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At least 19 recordsLinked to original sources
PubMed1978

Genetic analysis of the G1 period: isolation of mutants (or variants) with a G1 perior from a Chinese hamster cell line lacking G1.

Cells of the Chinese hamster line V79-8 multiply without a G1 period (i.e., they are G1(-)) and have an average generation time of 9.5 hr. After mutagenesis and selection we have derived five stable mutants (or variants) of this line that have longer generation times. In each case the increase in generation time is due solely to the introduction of a G1 period into the cell cycle, with no measurable effect on S, G2, or M. Fusions among these five G1(+) mutant lines and another presumably nonmutant G1(+) line (V79-743) produce hybrid cells lacking a G1 period in all but one case. These complementation tests define five complementation groups among these six G1(+) cell lines. The six G1(+) lines represent five different causes or bases for the presence of a G1 period. The two G1(+) mutants belonging to complementation group V are temperature sensitive for expression of the G1(+) phenotype (G1 congruent with 0, 4, and 6 hr at 33 degrees , 37 degrees , and 39 degrees , respectively). In all cases the G1(-) state is dominant over the G1(+) state, suggesting that the presence of G1 represents a "deficient" condition. Mutants of this type may be useful in the analysis of the switch from G1(-) to G1(+) that occurs normally in cleaving embryos and in elucidation of the genetic mechanism(s) responsible for the presence of a measurable G1 in most cells.

Cell Cycle↗
PubMed1992

Characterization of centromere arrangements and test for random distribution in G0, G1, S, G2, G1, and early S' phase in human lymphocytes.

The arrangement of centromeres, cluster formation and association with the nucleolus and the nuclear membrane were characterized in human lymphocytes during the course of interphase in a cell-phase-dependent manner. We evaluated 3,893 cell nuclei categorized by five parameters. The centromeres were visualized by means of indirect immunofluorescent labeling with anti-centromere antibodies (ACA) contained in serum of patients with CREST syndrome. The cell nuclei were classified as G0, G1, S, G2, G1' and early S' phase by comparing microscopically identified groups of cell nuclei with flow cytometric determination of cell cycle stage of synchronized and unsynchronized lymphocyte cell cultures. Based on a discrimination analysis, a program was devised that calculated the probability for any cell nucleus belonging to the G0, G1, S, G2, G1' and early S' phase using only two microscopic parameters. Various characteristics were determined in the G0, S, and G2 stages. A transition stage to S phase within G1 was detected. This stage shows centromere arrangements not repeated in later cell cycles and which develop from the dissolution of centromere clusters in the periphery of the nucleus during G0 and G1. S phase exhibits various non-random centromere arrangements and associations of centromeres with the nucleolus. G1' and early S' phase of the second cell cycle display no characteristic centromere arrangement. The duplication of centromeres in G2 is asynchronous in two phases. For all cell phases a test for random distribution of the centromeres in the cell nucleus was performed. There is a distinct tendency for centromeres to be in a peripheral position during G0 and G1; this tendency becomes weaker in S phase. Although the visual impression is a seemingly random distribution of centromeres in G2 and G1', statistical analysis still demonstrates a significant deviation from random distribution in favor of a peripheral location. Only the early S phase of the second cell cycle shows no significant deviation from a random distribution.

Cell Cycle↗
PubMed1979

Inhibition of cellular transition from G1-resting to G1-prereplicative phase by aminonucleoside of puromycin.

Human embryonic lung fibroblasts (IMR-90 and WI-38) were arrested in the G1 phase of the cell cycle by serum deprivation and high population density. Within 1 hr after the addition of medium containing fresh serum, these cells showed an increase in rRNA synthesis. The inclusion of 100 micrograms per ml aminonucleoside of puromycin (AMS) in the fresh medium eliminated the serum stimulation of rRNA synthesis and prevented the cells from making the G1-resting phase to G1-prereplicative phase transition. AMS also prevented the synthesis of HnRNA normally found within 10 hr after serum stimulation. Serum-stimulated RNA synthesis in starved, SV-40 transformed fibroblasts (WI-38-VA-13 cells) was inhibited, but not completely prevented, by AMS indicating that transformed cells may produce specific RNA's that are not AMS-sensitive and that may be responsible for the failure of transformed cells to be arrested in G1.

Animals↗
PubMed1992

The retinoblastoma protein is partially phosphorylated during early G1 in cycling cells but not in G1 cells arrested with alpha-interferon.

