Plasma Membrane MCC/Eisosome Domains Promote Stress Resistance in Fungi

Deletion of core septin gene aspB in Aspergillus fumigatus results in fungicidal activity of caspofungin

Septins are a family of GTP-binding proteins found in many eukaryotic lineages. Although highly conserved throughout many eukaryotes, their functions vary across species. In Aspergillus fumigatus, the etiological agent of invasive aspergillosis, septins participate in a variety of processes such as cell wall organization of conidia, septation, and response to cell wall stress. Previous studies determined that the ΔaspB strain had a greater sensitivity to anti-cell wall drugs, especially the echinocandin caspofungin, yet mechanisms behind this augmented sensitivity are unknown. We performed cell viability staining of the deletion strains post-caspofungin exposure and found that the ΔaspA, ΔaspB, and ΔaspC strains have significantly lower cell viability. Concomitant with the reduced viability, deletion strains are more susceptible to caspofungin on solid media. These results indicate that the septin cytoskeleton is important for A. fumigatus survival in the presence of caspofungin. Due to the potential of improved therapeutic outcome, we followed up using a neutropenic murine model of invasive aspergillosis. Animals infected with the ΔaspB strain and treated with caspofungin showed improved survival compared to the animals infected with akuB KU80 wild-type or complemented strains. Additionally, histological analysis showed reduced fungal burden and inflammation in the ΔaspB infected, caspofungin-treated group. Affinity purification coupled with quantitative proteomics identified proteins involved in the septin-dependent response to caspofungin, includng four candidate interactors involved in cell wall stress response. Deletion of these candidate genes resulted in increased susceptibility to caspofungin and moderately reduced viability post-drug exposure. Taken together, these data suggest that septin AspB contributes to the fungistatic response to caspofungin.

The curtain model as an alternative and complementary to the classic turgor concept of filamentous fungi

Turgor pressure is critically important for all organisms with the cell wall. In fungi, turgor is involved in the apical growth of hyphae, affects cell size, provides tension to the plasma membrane, creates the necessary rigidity for hyphae to penetrate the substrate, and has many other functions. However, there is increasing evidence that turgor pressure is not always the sole or main factor influencing some of these processes. This review characterizes the curtain model, previously proposed to describe the regulation of plasma membrane tension in the hyphae of basidiomycetes. The current understanding of the four main components of the model is outlined: the driving actin cytoskeleton, the elastic cell wall, tight adhesion of the plasma membrane to the cell wall, and macroinvaginations of the plasma membrane. All four elements, as a single model, complement or replace some physiological functions of turgor and allow us to understand how a non-apical fungal cell maintains its physiological functionality under changing environmental conditions. Further experimental confirmation of this model is fundamentally important for mycology and applied sciences.

The Cwr1 protein kinase localizes to the plasma membrane and mediates resistance to cell wall stress in Candida albicans

The plasma membrane is critical for the virulence of the human fungal pathogen Candida albicans. In addition to functioning as a protective barrier, the plasma membrane plays dynamic roles in a wide range of functions needed for virulence including nutrient uptake, cell wall synthesis, morphogenesis, resistance to stress, and invasive hyphal growth. Screening a collection of C. albicans mutants identified an understudied gene that is important for invasive hyphal growth, which we have termed CWR1 (Cell Wall Regulatory kinase). A mutant strain lacking CWR1 displayed defects in resisting stressful conditions that exacerbate cell wall defects. The Cwr1 protein shows strong similarity to protein kinases, suggesting it plays a regulatory role in coordinating plasma membrane and cell wall functions. A Cwr1-green fluorescent protein (GFP) fusion protein localized to punctate patches associated with the plasma membrane that partially overlapped Membrane Compartment of Can1 (MCC)/eisosome domains. In contrast to the static MCC/eisosome domains, the Cwr1-GFP patches were very dynamic. Truncation mutants lacking C-terminal sequences distal to the protein kinase domain failed to show detectable localization at the plasma membrane. Surprisingly, these mutant strains did not show the defects of a cwr1Δ mutant, suggesting that localization to punctate patches associated with the plasma membrane is not essential for Cwr1 function. Altogether, these data indicate that Cwr1 contributes to the regulation of plasma membrane functions that promote proper morphogenesis and resistance to cell wall stress, both of which are important for C. albicans virulence.

IMPORTANCE

The ability of Candida albicans to grow invasively in the host and resist stress is critical for it to be an effective human pathogen. Identifying the genes that promote these processes is important for developing new strategies to block infection. Therefore, genetic methods were used in this study to identify a novel gene that is needed for invasive growth and stress resistance (Cell Wall Regulatory kinase [CWR1]). Interestingly, the Cwr1 protein localized to punctate patches in the plasma membrane, some of which co-localized with specialized subdomains of the plasma membrane known as eisosomes that are known to promote stress resistance and invasive growth in the host. Thus, these studies identified a novel regulator of traits that are critical for C. albicans pathogenesis.

