Lammens, K. | bioRxiv Science

bioRxiv2025

Structural basis for DNA double-strand break sensing by human MRE11-RAD50-NBS1 and its TRF2 complex

The MRE11-RAD50-NBS1 (MRN) complex is a central, multifunctional factor in the detection, signaling and nucleolytic processing of DNA double-strand breaks (DSBs). To clarify how human MRN binds generic and telomeric DNA ends and can separate DNA end sensing from nuclease activities, we determined cryo-electron microscopy structures of human MRN bound to DNA and to DNA and the telomere protection factor TRF2. MRN senses DSBs through a tight clamp-like sensing state with closed coiled-coil domains, but auto-inhibited MRE11 nuclease. NBS1 wraps around the MRE11 dimer, with NBS1s ATM recruitment motif sequestered by binding to the regulatory RAD50 S site, necessitating an allosteric switch for ATM activation. At telomeric DNA, TRF2 blocks the second S site via the iDDR motif to prevent nuclease and ATM activation. Our results provide a structural framework for topological DNA sensing and separation of sensing, signaling and processing activities of mammalian MRN. Highlights- Human MRN senses DNA ends with an autoinhibited nuclease - NBS1s C-terminus binds one RAD50 S site in the sensing state - TRF2 binds MRNs second S site at telomeres - RAD50 and ATM compete for the NBS1 C-terminus

biochemistry↗

bioRxiv2025

Structural Basis for OAS2 Regulation and its Antiviral Function

Oligoadenylate synthetase (OAS) proteins are immune sensors for double-stranded RNA and critical for restricting viruses. OAS2 comprises two OAS domains, only one of which can synthesize 2-5-oligoadenylates for RNase L activation. Existing structures of OAS1 provide a model for enzyme activation, but do not explain how multiple OAS domains discriminate RNA length. Here, we discover that OAS2 exists in an autoinhibited state as a zinc-mediated dimer and present a mechanism for RNA length discrimination: the catalytically deficient domain acts as a molecular ruler that prevents autoreactivity to short RNAs. We demonstrate that dimerization and myristoylation localize OAS2 to Golgi membranes and that this is required for OAS2 activation and restriction of viruses that exploit the endomembrane system for replication, e.g. coronaviruses. Finally, our results highlight the non-redundant role of OAS proteins and emphasize the clinical relevance of OAS2 by identifying a patient with a loss-of-function mutation leading to autoimmune disease.

biochemistry↗