Biology subjects

Shiozaki, K.

Publications and source records attributed to Shiozaki, K..

5 recordsLinked to original sources

bioRxiv2025

Multiple ammonium transporters in fission yeast are coordinated by transcriptional and localization regulation in response to nitrogen starvation

Ammonium is a preferred nitrogen source for microorganisms. We investigated the regulation of ammonium transporters Amt1, Amt2, and Amt3 in the fission yeast Schizosaccharomyces pombe. Expression of the amt1+ gene increases under nitrogen starvation as well as in the TORC1-deficient mutant tor2-287, suggesting the negative regulation of amt1+ by TORC1. This regulation depends on the GATA transcription factor Gaf1, whose phosphorylation is regulated by Ppe1 and PP2A phosphatases. Ppe1 is required for the nuclear accumulation of Gaf1, partly contributing to the controlled expression of amt1+. On the other hand, PP2A phosphatase is essential for the amt1+ induction upon starvation, indicating distinct roles for Ppe1 and PP2A in Gaf1 regulation. Nitrogen starvation also promotes the plasma membrane translocation of Amt1 and Amt2 in a manner dependent on the Tsc-Rhb1 pathway. Intriguingly, the amt1{Delta} amt2{Delta} tsc{Delta} mutant is more sensitive to low-ammonium conditions than the amt1{Delta} amt2{Delta} amt3{Delta} triple mutant. Thus, there may be an unidentified ammonium transporter whose plasma membrane localization is also dependent on the Tsc-Rhb1 pathway.

cell biology↗

bioRxiv2025

Thermosensitivity of cellular translation restricts the growth of fission yeast at high temperatures

Living organisms have thermal limits above which they are unable to operate and survive. Our previous genetic screen identified proteins that impede the high-temperature growth of fission yeast, including the RNA-binding protein Dri1 and a fission yeast-specific protein termed Rhs1. Here, we show that Dri1 and Rhs1 form a complex and physically interact with the Ccr4-Not complex, a master regulator of mRNA metabolism. Gene expression analysis revealed that the Dri1-Rhs1 and Ccr4-Not complexes negatively regulate a set of genes implicated in ribosome biogenesis (Ribi genes). Loss of the Dri1-Rhs1 complex results in the augmented expression of Ribi genes, thereby suppressing the accumulation of 80S monosomes and the growth inhibition under high-temperature conditions. The thermosensitivity of the translational processes may be a determinant of the upper limit of the growth temperature in fission yeast.

cell biology↗

bioRxiv2025

Exomer cooperates with Ryh1RAB6 in regulating TORC2 activity

Exomer is a trans-Golgi protein complex involved in multiple biological processes, including lipid homeostasis and stress response. We have found that in fission yeast, the absence of exomer leads to a defect in the recovery of "target of rapamycin complex 2" (TORC2) activity in response to high concentrations of KCl, but not to sorbitol, which indicates that the mutants are impaired in their ability to respond to salt stress. Changes in lipid homeostasis did not suppress this defect. Ryh1RAB6 is a Rab GTPase that resides in the Golgi and endosomes and facilitates or stabilizes the interaction between TORC2 and its substrate, the AGC kinase Gad8. We have found genetic and functional interactions between exomer and Ryh1RAB6. The recovery of TORC2 activity in response to KCl requires Ryh1RAB6 and exomer, and the localization of Ryh1RAB6 is aberrant in exomer mutants. Co-immunoprecipitation experiments indicate that the interaction between TORC2 and Gad8 is weaker in the absence of exomer. We propose that exomer regulates TORC2 by facilitating appropriate Ryh1 localization.

cell biology↗

bioRxiv2023

Rapamycin-sensitive mechanisms confine the growth of fission yeast below the temperatures detrimental to cell physiology

