Affinage

CLSPN

Claspin · UniProt Q9HAW4

Round 2 corrected
Length
1339 aa
Mass
151.1 kDa
Annotated
2026-04-28
39 papers in source corpus 13 papers cited in narrative 13 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

Claspin is a nuclear adaptor protein essential for activating the ATR–Chk1 replication checkpoint in response to DNA replication stress and genotoxic damage. It binds Chk1 in a phosphorylation-dependent manner downstream of TopBP1 and ATR, selectively mediating Chk1 phosphorylation without affecting other ATR substrates, and also recruits BRCA1 to coordinate checkpoint signaling (PMID:11090622, PMID:12766152, PMID:16880517, PMID:15096610). Beyond checkpoint mediation, Claspin protects replication forks from oncogene-induced stress in a checkpoint-independent manner, a function upregulated in cancer cells (PMID:30796221). Claspin protein levels are tightly controlled through Plk1-dependent phosphorylation of a DSGxxS degron that triggers SCF-βTrCP ubiquitylation and proteasomal degradation at mitotic entry—a process counteracted during DNA damage by USP28 deubiquitylation and APC/C(Cdh1)-mediated Plk1 downregulation—and at the mRNA level by an ALKBH5–IGF2BP2 m6A regulatory axis (PMID:16885022, PMID:18662541, PMID:41069850).

Mechanistic history

Synthesis pass · year-by-year structured walk · 11 steps
  1. 2000 High

    The identification of Claspin as a Chk1-binding protein whose immunodepletion abolished Chk1 activation in Xenopus egg extracts established it as the first dedicated mediator of the replication checkpoint, answering how the ATR signal is transmitted specifically to Chk1.

    Evidence Co-immunoprecipitation and immunodepletion in Xenopus egg extracts with kinase activity assays

    PMID:11090622

    Open questions at the time
    • Mechanism of Claspin–Chk1 interaction remained undefined
    • Human ortholog function not yet confirmed
    • Whether Claspin acted on other checkpoint effectors was unknown
  2. 2003 High

    Extension to human cells revealed that Claspin is a cell-cycle-regulated nuclear protein that associates with Chk1 only after replication stress, and interacts with ATR and Rad9, establishing it as a damage-induced adaptor bridging the sensor and effector kinases.

    Evidence Co-immunoprecipitation, siRNA knockdown, and checkpoint assays in human cells

    PMID:12766152

    Open questions at the time
    • Structural basis for phosphorylation-dependent Chk1 binding unknown
    • Relationship to other mediators such as TopBP1 undefined
  3. 2004 High

    Discovery that ATR-dependent phosphorylation of Claspin induces a Claspin–BRCA1 complex controlling BRCA1 Ser1524 phosphorylation expanded the model from a linear ATR→Claspin→Chk1 relay to a branched signaling network coordinating BRCA1 and Chk1 activation.

    Evidence Reciprocal co-immunoprecipitation, phosphorylation mapping, and cell proliferation assays

    PMID:15096610

    Open questions at the time
    • Whether the Claspin–BRCA1 interaction is direct or scaffold-mediated was not resolved
    • Functional significance of BRCA1 Ser1524 phosphorylation downstream unclear
  4. 2006 High

    Epistasis experiments placed Claspin downstream of TopBP1 and showed it selectively mediates Chk1 phosphorylation without affecting other ATR substrates, resolving the pathway hierarchy and demonstrating substrate specificity in checkpoint signaling.

    Evidence RNAi-based epistasis, systematic ATR substrate phosphorylation analysis, immunofluorescence

    PMID:16880517

    Open questions at the time
    • How TopBP1 enables the Claspin–Chk1 interaction mechanistically was not defined
    • Whether Claspin has checkpoint-independent roles was unknown
  5. 2006 High

    Three concurrent studies demonstrated that Plk1 phosphorylates a DSGxxS degron in Claspin, enabling SCF-βTrCP-mediated ubiquitylation and proteasomal degradation at mitotic entry; non-degradable Claspin mutants prolonged Chk1 activation and delayed mitosis, answering how checkpoint signaling is terminated during recovery.

