Flap endonuclease 1 (FEN-1) is a structure-specific nuclease in the XPG/RAD2 family with dual 5'-flap endonuclease and 5'-3' exonuclease activities. It is essential for Okazaki fragment maturation during lagging-strand DNA replication (cleaving displaced 5'-flaps to produce ligatable nicks) and for long-patch base excision repair (trimming flaps generated by strand displacement synthesis). FEN1 also exhibits RNase H activity on RNA-DNA hybrids, contributing to RNA primer removal. The enzyme requires two magnesium ions for catalysis and interacts with PCNA to coordinate its activity at replication forks and repair sites. FEN1 localizes to the nucleolus (for rDNA maintenance), the nucleoplasm (upon DNA damage), and mitochondria (for mtDNA repair). In zebrafish, fen1 is essential for early development, and insertional mutants display severe retinal defects with reduced and disorganized retinal neurons, reflecting the high proliferative demand of retinal progenitor cells.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: FEN1 is a nuclear protein. The IBA annotation is supported by phylogenetic inference from well-characterized orthologs in yeast (Rad27) and mammals. UniProt confirms nuclear localization (nucleolus and nucleoplasm) based on HAMAP rule transfer. Reason: The enzyme acts on replication and repair intermediates in chromosomal DNA, so nuclear localization follows from the function; UniProt (HAMAP MF_03140) and the human localization study agree. Core location. Supporting Evidence: file:DANRE/fen1/fen1-uniprot.txt SUBCELLULAR LOCATION: Nucleus, nucleolus file:DANRE/fen1/fen1-uniprot.txt Nucleus, nucleoplasm PMID:18443037 FEN1 is superaccumulated in the nucleolus and plays a role in the resolution of stalled DNA replication forks |
| GO:0017108 5'-flap endonuclease activity | IBA GO_REF:0000033 | ACCEPT | Summary: 5'-flap endonuclease activity is the defining molecular function of FEN1. The enzyme enters the 5'-end of a flap, tracks to the flap base, and makes a single endonucleolytic cut at the junction between single- and double-stranded DNA. This is the core catalytic activity of the protein. Reason: Defining catalytic activity of the XPG/RAD2 FEN1 subfamily; PAINT node PTN000118612 is grounded by human, mouse, yeast, fly and plant FEN1 experimental data and zebrafish fen1 retains the full nuclease domain and the two-metal active site (UniProt binding-site features at residues 34, 47, 70, 86, 158, 160, 179, 181, 231, 233). Core function. Supporting Evidence: PMID:23451868 First discovered as a structure-specific endonuclease that evolved to cut at the base of single-stranded flaps PMID:23451868 FEN1 recognizes this structure, binds to the base of the flap, and precisely cleaves it, removing the RNA and some portion of the initiator DNA to make a nick file:DANRE/fen1/fen1-uniprot.txt Structure-specific nuclease with 5'-flap endonuclease and 5'- |
| GO:0008409 5'-3' exonuclease activity | IBA GO_REF:0000033 | ACCEPT | Summary: FEN1 possesses 5'-3' exonuclease activity on nicked or gapped double-stranded DNA, which is a well-characterized second catalytic mode of the enzyme. This activity requires occupancy of both divalent metal binding sites. Supporting Evidence: PMID:23451868 The polymerase and 5β² nuclease act together to carry out a process called nick translation, in which the 5β² side of a nick in DNA is degraded while the 3β² side is extended file:DANRE/fen1/fen1-deep-research-falcon.md three biochemically distinguishable nuclease activities described in the literature: **flap endonuclease (FEN)**, **5β² exonuclease (EXO)**, and **gap endonuclease (GEN)** file:DANRE/fen1/fen1-deep-research-falcon.md exonuclease activity contributes to editing/removal of Pol Ξ± errors during Okazaki fragment maturation, helping prevent mutagenesis when nascent lagging strands are processed |
| GO:0000287 magnesium ion binding | IBA GO_REF:0000033 | ACCEPT | Summary: FEN1 binds two magnesium ions per subunit that are essential for catalysis. The active site acidic residues coordinate these ions to hydrolyze phosphodiester bonds. A third magnesium ion may bind after substrate engagement. Supporting Evidence: PMID:18697748 the T5FEN-catalyzed reaction requires at least three magnesium ions, implying that an additional metal ion is bound file:DANRE/fen1/fen1-deep-research-falcon.md hydrolyzes phosphodiester bonds using a **two-metal (Mg2+) catalytic center** typical for this nuclease class file:DANRE/fen1/fen1-deep-research-falcon.md catalytic metal-binding residues include E158, E160, D179, D181 |
