Cross-talk between autophagy and sporulation in Saccharomyces cerevisiae

Meiotic Cytokinesis in Saccharomyces cerevisiae: Spores That Just Need Closure

In the budding yeast Saccharomyces cerevisiae, sporulation occurs during starvation of a diploid cell and results in the formation of four haploid spores forming within the mother cell ascus. Meiosis divides the genetic material that is encapsulated by the prospore membrane that grows to surround the haploid nuclei; this membrane will eventually become the plasma membrane of the haploid spore. Cellularization of the spores occurs when the prospore membrane closes to capture the haploid nucleus along with some cytoplasmic material from the mother cell, and thus, closure of the prospore membrane is the meiotic cytokinetic event. This cytokinetic event involves the removal of the leading-edge protein complex, a complex of proteins that localizes to the leading edge of the growing prospore membrane. The development and closure of the prospore membrane must be coordinated with other meiotic exit events such as spindle disassembly. Timing of the closure of the prospore membrane depends on the meiotic exit pathway, which utilizes Cdc15, a Hippo-like kinase, and Sps1, an STE20 family GCKIII kinase, acting in parallel to the E3 ligase Ama1-APC/C. This review describes the sporulation process and focuses on the development of the prospore membrane and the regulation of prospore membrane closure.

Heterozygous Mutations in Aromatic Amino Acid Synthesis Genes Trigger TOR Pathway Activation in Saccharomyces cerevisiae

The highly conserved complexes of Target of Rapamycin (TORC1 and TORC2) are central regulators to many vital cellular processes including growth and autophagy in response to nutrient availability. Previous research has extensively elucidated exogenous nutrient control on TORC1/TORC2; however, little is known about the potential alteration of nutrient pools from mutations in biosynthesis pathways and their impact on Tor pathway activity. Here, we analyze the impacts of heterozygous mutations in aromatic amino acid biosynthesis genes on TOR signaling via differential expression of genes downstream of TORC1 and autophagy induction for TORC1 and TORC2 activity.

Actin dynamics in protein homeostasis

Cell homeostasis is maintained in all organisms by the constant adjustment of cell constituents and organisation to account for environmental context. Fine-tuning of the optimal balance of proteins for the conditions, or protein homeostasis, is critical to maintaining cell homeostasis. Actin, a major constituent of the cytoskeleton, forms many different structures which are acutely sensitive to the cell environment. Furthermore, actin structures interact with and are critically important for the function and regulation of multiple factors involved with mRNA and protein production and degradation, and protein regulation. Altogether, actin is a key, if often overlooked, regulator of protein homeostasis across eukaryotes. In this review, we highlight these roles and how they are altered following cell stress, from mRNA transcription to protein degradation.

Yeast phospholipase D, Spo14, is not required for macroautophagy

Autophagy-related gene (Atg) proteins are key players in autophagy. Some proteins that function in vesicle trafficking and lipid metabolism are also involved in autophagy. The SPO14 in yeast, which encodes phospholipase D (PLD), is involved in membrane trafficking and plays a vital role in sporulation during meiosis. Crosstalk has been identified between autophagy and sporulation. Although the PLD is required for macroautophagy in mammals, its role in yeast macroautophagy remains unclear. We observed that Spo14 is not required for macroautophagy in yeast and that it is dispensable for Atg8 lipidation, which plays an important role in phagophore extension. Our results also revealed that green fluorescent protein (GFP)-Atg8 degradation is not completely blocked in atg1Δ/atg1Δ cells under sporulation condition. Therefore, Spo14 is not required for macroautophagy in yeast.

A High-Copy Suppressor Screen Reveals a Broad Role of Prefoldin-like Bud27 in the TOR Signaling Pathway in Saccharomyces cerevisiae

Bud27 is a prefoldin-like, a member of the family of ATP-independent molecular chaperones that associates with RNA polymerases I, II, and III in Saccharomyces cerevisiae. Bud27 and its human ortholog URI perform several functions in the cytoplasm and the nucleus. Both proteins participate in the TOR signaling cascade by coordinating nutrient availability with gene expression, and lack of Bud27 partially mimics TOR pathway inactivation. Bud27 regulates the transcription of the three RNA polymerases to mediate the synthesis of ribosomal components for ribosome biogenesis through the TOR cascade. This work presents a high-copy suppression screening of the temperature sensitivity of the bud27Δ mutant. It shows that Bud27 influences different TOR-dependent processes. Our data also suggest that Bud27 can impact some of these TOR-dependent processes: cell wall integrity and autophagy induction.

Molecular Mechanism of Overcoming Host Resistance by the Target of Rapamycin Gene in Leptographium qinlingensis

Leptographium qinlingensis is a fungal symbiont of the Chinese white pine beetle (Dendroctonus armandi) and a pathogen of the Chinese white pine (Pinus armandii) that must overcome the terpenoid oleoresin defenses of host trees to invade and colonize. L. qinlingensis responds to monoterpene flow with abundant mechanisms that include the decomposing and use of these compounds as a nitrogen source. Target of Rapamycin (TOR) is an evolutionarily conserved protein kinase that plays a central role in both plants and animals through integration of nutrients, energies, hormones, growth factors and environmental inputs to control proliferation, growth and metabolism in diverse multicellular organisms. In this study, in order to explore the relationship between TOR gene and carbon sources, nitrogen sources, host nutrients and host volatiles (monoterpenoids) in L. qinlingensis, we set up eight carbon source treatments, ten nitrogen source treatments, two host nutrients and six monoterpenoids (5%, 10% and 20%) treatments, and prepared different media conditions. By measuring the biomass and growth rate of mycelium, the results revealed that, on the whole, the response of L. qinlingensis to nitrogen sources was better than carbon sources, and the fungus grew well in maltose (carbon source), (NH₄)₂C₂O₄ (inorganic nitrogen source), asparagine (organic nitrogen source) and P. armandii (host nutrient) versus other treatments. Then, by analyzing the relationship between TOR expression and different nutrients, the data showed that: (i) TOR expression exhibited negative regulation in response to carbon sources and host nutrition. (ii) The treatments of nitrogen sources and terpenoids had positively regulatory effects on TOR gene; moreover, the fungus was most sensitive to β-pinene and 3-carene. In conclusion, our findings reveal that TOR in L. qinlingensis plays a key role in the utilization of host volatiles as nutrient intake, overcoming the physical and chemical host resistances and successful colonization.

Casting iron into the cell fate mold

This commentary discusses general concepts introduced in the article 'Bulk autophagy induction and life extension is achieved when iron is the only limited nutrient in Saccharomyces cerevisiae' by Montella-Manuel et al. (Biochem J (2021) 478: 811-837). Montella-Manuel et al. show that like central carbon metabolism, iron metabolism is also closely implicated in autophagy-mediated life extension via the TORC2 activator Ypk1p and the iron regulator Aft1p. While not being an iron-sulfur cluster protein, Aft1p interacts with such proteins and thus senses the redox status of the cell, which, similar to amino acids and AMP, reports its energetic status. Furthermore, glucose and iron deficiencies are interrelated as the diauxic shift in glucose depleted cells requires iron uptake for activating respiration in the absence of fermentation.