POLE4 (DNA Polymerase Epsilon Subunit 4): A Comprehensive Functional Review

Introduction and Summary

POLE4 (DNA polymerase epsilon subunit 4, also known as p12) is an accessory subunit of the eukaryotic DNA polymerase epsilon (Polε) complex, the primary enzyme responsible for leading strand DNA synthesis during chromosomal replication [li-2000-pole4-identification-abstract]. The human POLE4 gene is located on chromosome 2p12 and encodes a small 117 amino acid protein (approximately 12 kDa) that contains a C-terminal histone fold domain structurally similar to histone H2A [li-2000-pole4-identification-abstract]. POLE4 forms an obligate heterodimer with POLE3 (p17) through their complementary histone fold motifs, creating a structure that mimics the H2A-H2B histone dimer [bellelli-2018-histone-chaperone-abstract].

The primary functions of POLE4 are threefold: first, it serves as a structural scaffold essential for maintaining the stability of the entire Polε holoenzyme complex; second, as part of the POLE3-POLE4 heterodimer, it acts as a histone H3-H4 chaperone that facilitates the recycling of parental histones to the leading strand during DNA replication; and third, it contributes to the processivity of DNA polymerase epsilon by stabilizing its association with newly synthesized double-stranded DNA [bellelli-2018-pole-instability-abstract][bellelli-2018-histone-chaperone-abstract]. Unlike the catalytic subunit POLE1, POLE4 has no enzymatic activity of its own; rather, its role is regulatory and structural, coupling the DNA replication machinery to chromatin maintenance.

Structural Organization and Domain Architecture

POLE4 is the smallest subunit of the tetrameric DNA polymerase epsilon complex. The complete Polε holoenzyme consists of four subunits: POLE1 (p261, the catalytic subunit containing the DNA polymerase and 3'-5' exonuclease activities), POLE2 (p59), POLE3 (p17/CHRAC17), and POLE4 (p12) [li-2000-pole4-identification-abstract]. The nomenclature follows a parallel system in budding yeast Saccharomyces cerevisiae, where the orthologous proteins are designated Pol2, Dpb2, Dpb4, and Dpb3, respectively (notably, POLE3 corresponds to Dpb4 and POLE4 to Dpb3) [bellelli-2018-histone-chaperone-abstract].

The defining structural feature of POLE4 is its C-terminal histone fold domain, which spans approximately residues 45-117 and comprises three alpha helices connected by two loops in an arrangement characteristic of the histone superfamily. The POLE4 histone fold exhibits 34% sequence identity with the yeast Dpb3 subunit and structural similarity to histone H2A [li-2000-pole4-identification-abstract]. The N-terminal region preceding the histone fold is predicted to be largely unstructured and may provide flexibility for interactions with other complex components.

POLE3 contains a complementary H2B-like histone fold domain at its N-terminus, followed by two additional alpha helices and an acidic C-terminal region [bellelli-2018-histone-chaperone-abstract]. The POLE3-POLE4 heterodimer forms through conserved hydrophobic interactions between residues in the central alpha helix (α2) of each histone fold domain; specifically, Phe44 in POLE3 and Phe74 in POLE4 engage in pi-stacking interactions that stabilize the dimer interface. Mutation of these residues disrupts heterodimer formation and consequently impairs both complex stability and histone chaperone function [bellelli-2018-histone-chaperone-abstract].

Cryo-electron microscopy studies of the yeast and human Polε holoenzyme have revealed that the enzyme adopts a bilobal architecture: the N-terminal catalytic domain of POLE1 forms the active polymerase core, while the C-terminal non-catalytic domain of POLE1 together with POLE2 forms a second lobe that mediates interaction with the CMG (Cdc45-MCM-GINS) helicase at the replication fork. The POLE3-POLE4 heterodimer binds to a linker region connecting these two lobes via a conserved "mooring helix" in POLE1, positioning the histone chaperone module adjacent to the nascent DNA as it emerges from the polymerase active site [bellelli-2018-histone-chaperone-abstract].

Molecular Function: Role in DNA Replication

DNA polymerase epsilon is the principal enzyme synthesizing the leading strand during eukaryotic chromosomal DNA replication. The enzyme functions as part of the replisome, a large macromolecular machine that coordinates DNA unwinding by the CMG helicase with concurrent DNA synthesis by Polε on the leading strand and DNA polymerase delta (Polδ) on the lagging strand [bellelli-2018-pole-instability-abstract].

While the catalytic functions of Polε reside entirely within POLE1, the accessory subunits POLE3 and POLE4 perform essential regulatory roles. Studies in yeast demonstrated that deletion of Dpb3 (POLE4) and Dpb4 (POLE3) does not prevent DNA replication but reduces the processivity of the Pol2-Dpb2 subcomplex, leading to frequent dissociation from the DNA template [bellelli-2018-pole-instability-abstract]. This processivity defect manifests as gaps in the newly synthesized leading strand, elevated mutagenesis, and replication stress [hill-2024-parpinhibitor-abstract].

Recent work has defined a two-tier regulatory system governing Polε processivity during leading strand synthesis. The first tier involves PCNA (proliferating cell nuclear antigen), which is loaded specifically onto the leading strand by the CHTF18-RFC2/5 complex and serves as a sliding clamp to tether the polymerase to DNA. The second tier involves the POLE3-POLE4 heterodimer, which "grips" newly synthesized double-stranded DNA as it emerges from the polymerase, further stabilizing the enzyme-DNA association. Combined loss of both CHTF18-RFC loading and POLE3-POLE4 function is synthetically lethal, demonstrating that these represent parallel pathways essential for processive leading strand synthesis [hill-2024-parpinhibitor-abstract].

A critical finding from mammalian studies is that POLE4 is required for maintaining the stability of the entire Polε complex. In Pole4 knockout mice, levels of both POLE1 and POLE2 are substantially reduced, indicating that the POLE3-POLE4 subcomplex serves as a structural scaffold for holoenzyme assembly [bellelli-2018-pole-instability-abstract]. This contrasts with yeast, where Dpb3 deletion reduces processivity but does not destabilize the core enzyme. The enhanced requirement for POLE4 in mammals may reflect the greater complexity of mammalian genome replication or evolutionary divergence in complex assembly mechanisms.

Histone Chaperone Activity and Epigenetic Maintenance

Beyond its structural role in the Polε complex, the POLE3-POLE4 heterodimer functions as a bona fide histone H3-H4 chaperone, directly coupling DNA replication to the re-establishment of chromatin structure on newly synthesized DNA [bellelli-2018-histone-chaperone-abstract]. This function has important implications for epigenetic inheritance, as the faithful transmission of histone modifications from parental to daughter cells depends on the accurate recycling and partitioning of parental histones during DNA replication.

Biochemical studies established that purified POLE3-POLE4 complex directly binds histones H3 and H4 in the absence of other replisome components. Hydrogen-deuterium exchange mass spectrometry mapped the interaction interface to the C-terminal region of POLE3 (including the acidic domain) and the histone fold of H4 (particularly the α2-L2 region) [bellelli-2018-histone-chaperone-abstract]. The POLE3-POLE4 complex can bind both H3-H4 dimers and tetramers, with a preference for the tetrameric form, and promotes tetrasome (H3-H4 tetramer-DNA complex) assembly on linear DNA substrates in vitro. It also induces negative supercoiling of plasmid DNA in the presence of topoisomerase I, a hallmark of histone deposition activity.

At the replication fork, parental histones must be disassembled ahead of the advancing replisome and then reassembled onto the two daughter DNA duplexes behind the fork. The POLE3-POLE4 complex participates specifically in recycling parental H3-H4 tetramers to the leading strand. This pathway operates in parallel with the MCM2-dependent pathway that mediates parental histone transfer to the lagging strand via interactions with DNA polymerase alpha and the adapter protein Ctf4 [bellelli-2018-histone-chaperone-abstract].

Cellular experiments confirmed that POLE3-POLE4 binds both newly synthesized and parental histones in chromatin. Depletion of POLE3-POLE4 function impairs both parental histone retention and new histone deposition, resulting in altered chromatin maturation kinetics at replication forks. Specifically, POLE3-POLE4 deficiency leads to delayed PCNA unloading from chromatin and reduced RPA (replication protein A) accumulation at forks, indicating defects in the coordination of DNA synthesis with chromatin reassembly [bellelli-2018-histone-chaperone-abstract].

Recent studies have extended these findings by examining the consequences of disrupting both leading and lagging strand histone recycling pathways. While POLE4 knockout alone causes moderate asymmetry in parental histone inheritance (with reduced recycling to the leading strand), combined loss of POLE4 and MCM2-mediated pathways results in substantial parental histone loss to the soluble pool and aberrant accumulation of the repressive histone modification H3K27me3, which precedes changes in gene expression [bellelli-2018-histone-chaperone-abstract].

Subcellular Localization

Consistent with its role in DNA replication and chromatin maintenance, POLE4 is localized to the nucleus, specifically within the nucleoplasm where DNA replication occurs. The Human Protein Atlas reports nuclear/nucleoplasmic localization based on immunofluorescence studies, with additional evidence for cytosolic presence at lower levels. During S phase, POLE4 is recruited to replication foci as part of the Polε holoenzyme, where it associates with replicating chromatin.

The subcellular distribution of POLE4 is expected to be cell cycle-regulated, with peak chromatin association during S phase when DNA replication occurs. The protein lacks a classical nuclear localization signal, and nuclear import likely occurs through association with other Polε subunits or via the histone chaperone pathway. Upon depletion of POLE4, its partner POLE3 is rapidly destabilized, suggesting that the heterodimer must form in the cytoplasm prior to nuclear import [bellelli-2018-pole-instability-abstract].

Biological Processes and Disease Associations

Developmental Phenotypes

The biological importance of POLE4 is dramatically illustrated by mouse knockout studies. Pole4-/- mice exhibit embryonic lethality in the inbred C57BL/6 genetic background, while in outbred strains the knockout animals survive but display severe developmental abnormalities [bellelli-2018-pole-instability-abstract]. These include intrauterine growth restriction with low birth weight, craniofacial defects, skeletal abnormalities, cerebellar hypoplasia leading to ataxia, and profound immune system defects characterized by a 5-fold reduction in T cells and impaired thymic development.

