Practice question

Replication stress exposes extended single-stranded DNA regions coated by replication protein A, forming a key signaling platform. ATR kinase, recruited via ATRIP-interacting domain to RPA-ssDNA, is the principal sensor of this structure. At stalled forks, factors TopBP1, Claspin, Timeless-Tipin and Rad9-Rad1-Hus1 clamp augment ATR activation through direct binding and allosteric stimulation. Once activated, ATR directly phosphorylates checkpoint kinase Chk1 on serine residues 317 and 345 within its C-terminal regulatory domain. This modification relieves autoinhibition, enabling Chk1 catalytic activity through autophosphorylation at Ser296. Activated Chk1 diffuses away from chromatin to phosphorylate effectors: Cdc25A phosphatase marked for SCF-betaTrCP mediated degradation, Cdc25C generating 14-3-3 binding and cytoplasmic sequestration, and downstream targets like Wee1, Rad51 and polymerase eta. Consequences include inhibition of CDK2-Cyclin E and CDK1-Cyclin B, suppression of late origin firing, stabilization of replisome and arrest in intra-S and G2/M phases. This ATR-Chk1 axis provides time for fork repair and prevents entry into mitosis with under-replicated genomes preserving stability. Additional feedback loops involving polo-like kinases, phosphatases and SCF-mediated degradation reinforce irreversibility and protect against premature progression that would compromise genome integrity and viability.