Tetrahymena: a system for studying dynamic membrane alterations within the eukaryotic cell

The bifunctional protein TtFARAT from Tetrahymena thermophila catalyzes the formation of both precursors required to initiate ether lipid biosynthesis

The biosynthesis of ether lipids and wax esters requires as precursors fatty alcohols, which are synthesized by fatty acyl reductases (FARs). The presence of ether glycerolipids as well as branched wax esters has been reported in several free-living ciliate protozoa. In the genome of Tetrahymena thermophila, the only ORF sharing similarities with FARs is fused to an acyltransferase-like domain, whereas, in most other organisms, FARs are monofunctional proteins of similar size and domain structure. Here, we used heterologous expression in plant and yeast to functionally characterize the activities catalyzed by this protozoan protein. Transient expression in tobacco epidermis of a truncated form fused to the green fluorescence protein followed by confocal microscopy analysis suggested peroxisomal localization. In vivo approaches conducted in yeast indicated that the N-terminal FAR-like domain produced both 16:0 and 18:0 fatty alcohols, whereas the C-terminal acyltransferase-like domain was able to rescue the lethal phenotype of the yeast double mutant gat1Δ gat2Δ. Using in vitro approaches, we further demonstrated that this domain is a dihydroxyacetone phosphate acyltransferase that uses preferentially 16:0-coenzyme A as an acyl donor. Finally, coexpression in yeast with the alkyl-dihydroxyacetone phosphate synthase from T. thermophila resulted the detection of various glycerolipids with an ether bond, indicating reconstitution of the ether lipid biosynthetic pathway. Together, these results demonstrate that this FAR-like protein is peroxisomal and bifunctional, providing both substrates required by alkyl-dihydroxyacetone phosphate synthase to initiate ether lipid biosynthesis.

Comprehensive analysis reveals dynamic and evolutionary plasticity of Rab GTPases and membrane traffic in Tetrahymena thermophila

Cellular sophistication is not exclusive to multicellular organisms, and unicellular eukaryotes can resemble differentiated animal cells in their complex network of membrane-bound structures. These comparisons can be illuminated by genome-wide surveys of key gene families. We report a systematic analysis of Rabs in a complex unicellular Ciliate, including gene prediction and phylogenetic clustering, expression profiling based on public data, and Green Fluorescent Protein (GFP) tagging. Rabs are monomeric GTPases that regulate membrane traffic. Because Rabs act as compartment-specific determinants, the number of Rabs in an organism reflects intracellular complexity. The Tetrahymena Rab family is similar in size to that in humans and includes both expansions in conserved Rab clades as well as many divergent Rabs. Importantly, more than 90% of Rabs are expressed concurrently in growing cells, while only a small subset appears specialized for other conditions. By localizing most Rabs in living cells, we could assign the majority to specific compartments. These results validated most phylogenetic assignments, but also indicated that some sequence-conserved Rabs were co-opted for novel functions. Our survey uncovered a rare example of a nuclear Rab and substantiated the existence of a previously unrecognized core Rab clade in eukaryotes. Strikingly, several functionally conserved pathways or structures were found to be associated entirely with divergent Rabs. These pathways may have permitted rapid evolution of the associated Rabs or may have arisen independently in diverse lineages and then converged. Thus, characterizing entire gene families can provide insight into the evolutionary flexibility of fundamental cellular pathways.

Single-celled organisms appear simple compared to multicellular organisms, but this may not be true at the level of the individual cell. In fact, microscopic observations suggest that protists can possess networks of organelles just as elaborate as those in animal cells. Consistent with this idea, recent analysis has identified large families of genes in protists that are predicted to act as determinants for complex membrane networks. To test these predictions and to probe relationships between cellular structures across a wide swath of evolution, we focused on one gene family in the single-celled organism Tetrahymena. These genes control the traffic between organelles, with each gene controlling a single step in this traffic. We asked three questions about each of 56 genes in the family. First, what is the gene related to in humans? Second, under what conditions is the gene being used in Tetrahymena? Third, what is the role of each gene? The results provide insights into both the dynamics and evolution of membrane traffic, including the finding that some pathways appearing both structurally and functionally similar in protists and animals are likely to have arisen independently in the two lineages.

Mass spectrometric imaging of highly curved membranes during Tetrahymena mating

Biological membrane fusion is crucial to numerous cellular events, including sexual reproduction and exocytosis. Here, mass spectrometry images demonstrate that the low-curvature lipid phosphatidylcholine is diminished in the membrane regions between fusing Tetrahymena, where a multitude of highly curved fusion pores exist. Additionally, mass spectra and principal component analysis indicate that the fusion region contains elevated amounts of 2-aminoethylphosphonolipid, a high-curvature lipid. This evidence suggests that biological fusion involves and might in fact be driven by a heterogeneous redistribution of lipids at the fusion site.

