The eisosome core is composed of BAR domain proteins

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.

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.

Lsp1 partially substitutes for Pil1 function in eisosome assembly under stress conditions

Eisosomes are large hemitubular structures that underlie the invaginated microdomains in the plasma membrane of various ascomycetous fungi, lichens and unicellular algae. In fungi, they are organized by BAR-domain containing proteins of the Pil1 family. Two such proteins, Pil1 and Lsp1, participate in eisosome formation in the yeast Saccharomyces cerevisiae. Under normal laboratory conditions, deletion of the PIL1 gene results in the inability of cells to assemble wild-type-like eisosomes. We found that under certain stress conditions, Lsp1 partially substitutes for the Pil1 function and mediates assembly of eisosomes, specifically following a decrease in the activity of serine palmitoyltransferase, for example, in response to hyperosmotic stress. Besides Lsp1, the assembly of eisosomes lacking Pil1 also requires Seg1 and Nce102 proteins. Using next-generation sequencing, we found that the seg1Δnce102Δpil1Δ strain, which is unable to form eisosomes, overexpresses genes coding for proteins of oxidative phosphorylation and tricarboxylic acid cycle. By contrast, genes involved in DNA repair, ribosome biogenesis and cell cycle are downregulated. Our results identify Lsp1 as a stress-responsive eisosome organizer and indicate several novel functional connections between the eisosome and essential cellular processes.

Complementation of an Eisosomal Yeast pil1 Mutant and Characteristics of Eisosomal Distribution in Hyphae of Neurospora crassa Germinating from Two Different Spore Types

Eisosomes are plasma-membrane-associated protein complexes of fungi and algae involved in various cellular processes. The eisosome composition of the budding yeast is well described, but there is a limited number of studies only about eisosomes in filamentous fungi. In our study, we examined the Neurospora crassa LSP-1 protein (NcLSP1). By complementing a Saccharomyces cerevisiae Δpil1 mutant strain with nclsp1, we show the functional homology of the NcLSP1 to yeast PIL1 rather than to yeast LSP1 and hereby confirm that the NcLSP1 is an eisosomal core protein and suitable eisosomal marker. The subsequent cloning and expression of the nclsp1::trfp reporter gene construct in N. crassa allowed for a systematical investigation of the characteristics of eisosome formation and distribution in different developmental stages. In N. crassa, the hyphae germinating from sexual and asexual spores are morphologically identical and have been historically recognized as the same type of cells. Here, we demonstrate the structural differences on the cellular level between the hyphae germinating from sexual and asexual spores.

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.

Eisosome protein Pil1 regulates mitochondrial morphology, mitophagy, and cell death in Saccharomyces cerevisiae

Mitochondrial morphology and dynamics maintain mitochondrial integrity by regulating its size, shape, distribution, and connectivity, thereby modulating various cellular processes. Several studies have established a functional link between mitochondrial dynamics, mitophagy, and cell death, but further investigation is needed to identify specific proteins involved in mitochondrial dynamics. Any alteration in the integrity of mitochondria has severe ramifications that include disorders like cancer and neurodegeneration. In this study, we used budding yeast as a model organism and found that Pil1, the major component of the eisosome complex, also localizes to the periphery of mitochondria. Interestingly, the absence of Pil1 causes the branched tubular morphology of mitochondria to be abnormally fused or aggregated, whereas its overexpression leads to mitochondrial fragmentation. Most importantly, pil1Δ cells are defective in mitophagy and bulk autophagy, resulting in elevated levels of reactive oxygen species and protein aggregates. In addition, we show that pil1Δ cells are more prone to cell death. Yeast two-hybrid analysis and co-immunoprecipitations show the interaction of Pil1 with two major proteins in mitochondrial fission, Fis1 and Dnm1. Additionally, our data suggest that the role of Pil1 in maintaining mitochondrial shape is dependent on Fis1 and Dnm1, but it functions independently in mitophagy and cell death pathways. Together, our data suggest that Pil1, an eisosome protein, is a novel regulator of mitochondrial morphology, mitophagy, and cell death.

Correlation analyses reveal differential diffusion behavior of eisosomal proteins between mother and daughter cells

Eisosomes are nanoscale plasma membrane domains shaped as furrow-like invaginations. In Saccharomyces cerevisiae these relatively immobile and uniform structures are mainly composed of two cytoplasmic proteins Pil1 and Lsp1. The present work uses fluctuation of fluorescence signals and analytical methods to determine Pil1 and Lsp1 dynamics at different subcellular locations. Using scanning techniques and autocorrelation analysis we determine that the cytoplasmic pools of Pil1 and Lsp1 behave mainly by passive diffusion. Single-point FCS experiments performed at several subcellular locations reveal that Pil1 mobility is faster in daughter cells. Furthermore, pair correlation function analysis indicates a rapid dynamic of Pil1 near the plasma membrane of growing yeast buds, where the membrane is expected to be actively assembling eisosomes.

Microdomain Protein Nce102 Is a Local Sensor of Plasma Membrane Sphingolipid Balance

Sphingolipids are essential building blocks of eukaryotic membranes and important signaling molecules that are regulated tightly in response to environmental and physiological inputs. While their biosynthetic pathway has been well-described, the mechanisms that facilitate the perception of sphingolipid levels at the plasma membrane remain to be uncovered. In Saccharomyces cerevisiae, the Nce102 protein has been proposed to function as a sphingolipid sensor as it changes its plasma membrane distribution in response to sphingolipid biosynthesis inhibition. We show that Nce102 redistributes specifically in regions of increased sphingolipid demand, e.g., membranes of nascent buds. Furthermore, we report that the production of Nce102 increases following sphingolipid biosynthesis inhibition and that Nce102 is internalized when excess sphingolipid precursors are supplied. This finding suggests that the total amount of Nce102 in the plasma membrane is a measure of the current need for sphingolipids, whereas its local distribution marks sites of high sphingolipid demand. The physiological role of Nce102 in the regulation of sphingolipid synthesis is demonstrated by mass spectrometry analysis showing reduced levels of hydroxylated complex sphingolipids in response to heat stress in the nce102Δ deletion mutant. We also demonstrate that Nce102 behaves analogously in the widespread human fungal pathogen Candida albicans, suggesting a conserved principle of local sphingolipid control across species. IMPORTANCE Microorganisms are challenged constantly by their rapidly changing environment. To survive, they have developed diverse mechanisms to quickly perceive stressful situations and adapt to them appropriately. The primary site of both stress sensing and adaptation is the plasma membrane. We identified the yeast protein Nce102 as a marker of local sphingolipid levels and fluidity in the plasma membrane. Nce102 is an important structural and functional component of the membrane compartment Can1 (MCC), a plasma membrane microdomain stabilized by a large cytosolic hemitubular protein scaffold, the eisosome. The MCC/eisosomes are widely conserved among fungi and unicellular algae. To determine if Nce102 carries out similar functions in other organisms, we analyzed the human fungal pathogen Candida albicans and found that Nce102 responds to sphingolipid levels also in this organism, which has potential applications for the development of novel therapeutic approaches. The presented study represents a valuable model for how organisms regulate plasma membrane sphingolipids.