The retinoblastoma protein (pRB) is thought to act as a tumour suppressor which is inactivated by phosphorylation. In quiescent (G0) cells pRB exists in a hypophosphorylated form (pRB110), but proliferating cells in G1 contain a significant proportion of phosphorylated pRB (pRB112-114). Studies of synchronized or elutriated cells have suggested that the phosphorylated forms of pRB disappear as cells pass from G2/M to G0/G1 and that pRB is phosphorylated again to pRB114 at the G1/S border. In this study we used two-parameter flow cytometry and cell sorting to isolate cycling cells in early and late G1 (G1A and G1B), and we show that partially phosphorylated pRB is present in cycling human lymphoid cells even in G1A. These G1A cells contain intermediate forms of pRB which become further phosphorylated to pRB112-114 as cells pass into G1B. Therefore pRB is at least partially phosphorylated from early G1 onwards. Cell cycle arrest by alpha-interferon (alpha-IFN) results in an accumulation of cells in both G1A and G1B, and these cells contain mainly pRB110. Since pRB110 is thought to prevent cell proliferation, the cytostatic effect of alpha-IFN may therefore occur by preventing the initial phosphorylation of pRB during or prior to G1A.

Cell Cycle↗
PubMed1977

Regulation of DNA synthesis: age-dependent cooperation among G1 cells upon fusion.

The object of this study was to determine whether the inducer(s) of DNA synthesis in mammalian cells accumulates gradually throughout the G1 period or becomes available suddenly at the G1-S transition. HeLa cells, synchronized at various points in the G1 period, were fused by using UV-inactivated Sendai virus. Early G1 cells were fused with mid-G1 or late G1 cells and late G1 cells were fused with mid-G1 cells. The G1 traverse of mono-, bi-, and trinucleated cells was studied. The bi- and trinucleated cells of mid-G1 and late G1 parents traversed the G1 period significantly faster than did their mononucleated counterparts. The reduction in the duration of the G1 period was proportional to the number and age of nuclei at the time of fusion. There was no significant difference between the mono- and binucleated cells of the early G1 parent in their rates of entry into S period. In light of these findings, a model is proposed in which the inducer(s) of DNA synthesis accumulates gradually throughout the G1 period, reaching a critical level at the G1-S boundary when DNA replication is initiated; after reaching a peak during early or mid-S period, it declines to below the critical level when DNA synthesis ceases.

Cell Division↗
PubMed1979

Different Chinese hamster cell lines express a G1 period for different reasons.

Previous studies from our laboratory have shown that the absence of G1(G1-condition) in two lines of Chinese hamster cells is dominant over the presence of G1(G1+condition) in a variety of intraspecific cell hybrids. G1+ mutants or variants cna be isolated from G1- cells following mutagenesis and selection. These G1+ mutants fall into multiple complementation groups based on their abilities to form G1- cell hybrids with one another. This is evidence that different mutants have G1 intervals for different reasons, possibly as the result of deficiencies in functions necessary for G1- cell cycles. In this report we have used cell hybrid analysis to ask whether cells of different, naturally occurring G1+ lines of Chinese hamster are able to complement to produce G1- hybrids. We have found three complementation groups among the four G1+ cell lines examined. Therefore, these lines define three different reasons or bases for the existence of a G1 interval. These results lead us to suggest that multiple requirements must be met for these cells to start the S period, but that failure to fulfill only a single and different requirement is responsible for the presence of a G1 interval in any given cell line.

Animals↗
PubMed1979

Dexamethasone induces irreversible G1 arrest and death of a human lymphoid cell line.

Growth of a human leukemic T-cell line (CEM C7) in 10(-6) M dexamethasone results in inhibition of growth and rapid loss of cell viability after a delay of approximately 18 to 24 hours. Analysis of dexamethasone-treated cells by flow-microfluorometry showed that they were arrested in the G1 phase of the cell cycle. Loss of cell viability began at the same time as G1 accumulation was first detectable, and 20% of all cells were found to be blocked in G1 at this time suggesting that loss of viability and G1 arrest were coincident events. Half-maximal and maximal effects on both viability and G1 arrest after 48 hours in steroid were nearly identical with respect to steroid concentration and corresponded to half-maximal and full occupancy of glucocorticoid specific receptor by hormone, consistent with a glucocorticoid receptor mediated mechanism for both phenomena. Most non-viable cells were arrested in G1, and accumulation of cells in G1 was irreversible; removal of steroid in the presence of colcemid did not result in a decreased fraction of G1 cells. Furthermore, dexamethasone treatment did not protect cells against the effects of 33258 Hoechst-amplified killing of bromodeoxyuridine substituted cells exposed to light. These results show that dexamethasone arrests these leukemic cells in G1 and strongly suggest that dexamethasone-treated cells are killed upon entry into G1.