| GO:0004523 RNA-DNA hybrid ribonuclease activity | IBA GO_REF:0000033 | ACCEPT | Summary: FEN1 exhibits RNase H-like activity, cleaving RNA from RNA-DNA hybrids. This activity is relevant to its role in Okazaki fragment processing where RNA primers are displaced into flap structures. Well supported by phylogenetic inference and biochemical studies on orthologs. Supporting Evidence: PMID:23451868 the RNA primer is displaced into a 5' flap and then cleaved off |
| GO:0030145 manganese ion binding | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Phylogenetic inference of manganese ion binding from PANTHER node PTN000871763 (seeded by MGI:MGI:102779, mouse Fen1). Mn2+ can substitute for Mg2+ in FEN1-family two-metal-ion active sites in vitro, but magnesium is the physiological cofactor. Reason: UniProt lists only Mg2+ as the cofactor for zebrafish fen1 (two ions per subunit, with a possible third after substrate binding, transferred by HAMAP rule MF_03140); manganese binding reflects in vitro divalent-cation substitution common to the XPG/RAD2 two-metal-ion nuclease fold rather than a distinct physiological manganese requirement. Harmonized with human FEN1 (MARK_AS_OVER_ANNOTATED on the same PTN000871763 node). Propagation Review Root cause: SOURCE WEAK OR INFERRED Failure modes: SOURCE EVIDENCE WEAK Sources checked: PANTHER:PTN000871763 Β· XPG/RAD2 nuclease node SUPPORTS SOURCE BUT NOT TARGET Manganese binding in this family is in vitro metal substitution at the two-metal-ion catalytic center; UniProt lists only Mg2+ as the FEN1 cofactor. MGI:MGI:102779 Β· Fen1 SOURCE WEAK OR INFERRED Mouse Fen1 PAINT seed reflecting in vitro divalent-metal substitution rather than a physiological manganese requirement. Supporting Evidence: file:DANRE/fen1/fen1-uniprot.txt Name=Mg(2+); Xref=ChEBI:CHEBI:18420; PMID:18697748 a requirement for two viable cofactors (Mg2+ or Mn2+) |
| GO:0000287 magnesium ion binding | IEA GO_REF:0000104 | ACCEPT | Summary: Duplicates the IBA annotation for magnesium ion binding. Correctly inferred by UniRule transfer from characterized orthologs. FEN1 requires two Mg2+ ions for catalysis. Supporting Evidence: PMID:18697748 The presence of at least two ions bound with differing affinity is required to catalyze phosphate diester hydrolysis |
| GO:0003677 DNA binding | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: FEN1 binds DNA through multiple contacts including the XPG N- and I-domains and the helix-hairpin-helix (HhH2) motif. DNA binding is integral to its nuclease function. The annotation is correct but generic; the more informative annotations are the specific nuclease activities. Reason: DNA binding is accurate but less informative than the specific 5'-flap endonuclease and exonuclease activity annotations. It is a prerequisite for catalysis rather than the core function itself. Supporting Evidence: PMID:23451868 FEN1 binds to the flap base and then threads the 5' end of the flap through its helical arch and active site to create a configuration for cleavage |
| GO:0003824 catalytic activity | IEA GO_REF:0000002 | MODIFY | Summary: This is a very generic InterPro2GO annotation from the HhH2 domain (IPR008918). FEN1 is indeed catalytically active, but the specific nuclease activities (5'-flap endonuclease, 5'-3' exonuclease) are far more informative. Reason: Too generic. The specific nuclease activities already annotated are more appropriate. Proposed replacements: 5'-flap endonuclease activity |
| GO:0004518 nuclease activity | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO annotation from XPG domains (IPR006085, IPR006086). FEN1 is a nuclease, but the specific 5'-flap endonuclease and 5'-3' exonuclease terms are more informative. Reason: Correct but too generic. More specific nuclease terms are already annotated. Proposed replacements: 5'-flap endonuclease activity |
| GO:0005634 nucleus | IEA GO_REF:0000104 | ACCEPT | Summary: Duplicates the IBA nucleus annotation. Correctly inferred by UniRule. FEN1 is a nuclear protein that localizes to the nucleolus and nucleoplasm. Supporting Evidence: file:DANRE/fen1/fen1-uniprot.txt SUBCELLULAR LOCATION: Nucleus, nucleolus PMID:18443037 FEN1 is superaccumulated in the nucleolus and plays a role in the resolution of stalled DNA replication forks |