Thermodynamics and Free Energy Landscape of BAR-Domain Dimerization from Molecular Simulations

Proteins with BAR domains function to bind to and remodel biological membranes, where the dimerization of BAR domains is a key step in this function. These domains can dimerize in solution or after localizing to the membrane surface. Here, we characterize the binding thermodynamics of homodimerization between the LSP1 BAR domain proteins in solution, using molecular dynamics (MD) simulations. By combining the MARTINI coarse-grained protein models with enhanced sampling through metadynamics, we construct a two-dimensional free energy surface quantifying the bound versus unbound ensembles as a function of two distance variables. With this methodology, our simulations can simultaneously characterize the structures and relative stabilities of a range of sampled dimers, portraying a heterogeneous and extraordinarily stable bound ensemble, where the proper crystal structure dimer is the most stable in a 100 mM NaCl solution. Nonspecific dimers that are sampled involve contacts that are consistent with experimental structures of higher-order oligomers formed by the LSP1 BAR domain. Because the BAR dimers and oligomers can assemble on membranes, we characterize the relative alignment of the known membrane binding patches, finding that only the specific dimer is aligned to form strong interactions with the membrane. Hence, we would predict a strong selection of the specific dimer in binding to or assembling when on the membrane. Establishing the pairwise stabilities of homodimer contacts is difficult experimentally when the proteins form stable oligomers, but through the method used here, we can isolate these contacts, providing a foundation to study the same interactions on the membrane.

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.

The Composition and the Structure of MCC/Eisosomes in Neurospora crassa

MCC/eisosomes are protein-organized domains in the plasma membrane of fungi and algae. However, the composition and function(s) of MCC/eisosomes in the filamentous fungus Neurospora crassa were previously unknown. To identify proteins that localize to MCC/eisosomes in N. crassa, we isolated proteins that co-purified with the core MCC/eisosome protein LSP-1, which was tagged with GFP. Proteins that co-fractionated with LSP-1:GFP were then identified by mass spectrometry. Eighteen proteins were GFP-tagged and used to identify six proteins that highly colocalized with the MCC/eisosome marker LSP-1:RFP, while five other proteins showed partial overlap with MCC/eisosomes. Seven of these proteins showed amino acid sequence homology with proteins known to localize to MCC/eisosomes in the yeast Saccharomyces cerevisiae. However, homologs of three proteins known to localize to MCC/eisosomes in S. cerevisiae (Can1, Pkh1/2, and Fhn1) were not found to colocalize with MCC/eisosome proteins in N. crassa by fluorescence microscopy. Interestingly, one new eisosome protein (glutamine-fructose-6-phosphate aminotransferase, gene ID: NCU07366) was detected in our studies. These findings demonstrate that there are interspecies differences of the protein composition of MCC/eisosomes. To gain further insight, molecular modeling and bioinformatics analysis of the identified proteins were used to propose the organization of MCC/eisosomes in N. crassa. A model will be discussed for how the broad range of functions predicted for the proteins localized to MCC/eisosomes, including cell wall synthesis, response and signaling, transmembrane transport, and actin organization, suggests that MCC/eisosomes act as organizing centers in the plasma membrane.

Plasma Membrane MCC/Eisosome Domains Promote Stress Resistance in Fungi

There is growing appreciation that the plasma membrane orchestrates a diverse array of functions by segregating different activities into specialized domains that vary in size, stability, and composition. Studies with the budding yeast Saccharomyces cerevisiae have identified a novel type of plasma membrane domain known as the MCC (membrane compartment of Can1)/eisosomes that correspond to stable furrows in the plasma membrane. MCC/eisosomes maintain proteins at the cell surface, such as nutrient transporters like the Can1 arginine symporter, by protecting them from endocytosis and degradation. Recent studies from several fungal species are now revealing new functional roles for MCC/eisosomes that enable cells to respond to a wide range of stressors, including changes in membrane tension, nutrition, cell wall integrity, oxidation, and copper toxicity. The different MCC/eisosome functions are often intertwined through the roles of these domains in lipid homeostasis, which is important for proper plasma membrane architecture and cell signaling. Therefore, this review will emphasize the emerging models that explain how MCC/eisosomes act as hubs to coordinate cellular responses to stress. The importance of MCC/eisosomes is underscored by their roles in virulence for fungal pathogens of plants, animals, and humans, which also highlights the potential of these domains to act as novel therapeutic targets.

Sphingolipids: Regulators of azole drug resistance and fungal pathogenicity

In recent years, the role of sphingolipids in pathogenic fungi, in terms of pathogenicity and resistance to azole drugs, has been a rapidly growing field. This review describes evidence about the roles of sphingolipids in azole resistance and fungal virulence. Sphingolipids can serve as signaling molecules that contribute to azole resistance through modulation of the expression of drug efflux pumps. They also contribute to azole resistance by participating in various microbial pathways such as the unfolded protein response (UPR), pH-responsive Rim pathway, and pleiotropic drug resistance (PDR) pathway. In addition, sphingolipid signaling and eisosomes also coordinately regulate sphingolipid biosynthesis in response to azole-induced membrane stress. Sphingolipids are important for fungal virulence, playing roles during growth in hosts under stressful conditions, maintenance of cell wall integrity, biofilm formation, and production of various virulence factors. Finally, we discuss the possibility of exploiting fungal sphingolipids for the development of new therapeutic strategies to treat infections caused by pathogenic fungi.

Fungal plasma membrane domains

The plasma membrane (PM) performs a plethora of physiological processes, the coordination of which requires spatial and temporal organization into specialized domains of different sizes, stability, protein/lipid composition and overall architecture. Compartmentalization of the PM has been particularly well studied in the yeast Saccharomyces cerevisiae, where five non-overlapping domains have been described: The Membrane Compartments containing the arginine permease Can1 (MCC), the H+-ATPase Pma1 (MCP), the TORC2 kinase (MCT), the sterol transporters Ltc3/4 (MCL), and the cell wall stress mechanosensor Wsc1 (MCW). Additional cortical foci at the fungal PM are the sites where clathrin-dependent endocytosis occurs, the sites where the external pH sensing complex PAL/Rim localizes, and sterol-rich domains found in apically grown regions of fungal membranes. In this review, we summarize knowledge from several fungal species regarding the organization of the lateral PM segregation. We discuss the mechanisms of formation of these domains, and the mechanisms of partitioning of proteins there. Finally, we discuss the physiological roles of the best-known membrane compartments, including the regulation of membrane and cell wall homeostasis, apical growth of fungal cells and the newly emerging role of MCCs as starvation-protective membrane domains.

