Studies of membrane formation in Tetrahymena pyriformis. II. Isolation and lipid analysis of cell fractions

Genome analysis of sphingolipid metabolism-related genes in Tetrahymena thermophila and identification of a fatty acid 2-hydroxylase involved in the sexual stage of conjugation

Sphingolipids are bioactive lipids present in all eukaryotes. Tetrahymena thermophila is a ciliate that displays remarkable sphingolipid moieties, that is, the unusual phosphonate-linked headgroup ceramides, present in membranes. To date, no identification has been made in this organism of the functions or related genes implicated in sphingolipid metabolism. By gathering information from the T. thermophila genome database together with sphingolipid moieties and enzymatic activities reported in other Tetrahymena species, we were able to reconstruct the putative de novo sphingolipid metabolic pathway in T. thermophila. Orthologous genes of 11 enzymatic steps involved in the biosynthesis and degradation pathways were retrieved. No genes related to glycosphingolipid or phosphonosphingolipid headgroup transfer were found, suggesting that both conserved and innovative mechanisms are used in ciliate. The knockout of gene TTHERM_00463850 allowed to identify the gene encoding a putative fatty acid 2-hydroxylase, which is involved in the biosynthesis pathway. Knockout cells have shown several impairments in the sexual stage of conjugation since different mating types of knockout strains failed to form cell pairs and complete the conjugation process. This fatty acid 2-hydroxylase gene is the first gene of a sphingolipid metabolic pathway to be identified in ciliates and have a critical role in their sexual stage.

Gene identification and functional characterization of a Δ12 fatty acid desaturase in Tetrahymena thermophila and its influence in homeoviscous adaptation to low temperature

Homeoviscous adaptation in poikilotherms is based in the regulation of the level of desaturation of fatty acids, variation in phospholipids head groups and sterol content in the membrane lipids, in order to maintain the membrane fluidity in response to changes in environmental temperature. Increased proportion of unsaturated fatty acids is thought to be the main response to low-temperature acclimation, which is mostly achieved by fatty acid desaturases. Genome analysis of the ciliate Tetrahymena thermophila and a gene knockout approach has allowed us to identify one Δ12 FAD and to study its activity in the original host and in a yeast heterologous expression system. The "PUFA index" -relative content of polyunsaturated fatty acids compared to the sum of saturated and monounsaturated fatty acid content- was ~57% lower at 15 °C and 35 °C in the Δ12 FAD gene knockout strain (KOΔ12) compared to WT strain. We characterized the role of T. thermophila Δ12 FAD on homeoviscous adaptation and analyzed its involvement in cellular growth, cold stress response, and membrane fluidity, as well as its expression pattern during temperature shifts. Although these alterations allowed normal growth in the KOΔ12 strain at 30 °C or higher temperatures, growth was impaired at temperatures of 20 °C or lower, where homeoviscous adaptation is impaired. These results stress the importance of Δ12 FAD in the regulation of cold adaptation processes, as well as the suitability of T. thermophila as a valuable model to investigate the regulation of membrane lipids and evolutionary conservation and divergence of the underlying mechanisms.

Two arginine kinases of Tetrahymena pyriformis: characterization and localization

Two cDNAs, one coding a typical 40-kDa arginine kinase (AK1) and the other coding a two-domain 80-kDa enzyme (AK2), were isolated from ciliate Tetrahymena pyriformis, and their recombinant enzymes were successfully expressed in Escherichia coli. Both enzymes had an activity comparable to those of typical invertebrate AKs. Interestingly, the amino acid sequence of T. pyriformis AK1, but not AK2, had a distinct myristoylation signal sequence at the N-terminus, suggesting that 40-kDa AK1 targets the membrane. Moreover, Western blot analysis showed that the AK1 is mainly localized in the ciliary fraction. Based on these results, we discuss the phosphoarginine shuttle, which enables a continuous energy flow to dynein for ciliary movement in T. pyriformis, and the role of AK1 in this model.