Conserved mechanism of Xrn1 regulation by glycolytic flux and protein aggregation

The regulation of gene expression in eukaryotes relies largely on the action of exoribonucleases, evolutionarily conserved enzymes that digest decapped messenger RNAs in the 5'-3' direction. The activity of Xrn1, the major yeast exoribonuclease, is regulated by targeted changes in its cellular localisation in direct response to the cell's metabolic state. When fermentable carbon sources are available, active Xrn1 is diffusely localised in the cytosol. Upon depletion of these sources, Xrn1 is sequestered at the plasma membrane-associated protein complex, the eisosome, and becomes inactive. Although this phenomenon has been described previously, the molecular mechanisms underlying these changes remain unknown. We report that the binding of Xrn1 to the plasma membrane is subject to glycolytic flux, rather than the availability of a fermentable carbon source, is independent of TORC1 activity and requires the core eisosomal proteins Pil1 and Lsp1. We identify the SH3-like domain of the Xrn1 protein as a putative interaction domain. In addition, we show that when expressed in Saccharomyces cerevisiae, the human orthologue of Xrn1 mirrors its yeast counterpart, i.e., it segregates to the eisosome under conditions of halted glycolysis. Our results not only advance our understanding of Xrn1 regulation but also indicate that this regulatory principle is conserved from yeast to humans.

Inducible degradation-coupled phosphoproteomics identifies PP2ARts1 as a novel eisosome regulator

Introduction

Reversible protein phosphorylation is an abundant post-translational modification dynamically regulated by opposing kinases and phosphatases. Protein phosphorylation has been extensively studied in cell division, where waves of cyclin-dependent kinase activity, peaking in mitosis, drive the sequential stages of the cell cycle. Here we developed and employed a strategy to specifically probe kinase or phosphatase substrates at desired times or experimental conditions in the model organism Saccharomyces cerevisiae.

Methods

We combined auxin-inducible degradation (AID) with mass spectrometry-based phosphoproteomics, which allowed us to arrest physiologically normal cultures in mitosis prior to rapid phosphatase degradation and phosphoproteome analysis.

Results and discussion

Our results revealed that protein phosphatase 2A coupled with its B56 regulatory subunit, Rts1 (PP2ARts1), is involved in dephosphorylation of numerous proteins in mitosis, highlighting the need for phosphatases to selectively maintain certain proteins in a hypophosphorylated state in the face of high mitotic kinase activity. Unexpectedly, we observed elevated phosphorylation at many sites on several subunits of the fungal eisosome complex following rapid Rts1 degradation. Eisosomes are dynamic polymeric assemblies that create furrows in the plasma membrane important in regulating nutrient import, lipid metabolism, and stress responses, among other things. We found that PP2ARts1-mediated dephosphorylation of eisosomes promotes their plasma membrane association and we provide evidence that this regulation impacts eisosome roles in metabolic homeostasis. The combination of rapid, inducible protein degradation with proteomic profiling offers several advantages over common protein disruption methods for characterizing substrates of regulatory enzymes involved in dynamic biological processes.

Cryo-EM architecture of a near-native stretch-sensitive membrane microdomain

Biological membranes are partitioned into functional zones termed membrane microdomains, which contain specific lipids and proteins1-3. The composition and organization of membrane microdomains remain controversial because few techniques are available that allow the visualization of lipids in situ without disrupting their native behaviour3,4. The yeast eisosome, composed of the BAR-domain proteins Pil1 and Lsp1 (hereafter, Pil1/Lsp1), scaffolds a membrane compartment that senses and responds to mechanical stress by flattening and releasing sequestered factors5-9. Here we isolated near-native eisosomes as helical tubules made up of a lattice of Pil1/Lsp1 bound to plasma membrane lipids, and solved their structures by helical reconstruction. Our structures reveal a striking organization of membrane lipids, and, using in vitro reconstitutions and molecular dynamics simulations, we confirmed the positioning of individual PI(4,5)P2, phosphatidylserine and sterol molecules sequestered beneath the Pil1/Lsp1 coat. Three-dimensional variability analysis of the native-source eisosomes revealed a dynamic stretching of the Pil1/Lsp1 lattice that affects the sequestration of these lipids. Collectively, our results support a mechanism in which stretching of the Pil1/Lsp1 lattice liberates lipids that would otherwise be anchored by the Pil1/Lsp1 coat, and thus provide mechanistic insight into how eisosome BAR-domain proteins create a mechanosensitive membrane microdomain.

Sur7 mediates a novel pathway for PI4,5P2 regulation in C. albicans that promotes stress resistance and cell wall morphogenesis

The human fungal pathogen Candida albicans can cause lethal systemic infections due to its ability to resist stress from the host and to undergo invasive hyphal growth. Previous studies showed that plasma membrane MCC/eisosome domains were important for virulence by promoting the ability of Sur7 to mediate normal cell wall morphogenesis and stress resistance. The sur7Δ mutant displayed abnormal clusters of PI4,5P2, suggesting that misregulation of this lipid underlies the sur7Δ phenotype. To test this, we increased PI4,5P2 levels by deleting combinations of the three PI4,5P2 5' phosphatase genes (INP51, INP52, and INP54) and found that some combinations, such as inp51Δ inp52Δ, gave phenotypes similar the sur7Δ mutant. In contrast, deleting one copy of MSS4, the gene that encodes the 5' kinase needed to create PI4,5P2, reduced the abnormal PI4,5P2 clusters and also decreased the abnormal cell wall and stress sensitive phenotypes of the sur7Δ mutant. Additional studies support a model that the abnormal PI4,5P2 patches recruit septin proteins, which in turn promote aberrant cell wall growth. These results identify Sur7 as a novel regulator of PI4,5P2 and highlight the critical role of PI4,5P2 in the regulation of C. albicans virulence properties.

