Molecular analysis of CaMnt1p, a mannosyl transferase important for adhesion and virulence of Candida albicans

An improved expression and purification protocol enables the structural characterization of Mnt1, an antifungal target from Candida albicans

BACKGROUND

Candida albicans is one of the most prevalent fungi causing infections in the world. Mnt1 is a mannosyltransferase that participates in both the cell wall biogenesis and biofilm growth of C. albicans. While the cell wall performs crucial functions in pathogenesis, biofilm growth is correlated with sequestration of drugs by the extracellular matrix. Therefore, antifungals targeting CaMnt1 can compromise fungal development and potentially also render Candida susceptible to drug therapy. Despite its importance, CaMnt1 has not yet been purified to high standards and its biophysical properties are lacking.

RESULTS

We describe a new protocol to obtain high yield of recombinant CaMnt1 in Komagataella phaffii using methanol induction. The purified protein's identity was confirmed by MALDI-TOF/TOF mass spectroscopy. The Far-UV circular dichroism (CD) spectra demonstrate that the secondary structure of CaMnt1 is compatible with a protein formed by α-helices and β-sheets at pH 7.0. The fluorescence spectroscopy results show that the tertiary structure of CaMnt1 is pH-dependent, with a greater intensity of fluorescence emission at pH 7.0. Using our molecular modeling protocol, we depict for the first time the ternary complex of CaMnt1 bound to its two substrates, which has enabled the identification of residues involved in substrate specificity and catalytic reaction. Our results corroborate the hypothesis that Tyr209 stabilizes the formation of an oxocarbenium ion-like intermediate during nucleophilic attack of the acceptor sugar, opposing the double displacement mechanism proposed by other reports.

CONCLUSIONS

The methodology presented here can substantially improve the yield of recombinant CaMnt1 expressed in flask-grown yeasts. In addition, the structural characterization of the fungal mannosyltransferase presents novelties that can be exploited for new antifungal drug's development.

In it together: Candida-bacterial oral biofilms and therapeutic strategies

Under natural environmental settings or in the human body, the majority of microorganisms exist in complex polymicrobial biofilms adhered to abiotic and biotic surfaces. These microorganisms exhibit symbiotic, mutualistic, synergistic, or antagonistic relationships with other species during biofilm colonization and development. These polymicrobial interactions are heterogeneous, complex and hard to control, thereby often yielding worse outcomes than monospecies infections. Concerning fungi, Candida spp., in particular, Candida albicans is often detected with various bacterial species in oral biofilms. These Candida-bacterial interactions may induce the transition of C. albicans from commensal to pathobiont or dysbiotic organism. Consequently, Candida-bacterial interactions are largely associated with various oral diseases, including dental caries, denture stomatitis, periodontitis, peri-implantitis, and oral cancer. Given the severity of oral diseases caused by cross-kingdom consortia that develop hard-to-remove and highly drug-resistant biofilms, fundamental research is warranted to strategically develop cost-effective and safe therapies to prevent and treat cross-kingdom interactions and subsequent biofilm development. While studies have shed some light, targeting fungal-involved polymicrobial biofilms has been limited. This mini-review outlines the key features of Candida-bacterial interactions and their impact on various oral diseases. In addition, current knowledge on therapeutic strategies to target Candida-bacterial polymicrobial biofilms is discussed.

Adhesins in the virulence of opportunistic fungal pathogens of human

Aspergillosis, candidiasis, and cryptococcosis are the most common cause of mycoses-related disease and death among immune-compromised patients. Adhesins are cell-surface exposed proteins or glycoproteins of pathogens that bind to the extracellular matrix (ECM) constituents or mucosal epithelial surfaces of the host cells. The forces of interaction between fungal adhesins and host tissues are accompanied by ligand binding, hydrophobic interactions and protein-protein aggregation. Adherence is the primary and critical step involved in the pathogenesis; however, there is limited information on fungal adhesins compared to that on the bacterial adhesins. Except a few studies based on screening of proteome for adhesin identification, majority are based on characterization of individual adhesins. Recently, based on their characteristic signatures, many putative novel fungal adhesins have been predicted using bioinformatics algorithms. Some of these novel adhesin candidates have been validated by in-vitro studies; though, most of them are yet to be characterised experimentally. Morphotype specific adhesin expression as well as tissue tropism are the crucial determinants for a successful adhesion process. This review presents a comprehensive overview of various studies on fungal adhesins and discusses the targetability of the adhesins and adherence phenomenon, for combating the fungal infection in a preventive or therapeutic mode.

Candida albicans PPG1, a serine/threonine phosphatase, plays a vital role in central carbon metabolisms under filament-inducing conditions: A multi-omics approach

Candida albicans is the leading cause of life-threatening bloodstream candidiasis, especially among immunocompromised patients. The reversible morphological transition from yeast to hyphal filaments in response to host environmental cues facilitates C. albicans tissue invasion, immune evasion, and dissemination. Hence, it is widely considered that filamentation represents one of the major virulence properties in C. albicans. We have previously characterized Ppg1, a PP2A-type protein phosphatase that controls filament extension and virulence in C. albicans. This study conducted RNA sequencing analysis of samples obtained from C. albicans wild type and ppg1Δ/Δ strains grown under filament-inducing conditions. Overall, ppg1Δ/Δ strain showed 1448 upregulated and 710 downregulated genes, representing approximately one-third of the entire annotated C. albicans genome. Transcriptomic analysis identified significant downregulation of well-characterized genes linked to filamentation and virulence, such as ALS3, HWP1, ECE1, and RBT1. Expression analysis showed that essential genes involved in C. albicans central carbon metabolisms, including GDH3, GPD1, GPD2, RHR2, INO1, AAH1, and MET14 were among the top upregulated genes. Subsequent metabolomics analysis of C. albicans ppg1Δ/Δ strain revealed a negative enrichment of metabolites with carboxylic acid substituents and a positive enrichment of metabolites with pyranose substituents. Altogether, Ppg1 in vitro analysis revealed a link between metabolites substituents and filament formation controlled by a phosphatase to regulate morphogenesis and virulence.

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.