Adaptive regulation of membrane lipids and fluidity during thermal acclimation in Tetrahymena

The free-living eukaryotic protozoan Tetrahymena is a potentially useful model for the thermoadaptive membrane regulation because of easy growth in the axenic culture, systematic isolation of subcellular organelles, and quick response to temperature stress. Exposure of Tetrahymena cells to the cold temperature induces marked alterations in the lipid composition and the physical properties (fluidity) of various membranes. The increase in fatty acid unsaturation of membrane phospholipids is required to preserve the proper fluidity. In this homeoviscous adaptive response, acyl-CoA desaturase plays a pivotal role and its activity is regulated by induction of the enzyme via transcriptional activation.

An effective hierarchical model for the biomolecular covalent bond: an approach integrating artificial chemistry and an actual terrestrial life system

Under the AChem paradigm and the programmed self-decomposition (PSD) model, we propose a hierarchical model for the biomolecular covalent bond (HBCB model). This model assumes that terrestrial organisms arrange their biomolecules in a hierarchical structure according to the energy strength of their covalent bonds. It also assumes that they have evolutionarily selected the PSD mechanism of turning biological polymers (BPs) into biological monomers (BMs) as an efficient biomolecular recycling strategy We have examined the validity and effectiveness of the HBCB model by coordinating two complementary approaches: biological experiments using existent terrestrial life, and simulation experiments using an AChem system. Biological experiments have shown that terrestrial life possesses a PSD mechanism as an endergonic, genetically regulated process and that hydrolysis, which decomposes a BP into BMs, is one of the main processes of such a mechanism. In simulation experiments, we compared different virtual self-decomposition processes. The virtual species in which the self-decomposition process mainly involved covalent bond cleavage from a BP to BMs showed evolutionary superiority over other species in which the self-decomposition process involved cleavage from BP to classes lower than BM. These converging findings strongly support the existence of PSD and the validity and effectiveness of the HBCB model.

Phosphatidylinositol synthase of Tetrahymena: inositol isomers as substrates in phosphatidylinositol biosynthesis and headgroup exchange reactions

Phosphatidylinositol (PtdIns) synthase in microsomal fractions derived from Tetrahymena vorax was studied to determine its activity requirements. The suitability of inositol isomers as substrates for the synthase and in headgroup exchange reactions also was investigated. Tetrahymena PtdIn synthase activity was optimum in the presence of 2 mM MgCl2 plus 2 mM MnCl2, a pH of 7.8, and a temperature of 30 degrees C. The enzyme retained approximately 80% of its activity after incubation at 70 degrees C for 10 min. PtdIns headgroup exchange activity was maximal in the presence of cytidine monophosphate. By following either the accumulation of radiolabeled reaction products or the loss of radiolabel from precursors, each of the inositol isomers tested appeared to serve as substrates for both the PtdIns synthase and PtdIns:inositol phosphatidyl transferase activities. In each case, myo-inositol and scyllo-inositol were the preferred substrates. The data suggest two routes for the formation of phosphatidyl-non-myo-inositols in Tetrahymena and the potential for the production of novel, non-myo-inositol-containing second messengers.

New Tetrahymena basal body protein components identify basal body domain structure

Basal bodies organize the nine doublet microtubules found in cilia. Cilia are required for a variety of cellular functions, including motility and sensing stimuli. Understanding this biochemically complex organelle requires an inventory of the molecular components and the contribution each makes to the overall structure. We define a basal body proteome and determine the specific localization of basal body components in the ciliated protozoan Tetrahymena thermophila. Using a biochemical, bioinformatic, and genetic approach, we identify 97 known and candidate basal body proteins. 24 novel T. thermophila basal body proteins were identified, 19 of which were localized to the ultrastructural level, as seen by immunoelectron microscopy. Importantly, we find proteins from several structural domains within the basal body, allowing us to reveal how each component contributes to the overall organization. Thus, we present a high resolution localization map of basal body structure highlighting important new components for future functional studies.

Secondary metabolites of slime molds (myxomycetes)

The compounds reported from the slime molds (myxomycetes) species are described. Almost 100 natural compounds including their chemical structures and biological activities are described in this review article. Only metabolites with a well-defined structure are included.

