Cell cycle-dependent phosphorylation of human DNA polymerase α

The expression of DNA polymerase α, a principal chromosome replication enzyme, is constitutive during the cell cycle. We show in this report that DNA polymerase a catalytic polypeptide p180 is phosphorylated throughout the cell cycle and is hyperphosphorylated in G₂/M phase. The p70 subunit is phosphorylated only in G₂/M phase. This cell cycle-dependent phosphorylation is due to cell cycle-dependent kinase(s) and not to phosphatase(s). In vitro evidence indicates the involvement of p34^cdc2 kinase in the mitotic phosphorylation of DNA polymerase α. Tryptic phosphopeptide maps demonstrate that peptides phosphorylated in vitro are identical to those phosphorylated in vivo. DNA polymerase a from mitotic cells is found to have lower affinity for single-stranded DNA than does polymerase a from G₁/S phase cells. These results imply that the mitotic phosphorylation of polymerase a may affect its physical interaction with other replicative proteins and/or with DNA at the replication fork.