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Yao R, Li R, Wu X, Jin T, Luo Y, Li R, Huang Y. E3 ubiquitin ligase Hul6 modulates iron-dependent metabolism by regulating Php4 stability. J Biol Chem 2024; 300:105670. [PMID: 38272226 PMCID: PMC10882131 DOI: 10.1016/j.jbc.2024.105670] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2023] [Revised: 12/28/2023] [Accepted: 01/12/2024] [Indexed: 01/27/2024] Open
Abstract
Schizosaccharomyces pombe Php4 is the regulatory subunit of the CCAAT-binding complexes and plays an important role in the regulation of iron homeostasis and iron-dependent metabolism. Here, we show that Php4 undergoes ubiquitin-dependent degradation in the late logarithmic and stationary phases. The degradation and ubiquitination of Php4 could be attenuated by deletion of hul6, a gene encoding a putative HECT-type E3 ubiquitin ligase. The expression levels of Hul6 and Php4 are oppositely regulated during cell growth. Hul6 interacts with the C-terminal region of Php4. Two lysine residues (K217 and K274) located in the C-terminal region of Php4 are required for its polyubiquitination. Increasing the levels of Php4 by deletion of hul6 or overexpression of php4 decreased expression of Php4 target proteins involved in iron-dependent metabolic pathways such as the tricarboxylic cycle and mitochondrial oxidative phosphorylation, thus causing increased sensitivity to high-iron and reductions in succinate dehydrogenase and mitochondrial complex II activities. Hul6 is located primarily in the mitochondrial outer membrane and most likely targets cytosolic Php4 for ubiquitination and degradation. Taken together, our data suggest that Hul6 regulates iron-dependent metabolism through degradation of Php4 under normal growth conditions. Our results also suggest that Hul6 promotes iron-dependent metabolism to help the cell to adapt to a nutrient-starved growth phase.
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Affiliation(s)
- Rui Yao
- Jiangsu Key Laboratory for Microbes and Functional Genomics, School of Life Sciences, Nanjing Normal University, Nanjing, China
| | - Rongrong Li
- Jiangsu Key Laboratory for Microbes and Functional Genomics, School of Life Sciences, Nanjing Normal University, Nanjing, China
| | - Xiaoyu Wu
- Jiangsu Key Laboratory for Microbes and Functional Genomics, School of Life Sciences, Nanjing Normal University, Nanjing, China
| | - Ting Jin
- Jiangsu Key Laboratory for Microbes and Functional Genomics, School of Life Sciences, Nanjing Normal University, Nanjing, China
| | - Ying Luo
- Jiangsu Key Laboratory for Microbes and Functional Genomics, School of Life Sciences, Nanjing Normal University, Nanjing, China
| | - Rong Li
- Jiangsu Key Laboratory for Microbes and Functional Genomics, School of Life Sciences, Nanjing Normal University, Nanjing, China
| | - Ying Huang
- Jiangsu Key Laboratory for Microbes and Functional Genomics, School of Life Sciences, Nanjing Normal University, Nanjing, China.
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Chatfield-Reed K, Marno Jones K, Shah F, Chua G. Genetic-interaction screens uncover novel biological roles and regulators of transcription factors in fission yeast. G3 GENES|GENOMES|GENETICS 2022; 12:6655692. [PMID: 35924983 PMCID: PMC9434175 DOI: 10.1093/g3journal/jkac194] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 05/30/2022] [Accepted: 07/20/2022] [Indexed: 12/05/2022]
Abstract
In Schizosaccharomyces pombe, systematic analyses of single transcription factor deletion or overexpression strains have made substantial advances in determining the biological roles and target genes of transcription factors, yet these characteristics are still relatively unknown for over a quarter of them. Moreover, the comprehensive list of proteins that regulate transcription factors remains incomplete. To further characterize Schizosaccharomyces pombe transcription factors, we performed synthetic sick/lethality and synthetic dosage lethality screens by synthetic genetic array. Examination of 2,672 transcription factor double deletion strains revealed a sick/lethality interaction frequency of 1.72%. Phenotypic analysis of these sick/lethality strains revealed potential cell cycle roles for several poorly characterized transcription factors, including SPBC56F2.05, SPCC320.03, and SPAC3C7.04. In addition, we examined synthetic dosage lethality interactions between 14 transcription factors and a miniarray of 279 deletion strains, observing a synthetic dosage lethality frequency of 4.99%, which consisted of known and novel transcription factor regulators. The miniarray contained deletions of genes that encode primarily posttranslational-modifying enzymes to identify putative upstream regulators of the transcription factor query strains. We discovered that ubiquitin ligase Ubr1 and its E2/E3-interacting protein, Mub1, degrade the glucose-responsive transcriptional repressor Scr1. Loss of ubr1+ or mub1+ increased Scr1 protein expression, which resulted in enhanced repression of flocculation through Scr1. The synthetic dosage lethality screen also captured interactions between Scr1 and 2 of its known repressors, Sds23 and Amk2, each affecting flocculation through Scr1 by influencing its nuclear localization. Our study demonstrates that sick/lethality and synthetic dosage lethality screens can be effective in uncovering novel functions and regulators of Schizosaccharomyces pombe transcription factors.