Remarkably, many of these phenotypes can be rescued by inactivation of p53, the guardian of genome integrity. This finding indicates that developmental defects in Pole4-/- mice result primarily from p53-mediated cell death or senescence in response to replication stress, rather than from direct loss of an essential cellular function [bellelli-2018-pole-instability-abstract]. Polε deficiency leads to inefficient replication origin firing and increased replication fork stalling, which activates the ATR-p53 checkpoint pathway and eliminates cells with compromised genome replication.

Human IMAGe-like Syndrome

Human patients with biallelic hypomorphic mutations in POLE1 (the catalytic subunit) exhibit a syndrome closely resembling the Pole4-/- mouse phenotype [logan-2018-pole-image-syndrome-abstract]. These patients present with intrauterine growth restriction, metaphyseal dysplasia, adrenal hypoplasia congenita, and genitourinary abnormalities in males—features that define IMAGe syndrome, which was previously associated only with gain-of-function mutations in the cell cycle inhibitor CDKN1C. POLE1-deficient patients also display distinctive facial features and variable immunodeficiency with lymphocyte deficiency.

The parallel phenotypes between human POLE1 deficiency and mouse Pole4 deficiency underscore the functional interdependence of Polε subunits. Because POLE4 is required for the stability of the entire complex, Pole4 knockout effectively phenocopies hypomorphic POLE1 deficiency [logan-2018-pole-image-syndrome-abstract].

Cancer Predisposition and Tumorigenesis

Pole4-/- mice that survive to adulthood exhibit increased cancer susceptibility, particularly lymphomas. The incidence of thymic lymphoma is approximately 12% in Pole4-deficient mice compared to 4% in wild-type controls, while mesenteric lymph node lymphomas occur in 23.5% of knockouts versus 7.7% of controls [bellelli-2018-pole-instability-abstract]. This cancer predisposition is consistent with the role of Polε in maintaining genome stability during DNA replication.

Intriguingly, while p53 inactivation rescues developmental defects in Pole4-/- mice, it dramatically accelerates lymphomagenesis, with Pole4-/- p53+/- double mutant mice succumbing rapidly to T-cell lymphomas [bellelli-2018-pole-instability-abstract]. This illustrates the dual role of p53: it protects against cancer by eliminating cells with replication stress, but in so doing it also causes the developmental abnormalities observed in Polε-deficient animals. Two human patients with POLE1 deficiency also developed lymphomas (T-cell lymphoma at age 11 and Hodgkin lymphoma at age 28), suggesting that POLE deficiency confers lymphoma susceptibility in humans as well [logan-2018-pole-image-syndrome-abstract].

PARP Inhibitor Sensitivity

A therapeutically relevant discovery is that loss of POLE3 or POLE4 sensitizes cancer cells to PARP (poly-ADP-ribose polymerase) inhibitors, a class of drugs approved for treatment of BRCA1/2-deficient cancers [hill-2024-parpinhibitor-abstract][mamar-2024-parpinhibitor-abstract]. Two independent studies in 2024 demonstrated that POLE3-POLE4 knockout cells are hypersensitive to PARP inhibitors through a mechanism distinct from homologous recombination deficiency.

POLE4 knockout cells treated with PARP inhibitors accumulate single-stranded DNA gaps behind replication forks due to impaired post-replicative repair [mamar-2024-parpinhibitor-abstract]. This gap accumulation depends on the translesion polymerase PRIMPOL and leads to elevated replication stress signaling through ATR and DNA-PK kinases. Importantly, POLE4 loss sensitizes cells to PARP inhibitors even when resistance mechanisms associated with restoration of homologous recombination (such as 53BP1 knockdown) are operative, suggesting that targeting POLE4 could overcome acquired PARP inhibitor resistance in cancer therapy [hill-2024-parpinhibitor-abstract].

Additional Interactions: The ATAC Complex

POLE4 has been identified as a component of the ATAC (ADA-two-A-containing) complex, a histone acetyltransferase complex involved in transcriptional regulation. ATAC contains the acetyltransferases GCN5 and ATAC2 along with numerous other subunits, and acetylates histones H3 and H4 to promote transcriptionally active chromatin states. However, the significance of POLE4's association with ATAC remains unclear, and some studies have failed to detect POLE4 in ATAC preparations, possibly reflecting differences in purification stringency or cell type [bellelli-2018-histone-chaperone-abstract]. Whether POLE4 plays a functional role in ATAC-mediated transcription or whether this association reflects promiscuous histone fold interactions remains to be determined.

Role in Heterochromatin Maintenance

Beyond its roles in DNA replication processivity and histone chaperoning, the POLE4 ortholog in fission yeast (Dpb3) plays a critical role in the inheritance of heterochromatin states during DNA replication. Crystallographic studies determined a 1.9-Å structure of the fission yeast Dpb3-Dpb4 complex, confirming that these proteins form a canonical H2A-H2B-like histone fold dimer [he-2017-heterochromatin-abstract]. Disruption of the Dpb3-Dpb4 dimerization interface results in loss of heterochromatin silencing, demonstrating that the intact heterodimer is essential for epigenetic maintenance.

Mechanistically, the Dpb3-Dpb4 complex serves as a platform for recruiting chromatin-modifying enzymes to nascent DNA during replication. In fission yeast, this complex associates with Sir2 (a NAD-dependent histone deacetylase), chromatin remodelers, and Clr4 (the H3K9 methyltransferase responsible for heterochromatin establishment) [he-2017-heterochromatin-abstract]. Cells lacking Dpb3 fail to recruit Sir2 to heterochromatin regions and display strong silencing defects. These findings suggest that the POLE3-POLE4 heterodimer coordinates two key processes during replication-coupled heterochromatin assembly: histone hypoacetylation (via Sir2 recruitment) and H3K9 methylation (via Clr4 association).

This function provides a mechanistic explanation for the epigenetic defects observed in yeast mutants lacking these subunits, and suggests that the mammalian POLE3-POLE4 complex may similarly participate in recruiting chromatin modifiers during DNA replication, although direct evidence for this in mammalian cells remains limited.

Evolutionary Conservation

The POLE3-POLE4 heterodimer is conserved from yeast to humans, although with some notable differences in essentiality and function. In budding yeast, Dpb3 and Dpb4 deletion mutants are viable and show relatively modest phenotypes including reduced Polε processivity, increased mutagenesis, and defects in heterochromatin maintenance. In fission yeast, Dpb3-Dpb4 has been structurally characterized and shown to be important for recruiting chromatin modifiers that maintain heterochromatin silencing [he-2017-heterochromatin-abstract]. In mammals, however, POLE4 has acquired an essential role in maintaining Polε complex stability, making it indispensable for normal development [bellelli-2018-pole-instability-abstract].

The histone fold domains of POLE4 belong to a superfamily that includes not only core histones but also transcription factors (NF-Y subunits, TAF subunits of TFIID) and other DNA-associated proteins. This domain architecture appears repeatedly in proteins that need to bind DNA in a sequence-independent manner or that mediate protein-protein interactions within large complexes. The evolutionary recruitment of histone-fold proteins into the DNA replication machinery likely reflects the ancient coordination between DNA synthesis and chromatin assembly.

Open Questions

Several important questions about POLE4 function remain unresolved:

1. Structural dynamics at the replication fork: How does the POLE3-POLE4 heterodimer coordinate histone binding and release with the polymerase catalytic cycle? High-resolution structures of the complete replisome with bound histones would illuminate this mechanism.

2. Relative contributions to leading strand synthesis versus histone chaperoning: Can the processivity and histone chaperone functions of POLE3-POLE4 be genetically separated, and if so, which function is more critical for the observed phenotypes?

3. Therapeutic targeting: Can POLE4 be selectively targeted in cancer therapy, and what would be the consequences of inhibiting POLE4 function in normal proliferating tissues?

4. Interaction with DNA repair pathways: The synthetic interaction between POLE4 loss and PARP inhibition suggests that POLE4 may have roles in DNA repair beyond its established replication functions. The nature of post-replicative repair pathways requiring POLE4 deserves further investigation.

5. Epigenetic consequences: What are the long-term epigenetic consequences of POLE4 deficiency in surviving cells? Do defects in parental histone recycling lead to altered gene expression programs or cellular identity changes?

6. ATAC complex function: Does POLE4 play a genuine functional role in the ATAC transcriptional coactivator complex, and if so, how are its replication and transcription functions coordinated?

References

• [li-2000-pole4-identification-abstract] Li Y, Pursell ZF, Linn S. Identification and cloning of two histone fold motif-containing subunits of HeLa DNA polymerase epsilon. J Biol Chem. 2000;275(30):23247-52. PMID: 10801849. DOI: 10.1074/jbc.M002548200

• [bellelli-2018-pole-instability-abstract] Bellelli R, Borel V, et al. Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis. Mol Cell. 2018;70(4):707-721.e7. PMID: 29754823. PMCID: PMC5972231. DOI: 10.1016/j.molcel.2018.04.008

• [bellelli-2018-histone-chaperone-abstract] Bellelli R, Belan O, Pye VE, Clement C, et al. POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication. Mol Cell. 2018;72(1):112-126.e5. PMID: 30217558. PMCID: PMC6179962. DOI: 10.1016/j.molcel.2018.08.043

• [logan-2018-pole-image-syndrome-abstract] Logan CV, Murray JE, Jackson AP, et al. DNA Polymerase Epsilon Deficiency Causes IMAGe Syndrome with Variable Immunodeficiency. Am J Hum Genet. 2018;103(6):1038-1044. PMID: 30503519. PMCID: PMC6288413. DOI: 10.1016/j.ajhg.2018.10.024

• [hill-2024-parpinhibitor-abstract] Hill BR, Ozgencil M, Buckley-Benbow L, et al. Loss of POLE3-POLE4 unleashes replicative gap accumulation upon treatment with PARP inhibitors. Cell Rep. 2024;43(5):114205. PMID: 38753485. DOI: 10.1016/j.celrep.2024.114205

• [mamar-2024-parpinhibitor-abstract] Mamar H, Fajka-Boja R, Mórocz M, et al. The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity. Nucleic Acids Res. 2024;52(12):6994-7011. PMID: 38828775. PMCID: PMC11229324. DOI: 10.1093/nar/gkae439

• [he-2017-heterochromatin-abstract] He H, Li Y, Dong Q, et al. Coordinated regulation of heterochromatin inheritance by Dpb3-Dpb4 complex. Proc Natl Acad Sci U S A. 2017;114(47):12524-12529. PMID: 29109278. PMCID: PMC5703312. DOI: 10.1073/pnas.1712961114

Citations

1. bellelli-2018-histone-chaperone-abstract.md
2. bellelli-2018-histone-chaperone-summary.md
3. bellelli-2018-pole-instability-abstract.md
4. bellelli-2018-pole-instability-summary.md
5. he-2017-heterochromatin-abstract.md
6. he-2017-heterochromatin-summary.md
7. hill-2024-parpinhibitor-abstract.md
8. hill-2024-parpinhibitor-summary.md
9. li-2000-pole4-identification-abstract.md
10. li-2000-pole4-identification-summary.md
11. logan-2018-pole-image-syndrome-abstract.md
12. logan-2018-pole-image-syndrome-summary.md
13. mamar-2024-parpinhibitor-abstract.md
14. mamar-2024-parpinhibitor-summary.md
15. moiseeva-2016-pole1-phosphorylation-abstract.md

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Plan overview
- We verified the target identity (POLE4; UniProt Q9NR33) and matched it to the human DNA polymerase epsilon (Pol ε) accessory subunit carrying a histone-fold/CBFA_NFYB-like domain. We then gathered recent mechanistic and structural literature (prioritizing 2023–2024), classic foundational studies, and therapeutic-sensitivity screens. We synthesized function, pathways, localization, disease links, and quantitative details, and compiled a table of recent advances.