One Century of Study: What We Learned about Paracoccidioides and How This Pathogen Contributed to Advances in Antifungal Therapy

Paracoccidioidomycosis (PCM) is a notable fungal infection restricted to Latin America. Since the first description of the disease by Lutz up to the present day, Brazilian researchers have contributed to the understanding of the life cycle of this pathogen and provided the possibility of new targets for antifungal therapy based on the structural and functional genomics of Paracoccidioides. In this context, in silico approaches have selected molecules that act on specific targets, such as the thioredoxin system, with promising antifungal activity against Paracoccidioides. Some of these are already in advanced development stages. In addition, the application of nanostructured systems has addressed issues related to the high toxicity of conventional PCM therapy. Thus, the contribution of molecular biology and biotechnology to the advances achieved is unquestionable. However, it is still necessary to transcend the boundaries of synthetic chemistry, pharmaco-technics, and pharmacodynamics, aiming to turn promising molecules into newly available drugs for the treatment of fungal diseases.

Candida Auris Urinary Tract Infection in a Nursing Home Patient With Multicomorbidities

Candida (C.) auris is an opportunistic ascomycetous budding yeast that has been emerging as an invasive, multidrug-resistant pathogen over the past 11 years since its discovery. Candida auris infection has raised considerable attention in public health organizations due to its rising number of cases, virulence, and unique resistance to commonly used mycofungal therapy. This case follows a 64-year-old male with multiple comorbidities from the nursing home presenting with polybacterial sepsis along with a urinary tract infection growing Candida auris. Along with treatment for sepsis, the patient was placed on the Centers for Disease Control and Prevention's (CDC’s) recommended regimen of micafungin to eradicate C. auris infection and isolation precautions. Cases should be approached carefully and reported to public agencies such as the CDC and state health department.

Candida auris in the Age of Resistance

Candida auris (C. auris) is an opportunistic ascomycetous budding yeast that has been emerging as an invasive, multidrug-resistant pathogen over the past 10 years since its discovery. This fungi is the first to be labeled as a public health threat according to the Centers for Disease Control (CDC) and has since become a major problem in the United States. This serves as a detailed overview of the various factors contributing to the pathogenicity of C. auris.

Coronary Vasculitis Induced in Mice by the Cell Wall Mannoprotein of Candida krusei

Kawasaki disease (KD) is an inflammatory disease that was identified by Professor Tomisaku Kawasaki in 1961. Candida albicans-derived substances, such as the hot water extract of C. albicans (CADS) and Candida water-soluble fraction (CAWS), induced coronary vasculitis similar to KD in mice. An increasing proportion of deep-seated candidiasis cases are caused by non-albicans Candida and are often resistant to antifungal drugs. We herein investigated whether the hot water extract of C. krusei, inherently resistant to fluconazole, induces vasculitis in mice. Three strains of C. krusei, NBRC1395, NBRC1162, and NBRC10737, were cultured in natural (Y) and chemically defined (C) media and cell wall mannoprotein (MN) fractions were prepared by autoclaving cells (CKY1395MN, CKC1395MN, CKY1162MN, CKC1162MN, CKY10737MN, and CKC10737MN). All MN fractions reacted strongly with Concanavalin A (Con A) and dectin-2 and induced anaphylactoid shock in ICR mice. MNs induced severe coronary vasculitis in DBA/2 mice, resulting in cardiac hypertrophy. MNs also induced coronary vasculitis in C57Bl/6 mice. These results suggest that the MNs of non-albicans Candida, such as C. krusei, induce similar toxicity to those of C. albicans.

Presence of O-glycosidically linked oligosaccharides in the cell wall mannan of Candida krusei purified with Benanomicin A

Cell wall mannan of the pathogenic yeast Candida krusei was prepared using the antibiotic Benanomicin A, which has a lectin-like function. The chemical structure of this molecule was found to be similar to that of mannan prepared from the same yeast by the conventional method using Fehling reagent. Only a few degradation products were detected when the mannan prepared using Fehling reagent was subjected to alkali treatment (β-elimination), but multiple α-1,2-linked oligosaccharides were detected when the mannan purified with Benanomicin A was treated with alkali. These results indicate that most of the O-linked sugar chains in mannan were lost under conventional conditions when exposed to the strongly alkaline Fehling reagent. In contrast, the O-glycosidic bond in mannan was not cleaved and the O-linked sugar chains were maintained and almost intact following treatment with the mild novel preparation method using Benanomicin A. Therefore, we argue that the new mannan preparation method using Benanomicin A is superior to conventional methods. In addition, our study suggests that some yeast mannans, whose overall structure has already been reported, may contain more O-linked sugar chains than previously recognized.

Impact of the Environment upon the Candida albicans Cell Wall and Resultant Effects upon Immune Surveillance

The fungal cell wall is an essential organelle that maintains cellular morphology and protects the fungus from environmental insults. For fungal pathogens such as Candida albicans, it provides a degree of protection against attack by host immune defences. However, the cell wall also presents key epitopes that trigger host immunity and attractive targets for antifungal drugs. Rather than being a rigid shield, it has become clear that the fungal cell wall is an elastic organelle that permits rapid changes in cell volume and the transit of large liposomal particles such as extracellular vesicles. The fungal cell wall is also flexible in that it adapts to local environmental inputs, thereby enhancing the fitness of the fungus in these microenvironments. Recent evidence indicates that this cell wall adaptation affects host-fungus interactions by altering the exposure of major cell wall epitopes that are recognised by innate immune cells. Therefore, we discuss the impact of environmental adaptation upon fungal cell wall structure, and how this affects immune recognition, focussing on C. albicans and drawing parallels with other fungal pathogens.

Candida innate immunity at the mucosa

The tremendous diversity in microbial species that colonise the mucosal surfaces of the human body is only now beginning to be fully appreciated. Distinguishing between the behaviour of commensal microbes and harmful pathogens that reside at mucosal sites in the body is a complex, and exquisitely fine-tuned process central to mucosal health. The fungal pathobiont Candida albicans is frequently isolated from mucosal surfaces with an asymptomatic carriage rate of approximately 60% in the human population. While normally a benign member of the microbiota, overgrowth of C. albicans often results in localised mucosal infection causing morbidity in otherwise healthy individuals, and invasive infection that often causes death in the absence of effective immune defence. C. albicans triggers numerous innate immune responses at mucosal surfaces, and detection of C. albicans hyphae in particular, stimulates the production of antimicrobial peptides, danger-associated molecular patterns and cytokines that function to reduce fungal burdens during infection. This review will summarise our current understanding of innate immune responses to C. albicans at mucosal surfaces.

