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Abstract Fungal β-1,3-glucan synthase (Fks) plays a central role in synthesizing β-1,3-glucan, the main structural polysaccharide of fungal cell walls, and serves as a key target for antifungal drugs, such as echinocandins and ibrexafungerp. Recent cryo-electron microscopy (cryo-EM) studies have revealed the architecture of the Fks1 and Fks1–Rho1 complex and provided new insights into its catalytic and regulatory mechanisms. This review summarizes current understanding of Fks, including its domain organization, transmembrane topology, conformational dynamics, and evolutionary comparison with structurally resolved glycosyltransferases (GTs), including bacterial cellulose synthase (BcsA), plant cellulose synthase (CesA), and other eukaryotic GTs. Through comparison of publicly available cryo-EM structures of Fks in both the apo-state and Rho1-bound state, a working mechanism of the activated Fks has been discussed. In addition, we present a potential gating model of β-glucan translocation and drug-inhibition by integrating literature with structure-based analyses. This review provides a structure-based functional model of fungal β-1,3-glucan synthase and the putative binding mechanism of its inhibitor, aiming to support future antifungal drug discovery. This review combines cryo-EM structures and molecular modeling to reveal how fungal enzymes build cell walls, resist antifungal drugs like echinocandins by altering specific binding sites, and diverge evolutionarily, guiding future therapies against resistant infections.

In the face of interindividual variability and complexity of gut microbial communities, prediction of outcomes from a given fiber utilized by many microbes would require a sophisticated comprehension of all competitive interactions that occur in the gut. Results presented here suggest that high-specificity fibers potentially circumvent the competitive scope in the gut for fiber utilization, providing a promising path to targeted and predictable microbial shifts in different individuals. Most dietary fibers used to shape the gut microbiota present different and unpredictable responses, presumably due to the diverse microbial communities of people. Recently, we proposed that fibers can be classified in a hierarchical way where fibers of high specificity (i.e., structurally complex and utilized by a narrow group of gut bacteria) could have more similar interindividual responses than those of low specificity (i.e., structurally simple and utilized by many gut bacteria). To test this hypothesis, we evaluated microbiota fermentation of fibers tentatively classified as low (fructooligosaccharides), low-to-intermediate (type 2 resistant starch), intermediate (pectin), and high (insoluble β-1,3-glucan) specificity, utilizing fecal inoculum from distinct subjects, regarding interindividual similarity/dissimilarity in fiber responses. Individual shifts in target bacteria (as determined by linear discriminant analysis) confirmed that divergent fiber responses occur when utilizing both of the low-specificity dietary fibers, but fibers of intermediate and high specificity lead to more similar responses across subjects in support of targeted bacteria. The high-specificity insoluble β-glucan promoted a large increase of the target bacteria (from 0.3 to 16.5% average for Anaerostipes sp. and 2.5 to 17.9% average for Bacteroides uniformis ), which were associated with increases in ratios of related metabolites (butyrate and propionate, respectively) in every microbial community in which these bacteria were present. Also, high-specificity dietary fibers promoted more dramatic changes in microbial community structure than low-specificity ones relative to the initial microbial communities. IMPORTANCE In the face of interindividual variability and complexity of gut microbial communities, prediction of outcomes from a given fiber utilized by many microbes would require a sophisticated comprehension of all competitive interactions that occur in the gut. Results presented here suggest that high-specificity fibers potentially circumvent the competitive scope in the gut for fiber utilization, providing a promising path to targeted and predictable microbial shifts in different individuals. These findings are the first to indicate that fiber specificity is related to similarity and intensity of response in distinct human gut microbiota communities.

The membrane-integrated synthase FKS is involved in the biosynthesis of β-1,3-glucan, the core component of the fungal cell wall 1 , 2 . FKS is the target of widely prescribed antifungal drugs, including echinocandin and ibrexafungerp 3 , 4 . Unfortunately, the mechanism of action of FKS remains enigmatic and this has hampered development of more effective medicines targeting the enzyme. Here we present the cryo-electron microscopy structures of Saccharomyces cerevisiae FKS1 and the echinocandin-resistant mutant FKS1(S643P). These structures reveal the active site of the enzyme at the membrane–cytoplasm interface and a glucan translocation path spanning the membrane bilayer. Multiple bound lipids and notable membrane distortions are observed in the FKS1 structures, suggesting active FKS1–membrane interactions. Echinocandin-resistant mutations are clustered at a region near TM5–6 and TM8 of FKS1. The structure of FKS1(S643P) reveals altered lipid arrangements in this region, suggesting a drug-resistant mechanism of the mutant enzyme. The structures, the catalytic mechanism and the molecular insights into drug-resistant mutations of FKS1 revealed in this study advance the mechanistic understanding of fungal β-1,3-glucan biosynthesis and establish a foundation for developing new antifungal drugs by targeting FKS. Using cryo-electron microscopy, the molecular architecture and catalytic mechanism of action of the fungal β-1,3-glucan synthase FKS1 are determined.

