The MEP2 ammonium permease regulates pseudohyphal differentiation in Saccharomyces cerevisiae

Ammonia transporter RhBG initiates downstream signaling and functional responses by activating NFκB

Transceptors, solute transporters that facilitate intracellular entry of molecules and also initiate intracellular signaling events, have been primarily studied in lower-order species. Ammonia, a cytotoxic endogenous metabolite, is converted to urea in hepatocytes for urinary excretion in mammals. During hyperammonemia, when hepatic metabolism is impaired, nonureagenic ammonia disposal occurs primarily in skeletal muscle. Increased ammonia uptake in skeletal muscle is mediated by a membrane-bound, 12 transmembrane domain solute transporter, Rhesus blood group-associated B glycoprotein (RhBG). We show that in addition to its transport function, RhBG interacts with myeloid differentiation primary response-88 (MyD88) to initiate an intracellular signaling cascade that culminates in activation of NFκB. We also show that ammonia-induced MyD88 signaling is independent of the canonical toll-like receptor-initiated mechanism of MyD88-dependent NFκB activation. In silico, in vitro, and in situ experiments show that the conserved cytosolic J-domain of the RhBG protein interacts with the Toll-interleukin-1 receptor (TIR) domain of MyD88. In skeletal muscle from human patients, human-induced pluripotent stem cell-derived myotubes, and myobundles show an interaction of RhBG-MyD88 during hyperammonemia. Using complementary experimental and multiomics analyses in murine myotubes and mice with muscle-specific RhBG or MyD88 deletion, we show that the RhBG-MyD88 interaction is essential for the activation of NFkB but not ammonia transport. Our studies show a paradigm of substrate-dependent regulation of transceptor function with the potential for modulation of cellular responses in mammalian systems by decoupling transport and signaling functions of transceptors.

To each its own: Mechanisms of cross-talk between GPI biosynthesis and cAMP-PKA signaling in Candida albicans versus Saccharomyces cerevisiae

Candida albicans is an opportunistic fungal pathogen that can switch between yeast and hyphal morphologies depending on the environmental cues it receives. The switch to hyphal form is crucial for the establishment of invasive infections. The hyphal form is also characterized by the cell surface expression of hyphae-specific proteins, many of which are GPI-anchored and important determinants of its virulence. The coordination between hyphal morphogenesis and the expression of GPI-anchored proteins is made possible by an interesting cross-talk between GPI biosynthesis and the cAMP-PKA signaling cascade in the fungus; a parallel interaction is not found in its human host. On the other hand, in the nonpathogenic yeast, Saccharomyces cerevisiae, GPI biosynthesis is shut down when filamentation is activated and vice versa. This too is achieved by a cross-talk between GPI biosynthesis and cAMP-PKA signaling. How are diametrically opposite effects obtained from the cross-talk between two reasonably well-conserved pathways present ubiquitously across eukarya? This Review attempts to provide a model to explain these differences. In order to do so, it first provides an overview of the two pathways for the interested reader, highlighting the similarities and differences that are observed in C. albicans versus the well-studied S. cerevisiae model, before going on to explain how the different mechanisms of regulation are effected. While commonalities enable the development of generalized theories, it is hoped that a more nuanced approach, that takes into consideration species-specific differences, will enable organism-specific understanding of these processes and contribute to the development of targeted therapies.

How sensor Amt-like proteins integrate ammonium signals

Unlike aquaporins or potassium channels, ammonium transporters (Amts) uniquely discriminate ammonium from potassium and water. This feature has certainly contributed to their repurposing as ammonium receptors during evolution. Here, we describe the ammonium receptor Sd-Amt1, where an Amt module connects to a cytoplasmic diguanylate cyclase transducer module via an HAMP domain. Structures of the protein with and without bound ammonium were determined to 1.7- and 1.9-Ångstrom resolution, depicting the ON and OFF states of the receptor and confirming the presence of a binding site for two ammonium cations that is pivotal for signal perception and receptor activation. The transducer domain was disordered in the crystals, and an AlphaFold2 prediction suggests that the helices linking both domains are flexible. While the sensor domain retains the trimeric fold formed by all Amt family members, the HAMP domains interact as pairs and serve to dimerize the transducer domain upon activation.

The S-component fold: a link between bacterial transporters and receptors

The processes of nutrient uptake and signal sensing are crucial for microbial survival and adaptation. Membrane-embedded proteins involved in these functions (transporters and receptors) are commonly regarded as unrelated in terms of sequence, structure, mechanism of action and evolutionary history. Here, we analyze the protein structural universe using recently developed artificial intelligence-based structure prediction tools, and find an unexpected link between prominent groups of microbial transporters and receptors. The so-called S-components of Energy-Coupling Factor (ECF) transporters, and the membrane domains of sensor histidine kinases of the 5TMR cluster share a structural fold. The discovery of their relatedness manifests a widespread case of prokaryotic "transceptors" (related proteins with transport or receptor function), showcases how artificial intelligence-based structure predictions reveal unchartered evolutionary connections between proteins, and provides new avenues for engineering transport and signaling functions in bacteria.

Comparative transcriptome analysis provides insights into the resistance regulation mechanism and inhibitory effect of fungicide phenamacril in Fusarium asiaticum

Fusarium asiaticum is a destructive phytopathogenic fungus that causes Fusarium head blight of wheat (FHB), leading to serious yield and economic losses to cereal crops worldwide. Our previous studies indicated that target-site mutations (K216R/E, S217P/L, or E420K/G/D) of Type I myosin FaMyo5 conferred high resistance to phenamacril. Here, we first constructed one sensitive strain H1S and three point mutation resistant strains HA, HC and H1R. Then we conducted comparative transcriptome analysis of these F. asiaticum strains after 1 and 10 μg·mL-1 phenamacril treatment. Results indicated that 2135 genes were differentially expressed (DEGs) among the sensitive and resistant strains. The DEGs encoding ammonium transporter MEP1/MEP2, nitrate reductase, copper amine oxidase 1, 4-aminobutyrate aminotransferase, amino-acid permease inda1, succinate-semialdehyde dehydrogenase, 2, 3-dihydroxybenzoic acid decarboxylase, etc., were significantly up-regulated in all the phenamacril-resistant strains. Compared to the control group, a total of 1778 and 2097 DEGs were identified in these strains after 1 and 10 μg·mL-1 phenamacril treatment, respectively. These DEGs involved in 4-aminobutyrate aminotransferase, chitin synthase 1, multiprotein-bridging factor 1, transcriptional regulatory protein pro-1, amino-acid permease inda1, ATP-dependent RNA helicase DED1, acetyl-coenzyme A synthetase, sarcoplasmic/endoplasmic reticulum calcium ATPase 2, etc., showed significantly down-regulated expression in phenamacril-sensitive strain but not in resistant strains after phenamacril treatment. In addition, cyanide hydratase, mating-type protein MAT-1, putative purine nucleoside permease, plasma membrane protein yro2, etc., showed significantly co-down-regulated expression in all the strains after phenamacril treatment. Taken together, This study provides deep insights into the resistance regulation mechanism and the inhibitory effect of fungicide phenamacril and these new annotated proteins or enzymes are worth for the discovery of new fungicide targets.

Ecological inducers of the yeast filamentous growth pathway reveal environment-dependent roles for pathway components

Signaling modules, such as mitogen-activated protein kinase (MAPK) pathways, are evolutionarily conserved drivers of cell differentiation and stress responses. In many fungal species including pathogens, MAPK pathways control filamentous growth, where cells differentiate into an elongated cell type. The convenient model budding yeast Saccharomyces cerevisiae undergoes filamentous growth by the filamentous growth (fMAPK) pathway; however, the inducers of the pathway remain unclear, perhaps because pathway activity has been mainly studied in laboratory conditions. To address this knowledge gap, an ecological framework was used, which uncovered new fMAPK pathway inducers, including pectin, a material found in plants, and the metabolic byproduct ethanol. We also show that induction by a known inducer of the pathway, the non-preferred carbon source galactose, required galactose metabolism and induced the pathway differently than glucose limitation or other non-preferred carbon sources. By exploring fMAPK pathway function in fruit, we found that induction of the pathway led to visible digestion of fruit rind through a known target, PGU1, which encodes a pectolytic enzyme. Combinations of inducers (galactose and ethanol) stimulated the pathway to near-maximal levels, which showed dispensability of several fMAPK pathway components (e.g., mucin sensor, p21-activated kinase), but not others (e.g., adaptor, MAPKKK) and required the Ras2-protein kinase A pathway. This included a difference between the transcription factor binding partners for the pathway, as Tec1p, but not Ste12p, was partly dispensable for fMAPK pathway activity. Thus, by exploring ecologically relevant stimuli, new modes of MAPK pathway signaling were uncovered, perhaps revealing how a pathway can respond differently to specific environments. IMPORTANCE Filamentous growth is a cell differentiation response and important aspect of fungal biology. In plant and animal fungal pathogens, filamentous growth contributes to virulence. One signaling pathway that regulates filamentous growth is an evolutionarily conserved MAPK pathway. The yeast Saccharomyces cerevisiae is a convenient model to study MAPK-dependent regulation of filamentous growth, although the inducers of the pathway are not clear. Here, we exposed yeast cells to ecologically relevant compounds (e.g., plant compounds), which identified new inducers of the MAPK pathway. In combination, the inducers activated the pathway to near-maximal levels but did not cause detrimental phenotypes associated with previously identified hyperactive alleles. This context allowed us to identify conditional bypass for multiple pathway components. Thus, near-maximal induction of a MAPK pathway by ecologically relevant inducers provides a powerful tool to assess cellular signaling during a fungal differentiation response.