Targeting Candida spp. to develop antifungal agents

Invasive fungal infections are a complex challenge throughout the world because of their high incidence, mainly in critically ill patients, and high mortality rates. The antifungal agents currently available are limited; thus, there is a need for the rapid development of new drugs. In silico methods are a modern strategy to explore interactions between new compounds and specific fungal targets, but they depend on precise genetic information. Here, we discuss the main Candida spp. target genes, including information about null mutants, virulence, cytolocalization, co-regulatory genes, and compounds that are related to protein expression. These data will provide a basis for the future in silico development of antifungal drugs.

O-Mannosylation of Proteins Enables Histoplasma Yeast Survival at Mammalian Body Temperatures

The ability to grow at mammalian body temperatures is critical for pathogen infection of humans. For the thermally dimorphic fungal pathogen Histoplasma capsulatum, elevated temperature is required for differentiation of mycelia or conidia into yeast cells, a step critical for invasion and replication within phagocytic immune cells. Posttranslational glycosylation of extracellular proteins characterizes factors produced by the pathogenic yeast cells but not those of avirulent mycelia, correlating glycosylation with infection. Histoplasma yeast cells lacking the Pmt1 and Pmt2 protein mannosyltransferases, which catalyze O-linked mannosylation of proteins, are severely attenuated during infection of mammalian hosts. Cells lacking Pmt2 have altered surface characteristics that increase recognition of yeast cells by the macrophage mannose receptor and reduce recognition by the β-glucan receptor Dectin-1. Despite these changes, yeast cells lacking these factors still associate with and survive within phagocytes. Depletion of macrophages or neutrophils in vivo does not recover the virulence of the mutant yeast cells. We show that yeast cells lacking Pmt functions are more sensitive to thermal stress in vitro and consequently are unable to productively infect mice, even in the absence of fever. Treatment of mice with cyclophosphamide reduces the normal core body temperature of mice, and this decrease is sufficient to restore the infectivity of O-mannosylation-deficient yeast cells. These findings demonstrate that O-mannosylation of proteins increases the thermotolerance of Histoplasma yeast cells, which facilitates infection of mammalian hosts.IMPORTANCE For dimorphic fungal pathogens, mammalian body temperature can have contrasting roles. Mammalian body temperature induces differentiation of the fungal pathogen Histoplasma capsulatum into a pathogenic state characterized by infection of host phagocytes. On the other hand, elevated temperatures represent a significant barrier to infection by many microbes. By functionally characterizing cells lacking O-linked mannosylation enzymes, we show that protein mannosylation confers thermotolerance on H. capsulatum, enabling infection of mammalian hosts.

Candida albicans mannans mediate Streptococcus mutans exoenzyme GtfB binding to modulate cross-kingdom biofilm development in vivo

Candida albicans is frequently detected with heavy infection by Streptococcus mutans in plaque-biofilms from children with early-childhood caries (ECC). This cross-kingdom biofilm contains an extensive matrix of extracellular α-glucans that is produced by an exoenzyme (GtfB) secreted by S. mutans. Here, we report that mannans located on the outer surface of C. albicans cell-wall mediates GtfB binding, enhancing glucan-matrix production and modulating bacterial-fungal association within biofilms formed in vivo. Using single-molecule atomic force microscopy, we determined that GtfB binds with remarkable affinity to mannans and to the C. albicans surface, forming a highly stable and strong bond (1-2 nN). However, GtfB binding properties to C. albicans was compromised in strains defective in O-mannan (pmt4ΔΔ) or N-mannan outer chain (och1ΔΔ). In particular, the binding strength of GtfB on och1ΔΔ strain was severely disrupted (>3-fold reduction vs. parental strain). In turn, the GtfB amount on the fungal surface was significantly reduced, and the ability of C. albicans mutant strains to develop mixed-species biofilms with S. mutans was impaired. This phenotype was independent of hyphae or established fungal-biofilm regulators (EFG1, BCR1). Notably, the mechanical stability of the defective biofilms was weakened, resulting in near complete biomass removal by shear forces. In addition, these in vitro findings were confirmed in vivo using a rodent biofilm model. Specifically, we observed that C. albicans och1ΔΔ was unable to form cross-kingdom biofilms on the tooth surface of rats co-infected with S. mutans. Likewise, co-infection with S. mutans defective in GtfB was also incapable of forming mixed-species biofilms. Taken together, the data support a mechanism whereby S. mutans-secreted GtfB binds to the mannan layer of C. albicans to promote extracellular matrix formation and their co-existence within biofilms. Enhanced understanding of GtfB-Candida interactions may provide new perspectives for devising effective therapies to disrupt this cross-kingdom relationship associated with an important childhood oral disease.

Role of Candida albicans secreted aspartyl protease Sap9 in interkingdom biofilm formation

The fungus Candida albicans colonizes oral cavity surfaces and is carried by up to 60% of human populations. Biofilm development by C. albicans may be modulated by oral streptococci, such as Streptococcus gordonii, S. oralis or S. mutans, so as to augment pathogenicity. In this study we sought to determine if the cell wall-associated secreted aspartyl proteinase Sap9 was necessary for hyphal adhesin functions associated with biofilm community development. A sap9Δ mutant of C. albicans SC5314 formed biofilms that were flatter, and contained fewer blastospores and more hyphal filaments than the parent strain. This phenotypic difference was accentuated under flow (shear) conditions and in the presence of S. gordonii. Dual-species biofilms of C. albicans sap9Δ with S. oralis, S. sanguinis, S. parasanguinis, S. mutans and Enterococcus faecalis all contained more matted hyphae and more bacteria bound to substratum compared to C. albicans wild type. sap9Δ mutant hyphae showed significantly increased cell surface hydrophobicity, ∼25% increased levels of binding C. albicans cell wall protein Als3, and reduced interaction with Eap1, implicating Sap9 in fungal cell-cell recognition. These observations suggest that Sap9 is associated with protein-receptor interactions between fungal cells, and with interkingdom communication in the formation of polymicrobial biofilm communities.

