| Phospho-RPA32/RPA2 (Ser8) refers to the DNA damage–inducible modification of the 32 kDa subunit of replication protein A within the heterotrimeric RPA complex, a central single-stranded DNA–binding factor required for replication, recombination, checkpoint signaling and all major DNA repair pathways. RPA32 carries multiple N‑terminal serine and threonine sites that form a phospho-code; Ser4 and Ser8, together with Ser33 and other residues, become hyperphosphorylated in response to replication stress and genotoxic insults by phosphatidylinositol 3‑kinase–related kinases including DNA‑PK, ATR and ATM, on a background of prior cell cycle–linked phosphorylation at Ser23 and Ser29 by cyclin-dependent kinases. Phosphorylation of Ser4/Ser8 by DNA‑PK occurs when RPA accumulates on single-stranded DNA at stalled forks and acts as an early replication stress signal that modifies RPA–DNA and RPA–protein interactions; cells lacking DNA‑PK activity or expressing RPA32 mutants in which Ser4/Ser8 cannot be phosphorylated show defective replication checkpoint arrest, premature restart of stalled forks, failure to block late origin firing and increased mitotic catastrophe, indicating that the phospho‑Ser4/Ser8 state is required to enforce ATR–Chk1 checkpoint outputs and protect genome integrity. In this context, replication stress–induced hyperrecombination in Ser4/Ser8 mutants is ATM‑dependent, while other defects are ATM‑independent, placing DNA‑PK–RPA32 Ser4/Ser8 phosphorylation as a key branch point coordinating ATR- and ATM-mediated responses to stalled replication. Phosphorylated RPA32 at Ser4/Ser8 also participates in the regulation of homologous recombination repair: DNA‑PK, ATR and ATM collaboratively modify the RPA32 N‑terminus and p53 in a way that releases p53–RPA complexes, facilitates Rad51 loading and promotes HR repair of double-strand breaks, providing a mechanism for crosstalk between nonhomologous end joining and homologous recombination through coordinated control of p53–RPA interaction. RPA32 Ser4/Ser8 phosphorylation in G2 cells influences TopBP1 and Rad9 accumulation on chromatin, ATM-dependent KAP1 phosphorylation and Rad51 chromatin loading, reinforcing its role as a modulator of checkpoint signaling and recombination factor recruitment beyond the S phase. Phospho-RPA32 Ser8—typically assessed in combination with Ser4—serves as a mechanistic marker of replication stress and DNA damage response, where its status reports on DNA‑PK/ATR/ATM axis activation and functionally tunes replication checkpoint enforcement, fork restart timing, origin usage and the balance between recombination and mitotic catastrophe. |