Ammonia and Nematode Ascaroside Are Synergistic in Trap Formation in Arthrobotrys oligospora

Nematode-trapping (NT) fungi are natural predators of the soil living nematodes. Diverse external signals mediate the generation of predatory devices of NT fungi. Among these, broad ascarosides and nitrogenous ammonia are highly efficient inducers for trap structure initiation. However, the overlay effect of ammonia and ascaroside on the trap morphogenesis remains unclear. This study demonstrated that the combination of nitrogenous substances with nematode-derived ascarosides led to higher trap production compared to the single inducing cues; notably, ammonia and Ascr#18 had the most synergistic effect on the trap in A. oligospora. Further, the deletion of ammonia transceptor Amt43 blocked trap formation against ammonia addition in A. oligospora but not for the ascaroside Ascr#18 induction. Moreover, ammonia addition could promote plasma endocytosis in the process of trap formation. In contrast, ascaroside addition would facilitate the stability of intracellular organization away from endocytosis. Therefore, there is a synergistic effect on trap induction from different nitrogenous and ascaroside signals.

Prokaryotic ammonium transporters: what has three decades of research revealed?

The exchange of ammonium across cellular membranes is a fundamental process in all domains of life. In plants, bacteria and fungi, ammonium represents a vital source of nitrogen, which is scavenged from the external environment. In contrast, in animal cells ammonium is a cytotoxic metabolic waste product and must be excreted to prevent cell death. Transport of ammonium is facilitated by the ubiquitous Amt/Mep/Rh transporter superfamily. In addition to their function as transporters, Amt/Mep/Rh proteins play roles in a diverse array of biological processes and human physiopathology. Despite this clear physiological importance and medical relevance, the molecular mechanism of Amt/Mep/Rh proteins has remained elusive. Crystal structures of bacterial Amt/Rh proteins suggest electroneutral transport, whilst functional evidence supports an electrogenic mechanism. Here, focusing on bacterial members of the family, we summarize the structure of Amt/Rh proteins and what three decades of research tells us concerning the general mechanisms of ammonium translocation, in particular the possibility that the transport mechanism might differ in various members of the Amt/Mep/Rh superfamily.

Functional genome annotation and transcriptome analysis of Pseudozyma hubeiensis BOT-O, an oleaginous yeast that utilizes glucose and xylose at equal rates

Pseudozyma hubeiensis is a basidiomycete yeast that has the highly desirable traits for lignocellulose valorisation of being equally efficient at utilization of glucose and xylose, and capable of their co-utilization. The species has previously mainly been studied for its capacity to produce secreted biosurfactants in the form of mannosylerythritol lipids, but it is also an oleaginous species capable of accumulating high levels of triacylglycerol storage lipids during nutrient starvation. In this study, we aimed to further characterize the oleaginous nature of P. hubeiensis by evaluating metabolism and gene expression responses during storage lipid formation conditions with glucose or xylose as a carbon source. The genome of the recently isolated P. hubeiensis BOT-O strain was sequenced using MinION long-read sequencing and resulted in the most contiguous P. hubeiensis assembly to date with 18.95 Mb in 31 contigs. Using transcriptome data as experimental support, we generated the first mRNA-supported P. hubeiensis genome annotation and identified 6540 genes. 80% of the predicted genes were assigned functional annotations based on protein homology to other yeasts. Based on the annotation, key metabolic pathways in BOT-O were reconstructed, including pathways for storage lipids, mannosylerythritol lipids and xylose assimilation. BOT-O was confirmed to consume glucose and xylose at equal rates, but during mixed glucose-xylose cultivation glucose was found to be taken up faster. Differential expression analysis revealed that only a total of 122 genes were significantly differentially expressed at a cut-off of |log₂ fold change| ≥ 2 when comparing cultivation on xylose with glucose, during exponential growth and during nitrogen-starvation. Of these 122 genes, a core-set of 24 genes was identified that were differentially expressed at all time points. Nitrogen-starvation resulted in a larger transcriptional effect, with a total of 1179 genes with significant expression changes at the designated fold change cut-off compared with exponential growth on either glucose or xylose.

Synonymous and Nonsynonymous Substitutions in Dictyostelium discoideum Ammonium Transporter amtA Are Necessary for Functional Complementation in Saccharomyces cerevisiae

Ammonium transporters are present in all three domains of life. They have undergone extensive horizontal gene transfer (HGT), gene duplication, and functional diversification and therefore offer an excellent paradigm to study protein evolution. We attempted to complement a mep1Δmep2Δmep3Δ strain of Saccharomyces cerevisiae (triple-deletion strain), which otherwise cannot grow on ammonium as a sole nitrogen source at concentrations of <3 mM, with amtA of Dictyostelium discoideum, an orthologue of S. cerevisiae MEP2. We observed that amtA did not complement the triple-deletion strain of S. cerevisiae for growth on low-ammonium medium. We isolated two mutant derivatives of amtA (amtA M1 and amtA M2) from a PCR-generated mutant plasmid library that complemented the triple-deletion strain of S. cerevisiae. amtA M1 bears three nonsynonymous and two synonymous substitutions, which are necessary for its functionality. amtA M2 bears two nonsynonymous substitutions and one synonymous substitution, all of which are necessary for functionality. Interestingly, AmtA M1 transports ammonium but does not confer methylamine toxicity, while AmtA M2 transports ammonium and confers methylamine toxicity, demonstrating functional diversification. Preliminary biochemical analyses indicated that the mutants differ in their conformations as well as their mechanisms of ammonium transport. These intriguing results clearly point out that protein evolution cannot be fathomed by studying nonsynonymous and synonymous substitutions in isolation. The above-described observations have significant implications for various facets of biological processes and are discussed in detail. IMPORTANCE Functional diversification following gene duplication is one of the major driving forces of protein evolution. While the role of nonsynonymous substitutions in the functional diversification of proteins is well recognized, knowledge of the role of synonymous substitutions in protein evolution is in its infancy. Using functional complementation, we isolated two functional alleles of the D. discoideum ammonium transporter gene (amtA), which otherwise does not function in S. cerevisiae as an ammonium transporters. One of them is an ammonium transporter, while the other is an ammonium transporter that also confers methylammonium (ammonium analogue) toxicity, suggesting functional diversification. Surprisingly, both alleles require a combination of synonymous and nonsynonymous substitutions for their functionality. These results bring out a hitherto-unknown pathway of protein evolution and pave the way for not only understanding protein evolution but also interpreting single nucleotide polymorphisms (SNPs).

Lessons learned: from mentored to mentor

This Viewpoint was written in association with the 25th anniversary of the American Society for Clinical Investigation's (ASCI's) Stanley J. Korsmeyer Award, which honors the highest standards of scientific excellence, meritorious research, intellectual integrity, and the mentoring of future life-science researchers. In 2018, the award recognized Joseph Heitman (Figure 1), for his key contributions to our understanding of how eukaryotic microbial pathogens evolve, cause disease, and develop drug resistance and his discovery of TOR and FKBP12 as targets of the immunosuppressive chemotherapeutic drug rapamycin. Dr. Heitman has mentored numerous undergraduates, medical students, graduate students, and postdoctoral and medical fellows, many of whom have developed independent careers in medicine and basic biomedical research.

