Induction of choline transport and its role in the stimulation of the incorporation of choline into phosphatidylcholine by polyamines in a polyamine auxotroph of Saccharomyces cerevisiae

AGP2 encodes the major permease for high affinity polyamine import in Saccharomyces cerevisiae

Polyamines play essential functions in many aspects of cell biology. Plasma membrane transport systems for the specific uptake of polyamines exist in most eukaryotic cells but have been very recently identified at the molecular level only in the parasite Leishmania. We now report that the high affinity polyamine permease in Saccharomyces cerevisiae is identical to Agp2p, a member of the yeast amino acid transporter family that was previously identified as a carnitine transporter. Deletion of AGP2 dramatically reduces the initial velocity of spermidine and putrescine uptake and confers strong resistance to the toxicity of exogenous polyamines, and transformation with an AGP2 expression vector restored polyamine transport in agp2delta mutants. Yeast mutants deficient in polyamine biosynthesis required >10-fold higher concentrations of exogenous putrescine to restore cell proliferation upon deletion of the AGP2 gene. Disruption of END3, a gene required for an early step of endocytosis, increased the abundance of Agp2p, an effect that was paralleled by a marked up-regulation of spermidine transport velocity. Thus, AGP2 encodes the first eukaryotic permease that preferentially uses spermidine over putrescine as a high affinity substrate and plays a central role in the uptake of polyamines in yeast.

Regulatory enzymes of phosphatidylcholine biosynthesis: a personal perspective

Phosphatidylcholine is a prominent constituent of eukaryotic and some prokaryotic membranes. This Perspective focuses on the two enzymes that regulate its biosynthesis, choline kinase and CTP:phosphocholine cytidylyltransferase. These enzymes are discussed with respect to their molecular properties, isoforms, enzymatic activities, and structures, and the possible molecular mechanisms by which they participate in regulation of phosphatidylcholine levels in the cell.

Procyclic Trypanosoma brucei cell lines deficient in ornithine decarboxylase activity

Ornithine decarboxylase (ODC) is a rate limiting enzyme in the biosynthesis of polyamines. We report here the construction of ODC gene deficient Trypanosoma brucei brucei cell lines by homologous recombination and disruption of the two alleles of the ODC gene. With our first stable transfection vector, we replaced the 2.8 kb SacII ODC gene-containing fragment with a hygromycin-B-phosphotransferase gene (hph) cassette transcribed under the control of the endogenous promoter. For the second ODC allele knock-out, we stably transfected similar constructs that contained either the phleomycin or G418 resistance gene cassette, and included 1 mM putrescine in the media. These experiments resulted in two separate ODC- lines: one hygromycin and phleomycin resistant, the other hygromycin and G418 resistant. The two ODC gene knockout lines were verified by Southern and Northern hybridization, and confirmed by Western blot and enzymatic activity assay. There is no ODC expression in the two ODC- lines and the ODC messages in the single ODC gene knockouts were only half of that of the wild type. When grown in the presence of putrescine, the ODC- lines showed little difference, morphologically, from wild type trypanosomes. The growth rate of these lines varied greatly, depending on the concentration of the putrescine. Interestingly, when putrescine was completely withdrawn from the media, the ODC- trypanosomes soon reached a plateau phase and some cells remained viable for 7-8 weeks. The starved cells could be rescued by the addition of putrescine or introducing back the ODC gene. Cell cycle analysis suggested that putrescine is required for G1-S transition in the procyclic form T. brucei.

Stimulation of phosphatidylcholine biosynthesis by hemicholinium-3, a potent inhibitor of choline uptake in human leukemic monocyte-like U937 cells