Cold-regulated genes under control of the cold sensor Hik33 in Synechocystis

A histidine kinase, Hik33, appears to sense decreases in temperature and to regulate the expression of certain cold-inducible genes in the cyanobacterium Synechocystis sp. PCC6803. To examine the role of Hik33 in the regulation of gene expression, we analysed a DeltaHik33 mutant using the DNA microarray technique. In wild-type cells, genes that were strongly induced at low temperature encoded proteins that were predominantly subunits of the transcriptional and translational machinery. Most cold-repressible genes encoded components of the photosynthetic machinery. Mutation of the hik33 gene suppressed the expression of some of these cold-regulated genes, which could be divided into three groups according to the effect of the mutation of hik33. In the first group, regulation of gene expression by low temperature was totally abolished; in the second group, the extent of such regulation was reduced by half; and, in the third group, such regulation was totally unaffected. These results suggest that expression of the genes in the first group is regulated solely by Hik33, expression of genes in the third group is regulated by an as yet unidentified cold sensor, and expression of genes in the second group is regulated by both these cold sensors.

Characterization of inositol phospholipids and identification of a mastoparan-induced polyphosphoinositide response in Tetrahymena pyriformis

The unicellular eukaryote Tetrahymena is a popular model for the study of lipid metabolism. Less attention, however, has been given to the inositol phospholipids of the cell, although it is known that this class of lipids plays an important role in eukaryotic cell signaling. Tetrahymena pyriformis phosphatidylinositol was isolated, purified, and characterized by proton nuclear magnetic resonance analysis and [2-(3)H]myoinositol labeling. Labeling was also used for polyphosphoinositide (phosphatidylinositol phosphate and phosphatidylinositol bisphosphate) identification. Tetrahymena inositol phospholipids were found to belong to the diacylglycerol group, although major Tetrahymena phospholipids, phosphatidylcholine and aminoethylphosphonoglycerides, have been found to be mainly alkylacylglyceroderivatives. Further characterization of Tetrahymena phosphatidylinositol by gas chromatographic analysis indicated that 80% of fatty acids were myristic acid and palmitic acid. This is also in contrast to the fatty acid profile of Tetrahymena phosphatidylcholine and phosphatidylethanolamine, with respect both to the fatty acid length and degree of unsaturation, and may indicate that specific diacylglycerol species are connected with the phosphatidylinositol metabolism in this cell. Treatment of [3H]inositol-labeled Tetrahymena cells with mastoparan, a G-protein-activating peptide, induced changes in the polyphosphoinositide levels, suggesting that inositol phospholipids may form in Tetrahymena a functional signaling system similar to that of higher eukaryotes. Addition of 10 microM mastoparan resulted in a rapid and transient increase in [3H]phosphatidylinositol phosphate followed by a decrease in [3H]phosphatidylinositol bisphosphate. Similar changes in lipids have been reported when phosphoinositide-phospholipase C pathway is activated in both animal and plant cells.

1-O-alkyl-2-acetyl-sn-glycero-3-phosphate:phosphohydrolase activity in Tetrahymena pyriformis

Within the frame of the de novo formation of Platelet-Activating Factor in Tetrahymena, the occurrence as well as the properties of a lipid phosphate phosphohydrolase activity catalyzing the dephosphorylation of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphate was investigated. The activity was distributed in all the membrane fractions of the cell and in the cytosol. It showed preference for acyl-acetyl-sn-glycero-phosphate as well, and at a much lower level, for dipalmitoyl-glycero-phosphate. Mg2+ and Ca2+ caused a dose-dependent inhibition, while F-, EDTA and EGTA had no effect. The enzymic activity was linear for at least up to 60 min incubation time and up to 150 microg protein. Microsomal activity exhibited two optimal pH areas, around 7.0 and 9.0, while mitochondrial activity showed one peak, at pH 7.0. Acyl-GP, acyl-acetyl-GP and alkyl-GP could replace alkyl-acetyl-GP in significant rates, while dipalmitoyl-GP, beta-GP, fructose-6-GP, p-nitrophenylphosphate, creatine phosphate or ATP had no effect. Side phospholipase A2 and C activities were also detected. Taking into account the presence of PAF and alkylacetylglycerol in the protozoan as well as the presence of a dithiothrcitol-insensitive CDP-choline:cholinephosphotransferase activity that converts alkylacetylglycerol to PAF, we suggest that the present phosphohydrolase activity may be involved in the de novo production of PAF within Tetrahymena.

Does the membrane's physical state control the expression of heat shock and other genes?