Activation of the cell wall integrity pathway negatively regulates TORC2-Ypk1/2 signaling through blocking eisosome disassembly in Saccharomyces cerevisiae

The target of rapamycin complex 2 (TORC2) signaling is associated with plasma membrane (PM) integrity. In Saccharomyces cerevisiae, TORC2-Ypk1/2 signaling controls sphingolipid biosynthesis, and Ypk1/2 phosphorylation by TORC2 under PM stress conditions is increased in a Slm1/2-dependent manner, under which Slm1 is known to be released from an eisosome, a furrow-like invagination PM structure. However, it remains unsolved how the activation machinery of TORC2-Ypk1/2 signaling is regulated. Here we show that edelfosine, a synthetic lysophospholipid analog, inhibits the activation of TORC2-Ypk1/2 signaling, and the cell wall integrity (CWI) pathway is involved in this inhibitory effect. The activation of CWI pathway blocked the eisosome disassembly promoted by PM stress and the release of Slm1 from eisosomes. Constitutive activation of TORC2-Ypk1/2 signaling exhibited increased sensitivity to cell wall stress. We propose that the CWI pathway negatively regulates the TORC2-Ypk1/2 signaling, which is involved in the regulatory mechanism to ensure the proper stress response to cell wall damage.

The arginine transporter Can1 negatively regulates biofilm formation in yeasts

The arginine transporter Can1 is a multifunctional protein of the conventional yeast Saccharomyces cerevisiae. Apart from facilitating arginine uptake, Can1 plays a pivotal role in regulating proline metabolism and maintaining cellular redox balance. Here, we report a novel function of Can1 in the control of yeast biofilm formation. First, the S. cerevisiae CAN1 gene knockout strain displayed a significant growth delay compared to the wild-type strain. Our genetic screening revealed that the slow growth of the CAN1 knockout strain is rescued by a functional deficiency of the FLO8 gene, which encodes the master transcription factor associated with biofilm formation, indicating that Can1 is involved in biofilm formation. Intriguingly, the CAN1 knockout strain promoted the Flo11-dependent aggregation, leading to higher biofilm formation. Furthermore, the CAN1 knockout strain of the pathogenic yeast Candida glabrata exhibited slower growth and higher biofilm formation, similar to S. cerevisiae. More importantly, the C. glabrata CAN1 gene knockout strain showed severe toxicity to macrophage-like cells and nematodes. The present results could help to elucidate both the molecular mechanism underlying yeast biofilm formation and the role it plays. Future investigations may offer insights that contribute to development of antibiofilm agents.

The joint action of yeast eisosomes and membraneless organelles in response to ethanol stress

Elevated ethanol concentrations in yeast affect the plasma membrane. The plasma membrane in yeast has many lipid-protein complexes, such as Pma1 (MCP), Can1 (MCC), and the eisosome complex. We investigated the response of eisosomes, MCPs, and membraneless structures to ethanol stress. We found a correlation between ethanol stress and proton flux with quick acidification of the medium. Moreover, ethanol stress influences the symporter expression in stressed cells. We also suggest that acute stress from ethanol leads to increases in eisosome size and SG number: we hypothesized that eisosomes may protect APC symporters and accumulate an mRNA decay protein in ethanol-stressed cells. Our findings suggest that the joint action of these factors may provide a protective effect on cells under ethanol stress.

Riboproteome remodeling during quiescence exit in Saccharomyces cerevisiae

The quiescent state is the prevalent mode of cellular life in most cells. Saccharomyces cerevisiae is a useful model for studying the molecular basis of the cell cycle, quiescence, and aging. Previous studies indicate that heterogeneous ribosomes show a specialized translation function to adjust the cellular proteome upon a specific stimulus. Using nano LC-MS/MS, we identified 69 of the 79 ribosomal proteins (RPs) that constitute the eukaryotic 80S ribosome during quiescence. Our study shows that the riboproteome is composed of 444 accessory proteins comprising cellular functions such as translation, protein folding, amino acid and glucose metabolism, cellular responses to oxidative stress, and protein degradation. Furthermore, the stoichiometry of both RPs and accessory proteins on ribosome particles is different depending on growth conditions and among monosome and polysome fractions. Deficiency of different RPs resulted in defects of translational capacity, suggesting that ribosome composition can result in changes in translational activity during quiescence.

General mechanisms of weak acid-tolerance and current strategies for the development of tolerant yeasts

Yeast cells are often subjected to various types of weak acid stress in the process of industrial production, food processing, and preservation, resulting in growth inhibition and reduced fermentation performance. Under acidic conditions, weak acids enter the near-neutral yeast cytoplasm and dissociate into protons and anions, leading to cytoplasmic acidification and cell damage. Although some yeast strains have developed the ability to survive weak acids, the complexity and diversity of stresses during industrial production still require the application of appropriate strategies for phenotypes improvement. In this review, we summarized current knowledge concerning weak acid stress response and resistance, which may suggest important targets for further construction of more robust strains. We also highlight current feasible strategies for improving the weak acid resistance of yeasts, such as adaptive laboratory evolution, transcription factors engineering, and cell membrane/wall engineering. Moreover, the challenges and perspectives associated with improving the competitiveness of industrial strains are also discussed. This review provides effective strategies for improving the industrial phenotypes of yeast from multiple dimensions in future studies.