Localization of an alkyl-acetyl-glycerol-CDP-choline: cholinephosphotransferase activity in submitochondrial fractions of Tetrahymena pyriformis

Tetrahymena pyriformis contains platelet-activating factor (PAF) as a minor lipid, which is biosynthesized de novo. A dithiothreitol-insensitive CDP-choline:cholinephosphotransferase (AAG-CPT), which utilizes alkyl-acetyl-glycerol as a substrate, had been detected in both the mitochondrial and microsomal fractions of the protozoan. In the present report, localization of this enzyme in submitochondrial fractions was studied. Cell fractionation was evaluated with enzyme and morphological markers. In this respect, succinate dehydrogenase, NADPH:cytochrome c reductase, glucose-6-phosphatase, alkaline phosphatase, monoaminoxidase, and cytochrome c oxidase activities were investigated. In the presence of antimycin A, mitochondrial activity of NADPH-cytochrome c reductase, was increased, while the microsomal one was reduced. Cardiolipin was distributed in the inner mitochondrial membrane. Alkaline phosphatase was found exclusively in the cytosol of the protozoan. The main portion of the dithiothreitol-insensitive AAG-CPT was localized in the inner mitochondrial membrane. Our data indicate that mitochondria are able to produce PAF, which might be associated with their function.

Enzymatic characterization of phospholipase D of protozoan Tetrahymena cells

Phospholipase D (PLD), which is present in plant, bacterial, and mammalian cells, has been proposed to be involved in a number of cellular processes including transmembrane signaling and membrane deterioration. We demonstrated the existence of evolutionally related PLD activity in the unicellular eukaryotic protozoan Tetrahymena. The partial characterization of this enzyme showed that PLD in Tetrahymena cells was a neutral phospholipase, which catalyzed both transphosphatidylation and hydrolysis reac tions. The activity was markedly stimulated by phosphatidylinositol 4, 5-bisphosphate (PIP2) but was insensitive to phorbol 12-myristate 13-acetate (PMA) and guanosine 5'-3-O-(thio)triphosphate (GTPgammaS), suggesting that it is a PIP2-dependent PLD and that protein kinase C (PKC) and GTP-binding proteins are not implicated in the regulation of this enzyme. For its maximal activity Ca2+ was not required. This enzyme was also capable of hydrolyzing phosphatidylcholine (PC) but not phosphatidylethanolamine (PE), implying that PC was a preferred substrate. Subcellular fractionation showed that PLD-like activity localized mainly to the membrane fraction, especially microsomes. As an initial step to explore the functions of PLD in Tetrahymena, the PLD-like activity was determined during the different culture phases, and it was found to be significantly and transiently elevated in the early logarithmic phase, indicating its possible role in the development of Tetrahymena.

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.

Enzymes related to catecholamine biosynthesis in Tetrahymena pyriformis. Presence of GTP cyclohydrolase I

We first identified GTP cyclohydrolase I activity (EC 3.5.4.16) in the ciliated protozoa, Tetrahymena pyriformis. The Vmax value of the enzyme in the cellular extract of T. pyriformis was 255 pmol mg-1 protein h-1. Michaelis-Menten kinetics indicated a positive cooperative binding of GTP to the enzyme. The GTP concentration producing half-maximal velocity was 0.8 mM. By high-performance liquid chromatography (HPLC) with fluorescence detection, a major peak corresponding to D-monapterin (2-amino-4-hydroxy-6-[(1'R,2'R)-1',2',3'-trihydroxypropyl]pteridin e, D-threo-neopterin) and minor peaks of D-erythro-neopterin and L-erythro-biopterin were found to be present in the cellular extract of Tetrahymena. Thus, it is strongly suggested that Tetrahymena converts GTP into unconjugated pteridine derivatives. In this study, dopamine was detected as the major catecholamine, while neither epinephrine nor norepinephrine was identified. Indeed, this protozoa was shown to possess the activity of a dopamine synthesizing enzyme, aromatic L-amino acid decarboxylase. On the other hand, activities of tyrosine hydroxylase or tyrosinase which converts tyrosine into dopa, the substrate of aromatic L-amino acid decarboxylase, could not be detected in this protozoa. Furthermore, neither dopamine beta-hydroxylase activity nor phenylethanolamine N-methyltransferase activity could be identified by the HPLC methods.