    Evidence In vitro ubiquitylation, degron mutagenesis, co-immunoprecipitation, flow cytometry, and live-cell imaging across three independent laboratories

    PMID:16885021 PMID:16885022 PMID:16934469

    Open questions at the time
    • Additional post-translational modifications controlling Claspin turnover beyond the degron were not mapped
    • Whether Claspin degradation contributes to unperturbed cell-cycle progression was unclear
  6. 2006 Medium

    Identification of USP28 as a deubiquitylase that stabilizes Claspin during DNA damage revealed a counteracting mechanism to SCF-βTrCP-driven degradation, explaining how Claspin levels are maintained to sustain checkpoint signaling.

    Evidence 53BP1 complex purification, siRNA knockdown, Claspin protein level measurement

    PMID:16901786

    Open questions at the time
    • Direct deubiquitylation of Claspin by USP28 was not reconstituted in vitro
    • Specificity of USP28 for Claspin versus other checkpoint mediators not resolved
  7. 2006 Medium

    The Tim–Tipin complex was shown to be required for Claspin nuclear accumulation under replication stress, identifying an upstream regulatory step that ensures proper Claspin localization for checkpoint activation.

    Evidence siRNA knockdown, subcellular fractionation, immunofluorescence

    PMID:17102137

    Open questions at the time
    • Mechanism by which Tim–Tipin promotes Claspin nuclear retention unknown
    • Whether Tim–Tipin directly binds Claspin was not demonstrated
  8. 2008 High

    Discovery that APC/C(Cdh1)—activated via Cdc14B-mediated Plk1 degradation—stabilizes Claspin in G2 damage responses by removing the Plk1-driven degradation signal integrated the Claspin turnover circuit into the broader cell-cycle E3 ligase network, explaining how Claspin is protected during G2 checkpoint activation.

    Evidence In vivo ubiquitylation assays, siRNA epistasis, substrate degradation assays, cell-cycle synchronization

    PMID:18662541

    Open questions at the time
    • Whether Claspin is a direct APC/C(Cdh1) substrate (via a D-box or KEN-box) was not fully mapped
    • Relative contributions of USP28 versus APC/C(Cdh1)-Plk1 axis to Claspin stabilization were not quantified
  9. 2019 High

    Demonstration that Claspin protects replication forks from oncogene-induced stress independently of ATR–Chk1 checkpoint signaling revealed a second, mechanistically distinct function—fork stabilization—explaining why Claspin and Timeless are upregulated in tumors beyond their checkpoint roles.

    Evidence DNA fiber assays, siRNA titration to pretumoral levels, checkpoint signaling western blots, primary tumor expression analysis

    PMID:30796221

    Open questions at the time
    • Molecular mechanism by which Claspin stabilizes forks independently of Chk1 was not identified
    • Whether fork protection requires Claspin's known interaction domains is unknown
  10. 2020 Medium

    Functional analysis of CLSPN coding and promoter variants linked to breast cancer showed that a missense/splicing variant reduces Claspin expression and Chk1 activation, connecting CLSPN genetic variation to checkpoint competence and cancer susceptibility.

    Evidence Minigene splicing assay, luciferase reporter, western blotting for Chk1 phosphorylation, association study

    PMID:32847043

    Open questions at the time
    • Association-level evidence; not yet validated by family-based segregation or CRISPR knock-in
    • Effect size of individual variants on cancer risk not precisely determined
  11. 2025 Medium

    Identification of the ALKBH5–IGF2BP2 m6A axis as an epitranscriptomic regulator of CLSPN mRNA stability added a new layer of post-transcriptional control, explaining elevated Claspin expression and docetaxel resistance in castration-resistant prostate cancer.

    Evidence m6A-seq, RNA stability assays, siRNA knockdown of IGF2BP2, organoid models, clinical sample validation

    PMID:41069850

    Open questions at the time
    • m6A sites on CLSPN mRNA were not individually mutated to confirm causality
    • Whether this regulatory axis operates in non-prostate cancer contexts is unknown

Open questions

Synthesis pass · forward-looking unresolved questions
  • The molecular mechanism by which Claspin stabilizes replication forks independently of Chk1 remains undefined, as does the structural basis for its selective bridging of ATR to Chk1; whether disease-associated CLSPN variants confer clinically actionable cancer risk is also unresolved.
  • No structural model of Claspin in complex with Chk1 or replication fork substrates
  • Checkpoint-independent fork protection mechanism uncharacterized
  • Clinical significance of CLSPN variants not established by prospective studies

