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this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.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 target is correctly identified: CLB5/YPR120C of Saccharomyces cerevisiae S288c, encoding the S-phase entry cyclin Clb5 (UniProt P30283). The literature consistently describes this protein as a B-type, cyclin A/B-like regulatory subunit of the sole major cell-cycle CDK, Cdc28/Cdk1. Its cyclin fold and experimentally characterized hydrophobic substrate-docking patch agree with the supplied InterPro cyclin-domain annotations. No evidence concerning a similarly named gene from another organism was used. Clb5 is closely related to Clb6—reported sequence identity is approximately 50%—but the two proteins are not completely equivalent (schwob1993clb5andclb6 pages 10-11, zheng2024probingcellcycle pages 26-30).
Clb5 is not an enzyme by itself. Its primary function is to bind and activate Cdc28/Cdk1 and to direct the resulting serine/threonine kinase toward S-phase substrates. In chemical terms, the Clb5–Cdk1 complex catalyzes ATP-dependent phosphorylation of selected substrate Ser/Thr residues. Functionally, it promotes replication-origin activation, efficient progression through S phase, and mechanisms that prevent already replicated origins from being licensed or fired again. Cyclin-dependent docking is central to substrate selection: Clb5’s hydrophobic patch recognizes short docking motifs such as RXL/Cy motifs in substrates including Orc6 (archambault2005twofacedcyclinswith pages 1-2, bloom2007multiplelevelsof pages 7-8).
| Annotation topic | Best-supported conclusion | Evidence type | Representative source/date/DOI URL |
|---|---|---|---|
| Identity and family | Target is CLB5/YPR120C, encoding the S. cerevisiae S-phase B-type cyclin Clb5 (UniProt P30283). Clb5 belongs to the cyclin A/B-like family, activates Cdc28/Cdk1, and is approximately 50% identical to Clb6; this matches the supplied cyclin-domain annotations. | Direct genetic/sequence evidence | Schwob & Nasmyth, July 1993, 10.1101/gad.7.7a.1160 (schwob1993clb5andclb6 pages 10-11, schwob1993clb5andclb6 pages 1-2) |
| Primary molecular role | Clb5 is a regulatory—not independently catalytic—subunit of Clb5–Cdc28/Cdk1, an S-phase Ser/Thr protein-kinase complex that transfers phosphate from ATP to selected protein substrates and promotes DNA-replication initiation. | Direct genetics plus biochemical interpretation | Schwob & Nasmyth, July 1993, 10.1101/gad.7.7a.1160; Bloom & Cross, February 2007, 10.1038/nrm2105 (schwob1993clb5andclb6 pages 10-11, bloom2007multiplelevelsof pages 7-8) |
| Substrate recognition | A conserved Clb5 hydrophobic patch recognizes cyclin-docking motifs such as RXL/Cy. Hydrophobic-patch mutation impairs biological activity without simply abolishing intrinsic kinase activity; Clb5’s nonconserved amino terminus also contributes to functional specificity. | Direct mutagenesis, binding, kinase, and domain-swap experiments | Archambault et al., January 2005, 10.4161/cc.4.1.1402; DeCesare & Stuart, March 2012, 10.1534/genetics.111.134684 (archambault2005twofacedcyclinswith pages 1-2, decesare2012amongbtypecyclins pages 1-2) |
| Replication substrates | Supported targets include Sld2, whose phosphorylation promotes Dpb11 binding and polymerase recruitment, and the replication-control proteins Orc6 and Cdc6. Clb5 preferentially phosphorylates a subset enriched for S-phase proteins relative to Clb2. | Biochemical experiments summarized in authoritative review | Loog & Morgan, March 2005, 10.1038/nature03329; Bloom & Cross, February 2007, 10.1038/nrm2105 (ofir2010candidaalbicanscyclin pages 1-2, bloom2007multiplelevelsof pages 7-8) |
| Replication initiation | CLB5 transcription rises in late G1. clb5Δ cells initiate replication approximately on schedule but take more than twice the normal time to complete S phase, consistent with inefficient activation of later origins; clb5Δ clb6Δ further delays initiation. | Direct cell-cycle genetics and DNA-content analysis | Schwob & Nasmyth, July 1993, 10.1101/gad.7.7a.1160 (schwob1993clb5andclb6 pages 10-11, schwob1993clb5andclb6 pages 1-2) |
