Repression of ergosterol level during oxidative stress by fission yeast F-box protein Pof14 independently of SCF

Curcumin inhibits Aspergillus flavus infection and aflatoxin production possibly by inducing ROS burst

Aspergillus flavus contamination is common in various food and feed ingredients, and it poses to serious threats to human and animal health. Curcumin is a plant-derived polyphenol that exhibits antifungal activity. In this study, the antifungal effect of curcumin on A. flavus was evaluated, and the underlying mechanism was investigated. Curcumin effectively decreased aflatoxin B1 synthesis and suppressed A. flavus infection in peanut. Curcumin inhibited the mycelial growth and sporulation of A. flavus. Ergosterol biosynthesis in A. flavus was suppressed, and cell membrane permeability was enhanced. The pathogenicity of A. flavus was also reduced by curcumin treatment. Curcumin induced ROS burst in the hyphae of A. flavus, and those damages could be reversed by exogenous superoxide dismutase, suggesting that curcumin inhibited A. flavus possibly via inducing oxidative stress. These results indicate that curcumin has the potential to be used as a preservative to control A. flavus contamination in food and feedstuff.

Correlation between Perturbation of Redox Homeostasis and Antibiofilm Capacity of Phytochemicals at Non-Lethal Concentrations

Biofilms are the multicellular lifestyle of microorganisms and are present on potentially every type of biotic or abiotic surface. Detrimental biofilms are generally targeted with antimicrobial compounds. Phytochemicals at sub-lethal concentrations seem to be an exciting alternative strategy to control biofilms, as they are less likely to impose selective pressure leading to resistance. This overview gathers the literature on individual phytocompounds rather than on extracts of which the use is difficult to reproduce. To the best of our knowledge, this is the first review to target only individual phytochemicals below inhibitory concentrations against biofilm formation. We explored whether there is an overall mechanism that can explain the effects of individual phytochemicals at sub-lethal concentrations. Interestingly, in all experiments reported here in which oxidative stress was investigated, a modest increase in intracellular reactive oxygen species was reported in treated cells compared to untreated specimens. At sub-lethal concentrations, polyphenolic substances likely act as pro-oxidants by disturbing the healthy redox cycle and causing an accumulation of reactive oxygen species.

The Gβ-like Protein AfCpcB Affects Sexual Development, Response to Oxidative Stress and Phagocytosis by Alveolar Macrophages in Aspergillus fumigatus

G-protein signaling is important for signal transduction, allowing various stimuli that are external to a cell to affect its internal molecules. In Aspergillus fumigatus, the roles of Gβ-like protein CpcB on growth, asexual development, drug sensitivity, and virulence in a mouse model have been previously reported. To gain a deeper insight into Aspergillus fumigatus sexual development, the ΔAfcpcB strain was generated using the supermater AFB62 strain and crossed with AFIR928. This cross yields a decreased number of cleistothecia, including few ascospores. The sexual reproductive organ-specific transcriptional analysis using RNAs from the cleistothecia (sexual fruiting bodies) indicated that the CpcB is essential for the completion of sexual development by regulating the transcription of sexual genes, such as veA, steA, and vosA. The ΔAfcpcB strain revealed increased resistance to oxidative stress by regulating genes for catalase, peroxiredoxin, and ergosterol biosynthesis. The ΔAfcpcB strain showed decreased uptake by alveolar macrophages in vitro, decreased sensitivity to Congo red, decreased expression of cell wall genes, and increased expression of the hydrophobin genes. Taken together, these findings indicate that AfCpcB plays important roles in sexual development, phagocytosis by alveolar macrophages, biosynthesis of the cell wall, and oxidative stress response.

