SET 8 is degraded via PCNA-coupled CRL 4 ( CDT 2 ) ubiquitylation in S phase and after UV irradiation

Eukaryotic cell cycle progression requires faithful DNA duplication and division of the chromosomes between daughter cells. These processes are tightly controlled and require massive restructuring of chromatin, which primarily consists of DNA repetitively wrapped around histone cores (Margueron and Reinberg, 2010). Posttranslational modifications of histones by phosphorylation, acetylation, methylation, and ubiquitylation are involved in the regulation of processes such as DNA replication, mitosis, DNA damage signaling, and repair (Kouzarides, 2007). Certain modifications can change the compaction of chromatin and thereby the accessibility of the surrounding DNA. Histone modifications can also serve as docking sites for effector proteins. Several proteins have been implicated in recognition of specific histone modifications that can facilitate recruitment of effector protein complexes to chromatin, where they elicit their cellular functions (Kouzarides, 2007). A well-characterized example of this is the dynamic phosphorylation of Ser139 in histone H2AX (H2AX; Rogakou et al., 1998). This phosphorylation occurs near sites of DNA double-strand break lesions and is catalyzed by the ATM, DNA-PK, and ATR kinases. H2AX recruits the key DDR protein MDC1, which subsequently facilitates the localization of a number of effector proteins to the sites of DNA damage (Jackson and Bartek, 2009). Another highly dynamic chromatin modification is the methylated form of histone H4 lysine 20 (H4K20; Margueron and Reinberg, 2010). This residue can be mono-, di-, and trimethylated. Monomethylation (H4K20me1) is catalyzed by the histone methyltransferase SET8 (Fang et al., 2002; Nishioka et al., 2002). SET8 plays a critical role in cell cycle progression, as its depletion in mammalian cells leads to aberrant progression through mitosis, DNA condensation failure, and slow progression through S phase accompanied by massive DNA damage (Jørgensen et al., 2007; Tardat et al., 2007; Houston et al., 2008; The eukaryotic cell cycle is regulated by multiple ubiquitin-mediated events, such as the timely destruction of cyclins and replication licensing factors. The histone H4 methyltransferase SET8 (Pr-Set7) is required for chromosome compaction in mitosis and for maintenance of genome integrity. In this study, we show that SET8 is targeted for degradation during S phase by the CRL4(CDT2) ubiquitin ligase in a proliferating cell nuclear antigen (PCNA)–dependent manner. SET8 degradation requires a conserved degron responsible for its interaction with PCNA and recruitment to chromatin where ubiquitylation occurs. Efficient degradation of SET8 at the onset of S phase is required for the regulation of chromatin compaction status and cell cycle progression. Moreover, the turnover of SET8 is accelerated after ultraviolet irradiation dependent on the CRL4(CDT2) ubiquitin ligase and PCNA. Removal of SET8 supports the modulation of chromatin structure after DNA damage. These results demonstrate a novel regulatory mechanism, linking for the first time the ubiquitin–proteasome system with rapid degradation of a histone methyltransferase to control cell proliferation. SET8 is degraded via PCNA-coupled CRL4(CDT2) ubiquitylation in S phase and after UV irradiation