late S phase and continuing up to mitosis. |
| GO:0000086 G2/M transition of mitotic cell cycle | IMP PMID:1849457 The role of CDC28 and cyclins during mitosis in the budding ... | ACCEPT | Summary: Founding genetic evidence: CLB2 was cloned as a high-copy suppressor of the mitosis-defective (but Start-competent) cdc28-1N allele; clb1 clb2 double disruption is lethal with a mitotic defect, and cdc28-1N clb2 is inviable, placing Clb2-Cdc28 at the G2/M transition. This is the defining function of the gene. Reason: Clb2 is the principal mitotic B-type cyclin; the Clb2-Cdc28 holoenzyme supplies the M-phase CDK activity whose rise commits the cell to mitosis (the definition of GO:0000086). Genetic (this paper), biochemical (Clb2-associated H1 kinase peaking at G2/M) and localization data all converge on this core role. Supporting Evidence: PMID:1849457 Neither CLB1 nor CLB2 is essential; however, disruption of both is lethal and causes a mitotic defect. PMID:1849457 the double mutant cdc28-1N clb2::LEU2 is nonviable, whereas cdc28-4 clb2::LEU2 is viable, suggesting that the cdc28-1N protein may be defective in its interaction with B-type cyclins. |
| GO:0000086 G2/M transition of mitotic cell cycle | IMP PMID:8455600 Differential function and expression of Saccharomyces cerevi... | ACCEPT | Summary: Grandin & Reed profiled expression and Cdc28-associated kinase activity of all four Clbs: Clb2 is expressed and activates Cdc28 from late S phase to mitosis, is the primary mitotic cyclin, and is dispensable for (and undetectable in) meiosis. Reason: Direct biochemical timing of Clb2-Cdc28 kinase activity plus deletion phenotypes support the G2/M transition as the core process; the same paper shows Clb2 has no meiotic role, which sharpens the annotation to the mitotic cell cycle. Supporting Evidence: PMID:8455600 Clb1 and Clb2 were expressed and activated p34cdc28 later in the mitotic cell cycle, starting in late S phase and continuing up to mitosis. PMID:8455600 Clb2, the primary mitotic cyclin in S. cerevisiae, was not detectable during meiosis. |
| GO:0000307 cyclin-dependent protein kinase holoenzyme complex | IBA GO_REF:0000033 | ACCEPT | Summary: IBA part_of the cyclin-dependent protein kinase holoenzyme complex. Clb2 is the regulatory subunit of the Clb2-Cdc28 (Cdk1) kinase complex (ComplexPortal CPX-1701), which also carries Cks1; this is exactly what the term describes. Reason: The cyclin fold is the defining component of a CDK holoenzyme; every experimental row for Clb2 (kinase activation, Y19 regulation by Swe1/Mih1, Mck1 inhibition) assays the Clb2-Cdc28 complex. Sound phylogenetic transfer from a node in which all descendants are CDK partners. Supporting Evidence: PMID:11171327 The cyclin-dependent kinase Cdc28p associates with the cyclin Clb2p to induce mitosis in the yeast Saccharomyces cerevisiae. PMID:22918234 Clb2-Cdk1, the major mitotic cyclin-Cdk complex in yeast, accumulates before anaphase and must be inhibited in telophase for cells to exit mitosis and enter into the next cell cycle. |
| GO:0005515 protein binding | IPI PMID:10688190 A comprehensive analysis of protein-protein interactions in ... | REMOVE | Summary: Generic protein binding from the Uetz et al. genome-wide two-hybrid screen (interactor: Cks1, the Cdc28 accessory/phospho-adaptor subunit). The Clb2-Cks1 association is real (Cks1 is part of the Clb2-Cdc28-Cks1 holoenzyme) but GO:0005515 conveys no molecular function, and the complex membership is already captured by GO:0000307. Reason: Per project curation policy, a bare GO:0005515 protein binding annotation carries no functional information and is removed unless the cited paper supports a more informative molecular function. Clb2's informative activity (activation and substrate/location targeting of the Cdc28 kinase) is captured by GO:0016538/GO:0061575 and the CDK holoenzyme complex annotation. Removal does not dispute the reported interaction. Cks1 is not a kinase, so no more specific binding term is supported; the Clb2-Cdc28-Cks1 assembly is represented by the CDK holoenzyme complex row. Supporting Evidence: PMID:10688190 These approaches resulted in the detection of 957 putative interactions involving 1,004 S. cerevisiae proteins. |