Mevalonate kinase of Leishmania donovani protects parasite against oxidative stress by modulating ergosterol biosynthesis

Leishmaniasis comprises of a wide variety of diseases, caused by protozoan parasite belonging to the genus Leishmania. Leishmania parasites undergo different types of stress during their lifetime and have developed strategies to overcome this damage. Identifying the mechanistic approach used by the parasite in dealing with the stress is of immense importance for unfolding the survival strategy adopted by the parasite. Mevalonate kinase (MVK) is an important regulatory factor in the mevalonate pathway in both bacteria and eukaryotes. In this study, we explored the role of Leishmania donovani mevalonate kinase (LdMVK) in parasite survival under stress condition. Hydrogen peroxide (H₂O₂) and menadione, the two known oxidants were used to carry out the experiments. The MVK expression was found to be up regulated ∼2.1 fold and ∼2.3 fold under oxidative stress condition and under the effect of anti-Leishmania drug, AmBisome respectively. The cell viability declined under the effect of MVK inhibitor viz: vanadyl sulfate (VS). The level of intracellular ROS was also found to be increased under the effect of MVK inhibitor. To confirm the findings, LdMVK over expression (LdMVK OE) and LdMVK knockdown (LdMVK KD) parasites were generated. The level of ergosterol, an important component of plasma membrane in L. donovani, was observed and found to be reduced by nearly 60 % in LdMVK KD parasite and increased by nearly 30 % in LdMVK OE parasites as compared to wild type. However, the ergosterol content was found to be elevated under oxidative stress. Furthermore, LdMVK was also found to be associated with maintaining the plasma membrane integrity and also in preventing the peroxidation of cellular lipids when exposed to oxidative stress. The above data clearly suggests that MVK has a vital role in protecting the parasite from oxidative stress. These findings may also explore the contribution of LdMVK in drug unresponsiveness which may help in future rational drug designing for leishmaniasis.

PoRal2 Is Involved in Appressorium Formation and Virulence via Pmk1 MAPK Pathways in the Rice Blast Fungus Pyricularia oryzae

Pyricularia oryzae is an important plant pathogenic fungus that can severely damage rice and wheat crops, leading to significant reductions in crop productivity. To penetrate into and invade tissues of its plant host, this fungus relies on an invasive structure known as an appressorium. Appressorium formation is rigorously regulated by the cAMP-PKA and Pmk1 MAPK pathways. Here, we identified PoRal2, a homologous protein of Schizosaccharomyces pombe Ral2, and characterized its roles in fungal development and virulence in P. oryzae. PoRal2 contains N-terminal kelch repeats and C-terminal BTB domains. PoRal2 is involved in sporulation, aerial hypha and conidiophore differentiation, appressorium formation, plant penetration, and virulence. During appressorium formation, ∆Poral2 mutants generate appressoria with long germ tubes on hydrophobic surfaces. ∆Poral2 mutants exhibited a defective response to exogenous cAMP and the activated RAS2 ^G18V on a hydrophilic surface, indicating impairment in the cAMP-PKA or Pmk1 MAPK signaling pathways. Deletion of PoRAL2 leads to lowered Pmk1 phosphorylation level in the mutant. Moreover, PoRal2 is found to interact with Scd1, Smo1, and Mst50, which are involved in activation of Pmk1. In addition, the expression levels of MPG1, WISH, and PDEH in the cAMP-PKA pathway, RAS2 in both the cAMP-PKA and Pmk1 MAPK pathways, and melanin biosynthesis genes (ALB1, BUF1, and RSY1) were significantly down-regulated in the ∆Poral2. Therefore, PoRal2 is involved in fungal development and virulence by its crosstalk in the cAMP-PKA and Pmk1 MAPK signaling pathways.

Synthetic Pesticides Used in Agricultural Production Promote Genetic Instability and Metabolic Variability in Candida spp

The effects of triazole fungicide Tango^® (epoxiconazole) and two neonicotinoid insecticide formulations Mospilan^® (acetamiprid) and Calypso^® (thiacloprid) were investigated in Candida albicans and three non-albicans species Candida pulcherrima, Candida glabrata and Candida tropicalis to assess the range of morphological, metabolic and genetic changes after their exposure to pesticides. Moreover, the bioavailability of pesticides, which gives us information about their metabolization was assessed using gas chromatography-mass spectrophotometry (GC-MS). The tested pesticides caused differences between the cells of the same species in the studied populations in response to ROS accumulation, the level of DNA damage, changes in fatty acids (FAs) and phospholipid profiles, change in the percentage of unsaturated to saturated FAs or the ability to biofilm. In addition, for the first time, the effect of tested neonicotinoid insecticides on the change of metabolic profile of colony cells during aging was demonstrated. Our data suggest that widely used pesticides, including insecticides, may increase cellular diversity in the Candida species population-known as clonal heterogeneity-and thus play an important role in acquiring resistance to antifungal agents.