A Snf1-related nutrient-responsive kinase antagonizes endocytosis in yeast

Endocytosis is regulated in response to changing environmental conditions to adjust plasma membrane (PM) protein composition for optimal cell growth. Protein networks involved in cargo capture and sorting, membrane sculpting and deformation, and vesicle scission have been well-characterized, but less is known about the networks that sense extracellular cues and relay signals to trigger endocytosis of specific cargo. Hal4 and Hal5 are yeast Snf1-related kinases that were previously reported to regulate nutrient transporter stability by an unknown mechanism. Here we demonstrate that loss of Hal4 and Hal5 activates endocytosis of many different kinds of PM proteins, including Art1-mediated and Art1-independent endocytic events. Acute inhibition of Hal5 in the absence of Hal4 triggers rapid endocytosis, suggesting that Hal kinases function in a nutrient-sensing relay upstream of the endocytic response. Interestingly, Hal5 localizes to the PM, but shifts away from the cell surface in response to stimulation with specific nutrients. We propose that Hal5 functions as a nutrient-responsive regulator of PM protein stability, antagonizing endocytosis and promoting stability of endocytic cargos at the PM in nutrient-limiting conditions.

Cellular homeostasis, a fundamental requirement for all living organisms, is maintained in part through evolutionarily conserved mechanisms that regulate the abundance and activity of ion and nutrient transporters at the cell surface. These mechanisms often incorporate signaling networks that sense changes in the environment and relay signals to alter protein composition at the plasma membrane, often by inducing endocytosis of specific transporters in order to adjust and optimize transport activities at the cell surface. Here, we investigate two kinases in yeast–Hal4 and Hal5 –that are related to the yeast and human AMP sensing kinases. Loss of both Hal4 and Hal5 was previously reported to result in destabilization of ion and nutrient transporters by an unknown mechanism. Our data indicates that Hal kinases function broadly in the regulation of many different classes of endocytic cargo. Hal5 localizes to the plasma membrane in a manner that is responsive to nutrient availability and acute loss of Hal5 activity triggers rapid internalization of endocytic cargo. By uncovering a role for Hal5 as a nutrient-responsive regulator of endocytosis, this research sheds light on how signaling molecules regulate membrane trafficking events to coordinate adaptive growth responses.

An Abl-FBP17 mechanosensing system couples local plasma membrane curvature and stress fiber remodeling during mechanoadaptation

Cells remodel their structure in response to mechanical strain. However, how mechanical forces are translated into biochemical signals that coordinate the structural changes observed at the plasma membrane (PM) and the underlying cytoskeleton during mechanoadaptation is unclear. Here, we show that PM mechanoadaptation is controlled by a tension-sensing pathway composed of c-Abl tyrosine kinase and membrane curvature regulator FBP17. FBP17 is recruited to caveolae to induce the formation of caveolar rosettes. FBP17 deficient cells have reduced rosette density, lack PM tension buffering capacity under osmotic shock, and cannot adapt to mechanical strain. Mechanistically, tension is transduced to the FBP17 F-BAR domain by direct phosphorylation mediated by c-Abl, a mechanosensitive molecule. This modification inhibits FBP17 membrane bending activity and releases FBP17-controlled inhibition of mDia1-dependent stress fibers, favoring membrane adaptation to increased tension. This mechanoprotective mechanism adapts the cell to changes in mechanical tension by coupling PM and actin cytoskeleton remodeling.

Mechanical forces are sensed by cells and can alter plasma membrane properties, but biochemical changes underlying this are not clear. Here the authors show tension is sensed by c-Abl and FBP17, which couples changes in mechanical tension to remodelling of the plasma membrane and actin cytoskeleton.

Plasma membrane tension regulates eisosome structure and function

Eisosomes are membrane furrows at the cell surface of yeast that have been shown to function in two seemingly distinct pathways, membrane stress response and regulation of nutrient transporters. We found that many stress conditions affect both of these pathways by changing plasma membrane tension and thus the morphology and composition of eisosomes. For example, alkaline stress causes swelling of the cell and an endocytic response, which together increase membrane tension, thereby flattening the eisosomes. The flattened eisosomes affect membrane stress pathways and release nutrient transporters, which aids in their down-regulation. In contrast, glucose starvation or hyperosmotic shock causes cell shrinking, which results in membrane slack and the deepening of eisosomes. Deepened eisosomes are able to trap nutrient transporters and protect them from rapid endocytosis. Therefore, eisosomes seem to coordinate the regulation of both membrane tension and nutrient transporter stability.

Role of MCC/Eisosome in Fungal Lipid Homeostasis

One of the best characterized fungal membrane microdomains is the MCC/eisosome. The MCC (membrane compartment of Can1) is an evolutionarily conserved ergosterol-rich plasma membrane domain. It is stabilized on its cytosolic face by the eisosome, a hemitubular protein complex composed of Bin/Amphiphysin/Rvs (BAR) domain-containing Pil1 and Lsp1. These two proteins bind directly to phosphatidylinositol 4,5-bisphosphate and promote the typical furrow-like shape of the microdomain, with highly curved edges and bottom. While some proteins display stable localization in the MCC/eisosome, others enter or leave it under particular conditions, such as misbalance in membrane lipid composition, changes in membrane tension, or availability of specific nutrients. These findings reveal that the MCC/eisosome, a plasma membrane microdomain with distinct morphology and lipid composition, acts as a multifaceted regulator of various cellular processes including metabolic pathways, cellular morphogenesis, signalling cascades, and mRNA decay. In this minireview, we focus on the MCC/eisosome's proposed role in the regulation of lipid metabolism. While the molecular mechanisms of the MCC/eisosome function are not completely understood, the idea of intracellular processes being regulated at the plasma membrane, the foremost barrier exposed to environmental challenges, is truly exciting.

Eisosomes at the intersection of TORC1 and TORC2 regulation

Eisosomes are furrows in the yeast plasma membrane that form a membrane domain with distinct lipid and protein composition. Recent studies highlighted the importance of this domain for the regulation of proton-nutrient symporters. The amino acids and other nutrients, which these transporters deliver to the cytoplasm not only feed into metabolic pathways but also activate the metabolic regulator TORC1. Eisosomes have also been shown to harbor the membrane stress sensors Slm1 and Slm2. Membrane tension caused by hypoosmotic shock results in the redistribution of Slm1/2 from eisosomes to TORC2 which in turn regulates lipid synthesis. Therefore, eisosomes function upstream of both TORC1 and TORC2 regulation.

Analysis of the roles of phosphatidylinositol-4,5-bisphosphate and individual subunits in assembly, localization, and function of Saccharomyces cerevisiae target of rapamycin complex 2

Eukaryotic cell survival requires maintenance of plasma membrane (PM) homeostasis in response to environmental insults and changes in lipid metabolism. In yeast, a key regulator of PM homeostasis is target of rapamycin (TOR) complex 2 (TORC2), a multiprotein complex containing the evolutionarily conserved TOR protein kinase isoform Tor2. PM localization is essential for TORC2 function. One core TORC2 subunit (Avo1) and two TORC2-­associated regulators (Slm1 and Slm2) contain pleckstrin homology (PH) domains that exhibit specificity for binding phosphatidylinositol-4,5-bisphosphate (PtdIns4,5P2). To investigate the roles of PtdIns4,5P2 and constituent subunits of TORC2, we used auxin-inducible degradation to systematically eliminate these factors and then examined localization, association, and function of the remaining TORC2 components. We found that PtdIns4,5P2 depletion significantly reduced TORC2 activity, yet did not prevent PM localization or disassembly of TORC2. Moreover, truncated Avo1 (lacking its C-terminal PH domain) was still recruited to the PM and supported growth. Even when all three PH-containing proteins were absent, the remaining TORC2 subunits were PM-bound. Revealingly, Avo3 localized to the PM independent of both Avo1 and Tor2, whereas both Tor2 and Avo1 required Avo3 for their PM anchoring. Our findings provide new mechanistic information about TORC2 and pinpoint Avo3 as pivotal for TORC2 PM localization and assembly in vivo.

