Unphosphorylatable mutants of Cdc6 disrupt its nuclear export but still support DNA replication once per cell cycle

Cdc6 is essential for eukaryotic DNA replication. We have mutated highly co nserved CDK phosphorylation sites in Cdc6. Contrary to their reported pheno types in human cells, unphosphorylatable Delta CDK mutants fully support DN A replication in Xenopus eggs. WtCdc6 is actively exported from the nucleus , which could explain why nuclear permeabilization is required for reinitia tion within one cell cycle. However, Delta CDK mutants are retained in the nucleus, yet surprisingly they still support only one round of replication. As these highly conserved CDK sites are unnecessary for replication once p er cell cycle, an alternative checkpoint role for monitoring completion of the S phase is suggested.