| GO:0005515 protein binding | IPI PMID:11805837 Systematic identification of protein complexes in Saccharomy... | REMOVE | Summary: Generic protein binding from the Ho et al. HMS-PCI mass-spectrometry complex survey (interactor: Cks1). High-throughput co-purification consistent with the Clb2-Cdc28-Cks1 holoenzyme; uninformative as a molecular function. Reason: Per project curation policy, a bare GO:0005515 protein binding annotation carries no functional information and is removed unless the cited paper supports a more informative molecular function. Clb2's informative activity (activation and substrate/location targeting of the Cdc28 kinase) is captured by GO:0016538/GO:0061575 and the CDK holoenzyme complex annotation. Removal does not dispute the reported interaction. The Cks1 co-purification is already reflected in the CDK holoenzyme complex annotation (GO:0000307). Supporting Evidence: PMID:11805837 Beginning with 10% of predicted yeast proteins as baits, we detected 3,617 associated proteins covering 25% of the yeast proteome. |
| GO:0005515 protein binding | IPI PMID:22078879 A quantitative model for ordered Cdk substrate dephosphoryla... | REMOVE | Summary: Generic protein binding derived from Bouchoux & Uhlmann's kinetic analysis of Cdk substrate phosphorylation/dephosphorylation during mitotic exit (interactor: Ask1 (DASH complex)). The partner is a Cdk1 substrate assayed with Clb2-Cdk1; the relationship is kinase-complex-to-substrate, not a distinct molecular function of the cyclin. Reason: Per project curation policy, a bare GO:0005515 protein binding annotation carries no functional information and is removed unless the cited paper supports a more informative molecular function. Clb2's informative activity (activation and substrate/location targeting of the Cdc28 kinase) is captured by GO:0016538/GO:0061575 and the CDK holoenzyme complex annotation. Removal does not dispute the reported interaction. The cached record is abstract-only, but the study's design (in vitro Cdk substrate phosphorylation/dephosphorylation kinetics) makes clear the interaction reflects Clb2-Cdk1 acting on a substrate; that biology is carried by the CDK regulator/activator activity, not by bare protein binding. Supporting Evidence: PMID:22078879 kinetic analysis of Cdk substrate phosphorylation and dephosphorylation in vitro |
| GO:0005515 protein binding | IPI PMID:22078879 A quantitative model for ordered Cdk substrate dephosphoryla... | REMOVE | Summary: Generic protein binding derived from Bouchoux & Uhlmann's kinetic analysis of Cdk substrate phosphorylation/dephosphorylation during mitotic exit (interactor: Sli15 (chromosomal passenger/INCENP)). The partner is a Cdk1 substrate assayed with Clb2-Cdk1; the relationship is kinase-complex-to-substrate, not a distinct molecular function of the cyclin. Reason: Per project curation policy, a bare GO:0005515 protein binding annotation carries no functional information and is removed unless the cited paper supports a more informative molecular function. Clb2's informative activity (activation and substrate/location targeting of the Cdc28 kinase) is captured by GO:0016538/GO:0061575 and the CDK holoenzyme complex annotation. Removal does not dispute the reported interaction. The cached record is abstract-only, but the study's design (in vitro Cdk substrate phosphorylation/dephosphorylation kinetics) makes clear the interaction reflects Clb2-Cdk1 acting on a substrate; that biology is carried by the CDK regulator/activator activity, not by bare protein binding. Supporting Evidence: PMID:22078879 kinetic analysis of Cdk substrate phosphorylation and dephosphorylation in vitro |
| GO:0005515 protein binding | IPI PMID:22078879 A quantitative model for ordered Cdk substrate dephosphoryla... | REMOVE | Summary: Generic protein binding derived from Bouchoux & Uhlmann's kinetic analysis of Cdk substrate phosphorylation/dephosphorylation during mitotic exit (interactor: Orc6 (origin recognition complex)). The partner is a Cdk1 substrate assayed with Clb2-Cdk1; the relationship is kinase-complex-to-substrate, not a distinct molecular function of the cyclin. Reason: Per project curation policy, a bare GO:0005515 protein binding annotation carries no functional information and is removed unless the cited paper supports a more informative molecular function. Clb2's informative activity (activation and substrate/location targeting of the Cdc28 kinase) is captured by GO:0016538/GO:0061575 and the CDK holoenzyme complex annotation. Removal does not dispute the reported interaction. The cached record is abstract-only, but the study's design (in vitro Cdk substrate phosphorylation/dephosphorylation kinetics) makes clear the interaction reflects Clb2-Cdk1 acting on a substrate; that biology is carried by the CDK regulator/activator activity, not by bare protein binding. Supporting Evidence: PMID:22078879 kinetic analysis of Cdk substrate phosphorylation and dephosphorylation in vitro |
