Autophosphorylation and ATM activation: additional sites add to the complexity

Unique and redundant functions of ATM and DNA-PKcs during V(D)J recombination

Lymphocyte antigen receptor genes are assembled through the process of V(D)J recombination, during which pairwise DNA cleavage of gene segments results in the formation of four DNA ends that are resolved into a coding joint and a signal joint. The joining of these DNA ends occurs in G1-phase lymphocytes and is mediated by the non-homologous end-joining (NHEJ) pathway of DNA double-strand break (DSB) repair. The ataxia telangiectasia mutated (ATM) and the DNA-dependent protein kinase catalytic subunit (DNA-PKcs), two related kinases, both function in the repair of DNA breaks generated during antigen receptor gene assembly. Although these proteins have unique functions during coding joint formation, their activities in signal joint formation, if any, have been less clear. However, two recent studies demonstrated that ATM and DNA-PKcs have overlapping activities important for signal joint formation. Here, we discuss the unique and shared activities of the ATM and DNA-PKcs kinases during V(D)J recombination, a process that is essential for lymphocyte development and the diversification of antigen receptors.

Phosphorylation: the molecular switch of double-strand break repair

Repair of double-stranded breaks (DSBs) is vital to maintaining genomic stability. In mammalian cells, DSBs are resolved in one of the following complex repair pathways: nonhomologous end-joining (NHEJ), homologous recombination (HR), or the inclusive DNA damage response (DDR). These repair pathways rely on factors that utilize reversible phosphorylation of proteins as molecular switches to regulate DNA repair. Many of these molecular switches overlap and play key roles in multiple pathways. For example, the NHEJ pathway and the DDR both utilize DNA-PK phosphorylation, whereas the HR pathway mediates repair with phosphorylation of RPA2, BRCA1, and BRCA2. Also, the DDR pathway utilizes the kinases ATM and ATR, as well as the phosphorylation of H2AX and MDC1. Together, these molecular switches regulate repair of DSBs by aiding in DSB recognition, pathway initiation, recruitment of repair factors, and the maintenance of repair mechanisms.