Biosynthesis and function of GPI proteins in the yeast Saccharomyces cerevisiae

Generation and characterization of temperature-sensitive alleles encoding GPI anchored proteins Psu1 and Dfg502 in Schizosaccharomyces pombe

Glycosyl-phosphatidylinositol (GPI) anchored proteins are implicated in remodeling of the yeast cell wall during growth and division. Schizosaccharomyces pombe proteins, Psu1 , Dfg501 , and Dfg502 are predicted GPI anchored proteins with likely cell wall modifying activity. Here, we isolated and characterized null and temperature-sensitive alleles that will allow further analysis of the function of these proteins and S. pombe cell wall formation. Our data confirm that Psu1 is necessary for cell separation, maintaining proper cell shape, and viability. Additionally, we found that Dfg501 and Dfg502 share a redundant and essential function necessary for cell separation and viability.

Protein sorting upon exit from the endoplasmic reticulum dominates Golgi biogenesis in budding yeast

Cells sense and control the number and quality of their organelles, but the underlying mechanisms of this regulation are not understood. Our recent research in the yeast Saccharomyces cerevisiae has shown that long acyl chain ceramides in the endoplasmic reticulum (ER) membrane and the lipid moiety of glycosylphosphatidylinositol (GPI) anchor determine the sorting of GPI-anchored proteins in the ER. Here, we show that a mutant strain, which produces shorter ceramides than the wild-type strain, displays a different count of Golgi cisternae. Moreover, deletions of proteins that remodel the lipid portion of GPI anchors resulted in an abnormal number of Golgi cisternae. Thus, our study reveals that protein sorting in the ER plays a critical role in maintaining Golgi biogenesis.

Fungal Cell Wall-Associated Effectors: Sensing, Integration, Suppression, and Protection

The cell wall (CW) of plant-interacting fungi, as the direct interface with host plants, plays a crucial role in fungal development. A number of secreted proteins are directly associated with the fungal CW, either through covalent or non-covalent interactions, and serve a range of important functions. In the context of plant-fungal interactions many are important for fungal development in the host environment and may therefore be considered fungal CW-associated effectors (CWAEs). Key CWAE functions include integrating chemical/physical signals to direct hyphal growth, interfering with plant immunity, and providing protection against plant defenses. In recent years, a diverse range of mechanisms have been reported that underpin their roles, with some CWAEs harboring conserved motifs or functional domains, while others are reported to have novel features. As such, the current understanding regarding fungal CWAEs is systematically presented here from the perspective of their biological functions in plant-fungal interactions. An overview of the fungal CW architecture and the mechanisms by which proteins are secreted, modified, and incorporated into the CW is first presented to provide context for their biological roles. Some CWAE functions are reported across a broad range of pathosystems or symbiotic/mutualistic associations. Prominent are the chitin interacting-effectors that facilitate fungal CW modification, protection, or suppression of host immune responses. However, several alternative functions are now reported and are presented and discussed. CWAEs can play diverse roles, some possibly unique to fungal lineages and others conserved across a broad range of plant-interacting fungi. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

A Biotinylated Glycosylphosphatidylinositol (GPI) as the Universal Platform To Access GPI-Anchored Protein Analogues

A glycosylphosphatidylinositol (GPI) derivative with biotin linked to its mannose III 6-O-position was prepared by a convergent strategy. This biotinylated GPI was demonstrated to bind avidinated proteins readily through biotin-avidin interaction and, therefore, can serve as a universal platform to access various biologically significant GPI-anchored protein analogues.

Molecular basis of the inositol deacylase PGAP1 involved in quality control of GPI-AP biogenesis

The secretion and quality control of glycosylphosphatidylinositol-anchored proteins (GPI-APs) necessitates post-attachment remodeling initiated by the evolutionarily conserved PGAP1, which deacylates the inositol in nascent GPI-APs. Impairment of PGAP1 activity leads to developmental diseases in humans and fatality and infertility in animals. Here, we present three PGAP1 structures (2.66-2.84 Å), revealing its 10-transmembrane architecture and product-enzyme interaction details. PGAP1 holds GPI-AP acyl chains in an optimally organized, guitar-shaped cavity with apparent energetic penalties from hydrophobic-hydrophilic mismatches. However, abundant glycan-mediated interactions in the lumen counterbalance these repulsions, likely conferring substrate fidelity and preventing off-target hydrolysis of bulk membrane lipids. Structural and biochemical analyses uncover a serine hydrolase-type catalysis with atypical features and imply mechanisms for substrate entrance and product release involving a drawing compass movement of GPI-APs. Our findings advance the mechanistic understanding of GPI-AP remodeling.

Structure and Function of the Glycosylphosphatidylinositol Transamidase, a Transmembrane Complex Catalyzing GPI Anchoring of Proteins

Glycosylphosphatidylinositol (GPI) anchoring of proteins is a ubiquitous posttranslational modification in eukaryotic cells. GPI-anchored proteins (GPI-APs) play critical roles in enzymatic, signaling, regulatory, and adhesion processes. Over 20 enzymes are involved in GPI synthesis, attachment to client proteins, and remodeling after attachment. The GPI transamidase (GPI-T), a large complex located in the endoplasmic reticulum membrane, catalyzes the attachment step by replacing a C-terminal signal peptide of proproteins with GPI. In the last three decades, extensive research has been conducted on the mechanism of the transamidation reaction, the components of the GPI-T complex, the role of each subunit, and the substrate specificity. Two recent studies have reported the three-dimensional architecture of GPI-T, which represent the first structures of the pathway. The structures provide detailed mechanisms for assembly that rationalizes previous biochemical results and subunit-dependent stability data. While the structural data confirm the catalytic role of PIGK, which likely uses a caspase-like mechanism to cleave the proproteins, they suggest that unlike previously proposed, GPAA1 is not a catalytic subunit. The structures also reveal a shared cavity for GPI binding. Somewhat unexpectedly, PIGT, a single-pass membrane protein, plays a crucial role in GPI recognition. Consistent with the assembly mechanisms and the active site architecture, most of the disease mutations occur near the active site or the subunit interfaces. Finally, the catalytic dyad is located ~22 Å away from the membrane interface of the GPI-binding site, and this architecture may confer substrate specificity through topological matching between the substrates and the elongated active site. The research conducted thus far sheds light on the intricate processes involved in GPI anchoring and paves the way for further mechanistic studies of GPI-T.

Effector secretion and stability in the maize anthracnose pathogen Colletotrichum graminicola requires N-linked protein glycosylation and the ER chaperone pathway

N-linked protein glycosylation is a conserved and essential modification mediating protein processing and quality control in the endoplasmic reticulum (ER), but how this contributes to the infection cycle of phytopathogenic fungi is largely unknown. In this study, we discovered that inhibition of protein N-glycosylation severely affected vegetative growth, hyphal tip development, conidial germination, appressorium formation, and, ultimately, the ability of the maize (Zea mays) anthracnose pathogen Colletotrichum graminicola to infect its host. Quantitative proteomics analysis showed that N-glycosylation can coordinate protein O-glycosylation, glycosylphosphatidylinositol anchor modification, and endoplasmic reticulum quality control (ERQC) by directly targeting the proteins from the corresponding pathway in the ER. We performed a functional study of the N-glycosylation pathway-related protein CgALG3 and of the ERQC pathway-related protein CgCNX1, which demonstrated that N-glycosylation of ER chaperone proteins is essential for effector stability, secretion, and pathogenicity of C. graminicola. Our study provides concrete evidence for the regulation of effector protein stability and secretion by N-glycosylation.

