Inhibition of Mcm4,6,7 helicase activity by phosphorylation with cyclin A/Cdk2

A strong body of evidence indicates that cyclin-dependent protein kinases a re required not only for the initiation of DNA replication but also for pre venting over-replication in eukaryotic cells. Mcm proteins are one of the c omponents of the replication licensing system that permits only a single ro und of DNA replication per cell cycle. It has been reported that Mcm protei ns are phosphorylated by the cyclin-dependent kinases in vivo, suggesting t hat these two factors are cooperatively involved in the regulation of DNA r eplication. Our group has reported that a 600-kDa Mcm4,6,7 complex has a DN A helicase activity that is probably necessary for the initiation of DNA re plication. Here, we examined the in vitro phosphorylation of the Mcm comple xes with cyclin A/Cdk2 to understand the interplay between Mcm proteins and cyclin-dependent kinases, The cyclin A/Cdk2 mainly phosphorylated the amin o-terminal region of Mcm4 in the Mcm4,6,7 complex. The phosphorylation was associated with the inactivation of its DNA helicase activity. These result s raise the possibility that the inactivation of Mcm4,6,7 helicase activity by Cdk2 is a part of the system for regulating DNA replication.