Verification of target identity and domain architecture
- Identity and organism: POLE4 encodes the fourth, small accessory subunit of human DNA polymerase ε (Pol ε), part of a four-subunit complex with the catalytic POLE1 and accessory subunits POLE2 and POLE3; the small subunits POLE3/POLE4 are orthologous to yeast Dpb3/Dpb4. These small subunits carry histone-fold motifs and form a heterodimeric module associated with Pol ε (human; Homo sapiens) (bellelli2018polεinstabilitydrives pages 1-3, bellelli2018pole3pole4isa pages 4-5).
- Domains: Biochemical mapping established that POLE3–POLE4 forms a histone-fold heterodimer that binds histone H3–H4, consistent with annotations to CBFA_NFYB/histone-fold superfamilies. Disrupting interface residues (e.g., POLE4 Phe74, POLE3 Phe44) destabilizes the dimer and Pol ε association, supporting the histone-fold and CBFA_NFYB-like domain assignments functionally (bellelli2018pole3pole4isa pages 4-5, bellelli2018pole3pole4isa pages 18-20).
- Ambiguity check: The literature cited here explicitly studies human POLE4 as a Pol ε subunit. No conflicting alternative gene/protein with the same symbol in a different organism is used in our evidence; therefore, we proceed with human POLE4 (Q9NR33).

Key concepts: molecular function, complex membership, and localization
- Complex membership and architecture: Human Pol ε comprises POLE1 (catalytic), POLE2, POLE3, and POLE4. POLE3–POLE4 forms a stable heterodimeric histone-fold module that associates with the Pol ε holoenzyme, stabilizing the complex and connecting the replisome to chromatin assembly (bellelli2018polεinstabilitydrives pages 1-3, bellelli2018pole3pole4isa pages 4-5, bellelli2018pole3pole4isa pages 1-3).
- Molecular function: POLE4, as part of the POLE3–POLE4 module, functions as an H3–H4 histone chaperone. The module binds both newly synthesized and parental H3–H4 in chromatin, promotes tetrasome formation in vitro, and supports replication-coupled nucleosome assembly and maturation during S phase. Loss of POLE3/POLE4 disrupts histone dynamics and nucleosome maturation at replication forks (bellelli2018pole3pole4isa pages 1-3, bellelli2018pole3pole4isa pages 12-14).
- Cellular localization: POLE4 functions within the nuclear replisome at replication forks, associated with Pol ε on the leading strand; the POLE3–POLE4 module is proposed to bias parental histone H3–H4 recycling to the leading strand, complementing MCM2 on the lagging strand (bellelli2018pole3pole4isa pages 12-14).

Recent developments and latest research (prioritizing 2023–2024)
- 2024 human Pol ε–PCNA cryo-EM: He et al. solved structures of human Pol ε–PCNA–DNA in incoming-nucleotide and nucleotide-exchange states. The catalytic domain engages PCNA via a three-point interface (PIP motif, unique P-domain, and thumb contacting different PCNA protomers). The non-catalytic lobe (including POLE2–POLE3–POLE4) was not resolved, consistent with conformational flexibility in human Pol ε. Assembly conditions included 1 μM Pol ε, 3 μM PCNA, 1.2 μM primer/template; Pol ε was concentrated to 3.7 mg/mL for cryo-EM (Nature Communications; published September 2024; https://doi.org/10.1038/s41467-024-52257-x) (he2024structuresofthe pages 9-10).
- 2023 mechanistic insights into DNA damage repair coupling: Mao et al. showed that POLE3 (POLE4’s obligate dimer partner) cooperates with CHRAC1 to promote double-strand break (DSB) repair and KU80 recruitment; WDR70 regulates POLE3 expression via H2BK120ub1/H3K79me2 marks at the POLE3 locus. POLE3 loss increases camptothecin (CPT)-induced DNA damage (γH2AX, p-RPA32), indicating a chromatin-coupled repair role for the POLE3–POLE4 axis (Science Advances; September 2023; https://doi.org/10.1126/sciadv.adh2358) (mao2023requirementofwdr70 pages 8-11).
- Foundational and integrative updates: Multiple studies consistently support a model in which the POLE3–POLE4 histone-fold dimer chaperones H3–H4 at forks to maintain chromatin integrity and promote faithful histone recycling, complementing leading/lagging asymmetry observed in replication-coupled histone dynamics (bellelli2018pole3pole4isa pages 4-5, bellelli2018pole3pole4isa pages 1-3, bellelli2018pole3pole4isa pages 12-14).

Current applications and real-world implementations
- Structural and mechanistic frameworks: The 2024 human Pol ε–PCNA cryo-EM structures inform how Pol ε engages PCNA and occludes other PIP-containing factors, shaping models of leading-strand processivity and replisome factor exchange. These structures provide a template for interpreting how the flexible non-catalytic lobe (POLE2–POLE3–POLE4) rearranges during synthesis and could be leveraged in structure-guided hypotheses of fork regulation and vulnerability (he2024structuresofthe pages 9-10).
- Therapeutic sensitivity biomarkers: Genome-scale CRISPR screening identified POLE3 and POLE4 loss as genetic vulnerabilities that confer marked hypersensitivity to ATR inhibition (e.g., AZD6738), nominating POLE3/POLE4 deficiency as a putative biomarker for ATR inhibitor response and as an entry point for synthetic-lethality strategies under replication stress (Open Biology; September 2019; https://doi.org/10.1098/rsob.190156) (hustedt2019aconsensusset pages 7-8).
- DNA damage response coupling: Experimental assays linking POLE3–CHRAC1–KU80 and WDR70-dependent chromatin signaling to DSB repair suggest actionable nodes for modulating repair capacity in cancer models with replication stress, potentially in combination with topoisomerase poisons (e.g., CPT) or checkpoint inhibitors (mao2023requirementofwdr70 pages 8-11).

Expert opinions and analysis from authoritative sources
- Bellelli et al. (Molecular Cell, 2018) established POLE3–POLE4 as an intrinsic H3–H4 histone chaperone within Pol ε, bridging DNA synthesis to chromatin assembly. They proposed strand-biased parental histone recycling with POLE3–POLE4 acting on the leading strand, a view supported by subsequent chromatin inheritance literature (bellelli2018pole3pole4isa pages 1-3, bellelli2018pole3pole4isa pages 12-14).
- He et al. (Nature Communications, 2024) interpret the absence of density for the non-catalytic lobe in human Pol ε–PCNA structures as evidence of flexibility, implying transient or dynamic association of POLE2–POLE3–POLE4 relative to the catalytically engaged Pol ε core at PCNA. This contrasts with more rigid architectures described in yeast and informs expert discussions of human replisome plasticity (he2024structuresofthe pages 9-10).
- Hustedt et al. (Open Biology, 2019) argued that ATR inhibitor hypersensitivity in POLE3−/− or POLE4−/− cells is more consistent with a defect in DNA synthesis rather than a global failure of histone deposition, refining interpretations of how the POLE3–POLE4 module safeguards replication (hustedt2019aconsensusset pages 7-8).

Relevant statistics and quantitative data
- Structural parameters: human Pol ε–PCNA–DNA assembled at 1 μM Pol ε + 3 μM PCNA + 1.2 μM primer/template; Pol ε concentration for EM at 3.7 mg/mL; two states visualized (incoming nucleotide-bound and nucleotide-exchange), showing a three-point Pol ε–PCNA interface (he2024structuresofthe pages 9-10).
- Biochemistry of histone binding: The POLE3–POLE4 dimer robustly binds H3–H4 in a salt-dependent manner in vitro (binding strong at ~150 mM NaCl; reduced by ~50% at 300 mM; near-loss at 500 mM), supporting specific histone chaperone activity and multiple interaction surfaces on each subunit (bellelli2018pole3pole4isa pages 4-5).
- Genetic screening: POLE3/POLE4 were among a curated set of 117 genes conferring ATR inhibitor sensitivity in genome-scale CRISPR screens; clonogenic validations used AZD6738 (hustedt2019aconsensusset pages 7-8).
- DNA damage assays: POLE3 knockdown increased CPT-induced DNA damage within 4–12 h (γH2AX and p-RPA32 foci; comet assays), and impaired DSB repair readouts (HR/NHEJ reporters). A cancer-relevant CHRAC1 D121Y mutant failed to rescue repair defects due to weakened interaction with POLE3 (mao2023requirementofwdr70 pages 8-11).

Disease and phenotype associations
- Replication stress and development: Pol ε instability due to loss of POLE4 (and reduced Pol ε levels) drives replication stress, p53 activation, growth and developmental defects in mice, and tumor predisposition. Similar cellular phenotypes occur in human cells bearing destabilizing POLE1 mutations, linking Pol ε hypomorphy to human developmental disease spectra with microcephaly and growth restriction features (Molecular Cell, 2018; https://doi.org/10.1016/j.molcel.2018.04.008) (bellelli2018polεinstabilitydrives pages 1-3).
- Chromatin inheritance defects: Perturbing POLE3/POLE4 disrupts histone recycling and nucleosome maturation at forks, a mechanism plausibly underlying aspects of microcephaly/growth phenotypes observed in Pol ε deficiency models (bellelli2018pole3pole4isa pages 1-3, bellelli2018pole3pole4isa pages 12-14).
- Cancer relevance: Pol ε instability promotes tumorigenesis in mouse models; DNA repair coupling via POLE3–CHRAC1 and chromatin signaling via WDR70 at the POLE3 locus underscore potential mutational and epigenetic routes to genome instability in human cancers (bellelli2018polεinstabilitydrives pages 1-3, mao2023requirementofwdr70 pages 8-11).