The Mechanism of Transcription Factor Swi6 in Regulating Growth and Pathogenicity of Ceratocystis fimbriata: Insights from Non-Targeted Metabolomics

Ceratocystis fimbriata (C. fimbriata) is a notorious pathogenic fungus that causes sweet potato black rot disease. The APSES transcription factor Swi6 in fungi is located downstream of the cell wall integrity (CWI)-mitogen-activated protein kinase (MAPK) signaling pathway and has been identified to be involved in cell wall integrity and virulence in several filamentous pathogenic fungi. However, the specific mechanisms by which Swi6 regulates the growth and pathogenicity of plant pathogenic fungi remain elusive. In this study, the SWI6 deletion mutants and complemented strains of C. fimbriata were generated. Deletion of Swi6 in C. fimbriata resulted in aberrant growth patterns. Pathogenicity assays on sweet potato storage roots revealed a significant decrease in virulence in the mutant. Non-targeted metabolomic analysis using LC-MS identified a total of 692 potential differentially accumulated metabolites (PDAMs) in the ∆Cfswi6 mutant compared to the wild type, and the results of KEGG enrichment analysis demonstrated significant enrichment of PDAMs within various metabolic pathways, including amino acid metabolism, lipid metabolism, nucleotide metabolism, GPI-anchored protein synthesis, and ABC transporter metabolism. These metabolic pathways were believed to play a crucial role in mediating the growth and pathogenicity of C. fimbriata through the regulation of CWI. Firstly, the deletion of the SWI6 gene led to abnormal amino acid and lipid metabolism, potentially exacerbating energy storage imbalance. Secondly, significant enrichment of metabolites related to GPI-anchored protein biosynthesis implied compromised cell wall integrity. Lastly, disruption of ABC transport protein metabolism may hinder intracellular transmembrane transport. Importantly, this study represents the first investigation into the potential regulatory mechanisms of SWI6 in plant filamentous pathogenic fungi from a metabolic perspective. The findings provide novel insights into the role of SWI6 in the growth and virulence of C. fimbriata, highlighting its potential as a target for controlling this pathogen.

Elevated levels of sphingolipid MIPC in the plasma membrane disrupt the coordination of cell growth with cell wall formation in fission yeast

Coupling cell wall expansion with cell growth is a universal challenge faced by walled organisms. Mutations in Schizosaccharomyces pombe css1, which encodes a PM inositol phosphosphingolipid phospholipase C, prevent cell wall expansion but not synthesis of cell wall material. To probe how Css1 modulates cell wall formation we used classical and chemical genetics coupled with quantitative mass spectrometry. We found that elevated levels of the sphingolipid biosynthetic pathway's final product, mannosylinositol phosphorylceramide (MIPC), specifically correlated with the css1-3 phenotype. We also found that an apparent indicator of sphingolipids and a sterol biosensor accumulated at the cytosolic face of the PM at cell tips and the division site of css1-3 cells and, in accord, the PM in css1-3 was less dynamic than in wildtype cells. Interestingly, disrupting the protein glycosylation machinery recapitulated the css1-3 phenotype and led us to investigate Ghs2, a glycosylated PM protein predicted to modify cell wall material. Disrupting Ghs2 function led to aberrant cell wall material accumulation suggesting Ghs2 is dysfunctional in css1-3. We conclude that preventing an excess of MIPC in the S. pombe PM is critical to the function of key PM-localized proteins necessary for coupling growth with cell wall formation.

Emerging relevance of cell wall components from non-conventional yeasts as functional ingredients for the food and feed industry

Non-conventional yeast species, or non-Saccharomyces yeasts, are increasingly recognized for their involvement in fermented foods. Many of them exhibit probiotic characteristics that are mainly due to direct contacts with other cell types through various molecular components of their cell wall. The biochemical composition and/or the molecular structure of the cell wall components are currently considered the primary determinant of their probiotic properties. Here we first present the techniques that are used to extract and analyze the cell wall components of food industry-related non-Saccharomyces yeasts. We then review the current understanding of the cell wall composition and structure of each polysaccharide from these yeasts. Finally, the data exploring the potential beneficial role of their cell wall components, which could be a source of innovative functional ingredients, are discussed. Such research would allow the development of high value-added products and provide the food industry with novel inputs beyond the well-established S. cerevisiae.

Structures of liganded glycosylphosphatidylinositol transamidase illuminate GPI-AP biogenesis

Many eukaryotic receptors and enzymes rely on glycosylphosphatidylinositol (GPI) anchors for membrane localization and function. The transmembrane complex GPI-T recognizes diverse proproteins at a signal peptide region that lacks consensus sequence and replaces it with GPI via a transamidation reaction. How GPI-T maintains broad specificity while preventing unintentional cleavage is unclear. Here, substrates- and products-bound human GPI-T structures identify subsite features that enable broad proprotein specificity, inform catalytic mechanism, and reveal a multilevel safeguard mechanism against its promiscuity. In the absence of proproteins, the catalytic site is invaded by a locally stabilized loop. Activation requires energetically unfavorable rearrangements that transform the autoinhibitory loop into crucial catalytic cleft elements. Enzyme-proprotein binding in the transmembrane and luminal domains respectively powers the conformational rearrangement and induces a competent cleft. GPI-T thus integrates various weak specificity regions to form strong selectivity and prevent accidental activation. These findings provide important mechanistic insights into GPI-anchored protein biogenesis.

Neuroprotective effects of Longxue Tongluo Capsule on ischemic stroke rats revealed by LC-MS/MS-based metabolomics approach

Objective

The present study aimed to evaluate the therapeutic effect and explore the underlying mechanisms of Longxue Tongluo Capsule (LTC) on ischemic stroke rats.

Methods

Twenty-six rats were randomly divided into four groups, including sham group, sham + LTC group, MCAO group, and MCAO + LTC group. Ischemic stroke rats were simulated by middle cerebral artery occlusion (MCAO), and LTC treatment group were orally administrated with 300 mg/kg of LTC once daily for seven consecutive days. LTC therapy was validated in terms of neurobehavioral abnormality evaluation, cerebral infarct area, and histological assessments. The plasma metabolome comparisons amongst different groups were conducted by UHPLC-Q Exactive MS in combination with subsequent multivariate statistical analysis, aiming to finding the molecules in respond to the surgery or LTC treatment.

Results

Intragastric administration of LTC significantly decreased not only the neurobehavioral abnormality scores but also the cerebral infarct area of MCAO rats. The interstitial edema, atrophy, and pyknosis of glial and neuronal cells occurred in the infarcted area, core area, and marginal area of cerebral cortex were improved after LTC treatment. A total of 13 potential biomarkers were observed, and Youden index of 11 biomarkers such as LysoPC, SM, and PE were more than 0.7, which were involved in neuroprotective process. The correlation and pathway analysis showed that LTC was beneficial to ischemic stroke rats via regulating glycerophospholipid and sphingolipid metabolism, together with nicotinate and nicotinamide metabolism. Heatmap and ternary analysis indicated the synergistic effect of carbohydrates and lipids may be induced by flavonoid intake from LTC.

Conclusion

The present study could provide evidence that metabolomics, as systematic approach, revealed its capacity to evaluate the holistic efficacy of TCM, and investigate the molecular mechanism underlying the clinical treatment of LTC on ischemic stroke.

