10-S DNA polymerase from calf thymus which copies both poly(rA) . oligo(dT) and activated DNA

Terminal differentiation of human erythroleukemia cell line K562 induced by aphidicolin

To analyze the relationship between differentiation and DNA replication, the effect of aphidicolin, a specific inhibitor for DNA polymerase alpha, was measured with respect to erythroid differentiation and activities of DNA polymerases alpha, beta, and gamma. Five micromolar aphidicolin completely blocked the growth of K562 cells and caused 80% of cells to become hemoglobin positive after 5 days exposure. The cessation of K562 cell growth induced by aphidicolin was irreversible, whereas the inhibition of HeLa cell growth was completely reversible. The enzyme activity of DNA polymerase alpha of K562 cells showed a 50-110% increase with aphidicolin treatment as compared to control K562 cells; activities of DNA polymerases beta and gamma were not affected. These features sharply contrasted with the erythroid induction of the same cells by hemin, where cell growth was not suppressed and DNA polymerase alpha was not increased but rather decreased. The enzyme activity of DNA polymerase alpha remained high even after removal of aphidicolin from the culture medium. These results suggest that treatment with aphidicolin might induce an accumulation of protein factors for replication and/or differentiation, causing rapid cell differentiation of cells without cell division.

Structural study of immunoaffinity-purified DNA polymerase alpha-DNA primase complex from calf thymus

The DNA polymerase alpha-DNA primase complex was purified over 17,000-fold to near homogeneity from calf thymus using an immunoaffinity column. Sodium dodecyl sulfate gel electrophoresis revealed three polypeptides with molecular weights of 140, 50 and 47 kDa, in a ratio of 1:2:0.25. The complex showed a sedimentation coefficient of 9.7 S, a Stokes radius of 56 A and a native molecular weight of 250-260 kDa. Taken together, the data suggest that the calf thymus dNA polymerase alpha-DNA primase complex is essentially a heterotrimer of large (140 kDa) and small (50 kDa) subunits in a ratio of 1:2, with a globular conformation. Electron-microscopic studies of the complex revealed a spherical particle of 120 A in diameter, in agreement with the physiochemical results. The binding of the complex to DNA was also demonstrated.

A DNA polymerase with unusual properties from the slime mold Physarum polycephalum

Two forms of a DNA polymerase have been purified from microplasmodia of Physarum polycephalum by poly(ethyleneimine) precipitation and chromatography on DEAE-Sephacel, phosphocellulose, heparin Sepharose, hydroxyapatite, DNA-agarose, blue-Sepharose. They were separated from DNA polymerase alpha on phosphocellulose and from each other on heparin-Sepharose. Form HS1 enzyme was 30-40% pure and form HS2 enzyme 60% with regard to protein contents of the preparations. Form HS2 enzyme was generated from form HS1 enzyme on prolonged standing of enzyme preparations. The DNA polymerases were obtained as complexes of a 60-kDa protein associated with either a 135-kDa (HS1) or a 110-kDa (HS2) DNA-polymerizing polypeptide in a 1:1 molar stoichiometry. The biochemical function of the 60-kDa protein remained unknown. The complexes tended to dissociate during gradient centrifugation and during partition chromatography as well as during polyacrylamide gradient gel electrophoresis under nondenaturing conditions at high dilutions of samples. Both forms existed in plasmodia extracts, their proportions depending on several factors including those which promoted proteolysis. The DNA polymerases resembled eucaryotic DNA polymerase beta by several criteria and were functionally indistinguishable from each other. It is suggested that lower eucaryotes contain repair DNA polymerases, which are similar to those of eubacteria on a molecular mass basis.

Inhibition of DNA polymerase alpha by gossypol

Our earlier studies have shown that gossypol is a specific inhibitor of DNA synthesis in cultured cells at low doses. In an attempt to determine the mechanism for the inhibition of DNA synthesis by gossypol we observed that gossypol does not interact with DNA per se but may affect some of the enzymes involved in DNA replication. These studies indicated that gossypol inhibits both in vivo and in vitro the activity of DNA polymerase alpha (EC 2.7.7.7), a major enzyme involved in DNA replication, in a time- and dose-dependent manner. Kinetic analysis revealed that gossypol acts as a noncompetitive inhibitor of DNA polymerase alpha with respect to all four deoxynucleotide triphosphates and to the activated DNA template. Inhibition of DNA polymerase alpha does not appear to be due to either metal chelation or reduction of sulfhydryl groups on the enzyme. Gossypol also inhibited HeLa DNA polymerase beta in a dose-dependent manner, but had no effect on DNA polymerase gamma. These results suggest that inhibition of DNA polymerase alpha may account in part for the inhibition of DNA synthesis and the S-phase block caused by gossypol. The data also raise the possibility that gossypol may interfere with DNA repair processes as well.