| GO:0005515 protein binding | IPI PMID:22078879 A quantitative model for ordered Cdk substrate dephosphoryla... | REMOVE | Summary: Generic protein binding derived from Bouchoux & Uhlmann's kinetic analysis of Cdk substrate phosphorylation/dephosphorylation during mitotic exit (interactor: Fin1 (spindle/PP1-targeting filament protein)). The partner is a Cdk1 substrate assayed with Clb2-Cdk1; the relationship is kinase-complex-to-substrate, not a distinct molecular function of the cyclin. Reason: Per project curation policy, a bare GO:0005515 protein binding annotation carries no functional information and is removed unless the cited paper supports a more informative molecular function. Clb2's informative activity (activation and substrate/location targeting of the Cdc28 kinase) is captured by GO:0016538/GO:0061575 and the CDK holoenzyme complex annotation. Removal does not dispute the reported interaction. The cached record is abstract-only, but the study's design (in vitro Cdk substrate phosphorylation/dephosphorylation kinetics) makes clear the interaction reflects Clb2-Cdk1 acting on a substrate; that biology is carried by the CDK regulator/activator activity, not by bare protein binding. Supporting Evidence: PMID:22078879 kinetic analysis of Cdk substrate phosphorylation and dephosphorylation in vitro |
| GO:0005515 protein binding | IPI PMID:22918234 The Mck1 GSK-3 kinase inhibits the activity of Clb2-Cdk1 pos... | MODIFY | Summary: Co-immunoprecipitation of Clb2 with the GSK-3 kinase Mck1; Mck1 binds Clb2 (not Cdk1) and inhibits Clb2-Cdk1 kinase activity in vitro without phosphorylating Clb2, acting as a post-nuclear-division inhibitor of the mitotic kinase. Because the partner is a protein kinase and the binding is the proposed inhibitory mechanism, protein kinase binding is a more informative term than bare protein binding. Reason: The paper supports an informative molecular-function refinement: Clb2 directly binds the Mck1 protein kinase, and this binding (rather than phosphorylation) is the likely route by which Mck1 dampens Clb2-Cdk1 activity at the end of mitosis. GO:0019901 protein kinase binding captures this; GO:0005515 does not. Proposed replacements: protein kinase binding Supporting Evidence: PMID:22918234 We observe a direct interaction between Mck1 and Clb2 but not Cdk1 (data not shown); therefore, it is most plausible that Mck1 inhibits Clb2-Cdk1 activity through binding of Clb2. PMID:22918234 Clb2-HA was present in the Mck1-Myc IPs but not in the IP from an untagged strain |
| GO:0005515 protein binding | IPI PMID:37968396 The social and structural architecture of the yeast protein ... | REMOVE | Summary: Generic protein binding from the Michaelis et al. proteome-wide affinity-enrichment MS interactome (interactor: Cks1). Again consistent with the Clb2-Cdc28-Cks1 holoenzyme; no molecular function conveyed. Reason: Per project curation policy, a bare GO:0005515 protein binding annotation carries no functional information and is removed unless the cited paper supports a more informative molecular function. Clb2's informative activity (activation and substrate/location targeting of the Cdc28 kinase) is captured by GO:0016538/GO:0061575 and the CDK holoenzyme complex annotation. Removal does not dispute the reported interaction. Complex membership with Cks1/Cdc28 is represented by GO:0000307. Supporting Evidence: PMID:37968396 The 4,159 pull-downs generated a highly structured network of 3,927 proteins connected by 31,004 interactions |
| GO:0005515 protein binding | IPI PMID:7622566 Members of the NAP/SET family of proteins interact specifica... | REMOVE | Summary: Affinity-chromatography identification of Nap1 as a B-type-cyclin-specific Clb2 interactor (Nap1 binds Clb2 but not Clb3), with genetic evidence that NAP1 contributes to Clb2 function (bud growth/ mitotic events). Real and specific interaction, but GO:0005515 says nothing about the function; Nap1 is a histone/protein chaperone and no more specific binding term is supported. Reason: Per project curation policy, a bare GO:0005515 protein binding annotation carries no functional information and is removed unless the cited paper supports a more informative molecular function. Clb2's informative activity (activation and substrate/location targeting of the Cdc28 kinase) is captured by GO:0016538/GO:0061575 and the CDK holoenzyme complex annotation. Removal does not dispute the reported interaction. The Nap1 interaction is biologically meaningful (Nap1 is required for full Clb2-dependent switching to isotropic bud growth) but is best captured in notes and as a suggested question, not as a bare binding annotation. Supporting Evidence: PMID:7622566 the S. cerevisiae homolog of NAP1 interacts with Clb2 but not with Clb3 PMID:7622566 Genetic experiments in budding yeast indicate that NAP1 plays an important role in the function of Clb2 |