Fungal Drug Response and Antimicrobial Resistance

Antifungal resistance is a growing concern as it poses a significant threat to public health. Fungal infections are a significant cause of morbidity and mortality, especially in immunocompromised individuals. The limited number of antifungal agents and the emergence of resistance have led to a critical need to understand the mechanisms of antifungal drug resistance. This review provides an overview of the importance of antifungal resistance, the classes of antifungal agents, and their mode of action. It highlights the molecular mechanisms of antifungal drug resistance, including alterations in drug modification, activation, and availability. In addition, the review discusses the response to drugs via the regulation of multidrug efflux systems and antifungal drug-target interactions. We emphasize the importance of understanding the molecular mechanisms of antifungal drug resistance to develop effective strategies to combat the emergence of resistance and highlight the need for continued research to identify new targets for antifungal drug development and explore alternative therapeutic options to overcome resistance. Overall, an understanding of antifungal drug resistance and its mechanisms will be indispensable for the field of antifungal drug development and clinical management of fungal infections.

Accumulated precursors of specific GPI-anchored proteins upregulate GPI biosynthesis with ARV1

We previously reported that glycosylphosphatidylinositol (GPI) biosynthesis is upregulated when endoplasmic reticulum-associated degradation (ERAD) is defective; however, the underlying mechanistic basis remains unclear. Based on a genome-wide CRISPR-Cas9 screen, we show that a widely expressed GPI-anchored protein CD55 precursor and ER-resident ARV1 are involved in upregulation of GPI biosynthesis under ERAD-deficient conditions. In cells defective in GPI transamidase, GPI-anchored protein precursors fail to obtain GPI, with the remaining uncleaved GPI-attachment signal at the C-termini. We show that ERAD deficiency causes accumulation of the CD55 precursor, which in turn upregulates GPI biosynthesis, where the GPI-attachment signal peptide is the active element. Among the 31 GPI-anchored proteins tested, only the GPI-attachment signal peptides of CD55, CD48, and PLET1 enhance GPI biosynthesis. ARV1 is prerequisite for the GPI upregulation by CD55 precursor. Our data indicate that GPI biosynthesis is balanced to need by ARV1 and precursors of specific GPI-anchored proteins.

Genome-Wide Analysis and Abiotic Stress-Responsive Patterns of COBRA-like Gene Family in Liriodendron chinense

The COBRA gene encodes a plant-specific glycosylphosphatidylinositol (GPI)-anchored protein (GAP), which plays an important role in cell wall cellulose deposition. In this study, a total of 7 COBRA-like (COBL) genes were identified in the genome of the rare and endangered woody plant Liriodendron chinense (L. chinense). Phylogenetic analysis showed that these LcCOBL genes can be divided into two subfamilies, i.e., SF I and II. In the conserved motif analysis of two subfamilies, SF I contained 10 predicted motifs, while SF II contained 4-6 motifs. The tissue-specific expression patterns showed that LcCOBL5 was highly expressed in the phloem and xylem, indicating its potential role in cellulose biosynthesis. In addition, the cis-element analysis and abiotic stress transcriptomes showed that three LcCOBLs, LcCOBL3, LcCOBL4 and LcCOBL5, transcriptionally responded to abiotic stresses, including cold, drought and heat stress. In particular, the quantitative reverse-transcription PCR (qRT-PCR) analysis further confirmed that the LcCOBL3 gene was significantly upregulated in response to cold stress and peaked at 24-48 h, hinting at its potential role in the mechanism of cold resistance in L. chinense. Moreover, GFP-fused LcCOBL2, LcCOBL4 and LcCOBL5 were found to be localized in the cytomembrane. In summary, we expect these results to be beneficial for research on both the functions of LcCOBL genes and resistance breeding in L. chinense.

Diversity-Oriented Synthesis of Glycosylphosphatidylinositol Probes Based on an Orthogonally Protected Pentasaccharide

Two glycosylphosphatidylinositol (GPI) derivatives having an alkynyl group at different positions were derived from the same orthogonally protected pentasaccharide that in turn was assembled by a convergent [3+2] glycosylation strategy. The resultant alkynylated GPIs are useful biological probes and are suitable for further modification by click reaction to obtain other GPI probes. The pentasaccharide is a versatile platform for the synthesis of various uniquely functionalized GPI probes.

Labeling cell surface glycosylphosphatidylinositol-anchored proteins through metabolic engineering using an azide-modified phosphatidylinositol

Glycosylphosphatidylinositol (GPI) anchorage is one of the most common mechanisms to attach proteins to the plasma membrane of eukaryotic cells. GPI-anchored proteins (GPI-APs) play a critical role in many biological processes but are difficult to study. Here, a new method was developed for the effective and selective metabolic engineering and labeling of cell surface GPI-APs with an azide-modified phosphatidylinositol (PI) as the biosynthetic precursor of GPIs. It was demonstrated that this azido-PI derivative was taken up by HeLa cells and incorporated into the biosynthetic pathway of GPIs to present azide-labeled GPI-APs on the live cell surface. The azido group was used as a molecular handle to install other labels through a biocompatible click reaction to enable various biological studies, e.g., fluorescent imaging and protein pull-down, which can help explore the functions of GPI-APs and discover new GPI-APs.

FT-GPI, a highly sensitive and accurate predictor of GPI-anchored proteins, reveals the composition and evolution of the GPI proteome in Plasmodium species

BACKGROUND

Protozoan parasites are known to attach specific and diverse group of proteins to their plasma membrane via a GPI anchor. In malaria parasites, GPI-anchored proteins (GPI-APs) have been shown to play an important role in host-pathogen interactions and a key function in host cell invasion and immune evasion. Because of their immunogenic properties, some of these proteins have been considered as malaria vaccine candidates. However, identification of all possible GPI-APs encoded by these parasites remains challenging due to their sequence diversity and limitations of the tools used for their characterization.

METHODS

The FT-GPI software was developed to detect GPI-APs based on the presence of a hydrophobic helix at both ends of the premature peptide. FT-GPI was implemented in C ++and applied to study the GPI-proteome of 46 isolates of the order Haemosporida. Using the GPI proteome of Plasmodium falciparum strain 3D7 and Plasmodium vivax strain Sal-1, a heuristic method was defined to select the most sensitive and specific FT-GPI software parameters.

RESULTS

FT-GPI enabled revision of the GPI-proteome of P. falciparum and P. vivax, including the identification of novel GPI-APs. Orthology- and synteny-based analyses showed that 19 of the 37 GPI-APs found in the order Haemosporida are conserved among Plasmodium species. Our analyses suggest that gene duplication and deletion events may have contributed significantly to the evolution of the GPI proteome, and its composition correlates with speciation.

CONCLUSION

FT-GPI-based prediction is a useful tool for mining GPI-APs and gaining further insights into their evolution and sequence diversity. This resource may also help identify new protein candidates for the development of vaccines for malaria and other parasitic diseases.