Tetraspanner-based nanodomains modulate BAR domain-induced membrane curvature

The topography of biological membranes is critical for formation of protein and lipid microdomains. One prominent example in the yeast plasma membrane (PM) are BAR domain-induced PM furrows. Here we report a novel function for the Sur7 family of tetraspanner proteins in the regulation of local PM topography. Combining TIRF imaging, STED nanoscopy, freeze-fracture EM and membrane simulations we find that Sur7 tetraspanners form multimeric strands at the edges of PM furrows, where they modulate forces exerted by BAR domain proteins at the furrow base. Loss of Sur7 tetraspanners or Sur7 displacement due to altered PIP2 homeostasis leads to increased PM invagination and a distinct form of membrane tubulation. Physiological defects associated with PM tubulation are rescued by synthetic anchoring of Sur7 to furrows. Our findings suggest a key role for tetraspanner proteins in sculpting local membrane domains. The maintenance of stable PM furrows depends on a balance between negative curvature at the base which is generated by BAR domains and positive curvature at the furrows' edges which is stabilized by strands of Sur7 tetraspanners.

The biological functions of sphingolipids in plant pathogenic fungi

Sphingolipids are critically significant in a range of biological processes in animals, plants, and fungi. In mammalian cells, they serve as vital components of the plasma membrane (PM) in maintaining its structure, tension, and fluidity. They also play a key role in a wide variety of biological processes, such as intracellular signal transduction, cell polarization, differentiation, and migration. In plants, sphingolipids are important for cell development and for cell response to environmental stresses. In pathogenic fungi, sphingolipids are crucial for the initiation and the development of infection processes afflicting humans. However, our knowledge on the metabolism and function of the sphingolipid metabolic pathway of pathogenic fungi affecting plants is still very limited. In this review, we discuss recent developments on sphingolipid pathways of plant pathogenic fungi, highlighting their uniqueness and similarity with plants and animals. In addition, we discuss recent advances in the research and development of fungal-targeted inhibitors of the sphingolipid pathway, to gain insights on how we can better control the infection process occurring in plants to prevent or/and to treat fungal infections in crops.

Inducible degradation-coupled phosphoproteomics identifies PP2ARts1 as a novel eisosome regulator

Reversible protein phosphorylation is an abundant post-translational modification dynamically regulated by opposing kinases and phosphatases. Protein phosphorylation has been extensively studied in cell division, where waves of cyclin-dependent kinase activity, peaking in mitosis, drive the sequential stages of the cell cycle. Here we developed and employed a strategy to specifically probe kinase or phosphatase substrates at desired times or experimental conditions in the model organism Saccharomyces cerevisiae. We combined auxin-inducible degradation (AID) with mass spectrometry-based phosphoproteomics, which allowed us to arrest physiologically normal cultures in mitosis prior to rapid phosphatase degradation and phosphoproteome analysis. Our results revealed that protein phosphatase 2A coupled with its B56 regulatory subunit, Rts1 (PP2ARts1), is involved in dephosphorylation of numerous proteins in mitosis, highlighting the need for phosphatases to selectively maintain certain proteins in a hypophosphorylated state in the face of high mitotic kinase activity. Unexpectedly, we observed elevated phosphorylation at many sites on several subunits of the fungal eisosome complex following rapid Rts1 degradation. Eisosomes are dynamic polymeric assemblies that create furrows in the plasma membrane important in regulating nutrient import, lipid metabolism, and stress responses, among other things. We found that PP2ARts1-mediated dephosphorylation of eisosomes promotes their plasma membrane association and we provide evidence that this regulation impacts eisosome roles in metabolic homeostasis. The combination of rapid, inducible protein degradation with proteomic profiling offers several advantages over common protein disruption methods for characterizing substrates of regulatory enzymes involved in dynamic biological processes.

Analysis of acid-tolerance mechanism based on membrane microdomains in Saccharomyces cerevisiae

BACKGROUND

Saccharomyces cerevisiae has been used in the biosynthesis of acid products such as organic acids owing to its acid tolerance. Improving the acid tolerance of S. cerevisiae is beneficial for expanding its application range. Our previous study isolated the TAMC strain that was tolerant to a pH 2.3 through adaptive laboratory evolution; however, its mechanism underlying tolerance to low pH environment remains unclear.

RESULTS

In this study, through visual observation and order analysis of plasma membrane and membrane microdomains, we revealed that the membrane microdomains of TAMC strain play an indispensable role in acid tolerance. Transcriptomic analysis showed an increase in the expression of genes related to key components of membrane microdomains in TAMC strain. Furthermore, an obvious reduction was observed in the acid tolerance of the strain with sterol C-24 methyltransferase encoding gene ERG6 knockout for inhibiting membrane microdomain formation. Finally, colocalization analysis of H+-ATPase PMA1 and plasma membrane protein PMP1 showed that disruption of membrane microdomains could inhibit the formation of the H+-ATPase complex.