Cytotoxic Action of Artemisinin and Scopoletin on Planktonic Forms and on Biofilms of Candida Species

We investigated the antifungal activities of purified plant metabolites artemisinin (Ar) and scopoletin (Sc) including inhibition, effects on metabolic activities, viability, and oxidative stress on planktonic forms and on preformed biofilms of seven Candida species. The characteristic minimum inhibitory concentration (MIC90) of Ar and Sc against Candida species ranged from 21.83-142.1 µg/mL and 67.22-119.4 µg/mL, respectively. Drug concentrations causing ≈10% CFU decrease within 60 minutes of treatments were also determined (minimum effective concentration, MEC10) using 100-fold higher CFUs than in the case of MIC90 studies. Cytotoxic effects on planktonic and on mature biofilms of Candida species at MEC10 concentrations were further evaluated with fluorescent live/dead discrimination techniques. Candida glabrata, Candida guilliermondii, and Candida parapsilosis were the species most sensitive to Ar and Sc. Ar and Sc were also found to promote the accumulation of intracellular reactive oxygen species (ROS) by increasing oxidative stress at their respective MEC10 concentrations against the tested planktonic Candida species. Ar and Sc possess dose-dependent antifungal action but the underlying mechanism type (fungistatic and fungicidal) is not clear yet. Our data suggest that Ar and Sc found in herbal plants might have potential usage in the fight against Candida biofilms.

The hypoxic transcription factor KlMga2 mediates the response to oxidative stress and influences longevity in the yeast Kluyveromyces lactis

Hypoxia is defined as the decline of oxygen availability, depending on environmental supply and cellular consumption rate. The decrease in O2 results in reduction of available energy in facultative aerobes. The response and/or adaptation to hypoxia and other changing environmental conditions can influence the properties and functions of membranes by modifying lipid composition. In the yeast Kluyveromyces lactis, the KlMga2 gene is a hypoxic regulatory factor for lipid biosynthesis—fatty acids and sterols—and is also involved in glucose signaling, glucose catabolism and is generally important for cellular fitness.

In this work we show that, in addition to the above defects, the absence of the KlMGA2 gene caused increased resistance to oxidative stress and extended lifespan of the yeast, associated with increased expression levels of catalase and SOD genes. We propose that KlMga2 might also act as a mediator of the oxidative stress response/adaptation, thus revealing connections among hypoxia, glucose signaling, fatty acid biosynthesis and ROS metabolism in K. lactis.

Repression of Cell Differentiation by a cis-Acting lincRNA in Fission Yeast

The cell fate decision leading to gametogenesis requires the convergence of multiple signals on the promoter of a master regulator. In fission yeast, starvation-induced signaling leads to the transcriptional induction of the ste11 gene, which encodes the central inducer of mating and gametogenesis, known as sporulation. We find that the long intergenic non-coding (linc) RNA rse1 is transcribed divergently upstream of the ste11 gene. During vegetative growth, rse1 directly recruits a Mug187-Lid2-Set1 complex that mediates cis repression at the ste11 promoter through SET3C-dependent histone deacetylation. The absence of rse1 bypasses the starvation-induced signaling and induces gametogenesis in the presence of nutrients. Our data reveal that the remodeling of chromatin through ncRNA scaffolding of repressive complexes that is observed in higher eukaryotes is a conserved, likely very ancient mechanism for tight control of cell differentiation.

