Mcm2 and Mcm3 are constitutive nuclear proteins that exhibit distinct isoforms and bind chromatin during specific cell cycle stages of Saccharomyces cerevisiae

The Mcm2-7 proteins are a family of conserved proteins whose functions are essential for the initiation of DNA synthesis in all eukaryotes. These proteins are constitutively present in high abundance in actively proliferating cells. In Saccharomyces cerevisiae, the intracellular concentrations of Mcms are between 100 and 500 times the number of replication origins. However, these proteins are limiting for the initiation of DNA synthesis at replication origins. Our studies indicate that only a small fraction of Mcm2 and Mcm3 tightly associates with chromatin, from late M phase to the beginning of the S phase. The rest of the Mcm2 and Mcm3 proteins are distributed to both the cytoplasm and nucleoplasm in relatively constant levels throughout the cell cycle. We also show that S. cerevisiae Mcm3 is a phosphoprotein that exists in multiple isoforms and that distinct isoforms of Mcm2 and Mcm3 can be detected at specific stages of the cell cycle. These results suggest that the localization and function of the Mcm proteins are regulated by posttranslational phosphorylation in a manner that is consistent with a role for the Mcm proteins in restricting DNA replication to once per cell cycle.