Endocytosis in filamentous Fungi: Coordinating polarized hyphal growth and membrane recycling

Filamentous fungi rely on a finely tuned balance between exocytosis and endocytosis to maintain polarized growth. This review highlights the essential role of the subapical endocytic collar in recycling excess plasma membrane and key proteins, enabling sustained hyphal extension. It distinguishes between clathrin-mediated and AP-2-dependent clathrin-independent pathways, emphasizing their unique contributions to membrane homeostasis and cargo trafficking. The synthesis of quantitative imaging and genetic analyses provides a comprehensive framework for understanding vesicle dynamics, with implications for addressing fungal pathogenicity and industrial applications.

The phosphatase Glc7 controls the eisosomal response to starvation via post-translational modification of Pil1

The yeast (Saccharomyces cerevisiae) plasma membrane (PM) is organised into specific subdomains that regulate surface membrane proteins. Surface transporters actively uptake nutrients in particular regions of the PM where they are also susceptible to substrate-induced endocytosis. However, transporters also diffuse into distinct subdomains termed eisosomes, where they are protected from endocytosis. Although most nutrient transporter populations are downregulated in the vacuole following glucose starvation, a small pool is retained in eisosomes to provide efficient recovery from starvation. We find the core eisosome subunit Pil1, a Bin, Amphiphysin and Rvs (BAR) domain protein required for eisosome biogenesis, is phosphorylated primarily by the kinase Pkh2. In response to acute glucose starvation, Pil1 is rapidly dephosphorylated. Enzyme localisation and activity screens suggest that the phosphatase Glc7 is the primary enzyme responsible for Pil1 dephosphorylation. Defects in Pil1 phosphorylation, achieved by depletion of GLC7 or expression of phospho-ablative or phospho-mimetic mutants, correlate with reduced retention of transporters in eisosomes and inefficient starvation recovery. We propose that precise post-translational control of Pil1 modulates nutrient transporter retention within eisosomes, depending on extracellular nutrient levels, to maximise recovery following starvation.

Using fimbrin to quantify the endocytic subapical collar during polarized growth in three filamentous fungi

Filamentous fungi produce specialized cells called hyphae. These cells grow by polarized extension at their apex, which is maintained by the balance of endocytosis and exocytosis at the apex. Although endocytosis has been well characterized in other organisms, the details of endocytosis and its role in maintaining polarity during hyphal growth in filamentous fungi is comparatively sparsely studied. In recent years, a concentrated region of protein activity that trails the growing apex of hyphal cells has been discovered. This region, dubbed the "endocytic collar" (EC), is a dynamic 3-dimensional region of concentrated endocytic activity, the disruption of which results in the loss of hyphal polarity. Here, fluorescent protein-tagged fimbrin was used as a marker to map the collar during growth of hyphae in three fungi: Aspergillus nidulans, Colletotrichum graminicola, and Neurospora crassa. Advanced microscopy techniques and novel quantification strategies were then utilized to quantify the spatiotemporal localization and recovery rates of fimbrin in the EC during hyphal growth. Correlating these variables with hyphal growth rate revealed that the strongest observed relationship with hyphal growth is the distance by which the EC trails the apex, and that measured endocytic rate does not correlate strongly with hyphal growth rate. This supports the hypothesis that endocytic influence on hyphal growth rate is better explained by spatiotemporal regulation of the EC than by the raw rate of endocytosis.

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.

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.