Primary function distilled
- POLE4 is a small, non-catalytic Pol ε subunit that, together with POLE3, forms a histone-fold heterodimer acting as an H3–H4 histone chaperone at the leading-strand replisome. It stabilizes Pol ε, facilitates parental histone recycling and new histone deposition, and contributes to genome and epigenome integrity during S phase. The catalytic polymerase activity resides in POLE1; POLE4 does not catalyze nucleotide addition but enables chromatin-coupled replication (bellelli2018polεinstabilitydrives pages 1-3, bellelli2018pole3pole4isa pages 4-5, bellelli2018pole3pole4isa pages 1-3, bellelli2018pole3pole4isa pages 12-14).

Recent POLE4-related advances (compact table)
| Year | Study (first author) | System / Method | Key finding about POLE4 / POLE3–POLE4 or Pol ε | Quantitative detail | Journal | URL |
|------|----------------------|------------------|-------------------------------------------------|--------------------|---------|-----|
| 2024 | He (he2024structuresofthe pages 9-10) | Human Pol ε–PCNA–DNA cryo-EM; in vitro assembly (biochemistry + cryo-EM) | Human Pol ε–PCNA structure shows a flexible non-catalytic lobe; POLE3/POLE4 density not resolved; catalytic domain engages PCNA via a 3‑point interface (PIP, P‑domain, thumb). | Pol ε concentrated to 3.7 mg/mL; assembly: 1 μM Pol ε + 3 μM PCNA + 1.2 μM primer/template; two cryo-EM states reported (incoming nucleotide and nucleotide-exchange). | Nature Communications (2024) | https://doi.org/10.1038/s41467-024-52257-x (he2024structuresofthe pages 9-10) |
| 2023 | Mao (mao2023requirementofwdr70 pages 8-11) | Cell-based assays (HCT116/HT29), co-IP, ChIP, laser microirradiation, CPT treatment | POLE3 (partner of POLE4) promotes DSB repair via interaction with CHRAC1 and recruitment of KU80; WDR70 regulates POLE3 expression through chromatin marks; POLE3 loss increases CPT-induced DNA damage. | CPT treatment (4–12 h) produced increased γH2AX and p‑RPA32 foci in shPOLE3 cells; CHRAC1 D121Y fails to rescue repair phenotypes. | Science Advances (2023) | https://doi.org/10.1126/sciadv.adh2358 (mao2023requirementofwdr70 pages 8-11) |
| 2023 | Ahmad () | Genetic cell models (POLE1 exonuclease mutant, CTF18−/−, PARP1−/−), drug sensitivity assays | Pol ε exonuclease and CTF18 promote tolerance to camptothecin (CPT); PARP1 is required for this tolerance, linking Pol ε exonuclease activity to fork stability and drug response. | POLE1exo−/− cells show CPT sensitivity comparable to BRCA1−/−; combining POLE1exo−/− with BRCA1−/− synergistically increases CPT sensitivity. | Nucleic Acids Research (2023) | https://doi.org/10.1093/nar/gkad999 () |
| 2019 | Hustedt (hustedt2019aconsensusset pages 7-8) | Genome-scale CRISPR screens for ATR inhibitor (AZD6738) vulnerabilities; clonogenic assays | Loss of POLE3 or POLE4 causes marked hypersensitivity to ATR inhibition (AZD6738), implicating POLE3/POLE4 in replication-associated processes required for ATR inhibitor tolerance. | Identified within a consensus set of 117 genes whose loss confers ATRi sensitivity; POLE3/POLE4 among validated hypersensitivity hits using AZD6738. | Open Biology (2019) | https://doi.org/10.1098/rsob.190156 (hustedt2019aconsensusset pages 7-8) |

Table: Compact summary table of key 2019–2024 studies linking POLE4/POLE3–POLE4 to Pol ε structure, parental histone/chromatin roles, DNA repair and therapeutic sensitivities (ATRi/PARPi/CPT); includes methods, quantitative details and provenance for quick reference.

Citations (URLs and dates)
- He Q, Wang F, Yao NY, O’Donnell ME, Li H. Structures of the human leading strand Pol ε–PCNA holoenzyme. Nature Communications. 2024-09. https://doi.org/10.1038/s41467-024-52257-x (he2024structuresofthe pages 9-10).
- Mao X, Wu J, Zhang Q, et al. Requirement of WDR70 for POLE3-mediated DNA double-strand breaks repair. Science Advances. 2023-09. https://doi.org/10.1126/sciadv.adh2358 (mao2023requirementofwdr70 pages 8-11).
- Bellelli R, Belan O, Pye VE, et al. POLE3–POLE4 is a histone H3–H4 chaperone that maintains chromatin integrity during DNA replication. Molecular Cell. 2018-10. https://doi.org/10.1016/j.molcel.2018.08.043 (bellelli2018pole3pole4isa pages 4-5, bellelli2018pole3pole4isa pages 1-3, bellelli2018pole3pole4isa pages 18-20, bellelli2018pole3pole4isa pages 12-14).
- Bellelli R, Borel V, Logan C, et al. Pol ε instability drives replication stress, abnormal development, and tumorigenesis. Molecular Cell. 2018-05. https://doi.org/10.1016/j.molcel.2018.04.008 (bellelli2018polεinstabilitydrives pages 1-3).
- Hustedt N, Álvarez-Quilón A, McEwan A, et al. A consensus set of genetic vulnerabilities to ATR inhibition. Open Biology. 2019-09. https://doi.org/10.1098/rsob.190156 (hustedt2019aconsensusset pages 7-8).
- Zakharenko AL, Malakhova AA, Dyrkheeva NS, et al. PARP1 Gene Knockout Suppresses Expression of DNA Base Excision Repair Genes. Doklady Biochemistry and Biophysics. 2023-01. https://doi.org/10.1134/S1607672922700028 (zakharenko2023parp1geneknockout pages 6-6).

Notes and open questions
- The 2024 human Pol ε–PCNA structures do not resolve the non-catalytic lobe (POLE2–POLE3–POLE4), highlighting a current frontier: capturing POLE3–POLE4 placement relative to PCNA and DNA in human holoenzyme states. This will refine mechanistic models of how histone chaperoning is physically coupled to synthesis in the human replisome (he2024structuresofthe pages 9-10).

References

1. (bellelli2018polεinstabilitydrives pages 1-3): Roberto Bellelli, Valerie Borel, Clare Logan, Jennifer Svendsen, Danielle E. Cox, Emma Nye, Kay Metcalfe, Susan M. O’Connell, Gordon Stamp, Helen R. Flynn, Ambrosius P. Snijders, François Lassailly, Andrew Jackson, and Simon J. Boulton. Polε instability drives replication stress, abnormal development, and tumorigenesis. Molecular Cell, 70:707-721.e7, May 2018. URL: https://doi.org/10.1016/j.molcel.2018.04.008, doi:10.1016/j.molcel.2018.04.008. This article has 88 citations and is from a highest quality peer-reviewed journal.

2. (bellelli2018pole3pole4isa pages 4-5): Roberto Bellelli, Ondrej Belan, Valerie E. Pye, Camille Clement, Sarah L. Maslen, J. Mark Skehel, Peter Cherepanov, Genevieve Almouzni, and Simon J. Boulton. Pole3-pole4 is a histone h3-h4 chaperone that maintains chromatin integrity during dna replication. Molecular Cell, 72:112-126.e5, Oct 2018. URL: https://doi.org/10.1016/j.molcel.2018.08.043, doi:10.1016/j.molcel.2018.08.043. This article has 135 citations and is from a highest quality peer-reviewed journal.

3. (bellelli2018pole3pole4isa pages 18-20): Roberto Bellelli, Ondrej Belan, Valerie E. Pye, Camille Clement, Sarah L. Maslen, J. Mark Skehel, Peter Cherepanov, Genevieve Almouzni, and Simon J. Boulton. Pole3-pole4 is a histone h3-h4 chaperone that maintains chromatin integrity during dna replication. Molecular Cell, 72:112-126.e5, Oct 2018. URL: https://doi.org/10.1016/j.molcel.2018.08.043, doi:10.1016/j.molcel.2018.08.043. This article has 135 citations and is from a highest quality peer-reviewed journal.

4. (bellelli2018pole3pole4isa pages 1-3): Roberto Bellelli, Ondrej Belan, Valerie E. Pye, Camille Clement, Sarah L. Maslen, J. Mark Skehel, Peter Cherepanov, Genevieve Almouzni, and Simon J. Boulton. Pole3-pole4 is a histone h3-h4 chaperone that maintains chromatin integrity during dna replication. Molecular Cell, 72:112-126.e5, Oct 2018. URL: https://doi.org/10.1016/j.molcel.2018.08.043, doi:10.1016/j.molcel.2018.08.043. This article has 135 citations and is from a highest quality peer-reviewed journal.

5. (bellelli2018pole3pole4isa pages 12-14): Roberto Bellelli, Ondrej Belan, Valerie E. Pye, Camille Clement, Sarah L. Maslen, J. Mark Skehel, Peter Cherepanov, Genevieve Almouzni, and Simon J. Boulton. Pole3-pole4 is a histone h3-h4 chaperone that maintains chromatin integrity during dna replication. Molecular Cell, 72:112-126.e5, Oct 2018. URL: https://doi.org/10.1016/j.molcel.2018.08.043, doi:10.1016/j.molcel.2018.08.043. This article has 135 citations and is from a highest quality peer-reviewed journal.

6. (he2024structuresofthe pages 9-10): Qing He, Feng Wang, Nina Y. Yao, Michael E. O’Donnell, and Huilin Li. Structures of the human leading strand polε–pcna holoenzyme. Nature Communications, Sep 2024. URL: https://doi.org/10.1038/s41467-024-52257-x, doi:10.1038/s41467-024-52257-x. This article has 12 citations and is from a highest quality peer-reviewed journal.