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Affiliation(s)
- Kate Chatfield-Reed
- Department of Biological Sciences, University of Calgary , Calgary, Alberta T2N 1N4, Canada
| | - Kurtis Marno Jones
- Department of Biological Sciences, University of Calgary , Calgary, Alberta T2N 1N4, Canada
| | - Farah Shah
- Department of Biological Sciences, University of Calgary , Calgary, Alberta T2N 1N4, Canada
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3
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Borgert L, Mishra S, den Brave F. Quality control of cytoplasmic proteins inside the nucleus. Comput Struct Biotechnol J 2022; 20:4618-4625. [PMID: 36090811 PMCID: PMC9440239 DOI: 10.1016/j.csbj.2022.08.033] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2022] [Revised: 08/13/2022] [Accepted: 08/15/2022] [Indexed: 11/03/2022] Open
Abstract
A complex network of molecular chaperones and proteolytic machinery safeguards the proteins which comprise the proteome, from the time they are synthesized on ribosomes to their destruction via proteolysis. Impaired protein quality control results in the accumulation of aberrant proteins, which may undergo unwanted spurious interactions with other proteins, thereby interfering with a broad range of cellular functions. To protect the cellular environment, such proteins are degraded or sequestered into inclusions in different subcellular compartments. Recent findings demonstrate that aberrant or mistargeted proteins from different cytoplasmic compartments are removed from their environment by transporting them into the nucleus. These proteins are degraded by the nuclear ubiquitin–proteasome system or sequestered into intra-nuclear inclusions. Here, we discuss the emerging role of the nucleus as a cellular quality compartment based on recent findings in the yeast Saccharomyces cerevisiae. We describe the current knowledge on cytoplasmic substrates of nuclear protein quality control, the mechanism of nuclear import of such proteins, as well as possible advantages and risks of nuclear sequestration of aberrant proteins.
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Ubiquitin Ligase Redundancy and Nuclear-Cytoplasmic Localization in Yeast Protein Quality Control. Biomolecules 2021; 11:biom11121821. [PMID: 34944465 PMCID: PMC8698790 DOI: 10.3390/biom11121821] [Citation(s) in RCA: 17] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/20/2021] [Revised: 11/30/2021] [Accepted: 12/01/2021] [Indexed: 12/12/2022] Open
Abstract
The diverse functions of proteins depend on their proper three-dimensional folding and assembly. Misfolded cellular proteins can potentially harm cells by forming aggregates in their resident compartments that can interfere with vital cellular processes or sequester important factors. Protein quality control (PQC) pathways are responsible for the repair or destruction of these abnormal proteins. Most commonly, the ubiquitin-proteasome system (UPS) is employed to recognize and degrade those proteins that cannot be refolded by molecular chaperones. Misfolded substrates are ubiquitylated by a subset of ubiquitin ligases (also called E3s) that operate in different cellular compartments. Recent research in Saccharomyces cerevisiae has shown that the most prominent ligases mediating cytoplasmic and nuclear PQC have overlapping yet distinct substrate specificities. Many substrates have been characterized that can be targeted by more than one ubiquitin ligase depending on their localization, and cytoplasmic PQC substrates can be directed to the nucleus for ubiquitylation and degradation. Here, we review some of the major yeast PQC ubiquitin ligases operating in the nucleus and cytoplasm, as well as current evidence indicating how these ligases can often function redundantly toward substrates in these compartments.
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5
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Wang DY, Mou YN, Du X, Guan Y, Feng MG. Ubr1-mediated ubiquitylation orchestrates asexual development, polar growth, and virulence-related cellular events in Beauveria bassiana. Appl Microbiol Biotechnol 2021; 105:2747-2758. [PMID: 33686455 DOI: 10.1007/s00253-021-11222-0] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/06/2021] [Revised: 02/16/2021] [Accepted: 03/04/2021] [Indexed: 01/14/2023]
Abstract
The E3 ubiquitin ligase Ubr1 is a core player in yeast ubiquitylation and protein quality control required for cellular events including proteasomal degradation and gene activity but has been rarely explored in filamentous fungi. We show here an essentiality of orthologous Ubr1-mediated ubiquitylation for the activation of central developmental pathway (CPD) and the CPD-controlled cellular events in Beauveria bassiana, a filamentous fungal insect pathogen that undergoes an asexual cycle in vitro or in vivo. As a result of ubr1 disruption, intracellular free ubiquitin accumulation increased by 1.4-fold, indicating an impaired ability for the disruptant to transfer ubiquitin to target proteins. Consequently, the disruptant was compromised in polar growth featured with curved or hook-like germ tubes and abnormally branched hyphae, leading to impeded propagation of aberrant hyphal bodies in infected insect hemocoel and attenuated virulence. In the mutant, sharply repressed expression of three CDP activator genes (brlA, abaA, and wetA) correlated well with severe defects in aerial conidiation and submerged blastospore (hyphal body) production in insect hemolymph or a mimicking medium. Moreover, the disruptant was sensitive to cell wall perturbation or lysing and showed increased catalase activity and resistance to hydrogen peroxide despite null response to high osmolarity or heat shock. Most of the examined genes involved in polar growth and cell wall integrity were down-regulated in the disruptant. These findings uncover that the Ubr1-mediated ubiquitylation orchestrates polar growth and the CDP-regulated asexual cycle in vitro and in vivo in B. bassiana. KEY POINTS: • Ubr1 is an E3 ubiquitin ligase essential for ubiquitylation in Beauveria bassiana. • Ubr1-mediated ubiquitylation is required for activation of central development pathway. • Ubr1 orchestrates polar growth and asexual cycle in vitro and in vivo.
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Affiliation(s)
- Ding-Yi Wang
- Key Laboratory of Subtropical Mountain Ecology, School of Geographical Sciences, Fujian Normal University, Fuzhou, 350007, China
| | - Ya-Ni Mou
- Institute of Microbiology, College of Life Sciences, Zhejiang University, Zhejiang, 310058, Hangzhou, China
| | - Xi Du
- Fujian Key Laboratory of Marine Enzyme Engineering, Fuzhou University, Fuzhou, 350108, Fujian, China
| | - Yi Guan
- Fujian Key Laboratory of Marine Enzyme Engineering, Fuzhou University, Fuzhou, 350108, Fujian, China
| | - Ming-Guang Feng
- Institute of Microbiology, College of Life Sciences, Zhejiang University, Zhejiang, 310058, Hangzhou, China.
