Immunoaffinity-purified DNA polymerase alpha displays novel properties

The purification and characterization of a novel and more intact form of the DNA polymerase alpha-primase complex from calf thymus are described. The polymerase-primase was enriched 10,000-fold to apparent homogeneity by chromatography on phosphocellulose, heparin-Sepharose, and an immobilized anti-human DNA polymerase alpha monoclonal antibody [SJK287-38; Tanaka, S., Hu, S., Wang, T. S.-F., & Korn, D. (1982) J. Biol. Chem. 257, 8386-8390]. A quantitative elution from the antibody column was achieved by shifting the pH from neutrality to between 12.5 and 13. From 1 kg of calf thymus, the procedure yields 1-2 mg of polymerase-primase with a specific activity of 30,000-40,000 units/mg for the polymerase and 15,000-20,000 units/mg for the primase. The complex sediments at 9 S through a sucrose gradient and exhibits a Stokes radius of 6.0 nm, yielding a native molecular mass of 335,000. Denaturing gel electrophoresis of the complex gives bands of Mrs 180,000, 155,000, 148,000, 73,000, 59,000, and 48,000 with a relative abundance of the two smallest subunits. Primase activity was partially resolved from the complex by centrifugation through sucrose gradients. The primer-forming activity was found to be associated with the Mr 59,000 and 48,000 polypeptides. In contrast to conventional preparations, the immunopurified polymerase displays several features which show it is the most intact form of the enzyme known to date. The deoxynucleoside triphosphate Km values are all within the range of 0.6-0.9 microM. The Km for binding to a single RNA primer on M13 DNA is 3.5 nM; the Ki for nonspecific binding to unprimed DNA is 70 microM (nucleotide).(ABSTRACT TRUNCATED AT 250 WORDS)     

DNA polymerase alpha-DNA primase from human lymphoblasts

The DNA polymerase alpha-DNA primase complex from the human lymphoblast line HSC93 has been enriched to near homogeneity by using an immunoaffinity purification protocol which was developed earlier for the purification of the calf thymus enzyme (Nasheuer, H.-P. and Grosse, F. (1987) Biochemistry 26, 8458-8466). Immunoaffinity purified polymerase-primase from human cells consisted of four subunits displaying molecular weights of 195,000 and 180,000 for the DNA synthesizing alpha-subunit, of 68,000 for the beta-subunit, and of 55,000 and 48,000 for the primase-carrying gamma- and delta-subunit, respectively. The isoelectric pH values for the individual subunits were estimated from non-equilibrium pH gradients to be between 5.9 and 5.7 for the alpha-subunit, at 5.5 for the beta-subunit, and at 7.5 and 8.0 for the gamma- and delta-subunit, respectively. The purified polymerase-primase converted single-stranded phi X174 DNA into the double-stranded form in a primase-initiated reaction. During this process, 3-10 RNA primers were formed. RNA primers were about 11 nucleotides long. Elongation of existing RNA primers by the human polymerase-primase was semi-processive; following primer binding the DNA polymerase continuously incorporated 20 to 50 nucleotides, then it dissociated from the template DNA.     

The Chk1-mediated S-phase checkpoint targets initiation factor Cdc45 via a Cdc25A/Cdk2-independent mechanism

DNA damage induced by the carcinogen benzo[a]pyrene dihydrodiol epoxide (BPDE) induces a Chk1-dependent S-phase checkpoint. Here, we have investigated the molecular basis of BPDE-induced S-phase arrest. Chk1-dependent inhibition of DNA synthesis in BPDE-treated cells occurred without detectable changes in Cdc25A levels, Cdk2 activity, or Cdc7/Dbf4 interaction. Overexpression studies showed that Cdc25A, cyclin A/Cdk2, and Cdc7/Dbf4 were not rate-limiting for DNA synthesis when the BPDE-induced S-phase checkpoint was active. To investigate other potential targets of the S-phase checkpoint, we tested the effects of BPDE on the chromatin association of DNA replication factors. The levels of chromatin-associated Cdc45 (but not soluble Cdc45) were reduced concomitantly with BPDE-induced Chk1 activation and inhibition of DNA synthesis. The chromatin association of Mcm7, Mcm10, and proliferating cell nuclear antigen was unaffected by BPDE treatment. However, the association between Mcm7 and Cdc45 in the chromatin fraction was inhibited in BPDE-treated cells. Chromatin immunoprecipitation analyses demonstrated reduced association of Cdc45 with the beta-globin origin of replication in BPDE-treated cells. The inhibitory effects of BPDE on DNA synthesis, Cdc45/Mcm7 associations, and interactions between Cdc45 and the beta-globin locus were abrogated by the Chk1 inhibitor UCN-01. Taken together, our results show that the association between Cdc45 and Mcm7 at origins of replication is negatively regulated by Chk1 in a Cdk2-independent manner. Therefore, Cdc45 is likely to be an important target of the Chk1-mediated S-phase checkpoint.     