A virulence-related lectin traffics into eisosome and contributes to functionality of cytomembrane and cell-wall in the insect-pathogenic fungus Beauveria bassiana

Lectins are characterized of the carbohydrate-binding ability and play comprehensive roles in fungal physiology (e.g., defense response, development and host-pathogen interaction). Beauveria bassiana, a filamentous entomopathogenic fungus, has a lectin-like protein containing a Fruit Body_domain (BbLec1). BbLec1 could bind to chitobiose and chitin in fungal cell wall. BbLec1 proteins interacted with each other to form multimers, and translocated into eisosomes. Further, the interdependence between BbLec1 and the eisosome protein PliA was essential for stabilizing the eisosome architecture. To test the BbLec1 roles in B. bassiana, we constructed the gene disruption and complementation mutants. Notably, the BbLec1 loss resulted in the impaired cell wall in mycelia and conidia as well as conidial formation capacity. In addition, disruption of BbLec1 led to the reduced cytomembrane integrity and the enhanced sensitivity to osmotic stress. Finally, ΔBbLec1 mutant strain displayed the weakened virulence when compared with the wild-type strain. Taken together, BbLec1 traffics into eisosome and links the functionality of eisosome to development and virulence of B. bassiana.

The cytoskeleton influences the formation and distribution of eisosomes in Neurospora crassa

Eisosomes are stable protein complexes at the plasma membrane, with punctate distributional patterns. Their formation and how their locations are determined remain unclear. The current study discovered that the formation and distribution of eisosomes are influenced by the cytoskeleton. Disassembly of either the F-actin or the microtubules leads to eisosome localization at hyphal tips of germinated macroconidia in Neurospora crassa, and treatment with a high concentration of the microtubule-inhibitor benomyl results in the production of filamentous eisosome patterns. The defect in the cytoskeleton caused by the disassembly of microtubules or F-actin leads to an increased formation of eisosomes.

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.

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.

Role of membrane compartment occupied by Can1 (MCC) and eisosome subdomains in plant pathogenicity of the necrotrophic fungus Alternaria brassicicola

BACKGROUND

MCC/eisosomes are membrane microdomains that have been proposed to participate in the plasma membrane function in particular by regulating the homeostasis of lipids, promoting the recruitment of specific proteins and acting as provider of membrane reservoirs.

RESULTS

Here we showed that several potential MCC/eisosomal protein encoding genes in the necrotrophic fungus A. brassicicola were overexpressed when germinated spores were exposed to antimicrobial defence compounds, osmotic and hydric stresses, which are major constraints encountered by the fungus during the plant colonization process. Mutants deficient for key MCC/eisosome components did not exhibit any enhanced susceptibility to phytoalexins and to applied stress conditions compared to the reference strain, except for a slight hypersensitivity of the ∆∆abpil1a-abpil1b strain to 2 M sorbitol. Depending on the considered mutants, we showed that the leaf and silique colonization processes were impaired by comparison to the wild-type, and assumed that these defects in aggressiveness were probably caused by a reduced appressorium formation rate.

CONCLUSIONS

This is the first study on the role of MCC/eisosomes in the pathogenic process of a plant pathogenic fungus. A link between these membrane domains and the fungus ability to form functional penetration structures was shown, providing new potential directions for plant disease control strategies.

Correlative Microscopy of Vitreous Sections Provides Insights into BAR-Domain Organization In Situ

Electron microscopy imaging of macromolecular complexes in their native cellular context is limited by the inherent difficulty to acquire high-resolution tomographic data from thick cells and to specifically identify elusive structures within crowded cellular environments. Here, we combined cryo-fluorescence microscopy with electron cryo-tomography of vitreous sections into a coherent correlative microscopy workflow, ideal for detection and structural analysis of elusive protein assemblies in situ. We used this workflow to address an open question on BAR-domain coating of yeast plasma membrane compartments known as eisosomes. BAR domains can sense or induce membrane curvature, and form scaffold-like membrane coats in vitro. Our results demonstrate that in cells, the BAR protein Pil1 localizes to eisosomes of varying membrane curvature. Sub-tomogram analysis revealed a dense protein coat on curved eisosomes, which was not present on shallow eisosomes, indicating that while BAR domains can assemble at shallow membranes in vivo, scaffold formation is tightly coupled to curvature generation.