Analysis of CFB, a cytokinin-responsive gene of Arabidopsis thaliana encoding a novel F-box protein regulating sterol biosynthesis

Protein degradation by the ubiquitin-26S proteasome pathway is important for the regulation of cellular processes, but the function of most F-box proteins relevant to substrate recognition is unknown. We describe the analysis of the gene Cytokinin-induced F-box encoding (CFB, AT3G44326), identified in a meta-analysis of cytokinin-related transcriptome studies as one of the most robust cytokinin response genes. F-box domain-dependent interaction with the E3 ubiquitin ligase complex component ASK1 classifies CFB as a functional F-box protein. Apart from F-box and transmembrane domains, CFB contains no known functional domains. CFB is expressed in all plant tissues, predominantly in root tissue. A ProCFB:GFP-GUS fusion gene showed strongest expression in the lateral root cap and during lateral root formation. CFB-GFP fusion proteins were mainly localized in the nucleus and the cytosol but also at the plasma membrane. cfb mutants had no discernible phenotype, but CFB overexpressing plants showed several defects, such as a white upper inflorescence stem, similar to the hypomorphic cycloartenol synthase mutant cas1-1. Both CFB overexpressing plants and cas1-1 mutants accumulated the CAS1 substrate 2,3-oxidosqualene in the white stem tissue, the latter even more after cytokinin treatment, indicating impairment of CAS1 function. This suggests that CFB may link cytokinin and the sterol biosynthesis pathway.

Fission yeast Cdk7 controls gene expression through both its CAK and C-terminal domain kinase activities

Cyclin-dependent kinase (Cdk) activation and RNA polymerase II transcription are linked by the Cdk7 kinase, which phosphorylates Cdks as a trimeric Cdk-activating kinase (CAK) complex, and serine 5 within the polymerase II (Pol II) C-terminal domain (CTD) as transcription factor TFIIH-bound CAK. However, the physiological importance of integrating these processes is not understood. Besides the Cdk7 ortholog Mcs6, fission yeast possesses a second CAK, Csk1. The two enzymes have been proposed to act redundantly to activate Cdc2. Using an improved analogue-sensitive Mcs6-as kinase, we show that Csk1 is not a relevant CAK for Cdc2. Further analyses revealed that Csk1 lacks a 20-amino-acid sequence required for its budding yeast counterpart, Cak1, to bind Cdc2. Transcriptome profiling of the Mcs6-as mutant in the presence or absence of the budding yeast Cak1 kinase, in order to uncouple the CTD kinase and CAK activities of Mcs6, revealed an unanticipated role of the CAK branch in the transcriptional control of the cluster of genes implicated in ribosome biogenesis and cell growth. The analysis of a Cdc2 CAK site mutant confirmed these data. Our data show that the Cdk7 kinase modulates transcription through its well-described RNA Pol II CTD kinase activity and also through the Cdc2-activating kinase activity.

Beyond ubiquitination: the atypical functions of Fbxo7 and other F-box proteins

F-box proteins (FBPs) are substrate-recruiting subunits of Skp1-cullin1-FBP (SCF)-type E3 ubiquitin ligases. To date, 69 FBPs have been identified in humans, but ubiquitinated substrates have only been identified for a few, with the majority of FBPs remaining ‘orphans’. In recent years, a growing body of work has identified non-canonical, SCF-independent roles for about 12% of the human FBPs. These atypical FBPs affect processes as diverse as transcription, cell cycle regulation, mitochondrial dynamics and intracellular trafficking. Here, we provide a general review of FBPs, with a particular emphasis on these expanded functions. We review Fbxo7 as an exemplar of this special group as it has well-defined roles in both SCF and non-SCF complexes. We review its function as a cell cycle regulator, via its ability to stabilize p27 protein and Cdk6 complexes, and as a proteasome regulator, owing to its high affinity binding to PI31. We also highlight recent advances in our understanding of Fbxo7 function in Parkinson's disease, where it functions in the regulation of mitophagy with PINK1 and Parkin. We postulate that a few extraordinary FBPs act as platforms that seamlessly segue their canonical and non-canonical functions to integrate different cellular pathways and link their regulation.