The effect of hemicholinium-3 (HC-3) on choline uptake and phosphatidylcholine (PC) biosynthesis was examined in human leukemic monocyte-like U937 cells. HC-3 inhibited [3H]choline uptake in a dose- and time-dependent manner. After a 3 h treatment, HC-3 (100 microM) decreased choline uptake by as much as 80 per cent (p < 0.0001; n = 4). Reduction of incorporation of label into PC was also detected in a dose-dependent manner; the extent of inhibition, however, was always 10-20 per cent less than that observed in the total uptake. At 3 h HC-3 decreased the incorporation into PC by 65 per cent (p < 0.0001; n = 5). Kinetic studies in vivo showed that HC-3 inhibited total uptake and incorporation into PC differently, suggesting that the labelling of PC is not simply dictated by [3H]choline uptake. In separate experiments, cells were pretreated with 100 microM HC-3 for 3 h. After washing, the inhibitory effect on total uptake was no longer observed, while a 20 per cent stimulation of the incorporation into PC was obtained in these pretreated cells. In pulse-chase studies, the cells were prelabelled with [3H]choline for 30 min and chased with HC-3 for up to 3 h; the results showed a significant stimulation of incorporation into PC in a longer chase with 100 microM HC-3. After a 3 h treatment, the cytosolic CTP:cholinephosphate cytidylyltransferase (CT) was activated by 56 per cent, while choline kinase (CK) was inhibited slightly. The stimulation of CT was not simply due to the intact HC-3 molecule, and there was no redistribution of CT between cytosol and microsomes. Taken together, the results suggest that HC-3 activates PC biosynthesis apart from the inhibitory effect on choline uptake.

Oxygen toxicity in a polyamine-depleted spe2 delta mutant of Saccharomyces cerevisiae

When a mutant of Saccharomyces cerevisiae (spe2 delta) that cannot make spermidine or spermine was incubated in a polyamine-deficient medium in oxygen, there was a rapid cessation of cell growth and associated cell death. In contrast, when the mutant cells were incubated in the polyamine-deficient medium in air or anaerobically, the culture stopped growing more gradually, and there was no significant loss of cell viability. We also found that the polyamine-deficient cells grown in air, but not those grown anaerobically, showed a permanent loss of functional mitochondria ("respiratory competency"), as evidenced by their inability to grow on glycerol as the sole carbon source. These data support the postulation that polyamines act, in part, by protecting cell components from damage resulting from oxidation. However, since the mutant cells still required spermidine or spermine for growth when incubated under strictly anaerobic conditions, polyamines must also have other essential functions.

Polyamines and cell wall organization in Saccharomyces cerevisiae

Cells of Saccharomyces cerevisiae 179-5, an ornithine decarboxylase mutant (spe-1), showed several ultrastructural abnormalities when cultivated in the absence of polyamines. Besides the appearance of microvacuole-like spaces in the cytoplasm and of deformed nuclei, the most important alterations seemed to be located in the cell wall, which was thicker and of heterogeneous texture, and in the cell membrane, of irregular contour. These modifications could not be evoked by general stress conditions elicited by lack of nutrients. The relative levels of cell wall polysaccharides were altered in polyamine-deprived organisms, giving an envelope with increased mannan and decreased glucan content; this cell wall was incompletely attacked by the lytic enzyme zymolyase. Polyamine depletion led also to some abnormalities in the budding pattern. The above observations suggest the involvement of polyamines in the correct structure and organization of the yeast cell.

Comparison of the lipid regulation of yeast and rat CTP: phosphocholine cytidylyltransferase expressed in COS cells

The CTP: phosphocholine cytidylyltransferase (CT) gene from yeast and cDNA from rat liver were over-expressed 20-30-fold in COS cells. Most of the CT activities were found in the cytosolic fraction. The regulation of the yeast CT activity (Y-CT) by lipids was characterized for the first time in comparison with the regulation of the well-studied rat CT (R-CT). Sonicated vesicles composed of egg phosphatidylcholine (PC) or 1-stearoyl-2-oleoyl PC had no effect on Y-CT and only slightly stimulated R-CT activity. Both CTs were activated 10-50-fold by the anionic lipids cardiolipin, phosphatidyl-glycerol, phosphatidylinositol and oleic acid. The effects of varying the vesicle concentration and the mol% of anionic lipid in PC vesicles were tested. The concentration optima for the activation of Y-CT by oleic acid or anionic phospholipids were 5-10-fold lower than those for R-CT. For example, the stimulation of Y-CT activity by phosphatidylglycerol vesicles was optimal between 5 and 15 microM and declined at higher concentrations, but R-CT activation by these vesicles saturated at approximately 25 microM. The positively charged aminolipid sphingosine antagonized the stimulation by oleic acid of both Y-CT and R-CT. Y-CT activity was insensitive to PC vesicles containing the neutral lipids diacylglycerol, monoacylglycerol or oleyl alcohol. However, R-CT was stimulated 10-20-fold by vesicles containing these neutral lipids. Translocation of the CTs to microsomal membranes enriched with anionic or neutral lipids was compared. Oleic acid enrichment promoted translocation of Y-CT and R-CT, whereas diacylglycerol promoted only R-CT translocation. These data show that the activity of Y-CT is lipid-sensitive. Y-CT is affected only by charged lipids, whereas R-CT responds to charged and neutral lipid activators. The data are consistent with different modes of interaction of the two CTs with lipids.

