Control of DNA polymerase λ stability by phosphorylation and ubiquitination during the cell cycle

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Abstract
DNA polymerase (Pol) λ is a DNA repair enzyme involved in base excision repair, non‐homologous end joining and translesion synthesis. Recently, we identified Pol λ as an interaction partner of cyclin‐dependent kinase 2 (CDK2) that is central to the cell cycle G1/S transition and S‐phase progression. This interaction leads to in vitro phosphorylation of Pol λ, and its in vivo phosphorylation pattern during cell cycle progression mimics the modulation of CDK2/cyclin A. Here, we identify several phosphorylation sites of Pol λ. Experiments with phosphorylation‐defective mutants suggest that phosphorylation of Thr 553 is important for maintaining Pol λ stability, as it is targeted to the proteasomal degradation pathway through ubiquitination unless this residue is phosphorylated. In particular, Pol λ is stabilized during cell cycle progression in the late S and G2 phases. This most likely allows Pol λ to correctly conduct repair of damaged DNA during and after S phase.