An in-planta comparative study of Plasmopara viticola proteome reveals different infection strategies towards susceptible and Rpv3-mediated resistance hosts

Plasmopara viticola, an obligate biotrophic oomycete, is the causal agent of one of the most harmful grapevine diseases, downy mildew. Within this pathosystem, much information is gathered on the host, as characterization of pathogenicity and infection strategy of a biotrophic pathogen is quite challenging. Molecular insights into P. viticola development and pathogenicity are just beginning to be uncovered, mainly by transcriptomic studies. Plasmopara viticola proteome and secretome were only predicted based on transcriptome data. In this study, we have identified the in-planta proteome of P. viticola during infection of a susceptible ('Trincadeira') and a Rpv3-mediated resistance ('Regent') grapevine cultivar. Four hundred and twenty P. viticola proteins were identified on a label-free mass spectrometry-based approach of the apoplastic fluid of grapevine leaves. Overall, our study suggests that, in the compatible interaction, P. viticola manipulates salicylic-acid pathway and isoprenoid biosynthesis to enhance plant colonization. Furthermore, during the incompatible interaction, development-associated proteins increased while oxidoreductases protect P. viticola from ROS-associated plant defence mechanism. Up to our knowledge this is the first in-planta proteome characterization of this biotrophic pathogen, thus this study will open new insights into our understanding of this pathogen colonization strategy of both susceptible and Rpv3-mediated resistance grapevine genotypes.

Overexpression of LAS21 in Cellulase-Displaying Saccharomyces cerevisiae for High-Yield Ethanol Production from Pretreated Sugarcane Bagasse

The valorization of lignocellulosic feedstocks into biofuels and biochemicals has received much attention due to its environmental friendliness and sustainability. However, engineering an ideal microorganism that can both produce sufficient cellulases and ferment ethanol is highly challenging. In this study, we have tested seven different genes that are involved in glycosylphosphatidylinositol (GPI) biosynthesis and remodeling for the improvement of cellulase activity tethered on the S. cerevisiae cell surface. It was found that the overexpression of LAS21 can improve β-glucosidase activity by 48.8% compared to the original strain. Then, the three cellulase genes (cellobiohydrolase, endoglucanase, and β-glucosidase) and the LAS21 gene were co-introduced into a diploid thermotolerant S. cerevisiae strain by a multiple-round transformation approach, resulting in the cellulolytic ECBLCCE5 strain. Further optimization of the bioprocess parameters was found to enhance the ethanol yield of the ECBLCCE5 strain. Scaling up the valorization of pretreated sugarcane bagasses in a 1 L bioreactor resulted in a maximum ethanol concentration of 28.0 g/L (86.5% of theoretical yield). Our study provides a promising way to improve the economic viability of second-generation ethanol production. Moreover, the engineering of genes involved in GPI biosynthesis and remodeling can be applied to other yeast cell surface display applications.

Calcineurin Inhibitors Synergize with Manogepix to Kill Diverse Human Fungal Pathogens

Invasive fungal infections have mortality rates of 30–90%, depending on patient co-morbidities and the causative pathogen. The frequent emergence of drug resistance reduces the efficacy of currently approved treatment options, highlighting an urgent need for antifungals with new modes of action. Addressing this need, fosmanogepix (N-phosphonooxymethylene prodrug of manogepix; MGX) is the first in a new class of gepix drugs, and acts as a broad-spectrum, orally bioavailable inhibitor of the essential fungal glycosylphosphatidylinositol (GPI) acyltransferase Gwt1. MGX inhibits the growth of diverse fungal pathogens and causes accumulation of immature GPI-anchored proteins in the fungal endoplasmic reticulum. Relevant to the ongoing clinical development of fosmanogepix, we report a synergistic, fungicidal interaction between MGX and inhibitors of the protein phosphatase calcineurin against important human fungal pathogens. To investigate this synergy further, we evaluated a library of 124 conditional expression mutants covering 95% of the genes encoding proteins involved in GPI-anchor biosynthesis or proteins predicted to be GPI-anchored. Strong negative chemical-genetic interactions between the calcineurin inhibitor FK506 and eleven GPI-anchor biosynthesis genes were identified, indicating that calcineurin signalling is required for fungal tolerance to not only MGX, but to inhibition of the GPI-anchor biosynthesis pathway more broadly. Depletion of these GPI-anchor biosynthesis genes, like MGX treatment, also exposed fungal cell wall (1→3)-β-D-glucans. Taken together, these findings suggest the increased risk of invasive fungal infections associated with use of calcineurin inhibitors as immunosuppressants may be mitigated by their synergistic fungicidal interaction with (fos)manogepix and its ability to enhance exposure of immunostimulatory glucans.

Systematic Identification of Essential Genes Required for Yeast Cell Wall Integrity: Involvement of the RSC Remodelling Complex

Conditions altering the yeast cell wall lead to the activation of an adaptive transcriptional response mainly governed by the cell wall integrity (CWI) mitogen-activated protein kinase (MAPK) pathway. Two high-throughput screenings were developed using the yTHC collection of yeast conditional mutant strains to systematically identify essential genes related to cell wall integrity, and those required for the transcriptional program elicited by cell wall stress. Depleted expression of 52 essential genes resulted in hypersensitivity to the dye Calcofluor white, with chromatin organization, Golgi vesicle transport, rRNA processing, and protein glycosylation processes, as the most highly representative functional groups. Via a flow cytometry-based quantitative assay using a CWI reporter plasmid, 97 strains exhibiting reduced gene-reporter expression levels upon stress were uncovered, highlighting genes associated with RNA metabolism, transcription/translation, protein degradation, and chromatin organization. This screening also led to the discovery of 41 strains displaying a basal increase in CWI-associated gene expression, including mainly putative cell wall-related genes. Interestingly, several members of the RSC chromatin remodelling complex were uncovered in both screenings. Notably, Rsc9 was necessary to regulate the gene expression of CWI-related genes both under stress and non-stress conditions, suggesting distinct requirements of the RSC complex for remodelling particular genes.

Novel Promising Antifungal Target Proteins for Conquering Invasive Fungal Infections

Invasive fungal infections (IFIs) pose a serious clinical problem, but the antifungal arsenal is limited and has many disadvantages, such as drug resistance and toxicity. Hence, there is an urgent need to develop antifungal compounds that target novel target proteins of pathogenic fungi for treating IFIs. This review provides a comprehensive summary of the biological functions of novel promising target proteins for treating IFIs in pathogenic fungi and their inhibitors. Inhibitors of inositol phosphoramide (IPC) synthases (such as Aureobasidin A, Khafrefungin, Galbonolide A, and Pleofungin A) have potent antifungal activities by inhibiting sphingolipid synthesis. Disrupting glycosylphosphatidylinositol (GPI) biosynthesis by Jawsamycin (an inhibitor of Spt14), M720 (an inhibitor of Mcd4), and APX001A (an inhibitor of Gwt1) is a promising strategy for treating IFIs. Turbinmicin is a natural-compound inhibitor of Sec14 and has extraordinary antifungal efficacy, broad-antifungal spectrum, low toxicity, and is a promising new compound for treating IFIs. CMLD013075 targets fungal heat shock protein 90 (Hsp90) and has remarkable antifungal efficacy. Olorofim, as an inhibitor of dihydrolactate dehydrogenase, is a breakthrough drug treatment for IFIs. These novel target proteins and their inhibitors may overcome the limitations of currently available antifungal drugs and improve patient outcomes in the treatment of IFIs.

