Monitoring dynamics of gene expression in yeast during stationary phase

Quiescent cells maintain active degradation-mediated protein quality control requiring proteasome, autophagy, and nucleus-vacuole junctions

Many cells spend a major part of their life in quiescence, a reversible state characterized by a distinct cellular organization and metabolism. In glucose-depleted quiescent yeast cells, there is a metabolic shift from glycolysis to mitochondrial respiration, and a large fraction of proteasomes are reorganized into cytoplasmic granules containing disassembled particles. Given these changes, the operation of protein quality control (PQC) in quiescent cells, in particular the reliance on degradation-mediated PQC and the specific pathways involved, remains unclear. By examining model misfolded proteins expressed in glucose-depleted quiescent yeast cells, we found that misfolded proteins are targeted for selective degradation requiring functional 26S proteasomes. This indicates that a significant pool of proteasomes remains active in degrading quality control substrates. Misfolded proteins were degraded in a manner dependent on the E3 ubiquitin ligases Ubr1 and San1, with Ubr1 playing a dominant role. In contrast to exponentially growing cells, the efficient clearance of certain misfolded proteins additionally required intact nucleus-vacuole junctions (NVJ) and Cue5-independent selective autophagy. Our findings suggest that proteasome activity, autophagy, and NVJ-dependent degradation operate in parallel. Together, the data demonstrate that quiescent cells maintain active PQC that relies primarily on selective protein degradation. The necessity of multiple degradation pathways for the removal of misfolded proteins during quiescence underscores the importance of misfolded protein clearance in this cellular state.

A Systematic Review on Quiescent State Research Approaches in S. cerevisiae

Quiescence, the temporary and reversible arrest of cell growth, is a fundamental biological process. However, the lack of standardization in terms of reporting the experimental details of quiescent cells and populations can cause confusion and hinder knowledge transfer. We employ the systematic review methodology to comprehensively analyze the diversity of approaches used to study the quiescent state, focusing on all published research addressing the budding yeast Saccharomyces cerevisiae. We group research articles into those that consider all cells comprising the stationary-phase (SP) population as quiescent and those that recognize heterogeneity within the SP by distinguishing phenotypically distinct subpopulations. Furthermore, we investigate the chronological age of the quiescent populations under study and the methods used to induce the quiescent state, such as gradual starvation or abrupt environmental change. We also assess whether the strains used in research are prototrophic or auxotrophic. By combining the above features, we identify 48 possible experimental setups that can be used to study quiescence, which can be misleading when drawing general conclusions. We therefore summarize our review by proposing guidelines and recommendations pertaining to the information included in research articles. We believe that more rigorous reporting on the features of quiescent populations will facilitate knowledge transfer within and between disciplines, thereby stimulating valuable scientific discussion.

Quiescence in Saccharomyces cerevisiae

Most cells live in environments that are permissive for proliferation only a small fraction of the time. Entering quiescence enables cells to survive long periods of nondivision and reenter the cell cycle when signaled to do so. Here, we describe what is known about the molecular basis for quiescence in Saccharomyces cerevisiae, with emphasis on the progress made in the last decade. Quiescence is triggered by depletion of an essential nutrient. It begins well before nutrient exhaustion, and there is extensive crosstalk between signaling pathways to ensure that all proliferation-specific activities are stopped when any one essential nutrient is limiting. Every aspect of gene expression is modified to redirect and conserve resources. Chromatin structure and composition change on a global scale, from histone modifications to three-dimensional chromatin structure. Thousands of proteins and RNAs aggregate, forming unique structures with unique fates, and the cytoplasm transitions to a glass-like state.

A Geometric Clustering Tool (AGCT) to robustly unravel the inner cluster structures of time-series gene expressions

