Involvement of CUL4 Ubiquitin E3 Ligases in Regulating CDK Inhibitors Dacapo/p27Kip1 and Cyclin E Degradation

Abstract

The CUL4 (cullin 4) proteins are the core components of a new class of ubiquitinE3 ligases that regulate replication and transcription. To examine the roles of CUL4 incell cycle regulation, we analyzed the effect of inactivation of CUL4 in both Drosophilaand human cells. We found that loss of CUL4 in Drosophila cells causes G1 cell cyclearrest and an increased protein level of the CDK inhibitor Dacapo. Co-elimination ofDacapo with CUL4 abolishes the G1 cell arrest. In human cells, inactivation of CUL4Ainduces CDK inhibitor p27Kip1 stabilization and G1 cell cycle arrest which is dependenton the presence of p27, suggesting that this regulatory pathway is evolutionarilyconserved. In addition, we found that the Drosophila CUL4 also regulates the proteinlevel of cyclin E independent of Dacapo. We provide evidence that human CUL4B, aparalogue of human CUL4A, is involved in cyclin E regulation. Loss of CUL4B causesthe accumulation of cyclin E without a concomitant increase of p27. The human CUL4Band cyclin E proteins also interact with each other and the CUL4B complexes canpolyubiquitinate the CUL4B-associated cyclin E. Our studies suggest that the CUL4-containing ubiquitin E3 ligases plays a critical role in regulating G1 cell cycleprogression in both Drosophila and human cells.