CRL4(CDT2) targets SET8 for degradation after UV. (A) CDT2 depletion stabilizes SET8 after UV. U2-OS cells were depleted for CDT2 and synchronized with aphidicolin. Cells were released into mid–S phase and treated with UV, harvested after 2 h, and processed for immunoblotting with the indicated antibodies. (B) Knockdown of CDT2 decreases SET8 turnover after UV. U2-OS cells were treated with siRNA 12 h before aphidicolin treatment. 24 h after aphidicolin treatment, cells were washed and released into fresh medium for 3 h. Cells were then treated with cycloheximide and UV as indicated and processed for immunoblotting with the indicated antibodies. The blots were quantified using an image reader. This is representative data from three similar experiments. Data are presented as mean + SD. (C) UV increases in vivo ubiquitylation of SET8. HEK293 cells were depleted for CDT2 using siRNA 8 h before transfection with Flag-HA-SET8 and HIS-ubiquitin as indicated. Cells were treated with UV or a combination of UV and MG132 2 h before harvest followed by immunoprecipitation using cobalt beads and immunoblotted with the indicated antibodies. (D) Mutation of PIP2 but not PIP1 increases stability of SET8 after UV. HEK293 cells were transfected with either Flag-HA-SET8, -*PIP1, or -*PIP2. They were treated with a combination of cycloheximide and UV or cycloheximide, UV, and MG132. Cells were processed for immunoblotting with the indicated antibodies. (E) WT SET8 but not SET8-*PIP2 is ubiquitylated in vivo after UV treatment. Cells were transfected with Flag-HA-SET8, Flag-HA-SET8-*PIP2, and HIS-ubiquitin as indicated. Cells were processed for immunoprecipitation using cobalt beads and immunoblotted with the indicated antibodies.