Different activities of the largest subunit of replication protein A cooperate during SV40 DNA replication

Replication protein A (RPA) is a stable heterotrimeric complex consisting of p70, p32 and p14 subunits. The protein plays a crucial role in SV40 minichromosome replication. Peptides of p70 representing interaction sites for the smaller two subunits, DNA as well as the viral initiator protein large T-antigen (Tag) and the cellular DNA polymerase alpha-primase (Pol) all interfered with the replication process indicating the importance of the different p70 activities in this process. Inhibition by the peptide disrupting protein-protein interactions was observed only during the pre-initiation stage prior to primer synthesis, suggesting the formation of a stable initiation complex between RPA, Tag and Pol at the primer end.     

Human Cdc45 is a proliferation-associated antigen

Cell division cycle protein 45 (Cdc45) plays a critical role in DNA replication to ensure that chromosomal DNA is replicated only once per cell cycle. We analysed the expression of human Cdc45 in proliferating and nonproliferating cells. Our findings show that Cdc45 protein is absent from long-term quiescent, terminally differentiated and senescent human cells, although it is present throughout the cell cycle of proliferating cells. Moreover, Cdc45 is much less abundant than the minichromosome maintenance (Mcm) proteins in human cells, supporting the concept that origin binding of Cdc45 is rate limiting for replication initiation. We also show that the Cdc45 protein level is consistently higher in human cancer-derived cells compared with primary human cells. Consequently, tumour tissue is preferentially stained using Cdc45-specific antibodies. Thus, Cdc45 expression is tightly associated with proliferating cell populations and Cdc45 seems to be a promising candidate for a novel proliferation marker in cancer cell biology.     

Amino acids 257 to 288 of mouse p48 control the cooperation of polyomavirus large T antigen, replication protein A, and DNA polymerase alpha-primase to synthesize DNA in vitro

Although p48 is the most conserved subunit of mammalian DNA polymerase alpha-primase (pol-prim), the polypeptide is the major species-specific factor for mouse polyomavirus (PyV) DNA replication. Human and murine p48 contain two regions (A and B) that show significantly lower homology than the rest of the protein. Chimerical human-murine p48 was prepared and coexpressed with three wild-type subunits of pol-prim, and four subunit protein complexes were purified. All enzyme complexes synthesized DNA on single-stranded (ss) DNA and replicated simian virus 40 DNA. Although the recombinant protein complexes physically interacted with PyV T antigen (Tag), we determined that the murine region A mediates the species specificity of PyV DNA replication in vitro. More precisely, the nonconserved phenylalanine 262 of mouse p48 is crucial for this activity, and pol-prim with mutant p48, h-S262F, supports PyV DNA replication in vitro. DNA synthesis on RPA-bound ssDNA revealed that amino acid (aa) 262, aa 266, and aa 273 to 288 are involved in the functional cooperation of RPA, pol-prim, and PyV Tag.     

Species specificity of simian virus 40 DNA replication in vitro requires multiple functions of human DNA polymerase alpha

Human cell extracts support the replication of SV40 DNA, whereas mouse cell extracts do not. Species specificity is determined at the level of initiation of DNA replication, and it was previously found that this requires the large subunit, p180, of DNA polymerase alpha-primase to be of human origin. Furthermore, a functional interaction between SV40 large T antigen (TAg) and p180 is essential for viral DNA replication. In this study we determined that the N-terminal regions of human p180, which contain the TAg-binding sites, can be replaced with those of murine origin without losing the ability to support SV40 DNA replication in vitro. The same substitutions do not prevent SV40 TAg from stimulating the activity of DNA polymerase alpha-primase on single-stranded DNA in the presence of replication protein A. Furthermore, biophysical studies show that the interactions of human and murine DNA polymerase alpha-primase with SV40 TAg are of a similar magnitude. These studies strongly suggest that requirement of SV40 DNA replication for human DNA polymerase alpha depends neither on the TAg-binding site being of human origin nor on the strength of the binary interaction between SV40 TAg and DNA polymerase alpha-primase but rather on sequences in the C-terminal region of human p180.     

Two immunologically distinct human DNA polymerase alpha-primase subpopulations are involved in cellular DNA replication

Metabolic labeling of primate cells revealed the existence of phosphorylated and hypophosphorylated DNA polymerase alpha-primase (Pol-Prim) populations that are distinguishable by monoclonal antibodies. Cell cycle studies showed that the hypophosphorylated form was found in a complex with PP2A and cyclin E-Cdk2 in G1, whereas the phosphorylated enzyme was associated with a cyclin A kinase in S and G2. Modification of Pol-Prim by PP2A and Cdks regulated the interaction with the simian virus 40 origin-binding protein large T antigen and thus initiation of DNA replication. Confocal microscopy demonstrated nuclear colocalization of hypophosphorylated Pol-Prim with MCM2 in S phase nuclei, but its presence preceded 5-bromo-2'-deoxyuridine (BrdU) incorporation. The phosphorylated replicase exclusively colocalized with the BrdU signal, but not with MCM2. Immunoprecipitation experiments proved that only hypophosphorylated Pol-Prim associated with MCM2. The data indicate that the hypophosphorylated enzyme initiates DNA replication at origins, and the phosphorylated form synthesizes the primers for the lagging strand of the replication fork.