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0060090 molecular adaptor activity 3
Localization
GO:0005634 nucleus 3
Pathway
R-HSA-1640170 Cell Cycle 4 R-HSA-73894 DNA Repair 3 R-HSA-69306 DNA Replication 2
Partners
ATRBRCA1BTRCCHEK1PLK1RAD9ATIMELESSTOPBP1
Complex memberships
APC/C(Cdh1) (substrate)SCF-βTrCP (substrate)

Evidence

Reading pass · 13 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2000 Claspin (identified in Xenopus egg extracts) is a novel protein that binds to Chk1; this binding is elevated in the presence of checkpoint-activating DNA templates. Immunodepletion of Claspin abolishes both phosphorylation and activation of Chk1 and prevents cell-cycle arrest in response to DNA replication blocks, establishing Claspin as an essential upstream regulator of Chk1. Protein identification, co-immunoprecipitation, immunodepletion from Xenopus egg extracts, kinase activity assay Molecular cell High 11090622
2003 Human Claspin is a cell-cycle regulated protein peaking at S/G2 phase that localizes to the nucleus and associates with Chk1 only following replication stress or DNA damage. Claspin is phosphorylated in response to replication stress, and this phosphorylation is required for its association with Chk1. Claspin also interacts with ATR and Rad9 (of the 9-1-1 complex), suggesting it acts as an adaptor bringing these checkpoint components together. siRNA-mediated knockdown of Claspin inhibits Chk1 activation and impairs the replication checkpoint. Co-immunoprecipitation, siRNA knockdown, cell-cycle analysis, premature chromatin condensation assay, DNA synthesis assay The Journal of biological chemistry High 12766152
2004 Human Claspin is required for resistance to multiple genotoxic stresses (UV, IR, hydroxyurea). ATR-dependent phosphorylation of Claspin induces formation of a Claspin-BRCA1 complex. Claspin controls BRCA1 phosphorylation on serine 1524, suggesting a model where ATR phosphorylates Claspin, which recruits BRCA1 to jointly activate Chk1. Additionally, Claspin overexpression promotes cell proliferation, indicating a dual role as both a checkpoint mediator and a positive cell-cycle regulator. Co-immunoprecipitation, siRNA knockdown, phosphorylation mapping, cell proliferation assay Proceedings of the National Academy of Sciences of the United States of America High 15096610
2006 Claspin operates downstream of TopBP1 in the ATR signaling pathway and selectively mediates ATR-dependent phosphorylation of Chk1, but not other ATR substrates (Nbs1, Smc1, H2AX). Unlike TopBP1, Claspin remains distributed throughout the nucleus after DNA damage. TopBP1 is required for the DNA damage-induced interaction between Claspin and Chk1. Claspin depletion mimics Chk1 inactivation by inducing spontaneous DNA damage. RNAi knockdown, immunofluorescence, co-immunoprecipitation, western blotting for ATR substrate phosphorylation Molecular and cellular biology High 16880517
2006 During recovery from the DNA replication checkpoint, Claspin is degraded via the SCF-βTrCP ubiquitin ligase. Plk1 phosphorylates a canonical DSGxxS degron in Claspin, enabling βTrCP binding and ubiquitylation. A stable Claspin mutant unable to bind βTrCP prolongs Chk1 activation and delays mitotic entry. In G2 DNA damage responses, Claspin proteolysis is inhibited to allow checkpoint maintenance. In vitro ubiquitylation assay, degron mutagenesis, co-immunoprecipitation, stable mutant expression, flow cytometry Molecular cell High 16885022
2006 Claspin is degraded at the onset of mitosis via its interaction with SCF-βTrCP; this interaction is phosphorylation-dependent, requires Plk1 activity, and depends on an intact phosphodegron in the N-terminus of Claspin. Stabilized Claspin (phosphodegron mutant or βTrCP knockdown) impairs Chk1 dephosphorylation and delays G2/M transition during recovery from checkpoint arrest. Thus, Plk1-driven Claspin degradation is essential for timely checkpoint recovery. Ubiquitylation assay, phosphodegron mutagenesis, siRNA, co-immunoprecipitation, live-cell imaging, flow cytometry Molecular cell High 16885021