Spermidine or spermine is essential for the aerobic growth of Saccharomyces cerevisiae

A null mutation in the SPE2 gene of Saccharomyces cerevisiae, encoding S-adenosylmethionine decarboxylase, results in cells with no detectable S-adenosylmethionine decarboxylase, spermidine, and spermine. This mutant has an absolute requirement for spermidine or spermine for growth; this requirement is not satisfied by putrescine. Polyamine-depleted cells show a number of microscopic abnormalities that are similar to those reported for several cell division cycle (cdc) and actin mutants. These include a striking increase in cell size, a marked decrease in budding, accumulation of vesicle-like bodies, absence of specific localization of chitin-like material, and abnormal distribution of actin-like material. The absolute requirement for polyamines for growth and the microscopic abnormalities are not seen if the cultures are grown under anaerobic conditions.

Ornithine decarboxylase in Saccharomyces cerevisiae: chromosomal assignment and genetic mapping of the SPE1 gene

The gene for ornithine decarboxylase in Saccharomyces cerevisiae, SPE1, has been assigned to chromosome XI by the technique of transverse alternating pulsed field electrophoresis and DNA-DNA hybridization. Genetic mapping by tetrad analysis shows that the SPE1 gene is located on the left arm of chromosome XI, 6 cM from the LAP1 gene and 43 cM from the TRP3 gene. The spe10 mutation previously isolated in this laboratory is mapped to the N-terminal region of the SPE1 gene, and therefore should be designated as a spe1 allele.

Molecular cloning and characterization of the gene encoding cholinephosphate cytidylyltransferase in Saccharomyces cerevisiae

1. The structural gene for cholinephosphate cytidylyltransferase (CCT) was isolated from a Saccharomyces cerevisiae genomic library by means of complementation in a mutant of the yeast defective in the enzyme. The cloned DNA restored both the growth and cholinephosphate cytidylyltransferase activity of the mutant. Whereas the enzyme of the mutant was thermolabile, the enzyme produced by the transformant was indistinguishable in heat stability from that produced by the wild type. 2. Strains carrying a multicopy recombinant plasmid overproduced cholinephosphate cytidylyltransferase. The overproduction of the enzyme brought about an increase in the synthesis of CDPcholine in the transformant, but there was no increase in the overall rate of phosphatidylcholine synthesis. 3. The cloned DNA was subcloned into a 2.5-kb DNA fragment. The nucleotide sequence which contained CCT was determined by the dideoxy chain-termination method. The sequence contained an open reading frame capable of encoding a protein of 424 amino acid residues with a calculated relative molecular mass of 49,379.31. Northern blot analysis showed that this DNA segment is transcribed in yeast cells and the length of the transcript is consistent with the putative translation product. 4. Hydropathy analysis according to Kyte and Doolittle indicated that the primary translation product contains extended hydrophilic stretches in its N- and C-terminal regions. 5. The primary translation product contains a region showing local sequence homology with nucleotidyl-transfer enzymes such as DNA polymerase (Escherichia coli), CDPdiacylglycerol pyrophosphatase (E. coli), 3-deoxy-manno-octulosonate cytidylyltransferase (E. coli) and DNA ligase (T4 phage), suggesting that these five enzymes are evolutionarily related. Statistically significant sequence homology was also noted between the human c-fos gene product and the enzyme.