CONCLUSIONS

Membrane microdomains could provide a platform for forming H+-ATPase complexes to facilitate intracellular H+ homeostasis, and thereby improve cell acid resistance. This study proposed a novel acid tolerance mechanism, providing a new direction for the rational engineering of acid-tolerant strains.

Activation of CWI pathway through high hydrostatic pressure, enhancing glycerol efflux via the aquaglyceroporin Fps1 in Saccharomyces cerevisiae

The fungal cell wall is the initial barrier for the fungi against diverse external stresses, such as osmolarity changes, harmful drugs, and mechanical injuries. This study explores the roles of osmoregulation and the cell-wall integrity (CWI) pathway in response to high hydrostatic pressure in the yeast Saccharomyces cerevisiae. We demonstrate the roles of the transmembrane mechanosensor Wsc1 and aquaglyceroporin Fps1 in a general mechanism to maintain cell growth under high-pressure regimes. The promotion of water influx into cells at 25 MPa, as evident by an increase in cell volume and a loss of the plasma membrane eisosome structure, activates the CWI pathway through the function of Wsc1. Phosphorylation of Slt2, the downstream mitogen-activated protein kinase, was increased at 25 MPa. Glycerol efflux increases via Fps1 phosphorylation, which is initiated by downstream components of the CWI pathway, and contributes to the reduction in intracellular osmolarity under high pressure. The elucidation of the mechanisms underlying adaptation to high pressure through the well-established CWI pathway could potentially translate to mammalian cells and provide novel insights into cellular mechanosensation.

The arginine transporter Can1 acts as a transceptor for regulation of proline utilization in the yeast Saccharomyces cerevisiae

Proline is the most abundant amino acid in wine and beer, because the yeast Saccharomyces cerevisiae hardly assimilates proline during fermentation processes. Our previous studies showed that arginine induces endocytosis of the proline transporter Put4, resulting in inhibition of proline utilization. We here report a possible role of arginine sensing in the inhibition of proline utilization. We first found that two basic amino acids, ornithine, and lysine, inhibit proline utilization by inducing Put4 endocytosis in a manner similar to arginine, but citrulline does not. Our genetic screening revealed that the arginine transporter Can1 is involved in the inhibition of proline utilization by arginine. Intriguingly, the arginine uptake activity of Can1 was not required for the arginine-dependent inhibition of proline utilization, suggesting that Can1 has a function beyond its commonly known function of transporting arginine. More importantly, our biochemical analyses revealed that Can1 activates signaling cascades of protein kinase A in response to extracellular arginine. Hence, we proposed that Can1 regulates proline utilization by functioning as a transceptor possessing the activity of both a transporter and receptor of arginine.

SUR7 deletion in Candida albicans impacts extracellular vesicle features and delivery of virulence factors

Extracellular vesicles (EVs) from human fungal pathogens have been implicated in fungal virulence, yet little is known about their role in the host-pathogen interaction. Progress has been hampered by the lack of a specific marker for fungal EVs that can be used to monitor EV isolation and tracking in biological systems. Here we report the effect of a SUR7 gene knockout on the production, properties, and role of EVs in the virulence of Candida albicans. Sur7 is a component of the membrane compartment of Can1 (MCC) complex and is enriched in the EVs from C. albicans and other fungal species. MCC is a plasma membrane complex which together with the eisosome, a cytoplasmic protein complex, is a key regulator in plasma membrane organisation and plasma membrane associated processes. The SUR7 knockout strain produces smaller EVs than the wild-type (WT) with different protein and carbohydrate cargos. Furthermore, proteins with known roles in Candida pathogenesis were present in WT EVs and absent or diminished in the sur7Δ EVs. We demonstrate that the reduced virulence of the sur7Δ cells can be partially restored with EVs from a WT strain. These findings demonstrate the importance of Sur7-like proteins in the biogenesis of EVs in fungi and enhance our understanding of the role of fungal EVs in human pathogenesis.

Meisosomes, folded membrane microdomains between the apical extracellular matrix and epidermis

Apical extracellular matrices (aECMs) form a physical barrier to the environment. In Caenorhabditis elegans, the epidermal aECM, the cuticle, is composed mainly of different types of collagen, associated in circumferential ridges separated by furrows. Here, we show that in mutants lacking furrows, the normal intimate connection between the epidermis and the cuticle is lost, specifically at the lateral epidermis, where, in contrast to the dorsal and ventral epidermis, there are no hemidesmosomes. At the ultrastructural level, there is a profound alteration of structures that we term 'meisosomes,' in reference to eisosomes in yeast. We show that meisosomes are composed of stacked parallel folds of the epidermal plasma membrane, alternately filled with cuticle. We propose that just as hemidesmosomes connect the dorsal and ventral epidermis, above the muscles, to the cuticle, meisosomes connect the lateral epidermis to it. Moreover, furrow mutants present marked modifications of the biomechanical properties of their skin and exhibit a constitutive damage response in the epidermis. As meisosomes co-localise to macrodomains enriched in phosphatidylinositol (4,5) bisphosphate, they could conceivably act, like eisosomes, as signalling platforms, to relay tensile information from the aECM to the underlying epidermis, as part of an integrated stress response to damage.