| Rereplication prevention and localization | Clb5 associates with chromosomal replication origins as replication begins and remains origin-associated until mitosis. Its hydrophobic patch binds the RXL motif of origin-bound Orc6; disrupting this docking interaction, together with other licensing-control defects, markedly increases DNA rereplication. This supports a local origin-control function, but does not by itself establish an exclusively nuclear distribution. | Direct origin-association, co-purification, gel-shift, and genetic experiments; mechanistic interpretation | Archambault et al., January 2005, 10.4161/cc.4.1.1402; Bloom & Cross, February 2007, 10.1038/nrm2105 (archambault2005twofacedcyclinswith pages 1-2, bloom2007multiplelevelsof pages 7-8) |
| Cell-cycle regulation | SBF/MBF-dependent late-G1 transcription produces Clb5, but Sic1 initially inhibits Clb5–Cdc28. Cln–Cdc28-dependent Sic1 phosphorylation and degradation release S-CDK activity. During metaphase, APC/C–Cdc20 targets Clb5 for degradation; Clb5 is comparatively resistant to APC/C–Cdh1. | Established experiments synthesized in review/model; grouped Clb5/6 behavior in the 2024 model | Bloom & Cross, February 2007, 10.1038/nrm2105; Ravi et al., August 2, 2024, 10.1371/journal.pcbi.1012048 (bloom2007multiplelevelsof pages 3-4, ravi2024modelingthestart pages 2-3) |
| Deletion and redundancy | CLB5 is not essential for mitotic proliferation because Clb6 and mitotic B-type cyclins provide partial compensation. Nevertheless, clb5Δ markedly prolongs S phase; clb5Δ clb6Δ remains viable but shows delayed replication initiation. Clb5 also shares a spindle-assembly function with Clb3/Clb4. | Direct deletion and suppression genetics | Schwob & Nasmyth, July 1993, 10.1101/gad.7.7a.1160 (schwob1993clb5andclb6 pages 10-11, schwob1993clb5andclb6 pages 1-2) |
| Meiosis | Clb5 is required for efficient premeiotic DNA replication, meiotic recombination, aspects of synaptonemal-complex formation, activation of the S–M checkpoint, and successful meiosis. Clb1, Clb3, and Clb4 cannot substitute even when expressed early, demonstrating genuine cyclin-specific targeting. | Direct genetic, expression, kinase, and domain-swap experiments | DeCesare & Stuart, March 2012, 10.1534/genetics.111.134684 (decesare2012amongbtypecyclins pages 1-2) |
| 2023 development | Current CDK research emphasizes that cyclin-docking motifs and Cks1-assisted priming can enable phosphorylation of nonconventional CDK sites. An S-phase-cyclin docking motif, NLxxxL, supports Far1-S91 phosphorylation in budding yeast. This refines the conceptual model of Clb5 substrate selection but is not a new Clb5-specific structural study. | Expert perspective integrating prior biochemical evidence | Valk et al., November 2023, 10.1091/mbc.e22-06-0196 (valk2023cdksignalingvia pages 2-3) |
| 2024 development and application | The executable START-BYCC systems model places Clb5/6 downstream of SBF/MBF and Sic1 degradation, coupling START to DNA synthesis and later Cdh1 inhibition. It reproduces phenotypes of approximately 150 START mutants, but treats Clb5/6 as a module rather than experimentally resolving Clb5 alone. | Computational model and application; not new Clb5-specific wet-lab evidence | Ravi et al., August 2, 2024, 10.1371/journal.pcbi.1012048 (ravi2024modelingthestart pages 2-3) |
Table: Evidence matrix for the verified S. cerevisiae Clb5/P30283 target, separating direct experiments from review-based interpretation and computational modeling. It summarizes molecular function, localization, regulation, phenotypes, meiosis, and recent developments.
The supplied identifiers are mutually consistent:
The foundational genetic literature identified CLB5 and CLB6 as a pair of B-type cyclin genes involved in budding-yeast DNA replication. Clb5/Clb6 transcripts rise in late G1, together with G1 cyclins and DNA-replication genes. CLB5 overexpression partially suppresses a temperature-sensitive cdc28 defect, and ectopic CLB5 can bypass the normal requirement for G1 cyclins sufficiently to permit S-phase entry, supporting its assignment as a Cdc28 activator (Schwob and Nasmyth, July 1993, DOI: 10.1101/gad.7.7a.1160) (schwob1993clb5andclb6 pages 10-11, schwob1993clb5andclb6 pages 1-2).
The domain annotation is also experimentally coherent. Clb5 contains the cyclin fold required for Cdk association and a conserved hydrophobic patch involved in substrate docking. Mutation of this surface reduces Clb5 function without simply eliminating Cdk1-associated catalytic activity, showing that the patch helps select or position substrates rather than serving as the catalytic center (archambault2005twofacedcyclinswith pages 1-2, decesare2012amongbtypecyclins pages 1-2).