Membranes provide the structural framework that divides cells from their environment and that, in eukaryotic cells, permits compartmentation. They are not simply passive barriers that are liable to be damaged during environmental challenge or pathological states, but are involved in cellular responses and in modulating intracellular signalling. Recent data show that the expression of several genes, particularly those that respond to changes in temperature, ageing or disease, is influenced and/or controlled by the membrane's physical state.

Molecular cloning and cell-cycle-dependent expression of a novel NIMA (never-in-mitosis in Aspergillus nidulans)-related protein kinase (TpNrk) in Tetrahymena cells

With the intention of investigating the signal-transduction pathway that mediates the cold-stress response in Tetrahymena, we isolated a gene that encodes a novel protein kinase of 561 amino acids, termed Tetrahymena pyriformis NIMA (never-in-mitosis in Aspergillus nidulans)-related protein kinase (TpNrk), by differential display from Tetrahymena cells exposed to temperature shift-down. TpNrk possesses an N-terminal protein kinase domain that is highly homologous with other NIMA-related protein kinases (Neks) involved in the control of the cell cycle. The TpNrk protein is 42% identical in its catalytic domain with human Nek2, 41% identical with mouse Nek1 and 37% with A. nidulans NIMA. In addition, TpNrk and these NIMA-related kinases have long, basic C-terminal extensions and are therefore similar in overall structure. In order to further explore the function of the TpNrk gene and the association of the cold stress with the cell cycle of Tetrahymena, changes of TpNrk mRNA were determined during the course of the synchronous cell division induced by the intermittent heat treatment. The level of TpNrk transcription increased immediately after the end of the heat treatment, with a peak at 30 min, and declined thereafter reaching the minimum level when nearly 80% of the cells synchronously entered cell division (75 min after the end of heat treatment). The accumulation of TpNrk mRNA starting from 0 min to 30 min after the end of the heat treatment was assumed to be a prerequisite for the start of synchronous cell division. These results suggest that TpNrk may have a role in the cell cycle of Tetrahymena, and that mRNA expression, at least, is under tight cell-cycle control.

Cloning and sequencing of a cDNA encoding glyceraldehyde-3-phosphate dehydrogenase from Tetrahymena thermophila: growth-associated changes in its mRNA expression

Glyceraldehyde-3-phosphate dehydrogenase is a key enzyme in the glycolytic pathway. Since its transcript levels do not vary in most experimental conditions, it has been often used as a control in northern blot or reverse transcriptase-polymerase chain reaction analysis. We have cloned and sequenced a gene encoding glyceraldehyde-3-phosphate dehydrogenase (Tthgapdh) from Tetrahymena thermophila cDNA library and determined whether the Tthgapdh mRNA is a loading control in gene expression studies of T. thermophila cell. The open reading frame encoded a protein of 341 amino acid residues (36.8 kDa) containing a nicotinamide adenine dinucleotide-binding domain and a catalytic domain, which was highly similar to those of other organisms. Its mRNA levels at different growth stages were examined by northern blot analysis. The fragment of the isolated cDNA was hybridized to a 1.3-kb mRNA transcript. There was a marked increase in Tthgapdh mRNA level at the mid-exponential phase, followed by a gradual decrease. Therefore, much caution should be made to use Tthgapdh mRNA as an internal standard for northern blot analysis in Tetrahymena.

Effect of growth temperature on the biosynthesis of eukaryotic lipid molecular species by the cyanobacterium Spirulina platensis

The incorporation of linoleic acid added at mmolar concentrations to the culture medium of the photosynthetic prokaryote Spirulina platensis results in the synthesis of membrane glycerolipids with a eukaryotic distribution of fatty acid chain length on the glycerol backbone (Pham Quoc et al., Biochim. Biophys. Acta [1993] 1168, 94-99). This distribution contrasts with the usual prokaryotic one found in lipids of cyanobacteria. A subsequent desaturation of the exogenously supplied fatty acid resulted in a large increase of gamma-linolenic acid. In order to estimate the capacities of S. platensis for bioconversion of fatty acids in lipid classes, the effects of different temperatures of growth were studied in linoleic acid-supplemented cultures. The lipid composition was affected by growth temperature, the synthesis of SQDG was stimulated at low temperature. The molecular species of each lipid were isolated and analyzed. Whatever the temperature of growth, the biosynthesis of eukaryotic C18/C18 lipid molecular species was observed in all lipid classes. Furthermore, the proportion of eukaryotic lipids increased at low temperature (24 degrees C). The desaturation of C18 fatty acids at C1 and C2 positions of the glycerol moiety occurred and was further stimulated when the growth temperature was lowered. The resulting proportion of gamma-linolenic acid increased significantly in cultures supplemented with linoleate at low temperatures. Finally a pathway for the synthesis of eukaryotic lipids and the desaturation of fatty acids esterified to the acyl lipids of linoleate-supplemented S. platensis can be suggested.

