The pleiotropic effect of the GTS1 gene product on heat tolerance, sporulation and the life span of Saccharomyces cerevisiae

Overexpression of Q-rich prion-like proteins suppresses polyQ cytotoxicity and alters the polyQ interactome

Expansion of a poly-glutamine (polyQ) repeat in a group of functionally unrelated proteins is the cause of several inherited neurodegenerative disorders, including Huntington's disease. The polyQ length-dependent aggregation and toxicity of these disease proteins can be reproduced in Saccharomyces cerevisiae. This system allowed us to screen for genes that when overexpressed reduce the toxic effects of an N-terminal fragment of mutant huntingtin with 103 Q. Surprisingly, among the identified suppressors were three proteins with Q-rich, prion-like domains (PrDs): glycine threonine serine repeat protein (Gts1p), nuclear polyadenylated RNA-binding protein 3, and minichromosome maintenance protein 1. Overexpression of the PrD of Gts1p, containing an imperfect 28 residue glutamine-alanine repeat, was sufficient for suppression of toxicity. Association with this discontinuous polyQ domain did not prevent 103Q aggregation, but altered the physical properties of the aggregates, most likely early in the assembly pathway, as reflected in their increased SDS solubility. Molecular simulations suggested that Gts1p arrests the aggregation of polyQ molecules at the level of nonfibrillar species, acting as a cap that destabilizes intermediates on path to form large fibrils. Quantitative proteomic analysis of polyQ interactors showed that expression of Gts1p reduced the interaction between polyQ and other prion-like proteins, and enhanced the association of molecular chaperones with the aggregates. These findings demonstrate that short, Q-rich peptides are able to shield the interactive surfaces of toxic forms of polyQ proteins and direct them into nontoxic aggregates.

ROS production and apoptosis induction by formation of Gts1p-mediated protein aggregates

GTS1 of Saccharomyces cerevisiae is a pleiotropic gene. Its induction leads to a variety of biological phenomena represented by cell aggregation. The C-terminal polyglutamine sequence in Gts1p is indispensable for its pleiotropy and nuclear localization. This sequence is often observed in polyglutamine diseases, such as Huntington disease, and is believed to induce protein aggregation, leading to cell death. In this study, protein aggregates were formed in a polyglutamine-dependent manner in cells inducing GTS1, and heat-shock protein family, translation elongation factor, and mitochondrial proteins were trapped in Gts1p-mediated protein aggregates. Moreover, the polyglutamine sequence of Gts1p was indispensable to the induction of reactive oxygen species (ROS) production and apoptosis. Deletion of the genes encoding Por1p and Yhb1p altered the profiles of ROS production and apoptosis caused by GTS1 induction, suggesting that the trapping of these proteins in Gts1p-mediated protein aggregates inhibits the intrinsic functions of these proteins.

New insights into the Saccharomyces cerevisiae fermentation switch: dynamic transcriptional response to anaerobicity and glucose-excess

Background

The capacity of respiring cultures of Saccharomyces cerevisiae to immediately switch to fast alcoholic fermentation upon a transfer to anaerobic sugar-excess conditions is a key characteristic of Saccharomyces cerevisiae in many of its industrial applications. This transition was studied by exposing aerobic glucose-limited chemostat cultures grown at a low specific growth rate to two simultaneous perturbations: oxygen depletion and relief of glucose limitation.

Results

The shift towards fully fermentative conditions caused a massive transcriptional reprogramming, where one third of all genes within the genome were transcribed differentially. The changes in transcript levels were mostly driven by relief from glucose-limitation. After an initial strong response to the addition of glucose, the expression profile of most transcriptionally regulated genes displayed a clear switch at 30 minutes. In this respect, a striking difference was observed between the transcript profiles of genes encoding ribosomal proteins and those encoding ribosomal biogenesis components. Not all regulated genes responded with this binary profile. A group of 87 genes showed a delayed and steady increase in expression that specifically responded to anaerobiosis.

