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

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

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.

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.

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.

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

The Shape of Vesicle-Containing Organelles Is Critical for Their Functions in Vesicle Endocytosis

Exosomes are small vesicles secreted by a variety of cell types under physiological and pathological conditions. When Saccharomyces cerevisiae are grown in low glucose, small vesicles carrying more than 300 proteins with diverse biological functions are secreted. Upon glucose addition, secreted vesicles are endocytosed that requires cup-shaped organelles containing the major eisosome protein Pil1p at the rims. We aim to identify genes that regulate the function of cup-shaped organelles in vesicle endocytosis. In cells lacking either VID27 or VID21, Pil1p distribution was altered and cup-shaped organelles became elongated with narrower openings. Change in shape reduced the number of vesicles in the deeper areas and impaired vesicle endocytosis. Vid21p and Vid27p were localized to vesicle clusters and interacted with other Vid proteins. In the absence of these genes, these vesicles failed to aggregate and were secreted. Vid21p and Vid27p are required for the aggregation and retention of vesicles that contain Vid proteins in the cytoplasm. Increased vesicles near the plasma membrane in mutant strains correlate with an increased Pil1p movement resulting in the fusion of cup-shaped organelles. We conclude that the shape of vesicle-containing organelles is critical for their functions in vesicle endocytosis.

The Plasma Membrane Protein Nce102 Implicated in Eisosome Formation Rescues a Heme Defect in Mitochondria

The cellular transport of the cofactor heme and its biosynthetic intermediates such as protoporphyrin IX is a complex and highly coordinated process. To investigate the molecular details of this trafficking pathway, we created a synthetic lesion in the heme biosynthetic pathway by deleting the gene HEM15 encoding the enzyme ferrochelatase in S. cerevisiae and performed a genetic suppressor screen. Cells lacking Hem15 are respiratory-defective because of an inefficient heme delivery to the mitochondria. Thus, the biogenesis of mitochondrial cytochromes is negatively affected. The suppressor screen resulted in the isolation of respiratory-competent colonies containing two distinct missense mutations in Nce102, a protein that localizes to plasma membrane invaginations designated as eisosomes. The presence of the Nce102 mutant alleles enabled formation of the mitochondrial respiratory complexes and respiratory growth in hem15Δ cells cultured in supplemental hemin. Respiratory function in hem15Δ cells can also be restored by the presence of a heterologous plasma membrane heme permease (HRG-4), but the mode of suppression mediated by the Nce102 mutant is more efficient. Attenuation of the endocytic pathway through deletion of the gene END3 impaired the Nce102-mediated rescue, suggesting that the Nce102 mutants lead to suppression through the yeast endocytic pathway.

Eisosomes and membrane compartments in the ascomycetes: A view from Aspergillus nidulans

Eisosomes are punctate structures located in the cytoplasmic side of the cell membrane of ascomycetes. In Saccharomyces cerevisiae they coincide topologically with and are necessary for the organisation of specific membrane domains. The eisosomal proteins are universally and quite strictly conserved in the sub-phylum, however this evolutionary conservation is in apparent contradiction with an elusive functional significance. The comparative analysis of the eisosomes of S. cerevisiae and Aspergillus nidulans reveal striking differences in the assembly and developmental fate of these structures between these two model organisms.