fan-1 (C. elegans) - Research Notes

Gene Identity

• Gene: fan-1 (C01G5.8)
• UniProt: P90740 (FAN1_CAEEL)
• WormBase: WBGene00015310
• 865 amino acids, single-pass nuclear protein
• Human ortholog: FAN1/KIAA1018 (Q9Y2M0)

Discovery

FAN1 was identified simultaneously in 2010 by three independent groups:
- MacKay et al. (Cell, 2010) PMID:20603015
- Kratz et al. (Cell, 2010) PMID:20603016
- Smogorzewska et al. (Mol Cell, 2010) PMID:20603073

All three papers used C. elegans fan-1 mutants to demonstrate the conserved role in ICL repair.

Domain Architecture

• N-terminal UBZ4-type zinc finger (residues 35-62): binds ubiquitin, specifically monoubiquitinated FANCD2
• Winged-helix domain (approx 245-305): DNA binding module
• TPR domain (approx 457-609): protein-protein interactions
• C-terminal VRR-NUC domain (742-858): catalytic nuclease core, metal-dependent (Mn2+/Mg2+)

Key Molecular Functions

1. 5'-3' exonuclease activity: FAN1 cleaves DNA successively at every third nucleotide from a nick or 5' flap PMID:25430771
2. 5'-flap endonuclease activity: Cleaves branched DNA structures, particularly 5' flap substrates PMID:20603015
3. DNA binding: Binds flap-structured DNA and duplex DNA via winged-helix domain
4. Ubiquitin binding: UBZ4 domain binds monoubiquitinated FANCD2 PMID:20603016

Biological Process

• Interstrand cross-link repair: Primary function. fan-1 mutant worms show hypersensitivity to cisplatin, nitrogen mustard, and mitomycin C [PMID:20603015, PMID:20603016, PMID:20603073]
• DNA repair: General role in genome maintenance

C. elegans-specific Findings

Disruption phenotype

• No visible phenotype under normal conditions
• Strong increase in embryonic lethality following cisplatin, nitrogen mustard, or MMC treatment [PMID:20603015, PMID:20603016, PMID:20603073]

Systematic ICL repair analysis (Wilson et al. 2017)

PMID:28934497
- FAN-1 contributes to ICL resolution in a replication-dependent manner
- No critical role seen for FCD-2 (FANCD2) in this study, suggesting FAN-1 can act partially independently of the canonical FA pathway

Mutagenic consequences of ICL repair (Verschuren et al. 2025)

PMID:40082407
- FAN1 mediates translesion synthesis leading to single nucleotide polymorphisms during psoralen ICL repair
- Disruption of FAN1 eliminates SNV formation from ICL repair
- Suggests FAN-1 is required for error-prone TLS-mediated bypass of ICLs

Protein interactions

• Interacts with SMO-1 (SUMO) by IntAct data (3 experiments) [UniProt entry]
• High-throughput Y2H interactions from interactome mapping [PMID:14704431, PMID:19123269] - these are generic interactome screens, not FAN-1-specific studies

Structural Biology (human FAN1)

• Crystal structure solved: Wang et al. 2014 PMID:25430771 - mechanism of ICL unhooking by 3-nt interval cleavage
• Zhao et al. 2014 PMID:25500724 - dimeric FAN1 unwinds 5' flap DNA before incision
• Rao et al. 2018 PMID:29518739 - homodimerization important for long 5' flap cleavage
• Jin et al. 2018 PMID:29514982 - bacterial FAN1 structure reveals conserved ICL unhooking mechanism

Subcellular Localization

• Nucleus: IDA evidence from Kratz et al. 2010 PMID:20603016 - GFP-tagged C. elegans FAN-1 localizes to the nucleus

Verification of BioReason References

The BioReason deep research file does not cite specific PMIDs. It references domain architecture from InterPro, which is accurate. The reasoning trace is largely sound for this well-characterized protein, though some partner predictions (ERCC4/XPF homologs, PMS family, WRN helicase, DNA polymerase kappa) are speculative extrapolations from human pathway knowledge and are not demonstrated in C. elegans.

GO Annotation Notes

• GO:0008270 (zinc ion binding) appears in UniProt DR lines but NOT in the GOA TSV. It is a reasonable IEA annotation based on the UBZ4 zinc finger.
• The protein binding (GO:0005515) annotations from PMID:14704431 and PMID:19123269 are from high-throughput interactome screens detecting interaction with SMO-1/SUMO. Per curation guidelines, "protein binding" is uninformative - the interaction with SUMO is more relevant as it may indicate SUMOylation plays a role in FAN-1 regulation.
• Several annotations are duplicated across evidence types (e.g., nucleus from IBA, IEA, and IDA; DNA repair from IEA and IMP; ICL repair from IBA and IEA).