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Cryo-fluorescence microscopy eases electron cryo-tomography of vitreous sections

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Elusive protein assemblies are localized in situ by correlative microscopy

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Yeast BAR-domain protein Pil1 binds to plasma membrane with varying curvature

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Scaffold-like coats are only seen when Pil1 is bound to high curvature membranes

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.

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.

Transmembrane voltage: Potential to induce lateral microdomains

Lateral segregation of plasma membrane lipids is a generally accepted phenomenon. Lateral lipid microdomains of specific composition, structure and biological functions are established as a result of simultaneous action of several competing mechanisms which contribute to membrane organization. Various lines of evidence support the conclusion that among those mechanisms, the membrane potential plays significant and to some extent unique role. Above all, clear differences in the microdomain structure as revealed by fluorescence microscopy could be recognized between polarized and depolarized membranes. In addition, recent fluorescence spectroscopy experiments reported depolarization-induced changes in a membrane lipid order. In the context of earlier findings showing that plasma membranes of depolarized cells are less susceptible to detergents and the cells less sensitive to antibiotics or antimycotics treatment we discuss a model, in which membrane potential-driven re-organization of the microdomain structure contributes to maintaining membrane integrity during response to stress, pathogen attack and other challenges involving partial depolarization of the plasma membrane. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Characterization of AnNce102 and its role in eisosome stability and sphingolipid biosynthesis

The plasma membrane is implicated in a variety of functions, whose coordination necessitates highly dynamic organization of its constituents into domains of distinct protein and lipid composition. Eisosomes, at least partially, mediate this lateral plasma membrane compartmentalization. In this work, we show that the Nce102 homologue of Aspergillus nidulans colocalizes with eisosomes and plays a crucial role in density/number of PilA/SurG foci in the head of germlings. In addition we demonstrate that AnNce102 and PilA negatively regulate sphingolipid biosynthesis, since their deletions partially suppress the thermosensitivity of basA mutant encoding sphingolipid C4-hydroxylase and the growth defects observed upon treatment with inhibitors of sphingolipid biosynthesis, myriocin and Aureobasidin A. Moreover, we show that YpkA repression mimics genetic or pharmacological depletion of sphingolipids, conditions that induce the production of Reactive Oxygen Species (ROS), and can be partially overcome by deletion of pilA and/or annce102 at high temperatures. Consistent with these findings, pilAΔ and annce102Δ also show differential sensitivity to various oxidative agents, while AnNce102 overexpression can bypass sphingolipid depletion regarding the PilA/SurG foci number and organization, also leading to the mislocalization of PilA to septa.

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.

Membrane Compartment Occupied by Can1 (MCC) and Eisosome Subdomains of the Fungal Plasma Membrane

Studies on the budding yeast Saccharomyces cerevisiae have revealed that fungal plasma membranes are organized into different subdomains. One new domain termed MCC/eisosomes consists of stable punctate patches that are distinct from lipid rafts. The MCC/eisosome domains correspond to furrows in the plasma membrane that are about 300 nm long and 50 nm deep. The MCC portion includes integral membrane proteins, such as the tetraspanners Sur7 and Nce102. The adjacent eisosome includes proteins that are peripherally associated with the membrane, including the BAR domains proteins Pil1 and Lsp1 that are thought to promote membrane curvature. Genetic analysis of the MCC/eisosome components indicates these domains broadly affect overall plasma membrane organization. The mechanisms regulating the formation of MCC/eisosomes in model organisms will be reviewed as well as the role of these plasma membrane domains in fungal pathogenesis and response to antifungal drugs.