An apparent association between glycosylphosphatidylinositol-anchored proteins and a sphingolipid in Tetrahymena mimbres

Sphingolipids are thought to stabilize glycosylphosphatidylinositol (GPI)-anchored protein-rich membrane domains of yeast and polarized higher animal cells during the processing and targeting of these proteins to the plasma membrane. A widely used criterion for identifying the stable sphingolipid- and GPI-anchored protein-enriched membrane domains is the resistance of these lipid-modified proteins to solubilization by the detergent Triton X-100 (TX-100) at low temperature. Surprisingly, there have been no reports of sphingolipid/GPI-anchored protein association in protozoans, despite the fact that these cells contain considerably higher levels of GPI-anchored proteins than does any other organism. We report here the presence in Tetrahymena mimbres of a significant pool of GPI-anchored proteins which resisted extraction by 1% TX-100 at 4 degrees C but not at 37 degrees C. Of the total cellular complement of GPI-anchored proteins, which together accounted for more than 2% of whole-cell protein and were especially enriched in surface membranes, 10% of the major 63kDa component (gpi63) and 23% of a somewhat less abundant component (gpi23) were insoluble in TX-100 at 4 degrees C. A substantial proportion of the cell's only abundant sphingolipid, ceramideaminoethylphosphonate (CAEP), was also insoluble in 1% TX-100 at 4 degrees C. Radiolabelling studies involving [3H]leucine incorporation into proteins and [3H]palmitic acid incorporation into lipids revealed that the TX-100-resistant gpi63, gpi23 and CAEP molecules were all metabolically distinct from their TX-100-soluble counterparts in other compartments of the cell. The presence of detergent-resistant sphingolipid/GPI-anchored protein domains in non-polarized ciliate and trypanosomatid cells was probably obscured in previous studies by the profusion of accompanying detergent-soluble molecules.

Monoclonal antibodies reveal complex structure in the membrane skeleton of Tetrahymena

Twelve monoclonal antibodies were raised that are specific for the membrane skeleton of Tetrahymena. Five were directed against T. pyriformis and seven were directed against T. thermophila. Some cross-reactivity between species was found. Each monoclonal antibody recognized one of the three major components of epiplasm, i.e. the bands A, B, and C identified in electrophoretic separations of epiplasmic proteins. It was found, using these antibodies, that the epiplasmic proteins A, B and C have overlapping but independent distributions within the cell.

Phytosterols are present in Pneumocystis carinii

Although originally classified as a protozoan, Pneumocystis carinii is now considered to have fungal characteristics. Drugs typically used for the treatment of fungal infections target ergosterol. Because P. carinii is an important pathogen in AIDS and other immunocompromised patients, knowledge of the sterol content of this organism may be useful as a basis for developing new treatment strategies or for improving diagnosis. P. carinii organisms were harvested from infected rat lungs and were purified by filtration. Control preparations from uninfected animals were identically prepared. Lipids were extracted from the organisms and control preparations and were separated into neutral lipid, glycolipid, and phospholipid fractions by silicic acid chromatography. The neutral lipid fraction was further treated by alkaline hydrolysis and was analyzed by reversed-phase high-pressure liquid chromatography (HPLC), gas chromatography (GC), and GC-mass spectrometry (GC-MS). As shown by HPLC, the neutral lipid fraction from infected rats contained a minimum of six peaks, while in control preparations a single peak with a retention time identical to that of cholesterol was observed. The predominant sterol in these preparations was positively identified by GC-MS as cholesterol and constituted 80 to 90% of the total. The remaining peaks had relative retention times similar to those of phytosterols by both HPLC and GC, and the similarity of these sterols to those derived from plants and fungi was confirmed by MS. Ergosterol, however, was not present. These results provide further evidence for a close phylogenetic relationship between P. carinii and fungi and suggest that these sterols could be used as targets for drug development and for improving diagnosis.

