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. Author manuscript; available in PMC: 2023 Dec 15.
Published in final edited form as: J Mol Biol. 2022 Oct 19;434(23):167858. doi: 10.1016/j.jmb.2022.167858

Figure 1.

Figure 1.

NHEJ mechanism and its role in V(D)J recombination.

(A) NHEJ mechanism. The general steps of nonhomologous DNA end joining (NHEJ) are shown. When double-strand DNA breaks (DSBs) occur by ionizing radiation, reactive oxygen species, nicks at replication forks or enzymatic causes of a DSB (including RAG complex and topoisomerases), NHEJ begins with Ku70/80 binding to the dsDNA ends at DSB ends. Then, the nuclease complex (Artemis:DNA-PKcs complex), the DNA polymerases (μ, λ and TdT), and the ligase complex (XFL:XRCC4:DNA ligase IV and PAXX) can bind act. These proteins work on either of the two DNA ends independently, iteratively (loading repeatedly) and in any order until both strands are ligated. In addition to this mechanistic flexibility, each component exhibits enzymatic flexibility.

(B) The role of the Artemis:DNA-PKcs complex in V(D)J recombination during antigen receptor development. The basal state of the Artemis:DNA-PKcs complex, where the catalytic region of Artemis is external to DNA-PKcs, binds to the hairpin ends generated by the RAG complex. Next, the activated Artemis:DNA-PKcs complex, where the catalytic region of Artemis is now inside of the DNA-PKcs HEAT cradle, opens the hairpins. After modification by the nuclease complex and the NHEJ polymerases, the XRCC4:DNA ligase IV complex can join the coding ends. Hairpin opening is essential for completion of V(D)J recombination. Patients with failure to this hairpin opening leads to radiosensitive T−B− severe combined immunodeficiency (RS-SCID).