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The Regulatory Proteins Rtg1/3 Govern Sphingolipid Homeostasis in the Human-Associated Yeast Candida albicans. Cell Rep 2020; 30:620-629.e6. [DOI: 10.1016/j.celrep.2019.12.022] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2019] [Revised: 09/09/2019] [Accepted: 12/06/2019] [Indexed: 01/26/2023] Open
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7
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Marte L, Boronat S, García-Santamarina S, Ayté J, Kitamura K, Hidalgo E. Identification of ubiquitin-proteasome system components affecting the degradation of the transcription factor Pap1. Redox Biol 2019; 28:101305. [PMID: 31514053 PMCID: PMC6742857 DOI: 10.1016/j.redox.2019.101305] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/14/2019] [Revised: 08/13/2019] [Accepted: 08/20/2019] [Indexed: 01/06/2023] Open
Abstract
Signaling cascades respond to specific inputs, but also require active interventions to be maintained in their basal/inactive levels in the absence of the activating signal(s). In a screen to search for protein quality control components required for wild-type tolerance to oxidative stress in fission yeast, we have isolated eight gene deletions conferring resistance not only to H2O2 but also to caffeine. We show that dual resistance acquisition is totally or partially dependent on the transcription factor Pap1. Some gene products, such as the ribosomal-ubiquitin fusion protein Ubi1, the E2 conjugating enzyme Ubc2 or the E3 ligase Ubr1, participate in basal ubiquitin labeling of Pap1, and others, such as Rpt4, are non-essential constituents of the proteasome. We demonstrate here that basal nucleo-cytoplasmic shuttling of Pap1, occurring even in the absence of stress, is sufficient for the interaction of the transcription factor with nuclear Ubr1, and we identify a 30 amino acids peptide in Pap1 as the degron for this important E3 ligase. The isolated gene deletions increase only moderately the concentration of the transcription factor, but it is sufficient to enhance basal tolerance to stress, probably by disturbing the inactive stage of this signaling cascade.
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Affiliation(s)
- Luis Marte
- Oxidative Stress and Cell Cycle Group, Universitat Pompeu Fabra, C/ Doctor Aiguader 88, 08003, Barcelona, Spain
| | - Susanna Boronat
- Oxidative Stress and Cell Cycle Group, Universitat Pompeu Fabra, C/ Doctor Aiguader 88, 08003, Barcelona, Spain
| | - Sarela García-Santamarina
- Oxidative Stress and Cell Cycle Group, Universitat Pompeu Fabra, C/ Doctor Aiguader 88, 08003, Barcelona, Spain
| | - José Ayté
- Oxidative Stress and Cell Cycle Group, Universitat Pompeu Fabra, C/ Doctor Aiguader 88, 08003, Barcelona, Spain
| | - Kenji Kitamura
- Center for Gene Science, Hiroshima University, 1-4-2 Kagamiyama, Higashi-Hiroshima, 739-8527, Japan
| | - Elena Hidalgo
- Oxidative Stress and Cell Cycle Group, Universitat Pompeu Fabra, C/ Doctor Aiguader 88, 08003, Barcelona, Spain.
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8
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Dysfunction of Prohibitin 2 Results in Reduced Susceptibility to Multiple Antifungal Drugs via Activation of the Oxidative Stress-Responsive Transcription Factor Pap1 in Fission Yeast. Antimicrob Agents Chemother 2018; 62:AAC.00860-18. [PMID: 30181366 DOI: 10.1128/aac.00860-18] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/02/2018] [Accepted: 08/25/2018] [Indexed: 12/22/2022] Open
Abstract
The fight against resistance to antifungal drugs requires a better understanding of the underlying cellular mechanisms. In order to gain insight into the mechanisms leading to antifungal drug resistance, we performed a genetic screen on a model organism, Schizosaccharomyces pombe, to identify genes whose overexpression caused resistance to antifungal drugs, including clotrimazole and terbinafine. We identified the phb2 + gene, encoding a highly conserved mitochondrial protein, prohibitin (Phb2), as a novel determinant of reduced susceptibility to multiple antifungal drugs. Unexpectedly, deletion of the phb2 + gene also exhibited antifungal drug resistance. Overexpression of the phb2 + gene failed to cause drug resistance when the pap1 + gene, encoding an oxidative stress-responsive transcription factor, was deleted. Furthermore, pap1+ mRNA expression was significantly increased when the phb2 + gene was overexpressed or deleted. Importantly, either overexpression or deletion of the phb2 + gene stimulated the synthesis of NO and reactive oxygen species (ROS), as measured by the cell-permeant fluorescent NO probe DAF-FM DA (4-amino-5-methylamino-2',7'-difluorofluorescein diacetate) and the ROS probe DCFH-DA (2',7'-dichlorodihydrofluorescein diacetate), respectively. Taken together, these results suggest that Phb2 dysfunction results in reduced susceptibility to multiple antifungal drugs by increasing NO and ROS synthesis due to dysfunctional mitochondria, thereby activating the transcription factor Pap1 in fission yeast.
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9
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A novel bZIP protein, Gsb1, is required for oxidative stress response, mating, and virulence in the human pathogen Cryptococcus neoformans. Sci Rep 2017. [PMID: 28642475 PMCID: PMC5481450 DOI: 10.1038/s41598-017-04290-8] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
The human pathogen Cryptococcus neoformans, which causes life-threatening meningoencephalitis in immunocompromised individuals, normally faces diverse stresses in the human host. Here, we report that a novel, basic, leucine-zipper (bZIP) protein, designated Gsb1 (general stress-related bZIP protein 1), is required for its normal growth and diverse stress responses. C. neoformans gsb1Δ mutants grew slowly even under non-stressed conditions and showed increased sensitivity to high or low temperatures. The hypersensitivity of gsb1Δ to oxidative and nitrosative stresses was reversed by addition of a ROS scavenger. RNA-Seq analysis during normal growth revealed increased expression of a number of genes involved in mitochondrial respiration and cell cycle, but decreased expression of several genes involved in the mating-pheromone-responsive MAPK signaling pathway. Accordingly, gsb1Δ showed defective mating and abnormal cell-cycle progression. Reflecting these pleiotropic phenotypes, gsb1Δ exhibited attenuated virulence in a murine model of cryptococcosis. Moreover, RNA-Seq analysis under oxidative stress revealed that several genes involved in ROS defense, cell-wall remodeling, and protein glycosylation were highly induced in the wild-type strain but not in gsb1Δ. Gsb1 localized exclusively in the nucleus in response to oxidative stress. In conclusion, Gsb1 is a key transcription factor modulating growth, stress responses, differentiation, and virulence in C. neoformans.