"Fenestrin" and conjugation in Tetrahymena thermophila

Certain monoclonal antibodies interact with proteins of Tetrahymena thermophila found in the conjugation junction as well as around the gametic nuclei (pronuclei) of conjugating cells; they also react with the oral primordium and fission zone of vegetative cells and with the cytoproct and contractile vacuole pores of all cells. One of these (FXIX-3A7) was investigated in detail. Immunogold labelling suggests that the material labelled by the 3A7 monoclonal antibody, which we call "fenestrin," is located beneath the epiplasm (membrane skeleton). Immunoblots reveal that the major and perhaps sole antigen is a 64 kDa polypeptide, found in two isoelectric variants. Developmental studies implicate fenestrin in two processes involved in conjugation. The first is "tip transformation." During preliminary starvation ("initiation"), labelling of fenestrin first appeared as a spot at the anterior end of starved mature cells, then after mixing of different mating types ("costimulation") it extended posteriorly along the anterior suture. After pairing, this region spread to form a widened plate. The second process is pronuclear transfer. Fenestrations representing channels between the conjugating cells began to appear 0.5 to 1 h after the conjugants united, and eventually merged to form a small number of temporary large holes during exchange of the transfer pronuclei. A fenestrin envelope also enclosed both the transfer and resident pronuclei; a strand of fenestrin connected the two. Shortly after pronuclear transfer, both transfer and resident pronuclei were released from fenestrin caps and fused to produce a zygotic nucleus (synkaryon) not associated with fenestrin Fenestrin thus appears to be intimately involved in the process of pronuclear exchange.

Studies on the subcellular distribution of 1-O-alkyl-2-acetyl-sn-glycero phosphocholine (PAF) and on the enzymic activities involved in its biosynthesis within the ciliate Tetrahymena pyriformis

The ciliated protozoan Tetrahymena pyriformis contains platelet-activating factor (PAF) as a physiological minor lipid. Its subcellular localization was found as follows: 13.7% in the pellicles, 24.9% in mitochondria, 56.5% in microsomes and 7.1% in the cytosol. Succinate dehydrogenase was used as marker enzyme. PAF remains cell-associated unless bovine serum albumin is included in the extracellular medium. In this case 15% of total PAF, portion comparable to that found in the pellicles, is released. Investigation of the principal enzymic activities involved in PAF formation showed that PAF-acetyltransferase (2.3.167) is totally absent from the protozoan. This means that the 'remodelling' pathway occurring in pro-inflammatory cells does not contribute in PAF formation in our system. A dithiothreitol (DTT)-insensitive CDPcholine phosphocholinetransferase activity involved in PAF biosynthesis is shown for the first time to be responsible for PAF production in T. pyriformis. It uses exogenous alkyl-acetyl-glycerol as substrate and is saturated over substrate concentration 250 microM. It can also use endogenous lipids as substrate. It is distributed mainly in mitochondria and microsomes, much less is found in the pellicles and it is totally absent from the cytosol. Its insensitivity to DTT, its selectivity to alkyl-acetyl-G and its different distribution compared to the enzymic activity involved in PC formation (EC 2.7.8.2) suggest that a different enzyme, specific for PAF formation (EC2.7.8.16) via the de novo pathway exists in the protozoan.

Purification and characterization of phospholipase C preferentially hydrolysing phosphatidylcholine in Tetrahymena membranes

A phospholipase C (PLC) activity that preferentially hydrolyses phosphatidylcholine to diacylglycerol and phosphorylcholine was found to be present in Tetrahymena pyriformis, strain W and most of its activity was recovered in the membrane fraction. This enzyme was extracted with 1% Triton X-100 from the membrane fraction and purified to apparent homogeneity by sequential chromatographies on Fast Q-Sepharose, hydroxyapatite HCA-100S, Mono Q and Superose 12 gel filtration columns. The purified enzyme had specific activity of 2083 nmol of diacylglycerol released/mg of protein/min for dipalmitoylphosphatidylcholine hydrolysis. Its apparent molecular mass was 128 kDa as determined by SDS-polyacrylamide gel electrophoresis and was 127 kDa by gel filtration chromatography, indicating that the enzyme is present in a monomeric form. The enzyme exhibited an optimum pH 7.0 and the apparent Km value was determined to be 166 microM for dipalmitoylphosphatidylcholine. A marked increase was observed in phosphatidylcholine hydrolytic activity in the presence of 0.05% (1.2 mM) deoxycholate. Ca2+ but not Mg2+ enhanced the activity at a concentration of 2 mM. This purified phospholipase C exhibited a preferential hydrolytic activity for phosphatidylcholine but much less activity was observed for phosphatidylinositol (approximately 9%) and phosphatidylethanolamine (approximately 2%).