Molecular cloning of delta 9 fatty acid desaturase from the protozoan Tetrahymena thermophila and its mRNA expression during thermal membrane adaptation

In response to a decrease in its growth temperature, the protozoan Tetrahymena is known to increase the level of unsaturated fatty acids in its membrane phospholipids so as to maintain the correct physical state (fluidity) of the membranes. In this organism, synthesis of unsaturated fatty acids is initiated by delta 9 acyl-CoA desaturase. Our previous studies have shown that, during cold adaptation, the activity of microsomal palmitoyl- and stearoyl-CoA desaturase increases, reaching a maximal level at 2 h after a temperature down-shift to 15 degrees C. Two hypotheses have been proposed to explain this increase in desaturase activity: (1) self-regulation via a direct effect of reduced membrane fluidity, and (2) induction of desaturase mRNA. However, the precise mechanism is not clearly understood. In order to obtain further insight into the mechanism of regulation of the desaturase, we have isolated a gene that encodes delta 9 fatty acid desaturase from T. thermophila and examined its expression during cold adaptation. The nucleotide sequence indicates that the 1.4 kbp gene encodes a polypeptide of 292 amino acid residues which shows marked sequence similarity to delta 9 acyl-CoA desaturases from other sources, e.g. rat, mouse, Amblyomma americanum and Saccharomyces cerevisiae. This protein has three histidine-cluster motifs (one HXXXXH and two HXXHH), and two hydrophobic regions which are conserved among delta 9 acyl-CoA desaturases. The level of desaturase mRNA was sensitive to decreasing the temperature of the culture media, and was close to maximal immediately after the temperature was shifted down from 35 degrees C to 15 degrees C (0.8 degrees C/min). Thereafter, the amount of mRNA gradually decreased with time, but remained above the control level for at least 5 h. Furthermore, during the course of the cooling process to 15 degrees C, the increased expression of desaturase mRNA became evident at 27 degrees C. Nuclear run-on analysis and actinomycin D chase experiments revealed that the elevation of the mRNA level was due to increases in both transcription and mRNA stability. These results suggest that the enhanced desaturase activity is controlled, at least in part, at the transcriptional level.

A voltage-dependent chloride channel from Tetrahymena ciliary membrane incorporated into planar lipid bilayers

Membrane vesicles from cilia of Tetrahymena thermophila were incorporated into a planar phospholipid bilayer membrane, and single-channel currents across the planar membrane were recorded under voltage-clamp conditions. A novel and reproducible chloride channel was observed when a mixture of phosphatidylethanolamine and phosphatidylcholine was used to form the planar lipid membrane but not when acidic phospholipid mixtures such as asolectin or a mixture containing phosphatidylserine. Using symmetrical 100 mM KCl solutions, the single-channel conductance of the fully open state (O1) was 73.1 pS, with sub-level (O2) conductance of 9.0 pS. The permeability ratio Pc1/Pk was calculated as 3.7, according to the Goldman-Hodgkin-Katz current equation. This channel exhibited characteristic voltage-dependent burst activities. With an increase in membrane potential, the lifetimes of both the burst and interburst states decreased. In the burst state, the frequency of transition between the O1 and O2 states was also voltage-dependent, mainly due to the decrease in the lifetime of the O1 state, with an increase in membrane potential. In addition, channel activity was inhibited by indanyloxyacetic acid-94 (IAA-94), an inhibitor of epithelial chloride channels.

Purinergically induced membrane fluidization in ciliary cells: characterization and control by calcium and membrane potential

To examine the role of membrane dynamics in transmembrane signal transduction, we studied changes in membrane fluidity in mucociliary tissues from frog palate and esophagus epithelia stimulated by extracellular ATP. Micromolar concentrations of ATP induced strong changes in fluorescence polarization, possibly indicating membrane fluidization. This effect was dosage dependent, reaching a maximum at 10-microM ATP. It was dependent on the presence of extracellular Ca2+ (or Mg2+), though it was insensitive to inhibitors of voltage-gated calcium channels. It was inhibited by thapsigargin and by ionomycin (at low extracellular Ca2+ concentration), both of which deplete Ca2+ stores. It was inhibited by the calcium-activated potassium channel inhibitors quinidine, charybdotoxin, and apamine and was reduced considerably by replacement of extracellular Na+ with K+. Hyperpolarization, or depolarization, of the mucociliary membrane induced membrane fluidization. The degree of membrane fluidization depended on the degree of hyperpolarization or depolarization of the ciliary membrane potential and was considerably lower than the effect induced by extracellular ATP. These results indicate that appreciable membrane fluidization induced by extracellular ATP depends both on an increase in intracellular Ca2+, mainly from its internal stores, and on hyperpolarization of the membrane. Calcium-dependent potassium channels couple the two effects. In light of recent results on the enhancement of ciliary beat frequency, it would appear that extracellular ATP-induced changes both in ciliary beat frequency and in membrane fluidity are triggered by similar signal transduction pathways.

