Abstract
We isolated a murine gene for the DNA polymerase alpha associated protein P1, which shares high homology with the budding yeast MCM3 protein, which is a member of a protein family involved in the early event of DNA replication having a putative DNA-dependent ATPase motif. Using a polyclonal anti-P1 antibody raised against a beta-galactosidase-P1 fusion protein, we identified at least two forms of P1 protein in the nucleus of a mouse cell line, an underphosphorylated form that was associated with a particular nuclear structure and a hyperphosphorylated form loosely bound to the nucleus. During progression through S phase, P1 disappeared, first from the euchromatic region, then from the heterochromatic region, apparently in parallel with temporally ordered DNA replication. Thus, it is likely that the underphosphorylated P1 is dissociated from the nuclear structure after DNA replication by cell cycle-dependent phosphorylation. This is the first direct observation of a protein whose behavior is consistent with that of a hypothetical factor which restricts the chromatin to replicate once per cell cycle in higher eukaryotes.