Cell Cycle↗
PubMed1979

Induction of G1- cells at high frequency.

Experimental conditions are reported on the induction of G1- cells at high frequency by ultraviolet irradiation of temperature-sensitive cells arrested in G1 phase at the restricted temperature. Neither the wild type cells of Balb/C-3T3 nor the temperature-sensitive derivative of tsA83 grown at the permissive temperature could be 'mutated' under similar conditions. One such 'mutant' (RI8) was 'inducible' from a G1+ phenotype at 33 degrees C to G1- state at 38 degrees C within one cell generation time. The 'inducible' property of RI8 from G1+ to G1- lends support to the theory that the duration of the G1+ phenotype is determined by the level of precursors for DNA synthesis. Since the G1- variants were isolated from revertants, the frequency of 'mutagenesis' appeared to be artifically high.

Animals↗
PubMed1992

SIT4 protein phosphatase is required for the normal accumulation of SWI4, CLN1, CLN2, and HCS26 RNAs during late G1.

In Saccharomyces cerevisiae, the RNA levels of the G1 cyclins CLN1, CLN2, and HCS26 increase dramatically during the late G1 phase of the cell cycle. The SIT4 gene, which encodes a serine/threonine protein phosphatase, is required for the normal accumulation of CLN1, CLN2, and HCS26 RNAs during late G1. This requirement for SIT4 in normal G1 cyclin RNA accumulation is at least partly via SWI4. Strains containing mutations in SIT4 are sensitive to the loss of either CLN2 or CLN3 function. At the nonpermissive temperature, temperature-sensitive sit4 strains are blocked for both bud emergence and DNA synthesis. Heterologous expression of CLN2 in the absence of SIT4 function results in DNA synthesis, but most of the cells are still blocked for bud emergence. Therefore, SIT4 is required for at least two late G1 or G1/S functions: the normal accumulation of G1 cyclin RNAs (which is required for DNA synthesis) and some additional function that is required for bud emergence or cell cycle progression through late G1 or G1/S.

Cyclins↗
PubMed1976

Non-histone protein synthesis during G1 phase and its relation to DNA replication.

The kinetics of non-histone chromosomal protein (NHCP) synthesis were studied in Chinese hamster ovary (CHO) plateau phase cells stimulated to proliferate and were compared to NHCP synthesis kinetics in two populations of synchronous G1 traversing cells. In all cases, NHCP synthesis rates increase 3- to 5-fold as cells traversed G1 and attained maximum values one hour before semi-conservative DNA replication began. Similar to results in synchronous G1 cells, the molecular weight distributions of the NHCP fraction from stimulated plateau phase cells underwent only minor changes, measured by sodium dodecylsulfate (SDS) polyacrylamide gel electrophoresis, as these cells moved toward S phase. Yet, during this progression after plateau phase and in the transition from early G1 to late G1 in synchronous cells, the total NHCP fraction increased significantly (1.5-2-fold) in amount per cell. These data indicate that plateau phase cells are similar to early G1 cells both in terms of their amounts of non-histone per cell and in their subsequent NHCP synthesis kinetics as they move toward S phase. These results extend previous findings which suggested that NHCP synthesis was coupled to DNA replication and demonstrate that the increased NHCP synthesis and accumulation in chromatin may be a biochemical marker for G1 progression.

Cell Division↗
PubMed1979

A probe into nuclear events during the cell cycle of Saccharomyces cerevisiae: studies of folded chromosomes in cdc mutants which arrest in G1.

The sedimentation behavior of folded chromosomes from cell-division-cycle (cdc) mutants which arrest in G1 was examined. At the restrictive temperature the folded genome of cdc 7, which arrests after spindle pole body (SPB) separation and spindle formation, cosediments with a standard g1 structure, indicating that by the cdc 7 step the g1 form of the folded genome has been assembled. In the mutant, cdc 4, which arrests before SPB separation but after SPB duplication, a standard g1 structure is not formed. cdc 4 cells, however, are able to enter G0 at the restrictive temperature, and the corresponding go structure is stable. These results indicate that the cdc 4 gene product may be involved in the development of folded genome conformation which leads to the g1 structure. Since the cdc 4 gene product is required for SPB separation, the g1 structure may be defined by an association between chromosomes and spindle components. The folded chromosomes of the "start" mutants cdc 25 and cdc 28 are unstable at the restrictive temperature. In contrast to cdc 4, neither cdc 25 nor cdc 28 are able to enter the G0 stage in a normal manner, i.e., the g0 structure is unstable at the restrictive temperature. The inference is that both the cdc 25 and cdc 28 gene products are required for the functional integrity of the folded genome at both a stage early in G1 and in the pathway to G0.