The Yin and Yang of Current Antifungal Therapeutic Strategies: How Can We Harness Our Natural Defenses?

Fungal infections have aroused much interest over the last years because of their involvement in several human diseases. Immunocompromission due to transplant-related therapies and malignant cancer treatments are risk factors for invasive fungal infections, but also aggressive surgery, broad-spectrum antibiotics and prosthetic devices are frequently associated with infectious diseases. Current therapy is based on the administration of antifungal drugs, but the occurrence of resistant strains to the most common molecules has become a serious health-care problem. New antifungal agents are urgently needed and it is essential to identify fungal molecular targets that could offer alternatives for development of treatments. The fungal cell wall and plasma membrane are the most important structures that offer putative new targets which can be modulated in order to fight microbial infections. The development of monoclonal antibodies against new targets is a valid therapeutic strategy, both to solve resistance problems and to support the immune response, especially in immunocompromised hosts. In this review, we summarize currently used antifungal agents and propose novel therapeutic approaches, including new fungal molecular targets to be considered for drug development.

Phosphorylation by the stress-activated MAPK Slt2 down-regulates the yeast TOR complex 2

Here, Leskoske et al. studied how TORC2 activity is modulated in response to changes in the status of the cell envelope. They demonstrate that TORC2 subunit Avo2 is a direct target of Slt2, the MAPK of the cell wall integrity pathway, and their findings provide new insights into TORC2 function and regulation.

Saccharomyces cerevisiae target of rapamycin (TOR) complex 2 (TORC2) is an essential regulator of plasma membrane lipid and protein homeostasis. How TORC2 activity is modulated in response to changes in the status of the cell envelope is unclear. Here we document that TORC2 subunit Avo2 is a direct target of Slt2, the mitogen-activated protein kinase (MAPK) of the cell wall integrity pathway. Activation of Slt2 by overexpression of a constitutively active allele of an upstream Slt2 activator (Pkc1) or by auxin-induced degradation of a negative Slt2 regulator (Sln1) caused hyperphosphorylation of Avo2 at its MAPK phosphoacceptor sites in a Slt2-dependent manner and diminished TORC2-mediated phosphorylation of its major downstream effector, protein kinase Ypk1. Deletion of Avo2 or expression of a phosphomimetic Avo2 allele rendered cells sensitive to two stresses (myriocin treatment and elevated exogenous acetic acid) that the cell requires Ypk1 activation by TORC2 to survive. Thus, Avo2 is necessary for optimal TORC2 activity, and Slt2-mediated phosphorylation of Avo2 down-regulates TORC2 signaling. Compared with wild-type Avo2, phosphomimetic Avo2 shows significant displacement from the plasma membrane, suggesting that Slt2 inhibits TORC2 by promoting Avo2 dissociation. Our findings are the first demonstration that TORC2 function is regulated by MAPK-mediated phosphorylation.

A humanized yeast-based toolkit for monitoring phosphatidylinositol 3-kinase activity at both single cell and population levels

Phosphatidylinositol 3-kinase (PI3K) is a key regulator of phosphoinositide-dependent signaling in mammalian cells and its dysfunction is related to multiple syndromes, including cancer. By heterologous expression in Saccharomyces cerevisiae, we have developed a humanized yeast system as a tool for functional studies on higher eukaryotic PI3K. Here we restrict PI3K activity in yeast to specific plasma membrane (PM) microdomains by fusing the p110α PI3K catalytic subunit to either a septin or an eisosome component. We engineered a Dual Reporter for PI3K (DRAPIK), useful to monitor activity on cellular membranes in vivo at a single-cell level, by simultaneous PM staining of the enzyme substrate (PtdIns4,5P₂) with GFP and its product (PtdIns3,4,5P₃) with mCherry. We also developed a sensitive FLUorescence by PI3K Inhibition (FLUPI) assay based on a GFP transcriptional reporter that is turned off by PI3K activity. This reporter system proved useful to monitor PI3K inhibition in vivo by active compounds. Such novel tools were used to study the performance of yeast PM microdomain-directed PI3K. Our results show that tethering heterologous PI3K to discrete PM domains potentiates its activity on PtdIns4,5P₂ but different locations display distinct effects on yeast growth and endocytosis.

Sphingolipids inhibit endosomal recycling of nutrient transporters by inactivating ARF6

Endogenous sphingolipids (ceramide) and related synthetic molecules (FTY720, SH-BC-893) reduce nutrient access by decreasing cell surface expression of a subset of nutrient transporter proteins. Here, we report that these sphingolipids disrupt endocytic recycling by inactivating the small GTPase ARF6. Consistent with reported roles for ARF6 in maintaining the tubular recycling endosome, MICAL-L1-positive tubules were lost from sphingolipid-treated cells. We propose that ARF6 inactivation may occur downstream of PP2A activation since: (1) sphingolipids that fail to activate PP2A did not reduce ARF6-GTP levels; (2) a structurally unrelated PP2A activator disrupted tubular recycling endosome morphology and transporter localization; and (3) overexpression of a phosphomimetic mutant of the ARF6 GEF GRP1 prevented nutrient transporter loss. ARF6 inhibition alone was not toxic; however, the ARF6 inhibitors SecinH3 and NAV2729 dramatically enhanced the killing of cancer cells by SH-BC-893 without increasing toxicity to peripheral blood mononuclear cells, suggesting that ARF6 inactivation contributes to the anti-neoplastic actions of sphingolipids. Taken together, these studies provide mechanistic insight into how ceramide and sphingolipid-like molecules limit nutrient access and suppress tumor cell growth and survival.

Heterologous mammalian Akt disrupts plasma membrane homeostasis by taking over TORC2 signaling in Saccharomyces cerevisiae

The Akt protein kinase is the main transducer of phosphatidylinositol-3,4,5-trisphosphate (PtdIns3,4,5P₃) signaling in higher eukaryotes, controlling cell growth, motility, proliferation and survival. By co-expression of mammalian class I phosphatidylinositol 3-kinase (PI3K) and Akt in the Saccharomyces cerevisiae heterologous model, we previously described an inhibitory effect on yeast growth that relied on Akt kinase activity. Here we report that PI3K-Akt expression in yeast triggers the formation of large plasma membrane (PM) invaginations that were marked by actin patches, enriched in PtdIns4,5P₂ and associated to abnormal intracellular cell wall deposits. These effects of Akt were mimicked by overproduction of the PtdIns4,5P₂ effector Slm1, an adaptor of the Ypk1 and Ypk2 kinases in the TORC2 pathway. Although Slm1 was phosphorylated in vivo by Akt, TORC2-dependent Ypk1 activation did not occur. However, PI3K-activated Akt suppressed the lethality derived from inactivation of either TORC2 or Ypk protein kinases. Thus, heterologous co-expression of PI3K and Akt in yeast short-circuits PtdIns4,5P₂- and TORC2-signaling at the level of the Slm-Ypk complex, overriding some of its functions. Our results underscore the importance of phosphoinositide-dependent kinases as key actors in the homeostasis and dynamics of the PM.

