WRN, Sirtuin, Ku70/80, DNA ligase IV, PNKP, XRCC4, Rad50, Artemis, Brca1, MRE11, Casein kinase II, alpha chains, Ku80, FEN1, Casein kinase II, beta chain (Phosvitin), Ku70, DNA polymerase mu, DNA-PK, Nibrin, MRN complex
NHEJ mechanisms of DSBs repair
DNA double-strand breaks (DSBs) result from disruption of the phosphodiester backbone on both strands of the DNA double helix. Non-homologous end joining (NHEJ) seems to be the primary mechanism of DSBs repair in mammalian cells. This pathway does not require homology and can rejoin broken DNA ends directly end-to-end. It was suggested that DSBs repair via NHEJ is carried out in three steps: end-binding and bridging, terminal processing, and ligation .
In the first step, Ku70/80 heterodimer binds the DNA ends (the end-binding activity of Ku70/80 heterodimer suggests that it may be the primary damage detector in NHEJ), aligns them and thus prepares the ends for ligation and protects from degradation. Ku70/80 consists of two ATP-dependent DNA helicases II subunits, 70 kDa and 80 kDa ( Ku70 and Ku80 ). This complex recruits DNA-activated protein kinase ( DNA-PK ) to the DSBs, activating its kinase function .
Finally, DNA-PK binds to DNA ligase IV/ X-ray repair cross complementing protein 4 ( XRCC4 ) complex and phosphorylates it. Hereinafter, Casein kinase II - phosphorylated XRCC4 interacts with polynucleotide kinase ( PNKP ), which acts as a 5'-kinase/3'-phosphatase to create 5'-phosphate/3'-hydroxyl termini, which are a necessary prerequisite for ligation during repair .
The nuclease MRN complex also can participate in terminal processing of NHEJ, as well as in damage signaling and protection of the ends from degradation. MRN complex consists of double-strand break repair protein ( Mre11), Rad50 homolog (S. cerevisiae) ( Rad50 ) and Nijmegen breakage syndrome 1 protein ( Nibrin ). MRN complex may be activated via Brca1/ Rad50 pathway .
Other proteins that are involved in the end-processing are DNA polymerase mu , exonuclease flap structure-specific endonuclease 1 ( FEN1 ) and Werner syndrome helicase ( WRN ) . Ku70/80 interacts with WRN and stimulates WRN exonuclease activity . The ability of WRN to facilitate FEN1 cleavage of DNA replication/repair intermediates may be important for the role of WRN in the maintenance of genomic stability .
A significant fraction of DNA cross-link repair 1C protein, Artemis, exists in the cell in complex with DNA-PK, which becomes an endonuclease after it is phosphorylated by DNA-PK . Upon trimming off an excess or damaged DNA, Artemis/ DNA-PK complex may disassemble which permits binding of the ligase complex, XRCC4/ DNA ligase IV, which completes the joining .
In addition, silent mating type information regulation horologes ( Sirtuin s) may participate in DSB repair. Presence of Sirtuin s at DNA damage sites and its interaction with Ku70/80 indicate that they might influence the accessibility of the broken ends to DNA processing enzymes and/or to the Ku70/80 in NHEJ , .