Cdc28 supplies the kinase active site; Clb5 activates Cdc28 and changes when, where, and toward which substrates that kinase acts. Accordingly, it is more accurate to annotate Clb5 as an S-phase CDK regulatory/targeting cyclin than as a kinase. The preferred kinase recognition sequence includes conventional CDK Ser/Thr-Pro sites, but efficient phosphorylation also depends on docking interactions outside the phosphoacceptor motif (bloom2007multiplelevelsof pages 7-8, valk2023cdksignalingvia pages 2-3).
Clb5’s hydrophobic patch recognizes RXL/Cy docking motifs. A particularly strong example is Orc6, an origin-recognition-complex subunit: Orc6 contains a conserved RXL motif, and Clb5–Orc6 association was supported by in-vivo origin-associated interaction, co-affinity purification, and gel-retardation experiments. Mutating the Clb5 docking surface or substrate docking motif strongly reduces phosphorylation of Orc6 and Cdc6 in vitro (Archambault et al., January 2005, DOI: 10.4161/cc.4.1.1402) (bloom2007multiplelevelsof pages 7-8, archambault2005twofacedcyclinswith pages 1-2).
This specificity is broader than one motif. During meiosis, replacing the Clb5 hydrophobic patch alone does not transfer full Clb5 activity to another cyclin. Domain-swap experiments showed that a nonconserved amino-terminal region of Clb5 can confer premeiotic-S-phase activity on Clb3. Some chimeras accumulated to higher levels and had greater histone-H1 kinase activity than native Clb5 yet were less effective in initiating premeiotic replication. Thus, bulk kinase activity is not sufficient: substrate docking, cyclin structure, and pathway targeting determine biological output (DeCesare and Stuart, March 2012, DOI: 10.1534/genetics.111.134684) (decesare2012amongbtypecyclins pages 1-2).
Evidence supports the following replication-associated targets or outputs:
Clb5–Cdk1 has greater relative activity than Clb2–Cdk1 toward a subset of CDK substrates enriched in S-phase proteins. Conversely, mitotic targets are often better phosphorylated by mitotic cyclin complexes. This is evidence for graded cyclin specificity rather than an absolute rule that only Clb5 can phosphorylate replication proteins (ofir2010candidaalbicanscyclin pages 1-2).
CLB5 transcription rises in late G1. Newly synthesized Clb5–Cdk1 is initially restrained by the stoichiometric inhibitor Sic1. Cln–Cdk-dependent multisite phosphorylation promotes Sic1 ubiquitylation and degradation, releasing Clb5/6–Cdk1 activity near START and triggering DNA synthesis. This creates an ordered sequence: G1 transcription first produces Cln1/2 and Clb5/6; Cln activity removes the inhibitor; S-CDK then activates replication (vigano2011yeastcellsize pages 78-82, ravi2024modelingthestart pages 2-3).
The clearest quantitative phenotype is that clb5Δ cells begin DNA replication at approximately the expected time but take more than twice as long as wild type to complete S phase. The deletion preferentially preserves early-origin activation while compromising later origin use. The favored interpretation is that Clb6 can support early firing but disappears too soon to efficiently substitute at late origins. Combined deletion of CLB5 and CLB6 causes a stronger delay in replication initiation, although the double mutant remains viable because other cyclins can provide inefficient backup activity (schwob1993clb5andclb6 pages 10-11, schwob1993clb5andclb6 pages 1-2, bloom2007multiplelevelsof pages 3-4).
Clb5 has a dual role: it helps initiate replication and then helps prevent re-initiation. It associates with replication origins as firing begins and remains origin-associated until mitosis. Binding of the Clb5 hydrophobic patch to origin-bound Orc6 creates an origin-localized control mechanism. Disrupting the Orc6 RXL motif causes a severe proliferation defect when Cdc6 is stabilized; combining the docking mutation with defects in other licensing-control pathways produces marked rereplication and DNA contents substantially above 2C (archambault2005twofacedcyclinswith pages 1-2).
The most defensible mechanistic model is that Clb5–Cdk1 locally phosphorylates replication-licensing factors and may also sterically interfere with pre-replicative-complex reassembly. It acts alongside Cdc6 degradation, ORC phosphorylation, and Mcm2–7 nuclear exclusion. Because disruption of any single control is generally insufficient to produce extensive rereplication, these mechanisms should be considered redundant safeguards rather than separate all-or-none switches (bloom2007multiplelevelsof pages 7-8).
