E3 ubiquitin ligase Pub1 is implicated in endocytosis of a GPI-anchored protein Ecm33 in fission yeast

Role of mitochondrial complex III/IV in the activation of transcription factor Rst2 in Schizosaccharomyces pombe

Mitochondria play essential roles in eukaryotic cells for glucose metabolism to produce ATP. In Schizosaccharomyces pombe, transcription factor Rst2 can be activated upon glucose deprivation. However, the link between Rst2 and mitochondrial function remains elusive. Here, we monitored Rst2 transcriptional activity in living cells using a Renilla luciferase reporter system, and found that inhibition of mitochondrial complex III/IV caused cells to produce reactive oxygen species (ROS) and nitric oxide (NO), which in turn activated Rst2. Furthermore, Rst2-GFP was observed to translocate from cytoplasm to nucleus upon mitochondrial complex III/IV inhibitors treatment, and deletion of genes associated with complex III/IV resulted in delayed process of Rst2-GFP nuclear exportation under glucose-rich condition. In particular, we found that Rst2 was phosphorylated following the treatment of complex III/IV inhibitors or SNAP. Altogether, our findings suggest that mitochondrial complex III/IV participates in the activation of Rst2 through ROS and NO generation in Schizosaccharomyces pombe.

Pleiotropic effects of Ubi4, a polyubiquitin precursor required for ubiquitin accumulation, conidiation and pathogenicity of a fungal insect pathogen

Ubi4 is a polyubiquitin precursor well characterized in yeasts but unexplored in insect mycopathogens. Here, we report that orthologous Ubi4 plays a core role in ubiquitin- and asexual lifestyle-required cellular events in Beauveria bassiana. Deletion of ubi4 led to abolished ubiquitin accumulation, blocked autophagic process, severe defects in conidiation and conidial quality, reduced cell tolerance to oxidative, osmotic, cell wall perturbing and heat-shock stresses, decreased transcript levels of development-activating and antioxidant genes, but light effect on radial growth under normal conditions. The deletion mutant lost insect pathogenicity via normal cuticle infection and was severely compromised in virulence via cuticle-bypassing infection due to a block of dimorphic transition critical for acceleration of host mummification. Proteomic and ubiquitylomic analyses revealed 1081 proteins differentially expressed and 639 lysine residues significantly hyper- or hypo-ubiquitylated in the deletion mutant, including dozens of ubiquitin-activating, conjugating and ligating enzymes, core histones, and many more involved in proteasomes, autophagy-lysosome process and protein degradation. Singular deletions of seven ubiquitin-conjugating enzyme genes exerted differential Ubi4-like effects on conidiation level and conidial traits. These findings uncover an essential role of Ubi4 in ubiquitin transfer cascade and its pleiotropic effects on the in vitro and in vivo asexual cycle of B. bassiana.

Heme acquisition by Shu1 requires Nbr1 and proteins of the ESCRT complex in Schizosaccharomyces pombe

Assimilation of heme is mediated by the cell surface protein Shu1 in Schizosaccharomyces pombe. Shu1 undergoes internalization from the cell surface to the vacuole in response to high concentrations of hemin. Here, we have identified cellular components that are involved in mediating vacuolar targeting of Shu1. Cells deficient in heme biosynthesis and lacking the polyubiquitin gene ubi4⁺ exhibit poor growth in the presence of exogenous hemin as a sole source of heme. Microscopic analyses of hem1Δ shu1Δ ubi4Δ cells expressing a functional HA₄ -tagged Shu1 show that Shu1 localizes to the cell surface. Ubiquitinated Nbr1 functions as a receptor for the endosomal sorting complexes required for transport (ESCRT) that delivers cargos to the vacuole. Inactivation of nbr1⁺ , ESCRT-0 hse1⁺ or ESCRT-I sst6⁺ results in hem1Δ cells being unable to use exogenous hemin for the growth. Using lysate preparations from hemin-treated cells, Shu1-Nbr1 and Shu1-Hse1 complexes are detected by coimmunoprecipitation experiments. Further analysis by immunofluorescence microscopy shows that Shu1 is unable to reach vacuoles of hemin-treated cells harboring a deletion for one of the following genes: ubi4⁺ , nbr1⁺ , hse1⁺ and sst6⁺ . Together, these results reveal that hemin-mediated vacuolar targeting of Shu1 requires Ubi4-dependent ubiquitination, the receptor Nbr1 and the ESCRT proteins Hse1 and Sst6.