Cell Cycle↗
PubMed1976

G1 specific increases in cyclic AMP levels and protein kinase activity in Chinese hamster ovary cells.

Chinese hamster ovary cells were synchronized by selective detachment of cells in mitosis. The adenosine 3':5'-cyclic monophosphate (cyclic AMP) intracellular concentrations and cyclic AMP-dependent protein kinase activities were measured as these cells traversed G1 phase and entered S phase. Protein kinase activity, assayed in the presence or absence of saturating exogenous cyclic AMP in the reaction mixture, was lowest in early G1 phase (2 h after mitosis), increased 2-fold (plus exogenous cyclic AMP in reaction mixture) or 3.5-fold (minus cyclic AMP in reaction mixture) to maximum values in mid to late G1 phase (4-5 h after mitosis), and then decreased as cells entered S phase. Intracellular cyclic AMP concentrations were minimal 1 h after mitosis, increased 5-fold to maximum levels at 4-6 after mitosis, and decreased as cells entered S phase. Similar to the fluctuations in intracellular cyclic AMP, the cyclic AMP-dependent protein kinase activity ratio increased more than 40% in late G1 or early S phase. Puromycin (either 10 mug/ml or 50 mug/ml) administered 1 h after mitosis inhibited cyclic AMP-dependent protein kinase activity up to 50% by 5 h after mitosis, while similar treatment (10 mug/ml) had no effect on the increase in cyclic AMP formation. These data demonstrate that: (1) total protein kinase activity changed during G1 phase and this increase was dependent on new protein synthesis; (2) the increased intracellular concentrations of cyclic AMP were not dependent on new protein synthesis; and (3) the activation of cyclic AMP-dependent protein kinase was temporally coordinated with increased intracellular concentration of cycli AMP as Chinese hamster ovary cells traversed G1 phase and entered S phase. These results suggest that cyclic AMP acts during G1 phase to regulate the activation of cyclic AMP-dependent protein kinase.

Cell Line↗
PubMed1992

Maturation of Hantaan virus glycoproteins G1 and G2.

Hantaan virus-infected Vero E6 cell lysates were used for immunoprecipitation with monoclonal antibodies against glycoprotein G1 (MAbG1) or G2 (MAbG2). When cell lysates were prepared with buffer containing nonionic detergent, both G1 and G2 glycoproteins were precipitated with either MAbG1 or MAbG2. In contrast, when cell lysates were prepared with a buffer containing ionic detergents MAbG1 precipitated only glycoprotein G1 and MAbG2 precipitated only glycoprotein G2. Heterodimers and possibly higher oligomeric forms of the glycoproteins were detected on nonreducing SDS-polyacrylamide gels only after chemical cross-linking and immunoprecipitation with either MAbG1 or MAbG2. In order to determine the sites of Hantaan virus glycoproteins maturation and the G1-G2 complex formation, infected cells were treated with inhibitors that prevent specific steps of oligosaccharide processing. Furthermore, glycoproteins G1 and G2 immunoprecipitated from infected cell lysates or from isolated virus particles were tested for sensitivity to endoglycosidase H, endoglycosidase F, and endoglycosidase D. The results of these experiments show that maturation of both G1 and G2 takes place in the endoplasmic reticulum (ER). Furthermore, G1-G2 complex formation occurs in the ER as well, since the two glycoproteins co-precipitated with either MAbG1 or MAbG2 from infected cell lysates treated with brefeldin A and prepared with buffer containing nonionic detergent.

Animals↗
PubMed1975

Detection of G1 proteins in Chinese hamster cells synchronized by isoleucine deprivation or mitotic selection.

Examination of labeling patterns of proteins in Chinese hamster cells(line CHO) revealed the presence of a class of protein(s) that is synthesized during G1 phase of the cell cycle. Cells arrested in G1 by isoleucine (Ile) deprivation were prelabeded with [14-C]Ile, induced to traverse G1 by addition of unlabeled Ile, and labeled with [3-H]Ile at hourly intervals. Cells were fractionated into neclear and cytoplasmic portions, and proteins were separated by sodium dodecyl sulfate-polyacrylamide get electrophoresis. Gel profiles of proteins in the 45,000-160,000 mol wt range from the cytoplasm of cells in G1 were similar to those from cells arrested in G1 except for the presence of a mojor peak of [1-H]Ile incorporated into a protein(s) of approximately 80,000 mol wt. Peaks of net [3-H]Ile incorporation were not detected in neclear preparations. Cellular fractionation by differential centrifugation showed the peak I protein was located in the soluble supernatant fraction of the cytoplasm. Time-course studies showed that synthesis of this protein began 1-2 h after initiation of G1 traverse; the protein reached maximum levels in 4-6 h and was reduced to undetectable levels by 9 h. A cytoplasmic protein with similar electrophoretic mobility was found in G1 phase of cells synchronized by mitotic selection. This class of proteins is synthesized by cells before entry into S phase and may be involved in initiation of DNA synthesis.