Characterization of a Mr = 56,000 polypeptide associated with 10S DNA polymerase alpha purified from calf thymus using monoclonal antibody

Existence of a Mr = 56,000 polypeptide associated with 10S DNA polymerase alpha was shown by production of a monoclonal anti-calf thymus 10S DNA polymerase alpha antibody secreted from a hybridoma line named 3H1. The antibody bound three polypeptides with Mr = 180,000, 56,000 and 32,000 in hydroxylapatite fraction of 10S DNA polymerase alpha by immunoblot. The antibody co-precipitated the polypeptides with the large polypeptide (Mr = 150,000-140,000) of 10S DNA polymerase alpha with the aid of second antibody. Among three polypeptides, the Mr = 56,000 polypeptide was co-purified with DNA polymerase alpha through DNA-cellulose chromatography and repeated sucrose rate-zonal centrifugations. The Mr = 56,000 polypeptide was still associated with 10S DNA polymerase alpha after second sucrose rate-zonal centrifugation, but the amount of it was reduced. The polypeptide was banded at pH 7.2-8.0 and displayed microheterogeneity in respect of isoelectric point by isoelectrofocusing with 7 M urea, and showed weak DNA-binding property after blotting onto a nitrocellulose. The antibody against the polypeptide precipitated DNA polymerase alpha from human, rat, and mouse, and Mr = 56,000 and 32,000 polypeptides were detected in these DNA polymerase alpha fractions by immunoblot. These results suggest that the polypeptide with Mr = 56,000 may take part in the DNA polymerase reaction.

Large polypeptides of 10S DNA polymerase alpha from calf thymus: rapid isolation using monoclonal antibody and tryptic peptide mapping analysis

The polypeptides recognized by a monoclonal antibody against calf thymus DNA polymerase alpha (secreted from a hybridoma CL22 -2- 42B , Nucleic Acids Res. (1982) 10, 4703-4713) were identified by the immunoblot method as the large polypeptides of the partially-purified 10S DNA polymerase alpha fraction. Using an immunoprecipitation technique with the monoclonal antibody, a rapid immunological isolation of the polypeptides has been achieved. By this method, the large polypeptides with Mr = 140,000, 145,000, and 150,000 were isolated from a partially-purified preparation of 10S DNA polymerase alpha. On the other hand, the polypeptides with Mr = 150,000, 180,000, and 240,000 were obtained from a crude extract of calf thymus. Tryptic peptide maps showed that the large polypeptides with Mr = 150,000, and 180,000 were very similar in primary structure and that the structures of Mr = 180,000 and 240,000 polypeptides contained partially common sequences. Among these polypeptides, the Mr = 150,000 polypeptide was shown to correlate with the enzyme activity. These results suggest that the large polypeptide of 10S DNA polymerase alpha is initially synthesized as Mr = 180,000 or larger polypeptide, then converted to the form with Mr = 150,000. The Mr = 140,000 and 145,000 polypeptides in the purified preparation may be artificial products formed during purification.

Intracellular localization and metabolism of DNA polymerase alpha in human cells visualized with monoclonal antibody

Indirect immunofluorescence microscopy with monoclonal antibody against DNA polymerase alpha revealed the intranuclear localization of DNA polymerase alpha in G1, S, and G2 phases of transformed human cells, and dispersed cytoplasmic distribution during mitosis. In the quiescent, G0 phase of normal human skin fibroblasts or lymphocytes, the alpha-enzyme was barely detectable by either immunofluorescence or enzyme activity. By exposing cells to proliferation stimuli, however, DNA polymerase alpha appeared in the nuclei just prior to onset of DNA synthesis, increased rapidly during S phase, reached the maximum level at late S and G2 phases, and was then redistributed to the daughter cells through mitosis. It was also found that the increase in the amount of DNA polymerase alpha by proliferation stimuli was not affected by inhibition of DNA synthesis with aphidicolin or hydroxyurea.