7. (mao2023requirementofwdr70 pages 8-11): Xiaobing Mao, Jian Wu, Qin Zhang, Su Zhang, Xiaoshuang Chen, Xueqin Liu, Mingtian Wei, Xiaowen Wan, Lei Qiu, Ming Zeng, Xue Lei, Cong Liu, and Junhong Han. Requirement of wdr70 for pole3-mediated dna double-strand breaks repair. Science Advances, Sep 2023. URL: https://doi.org/10.1126/sciadv.adh2358, doi:10.1126/sciadv.adh2358. This article has 11 citations and is from a highest quality peer-reviewed journal.

8. (hustedt2019aconsensusset pages 7-8): Nicole Hustedt, Alejandro Álvarez-Quilón, Andrea McEwan, Jing Yi Yuan, Tiffany Cho, Lisa Koob, Traver Hart, and Daniel Durocher. A consensus set of genetic vulnerabilities to atr inhibition. Open Biology, 9:190156, Sep 2019. URL: https://doi.org/10.1098/rsob.190156, doi:10.1098/rsob.190156. This article has 116 citations and is from a peer-reviewed journal.

9. (zakharenko2023parp1geneknockout pages 6-6): A. L. Zakharenko, A. A. Malakhova, N. S. Dyrkheeva, L. S. Okorokova, S. P. Medvedev, S. M. Zakian, M. R. Kabilov, A. A. Tupikin, and O. I. Lavrik. Parp1 gene knockout suppresses expression of dna base excision repair genes. Doklady. Biochemistry and Biophysics, 508:6-11, Jan 2023. URL: https://doi.org/10.1134/s1607672922700028, doi:10.1134/s1607672922700028. This article has 3 citations.

Citations

1. he2024structuresofthe pages 9-10
2. hustedt2019aconsensusset pages 7-8
3. https://doi.org/10.1038/s41467-024-52257-x
4. https://doi.org/10.1126/sciadv.adh2358
5. https://doi.org/10.1098/rsob.190156
6. https://doi.org/10.1016/j.molcel.2018.04.008
7. https://doi.org/10.1093/nar/gkad999
8. https://doi.org/10.1016/j.molcel.2018.08.043
9. https://doi.org/10.1134/S1607672922700028
10. https://doi.org/10.1016/j.molcel.2018.04.008,
11. https://doi.org/10.1016/j.molcel.2018.08.043,
12. https://doi.org/10.1038/s41467-024-52257-x,
13. https://doi.org/10.1126/sciadv.adh2358,
14. https://doi.org/10.1098/rsob.190156,
15. https://doi.org/10.1134/s1607672922700028,

POLE4 (DNA Polymerase Epsilon Subunit 4) – Function, Processes, and Localization

Overview:
POLE4 (UniProt ID Q9NR33) encodes the smallest accessory subunit of DNA polymerase epsilon (Pol ε) in humans (www.abcam.com). Pol ε is a tetrameric enzyme complex and one of the core replicative DNA polymerases responsible for leading-strand DNA synthesis during S-phase (pmc.ncbi.nlm.nih.gov). The Pol ε holoenzyme consists of a large catalytic subunit (POLE, also called POLE1) with DNA polymerase and exonuclease (proofreading) activities, a second subunit (POLE2) that links Pol ε to the CMG helicase, and two small accessory subunits, POLE3 and POLE4, which lack catalytic activity but play critical structural and regulatory roles (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). POLE4 is also known as the p12 subunit of Pol ε (reflecting its ~12 kDa size) and has been identified in other nuclear complexes (historically termed CHRAC15 in the chromatin accessibility complex) due to its unique structural motif (pmc.ncbi.nlm.nih.gov) (www.abcam.com). Crucially, POLE4 functions within the nucleus at DNA replication forks, where it supports high-fidelity DNA replication and coordinates chromatin assembly, as detailed below.

Structural Features and Complex Assembly

POLE4 is a histone-fold protein, meaning it contains a helix-loop-helix motif structurally analogous to histone proteins. The C-terminal region of POLE4 adopts an H2A-like histone fold, while its partner POLE3 provides an H2B-like histone fold (pmc.ncbi.nlm.nih.gov). These two subunits form a stable POLE3–POLE4 heterodimer via conserved hydrophobic interactions (analogous to the H2A–H2B dimer in nucleosomes) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Biochemical reconstitution experiments have confirmed that human POLE3 and POLE4 directly bind each other to form a tight complex, remaining associated even in high-salt conditions (pmc.ncbi.nlm.nih.gov). This heterodimer interfaces with the Pol ε holoenzyme: POLE3–POLE4 docks onto the catalytic subunit (POLE1), enhancing the stability and DNA-binding capacity of the polymerase complex (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Indeed, structural modeling shows the POLE3–POLE4 dimer superimposes closely with the crystal structure of the yeast Pol ε small-subunit complex (Dpb3–Dpb4) (pmc.ncbi.nlm.nih.gov), highlighting the evolutionary conservation of this assembly.

Complex Stability: In higher eukaryotes, POLE4 is critical for maintaining Pol ε complex integrity. A targeted mouse knockout of Pole4 demonstrated that without POLE4, the entire Pol ε complex becomes destabilized, leading to greatly reduced levels of the other subunits (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Pole4^−/−^ mouse embryonic fibroblasts show loss of POLE3 and marked reduction of POLE1/POLE2 protein levels, indicating that POLE3–POLE4 normally stabilize the catalytic core (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Consistently, human cell studies have found that depleting POLE4 triggers co-depletion of POLE3, implying the two exist as an interdependent subcomplex in vivo (pmc.ncbi.nlm.nih.gov). In budding yeast (S. cerevisiae), the homologous small subunits (Dpb3 and Dpb4) are not essential for viability, and Pol ε can assemble without them (pmc.ncbi.nlm.nih.gov). However, the dispensability in yeast is likely due to compensatory mechanisms, as yeast Pol ε lacking Dpb3/4 shows reduced DNA-binding processivity in vitro (pmc.ncbi.nlm.nih.gov) and increased mutation rates in vivo (pmc.ncbi.nlm.nih.gov). In mammals, by contrast, POLE4 is required for normal development – loss of POLE4 in mice causes embryonic lethality on certain genetic backgrounds and severe growth defects with genomic instability in survivors (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This highlights that POLE4’s stabilizing role became crucial in higher eukaryotes, likely due to added demands on replication fidelity and genome maintenance.

Function in DNA Replication

Pol ε’s primary function is high-fidelity DNA synthesis on the leading strand of the replication fork. The POLE catalytic subunit polymerizes nucleotides in the 5′→3′ direction, with an intrinsic 3′→5′ exonuclease activity for proofreading errors. While POLE4 itself does not catalyze DNA synthesis, it contributes to the efficiency and regulation of replication through its role in the Pol ε complex. Pol ε is considered the major leading-strand DNA polymerase in eukaryotes (pmc.ncbi.nlm.nih.gov), working in concert with the replisome (the CMG helicase, primase, clamp loader, etc.) to duplicate the genome each S-phase. Importantly, emerging evidence shows that Pol ε has multiple, stage-specific roles in replication beyond just nucleotide incorporation:

• Replication Initiation: Pol ε is required to initiate leading-strand synthesis and is also involved in origin firing. The non-catalytic N-terminal domains of Pol ε (on POLE1/POLE2) help assemble the active CMG helicase at origins and trigger origin activation (pmc.ncbi.nlm.nih.gov). Even if the polymerase activity of Pol ε is experimentally bypassed, these assembly functions are essential for viability (pmc.ncbi.nlm.nih.gov). POLE4, by stabilizing Pol ε, indirectly supports this process; without POLE4, cells have inefficient replication origin firing and must rely on fewer active forks (pmc.ncbi.nlm.nih.gov). Pole4-deficient mouse cells showed abnormally large inter-origin distances, indicative of fewer active origins, and a compensatory increase in fork speed at remaining forks (pmc.ncbi.nlm.nih.gov). This suggests that a fully intact Pol ε (including POLE4) is needed for proper origin density and replication timing.

• Polymerase Processivity and Fork Progression: The POLE3/POLE4 subunits are thought to enhance Pol ε’s ability to remain engaged with DNA. Early studies in yeast demonstrated that a Pol ε lacking Dpb3/4 synthesizes DNA with lower processivity (falling off the template more readily) (pmc.ncbi.nlm.nih.gov). The histone-fold dimer of POLE3–POLE4 can bind double-stranded DNA in a sequence-independent manner, much like histone proteins binding DNA (www.ncbi.nlm.nih.gov). This binding may serve to tether Pol ε to the newly synthesized duplex behind the fork, thereby supporting continuous, processive replication (pmc.ncbi.nlm.nih.gov). (Notably, one study reconstituting human Pol ε found that removing POLE3/4 had little effect on polymerization rate in vitro, hinting that their functions in higher eukaryotes might be specialized beyond simple rate enhancement (pmc.ncbi.nlm.nih.gov).) In vivo, however, loss of POLE4 clearly slows replication fork progression. A recent 2024 study reported that POLE4-knockout human cells exhibit reduced fork speed and altered replication dynamics (pubmed.ncbi.nlm.nih.gov) (academic.oup.com). The absence of POLE3/4 led to replication stress markers despite normal checkpoint activation, underscoring that these subunits are needed for optimal fork movement and completion (pubmed.ncbi.nlm.nih.gov).

• Coordination with the Replisome: Pol ε also acts as a structural hub at the fork. It physically links with the Ctf18-RFC clamp loader complex, which helps load PCNA clamps during replication and is involved in establishing sister chromatid cohesion (pmc.ncbi.nlm.nih.gov). The Pol ε complex (likely via POLE2 or associated domains) attaches the Ctf18 complex to the replisome, supporting processive DNA synthesis and proper cohesion of sister chromatids (pmc.ncbi.nlm.nih.gov). While this role is primarily attributed to Pol ε’s larger subunits, the entire complex’s stability (which depends on POLE4) is necessary to maintain such interactions. Therefore, POLE4 indirectly contributes to sister chromatid cohesion and genome stability by keeping Pol ε anchored in the replisome architecture (pmc.ncbi.nlm.nih.gov).

High-Fidelity DNA Synthesis: Through the above mechanisms, Pol ε (with POLE4 as part of the complex) ensures that leading-strand DNA is replicated accurately and efficiently. POLE4’s importance is evident in how its loss increases replication errors and genome instability. Yeast lacking Dpb3/4 show elevated spontaneous mutagenesis (pmc.ncbi.nlm.nih.gov), and in mice, Pole4 deficiency triggers p53-dependent stress responses due to replication difficulties (pmc.ncbi.nlm.nih.gov). Thus, while POLE4 does not catalyze nucleotide addition, it guards replication fidelity by stabilizing Pol ε and preventing replication slippage or fork collapse.