Knockdown of the Sodium/Potassium ATPase Subunit Beta 2 Reduces Egg Production in the Dengue Vector, Aedes aegypti

The Na+/K+ ATPase (NKA) is present in the cellular membrane of most eukaryotic cells. It utilizes energy released by ATP hydrolysis to pump sodium ions out of the cell and potassium ions into the cell, which establishes and controls ion gradients. Functional NKA pumps consist of three subunits, alpha, beta, and FXYD. The alpha subunit serves as the catalytic subunit while the beta and FXYD subunits regulate the proper folding and localization, and ion affinity of the alpha subunit, respectively. Here we demonstrate that knockdown of NKA beta subunit 2 mRNA (nkaβ2) reduces fecundity in female Ae. aegypti. We determined the expression pattern of nkaβ2 in several adult mosquito organs using qRT-PCR. We performed RNAi-mediated knockdown of nkaβ2 and assayed for lethality, and effects on female fecundity. Tissue expression levels of nkaβ2 mRNA were highest in the ovaries with the fat body, midgut and thorax having similar expression levels, while Malpighian tubules had significantly lower expression. Survival curves recorded post dsRNA injection showed a non-significant decrease in survival of nkaβ2 dsRNA-injected mosquitoes compared to GFP dsRNA-injected mosquitoes. We observed a significant reduction in the number of eggs laid by nkaβ2 dsRNA-injected mosquitoes compared to control mosquitoes. These results, coupled with the tissue expression profile of nkaβ2, indicate that this subunit plays a role in normal female Ae. aegypti fecundity. Additional research needs to be conducted to determine the exact role played by NKAβ2 in mosquito post-blood meal nutrient sensing, transport, yolk precursor protein (YPP) synthesis and yolk deposition.

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

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

The NPR/Hal family of protein kinases in yeasts: biological role, phylogeny and regulation under environmental challenges

Protein phosphorylation is the most common and versatile post-translational modification occurring in eukaryotes. In yeast, protein phosphorylation is fundamental for maintaining cell growth and adapting to sudden changes in environmental conditions by regulating cellular processes and activating signal transduction pathways. Protein kinases catalyze the reversible addition of phosphate groups to target proteins, thereby regulating their activity. In Saccharomyces cerevisiae, kinases are classified into six major groups based on structural and functional similarities. The NPR/Hal family of kinases comprises nine fungal-specific kinases that, due to lack of similarity with the remaining kinases, were classified to the “Other” group. These kinases are primarily implicated in regulating fundamental cellular processes such as maintaining ion homeostasis and controlling nutrient transporters’ concentration at the plasma membrane. Despite their biological relevance, these kinases remain poorly characterized and explored. This review provides an overview of the information available regarding each of the kinases from the NPR/Hal family, including their known biological functions, mechanisms of regulation, and integration in signaling pathways in S. cerevisiae. Information gathered for non-Saccharomyces species of biotechnological or clinical relevance is also included.

Trehalose biosynthetic pathway regulates filamentation response in Saccharomyces cerevisiae

BACKGROUND

Diploid cells of Saccharomyces cerevisiae undergo either pseudohyphal differentiation or sporulation in response to depletion of carbon and nitrogen sources. Distinct signaling pathways regulate filamentation and sporulation in response to nutrient limitation. How these pathways are coordinated for implementing distinct cell fate decisions in response to similar nutritional cues is an enigma. Although the role of trehalose pathway in sporulation has been extensively studied, it's possible role in pseudohyphal differentiation has been unexplored.

METHODS AND RESULTS

Briefly, tps1 and tps2 mutants were tested for their ability to form pseudohyphae independently as well as in the background of GPR1 and RAS2 mutations. Here, we demonstrate that disruption of TPS1 but not TPS2 inhibits pseudohyphae formation. Interestingly, deletion of GPR1 suppresses the above defect. Further genetic analysis revealed that TPS1 and TPS2 exert opposing effects in triggering filamentation.

CONCLUSION

We provide new insights into the role of an otherwise well-known pathway of trehalose biosynthesis in pseudohyphal differentiation. Based on additional data we propose that downstream signaling, mediated by cAMP may be modulated by nutrient mediated differential regulation of RAS2 by TPS1 and TPS2.

Coexistence of Ammonium Transporter and Channel Mechanisms in Amt-Mep-Rh Twin-His Variants Impairs the Filamentation Signaling Capacity of Fungal Mep2 Transceptors

Ammonium translocation through biological membranes, by the ubiquitous Amt-Mep-Rh family of transporters, plays a key role in all domains of life. Two highly conserved histidine residues protrude into the lumen of the pore of these transporters, forming the family's characteristic Twin-His motif. It has been hypothesized that the motif is essential to confer the selectivity of the transport mechanism. Here, using a combination of in vitro electrophysiology on Escherichia coli AmtB, in silico molecular dynamics simulations, and in vivo yeast functional complementation assays, we demonstrate that variations in the Twin-His motif trigger a mechanistic switch between a specific transporter, depending on ammonium deprotonation, to an unspecific ion channel activity. We therefore propose that there is no selective filter that governs specificity in Amt-Mep-Rh transporters, but the inherent mechanism of translocation, dependent on the fragmentation of the substrate, ensures the high specificity of the translocation. We show that coexistence of both mechanisms in single Twin-His variants of yeast Mep2 transceptors disrupts the signaling function and so impairs fungal filamentation. These data support a signaling process driven by the transport mechanism of the fungal Mep2 transceptors. IMPORTANCE Fungal infections represent a significant threat to human health and cause huge damage to crop yields worldwide. The dimorphic switch between yeast and filamentous growth is associated with the virulence of pathogenic fungi. Of note, fungal Mep2 proteins of the conserved Amt-Mep-Rh family play a transceptor role in the induction of filamentation; however, the signaling mechanism remains largely unknown. Amt-Mep-Rh proteins ensure the specific scavenging of NH4+ through a mechanism relying on substrate deprotonation, thereby preventing competition and translocation of similar-sized K+. Our multidisciplinary approaches using E. coli AmtB, Saccharomyces cerevisiae, and Candida albicans Mep2 show that double variation of the family-defining Twin-His motif triggers a mechanistic switch from a specific transporter to an unspecific ion channel with both mechanisms still coexisting in single variants. Moreover, we show that this mechanistic alteration is associated with loss of signaling ability of Mep2, supporting a transport mechanism-driven process in filamentation induction.

Regulation of Copper Metabolism by Nitrogen Utilization in Saccharomyces cerevisiae

To understand the relationship between carbon or nitrogen utilization and iron homeostasis, we performed an iron uptake assay with several deletion mutants with partial defects in carbon or nitrogen metabolism. Among them, some deletion mutants defective in carbon metabolism partially and the MEP2 deletion mutant showed lower iron uptake activity than the wild type. Mep2 is known as a high-affinity ammonia transporter in Saccharomyces cerevisiae. Interestingly, we found that nitrogen starvation resulted in lower iron uptake activity than that of wild-type cells without downregulation of the genes involved in the high-affinity iron uptake system FET3/FTR1. However, the gene expression of FRE1 and CTR1 was downregulated by nitrogen starvation. The protein level of Ctr1 was also decreased by nitrogen starvation, and addition of copper to the nitrogen starvation medium partially restored iron uptake activity. However, the expression of MAC1, which is a copper-responsive transcriptional activator, was not downregulated by nitrogen starvation at the transcriptional level but was highly downregulated at the translational level. Mac1 was downregulated dramatically under nitrogen starvation, and treatment with MG132, which is an inhibitor of proteasome-dependent protein degradation, partially attenuated the downregulation of Mac1. Taken together, these results suggest that nitrogen starvation downregulates the high-affinity iron uptake system by degrading Mac1 in a proteasome-dependent manner and eventually downregulates copper metabolism.

Mechanisms of Metabolic Adaptation in Wine Yeasts: Role of Gln3 Transcription Factor

Wine strains of Saccharomyces cerevisiae have to adapt their metabolism to the changing conditions during their biotechnological use, from the aerobic growth in sucrose-rich molasses for biomass propagation to the anaerobic fermentation of monosaccharides of grape juice during winemaking. Yeast have molecular mechanisms that favor the use of preferred carbon and nitrogen sources to achieve such adaptation. By using specific inhibitors, it was determined that commercial strains offer a wide variety of glucose repression profiles. Transcription factor Gln3 has been involved in glucose and nitrogen repression. Deletion of GLN3 in two commercial wine strains produced different mutant phenotypes and only one of them displayed higher glucose repression and was unable to grow using a respiratory carbon source. Therefore, the role of this transcription factor contributes to the variety of phenotypic behaviors seen in wine strains. This variability is also reflected in the impact of GLN3 deletion in fermentation, although the mutants are always more tolerant to inhibition of the nutrient signaling complex TORC1 by rapamycin, both in laboratory medium and in grape juice fermentation. Therefore, most aspects of nitrogen catabolite repression controlled by TORC1 are conserved in winemaking conditions.