| GO:0005515 protein binding | IPI PMID:7622566 Members of the NAP/SET family of proteins interact specifica... | REMOVE | Summary: Generic protein binding with a Xenopus laevis NAP1 protein (nap1l1-a (Q4U0Y4)) as the WITH partner, from the same Kellogg et al. study. The abstract reports Xenopus NAP1 binding Xenopus cyclins B1/B2 and yeast Nap1 binding yeast Clb2; a cross-species yeast-Clb2/Xenopus-NAP1 pairing may be in the full text (not cached) and is not disputed here, but the term is uninformative either way. Reason: Per project curation policy, a bare GO:0005515 protein binding annotation carries no functional information and is removed unless the cited paper supports a more informative molecular function. Clb2's informative activity (activation and substrate/location targeting of the Cdc28 kinase) is captured by GO:0016538/GO:0061575 and the CDK holoenzyme complex annotation. Removal does not dispute the reported interaction. The cross-species pairing cannot be verified from the cached abstract; whether or not it was assayed, GO:0005515 adds no functional information beyond the yeast Nap1 row. Supporting Evidence: PMID:7622566 NAP1 in the Xenopus embryo interacts with cyclins B1 and B2, but not with cyclin A, and the S. cerevisiae homolog of NAP1 interacts with Clb2 but not with Clb3. |
| GO:0005515 protein binding | IPI PMID:7622566 Members of the NAP/SET family of proteins interact specifica... | REMOVE | Summary: Generic protein binding with a Xenopus laevis NAP1 protein (nap1l1-b (Q7ZY81)) as the WITH partner, from the same Kellogg et al. study. The abstract reports Xenopus NAP1 binding Xenopus cyclins B1/B2 and yeast Nap1 binding yeast Clb2; a cross-species yeast-Clb2/Xenopus-NAP1 pairing may be in the full text (not cached) and is not disputed here, but the term is uninformative either way. Reason: Per project curation policy, a bare GO:0005515 protein binding annotation carries no functional information and is removed unless the cited paper supports a more informative molecular function. Clb2's informative activity (activation and substrate/location targeting of the Cdc28 kinase) is captured by GO:0016538/GO:0061575 and the CDK holoenzyme complex annotation. Removal does not dispute the reported interaction. The cross-species pairing cannot be verified from the cached abstract; whether or not it was assayed, GO:0005515 adds no functional information beyond the yeast Nap1 row. Supporting Evidence: PMID:7622566 NAP1 in the Xenopus embryo interacts with cyclins B1 and B2, but not with cyclin A, and the S. cerevisiae homolog of NAP1 interacts with Clb2 but not with Clb3. |
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: IBA is_active_in nucleus. Clb2 carries a bipartite NLS and a leucine-rich NES, is primarily nuclear throughout the cycle, and its best-characterised substrates (Ndd1/Fkh2, Net1, Cdc6, Swe1, Fun30) are nuclear. Reason: Concordant with two IDA rows and with the functional evidence; nuclear Clb2-Cdc28 drives mitotic transcription, spindle events and origin-licensing control. Supporting Evidence: PMID:11171327 Wild-type Clb2p is primarily nuclear at all points of the cell. |
| GO:0005634 nucleus | IDA PMID:11171327 The Saccharomyces cerevisiae cyclin Clb2p is targeted to mul... | ACCEPT | Summary: GFP-Clb2 imaging: wild-type Clb2 is primarily nuclear at all cell-cycle stages; nuclear import requires a bipartite NLS, Srp1/Kap95 importins and the Ran system, and a leucine-rich NES/Yrb2-dependent export pathway shuttles it out. Reason: Direct localization of the endogenous-promoter fusion; the nucleus is the principal site of Clb2-Cdc28 action. Supporting Evidence: PMID:11171327 Wild-type Clb2p is primarily nuclear at all points of the cell. PMID:11171327 Clb2p contains a bipartite nuclear localization signal (NLS), and its nuclear localization requires the alpha and beta importins (Srp1p and Kap95p), as well as the yeast Ran GTPase and its regulators. |
