The use of zonal centrifugation to study membrane formation during the life cycle of mammalian cells. Synthesis of 'marker' enzymes and other components of cellular organelles

Cell-cycle-specific biosynthesis of the photosynthetic membrane of Rhodopseudomonas sphaeroides. Structural implications

Structural changes association with the intracytoplasmic membrane during the cell cycle of the photosynthetic bacterium Rhodopseudomonas sphaeroides have been studied by freeze-fracture electron microscopy. The isolated intracytoplasmic membrane vesicles, chromatophores, were fused in order to obtain large fracture faces, allowing more precise measurements and statistical analysis of both intramembrane particle density and size determinations. The intramembrane particle density of the protoplasmic face (PF) of the intracytoplasmic membrane, (from 4970 to 8290/micrometers 2), was shown to be a linear function of the protein/phospholipid ratio (from 2.5 to 5.1, w/w) of the intracytoplasmic membrane. Under constant light intensity, both the average particle size and particle size distribution remained unchanged during the cell cycle. These results provide the structural basis for the earlier reported cell-cycle-specific variations in both protein/phospholipid ratio and alternation in phospholipid structure of the intracytoplasmic membrane of R. sphaeroides during photosynthetic growth. The average particle diameter in the PF face of the intracytoplasmic membrane was 8.25, 9.08 and 9.75 nm at incident light intensities of 4000, 500 and 30 ft X cd, respectively. When chromatophores were fused with small, unilamellar liposomes, the intramembrane particle density decreased as input liposome phospholipid increased, whereas the particle size remained constant and particle distribution became random.

Characterization of subcellular components in synchronized hepatoma cells as a function of the cell cycle

The specific activity and subcellular distribution of marker enzymes for the main subcellular components were analysed in homogenates of synchronized hepatoma cells (Morris 7288c), obtained by selective detachment at mitosis combined with a metaphase block with Colcemid. Markers for lysosomes, mitochondrial outer membrane, plasma membrane and cytosol are synthesized throughout the cycle at the same rate as the bulk of cellular protein. Larger variations are observed for a Golgi marker; after a decrease around mitosis, the specific activity of galactosyltransferase increases steadily from middle G(1)-phase on, and at the end of G(2)-phase it is nearly twice that observed at the beginning of G(1)-phase. Our results show that synthesis of cytochrome oxidase may occur preferentially in G(2)-phase. Large modifications of the density distribution of lysosomes are observed during the cell cycle; the median equilibrium density of lysosomal markers decreases in G(1)-phase, and some increase in soluble activity occurs at the same time. Reverse changes occur progressively during S- and G(2)-phases. At mitosis, Golgi galactosyltransferase shows a more dispersed distribution, and modifications in the density distribution of endoplasmic-reticulum NADPH-cytochrome c reductase are observed. The latter can be most easily explained by a detachment of ribosomes from endoplasmic-reticulum membranes. No significant modifications occur in mitochondrial and plasma-membrane markers.

Quantized generation time in mammalian cells as an expression of the cellular clock

The distribution of possible generation times in mammalian cells does not appear to be continous within the limits of range for each cell type; rather, generation time is quantized in multiples of 3-4 hr. Synchronous cultures of Chinese hamster V79 cells were prepared using manual and automated methods to select and stage mitotic cells. Using synchronous cultures and time-lapse video tape microscopy, it was possible to show that generation times within a population of mitotically selected cells normally disperse in a quantized fashion, with intervals of 3-4 hr occurring between bursts in division. In addition, at temperatures above 37 degrees, V79 cells have a 7.5-8.5 hr modal cell cycle, while at temperatures from 36.5 degrees to 33.5 degrees the modal cell cycle is 11-12 hr long. A survey of the synchrony literature reveals that the tendency to preferred generation times holds between cell lines. The distribution of modal generation times from a variety of different cell types forms a series with a similar interval but with a greater range of values than that observed here for V79 cells. To satisfy the published data and the work presented here, I propose a subcycle, Gq, which has a traverse time equal to the period of the clock. The period appears to be fixed at close to the same value in all mammalian somatic cells. The timekeeping mechanism appears to be temperature compensated, since the time required to traverse Gq is constant at temperatures between 34 degrees and 39 degrees. It is suggested that cell cycle time increases at lower temperatures, lower serum concentration, and high cell densitite because the number of rounds of traverse through Gq increases.

