A structural model for the cholesterol-phosphatidylcholine complexes in bilayer membranes

Based on the structural properties of phospholipid and cholesterol molecules, and making use of the known structural and motional effects of cholesterol and its analogs on phospholipid bilayers, a model for the cholesterol-phosphatidylcholine complex is proposed. In this model, the 3beta-hydroxyl group of cholesterol is assumed to engage in hydrogen bonding with the carbonyl oxygen of the fatty acyl groups in phospholipids. Some specific configurations of the saturated and unsaturated fatty acyl chains of the phospholipid are suggested to participate in van der Waals attractive interactions with the apha and beta surface of the steroid nucleus.

Isolation of transcriptionally active chromatin from mammalian nucleoli

Nucleoli isolated from HeLa cells are functionally active but contain large amounts of RNA and proteins (RNA/DNA ratio 1:1; protein/DNA ratio 7:1). We have isolated from the nucleolus a DNA-protein complex that has the characteristics of nucleolar chromatin (RNA/DNA ratio less than 0.05:1; protein/DNA ratio 1.7:1). This nucleolar chromatin has most of the transcriptional activity of the intact nucleolus and, as assayed by circular dichroism and dye binding, has largely preserved its structure. The isolation of a transcriptionally active, fragment of chromatin, which constitutes only a small part of the total genome and codes for only one recognizable product, offers several advantages for the study of chromatin structure and function.

The role of nuclei and nucleoli in the control of cell proliferation

A number of changes have been reported to occur in chromatin of quiescent cells stimulated to proliferate. These changes, indicative of increased transcriptional activity, occur in the early prereplicative phase, several hours before the onset of DNA replication, and are detectable not only in chromatin, but also in isolated nuclei and in whole cells. Most of the increased transcriptional activity can be attributed to an increased activity of the nucleolus; however, extranucleolar genes are also important in the regulation of the cell cycle flow, from G0 to S.

Circular dichroism studies of ethidium bromide binding to the isolated nucleolus

Circular dichroism in the 300-360 nm region and fluorescence induced by intercaltating binding of ethidum bromide to both DNA and RNA components were studied in isolated HeLa nucleoli. Both DNA and RNA compoents contribute to the induced dichroic elliticity. Digestion of nucleoli by RNase or DNase shows that most of the induced ellipticity comes from the DNA component. In nucleoli with an RNA/DNA = 0.8/1.0 the RNA component gives only 20% of the total ellipticity when measured at an ethidium bromide/DNA = 0.25. Spectro-fluorometric titration shows that ethidium bromide intercalates mostly into DNA in nucleoli. Both circular dichroism and fluorescence studies indicate that both DNA and RNA components in isolated nucleoli are less accessible to intercalating binding by ethidium bromide when compared to purified nucleolar DNA, DNA in chromatin or purified ribosomal RNA. Circular dichroic measurements of intercalating binding of ethidium bromide to to nucleoli may be used to study changes in nucleoli under different physiological or pathological conditions.

Circular dichroic studies of the DNA and RNA of nucleoli

Circular dichroism (CD) in the 240-300-nm region was used to study the conformation of DNA and RNA complexed with proteins in isolated nucleoli form HeLa cells. Deoxyribonuclease or ribonuclease digestion was employed to obtain (1) the individual CD spectra of nucleolar DNA or RNA in complex form with proteins, or in free form; and (2) the experimental CD baseline correction to exclude contributions from nonnucleic acid sources such as light scattering artifacts and proteins. The CD spectrum of nucleolar DNA in DNA-protein complexes was highly reduced in ellipticity in comparison with protein-free DNA. It showed a positive peak at 283 nm with a molar ellipticity [theta]283 = 1200 deg cm2 dmol-1 and a crossover at 262 nm. Addition of sodium dodecylsulfate shifted the peak to 276 nm with [theta]276 8000 deg cm2 dmol-1 and a crossover at 254 nm. The CD spectrum of nucleolar RNA in RNA-protein complexes was also reduced in comparison with protein-free RNA, showing a peak at 269 nm ([theta]269 = 6900 deg cm2 dmol-1), and a crossover at 250 nm. Addition of sodium dodecyl sulfate shifted the peak to 265 nm with [theta]265 = 18 000 deg cm2 dmol-1 and a crossover at 246 nm. The low ellipticity of both nucleolar DNA and RNA when complexed with proteins was increased by treatment with sodium chloride, urea, or heparin. This suggests that some ionic, hydrophobic, and hydrogen bondings are involved in the nucleic acid-protein interaction in nucleolar chromatin similar to that observed in nuclear chromatin.