Conclusion

Our study demonstrated that, despite the complexity of this multiple-input perturbation, the transcriptional responses could be categorized and biologically interpreted. By comparing this study with public datasets representing dynamic and steady conditions, 14 up-regulated and 11 down-regulated genes were determined to be anaerobic specific. Therefore, these can be seen as true "signature" transcripts for anaerobicity under dynamic as well as under steady state conditions.

Role of Gts1p in regulation of energy-metabolism oscillation in continuous cultures of the yeast Saccharomyces cerevisiae

Energy-metabolism oscillation (EMO) in an aerobic chemostat culture of yeast is basically regulated by a feedback loop of redox reactions in energy metabolism and modulated by metabolism of storage carbohydrates. In this study, we investigated the role of Gts1p in the stabilization of EMO, using the GTS1-deleted transformant gts1Delta. We found that fluctuations in the redox state of the NAD co-factor and levels of redox-regulated metabolites in glycolysis, especially of ethanol, are markedly reduced in amplitude during EMO of gts1Delta, while respiration indicated by the oxygen uptake rate (OUR) and energy charge is not so affected throughout EMO in gts1Delta. Further, the transitions of the levels of OUR, NAD(+) : NADH ratio and intracellular pH between the two phases were apparently retarded compared with those in the wild-type, suggesting attenuation of EMO in gts1Delta. Furthermore, the mRNA levels of genes encoding enzymes for the synthesis of trehalose and glycogen are fairly reduced in gts1Delta, consistent with the decreased synthesis of storage carbohydrates. In addition, the level of inorganic phosphate, which is required for the reduction of NAD(+) and mainly supplied from trehalose synthesis, was decreased in the early respiro-fermentative phase in gts1Delta. Thus, we suggested that the deletion of GTS1 as a transcriptional co-activator for these genes inhibited the metabolism of storage carbohydrates, which causes attenuation of the feedback loop of dehydrogenase reactions in glycolysis with the restricted fluctuation of ethanol as a main synchronizing agent for EMO in a cell population.

Destabilization of energy-metabolism oscillation in the absence of trehalose synthesis in the chemostat culture of yeast

Energy-metabolism oscillation (EMO) in yeast is basically regulated by a feedback-loop of redox reactions and modulated by the metabolism of storage carbohydrates like glycogen and trehalose. We found that EMO of the transformant tps1Delta deleted of TPS1 encoding trehalose-6-phosphate synthase fluctuated unsteadily with a short wavelength in the absence of trehalose synthesis, while EMO was gradually destabilized with the wavelength increasing as storage in a frozen state was prolonged. During EMO, whereas the fluctuations in levels of the oxygen uptake rate, NAD(P)H and cAMP were attenuated, the glycerol level fluctuated with high amplitude and the levels of glycogen and ethanol fluctuated with similar amplitudes to those in the wild type. Thus, EMO barely operated in tps1Delta depending on the increase of glycerol synthesis as a source of inorganic phosphate in place of trehalose synthesis and fairly conserved fluctuation in the level of ethanol as a synchronizing agent.

Localization of Gts1p in cortical actin patches of yeast and its possible role in endocytosis

Herein we report that Gts1p fused with green-fluorescent protein (GFP) is localized in the cortical actin patch besides nuclei in yeast and the cortical Gts1p changed its position together with the patch depending on the cell-cycle phase, while nuclear Gts1p accumulated predominantly in the budding phase. Whereas Gts1p does not directly bind to actin, it associated mainly with the actin-associated protein Pan1p. In the GTS1-deleted transformant gts1Delta, the number of cells containing either a fragmented vacuole or an enlarged single central vacuole increased and the uptake of the hydrophilic dye Lucifer yellow (LY) in the vacuole decreased. Further, gts1Delta transformed with a mutant Gts1p having two cysteine-to-alanine substitutions in a zinc finger resembling that of GTPase-activating proteins of ADP-ribosylation factors (ARF-GAP) neither recovered the LY uptake unlike gts1Delta transformed with the wild-type GTS1, nor reduced the average size of central vacuoles as much as the latter did. These results suggested that Gts1p in the actin patch is involved in the fluid-phase endocytosis and membrane trafficking for vacuole formation and that the putative ARF-GAP domain in Gts1p plays an important role in these functions.