Uncovering Divergence in Gene Expression Regulation in the Adaptation of Yeast to Nitrogen Scarcity

Saccharomyces cerevisiae rewires its transcriptional output to survive stressful environments, such as nitrogen scarcity under fermentative conditions. Although divergence in nitrogen metabolism among natural yeast populations has been reported, the impact of regulatory genetic variants modulating gene expression and nitrogen consumption remains to be investigated. Here, we employed an F1 hybrid from two contrasting S. cerevisiae strains, providing a controlled genetic environment to map cis factors involved in the divergence of gene expression regulation in response to nitrogen scarcity. We used a dual approach to obtain genome-wide allele-specific profiles of chromatin accessibility, transcription factor binding, and gene expression through ATAC-seq (assay for transposase accessible chromatin) and RNA-seq (transcriptome sequencing). We observed large variability in allele-specific expression and accessibility between the two genetic backgrounds, with a third of these differences specific to a deficient nitrogen environment. Furthermore, we discovered events of allelic bias in gene expression correlating with allelic bias in transcription factor binding solely under nitrogen scarcity, where the majority of these transcription factors orchestrates the nitrogen catabolite repression regulatory pathway and demonstrates a cis × environment-specific response. Our approach allowed us to find cis variants modulating gene expression, chromatin accessibility, and allelic differences in transcription factor binding in response to low nitrogen culture conditions. IMPORTANCE Historically, coding variants were prioritized when searching for causal mechanisms driving adaptation of natural populations to stressful environments. However, the recent focus on noncoding variants demonstrated their ubiquitous role in adaptation. Here, we performed genome-wide regulatory variation profiles between two divergent yeast strains when facing nitrogen nutritional stress. The open chromatin availability of several regulatory regions changes in response to nitrogen scarcity. Importantly, we describe regulatory events that deviate between strains. Our results demonstrate a widespread variation in gene expression regulation between naturally occurring populations in response to stressful environments.

Cation Transporters of Candida albicans-New Targets to Fight Candidiasis?

Candidiasis is the wide-spread fungal infection caused by numerous strains of yeast, with the prevalence of Candida albicans. The current treatment of candidiasis is becoming rather ineffective and costly owing to the emergence of resistant strains; hence, the exploration of new possible drug targets is necessary. The most promising route is the development of novel antibiotics targeting this pathogen. In this review, we summarize such candidates found in C. albicans and those involved in the transport of (metal) cations, as the latter are essential for numerous processes within the cell; hence, disruption of their fluxes can be fatal for C. albicans.

Dosage sensitivity of JDPs, a valuable tool for understanding their function: a case study on Caj1 overexpression-mediated filamentous growth in budding yeast

J-domain proteins (JDPs) partner with Hsp70s to oversee proper synthesis, folding, transport and turnover of proteins in the cell. In any subcellular compartment, often multiple JDPs collaborate with a single Hsp70 to perform a variety of functions. Being co-localized, JDPs may exhibit complex genetic and physical interactions with each other, their clients as well as the Hsp70 partners. Even though most JDPs are highly specialized, redundancy between them is possible, making their functional analysis challenging. In the absence of assayable deletion phenotypes, protein overexpression appears to be a powerful alternative strategy to study JDP function. Here, we show that high levels of Caj1, one of the cytosolic JDPs, cause filamentous growth and G2/M arrest in yeast cells. Mutation in the critical HPD motif in the J-domain of Caj1 completely abolished these phenotypes, suggesting that Hsp70 co-chaperone function is important for the dominant-negative phenotypes exhibited by Caj1 overexpression. In this paper, we discuss the possible underlying mechanisms responsible for the pleiotropic phenotypes displayed by Caj1 overexpression in the light of current models proposed for dosage-sensitive genes (DSGs). Finally, we present generalized mechanisms of JDP overexpression-mediated dominant-negative phenotypes in budding yeast.

NRT2.1 C-terminus phosphorylation prevents root high affinity nitrate uptake activity in Arabidopsis thaliana

In Arabidopsis thaliana, NRT2.1 codes for a main component of the root nitrate high-affinity transport system. Previous studies revealed that post-translational regulation of NRT2.1 plays an important role in the control of root nitrate uptake and that one mechanism could correspond to NRT2.1 C-terminus processing. To further investigate this hypothesis, we produced transgenic plants with truncated forms of NRT2.1. This revealed an essential sequence for NRT2.1 activity, located between the residues 494 and 513. Using a phospho-proteomic approach, we found that this sequence contains one phosphorylation site, at serine 501, which can inactivate NRT2.1 function when mimicking the constitutive phosphorylation of this residue in transgenic plants. This phenotype could neither be explained by changes in abundance of NRT2.1 and NAR2.1, a partner protein of NRT2.1, nor by a lack of interaction between these two proteins. Finally, the relative level of serine 501 phosphorylation was found to be increased by ammonium nitrate in wild-type plants, leading to the inactivation of NRT2.1 and to a decrease in high affinity nitrate transport into roots. Altogether, these observations reveal a new and essential mechanism for the regulation of NRT2.1 activity.

New Aspects of Invasive Growth Regulation Identified by Functional Profiling of MAPK Pathway Targets in Saccharomyces cerevisiae

MAPK pathways are drivers of morphogenesis and stress responses in eukaryotes. A major function of MAPK pathways is the transcriptional induction of target genes, which produce proteins that collectively generate a cellular response. One approach to comprehensively understand how MAPK pathways regulate cellular responses is to characterize the individual functions of their transcriptional targets. Here, by examining uncharacterized targets of the MAPK pathway that positively regulates filamentous growth in Saccharomyces cerevisiae (fMAPK pathway), we identified a new role for the pathway in negatively regulating invasive growth. Specifically, four targets were identified that had an inhibitory role in invasive growth: RPI1, RGD2, TIP1, and NFG1/YLR042cNFG1 was a highly induced unknown open reading frame that negatively regulated the filamentous growth MAPK pathway. We also identified SFG1, which encodes a transcription factor, as a target of the fMAPK pathway. Sfg1p promoted cell adhesion independently from the fMAPK pathway target and major cell adhesion flocculin Flo11p, by repressing genes encoding presumptive cell-wall-degrading enzymes. Sfg1p also contributed to FLO11 expression. Sfg1p and Flo11p regulated different aspects of cell adhesion, and their roles varied based on the environment. Sfg1p also induced an elongated cell morphology, presumably through a cell-cycle delay. Thus, the fMAPK pathway coordinates positive and negative regulatory proteins to fine-tune filamentous growth resulting in a nuanced response. Functional analysis of other pathways' targets may lead to a more comprehensive understanding of how signaling cascades generate biological responses.

Nitrogen Catabolite Repression in members of Paracoccidioides complex

Paracoccidioides complex is a genus that comprises pathogenic fungi which are responsible by systemic disease Paracoccidioidomycosis. In host tissues, pathogenic fungi need to acquire nutrients in order to survive, making the uptake of nitrogen essential for their establishment and dissemination. Nitrogen utilization is employed by the alleviation of Nitrogen Catabolite Repression (NCR) which ensures the use of non-preferential or alternative nitrogen sources when preferential sources are not available. NCR is controlled by GATA transcription factors which act through GATA binding sites on promoter regions in NCR-sensitive genes. This process is responsible for encoding proteins involved with the scavenge, uptake and catabolism of a wide variety of non-preferential nitrogen sources. In this work, we predict the existence of AreA GATA transcription factor and feature the zinc finger domain by three-dimensional structure in Paracoccidioides. Furthermore, we demonstrate the putative genes involved with NCR response by means of in silico analysis. The gene expression profile under NCR conditions was evaluated. Demonstrating that P. lutzii supported transcriptional regulation and alleviated NCR in non-preferential nitrogen-dependent medium. The elucidation of NCR in members of Paracoccidioides complex will provide new knowledge about survival, dissemination and virulence for these pathogens with regard to nitrogen-scavenging strategies in the interactions of host-pathogens.

Molecular Mechanisms Controlling the Disease Cycle in the Vascular Pathogen Verticillium dahliae Characterized Through Forward Genetics and Transcriptomics

The soil-borne pathogen Verticillium dahliae has a worldwide distribution and a plethora of hosts of agronomic value. Molecular analysis of virulence processes can identify targets for disease control. In this work, we compared the global gene transcription profile of random T-DNA insertion mutant strain D-10-8F, which exhibits reduced virulence and alterations in microsclerotium formation and polar growth, with that of the wild-type strain. Three genes identified as differentially expressed were selected for functional characterization. To produce deletion mutants, we developed an updated version of one-step construction of Agrobacterium-recombination-ready plasmids (OSCAR) that included the negative selection marker HSVtk (herpes simplex virus thymidine kinase gene) to prevent ectopic integration of the deletion constructs. Deletion of VdRGS1 (VDAG_00683), encoding a regulator of G protein signaling (RGS) protein and highly upregulated in the wild type versus D-10-8F, resulted in phenotypic alterations in development and virulence that were indistinguishable from those of the random T-DNA insertion mutant. In contrast, deletion of the other two genes selected, vrg1 (VDAG_07039) and vvs1 (VDAG_01858), showed that they do not play major roles in morphogenesis or virulence in V. dahliae. Taken together the results presented here on the transcriptomic analysis and phenotypic characterization of D-10-8F and ∆VdRGS1 strains provide evidence that variations in G protein signaling control the progression of the disease cycle in V. dahliae. We propose that G protein-mediated signals induce the expression of multiple virulence factors during biotrophic growth, whereas massive production of microsclerotia at late stages of infection requires repression of G protein signaling via upregulation of VdRGS1 activity.