Structure in the polar head region of phospholipid bilayers: A 31P [1H] nuclear Overhauser effect study

The structure of the head-group region of some phospholipid bilayers in vesicle form has been studied and an intermolecular association of the N-methyl protons of phosphatidylcholine (PC) with the phosphate of phosphatidylethanolamine (PE) in mixed vesicles has been identified. Observation of a 31P[1H] nuclear Overhauser effect (NOE) in the phosphorus nuclear magnetic resonances of both PC and PE in mixed vesicles demonstrates an intimate dipolar interaction between some protons and the phosphorus nuclei. Substitution of deuterium for the N-methyl protons of PC eliminated the majority of the effect and necessitated the construction of a model of the bilayer surface in which the N-methyl protons of PC could interact closely with the phosphates of neighboring PE molecules. The predominant orientation of the head group must then be parallel to the bilayer surface. The amino protons of PE do not contribute significantly to the observed NOE. A corollary of these results is that there is little if any tendency for either PC or PE in the mixed vesicles to segregate into separate domains. A decrease in NOE in sphingomyelin vesicles on going from H2O to D2O suggests that an exchangeable proton contributes to the NOE. In addition the low value of the NOE observed in D2O suggests that the head-group conformation of sphingomyelin differs from that of PC.

Preferential inhibition of herpes-group viruses by phosphonoacetic acid: effect on virus DNA synthesis and virus-induced DNA polymerase activity

In tissue culture phosphonoacetic acid (PAA) specifically inhibited DNA synthesis of human cytomegalovirus (CMV), murine CMV, simian CMV, Epstein-Barr virus, and Herpesvirus saimiri. Fifty to one hundred micrograms per milliliter PAA completely inhibited viral DNA synthesis with no significant damage to host cell DNA synthesis. In vitro DNA polymerization assays showed that 10 μg/ml of PAA specifically inhibited partially purified human CMV-induced DNA polymerase, while little inhibition of host-cell DNA polymerase activity was found. The specific inhibition of herpes-group virus DNA synthesis with little toxicity to host cells suggests that PAA has great potential as an antiherpesvirus therapeutic agent.

Headgroup conformation and lipid--cholesterol association in phosphatidylcholine vesicles: a 31P(1H) nuclear Overhauser effect study

The nuclear Overhauser effect has been observed in the nuclear magnetic resonance spectra of 31P. The information content of the nuclear Overhauser effect has been applied to the structure and dynamic properties of phosphatidylcholine vesicles. In the vesicles only 1/3 of the theoretical maximum nuclear Overhauser effect enhancement is observed. This result is accounted for by dipolar interactions between the N-methyl protons and the phosphate of phosphatidylcholine, and a correlation time for internal motion of 1.4 X 10(-9) sec. Addition of up to 30% cholesterol does not change the nuclear Overhauser effect enhancement or spin-lattice relaxation time of the vesicles. It is argued that the OH group of cholesterol is hydrogen bonded to the ester carbonyl oxygen of the phosphatidylcholine molecules.

