C. elegans CUL-4 Prevents Rereplication by Promoting the Nuclear Export of CDC-6 via a CKI-1-Dependent Pathway

SummaryGenome stability requires that genomic DNA is replicated only once per cell cycle. The replication-licensing system ensures that the formation of prereplicative complexes is temporally separated from the initiation of DNA replication 1, 2, 3 and 4. The replication-licensing factors Cdc6 and Cdt1 are required for the assembly of prereplicative complexes during G1 phase. During S phase, metazoan Cdt1 is targeted for degradation by the CUL4 ubiquitin ligase 5, 6, 7 and 8, and vertebrate Cdc6 is translocated from the nucleus to the cytoplasm 9 and 10. However, because residual vertebrate Cdc6 remains in the nucleus throughout S phase 10, 11, 12 and 13, it has been unclear whether Cdc6 translocation to the cytoplasm prevents rereplication 1, 2 and 14. The inactivation of C. elegans CUL-4 is associated with dramatic levels of DNA rereplication [5]. Here, we show that C. elegans CDC-6 is exported from the nucleus during S phase in response to the phosphorylation of multiple CDK sites. CUL-4 promotes the phosphorylation and subsequent translocation of CDC-6 via negative regulation of the CDK-inhibitor CKI-1. Rereplication can be induced by coexpression of nonexportable CDC-6 with nondegradable CDT-1, indicating that redundant regulation of CDC-6 and CDT-1 prevents rereplication. This demonstrates that CDC-6 translocation is critical for preventing rereplication and that CUL-4 independently controls both replication-licensing factors.