Transcriptome analyses of the ginseng root rot pathogens Cylindrocarpon destructans and Fusarium solani to identify radicicol resistance mechanisms

Background

The ascomycete fungi Cylindrocarpon destructans (Cd) and Fusarium solani (Fs) cause ginseng root rot and significantly reduce the quality and yield of ginseng. Cd produces the secondary metabolite radicicol, which targets the molecular chaperone Hsp90. Fs is resistant to radicicol, whereas other fungal genera associated with ginseng disease are sensitive to it. Radicicol resistance mechanisms have not yet been elucidated.

Methods

Transcriptome analyses of Fs and Cd mycelia treated with or without radicicol were conducted using RNA-seq. All of the differentially expressed genes (DEGs) were functionally annotated using the Fusarium graminearum transcript database. In addition, deletions of two transporter genes identified by RNA-seq were created to confirm their contributions to radicicol resistance.

Results

Treatment with radicicol resulted in upregulation of chitin synthase and cell wall integrity genes in Fs and upregulation of nicotinamide adenine dinucleotide dehydrogenase and sugar transporter genes in Cd. Genes encoding an ATP-binding cassette transporter, an aflatoxin efflux pump, ammonium permease 1 (mep1), and nitrilase were differentially expressed in both Fs and Cd. Among these four genes, only the ABC transporter was upregulated in both Fs and Cd. The aflatoxin efflux pump and mep1 were upregulated in Cd, but downregulated in Fs, whereas nitrilase was downregulated in both Fs and Cd.

Conclusion

The transcriptome analyses suggested radicicol resistance pathways, and deletions of the transporter genes indicated that they contribute to radicicol resistance.

Yeast filamentation signaling is connected to a specific substrate translocation mechanism of the Mep2 transceptor

The dimorphic transition from the yeast to the filamentous form of growth allows cells to explore their environment for more suitable niches and is often crucial for the virulence of pathogenic fungi. In contrast to their Mep1/3 paralogues, fungal Mep2-type ammonium transport proteins of the conserved Mep-Amt-Rh family have been assigned an additional receptor role required to trigger the filamentation signal in response to ammonium scarcity. Here, genetic, kinetic and structure-function analyses were used to shed light on the poorly characterized signaling role of Saccharomyces cerevisiae Mep2. We show that Mep2 variants lacking the C-terminal tail conserve the ability to induce filamentation, revealing that signaling can proceed in the absence of exclusive binding of a putative partner to the largest cytosolic domain of the protein. Our data support that filamentation signaling requires the conformational changes accompanying substrate translocation through the pore crossing the hydrophobic core of Mep2. pHluorin reporter assays show that the transport activity of Mep2 and of non-signaling Mep1 differently affect yeast cytosolic pH in vivo, and that the unique pore variant Mep2^H194E, with apparent uncoupling of transport and signaling functions, acquires increased ability of acidification. Functional characterization in Xenopus oocytes reveals that Mep2 mediates electroneutral substrate translocation while Mep1 performs electrogenic transport. Our findings highlight that the Mep2-dependent filamentation induction is connected to its specific transport mechanism, suggesting a role of pH in signal mediation. Finally, we show that the signaling process is conserved for the Mep2 protein from the human pathogen Candida albicans.

Fungal Mep2-type ammonium transport proteins of the conserved Mep-Amt-Rh family that includes human Rhesus factors are specifically required to allow filamentation in response to ammonium limitation. These proteins were therefore assigned a receptor role while the underlying mechanism of signal transduction remains poorly understood. The “transceptor” property has subsequently been proposed to concern transporters of all kind of micro- and macro- nutrients in eukaryotes, from fungi to human. However, bringing the firm demonstration of their existence remains challenging as variants with full uncoupling of transport and receptor functions are difficult to obtain. Our data question the involvement of the C-terminal extremity of Saccharomyces cerevisiae Mep2 in the signal mediation leading to filamentation. If signaling partners exist, they should also bind to cytosolic loops and/or membrane-embedded domains. The capacity of Mep2 to enable filamentation is closely intertwined to the mechanism of substrate translocation through the pore of the hydrophobic core of the protein. In Xenopus oocytes, the transport activity of non-signaling Mep1 is electrogenic while it is electroneutral for Mep2, the latter likely translocating the weak base NH₃, but not the proton released after NH₄⁺ recognition and depronotation. We propose that given consequences of a Mep2-specific transport process, such as an intracellular pH modification, could be the underlying cause of the filamentation signal ensured by Mep2-type proteins.

Stress-seventy subfamily A 4, A member of HSP70, confers yeast cadmium tolerance in the loss of mitochondria pyruvate carrier 1

Mitochondrial pyruvate carrier (MPC), which transports pyruvate into mitochondria, is a key regulatory element in the material metabolism and energy metabolism. Since MPC was firstly identified in yeast in 2012, many groups have investigated the function of MPC. As MPC is a classic material transporter, the focus of previous studies has been placed on its role in pyruvate transport. In this study, we discovered a novel Cd resistant gene, stress-seventy subfamily A 4 (SSA4), which can recover the Cd sensitive phenotype in the yeast MPC1 mutant strain. It is suggested that, except for adjusting metabolism, MPC can regulate stress tolerance by regulating downstream genes in yeast. Previously, we discovered a Cd related gene, AGP30, which is associated with MPC1 in Arabidopsis. These results indicate that MPC can regulate Cd tolerance through downstream genes in both Arabidopsis and yeast. This study will pave the way for further exploring the bypass pathways of MPC at the molecular level, and the interaction between MPC and the downstream genes in biology.

Two 14-3-3 proteins contribute to nitrogen sensing through the TOR and glutamine synthetase-dependent pathways in Fusarium graminearum

Fusarium graminearum responds to environmental cues to modulate its growth and metabolism during wheat pathogenesis. Nitrogen limitation activates virulence-associated behaviours in F. graminearum including mycotoxin production and penetrative growth. In other filamentous fungi, nitrogen sensing is mediated by both the Target of Rapamycin (TOR) and the glutamine synthetase (GS)-dependent signaling pathways. While TOR-dependent nitrogen responses have been demonstrated in F. graminearum, the involvement of GS remains unclear. Our study indicates that both the TOR and GS signalling pathways are involved in nitrogen sensing in F. graminearum and contribute to glutamine-induced mycelial growth. However, neither pathway is required for glutamine-induced repression of the mycotoxin deoxynivalenol (DON) indicating that an additional nitrogen sensing pathway must exist. Further, two genes FgBMH1 and FgBMH2 encoding 14-3-3 proteins regulate nitrogen responses with effects on gene expression, DON production and mycelial growth. Unlike yeast, where 14-3-3s function redundantly in regulating nitrogen sensing, the 14-3-3 proteins have differing functions in F. graminearum. While both FgBMH1 and FgBMH2 regulate early glutamine-induced DON repression, only FgBMH2 is involved in regulating reproduction, virulence and glutamine-induced AreA repression. Together, our findings help to clarify the nitrogen sensing pathways in F. graminearum and highlight the involvement of 14-3-3s in the nitrogen response of filamentous fungi.

A cell-based screening system for RNA polymerase I inhibitors

RNA polymerase I (RNA Pol I) is a "factory" that orchestrates the transcription of ribosomal RNA for constructing ribosomes as a primary workshop for protein translation to sustain cell growth. The deregulation of RNA Pol I often causes uncontrolled cell proliferation, leading to cancer. Efficient and reliable methods are needed for the identification of selective inhibitors of RNA Pol I. Yeast (Saccharomyces cerevisiae) is eukaryotic and represents a valuable model system to study RNA Pol I, especially with the availability of the X-ray crystal structure of the yeast homologue of RNA Pol I, offering a structural basis to selectively target this transcriptional machinery. Herein, we developed a cell-based screening strategy by establishing a stable yeast cell line with a stably integrated human RNA Pol I promoter and ribosomal DNA. The model system was validated using the well-known RNA Pol I inhibitor CX-5461 by measuring transcribed human rRNA as readout. Virtual screening coupled with compound library screening using this cell line enabled the identification of a new candidate inhibitor of RNA Pol I, namely, cerivastatin sodium. Furthermore, we used growth and transcription activity assays to biologically evaluate the hit compound. Preliminary studies demonstrated antiproliferative effects of cerivastatin sodium against human cancer cells, namely, A2780 and H460 cell lines. These results implicated cerivastatin sodium as a selective RNA Pol I inhibitor worthy of further development together with potential as a targeted anticancer therapeutic.

