In vivo phosphorylation, mitotic behavior, and nuclear binding of the catalytic subunit of DNA polymerase alpha in fission yeast.

We studied the phosphorylation of fission yeast p170 (the catalytic subunit of DNA polymerase alpha) and its relationship to the cell cycle. In exponentially growing cells, p170 was phosphorylated at serine residues. Its phosphorylation level did not quantitatively change when cell strains carrying conditional cell division cycle (cdc) mutations arrested at different stages of the cell cycle, under restrictive growth conditions. Especially, phosphorylation did not significantly vary when cells carrying the temperature-sensitive cdc2-33 mutation were shifted to the restrictive temperature, which indicates a minor role, if any, of p34cdc2 in this process. Also, the extent of p170 phosphorylation did not remarkably change during mitosis, a situation which differs from that reported for human DNA polymerase alpha. We used immunofluorescence microscopy and cell fractionation to study the intracellular distribution of p170. We here provide evidence that the protein remains tenaciously associated with nuclear structures throughout the cell cycle and is not redistributed into the cytoplasm at mitosis, as it is in human cells. A possible correlation between phosphorylation, nuclear binding, and mitotic behavior of DNA polymerase alpha catalytic subunits in eukaryotes is therefore conceivable.