Phospholipid biosynthesis in protozoa

Because of the diverse nature of the organisms which are all classed as 'protozoa' (and because of the lack of detailed information on phospholipid metabolism about most of them), it will probably never be possible to generalize phospholipid metabolism to the degree that it has been possible to characterize a mammalian metabolism. Nonetheless, patterns have begun to emerge (i.e. the similarities among the ciliates Entodinium, Paramecium and Tetrahymena) and will not doubt be expanded upon in the future.

Immunocytochemical analysis of secretion mutants of Tetrahymena using a mucocyst-specific monoclonal antibody

Dense-core granules represent an adaptation of specialized secretory cells to facilitate stimulus-regulated release of stored proteins. Such granules are a prominent feature of mammalian neuroendocrine and exocrine cells and are also well developed in the ciliates. In Tetrahymena thermophila, the ability to generate mutants in dense-core granule biosynthesis and fusion presents a versatile system for dissecting steps in regulated exocytosis. We have previously shown that defective granules in such mutants could be characterized by several biochemical criteria, including buoyant density, which increases during maturation, and the degree of proteolytic processing of the content precursors. We have now used indirect immunofluorescence, taking advantage of a monoclonal antibody directed against a granule protein, to visualize the morphology and distribution of both granules and putative granule intermediates in mutant and wild-type cells. The results are consistent with the biochemical analysis and extend our characterization of the mutants, allowing us to distinguish four classes. In addition, the assay represents a powerful technique for diagnosis of new mutants.

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.

Presence of benzodiazepine binding sites (receptors) and amplification thereof by imprinting in Tetrahymena

Live Tetrahymena cells bound 3H-diazepam specifically, as demonstrated by autoradiographic evidence of displacement of about 25% of labeled diazepam in the presence of a 1000-fold amount of cold diazepam. The 3H-diazepam bound to membrane preparations isolated from untreated (control) cells was not displaced by cold diazepam, whereas cells involved in primary interaction (imprinting) with diazepam showed amplification and specificity of diazepam binding in both in vivo (cell suspension) and in vitro (pellicle) systems, as well as displacement of bound label in the presence of 1000-fold cold diazepam. It appears that diazepam induced imprinting and, consequently, also the formation of specific receptors in Tetrahymena.

Purification and properties of a membrane-bound L-asparaginase of Tetrahymena pyriformis

L-Asparaginase activity reaches maximal values at the stationary phase of growth of Tetrahymena pyriformis and fluctuates upon the growth conditions and the composition of the medium. Most of the L-asparaginase activity (80%) is associated with the endoplasmic reticulum, and the remaining with the pellicles. Detergents either alone or in combination with NaCl up to 0.5 M concentration failed to solubilize L-asparaginase. Solubilization can be accomplished by means of either the chaotropic agents KSCN and NaClO4, or 0.1 M sodium phosphate buffer pH 8.0, following pretreatment of the particulates with 2% w/v Triton X100. L-Asparaginase has been purified to near homogeneity by hydrophobic and gel filtration chromatography. The native enzyme has a relative molecular weight of 230,000. It is a multiple subunit enzyme, with subunit size of 39,000. Its isoelectric point is at pH 6.8. It acts optimally at pH 8.6 with a Km of 2.2 mM. It does not hydrolyse L-glutamine and its reaction is inhibited competitively by D-aspartic acid and D-asparagine as well as by L-asparagine analogues with substituents at the beta position.

Alterations of the composition and size of the free fatty acid pool of Tetrahymena responding to low-temperature stress