Structure of the Glycosyltransferase Ktr4p from Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, members of the Kre2/Mnt1 protein family have been shown to be α-1,2-mannosyltransferases or α-1,2-mannosylphosphate transferases, utilising an Mn2+-coordinated GDP-mannose as the sugar donor and a variety of mannose derivatives as acceptors. Enzymes in this family are localised to the Golgi apparatus, and have been shown to be involved in both N- and O-linked glycosylation of newly-synthesised proteins, including cell wall glycoproteins. Our knowledge of the nine proteins in this family is however very incomplete at present. Only one family member, Kre2p/Mnt1p, has been studied by structural methods, and three (Ktr4p, Ktr5p, Ktr7p) are completely uncharacterised and remain classified only as putative glycosyltransferases. Here we use in vitro enzyme activity assays to provide experimental confirmation of the predicted glycosyltransferase activity of Ktr4p. Using GDP-mannose as the donor, we observe activity towards the acceptor methyl-α-mannoside, but little or no activity towards mannose or α-1,2-mannobiose. We also present the structure of the lumenal catalytic domain of S. cerevisiae Ktr4p, determined by X-ray crystallography to a resolution of 2.2 Å, and the complex of the enzyme with GDP to 1.9 Å resolution.

O-mannosylation in Candida albicans enables development of interkingdom biofilm communities

Candida albicans is a fungus that colonizes oral cavity surfaces, the gut, and the genital tract. Streptococcus gordonii is a ubiquitous oral bacterium that has been shown to form biofilm communities with C. albicans. Formation of dual-species S. gordonii-C. albicans biofilm communities involves interaction of the S. gordonii SspB protein with the Als3 protein on the hyphal filament surface of C. albicans. Mannoproteins comprise a major component of the C. albicans cell wall, and in this study we sought to determine if mannosylation in cell wall biogenesis of C. albicans was necessary for hyphal adhesin functions associated with interkingdom biofilm development. A C. albicans mnt1Δ mnt2Δ mutant, with deleted α-1,2-mannosyltransferase genes and thus defective in O-mannosylation, was abrogated in biofilm formation under various growth conditions and produced hyphal filaments that were not recognized by S. gordonii. Cell wall proteomes of hypha-forming mnt1Δ mnt2Δ mutant cells showed growth medium-dependent alterations, compared to findings for the wild type, in a range of protein components, including Als1, Als3, Rbt1, Scw1, and Sap9. Hyphal filaments formed by mnt1Δ mnt2Δ mutant cells, unlike wild-type hyphae, did not interact with C. albicans Als3 or Hwp1 partner cell wall proteins or with S. gordonii SspB partner adhesin, suggesting defective functionality of adhesins on the mnt1Δ mnt2Δ mutant. These observations imply that early stage O-mannosylation is critical for activation of hyphal adhesin functions required for biofilm formation, recognition by bacteria such as S. gordonii, and microbial community development. IMPORTANCE In the human mouth, microorganisms form communities known as biofilms that adhere to the surfaces present. Candida albicans is a fungus that is often found within these biofilms. We have focused on the mechanisms by which C. albicans becomes incorporated into communities containing bacteria, such as Streptococcus. We find that impairment of early stage addition of mannose sugars to C. albicans hyphal filament proteins deleteriously affects their subsequent performance in mediating formation of polymicrobial biofilms. Our analyses provide new understanding of the way that microbial communities develop, and of potential means to control C. albicans infections.

Mannosylation in Candida albicans: role in cell wall function and immune recognition

The fungal cell wall is a dynamic organelle required for cell shape, protection against the environment and, in pathogenic species, recognition by the innate immune system. The outer layer of the cell wall is comprised of glycosylated mannoproteins with the majority of these post‐translational modifications being the addition of O‐ and N‐linked mannosides. These polysaccharides are exposed on the outer surface of the fungal cell wall and are, therefore, the first point of contact between the fungus and the host immune system. This review focuses on O‐ and N‐linked mannan biosynthesis in the fungal pathogen Candida albicans and highlights new insights gained from the characterization of mannosylation mutants into the role of these cell wall components in host–fungus interactions. In addition, we discuss the use of fungal mannan as a diagnostic marker of fungal disease.

The Mnn2 mannosyltransferase family modulates mannoprotein fibril length, immune recognition and virulence of Candida albicans

The fungal cell wall is the first point of interaction between an invading fungal pathogen and the host immune system. The outer layer of the cell wall is comprised of GPI anchored proteins, which are post-translationally modified by both N- and O-linked glycans. These glycans are important pathogen associated molecular patterns (PAMPs) recognised by the innate immune system. Glycan synthesis is mediated by a series of glycosyl transferases, located in the endoplasmic reticulum and Golgi apparatus. Mnn2 is responsible for the addition of the initial α1,2-mannose residue onto the α1,6-mannose backbone, forming the N-mannan outer chain branches. In Candida albicans, the MNN2 gene family is comprised of six members (MNN2, MNN21, MNN22, MNN23, MNN24 and MNN26). Using a series of single, double, triple, quintuple and sextuple mutants, we show, for the first time, that addition of α1,2-mannose is required for stabilisation of the α1,6-mannose backbone and hence regulates mannan fibril length. Sequential deletion of members of the MNN2 gene family resulted in the synthesis of lower molecular weight, less complex and more uniform N-glycans, with the sextuple mutant displaying only un-substituted α1,6-mannose. TEM images confirmed that the sextuple mutant was completely devoid of the outer mannan fibril layer, while deletion of two MNN2 orthologues resulted in short mannan fibrils. These changes in cell wall architecture correlated with decreased proinflammatory cytokine induction from monocytes and a decrease in fungal virulence in two animal models. Therefore, α1,2-mannose of N-mannan is important for both immune recognition and virulence of C. albicans.

Reporters for the analysis of N-glycosylation in Candida albicans

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Reporters for dissection of N-glycosylation in Candida albicans.

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Detection of glycosylation at the single site on epitope-tagged reporter.

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Reporter faithfully reflects glycosylation defects in cell wall mutants.

