PHOSPHORYLATION OF MCM4 BY CDC2 PROTEIN-KINASE INHIBITS THE ACTIVITY OF THE MINICHROMOSOME MAINTENANCE COMPLEX

In eukaryotes, tight regulatory mechanisms ensure the ordered progress ion through the cell cycle phases. The mechanisms that prevent chromos omal DNA replication from taking place more than once each cell cycle are thought to involve the function of proteins of the minichromosome maintenance (MCM) family, Here, pie demonstrate that Xenopus MCM4, a m ember of the MCM protein family related to Spcdc21/ScCDC54, is part of a large protein complex comprising several other MCM proteins. MCM4 u ndergoes cell cycle-dependent phosphorylation both in cleaving embryos and in cell-free extracts. MCM4 phosphorylation starts concomitantly with the clearing of the MCM complex from the chromatin during S phase . Phosphorylation is carried out by cdc2/cyclinB protein kinase, which phosphorylates MCM4 in vitro at identical sites as the ones phosphory lated in vivo. Phosphorylation is specific for cdc2 protein kinase sin ce MCM4 is not a substrate for other members of the cdk family, Furthe rmore, phosphorylation of MCM4 dramatically reduces its affinity for t he chromatin, We propose that the cell cycle-dependent phosphorylation of MCM4 is a mechanism which inactivates the MCM complex from late S phase through mitosis, thus preventing illegitimate DNA replication du ring that period of the cell cycle.