The suppression of the synthesis of a nuclear protein in cells blocked in G2 phase: identification of NP-170 as topoisomerase II.

Previous studies of a nuclear protein of molecular weight 170 kDa (NP-170) have shown it to have two interesting properties. First, NP-170 synthesis began in mid- to late S phase and became maximal in G2 phase. Second, the synthesis of NP-170 was suppressed in cells blocked in G2 phase following irradiation with 6.8 Gy (J. M. Holland et al., Radiat. Res. 122, 197-208, 1990). The molecular weight of NP-170 is the same as that of Topoisomerase II (Topo II), an enzyme involved in the alteration of DNA supercoiling status with a double-strand passing function. This study was undertaken to determine whether NP-170 could be Topo II. The results from the present study show that both the proteins have identical cell cycle synthesis patterns. The synthesis of both these proteins is suppressed following irradiation. NP-170 was found to be recognized by a Topo II antibody in both Western blots and immunoprecipitation. This study characterizes NP-170 as Topo II.     

Periodic fluctuations of nuclear high mobility group like proteins during the cell cycle of Physarum polycephalum

High mobility group like (HMG-like) nuclear proteins were isolated from plasmodia of the lower eucaryote Physarum polycephalum and characterized by different types of polyacrylamide gel electrophoresis. The synthesis of these proteins was measured during the naturally synchronous cell cycle of Physarum. The four HMG-like proteins (AS1-4) exhibit a pronounced cell cycle dependent pattern of synthesis: AS1 and AS4 have a clear maximum of synthesis in mid S phase with a basal synthesis during the entire G2 period. In contrast, AS2 and AS3 have little synthesis in S phase but a broad maximum in mid G2 period. The four HMG-like proteins have a very low synthesis in early S phase and late G2 period. In addition, other non-histone proteins, which are coextracted with the HMG proteins, exhibit distinct periodic synthesis patterns. A novel non-histone protein, which is the most abundant protein species in 0.35 M NaCl extracts, was detected. It exhibits a high rate of synthesis around the time of mitosis. In general, the results indicate that, in contrast to the main cytoplasmic proteins, most nuclear proteins are phase-specific with respect to their synthesis in the cell cycle.     

Patterns of protein synthesis during the cell cycle of Chinese hamster ovary cells

The protein synthesis patterns at various stages of the cell cycle of Chinese hamster ovary cells were examined by labelling cells with [35S]methionine and then separating the proteins by isoelectric focussing and two-dimensional, nonequilibrium pH gradient gel electrophoresis. We have observed a number of proteins which display quantitative differences in synthesis at specific cell cycle stages and of these the alpha- and beta-tubulins have been identified. A few proteins appear to be uniquely synthesized at specific times during the cell cycle. These include the histones and a modified version of them, which are synthesized only in S phase, and a pair of 21 kilodalton (kDa), pI 5.5 proteins, which appear only in late G2 and mitosis. We have also identified a 58-kDa, pI 7.5 protein which is present at all cell cycle stages except during late G2. This protein appears to have the same temporal properties as a 57-kDa protein called "cyclin" originally described in sea urchin embryos.     

Analysis of the interphase accumulation induced by hydroxyurea on proliferating plant cells.

Cell distribution in different compartments of the cell cycle (G1, early, middle and late S, G2 and mitosis) has been studied during continuous treatments with hydroxyurea (HU) in onion root meristems by cytophotometric and autoradiographic methods. A sublethal dosis of HU (0.75 mM) has been chosen to allow a good wave of mitotic synchrony during recovery, with a negligible level of chromosomal aberrations. Proliferating cells begin the S period in the presence of HU and are accumulated in early S, where the maximum value (60%) is reached after 8 h of treatment; at the same time middle and late S are practically empty. In the presence of the drug, residual DNA synthesis allows a slow but continuous progress of cells throughout the S period. Differential sensitivity of S cells to HU can be observed; replication is more affected in early S (85% inhibition) than in the second half of the period (70% inhibition). On the other hand, G1 cells are not apparently affected by HU, while cells in G2 show a delay in their entrance into mitosis.     

Cyclic AMP-dependent and -independent protein phosphokinase activity in subcellular fractions of synchronously growing leydig I-10 cells.

