Nutrient transceptors physically interact with the yeast S6/protein kinase B homolog, Sch9, a TOR kinase target

Regulations and functions of proline utilization in yeast Saccharomyces cerevisiae

The quality of alcoholic beverages strongly depends on the metabolic characteristics of the yeast cells being used. To control the aroma and the taste of alcoholic beverages, as well as the production of ethanol in them, it is thus crucial to select yeast cells with the proper characteristics. Grape must contain a high concentration of proline, an amino acid that can potentially be a useful nitrogen source. However, Saccharomyces cerevisiae cannot utilize proline during the wine-making process, resulting in the elevated levels of proline in wine and consequent negative effects on wine quality. In this article, I review and discuss recent discoveries about the inhibitory mechanisms and roles of proline utilization in yeast. The information can help in developing novel yeast strains that can improve fermentation and enhance the quality and production efficiency of wine.

Crucial aspects of metabolism and cell biology relating to industrial production and processing of Saccharomyces biomass

The multitude of applications to which Saccharomyces spp. are put makes these yeasts the most prolific of industrial microorganisms. This review considers biological aspects pertaining to the manufacture of industrial yeast biomass. It is proposed that the production of yeast biomass can be considered in two distinct but interdependent phases. Firstly, there is a cell replication phase that involves reproduction of cells by their transitions through multiple budding and metabolic cycles. Secondly, there needs to be a cell conditioning phase that enables the accrued biomass to withstand the physicochemical challenges associated with downstream processing and storage. The production of yeast biomass is not simply a case of providing sugar, nutrients, and other growth conditions to enable multiple budding cycles to occur. In the latter stages of culturing, it is important that all cells are induced to complete their current budding cycle and subsequently enter into a quiescent state engendering robustness. Both the cell replication and conditioning phases need to be optimized and considered in concert to ensure good biomass production economics, and optimum performance of industrial yeasts in food and fermentation applications. Key features of metabolism and cell biology affecting replication and conditioning of industrial Saccharomyces are presented. Alternatives for growth substrates are discussed, along with the challenges and prospects associated with defining the genetic bases of industrially important phenotypes, and the generation of new yeast strains."I must be cruel only to be kind: Thus bad begins, and worse remains behind." William Shakespeare: Hamlet, Act 3, Scene 4.

The arginine transporter Can1 acts as a transceptor for regulation of proline utilization in the yeast Saccharomyces cerevisiae

Proline is the most abundant amino acid in wine and beer, because the yeast Saccharomyces cerevisiae hardly assimilates proline during fermentation processes. Our previous studies showed that arginine induces endocytosis of the proline transporter Put4, resulting in inhibition of proline utilization. We here report a possible role of arginine sensing in the inhibition of proline utilization. We first found that two basic amino acids, ornithine, and lysine, inhibit proline utilization by inducing Put4 endocytosis in a manner similar to arginine, but citrulline does not. Our genetic screening revealed that the arginine transporter Can1 is involved in the inhibition of proline utilization by arginine. Intriguingly, the arginine uptake activity of Can1 was not required for the arginine-dependent inhibition of proline utilization, suggesting that Can1 has a function beyond its commonly known function of transporting arginine. More importantly, our biochemical analyses revealed that Can1 activates signaling cascades of protein kinase A in response to extracellular arginine. Hence, we proposed that Can1 regulates proline utilization by functioning as a transceptor possessing the activity of both a transporter and receptor of arginine.

Effect of environmental stresses during fermentation on brewing yeast and exploration on the novel flocculation-associated function of RIM15 gene

Flocculation of brewer's yeast is an environment-friendly and cost-effective way to separate yeast cells from fermentation broth for subsequent production. Diverse genetic background and complex fermentation environment cause difficulty to explore flocculation mechanism and regulate yeast flocculation. In this study, comparative transcriptome analysis was carried out between an industrial brewing yeast and its flocculation-enhanced mutant strain, unveiling the differentially-expressed genes were enriched in response to stresses. The expression level of Lg-FLO1 was the highest among all FLO genes. Environmental stresses of fermentation were simulated to stimulated yeast cells and it was found that nitrogen and amino acid starvation promoted the process of flocculation. It is the first time to reveal the nutrient-responsive gene RIM15 has a novel genetic function regulating flocculation. The study provides novel direction and strategies to manage yeast flocculation and achieve effective cell utilization in fermentation.

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.

Interaction of TOR and PKA Signaling in S. cerevisiae

TOR and PKA signaling are the major growth-regulatory nutrient-sensing pathways in S. cerevisiae. A number of experimental findings demonstrated a close relationship between these pathways: Both are responsive to glucose availability. Both regulate ribosome production on the transcriptional level and repress autophagy and the cellular stress response. Sch9, a major downstream effector of TORC1 presumably shares its kinase consensus motif with PKA, and genetic rescue and synthetic defects between PKA and Sch9 have been known for a long time. Further, studies in the first decade of this century have suggested direct regulation of PKA by TORC1. Nonetheless, the contribution of a potential direct cross-talk vs. potential sharing of targets between the pathways has still not been completely resolved. What is more, other findings have in contrast highlighted an antagonistic relationship between the two pathways. In this review, I explore the association between TOR and PKA signaling, mainly by focusing on proteins that are commonly referred to as shared TOR and PKA targets. Most of these proteins are transcription factors which to a large part explain the major transcriptional responses elicited by TOR and PKA upon nutrient shifts. I examine the evidence that these proteins are indeed direct targets of both pathways and which aspects of their regulation are targeted by TOR and PKA. I further explore if they are phosphorylated on shared sites by PKA and Sch9 or when experimental findings point towards regulation via the PP2ASit4/PP2A branch downstream of TORC1. Finally, I critically review data suggesting direct cross-talk between the pathways and its potential mechanism.

Acetyl-CoA Metabolism and Histone Acetylation in the Regulation of Aging and Lifespan

Acetyl-CoA is a metabolite at the crossroads of central metabolism and the substrate of histone acetyltransferases regulating gene expression. In many tissues fasting or lifespan extending calorie restriction (CR) decreases glucose-derived metabolic flux through ATP-citrate lyase (ACLY) to reduce cytoplasmic acetyl-CoA levels to decrease activity of the p300 histone acetyltransferase (HAT) stimulating pro-longevity autophagy. Because of this, compounds that decrease cytoplasmic acetyl-CoA have been described as CR mimetics. But few authors have highlighted the potential longevity promoting roles of nuclear acetyl-CoA. For example, increasing nuclear acetyl-CoA levels increases histone acetylation and administration of class I histone deacetylase (HDAC) inhibitors increases longevity through increased histone acetylation. Therefore, increased nuclear acetyl-CoA likely plays an important role in promoting longevity. Although cytoplasmic acetyl-CoA synthetase 2 (ACSS2) promotes aging by decreasing autophagy in some peripheral tissues, increased glial AMPK activity or neuronal differentiation can stimulate ACSS2 nuclear translocation and chromatin association. ACSS2 nuclear translocation can result in increased activity of CREB binding protein (CBP), p300/CBP-associated factor (PCAF), and other HATs to increase histone acetylation on the promoter of neuroprotective genes including transcription factor EB (TFEB) target genes resulting in increased lysosomal biogenesis and autophagy. Much of what is known regarding acetyl-CoA metabolism and aging has come from pioneering studies with yeast, fruit flies, and nematodes. These studies have identified evolutionary conserved roles for histone acetylation in promoting longevity. Future studies should focus on the role of nuclear acetyl-CoA and histone acetylation in the control of hypothalamic inflammation, an important driver of organismal aging.