Effect of growth phase on phospholipid biosynthesis in Saccharomyces cerevisiae

The effect of growth phase on the membrane-associated phospholipid biosynthetic enzymes CDP-diacylglycerol synthase, phosphatidylserine synthase, phosphatidylinositol synthase, and the phospholipid N-methyltransferases in wild-type Saccharomyces cerevisiae was examined. Maximum activities were found in the exponential phase of cells grown in complete synthetic medium. As cells entered the stationary phase of growth, the activities of the CDP-diacylglycerol synthase, phosphatidylserine synthase, and the phospholipid N-methyltransferases decreased 2.5- to 5-fold. The subunit levels of phosphatidylserine synthase and the cytoplasmic-associated enzyme inositol-1-phosphate synthase were not significantly affected by the growth phase. When grown in medium supplemented with inositol-choline, cells in the exponential phase of growth had reduced CDP-diacylglycerol synthase, phosphatidylserine synthase, and phospholipid N-methyltransferase activities, with repressed subunit levels of phosphatidylserine synthase and inositol-1-phosphate synthase compared with cells grown without inositol-choline. Enzyme activity levels remained reduced in the stationary phase of growth of cells supplemented with inositol-choline. The phosphatidylserine synthase and inositol-1-phosphate synthase subunit levels, however, were depressed. Phosphatidylinositol synthase (activity and subunit) was not affected by growth in medium supplemented with or without inositol-choline or the growth phase of the culture. The phospholipid composition of cells in the exponential and stationary phase of growth was also examined. The phosphatidylinositol to phosphatidylserine ratio doubled in stationary-phase cells. The phosphatidylcholine to phosphatidylethanolamine ratio was not significantly affected by the growth phase of cells.

Cloning of a gene encoding choline transport in Saccharomyces cerevisiae

By genetic complementation in a yeast choline transport mutant from a yeast gene library, we isolated plasmids encoding choline transport. The cloned plasmids contained a common 4.0-kilobase DNA fragment and also complemented an ethanolamine transport defect. The cloned sequence present in the yeast genome was possibly unique.

Coupling between phosphatidylinositol metabolism and cdc 28 gene product of Saccharomyces cerevisiae. On the possible mechanism of cdc 28 gene action

It was shown that the decrease in phosphatidylinositol (PI) content in cdc 28 G1-cells was due to a defect in inositol transport. This decrease in inositol transport was linked to microtubular function which was evident by the effect of a microtubular disrupting agent (colcemid) on inositol transport in stationary phase A364A cells. The involvement of PI in yeast G1 phase was further substantiated by the observation that o-phenanthroline, which blocks yeast cells in G1 phase, could inhibit inositol transport and PI levels as well. It is proposed that the regulation of PI metabolism is mediated by the gene cdc 28 and that microtubules may play a major role in the mechanism of action of this gene product.

Myo-inositol transport in Saccharomyces cerevisiae

myo-Inositol uptake in Saccharomyces cerevisiae was dependent on temperature, time, and substrate concentration. The transport obeyed saturation kinetics with an apparent Km for myo-inositol of 0.1 mM, myo-Inositol analogs, such as scyllo-inositol, 2-inosose, mannitol, and 1,2-cyclohexanediol, had no effect on myo-inositol uptake, myo-Inositol uptake required metabolic energy. Removal of D-glucose resulted in a loss of activity, and azide and cyanide ions were inhibitory. In the presence of D-glucose, myo-inositol was accumulated in the cells against a concentration gradient. A myo-inositol transport mutant was isolated from UV-mutagenized S. cerevisiae cells using the replica-printing technique. The defect in myo-inositol uptake was due to a single nuclear gene mutation. The activities of L-serine and D-glucose transport were not affected by the mutation. Thus it was shown that S. cerevisiae grown under the present culture conditions possessed a single and specific myo-inositol transport system. myo-Inositol transport activity was reduced by the addition of myo-inositol to the culture medium. The activity was reversibly restored by the removal of myo-inositol from the medium. This restoration of activity was completely abolished by cycloheximide.