Rapid effects of insulin on cyclic GMP location in an intact protozoan

We studied rapid changes in location of cyclic GMP in Tetrahymena pyriformis. Insulin caused cGMP localization in cilia and near the plasma membrane (0.5-1 min). Later (1-5 min) cGMP localization was diffuse in cytoplasm with perinuclear accentuation. Inactive insulin analogs did not elicit these changes.

Two separate pools of sphingomyelin in BHK cells

In BHK cells labelled to equilibrium with [3H]choline and treated with sphingomyelinase the surface pool of sphingomyelin is degraded very rapidly (half-time 10 min) but the internal pool of sphingomyelin which accounts for about 30% of the total is only degraded slowly (half-time about 80 h) showing that the internal pool does not normally reach the surface. In [3H]choline incorporation experiments the internal pool begins to accumulate radioactivity at about the same time as phosphatidylcholine (30 min) but label does not enter the surface pool of sphingomyelin for a further 90 min. The internal and external pools reach the same specific activity only after about 20 h. Pulse-chase analysis with [3H]choline shows that radioactivity in each pool of sphingomyelin continues to increase when the specific radioactivity of phosphatidylcholine is decreasing, consistent with both pools being synthesised from a phosphatidylcholine precursor. The results suggest that sphingomyelin in BHK cells is present not only in the plasma membrane but also in a more rapidly labelling pool which does not mix with the surface pool.

A PAF-acetylhydrolase activity in Tetrahymena pyriformis cells

Our study provides evidence for the existence of an acylhydrolase activity in Tetrahymena pyriformis cells, capable of hydrolyzing the sn-2 ester bond of the PAF molecule. This activity is mainly distributed in the microsomal fraction (76.5% of total) and has properties similar to the mammalian PAF-acetylhydrolase since it is Ca(2+)-independent, acid-labile, is inhibited by DFP and PMSF but it is not affected by egg yolk phosphatidylcholine. This microsomal acylhydrolase has apparent Km and Vmax values of 1.56 microM and 373 pmols.mg.min respectively. This is the first report of the existence of a PAF-acetylhydrolase activity in a non-mammalian cell.

Action of a homogeneous hydrogenation catalyst on living Tetrahymena mimbres cells

Various conditions were tested in an attempt to hydrogenate the unsaturated fatty acids of living Tetrahymena mimbres with the homogeneous catalyst palladium di-(sodium alizarine monosulfonate) without causing serious damage to the cells. Using a low (20 micrograms/ml) catalyst concentration in the external medium, hydrogenation of greater than 20% of surface membrane lipid double bonds were obtained, but hydrogenation of intracellular membranes was minimal. When exposed to H2, cells preincubated with inactive catalyst for several hours and visibly loaded with the catalyst lost viability as soon as hydrogenation exceeded trace levels. Material secreted by Tetrahymena into their medium effectively inhibited hydrogenation of added oleic acid, normally a good substrate. Mucus secreted by the cells, soluble proteins isolated from cell homogenates, bovine serum albumin, and cysteine were also inhibitory, but the inhibition could be overcome by employing higher catalyst concentrations. Although some enzymatic retroconversion of saturated lipids back to unsaturated lipids appeared to take place, the scale of the conversion was small, and further experimentation will be required to understand the mechanism involved. The selective hydrogenation of surface membranes achieved by these methods may be especially useful to those interested in fluidity effects on plasma membrane properties.

In vivo metabolism of platelet-activating-factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) by the protozoan Tetrahymena pyriformis

The metabolism of exogenous platelet-activating-factor was studied in the protozoan Tetrahymena pyriformis in vivo. When the cells are exposed to 1.10(-6) M PAF, the molecule is rapidly metabolized to 1-O-alkyl-2-acyl(long chain)-GPC, a major component of the protozoan membranes. The appearance of lyso-PAF from the first minutes even in low levels provides evidence that deacetylation is an intermediate step. After incubation for 30 min, transformation to aminoethyl phosphonolipids is also observed. The fate of PAF in concentrations 1.5.10(-11) M or 1.10(-8) M PAF, was the same. An amount of PAF depending on the external PAF concentration remained intact in the cell even after 1 h incubation. Our results suggest that the easily cultured protozoan can be a useful model for studying PAF's metabolism.