Systems biology aims at holistically understanding the complexity of biological systems. In particular, nowadays with the broad availability of gene expression measurements, systems biology challenges the deciphering of the genetic cell machinery from them. In order to help researchers, reverse engineer the genetic cell machinery from these noisy datasets, interactive exploratory clustering methods, pipelines and gene clustering tools have to be specifically developed. Prior methods/tools for time series data, however, do not have the following four major ingredients in analytic and methodological view point: (i) principled time-series feature extraction methods, (ii) variety of manifold learning methods for capturing high-level view of the dataset, (iii) high-end automatic structure extraction, and (iv) friendliness to the biological user community. With a view to meet the requirements, we present AGCT (A Geometric Clustering Tool), a software package used to unravel the complex architecture of large-scale, non-necessarily synchronized time-series gene expression data. AGCT capture signals on exhaustive wavelet expansions of the data, which are then embedded on a low-dimensional non-linear map using manifold learning algorithms, where geometric proximity captures potential interactions. Post-processing techniques, including hard and soft information geometric clustering algorithms, facilitate the summarizing of the complete map as a smaller number of principal factors which can then be formally identified using embedded statistical inference techniques. Three-dimension interactive visualization and scenario recording over the processing helps to reproduce data analysis results without additional time. Analysis of the whole-cell Yeast Metabolic Cycle (YMC) moreover, Yeast Cell Cycle (YCC) datasets demonstrate AGCT's ability to accurately dissect all stages of metabolism and the cell cycle progression, independently of the time course and the number of patterns related to the signal. Analysis of Pentachlorophenol iduced dataset demonstrat how AGCT dissects data to identify two networks: Interferon signaling and NRF2-signaling networks.

Gene expression is circular: factors for mRNA degradation also foster mRNA synthesis

Maintaining proper mRNA levels is a key aspect in the regulation of gene expression. The balance between mRNA synthesis and decay determines these levels. We demonstrate that most yeast mRNAs are degraded by the cytoplasmic 5'-to-3' pathway (the "decaysome"), as proposed previously. Unexpectedly, the level of these mRNAs is highly robust to perturbations in this major pathway because defects in various decaysome components lead to transcription downregulation. Moreover, these components shuttle between the cytoplasm and the nucleus, in a manner dependent on proper mRNA degradation. In the nucleus, they associate with chromatin-preferentially ∼30 bp upstream of transcription start-sites-and directly stimulate transcription initiation and elongation. The nuclear role of the decaysome in transcription is linked to its cytoplasmic role in mRNA decay; linkage, in turn, seems to depend on proper shuttling of its components. The gene expression process is therefore circular, whereby the hitherto first and last stages are interconnected.

Dual role of NRSF/REST in activation and repression of the glucocorticoid response

Restriction of glutamine synthetase to the nervous system is mainly achieved through the mutual function of the glucocorticoid receptor and the neural restrictive silencing factor, NRSF/REST. Glucocorticoids induce glutamine synthetase expression in neural tissues while NRSF/REST represses the hormonal response in non-neural cells. NRSF/REST is a modular protein that contains two independent repression domains, at the N and C termini of the molecule, and is dominantly expressed in nonneural cells. Neural tissues express however splice variants, REST4/5, which contain the repression domain at the N, but not at the C terminus of the molecule. Here we show that full-length NRSF/REST or its C-terminal domain can inhibit almost completely the induction of gene transcription by glucocorticoids. By contrast, the N-terminal domain not only fails to repress the hormonal response but rather stimulates it markedly. The inductive activity of the N-terminal domain is mediated by hBrm, which is recruited to the promoter only in the concomitant presence of GR. Importantly, a similar inductive activity is also exerted by the splice variant REST4. These findings raise the possibility that NRSF/REST exhibits a dual role in regulation of glutamine synthetase. It represses gene induction in nonneural cells and enhances the hormonal response, via its splice variant, in the nervous system.

Mat formation in Saccharomyces cerevisiae requires nutrient and pH gradients

The ability of Saccharomyces cerevisiae to form morphologically complex colony-like structures called mats requires expression of the cell surface glycoprotein Flo11p and growth on a semisolid surface. As the mat grows, it forms two visually distinct populations called the rim (edge of the mat) and the hub (interior of the mat), which can be physically separated from one another based on their agar adherence properties. Here, we show that growth of the mat on a semisolid agar surface creates concentric glucose and pH gradients in the medium that are required for the differentiation of the hub and rim. Disruption of the pathways that respond to changing levels of glucose block mat formation by decreasing FLO11 expression. However, in wild-type cells, Flo11p is expressed in both portions of the structure. The difference in adherence between the rim and hub appears to be a consequence of the reduced adherence of Flo11p at the elevated pH of the rim.

Exploring the ubiquitin-proteasome protein degradation pathway in yeast

This article describes an undergraduate biochemistry laboratory investigating the ubiquitin-proteasome pathway in yeast. In this exercise, the enzyme β-galactosidase (β-gal) is expressed in yeast under the control of a stress response promoter. Following exposure to heat stress to induce β-gal expression, cycloheximide is added to halt translation, and β-gal degradation is monitored by measuring enzyme activity as a function of time. Students observe that an N-Ile-β-gal variant with an N-terminal isoleucine has a significantly lower stability than wild-type β-gal, whose N-terminal residue is methionine. This strong dependence of protein stability on the N-terminal residue is known as the "N-end rule." To corroborate the enzyme activity assay results, students perform denaturing protein electrophoresis and immunoblotting of lysates, observing that the time-dependent loss of enzyme activity is coincident with the disappearance of the β-gal protein.