The Hect domain E3 ligase Tom1 and the F-box protein Dia2 control Cdc6 degradation in G1 phase

The accurate replication of genetic information is critical to maintaining chromosomal integrity. Cdc6 functions in the assembly of pre-replicative complexes and is specifically required to load the Mcm2-7 replicative helicase complex at replication origins. Cdc6 is targeted for protein degradation by multiple mechanisms in Saccharomyces cerevisiae, although only a single pathway and E3 ubiquitin ligase for Cdc6 has been identified, the SCF(Cdc4) (Skp1/Cdc53/F-box protein) complex. Notably, Cdc6 is unstable during the G(1) phase of the cell cycle, but the ubiquitination pathway has not been previously identified. Using a genetic approach, we identified two additional E3 ubiquitin ligase components required for Cdc6 degradation, the F-box protein Dia2 and the Hect domain E3 Tom1. Both Dia2 and Tom1 control Cdc6 turnover during G(1) phase of the cell cycle and act separately from SCF(Cdc4). Ubiquitination of Cdc6 is significantly reduced in dia2Δ and tom1Δ cells. Tom1 and Dia2 each independently immunoprecipitate Cdc6, binding to a C-terminal region of the protein. Tom1 and Dia2 cannot compensate for each other in Cdc6 degradation. Cdc6 and Mcm4 chromatin association is aberrant in tom1Δ and dia2Δ cells in G(1) phase. Together, these results present evidence for a novel degradation pathway that controls Cdc6 turnover in G(1) that may regulate pre-replicative complex assembly.

Proteomic analysis reveals resistance mechanism against biofuel hexane in Synechocystis sp. PCC 6803

BACKGROUND

Recent studies have demonstrated that photosynthetic cyanobacteria could be an excellent cell factory to produce renewable biofuels and chemicals due to their capability to utilize solar energy and CO2 as the sole energy and carbon sources. Biosynthesis of carbon-neutral biofuel alkanes with good chemical and physical properties has been proposed. However, to make the process economically feasible, one major hurdle to improve the low cell tolerance to alkanes needed to be overcome.

RESULTS

Towards the goal to develop robust and high-alkane-tolerant hosts, in this study, the responses of model cyanobacterial Synechocystis PCC 6803 to hexane, a representative of alkane, were investigated using a quantitative proteomics approach with iTRAQ - LC-MS/MS technologies. In total, 1,492 unique proteins were identified, representing about 42% of all predicted protein in the Synechocystis genome. Among all proteins identified, a total of 164 and 77 proteins were found up- and down-regulated, respectively. Functional annotation and KEGG pathway enrichment analyses showed that common stress responses were induced by hexane in Synechocystis. Notably, a large number of transporters and membrane-bound proteins, proteins against oxidative stress and proteins related to sulfur relay system and photosynthesis were induced, suggesting that they are possibly the major protection mechanisms against hexane toxicity.

CONCLUSION

The study provided the first comprehensive view of the complicated molecular mechanism employed by cyanobacterial model species, Synechocystis to defend against hexane stress. The study also provided a list of potential targets to engineer Synechocystis against hexane stress.

Amphotericin B membrane action: role for two types of ion channels in eliciting cell survival and lethal effects

The formation of aqueous pores by the polyene antibiotic amphotericin B (AmB) is at the basis of its fungicidal and leishmanicidal action. However, other types of nonlethal and dose-dependent biphasic effects that have been associated with the AmB action in different cells, including a variety of survival responses, are difficult to reconcile with the formation of a unique type of ion channel by the antibiotic. In this respect, there is increasing evidence indicating that AmB forms nonaqueous (cation-selective) channels at concentrations below the threshold at which aqueous pores are formed. The main foci of this review will be (1) to provide a summary of the evidence supporting the formation of cation-selective ion channels and aqueous pores by AmB in lipid membrane models and in the membranes of eukaryotic cells; (2) to discuss the influence of membrane parameters such as thickness fluctuations, the type of sterol present and the existence of sterol-rich specialized lipid raft microdomains in the formation process of such channels; and (3) to develop a cell model that serves as a framework for understanding how the intracellular K(+) and Na(+) concentration changes induced by the cation-selective AmB channels enhance multiple survival response pathways before they are overcome by the more sustained ion fluxes, Ca(2+)-dependent apoptotic events and cell lysis effects that are associated with the formation of AmB aqueous pores.