Multicellular group formation in Saccharomyces cerevisiae

Understanding how and why cells cooperate to form multicellular organisms is a central aim of evolutionary biology. Multicellular groups can form through clonal development (where daughter cells stick to mother cells after division) or by aggregation (where cells aggregate to form groups). These different ways of forming groups directly affect relatedness between individual cells, which in turn can influence the degree of cooperation and conflict within the multicellular group. It is hard to study the evolution of multicellularity by focusing only on obligately multicellular organisms, like complex animals and plants, because the factors that favour multicellular cooperation cannot be disentangled, as cells cannot survive and reproduce independently. We support the use of Saccharomyces cerevisiae as an ideal model for studying the very first stages of the evolution of multicellularity. This is because it can form multicellular groups both clonally and through aggregation and uses a family of proteins called ‘flocculins’ that determine the way in which groups form, making it particularly amenable to laboratory experiments. We briefly review current knowledge about multicellularity in S. cerevisiae and then propose a framework for making predictions about the evolution of multicellular phenotypes in yeast based on social evolution theory. We finish by explaining how S. cerevisiae is a particularly useful experimental model for the analysis of open questions concerning multicellularity.

Drosophila ammonium transporter Rh50 is required for integrity of larval muscles and neuromuscular system

Rhesus glycoproteins (Rh50) have been shown to be ammonia transporters in many species from bacteria to human. They are involved in various physiological processes including acid excretion and pH regulation. Rh50 proteins can also provide a structural link between the cytoskeleton and the plasma membranes that maintain cellular integrity. Although ammonia plays essential roles in the nervous system, in particular at glutamatergic synapses, a potential role for Rh50 proteins at synapses has not yet been investigated. To better understand the function of these proteins in vivo, we studied the unique Rh50 gene of Drosophila melanogaster, which encodes two isoforms, Rh50A and Rh50BC. We found that Drosophila Rh50A is expressed in larval muscles and enriched in the postsynaptic regions of the glutamatergic neuromuscular junctions. Rh50 inactivation by RNA interference selectively in muscle cells caused muscular atrophy in larval stages and pupal lethality. Interestingly, Rh50-deficiency in muscles specifically increased glutamate receptor subunit IIA (GluRIIA) level and the frequency of spontaneous excitatory postsynaptic potentials. Our work therefore highlights a new role for Rh50 proteins in the maintenance of Drosophila muscle architecture and synaptic physiology, which could be conserved in other species.

The administration of L-cysteine and L-arginine inhibits biofilm formation in wild-type biofilm-forming yeast by modulating FLO11 gene expression

Microbial biofilms are undesired in food manufacturing, drinking water distribution systems, and clinical realms. Yeast biofilms are particularly problematic because of the strong capacity of yeast cells to adhere to abiotic surfaces, cells, and tissues. Novel approaches have been developed over recent years to prevent the establishment of microbial biofilms, such as through the use of small molecules with inhibiting and dispersing properties. Here, we studied the inhibitory activity of 11 different amino acids on the biofilm formation ability of three wild-type Saccharomyces cerevisiae strains and the reference strain ∑1278b. Subsequent evaluation of different concentrations of the two most effective amino acids, namely, arginine and cysteine, revealed that they acted in different ways. Arginine prevented biofilm formation by reducing FLO11 gene expression; its addition did not affect cell viability and was even found to enhance cell metabolism (vitality marker) as determined by phenotype microarray (PM) analysis. On the contrary, the addition of cysteine reduced both cell viability and vitality as well as FLO11 expression. Thus, the use of cysteine and arginine as agents against biofilm formation can be diversified depending on the most desired action towards yeast growth.

Nutrient and Stress Sensing in Pathogenic Yeasts

More than 1.5 million fungal species are estimated to live in vastly different environmental niches. Despite each unique host environment, fungal cells sense certain fundamentally conserved elements, such as nutrients, pheromones and stress, for adaptation to their niches. Sensing these extracellular signals is critical for pathogens to adapt to the hostile host environment and cause disease. Hence, dissecting the complex extracellular signal-sensing mechanisms that aid in this is pivotal and may facilitate the development of new therapeutic approaches to control fungal infections. In this review, we summarize the current knowledge on how two important pathogenic yeasts, Candida albicans and Cryptococcus neoformans, sense nutrient availability, such as carbon sources, amino acids, and ammonium, and different stress signals to regulate their morphogenesis and pathogenicity in comparison with the non-pathogenic model yeast Saccharomyces cerevisiae. The molecular interactions between extracellular signals and their respective sensory systems are described in detail. The potential implication of analyzing nutrient and stress-sensing systems in antifungal drug development is also discussed.

The lipid environment determines the activity of the Escherichia coli ammonium transporter AmtB

The movement of ammonium across biologic membranes is a fundamental process in all living organisms and is mediated by the ubiquitous ammonium transporter/methylammonium permease/rhesus protein (Amt/Mep/Rh) family of transporters. Recent structural analysis and coupled mass spectrometry studies have shown that the Escherichia coli ammonium transporter AmtB specifically binds 1-palmitoyl-2-oleoyl phosphatidylglycerol (POPG). Upon POPG binding, several residues of AmtB undergo a small conformational change, which stabilizes the protein against unfolding. However, no studies have so far been conducted, to our knowledge, to explore whether POPG binding to AmtB has functional consequences. Here, we used an in vitro experimental assay with purified components, together with molecular dynamics simulations, to characterize the relation between POPG binding and AmtB activity. We show that the AmtB activity is electrogenic. Our results indicate that the activity, at the molecular level, of Amt in archaebacteria and eubacteria may differ. We also show that POPG is an important cofactor for AmtB activity and that, in the absence of POPG, AmtB cannot complete the full translocation cycle. Furthermore, our simulations reveal previously undiscovered POPG binding sites on the intracellular side of the lipid bilayer between the AmtB subunits. Possible molecular mechanisms explaining the functional role of POPG are discussed.-Mirandela, G. D., Tamburrino, G., Hoskisson, P. A., Zachariae, U., Javelle, A. The lipid environment determines the activity of the Escherichia coli ammonium transporter AmtB.

Coordinate regulation of Ustilago maydis ammonium transporters and genes involved in mating and pathogenicity

The dimorphic switch from budding to filamentous growth is an essential morphogenetic transition many fungi utilize to cause disease in the host. Although different environmental signals can induce filamentous growth, the developmental programs associated with transmitting these different signals may differ. Here, we explore the relationship between filamentation and expression levels of ammonium transporters (AMTs) that also sense low ammonium for Ustilago maydis, the pathogen of maize. Overexpression of the high affinity ammonium transporter, Ump2, under normally non-inducing conditions, results in filamentous growth. Furthermore, ump2 expression levels are correlated with expression of genes involved in the mating response pathway and in pathogenicity. Ump1 and Ump2 transcription levels also tracked expression of genes normally up-regulated during either filamentous growth or during growth of the fungus inside the host. Interestingly, haploid strains deleted for the b mating-type locus, like those deleted for ump2, failed to filament on low ammonium; they also shared some alterations in gene expression patterns with cells deleted for ump2 or over-expressing this gene. Deletion of ump2 either in both mating partners or in a solopathogenic haploid strain resulted in a dramatic reduction in disease severity for infected plants, suggesting some importance of this transceptor in the pathogenesis program.