Release of extraction-resistant mRNA in stationary phase Saccharomyces cerevisiae produces a massive increase in transcript abundance in response to stress

A rapid transcript increase due to the release of extraction-resistant mRNAs from yeast cells in response to stress is described.

Background

As carbon sources are exhausted, Saccharomyces cerevisiae cells exhibit reduced metabolic activity and cultures enter the stationary phase. We asked whether cells in stationary phase cultures respond to additional stress at the level of transcript abundance.

Results

Microarrays were used to quantify changes in transcript abundance in cells from stationary phase cultures in response to stress. More than 800 mRNAs increased in abundance by one minute after oxidative stress. A significant number of these mRNAs encode proteins involved in stress responses. We tested whether mRNA increases were due to new transcription, rapid poly-adenylation of message (which would not be detected by microarrays), or potential release of mature mRNA present in the cell but resistant to extraction during RNA isolation. Examination of the response to oxidative stress in an RNA polymerase II mutant, rpb1-1, suggested that new transcription was not required. Quantitative RT-PCR analysis of a subset of these transcripts further suggested that the transcripts present in isolated total RNA from stationary phase cultures were polyadenylated. In contrast, over 2,000 transcripts increased after protease treatment of cell-free lysates from stationary phase but not exponentially growing cultures. Different subsets of transcripts were released by oxidative stress and temperature upshift, suggesting that mRNA release is stress-specific.

Conclusions

Cells in stationary phase cultures contain a large number of extraction-resistant mRNAs in a protease-labile, rapidly releasable form. The transcript release appears to be stress-specific. We hypothesize that these transcripts are associated with P-bodies.

Evidence that Spt10 and Spt21 of Saccharomyces cerevisiae play distinct roles in vivo and functionally interact with MCB-binding factor, SCB-binding factor and Snf1

Mutations in SPT10 and SPT21 of Saccharomyces cerevisiae have been previously shown to cause two prominent mutant phenotypes: (1) defects in transcription of particular histone genes and (2) suppression of Ty and delta-insertion mutations (Spt(-) phenotype). The requirement for Spt10 and Spt21 for transcription of particular histone genes suggested that they may interact with two factors previously shown to be present at histone loci, SBF (Swi4 and Swi6) and MBF (Mbp1 and Swi6). Therefore, we have studied swi4Delta, mbp1Delta, and swi6Delta mutants with respect to histone gene transcription and for interactions with spt10Delta and spt21Delta. Our results suggest that MBF and SBF play only modest roles in activation of histone gene transcription. In addition, we were surprised to find that swi4Delta, mbp1Delta, and swi6Delta mutations suppress the spt21Delta Spt(-) phenotype, but not the spt21Delta defect in histone gene transcription. In contrast, both swi4Delta and mbp1Delta cause lethality when combined with spt10Delta. To learn more about mutations that can suppress the spt21Delta Spt(-) phenotype, we performed a genetic screen and identified spt21Delta suppressors in seven additional genes. Three of these spt21Delta suppressors also cause lethality when combined with spt10Delta. Analysis of one spt21Delta suppressor, reg1, led to the finding that hyperactivation of Snf1 kinase, as caused by reg1Delta, suppresses the Spt(-) phenotype of spt21Delta. Taken together, these genetic interactions suggest distinct roles for Spt21 and Spt10 in vivo that are sensitive to multiple perturbations in transcription networks.

Ubiquitin fusion technique and related methods

The ubiquitin fusion technique, developed in 1986, is still the method of choice for producing a desired N-terminal residue in a protein of interest in vivo. This technique is also used as a tool for protein expression. Over the past two decades, several otherwise unrelated methods were invented that have in common the use of ubiquitin fusions as a component of design. I describe the original ubiquitin fusion technique, its current applications, and other methods that use the properties of ubiquitin fusions.