Gts1p stabilizes oscillations in energy metabolism by activating the transcription of TPS1 encoding trehalose-6-phosphate synthase 1 in the yeast Saccharomyces cerevisiae

We reported previously that Gts1p regulates oscillations of heat resistance in concert with those of energy metabolism in continuous cultures of the yeast Saccharomyces cerevisiae by inducing fluctuations in the levels of trehalose, but not in those of Hsp104 (heat shock protein 104). Further, the expression of TPS1, encoding trehalose-6-phosphate synthase 1, and HSP104 was activated by Gts1p in combination with Snf1 kinase, a transcriptional activator of glucose-repressible genes, in batch cultures under derepressed conditions. Here we show that, in continuous cultures, the mRNA level of TPS1 increased 6-fold in the early respiro-fermentative phase, while that of HSP104 did not change. The expression of SUC2, a representative glucose-repressible gene encoding invertase, also fluctuated, suggesting the involvement of the Snf1 kinase in the periodic activation of these genes. However, this possibility was proven to be unlikely, since the oscillations in both TPS1 and SUC2 mRNA expression were reduced by approx. 3-fold during the transient oscillation in gts1Delta (GTS1-deleted) cells, in which the energy state determined by extracellular glucose and intracellular adenine nucleotide levels was comparable with that in wild-type cells. Furthermore, neither the mRNA level nor the phosphorylation status of Snf1p changed significantly during the oscillation. Thus we suggest that Gts1p plays a major role in the oscillatory expression of TPS1 and SUC2 in continuous cultures of Saccharomyces cerevisiae, and hypothesized that Gts1p stabilizes oscillations in energy metabolism by activating trehalose synthesis to facilitate glycolysis at the shift from the respiratory to the respiro-fermentative phase.

Gts1p activates SNF1-dependent derepression of HSP104 and TPS1 in the stationary phase of yeast growth

We previously reported that the GTS1 product, Gts1p, plays an important role in the regulation of heat tolerance of yeast under glucose-limited conditions in either batch or continuous culture. Here we show that heat tolerance was decreased in GTS1-deleted and increased in GTS1-overexpressing cells under glucose-derepressed conditions during the batch culture and that the disruption of SNF1, a transcriptional activator of glucose-repressible genes, diminished this effect of GTS1. Intracellular levels of Hsp104 and trehalose, which were reportedly required for the acquisition of heat tolerance in the stationary phase of cell growth, were affected in both GTS1 mutants roughly in proportion to the gene dosage of GTS1, whereas those of other Hsps were less affected. The mRNA levels of genes for Hsp104 and trehalose-6-phosphate synthase 1 changed as a function of GTS1 gene dosage. The Q-rich domain of Gts1p fused with the DNA-binding domain of LexA activated the transcription of the reporter gene LacZ, and Gts1p lacking the Q-rich domain lost the activation activity of HSP104 and TPS1. Furthermore, Gts1p bound to subunits of Snf1 kinase, whereas it did not bind to DNA. Therefore, we suggested that GTS1 increases heat tolerance by mainly activating Snf1 kinase-dependent derepression of HSP104 and TPS1 in the stationary phase of yeast growth.