Molecular insights into biogenesis of glycosylphosphatidylinositol anchor proteins

Eukaryotic cells are coated with an abundance of glycosylphosphatidylinositol anchor proteins (GPI-APs) that play crucial roles in fertilization, neurogenesis, and immunity. The removal of a hydrophobic signal peptide and covalent attachment of GPI at the new carboxyl terminus are catalyzed by an endoplasmic reticulum membrane GPI transamidase complex (GPI-T) conserved among all eukaryotes. Here, we report the cryo-electron microscopy (cryo-EM) structure of the human GPI-T at a global 2.53-Å resolution, revealing an equimolar heteropentameric assembly. Structure-based mutagenesis suggests a legumain-like mechanism for the recognition and cleavage of proprotein substrates, and an endogenous GPI in the structure defines a composite cavity for the lipid substrate. This elongated active site, stemming from the membrane and spanning an additional ~22-Å space toward the catalytic dyad, is structurally suited for both substrates which feature an amphipathic pattern that matches this geometry. Our work presents an important step towards the mechanistic understanding of GPI-AP biosynthesis.

GPI-T catalyzes the committed step in GPI anchor protein biogenesis. Here, Xu et al. report the cryo-EM structure of the human GPI-T, revealing critical elements within an elongated, shared active site which is topologically arranged for substrate specificity.

The Leishmania donovani Ortholog of the Glycosylphosphatidylinositol Anchor Biosynthesis Cofactor PBN1 Is Essential for Host Infection

Visceral leishmaniasis is a deadly infectious disease caused by Leishmania donovani, a kinetoplastid parasite for which no licensed vaccine is available. To identify potential vaccine candidates, we systematically identified genes encoding putative cell surface and secreted proteins essential for parasite viability and host infection. We identified a protein encoded by LdBPK_061160 which, when ablated, resulted in a remarkable increase in parasite adhesion to tissue culture flasks. Here, we show that this phenotype is caused by the loss of glycosylphosphatidylinositol (GPI)-anchored surface molecules and that LdBPK_061160 encodes a noncatalytic component of the L. donovani GPI-mannosyltransferase I (GPI-MT I) complex. GPI-anchored surface molecules were rescued in the LdBPK_061160 mutant by the ectopic expression of both human genes PIG-X and PIG-M, but neither gene could complement the phenotype alone. From further sequence comparisons, we conclude that LdBPK_061160 is the functional orthologue of yeast PBN1 and mammalian PIG-X, which encode the noncatalytic subunits of their respective GPI-MT I complexes, and we assign LdBPK_061160 as LdPBN1. The LdPBN1 mutants could not establish a visceral infection in mice, a phenotype that was rescued by constitutive expression of LdPBN1. Although mice infected with the null mutant did not develop an infection, exposure to these parasites provided significant protection against subsequent infection with a virulent strain. In summary, we have identified the orthologue of the PBN1/PIG-X noncatalytic subunit of GPI-MT I in trypanosomatids, shown that it is essential for infection in a murine model of visceral leishmaniasis, and demonstrated that the LdPBN1 mutant shows promise for the development of an attenuated live vaccine.

Comparison of the Unfolded Protein Response in Cellobiose Utilization of Recombinant Angel- and W303-1A-Derived Yeast Expressing β-Glucosidase

The unfolded protein response (UPR) is one of the most important protein quality control mechanisms in cells. At least, three factors are predicted to activate the UPR in yeast cells during fermentation. Using UPRE-lacZ as a reporter, we constructed two indicator strains, KZ and WZ, based on Angel-derived K-a and W303-1A strains, respectively, and investigated their UPR response to tunicamycin, ethanol, and acetic acid. Then, four strains carrying plasmids BG-cwp2 and BG were obtained to realize the displaying and secretion of β-glucosidase, respectively. The results of cellobiose utilization assays indicated interactions between the UPR and the metabolic burden between the strain source, anchoring moiety, oxygen supply, and cellobiose concentration. Meanwhile, as expected, growth (OD600), β-glucosidase, and β-galactosidase activities were shown to have a positive inter-relationship, in which the values of the KZ-derived strains were far lower than those of the WZ-derived strains. Additionally, extra metabolic burden by displaying over secreting was also much more serious in strain KZ than in strain WZ. The maximum ethanol titer of the four strains (KZ (BG-cwp2), KZ (BG), WZ (BG-cwp2), and WZ (BG)) in oxygen-limited 10% cellobiose fermentation was 3.173, 5.307, 5.495, and 5.486% (v/v), respectively, and the acetic acid titer ranged from 0.038 to 0.060% (v/v). The corresponding maximum values of the ratio of β-galactosidase activity to that of the control were 3.30, 5.29, 6.45, and 8.72, respectively. Under aerobic conditions with 2% cellobiose, those values were 3.79, 4.97, 6.99, and 7.67, respectively. A comparison of the results implied that β-glucosidase expression durably induced the UPR, and the effect of ethanol and acetic acid depended on the titer produced. Further study is necessary to identify ethanol- or acid-specific target gene expression. Taken together, our results indicated that the host strain W303-1A is a better secretory protein producer, and the first step to modify strain K-a for cellulosic ethanol fermentation would be to relieve the bottleneck of UPR capacity. The results of the present study will help to identify candidate host strains and optimize expression and fermentation by quantifying UPR induction.

Glycosylphosphatidylinositol Anchor Biosynthesis Pathway-Related Protein GPI7 Is Required for the Vegetative Growth and Pathogenicity of Colletotrichum graminicola

Glycosylphosphatidylinositol (GPI) anchoring is a common post-translational modification in eukaryotic cells and has been demonstrated to have a wide range of biological functions, such as signal transduction, cellular adhesion, protein transport, immune response, and maintaining cell wall integrity. More than 25 proteins have been proven to participate in the GPI anchor synthesis pathway which occurs in the cytoplasmic and the luminal face of the ER membrane. However, the essential proteins of the GPI anchor synthesis pathway are still less characterized in maize pathogen Colletotrichum graminicola. In the present study, we analyzed the biological function of the GPI anchor synthesis pathway-related gene, CgGPI7, that encodes an ethanolamine phosphate transferase, which is localized in ER. The vegetative growth and conidia development of the ΔCgGPI7 mutant was significantly impaired in C. graminicola. and qRT-PCR results showed that the transcriptional level of CgGPI7 was specifically induced in the initial infection stage and that the pathogenicity of ΔCgGPI7 mutant was also significantly decreased compared with the wild type. Furthermore, the ΔCgGPI7 mutant displayed more sensitivity to cell wall stresses, suggesting that CgGPI7 may play a role in the cell wall integrity of C. graminicola. Cell wall synthesis-associated genes were also quantified in the ΔCgGPI7 mutant, and the results showed that chitin and β-1,3-glucans synthesis genes were significantly up-regulated in ΔCgGPI7 mutants. Our results suggested that CgGPI7 is required for vegetative growth and pathogenicity and might depend on the cell wall integrity of C. graminicola.