The Leydig I-10 tumor cell line was synchronized by the double thymidine block method using 1.0 mM thymidine. Protein phosphokinase activity of subcellular fractions was determined at various times throughout the cell cycle. Microsomal cAMP-independent kinase activity increased in G2 and decreased during the S and G1 phases. Except for relatively small increases during the G1 and late S phases, microsomal cAMP-dependent kinase activity remained unchanged throughout most of the cycle. In the lysosomal-mitochondrial fraction, cAMP-dependent and cAMP-independent protein kinase activity increased during the S phase. Independent kinase activity peaked again during G1, while the dependent kinase became depressed. Phosphokinase activity increased in the nuclear fraction in late G2 and during mitosis, and was due to increases in both cAMP-independent and cAMP-dependent kinase activity. Cytosol cAMP-dependent kinase activity increased in G2 and during mitosis; cAMP-independent kinase activity showed some increased activity during late G2 and mitosis. These temporal variations in the subcellular kinase activities throughout the cell cycle may act to phosphorylate subcellular protein substrates in a cell cycle-specific fashion.     

DNA polymerase alpha in the fission yeast Schizosaccharomyces pombe: identification and tracing of the catalytic subunit during the cell cycle.

A recombinant protein was obtained in Escherichia coli by subcloning part of the Schizosaccharomyces pombe POL1 gene at the 3'-end of lacZ. Antibodies raised against this protein were used to identify the POL1 gene product in extracts of exponentially growing S. pombe cells. A major 170-kDa protein, whose structure and properties were typical of the catalytic subunit of eukaryotic DNA polymerases alpha (pol alpha), was detected. The same antibodies were used to trace pol alpha and to quantify its level during the S. pombe cell cycle. We found that pol alpha was present at all stages of the cycle and that its cellular pool was subject to limited (three-fold) increase in G1 and S phases, with a decline to the initial level soon after. In addition, we found that a second form of pol alpha with slightly lower molecular weight (165 kDa) existed only during late G1 and S phases. Moreover, absence of initiation or perturbations in the course of DNA replication induced overproduction of the 165-kDa form.     

Cycle specific association of nascent chromatin with nuclear envelope components in Physarum polycephalum.

The action of heparin on isolated nuclei derived from different phases of the mitotic cycle in plasmodia of Physarum polycephalum was studied. Heparin addition at two-fold excess over DNA concentration to nuclei in Mg-free low ionic strength buffer (10 mM TRIS-HC1, 10 mM Na2 HPO4, pH = 8) releases 60-80% of chromatin from S, G2, and mitotic phase nuclei. The RNA/protein ratio of herparin-solubilized cromatin is constant through S and G2 phases, but rises about two-fold at early prophase coincident with nucleolar breakdown. Purified nuclear envelopes were obtained from heparin-treated nuclei by sedimentation according to Bornens procedures (Nature 244, 28, 1973), and examined by transmission electron microscopy. Residual chromatin is seen at all stages with fine network of DNA fibrils in contact with the envelop. Regardless of time in S, 80% of 3H-labeled DNA was released into soluble chromatin with identical 3H/14C ratios. The residual chromatin in nuclear envelopes exhibited a preferential association of early S-DNA in nuclei engaged in early S replication, and late S preferential association in nuclei engaged in late S replication.     

The forkhead-associated domain protein Cep170 interacts with Polo-like kinase 1 and serves as a marker for mature centrioles

We report the characterization of Cep170, a forkhead-associated (FHA) domain protein of previously unknown function. Cep170 was identified in a yeast two-hybrid screen for interactors of Polo-like kinase 1 (Plk1). In human cells, Cep170 is constantly expressed throughout the cell cycle but phosphorylated during mitosis. It interacts with Plk1 in vivo and can be phosphorylated by Plk1 in vitro, suggesting that it is a physiological substrate of this kinase. Both overexpression and small interfering RNA (siRNA)-mediated depletion studies suggest a role for Cep170 in microtuble organization and cell morphology. Cep170 associates with centrosomes during interphase and with spindle microtubules during mitosis. As shown by immunoelectron microscopy, Cep170 associates with subdistal appendages, typical of the mature mother centriole. Thus, anti-Cep170 antibodies stain only one centriole during G1, S, and early G2, but two centrioles during late G2 phase of the cell cycle. We show that Cep170 labeling can be used to discriminate bona fide centriole overduplication from centriole amplification that results from aborted cell division.     