Multiple Transceptors for Macro- and Micro-Nutrients Control Diverse Cellular Properties Through the PKA Pathway in Yeast: A Paradigm for the Rapidly Expanding World of Eukaryotic Nutrient Transceptors Up to Those in Human Cells

The nutrient composition of the medium has dramatic effects on many cellular properties in the yeast Saccharomyces cerevisiae. In addition to the well-known specific responses to starvation for an essential nutrient, like nitrogen or phosphate, the presence of fermentable sugar or a respirative carbon source leads to predominance of fermentation or respiration, respectively. Fermenting and respiring cells also show strong differences in other properties, like storage carbohydrate levels, general stress tolerance and cellular growth rate. However, the main glucose repression pathway, which controls the switch between respiration and fermentation, is not involved in control of these properties. They are controlled by the protein kinase A (PKA) pathway. Addition of glucose to respiring yeast cells triggers cAMP synthesis, activation of PKA and rapid modification of its targets, like storage carbohydrate levels, general stress tolerance and growth rate. However, starvation of fermenting cells in a glucose medium for any essential macro- or micro-nutrient counteracts this effect, leading to downregulation of PKA and its targets concomitant with growth arrest and entrance into G0. Re-addition of the lacking nutrient triggers rapid activation of the PKA pathway, without involvement of cAMP as second messenger. Investigation of the sensing mechanism has revealed that the specific high-affinity nutrient transporter(s) induced during starvation function as transporter-receptors or transceptors for rapid activation of PKA upon re-addition of the missing substrate. In this way, transceptors have been identified for amino acids, ammonium, phosphate, sulfate, iron, and zinc. We propose a hypothesis for regulation of PKA activity by nutrient transceptors to serve as a conceptual framework for future experimentation. Many properties of transceptors appear to be similar to those of classical receptors and nutrient transceptors may constitute intermediate forms in the development of receptors from nutrient transporters during evolution. The nutrient-sensing transceptor system in yeast for activation of the PKA pathway has served as a paradigm for similar studies on candidate nutrient transceptors in other eukaryotes and we succinctly discuss the many examples of transceptors that have already been documented in other yeast species, filamentous fungi, plants, and animals, including the examples in human cells.

Dal81 Regulates Expression of Arginine Metabolism Genes in Candida parapsilosis

Fungi can use a wide variety of nitrogen sources. In the absence of preferred sources such as ammonium, glutamate, and glutamine, secondary sources, including most other amino acids, are used. Expression of the nitrogen utilization pathways is very strongly controlled at the transcriptional level. Here, we investigated the regulation of nitrogen utilization in the pathogenic yeast Candida parapsilosis. We found that the functions of many regulators are conserved with respect to Saccharomyces cerevisiae and other fungi. For example, the core GATA activators GAT1 and GLN3 have a conserved role in nitrogen catabolite repression (NCR). There is one ortholog of GZF3 and DAL80, which represses expression of genes in preferred nitrogen sources. The regulators PUT3 and UGA3 are required for metabolism of proline and γ-aminobutyric acid (GABA), respectively. However, the role of the Dal81 transcription factor is distinctly different. In S. cerevisiae, Dal81 is a positive regulator of acquisition of nitrogen from GABA, allantoin, urea, and leucine, and it is required for maximal induction of expression of the relevant pathway genes. In C. parapsilosis, induction of GABA genes is independent of Dal81, and deleting DAL81 has no effect on acquisition of nitrogen from GABA or allantoin. Instead, Dal81 represses arginine synthesis during growth under preferred nitrogen conditions. IMPORTANCE Utilization of nitrogen by fungi is controlled by nitrogen catabolite repression (NCR). Expression of many genes is switched off during growth on nonpreferred nitrogen sources. Gene expression is regulated through a combination of activation and repression. Nitrogen regulation has been studied best in the model yeast Saccharomyces cerevisiae. We found that although many nitrogen regulators have a conserved function in Saccharomyces species, some do not. The Dal81 transcriptional regulator has distinctly different functions in S. cerevisiae and C. parapsilosis. In the former, it regulates utilization of nitrogen from GABA and allantoin, whereas in the latter, it regulates expression of arginine synthesis genes. Our findings make an important contribution to our understanding of nitrogen regulation in a human-pathogenic fungus.

The Biotrophic Development of Ustilago maydis Studied by RNA-Seq Analysis

The maize smut fungus Ustilago maydis is a model organism for elucidating host colonization strategies of biotrophic fungi. Here, we performed an in depth transcriptional profiling of the entire plant-associated development of U. maydis wild-type strains. In our analysis, we focused on fungal metabolism, nutritional strategies, secreted effectors, and regulatory networks. Secreted proteins were enriched in three distinct expression modules corresponding to stages on the plant surface, establishment of biotrophy, and induction of tumors. These modules are likely the key determinants for U. maydis virulence. With respect to nutrient utilization, we observed that expression of several nutrient transporters was tied to these virulence modules rather than being controlled by nutrient availability. We show that oligopeptide transporters likely involved in nitrogen assimilation are important virulence factors. By measuring the intramodular connectivity of transcription factors, we identified the potential drivers for the virulence modules. While known components of the b-mating type cascade emerged as inducers for the plant surface and biotrophy module, we identified a set of yet uncharacterized transcription factors as likely responsible for expression of the tumor module. We demonstrate a crucial role for leaf tumor formation and effector gene expression for one of these transcription factors.

Filamentous invasive growth of mutants of the genes encoding ammonia-metabolizing enzymes in the fission yeast Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe undergoes a switch from yeast to filamentous invasive growth in response to certain environmental stimuli. Among them is ammonium limitation. Amt1, one of the three ammonium transporters in this yeast, is required for the ammonium limitation-induced morphological transition; however, the underlying molecular mechanism remains to be understood. Cells lacking Amt1 became capable of invasive growth upon increasing concentrations of ammonium in the medium, suggesting that the ammonium taken up into the cell or a metabolic intermediate in ammonium assimilation might serve as a signal for the ammonium limitation-induced morphological transition. To investigate the possible role of ammonium-metabolizing enzymes in the signaling process, deletion mutants were constructed for the gdh1, gdh2, gln1, and glt1 genes, which were demonstrated by enzyme assays to encode NADP-specific glutamate dehydrogenase, NAD-specific glutamate dehydrogenase, glutamine synthetase, and glutamate synthase, respectively. Growth tests on various nitrogen sources revealed that a gln1Δ mutant was a glutamine auxotroph and that a gdh1Δ mutant had a defect in growth on ammonium, particularly at high concentrations. The latter observation indicates that the NADP-specific glutamate dehydrogenase of S. pombe plays a major role in ammonium assimilation under high ammonium concentrations. Invasive growth assays showed that gdh1Δ and glt1Δ mutants underwent invasive growth to a lesser extent than did wild-type strains. Increasing the ammonium concentration in the medium suppressed the invasive growth defect of the glt1Δ mutant, but not the gdh1Δ mutant. These results suggest that the nitrogen status of the cell is important in the induction of filamentous invasive growth in S. pombe.

Nutrient Sensing at the Plasma Membrane of Fungal Cells

To respond to the changing environment, cells must be able to sense external conditions. This is important for many processes including growth, mating, the expression of virulence factors, and several other regulatory effects. Nutrient sensing at the plasma membrane is mediated by different classes of membrane proteins that activate downstream signaling pathways: nontransporting receptors, transceptors, classical and nonclassical G-protein-coupled receptors, and the newly defined extracellular mucin receptors. Nontransporting receptors have the same structure as transport proteins, but have lost the capacity to transport while gaining a receptor function. Transceptors are transporters that also function as a receptor, because they can rapidly activate downstream signaling pathways. In this review, we focus on these four types of fungal membrane proteins. We mainly discuss the sensing mechanisms relating to sugars, ammonium, and amino acids. Mechanisms for other nutrients, such as phosphate and sulfate, are discussed briefly. Because the model yeast Saccharomyces cerevisiae has been the most studied, especially regarding these nutrient-sensing systems, each subsection will commence with what is known in this species.

Post-translational regulation of nitrogen transporters in plants and microorganisms

For microorganisms and plants, nitrate and ammonium are the main nitrogen sources and they are also important signaling molecules controlling several aspects of metabolism and development. Over the past decade, numerous studies revealed that nitrogen transporters are strongly regulated at the transcriptional level. However, more and more reports are now showing that nitrate and ammonium transporters are also subjected to post-translational regulations in response to nitrogen availability. Phosphorylation is so far the most well studied post-translational modification for these transporters and it affects both the regulation of nitrogen uptake and nitrogen sensing. For example, in Arabidopsis thaliana, phosphorylation was shown to activate the sensing function of the root nitrate transporter NRT1.1 and to switch the transport affinity. Also, for ammonium transporters, a phosphorylation-dependent activation/inactivation mechanism was elucidated in recent years in both plants and microorganisms. However, despite the fact that these regulatory mechanisms are starting to be thoroughly described, the signaling pathways involved and their action on nitrogen transporters remain largely unknown. In this review, we highlight the inorganic nitrogen transporters regulated at the post-translational level and we compare the known mechanisms in plants and microorganisms. We then discuss how these mechanisms could contribute to the regulation of nitrogen uptake and/or nitrogen sensing.