Animals↗
PubMed1992

The Hantaan virus M-segment glycoproteins G1 and G2 can be expressed independently.

The two glycoproteins of Hantaan virus (HTV), G1 and G2, are encoded as a continuous single open reading frame in the M segment of the virion RNA. They are believed to be synthesized contemporaneously via a polypeptide precursor which is then processed to yield two glycoproteins, both of which appear in the Golgi complex of the cell. To study the properties of G1 and G2 as separate entities, we have constructed vaccinia virus recombinants which contain the sequences for each glycoprotein individually. Both glycoproteins made from these recombinants appear normal on sodium dodecyl sulfate-polyacrylamide gels compared with HTV products made in virus-infected cells. Interestingly, in the independently expressed G2 recombinant, a stretch of hydrophobic amino acids preceding the mature G2 N terminus appears to contain the signals necessary for translocation across membranes and proper glycosylation; partial deletion of this hydrophobic sequence results in production of an nonglycosylated form of G2. Thus, both G1 and G2 appear able to be expressed in an authentic fashion quite independently of each other, using their own signal sequences. In addition, it appears that the G1 from vaccinia virus recombinants contains the motif(s) necessary for cellular targeting of the HTV glycoproteins, while G2 from vaccinia virus recombinants remains strongly associated with the endoplasmic reticulum. In contrast, cells doubly infected with G1-vaccinia virus and G2-vaccinia virus recombinants show the G2 in a predominantly perinuclear (Golgi-like) distribution, presumably targeted there through association with G1. A carboxy-terminal deletion of G1 (2-43-Vac), which lacks 82 amino acids proximal to the start of the mature G2, retains a Golgi-like distribution.

Biological Transport↗
PubMed1992

Localization to the Golgi complex of Uukuniemi virus glycoproteins G1 and G2 expressed from cloned cDNAs.

The membrane glycoproteins G1 and G2 of Uukuniemi virus, a bunyavirus, accumulate in the Golgi complex (GC) during virus infection. These proteins have therefore been considered to be good models for studying the intracellular transport to and retention in the GC. In this study, I have used indirect immunofluorescence to localize in COS cells the Uukuniemi virus glycoproteins G1 and G2 expressed together or separately from cloned cDNAs with use of simian virus 40-based vectors. When expressed together from the full-length cDNA, G1 and G2 were correctly translocated, processed, and targeted to the GC, indicating that the information for GC targeting resides in the proteins. When the proteins were expressed separately, G1 was transported to the GC and retained there. In contrast, G2 could not be detected in the GC but was most probably retained and finally degraded in the endoplasmic reticulum. However, in cells cotransfected with G1 and G2 cDNAs, the proteins could both again be found in the GC. These results suggest that G1 is a responsible for targeting to and retention of the Uukuniemi virus glycoproteins in the GC. G2 would thus accumulate in the GC by virtue of its binding to G1.

Amino Acid Sequence↗
PubMed1976

Characterization of a G1 inhibitor from old JB-1 ascites tumor fluid. Interaction with polyions and ion exchangers.

In most experimental ascites tumors the growth rate decreases with increasing age and cell number. This decrease is caused by a prolongation of the cell cycle and an increasing accumulation of noncycling cells in resting (or quiescent) G1 and G2 compartments. In cell-free ascitic fluid from the JB-1 ascites tumor in the plateau phase of growth, low molecular weight substances have been found which reversibly and specifically arrest JB-1 cells in G1 and G2. In order to characterize the JB-1 G1 inhibitor we have investigated the effect of ion exchangers and polyions on the activity of this inhibitor assayed in vitro by means of a partially synchronized JB-1 cell population analyzed by flow microfluorometry. The results indicate that polyanions and cation exchangers (immobilized polyanions) bind and abolish the G1-inhibitory activity. From this it is suggested that the G1 inhibitor is of a basic or polycationic nature. Since anion exchangers (immobilized polycations) are without effect on this activity it was surprising to find that polycations also neutralize the activity. The results indicate that this occurs by blocking an anionic G2-inhibitor receptor on the cell, thus preventing the polycationic G1 inhibitor from being bound to this receptor.

Animals↗