DNA primase associated with 10 S DNA polymerase alpha from calf thymus

Among multiple subspecies of DNA polymerase alpha of calf thymus, only 10 S DNA polymerase alpha had a capacity to initiate DNA synthesis on an unprimed single-stranded, circular M13 phage DNA in the presence of ribonucleoside triphosphates (DNA primase activity). The primase was copurified with 10 S DNA polymerase alpha through the purification and both activities cosedimented at 10 S through gradients of either sucrose or glycerol. Furthermore, these two activities were immunoprecipitated at a similar efficiency by a monoclonal antibody directed against calf thymus DNA polymerase alpha. These results indicate that the primase is tightly bound to 10 S DNA polymerase alpha. The RNA polymerizing activity was resistant to alpha-amanitin, required high concentration of all four ribonucleoside triphosphates (800 microM) for its maximal activity, and produced the limited length of oligonucleotides (around 10 nucleotides long) which were necessary to serve as a primer for DNA synthesis. Covalent bonding to RNA to DNA was strongly suggested by the nearest neighbour frequency analysis and the DNAase treatment. The DNA synthesis primed by the RNA oligomers may be carried out by the associating DNA polymerase alpha because it was strongly inhibited by araCTP, resistant to d2TTP, and was also inhibited by aphidicolin but at relatively high concentration. The primase preferred single-stranded DNA as a template, but it also showed an activity on the double-stranded DNA from calf thymus at an efficiency of approx. 10% of that with single-stranded DNA.

Production and characterization of monoclonal antibody against 10S DNA polymerase alpha from calf thymus

One hybridoma cell line that produces an antibody directed against 10S DNA polymerase alpha purified from calf thymus was obtained. The monoclonality of the antibody was tested by sodium dodecyl sulfate polyacrylamide gel electrophoresis, isoelectrofocusing and antibody subclass determination. The antibody specifically recognized the 10S DNA polymerase alpha and 6.5S DNA polymerase alpha-2 from calf thymus, but not 6.5S DNA polymerase alpha-1. The antibody precipitated both polypeptides of 140-150,000 and 46-50,000 dalton of 10S DNA polymerase alpha. The antibody also recognized the DNA polymerase alpha purified from human cells, but did pig DNA polymerase alpha only partially. The antibody did not crossreact with rat DNA polymerase alpha, calf DNA polymerase beta, virus DNA polymerase and E. coli DNA polymerase I. This antibody will be a useful tool for studying the mechanism of DNA replication in eukaryotic cells.

Purification of a 9S DNA polymerase alpha species from calf thymus

a DNA polymerase alpha species from calf thymus has been purified 12 000-fold to near homogeneity. The enzyme sediments under high salt conditions in the preparative ultracentrifuge as a homogeneous band at 9S. The specific activity is 50 000-70 000 units/mg of protein. Polypeptides of 148 000, 59 000, and 48 000 daltons are detectable. The molecular weight as estimated from gradient gel electrophoresis is about 500 000. The 9S DNA polymerase is free from terminal nucleotidyl transferase activity and does not exhibit endonuclease or exonuclease activity. It is inhibited by low concentrations of salt, aphidicolin, and N-ethylmaleimide.

Further characterization of a poly(rA) . oligo(dT)-dependent activity of multiple DNA polymerase alpha from calf thymus