A large proportion of Candida albicans cell surface proteins are decorated post-translationally by glycosylation. Indeed N-glycosylation is critical for cell wall biogenesis in this major fungal pathogen and for its interactions with host cells. A detailed understanding of N-glycosylation will yield deeper insights into host-pathogen interactions. However, the analysis of N-glycosylation is extremely challenging because of the complexity and heterogeneity of these structures. Therefore, in an attempt to reduce this complexity and facilitate the analysis of N-glycosylation, we have developed new synthetic C. albicans reporters that carry a single N-linked glycosylation site derived from Saccharomyces cerevisiae Suc2. These glycosylation reporters, which carry C. albicans Hex1 or Sap2 signal sequences plus carboxy-terminal FLAG₃ and His₆ tags, were expressed in C. albicans from the ACT1 promoter. The reporter proteins were successfully secreted and hyperglycosylated by C. albicans cells, and their outer chain glycosylation was dependent on Och1 and Pmr1, which are required for N-mannan synthesis, but not on Mnt1 and Mnt2 which are only required for O-mannosylation. These reporters are useful tools for the experimental dissection of N-glycosylation and other related processes in C. albicans, such as secretion.

Fungal cell wall organization and biosynthesis

The composition and organization of the cell walls from Saccharomyces cerevisiae, Candida albicans, Aspergillus fumigatus, Schizosaccharomyces pombe, Neurospora crassa, and Cryptococcus neoformans are compared and contrasted. These cell walls contain chitin, chitosan, β-1,3-glucan, β-1,6-glucan, mixed β-1,3-/β-1,4-glucan, α-1,3-glucan, melanin, and glycoproteins as major constituents. A comparison of these cell walls shows that there is a great deal of variability in fungal cell wall composition and organization. However, in all cases, the cell wall components are cross-linked together to generate a cell wall matrix. The biosynthesis and properties of each of the major cell wall components are discussed. The chitin and glucans are synthesized and extruded into the cell wall space by plasma membrane-associated chitin synthases and glucan synthases. The glycoproteins are synthesized by ER-associated ribosomes and pass through the canonical secretory pathway. Over half of the major cell wall proteins are modified by the addition of a glycosylphosphatidylinositol anchor. The cell wall glycoproteins are also modified by the addition of O-linked oligosaccharides, and their N-linked oligosaccharides are extensively modified during their passage through the secretory pathway. These cell wall glycoprotein posttranslational modifications are essential for cross-linking the proteins into the cell wall matrix. Cross-linking the cell wall components together is essential for cell wall integrity. The activities of four groups of cross-linking enzymes are discussed. Cell wall proteins function as cross-linking enzymes, structural elements, adhesins, and environmental stress sensors and protect the cell from environmental changes.

The impact of protein glycosylation on Flo11-dependent adherence in Saccharomyces cerevisiae

Fungal cell adhesion molecules are critical for the attachment of cells to each other and to surfaces and in pathogens contribute to virulence. Fungal adhesins are typically heavily glycosylated. The impact of protein glycosylation on the function and regulation of adhesion glycoproteins is not clear. We examined the role of protein glycosylation on the adherence properties of the major adhesion molecule Muc1/Flo11 in the budding yeast Saccharomyces cerevisiae. Using a conditional mutant required for an early step in protein glycosylation, pmi40-101, we show that the glycosylation of Flo11 is required for invasive growth and biofilm/mat formation. Underglycosylated Flo11 was not defective in cell-surface localization or binding to wild-type cells in trans. However, wild-type Flo11 was defective for binding to the surface of cells undergoing a glycosylation stress. Shed Flo11 and other shed glycoproteins (Msb2 and Hkr1) were extremely stable with half-lives on the order of days. The glycosylation of Flo11 contributed to its stability. Moreover, the overall balance between Flo11 production, shedding, and turnover favored accumulation of the shed protein over time. Our findings may be applicable to fungal adhesion molecules in other species including pathogens.

Stage specific assessment of Candida albicans phagocytosis by macrophages identifies cell wall composition and morphogenesis as key determinants

Candida albicans is a major life-threatening human fungal pathogen. Host defence against systemic Candida infection relies mainly on phagocytosis of fungal cells by cells of the innate immune system. In this study, we have employed video microscopy, coupled with sophisticated image analysis tools, to assess the contribution of distinct C. albicans cell wall components and yeast-hypha morphogenesis to specific stages of phagocytosis by macrophages. We show that macrophage migration towards C. albicans was dependent on the glycosylation status of the fungal cell wall, but not cell viability or morphogenic switching from yeast to hyphal forms. This was not a consequence of differences in maximal macrophage track velocity, but stems from a greater percentage of macrophages pursuing glycosylation deficient C. albicans during the first hour of the phagocytosis assay. The rate of engulfment of C. albicans attached to the macrophage surface was significantly delayed for glycosylation and yeast-locked morphogenetic mutant strains, but enhanced for non-viable cells. Hyphal cells were engulfed at a slower rate than yeast cells, especially those with hyphae in excess of 20 µm, but there was no correlation between hyphal length and the rate of engulfment below this threshold. We show that spatial orientation of the hypha and whether hyphal C. albicans attached to the macrophage via the yeast or hyphal end were also important determinants of the rate of engulfment. Breaking down the overall phagocytic process into its individual components revealed novel insights into what determines the speed and effectiveness of C. albicans phagocytosis by macrophages.

Host defence against systemic candidiasis relies mainly on the ingestion and elimination of fungal cells by cells of the innate immune system, especially neutrophils and macrophages. Here we have used live cell video microscopy coupled with sophisticated image analysis to generate a temporal and spatial analysis in unprecedented detail of the specific effects of C. albicans viability, cell wall composition, morphogenesis and spatial orientation on two distinct stages (macrophage migration and engulfment of bound C. albicans) of the phagocytosis process. The novel methods employed here to study phagocytosis of C. albicans could be applied to study other pathogens and uptake of dying host cells. Thus, our studies have direct implications for a much broader community and provide a blueprint for future studies with other phagocytes/microorganisms that would significantly enhance our understanding of the mechanisms that govern effective phagocytosis and ultimately the innate immune response to infection.