Purification of membranes and identification of phase-specific proteins of the dimorphic pathogenic fungus Histoplasma capsulatum

Plasma membrane vesicles from the yeast and mycelial phases of Histoplasma capsulatum have been purified and characterized. The method of purification involved differential centrifugation of ballistically fractured cells followed by sedimentation through discontinuous sucrose density gradient and equilibrium centrifugation. Purity of the preparation was assessed by electron microscopy. The protein composition of the membrane preparations from the yeast and mycelial phases of the fungus was analyzed by polyacrylamide gels. A comparison of the two morphologic phases revealed quantitative and qualitative differences in the expressions of several membrane-specific proteins. Physical differences in the appearance of the membranes were also observed by electron micrography of membrane preparations. Alteration in membrane fluidity may be one of the many causes for differences in the appearance of membrane vesicles in the two phases.

Trypanosoma cruzi: changes in lipid composition during aging in culture

Changes in lipid composition and fatty acid distribution in lipid fractions from total extracts of Trypanosoma cruzi were studied in culture from Day 2 to Day 14. This comprises the phases of exponential, stationary, and declining growth. Total phospholipid content decreased steadily during the three culture phases due to the marked reduction of phosphatidylcholine. Phosphatidylethanolamine increased during the exponential and declining phases. Thus, the final phosphatidylethanolamine/phosphatidylcholine ratio was higher than that determined on the second day. Sterols and acylglycerides increased as cultures aged. Fatty acid composition of different fractions varied during aging: phosphatidylcholine and phosphatidylethanolamine presented an increase of saturated and reduction of polyunsaturated (linoleic) acids, while for lysophosphatidylcholine and acylglycerides, the opposite change occurred. The modifications described may produce reduction of membrane fluidity and indicate that lipids participate actively in the adaptation of T. cruzi to the environmental changes produced by aging in culture.

Lipid analyses of isolated surface membranes of Leishmania donovani promastigotes

Constituent lipids of surface membranes (SM) isolated from Leishmania donovani promastigotes were analyzed and compared with those obtained from whole cells and an isolated kinetoplast-mitochondrion fraction (KM). On a dry weight basis, the total extractable lipids constituted approximately 47%, 12% and 24% of the SM, cells and KM, respectively. The total lipids of SM, cells and KM all were composed of approximately 70% phospholipids (PL), 20-25% neutral lipids and 5-10% glycolipids. Sterols and diglycerides composed 60% and 30%, respectively, of the various neutral lipid fractions. Several mannose- and galactose-containing glycolipids were fractionated but not identified. The glycolipid fractions from cells and SM had demonstrable antigenic activities with rabbit anti-SM sera. Striking quantitative differences were apparent between the PL profiles of the 3 cellular components examined. The PL of SM, whole cells and KM, respectively, were composed of: 15%, 51% and 24% phosphatidylcholine; 37%, 13% and 11% phosphatidylethanolamine (PE); 18%, 10% and 14% phosphatidylinositol; 10%, 1% and 4% phosphatidylserine and traces of cardiolipin, phosphatidylglycerol and phosphatidic acid. An unknown PL containing sphingosine, choline and vicinal hydroxyl groups but no free amino moieties made up approximately 19% of the PL of SM and whole cells, but it constituted approximately 27% of the PL of KM. The PL side chain constituents of whole cells and SM were composed mainly of longchain fatty acids (C18-20). Further, over 50% of the PE of SM was in the alkyl and alK-1-enyl ether forms. These SM properties might contribute to the organism's resistance to digestion in the hydrolytic environs of both its insect vector and mammalian hosts.

Changes in lipid fluidity and fatty acid composition with altered culture temperature in Tetrahymena pyriformis-NT1

Cultures of T. pyriformis-NT1 were grown at 20 degrees C (Tg 20 degrees C) and 38 degrees C (Tg 38 degrees C). G.L.C. analysis and D.P.H. fluorescence polarization measurements in extracted phospholipids indicated that there was increased saturation of fatty acids and relatively reduced fluidity as growth temperature was increased. Breakpoints occurred in the Arrhenius plots of fluorescence polarization at 16 degrees C for Tg 38 degrees C total extracted phospholipids and 9 degrees C for Tg 20 degrees C lipids.