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10
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Amm I, Wolf DH. Molecular mass as a determinant for nuclear San1-dependent targeting of misfolded cytosolic proteins to proteasomal degradation. FEBS Lett 2016; 590:1765-75. [PMID: 27173001 DOI: 10.1002/1873-3468.12213] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/13/2015] [Revised: 05/04/2016] [Accepted: 05/08/2016] [Indexed: 11/08/2022]
Abstract
Most misfolded cytosolic proteins in the cell are eliminated by the ubiquitin-proteasome system. In yeast, polyubiquitination of misfolded cytosolic proteins is triggered mainly by the action of two ubiquitin ligases Ubr1, formerly discovered as recognition component of the N-end rule pathway, and the nuclear ubiquitin ligase San1. For San1-mediated targeting to proteasomal degradation, cytosolic proteins have to be imported into the nucleus. Selection of misfolded substrates for import into the nucleus had remained elusive. This study shows that an increasing molecular mass of substrates prevents nuclear San1-triggered proteasomal degradation but renders them susceptible to cytoplasmic Ubr1-triggered degradation.
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Affiliation(s)
- Ingo Amm
- Institut für Biochemie, Universität Stuttgart, Germany
| | - Dieter H Wolf
- Institut für Biochemie, Universität Stuttgart, Germany
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11
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Kitamura K. Inhibition of the Arg/N-end rule pathway-mediated proteolysis by dipeptide-mimetic molecules. Amino Acids 2015; 48:235-43. [PMID: 26334349 DOI: 10.1007/s00726-015-2083-1] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2015] [Accepted: 08/21/2015] [Indexed: 11/30/2022]
Abstract
Ubr11 in the fission yeast Schizosaccharomyces pombe is an evolutionarily conserved ubiquitin ligase functioning in the Arg/N-end rule pathway, which promotes degradation of substrate proteins via the proteasome. Ubr11 recognizes the N-degron sequence in substrates. The primary N-degron contains a destabilization-inducing N-terminal amino acid, which is either a basic (type 1) or bulky hydrophobic (type 2) residue. Dipeptides are known to inhibit proteolytic degradation via the Arg/N-end rule pathway. Here, I examined the potency of some amino acid- or dipeptide-related molecules in their inhibition of Ubr11/N-end rule-mediated degradation. An amide form of L-arginine and L-tryptophan had weak inhibitory activity for type 1 and type 2 substrates, respectively, although the unmodified amino acid monomer and its carboxymethylated ester were ineffective. Among the naturally occurring dipeptides tested, Lys-Leu and Tyr-Leu showed potent inhibitory activity, but their effect was transient, especially at submillimolar concentrations. L-arginine-β-naphthylamide (Arg-βNA) showed stronger activity than several dipeptides for type 1 substrates, but all Lys-Leu, Tyr-Leu, and Arg-βNA caused growth retardation. The inhibitory activity of the L-phenylalanine carbobenzoxy-hydrazide for type 2 substrates was not very strong, but it prolonged the action of Tyr-Leu at low concentrations and, importantly, did not interfere with cell growth. Apart from their utility, these dipeptidomimetics provide a clue for understanding the determinants of recognition by Ubr ubiquitin ligase and further designing novel inhibitors of the Arg/N-end rule pathway.
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Affiliation(s)
- Kenji Kitamura
- Center for Gene Science, Hiroshima University, 1-4-2 Kagamiyama, Higashi-Hiroshima, 739-8527, Japan.
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12
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Oxidative stress responses in the human fungal pathogen, Candida albicans. Biomolecules 2015; 5:142-65. [PMID: 25723552 PMCID: PMC4384116 DOI: 10.3390/biom5010142] [Citation(s) in RCA: 166] [Impact Index Per Article: 18.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/12/2015] [Revised: 02/11/2015] [Accepted: 02/12/2015] [Indexed: 02/07/2023] Open
Abstract
Candida albicans is a major fungal pathogen of humans, causing approximately 400,000 life-threatening systemic infections world-wide each year in severely immunocompromised patients. An important fungicidal mechanism employed by innate immune cells involves the generation of toxic reactive oxygen species (ROS), such as superoxide and hydrogen peroxide. Consequently, there is much interest in the strategies employed by C. albicans to evade the oxidative killing by macrophages and neutrophils. Our understanding of how C. albicans senses and responds to ROS has significantly increased in recent years. Key findings include the observations that hydrogen peroxide triggers the filamentation of this polymorphic fungus and that a superoxide dismutase enzyme with a novel mode of action is expressed at the cell surface of C. albicans. Furthermore, recent studies have indicated that combinations of the chemical stresses generated by phagocytes can actively prevent C. albicans oxidative stress responses through a mechanism termed the stress pathway interference. In this review, we present an up-date of our current understanding of the role and regulation of oxidative stress responses in this important human fungal pathogen.
