Purification and characterization of DNA polymerase alpha of Chinese hamster ovary cells

Aphidicolin-resistant Chinese hamster ovary cells possess altered DNA polymerases of the alpha-family

DNA polymerases alpha, delta and epsilon were partially purified and characterized from a wild type Chinese hamster ovary (CHO) cell line and from two aphidicolin-resistant mutant CHO cell lines (BR5 and BR5-20a). The main characteristics of the wild type and mutant DNA polymerases were compared in order to reveal differences in the properties of these enzymes responsible for the aphidicolin resistance of the mutant cell lines. Pol alpha's of the mutant cells show: (1) in vitro aphidicolin-resistance, (2) 1.5-3-fold lower specific activity than that of the wild type, (3) resistance to cytosine and adenosine arabinofuranoside 5'-triphosphate (araCTP and araATP), (4) altered resistance to carbonyldiphosphonate (COMDP) and to alkylphenyl nucleotide analogs (butylphenyl-dGTP and butylanilino-dATP), and (5) lower activity on poly(dA)/oligo(dT) template-primers. These changes in the biochemical properties of this enzyme may result from a mutation in pol alpha gene. Pol epsilon and delta of the mutant cells did not differ from the wild type enzymes with respect to aphidicolin resistance. However, the specific activities of these mutant enzymes were much higher (1.5 to 8-fold for pol epsilon and 4 to 20-fold for pol delta) in comparison to that of the wild type enzymes. Also in comparison to the wild type enzymes, the mutant pol epsilon showed changes in the template-primer preference; whereas the mutant pol delta was found to have altered sensitivity to other inhibitors. These results indicate that pol epsilon and pol delta are also altered as a secondary effect of mutation in the aphidicolin-resistant cells. It is suggested that these altered properties of the DNA pols of the alpha family are responsible for the in vivo aphidicolin resistance of the mutant cells.

Plasmodium falciparum: properties of an alpha-like DNA polymerase, the key enzyme in DNA synthesis

An alpha-like DNA polymerase has been identified and characterized in the extracts from the malarial parasite Plasmodium falciparum. The enzyme is sensitive to the specific inhibitors of alpha-DNA polymerase, N-ethylmaleimide and aphidicolin, and is cell-cycle specific. High activity has been found in the schizont, is lower in trophozoites, and has only negligible activity in the ring form. The enzyme has a molecular weight of about Mr 100,000-103,000 estimated by detecting activity in SDS-polyacrylamide electrophoresis and by Bio-Gel filtration. Another active band of a molecular Mr 68,000 was detected by SDS electrophoresis when the enzyme was stored for 2 months at -20 degrees C. The catalytic activity of parasite enzyme was optimal between pH 8 and pH 9. The apparent Michaelis constant for dTTP was 4.3 microM.

Calcium-dependent calmodulin-binding proteins associated with mammalian DNA polymerase alpha

Complex, multiprotein forms of bovine (calf thymus), hamster (Chinese hamster ovary cell), and human (HeLa) cell DNA polymerase alpha (Pol alpha) were analyzed for their content of calmodulin-binding proteins. The approach used an established autoradiographic technique employing 125I-labeled calmodulin to probe proteins in denaturing SDS-polyacrylamide gel electropherograms. All three Pol alpha enzymes were associated with discrete, Ca2+-dependent calmodulin-binding proteins. Conventionally purified calf thymus Pol alpha holoenzyme contained three prominent, trifluoperazine-sensitive species with apparent molecular masses of approx. 120, 80 and 48 kDa. The 120 and 48 kDa species remained associated with the polymerase.primase core of the calf enzyme during immunopurification with monoclonal antibodies directed specifically against the polymerase subunit. The patterns of the calmodulin-binding proteins displayed by conventionally purified preparations of hamster and human Pol alpha enzymes were similar to each other and distinctly different from the pattern of comparable preparations of calf thymus Pol alpha. Immunopurified preparations of the human and hamster Pol alphas retained significant calmodulin-binding activity of apparent molecular masses of approx. 55, 80 and 150-200 kDa.

Inhibition of DNA polymerase alpha by aphidicolin derivatives

17-Acetylaphidicolin was 10-fold weaker and two derivatives lacking hydroxyl groups at the 16 and 17 positions were 100-fold weaker than aphidicolin as inhibitors of DNA polymerase alpha from HeLa and Chinese hamster ovary cells. 17,18-Diacetyl, 3,17,18-triacetyl and 3-epi derivatives of aphidicolin were inactive. Active compounds were, like aphidicolin, competitive with dCTP and did not inhibit aphidicolin-resistant DNA polymerases.

Phosphorylation of a high molecular weight DNA polymerase alpha

Anti-human DNA polymerase alpha murine IgG SJK-287-38 [Tanaka, S., Hu, S.-Z., Wang, T. S.-F. & Korn, D. (1982) J. Biol. Chem. 257, 8386-8390] neutralized DNA polymerase alpha activity from rat embryonic fibroblasts infected with a temperature-sensitive transformation mutant of Rous sarcoma virus (tsLA24). After centrifugation of a crude cytosol fraction from log-phase cells in a 5-20% linear sucrose gradient, polypeptides of Mr approximately equal to 185,000 and 220,000 were immunoprecipitated only from gradient fractions containing DNA polymerase alpha activity. When similar cultures were incubated in medium containing [32P]orthophosphate, it was found that the Mr 220,000 protein was phosphorylated but that the other peptides specific for polymerase alpha activity did not contain detectable amounts of phosphate. Phospho amino acid analysis of the high molecular weight immunoprecipitable proteins indicated that the labeled amino acid was phosphoserine. Incubation of 2.5 units of crude DNA polymerase alpha with 4 units of agarose-immobilized alkaline phosphatase resulted in a nearly complete inhibition of DNA polymerase alpha activity. Subsequent incubation of this preparation with 5 or 50 microM ATP, but not the nonhydrolyzable analog adenosine 5'-[gamma-thio]triphosphate, restored the in vitro DNA polymerizing activity. These results demonstrate that a high molecular weight DNA polymerase alpha (Mr approximately equal to 220,000) is phosphorylated in cultured cells and that this protein is a substrate for a serine kinase rather than the tyrosine-specific protein kinase of Rous sarcoma virus. The results suggest that phosphorylation/dephosphorylation reactions modulate the activity of this polymerase.