The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
The identity is verified: cig2, also called cyc17, encodes a B-type cyclin of Schizosaccharomyces pombe. This agrees with UniProt P36630 and its cyclin A/B-like and cyclin C-terminal domains. No evidence concerning a similarly named gene from another organism was included.
The strongest functional annotation is: Cig2 is a noncatalytic regulatory cyclin that binds and activates the cyclin-dependent kinase Cdc2 during G1/S, raising CDK activity sufficiently for passage through Start and initiation of DNA replication. It is not an enzyme by itself, transporter, or structural protein. Rather, substrate phosphorylation is catalyzed by Cdc2 in the Cig2–Cdc2 complex. Its normal physiological specialization is G1/S and S phase, notwithstanding the UniProt description “G2/mitotic-specific cyclin.” Under engineered conditions, Cig2–Cdc2 can also drive mitosis, showing that its specialization is quantitative and regulatory rather than an absolute inability to phosphorylate mitotic targets. (martincastellanos1996b‐typecyclinsregulate pages 4-5, pickering2017thefissionyeast pages 1-3, martincastellanos1996b‐typecyclinsregulate pages 8-9)
| Topic | Best-supported conclusion | Evidence type/experiment | Key quantitative detail | Source/date/DOI |
|---|---|---|---|---|
| Identity and classification | cig2/cyc17 encodes the S. pombe B-type cyclin Cig2, corresponding to P36630. Physiologically, literature most often describes it as a G1/S or S-phase cyclin rather than a conventional mitotic cyclin. | Genetic analysis, cyclin-associated kinase assays, and cell-cycle synchronization. | Cig2 protein and associated kinase activity peak around G1/S and are low in dividing cells. | Martín-Castellanos et al., Feb. 1996, 10.1002/j.1460-2075.1996.tb00419.x (martincastellanos1996b‐typecyclinsregulate pages 4-5, martincastellanos1996b‐typecyclinsregulate pages 1-2) |
| Cdc2 association and activity | Cig2 forms an active complex with the CDK Cdc2; Cig2 is a regulatory cyclin, not an enzyme with an independent catalytic reaction. The complex phosphorylates protein substrates using ATP. | Anti-HA immunoprecipitation of Cig2-HA recovered histone-H1 kinase activity; activity was lost in a cdc2-33 background. | Activity accumulates in G1 and peaks at S-phase entry. No Cig2–Cdc2 binding constant or substrate-specificity constant was reported. | Martín-Castellanos et al., Feb. 1996, 10.1002/j.1460-2075.1996.tb00419.x (martincastellanos1996b‐typecyclinsregulate pages 4-5) |
| G1/S transition and DNA replication | The primary physiological function is to raise Cdc2 activity during G1, promote passage through Start, and facilitate initiation of DNA replication. Loss delays entry into S phase more than progression through S phase itself. | Synchronized-cell kinase/protein measurements, DNA-content analysis, deletion genetics, and rereplication assays. | In cdc13Δ cig2Δ, only one rereplication round occurred in 8 h; deleting both cig1 and cig2 abolished rereplication and maintained G1 arrest for at least 8 h. | Fisher & Nurse, Feb. 1996, 10.1002/j.1460-2075.1996.tb00420.x; Martín-Castellanos et al., Feb. 1996 (martincastellanos1996b‐typecyclinsregulate pages 2-3, fisher1996asinglefission pages 2-3) |
| Redundancy and nonessentiality | Cig2 is the principal G1/S cyclin under normal conditions but is not individually essential: Cig1 and especially Cdc13 can provide enough Cdc2 activity for G1/S progression when Cig2 is absent. | Single- and multiple-cyclin deletion genetics and DNA-content analysis. | In cdc2-56 cig2Δ, about 20% of cells had 1C DNA at permissive temperature and about 40% arrested in G1 at 36.5°C for roughly 3 h. | Martín-Castellanos et al., Feb. 1996, 10.1002/j.1460-2075.1996.tb00419.x (martincastellanos1996b‐typecyclinsregulate pages 2-3, martincastellanos1996b‐typecyclinsregulate pages 8-9) |
| Rum1 regulation | The CDK inhibitor Rum1 restrains Cig2–Cdc2 in G1, helping maintain low CDK activity until Start and during starvation-associated G1 arrest. | Purified-Rum1 inhibition assays, genetics, and synthesis in a 2024 review. | Rum1 inhibited Cig2-associated Cdc2 kinase in vitro, although less strongly than Cdc13–Cdc2; no inhibition constant was reported. | Martín-Castellanos et al., Feb. 1996, 10.1002/j.1460-2075.1996.tb00419.x; Kawamukai, Mar. 2024, 10.1093/bbb/zbae019 (martincastellanos1996b‐typecyclinsregulate pages 7-8, kawamukai2024regulationofsexual pages 1-2) |