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Kitamura K. The ClpS-like N-domain is essential for the functioning of Ubr11, an N-recognin in Schizosaccharomyces pombe. SPRINGERPLUS 2014; 3:257. [PMID: 26034658 PMCID: PMC4447728 DOI: 10.1186/2193-1801-3-257] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 02/28/2014] [Accepted: 05/16/2014] [Indexed: 12/02/2022]
Abstract
Several Ubr ubiquitin ligases recognize the N-terminal amino acid of substrate proteins and promote their degradation via the Arg/N-end rule pathway. The primary destabilizing N-terminal amino acids in yeast are classified into type 1 (Arg, Lys, and His) and type 2 (Phe, Trp, Tyr, Leu, Ile, and Met-Ф) residues. The type 1 and type 2 residues bind to the UBR box and the ClpS/N-domain, respectively, in canonical Ubr ubiquitin ligases that act as N-recognins. In this study, the requirement for type 1 and type 2 amino acid recognition by Schizosaccharomyces pombe Ubr11 was examined in vivo. Consistent with the results of previous studies, the ubr11∆ null mutant was found to be defective in oligopeptide uptake and resistant to ergosterol synthesis inhibitors. Furthermore, the ubr11∆ mutant was also less sensitive to some protein synthesis inhibitors. A ubr11 ClpS/N-domain mutant, which retained ubiquitin ligase activity but could not recognize type 2 amino acids, phenocopied all known defects of the ubr11∆ mutant. However, the recognition of type 1 residues by Ubr11 was not required for its functioning, and no severe physiological abnormalities were observed in a ubr11 mutant defective in the recognition of type 1 residues. These results reinforce the fundamental importance of the ClpS/N-domain for the functioning of the N-recognin, Ubr11.
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Gallagher PS, Oeser ML, Abraham AC, Kaganovich D, Gardner RG. Cellular maintenance of nuclear protein homeostasis. Cell Mol Life Sci 2014; 71:1865-79. [PMID: 24305949 PMCID: PMC3999211 DOI: 10.1007/s00018-013-1530-y] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/23/2013] [Revised: 11/04/2013] [Accepted: 11/19/2013] [Indexed: 12/11/2022]
Abstract
The accumulation and aggregation of misfolded proteins is the primary hallmark for more than 45 human degenerative diseases. These devastating disorders include Alzheimer's, Parkinson's, Huntington's, and amyotrophic lateral sclerosis. Over 15 degenerative diseases are associated with the aggregation of misfolded proteins specifically in the nucleus of cells. However, how the cell safeguards the nucleus from misfolded proteins is not entirely clear. In this review, we discuss what is currently known about the cellular mechanisms that maintain protein homeostasis in the nucleus and protect the nucleus from misfolded protein accumulation and aggregation. In particular, we focus on the chaperones found to localize to the nucleus during stress, the ubiquitin-proteasome components enriched in the nucleus, the signaling systems that might be present in the nucleus to coordinate folding and degradation, and the sites of misfolded protein deposition associated with the nucleus.
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Affiliation(s)
- Pamela S Gallagher
- Department of Pharmacology, University of Washington, Seattle, WA, 98195, USA
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15
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Belfield C, Queenan C, Rao H, Kitamura K, Walworth NC. The oxidative stress responsive transcription factor Pap1 confers DNA damage resistance on checkpoint-deficient fission yeast cells. PLoS One 2014; 9:e89936. [PMID: 24587136 PMCID: PMC3934961 DOI: 10.1371/journal.pone.0089936] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2013] [Accepted: 01/28/2014] [Indexed: 11/19/2022] Open
Abstract
Eukaryotic cells invoke mechanisms to promote survival when confronted with cellular stress or damage to the genome. The protein kinase Chk1 is an integral and conserved component of the DNA damage response pathway. Mutation or inhibition of Chk1 results in mitotic death when cells are exposed to DNA damage. Oxidative stress activates a pathway that results in nuclear accumulation of the bZIP transcription factor Pap1. We report the novel finding that fission yeast Pap1 confers resistance to drug- and non-drug-induced DNA damage even when the DNA damage checkpoint is compromised. Multi-copy expression of Pap1 restores growth to chk1-deficient cells exposed to camptothecin or hydroxyurea. Unexpectedly, increased Pap1 expression also promotes survival of chk1-deficient cells with mutations in genes encoding DNA ligase (cdc17) or DNA polymerase δ (cdc6), but not DNA replication initiation mutants. The ability of Pap1 to confer resistance to DNA damage was not specific to chk1 mutants, as it also improved survival of rad1- and rad9-deficient cells in the presence of CPT. To confer resistance to DNA damage Pap1 must localize to the nucleus and be transcriptionally active.