Regulation of leucine uptake by tor1+ in Schizosaccharomyces pombe is sensitive to rapamycin

TOR protein kinases are key regulators of cell growth in eukaryotes. TOR is also known as the target protein for the immunosuppressive and potentially anticancer drug rapamycin. The fission yeast Schizosaccharomyces pombe has two TOR homologs. tor1+ is required under starvation and a variety of stresses, while tor2+ is an essential gene. Surprisingly, to date no rapamycin-sensitive TOR-dependent function has been identified in S. pombe. Herein, we show that S. pombe auxotrophs, in particular leucine auxotrophs, are sensitive to rapamycin. This sensitivity is suppressed by deletion of the S. pombe FKBP12 or by introducing a rapamycin-binding defective tor1 allele, suggesting that rapamycin inhibits a tor1p-dependent function. Sensitivity of leucine auxotrophs to rapamycin is observed when ammonia is used as the nitrogen source and can be suppressed by its replacement with proline. Consistently, using radioactive labeled leucine, we show that cells treated with rapamycin or disrupted for tor1+ are defective in leucine uptake when the nitrogen source is ammonia but not proline. Recently, it has been reported that tsc1+ and tsc2+, the S. pombe homologs for the mammalian TSC1 and TSC2, are also defective in leucine uptake. TSC1 and TSC2 may antagonize TOR signaling in mammalian cells and Drosophila. We show that reduction of leucine uptake in tor1 mutants is correlated with decreased expression of three putative amino acid permeases that are also downregulated in tsc1 or tsc2. These findings suggest a possible mechanism for regulation of leucine uptake by tor1p and indicate that tor1p, as well as tsc1p and tsc2p, positively regulates leucine uptake in S. pombe.

Proteasome disassembly and downregulation is correlated with viability during stationary phase

During prolonged starvation, yeast cells enter a stationary phase (SP) during which the synthesis of many proteins is dramatically decreased. We show that a parallel decrease in proteasome-dependent proteolysis also occurs. The reduction in proteolysis is correlated with disassembly of 26S proteasome holoenzymes into their 20S core particle (CP) and 19S regulatory particle (RP) components. Proteasomes are reassembled, and proteolysis resumes prior to cell cycle reentry. Free 20S CPs are found in an autoinhibited state in which the N-terminal tails from neighboring alpha subunits are anchored by an intricate lattice of interactions blocking the channel that leads into the 20S CPs. By deleting channel gating residues of CP alpha subunits, we generated an "open channel" proteasome that exhibits faster rates of protein degradation both in vivo and in vitro, indicating that gating contributes to regulation of proteasome activity. This open channel mutant is delayed in outgrowth from SP and cannot survive following prolonged starvation. In summary, we have found that the ubiquitin-proteasome pathway can be subjected to global downregulation, that the proteasome is a target of this regulation, and that proteasome downregulation is linked to survival of SP cells. Maintaining high viability during SP is essential for evolutionary fitness, which may explain the extreme conservation of channel gating residues in eukaryotic proteasomes.

Eukaryotic translation initiation factor 4E-dependent translation is not essential for survival of starved yeast cells

The eukaryotic translation initiation factor 4E (eIF4E) interacts with the mRNA 5' cap structure (m(7)GpppX) and is essential for the appropriate translation of the vast majority of eukaryotic mRNAs. Most studies of the yeast Saccharomyces cerevisiae CDC33 gene product, eIF4E, have been carried out with logarithmically growing cells, and little is known about its role in starved, nonproliferating cells that enter the stationary phase (SP). It has previously been found that the rate of translation in SP cells is more than 2 orders of magnitude lower than it is in dividing yeast cells. Here we show that this low rate of translation is essential for maintaining the viability of starved yeast cells that enter SP. Specifically, starved cells whose eIF4A is inactive or treated with cycloheximide rapidly lose viability. Moreover, after heat inactivation of the cdc33 temperature-sensitive product, the synthesis of most proteins is abolished and only a small group of proteins is still produced. Unexpectedly, starved cdc33 mutant cells whose eIF4E is inactive and which therefore fail to synthesize the bulk of their proteins remain viable for long periods of time, indistinguishable from their isogenic wild-type counterparts. Taken together, our results indicate that eIF4E-independent translation is necessary and sufficient for survival of yeast cells during long periods of starvation.

Organization and cell-cell interaction in starved Saccharomyces cerevisiae colonies

Cell growth in yeast colonies is a complex process, the control of which is largely unknown. Here we present scanning electron micrographs of Saccharomyces cerevisiae colonies, showing changes in the pattern of cell organization and cell-cell interactions during colony development. In young colonies (</=36 h), cell density is relatively low, and the cells seem to divide in a random orientation. However, as the colonies age, cell density increases and the cells seem to be oriented in a more orderly fashion. Unexpectedly, cells in starved colonies form connecting fibrils. A single connecting fibril 180 +/- 50 nm wide is observed between any two neighboring cells, and the fibrils appear to form a global network. The results suggest a novel type of communication between cells within a colony that may contribute to the ability of the community to cope with starvation.