Variation in susceptibility of a human lymphoid cell line to immune lysis during the cell cycle. Lack of correlation with antigen density and complement binding

Cultured human lymphoid cells RPMI 8866 at different stages of their growth cycle vary in their susceptibility to lysis by rabbit, human, and guinea pig complement activated by HL-A antibodies or heterologous antibodies directed to membrane antigens; cells in G(1) phase are the least sensitive to lysis. To investigate the cause of differential susceptibility of cells RPMI 8866 to lysis, the expression of HL-A determinants and the ability of cells to react with complement were investigated. No change was detected in the density of HL-A antigens on RPMI 8866 cells in synchronous growth as determined by quantitative microabsorption assays, isotopic antiglobulin tests and yields of soluble HL-A antigens. Cells did not vary during the growth cycle in their ability to interact with complement components and in their capacity to activate the complement system through the classical or alternate pathway. These data suggest that variability in lytic susceptibility is due to changes in the structure of the cell membrane or in its ability to repair complement induced damage at certain intervals during the cell cycle. Therefore, this cell line constitutes a useful model to investigate the final steps of the cytolytic reaction.

Cell mitotic cycle synthesis of NIL hamster glycolipids including the Forssman antigen

The synthesis of phospholipids and glycolipids during the cell mitotic cycle of an established hamster line, NIL, has been studied. Cells were synchronized with excess thymidine and mitotically harvested by shaking. Cells were radioactively labeled for 4 h with palmitate, glucosamine, or galactose. Lipids were analyzed by thin-layer chromatography. As cells progressed through the mitotic cycle, incorporation into phospholipids increased but the fraction represented by each remained constant. Similarly, ceramide monohexoside, dihexoside, and hematoside were labeled equally in all phases. Ceramide trihexoside and tetrahexoside were labeled only during G(1) and S. Ceramide pentahexoside (the Forssman antigen) shows density-dependent synthesis, accumulation, and reactivity. Ceramide pentahexoside was labeled during all phases of the mitotic cycle but the rate of incorporation decreased in S and G(2). The total amount of lipid assayed immunologically in cell extracts gradually increased. Exposure of the Forssman antigen in untreated or trypsin-treated cells was studied using binding of chemically labeled antiForssman antiserum. The amount of antigen detected in trypsinized cells increased during G(1) and early S but then remained constant. Mitotic cells exposed all detectable antigen. As cells progressed through the mitotic cycle, a large fraction of the Forssman antigen became cryptic.

Oscillations of enzyme activities during the cell-cycle of a glucose-repressed fission-yeast Schizosaccharomyces pombe 972h-

1. Increased specific activities of cytochrome c oxidase, catalase, succinate dehydrogenase, succinate-cytochrome c oxidoreductase, NADH-cytochrome c oxidoreductase and malate dehydrogenase were observed during glucose de-repression of Schizosaccharomyces pombe. 2. The cell-cycle of this organism was analysed by three different methods: (a) harvesting of cells at intervals from a synchronous culture, (b) separation of cells by rate-zonal centrifugation into different size classes and (c) separation of cells by isopycnic-zonal centrifugation into different density classes. 3. Measurement of enzyme activities during the cell-cycle showed that all the enzymes assayed [cytochrome c oxidase, catalase, acid p-nitrophenylphosphatase, NADH-dehydrogenase, NADH-cytochrome c oxidoreductase, NADPH-cytochrome c oxidoreductase, succinate dehydrogenase, malate dehydrogenase, isocitrate dehydrogenase (NADP) and fumarate hydratase] show periodic expression as ;peaks'. 4. Cytochrome c oxidase shows a single maximum at 0.67 of a cycle, whereas succinate dehydrogenase exhibits two maxima separated by 0.5 of a cell-cycle. 5. All other enzymes assayed showed two distinct maxima per cell-cycle; for catalase, malate dehydrogenase and NADPH-cytochrome c oxidoreductase there is the possibility of multiple fluctuations. 6. The single maximum of cytochrome c oxidase appears at a similar time in the cycle to one maximum of each of the other enzymes studied, except for NADH dehydrogenase. 7. These results are discussed with reference to previous observations on the expression of enzyme activities during the cell-cycle of yeasts.