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Minichromosome Maintenance Mobility

The minichromosome maintenance proteins (MCM) are essential for the initiation of eukaryotic DNA replication. The proteins form a helicase consisting of MCM2-MCM7. Activation of the MCM complex requires the removal of MCM2, MCM3, and MCM5, presumably via the phosphorylation of MCM subunits by various kinases that regulate the cell cycle. In addition, these kinases are needed for recruitment of Cdc45 and other replication initiation factors to origins of replication.

Overexpression of Cdc7 induces N-terminal phosphorylation on MCM4.

In this Paper of the Week Hisao Masai and colleagues analyzed the phosphorylation of MCM subunits during the cell cycle. They found that the protein kinase Cdc7 phosphorylates MCM4 on its N terminus. This N-terminal phosphorylation event stimulates the association of Cdc45 with chromatin, suggesting that it may be an important phosphorylation event for the activation of replication origins. The authors also found that deletion of N-terminal residues on MCM4 resulted in growth inhibition and that the combination of this deletion with the alteration of putative N-terminal Cdc7 target sites on MCM2 and MCM6 leads to a nonviable phenotype. These results indicate that the N-terminal phosphorylation of MCM2, MCM4, and MCM6 may play functionally redundant but essential roles in initiation of DNA replication.

FOOTNOTES

See referenced article, J. Biol. Chem. 2006, 281, 39249-39261

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