Characterization of Arabidopsis Post-Glycosylphosphatidylinositol Attachment to Proteins Phospholipase 3 Like Genes

Lipid remodeling of Glycosylphosphatidylinositol (GPI) anchors is required for their maturation and may influence the localization and function of GPI-anchored proteins (GPI-APs). Maturation of GPI-anchors is well characterized in animals and fungi but very little is known about this process in plants. In yeast, the GPI-lipid remodeling occurs entirely at the ER and is initiated by the remodeling enzyme Bst1p (Post-Glycosylphosphatidylinositol Attachment to Proteins inositol deacylase 1 -PGAP1- in mammals and Arabidopsis). Next, the remodeling enzyme Per1p (Post-Glycosylphosphatidylinositol Attachment to Proteins phospholipase 3 -PGAP3- in mammals) removes a short, unsaturated fatty acid of phosphatidylinositol (PI) that is replaced with a very long-chain saturated fatty acid or ceramide to complete lipid remodeling. In mammals, lipid remodeling starts at the ER and is completed at the Golgi apparatus. Studies of the Arabidopsis PGAP1 gene showed that the lipid remodeling of the GPI anchor is critical for the final localization of GPI-APs. Here we characterized loss-of-function mutants of Arabidopsis Per1/PGAP3 like genes (AtPGAP3A and AtPGAP3B). Our results suggest that PGAP3A function is required for the efficient transport of GPI-anchored proteins from the ER to the plasma membrane/cell wall. In addition, loss of function of PGAP3A increases susceptibility to salt and osmotic stresses that may be due to the altered localization of GPI-APs in this mutant. Furthermore, PGAP3B complements a yeast strain lacking PER1 gene suggesting that PGAP3B and Per1p are functional orthologs. Finally, subcellular localization studies suggest that PGAP3A and PGAP3B cycle between the ER and the Golgi apparatus.

Remodeling-defective GPI-anchored proteins on the plasma membrane activate the spindle assembly checkpoint

Newly synthesized glycosylphosphatidylinositol-anchored proteins (GPI-APs) undergo extensive remodeling prior to transport to the plasma membrane. GPI-AP remodeling events serve as quality assurance signatures, and complete remodeling of the anchor functions as a transport warrant. Using a genetic approach in yeast cells, we establish that one remodeling event, the removal of ethanolamine-phosphate from mannose 2 via Ted1p (yPGAP5), is essential for cell viability in the absence of the Golgi-localized putative phosphodiesterase Dcr2p. While GPI-APs in which mannose 2 has not been remodeled in dcr2 ted1-deficient cells can still be delivered to the plasma membrane, their presence elicits a unique stress response. Stress is sensed by Mid2p, a constituent of the cell wall integrity pathway, whereupon signal promulgation culminates in activation of the spindle assembly checkpoint. Our results are consistent with a model in which cellular stress response and chromosome segregation checkpoint pathways are functionally interconnected.

AtPGAP1 functions as a GPI inositol-deacylase required for efficient transport of GPI-anchored proteins

Glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) play an important role in a variety of plant biological processes including growth, stress response, morphogenesis, signaling, and cell wall biosynthesis. The GPI anchor contains a lipid-linked glycan backbone that is synthesized in the endoplasmic reticulum (ER) where it is subsequently transferred to the C-terminus of proteins containing a GPI signal peptide by a GPI transamidase. Once the GPI anchor is attached to the protein, the glycan and lipid moieties are remodeled. In mammals and yeast, this remodeling is required for GPI-APs to be included in Coat Protein II-coated vesicles for their ER export and subsequent transport to the cell surface. The first reaction of lipid remodeling is the removal of the acyl chain from the inositol group by Bst1p (yeast) and Post-GPI Attachment to Proteins Inositol Deacylase 1 (PGAP1, mammals). In this work, we have used a loss-of-function approach to study the role of PGAP1/Bst1 like genes in plants. We have found that Arabidopsis (Arabidopsis thaliana) PGAP1 localizes to the ER and likely functions as the GPI inositol-deacylase that cleaves the acyl chain from the inositol ring of the GPI anchor. In addition, we show that PGAP1 function is required for efficient ER export and transport to the cell surface of GPI-APs.

Isolation and Identification of Talaromyces sp. Strain Q2 and Its Biocontrol Mechanisms Involved in the Control of Fusarium Wilt

Fusarium wilt is an important disease of many food crops and often causes serious damages to yield and food quality. Consequently, numerous studies mainly focused on exploring the control strategy for Fusarium oxysporum as well as the mechanism of interaction between the F. oxysporum and other beneficial soil microorganisms. In this study, we have screened and identified an efficient biocontrol strain from the soil with infection of F. oxysporum f. sp. momordica (referred to as Fom), Talaromyces purpurogenus Q2 (referred to as TpQ2), which could be effective to reduce relative abundance of the rhizospheric Fom, leading to a significant decrease of Fusarium wilt disease incidence in bitter gourd during the greenhouse and field trails. TpQ2 can reduce the relative abundance of rhizospheric Fom through inhibition of growth and development of Fom. During the co-cultivation of TpQ2 and Fom, we confirmed that TpQ2 could significantly suppress the growth and development of Fom through disturbing the normal hyphae shape and function of the cell walls of Fom via secreting cell wall–degrading enzymes and suppression of the expression of cell wall biosynthesis genes, such as FomCFEM. In the meantime, TpQ2 showed a strong negative correlation with F. oxysporum in soil and positive correlation with beneficial indigenous microorganisms that had significant negative correlation with Fusarium populations, such as Streptomycetes, Lysobacter, and Sphingobium. To summarize, TpQ2 has a good biocontrol efficacy on Fusarium wilt of bitter gourd. The biocontrol mechanisms of TpQ2 on Fusarium wilt are complex and diverse.

Role of Protein Glycosylation in Interactions of Medically Relevant Fungi with the Host

Protein glycosylation is a highly conserved post-translational modification among organisms. It plays fundamental roles in many biological processes, ranging from protein trafficking and cell adhesion to host–pathogen interactions. According to the amino acid side chain atoms to which glycans are linked, protein glycosylation can be divided into two major categories: N-glycosylation and O-glycosylation. However, there are other types of modifications such as the addition of GPI to the C-terminal end of the protein. Besides the importance of glycoproteins in biological functions, they are a major component of the fungal cell wall and plasma membrane and contribute to pathogenicity, virulence, and recognition by the host immunity. Given that this structure is absent in host mammalian cells, it stands as an attractive target for developing selective compounds for the treatment of fungal infections. This review focuses on describing the relationship between protein glycosylation and the host–immune interaction in medically relevant fungal species.

Protein sorting: A new quality control pathway for GPI-anchored proteins

Glycosylphosphatidylinositol-anchored proteins are a class of lipid-anchored membrane proteins found at the surface of all eukaryotic cells. New work provides genome-wide insights into mechanisms that mediate quality control of the folding of this important protein family.

Structural analysis of the GPI glycan

Glycosylphosphatidylinositol (GPI) anchoring of proteins is an essential post-translational modification in all eukaryotes that occurs at the endoplasmic reticulum (ER) and serves to deliver GPI-anchored proteins (GPI-APs) to the cell surface where they play a wide variety of vital physiological roles. This paper describes a specialized method for purification and structural analysis of the GPI glycan of individual GPI-APs in yeast. The protocol involves the expression of a specific GPI-AP tagged with GFP, enzymatic release from the cellular membrane fraction, immunopurification, separation by electrophoresis and analysis of the peptides bearing GPI glycans by mass spectrometry after trypsin digestion. We used specifically this protocol to address the structural remodeling that undergoes the GPI glycan of a specific GPI-AP during its transport to the cell surface. This method can be also applied to investigate the GPI-AP biosynthetic pathway and to directly confirm predicted GPI-anchoring of individual proteins.