2006 Plk1-dependent proteasomal degradation of Claspin at mitotic entry controls termination of the Chk1-mediated checkpoint. Claspin interacts directly with both βTrCP and Plk1; inactivation of either or mutation of the βTrCP recognition motif in Claspin prevents mitotic degradation. A non-degradable Claspin mutant inhibits recovery from DNA-damage checkpoint arrest. Chk1 activity stabilizes Claspin during checkpoint response. Co-immunoprecipitation, proteasome inhibition, dominant-negative and RNAi approaches, flow cytometry, mitotic progression assay Current biology High 16934469
2006 The deubiquitylating enzyme USP28 is required to stabilize Claspin (and other checkpoint mediators including Mdc1 and TopBP1) in response to DNA damage, counteracting ubiquitin-mediated degradation and sustaining checkpoint signaling. Co-immunoprecipitation via 53BP1 complex purification, siRNA knockdown, western blotting for Claspin stability Cell Medium 16901786
2006 Tim (Timeless) and its interacting partner Tipin facilitate the accumulation of Claspin in the nucleus under replication stress conditions. Knockdown of Tipin or Tim causes mislocalization of Claspin to the cytoplasm and impairs Chk1 phosphorylation under replication stress, demonstrating that the Tim-Tipin complex regulates Claspin nuclear localization and is required for full checkpoint signaling. siRNA knockdown, subcellular fractionation, immunofluorescence, Chk1 phosphorylation assay The Journal of biological chemistry Medium 17102137
2008 In response to genotoxic stress in G2, APC/C(Cdh1) activation (triggered by Cdc14B-mediated Plk1 degradation) stabilizes Claspin by reducing Plk1-dependent phosphorylation required for βTrCP-mediated degradation. Claspin is also identified as an APC/C(Cdh1) substrate in G1. The deubiquitylating enzyme Usp28 counteracts APC/C(Cdh1)-mediated Claspin ubiquitylation to permit Claspin-mediated Chk1 activation in G2 damage response. In vivo ubiquitylation assay, co-immunoprecipitation, siRNA, substrate degradation assay, cell-cycle synchronization Cell High 18662541
2020 The CLSPN variant c.1574A>G (p.Asn525Ser) causes partial exon skipping and decreased Claspin expression, and reduces Chk1 activation in signaling experiments. This variant is significantly associated with breast cancer. A promoter variant c.-68C>T increases CLSPN transcriptional activity in a luciferase assay. These results demonstrate that CLSPN variants can modulate Claspin function by altering transcription, RNA processing, and Chk1 activation. Minigene splicing assay, luciferase reporter assay, western blotting for Chk1 activation, association study Cancers Medium 32847043
2019 Claspin (and Timeless) are overexpressed in cancer cells and function to protect replication forks from oncogene-induced replication stress in a checkpoint-independent manner. Reducing Claspin and Timeless to pretumoral levels impedes fork progression without affecting ATR-Chk1 checkpoint signaling. Primary fibroblasts also upregulate Claspin and Timeless in response to oncogene-induced replication stress independently of ATR signaling, indicating a fork-protective gain-of-function role beyond checkpoint activation. DNA fiber assay, siRNA knockdown, checkpoint signaling western blots, primary tumor sample expression analysis Nature communications High 30796221
2025 ALKBH5-mediated m6A demethylation reduces CLSPN mRNA stability in an IGF2BP2-dependent manner. When ALKBH5 is low (as in CRPC), IGF2BP2 reads m6A marks on CLSPN mRNA to stabilize it, increasing Claspin protein levels and promoting docetaxel resistance in prostate cancer. Knocking down IGF2BP2 reverses this resistance, establishing the ALKBH5-IGF2BP2 axis as an epitranscriptomic regulator of CLSPN expression and drug resistance. m6A sequencing, RNA stability assay, Co-IP, siRNA knockdown, organoid models, clinical sample validation iScience Medium 41069850

