RFC4

RFC4 is a conserved subunit of the replication factor C (RFC) clamp-loader complex that couples ATP-dependent recognition of primer-template DNA to loading of the PCNA sliding clamp during DNA replication (PMID:8063832, PMID:11432854). Its ATPase domain is functionally essential: the Walker A K55 residue is required for efficient DNA-dependent clamp loading, as a K55E mutant complex loads PCNA only at very high ATP concentrations (PMID:11432854). Beyond catalysis, RFC4 physically interacts with the N-terminal domain of Rpa1 (Rpa1N), and this contact is required for both replication and the G1/S, intra-S, and G2/M DNA damage checkpoints, placing RFC4 as a sensor that is epistatic with RAD24 (PMID:11340166); consistent with this, RFC4 localizes to replicating nuclei and its loss causes checkpoint failure with aberrant chromosome condensation and premature sister chromatid separation (PMID:11438670). RFC4 is required to maintain the integrity and expression of the assembled RFC complex, and bi-allelic loss-of-function variants disrupting its C-terminal domain destabilize RFC4 and the other RFC subunits and perturb DNA replication and cell-cycle progression, defining RFC4 as the basis of a human Mendelian disorder (PMID:39106866). In cancer settings RFC4 acts in additional protein-stabilizing roles: it promotes NHEJ-mediated repair through interaction with Ku70/Ku80 (PMID:30979744), directly binds NICD1 to block CDK8/FBXW7-mediated degradation and sustain Notch signaling in a positive feedback loop (PMID:33976158), and stabilizes an STK38-BECN1 complex to drive autophagy-based temozolomide resistance (PMID:41872171). The small molecule Platycodin D binds RFC4 and reduces nuclear Notch intracellular domain accumulation, downregulating Notch signaling (PMID:42142580).

1. 1994 Medium

   Established RFC4 as an essential structural subunit of the RFC clamp-loader and defined its first direct intra-complex contact, anchoring all later mechanism in a defined protein assembly.

   Evidence Cloning, E. coli expression, purification and interaction assay of yeast Rfc4p with Rfc3p

   PMID:8063832

   Open questions at the time

   • Does not resolve stoichiometry or architecture of the full RFC pentamer
   • No demonstration of clamp-loading activity for Rfc4 itself
2. 2001 High

   Showed that RFC4's ATPase domain is mechanistically required for ATP-dependent clamp loading, not merely structural.

   Evidence Walker A K55E site-directed mutagenesis with in vitro ATPase, DNA binding and PCNA loading assays on reconstituted RFC complexes

   PMID:11432854

   Open questions at the time

   • Does not define order of ATP hydrolysis among RFC subunits
   • No structural model of the K55 mutant in the loading reaction
3. 2001 High

   Linked RFC4 to checkpoint signaling by identifying a required Rpa1N interaction and placing RFC4 in the G1/S, intra-S, and G2/M checkpoints epistatic with RAD24.

   Evidence Allele-specific synthetic lethality (rfc4-2 / rfa1-t11), checkpoint and HU-sensitivity assays, RAD24 epistasis in yeast

   PMID:11340166

   Open questions at the time

   • Does not define the molecular signal RFC4 transmits to downstream checkpoint kinases
   • Interaction mapped genetically/two-hybrid, not structurally
4. 2001 High

   Tied RFC4 protein localization to checkpoint function in a metazoan, showing chromatin association during replication and checkpoint-specific (not kinetochore) failures upon loss.

   Evidence Immunofluorescence and loss-of-function alleles in Drosophila with replication-block and DNA-damage checkpoint assays

   PMID:11438670

   Open questions at the time

   • Does not address mammalian-specific checkpoint wiring
   • Mechanism of chromatin dispersal at mitosis unexplained
5. 1995 Medium

   Mapped the human RFC4 gene to 3q27, enabling later human genetic and disease studies.

   Evidence PCR from somatic hybrid panels and FISH

   PMID:7774928

   Open questions at the time

   • No functional information
   • No disease association at the time
6. 2019 Medium

   Extended RFC4 function beyond replication to DNA double-strand break repair, implicating it in NHEJ through Ku70/Ku80.

   Evidence Genome-wide RNAi screen, Co-IP with Ku70/Ku80, NHEJ vs HR repair and apoptosis assays in colorectal cancer cells and xenografts

   PMID:30979744

   Open questions at the time

   • Single lab Co-IP without reciprocal/structural validation
   • Whether NHEJ role depends on intact RFC complex unclear
7. 2021 Medium

   Revealed a non-replicative protein-stabilization role: RFC4 directly binds NICD1 to block its degradation, sustaining Notch signaling in a feedback loop driving metastasis.

   Evidence Co-IP, competitive binding and reporter assays, knockdown/overexpression, in vitro and in vivo NSCLC tumor assays

   PMID:33976158

   Open questions at the time

   • Binding interface on RFC4 not mapped
   • Whether this requires monomeric RFC4 or the RFC complex is unresolved
8. 2024 Medium

   Established RFC4 as the cause of a human Mendelian disorder by showing C-terminal loss-of-function variants destabilize RFC4 and the whole RFC complex with replication/cell-cycle defects.

   Evidence Patient fibroblasts and RFC4-deficient HeLa cells, Western blot of RFC subunit stability, complex-formation and cell-cycle assays, cryo-EM structural interpretation

   PMID:39106866

   Open questions at the time

   • Genotype-phenotype spectrum not fully defined
   • Direct structural data on patient variants not generated
9. 2026 Medium

   Showed RFC4 is transcriptionally inducible (via YY1) and stabilizes an STK38-BECN1 complex to activate autophagy and confer chemoresistance.

   Evidence Chromatin accessibility/ChIP-reporter for YY1, Co-IP of RFC4-STK38-BECN1, T444 phospho-deficient mutagenesis, autophagy assays and xenografts in GBM

   PMID:41872171

   Open questions at the time

   • Direct RFC4-STK38 binding interface unmapped
   • Single lab; relationship to RFC4's clamp-loader role unaddressed
10. 2026 Medium

    Identified RFC4 as a druggable target whose engagement by Platycodin D suppresses nuclear Notch intracellular domain accumulation and Notch signaling.

    Evidence Thermal proteome profiling, CETSA, PELSA binding assays, IP-WB, proteomic and ubiquitinomic profiling

    PMID:42142580

    Open questions at the time

    • Binding site of Platycodin D on RFC4 not defined
    • Mechanistic link between PD binding and Notch nuclear exclusion not fully resolved
11. 2026 Low

    Positioned RFC4 upstream of Wnt/beta-catenin-driven invasion and EMT in colorectal cancer.

    Evidence siRNA knockdown with proliferation/invasion/EMT assays and pharmacological BML-284 rescue, liver metastasis model

    PMID:41574400

    Open questions at the time

    • No direct binding between RFC4 and Wnt pathway components demonstrated
    • Epistasis rests on pharmacological rescue alone
    • Single lab, not independently confirmed

• How RFC4's canonical clamp-loader/checkpoint functions mechanistically relate to its many cancer-associated protein-stabilization roles (NICD1, Ku70/80, STK38-BECN1, Wnt) remains unresolved.
• Unclear whether non-replicative roles require the intact RFC complex or free RFC4
• No shared structural binding interface identified across partners