Role in Chromatin Assembly and Genome Maintenance

Beyond polymerization, POLE4 plays a key role in coupling DNA replication with chromatin assembly. This has emerged as a critical function of the POLE3–POLE4 subcomplex in higher eukaryotes (pmc.ncbi.nlm.nih.gov). As the replication fork progresses, parental nucleosomes are disassembled ahead of the fork and new nucleosomes must form promptly behind the fork to restore chromatin structure and preserve epigenetic information (pmc.ncbi.nlm.nih.gov). Several lines of evidence indicate POLE4 is directly involved in this replication-coupled nucleosome assembly process:

• Histone Chaperone Activity: In 2018, Bellelli et al. demonstrated that the POLE3–POLE4 dimer functions as a bona fide histone chaperone for H3–H4 histones (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Using purified proteins, they showed POLE3–POLE4 can bind core histones H3–H4 in vitro and facilitate the formation of tetrasomes (a DNA–H3/H4 intermediate in nucleosome assembly) accompanied by DNA supercoiling (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This is a hallmark of histone chaperone activity. The complex binds H3–H4 both in vitro and in vivo, indicating that POLE4 (with POLE3) escorts histone complexes during replication (pmc.ncbi.nlm.nih.gov). Notably, the histone-binding function was mapped to the C-terminus of POLE3 (which interacts with H3–H4), and mutations that disrupt the POLE3–POLE4 heterodimer also abolish histone binding (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This firmly establishes that POLE4 is part of a replisome-associated histone H3–H4 chaperone complex.

• Parental Histone Recycling: POLE4’s contribution to chromatin was foreshadowed by yeast studies. Deletion of Dpb3/Dpb4 in yeast causes defects in heterochromatin maintenance at telomeres and other regions (pmc.ncbi.nlm.nih.gov). Dpb3/Dpb4 mutants phenocopy mutations in known chromatin assembly factors (like CAF-1 and Asf1) with respect to losing transcriptional gene silencing at heterochromatic loci (pmc.ncbi.nlm.nih.gov). These observations suggested the small Pol ε subunits aid in redepositing parental histones onto daughter DNA strands, thereby preserving epigenetic marks (pmc.ncbi.nlm.nih.gov). The Bellelli et al. study confirmed this in mammalian cells: POLE3/POLE4 depletion leads to defective nucleosome re-assembly at forks, evidenced by abnormalities in replication-fork chromatin. Specifically, cells lacking POLE3/4 showed aberrant RPA accumulation and prolonged PCNA retention on chromatin, indicative of unprocessed single-stranded DNA and delayed maturation of newly replicated chromatin (pmc.ncbi.nlm.nih.gov). In normal cells, as new nucleosomes form, RPA (which binds single-stranded DNA) is displaced and PCNA (the sliding clamp) eventually unloads once Okazaki fragments are ligated and chromatin is in place. The persistence of RPA and PCNA in POLE4-deficient cells points to problems in chromatin reassembly behind the fork (pmc.ncbi.nlm.nih.gov). Thus, POLE4 is required for timely restoration of chromatin structure during replication, working in concert with other histone chaperones.

• Genome Stability and DNA Repair: By ensuring proper chromatin assembly, POLE4 helps maintain genome stability. In the absence of POLE3/4, newly replicated DNA is more prone to damage and irregularities. For example, loss of POLE4 triggers a DNA damage-like response: ATR (ATM and Rad3-related kinase) signaling is elevated, and p53 is activated due to replication stress (pubmed.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). A 2024 study showed that POLE4 knockout cells accumulate post-replicative single-strand DNA gaps – stretches of unreplicated or unligated DNA left behind replication forks (pubmed.ncbi.nlm.nih.gov). These gaps are normally repaired or filled in by post-replication repair pathways; however, POLE4-deficient cells exhibit impaired processing of such gaps, especially under stress conditions (pubmed.ncbi.nlm.nih.gov). Consequently, when treated with PARP inhibitors (which block a DNA single-strand break repair pathway), POLE4-null cells suffer catastrophic levels of DNA gaps and breaks, leading to hypersensitivity to the drug (pubmed.ncbi.nlm.nih.gov). This finding underscores POLE4’s role in facilitating the completion of DNA replication and repair of any residual nicks or gaps, likely by coordinating with repair polymerases or ligases after the replication fork has passed. Additionally, cells lacking POLE3/4 show heightened ATR checkpoint signaling and activation of DNA-PK (a kinase in double-strand break repair), consistent with the accumulation of replication-associated DNA lesions (pubmed.ncbi.nlm.nih.gov). Altogether, these data indicate that POLE4 is crucial for preventing replication stress – it helps Pol ε synthesize DNA continuously and couples that synthesis to immediate chromatin restoration, thereby avoiding abnormal single-stranded regions or DNA damage during S-phase.

It is worth noting that while Pol ε’s primary task is replication, it can also participate in certain DNA repair processes. Pol ε contributes to nucleotide excision repair and long-patch base-excision repair in some contexts, and its exonuclease activity can aid mismatch repair, although Pol δ plays the dominant role in the latter. POLE4’s direct involvement in repair is less well-characterized, but by similarity to yeast and other systems, Pol ε’s accessory subunits are thought to assist in repair-related DNA synthesis and nucleosome reassembly following repair (www.abcam.com) (www.ncbi.nlm.nih.gov). In summary, POLE4’s maintenance of Pol ε integrity and its histone chaperoning function combine to safeguard the genome during both DNA replication and the maturation of replicated DNA.

Cellular Localization and Context of Action

Subcellular localization: The POLE4 protein carries out its functions in the cell nucleus, in line with its role at replication forks. Immunocytochemistry and protein localization data confirm that POLE4 is predominantly found in the nucleoplasm (the interior nuclear space where DNA replication occurs) (www.proteinatlas.org). During S-phase, POLE4 localizes to replication foci – discrete nuclear sites of active DNA synthesis – as part of the Pol ε complex. Some studies and proteomic data have also detected a fraction of POLE4 in the cytosol (www.proteinatlas.org), but this likely represents unassembled protein or pre-import pools, since the functional action of POLE4 requires it to be in the nucleus and incorporated into Pol ε or other chromatin complexes. There is no signal peptide or transmembrane domain in POLE4 (v23.proteinatlas.org), and it is predicted to be an intracellular protein, consistent with a nuclear/chromatin role. Thus, inside the cell nucleus is the principal site of POLE4 activity, specifically at the replication machinery and chromatin interface.

Complex associations: Within the nucleus, POLE4 exists in at least two major complexes: (1) the Pol ε holoenzyme at replication forks, and (2) the CHRAC (Chromatin Accessibility Complex) involved in chromatin remodeling. The human CHRAC complex, which facilitates nucleosome spacing and chromatin assembly, contains the ATP-dependent remodeler ACF1 and two small histone-fold proteins originally termed CHRAC17 and CHRAC15 (pmc.ncbi.nlm.nih.gov). Notably, CHRAC17 is identical to POLE3, and CHRAC15 is the POLE4 protein (pmc.ncbi.nlm.nih.gov). In other words, POLE3–POLE4 double as subunits in a chromatin remodeling complex, reflecting their ability to bind DNA and histones. This dual presence suggests that POLE4 is not only integral to the replication fork, but also contributes to broader chromatin metabolism. In the CHRAC context, POLE3/POLE4 help the ISWI-family remodeler to organize nucleosomes and modulate DNA accessibility (pmc.ncbi.nlm.nih.gov). The interchangeable use of POLE4 in both replication and remodeling complexes underlines its general role as a DNA-binding adapter that can be recruited to different machinery where a histone-fold module is needed.

Pathway Integration and Biological Impact

Replication and Cell Cycle Pathways: POLE4 functions within the core DNA replication pathway, particularly the leading-strand synthesis arm of the eukaryotic replisome. Through Pol ε, it is connected to the cell cycle regulation of S-phase. For instance, proper Pol ε assembly (including POLE4) is required for the activation of replication origins in concert with S-phase kinase signaling (pmc.ncbi.nlm.nih.gov). If POLE4 or Pol ε is defective, intra-S phase checkpoints (like ATR) are activated due to replication stress (pubmed.ncbi.nlm.nih.gov). Indeed, Pol ε has been proposed to serve as a sensor in the S-phase DNA damage checkpoint, since stalls in leading strand synthesis rapidly trigger ATR/Chk1 signaling. Some of this sensing function is attributed to the large subunit (POLE1) and its interaction with checkpoint proteins, but maintaining Pol ε’s presence via POLE4 is necessary for these signals to be effective (pmc.ncbi.nlm.nih.gov). Therefore, POLE4 indirectly partakes in the DNA damage response (DDR) pathways: by preventing replication fork collapse it reduces the need for checkpoint activation, and when problems occur, a Pol ε complex with POLE4 can properly signal and recruit repair machinery (pmc.ncbi.nlm.nih.gov). The 2022 mini-review by Cvetkovic et al. encapsulates Pol ε’s multi-faceted roles, noting that Pol ε “supports processive DNA synthesis, DNA damage response signaling as well as sister chromatid cohesion” as part of its structural functions at the fork (pmc.ncbi.nlm.nih.gov). POLE4 is an essential contributor to these outcomes.

Development and Disease: The importance of POLE4 for normal cell physiology is evident from genetic studies. In vivo, knockout of Pole4 in mice is lethal on certain backgrounds or causes severe developmental abnormalities (pmc.ncbi.nlm.nih.gov). Pole4^−/−^ embryos that survive exhibit growth retardation, immune cell deficiencies (leukopenia), and a high incidence of developmental defects (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Many of these phenotypes (growth impairment, immunodeficiency) resemble those seen in human patients with hypomorphic mutations in POLE1/POLE2 (pmc.ncbi.nlm.nih.gov), underlining that a functional Pol ε complex is required for proliferative tissues and genome maintenance in mammals. The developmental arrest and cell death in Pole4-null conditions are largely due to unresolved replication stress activating p53-mediated apoptosis (pmc.ncbi.nlm.nih.gov). When p53 was removed in Pole4-deficient mice, viability improved, but these animals developed cancers at an accelerated rate (pmc.ncbi.nlm.nih.gov). This reveals that POLE4 has a tumor suppressor role via maintaining replication fidelity – in its absence, cells accumulate DNA errors and genomic instability that predispose to cancer (unless eliminated by p53-driven checkpoints) (pmc.ncbi.nlm.nih.gov).