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Affiliation(s)
- Carrie Belfield
- Department of Pharmacology, Rutgers Robert Wood Johnson Medical School, Piscataway, New Jersey, United States of America
- Graduate Program in Cellular and Molecular Pharmacology, Rutgers Graduate School of Biomedical Sciences, Piscataway, New Jersey, United States of America
| | - Craig Queenan
- Department of Pharmacology, Rutgers Robert Wood Johnson Medical School, Piscataway, New Jersey, United States of America
| | - Hui Rao
- Department of Pharmacology, Rutgers Robert Wood Johnson Medical School, Piscataway, New Jersey, United States of America
| | - Kenji Kitamura
- Center for Gene Science, Hiroshima University, Higashi-Hiroshima, Japan
| | - Nancy C. Walworth
- Department of Pharmacology, Rutgers Robert Wood Johnson Medical School, Piscataway, New Jersey, United States of America
- Graduate Program in Cellular and Molecular Pharmacology, Rutgers Graduate School of Biomedical Sciences, Piscataway, New Jersey, United States of America
- Member, Rutgers Cancer Institute of New Jersey, New Brunswick, New Jersey, United States of America
- * E-mail:
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Patterson MJ, McKenzie CG, Smith DA, da Silva Dantas A, Sherston S, Veal EA, Morgan BA, MacCallum DM, Erwig LP, Quinn J. Ybp1 and Gpx3 signaling in Candida albicans govern hydrogen peroxide-induced oxidation of the Cap1 transcription factor and macrophage escape. Antioxid Redox Signal 2013; 19:2244-60. [PMID: 23706023 PMCID: PMC3869436 DOI: 10.1089/ars.2013.5199] [Citation(s) in RCA: 56] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
AIMS As Candida albicans is the major fungal pathogen of humans, there is an urgent need to understand how this pathogen evades toxic reactive oxygen species (ROS) generated by the host immune system. A key regulator of antioxidant gene expression, and thus ROS resistance, in C. albicans is the AP-1-like transcription factor Cap1. Despite this, little is known regarding the intracellular signaling mechanisms that underlie the oxidation and activation of Cap1. Therefore, the aims of this study were; (i) to identify the regulatory proteins that govern Cap1 oxidation, and (ii) to investigate the importance of Cap1 oxidation in C. albicans pathogenesis. RESULTS In response to hydrogen peroxide (H2O2), but not glutathione-depleting/modifying oxidants, Cap1 oxidation, nuclear accumulation, phosphorylation, and Cap1-dependent gene expression, is mediated by a glutathione peroxidase-like enzyme, which we name Gpx3, and an orthologue of the Saccharomyces cerevisiae Yap1 binding protein, Ybp1. In addition, Ybp1 also functions to stabilise Cap1 and this novel function is conserved in S. cerevisiae. C. albicans cells lacking Cap1, Ybp1, or Gpx3, are unable to filament and thus, escape from murine macrophages after phagocytosis, and also display defective virulence in the Galleria mellonella infection model. INNOVATION Ybp1 is required to promote the stability of fungal AP-1-like transcription factors, and Ybp1 and Gpx3 mediated Cap1-dependent oxidative stress responses are essential for the effective killing of macrophages by C. albicans. CONCLUSION Activation of Cap1, specifically by H2O2, is a prerequisite for the subsequent filamentation and escape of this fungal pathogen from the macrophage.
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Affiliation(s)
- Miranda J. Patterson
- Faculty of Medical Sciences, Institute for Cell and Molecular Biosciences, Newcastle University, Newcastle upon Tyne, United Kingdom
| | | | - Deborah A. Smith
- Faculty of Medical Sciences, Institute for Cell and Molecular Biosciences, Newcastle University, Newcastle upon Tyne, United Kingdom
| | - Alessandra da Silva Dantas
- Faculty of Medical Sciences, Institute for Cell and Molecular Biosciences, Newcastle University, Newcastle upon Tyne, United Kingdom
| | - Sam Sherston
- Faculty of Medical Sciences, Institute for Cell and Molecular Biosciences, Newcastle University, Newcastle upon Tyne, United Kingdom
| | - Elizabeth A. Veal
- Faculty of Medical Sciences, Institute for Cell and Molecular Biosciences, Newcastle University, Newcastle upon Tyne, United Kingdom
| | - Brian A. Morgan
- Faculty of Medical Sciences, Institute for Cell and Molecular Biosciences, Newcastle University, Newcastle upon Tyne, United Kingdom
| | - Donna M. MacCallum
- Aberdeen Fungal Group, Institute of Medical Sciences, University of Aberdeen, Aberdeen, United Kingdom
| | - Lars-Peter Erwig
- Division of Applied Medicine, University of Aberdeen, Aberdeen, United Kingdom
- Aberdeen Fungal Group, Institute of Medical Sciences, University of Aberdeen, Aberdeen, United Kingdom
| | - Janet Quinn
- Faculty of Medical Sciences, Institute for Cell and Molecular Biosciences, Newcastle University, Newcastle upon Tyne, United Kingdom
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17
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Brown JD, Day AM, Taylor SR, Tomalin LE, Morgan BA, Veal EA. A peroxiredoxin promotes H2O2 signaling and oxidative stress resistance by oxidizing a thioredoxin family protein. Cell Rep 2013; 5:1425-35. [PMID: 24268782 PMCID: PMC3898613 DOI: 10.1016/j.celrep.2013.10.036] [Citation(s) in RCA: 51] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/14/2013] [Revised: 07/23/2013] [Accepted: 10/21/2013] [Indexed: 01/04/2023] Open
Abstract
H2O2 can cause oxidative damage associated with age-related diseases such as diabetes and cancer but is also used to initiate diverse responses, including increased antioxidant gene expression. Despite significant interest, H2O2-signaling mechanisms remain poorly understood. Here, we present a mechanism for the propagation of an H2O2 signal that is vital for the adaptation of the model yeast, Schizosaccharomyces pombe, to oxidative stress. Peroxiredoxins are abundant peroxidases with conserved antiaging and anticancer activities. Remarkably, we find that the only essential function for the thioredoxin peroxidase activity of the Prx Tpx1(hPrx1/2) in resistance to H2O2 is to inhibit a conserved thioredoxin family protein Txl1(hTxnl1/TRP32). Thioredoxins regulate many enzymes and signaling proteins. Thus, our discovery that a Prx amplifies an H2O2 signal by driving the oxidation of a thioredoxin-like protein has important implications, both for Prx function in oxidative stress resistance and for responses to H2O2. The thioredoxin-like protein Txl1 is oxidized in response to H2O2 The thioredoxin peroxidase activity of the Prx Tpx1 is required for oxidation of Txl1 The AP-1-like transcription factor Pap1 is an in vivo substrate for Txl1 Tpx1’s thioredoxin peroxidase activity provides H2O2 resistance by regulating Txl1
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Affiliation(s)
- Jonathon D Brown
- Institute for Cell and Molecular Biosciences, Newcastle University, Framlington Place, Newcastle NE2 4HH, Tyne and Wear, UK
| | - Alison M Day
- Institute for Cell and Molecular Biosciences, Newcastle University, Framlington Place, Newcastle NE2 4HH, Tyne and Wear, UK
| | - Sarah R Taylor
- Institute for Cell and Molecular Biosciences, Newcastle University, Framlington Place, Newcastle NE2 4HH, Tyne and Wear, UK
| | - Lewis E Tomalin
- Institute for Cell and Molecular Biosciences, Newcastle University, Framlington Place, Newcastle NE2 4HH, Tyne and Wear, UK
| | - Brian A Morgan
- Institute for Cell and Molecular Biosciences, Newcastle University, Framlington Place, Newcastle NE2 4HH, Tyne and Wear, UK.
| | - Elizabeth A Veal
- Institute for Cell and Molecular Biosciences, Newcastle University, Framlington Place, Newcastle NE2 4HH, Tyne and Wear, UK.