The MARVEL domain protein Nce102 regulates actin organization and invasive growth of Candida albicans

Invasive growth of the fungal pathogen Candida albicans into tissues promotes disseminated infections in humans. The plasma membrane is essential for pathogenesis because this important barrier mediates morphogenesis and invasive growth, as well as secretion of virulence factors, cell wall synthesis, nutrient import, and other processes. Previous studies showed that the Sur7 tetraspan protein that localizes to MCC (membrane compartment occupied by Can1)/eisosome subdomains of the plasma membrane regulates a broad range of key functions, including cell wall synthesis, morphogenesis, and resistance to copper. Therefore, a distinct tetraspan protein found in MCC/eisosomes, Nce102, was investigated. Nce102 belongs to the MARVEL domain protein family, which is implicated in regulating membrane structure and function. Deletion of NCE102 did not cause the broad defects seen in sur7Δ cells. Instead, the nce102Δ mutant displayed a unique phenotype in that it was defective in forming hyphae and invading low concentrations of agar but could invade well in higher agar concentrations. This phenotype was likely due to a defect in actin organization that was observed by phalloidin staining. In support of this, the invasive growth defect of a bni1Δ mutant that mislocalizes actin due to lack of the Bni1 formin was also reversed at high agar concentrations. This suggests that a denser matrix provides a signal that compensates for the actin defects. The nce102Δ mutant displayed decreased virulence and formed abnormal hyphae in mice. These studies identify novel ways that Nce102 and the physical environment surrounding C. albicans regulate morphogenesis and pathogenesis.

The plasma membrane promotes virulence of the human fungal pathogen Candida albicans by acting as a protective barrier around the cell and mediating dynamic activities, such as morphogenesis, cell wall synthesis, secretion of virulence factors, and nutrient uptake. To better understand how the plasma membrane contributes to virulence, we analyzed a set of eight genes encoding MARVEL family proteins that are predicted to function in membrane organization. Interestingly, deletion of one gene, NCE102, caused a strong defect in formation of invasive hyphal growth in vitro and decreased virulence in mice. The nce102Δ mutant cells showed defects in actin organization that underlie the morphogenesis defect, since mutation of a known regulator of actin organization caused a similar defect. These studies identify a novel way in which the plasma membrane regulates the actin cytoskeleton and contributes to pathogenesis.

Rax2 is important for directional establishment of growth sites, but not for reorientation of growth axes, during Candida albicans hyphal morphogenesis

Hyphae of filamentous fungi maintain generally linear growth over long distances. In Candida albicans, hyphae are able to reorient their growth in the direction of certain environmental cues. In previous work, the C. albicans bud-site selection proteins Rsr1 and Bud2 were identified as important for hyphae to maintain linear growth and were necessary for hyphal responses to directional cues in the environment (tropisms). To ask if hyphal directional responses are general functions of all yeast bud-site selection proteins, we studied the role of Rax2, ortholog of the Saccharomyces cerevisiae bud-site selection protein Rax2, in C. albicans hyphal morphogenesis. Rax2-YFP localized to the hyphal cell surface in puncta and at the hyphal tip in a crescent. Strains lacking Rax2 had hyphal morphologies that did not differ from control strains. In non-cued growth conditions, rax2 mutant strains had defects in both yeast (bud) and hyphal (branch) site selection and mutant hyphae exhibited non-linear growth trajectories as compared to control hyphae. In contrast, when encountering a directional environmental cue, hyphae lacking Rax2 retained the ability to reorient growth in response to both topographical (thigmotropism) and electric-field (galvanotropism) stimuli but exhibited a reduced ability to establish hyphal growth in the direction of a cathodal stimulus. In conclusion, these results indicate that C. albicans Rax2 is important for establishing sites of emergence of yeast and hyphal daughters and for maintaining the linearity of hyphal growth. In contrast to Rsr1 and Bud2, Rax2 is not involved in responses that require a reorientation of the direction of already established hyphal growth (tropisms). Thus, it appears that some hyphal directionality responses are separable in that they are mediated by a different set of polarity proteins.