| GO:0005654 nucleoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: FEN1 relocalizes to the nucleoplasm from the nucleolus upon DNA damage. This relocalization is mediated by phosphorylation. Correctly inferred from UniProt subcellular location vocabulary. Supporting Evidence: file:DANRE/fen1/fen1-uniprot.txt Nucleus, nucleoplasm file:DANRE/fen1/fen1-uniprot.txt the nucleoli and relocalizes to the nucleoplasm upon DNA damage PMID:18443037 In response to UV irradiation and upon phosphorylation, FEN1 migrates to nuclear plasma to participate in the resolution of UV cross-links on DNA |
| GO:0005730 nucleolus | IEA GO_REF:0000044 | ACCEPT | Summary: FEN1 super-accumulates in the nucleolus where it maintains stability of ribosomal DNA tandem repeats. This is its primary nuclear location under normal conditions. Supporting Evidence: file:DANRE/fen1/fen1-uniprot.txt SUBCELLULAR LOCATION: Nucleus, nucleolus PMID:18443037 FEN1 is superaccumulated in the nucleolus and plays a role in the resolution of stalled DNA replication forks formed at the sites of natural replication fork barriers |
| GO:0005739 mitochondrion | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: FEN1 localizes to mitochondria where it participates in mitochondrial DNA replication and repair. This has been demonstrated in both yeast (Rad27) and mammals. Reason: Mitochondrial localization is transferred by HAMAP rule from yeast and mouse, where it was shown directly (PMID:19699691, abstract-only cache); not demonstrated on the zebrafish protein but no reason to doubt the transfer, and the mtDNA-repair role is secondary to the nuclear replication/repair role. Accepted as a real, non-primary location. Supporting Evidence: file:DANRE/fen1/fen1-uniprot.txt Mitochondrion {ECO:0000255|HAMAP- file:DANRE/fen1/fen1-uniprot.txt mitochondrial DNA. PMID:19699691 Rad27p/FEN1 is localized in the mitochondrial compartment of both yeast and mice and that Rad27p has a significant role in maintaining mtDNA integrity |
| GO:0006284 base-excision repair | IEA GO_REF:0000104 | ACCEPT | Summary: The term claims fen1 acts in base-excision repair. FEN1 is the flap nuclease of long-patch BER: after AP-site incision, strand-displacement synthesis lifts the 5'-dRP-terminated strand into a flap that FEN1 cleaves to leave a ligatable nick. Transferred to zebrafish by UniRule/HAMAP MF_03140 from the mammalian and yeast experimental record. Reason: FEN1 catalyses a defined step of the LP-BER sub-pathway, so this is participation, not just necessity. A zebrafish larval toxicology study reports fen1 mRNA induction together with other BER genes under DNA-damaging exposure (deep-research file), which is consistent with but not proof of the role; the ACCEPT rests on the conserved biochemistry. Supporting Evidence: PMID:15189154 in long-patch base excision repair, a damaged nucleotide is displaced into a flap and removed by FEN1 PMID:23451868 Pol Ξ² performs strand-displacement synthesis, similar to Pol Ξ΄ during Okazaki fragment maturation, and lifts the 5β² dRP moiety into a flap structure, initiating LP-BER. The flap is a substrate for FEN1 file:DANRE/fen1/fen1-uniprot.txt ligation. Also involved in the long patch base excision repair (LP-BER) file:DANRE/fen1/fen1-deep-research-falcon.md show **significant fen1 upregulation** in zebrafish under multiple exposure conditions |
| GO:0008409 5'-3' exonuclease activity | IEA GO_REF:0000104 | ACCEPT | Summary: Duplicates the IBA annotation for 5'-3' exonuclease activity. Correctly inferred by UniRule transfer. This is a core catalytic activity of FEN1. Supporting Evidence: PMID:15189154 FEN1 is a genome stabilization factor that prevents flaps from equilibrating into structures that lead to duplications and deletions |
| GO:0016788 hydrolase activity, acting on ester bonds | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO annotation from XPG conserved site (IPR019974) and Flap endonuclease 1 family (IPR023426). FEN1 is a phosphodiesterase that hydrolyzes phosphoester bonds in DNA. Correct but very generic. Reason: Too generic. The specific nuclease activities are more informative and already annotated. Proposed replacements: 5'-flap endonuclease activity |
| GO:0017108 5'-flap endonuclease activity | IEA GO_REF:0000104 | ACCEPT | Summary: Duplicates the IBA annotation for 5'-flap endonuclease activity. Correctly inferred by UniRule transfer. This is the defining molecular function of FEN1. Supporting Evidence: PMID:23451868 Substrate specificity allows FEN1 to process intermediates of Okazaki fragment maturation, long-patch base excision repair, telomere maintenance, and stalled replication fork rescue file:DANRE/fen1/fen1-deep-research-falcon.md describes FEN1 as the **primary endonuclease** that cleaves these short RNAβDNA/DNA flaps to enable Okazaki fragment joining file:DANRE/fen1/fen1-deep-research-falcon.md PCNA is reported to increase FEN1 activity by approximately **~10β50-fold** |
