| RAD50 is an evolutionarily conserved structural maintenance of chromosomes (SMC)–related ATPase that functions as the central DNA tethering and signaling component of the MRE11–RAD50–NBS1 (MRN) complex, which orchestrates the detection, processing, and repair of DNA double‑strand breaks (DSBs), activation of ATM/ATR‑dependent checkpoint signaling, telomere maintenance, and meiotic recombination. The protein contains N‑ and C‑terminal Walker A and B ATPase motifs that fold into a globular ABC‑ATPase head, extended central coiled‑coil arms, and a C‑terminal Cys‑His‑His‑Cys zinc‑hook motif positioned at the apex of the coiled‑coils that coordinates a Zn²⁺ ion and mediates dimer–dimer interactions, allowing RAD50 dimers in the MRN complex to bridge distant DNA ends and form dynamic DNA loops important for end tethering and higher‑order chromatin architecture at sites of damage. ATP binding at the RAD50 head domains drives head‑to‑head engagement and converts MRN from an open conformation, in which MRE11 nuclease sites are solvent‑exposed, to a closed clamp in which broken DNA ends are juxtaposed and positioned into the MRE11 nuclease active site, providing ATP‑dependent control over end resection and conformational coupling between DNA tethering and nuclease activation. Within MRN, RAD50’s coiled‑coil and head domains bind DNA ends and hold them in close proximity, while MRE11 executes 3′–5′ exonuclease and endonuclease activities to initiate 5′‑end resection and generate 3′ single‑stranded DNA overhangs that become coated by RPA and later RAD51 during homologous recombination; NBS1 supplies phospho‑dependent interaction motifs that recruit ATM and other repair factors. MRN functions at the apex of the DNA damage response as a sensor and transducer: it recognizes DSBs, recruits and activates ATM through direct interaction, stimulates ATM autophosphorylation and monomerization, and supports ATR activation at resected ends, thereby triggering phosphorylation cascades that stabilize stalled replication forks, enforce intra‑S and G2/M checkpoints, and coordinate repair pathway choice between homologous recombination, classical non‑homologous end joining, and alternative end joining. RAD50 also contributes to telomere homeostasis by associating with chromosome ends, promoting t‑loop formation and end protection, and participating in the processing of telomeric DNA, while in meiosis MRN‑RAD50 facilitates programmed DSB processing required for homologous chromosome pairing and crossover formation. Germline biallelic hypomorphic mutations in RAD50 cause a Nijmegen breakage syndrome‑like disorder characterized by microcephaly, growth retardation, immunodeficiency, and radiosensitivity, with patient cells showing impaired DSB repair and defective checkpoint activation, highlighting RAD50’s essential role in genome stability. Somatic RAD50 alterations including missense, nonsense, and frameshift mutations are detected across multiple cancers, and MRN dysfunction associates with chromosomal instability and altered sensitivity to DNA‑damaging therapies. |
Lane 1: PANC-1, Lane 2: 293T, Lane 3: K562