| GO:0005634 nucleus | IDA PMID:12972503 Differential cellular localization among mitotic cyclins fro... | ACCEPT | Summary: Bailly et al. confirm nuclear localization of GFP-Clb2 expressed from its own promoter, alongside spindle, SPB and bud-neck pools. Reason: Independent IDA confirmation of nuclear localization. Supporting Evidence: PMID:12972503 GFP-tagged Clb2 expressed from its natural promoter localizes to various cellular compartments, including the nucleus, the mitotic spindle, the spindle pole bodies as well as the mother-bud neck. |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: IBA is_active_in cytoplasm. Clb2 is a nucleocytoplasmic shuttling cyclin (NES/Yrb2-dependent export) with a cytoplasmic pool that reaches the mother-bud neck, where Clb2-Cdc28 acts on morphogenesis and late mitotic events. Reason: A genuine, functionally relevant cytoplasmic pool exists (bud-neck targeting, control of the apical-to-isotropic growth switch), consistent with the IDA/IMP rows; broad but correct. Supporting Evidence: PMID:11171327 These data indicate that Clb2p exists in multiple places in the yeast cell, possibly allowing Cdc28p to locally phosphorylate substrates at distinct subcellular sites. |
| GO:0005737 cytoplasm | IDA PMID:12972503 Differential cellular localization among mitotic cyclins fro... | ACCEPT | Summary: Imaging of GFP-Clb2 shows a cytoplasmic fraction in addition to nucleus, spindle, SPB and bud neck. Reason: Direct observation; minor relative to the nuclear pool but real and functionally engaged at the neck. Supporting Evidence: PMID:12972503 GFP-tagged Clb2 expressed from its natural promoter localizes to various cellular compartments, including the nucleus, the mitotic spindle, the spindle pole bodies as well as the mother-bud neck. |
| GO:0005737 cytoplasm | IMP PMID:11171327 The Saccharomyces cerevisiae cyclin Clb2p is targeted to mul... | ACCEPT | Summary: IMP-based localization: deleting the Clb2 NLS increases cytoplasmic Clb2 and bud-neck accumulation, and NES/yrb2 mutants show a nuclear-export defect, implying wild-type Clb2 normally cycles through the cytoplasm. Reason: Mutant analysis establishes an export-dependent cytoplasmic pool; consistent with the IDA rows. Wild-type protein is predominantly nuclear, so this is a secondary location. Supporting Evidence: PMID:11171327 Deletion of the Clb2p NLS causes increased cytoplasmic localization of the protein, as well as accumulation at the bud neck. |
| GO:0005815 microtubule organizing center | IBA GO_REF:0000033 | ACCEPT | Summary: IBA is_active_in microtubule organizing center (node PTN007424001, seeded by pombe cdc13/cig2, human cyclin B1/B2 and Clb2 itself). In budding yeast the MTOC is the spindle pole body, where Clb2 is observed by IDA and where Clb2-Cdc28 phosphorylates kinesin-5 motors to drive SPB separation. Reason: Cyclin B/CDK localization to the centrosome/SPB is a conserved feature of mitotic cyclins (hydrophobic-patch dependent in cdc13); the SGD IDA for spindle pole body grounds the propagation on the target itself. Supporting Evidence: PMID:12972503 GFP-tagged Clb2 expressed from its natural promoter localizes to various cellular compartments, including the nucleus, the mitotic spindle, the spindle pole bodies as well as the mother-bud neck. |
| GO:0005816 spindle pole body | IDA PMID:12972503 Differential cellular localization among mitotic cyclins fro... | ACCEPT | Summary: GFP-Clb2 decorates the spindle pole bodies; functionally, Clb2-Cdc28 promotes SPB separation and spindle assembly via kinesin-5 (Kip1/Cin8) phosphorylation. Reason: Direct localization at a site where the complex has a well-defined substrate set (Kip1/Cin8, Spc110-associated events). Supporting Evidence: PMID:12972503 GFP-tagged Clb2 expressed from its natural promoter localizes to various cellular compartments, including the nucleus, the mitotic spindle, the spindle pole bodies as well as the mother-bud neck. |
| GO:0005819 spindle | IDA PMID:12972503 Differential cellular localization among mitotic cyclins fro... | ACCEPT | Summary: GFP-Clb2 localizes along the mitotic spindle. Reason: Direct localization; consistent with Clb2-Cdc28 control of spindle assembly and elongation. Supporting Evidence: PMID:12972503 GFP-tagged Clb2 expressed from its natural promoter localizes to various cellular compartments, including the nucleus, the mitotic spindle, the spindle pole bodies as well as the mother-bud neck. |