Biochemical characterization of recombinant Candida albicans mannosyltransferases Mnt1, Mnt2 and Mnt5 reveals new functions in O- and N-mannan biosynthesis

The cell surface of Candida albicans is enriched with highly glycosylated mannoproteins that are involved in the interaction with host tissues. N- and O-glycosylation are post-translational modifications that initiate in the endoplasmic reticulum, and finalize in the Golgi. The KRE2/MNT1 family encode a set of multifunctional mannosyltransferases that participate in O-, N- and phosphomannosylation. In order to gain insights into the substrate specificities of these enzymes, recombinant forms of Mnt1, Mnt2, and Mnt5 were expressed in Pichia pastoris and the enzyme activities characterized. Mnt1 and Mnt2 showed a high specificity for α-methylmannoside and α1,2-mannobiose as acceptor substrates. Notably, they also used Saccharomyces cerevisiaeO-mannans as acceptors and generated products with more than three mannose residues, suggesting than Mnt1 and Mnt2 could be the mannosyltransferases adding the fourth and fifth mannose residue to the O-mannans in C. albicans. Mnt5 only recognized α-methylmannoside as acceptor, suggesting that participates in the addition of the second mannose residues to the N-glycan outer chain.

Immunochemistry of pathogenic yeast, Candida species, focusing on mannan

This review describes recent findings based on structural and immunochemical analyses of the cell wall mannan of Candida albicans, and other medically important Candida species. Mannan has been shown to consist of α-1,2-, α-1,3-, α-1,6-, and β-1,2-linked mannopyranose units with few phosphate groups. Each Candida species has a unique mannan structure biosynthesized by sequential collaboration between species-specific mannosyltransferases. In particular, the β-1,2-linked mannose units have been shown to comprise a characteristic oligomannosyl side chain that is strongly antigenic. For these pathogenic Candida species, cell-surface mannan was also found to participate in the adhesion to the epithelial cells, recognition by innate immune receptors and development of pathogenicity. Therefore, clarification of the precise chemical structure of Candida mannan is indispensable for understanding the mechanism of pathogenicity, and for development of new antifungal drugs and immunotherapeutic procedures.

Candida albicans cell wall glycosylation may be indirectly required for activation of epithelial cell proinflammatory responses

Oral epithelial cells discriminate between the yeast and hyphal forms of Candida albicans via the mitogen-activated protein kinase (MAPK) signaling pathway. This occurs through phosphorylation of the MAPK phosphatase MKP1 and activation of the c-Fos transcription factor by the hyphal form. Given that fungal cell wall polysaccharides are critical in host recognition and immune activation in myeloid cells, we sought to determine whether β-glucan and N- or O-glycosylation was important in activating the MAPK/MKP1/c-Fos hypha-mediated response mechanism and proinflammatory cytokines in oral epithelial cells. Using a series of β-glucan and N- and O-mannan mutants, we found that N-mannosylation (via Δoch1 and Δpmr1 mutants) and O-mannosylation (via Δpmt1 and Δmnt1 Δmnt2 mutants), but not phosphomannan (via a Δmnn4 mutant) or β-1,2 mannosylation (via Δbmt1 to Δbmt6 mutants), were required for MKP1/c-Fos activation, proinflammatory cytokine production, and cell damage induction. However, the N- and O-mannan mutants showed reduced adhesion or lack of initial hypha formation at 2 h, resulting in little MKP1/c-Fos activation, or restricted hypha formation/pseudohyphal formation at 24 h, resulting in minimal proinflammatory cytokine production and cell damage. Further, the α-1,6-mannose backbone of the N-linked outer chain (corresponding to a Δmnn9 mutant) may be required for epithelial adhesion, while the α-1,2-mannose component of phospholipomannan (corresponding to a Δmit1 mutant) may contribute to epithelial cell damage. β-Glucan appeared to play no role in adhesion, epithelial activation, or cell damage. In summary, N- and O-mannosylation defects affect the ability of C. albicans to induce proinflammatory cytokines and damage in oral epithelial cells, but this may be due to indirect effects on fungal pathogenicity rather than mannose residues being direct activators of the MAPK/MKP1/c-Fos hypha-mediated immune response.

Damage to the glycoshield activates PMT-directed O-mannosylation via the Msb2-Cek1 pathway in Candida albicans

Protein-O-mannosyltransferases (Pmt) transfer mannosyl residues to secretory proteins. Five isoforms of Pmt proteins in the human fungal pathogen Candida albicans have distinct functions in growth, morphogenesis and antifungal resistance. We found that PMT genes encoding the major isoforms Pmt1, Pmt2, Pmt4 are regulated differently in response to impaired glycostructures. While the PMT1 transcript level increased in cell wall mutants and under inhibition of N-glycosylation by tunicamycin, PMT2 and PMT4 transcripts were upregulated only by inhibition of Pmt1 activity. Reporter fusions revealed specific promoter sequences to be required for PMT1 repression in undamaged cells, which was de-repressed by tunicamycin. Constitutive PMT1 de-repression was observed in mutants lacking the Cek1 MAP kinase and its upstream sensor Msb2. In contrast, in msb2 and cek1 mutants, upregulation of PMT2/PMT4 by Pmt1 inhibition did not occur and basal expression of both transcripts were decreased. We identified Ace2 as a novel transcription factor, which upregulates PMT basal expression and induction in response to glycostructure damage. Mutants lacking Msb2, Cek1 and Ace2 were supersensitive to glycosylation and cell wall inhibitors. We propose that a Msb2, Cek1 and Ace2 signalling pathway addresses PMT genes as downstream targets and that different modes of regulation have evolved for PMT1 and PMT2/PMT4 genes.

Comparative genomics allowed the identification of drug targets against human fungal pathogens

Background

The prevalence of invasive fungal infections (IFIs) has increased steadily worldwide in the last few decades. Particularly, there has been a global rise in the number of infections among immunosuppressed people. These patients present severe clinical forms of the infections, which are commonly fatal, and they are more susceptible to opportunistic fungal infections than non-immunocompromised people. IFIs have historically been associated with high morbidity and mortality, partly because of the limitations of available antifungal therapies, including side effects, toxicities, drug interactions and antifungal resistance. Thus, the search for alternative therapies and/or the development of more specific drugs is a challenge that needs to be met. Genomics has created new ways of examining genes, which open new strategies for drug development and control of human diseases.

