Requirements of Slm proteins for proper eisosome organization, endocytic trafficking and recycling in the yeast Saccharomyces cerevisiae

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.

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.

Deciphering the Dynamics of Signaling Cascades and Virulence Factors of B. cinerea during Tomato Cell Wall Degradation

The ascomycete Botrytis cinerea is one of the most relevant plant pathogenic fungi, affecting fruits, flowers, and greenhouse-grown crops. The infection strategy used by the fungus comprises a magnificent set of tools to penetrate and overcome plant defenses. In this context, the plant-pathogen communication through membrane receptors and signal transduction cascades is essential to trigger specific routes and the final success of the infection. In previous reports, proteomics approaches to B. cinerea signal transduction cascades changes in response to different carbon source and plant-based elicitors have been performed. Analyzing the secretome, membranome, phosphoproteome, and the phosphomembranome. Moreover, phenotypic changes in fungal biology was analyzed, specifically toxin production. To obtain the whole picture of the process and reveal the network from a system biology approach, this proteomic information has been merged with the phenotypic characterization, to be analyzed using several bioinformatics algorithms (GO, STRING, MCODE) in order to unravel key points in the signal transduction regulation crucial to overcome plant defenses, as well as new virulence/pathogenicity factors that could be used as therapeutic targets in the control of the gray mold rot disease. A total of 1721 and 663 exclusive or overexpressed proteins were identified under glucose (GLU) and deproteinized tomato cell walls (TCW), summarizing all of the protein identifications under phenotypic characterized stages. Under GO analysis, there are more biological process and molecular functions described in GLU, highlighting the increase in signaling related categories. These results agree with the high number of total identified proteins in GLU, probably indicating a more varied and active metabolism of the fungus. When analyzing only GO annotations related with signal transduction, it was revealed that there were proteins related to TOR signaling, the phosphorelay signal transduction system, and inositol lipid-mediated signaling, only under GLU conditions. On the contrary, calcium-mediated signaling GO annotation is only present between the proteins identified under TCW conditions. To establish a potential relationship between expressed proteins, cluster analyses showed 41 and 14 clusters under GLU and TCW conditions, confirming an increase in biological activity in GLU, where we identified a larger number of clusters related to transcription, translation, and cell division, between others. From these analyses, clusters related to signal transduction and clusters related to mycotoxin production were found, which correlated with the phenotypic characterization. The identification of the proteins encompassed in each condition and signal transduction cascade would provide the research community with new information about the B. cinerea infection process and potential candidates of pathogenicity/virulence factors, overcoming plant defenses, and new therapeutic targets.

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.

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.

Endocytosis Detection in Magnaporthe oryzae

Endocytosis is an intracellular trafficking pathway that occurs in nutrient uptake, signal transduction and reconstruction of cell polarity and is conserved in eukaryotic cells. In fungi, endocytosis plays crucial roles in the physiology of hyphal growth and pathogenicity. vidence for endocytosis in filamentous fungi is detected by the membrane-selective dyes FM4-64. Cells of a range of filamentous fungal species readily take up these dyes. However, the method for endocytosis detection has not been well established in Magnaporthe oryzae. Here, we provide a protocol for tracking endocytosis in Magnaporthe oryzae.

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.

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.

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.

Eisosomes are metabolically regulated storage compartments for APC-type nutrient transporters

Eisosomes are lipid domains of the yeast plasma membrane that share similarities to caveolae of higher eukaryotes. Eisosomes harbor APC-type nutrient transporters for reasons that are poorly understood. Our analyses support the model that eisosomes function as storage compartments, keeping APC transporters in a stable, inactive state. By regulating eisosomes, yeast is able to balance the number of proton-driven APC transporters with the proton-pumping activity of Pma1, thereby maintaining the plasma membrane proton gradient. Environmental or metabolic changes that disrupt the proton gradient cause the rapid restructuring of eisosomes and results in the removal of the APC transporters from the cell surface. Furthermore, we show evidence that eisosomes require the presence of APC transporters, suggesting that regulating activity of nutrient transporters is a major function of eisosomes.

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.

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.

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.

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.