Functional Analysis of the GPI Transamidase Complex by Screening for Amino Acid Mutations in Each Subunit

Glycosylphosphatidylinositol (GPI) anchor modification is a posttranslational modification of proteins that has been conserved in eukaryotes. The biosynthesis and transfer of GPI to proteins are carried out in the endoplasmic reticulum. Attachment of GPI to proteins is mediated by the GPI-transamidase (GPI-TA) complex, which recognizes and cleaves the C-terminal GPI attachment signal of precursor proteins. Then, GPI is transferred to the newly exposed C-terminus of the proteins. GPI-TA consists of five subunits: PIGK, GPAA1, PIGT, PIGS, and PIGU, and the absence of any subunit leads to the loss of activity. Here, we analyzed functionally important residues of the five subunits of GPI-TA by comparing conserved sequences among homologous proteins. In addition, we optimized the purification method for analyzing the structure of GPI-TA. Using purified GPI-TA, preliminary single particle images were obtained. Our results provide guidance for the structural and functional analysis of GPI-TA.

Diversity of GPI-anchored fungal adhesins

Selective adhesion of fungal cells to one another and to foreign surfaces is fundamental for the development of multicellular growth forms and the successful colonization of substrates and host organisms. Accordingly, fungi possess diverse cell wall-associated adhesins, mostly large glycoproteins, which present N-terminal adhesion domains at the cell surface for ligand recognition and binding. In order to function as robust adhesins, these glycoproteins must be covalently linkedto the cell wall via C-terminal glycosylphosphatidylinositol (GPI) anchors by transglycosylation. In this review, we summarize the current knowledge on the structural and functional diversity of so far characterized protein families of adhesion domains and set it into a broad context by an in-depth bioinformatics analysis using sequence similarity networks. In addition, we discuss possible mechanisms for the membrane-to-cell wall transfer of fungal adhesins by membrane-anchored Dfg5 transglycosidases.

A knockout cell library of GPI biosynthetic genes for functional studies of GPI-anchored proteins

Over 100 kinds of proteins are expressed as glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) on the cell surface in mammalian cells. GPI-APs possess unique properties in terms of their intracellular trafficking and association with lipid rafts. Although it is clear that GPI-APs play critical roles in various biological phenomena, it is poorly understood how the GPI moiety contributes to these mechanisms. More than 30 genes are involved in the correct biosynthesis of GPI-APs. We here constructed a cell library in which 32 genes involved in GPI biosynthesis were knocked out in human embryonic kidney 293 cells. Using the cell library, the surface expression and sensitivity to phosphatidylinositol-specific phospholipase C of GPI-APs were analyzed. Furthermore, we identified structural motifs of GPIs that are recognized by a GPI-binding toxin, aerolysin. The cell-based GPI-knockout library could be applied not only to basic researches, but also to applications and methodologies related to GPI-APs.

Proteomic identification of the UDP-GlcNAc: PI α1-6 GlcNAc-transferase subunits of the glycosylphosphatidylinositol biosynthetic pathway of Trypanosoma brucei

The first step of glycosylphosphatidylinositol (GPI) anchor biosynthesis in all eukaryotes is the addition of N-acetylglucosamine (GlcNAc) to phosphatidylinositol (PI) which is catalysed by a UDP-GlcNAc: PI α1-6 GlcNAc-transferase, also known as GPI GnT. This enzyme has been shown to be a complex of seven subunits in mammalian cells and a similar complex of six homologous subunits has been postulated in yeast. Homologs of these mammalian and yeast subunits were identified in the Trypanosoma brucei predicted protein database. The putative catalytic subunit of the T. brucei complex, TbGPI3, was epitope tagged with three consecutive c-Myc sequences at its C-terminus. Immunoprecipitation of TbGPI3-3Myc followed by native polyacrylamide gel electrophoresis and anti-Myc Western blot showed that it is present in a ~240 kDa complex. Label-free quantitative proteomics were performed to compare anti-Myc pull-downs from lysates of TbGPI-3Myc expressing and wild type cell lines. TbGPI3-3Myc was the most highly enriched protein in the TbGPI3-3Myc lysate pull-down and the expected partner proteins TbGPI15, TbGPI19, TbGPI2, TbGPI1 and TbERI1 were also identified with significant enrichment. Our proteomics data also suggest that an Arv1-like protein (TbArv1) is a subunit of the T. brucei complex. Yeast and mammalian Arv1 have been previously implicated in GPI biosynthesis, but here we present the first experimental evidence for physical association of Arv1 with GPI biosynthetic machinery. A putative E2-ligase has also been tentatively identified as part of the T. brucei UDP-GlcNAc: PI α1-6 GlcNAc-transferase complex.

Cell wall-associated effectors of plant-colonizing fungi

Plant-colonizing fungi secrete a cocktail of effector proteins during colonization. After secretion, some of these effectors are delivered into plant cells to directly dampen the plant immune system or redirect host processes benefitting fungal growth. Other effectors function in the apoplastic space either as released proteins modulating the activity of plant enzymes associated with plant defense or as proteins bound to the fungal cell wall. For such fungal cell wall-bound effectors, we know particularly little about their molecular function. In this review, we describe effectors that are associated with the fungal cell wall and discuss how they contribute to colonization.

Potential Physiological Relevance of ERAD to the Biosynthesis of GPI-Anchored Proteins in Yeast

Misfolded and/or unassembled secretory and membrane proteins in the endoplasmic reticulum (ER) may be retro-translocated into the cytoplasm, where they undergo ER-associated degradation, or ERAD. The mechanisms by which misfolded proteins are recognized and degraded through this pathway have been studied extensively; however, our understanding of the physiological role of ERAD remains limited. This review describes the biosynthesis and quality control of glycosylphosphatidylinositol (GPI)-anchored proteins and briefly summarizes the relevance of ERAD to these processes. While recent studies suggest that ERAD functions as a fail-safe mechanism for the degradation of misfolded GPI-anchored proteins, several pieces of evidence suggest an intimate interaction between ERAD and the biosynthesis of GPI-anchored proteins.

CaNRT2.1 Is Required for Nitrate but Not Nitrite Uptake in Chili Pepper Pathogen Colletotrichum acutatum

Chili peppers are an important food additive used in spicy cuisines worldwide. However, the yield and quality of chilis are threatened by anthracnose disease caused by Colletotrichum acutatum. Despite the impact of C. acutatum on chili production, the genes involved in fungal development and pathogenicity in this species have not been well characterized. In this study, through T-DNA insertional mutagenesis, we identified a mutant strain termed B7, which is defective for the growth of C. acutatum on a minimal nutrient medium. Our bioinformatics analysis revealed that a large fragment DNA (19.8 kb) is deleted from the B7 genome, thus resulting in the deletion of three genes, including CaGpiP1 encoding a glycosylphosphatidyl-inisotol (GPI)-anchored protein, CaNRT2.1 encoding a membrane-bound nitrate/nitrite transporter, and CaRQH1 encoding a RecQ helicase protein. In addition, T-DNA is inserted upstream of the CaHP1 gene encoding a hypothetical protein. Functional characterization of CaGpiP1, CaNRT2.1, and CaHP1 by targeted gene disruption and bioassays indicated that CaNRT2.1 is responsible for the growth-defective phenotype of B7. Both B7 and CaNRT2.1 mutant strains cannot utilize nitrate as nitrogen sources, thus restraining the fungal growth on a minimal nutrient medium. In addition to CaNRT2.1, our results showed that CaGpiP1 is a cell wall-associated GPI-anchored protein. However, after investigating the functions of CaGpiP1 and CaHP1 in fungal pathogenicity, growth, development and stress tolerance, we were unable to uncover the roles of these two genes in C. acutatum. Collectively, in this study, our results identify the growth-defective strain B7 via T-DNA insertion and reveal the critical role of CaNRT2.1 in nitrate transportation for the fungal growth of C. acutatum.