Fibroblast growth regulatory factor inhibits DNA synthesis in BALB/c 3T3 cells by arresting in G1

A cell surface macromolecular component from quiescent BALB/c 3T3 mouse cells (designated fibroblast growth regulatory factor, FGRF) inhibits DNA synthesis and cell division in growing 3T3 cells. Addition of FGRF to synchronized populations of growing 3T3 cells in the late G1 or early S phase did not inhibit DNA synthesis in the immediate S phase. However, a significant inhibition was observed in the S phase of the next round of cell cycle. Cells exposed to the regulatory factor in late S/early G2 or early G1 showed reduced DNA synthesis in the upcoming S phase; the late S/early G2 cells were more sensitive to inhibition than the cells in the G1. Further, the regulatory factor delayed the progression of G0/G1-arrested cells into the next S phase. These results suggest that the physiological effect of FGRF is to arrest cells in early G1, thus preventing their entry into a new round of cell cycle. In contrast to untransformed 3T3 cells, mouse cells transformed by SV40 were not subjected to growth-arrest by the regulatory factor, although the transformed cells contain active FGRF that inhibits DNA synthesis in growing 3T3 cells.     

Identification of proteins whose synthesis is modulated during the cell cycle of Saccharomyces cerevisiae

We examined the synthesis and turnover of individual proteins in the Saccharomyces cerevisiae cell cycle. Proteins were pulse-labeled with radioactive isotope (35S or 14C) in cells at discrete cycle stages and then resolved on two-dimensional gels and analyzed by a semiautomatic procedure for quantitating gel electropherogram-autoradiographs. The cells were obtained by one of three methods: (i) isolation of synchronous subpopulations of growing cells by zonal centrifugation.; (ii) fractionation of pulse-labeled steady-state cultures according to cell age; and (iii) synchronization of cells with the mating pheromone, alpha-factor. In confirmation of previous studies, we found that the histones H4, H2A, and H2B were synthesized almost exclusively in the late G1 and early S phases. In addition, we identified eight proteins whose rates of synthesis were modulated in the cell cycle, and nine proteins (of which five, which may well be related, were unstable, with half-lives of 10 to 15 min) that might be regulated in the cell cycle by periodic synthesis, modification, or degradation. Based on the time of maximal labeling in the cell cycle and on experiments with alpha-factor and hydroxyurea, we assigned the cell cycle proteins to two classes: proteins in class I were labeled principally in early G1 phase and at a late stage of the cycle, whereas those in class II were primarily synthesized at times ranging from late G1 to mid S phase. At least one major control point for the cell cycle proteins occurred between "start" and early S phase. A set of stress-responsive proteins was also identified and analyzed. The rates of synthesis of these proteins were affected by certain perturbations that resulted during selection of synchronous cell populations and by heat shock.     

Induction of 68,000-dalton heat shock proteins by cyclopentenone prostaglandins. Its association with prostaglandin-induced G1 block in cell cycle progression

Cyclopentenone prostaglandins (PGs) such as PGA2 and delta 12-PGJ2 act specifically on cells in the G1 phase and induce block of cell cycle progression (Ohno, K., Sakai, T., Fukushima, M., Narumiya, S., and Fujiwara, M. (1988) J. Pharmacol. Exp. Ther. 245, 294-298). In this study, we characterized proteins induced by these PGs in HeLa S3 cells of synchronized growth and examined its association with the cell cycle block. HeLa S3 cells transiently expressed two 68-kDa proteins of isoelectric points of 5.5 and 5.6 in the G1 phase of cell cycle. When G1-enriched cells were incubated with either PGA2 or delta 12-PGJ2, synthesis of these proteins was markedly enhanced. Enhancement by delta 12-PGJ2 was persistent and irreversible, whereas that by PGA2 was reversible. delta 12-PGJ2 also enhanced the synthesis of two additional 68-kDa proteins with isoelectric points of 5.8 and 5.9. On two-dimensional gel electrophoresis, these proteins overlapped exactly with the 68-kDa heat shock proteins induced in cells treated at 43 degrees C for 90 min. They were also indistinguishable from the heat shock proteins in limited proteolysis. When delta 12-PGJ2 was incubated with G2/M phase cells, it induced only a small and transient increase in the 68-kDa proteins. These results suggest that cyclopentenone PGs extensively induce 68-kDa heat shock proteins in the G1 phase HeLa S3 cells and this induction is closely associated with the G1 block of cell cycle progression caused by these PGs.     

