Cryo-EM visualization of DNA-PKcs structural intermediates in NHEJ

Abstract

DNA double-strand breaks (DSBs), one of the most cytotoxic forms of DNA damage, can be repaired by the tightly regulated nonhomologous end joining (NHEJ) machinery (Stinson and Loparo and Zhao <i>et al.</i>). Core NHEJ factors form an initial long-range (LR) synaptic complex that transitions into a DNA-PKcs (DNA-dependent protein kinase, catalytic subunit)-free, short-range state to align the DSB ends (Chen <i>et al.</i>). Using single-particle cryo-electron microscopy, we have visualized three additional key NHEJ complexes representing different transition states, with DNA-PKcs adopting distinct dimeric conformations within each of them. Upon DNA-PKcs autophosphorylation, the LR complex undergoes a substantial conformational change, with both Ku and DNA-PKcs rotating outward to promote DNA break exposure and DNA-PKcs dissociation. We also captured a dimeric state of catalytically inactive DNA-PKcs, which resembles structures of other PIKK (Phosphatidylinositol 3-kinase-related kinase) family kinases, revealing a model of the full regulatory cycle of DNA-PKcs during NHEJ.