Genetic characterization of genes encoding enzymes catalyzing addition of phospho-ethanolamine to the glycosylphosphatidylinositol anchor in Saccharomyces cerevisiae

Aspergillus fumigatus phosphoethanolamine transferase gene gpi7 is required for proper transportation of the cell wall GPI-anchored proteins and polarized growth

In fungi many proteins, which play important roles in maintaining the function of the cell wall and participating in pathogenic processes, are anchored to the cell surface by a glycosylphosphatidylinositol (GPI) anchor. It has been known that modification and removal of phosphoethanolamine (EtN-P) on the second mannose residue in GPI anchors is important for maturation and sorting of GPI anchored proteins in yeast and mammalian cells, but is a step absent from some protist parasites. In Aspergillus fumigatus, an opportunistic fungal pathogen causing invasive aspergillosis in humans, GPI-anchored proteins are known to be involved in cell wall synthesis and virulence. In this report the gene encoding A. fumigatus EtN-P transferase GPI7 was investigated. By deletion of the gpi7 gene, we evaluated the effects of EtN-P modification on the morphogenesis of A. fumigatus and localization of GPI proteins. Our results showed that deletion of the gpi7 gene led to reduced cell membrane GPI anchored proteins, the mis-localization of the cell wall GPI anchored protein Mp1, abnormal polarity, and autophagy in A. fumigatus. Our results suggest that addition of EtN-P of the second mannose on the GPI anchor is essential for transportation and localization of the cell wall GPI-anchored proteins.

Autophagy competes for a common phosphatidylethanolamine pool with major cellular PE-consuming pathways in Saccharomyces cerevisiae

Autophagy is a highly regulated pathway that selectively degrades cellular constituents such as protein aggregates and excessive or damaged organelles. This transport route is characterized by engulfment of the targeted cargo by autophagosomes. The formation of these double-membrane vesicles requires the covalent conjugation of the ubiquitin-like protein Atg8 to phosphatidylethanolamine (PE). However, the origin of PE and the regulation of lipid flux required for autophagy remain poorly understood. Using a genetic screen, we found that the temperature-sensitive growth and intracellular membrane organization defects of mcd4-174 and mcd4-P301L mutants are suppressed by deletion of essential autophagy genes such as ATG1 or ATG7. MCD4 encodes an ethanolamine phosphate transferase that uses PE as a precursor for an essential step in the synthesis of the glycosylphosphatidylinositol (GPI) anchor used to link a subset of plasma membrane proteins to lipid bilayers. Similar to the deletion of CHO2, a gene encoding the enzyme converting PE to phosphatidylcholine (PC), deletion of ATG7 was able to restore lipidation and plasma membrane localization of the GPI-anchored protein Gas1 and normal organization of intracellular membranes. Conversely, overexpression of Cho2 was lethal in mcd4-174 cells grown at restrictive temperature. Quantitative lipid analysis revealed that PE levels are substantially reduced in the mcd4-174 mutant but can be restored by deletion of ATG7 or CHO2. Taken together, these data suggest that autophagy competes for a common PE pool with major cellular PE-consuming pathways such as the GPI anchor and PC synthesis, highlighting the possible interplay between these pathways and the existence of signals that may coordinate PE flux.

A screen for deficiencies in GPI-anchorage of wall glycoproteins in yeast

Many of the genes and enzymes critical for assembly and biogenesis of yeast cell walls remain unidentified or poorly characterized. Therefore, we designed a high throughput genomic screen for defects in anchoring of GPI-cell wall proteins (GPI-CWPs), based on quantification of a secreted GFP-Sag1p fusion protein. Saccharomyces cerevisiae diploid deletion strains were transformed with a plasmid expressing the fusion protein under a GPD promoter, then GFP fluorescence was determined in culture supernatants after mid-exponential growth. Variability in the amount of fluorescent marker secreted into the medium was reduced by growth at 18 degrees C in buffered defined medium in the presence of sorbitol. Secondary screens included immunoblotting for GFP, fluorescence emission spectra, cell surface fluorescence, and cell integrity. Of 167 mutants deleted for genes affecting cell wall biogenesis or structure, eight showed consistent hyper-secretion of GFP relative to parental strain BY4743: tdh3 (glyceraldehyde-3-phosphate dehydrogenase), gda1 (guanosine diphosphatase), gpi13 and mcd4 (both ethanolamine phosphate-GPI-transferases), kre5 and kre1 (involved in synthesis of beta1,6 glucan), dcw1(implicated in GPI-CWP cross-linking to cell wall glucan), and cwp1 (a major cell wall protein). In addition, deletion of a number of genes caused decreased secretion of GFP. These results elucidate specific roles for specific genes in cell wall biogenesis, including differentiating among paralogous genes.

Biosynthesis and function of GPI proteins in the yeast Saccharomyces cerevisiae

Like most other eukaryotes, Saccharomyces cerevisiae harbors a GPI anchoring machinery and uses it to attach proteins to membranes. While a few GPI proteins reside permanently at the plasma membrane, a majority of them gets further processed and is integrated into the cell wall by a covalent attachment to cell wall glucans. The GPI biosynthetic pathway is necessary for growth and survival of yeast cells. The GPI lipids are synthesized in the ER and added onto proteins by a pathway comprising 12 steps, carried out by 23 gene products, 19 of which are essential. Some of the estimated 60 GPI proteins predicted from the genome sequence serve enzymatic functions required for the biosynthesis and the continuous shape adaptations of the cell wall, others seem to be structural elements of the cell wall and yet others mediate cell adhesion. Because of its genetic tractability S. cerevisiae is an attractive model organism not only for studying GPI biosynthesis in general, but equally for investigating the intracellular transport of GPI proteins and the peculiar role of GPI anchoring in the elaboration of fungal cell walls.

