Regulation of oxidative DNA damage repair by DNA polymerase λ and MutYH by cross-talk of phosphorylation and ubiquitination

It is of pivotal importance for genome stability that repair DNA polymerases (Pols), such as Pols lambda and beta, which all exhibit considerably reduced fidelity when replicating undamaged DNA, are tightly regulated, because their misregulation could lead to mutagenesis. Recently, we found that the correct repair of the abundant and highly miscoding oxidative DNA lesion 7,8-dihydro-8-oxo-2'-deoxyguanine (8-oxo-G) is performed by an accurate repair pathway that is coordinated by the MutY glycosylase homologue (MutYH) and Pol lambda in vitro and in vivo. Pol lambda is phosphorylated by Cdk2/cyclinA in late S and G2 phases of the cell cycle, promoting Pol lambda stability by preventing it from being targeted for proteasomal degradation by ubiquitination. However, it has remained a mystery how the levels of Pol lambda are controlled, how phosphorylation promotes its stability, and how the engagement of Pol lambda in active repair complexes is coordinated. Here, we show that the E3 ligase Mule mediates the degradation of Pol lambda and that the control of Pol lambda levels by Mule has functional consequences for the ability of mammalian cells to deal with 8-oxo-G lesions. Furthermore, we demonstrate that phosphorylation of Pol lambda by Cdk2/cyclinA counteracts its Mule-mediated degradation by promoting recruitment of Pol. to chromatin into active 8-oxo-G repair complexes through an increase in Pol lambda's affinity to chromatin-bound MutYH. Finally, MutYH appears to promote the stability of Pol lambda by binding it to chromatin. In contrast, Pol lambda not engaged in active repair on chromatin is subject for proteasomal degradation.