Severe reduction of superoxide dismutase activity in the yeast Saccharomyces cerevisae with the deletion or overexpression of GTS1

We report herein that the level of reactive oxygen species (ROS) observed using dihydrorhodamine is much higher in either GTS1-deleted (gts1Delta) or GTS1-overexpressing (TMpGTS1) transformants than in the wild-type and that the levels of protein carbonyls are increased and the glutathione levels are decreased in both transformants. Consistently, the activities of superoxide dismutases (SODs) in both gts1Delta and TMpGTS1 were severely weakened, while the protein levels of both Cu/Zn-SOD and Mn-SOD were not so changed. As the intracellular copper levels were significantly increased in both transformants, we hypothesized that, in either gts1Delta or TMpGTS1 cells, the imbalanced homeostasis of copper induced an accumulation of ROS which caused inactivation of SODs further increasing ROS levels.

The Gts1 protein stabilizes the autonomous oscillator in yeast

Continuous cultures of Saccharomyces cerevisiae show a robust autonomous temperature compensated oscillation in many metabolic functions. Respiratory activity, a convenient output to measure, oscillates with a period of 40 min. Deletion of GTS1, whose protein product has homology to the circadian per protein, has been implicated in temporal events within yeast, causes a reduction in periodicity to 18 min (wild-type period 40-60 min). The dilution rate was steadily increased from 0.04/h to 0.085/h and the oscillation stabilized after four to six dilutions. However, Gts1p's involvement in the maintenance and generation of metabolic synchrony, and in the central oscillating loop, appear to be minimal, as the mutant oscillation was robust and autonomous. Deletion of GTS1 did cause decreased temperature compensation of the period of the oscillation from Q(10) = 1.07 for the wild-type to Q(10) = 1.6 for the mutant. Also the degree of nutrient compensation observed for the wild-type was not observed in the GTS1-null mutant strain. It is postulated that Gts1p is involved in the mechanism that communicates external conditions, such as temperature, to the central oscillating loop.

Interaction of the GTS1 gene product with glyceraldehyde- 3-phosphate dehydrogenase 1 required for the maintenance of the metabolic oscillations of the yeast Saccharomyces cerevisiae

We previously reported that GTS1 is involved in regulating ultradian oscillations of the glycolytic pathway induced by cyanide in cell suspensions as well as oscillations of energy metabolism in aerobic continuous cultures. Here, we screened a yeast cDNA library for proteins that bind to Gts1p using the yeast two-hybrid system and cloned multiple TDH cDNAs encoding the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). We found that the zinc-finger and dimerization sites of Gts1p were required for full ability to bind GAPDH, and Gts1ps mutated at these sites lost the ability to regulate both aerobic and unaerobic ultradian oscillations of energy metabolism. Of the three TDH genes, only TDH1 fluctuated at the mRNA level in continuous culture and its deletion resulted in the disappearance of the oscillation without any affect on growth rate. This loss of biological rhythms in the TDH1-deleted mutant was rescued by the expression of TDH1 but not of TDH2 or TDH3 under the control of the TDH1 promoter. Thus, we hypothesized that Gts1p plays a role in the regulation of metabolic oscillation by interacting with the TDH1 product, GAPDH1, in yeast.

Ultradian rhythm of trehalose levels coupled to heat resistance in continuous cultures of the yeast Saccharomyces cerevisiae

Heat resistance appears to cycle in concert with energy metabolism in continuous culture of the yeast Saccharomyces cerevisiae. To study the mechanism of this oscillation, the authors first examined if heat shock proteins (Hsps) are involved. Neither the protein levels of major Hsps nor the expression of the beta-galactosidase gene as a reporter under the control of the promoter carrying heat-shock element oscillated during the metabolic oscillation. The level of trehalose in yeast cycled with the same periodicity, as did energy metabolism. This oscillation was not found in a GTS1-deleted mutant that also did not show cyclic changes in heat resistance. These results suggest that heat resistance oscillation is induced by fluctuations in trehalose level and not by an oscillatory expression of Hsps. The increase in trehalose began at the start of the respiro-fermentative phase and the decrease began after the elevation of the cyclic adenosine monophosphate (cAMP) level. The authors hypothesize that the synthesis of trehalose parallels the activation of the glycolytic pathway and that trehalose is degraded by trehalase activated by cAMP coupled with the metabolic oscillation in the continuous culture of yeast.