| GO:0043137 DNA replication, removal of RNA primer | IEA GO_REF:0000104 | ACCEPT | Summary: FEN1 participates in RNA primer removal during Okazaki fragment maturation, working in concert with RNase H. This is a core biological process directly linked to its 5'-flap endonuclease activity during lagging-strand DNA replication. Supporting Evidence: PMID:23451868 FEN1 recognizes this structure, binds to the base of the flap, and precisely cleaves it, removing the RNA and some portion of the initiator DNA to make a nick file:DANRE/fen1/fen1-deep-research-falcon.md FEN1 removes flap/nick intermediates iteratively in coordination with RNaseH2 and PCNA to complete primer removal and ligation-ready processing |
| GO:0060041 retina development in camera-type eye | IMP PMID:15716491 Identification of zebrafish insertional mutants with defects... | KEEP AS NON CORE | Summary: The term claims fen1 acts in retina development. ZFIN annotated this from the Hopkins-lab retroviral insertional mutant collection (genotype ZDB-GENO-040831-3): the visual-system screen of Gross et al. 2005 classified 40 insertional mutants by eye histology at 5 dpf, and the fen1 insertion falls among the loci with growth/central-retinal defects. The cached record is abstract-only and the abstract does not name fen1 individually; the ZFIN curator had the full text. Reason: This is a genuine zebrafish IMP and should not be removed. On the participation test, however, FEN1 does no retina-specific work: it is a ubiquitous Okazaki-fragment/LP-BER nuclease, and a small, disorganised retina in a null embryo is the expected downstream consequence of failed DNA replication in one of the most proliferative larval tissues (Fen1-null mouse blastocysts arrest in S phase, PMID:12861020). The same collection lists fen1 among 315 genes essential for early development generally (PMID:15256591), showing the defect is not eye-restricted. Kept as a non-core, phenotype-level annotation. Supporting Evidence: PMID:15716491 Forty loci whose disruption resulted in defects in eye development and/or visual function were identified PMID:15716491 (2) growth and central retinal development PMID:15256591 We completed a large insertional mutagenesis screen in zebrafish to identify genes essential for embryonic and early larval development PMID:12861020 complete depletion of FEN1 causes early embryonic lethality |
| GO:0006260 DNA replication | IBA GO_REF:0000033 | ACCEPT | Summary: The term claims fen1 does part of the work of DNA replication. FEN1 is the Okazaki-fragment flap nuclease of lagging-strand synthesis: it cleaves the 5'-flap that Pol delta displacement synthesis creates at the downstream Okazaki fragment, leaving the nick that DNA ligase I seals. The PAINT node PTN000118792 is seeded by yeast RAD27 (SGD:S000001596), mouse Fen1 (MGI:102779) and human FEN1 (P39748), all with direct experimental support; zebrafish fen1 is a one-to-one ortholog within that clade and UniProt transfers the same function to Q6TNU4 by HAMAP rule MF_03140. Reason: fen1 performs a catalytic step of lagging-strand replication (flap removal during Okazaki fragment maturation), so it passes the participation test, not merely a necessity test. The more specific child GO:0043137 (removal of RNA primer) is also annotated; this parent term is retained because the flap-cleavage step also removes displaced DNA (Pol alpha-synthesised initiator DNA), which is broader than primer removal alone. No zebrafish-specific loss or divergence is known; the insertional mutant is embryonic/larval lethal with proliferation-type defects, consistent with a conserved replication role. Supporting Evidence: PMID:23451868 Substrate specificity allows FEN1 to process intermediates of Okazaki fragment maturation, long-patch base excision repair, telomere maintenance, and stalled replication fork rescue PMID:23451868 FEN1 recognizes this structure, binds to the base of the flap, and precisely cleaves it, removing the RNA and some portion of the initiator DNA to make a nick file:DANRE/fen1/fen1-uniprot.txt During DNA replication, cleaves the 5'-overhanging flap structure that file:DANRE/fen1/fen1-uniprot.txt ligation. Also involved in the long patch base excision repair (LP-BER) |