| MBF and nutrient regulation | cig2 transcription is controlled by the G1/S MBF complex containing Cdc10, Res1, and Res2. Nutrient-responsive TORC2–Gad8 signaling can reduce MBF-dependent cig2 expression, favoring G1 arrest and differentiation. | Current expert review integrating genetic and signaling studies. | No Cig2-specific fold-change or rate constant was available in the gathered evidence. | Kawamukai, Mar. 2024, 10.1093/bbb/zbae019 (kawamukai2024regulationofsexual pages 1-2, kawamukai2024regulationofsexual pages 2-3) |
| Engineered mitotic capability | Although not the normal mitotic driver, sufficiently deregulated Cig2–Cdc2 can trigger mitosis, including lethal premature mitosis, supporting a quantitative CDK-activity model rather than absolute S-phase substrate specificity. | Removal of Wee1/Mik1-dependent inhibitory control, cig2 deletion, loss of Cdc13 activity, and constitutive Cig2 expression. | wee1-50ts mik1Δ cig2Δ cells remained viable at 30°C and divided at 8.4 ± 1.3 µm; the strain was inviable at 35°C. | Pickering et al., Nov. 2017 preprint, 10.1101/213330 (pickering2017thefissionyeast pages 1-3) |
| Localization | Evidence is limited. Cig2 performs its replication-control function in the nuclear cell-cycle system, and modeling literature explicitly invokes nuclear accumulation of Cdc2–Cig2; the gathered evidence does not establish a high-resolution localization pattern experimentally. | Mechanistic modeling informed by cell-cycle genetics; no direct microscopy evidence retrieved. | No nuclear/cytoplasmic concentration ratio or organelle-resolved measurement was available. | Sveiczer et al., Feb. 2004, 10.1093/bfgp/2.4.298 (sveiczer2004modellingthefission pages 7-8) |
| Sexual differentiation | Cig2 activity links vegetative G1/S progression to the decision to differentiate. Lower Cig2–Cdc2 activity promotes G1 arrest and mating; cig2Δ enhances conjugation and may permit differentiation even without starvation. | Deletion phenotypes and a 2024 review of nutrient-responsive differentiation pathways. | No mating-efficiency percentage was available in the gathered evidence. | Martín-Castellanos et al., Feb. 1996; Kawamukai, Mar. 2024, 10.1093/bbb/zbae019 (martincastellanos1996b‐typecyclinsregulate pages 2-3, kawamukai2024regulationofsexual pages 1-2) |
| Evidence gaps | Direct Cig2 substrates, structural data for P36630–Cdc2, binding/kinetic constants, experimentally resolved localization, and Cig2-specific 2023–2024 primary studies remain limited. The gathered evidence does not justify a specific APC/C- or SCF-dependent Cig2 degradation mechanism. | Critical assessment of the retrieved primary studies, modeling article, and 2024 review. | Recent source coverage is chiefly a 2024 pathway review; most direct mechanistic evidence dates from 1996–2017. | Evidence synthesis (martincastellanos1996b‐typecyclinsregulate pages 4-5, pickering2017thefissionyeast pages 1-3, sveiczer2004modellingthefission pages 7-8, kawamukai2024regulationofsexual pages 1-2) |
Table: Evidence matrix summarizing experimentally supported functions and regulatory relationships of S. pombe Cig2/P36630. It separates direct findings from model-based localization and unresolved annotation gaps.
Foundational genetic and biochemical work identifies S. pombe Cig2/Cyc17 as a B-type cyclin whose abundance and associated kinase activity peak around the G1/S transition. This matches the supplied UniProt family and domain annotations: cyclin family, cyclin AB subfamily, cyclin-like domain/superfamily, and cyclin C-terminal domain. (martincastellanos1996b‐typecyclinsregulate pages 4-5, martincastellanos1996b‐typecyclinsregulate pages 2-3)
There is, however, a terminology issue. The experimentally dominant description is G1/S cyclin or S-phase cyclin, not a cyclin whose principal normal function is G2/mitosis. Cdc13 is the major physiological mitotic B-type cyclin. The P36630 label should therefore be interpreted as a broad family-style description rather than the most precise statement of Cig2’s phase-specific role. Cig2 has latent mitotic capacity, but this is normally restrained by its expression pattern and CDK inhibitory circuitry. (pickering2017thefissionyeast pages 1-3, martincastellanos1996b‐typecyclinsregulate pages 8-9)
Cig2 supplies the regulatory cyclin subunit of a complex with Cdc2, the single major cell-cycle CDK in fission yeast. Immunoprecipitated Cig2-HA recovered histone-H1 kinase activity, and that activity disappeared in a cdc2-33 background. This establishes that the measured catalytic activity depends on Cdc2 rather than Cig2 itself. In synchronized cells, Cig2-associated kinase activity accumulated in G1, peaked as cells entered S phase, and was low in dividing cells. Cig2 protein showed similar periodicity. (martincastellanos1996b‐typecyclinsregulate pages 4-5)
The net biochemical reaction of the complex is the usual CDK reaction:
protein substrate + ATP → phosphorylated protein substrate + ADP.