DNA polymerase alpha (EC 2.7.7.7) from calf thymus has been separated into three molecular species, i.e., 10 S DNA polymerase alpha, 6.5 S DNA polymerase alpha-1 and 6.5 S DNA polymerase alpha-2 (Masaki, S. and Yoshida, S. (1978) Biochim, Biophys. Acta 531, 74-88; Yoshida, S., Yamada, M., Masaki S. and Seneyoshi, M. (1979) Cancer Res. 39, 3955-3958). Among these three, 10 S DNA polymerase alpha and 6.5 S DNA polymerase alpha-2 were found to copy efficiently poly(rA) . oligo(dT), a template-primer, which was thought to be specific for DNA polymerase gamma or beta. 6.5 S DNA polymerase alpha-1, however, could not use the ribopolymer as a template. The poly(rA) . oligo(dT)-dependent activities of DNA polymerase alpha species differed markedly from those with activated calf thymus DNA in sensitivity to various reagents: the former was inhibited more than 80% by 80 mM KCl, while the latter was stimulated somewhat. Furthermore, aphidicolin, a specific inhibitor of DNA polymerase alpha, did not inhibit the poly(rA) . oligo(dT)-dependent activity. 2',3'-DideoxyTTP, a potent inhibitor of DNA polymerase beta or gamma, slightly inhibited the reactions with poly(rA) . oligo(dT), while it did not inhibit the reactions with activated DNA. The apparent Km values for dTTP on poly(rA) . oligo(dT) template were 260 and 70 microM for 10 S alpha and 6.5 S alpha-2, respectively; these values were much higher than those obtained on activated DNA template (8-10 microM).

Differential inhibition of multiple forms of DNA polymerase alpha from IMR-32 human neuroblastoma cells

Three forms of DNA polymerase (pol) alpha from human neuroblastoma IMR-32 were separated by DEAE column chromatography. All sedimented at approximately 7 S in 5-20% continuous sucrose density gradients. All were heat labile, with pol alpha 2 the most (90% inactivated) and pol alpha 3 the least (50% inactivated) sensitive to heating for 5 min at 50 degrees C. pol alpha 1 and alpha 2 efficiently utilized activated calf thymus DNA as template. The most active form, pol alpha 2, used both poly(dA).(dT)12-18 and poly(dT).(dA)12-18 as template at equal rates. Differential inhibition of DNA polymerase alpha activities was examined in the presence of ricin, hemin, and a nonhistone chromatin protein. All three polymerases were inhibited by both ricin (nonreduced) and hemin, with pol alpha 2 the most (80-90%) and pol alpha 3 the least (60%) sensitive in each case. In contrast, only pol alpha 2 and alpha 3 activities were inhibited (80-85%) by rat liver nonhistone chromatin protein.

Cooperation of terminal deoxynucleotidyl transferase with DNA polymerase alpha in the replication of ultraviolet-irradiated DNA

The amount of DNA synthesis in vitro with the ultraviolet-irradiated poly-(dT) . oligo(rA) template initiators catalysed by DNA polymerase alpha (Masaki, S. and Yoshida, S., Biochim. Biophys. Acta 521, 74--88) decreased with the dose of ultraviolet-irradiation. The ultraviolet irradiation to the template, however did not affect the rate of incorporation of incorrect deoxynucleotides into the newly synthesized poly(dA). The addition of terminal deoxynucleotidyl transferase to this system enhanced the DNA synthesis to a level which is comparable to that of the control and it concomitantly increased the incorporation of the mismatched deoxynucleotide into the newly synthetized poly(dA) strands. On the other hand, with an unirradiated template initiator, the misincorporation was only slightly enhanced by the addition of terminal deoxynucleotidyl transferase. The sizes of newly synthetized DNA measured by sedimentation velocities were found to be smaller with the ultraviolet-irradiated templates but they increased to the control level with the addition of terminal deoxynucleotidyl transferase to the systems. These results suggest that terminal deoxynucleotidyl transferase can help DNA polymerase alpha to "bypass" thymine dimers in vitro by the formation of mismatched regions at the positions opposite to pyrimidine dimers on the template.

Differential inhibition of mammalian DNA polymerases by X-irradiated DNA

Using mammalian DNA polymerases alpha, beta and terminal deoxynucleotidyl transferase, we have examined the inhibitory action of X-irradiated DNA on in vitro DNA synthetic activities of these enzymes. It was found that DNA polymerase beta was highly sensitive inhibition by the irradiated DNA as well as DNA polymerases I of E. coli, while DNA polymerase alpha was at least two hundred times more resistant to inhibition than DNA ploymerase beta. Terminal deoxynucleotidyl transferase was inhibited moderately by the single-stranded form of the irradiated DNA. Since the inhibition was competitive with respect to a template-initiator for all DNA polymerases or an initiator for terminal deoxynucleotidyl transferase, the differences in sensitivities to the inhibition may be due to the different affinities of the enzymes to the X-ray-induced inhibitory sites on the DNA strand.