Results

In silico analyses and manual mining selected initially 57 potential drug targets, based on 55 genes experimentally confirmed as essential for Candida albicans or Aspergillus fumigatus and other 2 genes (kre2 and erg6) relevant for fungal survival within the host. Orthologs for those 57 potential targets were also identified in eight human fungal pathogens (C. albicans, A. fumigatus, Blastomyces dermatitidis, Paracoccidioides brasiliensis, Paracoccidioides lutzii, Coccidioides immitis, Cryptococcus neoformans and Histoplasma capsulatum). Of those, 10 genes were present in all pathogenic fungi analyzed and absent in the human genome. We focused on four candidates: trr1 that encodes for thioredoxin reductase, rim8 that encodes for a protein involved in the proteolytic activation of a transcriptional factor in response to alkaline pH, kre2 that encodes for α-1,2-mannosyltransferase and erg6 that encodes for Δ(24)-sterol C-methyltransferase.

Conclusions

Our data show that the comparative genomics analysis of eight fungal pathogens enabled the identification of four new potential drug targets. The preferred profile for fungal targets includes proteins conserved among fungi, but absent in the human genome. These characteristics potentially minimize toxic side effects exerted by pharmacological inhibition of the cellular targets. From this first step of post-genomic analysis, we obtained information relevant to future new drug development.

Enzymatic synthesis of new oligosaccharides using mannosyltransferases from Candida species and their NMR assignments

The outer layer of the cell wall of pathogenic fungi, Candida species, consists of mannan, which plays an important role in infection. In this study, we synthesized several oligosaccharides using mannosyltransferases obtained from Candida parapsilosis and Candida albicans. Namely, we synthesized mannotetraoses [Manalpha1--> 2Manalpha1-->3Manalpha1-->2Man and Manalpha1-->3(Manalpha1-->6)Manalpha1-->2Man] from mannotriose, Manalpha1-->3Manalpha1--> 2Man, and mannohexaoses [Manalpha1-->2Manalpha1-->3Manalpha1-->3Manalpha1-->2Manalpha1-->2Man and Manalpha1-->3(Manalpha1-->6)Manalpha1-->3Manalpha1-->2Manalpha1-->2Man] from mannopentaose, Manalpha1-->3Manalpha1-->3Manalpha1-->2Manalpha1-->2Man. The linkage sequence of these oligosaccharides was identified by a sequential (1)H-NMR assignment method combined with rotating frame nuclear Overhauser enhancement spectroscopy and relayed coherence transfer spectroscopy. The steric effect by the alpha-1,6-linked branching mannose residue to the H-1 proton chemical shift of the neighboring 3-O-substituted mannose residue was different from that of the 2-O-substituted mannose residue. These oligosaccharides having novel structures seem to be useful as the substrate or ligand for glycomics.

Candida infections of the genitourinary tract

All humans are colonized with Candida species, mostly Candida albicans, yet some develop diseases due to Candida, among which genitourinary manifestations are extremely common. The forms of genitourinary candidiasis are distinct from each other and affect different populations. While vulvovaginal candidiasis affects mostly healthy women, candiduria occurs typically in elderly, hospitalized, or immunocompromised patients and in neonates. Despite its high incidence and clinical relevance, genitourinary candidiasis is understudied, and therefore, important questions about pathogenesis and treatment guidelines remain to be resolved. In this review, we summarize the current knowledge about genitourinary candidiasis.

A multifunctional mannosyltransferase family in Candida albicans determines cell wall mannan structure and host-fungus interactions

The cell wall proteins of fungi are modified by N- and O-linked mannosylation and phosphomannosylation, resulting in changes to the physical and immunological properties of the cell. Glycosylation of cell wall proteins involves the activities of families of endoplasmic reticulum and Golgi-located glycosyl transferases whose activities are difficult to infer through bioinformatics. The Candida albicans MNT1/KRE2 mannosyl transferase family is represented by five members. We showed previously that Mnt1 and Mnt2 are involved in O-linked mannosylation and are required for virulence. Here, the role of C. albicans MNT3, MNT4, and MNT5 was determined by generating single and multiple MnTDelta null mutants and by functional complementation experiments in Saccharomyces cerevisiae. CaMnt3, CaMnt4, and CaMnt5 did not participate in O-linked mannosylation, but CaMnt3 and CaMnt5 had redundant activities in phosphomannosylation and were responsible for attachment of approximately half of the phosphomannan attached to N-linked mannans. CaMnt4 and CaMnt5 participated in N-mannan branching. Deletion of CaMNT3, CaMNT4, and CaMNT5 affected the growth rate and virulence of C. albicans, affected the recognition of the yeast by human monocytes and cytokine stimulation, and led to increased cell wall chitin content and exposure of beta-glucan at the cell wall surface. Therefore, the MNT1/KRE2 gene family participates in three types of protein mannosylation in C. albicans, and these modifications play vital roles in fungal cell wall structure and cell surface recognition by the innate immune system.

Protein glycosylation in Candida

Candidiasis is a significant cause of invasive human mycosis with associated mortality rates that are equivalent to, or worse than, those cited for most cases of bacterial septicemia. As a result, considerable efforts are being made to understand how the fungus invades host cells and to identify new targets for fungal chemotherapy. This has led to an increasing interest in Candida glycobiology, with an emphasis on the identification of enzymes essential for glycoprotein and adhesion metabolism, and the role of N- and O-linked glycans in host recognition and virulence. Here, we refer to studies dealing with the identification and characterization of enzymes such as dolichol phosphate mannose synthase, dolichol phosphate glucose synthase and processing glycosidases and synthesis, structure and recognition of mannans and discuss recent findings in the context of Candida albicans pathogenesis.

The O-mannosyltransferase PMT4 is essential for normal appressorium formation and penetration in Ustilago maydis

In Saccharomyces cerevisiae, the PMT, KRE2/MNT1, and MNN1 mannosyltransferase protein families catalyze the steps of the O-mannosylation pathway, sequentially adding mannoses to target proteins. We have identified members of all three families and analyzed their roles in pathogenesis of the maize smut fungus Ustilago maydis. Furthermore, we have shown that PMT4, one of the three PMT family members in U. maydis, is essential for tumor formation in Zea mays. Significantly, PMT4 seems to be required only for pathogenesis and is dispensable for other aspects of the U. maydis life cycle. We subsequently show that the deletion of pmt4 results in a strong reduction in the frequency of appressorium formation, with the few appressoria that do form lacking the capacity to penetrate the plant cuticle. Our findings suggest that the O-mannosylation pathway plays a key role in the posttranslational modification of proteins involved in the pathogenic development of U. maydis. The fact that PMT homologs are not found in plants may open new avenues for the development of fungal control strategies. Moreover, the discovery of a highly specific requirement for a single O-mannosyltransferase should aid in the identification of the proteins directly involved in fungal plant penetration, thus leading to a better understanding of plant-fungi interactions.