Thermal adaptation of Tetrahymena membranes with special reference to mitochondria. II. Preferential interaction of cardiolipin with specific molecular species of phospholipid

A specific effect of cardiolipin on fluidity of mitochondrial membranes was demonstrated in Tetrahymena cells acclimated to a lower temperature in the previous report (Yamauchi, T., Ohki, K., Maruyama, H. and Nozawa, Y. (1981) Biochim. Biophys. Acta 649, 385-392). This study was further confirmed by the experiment using fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH). Anisotropy of DPH for microsomal and pellicular total lipids from Tetrahymena cells showed that membrane fluidity of these lipids increased gradually as the cells were incubated at 15 degrees C after the shift down of growth temperature from 39 degrees C. However, membrane fluidity of mitochondrial total lipids was kept constant up to 10 h. This finding is compatible with the result obtained using spin probe in the previous report. Additionally, the break-point temperature of DPH anisotropy was not changed in mitochondrial lipids whereas those temperatures in pellicular and microsomal lipids lowered during the incubation at 15 degrees C. Interaction between cardiolipins and various phospholipids, which were isolated from Tetrahymena cells grown at 39 degrees C or 15 degrees C and synthesized chemically, was investigated extensively using a spin labeling technique. The addition of cardiolipins from Tetrahymena cells grown at either 39 degrees C or 15 degrees C did not change the membrane fluidity (measured at 15 degrees C) of phosphatidylcholine from whole cells grown at 39 degrees C. On the other hand, both cardiolipins of 39 degrees C-grown and 15 degrees C-grown cells decreased the membrane fluidity of phosphatidylcholine from Tetrahymena cells grown at 15 degrees C. The same results were obtained for phosphatidylcholines of mitochondria and microsomes. Membrane fluidity of phosphatidylethanolamine, isolated from cells grown at 15 degrees C, was reduced to a small extent by Tetrahymena cardiolipin whereas that of 39 degrees C-grown cells was not changed. Representative molecular species of phosphatidylcholines of cells grown at 39 degrees C and 15 degrees C were synthesized chemically; 1-palmitoyl-2-oleoylphosphatidylcholine for 39 degrees C-grown cells and dipalmitoleoylphosphatidylcholine for 15 degrees C-grown ones. By the addition of Tetrahymena cardiolipin, the membrane fluidity of 1-palmitoyl-2-oleoylphosphatidylcholine was not changed but that of dipalmitoleoylphosphatidylcholine was decreased markedly. These phenomena were caused by Tetrahymena cardiolipin. However, bovine heart cardiolipin, which has a different composition of fatty acyl chains from the Tetrahymena one, exerted only a small effect.

The effect of growth temperature on the membrane lipid environment of the psychrophilic bacterium Micrococcus cryophilus

The relationship between the delta 9-desaturase activity of the psychrophilic bacterium Micrococcus cryophilus grown at different temperatures and the physical state of its membrane lipids as measured by ESR spectroscopy has been studied. Arrhenius plots of desaturase activity were biphasic with a discontinuity at a temperature which depended upon the bacterial growth temperature. Changes in the desaturase activation energy, which increased as the growth temperature was lowered, are discussed in the context of membrane lipid fluidity adaptation to changing environmental temperature. The fluidity of membranes and isolated lipids was measured using nitroxide-labeled fatty acids. The spectra of 2-(10-carboxydecyl)-2-hexyl-4,4-dimethyl-3-oxazolidinoxyl in membranes indicated that there were two lipid environments within the membrane whose relative proportions were dependent both on temperature of measurement and on bacterial growth temperature. In contrast, 2-(3-carboxypropyl)-4,4-dimethyl-2-tridecyl-3-oxazolidinoxyl spectra showed a single lipid environment and plots of log order parameter (S3) vs 1/T were biphasic with inflexion temperatures which were closely related to the bacterial growth temperature. As with membranes, plots of log S3 vs 1/T for total lipids, phosphatidylglycerol and cardiolipin, but not phosphatidylethanolamine, were biphasic and showed inflexions which correlated well with bacterial growth temperature. These results are interpreted as being consistent with a location for the desaturase within the bulk lipid of the membrane rather than in association with specific lipid types.

The role of squalene synthetase in the inhibition of tetrahymanol biosynthesis by cholesterol in Tetrahymena pyriformis

The biosynthesis of the triterpenoid alcohol tetrahymanol by Tetrahymena pyriformis is rapidly inhibited by the addition of cholesterol to the growth medium. The primary site of this inhibition by cholesterol has been established to be at the level of the enzyme squalene synthetase. The protein synthesis inhibitor cycloheximide produces an identical decline in squalene synthetase activity to that of cholesterol and the half-life of the enzyme is about 50 minutes. No direct inhibition of the enzyme is observed and suggests that cholesterol inhibits the actual synthesis of the enzyme squalene synthetase. Farnesol is accumulated during in vitro incubations derived from cells grown in the presence of cholesterol or cycloheximide.