Roles for the E4 orf6, orf3, and E1B 55-Kilodalton Proteins in Cell Cycle-Independent Adenovirus Replication

Adenoviruses bearing lesions in the E1B 55-kDa protein (E1B 55-kDa) gene are restricted by the cell cycle such that mutant virus growth is most impaired in cells infected during G(1) and least restricted in cells infected during S phase (F. D. Goodrum and D. A. Ornelles, J. Virol. 71:548-561, 1997). A similar defect is reported here for E4 orf6-mutant viruses. An E4 orf3-mutant virus was not restricted for growth by the cell cycle. However, orf3 was required for enhanced growth of an E4 orf6-mutant virus in cells infected during S phase. The cell cycle restriction may be linked to virus-mediated mRNA transport because both E1B 55-kDa- and E4 orf6-mutant viruses are defective at regulating mRNA transport at late times of infection. Accordingly, the cytoplasmic-to-nuclear ratio of late viral mRNA was reduced in G(1) cells infected with the mutant viruses compared to that in G(1) cells infected with the wild-type virus. By contrast, this ratio was equivalent among cells infected during S phase with the wild-type or mutant viruses. Furthermore, cells infected during S phase with the E1B 55-kDa- or E4 orf6-mutant viruses synthesized more late viral protein than did cells infected during G(1). However, the total amount of cytoplasmic late viral mRNA was greater in cells infected during G(1) than in cells infected during S phase with either the wild-type or mutant viruses, indicating that enhanced transport of viral mRNA in cells infected during S phase cannot account for the difference in yields in cells infected during S phase and in cells infected during G(1). Thus, additional factors affect the cell cycle restriction. These results indicate that the E4 orf6 and orf3 proteins, in addition to the E1B 55-kDa protein, may cooperate to promote cell cycle-independent adenovirus growth.     

Cell cycle dependent expression and stability of the nuclear protein detected by Ki-67 antibody in HL-60 cells

The expression and stability of the proliferation-associated nuclear antigen detected by Ki-67 antibody have been investigated in human promyelocytic leukaemic HL-60 cells in relation to their progression through the cell cycle. Expression of this antigen was minimal in late G1 and early S phase cells. The antigen accumulated in the cells predominantly during S phase, and its rate of increase per cell accelerated during the second half of this phase. The accumulation of Ki-67 antigen during S exceeded the increase in DNA content, and thus the Ki-67/DNA ratio rose 80% from late G1 to G2 + M. This antigen rapidly disappeared from post-mitotic cells. The half-life of this protein estimated in post-mitotic cells during stathmokinesis induced by vinblastine appeared to be shorter than 1 h. This rapid turnover should be compared with the relatively long (6-8 h) duration of G1 of the studied cells. In cells in which de novo protein synthesis was inhibited by 0.1 microgram/ml cycloheximide, the half-life of the Ki-67 antigen was also found to be about 1 h regardless of the cell position in the cell cycle. Thus, the data suggest that variations in the level of this protein during the cell cycle are a consequence of its different synthesis rate rather than phase-specific changes in the rate of its degradation. Because the late G1 and very early S phase cells express the antigen at levels only slightly above background, it is possible that, when using Ki-67 antibody as a marker of the cell growth fraction, some late G1 cells can be erroneously classified as non-cycling cells.     