Functional analysis of Abp1p-interacting proteins involved in endocytosis of the MCC component in Aspergillus oryzae

We have investigated the functions of three endocytosis-related proteins in the filamentous fungus Aspergillus oryzae. Yeast two-hybrid screening using the endocytic marker protein AoAbp1 (A.oryzae homolog of Saccharomyces cerevisiae Abp1p) as a bait identified four interacting proteins named Aip (AoAbp1 interacting proteins). In mature hyphae, EGFP (enhanced green fluorescent protein) fused to Aips colocalized with AoAbp1 at the hyphal tip region and the plasma membrane, suggesting that Aips function in endocytosis. aipA is a putative AAA ATPase and its function has been dissected (Higuchi et al., 2011). aipB, the homolog of A. nidulans myoA, encodes an essential class I myosin and its conditional mutant showed a germination defect. aipC and aipD do not contain any recognizable domains except some proline-rich regions which may interact with two SH3 (Src homology 3) domains of AoAbp1. Neither aipC nor aipD disruptants showed any defects in their growth, but the aipC disruptant formed less conidia compared with the control strain. In addition, the aipC disruptant was resistant to the triazole antifungal drugs that inhibit ergosterol biosynthesis. Although no aip disruptants showed any defects in the uptake of the fluorescent dye FM4-64, the endocytosis of the arginine permease AoCan1, one of the MCC (membrane compartment of Can1p) components, was delayed in both aipC and aipD disruptants. In A. oryzae, AoCan1 localized mainly at the plasma membrane in the basal region of hyphae, suggesting that different endocytic mechanisms exist in apical and basal regions of highly polarized cells.

Membrane microdomains, rafts, and detergent-resistant membranes in plants and fungi

The existence of specialized microdomains in plasma membranes, postulated for almost 25 years, has been popularized by the concept of lipid or membrane rafts. The idea that detergent-resistant membranes are equivalent to lipid rafts, which was generally abandoned after a decade of vigorous data accumulation, contributed to intense discussions about the validity of the raft concept. The existence of membrane microdomains, meanwhile, has been verified by unequivocal independent evidence. This review summarizes the current state of research in plants and fungi with respect to common aspects of both kingdoms. In these organisms, principally immobile microdomains large enough for microscopic detection have been visualized. These microdomains are found in the context of cell-cell interactions (plant symbionts and pathogens), membrane transport, stress, and polarized growth, and the data corroborate at least three mechanisms of formation. As documented in this review, modern methods of visualization of lateral membrane compartments are also able to uncover the functional relevance of membrane microdomains.

Insights into eisosome assembly and organization

Eisosomes, large protein complexes that are predominantly composed of BAR-domain-containing proteins Pil1 and its homologs, are situated under the plasma membrane of ascomycetes. A successful targeting of Pil1 onto the future site of eisosome accompanies maturation of eisosome. During or after recruitment, Pil1 undergoes self-assembly into filaments that can serve as scaffolds to induce membrane furrows or invaginations. Although a consequence of the invagination is likely to redistribute particular proteins and lipids to a different location, the precise physiological role of membrane invagination and eisosome assembly awaits further investigation. The present review summarizes recent research findings within the field regarding the detailed structural and functional significance of Pil1 on eisosome organization.

Seg1 controls eisosome assembly and shape

Seg1 establishes a platform for the assembly of eisosomes and is important for determining their length.

Eisosomes are stable domains at the plasma membrane of the budding yeast Saccharomyces cerevisiae and have been proposed to function in endocytosis. Eisosomes are composed of two main cytoplasmic proteins, Pil1 and Lsp1, that form a scaffold around furrow-like plasma membrane invaginations. We show here that the poorly characterized eisosome protein Seg1/Ymr086w is important for eisosome biogenesis and architecture. Seg1 was required for efficient incorporation of Pil1 into eisosomes and the generation of normal plasma membrane furrows. Seg1 preceded Pil1 during eisosome formation and established a platform for the assembly of other eisosome components. This platform was further shaped and stabilized upon the arrival of Pil1 and Lsp1. Moreover, Seg1 abundance controlled the shape of eisosomes by determining their length. Similarly, the Schizosaccharomyces pombe Seg1-like protein Sle1 was necessary to generate the filamentous eisosomes present in fission yeast. The function of Seg1 in the stepwise biogenesis of eisosomes reveals striking architectural similarities between eisosomes in yeast and caveolae in mammals.