Coupling histone homeostasis to centromere integrity via the ubiquitin-proteasome system

In many eukaryotes, histone gene expression is regulated in a cell cycle-dependent manner, with a spike pattern at S phase. In fission yeast the GATA-type transcription factor Ams2 is required for transcriptional activation of all the core histone genes during S phase and Ams2 protein levels per se show concomitant periodic patterns. We have recently unveiled the molecular mechanisms underlying Ams2 fluctuation during the cell cycle. We have found that Ams2 stability varies during the cell cycle, and that the ubiquitin-proteasome pathway is responsible for Ams2 instability. Intriguingly, Ams2 proteolysis requires Hsk1-a Cdc7 homologue in fission yeast generally called Dbf4-dependent protein kinase (DDK)-and the SCF ubiquitin ligase containing the substrate receptor Pof3 F-box protein. Here, we discuss why histone synthesis has to occur only during S phase. Our results indicate that excess synthesis of core histones outside S phase results in deleterious effects on cell survival. In particular, functions of the centromere, in which the centromere-specific H3 variant CENP-A usually form centromeric nucleosomes, are greatly compromised. This defect is, at least in part, ascribable to abnormal incorporation of canonical histone H3 into these nucleosomes. Finally, we address the significance and potential implications of our work from an evolutionary point of view.

Characterization of a CuZn superoxide dismutase gene in the arbuscular mycorrhizal fungus Glomus intraradices

To gain further insights into the mechanisms of redox homeostasis in arbuscular mycorrhizal fungi, we characterized a Glomus intraradices gene (GintSOD1) showing high similarity to previously described genes encoding CuZn superoxide dismutases (SODs). The GintSOD1 gene consists of an open reading frame of 471 bp, predicted to encode a protein of 157 amino acids with an estimated molecular mass of 16.3 kDa. Functional complementation assays in a CuZnSOD-defective yeast mutant showed that GintSOD1 protects the yeast cells from oxygen toxicity and that it, therefore, encodes a protein that scavenges reactive oxygen species (ROS). GintSOD1 transcripts differentially accumulate during the fungal life cycle, reaching the highest expression levels in the intraradical mycelium. GintSOD1 expression is induced by the well known ROS-inducing agents paraquat and copper, and also by fenpropimorph, a sterol biosynthesis inhibitor (SBI) fungicide. These results suggest that GintSOD1 is involved in the detoxification of ROS generated from metabolic processes and by external agents. In particular, our data indicate that the antifungal effects of fenpropimorph might not be only due to the interference with sterol metabolism but also to the perturbation of other biological processes and that ROS production and scavenging systems are involved in the response to SBI fungicides.

Genome-wide fitness and expression profiling implicate Mga2 in adaptation to hydrogen peroxide

Caloric restriction extends lifespan, an effect once thought to involve attenuation of reactive oxygen species (ROS) generated by aerobic metabolism. However, recent evidence suggests that caloric restriction may in fact raise ROS levels, which in turn provides protection from acute doses of oxidant through a process called adaptation. To shed light on the molecular mechanisms of adaptation, we designed a series of genome-wide deletion fitness and mRNA expression screens to identify genes involved in adaptation to hydrogen peroxide. Combined with known transcriptional interactions, the integrated data implicate Yap1 and Skn7 as central transcription factors of both the adaptive and acute oxidative responses. They also identify the transcription factors Mga2 and Rox1 as active exclusively in the adaptive response and show that Mga2 is essential for adaptation. These findings are striking because Mga2 and Rox1 have been thought to control the response to hypoxic, not oxidative, conditions. Expression profiling of mga2Δ and rox1Δ knockouts shows that these factors most strongly regulate targets in ergosterol, fatty-acid, and zinc metabolic pathways. Direct quantitation of ergosterol reveals that its basal concentration indeed depends on Mga2, but that Mga2 is not required for the decrease in ergosterol observed during adaptation.