A calcineurin-dependent switch controls the trafficking function of α-arrestin Aly1/Art6

Proper regulation of plasma membrane protein endocytosis by external stimuli is required for cell growth and survival. In yeast, excess levels of certain nutrients induce endocytosis of the cognate permeases to prevent toxic accumulation of metabolites. The α-arrestins, a family of trafficking adaptors, stimulate ubiquitin-dependent and clathrin-mediated endocytosis by interacting with both a client permease and the ubiquitin ligase Rsp5. However, the molecular mechanisms that control α-arrestin function are not well understood. Here, we show that α-arrestin Aly1/Art6 is a phosphoprotein that specifically interacts with and is dephosphorylated by the Ca(2+)- and calmodulin-dependent phosphoprotein phosphatase calcineurin/PP2B. Dephosphorylation of Aly1 by calcineurin at a subset of phospho-sites is required for Aly1-mediated trafficking of the aspartic acid and glutamic acid transporter Dip5 to the vacuole, but it does not alter Rsp5 binding, ubiquitinylation, or stability of Aly1. In addition, dephosphorylation of Aly1 by calcineurin does not regulate the ability of Aly1 to promote the intracellular sorting of the general amino acid permease Gap1. These results suggest that phosphorylation of Aly1 inhibits its vacuolar trafficking function and, conversely, that dephosphorylation of Aly1 by calcineurin serves as a regulatory switch to promote Aly1-mediated trafficking to the vacuole.

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.

Plasma membrane recruitment and activation of the AGC kinase Ypk1 is mediated by target of rapamycin complex 2 (TORC2) and its effector proteins Slm1 and Slm2

The yeast AGC kinase orthologs Ypk1 and Ypk2 control several important cellular processes, including actin polarization, endocytosis, and sphingolipid metabolism. Activation of Ypk1/2 requires phosphorylation by kinases localized at the plasma membrane (PM), including the 3-phosphoinositide-dependent kinase 1 orthologs Pkh1/Pkh2 and the target of rapamycin complex 2 (TORC2). Unlike their mammalian counterparts SGK and Akt, Ypk1 and Ypk2 lack an identifiable lipid-targeting motif; therefore, how these proteins are recruited to the PM has remained elusive. To explore Ypk1/2 function, we constructed ATP analog-sensitive alleles of both kinases and monitored global changes in gene expression following their inhibition, where we observed increased expression of stress-responsive target genes controlled by Ca(2+)-dependent phosphatase calcineurin. TORC2 has been shown previously to negatively regulate calcineurin in part by phosphorylating two related proteins, Slm1 and Slm2, which associate with the PM via plextrin homology domains. We therefore investigated the relationship between Slm1 and Ypk1 and discovered that these proteins interact physically and that Slm1 recruits Ypk1 to the PM for phosphorylation by TORC2. We observed further that these steps facilitate subsequent phosphorylation of Ypk1 by Pkh1/2. Remarkably, a requirement for Slm1, can be bypassed by fusing the plextrin homology domain of Slm1 alone onto Ypk1, demonstrating that the essential function of Slm1 is largely attributable to its role in Ypk1 activation. These findings both extend the scope of cellular processes regulated by Ypk1/2 to include negative regulation of calcineurin and broaden the repertoire of mechanisms for membrane recruitment and activation of a protein kinase.

Pil1, an eisosome organizer, plays an important role in the recruitment of synaptojanins and amphiphysins to facilitate receptor-mediated endocytosis in yeast

The eisosome protein Pil1 is known to be implicated in the endocytosis of Ste3, but the precise biological function of it during endocytosis is poorly understood. Here, we present data to reveal Pil1's role in receptor-mediated endocytosis. Using live cell imaging, we show that endocytic patches carrying Abp1 and Las17 persisted much longer in PIL1-deficient cells. The loss of Pil1 also greatly affected both the scission efficiency and the frequency of formation of endocytic sites carrying Rvs161- and Rvs167-GFP. Furthermore, the mistargeting of the synaptojanins, Sjl1 and Sjl2, to the cytoplasm in pil1Δ cells suggests that Pil1 is required for the proper recruitment of the synaptojanins to endocytic sites. A severe motility defect of Abp1-GFP during its internalization in a codeletant of PIL1 and SJL2 indicates a functional interplay between them in endocytosis. Together, these results establish that Pil1 is involved in the recruitment of endocytic proteins to optimize endocytosis.

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.