In the context of human disease, POLE4 itself is not a well-known mutational hotspot (unlike POLE, whose exonuclease-domain mutations cause ultramutator tumors). However, the POLE3–POLE4 subunits have attracted interest as possible targets for therapy. Because cells lacking these subunits are vulnerable to PARP inhibitors, researchers suggest that inhibiting the POLE3–POLE4 function could be a strategy to induce lethal replication stress in cancer cells (pubmed.ncbi.nlm.nih.gov). A June 2024 Nucleic Acids Research study by Mamar et al. showed that removing POLE4 in cancer cells synergizes with PARP inhibition, even in tumors that are BRCA1-proficient (pubmed.ncbi.nlm.nih.gov). The loss of POLE4 slows DNA replication and leads to accumulation of single-stranded gaps, which PARP inhibitors convert into fatal DNA damage (pubmed.ncbi.nlm.nih.gov). Thus, POLE4 is emerging as a potential target to exploit replication stress in cancer therapy (pubmed.ncbi.nlm.nih.gov). These findings, while translational, reinforce the notion that POLE4’s normal role is to prevent pathological replication stress by aiding complete DNA synthesis and chromatin maturation.

Summary of Key Points

• Primary Function: POLE4 is a non-enzymatic subunit of DNA polymerase ε, essential for leading-strand DNA replication. It does not catalyze DNA synthesis itself, but it enables the Pol ε complex to function properly. The Pol ε enzyme (with POLE4 as a component) catalyzes the polymerization of deoxynucleotides onto the growing DNA strand with high fidelity, and includes proofreading capability in the POLE1 subunit (pmc.ncbi.nlm.nih.gov). POLE4’s broader role is structural and regulatory – it stabilizes the polymerase and connects it to DNA and chromatin substrates.

• Molecular Mechanism: POLE4 contains a histone-fold domain and forms a heterodimer with POLE3 (pmc.ncbi.nlm.nih.gov). This POLE3–POLE4 pair binds double-stranded DNA without sequence specificity (www.ncbi.nlm.nih.gov) and associates with histones H3–H4 (pmc.ncbi.nlm.nih.gov). Through these interactions, POLE4 increases Pol ε’s binding to DNA and promotes nucleosome reassembly behind the replication fork (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). It effectively acts as an adapter/chaperone, bridging the replication machinery with chromatin.

• Biological Processes: POLE4 is involved in chromosomal DNA replication – specifically initiation of replication (origin firing) and elongation on the leading strand (pmc.ncbi.nlm.nih.gov). It is also critical for replication-coupled nucleosome assembly, helping recycle parental histones and deposit new histones to maintain chromatin integrity (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). By ensuring proper chromatin restoration, POLE4 aids in preserving epigenetic information and heterochromatin structures during replication (pmc.ncbi.nlm.nih.gov). Additionally, POLE4 contributes to the cellular DNA damage response to stalled forks and influences post-replicative repair of DNA gaps (pubmed.ncbi.nlm.nih.gov). It thereby plays a role in maintaining overall genome stability during cell proliferation.

• Cellular Localization: The POLE4 protein functions in the nucleus, predominantly in the nucleoplasm and at sites of DNA replication (replication foci) (www.proteinatlas.org). It is a component of nuclear enzymatic complexes (Pol ε and CHRAC) and is not secreted or membrane-bound. A minor cytosolic presence may occur, but active POLE4 is nuclear, where it associates with DNA, histones, and replication factories (www.abcam.com).

• Pathway and Interactions: POLE4 operates within the DNA replication pathway and S-phase checkpoint network. It is required for proper assembly of the CMG helicase and the progression of replication forks (pmc.ncbi.nlm.nih.gov). It also has a role in linking the replication fork to chromatin cohesion mechanisms via the Ctf18 clamp loader (pmc.ncbi.nlm.nih.gov). In terms of signaling, a functional Pol ε (with POLE4 intact) is needed for normal ATR-dependent checkpoint activation when replication stress occurs (pmc.ncbi.nlm.nih.gov). Loss of POLE4 skews this balance, leading to heightened ATR signaling and reliance on p53 to halt the cell cycle (pmc.ncbi.nlm.nih.gov).

All these findings are supported by recent experimental evidence. For example, Bellelli et al. (2018) showed biochemically that POLE3–POLE4 binds histones and is required for nucleosome assembly during replication (pmc.ncbi.nlm.nih.gov). Cox et al. (2018) demonstrated in a Pole4^-/- mouse model that POLE4 is indispensable for Pol ε stability and normal development (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Mamar et al. (NAR, 2024) and Hill et al. (Cell Rep, 2024) independently found that cells lacking POLE4 experience slower replication and accumulate ssDNA gaps, linking POLE4 to replication-fork repair processes (pubmed.ncbi.nlm.nih.gov). Furthermore, a 2022 review by Bellelli and colleagues summarizes that Pol ε’s histone-fold subunits (POLE3/4), while not essential for the polymerase activity per se, serve a critical role in redepositing parental histones onto newly synthesized DNA and supporting fork progression (pmc.ncbi.nlm.nih.gov). These authoritative sources collectively portray POLE4 as a key adapter protein that connects the enzymatic function of DNA polymerase ε to the structural demands of chromatin replication, thereby ensuring that DNA duplication and nucleosome assembly proceed hand-in-hand.

References: Recent research and reviews were used to compile this information, including Molecular Cell (2018) (pmc.ncbi.nlm.nih.gov), Cell Reports (2018) (pmc.ncbi.nlm.nih.gov), Biochemical Society Transactions (2022) (pmc.ncbi.nlm.nih.gov), and Nucleic Acids Research (2024) (pubmed.ncbi.nlm.nih.gov), among others. These studies provide experimental evidence and expert analysis of POLE4’s function, localization, and involvement in DNA replication and genome maintenance. All claims are supported by the cited literature, reflecting the current understanding (as of 2023–2024) of the human POLE4 gene product.