Conformation-dependent partitioning of yeast nutrient transporters into starvation-protective membrane domains

The eukaryotic plasma membrane is compartmentalized into domains enriched in specific lipids and proteins. However, our understanding of the molecular bases and biological roles of this partitioning remains incomplete. The best-studied domain in yeast is the membrane compartment containing the arginine permease Can1 (MCC) and later found to cluster additional transporters. MCCs correspond to static, furrow-like invaginations of the plasma membrane and associate with subcortical structures named "eisosomes" that include upstream regulators of the target of rapamycin complex 2 (TORC2) in the sensing of sphingolipids and membrane stress. However, how and why Can1 and other nutrient transporters preferentially segregate in MCCs remains unknown. In this study we report that the clustering of Can1 in MCCs is dictated by its conformation, requires proper sphingolipid biosynthesis, and controls its ubiquitin-dependent endocytosis. In the substrate-free outward-open conformation, Can1 accumulates in MCCs in a manner dependent on sustained biogenesis of complex sphingolipids. An arginine transport-elicited shift to an inward-facing conformation promotes its cell-surface dissipation and makes it accessible to the ubiquitylation machinery triggering its endocytosis. We further show that under starvation conditions MCCs increase in number and size, this being dependent on the BAR domain-containing Lsp1 eisosome component. This expansion of MCCs provides protection for nutrient transporters from bulk endocytosis occurring in parallel with autophagy upon TORC1 inhibition. Our study reveals nutrient-regulated protection from endocytosis as an important role for protein partitioning into membrane domains.

Steric exclusion and protein conformation determine the localization of plasma membrane transporters

The plasma membrane (PM) of Saccharomyces cerevisiae contains membrane compartments, MCC/eisosomes and MCPs, named after the protein residents Can1 and Pma1, respectively. Using high-resolution fluorescence microscopy techniques we show that Can1 and the homologous transporter Lyp1 are able to diffuse into the MCC/eisosomes, where a limited number of proteins are conditionally trapped at the (outer) edge of the compartment. Upon addition of substrate, the immobilized proteins diffuse away from the MCC/eisosomes, presumably after taking a different conformation in the substrate-bound state. Our data indicate that the mobile fraction of all integral plasma membrane proteins tested shows extremely slow Brownian diffusion through most of the PM. We also show that proteins with large cytoplasmic domains, such as Pma1 and synthetic chimera of Can1 and Lyp1, are excluded from the MCC/eisosomes. We hypothesize that the distinct localization patterns found for these integral membrane proteins in S. cerevisiae arises from a combination of slow lateral diffusion, steric exclusion, and conditional trapping in membrane compartments.

MCC/Eisosomes Regulate Cell Wall Synthesis and Stress Responses in Fungi

The fungal plasma membrane is critical for cell wall synthesis and other important processes including nutrient uptake, secretion, endocytosis, morphogenesis, and response to stress. To coordinate these diverse functions, the plasma membrane is organized into specialized compartments that vary in size, stability, and composition. One recently identified domain known as the Membrane Compartment of Can1 (MCC)/eisosome is distinctive in that it corresponds to a furrow-like invagination in the plasma membrane. MCC/eisosomes have been shown to be formed by the Bin/Amphiphysin/Rvs (BAR) domain proteins Lsp1 and Pil1 in a range of fungi. MCC/eisosome domains influence multiple cellular functions; but a very pronounced defect in cell wall synthesis has been observed for mutants with defects in MCC/eisosomes in some yeast species. For example, Candida albicans MCC/eisosome mutants display abnormal spatial regulation of cell wall synthesis, including large invaginations and altered chemical composition of the walls. Recent studies indicate that MCC/eisosomes affect cell wall synthesis in part by regulating the levels of the key regulatory lipid phosphatidylinositol 4,5-bisphosphate (PI_4,5P₂) in the plasma membrane. One general way MCC/eisosomes function is by acting as protected islands in the plasma membrane, since these domains are very stable. They also act as scaffolds to recruit >20 proteins. Genetic studies aimed at defining the function of the MCC/eisosome proteins have identified important roles in resistance to stress, such as resistance to oxidative stress mediated by the flavodoxin-like proteins Pst1, Pst2, Pst3 and Ycp4. Thus, MCC/eisosomes play multiple roles in plasma membrane organization that protect fungal cells from the environment.

The TORC2-Dependent Signaling Network in the Yeast Saccharomyces cerevisiae

To grow, eukaryotic cells must expand by inserting glycerolipids, sphingolipids, sterols, and proteins into their plasma membrane, and maintain the proper levels and bilayer distribution. A fungal cell must coordinate growth with enlargement of its cell wall. In Saccharomyces cerevisiae, a plasma membrane-localized protein kinase complex, Target of Rapamicin (TOR) complex-2 (TORC2) (mammalian ortholog is mTORC2), serves as a sensor and master regulator of these plasma membrane- and cell wall-associated events by directly phosphorylating and thereby stimulating the activity of two types of effector protein kinases: Ypk1 (mammalian ortholog is SGK1), along with a paralog (Ypk2); and, Pkc1 (mammalian ortholog is PKN2/PRK2). Ypk1 is a central regulator of pathways and processes required for plasma membrane lipid and protein homeostasis, and requires phosphorylation on its T-loop by eisosome-associated protein kinase Pkh1 (mammalian ortholog is PDK1) and a paralog (Pkh2). For cell survival under various stresses, Ypk1 function requires TORC2-mediated phosphorylation at multiple sites near its C terminus. Pkc1 controls diverse processes, especially cell wall synthesis and integrity. Pkc1 is also regulated by Pkh1- and TORC2-dependent phosphorylation, but, in addition, by interaction with Rho1-GTP and lipids phosphatidylserine (PtdSer) and diacylglycerol (DAG). We also describe here what is currently known about the downstream substrates modulated by Ypk1-mediated and Pkc1-mediated phosphorylation.

