RFC2

RFC2 (RFC40) is an essential subunit of the heteropentameric Replication Factor C (RFC) clamp-loader complex that loads the PCNA sliding clamp onto primed DNA during chromosomal replication and supports the S-phase DNA damage checkpoint (PMID:8202350, PMID:9671499). Its conserved Walker A lysine (K71 in yeast) is required for ATPase activity, primed-template DNA binding, and clamp loading, while the arginine finger motif and central β-sheet are structurally critical for function (PMID:11432854, PMID:19664060). RFC2 acts within a network of replication factors, showing functional interactions with Rfc1, Rfc5, and DNA polymerases δ and ε (PMID:9671499). The protein is regulated by post-translational and protein-interaction controls: it is monoubiquitylated by the RAD6-RAD18 E2-E3 ligase complex following DNA damage, a modification suppressed by RPA binding to RFC2 (PMID:18245774), and in human cells its nuclear import depends on interaction with the PKA regulatory subunit RIα, a complex displaced by RFC3 (RFC37) and dissolved upon CDK2/Cyclin E phosphorylation of RIα to permit G1-to-S progression (PMID:15655353, PMID:15846072, PMID:16582606). Loss of RFC2 function causes chromosomal missegregation and aneuploidy, and rfc2 knockout in zebrafish produces craniofacial, brain, and vascular defects with altered neural survival/differentiation gene expression (PMID:22720015, PMID:39368701).

1. 1994 High

   Established that Rfc2 is a bona fide subunit of the RF-C clamp-loader complex required for DNA replication, defining its place in the replication machinery.

   Evidence Co-purification with RF-C activity, antibody inhibition of a polymerase III-dependent replication reaction, in vitro DNA/ATP binding, and lethal gene disruption in yeast

   PMID:8202350

   Open questions at the time

   • Did not resolve the stoichiometry or catalytic mechanism within the complex
   • ATP binding characterized only as weak with recombinant protein
2. 1998 High

   Connected RFC2 not only to bulk replication but to an S-phase DNA damage checkpoint and placed it genetically with Rfc1, Rfc5, and polymerases δ/ε.

   Evidence Thermosensitive rfc2-1 allele with DNA damage sensitivity, recombination/chromosome loss assays, synthetic lethality and multicopy suppression in yeast

   PMID:9671499

   Open questions at the time

   • Genetic interactions do not establish direct physical contacts
   • Molecular basis of the checkpoint defect not dissected
3. 2001 High

   Pinpointed the Walker A lysine K71 as essential for the ATPase, DNA-binding, and clamp-loading activities, separating catalytic function from PCNA association.

   Evidence Active-site mutagenesis (K71E/K71R) with in vitro ATPase, clamp-loading, DNA-binding, and PCNA-interaction assays on reconstituted RFC complexes

   PMID:11432854

   Open questions at the time

   • Did not map other catalytic residues to function
   • Mechanism of ATP-driven clamp opening not resolved
4. 2005 Medium

   Identified a non-replication regulatory axis: RFC40 binds PKA regulatory subunit RIα, which serves as its nuclear transport carrier, linking RFC40 nuclear import to cell cycle progression.

   Evidence Yeast two-hybrid, Co-IP, domain mapping (RIα N-terminus to RFC40 C-terminus), RFC3 competition, and G1-arrest cell cycle analysis

   PMID:15655353 PMID:15846072

   Open questions at the time

   • Whether RIα transports free RFC40 or assembled RFC unclear
   • Single lab with overlapping methods
5. 2006 Medium

   Showed CDK2/Cyclin E phosphorylation of RIα dissolves the RIα-RFC40 complex, providing a cell-cycle switch governing RFC40 availability.

   Evidence In vitro kinase assay, CDK inhibitor (olomoucine) and phosphatase inhibitor (Calyculin A) treatments, Co-IP of complex changes

   PMID:16582606

   Open questions at the time

   • Phosphosite not confirmed by mutagenesis
   • Single lab, pharmacological inference
6. 2008 High

   Defined a DNA-damage-responsive modification of RFC2: RAD6-RAD18 monoubiquitylation antagonized by RPA binding, coupling RFC2 to damage signaling.

   Evidence In vitro reconstituted ubiquitylation, D228A mutant causing constitutive ubiquitylation, in-cell RAD18-dependent damage response, RPA inhibition assay

   PMID:18245774

   Open questions at the time

   • Functional consequence of the ubiquitin mark on clamp loading not established
   • Ubiquitylation site(s) on RFC2 not mapped
7. 2009 Medium

   Mapped which structural elements are indispensable, confirming ATP-binding sites and the central β-sheet as functionally critical surfaces.

   Evidence Pentapeptide-scanning mutagenesis of fission yeast rfc2 with growth complementation and structural mapping onto budding yeast Rfc2

   PMID:19664060

   Open questions at the time

   • In vivo growth only; no in vitro biochemical activity measurements
   • Effects of individual insertions on clamp loading not measured
8. 2012 Low

   Demonstrated a direct cellular requirement for RFC40 in chromosomal integrity, with depletion causing missegregation and aneuploidy.

   Evidence siRNA knockdown in neonatal rat cardiac myocytes with chromosome-12 FISH and cell counting

   PMID:22720015

   Open questions at the time

   • Single lab phenotypic study without mechanistic dissection
   • Off-target effects of siRNA not controlled by rescue
9. 2020 Low

   Placed the RFC2-5 complex within damage-response signaling via Rad17, regulated by NELFA RNA, affecting CHK1/CHK2/BRCA1 phosphorylation.

   Evidence Co-IP, RNA immunoprecipitation, knockdown measuring checkpoint kinase phosphorylation in glioblastoma cells

   PMID:31845510

   Open questions at the time

   • RFC2-specific contribution within the complex not isolated
   • Indirect, single-lab evidence
10. 2024 Medium

    Established an organismal developmental requirement, with rfc2 knockout causing craniofacial, neural, and vascular defects resembling a human syndrome.

    Evidence CRISPR-Cas9 rfc2 knockout zebrafish, phenotyping, RNA-seq, and behavioral assays

    PMID:39368701

    Open questions at the time

    • No biochemical mechanism linking RFC2 loss to phenotypes
    • Causal disease relationship in humans not established by this model

• How DNA-damage monoubiquitylation, RIα/PKA-dependent trafficking, and clamp-loading activity are integrated into a single regulatory program for RFC2 across cell cycle and development remains unresolved.
• No structure of human RFC2 within an assembled clamp-loader in the corpus
• Functional consequence of RFC2 monoubiquitylation undefined
• Connection between developmental phenotypes and replication function unestablished