The retrieved evidence does not establish a comprehensive set of direct Cig2–Cdc2 substrates, a unique peptide-recognition motif attributable specifically to Cig2, a Cig2–Cdc2 dissociation constant, or catalytic constants. Histone H1 was used as an experimental kinase substrate, not demonstrated to be the defining physiological target. Consequently, a precise substrate-specificity annotation beyond “cell-cycle CDK substrates involved in G1/S and replication initiation” would overstate current direct evidence.
Cig2–Cdc2 is the principal normal G1/S CDK complex in S. pombe. Cig2 protein, transcript, and associated kinase activity rise during G1 and peak near G1/S. Genetic results show that Cig2 chiefly promotes passage through Start and the onset of S phase, rather than controlling elongation through S phase after replication has begun. (martincastellanos1996b‐typecyclinsregulate pages 2-3, martincastellanos1996b‐typecyclinsregulate pages 7-8)
Deletion of cig2 delays G1 exit, particularly in small cells or sensitized cdc2 backgrounds. In cdc2-56 cig2Δ cells, approximately 20% of cells had 1C DNA content at permissive temperature; following transfer to 36.5°C, approximately 40% arrested in G1 and remained arrested for about three hours. Nevertheless, cells lacking Cig2 can ultimately replicate because Cig1 and Cdc13 can provide compensating Cdc2 activity. Thus, Cig2 is the favored physiological G1/S cyclin but is not individually essential. (martincastellanos1996b‐typecyclinsregulate pages 2-3, martincastellanos1996b‐typecyclinsregulate pages 8-9)
Cyclin-deletion experiments in a rereplication system further demonstrate its contribution to replication initiation. In cdc13Δ cig2Δ cells, only one rereplication round was completed over eight hours. Deleting both cig1 and cig2 abolished rereplication and maintained cells in G1 for at least eight hours. These findings place Cig2 upstream of, or at, the CDK-dependent triggering of replication rather than assigning it a role in DNA synthesis chemistry itself. (fisher1996asinglefission pages 2-3)
cig2 is a periodically expressed G1/S gene regulated by the MBF transcription-factor complex, which includes Cdc10, Res1, and Res2. MBF activates genes needed for the G1/S transition. Cig2-dependent CDK activity participates in feedback within this regulatory system, helping convert a transcriptional G1/S program into kinase activation and replication entry. (kawamukai2024regulationofsexual pages 1-2, kawamukai2024regulationofsexual pages 2-3)
The CDK inhibitor Rum1 restrains Cig2–Cdc2 in G1. Purified Rum1 inhibited Cig2-associated Cdc2 kinase in vitro, although less strongly than Cdc13–Cdc2 in the reported comparison. The physiological model is that Rum1 keeps CDK activity low until cells attain conditions appropriate for Start. Removing or overcoming this inhibition permits G1/S progression; maintaining it supports G1 arrest. (martincastellanos1996b‐typecyclinsregulate pages 7-8, kawamukai2024regulationofsexual pages 1-2)
In fission yeast, nutrient limitation produces G1 arrest, which is required for mating and sexual differentiation. A 2024 authoritative review places Cig2 at this proliferation-versus-differentiation decision: TORC2–Gad8 signaling interfaces with MBF and can reduce cig2 expression, while Rum1 inhibits Cig2–Cdc2 activity. Reduced Cig2 function consequently favors G1 arrest and differentiation. cig2Δ enhances conjugation and has been reported to permit G1 arrest and sexual differentiation even without the usual starvation signal. (martincastellanos1996b‐typecyclinsregulate pages 2-3, kawamukai2024regulationofsexual pages 1-2, kawamukai2024regulationofsexual pages 2-3)
The retrieved evidence does not support assigning Cig2 a direct enzymatic role in nutrient sensing or pheromone signaling. Its role is to translate those upstream signals into altered G1/S CDK activity.