Cells of Tetrahymena mimbres (formerly T. pyriformis NT-1) in midlogarithmic growth under isothermal conditions (at 39 degrees C) contained a very small, compositionally discrete pool of free fatty acids, principally (60.6% of the total free fatty acid mass) palmitic and stearic acids. The composition, degree of unsaturation, and size of this free fatty acid pool were rapidly (15 min or less) altered in response to chilling. During the acclimation period following chilling to 15 degrees C, the size of the free fatty acid pool increased from a mean value of 15.5 nmol free fatty acid/mumol lipid phosphorus in 39 degrees C cells to 24.2 nmol free fatty acid/mumol lipid phosphorus. The degree of free fatty acid saturation rapidly increased over the initial hour following the onset of hypothermal conditions, but by 24 h the unsaturated free fatty acid/saturated free fatty acid ratio was 0.35 (equivalent to a 2.7-fold increase in unsaturation relative to 39 degrees C controls (unsaturated/saturated ratio = 0.13) and 4.4-fold greater than cells acclimated for 1 h (unsaturated/saturated ratio = 0.08)). By 24 h the percentage of palmitic and stearic acids had decreased to 45.6%. Similar, and in some instances more pronounced, changes were observed to occur in triacylglycerol-bound fatty acids. Modulation of steady-state free fatty acid composition could also be achieved by the addition of exogenous fatty acids to the growth medium. The ability to manipulate the level of intracellular free fatty acids should prove to be a valuable experimental tool in determining how specific fatty acids regulate various lipid-modifying enzymes.

Secretion heterogeneity of lysosomal enzymes in Tetrahymena pyriformis

A number of lysosomal enzymes are secreted from Tetrahymena pyriformis during growth and during starvation. The secretion is energy-dependent and kinetically different among hydrolases. On the basis of the secretion kinetics under starvation conditions, Tetrahymena hydrolases can be separated into three classes. The first group containing acid phosphatase, beta-glucosidase and alpha-galactosidase, are secreted slowly. Within this group about 4% of the initial cellular activity is released per hour. The second group of enzymes, including alpha-glucosidase, alpha-mannosidase and beta-galactosidase, exhibit moderate secretion (11-15% of the initial cellular activity per hour). The third group, N-acetyl-beta-hexosaminidase, has the highest rate of secretion (22% of the initial cellular activity per hour). N-Acetyl-beta-hexosaminidase shows a continuous increase in overall activity during starvation, which is completely blocked by adding cycloheximide; its secretion is also suppressed. Such involvement of enzyme biosynthesis was not seen in the first and second groups. Furthermore, treatment with weak bases caused inhibited secretion of differing degree among acid phosphatase (group I), alpha-glucosidase (group II) and N-acetyl-beta-hexosaminidase (group III).

Properties of digitonin-solubilized calmodulin-dependent guanylate cyclase from the plasma membranes of Tetrahymena pyriformis NT-1 cells

Calmodulin-dependent guanylate cyclase from Tetrahymena plasma membranes was solubilized in about a 22% yield by using digitonin in the presence of 0.2 mM CaCl2 and 20% glycerol. The detergent, when present in the assay at concentrations above 0.05%, diminished the basal and calmodulin-stimulated activity of the enzyme. Guanylate cyclase solubilized with digitonin was eluted from DEAE-cellulose with 200 mM KCl in a yield of 50%. Properties of the solubilized enzyme were similar to those of the native membrane-bound enzyme. The Kms for Mg-GTP and Mn-GTP were 140 and 30 microM, respectively. The enzyme required Mn2+ for maximum activity, the relative activity in the presence of Mg2+ being 30% of the activity with Mn2+. The solubilized enzyme retained the ability to be activated by calmodulin, with its extent being reduced as compared to the membrane-bound enzyme. The presence of a Ca2+-dependent calmodulin-binding site on the solubilized enzyme was shown by the Ca2+-dependent retention of the enzyme on a calmodulin-Sepharose-4B column.

Effect of di-n-octyl phthalate on fatty acid composition of phosphatidylcholine in Tetrahymena

We examined the effect of di-n-octyl phthalate (DOP) on fatty acid composition of phosphatidylcholine (PC) in Tetrahymena pyriformis NT-1. When Tetrahymena cells were grown in DOP-containing proteose peptone medium, the cell growth was repressed. This repression was attended by decreases in the PC content of the cells and decreases in oleic (18:1), linoleic (18:2) and linolenic (18:3) acids of PC and an increase in palmitoleic acid (16:1). The ratio of 18:1/stearic acid (18:0) of PC in cells grown in DOP-containing medium was lower than that of control cells, while the ratio of 16:1/palmitic acid (16:0) was higher than that of control. On the other hand, no changes in the ratios of 18:2/18:1 and 18:3/18:2 were observed. The activity of microsomal stearoyl-CoA desaturase from cells grown with DOP (0.63 mumol/ml medium) decreased to 27% of that from control cells, while the microsomal palmitoyl-CoA desaturase activity increased to 210% of the control value. By the addition of dioleoyl glyceride to the DOP-containing medium, the effects of DOP on Tetrahymena cells were completely blocked. These results suggest that the changes in fatty acid composition of PC may be due to the alteration of the substrate specificity of microsomal delta 9-desaturase, and the decrease in stearoyl-CoA desaturase activity may be a cause for the cell growth repression.