| GO:0005935 cellular bud neck | IDA PMID:12972503 Differential cellular localization among mitotic cyclins fro... | KEEP AS NON CORE | Summary: Clb2 (uniquely among the Clbs) accumulates at the mother-bud neck, targeted by Bud3 via a central region (residues 213-255) and the hydrophobic patch; loss of neck targeting delays cytokinesis/cell separation. Reason: Real, Clb2-specific localization with a plausible role in late mitotic events, but it is a peripheral site relative to the nuclear/SPB pools that execute the core G2/M function; graded non-core together with the morphogenesis outputs. Supporting Evidence: PMID:12972503 The neck localization is specific to Clb2 as Clb1, Clb3 and Clb4 are never observed there, even when over-expressed. PMID:12972503 Clb2 is shown to be specifically delocalized in Deltabud3 cells and in a bud3 mutant deleted for its C-terminal Clb2-interacting domain (bud3(Delta1221)), but not in a Deltabud10 mutant. |
| GO:0006974 DNA damage response | IDA PMID:26801641 Enrichment of Cdk1-cyclins at DNA double-strand breaks stimu... | KEEP AS NON CORE | Summary: ComplexPortal-derived IDA: Cdk1 and cyclins including Clb2 are enriched at HO-induced double-strand breaks; clb2 deletion impairs Rad9/Rad53 phosphorylation (checkpoint activation) and Rad9 recruitment, raises the rate of extensive resection, and purified Cdk1-Clb2 phosphorylates the Fun30 remodeler on S28 in vitro. Clb2 thus directs Cdk1 to DDR substrates locally at breaks. Reason: Participation is genuine (the Clb2-Cdk1 complex does the phosphorylation of Fun30 and checkpoint targets, and Clb2 itself is recruited), and the phenotypes are Clb2-specific, partly redundant with Clb5. It is nevertheless a context-dependent output of the mitotic kinase rather than the gene's core function, so it is retained as non-core. Supporting Evidence: PMID:26801641 the recruitment of Cdk1 and cyclins Clb2 and Clb5 ensures optimal Fun30 phosphorylation and checkpoint activation PMID:26801641 we found Rad53 phosphorylation is impaired in the clb2Ξ mutant and that the defect is more severe in the clb2Ξ clb5Ξ double mutant PMID:26801641 In contrast to clb5Ξ, clb2Ξ mutant cells have a higher rate of extensive resection (Figure 5C) |
| GO:0007089 traversing start control point of mitotic cell cycle | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: IBA to passage through Start (G1/S commitment) from node PTN000808767, whose only experimental donor is fission-yeast cdc13. In S. pombe that annotation rests on the engineered situation in which Cdc13-Cdc2 is the sole cyclin-CDK and can drive G1/S; in budding yeast Start is executed by Cln3 then Cln1/2-Cdc28 with SBF/MBF, Clb2 is not present in G1 (CLB2-cluster transcription begins in late S), and Clb2-Cdc28 in fact inhibits SBF and holds Cdc6 inactive. Reason: The native Clb2 function is at G2/M; there is no evidence Clb2 participates in the G1 CDK-transcription positive-feedback loop that defines GO:0007089, and the G1/S row (GO:0000082) is graded the same way. The fission-yeast cdc13 review in this repository also marks the source annotation as over-annotated for the endogenous protein. Propagation Review Root cause: PROPAGATION BAD Failure modes: FUNCTIONAL DIVERGENCE SOURCE EVIDENCE WEAK Sources checked: PANTHER:PTN000808767 SUPPORTS SOURCE BUT NOT TARGET Node seeded only by S. pombe cdc13, whose Start annotation reflects an engineered single-cyclin cell; in wild-type budding yeast the Start function is carried by the Cln cyclins and Clb2 is absent in G1. Supporting Evidence: PMID:8455600 Clb1 and Clb2 were expressed and activated p34cdc28 later in the mitotic cell cycle, starting in late S phase and continuing up to mitosis. |
| GO:0010696 positive regulation of mitotic spindle pole body separation | IGI PMID:1387566 Characterization of four B-type cyclin genes of the budding ... | ACCEPT | Summary: IGI with CLB1: Fitch et al. built all clb deletion combinations; every lethal combination included clb2, and conditional clb mutants arrested as large-budded G2 cells whose SPBs had duplicated but not separated, with no spindle. Clb2-Cdc28 later shown to phosphorylate the kinesin-5 motors Kip1/Cin8 that drive SPB separation, giving a direct mechanism. Reason: Genetic requirement for Clb/Cdc28 in SPB separation and spindle formation, with Clb2 the key contributor, and a direct substrate (Kip1 phosphosites required for SPB separation) reported subsequently. This is a core mitotic output of the Clb2-Cdc28 kinase; the IGI partner is a redundant B-type cyclin. Supporting Evidence: PMID:1387566 All lethal combinations included the clb2 deletion, whereas the clb1 clb3 clb4 triple mutant was viable, suggesting a key role for CLB2. PMID:1387566 Electron microscopy showed that the spindle pole bodies had duplicated but not separated, and no spindle had formed. This suggests that the Clb/Cdc28 kinase may have a relatively direct role in spindle formation. |