Reactive oxygen species (ROS) damage a variety of structures within the cell, resulting in disease and aging. In a seemingly paradoxical effect termed adaptation, it is possible to prevent damage caused by ROS by pre-treating the cell with a small amount of oxidant. We studied this process in order to identify the mechanisms that provide this protection. Our study identified a number of genes and processes with previously unappreciated roles in adaptation. The mechanisms we identified are remarkable because they are distinct from those previously known to protect the cell from ROS. Although this study is conducted in yeast, the wide conservation of adaptation among many organisms suggests that the results from this study may be widely applicable.

Modulation of plasma membrane lipid profile and microdomains by H2O2 in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the rate of hydrogen peroxide (H(2)O(2)) diffusion through the plasma membrane decreases during adaptation to H(2)O(2) by a still unknown mechanism. Here, adaptation to H(2)O(2) was observed to modulate rapidly the expression of genes coding for enzymes involved in ergosterol and lipid metabolism. Adaptation to H(2)O(2) also alters plasma membrane lipid composition. The main changes were the following: (a) there was a decrease in oleic acid (30%) and in the ratio between unsaturated and saturated long-chain fatty acids; (b) the phosphatidylcholine:phosphatidylethanolamine ratio increased threefold; (c) sterol levels were unaltered but there was an increased heterogeneity of sterol-rich microdomains and increased ordered domains; (d) the levels of the sterol precursor squalene increased twofold, in agreement with ERG1 gene down-regulation; and (e) C26:0 became the major very long chain fatty acid owing to an 80% decrease in 2-hydroxy-C26:0 levels and a 50% decrease in C20:0 levels, probably related to the down-regulation of fatty acid elongation (FAS1, FEN1, SUR4) and ceramide synthase (LIP1, LAC1) genes. Therefore, H(2)O(2) leads to a reorganization of the plasma membrane microdomains, which may explain the lower permeability to H(2)O(2), and emerges as an important regulator of lipid metabolism and plasma membrane lipid composition.

Roles of proteolysis in plant self-incompatibility

Self-incompatibility (SI) is a genetically controlled system adopted by many flowering plants to avoid inbreeding and thus to maintain species diversity. Generally, self-pollen rejection occurs through active pollen and pistil recognition and subsequent signaling responses. So far, three different molecular controls of pollen and pistil recognition have been characterized and are exemplified by three families: the Solanaceae, the Papaveraceae, and the Brassicaceae. With more components involved in these SI systems coming to light, recent studies have provided intriguing insights into the downstream reactions that follow the initial SI signal perception. The process of pollen rejection is closely associated with rapid and effective proteolytic events, including the ubiquitin-proteasome pathway and the vacuolar sorting pathway. Here, we review our current understanding of the roles of proteolysis in SI responses of flowering plants.

Arabidopsis thaliana contains a single gene encoding squalene synthase

Squalene synthase (SQS) catalyzes the condensation of two molecules of farnesyl diphosphate (FPP) to produce squalene (SQ), the first committed precursor for sterol, brassinosteroid, and triterpene biosynthesis. Arabidopsis thaliana contains two SQS-annotated genomic sequences, At4g34640 (SQS1) and At4g34650 (SQS2), organized in a tandem array. Here we report that the SQS1 gene is widely expressed in all tissues throughout plant development, whereas SQS2 is primarily expressed in the vascular tissue of leaf and cotyledon petioles, and the hypocotyl of seedlings. Neither the complete A. thaliana SQS2 protein nor the chimeric SQS resulting from the replacement of the 69 C-terminal residues of SQS2 by the 111 C-terminal residues of the Schizosaccharomyces pombe SQS were able to confer ergosterol prototrophy to a Saccharomyces cerevisiae erg9 mutant strain lacking SQS activity. A soluble form of SQS2 expressed in Escherichia coli and purified was unable to synthesize SQ from FPP in the presence of NADPH and either Mg2+ or Mn2+. These results demonstrated that SQS2 has no SQS activity, so that SQS1 is the only functional SQS in A. thaliana. Mutational studies revealed that the lack of SQS activity of SQS2 cannot be exclusively attributed to the presence of an unusual Ser replacing the highly conserved Phe at position 287. Expression of green fluorescent protein (GFP)-tagged versions of SQS1 in onion epidermal cells demonstrated that SQS1 is targeted to the endoplasmic reticulum (ER) membrane and that this location is exclusively dependent on the presence of the SQS1 C-terminal hydrophobic trans-membrane domain.