Clb5’s best-supported functional location is the nuclear chromatin/replication-origin compartment during S phase. The strongest direct spatial evidence is its association with chromosomal origins from replication initiation until mitosis and its interaction with origin-bound Orc6 (bloom2007multiplelevelsof pages 7-8, archambault2005twofacedcyclinswith pages 1-2).
Clb5 also contributes redundantly with Clb3 and Clb4 to mitotic-spindle functions. However, the retrieved evidence does not justify claiming that Clb5 is constitutively or exclusively localized to spindle pole bodies. Likewise, origin association establishes a nuclear site of action but should not be interpreted as proving that every Clb5 molecule is nuclear throughout the cell cycle. The precise annotation is therefore: a cell-cycle-regulated S-CDK cyclin acting principally in the nucleus, including at replication origins, with additional genetically supported spindle-related functions (schwob1993clb5andclb6 pages 1-2, zheng2024probingcellcycle pages 26-30).
Clb5 is regulated at several levels:
CLB5 is nonessential for ordinary mitotic proliferation because other B-type cyclins can eventually drive replication. Nevertheless, loss of CLB5 greatly lengthens S phase, and genetic combinations reveal overlapping roles with Clb3/Clb4 in spindle assembly. Its mitotic phenotype therefore reflects delayed and mistimed execution rather than complete loss of DNA synthesis (schwob1993clb5andclb6 pages 1-2, zheng2024probingcellcycle pages 26-30).
The meiotic requirement is much more specific. Clb5 is required for premeiotic DNA replication, efficient meiotic recombination, aspects of synaptonemal-complex formation, activation of the meiotic S–M checkpoint, and successful progression through meiosis. Clb1, Clb3, and Clb4 accumulate in clb5 clb6 meiotic cells but do not trigger premeiotic replication; even expression of Clb1 or Clb3 from the CLB5 promoter fails to restore it. These experiments argue that the phenotype is not explained solely by cyclin abundance or expression timing: Clb5 directs Cdk1 toward a uniquely appropriate substrate set (decesare2012amongbtypecyclins pages 1-2).
Recent research has refined the framework around Clb5 more than it has replaced its established annotation.
A 2023 perspective emphasized that biologically important CDK phosphorylation need not occur exclusively at canonical S/T-P sites. Distant cyclin-docking motifs and Cks1-dependent priming can enable phosphorylation of nonconventional sites. In budding yeast, an S-phase-cyclin docking motif, NLxxxL, helps promote phosphorylation of Far1 S91 as part of a phosphodegron. This finding broadens the conceptual model of how S-phase cyclins select substrates, although it is not a new structure or genome-wide substrate map specifically for Clb5 (Valk et al., November 2023, DOI: 10.1091/mbc.e22-06-0196) (valk2023cdksignalingvia pages 2-3).
The 2024 START-BYCC model incorporates Clb5/6 downstream of SBF/MBF transcription and Sic1/Cdc6 degradation, linking START to DNA synthesis and later Cdh1 inhibition. The published model reportedly reproduces phenotypic behavior for approximately 150 known START mutants and is available for simulation, making it a practical systems-biology implementation of the regulatory network. It treats Clb5 and Clb6 as a combined module, however, and therefore should not be cited as new experimental evidence for a Clb5-specific interaction (Ravi et al., published August 2, 2024, DOI: 10.1371/journal.pcbi.1012048) (ravi2024modelingthestart pages 2-3).
Accordingly, the current expert view is stable: Clb5 is an S-phase cyclin whose specificity emerges from expression timing, inhibitor removal, substrate docking, localization at replication origins, and selective proteolysis. The major high-resolution Clb5-specific experiments remain foundational studies rather than 2023–2024 papers.
CLB5 is principally a research and systems-biology target, not a clinical or industrial intervention target. Current applications include:
These are mechanistic model-system applications. Direct extrapolation to human cyclins or cancer therapeutics should be cautious because budding yeast uses one principal cell-cycle CDK paired sequentially with multiple cyclins, whereas metazoan systems contain several CDKs and additional regulatory layers.
Recommended primary annotation: Clb5 is a nuclear S-phase B-type cyclin that binds and activates Cdc28/Cdk1, directs the kinase toward replication-associated substrates through hydrophobic-patch-dependent docking and additional specificity determinants, promotes timely replication-origin firing and S-phase progression, and helps prevent origin relicensing/rereplication after firing.
Biological processes: mitotic DNA replication, replication-origin activation, negative regulation of origin relicensing, START/G1–S progression, premeiotic DNA replication, meiotic recombination/checkpoint progression, and partially redundant spindle functions.