| GO:0006281 DNA repair | IBA GO_REF:0000033 | ACCEPT | Summary: The term claims fen1 does part of the work of DNA repair. FEN1 is the flap nuclease of long-patch base excision repair (it removes the 5'-dRP-terminated flap lifted by strand-displacement synthesis), and its orthologs also act on flaps at stalled replication forks and in rDNA/mitochondrial DNA maintenance. The PAINT node PTN008960732 is a deep XPG/RAD2-family node; within it the FEN1 lineage itself carries experimental repair annotations (yeast RAD27 SGD:S000001596, S. pombe rad2 SPAC3G6.06c, mouse Fen1 MGI:102779), so the zebrafish gene sits inside the clade that inherited the function. Reason: LP-BER flap trimming is a catalytic step performed by FEN1 itself, so the participation test is met. The more specific child GO:0006284 (base-excision repair, IEA) is also present; the parent is retained because FEN1 repair activity extends beyond BER (fork rescue, rDNA and mtDNA repair per the UniProt FUNCTION line). No evidence of repair-function loss in the teleost lineage. Supporting Evidence: PMID:23451868 Pol Ξ² performs strand-displacement synthesis, similar to Pol Ξ΄ during Okazaki fragment maturation, and lifts the 5β² dRP moiety into a flap structure, initiating LP-BER. The flap is a substrate for FEN1 file:DANRE/fen1/fen1-uniprot.txt ligation. Also involved in the long patch base excision repair (LP-BER) file:DANRE/fen1/fen1-uniprot.txt involved in replication and repair of rDNA and in repairing file:DANRE/fen1/fen1-uniprot.txt mitochondrial DNA. |
| GO:0006310 DNA recombination | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: The term claims fen1 takes part in DNA recombination. The IBA is placed at the deep XPG/RAD2-family node PTN008960732, whose experimental donors are mostly EXO1 (human Q9UQ84, mouse MGI:1349427, yeast EXO1 SGD:S000005559) and GEN1 (Q17RS7) - nucleases with well-established roles in resection and Holliday-junction resolution. However the FEN1 lineage is represented among the donors by yeast RAD27 (SGD:S000001596), whose deletion causes recombination-related phenotypes and which processes flap intermediates in recombination pathways, so the function is not absent from the FEN1 clade. Human FEN1 additionally carries a Reactome-derived TAS for double-strand break repair via homologous recombination (GO:0000724). Reason: fen1 is inside the clade the PAINT curator judged to have inherited a recombination role, and the FEN1 branch itself has experimental grounding (RAD27), so there is no basis to challenge the node placement. But the recombination role is a minor use of the same flap/nick nuclease activity whose primary evolved context is Okazaki fragment maturation and LP-BER; the cached literature for FEN1 foregrounds replication and repair and mentions recombination only in passing. Kept as non-core rather than accepted as a defining function. Propagation Review Root cause: NO FAILURE NON CORE Sources checked: PANTHER:PTN008960732 SUPPORTS TRANSFER Deep XPG/RAD2 node covering FEN1, EXO1 and GEN1 subfamilies; recombination role placed here by PAINT. SGD:S000001596 Β· RAD27 SUPPORTS TRANSFER Yeast FEN1 ortholog; shows the function is retained in the FEN1 branch, not only in EXO1/GEN1. UniProtKB:Q9UQ84 Β· EXO1 SUPPORTS SOURCE BUT NOT TARGET Resection nuclease; its recombination role is subfamily-specific and does not by itself transfer to FEN1. Supporting Evidence: PMID:23451868 Substrate specificity allows FEN1 to process intermediates of Okazaki fragment maturation, long-patch base excision repair, telomere maintenance, and stalled replication fork rescue PMID:23451868 The absence of the cap in some nucleases possibly allows them to access DNA bubbles and recombination intermediates |
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Download this section (compressed HTML)Experiment: Direct biochemical characterization of zebrafish FEN1 nuclease activities (endonuclease and exonuclease) to confirm that the zebrafish protein retains the same catalytic properties as mammalian FEN1. Currently all molecular function annotations are transferred by homology (IBA/IEA) with no direct experimental evidence on the zebrafish protein.
Type: Biochemical characterization
Experiment: Spatiotemporal expression analysis of fen1 during zebrafish retinal development to understand why the retina is particularly sensitive to fen1 loss-of-function, given that fen1 is expected to be ubiquitously required for DNA replication.
Type: Expression analysis
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