Further characterization of gradient-fractionated submitochondrial membrane fragments from beef heart mitochondria

We have recently reported that with a linear sucrose density gradient centrifugation two distinct types of membrane fragments, designated as X- and Y-fragments are obtained (Huang, C. H., Keyhani, E. and Lee, C. P. (1973) Biochim. Biophys. Acta 305, 455-473). Further characterization of these two membranes fragments is reported. (1) Potassium chloride at the concentration of 0.15 m extracts 7% and 30% of cytochrome c from the X- and Y-fragments, respectively. (2) When cytochrome c was added to the mitochondrial suspension prior to sonication, the cytochrome c content was increased by 6-8-fold in both X- and Y-fragments. Subsequently KC1 extraction resulted in loss of cytochrome c by 1/4 in the X- and by 2/3 in the Y-fragments. (3) With partially inhibitory concentrations of KCN, cytochrome c in either the X- or the KC1 extracted X-fragments showed uncoupler-sensitive, biphasic reduction kinetics upon the addition of NADH to the oligomycin-supplemented system. Under identical conditions rapid first order reduction kinetics were seen for cytochrome c in Y-fragments supplemented with either oligomycin or oligomycin + carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP). (4) When cytochrome c was added to the mitochondrial suspension after sonication, a significant amount of cytochrome c was bound to both X- and Y-fragments, but was readily removed with a high ionic strength medium. (5) Lubrol had little effect on the ATPase activity of the X- and the Y-fragments, suggesting a lack of membrane-buried ATPase. (6) Partial depletion of ATPase in X-fragments did not induce an increase in reactivity towards externally added cytochrome c. (7) Both the X- and the Y-fragments showed an energy-linked fluorescence enhancement of 8-anilinonaphthalene-1-sulfonate and an energy-linked fluorescence decrease of quinacrine. (8) In the presence of K-+ nigericin alone or in combination with valinomycin exhibited a stimulating effect on the rate of NADH oxidase of the oligomycinsupplemented X- and Y-fragments.

Differential interaction of cholesterol with phosphatidylcholine on the inner and outer surfaces of lipid bilayer vesicles

The separate identification of the -N(+)-(CH(3))(3) groups located on the outside and inside of phospholipid bilayers observed in proton magnetic resonance spectroscopy upon addition of low concentrations of praseodymium ion was exploited to investigate the effects of incorporated cholesterol. Only about 7% of the phosphate groups on the outside of the bilayer belong to a class of strong binding or shifting sites. Upon addition of up to about 30% cholesterol to egg lecithin bilayers, no changes in chemical shift or ratio of areas of peaks due to outer and inner -N(+)(CH(3))(3) groups appear. At about 30% incorporated cholesterol, an abrupt decrease occurs in the chemical-shift difference between -N(+)(CH(3))(3) groups located on the outer and inner bilayer surfaces, and an abrupt increase occurs in the ratio of the areas of the two peaks. For L-alpha-dipalmitoyl lecithin bilayers, an abrupt change in chemical-shift difference occurring between 10 and 20% cholesterol is not accompanied by a change in the relative number of -N(+)(CH(3))(3) groups located on the outer and inner surfaces. These results are interpreted as due to the homogeneous distribution of up to 30% cholesterol in egg lecithin bilayers. Above 30%, cholesterol is asymmetrically distributed in favor of the inner layer. In egg lecithin with a variety of polyunsaturated side chains, the side chains with the greater number of double bonds are preferentially displaced by high concentrations of cholesterol, which accounts for the increase in the ratio of outer to inner -N(+)(CH(3))(3) groups. Such preferential displacement by cholesterol cannot occur with the saturated L-alpha-dipalmitoyl lecithin. It is suggested that modified phospholipid vesicles of low radii of curvature may provide high concentrations of "active sites" present in membranes.

DNA of Epstein-Barr virus detected in tissue of Burkitt's lymphoma and nasopharyngeal carcinoma

The association of Epstein-Barr virus (EBV) DNA with African tumors was examined by the method of hybridization with complementary RNA. 22 out of 23 biopsies of Burkitt's lymphoma and 18 out of 23 biopsies of nasopharyngeal carcinoma were found to contain EBV DNA. The number of EBV genome equivalents per cell ranged from four to 113. Five out of 24 other African tumors, two adenocarcinomas, and one each of melanoma, reticulum cell sarcoma, and carcinoma of the antrum, also contained EBV DNA.