Phosphorylation of a high molecular weight DNA polymerase alpha

Phosphorylation and dephosphorylation of the B subunit of DNA polymerase alpha-primase complex in the early embryogenesis of Drosophila

In the early embryos of Drosophila, the B subunit of the DNA polymerase alpha-primase complex was found to migrate more slowly during the first 13 mitotic cycles than that from cycle 14 using SDS-polyacrylamide gel electrophoresis. Lambda phosphatase treatment showed that the reduced migration was caused by phosphorylation of the B subunit. Detailed analysis using the partially purified B subunit indicated that most of the B subunit until cycle 13 was a phosphorylated form while the B subunit of cycle 14 was a dephosphorylated form.

Deficiencies in DNA replication and cell-cycle progression in polyamine-depleted HeLa cells

Synchronized HeLa cells depleted of polyamines by alpha-difluoromethylornithine exhibited substantially decreased DNA synthesis, and proliferation ceased after the release of the cells into S phase. Nuclei from these cells synthesized 70-80% less DNA than did nuclei from control cells. Extraction of isolated nuclei with 0.3 M-KCl decreased DNA synthesis by about 60%, which was recovered almost completely in control cell nuclei by reconstitution with the salt extracts of these nuclei. On the other hand, salt extracts of polyamine-depleted nuclei restored only 50% of DNA synthesis in extracted control nuclei. Salt extracts of control cell nuclei contained twice the DNA polymerase alpha activity of polyamine-depleted nuclear extracts. Extracts of cell lysates of both control and polyamine-depleted HeLa cells exhibited similar DNA polymerase alpha activity, suggesting that uptake of the enzyme or its retention by the nuclei of polyamine-depleted cells was decreased. Polyamine-depleted nuclei also showed altered phosphorylation of a 31 kDa protein as compared with control nuclei. Almost normal DNA synthesis, cell proliferation, DNA polymerase alpha activity and nuclear protein phosphorylation were restored in polyamine-depleted cells grown in medium supplemented with 20 microM-spermidine at least 10-12 h before S phase. Cultures in which proliferation was blocked by alpha-difluoromethylornithine did not exhibit synchronous growth after the block was removed. Thus it may be concluded that HeLa cells depleted of polyamines are not inhibited at a single control point in the cell cycle, but are arrested at diverse sites throughout G1 phase.

Casein kinase II phosphorylates DNA-polymerase-alpha--DNA-primase without affecting its basic enzymic properties

Immunoaffinity-purified DNA-polymerase-alpha--DNA-primase complex from calf thymus was phosphorylated in vitro by highly purified casein kinase II from the same tissue. Specific phosphorylation of the DNA-polymerizing alpha subunit and the primase-associated gamma subunit was observed. About 1 mol phosphate/mol polymerase--primase was incorporated. Despite this effect, neither the DNA polymerase nor the DNA primase activity were changed after phosphorylation by casein kinase II. Furthermore, dephosphorylation of polymerase--primase with alkaline phosphatase did not change the polymerase or the primase activity to a significant extent. Moreover, both alkaline phosphatase and casein kinase II had no effect on the processivity of DNA synthesis and on the lengths and amounts of primers formed by the DNA primase. Because DNA polymerase alpha maintained all its basic properties even after extensive treatment with alkaline phosphatase, it is unlikely that phosphorylation has a direct influence on the activities of the DNA-polymerase-alpha--DNA-primase complex. The possible influence of post-translational phosphorylation on the formation of a complex of polymerase alpha and its accessory proteins is discussed.

Constitutive expression of simian virus 40 large T antigen in monkey cells activates their capacity to support polyomavirus replication

Polyomavirus DNA replication is normally restricted to rodent cells, and simian virus 40 (SV40) DNA replication is restricted to primate cells. We demonstrate that DNAs containing the polyomavirus origin can be replicated in monkey cells which constitutively express SV40 large T antigen. Permissivity is most likely caused by SV40 T antigen modification of cellular protein(s) required to replicate the polyomavirus origin. A possible target for the T-antigen-induced modification is DNA polymerase alpha-DNA primase.

