Identification of RNF8 as a ubiquitin ligase involved in targeting the p12 subunit of DNA polymerase δ for degradation in response to DNA damage

DNA Polymerase δ consists of four subunits, one of which, p12, is degraded in response to DNA damage through the ubiquitin-proteasome pathway. However, the identities of the ubiquitin Ligase(s) that are responsible for the proximal biochemical events in triggering proteasomal degradation of p12 are unknown. We employed a classical approach to identifying a ubiquitin Ligase that is involved in p12 degradation. Using UbcH5c as ubiquitin-conjugating enzyme, a ubiquitin Ligase activity that polyubiquitinates p12 was purified from HeLa cells. Proteomic analysis revealed that RNF8, a RING finger ubiquitin Ligase that plays an important role in the DNA damage response, was the only ubiquitin Ligase present in the purified preparation. In vivo, DNA damage-induced p12 degradation was significantly reduced by shRNA knockdown of RNF8 in cultured human cells and in RNF8(-/-) mouse epithelial cells. These studies provide the first identification of a ubiquitin Ligase activity that is involved in the DNA damage-induced destruction of p12. The identification of RNF8 allows new insights into the integration of the control of p12 degradation by different DNA damage signaling pathways.