Human Orc2 localizes to centrosomes, centromeres and heterochromatin during chromosome inheritance

The initiation of DNA replication in S phase requires the prior assembly of an origin recognition complex (ORC)-dependent pre-replicative complex on chromatin during G1 phase of the cell division cycle. In human cells, the Orc2 subunit localized to the nucleus as expected, but it also localized to centrosomes throughout the entire cell cycle. Furthermore, Orc2 was tightly bound to heterochromatin and heterochromatin protein 1alpha (HP1alpha) and HP1beta in G1 and early S phase, but during late S, G2 and M phases tight chromatin association was restricted to centromeres. Depletion of Orc2 by siRNA caused multiple phenotypes. A population of cells showed an S-phase defect with little proliferating cell nuclear antigen (PCNA) on chromatin, although MCM proteins remained. Orc2 depletion also disrupted HP1 localization, but not histone-H3-lysine-9 methylation at prominent heterochromatic foci. Another subset of Orc2-depleted cells containing replicated DNA arrested with abnormally condensed chromosomes, failed chromosome congression and multiple centrosomes. These results implicate Orc2 protein in chromosome duplication, chromosome structure and centrosome copy number control, suggesting that it coordinates all stages of the chromosome inheritance cycle.     

Human DNA polymerase epsilon colocalizes with proliferating cell nuclear antigen and DNA replication late, but not early, in S phase.

DNA polymerase epsilon (pol epsilon) has been implicated in DNA replication, DNA repair, and cell cycle control, but its precise roles are unclear. When the subcellular localization of human pol epsilon was examined by indirect immunofluorescence, pol epsilon appeared in discrete nuclear foci that colocalized with proliferating cell nuclear antigen (PCNA) foci and sites of DNA synthesis only late in S phase. Early in S phase, pol epsilon foci were adjacent to PCNA foci. In contrast to PCNA foci that were only present in S phase, pol epsilon foci were present throughout mitosis and the G(1) phase of cycling cells. It is hypothesized from these observations that pol epsilon and PCNA have separate but associated functions early in S phase and that pol epsilon participates with PCNA in DNA replication late in S phase.     

Lipid biosynthesis and its coordination with cell cycle progression

The activation of cell cycle regulators at the G1/S boundary has been linked to the cellular protein synthesis rate. It is conceivable that regulatory mechanisms are required to allow cells to coordinate the synthesis of other macromolecules with cell cycle progression. The availability of highly synchronized cells and flow cytometric methods facilitates investigation of the dynamics of lipid synthesis in the entire cell cycle of the heterotrophic dinoflagellate Crypthecodinium cohnii. Flow cytograms of Nile red-stained cells revealed a stepwise increase in the polar lipid content and a continuous increase in neutral lipid content in the dinoflagellate cell cycle. A cell cycle delay at early G1, but not G2/M, was observed upon inhibition of lipid synthesis. However, lipid synthesis continued during cell cycle arrest at the G1/S transition. A cell cycle delay was not observed when inhibitors of cellulose synthesis and fatty acid synthesis were added after the late G1 phase of the cell cycle. This implicates a commitment point that monitors the synthesis of fatty acids at the late G1 phase of the dinoflagellate cell cycle. Reduction of the glucose concentration in the medium down-regulated the G1 cell size with a concomitant forward shift of the commitment point. Inhibition of lipid synthesis up-regulated cellulose synthesis and resulted in an increase in cellulosic contents, while an inhibition of cellulose synthesis had no effects on lipid synthesis. Fatty acid synthesis and cellulose synthesis are apparently coupled to the cell cycle via independent pathways.     

Control of nucleolar growth during interphase in higher plant meristem cells

Summary The evolution of nuclear and nucleolar sizes throughout interphase have been studied in synchronous caffeine-labeled binucleate cells of onion root meristems by using silver impregnation and stereological methods over semithin sections. Nucleus and nucleolus grow independently, since nucleolus enlarges at its fastest rate in G 1, while nucleus grows mostly in two periods: onset of replication and G 2. Nucleolar size in the cycle seems to be a genecontrolled function, hardly affected by protein synthesis inhibition. Hence, there is a biphasic response to cycloheximide (CHM) in the fast growing nucleoli of both early and late G 1 with an initial stimulation later counterbalanced by a depressed rate, so that nucleolar size in S was similar to control shortly afterwards the start of the CHM treatment. The initial enlargement under CHM was due to an increase of all nucleolar structural components, i.e., fibrillar, granular, vacuolar, and lacunar regions. No cycloheximide effect whatsoever was detected in S and G 2 nucleoli.Abbreviations CHM cycloheximide - F fibrillar component - G granular component - L lacunae - V vacuoles - VN nuclear volume - VNu nucleolar volume - VvNu volume density of the nucleoli