Cells cease to proliferate above their growth-permissible temperatures, a ubiquitous phenomenon generally attributed to protein denaturing and heat damage to other cellular macromolecules. We here report that, in the presence of the macrolide compound rapamycin, the fission yeast Schizosaccharomyces pombe can proliferate at high temperatures that normally arrest its growth. Rapamycin is a potent inhibitor of the protein kinase complex TOR Complex 1 (TORC1), and consistently, mutations to the TORC1 subunit RAPTOR/Mip1 and the TORC1 substrate Sck1 significantly improve cellular heat resistance. These results suggest that TORC1, a well-established growth promoter, restricts the high-temperature growth of fission yeast and that compromised TORC1 signaling allows cell proliferation at higher temperatures. Aiming for a more comprehensive understanding of the negative regulation of high-temperature growth, we conducted genome-wide screens in S. pombe, which identified Sck1 and additional factors that appear to suppress cell proliferation at high temperatures. Our study has uncovered unexpected mechanisms of growth restraint even below the temperatures deleterious to cell physiology. Thus, growth arrest at high temperatures may not directly result from heat damage to cellular components essential for proliferation and viability. Significance StatementThe immunosuppressant rapamycin is a specific inhibitor of the protein kinase Target Of Rapamycin (TOR), and the drug is known to extend the lifespan of diverse eukaryotic organisms. In this study, we have found that rapamycin confers heat resistance on fission yeast, allowing its proliferation above the normal permissive temperatures. This unexpected observation suggests that TOR, which is known as a growth-promoting kinase, is inhibitory to cell proliferation at high temperatures. Our genome-wide screens have identified additional genes whose deletion leads to improved growth under heat stress. Thus, cells may have mechanisms that restrict proliferation even below the temperatures deleterious to their physiology.

cell biology↗

bioRxiv2018

PP2AB55δ Responsible for the High Initial Rates of Alcoholic Fermentation in Sake Yeast Strains of Saccharomyces cerevisiae

Sake yeast strain Kyokai no. 7 (K7) and its Saccharomyces cerevisiae relatives carry a homozygous loss-of-function mutation in the RIM15 gene, which encodes a Greatwall-family protein kinase. Disruption of RIM15 in non-sake yeast strains leads to improved alcoholic fermentation, indicating that the defect in Rim15p is associated with the enhanced fermentation performance of sake yeast cells. In order to understand how Rim15p mediates fermentation control, we here focused on target-of-rapamycin protein kinase complex 1 (TORC1) and protein phosphatase 2A with the B55{Delta} regulatory subunit (PP2AB55{delta}), complexes that are known to act upstream and downstream of Rim15p, respectively. Several lines of evidence, including our previous transcriptomic analysis data, suggested enhanced TORC1 signaling in sake yeast cells during sake fermentation. Fermentation tests of the TORC1-related mutants using a laboratory strain revealed that TORC1 signaling positively regulates the initial fermentation rate in a Rim15p-dependent manner. Deletion of the CDC55 gene encoding B55{delta} abolished the high fermentation performance of Rim15p-deficient laboratory yeast and sake yeast cells, indicating that PP2AB55{delta} mediates the fermentation control by TORC1 and Rim15p. The TORC1-Greatwall-PP2AB55{delta} pathway similarly affected the fermentation rate in the fission yeast Schizosaccharomyces pombe, strongly suggested that the evolutionarily conserved pathway governs alcoholic fermentation in yeasts. It is likely that elevated PP2AB55{delta} activity accounts for the high fermentation performance of sake yeast cells. Heterozygous loss-of-function mutations in CDC55 found in K7-related sake strains may indicate that the Rim15p-deficient phenotypes are disadvantageous to cell survival.\n\nIMPORTANCEThe biochemical processes and enzymes responsible for glycolysis and alcoholic fermentation by the yeast S. cerevisiae have long been the subject of scientific research. Nevertheless, the factors determining fermentation performance in vivo are not fully understood. As a result, the industrial breeding of yeast strains has required empirical characterization of fermentation by screening numerous mutants through laborious fermentation tests. To establish a rational and efficient breeding strategy, key regulators of alcoholic fermentation need to be identified. In the present study, we focused on how sake yeast strains of S. cerevisiae have acquired high alcoholic fermentation performance. Our findings provide a rational molecular basis to design yeast strains with optimal fermentation performance for production of alcoholic beverages and bioethanol. In addition, as the evolutionarily conserved TORC1-Greatwall-PP2AB55{delta} pathway plays a major role in the glycolytic control, our work may contribute to research on carbohydrate metabolism in higher eukaryotes.

microbiology↗