Cdc1p is a Golgi-localized glycosylphosphatidylinositol-anchored protein remodelase

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) undergo extensive posttranslational modifications and remodeling, including the addition and subsequent removal of phosphoethanolamine (EtNP) from mannose 1 (Man1) and mannose 2 (Man2) of the glycan moiety. Removal of EtNP from Man1 is catalyzed by Cdc1p, an event that has previously been considered to occur in the endoplasmic reticulum (ER). We establish that Cdc1p is in fact a cis/medial Golgi membrane protein that relies on the COPI coatomer for its retention in this organelle. We also determine that Cdc1p does not cycle between the Golgi and the ER, and consistent with this finding, when expressed at endogenous levels ER-localized Cdc1p-HDEL is unable to support the growth of cdc1Δ cells. Our cdc1 temperature-sensitive alleles are defective in the transport of a prototypical GPI-AP-Gas1p to the cell surface, a finding we posit reveals a novel Golgi-localized quality control warrant. Thus, yeast cells scrutinize GPI-APs in the ER and also in the Golgi, where removal of EtNP from Man2 (via Ted1p in the ER) and from Man1 (by Cdc1p in the Golgi) functions as a quality assurance signal.

Three Decades of Advances in Arabinogalactan-Protein Biosynthesis

Arabinogalactan-proteins (AGPs) are a large, complex, and highly diverse class of heavily glycosylated proteins that belong to the family of cell wall hydroxyproline-rich glycoproteins. Approximately 90% of the molecules consist of arabinogalactan polysaccharides, which are composed of arabinose and galactose as major sugars and minor sugars such as glucuronic acid, fucose, and rhamnose. About half of the AGP family members contain a glycosylphosphatidylinositol (GPI) lipid anchor, which allows for an association with the outer leaflet of the plasma membrane. The mysterious AGP family has captivated the attention of plant biologists for several decades. This diverse family of glycoproteins is widely distributed in the plant kingdom, including many algae, where they play fundamental roles in growth and development processes. The journey of AGP biosynthesis begins with the assembly of amino acids into peptide chains of proteins. An N-terminal signal peptide directs AGPs toward the endoplasmic reticulum, where proline hydroxylation occurs and a GPI anchor may be added. GPI-anchored AGPs, as well as unanchored AGPs, are then transferred to the Golgi apparatus, where extensive glycosylation occurs by the action of a variety glycosyltransferase enzymes. Following glycosylation, AGPs are transported by secretory vesicles to the cell wall or to the extracellular face of the plasma membrane (in the case of GPI-anchored AGPs). GPI-anchored proteins can be released from the plasma membrane into the cell wall by phospholipases. In this review, we present an overview of the accumulated knowledge on AGP biosynthesis over the past three decades. Particular emphasis is placed on the glycosylation of AGPs as the sugar moiety is essential to their function. Recent genetics and genomics approaches have significantly contributed to a broader knowledge of AGP biosynthesis. However, many questions remain to be elucidated in the decades ahead.

The role of very long chain fatty acids in yeast physiology and human diseases

Fatty acids (FAs) are a highly diverse class of molecules that can have variable chain length, number of double bonds and hydroxylation sites. FAs with 22 or more carbon atoms are described as very long chain fatty acids (VLCFAs). VLCFAs are synthesized in the endoplasmic reticulum (ER) through a four-step elongation cycle by membrane embedded enzymes. VLCFAs are precursors for the synthesis of sphingolipids (SLs) and glycerophospholipids. Besides their role as lipid constituents, VLCFAs are also found as precursors of lipid mediators. Mis-regulation of VLCFA metabolism can result in a variety of inherited diseases ranging from ichthyosis, to myopathies and demyelination. The enzymes for VLCFA biosynthesis are evolutionary conserved and many of the pioneering studies were performed in the model organism Saccharomyces cerevisiae. A growing body of evidence suggests that VLCFA metabolism is intricately regulated to maintain lipid homeostasis. In this review we will describe the metabolism of VLCFAs, how they are synthesized, transported and degraded and how these processes are regulated, focusing on budding yeast. We will review how lipid metabolism and membrane properties are affected by VLCFAs and which impact mutations in the biosynthetic genes have on physiology. We will also briefly describe diseases caused by mis-regulation of VLCFAs in human cells.

Galactomannan Produced by Aspergillus fumigatus: An Update on the Structure, Biosynthesis and Biological Functions of an Emblematic Fungal Biomarker

The galactomannan (GM) that is produced by the human fungal pathogen Aspergillus fumigatus is an emblematic biomarker in medical mycology. The GM is composed of two monosaccharides: mannose and galactofuranose. The furanic configuration of galactose residues, absent in mammals, is responsible for the antigenicity of the GM and has favoured the development of ELISA tests to diagnose aspergillosis in immunocompromised patients. The GM that is produced by A. fumigatus is a unique fungal polysaccharide containing a tetramannoside repeat unit and having three different forms: (i) membrane bound through a glycosylphosphatidylinositol (GPI)-anchor, (ii) covalently linked to β-1,3-glucans in the cell wall, or (iii) released in the culture medium as a free polymer. Recent studies have revealed the crucial role of the GM during vegetative and polarized fungal growth. This review highlights these recent data on its biosynthetic pathway and its biological functions during the saprophytic and pathogenic life of this opportunistic human fungal pathogen.

Antifungal Activity of Gepinacin Scaffold Glycosylphosphatidylinositol Anchor Biosynthesis Inhibitors with Improved Metabolic Stability

The glycosylphosphatidylinositol anchor biosynthesis inhibitor gepinacin demonstrates broad-spectrum antifungal activity and negligible mammalian toxicity in culture but is metabolically labile. The stability and bioactivity of 39 analogs were tested in vitro to identify LCUT-8, a stabilized lead with increased potency and promising single-dose pharmacokinetics. Unfortunately, no antifungal activity was seen at the maximum dosing achievable in a neutropenic rabbit model. Nevertheless, structure-activity relationships identified here suggest strategies to further improve compound potency, solubility, and stability.

Construction and screening of a glycosylphosphatidylinositol protein deletion library in Pichia pastoris

Background

Glycosylphosphatidylinositol (GPI)-anchored glycoproteins have diverse intrinsic functions in yeasts, and they also have different uses in vitro. In this study, the functions of potential GPI proteins in Pichia pastoris were explored by gene knockout approaches.

Results

Through an extensive knockout of GPI proteins in P. pastoris, a single-gene deletion library was constructed for 45 predicted GPI proteins. The knockout of proteins may lead to the activation of a cellular response named the ‘compensatory mechanism’, which is characterized by changes in the content and relationship between cell wall polysaccharides and surface proteins. Among the 45 deletion strains, five showed obvious methanol tolerance, four owned high content of cell wall polysaccharides, and four had a high surface hydrophobicity. Some advantages of these strains as production hosts were revealed. Furthermore, the deletion strains with high surface hydrophobicity were used as hosts to display Candida antarctica lipase B (CALB). The strain gcw22Δ/CALB-GCW61 showed excellent fermentation characteristics, including a faster growth rate and higher hydrolytic activity.

Conclusions

This GPI deletion library has some potential applications for production strains and offers a valuable resource for studying the precise functions of GPI proteins, especially their putative functions.