| GO:0010696 positive regulation of mitotic spindle pole body separation | IGI PMID:1387566 Characterization of four B-type cyclin genes of the budding ... | ACCEPT | Summary: IGI with CLB3: Fitch et al. built all clb deletion combinations; every lethal combination included clb2, and conditional clb mutants arrested as large-budded G2 cells whose SPBs had duplicated but not separated, with no spindle. Clb2-Cdc28 later shown to phosphorylate the kinesin-5 motors Kip1/Cin8 that drive SPB separation, giving a direct mechanism. Reason: Genetic requirement for Clb/Cdc28 in SPB separation and spindle formation, with Clb2 the key contributor, and a direct substrate (Kip1 phosphosites required for SPB separation) reported subsequently. This is a core mitotic output of the Clb2-Cdc28 kinase; the IGI partner is a redundant B-type cyclin. Supporting Evidence: PMID:1387566 All lethal combinations included the clb2 deletion, whereas the clb1 clb3 clb4 triple mutant was viable, suggesting a key role for CLB2. PMID:1387566 Electron microscopy showed that the spindle pole bodies had duplicated but not separated, and no spindle had formed. This suggests that the Clb/Cdc28 kinase may have a relatively direct role in spindle formation. |
| GO:0010696 positive regulation of mitotic spindle pole body separation | IGI PMID:1387566 Characterization of four B-type cyclin genes of the budding ... | ACCEPT | Summary: IGI with CLB4: Fitch et al. built all clb deletion combinations; every lethal combination included clb2, and conditional clb mutants arrested as large-budded G2 cells whose SPBs had duplicated but not separated, with no spindle. Clb2-Cdc28 later shown to phosphorylate the kinesin-5 motors Kip1/Cin8 that drive SPB separation, giving a direct mechanism. Reason: Genetic requirement for Clb/Cdc28 in SPB separation and spindle formation, with Clb2 the key contributor, and a direct substrate (Kip1 phosphosites required for SPB separation) reported subsequently. This is a core mitotic output of the Clb2-Cdc28 kinase; the IGI partner is a redundant B-type cyclin. Supporting Evidence: PMID:1387566 All lethal combinations included the clb2 deletion, whereas the clb1 clb3 clb4 triple mutant was viable, suggesting a key role for CLB2. PMID:1387566 Electron microscopy showed that the spindle pole bodies had duplicated but not separated, and no spindle had formed. This suggests that the Clb/Cdc28 kinase may have a relatively direct role in spindle formation. |
| GO:0016538 cyclin-dependent protein serine/threonine kinase regulator activity | IBA GO_REF:0000033 | ACCEPT | Summary: IBA cyclin-dependent protein serine/threonine kinase regulator activity from the deep cyclin node PTN000019791; Clb2 is one of the experimental donors (SGD:S000006323). This is the molecular function of every cyclin and is well grounded on the target. Reason: Clb2 binds and activates Cdc28, determines its substrate preference (Swe1 via N260/K270, Kip1/Cin8, Ndd1/Fkh2, Cdc6) and its localization. The more specific child GO:0061575 (activator activity) is used in core_functions; the regulator term is correct and additionally covers substrate/location targeting. Supporting Evidence: PMID:8455600 Clb1 and Clb2 were expressed and activated p34cdc28 later in the mitotic cell cycle, starting in late S phase and continuing up to mitosis. |
| GO:0016538 cyclin-dependent protein serine/threonine kinase regulator activity | IDA PMID:1731251 Regulation of p34CDC28 tyrosine phosphorylation is not requi... | ACCEPT | Summary: Amon et al. showed that Cdc28 undergoes cell-cycle-regulated Y19 phosphorylation but that Y19 dephosphorylation is not required for activation of the CDC28/cyclin kinase; the Clb2-associated H1 kinase assays in the full text (not cached) are the direct evidence for Clb2 activating Cdc28. Reason: The cached abstract only names the CDC28/cyclin kinase, but the SGD curator read the full text, which assays Clb2-associated H1 kinase activity across the cycle and in the Y19F mutant; the regulator activity is unambiguously correct for Clb2 and is deferred to the curator per project rules. The direct assay (Clb2 activating Cdc28) also supports the more specific child GO:0061575 activator activity, which is used in core_functions; the regulator term is retained here for consistency with the IBA/IEA rows and because it additionally covers substrate and location targeting. Supporting Evidence: PMID:1731251 dephosphorylation of Y19 is not required for the activation of the CDC28/cyclin kinase |