Human fungal pathogen Candida albicans in the postgenomic era: an overview

Candida albicans is an opportunistic human fungal pathogen and is responsible for candidiasis. Owing to the improvement in healthcare, the number of immunocompromised patients in hospitals has increased worldwide and these individuals are susceptible to infections caused by many pathogenic microbes, among which C. albicans is one of the major players. Currently, the complete genome sequence of this pathogen is available and the size of this was estimated to be of 16 Mb. Annotation of C. albicans genome revealed that there are 6114 open reading frames (ORFs), of which 774 are specific to C. albicans. This poses a challenge as well as an opportunity to the Candida community to understand the functions of the unknown genes, especially those specific to C. albicans. Efforts have been made by the Candida community to systematically delete the ORFs and assign the functions. This will, in turn, help in understanding the biology of C. albicans and its interactions with animals as well as humans, and better drugs can be developed to treat Candida infections. In this article, we review updates on the Candida biology in the context of the availability of the genome sequence, its functional analysis and anti-Candida therapy. Finally, in the light of present trends in Candida research and current challenges, various opportunities are identified and suggestions are made.

The putative alpha-1,2-mannosyltransferase AfMnt1 of the opportunistic fungal pathogen Aspergillus fumigatus is required for cell wall stability and full virulence

Proteins entering the eukaryotic secretory pathway commonly are glycosylated. Important steps in this posttranslational modification are carried out by mannosyltransferases. In this study, we investigated the putative alpha-1,2-mannosyltransferase AfMnt1 of the human pathogenic mold Aspergillus fumigatus. AfMnt1 belongs to a family of enzymes that comprises nine members in Saccharomyces cerevisiae but only three in A. fumigatus. A Deltaafmnt1 mutant is viable and grows normally at 37 degrees C, but its hyphal cell wall appears to be thinner than that of the parental strain. The lack of AfMnt1 leads to a higher sensitivity to calcofluor white and Congo red but not to sodium dodecyl sulfate. The growth of the mutant is abrogated at 48 degrees C but can be restored by osmotic stabilization. The resulting colonies remain white due to a defect in the formation of conidia. Electron and immunofluorescence microscopy further revealed that the observed growth defect of the mutant at 48 degrees C can be attributed to cell wall instability resulting in leakage at the hyphal tips. Using a red fluorescence fusion protein, we localized AfMnt1 in compact, brefeldin A-sensitive organelles that most likely represent fungal Golgi equivalents. The tumor necrosis factor alpha response of murine macrophages to hyphae was not affected by the lack of the afmnt1 gene, but the corresponding mutant was attenuated in a mouse model of infection. This and the increased sensitivity of the Deltaafmnt1 mutant to azoles, antifungal agents that currently are used to treat Aspergillus infections, suggest that alpha-1,2-mannosyltransferases are interesting targets for novel antifungal drugs.

Animal models of mucosal Candida infection

Rodent models of oral, vaginal and gastrointestinal Candida infection are described and discussed in terms of their scientific merits. The common feature of all experimental mucosal Candida infections is the need for some level of host immunocompromise or exogenous treatment to ensure quantitatively reproducible disease. A growing literature describes the contributions of such candidiasis models to our understanding of certain aspects of fungal virulence and host response to mucosal Candida albicans challenge. Evidence to date shows that T-lymphocyte responses dominate host immune defences to oral and gastrointestinal challenge, while other, highly compartmentalized responses defend vaginal surfaces. By contrast the study of C. albicans virulence factors in mucosal infection models has only begun to unravel the complex of attributes required to define the difference between strongly and weakly muco-invasive strains.

Histone deacetylase inhibitors may reduce pathogenicity and virulence inCandida albicans

Candida albicans is able to establish mucosal and invasive diseases by means of different virulence factors that are frequently regulated by epigenetic mechanisms, including the acetylation-deacetylation of histones and of other regulatory proteins. The focus of our work was on understanding the possible effects of several histone deacetylase inhibitors (HDACi) on the expression of phenotypes that are associated with virulence and pathogenicity in C. albicans, such as adhesion to epithelial cells and the yeast to hypha transition. Some of the HDACi used for experiments caused a 90% reduction in the adherence of C. albicans to human cultured pneumocytes and significantly inhibited serum-induced germination. Inhibition of germination was correlated with a significant reduction in transcription of EFG1. Inhibition appeared less evident when an HDA1-deficient strain was tested. These results suggest that selective and specific HDACi could prove to be a valid approach for selected at-risk patients in the combined treatment of infections caused by C. albicans.

Endoplasmic reticulum alpha-glycosidases of Candida albicans are required for N glycosylation, cell wall integrity, and normal host-fungus interaction

The cell surface of Candida albicans is enriched in highly glycosylated mannoproteins that are involved in the interaction with the host tissues. N glycosylation is a posttranslational modification that is initiated in the endoplasmic reticulum (ER), where the Glc(3)Man(9)GlcNAc(2) N-glycan is processed by alpha-glucosidases I and II and alpha1,2-mannosidase to generate Man(8)GlcNAc(2). This N-oligosaccharide is then elaborated in the Golgi to form N-glycans with highly branched outer chains rich in mannose. In Saccharomyces cerevisiae, CWH41, ROT2, and MNS1 encode for alpha-glucosidase I, alpha-glucosidase II catalytic subunit, and alpha1,2-mannosidase, respectively. We disrupted the C. albicans CWH41, ROT2, and MNS1 homologs to determine the importance of N-oligosaccharide processing on the N-glycan outer-chain elongation and the host-fungus interaction. Yeast cells of Cacwh41Delta, Carot2Delta, and Camns1Delta null mutants tended to aggregate, displayed reduced growth rates, had a lower content of cell wall phosphomannan and other changes in cell wall composition, underglycosylated beta-N-acetylhexosaminidase, and had a constitutively activated PKC-Mkc1 cell wall integrity pathway. They were also attenuated in virulence in a murine model of systemic infection and stimulated an altered pro- and anti-inflammatory cytokine profile from human monocytes. Therefore, N-oligosaccharide processing by ER glycosidases is required for cell wall integrity and for host-fungus interactions.