Single-molecule imaging of the BAR-domain protein Pil1p reveals filament-end dynamics

Molecular assemblies can have highly heterogeneous dynamics within the cell, but the limitations of conventional fluorescence microscopy can mask nanometer-scale features. Here we adapt a single-molecule strategy to perform single-molecule recovery after photobleaching (SRAP) within dense macromolecular assemblies to reveal and characterize binding and unbinding dynamics within such assemblies. We applied this method to study the eisosome, a stable assembly of BAR-domain proteins on the cytoplasmic face of the plasma membrane in fungi. By fluorescently labeling only a small fraction of cellular Pil1p, the main eisosome BAR-domain protein in fission yeast, we visualized whole eisosomes and, after photobleaching, localized recruitment of new Pil1p molecules with ∼30-nm precision. Comparing our data to computer simulations, we show that Pil1p exchange occurs specifically at eisosome ends and not along their core, supporting a new model of the eisosome as a dynamic filament. This result is the first direct observation of any BAR-domain protein dynamics in vivo under physiological conditions consistent with the oligomeric filaments reported from in vitro experiments.

TORC2 and eisosomes are spatially interdependent, requiring optimal level of phosphatidylinositol 4, 5-bisphosphate for their integrity

The elucidation of the organization and maintenance of the plasma membrane has been sought due to its numerous roles in cellular function. In the budding yeast Saccharomyces cerevisiae, a novel paradigm has begun to emerge in the understanding of the distribution of plasma membrane microdomains and how they are regulated. We aimed to investigate the dynamic interdependence between the protein complexes eisosome and TORC2, representing microdomains MCC and MCT, respectively. In this study, we reveal that the eisosome organizer Pil1 colocalizes with the MCT marker Avo2. Furthermore, we provide evidence that the formation of MCT is dependent on both eisosome integrity and adequate levels of the plasma membrane phosphoinositide PI(4,5)P2. Taken together, our findings indicate that TORC2, eisosomes, and PI(4,5)P2 exist in an interconnected relationship, which supports the emerging model of the plasma membrane.

Sng1 associates with Nce102 to regulate the yeast Pkh-Ypk signalling module in response to sphingolipid status

All cells are delimited by biological membranes, which are consequently a primary target of stress-induced damage. Cold alters membrane functionality by decreasing lipid fluidity and the activity of membrane proteins. In Saccharomyces cerevisiae, evidence links sphingolipid homeostasis and membrane phospholipid asymmetry to the activity of the Ypk1/2 proteins, the yeast orthologous of the mammalian SGK1-3 kinases. Their regulation is mediated by different protein kinases, including the PDK1 orthologous Pkh1/2p, and requires the function of protein effectors, among them Nce102p, a component of the sphingolipid sensor machinery. Nevertheless, the mechanisms and the actors involved in Pkh/Ypk regulation remain poorly defined. Here, we demonstrate that Sng1, a transmembrane protein, is an effector of the Pkh/Ypk module and identify the phospholipid asymmetry as key for yeast cold adaptation. Overexpression of SNG1 impairs phospholipid flipping, reduces reactive oxygen species (ROS) and improves, in a Pkh-dependent manner, yeast growth in myriocin-treated cells, suggesting that excess Sng1p stimulates the Pkh/Ypk signalling. Furthermore, we link these effects to the association of Sng1p with Nce102p. Indeed, we found that Sng1p interacts with Nce102p both physically and genetically. Moreover, mutant nce102∆ sng1∆ cells show features of impaired Pkh/Ypk signalling, including increased ROS accumulation, reduced life span and defects in Pkh/Ypk-controlled regulatory pathways. Finally, myriocin-induced hyperphosphorylation of Ypk1p and Orm2p, which controls sphingolipid homeostasis, does not occur in nce102∆ sng1∆ cells. Hence, both Nce102p and Sng1p participate in a regulatory circuit that controls the activity of the Pkh/Ypk module and their function is required in response to sphingolipid status.

Eisosomes promote the ability of Sur7 to regulate plasma membrane organization in Candida albicans

The plasma membrane of the fungal pathogen Candida albicans forms a protective barrier that also mediates many processes needed for virulence, including cell wall synthesis, invasive hyphal morphogenesis, and nutrient uptake. Because compartmentalization of the plasma membrane is believed to coordinate these diverse activities, we examined plasma membrane microdomains termed eisosomes or membrane compartment of Can1 (MCC), which correspond to ∼200-nm-long furrows in the plasma membrane. A pil1∆ lsp1∆ mutant failed to form eisosomes and displayed strong defects in plasma membrane organization and morphogenesis, including extensive cell wall invaginations. Mutation of eisosome proteins Slm2, Pkh2, and Pkh3 did not cause similar cell wall defects, although pkh2∆ cells formed chains of furrows and pkh3∆ cells formed wider furrows, identifying novel roles for the Pkh protein kinases in regulating furrows. In contrast, the sur7∆ mutant formed cell wall invaginations similar to those for the pil1∆ lsp1∆ mutant even though it could form eisosomes and furrows. A PH-domain probe revealed that the regulatory lipid phosphatidylinositol 4,5-bisphosphate was enriched at sites of cell wall invaginations in both the sur7∆ and pil1∆ lsp1∆ cells, indicating that this contributes to the defects. The sur7∆ and pil1∆ lsp1∆ mutants displayed differential susceptibility to various types of stress, indicating that they affect overlapping but distinct functions. In support of this, many mutant phenotypes of the pil1∆ lsp1∆ cells were rescued by overexpressing SUR7 These results demonstrate that C. albicans eisosomes promote the ability of Sur7 to regulate plasma membrane organization.

New Insight Into the Roles of Membrane Microdomains in Physiological Activities of Fungal Cells

The organization of biological membranes into structurally and functionally distinct lateral microdomains is generally accepted. From bacteria to mammals, laterally compartmentalized membranes seem to be a vital attribute of life. The crucial fraction of our current knowledge about the membrane microdomains has been gained from studies on fungi. In this review we summarize the evidence of the microdomain organization of membranes from fungal cells, with accent on their enormous diversity in composition, temporal dynamics, modes of formation, and recognized engagement in the cell physiology. A special emphasis is laid on the fact that in addition to their other biological functions, membrane microdomains also mediate the communication among different membranes within a eukaryotic cell and coordinate their functions. Involvement of fungal membrane microdomains in stress sensing, regulation of lipid homeostasis, and cell differentiation is discussed more in detail.

Plasma membrane organization promotes virulence of the human fungal pathogen Candida albicans

Candida albicans is a human fungal pathogen capable of causing lethal systemic infections. The plasma membrane plays key roles in virulence because it not only functions as a protective barrier, it also mediates dynamic functions including secretion of virulence factors, cell wall synthesis, invasive hyphal morphogenesis, endocytosis, and nutrient uptake. Consistent with this functional complexity, the plasma membrane is composed of a wide array of lipids and proteins. These components are organized into distinct domains that will be the topic of this review. Some of the plasma membrane domains that will be described are known to act as scaffolds or barriers to diffusion, such as MCC/eisosomes, septins, and sites of contact with the endoplasmic reticulum. Other zones mediate dynamic processes, including secretion, endocytosis, and a special region at hyphal tips that facilitates rapid growth. The highly organized architecture of the plasma membrane facilitates the coordination of diverse functions and promotes the pathogenesis of C. albicans.