| GO:0016538 cyclin-dependent protein serine/threonine kinase regulator activity | IEA GO_REF:0000120 | ACCEPT | Summary: InterPro/ARBA electronic mapping of cyclin A/B-like domain (IPR046965) to CDK regulator activity. Reason: Correct family-level mapping, fully corroborated by the experimental and IBA rows. |
| GO:0016538 cyclin-dependent protein serine/threonine kinase regulator activity | IMP PMID:1731251 Regulation of p34CDC28 tyrosine phosphorylation is not requi... | ACCEPT | Summary: IMP from the same Amon et al. study (mutational analysis of Cdc28 Y19 and cyclin-dependent kinase activation). Reason: Same evidence base as the IDA row; the regulator activity is the defining function of Clb2. Supporting Evidence: PMID:1731251 dephosphorylation of Y19 is not required for the activation of the CDC28/cyclin kinase |
| GO:0016538 cyclin-dependent protein serine/threonine kinase regulator activity | IMP PMID:1849457 The role of CDC28 and cyclins during mitosis in the budding ... | ACCEPT | Summary: IMP: high-copy CLB2 suppresses cdc28-1N and cdc28-1N clb2 is inviable, showing that Clb2 regulates the mitotic activity of the Cdc28 kinase. Reason: Genetic evidence that Clb2 is the B-type cyclin partner through which Cdc28 executes its mitotic role. Supporting Evidence: PMID:1849457 Its mitotic role, we believe, involves interaction with a family of at least four G2-specific cyclins. |
| GO:0032888 regulation of mitotic spindle elongation | IMP PMID:23468650 Coordination of chromatid separation and spindle elongation ... | MODIFY | Summary: Liang et al.: overexpressed CLB2 in swe1Ξ cells (hyperactive mitotic CDK) causes premature anaphase spindle elongation, suppressed by FEAR mutants (net1-6Cdk, spo12Ξ) and cdc14-1; earlier work showed clb1Ξ clb2 cells fail to elongate the spindle. Mitotic Clb2-Cdk1 thus promotes spindle elongation by phosphorylating Net1 to release Cdc14, which reverses inhibitory S-phase-CDK phosphorylation (e.g. Spc110). Reason: The direction of the effect is unambiguous and mechanistically explained (Clb2-Cdk1 activates the FEAR pathway); the more specific child term positive regulation of mitotic spindle elongation (GO:1902846) should be used. Clb2-Cdk1 does part of the work (Net1 phosphorylation), so this passes the participation test. Proposed replacements: positive regulation of mitotic spindle elongation Supporting Evidence: PMID:23468650 In contrast, mitotic CDK promotes spindle elongation by activating Cdc14 phosphatase, which reverses the protein phosphorylation imposed by S-phase CDK. PMID:23468650 Here we show evidence indicating that overexpression of mitotic cyclin CLB2 induces premature spindle elongation in swe1Ξ mutant cells. PMID:23468650 Previous work validates the role of mitotic CDK in spindle elongation, as cells lacking both Clb1 and Clb2 fail to elongate the spindle. |
| GO:0044772 mitotic cell cycle phase transition | IEA GO_REF:0000120 | ACCEPT | Summary: InterPro/ARBA electronic mapping to mitotic cell cycle phase transition; broad ancestor of the experimentally supported G2/M transition. Reason: Correct, if general; the specific transition (G2/M) is captured by the IMP rows. |
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Download this section (compressed HTML)Q: Which Clb2-Cdc28 substrates strictly require the bud-neck (Bud3-targeted) or SPB pools of Clb2 rather than the nuclear pool, and does the neck pool have a genuine cytokinesis role or only a timing effect?
Q: What does the Nap1-Clb2 interaction do mechanistically (adaptor, chaperone, or substrate presentation), given that Nap1 binds Clb2 but not Clb3 and is required for full Clb2 function in bud morphogenesis?
Q: Is the recruitment of Clb2-Cdk1 to double-strand breaks a dedicated, regulated DNA-damage function or a consequence of general chromatin association of active mitotic CDK, and which checkpoint substrates besides Fun30 depend specifically on Clb2?
Experiment: Combine analog-sensitive cdc28-as1 with auxin-degron Clb2 (and Clb1) and quantitative phosphoproteomics across a synchronous G2/M window to define the Clb2-specific Cdc28 substrate set versus Clb1/Clb3/Clb5 complexes, using Clb2 hydrophobic-patch and N260/K270 mutants to separate docking-dependent targets.
Experiment: Use endogenously tagged Clb2 localization-signal mutants (NLS, NES, residues 213-255) with live imaging of spindle elongation, Cdc14 release and cytokinesis to assign each localized Clb2 pool to specific mitotic outputs.
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