Mutation of cyclin/cdk phosphorylation sites in HsCdc6 disrupts a late step in initiation of DNA replication in human cells

Cyclin-dependent kinases (Cdk) are essential for promoting the initiation o f DNA replication, presumably by phosphorylating key regulatory proteins th at are involved in triggering the G1/S transition. Human Cdc6 (HsCdc6), a p rotein required for initiation of DNA replication, is phosphorylated by Cdk in vitro and in vivo. Here we report that HsCdc6 with mutations at potenti al Cdk phosphorylation sites was poorly phosphorylated in vitro by Cdk, but retained all other biochemical activities of the wild-type protein tested. Microinjection of mutant HsCdc6 proteins into human cells blocked initiati on of DNA replication or slowed S phase progression. The inhibitory effect of mutant HsCdc6 was lost at the G1/S transition, indicating that phosphory lation of HsCdc6 by Cdk is critical for a late step in initiation of DNA re plication in human cells.