Glycosylphosphatidylinositol-anchored Ecm33p influences conidial cell wall biosynthesis in Aspergillus fumigatus

ECM33 encodes a glycosylphosphatidylinositol-anchored protein whose orthologs in yeast are essential for sporulation. Aspergillus fumigatus Ecm33p is unique and has an apparent mass of 55 kDa. Disruption of A. fumigatus ECM33 results in a mutant with several morphogenetic aberrations, including the following: (i) a defect in conidial separation, (ii) an increase in the diameter of the conidia of the mutant associated with an increase in the concentration of the cell wall chitin, (iii) conidia that were sensitive to the absence of aeration during long-term storage, and (iv) conidia that were more resistant to killing by phagocytes, whereas the mycelium was more easily killed by neutrophils.

Ethanolaminephosphate Side Chain Added to Glycosylphosphatidylinositol (GPI) Anchor by Mcd4p Is Required for Ceramide Remodeling and Forward Transport of GPI Proteins from Endoplasmic Reticulum to Golgi

Glycosylphosphatidylinositol (GPI) anchors of mammals as well as yeast contain ethanolaminephosphate side chains on the alpha1-4- and the alpha1-6-linked mannoses of the anchor core structure (protein-CO-NH-(CH(2))(2)-PO(4)-6Manalpha1-2Manalpha1-6Manalpha1-4GlcNH(2)-inositol-PO(4)-lipid). In yeast, the ethanolaminephosphate on the alpha1-4-linked mannose is added during the biosynthesis of the GPI lipid by Mcd4p. MCD4 is essential because Gpi10p, the mannosyltransferase adding the subsequent alpha1-2-linked mannose, requires substrates with an ethanolaminephosphate on the alpha1-4-linked mannose. The Gpi10p ortholog of Trypanosoma brucei has no such requirement. Here we show that the overexpression of this ortholog rescues mcd4Delta cells. Phenotypic analysis of the rescued mcd4Delta cells leads to the conclusion that the ethanolaminephosphate on the alpha1-4-linked mannose, beyond being an essential determinant for Gpi10p, is necessary for an efficient recognition of GPI lipids and GPI proteins by the GPI transamidase for the efficient transport of GPI-anchored proteins from the endoplasmic reticulum to Golgi and for the physiological incorporation of ceramides into GPI anchors by lipid remodeling. Furthermore, mcd4Delta cells have a marked defect in axial bud site selection, whereas this process is normal in gpi7Delta and gpi1. This also suggests that axial bud site selection specifically depends on the presence of the ethanolaminephosphate on the alpha1-4-linked mannose.

GPI7 Involved in Glycosylphosphatidylinositol Biosynthesis Is Essential for Yeast Cell Separation

GPI7 is involved in adding ethanolaminephosphate to the second mannose in the biosynthesis of glycosylphosphatidylinositol (GPI) in Saccharomyces cerevisiae. We isolated gpi7 mutants, which have defects in cell separation and a daughter cell-specific growth defect at the non-permissive temperature. WSC1, RHO2, ROM2, GFA1, and CDC5 genes were isolated as multicopy suppressors of gpi7-2 mutant. Multicopy suppressors could suppress the growth defect of gpi7 mutants but not the cell separation defect. Loss of function mutations of genes involved in the Cbk1p-Ace2p pathway, which activates the expression of daughter-specific genes for cell separation after cytokinesis, bypassed the temperature-sensitive growth defect of gpi7 mutants. Furthermore, deletion of EGT2, one of the genes controlled by Ace2p and encoding a GPI-anchored protein required for cell separation, ameliorated the temperature sensitivity of the gpi7 mutant. In this mutant, Egt2p was displaced from the septal region to the cell cortex, indicating that GPI7 plays an important role in cell separation via the GPI-based modification of daughter-specific proteins in S. cerevisiae.

Glycosylphosphatidylinositol (GPI) proteins of Saccharomyces cerevisiae contain ethanolamine phosphate groups on the alpha1,4-linked mannose of the GPI anchor

In humans and Saccharomyces cerevisiae the free glycosylphosphatidylinositol (GPI) lipid precursor contains several ethanolamine phosphate side chains, but these side chains had been found on the protein-bound GPI anchors only in humans, not yeast. Here we confirm that the ethanolamine phosphate side chain added by Mcd4p to the first mannose is a prerequisite for the addition of the third mannose to the GPI precursor lipid and demonstrate that, contrary to an earlier report, an ethanolamine phosphate can equally be found on the majority of yeast GPI protein anchors. Curiously, the stability of this substituent during preparation of anchors is much greater in gpi7Delta sec18 double mutants than in either single mutant or wild type cells, indicating that the lack of a substituent on the second mannose (caused by the deletion of GPI7) influences the stability of the one on the first mannose. The phosphodiester-linked substituent on the second mannose, probably a further ethanolamine phosphate, is added to GPI lipids by endoplasmic reticulum-derived microsomes in vitro but cannot be detected on GPI proteins of wild type cells and undergoes spontaneous hydrolysis in saline. Genetic manipulations to increase phosphatidylethanolamine levels in gpi7Delta cells by overexpression of PSD1 restore cell growth at 37 degrees C without restoring the addition of a substituent to Man2. The three putative ethanolamine-phosphate transferases Gpi13p, Gpi7p, and Mcd4p cannot replace each other even when overexpressed. Various models trying to explain how Gpi7p, a plasma membrane protein, directs the addition of ethanolamine phosphate to mannose 2 of the GPI core have been formulated and put to the test.