Source papers

Stage 0 corpus · 39 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2006 Global, in vivo, and site-specific phosphorylation dynamics in signaling networks. Cell 2861 17081983
2003 Sensing DNA damage through ATRIP recognition of RPA-ssDNA complexes. Science (New York, N.Y.) 2197 12791985
2002 Generation and initial analysis of more than 15,000 full-length human and mouse cDNA sequences. Proceedings of the National Academy of Sciences of the United States of America 1479 12477932
2015 The BioPlex Network: A Systematic Exploration of the Human Interactome. Cell 1118 26186194
2017 Architecture of the human interactome defines protein communities and disease networks. Nature 1085 28514442
2015 A human interactome in three quantitative dimensions organized by stoichiometries and abundances. Cell 1015 26496610
2021 Dual proteome-scale networks reveal cell-specific remodeling of the human interactome. Cell 705 33961781
2011 Phylogenetic-based propagation of functional annotations within the Gene Ontology consortium. Briefings in bioinformatics 656 21873635
2018 High-Density Proximity Mapping Reveals the Subcellular Organization of mRNA-Associated Granules and Bodies. Molecular cell 580 29395067
2004 The status, quality, and expansion of the NIH full-length cDNA project: the Mammalian Gene Collection (MGC). Genome research 438 15489334
2022 OpenCell: Endogenous tagging for the cartography of human cellular organization. Science (New York, N.Y.) 432 35271311
2000 Claspin, a novel protein required for the activation of Chk1 during a DNA replication checkpoint response in Xenopus egg extracts. Molecular cell 355 11090622
2021 A proximity-dependent biotinylation map of a human cell. Nature 339 34079125
2008 The Cdc14B-Cdh1-Plk1 axis controls the G2 DNA-damage-response checkpoint. Cell 334 18662541
2006 A role for the deubiquitinating enzyme USP28 in control of the DNA-damage response. Cell 291 16901786
2006 SCFbetaTrCP-mediated degradation of Claspin regulates recovery from the DNA replication checkpoint response. Molecular cell 247 16885022
2018 Mapping the Genetic Landscape of Human Cells. Cell 225 30033366
2011 mTOR drives its own activation via SCF(βTrCP)-dependent degradation of the mTOR inhibitor DEPTOR. Molecular cell 216 22017875
2006 Destruction of Claspin by SCFbetaTrCP restrains Chk1 activation and facilitates recovery from genotoxic stress. Molecular cell 215 16885021
2003 Human claspin is required for replication checkpoint control. The Journal of biological chemistry 213 12766152
2015 A deep proteomics perspective on CRM1-mediated nuclear export and nucleocytoplasmic partitioning. eLife 198 26673895
2006 Polo-like kinase-1 controls proteasome-dependent degradation of Claspin during checkpoint recovery. Current biology : CB 185 16934469
2019 Genetic Screens Reveal FEN1 and APEX2 as BRCA2 Synthetic Lethal Targets. Molecular cell 167 30686591
2013 hPrimpol1/CCDC111 is a human DNA primase-polymerase required for the maintenance of genome integrity. EMBO reports 160 24126761
2018 MYC Protein Interactome Profiling Reveals Functionally Distinct Regions that Cooperate to Drive Tumorigenesis. Molecular cell 152 30415952
2006 The DNA sequence and biological annotation of human chromosome 1. Nature 144 16710414
2006 Claspin operates downstream of TopBP1 to direct ATR signaling towards Chk1 activation. Molecular and cellular biology 141 16880517
2004 Human Claspin works with BRCA1 to both positively and negatively regulate cell proliferation. Proceedings of the National Academy of Sciences of the United States of America 125 15096610
2006 Human Tim/Timeless-interacting protein, Tipin, is required for efficient progression of S phase and DNA replication checkpoint. The Journal of biological chemistry 114 17102137
2019 Overexpression of Claspin and Timeless protects cancer cells from replication stress in a checkpoint-independent manner. Nature communications 110 30796221
2008 Germline alterations in the CLSPN gene in breast cancer families. Cancer letters 13 18077083
2024 CLSPN actives Wnt/β-catenin signaling to facilitate glycolysis and cell proliferation in oral squamous cell carcinoma. Experimental cell research 11 38237848
2021 circRNA derived from CLSPN (circCLSPN) is an oncogene in human glioblastoma multiforme by regulating cell growth, migration and invasion via ceRNA pathway. Journal of biosciences 10 34269180
2020 Implications of CLSPN Variants in Cellular Function and Susceptibility to Cancer. Cancers 6 32847043
2024 The Interaction between CLSPN Gene Polymorphisms and Alcohol Consumption Contributes to Oral Cancer Progression. International journal of molecular sciences 3 38256171
2025 Potential of CLSPN as a therapeutic target in melanoma: a key player in melanoma progression and tumor microenvironment. Journal of translational medicine 1 40275302
2025 ALKBH5-IGF2BP2 axis mediates prostate cancer progression and docetaxel resistance via m6A-stabilized CLSPN RNA. iScience 1 41069850
2026 Integrating multi-omics analysis identifies DNA damage-related gene CLSPN as a biomarker in gastric cancer. Scientific reports 0 41656416
2024 Establishment of potent TCR-T cells specific for cisplatin-resistance related tumor-associated antigen, CLSPN using codon-optimization. Human vaccines & immunotherapeutics 0 39539024