The exocyst subunit Sec3 is regulated by a protein quality control pathway

Exocytosis involves fusion of secretory vesicles with the plasma membrane, thereby delivering membrane proteins to the cell surface and releasing material into the extracellular space. The tethering of the secretory vesicles before membrane fusion is mediated by the exocyst, an essential phylogenetically conserved octameric protein complex. Exocyst biogenesis is regulated by several processes, but the mechanisms by which the exocyst is degraded are unknown. Here, to unravel the components of the exocyst degradation pathway, we screened for extragenic suppressors of a temperature-sensitive fission yeast strain mutated in the exocyst subunit Sec3 (sec3-913). One of the suppressing DNAs encoded a truncated dominant-negative variant of the 26S proteasome subunit, Rpt2, indicating that exocyst degradation is controlled by the ubiquitin-proteasome system. The temperature-dependent growth defect of the sec3-913 strain was gene dosage-dependent and suppressed by blocking the proteasome, Hsp70-type molecular chaperones, the Pib1 E3 ubiquitin-protein ligase, and the deubiquitylating enzyme Ubp3. Moreover, defects in cell septation, exocytosis, and endocytosis in sec3 mutant strains were similarly alleviated by mutation of components in this pathway. We also found that, particularly under stress conditions, wild-type Sec3 degradation is regulated by Pib1 and the 26S proteasome. In conclusion, our results suggest that a cytosolic protein quality control pathway monitors folding and proteasome-dependent turnover of an exocyst subunit and, thereby, controls exocytosis in fission yeast.

The Ubiquitin Ligase CHIP Integrates Proteostasis and Aging by Regulation of Insulin Receptor Turnover

Aging is attended by a progressive decline in protein homeostasis (proteostasis), aggravating the risk for protein aggregation diseases. To understand the coordination between proteome imbalance and longevity, we addressed the mechanistic role of the quality-control ubiquitin ligase CHIP, which is a key regulator of proteostasis. We observed that CHIP deficiency leads to increased levels of the insulin receptor (INSR) and reduced lifespan of worms and flies. The membrane-bound INSR regulates the insulin and IGF1 signaling (IIS) pathway and thereby defines metabolism and aging. INSR is a direct target of CHIP, which triggers receptor monoubiquitylation and endocytic-lysosomal turnover to promote longevity. However, upon proteotoxic stress conditions and during aging, CHIP is recruited toward disposal of misfolded proteins, reducing its capacity to degrade the INSR. Our study indicates a competitive relationship between proteostasis and longevity regulation through CHIP-assisted proteolysis, providing a mechanistic concept for understanding the impact of proteome imbalance on aging.

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The ubiquitin ligase CHIP triggers insulin receptor turnover

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Insulin receptor level is linked to insulin and IGF1 signaling and longevity

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Engagement of CHIP in protein quality control limits insulin receptor degradation

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Proteotoxic stress aggravates insulin receptor stability, drives aging, and shortens lifespan

Genetic evidence for involvement of membrane trafficking in the action of 5-fluorouracil

To identify novel genes that mediate cellular sensitivity and resistance to 5-fluorouracil (5-FU), we performed a genome-wide genetic screening to identify altered susceptibility to 5-FU by Schizosaccharomyces pombe haploid nonessential gene deletion library containing 3004 deletion mutants. We identified 50 hypersensitive and 12 resistant mutants to this drug. Mutants sensitive or resistant to 5-FU were classified into various categories based on their putative functions. The largest group of the genes whose disruption renders cells altered susceptibility to 5-FU is involved in nucleic acid metabolism, but to our surprise, the second largest group is involved in membrane trafficking. In addition, several other membrane traffic mutants examined including gdi1-i11, ypt3-i5, Δryh1, Δric1, and Δaps1 exhibited hypersensitivity to 5-FU. Furthermore, we found that 5-FU in low concentration that generally do not affect cell growth altered the localization of Syb1, a secretory vesicle SNARE synaptobrevin which is cycled between the plasma membrane and the endocytic pathway. Notably, 5-FU at such low concentration also significantly inhibited the secretion of acid phosphatase. Altogether, our findings revealed the first evidence that 5-FU influences membrane trafficking as the potential underlying mechanism of the drug action.

Cell Lysis in S. pombe ura4 Mutants Is Suppressed by Loss of Functional Pub1, Which Regulates the Uracil Transporter Fur4

Schizosaccharomyces pombe Δura4 cells lyse when grown on YPD medium. A S. pombe non-essential gene deletion library was screened to determine suppressors of the lysis phenotype. Deletion of the pub1 gene, which encoded E3 ubiquitin ligase, strongly suppressed cell lysis in Δura4 cells. The Δpub1 cells displayed high sensitivity to 5-fluorouracil, a toxic analog of uracil, and this sensitivity was suppressed by deletion of fur4, which encoded a uracil transporter. Fur4 localized primarily to the Golgi apparatus and vacuoles in wild-type cells, but localization was predominantly at the plasma membrane in Δpub1 cells. Fur4 was necessary for the utilization of extracellular uracil, cytosine, or UMP. Uracil uptake activity increased in the Δpub1 strain in a Fur4-dependent manner. In addition, uracil starvation was critical for induction of cell lysis of Δura4 strains and uracil supplementation suppressed lysis. In summary, the increased uracil uptake ability of Δpub1 cells, where Fur4 was predominantly localized to the plasma membrane, resulted in suppression of cell lysis in the Δura4 background.