The role of flotillin FloA and stomatin StoA in the maintenance of apical sterol-rich membrane domains and polarity in the filamentous fungus Aspergillus nidulans

Apical sterol-rich plasma membrane domains (SRDs), which can be viewed using the sterol-binding fluorescent dye filipin, are gaining attention for their important roles in polarized growth of filamentous fungi. The microdomain scaffolding protein flotillin/reggie and related stomatin were thought to be good candidates involved in the formation of SRDs. Here, we show that the flotillin/reggie orthologue FloA tagged with GFP localized as stable dots along the plasma membrane except hyphal tips. Deletion of floA reduced the growth rate, often resulted in irregularly shaped hyphae and impaired SRDs. In contrast, the stomatin orthologue StoA, tagged with GFP, localized at the cortex of young branch tips and at the subapical cortex in long hyphal tips, and was transported bi-directionally along microtubules on endosomes. Deletion of stoA resulted in irregular hyphal morphology and increased branching especially in young hyphae, but did not obviously affect SRDs. Double deletion of floA and stoA enhanced the defects of growth and hyphal morphology. Our data suggest that the plasma membrane of hyphal tips and in subapical regions are distinct and that FloA is involved in membrane compartmentalization and probably indirectly in SRD maintenance.

NCE102 homologue in Aspergillus fumigatus is required for normal sporulation, not hyphal growth or pathogenesis

In Saccharomyces cerevisiae, Nce102 encodes a 173 amino acid transmembrane protein, which acts as a key player in eisosome assembly and plasma membrane organization. Here, we describe the characterization of Nce102 homologue in the human pathogen, Aspergillus fumigatus. Our results demonstrated that AfuNce102 is continuously expressed during fungal growth. In addition, microscopic examination of an AfuNce102-GFP-expressing transformant confirmed the localization of the fusion protein to the endoplasmic reticulum with higher density fluorescence at the tip of the mycelium. During conidiogenesis, the protein was localized to the conidiophores and the conidia. Abnormal conidiation of AfuNce102 deletion mutant suggests a potential role for AfuNce102 in sporulation process.

Sur7 promotes plasma membrane organization and is needed for resistance to stressful conditions and to the invasive growth and virulence of Candida albicans

The human fungal pathogen Candida albicans causes lethal systemic infections because of its ability to grow and disseminate in a host. The C. albicans plasma membrane is essential for virulence by acting as a protective barrier and through its key roles in interfacing with the environment, secretion of virulence factors, morphogenesis, and cell wall synthesis. Difficulties in studying hydrophobic membranes have limited the understanding of how plasma membrane organization contributes to its function and to the actions of antifungal drugs. Therefore, the role of the recently discovered plasma membrane subdomains termed the membrane compartment containing Can1 (MCC) was analyzed by assessing the virulence of a sur7Δ mutant. Sur7 is an integral membrane protein component of the MCC that is needed for proper localization of actin, morphogenesis, cell wall synthesis, and responding to cell wall stress. MCC domains are stable 300-nm-sized punctate patches that associate with a complex of cytoplasmic proteins known as an eisosome. Analysis of virulence-related properties of a sur7Δ mutant revealed defects in intraphagosomal growth in macrophages that correlate with increased sensitivity to oxidation and copper. The sur7Δ mutant was also strongly defective in pathogenesis in a mouse model of systemic candidiasis. The mutant cells showed a decreased ability to initiate an infection and greatly diminished invasive growth into kidney tissues. These studies on Sur7 demonstrate that the plasma membrane MCC domains are critical for virulence and represent an important new target for the development of novel therapeutic strategies.