Expression of a cell surface immobilization antigen during serotype transformation in Tetrahymena thermophila

A temperature shift from 40 to 28 degrees C rapidly induced expression of a specific immobilization antigen at the cell surface in Tetrahymena thermophila. This transformation was inhibited by actinomycin D and cycloheximide but not by colchicine or cytochalasin B. The major surface antigen expressed at 28 degrees C in cells homozygous for the SerH3 allele was partially purified, and an antiserum against this preparation was raised in rabbits. Electrophoresis, immunoblot, and [35S]methionine incorporation studies are reported which support the conclusion that the H3 antigen is an acidic protein with an Mr of approximately 52,000 daltons. An induced synthesis of the H3 immobilization antigen was detected within 30 min after a shift from 40 to 28 degrees C. This protein appeared to be synthesized in the microsomal fraction and transferred without cleavage to the cell surface, where it was inserted first into nonciliated regions.

Effect of dimethylaminoethylphosphonate on phospholipid metabolism in Tetrahymena

Dimethylaminoethylphosphonate (DMAEP) was incorporated into the phospholipids of the ciliate protozoan Tetrahymena thermophila at the expense of both phosphatidylethanolamine and phosphatidylcholine, but it had no effect on the levels of the 2-aminoethylphosphonolipid. The newly formed DMAEP-lipid accounted for almost 50% of the phospholipids of the organism. The DMAEP was incorporated into the phospholipids using both the ethanolaminephosphotransferase and cholinephosphotransferase pathways. The DMAEP-lipid was not methylated to the trimethyl derivative, confirming the lack of methylation of phosphonolipids by Tetrahymena.

Expression of a cell surface immobilization antigen during serotype transformation in Tetrahymena thermophila

A temperature shift from 40 to 28 degrees C rapidly induced expression of a specific immobilization antigen at the cell surface in Tetrahymena thermophila. This transformation was inhibited by actinomycin D and cycloheximide but not by colchicine or cytochalasin B. The major surface antigen expressed at 28 degrees C in cells homozygous for the SerH3 allele was partially purified, and an antiserum against this preparation was raised in rabbits. Electrophoresis, immunoblot, and [35S]methionine incorporation studies are reported which support the conclusion that the H3 antigen is an acidic protein with an Mr of approximately 52,000 daltons. An induced synthesis of the H3 immobilization antigen was detected within 30 min after a shift from 40 to 28 degrees C. This protein appeared to be synthesized in the microsomal fraction and transferred without cleavage to the cell surface, where it was inserted first into nonciliated regions.

Growth-dependent changes of guanylate and adenylate cyclase activities in cilia and cell bodies of Tetrahymena pyriformis

The calmodulin-dependent guanylate cyclase of Tetrahymena pyriformis was shown previously to be localized in surface membranes (ciliary and pellicular membranes) (Kudo, S, Nakazawa, K, Nagao, S & Nozawa, Y, Japan j exp med 52 (1952) 193) [21], whereas in a recent report Schultz et al, (Schultz, J E, Schonefeld, U & Klumpp, S, Eur j biochem 137 (1983) 89) [12] demonstrated the localization of this enzyme in ciliary membrane, arguing against its presence in pellicular membrane. To examine the discrepancy, the activities of guanylate and adenylate cyclases were examined in cilia and cell bodies of Tetrahymena pyriformis during transition from early log to stationary growth phase. The guanylate cyclase activity in the cell bodies increased significantly with growth of age, while in cilia the activity was rather consistent. In contrast, adenylate cyclase did not show any growth-dependent activity changes in both cilia and cell bodies. The increase of guanylate cyclase activity was not related to the increase of its activator calmodulin, because the change in enzyme activity could not be negated by addition of a saturating amount of calmodulin. These results suggest that the content of guanylate cyclase itself would be increased in the cell bodies during growth.