Eisosomes are dynamic plasma membrane domains showing pil1-lsp1 heteroligomer binding equilibrium

Eisosomes are plasma membrane domains concentrating lipids, transporters, and signaling molecules. In the budding yeast Saccharomyces cerevisiae, these domains are structured by scaffolds composed mainly by two cytoplasmic proteins Pil1 and Lsp1. Eisosomes are immobile domains, have relatively uniform size, and encompass thousands of units of the core proteins Pil1 and Lsp1. In this work we used fluorescence fluctuation analytical methods to determine the dynamics of eisosome core proteins at different subcellular locations. Using a combination of scanning techniques with autocorrelation analysis, we show that Pil1 and Lsp1 cytoplasmic pools freely diffuse whereas an eisosome-associated fraction of these proteins exhibits slow dynamics that fit with a binding-unbinding equilibrium. Number and brightness analysis shows that the eisosome-associated fraction is oligomeric, while cytoplasmic pools have lower aggregation states. Fluorescence lifetime imaging results indicate that Pil1 and Lsp1 directly interact in the cytoplasm and within the eisosomes. These results support a model where Pil1-Lsp1 heterodimers are the minimal eisosomes building blocks. Moreover, individual-eisosome fluorescence fluctuation analysis shows that eisosomes in the same cell are not equal domains: while roughly half of them are mostly static, the other half is actively exchanging core protein subunits.

Form, Fabric, and Function of a Flagellum-Associated Cytoskeletal Structure

Trypanosoma brucei is a uniflagellated protist and the causative agent of African trypanosomiasis, a neglected tropical disease. The single flagellum of T. brucei is essential to a number of cellular processes such as motility, and has been a longstanding focus of scientific enquiry. A number of cytoskeletal structures are associated with the flagellum in T. brucei, and one such structure—a multiprotein complex containing the repeat motif protein TbMORN1—is the focus of this review. The TbMORN1-containing complex, which was discovered less than ten years ago, is essential for the viability of the mammalian-infective form of T. brucei. The complex has an unusual asymmetric morphology, and is coiled around the flagellum to form a hook shape. Proteomic analysis using the proximity-dependent biotin identification (BioID) technique has elucidated a number of its components. Recent work has uncovered a role for TbMORN1 in facilitating protein entry into the cell, thus providing a link between the cytoskeleton and the endomembrane system. This review summarises the extant data on the complex, highlights the outstanding questions for future enquiry, and provides speculation as to its possible role in a size-exclusion mechanism for regulating protein entry. The review additionally clarifies the nomenclature associated with this topic, and proposes the adoption of the term “hook complex” to replace the former name “bilobe” to describe the complex.

Eisosomes provide membrane reservoirs for rapid expansion of the yeast plasma membrane

Cell surface area rapidly increases during mechanical and hypoosmotic stresses. Such expansion of the plasma membrane requires 'membrane reservoirs' that provide surface area and buffer membrane tension, but the sources of this membrane remain poorly understood. In principle, the flattening of invaginations and buds within the plasma membrane could provide this additional surface area, as recently shown for caveolae in animal cells. Here, we used microfluidics to study the rapid expansion of the yeast plasma membrane in protoplasts, which lack the rigid cell wall. To survive hypoosmotic stress, yeast cell protoplasts required eisosomes, protein-based structures that generate long invaginations at the plasma membrane. Both budding yeast and fission yeast protoplasts lacking eisosomes were unable to expand like wild-type protoplasts during hypoosmotic stress, and subsequently lysed. By performing quantitative fluorescence microscopy on single protoplasts, we also found that eisosomes disassembled as surface area increased. During this process, invaginations generated by eisosomes at the plasma membrane became flattened, as visualized by scanning electron microscopy. We propose that eisosomes serve as tension-dependent membrane reservoirs for expansion of yeast cells in an analogous manner to caveolae in animal cells.

Eisosomes Regulate Phosphatidylinositol 4,5-Bisphosphate (PI(4,5)P2) Cortical Clusters and Mitogen-activated Protein (MAP) Kinase Signaling upon Osmotic Stress

Eisosomes are multiprotein structures that generate linear invaginations at the plasma membrane of yeast cells. The core component of eisosomes, the BAR domain protein Pil1, generates these invaginations through direct binding to lipids including phosphoinositides. Eisosomes promote hydrolysis of phosphatidylinositol 4,5 bisphosphate (PI(4,5)P2) by functioning with synaptojanin, but the cellular processes regulated by this pathway have been unknown. Here, we found that PI(4,5)P2 regulation by eisosomes inhibits the cell integrity pathway, a conserved MAPK signal transduction cascade. This pathway is activated by multiple environmental conditions including osmotic stress in the fission yeast Schizosaccharomyces pombe. Activation of the MAPK Pmk1 was impaired by mutations in the phosphatidylinositol (PI) 5-kinase Its3, but this defect was suppressed by removal of eisosomes. Using fluorescent biosensors, we found that osmotic stress induced the formation of PI(4,5)P2 clusters that were spatially organized by eisosomes in both fission yeast and budding yeast cells. These cortical clusters contained the PI 5-kinase Its3 and did not assemble in the its3-1 mutant. The GTPase Rho2, an upstream activator of Pmk1, also co-localized with PI(4,5)P2 clusters under osmotic stress, providing a molecular link between these novel clusters and MAPK activation. Our findings have revealed that eisosomes regulate activation of MAPK signal transduction through the organization of cortical lipid-based microdomains.

Yeast Pmp3p has an important role in plasma membrane organization

Pmp3p-related proteins are highly conserved proteins that exist in bacteria, yeast, nematodes and plants, and its transcript is regulated in response to abiotic stresses, such as low temperature or high salinity. Pmp3p was originally identified in Saccharomyces cerevisiae, and it belongs to the sensitive to Na(+) (SNA)-protein family, which comprises four members--Pmp3p/Sna1p, Sna2p, Sna3p and Sna4p. Deletion of the PMP3 gene conferred sensitivity to cytotoxic cations, whereas removal of the other SNA genes did not lead to clear phenotypic effects. It has long been believed that Pmp3p-related proteins have a common and important role in the modulation of plasma membrane potential and in the regulation of intracellular ion homeostasis. Here, we show that several growth phenotypes linked to PMP3 deletion can be modulated by the removal of specific genes involved in sphingolipid synthesis. These genetic interactions, together with lipid binding assays and epifluorescence microscopy, as well as other biochemical experiments, suggest that Pmp3p could be part of a phosphoinositide-regulated stress sensor.

Protein-induced membrane curvature alters local membrane tension

Adsorption of proteins onto membranes can alter the local membrane curvature. This phenomenon has been observed in biological processes such as endocytosis, tubulation, and vesiculation. However, it is not clear how the local surface properties of the membrane, such as membrane tension, change in response to protein adsorption. In this article, we show that the partial differential equations arising from classical elastic model of lipid membranes, which account for simultaneous changes in shape and membrane tension due to protein adsorption in a local region, cannot be solved for nonaxisymmetric geometries using straightforward numerical techniques; instead, a viscous-elastic formulation is necessary to fully describe the system. Therefore, we develop a viscous-elastic model for inhomogeneous membranes of the Helfrich type. Using the newly available viscous-elastic model, we find that the lipids flow to accommodate changes in membrane curvature during protein adsorption. We show that, at the end of protein adsorption process, the system sustains a residual local tension to balance the difference between the actual mean curvature and the imposed spontaneous curvature. We also show that this change in membrane tension can have a functional impact such as altered response to pulling forces in the presence of proteins.

