HUMAN CDC6 CDC18 ASSOCIATES WITH ORC1 AND CYCLIN-CDK AND IS SELECTIVELY ELIMINATED FROM THE NUCLEUS AT THE ONSET OF S-PHASE/

In a two-hybrid screen for proteins that interact with human PCNA, we identified and cloned a human protein (hCdc18) homologous to yeast CDC 6/Cdc18 and human Orc1. Unlike yeast, in which the rapid and total des truction of CDC6/Cdc18 protein in S phase is a central feature of DNA replication, the total level of the human protein is unchanged through out the cell cycle. Epitope-tagged protein is nuclear in G(1) and cyto plasmic in S-phase cells, suggesting that DNA replication may be regul ated by either the translocation of this protein between the nucleus a nd the cytoplasm or the selective degradation of the protein in the nu cleus. Mutation of the only nuclear localization signal of this protei n does not alter its nuclear localization, implying that the protein i s translocated to the nucleus through its association with other nucle ar proteins. Rapid elimination of the nuclear pool of this protein aft er the onset of DNA replication and its association with human Orc1 pr otein and cyclin-cdks supports its identification as human CDC6/Cdc18 protein.