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18
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Kitamura K, Fujiwara H. The type-2 N-end rule peptide recognition activity of Ubr11 ubiquitin ligase is required for the expression of peptide transporters. FEBS Lett 2012; 587:214-9. [PMID: 23219921 DOI: 10.1016/j.febslet.2012.11.028] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/03/2012] [Revised: 11/28/2012] [Accepted: 11/28/2012] [Indexed: 10/27/2022]
Abstract
The Ubr1-like canonical N-recognins, widely conserved ubiquitin ligases in eukaryotes, play a role in the N-end rule pathway-mediated degradation of substrates harboring basic (type-1) or bulky hydrophobic (type-2) amino acids at the N-terminus. In this study, the roles of conserved domains were studied in the Schizosaccharomyces pombe Ubr11 protein. Mutations in the UBR box and the autoinhibitory domain blocked degradation of both type-1 and type-2 substrates, expression of peptide transporter genes, and the uptake of oligopeptides. An N-domain mutant was normal for the type-1-related function, but nevertheless failed to express peptide transporters. These data suggest the importance of the type-2-related activity of Ubr11 for its in vivo function.
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Affiliation(s)
- Kenji Kitamura
- Center for Gene Science, Hiroshima University, Kagamiyama 1-4-2, Higashi-Hiroshima 739-8527, Japan.
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19
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Fujiwara H, Tanaka N, Yamashita I, Kitamura K. Essential role of Ubr11, but not Ubr1, as an N-end rule ubiquitin ligase in Schizosaccharomyces pombe. Yeast 2012; 30:1-11. [PMID: 23348717 DOI: 10.1002/yea.2936] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2012] [Accepted: 10/18/2012] [Indexed: 11/11/2022] Open
Abstract
The N-end rule pathway degrades proteins bearing a destabilization-inducing amino acid at the N-terminus. In this proteolytic system, Ubr ubiquitin ligases recognize and ubiquitylate substrates intended for degradation. Schizosaccharomyces pombe has two similar Ubr proteins, Ubr1 and Ubr11. Both proteins have unique roles in various cellular processes, although the ubr1∆ strain shows more severe defects. However, their involvement in the N-end rule pathway is unclear, and even the N-end rule pathway-dependent proteolytic activity has not been demonstrated in Sz. pombe. Here, we show that: (a) Sz. pombe has the N-end rule pathway in which only Ubr11, but not Ubr1, is responsible; and (b) the C-terminal fragment of the meiotic cohesin Rec8 (denoted as Rec8c) generated by separase-mediated cleavage is an endogenous substrate of the N-end rule pathway. Forced overexpression of stable Rec8c was deleterious in mitosis and caused a loss of the mini-chromosome. In unperturbed mitosis without overexpression, the rate of mini-chromosome loss was five-fold higher in the ubr11∆ strain. Since Rec8 is normally produced in meiosis, we examined whether meiosis and sporulation were affected in the ubr11∆ strain. In unperturbed meiosis, chromosome segregation occurred almost normally and viable spores were produced in the ubr11∆ cells, irrespective of the presence of undegraded endogenous Rec8c peptides.
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20
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Penney M, Samejima I, Wilkinson CR, McInerny CJ, Mathiassen SG, Wallace M, Toda T, Hartmann-Petersen R, Gordon C. Fission yeast 26S proteasome mutants are multi-drug resistant due to stabilization of the Pap1 transcription factor. PLoS One 2012; 7:e50796. [PMID: 23209828 PMCID: PMC3507774 DOI: 10.1371/journal.pone.0050796] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2012] [Accepted: 10/25/2012] [Indexed: 01/12/2023] Open
Abstract
Here we report the result of a genetic screen for mutants resistant to the microtubule poison methyl benzimidazol-2-yl carbamate (MBC) that were also temperature sensitive for growth. In total the isolated mutants were distributed in ten complementation groups. Cloning experiments revealed that most of the mutants were in essential genes encoding various 26S proteasome subunits. We found that the proteasome mutants are multi-drug resistant due to stabilization of the stress-activated transcription factor Pap1. We show that the ubiquitylation and ultimately the degradation of Pap1 depend on the Rhp6/Ubc2 E2 ubiquitin conjugating enzyme and the Ubr1 E3 ubiquitin-protein ligase. Accordingly, mutants lacking Rhp6 or Ubr1 display drug-resistant phenotypes.