Activity of deoxyribonucleic acid polymerase alpha stimulated by estrogen in the endometrium of the human uterus during the menstrual cycle

To provide some insight into how deoxyribonucleic acid (DNA) synthesis occurs in the endometrium of the human uterus during the menstrual cycle, the DNA polymerase activities (alpha and beta) in endometrial samples taken from normal cycling women, and the concentration of estradiol-17 beta (E2) and progesterone in the serum were measured. DNA polymerase alpha activity increased gradually from the beginning of the menstrual cycle, reaching a peak 2-3 days before ovulation, and then showed a decrease. Increase in this activity occurred in parallel with that of the concentration of E2, but not progesterone, in the serum sample in the proliferative phase (correlation coefficient r = 0.924, p less than 0.001). In contrast, DNA polymerase alpha activity stimulated by estrogen in the endometrium of the 2nd grade amenorrheal women decreased abruptly after an injection of 125 mg progesterone. DNA polymerase beta activity showed no significant change during the menstrual cycle or after estrogen and progesterone treatment. These results suggest that estrogen seems to stimulate the induction of DNA polymerase alpha activity during cell proliferation in the endometrium of the human uterus.

Regulation of Xenopus laevis DNA topoisomerase I activity by phosphorylation in vitro

DNA topoisomerase I has been purified to electrophoretic homogeneity from ovaries of the frog Xenopus laevis. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the most purified fraction revealed a single major band at 110 kDa and less abundant minor bands centered at 62 kDa. Incubation of the most purified fraction with immobilized calf intestinal alkaline phosphatase abolished all DNA topoisomerase enzymatic activity in a time-dependent reaction. Treatment of the dephosphorylated X. laevis DNA topoisomerase I with a X. laevis casein kinase type II activity and ATP restored DNA topoisomerase activity to a level higher than that observed in the most purified fraction. In vitro labeling experiments which employed the most purified DNA topoisomerase I fraction, [gamma-32P]ATP, and the casein kinase type II enzyme showed that both the 110- and 62-kDa bands became phosphorylated in approximately molar proportions. Phosphoamino acid analysis showed that only serine residues became phosphorylated. Phosphorylation was accompanied by an increase in DNA topoisomerase activity in vitro. Dephosphorylation of DNA topoisomerase I appears to block formation of the initial enzyme-substrate complex on the basis of the failure of the dephosphorylated enzyme to nick DNA in the presence of camptothecin. We conclude that X. laevis DNA topoisomerase I is partially phosphorylated as isolated and that this phosphorylation is essential for expression of enzymatic activity in vitro. On the basis of the ability of the casein kinase type II activity to reactivate dephosphorylated DNA topoisomerase I, we speculate that this kinase may contribute to the physiological regulation of DNA topoisomerase I activity.

CDC7-dependent protein kinase activity in yeast replicative-complex preparations

A protein kinase activity was identified in preparations of DNA-replicative complex from the budding yeast Saccharomyces cerevisiae. The activity phosphorylated only a few of the endogenous proteins in the replicative fraction, and it displayed a marked preference for a 48-kDa polypeptide. Despite this relative specificity, the protein kinase activity was capable of utilizing exogenously added histone as substrate. The 48-kDa polypeptide was phosphorylated on serine residue(s) exclusively by the endogenous activity in the replicative-complex preparation. The activity was not stimulated by cAMP, cGMP, Ca2+/phosphatidylserine/diacylglycerol, or Ca2+/calmodulin. It did not utilize Ca2+ or Zn2+ in the place of Mg2+, and Mn2+ was only 22% as effective in fulfilling the divalent-cation requirement. Most importantly, the protein kinase activity was heat-sensitive in replicative fractions from the cell division cycle 7 (cdc7) mutant, which arrests at or close to the G1/S boundary of the cell cycle at restrictive temperature. Thus, the activity is CDC7-dependent. An effect of heat treatment on replicating activity in the replicative fraction from cdc7 cells was also found. This result and the finding that the protein kinase activity copurified with replicating activity in the preparations suggest that the CDC7 gene product and the protein kinase activity, whether or not they are the same entity, interact with yeast replicative complex. All of these results raise the possibility that phosphorylation of components of the replication machinery may play a role in the control of initiation of DNA replication during the cell cycle. It is possible that the phosphorylation observed is part of a protein kinase cascade that regulates progress through the G1 phase of the cell cycle.