Cig2 abundance is strongly periodic and falls after its G1/S peak, so regulated synthesis and turnover clearly shape its activity. However, the evidence gathered here does not adequately establish a particular Cig2-specific APC/C or SCF ubiquitin-ligase mechanism. The 2024 review discusses APC-dependent degradation of other B-type cyclins and SCF-dependent turnover of Rum1, but those observations should not be automatically transferred to Cig2. A specific degradation pathway should therefore remain an evidence gap in this annotation. (martincastellanos1996b‐typecyclinsregulate pages 4-5, kawamukai2024regulationofsexual pages 1-2)
Under normal conditions, Cdc13–Cdc2 is the major mitotic driver, and Cig2 is restricted to the G1/S program. Experiments disrupting inhibitory tyrosine phosphorylation revealed that Cig2 nevertheless has substantial latent mitotic activity. Cig2–CDK can cause mitotic catastrophe, drive mitosis without normal Cdc13–CDK activity, and—when constitutively expressed—compensate for loss of Cdc13 activity. Deleting cig2 suppressed lethal premature division in wee1-50ts mik1Δ cells at 30°C; the rescued cells divided at 8.4 ± 1.3 μm, although the strain remained inviable at 35°C. (pickering2017thefissionyeast pages 1-3)
These findings support an expert interpretation based on a quantitative CDK-activity model. Cig2 is not necessarily endowed with completely S-phase-exclusive substrate recognition. Instead, its normal concentration, timing, degradation, inhibition, and the threshold of total CDK activity prevent it from triggering mitosis. When those controls are altered, Cig2–Cdc2 can cross the mitotic activity threshold. This reconciles the protein’s B-type cyclin architecture with its normal G1/S specialization. (pickering2017thefissionyeast pages 1-3)
Cig2 acts in the cellular compartment in which Start control and chromosome replication are executed—principally the nuclear cell-cycle regulatory system. Cell-cycle models explicitly invoke nuclear accumulation of Cig2–Cdc2 before replication. (sveiczer2004modellingthefission pages 7-8)
Nevertheless, localization confidence is lower than functional confidence. The retrieved primary evidence did not provide high-resolution Cig2 microscopy, nuclear-to-cytoplasmic concentration measurements, or evidence for localization to replication origins, spindle-pole bodies, or other subnuclear structures. The defensible annotation is therefore nuclear or nucleus-enriched during G1/S, with the important qualification that the evidence recovered here is mainly mechanistic/model-based rather than a definitive localization study.
The best-supported associated processes are:
The major pleiotropic phenotypes—G1 accumulation, enhanced conjugation, rereplication defects in sensitized backgrounds, and mitotic catastrophe under deregulated conditions—are all parsimoniously explained by changes in the timing or magnitude of Cdc2 activity.
Cig2 has no direct clinical or industrial application established by the retrieved literature. Its real-world use is as an experimental component of the S. pombe model system for:
These applications have broader relevance because cyclin–CDK architecture, inhibitory phosphorylation, CDK inhibitors, and activity thresholds are conserved principles of eukaryotic cell-cycle control. Extrapolation to a specific mammalian cyclin or disease target, however, should not be made solely from Cig2.
The most relevant recent authoritative source recovered was Kawamukai’s review, published March 2024, which integrates Cig2 into modern understanding of nutrient signaling and sexual-differentiation initiation (DOI 10.1093/bbb/zbae019). It reinforces, rather than overturns, the foundational model of Cig2 as the major G1/S cyclin controlled by MBF and Rum1. (kawamukai2024regulationofsexual pages 1-2, kawamukai2024regulationofsexual pages 2-3)
Most direct Cig2-specific mechanistic evidence remains older because the core function was established through precise genetics and biochemistry in the 1990s. The major later conceptual advance was the demonstration that Cig2 can drive mitosis when regulatory constraints are removed. There appears to be relatively little Cig2-specific primary work from 2023–2024; recent studies of fission-yeast CDK dynamics more often examine total CDK activity or Cdc13 rather than resolving Cig2-specific biochemistry.
Important unresolved areas include:
Cig2/P36630 is a nucleus-associated B-type regulatory cyclin of Schizosaccharomyces pombe that forms an active kinase complex with Cdc2. Its principal physiological function is to generate the G1/S pulse of CDK activity required for Start and efficient initiation of DNA replication. Its abundance and activity are controlled by MBF-dependent transcription, inhibition by Rum1, cell-cycle timing, and nutrient-responsive differentiation pathways. Cig2 is partially redundant with Cig1 and Cdc13 and is therefore not individually essential. Although normally specialized for G1/S, deregulated Cig2–Cdc2 can cross the activity threshold for mitosis, demonstrating latent mitotic competence.