Lomasomes and Other Fungal Plasma Membrane Macroinvaginations Have a Tubular and Lamellar Genesis

The plasma membrane of filamentous fungi forms large-sized invaginations, which are either tubes or parietal vesicles. Vesicular macroinvaginations at the ultrastructural level correspond to classical lomasomes. There is an assumption that vesicular macroinvaginations/lomasomes may be involved in macrovesicular endocytosis. The original aim of this study was to test for the presence of macroendocytosis in xylotrophic basidiomycetes using time-lapse and Z-stacks fluorescent microscopic technologies. However, the results were unexpected since most of the membrane structures labeled by the endocytic tracer (FM4-64 analog) are various types of plasma membrane macroinvaginations and not any endomembranes. All of these macroinvaginations have a tubular or lamellar genesis. Moreover, under specific conditions of a microscopic preparation, the diameter of the tubes forming the macroinvaginations increases with the time of the sample observation. In addition, the morphology and successive formation of the macroinvaginations mimic the endocytic pathway; these invaginations can easily be mistaken for endocytic vesicles, endosomes, and vacuole-lysosomes. The paper analyzes the various macroinvagination types, suggests their biological functions, and discusses some features of fungal endocytosis. This study is a next step toward understanding complex fungal physiology and is a presentation of a new intracellular tubular system in wood-decaying fungi.

TOR complex 2 is a master regulator of plasma membrane homeostasis

As first demonstrated in budding yeast (Saccharomyces cerevisiae), all eukaryotic cells contain two, distinct multi-component protein kinase complexes that each harbor the TOR (Target Of Rapamycin) polypeptide as the catalytic subunit. These ensembles, dubbed TORC1 and TORC2, function as universal, centrally important sensors, integrators, and controllers of eukaryotic cell growth and homeostasis. TORC1, activated on the cytosolic surface of the lysosome (or, in yeast, on the cytosolic surface of the vacuole), has emerged as a primary nutrient sensor that promotes cellular biosynthesis and suppresses autophagy. TORC2, located primarily at the plasma membrane, plays a major role in maintaining the proper levels and bilayer distribution of all plasma membrane components (sphingolipids, glycerophospholipids, sterols, and integral membrane proteins). This article surveys what we have learned about signaling via the TORC2 complex, largely through studies conducted in S. cerevisiae. In this yeast, conditions that challenge plasma membrane integrity can, depending on the nature of the stress, stimulate or inhibit TORC2, resulting in, respectively, up-regulation or down-regulation of the phosphorylation and thus the activity of its essential downstream effector the AGC family protein kinase Ypk1. Through the ensuing effect on the efficiency with which Ypk1 phosphorylates multiple substrates that control diverse processes, membrane homeostasis is maintained. Thus, the major focus here is on TORC2, Ypk1, and the multifarious targets of Ypk1 and how the functions of these substrates are regulated by their Ypk1-mediated phosphorylation, with emphasis on recent advances in our understanding of these processes.

Yeast osmoregulation - glycerol still in pole position

In response to osmotic dehydration cells sense, signal, alter gene expression, and metabolically counterbalance osmotic differences. The main compatible solute/osmolyte that accumulates in yeast cells is glycerol, which is produced from the glycolytic intermediate dihydroxyacetone phosphate. This review covers recent advancements in understanding mechanisms involved in sensing, signaling, cell-cycle delays, transcriptional responses as well as post-translational modifications on key proteins in osmoregulation. The protein kinase Hog1 is a key-player in many of these events, however, there is also a growing body of evidence for important Hog1-independent mechanisms playing vital roles. Several missing links in our understanding of osmoregulation will be discussed and future avenues for research proposed. The review highlights that this rather simple experimental system—salt/sorbitol and yeast—has developed into an enormously potent model system unravelling important fundamental aspects in biology.

Arginine inhibits Saccharomyces cerevisiae biofilm formation by inducing endocytosis of the arginine transporter Can1

Biofilms are formed by the aggregation of microorganisms into multicellular structures that adhere to surfaces. Biofilm formation by yeast is a critical issue in clinical and industrial fields because of the strong adhesion of yeast biofilm to abiotic surfaces and tissues. Here, we clarified the arginine-mediated inhibition of biofilm formation by yeast. First, we showed that arginine inhibits biofilm formation in fungi such as Saccharomyces cerevisiae, Candida glabrata, and Cladosporium cladosporioides, but not in bacteria. In regard to the underlying mechanism, biochemical analysis indicated that arginine inhibits biofilm formation by suppressing Flo11-dependent flocculation. Intriguingly, a strain with deletion of the arginine transporter-encoding CAN1 was insensitive to arginine-mediated inhibition of biofilm formation. Finally, Can1 endocytosis appeared to be required for the inhibitory mechanism of biofilm formation by arginine. The present results could help to elucidate the molecular mechanism of yeast biofilm formation and its control.