Clock control of ultradian respiratory oscillation found during yeast continuous culture

A short-period autonomous respiratory ultradian oscillation (period approximately 40 min) occurs during aerobic Saccharomyces cerevisiae continuous culture and is most conveniently studied by monitoring dissolved O(2) concentrations. The resulting data are high quality and reveal fundamental information regarding cellular dynamics. The phase diagram and discrete fast Fourier transformation of the dissolved O(2) values revealed a square waveform with at least eight harmonic peaks. Stepwise changes in temperature revealed that the oscillation was temperature compensated at temperatures ranging from 27 to 34 degrees C when either glucose (temperature quotient [Q(10)] = 1.02) or ethanol (Q(10) = 0.82) was used as a carbon source. After alteration of the temperature beyond the temperature compensation region, phase coherence events for individual cells were quickly lost. As the cell doubling rate decreased from 15.5 to 9.2 h (a factor of 1.68), the periodicity decreased by a factor of 1.26. This indicated that there was a degree of nutrient compensation. Outside the range of dilution rates at which stable oscillation occurred, the mode of oscillation changed. The oscillation in respiratory output is therefore under clock control.

Functional correlation between the nuclear localization of Fht1p and its flocculation and heat tolerance activities in budding yeast Saccharomyces cerevisiae

Fht1p is involved in the flocculation and heat tolerance machinery of budding yeast Saccharomyces cerevisiae. Despite knowledge of its involvement in those phenotypes, a precise mechanism has yet to be discovered. To this end, we monitored the relationship between subcellular localization of Fht1p and its flocculation or heat tolerance function using newly developed expression vectors with a recombinant green fluorescent protein (GFP; S65T/S147P) of Aequorea victoria added at both the N- and C-terminus of Fht1p. The main fluorescent signal of the GFP tagged with either a wild-type Fht1p or mutants which preserve their flocculation function was detected in the nucleus, whereas signals of functionless mutants were dispersed to the cytoplasm.

Evidence for the involvement of the GTS1 gene product in the regulation of biological rhythms in the continuous culture of the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, ultradian oscillations of energy metabolism have been observed in continuous cultures. Here, we found that the level of the GTS1 gene product oscillated in concert with the ultradian rhythm of energy metabolism. When GTS1 was inactivated by gene disruption, the metabolic oscillation was affected severely, mostly disappearing within a day, in the absence of synchronized stress-response oscillations throughout the continuous culture. The disappearance of biological rhythms in the GTS1-deleted mutant was substantially rescued by transformation with chimera plasmids carrying GTS1 under the control of GTS1's own promoter. On the other hand, this disappearance was not rescued by constitutive expression of GTS1 under the control of the triose phosphate isomerase promoter.

Phosphorylation of the GTS1 gene product of the yeast Saccharomyces cerevisiae and its effect on heat tolerance and flocculation

The GTS1 gene from the yeast Saccharomyces cerevisiae showed pleiotropic effects on yeast phenotypes, including an increase of heat tolerance in stationary-phase cells and an induction of flocculation. Here, we found that the GTS1 product, Gts1p, was partially phosphorylated at some serine residue(s) in cells grown on glucose. Studies using mutants of protein kinase A (PKA) and CDC25, the Ras-GTP exchange activator, showed that PKA positively regulated the phosphorylation level of Gts1p. Overexpression of Gts1p in a mutant with attenuated PKA activity did not show any increase of heat tolerance and partially decreased flocculation inducibility, suggesting that phosphorylation of Gts1p is required for induction of these phenomena.