Phase behavior of the major lipids of tetrahymena ciliary membranes

The major lipids of Tetrahymena membranes have been purified by thin-layer and high pressure liquid chromatography and the phosphatidylethanolamine and aminoethylphosphonate lipids were examined in detail. 31P-NMR, X-ray diffraction and freeze-fracture electron microscopy were employed to describe the phase behavior of these lipids. The phosphatidylethanolamine was found to form a hexagonal phase above 10 degrees C. The aminoethylphosphonate formed a lamellar phase up to 20 degrees C, but converted to a hexagonal phase structure at 40 degrees C. Small amounts of phosphatidylcholine stabilized the lamellar phase for the aminoethylphosphonate. 31P-NMR spectra of the intact ciliary membranes were consistent with a phospholipid bilayer at 30 degrees C, suggesting that phosphatidylcholine in the membrane stabilized the lamellar form, even though most of the lipid of that membrane prefers a hexagonal phase in pure form at 30 degrees C. 31P-NMR spectra also showed a distinctive difference in the chemical shift tensor of the aminoethylphosphonolipid, when compared to that of phosphatidylethanolamine, due to the difference in chemical structure of the polar headgroups of the two lipids.

Thermal adaptation of Tetrahymena membranes with special reference to mitochondria. Role of cardiolipin in fluidity of mitochondrial membranes

During temperature acclimation of Tetrahymena pyriformis, the changes in fluidity and composition of total lipids from three membrane fractions, mitochondria, pellicles and microsomes were studied by a spin-label technique using a stearate probe and thin-layer and gas-liquid chromatography. The increase of fluidity observed in microsomal and pellicular lipids following the temperature shift from 39 to 15 degrees C corresponds with the increase of the ratio of total unsaturated to saturated fatty acid content. However, despite the increase of this ratio, the fluidity of mitochondrial lipids was found to be constant up to 10 h after the temperature shift. The fluidity of total lipids of mitochondria isolated from Tetrahymena cells grown at 39 degrees C was not changed by removal of cardiolipin, whereas cardiolipin-depleted lipids of mitochondria from 15 degrees C-acclimated cells showed a decrease in fluidity. The re-addition of cardiolipin to the mitochondrial lipids depleted of cardiolipin restored the fluidity to the initial level, thereby confirming the rigidifying effect of cardiolipin in cold-acclimated cells. These results suggest that cardiolipin may be implicated in maintaining consistent fluidity of mitochondrial membranes against change in thermal environment.

Mutational alteration of membrane phospholipid composition and voltage-sensitive ion channel function in paramecium

A behavioral mutant of Paramecium tetraurelia (baA) has been isolated that has an abnormal response when placed in solutions containing Ba2+. This mutant is shown here to have a dramatic alteration of the sphingolipid and phosphonolipid composition of its ciliary membrane. This biochemical defect is present in independently isolated alleles at baA locus and segregates in crosses with the behavioral phenotype. Electrophysiologically, the mutation reduces significantly conductance of both voltage-sensitive Ca2+ channels and voltage-sensitive K+ channels. When the mutant is grown in sterol-supplemented medium, its behavior, electrophysiological properties, and lipid composition are hardly distinguishable from wild type grown under similar conditions. This mutant then, provides strong evidence that membrane lipids significantly influence the function of the membrane molecules responsible for the generation of action potentials.

Studies of diacylglycerol cholinephosphotransferase and diacylglycerol ethanolaminephosphotransferase activities in Tetrahymena microsomes

Microsomes isolated from Tetrahymena pyriformis synthesized phosphatidylcholine and phosphatidylethanolamine by CDPcholine: 1,2-diacylglycerol cholinephosphotransferase (EC 2.7.8.2) and CDPethanolamine: 1,2-diacylglycerol ethanolaminephosphotransferase (EC 2.7.8.1), utilizing ethanol-dispersed dioleoglycerol. Cholinephosphotransferase and ethanolaminephosphotransferase activities have similar dependences on MgCl2 and MnCl2, but the latter was more effective than the former for both enzyme activities. The V values for 1,2-dioleoylglycerol obtained at optimal conditions were 1.8 nmol/min per mg microsomal protein for cholinephosphotransferase and 0.6 nmol/min per mg microsomal protein for ethanolaminephosphotransferase. Both enzymes could not utilize 1,3-dioleoylglycerol or 1-oleoylglycerol as substrates. Cholinephosphotransferase had an apparent Km for CDPcholine of 11.7 microM with 1,2-dioleoylglycerol and was inhibited by CDPethanolamine competitively. On the other hand, ethanolaminephosphotransferase has an apparent Km for CDPethanolamine of 8 microM and CDPcholine was a noncompetitive inhibitor of ethanolaminephosphotransferase activity. Furthermore, despite the marked alteration of phospholipid composition occurring during the temperature acclimation of Tetrahymena cells, both enzyme activities showed similar dependences on growth and incubation temperatures. This may imply that the final step of de novo synthesis of two major phospholipids does not participate in the thermally induced modification of the profile of phospholipid polar head group in membranes.