Coordinated regulation of TORC2 signaling by MCC/eisosome-associated proteins, Pil1 and tetraspan membrane proteins during the stress response

MCCs are linear invaginations of the yeast plasma membrane that form stable membrane microdomains. Although over 20 proteins are localized in the MCCs, it is not well understood how these proteins coordinately maintain normal MCC function. Pil1 is a core eisosome protein and is responsible for MCC-invaginated structures. In addition, six-tetraspan membrane proteins (6-Tsp) are localized in the MCCs and classified into two families, the Sur7 family and Nce102 family. To understand the coordinated function of these MCC proteins, single and multiple deletion mutants of Pil1 and 6-Tsp were generated and their MCC structure and growth under various stresses were investigated. Genetic interaction analysis revealed that the Sur7 family and Nce102 function in stress tolerance and normal eisosome assembly, respectively, by cooperating with Pil1. To further understand the role of MCCs/eisosomes in stress tolerance, we screened for suppressor mutants using the SDS-sensitive phenotype of pil1Δ 6-tspΔ cells. This revealed that SDS sensitivity is caused by hyperactivation of Tor kinase complex 2 (TORC2)-Ypk1 signaling. Interestingly, inhibition of sphingolipid metabolism, a well-known downstream pathway of TORC2-Ypk1 signaling, did not rescue the SDS-sensitivity of pil1Δ 6-tspΔ cells. These results suggest that Pil1 and 6-Tsp cooperatively regulate TORC2 signaling during the stress response.

Substrates of the MAPK Slt2: Shaping Yeast Cell Integrity

The cell wall integrity (CWI) MAPK pathway of budding yeast Saccharomyces cerevisiae is specialized in responding to cell wall damage, but ongoing research shows that it participates in many other stressful conditions, suggesting that it has functional diversity. The output of this pathway is mainly driven by the activity of the MAPK Slt2, which regulates important processes for yeast physiology such as fine-tuning of signaling through the CWI and other pathways, transcriptional activation in response to cell wall damage, cell cycle, or determination of the fate of some organelles. To this end, Slt2 precisely phosphorylates protein substrates, modulating their activity, stability, protein interaction, and subcellular localization. Here, after recapitulating the methods that have been employed in the discovery of proteins phosphorylated by Slt2, we review the bona fide substrates of this MAPK and the growing set of candidates still to be confirmed. In the context of the complexity of MAPK signaling regulation, we discuss how Slt2 determines yeast cell integrity through phosphorylation of these substrates. Increasing data from large-scale analyses and the available methodological approaches pave the road to early identification of new Slt2 substrates and functions.

Substrate-induced differential degradation and partitioning of the two tryptophan permeases Tat1 and Tat2 into eisosomes in Saccharomyces cerevisiae

Tryptophan is a relatively rare amino acid whose influx is strictly controlled to meet cellular demands. The yeast Saccharomyces cerevisiae has two tryptophan permeases, namely Tat1 (low-affinity type) and Tat2 (high-affinity type). These permeases are differentially regulated through ubiquitination based on inducible conditions and dependence on arrestin-related trafficking adaptors, although the physiological significance of their degradation remain unclear. Here, we demonstrated that Tat2 was rapidly degraded in an Rsp5-Bul1-dependent manner upon the addition of tryptophan, phenylalanine, or tyrosine, whereas Tat1 was unaffected. The expression of the ubiquitination-deficient variant Tat2^5K>R led to a reduction in cell yield at 4 μg/mL tryptophan, suggesting the occurrence of an uncontrolled, excessive consumption of tryptophan at low tryptophan concentrations. Eisosomes are membrane furrows that are thought to be storage compartments for some nutrient permeases. Tryptophan addition caused rapid Tat2 dissociation from eisosomes, whereas Tat1 distribution was unaffected. The 5 K > R mutation had no marked effect on Tat2 dissociation, suggesting that dissociation is independent of ubiquitination. Interestingly, the D74R mutation, which was created within the N-terminal acidic patch, stabilized Tat2 while reducing the degree of partitioning into eisosomes. Moreover, the hyperactive I285V mutation in Tat2, which increases V_max/K_m for tryptophan import by 2-fold, reduced the degree of segregation into eisosomes. Our findings illustrate the coordinated activity of Tat1 and Tat2 in the regulation of tryptophan transport at various tryptophan concentrations and suggest the positive role of substrates in inducing a conformational transition in Tat2, resulting in its dissociation from eisosomes and subsequent ubiquitination-dependent degradation.

Exploring the biological function of efflux pumps for the development of superior industrial yeasts

Among the mechanisms used by yeasts to overcome the deleterious effects of chemical and other environmental stresses is the activity of plasma membrane efflux pumps involved in multidrug resistance (MDR), a role on the focus of intensive research for years in pathogenic yeasts. More recently, these active transporters belonging to the MFS (Drug: H⁺ antiporters) or the ABC superfamily have been involved in resistance to xenobiotic compounds and in the transport of substrates with a clear physiological role. This review paper focuses on these putative efflux pumps concerning their tolerance phenotypes towards bioprocess-specific multiple stress factors, expression levels, physiological roles, and mechanisms by which they may lead to multistress resistance. Their association with the increased secretion of metabolites and other bioproducts and in the development of more robust superior strains for Yeast Chemical Biotechnology is highlighted.