Candida albicans, the most common human fungal pathogen, causes lethal systemic infections by growing and disseminating in a host. The plasma membrane plays key roles in enabling C. albicans to grow in vivo, and it is also the target of the most commonly used antifungal drugs. However, plasma membrane organization is poorly understood because of the experimental difficulties in studying hydrophobic components. Interestingly, recent studies have identified a novel type of plasma membrane subdomain in fungi known as the membrane compartment containing Can1 (MCC). Cells lacking the MCC-localized protein Sur7 display broad defects in cellular organization and response to stress in vitro. Consistent with this, C. albicans cells lacking the SUR7 gene were more susceptible to attack by macrophages than cells with the gene and showed greatly reduced virulence in a mouse model of systemic infection. Thus, Sur7 and other MCC components represent novel targets for antifungal therapy.

Actin organization and dynamics in filamentous fungi

Growth and morphogenesis of filamentous fungi is underpinned by dynamic reorganization and polarization of the actin cytoskeleton. Actin has crucial roles in exocytosis, endocytosis, organelle movement and cytokinesis in fungi, and these processes are coupled to the production of distinct higher-order structures (actin patches, cables and rings) that generate forces or serve as tracks for intracellular transport. New approaches for imaging actin in living cells are revealing important similarities and differences in actin architecture and organization within the fungal kingdom, and have yielded key insights into cell polarity, tip growth and long-distance intracellular transport. In this Review, we discuss the contribution that recent live-cell imaging and mutational studies have made to our understanding of the dynamics and regulation of actin in filamentous fungi.

The filament-forming protein Pil1 assembles linear eisosomes in fission yeast

The cortical cytoskeleton mediates a range of cellular activities such as endocytosis, cell motility, and the maintenance of cell rigidity. Traditional polymers, including actin, microtubules, and septins, contribute to the cortical cytoskeleton, but additional filament systems may also exist. In yeast cells, cortical structures called eisosomes generate specialized domains termed MCCs to cluster specific proteins at sites of membrane invaginations. Here we show that the core eisosome protein Pil1 forms linear cortical filaments in fission yeast cells and that purified Pil1 assembles into filaments in vitro. In cells, Pil1 cortical filaments are excluded from regions of cell growth and are independent of the actin and microtubule cytoskeletons. Pil1 filaments assemble slowly at the cell cortex and appear stable by time-lapse microscopy and fluorescence recovery after photobleaching. This stability does not require the cell wall, but Pil1 and the transmembrane protein Fhn1 colocalize and are interdependent for localization to cortical filaments. Increased Pil1 expression leads to cytoplasmic Pil1 rods that are stable and span the length of cylindrical fission yeast cells. We propose that Pil1 is a novel component of the yeast cytoskeleton, with implications for the role of filament assembly in the spatial organization of cells.

Evolution of multinucleated Ashbya gossypii hyphae from a budding yeast-like ancestor

In the filamentous ascomycete Ashbya gossypii polarity establishment at sites of germ tube and lateral branch emergence depends on homologues of Saccharomyces cerevisiae factors controlling bud site selection and bud emergence. Maintenance of polar growth involves homologues of well-known polarity factors of budding yeast. To achieve the much higher rates of sustained polar surface expansion of hyphae compared to mainly non-polarly growing yeast buds five important alterations had to evolve. Permanent presence of the polarity machinery at a confined area in the rapidly expanding hyphal tip, increased cytoplasmic space with a much enlarged ER surface for generating secretory vesicles, efficient directed transport of secretory vesicles to and accumulation at the tip, increased capacity of the exocytosis system to process these vesicles, and an efficient endocytosis system for membrane and polarity factor recycling adjacent to the zone of exocytosis. Morphological, cell biological, and molecular aspects of this evolution are discussed based on experiments performed within the past 10 y.