Kinetics of hydrogen peroxide elimination by astrocytes and C6 glioma cells analysis based on a mathematical model

Oxidative stress is implicated in a variety of disorders including neurodegenerative diseases, and H(2)O(2) is important in the generation of reactive oxygen and oxidative stress. In this study, we have examined the rate of extracellular H(2)O(2) elimination and relevant enzyme activities in cultured astrocytes and C6 glioma cells and have analyzed the results based on a mathematical model. As compared with other types of cultured cells, astrocytes showed higher activity of glutathione peroxidase (GPx) but lower activities for GSH recycling. C6 cells showed relatively low GPx activity, and treatment of C6 cells with dibutyryl-cAMP, which induces astrocytic differentiation, increased catalase activity and H(2)O(2) permeation rate but exerted little effect on other enzyme activities. A mathematical model [N. Makino, K. Sasaki, N. Hashida, Y. Sakakura, A metabolic model describing the H(2)O(2) elimination by mammalian cells including H(2)O(2) permeation through cytoplasmic and peroxisomal membranes: comparison with experimental data, Biochim. Biophys. Acta 1673 (2004) 149-159.], which includes relevant enzymes and H(2)O(2) permeation through membranes, was found to be fitted well to the H(2)O(2) concentration dependences of removal reaction with the permeation rate constants as variable parameters. As compared with PC12 cells as a culture model for neuron, H(2)O(2) removal activity of astrocytes was considerably higher at physiological H(2)O(2) concentrations. The details of the mathematical model are presented in Appendix.

H2O2 induces rapid biophysical and permeability changes in the plasma membrane of Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the diffusion rate of hydrogen peroxide (H2O2) through the plasma membrane decreases during adaptation to H2O2 by means of a mechanism that is still unknown. Here, evidence is presented that during adaptation to H2O2 the anisotropy of the plasma membrane increases. Adaptation to H2O2 was studied at several times (15min up to 90min) by applying the steady-state H2O2 delivery model. For wild-type cells, the steady-state fluorescence anisotropy increased after 30min, or 60min, when using 2-(9-anthroyloxy) stearic acid (2-AS), or diphenylhexatriene (DPH) membrane probe, respectively. Moreover, a 40% decrease in plasma membrane permeability to H2O2 was observed at 15min with a concomitant two-fold increase in catalase activity. Disruption of the ergosterol pathway, by knocking out either ERG3 or ERG6, prevents the changes in anisotropy during H2O2 adaptation. H2O2 diffusion through the plasma membrane in S. cerevisiae cells is not mediated by aquaporins since the H2O2 permeability constant is not altered in the presence of the aquaporin inhibitor mercuric chloride. Altogether, these results indicate that the regulation of the plasma membrane permeability towards H2O2 is mediated by modulation of the biophysical properties of the plasma membrane.

F-box proteins: more than baits for the SCF?

Regulation of protein stability through the ubiquitin proteasome system is a key mechanism underlying numerous cellular processes. The ubiquitin protein ligases (or E3) are in charge of substrate specificity and therefore play a pivotal role in the pathway. Among the several different E3 enzyme families, the SCF (Skp1-Cullin-F box protein) is one of the largest and best characterized. F-box proteins, in addition to the loosely conserved F-box motif that binds Skp1, often carry typical protein interaction domains and are proposed to recruit the substrate to the SCF complex. Strikingly, genomes analysis revealed the presence of large numbers of F-box proteins topping to nearly 700 predicted in Arabidopsis thaliana.

Recent evidences in various species suggest that some F-box proteins have functions not directly related to the SCF complex raising questions about the actual connection between the large F-box protein family and protein degradation, but also about their origins and evolution.