Ammonium as a signal for physiological and morphological responses in plants

Ammonium is a major inorganic nitrogen source for plants. At low external supplies, ammonium promotes plant growth, while at high external supplies it causes toxicity. Ammonium triggers rapid changes in cytosolic pH, in gene expression, and in post-translational modifications of proteins, leading to apoplastic acidification, co-ordinated ammonium uptake, enhanced ammonium assimilation, altered oxidative and phytohormonal status, and reshaped root system architecture. Some of these responses are dependent on AMT-type ammonium transporters and are not linked to a nutritional effect, indicating that ammonium is perceived as a signaling molecule by plant cells. This review summarizes current knowledge of ammonium-triggered physiological and morphological responses and highlights existing and putative mechanisms mediating ammonium signaling and sensing events in plants. We put forward the hypothesis that sensing of ammonium takes place at multiple steps along its transport, storage, and assimilation pathways.

From Lipid Homeostasis to Differentiation: Old and New Functions of the Zinc Cluster Proteins Ecm22, Upc2, Sut1 and Sut2

Zinc cluster proteins are a large family of transcriptional regulators with a wide range of biological functions. The zinc cluster proteins Ecm22, Upc2, Sut1 and Sut2 have initially been identified as regulators of sterol import in the budding yeast Saccharomyces cerevisiae. These proteins also control adaptations to anaerobic growth, sterol biosynthesis as well as filamentation and mating. Orthologs of these zinc cluster proteins have been identified in several species of Candida. Upc2 plays a critical role in antifungal resistance in these important human fungal pathogens. Upc2 is therefore an interesting potential target for novel antifungals. In this review we discuss the functions, mode of actions and regulation of Ecm22, Upc2, Sut1 and Sut2 in budding yeast and Candida.

The metabolic waste ammonium regulates mTORC2 and mTORC1 signaling

Two structurally and functionally distinct mammalian TOR complexes control cell growth and metabolism in physiological and pathological contexts including cancer. Upregulated glutaminolysis is part of the metabolic reprogramming occurring in cancer, providing fuels for growth but also liberating ammonium, a potent neurotoxic waste product. Here, we identify ammonium as a novel dose-dependent signal mediating rapid mTORC2 activation and further regulating mTORC1. We show that ammonium induces rapid RICTOR-dependent phosphorylation of AKT-S473, a process requiring the PI3K pathway and further involving the Src-family kinase YES1, the FAK kinase and the ITGβ1 integrin. Release of calcium from the endoplasmic reticulum store triggers rapid mTORC2 activation, similar to ammonium-induced activation, the latter being conversely prevented by calcium chelation.Moreover, in analogy to growth factors, ammonium triggers the AKT-dependent phosphoinhibition of the TSC complex and of PRAS40, two negative regulators of mTORC1. Consistent with mTORC1 stimulation, ammonium induces the inhibitory phosphorylation of 4EBP1, a negative regulator of protein biogenesis. Ammonium however dually impacts on the phosphorylation of p70S6K1 triggering a transient AKT-independent decrease in the phosphorylation of this second mTORC1 readout. Finally, we reveal ammonium as a dose-dependent stimulator of proliferation. This study underscores an mTORC2 and mTORC1 response to the so-called ammonium waste.

Nitrate Assimilation in Fusarium fujikuroi Is Controlled by Multiple Levels of Regulation

Secondary metabolite production of the phytopathogenic ascomycete fungus Fusarium fujikuroi is greatly influenced by the availability of nitrogen. While favored nitrogen sources such as glutamine and ammonium are used preferentially, the uptake and utilization of nitrate is subject to a regulatory mechanism called nitrogen metabolite repression (NMR). In Aspergillus nidulans, the transcriptional control of the nitrate assimilatory system is carried out by the synergistic action of the nitrate-specific transcription factor NirA and the major nitrogen-responsive regulator AreA. In this study, we identified the main components of the nitrate assimilation system in F. fujikuroi and studied the role of each of them regarding the regulation of the remaining components. We analyzed mutants with deletions of the nitrate-specific activator NirA, the nitrate reductase (NR), the nitrite reductase (NiR) and the nitrate transporter NrtA. We show that NirA controls the transcription of the nitrate assimilatory genes NIAD, NIIA, and NRTA in the presence of nitrate, and that the global nitrogen regulator AreA is obligatory for expression of most, but not all NirA target genes (NIAD). By transforming a NirA-GFP fusion construct into the ΔNIAD, ΔNRTA, and ΔAREA mutant backgrounds we revealed that NirA was dispersed in the cytosol when grown in the presence of glutamine, but rapidly sorted to the nucleus when nitrate was added. Interestingly, the rapid and nitrate-induced nuclear translocation of NirA was observed also in the ΔAREA and ΔNRTA mutants, but not in ΔNIAD, suggesting that the fungus is able to directly sense nitrate in an AreA- and NrtA-independent, but NR-dependent manner.

Genomic signatures of adaptation to wine biological ageing conditions in biofilm-forming flor yeasts

The molecular and evolutionary processes underlying fungal domestication remain largely unknown despite the importance of fungi to bioindustry and for comparative adaptation genomics in eukaryotes. Wine fermentation and biological ageing are performed by strains of S. cerevisiae with, respectively, pelagic fermentative growth on glucose and biofilm aerobic growth utilizing ethanol. Here, we use environmental samples of wine and flor yeasts to investigate the genomic basis of yeast adaptation to contrasted anthropogenic environments. Phylogenetic inference and population structure analysis based on single nucleotide polymorphisms revealed a group of flor yeasts separated from wine yeasts. A combination of methods revealed several highly differentiated regions between wine and flor yeasts, and analyses using codon-substitution models for detecting molecular adaptation identified sites under positive selection in the high-affinity transporter gene ZRT1. The cross-population composite likelihood ratio revealed selective sweeps at three regions, including in the hexose transporter gene HXT7, the yapsin gene YPS6 and the membrane protein coding gene MTS27. Our analyses also revealed that the biological ageing environment has led to the accumulation of numerous mutations in proteins from several networks, including Flo11 regulation and divalent metal transport. Together, our findings suggest that the tuning of FLO11 expression and zinc transport networks are a distinctive feature of the genetic changes underlying the domestication of flor yeasts. Our study highlights the multiplicity of genomic changes underlying yeast adaptation to man-made habitats and reveals that flor/wine yeast lineage can serve as a useful model for studying the genomics of adaptive divergence.

KRH1 and KRH2 are functionally non-redundant in signaling for pseudohyphal differentiation in Saccharomyces cerevisiae

Diploid cells of Saccharomyces cerevisiae undergo pseudohyphal differentiation in response to nutrient depletion. Although this dimorphic transition occurs due to signals originating from carbon and nitrogen limitation, how these signals are coordinated and integrated is not understood. Results of this study indicate that the pseudohyphal defect of the mep2∆ mutant is overcome upon disruption of KRH2/GPB1 but not KRH1/GPB2. Further, the agar invasion defect observed in a mep2 mutant strain is suppressed only by deleting KRH2 and not KRH1. Thus, the results presented indicate that MEP2 functions by inhibiting KRH2 to trigger filamentation response when glucose becomes limiting. Biochemical data and phenotypic response to glucose replenishment reveal that KRH1 and KRH2 are differentially regulated by glucose and ammonium to induce pseudohyphae formation via the cAMP-PKA pathway. In contrast to the current view, this study clearly demonstrates that, KRH1 and KRH2 are not functionally redundant.

Dissecting Gene Expression Changes Accompanying a Ploidy-Based Phenotypic Switch

Aneuploidy, a state in which the chromosome number deviates from a multiple of the haploid count, significantly impacts human health. The phenotypic consequences of aneuploidy are believed to arise from gene expression changes associated with the altered copy number of genes on the aneuploid chromosomes. To dissect the mechanisms underlying altered gene expression in aneuploids, we used RNA-seq to measure transcript abundance in colonies of the haploid Saccharomyces cerevisiae strain F45 and two aneuploid derivatives harboring disomies of chromosomes XV and XVI. F45 colonies display complex “fluffy” morphologies, while the disomic colonies are smooth, resembling laboratory strains. Our two disomes displayed similar transcriptional profiles, a phenomenon not driven by their shared smooth colony morphology nor simply by their karyotype. Surprisingly, the environmental stress response (ESR) was induced in F45, relative to the two disomes. We also identified genes whose expression reflected a nonlinear interaction between the copy number of a transcriptional regulatory gene on chromosome XVI, DIG1, and the copy number of other chromosome XVI genes. DIG1 and the remaining chromosome XVI genes also demonstrated distinct contributions to the effect of the chromosome XVI disome on ESR gene expression. Expression changes in aneuploids appear to reflect a mixture of effects shared between different aneuploidies and effects unique to perturbing the copy number of particular chromosomes, including nonlinear copy number interactions between genes. The balance between these two phenomena is likely to be genotype- and environment-specific.