References
(martincastellanos1996b‐typecyclinsregulate pages 4-5): Cristina Martín-Castellanos, K. Labib, and Sergio Moreno. B‐type cyclins regulate g1 progression in fission yeast in opposition to the p25rum1 cdk inhibitor. The EMBO Journal, 15(4):839-849, Feb 1996. URL: https://doi.org/10.1002/j.1460-2075.1996.tb00419.x, doi:10.1002/j.1460-2075.1996.tb00419.x. This article has 154 citations.
(pickering2017thefissionyeast pages 1-3): M. Pickering, Mira Magner, Daniel Keifenheim, and Nicholas Rhind. The fission yeast s-phase cyclin cig2 can drive mitosis. bioRxiv, Nov 2017. URL: https://doi.org/10.1101/213330, doi:10.1101/213330. This article has 0 citations.
(martincastellanos1996b‐typecyclinsregulate pages 8-9): Cristina Martín-Castellanos, K. Labib, and Sergio Moreno. B‐type cyclins regulate g1 progression in fission yeast in opposition to the p25rum1 cdk inhibitor. The EMBO Journal, 15(4):839-849, Feb 1996. URL: https://doi.org/10.1002/j.1460-2075.1996.tb00419.x, doi:10.1002/j.1460-2075.1996.tb00419.x. This article has 154 citations.
(martincastellanos1996b‐typecyclinsregulate pages 1-2): Cristina Martín-Castellanos, K. Labib, and Sergio Moreno. B‐type cyclins regulate g1 progression in fission yeast in opposition to the p25rum1 cdk inhibitor. The EMBO Journal, 15(4):839-849, Feb 1996. URL: https://doi.org/10.1002/j.1460-2075.1996.tb00419.x, doi:10.1002/j.1460-2075.1996.tb00419.x. This article has 154 citations.
(martincastellanos1996b‐typecyclinsregulate pages 2-3): Cristina Martín-Castellanos, K. Labib, and Sergio Moreno. B‐type cyclins regulate g1 progression in fission yeast in opposition to the p25rum1 cdk inhibitor. The EMBO Journal, 15(4):839-849, Feb 1996. URL: https://doi.org/10.1002/j.1460-2075.1996.tb00419.x, doi:10.1002/j.1460-2075.1996.tb00419.x. This article has 154 citations.
(fisher1996asinglefission pages 2-3): D. L. Fisher and P. Nurse. A single fission yeast mitotic cyclin b p34cdc2 kinase promotes both s‐phase and mitosis in the absence of g1 cyclins. The EMBO Journal, 15:850-860, Feb 1996. URL: https://doi.org/10.1002/j.1460-2075.1996.tb00420.x, doi:10.1002/j.1460-2075.1996.tb00420.x. This article has 394 citations.
(martincastellanos1996b‐typecyclinsregulate pages 7-8): Cristina Martín-Castellanos, K. Labib, and Sergio Moreno. B‐type cyclins regulate g1 progression in fission yeast in opposition to the p25rum1 cdk inhibitor. The EMBO Journal, 15(4):839-849, Feb 1996. URL: https://doi.org/10.1002/j.1460-2075.1996.tb00419.x, doi:10.1002/j.1460-2075.1996.tb00419.x. This article has 154 citations.
(kawamukai2024regulationofsexual pages 1-2): Makoto Kawamukai. Regulation of sexual differentiation initiation in schizosaccharomyces pombe. Bioscience, biotechnology, and biochemistry, 88:475-492, Mar 2024. URL: https://doi.org/10.1093/bbb/zbae019, doi:10.1093/bbb/zbae019. This article has 17 citations.
(kawamukai2024regulationofsexual pages 2-3): Makoto Kawamukai. Regulation of sexual differentiation initiation in schizosaccharomyces pombe. Bioscience, biotechnology, and biochemistry, 88:475-492, Mar 2024. URL: https://doi.org/10.1093/bbb/zbae019, doi:10.1093/bbb/zbae019. This article has 17 citations.
(sveiczer2004modellingthefission pages 7-8): Á. Sveiczer, J. Tyson, and B. Novák. Modelling the fission yeast cell cycle. Briefings in functional genomics & proteomics, 2 4:298-307, Feb 2004. URL: https://doi.org/10.1093/bfgp/2.4.298, doi:10.1093/bfgp/2.4.298. This article has 67 citations.