Protein interactions of Gts1p of Saccharomyces cerevisiae throughout a region similar to a cytoplasmic portion of some ATP-binding cassette transporters

The GTS1 gene product, Gts1p, has pleiotropic effects on the timing of budding, cell size, heat tolerance, sporulation and the lifespan of the yeast Saccharomyces cerevisiae. In this study, we found (using the yeast two-hybrid system) that Gts1p forms homodimers throughout the 18-amino acid region 296-313 which has considerable similarity to a region downstream of the Walker nucleotide-binding motif A of some ATP-binding cassette (ABC) transporters. The region contains two aspartic acid residues at 301 and 310 preceded by hydrophobic amino acid residues, and Gts1p with an Asp310 to Ala substitution showed considerably reduced homodimerization, as shown by the two-hybrid assay. Overexpression of the point-mutated Gts1p did not efficiently induce the Gts1p-related phenotypes described above, suggesting that the homodimerization of Gts1p is required for it to function in vivo. The C-terminal cytoplasmic domain of the yeast ABC transporters Mdl1p (multidrug resistance-like transporter) and Ycf1p (yeast cadmium factor or glutathione S-conjugate pump) bound to Gts1p in the two-hybrid system, and the heterodimerization activity of the Gts1p with the Asp301 to Ala substitution was more affected than the Gts1p with the Asp310 to Ala substitution. Overexpression of GTS1 considerably reduced, and disruption of GTS1 slightly decreased, cellular resistance to cycloheximide, cadmium, cisplatin and 1-chloro-2,4-dinitrophenol, which (except for cycloheximide) are all substrates of Ycf1p. These results suggest that Gts1p interacts with some ABC transporters through the binding site overlapping that of homodimerization and modulates their activity.

The GTS1 gene product influences the ultradian oscillation of glycolysis in cell suspension of the yeast Saccharomyces cerevisiae

We tested the effect of the GTS1 gene of the yeast Saccharomyces cerevisiae on the cyanide-induced ultradian oscillation of the glycolytic metabolite NADH in cell suspension of strains with different copy numbers of the gene, that is, the wild-type, GTS1-disrupted and GTS1-overexpressing strains. The cells showed long-lasting oscillations when harvested between 1 and 2 hours after the diauxic shift from glucose to ethanol as a growth substrate. The frequencies of oscillation did not vary very much among the three strains tested. However, the amplitudes and durations of the oscillation were changed significantly as a function of the GTS1 gene-dosage. The effect of GTS1 on the amplitude was not caused by changing rates of glucose incorporation into cells as the rates were the same among the three strains during the macroscopic oscillation.

The ultradian clocks of eukaryotic microbes: timekeeping devices displaying a homeostasis of the period

Temperature compensation of their period is one of the canonical characteristics of circadian rhythms, yet it is not restricted to circadian rhythms. This short review summarizes the evidence for ultradian rhythms, with periods from 1 minute to several hours, that likewise display a strict temperature compensation. They have been observed mostly in unicellular organisms in which their constancy of period at different temperatures, as well as under different growth conditions (e.g., medium type, carbon source), indicates a general homeostasis of the period. Up to eight different parameters, including cell division, cell motility, and energy metabolism, were observed to oscillate with the same periodicity and therefore appear to be under the control of the same central pacemaker. This suggests that these ultradian clocks should be considered as cellular timekeeping devices that in fast-growing cells take over temporal control of cellular functions controlled by the circadian clock in slow-growing or nongrowing cells. Being potential relatives of circadian clocks, these ultradian rhythms may serve as model systems in chronobiological research. Indeed, mutations have been found that affect both circadian and ultradian periods, indicating that the respective oscillators share some mechanistic features. In the haploid yeast Schizosaccharomyces pombe, a number of genes have been identified where mutation, deletion, or overexpression affect the ultradian clock. Since most of these genes play roles in cellular metabolism and signaling, and mutations have pleiotropic effects, it has to be assumed that the clock is deeply embedded in cellular physiology. It is therefore suggested that mechanisms ensuring temperature compensation and general homeostasis of period are to be sought in a wider context.