Search for multienzyme complexes of DNA precursor pathways in uninfected mammalian cells and in cells infected with herpes simplex virus type I

Replitase: complete machinery for DNA synthesis

Replication of nuclear DNA in eukaryotes presents a tremendous challenge, not only due to the size and complexity of the genome, but also because of the time constraint imposed by a limited duration of S phase during which the entire genome has to be duplicated accurately and only once per cell division cycle. A challenge of this magnitude can only be met by the close coupling of DNA precursor synthesis to replication. Prokaryotic systems provide evidence for multienzyme and multiprotein complexes involved in DNA precursor synthesis and DNA replication. In addition, fractionation of nuclear proteins from proliferating mammalian cells shows co-sedimentation of enzymes involved in DNA replication with those required for synthesis of deoxynucleoside triphosphates (dNTPs). Such complexes can be isolated only from cells that are in S phase, but not from cells in G(0)/G(1) phases of cell cycle. The kinetics of deoxynucleotide metabolism supporting DNA replication in intact and permeabilized cells reveals close coupling and allosteric interaction between the enzymes of dNTP synthesis and DNA replication. These interactions contribute to channeling and compartmentation of deoxynucleotides in the microvicinity of DNA replication. A multienzyme and multiprotein megacomplex with these unique properties is called "replitase." In this article, we summarize some of the relevant evidence to date that supports the concept of replitase in mammalian cells, which originated from the observations in Dr. Pardee's laboratory. In addition, we show that androgen receptor (AR), which plays a critical role in proliferation and viability of prostate cancer cells, is associated with replitase, and that identification of constituents of replitase in androgen-dependent versus androgen-independent prostate cancer cells may provide insights into androgen-regulated events that control proliferation of prostate cancer cells and potentially offer an effective strategy for the treatment of prostate cancer.

NM23/nucleoside diphosphate kinase and signal transduction

NM23s (or NDP kinases) regulate a fascinating variety of cellular activities, including proliferation, development, and differentiation. All these processes are modulated by external stimuli, leading to the idea that this family of proteins modulates transmembrane signaling pathways. This review summarizes the evidence indicating that NM23/NDP kinases participate in transmembrane signaling in eukaryotic cells and discusses the molecular mechanisms proposed to account for these actions.

Adenovirus preterminal protein binds to the CAD enzyme at active sites of viral DNA replication on the nuclear matrix

Adenovirus (Ad) replicative complexes form at discrete sites on the nuclear matrix (NM) via an interaction mediated by the precursor of the terminal protein (pTP). The identities of cellular proteins involved in these complexes have remained obscure. We present evidence that pTP binds to a multifunctional pyrimidine biosynthesis enzyme found at replication domains on the NM. Far-Western blotting identified proteins of 150 and 240 kDa that had pTP binding activity. Amino acid sequencing of the 150-kDa band revealed sequence identity to carbamyl phosphate synthetase I (CPS I) and a high degree of homology to the related trifunctional enzyme known as CAD (for carbamyl phosphate synthetase, aspartate transcarbamylase, and dihydroorotase). Western blotting with an antibody directed against CAD detected a 240-kDa band that comigrated with that detected by pTP far-Western blotting. Binding experiments showed that a pTP-CAD complex was immunoprecipitable from cell extracts in which pTP was expressed by a vaccinia virus recombinant. Additionally, in vitro-translated epitope-tagged pTP and CAD were immunoprecipitable as a complex, indicating the occurrence of a protein-protein interaction. Confocal fluorescence microscopy of Ad-infected NM showed that pTP and CAD colocalized in nuclear foci. Both pTP and CAD were confirmed to colocalize with active sites of replication detected by bromodeoxyuridine incorporation. These data support the concept that the pTP-CAD interaction may allow anchorage of Ad replication complexes in the proximity of required cellular factors and may help to segregate replicated and unreplicated viral DNA.

Association of c-myc protein with enzymes of DNA replication in high molecular weight fractions from mammalian cells

Oncoprotein c-myc is expressed in proliferating but not quiescent mammalian cells, and its overexpression or inappropriate expression is associated with malignant transformation. However, in spite of an intense interest, the normal function of this protein has remained elusive. As a step towards the elucidation of the function of c-myc protein, we studied its distribution within several types of cells, including HL 60, K 562, COLO 320, and CHEF/18 cells. In all of the cells studied, c-myc protein was detected in high molecular weight protein fractions, in 350-600 Kd range, in gel-exclusion chromatography and sucrose gradient centrifugation. This distribution of c-myc protein coincided with the distribution of DNA polymerase alpha and several other enzymes necessary for DNA replication. The data suggest that c-myc product may be a component of the replitase complex of enzymes involved in nuclear DNA replication.

Transfection of mouse cells with thymidine kinase gene of herpes simplex virus

A mouse cell line (LP1-1) was established from the murine L cells deficient in thymidine kinase (L-M(TK-] by prolonged selective culture on the hypoxanthine-aminopterine-thymidine (HAT) medium following transfection with the thymidine kinase gene of herpes simplex virus type-I (HSVTK). Southern blot analysis has shown that the viral TK gene was integrated into one of the chromosomal loci by a single copy. From this established cell line, the 5-bromo-2-deoxyuridine (BrdU) resistant revertant was brought out at a frequency of 1 x 10(-6) and from these BrdU resistant revertants (LP1BU), one out of 1 x 10(5) cells could return to the HAT-resistant phenotype. The established LP1-1 cell line showed a typical biphasic nature of DNA synthesis as determined by the 3H-thymidine incorporation test. The activity of thymidine kinase was shown to be equivalent to that of the DNA polymerase-alpha when the whole nuclear fraction or the nuclear matrix were used for examination. These results indicate that the transfected viral TK gene can be expressed under the normal cell-cycle regulation and its gene product can act as a component of the multienzyme complex which is responsible for DNA replication.