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Affiliation(s)
- Mary Penney
- Medical Research Council, Human Genetics Unit, Western General Hospital, Edinburgh, United Kingdom
| | - Itaru Samejima
- Medical Research Council, Human Genetics Unit, Western General Hospital, Edinburgh, United Kingdom
| | - Caroline R. Wilkinson
- Cell Regulation Group, Paterson Institute for Cancer Research, University of Manchester, Manchester, United Kingdom
| | - Christopher J. McInerny
- Division of Molecular and Cellular Biology, School of Life Sciences, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, United Kingdom
| | - Søs G. Mathiassen
- Department of Biology, University of Copenhagen, Copenhagen, Denmark
| | - Mairi Wallace
- Medical Research Council, Human Genetics Unit, Western General Hospital, Edinburgh, United Kingdom
| | - Takashi Toda
- Laboratory of Cell Regulation, Cancer Research UK, London Research Institute, Lincoln's Inn Fields Laboratories, London, United Kingdom
| | | | - Colin Gordon
- Medical Research Council, Human Genetics Unit, Western General Hospital, Edinburgh, United Kingdom
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21
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Gulshan K, Thommandru B, Moye-Rowley WS. Proteolytic degradation of the Yap1 transcription factor is regulated by subcellular localization and the E3 ubiquitin ligase Not4. J Biol Chem 2012; 287:26796-805. [PMID: 22707721 DOI: 10.1074/jbc.m112.384719] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Saccharomyces cerevisiae Yap1 is a transcriptional regulatory protein that serves as a central determinant of oxidative stress tolerance. Activity of this factor is regulated in large part by control of its subcellular location. In the absence of oxidants, Yap1 is primarily located in the cytoplasm. Upon oxidant challenge, Yap1 accumulates rapidly in the nucleus where it activates expression of genes required for oxidative stress tolerance such as the thioredoxin TRX2. Here, we demonstrate that Yap1 degradation is accelerated in response to oxidative stress. Yap1 is folded differently depending on the oxidant used to induce its nuclear localization but is degraded similarly, irrespective of its folded status. Mutant forms of Yap1 that are constitutively trapped in the nucleus are degraded in the absence of an oxidant signal. Degradation requires the ability of the protein to bind DNA and a domain in the amino-terminal region of the factor. Inhibition of the proteasome prevents Yap1 turnover. Screening a variety of mutants involved in ubiquitin-mediated proteolysis demonstrated an important role for the nuclear ubiquitin ligase Not4 in Yap1 degradation. Not4 was found to bind to Yap1 in an oxidant-stimulated fashion. The Candida albicans Yap1 homologue (Cap1) also was degraded after oxidant challenge. These data uncover a new, conserved pathway for regulation of the oxidative stress response that serves to temporally limit the duration of Yap1-dependent transcriptional activation.
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Affiliation(s)
- Kailash Gulshan
- Department of Molecular Physiology and Biophysics, Carver College of Medicine, University of Iowa, Iowa City, Iowa 52242, USA
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22
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Calvo IA, García P, Ayté J, Hidalgo E. The transcription factors Pap1 and Prr1 collaborate to activate antioxidant, but not drug tolerance, genes in response to H2O2. Nucleic Acids Res 2012; 40:4816-24. [PMID: 22344694 PMCID: PMC3367182 DOI: 10.1093/nar/gks141] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
In response to hydrogen peroxide (H2O2), the transcription factor Pap1 from Schizosaccharomyces pombe regulates transcription of genes required for adaptation to oxidative stress and for tolerance to toxic drugs. H2O2 induces oxidation of Pap1, its nuclear accumulation and expression of more than fifty Pap1-dependent genes. Oxidation and nuclear accumulation of Pap1 can also be accomplished by genetic inhibition of thioredoxin reductase. Furthermore, genetic alteration of the nuclear export pathway, or mutations in Pap1 nuclear export signal trigger nuclear accumulation of reduced Pap1. We show here that a subset of Pap1-dependent genes, such as those coding for the efflux pump Caf5, the ubiquitin-like protein Obr1 or the dehydrogenase SPCC663.08c, only require nuclear Pap1 for activation, whereas another subset of genes, those coding for the antioxidants catalase, sulfiredoxin or thioredoxin reductase, do need oxidized Pap1 to form a heterodimer with the constitutively nuclear transcription factor Prr1. The ability of Pap1 to bind and activate drug tolerance promoters is independent on Prr1, whereas its affinity for the antioxidant promoters is significantly enhanced upon association with Prr1. This finding suggests that the activation of both antioxidant and drug resistance genes in response to oxidative stress share a common inducer, H2O2, but alternative effectors.
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Affiliation(s)
- Isabel A Calvo
- Oxidative Stress and Cell Cycle Group, Department de Ciències Experimentals i de la Salut, Universitat Pompeu Fabra, C/Dr. Aiguader 88, E-08003 Barcelona, Spain
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23
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The Ubiquitin ligase Ubr11 is essential for oligopeptide utilization in the fission yeast Schizosaccharomyces pombe. EUKARYOTIC CELL 2012; 11:302-10. [PMID: 22226946 DOI: 10.1128/ec.05253-11] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Uptake of extracellular oligopeptides in yeast is mediated mainly by specific transporters of the peptide transporter (PTR) and oligopeptide transporter (OPT) families. Here, we investigated the role of potential peptide transporters in the yeast Schizosaccharomyces pombe. Utilization of naturally occurring dipeptides required only Ptr2/SPBC13A2.04c and none of the other 3 OPT proteins (Isp4, Pgt1, and Opt3), whereas only Isp4 was indispensable for tetrapeptide utilization. Both Ptr2 and Isp4 localized to the cell surface, but under rich nutrient conditions Isp4 localized in the Golgi apparatus through the function of the ubiquitin ligase Pub1. Furthermore, the ubiquitin ligase Ubr11 played a significant role in oligopeptide utilization. The mRNA levels of both the ptr2 and isp4 genes were significantly reduced in ubr11Δ cells, and the dipeptide utilization defect in the ubr11Δ mutant was rescued by the forced expression of Ptr2. Consistent with its role in transcriptional regulation of peptide transporter genes, the Ubr11 protein was accumulated in the nucleus. Unlike the situation in Saccharomyces cerevisiae, the oligopeptide utilization defect in the S. pombe ubr11Δ mutant was not rescued by inactivation of the Tup11/12 transcriptional corepressors, suggesting that the requirement for the Ubr ubiquitin ligase in the upregulation of peptide transporter mRNA levels is conserved in both yeasts; however, the actual mechanism underlying the control appears to be different. We also found that the peptidomimetic proteasome inhibitor MG132 was still operative in a strain lacking all known PTR and OPT peptide transporters. Therefore, irrespective of its peptide-like structure, MG132 is carried into cells independently of the representative peptide transporters.
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