Eisosome Ultrastructure and Evolution in Fungi, Microalgae, and Lichens

Eisosomes are among the few remaining eukaryotic cellular differentations that lack a defined function(s). These trough-shaped invaginations of the plasma membrane have largely been studied in Saccharomyces cerevisiae, in which their associated proteins, including two BAR domain proteins, have been identified, and homologues have been found throughout the fungal radiation. Using quick-freeze deep-etch electron microscopy to generate high-resolution replicas of membrane fracture faces without the use of chemical fixation, we report that eisosomes are also present in a subset of red and green microalgae as well as in the cysts of the ciliate Euplotes. Eisosome assembly is closely correlated with both the presence and the nature of cell walls. Microalgal eisosomes vary extensively in topology and internal organization. Unlike fungi, their convex fracture faces can carry lineage-specific arrays of intramembranous particles, and their concave fracture faces usually display fine striations, also seen in fungi, that are pitched at lineage-specific angles and, in some cases, adopt a broad-banded patterning. The conserved genes that encode fungal eisosome-associated proteins are not found in sequenced algal genomes, but we identified genes encoding two algal lineage-specific families of predicted BAR domain proteins, called Green-BAR and Red-BAR, that are candidate eisosome organizers. We propose a model for eisosome formation wherein (i) positively charged recognition patches first establish contact with target membrane regions and (ii) a (partial) unwinding of the coiled-coil conformation of the BAR domains then allows interactions between the hydrophobic faces of their amphipathic helices and the lipid phase of the inner membrane leaflet, generating the striated patterns.

Effects of FSGS-associated mutations on the stability and function of myosin-1 in fission yeast

Point mutations in the human MYO1E gene, encoding class I myosin Myo1e, are associated with focal segmental glomerulosclerosis (FSGS), a primary kidney disorder that leads to end-stage kidney disease. In this study, we used a simple model organism, fission yeast Schizosaccharomyces pombe, to test the effects of FSGS-associated mutations on myosin activity. Fission yeast has only one class I myosin, Myo1, which is involved in actin patch assembly at the sites of endocytosis. The amino acid residues mutated in individuals with FSGS are conserved between human Myo1e and yeast Myo1, which allowed us to introduce equivalent mutations into yeast myosin and use the resulting mutant strains for functional analysis. Yeast strains expressing mutant Myo1 exhibited defects in growth and endocytosis similar to those observed in the myo1 deletion strain. These mutations also disrupted Myo1 localization to endocytic actin patches and resulted in mis-localization of Myo1 to eisosomes, linear membrane microdomains found in yeast cells. Although both mutants examined in this study exhibited loss of function, one of these mutants was also characterized by the decreased protein stability. Thus, using the yeast model system, we were able to determine that the kidney-disease-associated mutations impair myosin functional activity and have differential effects on protein stability.

Summary: In the fission yeast S. pombe, kidney disease-associated mutations in Myo1, a homolog of human Myo1e, disrupt myosin localization and function.

Ltc1 is an ER-localized sterol transporter and a component of ER-mitochondria and ER-vacuole contacts

Organelle contact sites perform fundamental functions in cells, including lipid and ion homeostasis, membrane dynamics, and signaling. Using a forward proteomics approach in yeast, we identified new ER-mitochondria and ER-vacuole contacts specified by an uncharacterized protein, Ylr072w. Ylr072w is a conserved protein with GRAM and VASt domains that selectively transports sterols and is thus termed Ltc1, for Lipid transfer at contact site 1. Ltc1 localized to ER-mitochondria and ER-vacuole contacts via the mitochondrial import receptors Tom70/71 and the vacuolar protein Vac8, respectively. At mitochondria, Ltc1 was required for cell viability in the absence of Mdm34, a subunit of the ER-mitochondria encounter structure. At vacuoles, Ltc1 was required for sterol-enriched membrane domain formation in response to stress. Increasing the proportion of Ltc1 at vacuoles was sufficient to induce sterol-enriched vacuolar domains without stress. Thus, our data support a model in which Ltc1 is a sterol-dependent regulator of organelle and cellular homeostasis via its dual localization to ER-mitochondria and ER-vacuole contact sites.

Evolutionarily conserved 5'-3' exoribonuclease Xrn1 accumulates at plasma membrane-associated eisosomes in post-diauxic yeast

Regulation of gene expression on the level of translation and mRNA turnover is widely conserved evolutionarily. We have found that the main mRNA decay enzyme, exoribonuclease Xrn1, accumulates at the plasma membrane-associated eisosomes after glucose exhaustion in a culture of the yeast S. cerevisiae. Eisosomal localization of Xrn1 is not achieved in cells lacking the main component of eisosomes, Pil1, or Sur7, the protein accumulating at the membrane compartment of Can1 (MCC) - the eisosome-organized plasma membrane microdomain. In contrast to the conditions of diauxic shift, when Xrn1 accumulates in processing bodies (P-bodies), or acute heat stress, in which these cytosolic accumulations of Xrn1 associate with eIF3a/Rpg1-containing stress granules, Xrn1 is not accompanied by other mRNA-decay machinery components when it accumulates at eisosomes in post-diauxic cells. It is important that Xrn1 is released from eisosomes after addition of fermentable substrate. We suggest that this spatial segregation of Xrn1 from the rest of the mRNA-decay machinery reflects a general regulatory mechanism, in which the key enzyme is kept separate from the rest of mRNA decay factors in resting cells but ready for immediate use when fermentable nutrients emerge and appropriate metabolism reprogramming is required. In particular, the localization of Xrn1 to the eisosome, together with previously published data, accents the relevance of this plasma membrane-associated compartment as a multipotent regulatory site.

Pil1 cytoplasmic rods contain bundles of crosslinked tubules

Cytoskeletal polymers are organized into a wide variety of higher-order structures in cells. The yeast BAR domain protein Pil1 self-assembles into tubules in vitro, and forms linear polymers at cortical eisosomes in cells. In the fission yeast S. pombe, over-expressed Pil1 forms thick rods that detach from the plasma membrane. In this study, we used thin-section electron microscopy to determine the ultrastructure of these cytoplasmic Pil1 rods. We found that cytoplasmic rods contained crosslinked Pil1 tubules that displayed regular, hexagonal spacing. These bundles were stained by filipin, a sterol-binding fluorescent dye, suggesting that they contained lipids. Cytoplasmic Pil1 rods were present but less abundant in sle1Δ and fhn1Δ mutant cells. We also found that endogenous Pil1 formed thick rods under saturated growth conditions. Taken together, our findings suggest the presence of cellular mechanisms that assemble Pil1 tubules into higher-order structures.