Citations

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19. AnnotationURLCitation(end_index=5640, start_index=5518, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=DNA%20polymerase%20%CE%B5%20,In%20both')
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42. AnnotationURLCitation(end_index=14536, start_index=14378, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=During%20S%20phase%20of%20the,deposition%20of%20newly%20synthesized%20ones')
43. AnnotationURLCitation(end_index=14961, start_index=14829, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=Mammalian%20POLE3,H2B%20histone%20fold%20complex')
44. AnnotationURLCitation(end_index=15124, start_index=14962, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=Bellelli%20et%C2%A0al,dismantling%2Fmaturation%20at%20the%20replication%20fork')
45. AnnotationURLCitation(end_index=15479, start_index=15344, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=The%20POLE3,H4%20in%C2%A0vitro%20and%20in%C2%A0vivo')
46. AnnotationURLCitation(end_index=15642, start_index=15480, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=Bellelli%20et%C2%A0al,dismantling%2Fmaturation%20at%20the%20replication%20fork')
47. AnnotationURLCitation(end_index=15961, start_index=15829, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=Mammalian%20POLE3,H2B%20histone%20fold%20complex')
48. AnnotationURLCitation(end_index=16327, start_index=16157, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=POLE3%20contains%20a%20histone%20fold,superimposes%20with%20the%20recently%20published')
49. AnnotationURLCitation(end_index=16516, start_index=16328, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=structural%20model%20almost%20completely%20superimposes,POLE4%2C%20validating%20our%20structural%20model')
50. AnnotationURLCitation(end_index=17022, start_index=16840, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=Observations%20in%20budding%20and%20fission,maintenance%20of%20heterochromatin%20remains%20unclear')
51. AnnotationURLCitation(end_index=17380, start_index=17198, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=Observations%20in%20budding%20and%20fission,maintenance%20of%20heterochromatin%20remains%20unclear')
52. AnnotationURLCitation(end_index=17723, start_index=17541, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=Observations%20in%20budding%20and%20fission,maintenance%20of%20heterochromatin%20remains%20unclear')
53. AnnotationURLCitation(end_index=18293, start_index=18143, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=chaperone,dismantling%2Fmaturation%20at%20the%20replication%20fork')
54. AnnotationURLCitation(end_index=18768, start_index=18618, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=chaperone,dismantling%2Fmaturation%20at%20the%20replication%20fork')
55. AnnotationURLCitation(end_index=19438, start_index=19288, title='The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38828775/#:~:text=recombination%20deficiency,and%20hamper%20acquired%20PARPi%20resistance')
56. AnnotationURLCitation(end_index=19615, start_index=19439, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=observed%20in%20human%20patients%20harboring,normal%20development%20and%20tumor%20prevention')
57. AnnotationURLCitation(end_index=19957, start_index=19793, title='The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38828775/#:~:text=POLE3%20and%20POLE4%2C%20sensitizes%20cells,promising%20target%20to%20improve%20PARPi')
58. AnnotationURLCitation(end_index=20318, start_index=20154, title='The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38828775/#:~:text=POLE3%20and%20POLE4%2C%20sensitizes%20cells,promising%20target%20to%20improve%20PARPi')
59. AnnotationURLCitation(end_index=20696, start_index=20532, title='The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38828775/#:~:text=POLE3%20and%20POLE4%2C%20sensitizes%20cells,promising%20target%20to%20improve%20PARPi')
60. AnnotationURLCitation(end_index=21298, start_index=21148, title='The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38828775/#:~:text=recombination%20deficiency,and%20hamper%20acquired%20PARPi%20resistance')
61. AnnotationURLCitation(end_index=22241, start_index=22158, title='POLE4 | Abcam', type='url_citation', url='https://www.abcam.com/en-us/targets/pole4/20801#:~:text=Function')
62. AnnotationURLCitation(end_index=22408, start_index=22242, title='POLE4 DNA polymerase epsilon 4, accessory subunit [Homo sapiens (human)] - Gene - NCBI', type='url_citation', url='https://www.ncbi.nlm.nih.gov/gene?Cmd=DetailsSearch&Db=gene&Term=56655#:~:text=POLE4%20is%20a%20histone,supplied%20by%20OMIM%2C%20Apr%202004')
63. AnnotationURLCitation(end_index=23122, start_index=22966, title='POLE4 protein expression summary - The Human Protein Atlas', type='url_citation', url='https://www.proteinatlas.org/ENSG00000115350-POLE4#:~:text=Subcellular%20location,i%7D%20Intracellular%20TISSUE%20RNA%20EXPRESSION')
64. AnnotationURLCitation(end_index=23507, start_index=23351, title='POLE4 protein expression summary - The Human Protein Atlas', type='url_citation', url='https://www.proteinatlas.org/ENSG00000115350-POLE4#:~:text=Subcellular%20location,i%7D%20Intracellular%20TISSUE%20RNA%20EXPRESSION')
65. AnnotationURLCitation(end_index=23913, start_index=23765, title='Protein structure - POLE4 - The Human Protein Atlas', type='url_citation', url='https://v23.proteinatlas.org/ENSG00000115350-POLE4/structure#:~:text=Structure%20prediction%20of%20Q9NR33%20from,predicted')
66. AnnotationURLCitation(end_index=24704, start_index=24591, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=match%20at%20L1811%20P,Google')
67. AnnotationURLCitation(end_index=24892, start_index=24779, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=match%20at%20L1811%20P,Google')
68. AnnotationURLCitation(end_index=25463, start_index=25294, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=Pol%CE%B5%20binding%20to%20double,studies%20using%20reconstituted%20human%20Pol%CE%B5')
69. AnnotationURLCitation(end_index=26294, start_index=26130, title='Multiple roles of Pol epsilon in eukaryotic chromosome replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC9022971/#:~:text=Pol%20epsilon%20is%20a%20tetrameric,this%20minireview%2C%20we%20discuss%20recent')
70. AnnotationURLCitation(end_index=26555, start_index=26405, title='The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38828775/#:~:text=recombination%20deficiency,and%20hamper%20acquired%20PARPi%20resistance')
71. AnnotationURLCitation(end_index=27114, start_index=26938, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=observed%20in%20human%20patients%20harboring,normal%20development%20and%20tumor%20prevention')
72. AnnotationURLCitation(end_index=27544, start_index=27388, title='Multiple roles of Pol epsilon in eukaryotic chromosome replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC9022971/#:~:text=activation%2C%20while%20non,this%20minireview%2C%20we%20discuss%20recent')
73. AnnotationURLCitation(end_index=27961, start_index=27805, title='Multiple roles of Pol epsilon in eukaryotic chromosome replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC9022971/#:~:text=activation%2C%20while%20non,this%20minireview%2C%20we%20discuss%20recent')
74. AnnotationURLCitation(end_index=28371, start_index=28249, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=DNA%20polymerase%20%CE%B5%20,In%20both')
75. AnnotationURLCitation(end_index=28664, start_index=28518, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=Pol%CE%B5%20processivity%20in%C2%A0vitro%20but%20are,In%20both')
76. AnnotationURLCitation(end_index=28850, start_index=28665, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=leading%20to%20embryonic%20lethality%20in,mice%20exhibit%20accelerated%20tumorigenesis%2C%20revealing')
77. AnnotationURLCitation(end_index=29180, start_index=28995, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=leading%20to%20embryonic%20lethality%20in,mice%20exhibit%20accelerated%20tumorigenesis%2C%20revealing')
78. AnnotationURLCitation(end_index=29627, start_index=29451, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=observed%20in%20human%20patients%20harboring,normal%20development%20and%20tumor%20prevention')
79. AnnotationURLCitation(end_index=29925, start_index=29754, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=with%20replication%20stress%20and%20p53,normal%20development%20and%20tumor%20prevention')
80. AnnotationURLCitation(end_index=30330, start_index=30159, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=with%20replication%20stress%20and%20p53,normal%20development%20and%20tumor%20prevention')
81. AnnotationURLCitation(end_index=30959, start_index=30795, title='The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38828775/#:~:text=POLE3%20and%20POLE4%2C%20sensitizes%20cells,promising%20target%20to%20improve%20PARPi')
82. AnnotationURLCitation(end_index=31326, start_index=31143, title='The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38828775/#:~:text=their%20applicability%20beyond%20homologous%20recombination,sensitivity%20to%20PARPi%20and%20counteracts')
83. AnnotationURLCitation(end_index=31635, start_index=31471, title='The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38828775/#:~:text=POLE3%20and%20POLE4%2C%20sensitizes%20cells,promising%20target%20to%20improve%20PARPi')
84. AnnotationURLCitation(end_index=31898, start_index=31735, title='The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38828775/#:~:text=PARPi%20treatment%2C%20due%20to%20impaired,promising%20target%20to%20improve%20PARPi')
85. AnnotationURLCitation(end_index=32680, start_index=32558, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=DNA%20polymerase%20%CE%B5%20,In%20both')
86. AnnotationURLCitation(end_index=33090, start_index=32920, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=POLE3%20contains%20a%20histone%20fold,superimposes%20with%20the%20recently%20published')
87. AnnotationURLCitation(end_index=33358, start_index=33169, title='POLE4 DNA polymerase epsilon 4, accessory subunit [Homo sapiens (human)] - Gene - NCBI', type='url_citation', url='https://www.ncbi.nlm.nih.gov/gene?Cmd=DetailsSearch&Db=gene&Term=56655#:~:text=Summary%20POLE4%20is%20a%20histone,enzymatic%20complexes%20for%20DNA%20transcription')
88. AnnotationURLCitation(end_index=33529, start_index=33394, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=The%20POLE3,H4%20in%C2%A0vitro%20and%20in%C2%A0vivo')
89. AnnotationURLCitation(end_index=33815, start_index=33665, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=Ohya%20et%C2%A0al,processivity%20on%20synthetic%20DNA%20substrates')
90. AnnotationURLCitation(end_index=33978, start_index=33816, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=Bellelli%20et%C2%A0al,dismantling%2Fmaturation%20at%20the%20replication%20fork')
91. AnnotationURLCitation(end_index=34435, start_index=34264, title='Multiple roles of Pol epsilon in eukaryotic chromosome replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC9022971/#:~:text=Pol%20epsilon%20is%20a%20tetrameric,synthesis%2C%20DNA%20damage%20response%20signalling')
92. AnnotationURLCitation(end_index=34779, start_index=34597, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=Observations%20in%20budding%20and%20fission,maintenance%20of%20heterochromatin%20remains%20unclear')
93. AnnotationURLCitation(end_index=34941, start_index=34780, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=Here%20we%20show%20that%20the,important%20role%20in%20chromatin%20maintenance')
94. AnnotationURLCitation(end_index=35265, start_index=35083, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=Observations%20in%20budding%20and%20fission,maintenance%20of%20heterochromatin%20remains%20unclear')
95. AnnotationURLCitation(end_index=35575, start_index=35411, title='The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38828775/#:~:text=POLE3%20and%20POLE4%2C%20sensitizes%20cells,promising%20target%20to%20improve%20PARPi')
96. AnnotationURLCitation(end_index=35990, start_index=35834, title='POLE4 protein expression summary - The Human Protein Atlas', type='url_citation', url='https://www.proteinatlas.org/ENSG00000115350-POLE4#:~:text=Subcellular%20location,i%7D%20Intracellular%20TISSUE%20RNA%20EXPRESSION')
97. AnnotationURLCitation(end_index=36333, start_index=36235, title='POLE4 | Abcam', type='url_citation', url='https://www.abcam.com/en-us/targets/pole4/20801#:~:text=Cellular%20localization')
98. AnnotationURLCitation(end_index=36713, start_index=36550, title='Multiple roles of Pol epsilon in eukaryotic chromosome replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC9022971/#:~:text=function%20is%20dispensable%20for%20viability,the%20multiple%20roles%20of%20Pol')
99. AnnotationURLCitation(end_index=36982, start_index=36826, title='Multiple roles of Pol epsilon in eukaryotic chromosome replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC9022971/#:~:text=activation%2C%20while%20non,this%20minireview%2C%20we%20discuss%20recent')
100. AnnotationURLCitation(end_index=37310, start_index=37134, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=observed%20in%20human%20patients%20harboring,normal%20development%20and%20tumor%20prevention')
101. AnnotationURLCitation(end_index=37601, start_index=37425, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=observed%20in%20human%20patients%20harboring,normal%20development%20and%20tumor%20prevention')
102. AnnotationURLCitation(end_index=37990, start_index=37828, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=Bellelli%20et%C2%A0al,dismantling%2Fmaturation%20at%20the%20replication%20fork')
103. AnnotationURLCitation(end_index=38249, start_index=38127, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=DNA%20polymerase%20%CE%B5%20,In%20both')
104. AnnotationURLCitation(end_index=38426, start_index=38250, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=observed%20in%20human%20patients%20harboring,normal%20development%20and%20tumor%20prevention')
105. AnnotationURLCitation(end_index=38815, start_index=38651, title='The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38828775/#:~:text=POLE3%20and%20POLE4%2C%20sensitizes%20cells,promising%20target%20to%20improve%20PARPi')
106. AnnotationURLCitation(end_index=39270, start_index=39107, title='Multiple roles of Pol epsilon in eukaryotic chromosome replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC9022971/#:~:text=function%20is%20dispensable%20for%20viability,the%20multiple%20roles%20of%20Pol')
107. AnnotationURLCitation(end_index=39830, start_index=39668, title='POLE3-POLE4 Is a Histone H3-H4 Chaperone that Maintains Chromatin Integrity during DNA Replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6179962/#:~:text=Bellelli%20et%C2%A0al,dismantling%2Fmaturation%20at%20the%20replication%20fork')
108. AnnotationURLCitation(end_index=39976, start_index=39854, title='Polε Instability Drives Replication Stress, Abnormal Development, and Tumorigenesis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC5972231/#:~:text=DNA%20polymerase%20%CE%B5%20,In%20both')
109. AnnotationURLCitation(end_index=40184, start_index=40020, title='Multiple roles of Pol epsilon in eukaryotic chromosome replication - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC9022971/#:~:text=Pol%20epsilon%20is%20a%20tetrameric,this%20minireview%2C%20we%20discuss%20recent')
110. AnnotationURLCitation(end_index=40386, start_index=40222, title='The loss of DNA polymerase epsilon accessory subunits POLE3-POLE4 leads to BRCA1-independent PARP inhibitor sensitivity - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/38828775/#:~:text=POLE3%20and%20POLE4%2C%20sensitizes%20cells,promising%20target%20to%20improve%20PARPi')