Functional location: principally the nucleus and chromosomal replication origins during S phase; additional spindle-related activity is genetically supported, but exclusive spindle-pole localization is not established by the evidence reviewed here.
Confidence: high for identity, Cdc28/Cdk1 partnership, S-phase function, hydrophobic-patch substrate targeting, origin association, delayed-S-phase deletion phenotype, and premeiotic function; moderate for individual substrate assignments where evidence is derived from broader Clb–Cdk studies; limited for a comprehensive, cell-cycle-resolved Clb5 localization map.
References
(schwob1993clb5andclb6 pages 10-11): E. Schwob and K. Nasmyth. Clb5 and clb6, a new pair of b cyclins involved in dna replication in saccharomyces cerevisiae. Genes & development, 7 7A:1160-75, Jul 1993. URL: https://doi.org/10.1101/gad.7.7a.1160, doi:10.1101/gad.7.7a.1160. This article has 703 citations and is from a highest quality peer-reviewed journal.
(zheng2024probingcellcycle pages 26-30): H Zheng. Probing cell cycle commitment and global transcription regulation at the single-molecule level in budding yeast. Unknown journal, 2024.
(archambault2005twofacedcyclinswith pages 1-2): Vincent Archambault, Nicolas E. Buchler, Gwendolyn M. Wilmes, Matthew D. Jacobson, and Frederick R. Cross. Two-faced cyclins with eyes on the targets. Cell Cycle, 4:125-130, Jan 2005. URL: https://doi.org/10.4161/cc.4.1.1402, doi:10.4161/cc.4.1.1402. This article has 46 citations and is from a peer-reviewed journal.
(bloom2007multiplelevelsof pages 7-8): Joanna Bloom and Frederick R. Cross. Multiple levels of cyclin specificity in cell-cycle control. Nature Reviews Molecular Cell Biology, 8:149-160, Feb 2007. URL: https://doi.org/10.1038/nrm2105, doi:10.1038/nrm2105. This article has 743 citations and is from a domain leading peer-reviewed journal.
(schwob1993clb5andclb6 pages 1-2): E. Schwob and K. Nasmyth. Clb5 and clb6, a new pair of b cyclins involved in dna replication in saccharomyces cerevisiae. Genes & development, 7 7A:1160-75, Jul 1993. URL: https://doi.org/10.1101/gad.7.7a.1160, doi:10.1101/gad.7.7a.1160. This article has 703 citations and is from a highest quality peer-reviewed journal.
(decesare2012amongbtypecyclins pages 1-2): James M DeCesare and David T Stuart. Among b-type cyclins only clb5 and clb6 promote premeiotic s phase in saccharomyces cerevisiae. Genetics, 190:1001-1016, Mar 2012. URL: https://doi.org/10.1534/genetics.111.134684, doi:10.1534/genetics.111.134684. This article has 12 citations and is from a domain leading peer-reviewed journal.
(ofir2010candidaalbicanscyclin pages 1-2): Ayala Ofir and Daniel Kornitzer. Candida albicans cyclin clb4 carries s-phase cyclin activity. Sep 2010. URL: https://doi.org/10.1128/ec.00038-10, doi:10.1128/ec.00038-10. This article has 19 citations and is from a peer-reviewed journal.
(bloom2007multiplelevelsof pages 3-4): Joanna Bloom and Frederick R. Cross. Multiple levels of cyclin specificity in cell-cycle control. Nature Reviews Molecular Cell Biology, 8:149-160, Feb 2007. URL: https://doi.org/10.1038/nrm2105, doi:10.1038/nrm2105. This article has 743 citations and is from a domain leading peer-reviewed journal.
(ravi2024modelingthestart pages 2-3): Janani Ravi, Kewalin Samart, and Jason Zwolak. Modeling the start transition in the budding yeast cell cycle. Aug 2024. URL: https://doi.org/10.1371/journal.pcbi.1012048, doi:10.1371/journal.pcbi.1012048. This article has 1 citations and is from a highest quality peer-reviewed journal.
(valk2023cdksignalingvia pages 2-3): Ervin Valk, Mihkel Örd, Ilona Faustova, and Mart Loog. Cdk signaling via nonconventional cdk phosphorylation sites. Molecular Biology of the Cell, Nov 2023. URL: https://doi.org/10.1091/mbc.e22-06-0196, doi:10.1091/mbc.e22-06-0196. This article has 26 citations and is from a domain leading peer-reviewed journal.
(vigano2011yeastcellsize pages 78-82): M Vigano. Yeast cell size control: an interplay among ribosome biogenesis, protein synthesis and mapk routes. Unknown journal, 2011.