Restricting the ligation step of non-homologous end-joining

Abstract

Non-homologous end-joining is an important pathway for repairing DNA double-strand breaks. The budding yeast Saccharomyces cerevisiae possesses two proteins, Nej1/Lif2 and Ntr1/Spp382, which play a role in restricting the activity of Dnl4-Lif1, the complex that executes the final ligation step of this process.

Fig. 1

Ways of restricting the activity of the ligase complex in Saccharomyces cerevisiae. (A) A global way. In diploid cells, the Mata1-Matα2 repressor binds to promoter region of the NEJ1 gene. Consequently, in these cells NEJ1 is not transcribed and the Nej1 protein is entirely absent. In contrast, haploid cells contain Nej1 due to unrepressed NEJ1 expression, where it interacts with the ligase complex and enhances NHEJ efficiency. This net effect is lower NHEJ efficiency in diploid compared to haploid cells. (B) A local way. The Ntr1 protein deposited at telomeres, possibly via its association with Rap1, assembles with Lif1 and disrupts the ligase complex, creating putative Ntr1-Lif1 and Ntr1-Lif1-Nej1 complexes as an outcome (only the situation in a haploid cell is shown). This process may lead to local inhibition of the ligation step of NHEJ at telomeres. In contrast, if Ntr1 does not or cannot interact with Lif1, for example due to decreased Rap1 binding, fully active ligase complex is retained. Such complex can support NHEJ at telomeres leading to unwanted end-to-end chromosomal fusions. In addition, at eroding telomeres Nej1 suppresses NHEJ by a mechanism which is not clearly understood yet. Therefore, if this protein is missing, eroded chromosome ends are fused in a Dnl4-dependent manner. Presumably, a novel, yet unidentified, role of Nej1 drives action of this protein in opposing direction compared to its role in canonical NHEJ. For references, see text.

Fig. 2

Interacting domains within the ligase complex in Saccharomyces cerevisiae. Dnl4- and Ntr1-interacting domains in Lif1 map to the regions 205–228 and 220–240, respectively, indicating a competition between Dnl4 and Ntr1 for the binding site in Lif1. The drawings are not to scale and are shown in a haploid cell. For details and references, see text.