The Sur7 cytoplasmic C terminus regulates morphogenesis and stress responses in Candida albicans

MCC/eisosome subdomains of the plasma membrane promote proper cell wall morphogenesis that is critical for the fungal pathogen Candida albicans to grow invasively and resist stressful environments in the host. Sur7 localizes to MCC/eisosomes and is needed for their function, so in this work, the role of this tetraspan membrane protein was studied by mutagenesis. Deletion mutant analysis showed that the N-terminal region containing the four transmembrane domains mediates Sur7 localization to MCC/eisosomes. Mutation of 32 conserved residues in the N-terminal region indicated that extracellular loop 1 is important, although these mutants generally displayed weak phenotypes. Surprisingly, two Cys residues in a conserved motif in extracellular loop 1 were not important. However, deletion of the entire 15 amino acid motif revealed that it was needed for proper membrane trafficking of Sur7. Deletion and substitution mutagenesis showed that the C terminus is important for resisting cell wall stress. This is significant as it indicates Sur7 carries out an important role in the cytoplasm. Altogether, these results indicate that the N-terminal region localizes Sur7 to MCC/eisosomes and that the C-terminal domain promotes responses in the cytoplasm needed for cell wall morphogenesis and stress resistance.

H2O2 Induces Major Phosphorylation Changes in Critical Regulators of Signal Transduction, Gene Expression, Metabolism and Developmental Networks in Aspergillus nidulans

Reactive oxygen species (ROS) regulate several aspects of cell physiology in filamentous fungi including the antioxidant response and development. However, little is known about the signaling pathways involved in these processes. Here, we report Aspergillus nidulans global phosphoproteome during mycelial growth and show that under these conditions, H2O2 induces major changes in protein phosphorylation. Among the 1964 phosphoproteins we identified, H2O2 induced the phosphorylation of 131 proteins at one or more sites as well as the dephosphorylation of a larger set of proteins. A detailed analysis of these phosphoproteins shows that H2O2 affected the phosphorylation of critical regulatory nodes of phosphoinositide, MAPK, and TOR signaling as well as the phosphorylation of multiple proteins involved in the regulation of gene expression, primary and secondary metabolism, and development. Our results provide a novel and extensive protein phosphorylation landscape in A. nidulans, indicating that H2O2 induces a shift in general metabolism from anabolic to catabolic, and the activation of multiple stress survival pathways. Our results expand the significance of H2O2 in eukaryotic cell signaling.

Differential Roles of a Family of Flavodoxin-Like Proteins That Promote Resistance to Quinone-Mediated Oxidative Stress in Candida albicans

Survival of the fungal pathogen Candida albicans within a mammalian host relies on its ability to resist oxidative stress. The four flavodoxin-like proteins (Pst1, Pst2, Pst3, and Ycp4) that reside on the inner surface of the C. albicans plasma membrane represent a recently discovered antioxidant mechanism that is essential for virulence. Flavodoxin-like proteins combat oxidative stress by promoting a two-electron reduction of quinone molecules, which prevents the formation of toxic semiquinone radicals. Previous studies indicated that Pst3 played a major role in promoting resistance to the small quinone molecules p-benzoquinone and menadione. Analysis of additional quinones confirmed this role for Pst3. To better define their function, antibodies were raised against each of the four flavodoxin-like proteins and used to quantify protein levels. Interestingly, the basal level of flavodoxin-like proteins differed, with Pst3 and Ycp4 being the most abundant. However, after induction with p-benzoquinone, Pst1 and Pst3 were the most highly induced, resulting in Pst3 becoming the most abundant. Constitutive expression of the flavodoxin-like protein genes from a TDH3 promoter resulted in similar protein levels and showed that Pst1 and Pst3 were better at protecting C. albicans against p-benzoquinone than Pst2 or Ycp4. In contrast, Pst1 and Ycp4 provided better protection against oxidative damage induced by tert-butyl hydroperoxide. Thus, both the functional properties and the relative abundance contribute to the distinct roles of the flavodoxin-like proteins in resisting oxidative stress. These results further define how C. albicans combats the host immune response and survives in an environment rich in oxidative stress.

Physical, genetic and functional interactions between the eisosome protein Pil1 and the MBOAT O-acyltransferase Gup1

The Saccharomyces cerevisiae MBOAT O-acyltransferase Gup1 is involved in many processes, including cell wall and membrane composition and integrity, and acetic acid-induced cell death. Gup1 was previously shown to interact physically with the mitochondrial membrane VDAC (Voltage-Dependent Anion Channel) protein Por1 and the ammonium transceptor Mep2. By co-immunoprecipitation, the eisosome core component Pil1 was identified as a novel physical interaction partner of Gup1. The expression of PIL1 and Pil1 protein levels were found to be unaffected by GUP1 deletion. In ∆gup1 cells, Pil1 was distributed in dots (likely representing eisosomes) in the membrane, identically to wt cells. However, ∆gup1 cells presented 50% less Pil1-GFP dots/eisosomes, suggesting that Gup1 is important for eisosome formation. The two proteins also interact genetically in the maintenance of cell wall integrity, and during arsenite and acetic acid exposure. We show that Δgup1 Δpil1 cells take up more arsenite than wt and are extremely sensitive to arsenite and to acetic acid treatments. The latter causes a severe apoptotic wt-like cell death phenotype, epistatically reverting the ∆gup1 necrotic type of death. Gup1 and Pil1 are thus physically, genetically and functionally connected.