Nuclear localization of 68 kDa calmodulin-binding protein is associated with the onset of DNA replication

In Chinese hamster embryo fibroblast cells, an increase in intracellular calmodulin levels coincided with the nuclear localization of a calmodulin-binding protein of about 68 kDa as the cells progressed from G1 to S phase. When cells were limited from entering into S phase, by omitting insulin a defined medium, intracellular CaM levels did not increase and the 68 kDa calmodulin-binding protein was completely absent from the nuclei. Corresponding to the nuclear localization of calmodulin and the 68 kDa calmodulin-binding protein in S phase cells, there was a dramatic increase in DNA polymerase and thymidine kinase activities in the nuclei of S phase cells as compared to G1 phase cells. In addition, the 68 kDa calmodulin-binding protein, along with calmodulin, is observed to be an integral component of replitase complex responsible for nuclear DNA replication in S phase cells. These observations point to the association of calmodulin and calmodulin-binding protein(s) with the replication machinery responsible for nuclear DNA replication during S phase. A possible regulatory role of these proteins in the onset of DNA replication and cell proliferation is discussed.

G1 events and regulation of cell proliferation

Cells prepare for S phase during the G1 phase of the cell cycle. Cell biological methods have provided knowledge of cycle kinetics and of substages of G1 that are determined by extracellular signals. Through the use of biochemical and molecular biological techniques to study effects of growth factors, oncogenes, and inhibitors, intracellular events during G1 that lead to DNA synthesis are rapidly being discovered. Many cells in vivo are in a quiescent state (G0), with unduplicated DNA. Cells can be activated to reenter the cycle during G1. Similarly, cells in culture can be shifted between G0 and G1. These switches in and out of G1 are the main determinants of post-embryonic cell proliferation rate and are defectively controlled in cancer cells.

Compartmentation of deoxypyrimidine nucleotides for nuclear DNA replication in S phase mammalian cells

DNA synthesis in S phase Chinese hamster embryo fibroblast cells in the presence of exogenous 3H-dUrd shows incorporation of the labeled precursor with very little dilution by the large unlabeled intracellular precursor pools. Full mixing would predict a specific activity 10-fold less than that measured. This coupled with the finding that 80% of the radioactivity derived from the exogenous 3H-dUrd appears in the karyoplasts implies a compartmentation where 3H-dUMP and 3H-dTTP derived from exogenous 3H-dUrd do not mix freely with endogenous cytoplasmic pools.

Association of 3'----5' exodeoxyribonuclease activity with DNA replitase complex from S-phase Chinese hamster embryo fibroblast cells

DNA replitase has been described as a complex of enzymes/proteins that are associated with both DNA precursor biosynthesis and DNA replication in mammalian cells [Reddy, G. P. V., and Pardee, A. B. (1980) Proc. Natl. Acad. Sci. USA 77, 3312-3316]. We demonstrate for the first time a 3'----5' exodeoxyribonuclease activity is associated with the replitase complex. As much as 60% of this exonuclease activity was similar to that associated with DNA polymerase delta based upon its sensitivity to inhibition by GMP and by butyl-phenyl-deoxyguanosine triphosphate (BuPdGTP). Association of 3'----5' exonuclease activity with the DNA polymerase in the replitase complex was also demonstrated by analyzing dTTP turnover to dTMP in an in vitro DNA polymerase assay system. The DNA polymerase activity in replitase complex exhibited a sensitivity to BuPdGTP which both was similar to that of DNA replication in permeable cells and was intermediate between the BuPdGTP inhibition of purified DNA polymerases alpha and delta. These studies suggest that the replitase complex contains 3'----5' exonuclease activity associated with the DNA polymerase activity responsible for nuclear DNA replication in mammalian cells. Further studies are required to determine if these activities are at least partially attributed to DNA polymerase delta.

Intracellular compartmentation of diadenosine tetraphosphate (Ap4A) and dTTP in rat liver

1. The intracellular compartmentation of diadenosine tetraphosphate (Ap4A) and of dTTP was studied in rat liver cells using non-aqueous glycerol for the isolation of cell nuclei. 2. This method allows a stepwise removal of cytoplasm from the nuclei. 3. The decrease in Ap4A or dTTP during the process was compared to the simultaneous decrease in RNA, which was taken to represent the cytoplasm. 4. In regenerating liver excised 24 hr after partial hepatectomy, Ap4A was almost equally distributed between the nucleus and cytoplasm. 5. In livers from unoperated control rats, the nuclear concentration of Ap4A was slightly elevated compared to that of whole cells. dTTP was only investigated in regenerating liver. 6. Significantly higher concentrations were found in the nuclear fractions. 7. The purest nuclei contained about 26% of whole cell levels of dTTP, while their RNA values had decreased to 7% of the whole cell RNA. 8. Considering that the liver cell nucleus comprises about 7% of the entire cell mass, a nuclear dTTP concentration of 26% indicates significantly higher dTTP levels in the nuclear compartment than in the cytoplasm of regenerating rat liver cells.