Posttranslational modifications regulate cellular responses upon UV-induced DNA damage

SUMOylation is a posttranslational protein modification which is characterized by the covalent attachment of a small 11kDa protein, called Small Ubiquitin-like MOdifier (SUMO). SUMOylation plays a pivotal role in a multitude of cellular pathways including cellular responses upon DNA damage. Here, we identified multiple proteins which are SUMOylated in U2OS cells in response to ultraviolet light (UV) irradiation and ionizing radiation (IR). We show that the SUMOylation response upon UV irradiation was more pronounced compared to the response upon IR. The major SUMOylation target upon UV-irradiation was the transcription-coupled nucleotide excision repair (TC-NER) protein, Cockayne Syndrome B (CSB). This protein plays an important role in the repair of UV-induced lesions in actively transcribed genes. In a second proteomic approach we identified SUMOylation-dependent and independent protein interactors of the N-terminus of CSB. Here, we uncovered that the affinity of multiple RNA polymerase-associated proteins towards CSB is influenced by SUMOylation. Finally, we set out to identify ubiquitination events upon UV-irradiation which are influenced by the CSA-ubiquitin ligase complex, which is also involved in TC-NER and is closely connected to CSB, because mutations in either CSA or CSB result in the same phenotype, Cockayne syndrome. We found that RPB1, the major subunit of RNA polymerase II, was ubiquitinated in a CSA-dependent manner upon UV which finally led to its degradation.