Practice question

Double-strand breaks caused by ionizing radiation or replication collapse activate upstream kinase ATM, recruited and stimulated by MRN complex bound to broken ends. Active ATM phosphorylates histone variant H2AX at Ser139 forming gamma-H2AX domains that recruit mediator proteins Mdc1, 53BP1 and effector kinase Chk2 through its FHA domain. ATM phosphorylates Chk2 at Thr68 within N-terminal SQ/TQ cluster, promoting dimerization via FHA domain exchange and autophosphorylation at Thr383/Thr387 in activation loop, yielding fully active kinase. Chk2 phosphorylates numerous downstream substrates, most notably tumor suppressor p53 at Ser20 within N-terminal transactivation domain, disrupting interaction with E3 ligase Mdm2 and stabilizing p53 tetramers. Accumulated p53 induces transcription of cyclin dependent kinase inhibitor p21, GADD45, 14-3-3 sigma and proapoptotic Bax, Puma, Noxa, producing G1/S and G2/M arrest or apoptosis depending on damage severity. Chk2 also phosphorylates Cdc25A, BRCA1 Ser988 facilitating homologous recombination, and PML, integrating break detection with transcriptional DNA damage program for genome preservation. 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.