Fungal cell walls undergo continual remodeling that generates β-1,3-glucan fragments as products of endo-glycosyl hydrolases (GHs), which can be recognized as pathogen-associated molecular patterns (PAMPs) and trigger plant immune responses. How fungal pathogens suppress those responses is often poorly understood. Here, we study mechanisms underlying the suppression of β-1,3-glucan-triggered plant immunity by the blast fungus Magnaporthe oryzae . We show that an exo-β-1,3-glucanase of the GH17 family, named Ebg1, is important for fungal cell wall integrity and virulence of M. oryzae . Ebg1 can hydrolyze β-1,3-glucan and laminarin into glucose, thus suppressing β-1,3-glucan-triggered plant immunity. However, in addition, Ebg1 seems to act as a PAMP, independent of its hydrolase activity. This Ebg1-induced immunity appears to be dampened by the secretion of an elongation factor 1 alpha protein (EF1α), which interacts and co-localizes with Ebg1 in the apoplast. Future work is needed to understand the mechanisms behind Ebg1-induced immunity and its suppression by EF1α. Fungal cell walls release β-1,3-glucan fragments that trigger plant immunity. Here, the authors show that a glucanase (Ebg1) of the blast fungus Magnaporthe oryzae suppresses plant immunity by hydrolyzing β-1,3-glucan. At the same time, Ebg1 induces plant immune responses that are dampened by a fungal protein that interacts with Ebg1.

Abstract Vast efforts have been devoted to the development of antifungal drugs targeting the cell wall, but the supramolecular architecture of this carbohydrate-rich composite remains insufficiently understood. Here we compare the cell wall structure of a fungal pathogen Aspergillus fumigatus and four mutants depleted of major structural polysaccharides. High-resolution solid-state NMR spectroscopy of intact cells reveals a rigid core formed by chitin, β-1,3-glucan, and α-1,3-glucan, with galactosaminogalactan and galactomannan present in the mobile phase. Gene deletion reshuffles the composition and spatial organization of polysaccharides, with significant changes in their dynamics and water accessibility. The distribution of α-1,3-glucan in chemically isolated and dynamically distinct domains supports its functional diversity. Identification of valines in the alkali-insoluble carbohydrate core suggests a putative function in stabilizing macromolecular complexes. We propose a revised model of cell wall architecture which will improve our understanding of the structural response of fungal pathogens to stresses.

A number of transcription factors are then activated including Cas5 and Rlm1 that switch on the expression of genes involved in cell wall construction and remodelling. CDREs, calcium-dependent response elements; CWPGs, cell wall protein genes; GPI, glycosylphosphatidylinositol. https://doi.org/10.1371/journal.ppat.1009470.g001 The development of antifungal therapies that target the cell wall has received great attention (Fig 1A) but with only limited success. Other chitin synthase inhibitors such as the 3-substituted amino-4-hydroxycoumarin derivatives have also been found to have antifungal activity [4], but none has made it to the clinic. β-1,3-glucan β-1,3-glucan is a polymer of glucose units and can exist in a stable triple helical structure which gives it some degree of elasticity and tensile strength making it the main structural polysaccharide in the cell wall of most fungi [5]. β-1,3-glucan has been the most attractive antifungal target due to its central structural role in the cell wall. β-1,3-glucan is synthesised by essential β-1,3-glucan synthase enzymes composed of an integral membrane protein catalytic subunit, Fks, and a regulatory subunit, Rho1 (Fig 1A). Significant success has been seen in developing β-1,3-glucan synthesis inhibitors partly because β-1,3-glucan is synthesised by a single synthase. β-1,6-glucan β-1,6-glucan plays a central role in cell wall organisation and structure by linking cell wall proteins (CWPs) to the cell wall matrix [15].

Biofilm-associated polymicrobial infections, particularly those involving fungi and bacteria, are responsible for significant morbidity and mortality and tend to be challenging to treat. Candida albicans and Staphylococcus aureus specifically are considered leading opportunistic fungal and bacterial pathogens, respectively, mainly due to their ability to form biofilms on catheters and indwelling medical devices. However, the impact of mixed-species biofilm growth on therapy remains largely understudied. In this study, we investigated the influence of C. albicans secreted cell wall polysaccharides on the response of S. aureus to antibacterial agents in biofilm. Results demonstrated significantly enhanced tolerance for S. aureus to drugs in the presence of C. albicans or its secreted cell wall polysaccharide material. Fluorescence confocal time-lapse microscopy revealed impairment of drug diffusion through the mixed biofilm matrix. Using C. albicans mutant strains with modulated cell wall polysaccharide expression, exogenous supplementation, and enzymatic degradation, the C. albicans -secreted β-1,3-glucan cell wall component was identified as the key matrix constituent providing the bacteria with enhanced drug tolerance. Further, antibody labeling demonstrated rapid coating of the bacteria by the C. albicans matrix material. Importantly, via its effect on the fungal biofilm matrix, the antifungal caspofungin sensitized the bacteria to the drugs. Understanding such symbiotic interactions with clinical relevance between microbial species in biofilms will greatly aid in overcoming the limitations of current therapies and in defining potential new targets for treating polymicrobial infections. IMPORTANCE The fungus Candida albicans and the bacterium Staphylococcus aureus are important microbial pathogens responsible for the majority of infections in hospitalized patients and are often coisolated from a host. In this study, we demonstrated that when grown together, the fungus provides the bacterium with enhanced tolerance to antimicrobial drugs. This process was mediated by polysaccharides secreted by the fungal cell into the environment. The biofilm matrix formed by these polysaccharides prevented penetration by the drugs and provided the bacteria with protection. Importantly, we show that by inhibiting the production of the fungal polysaccharides, a specific antifungal agent indirectly sensitized the bacteria to antimicrobials. Understanding the therapeutic implications of the interactions between these two diverse microbial species will aid in overcoming the limitations of current therapies and in defining new targets for treating complex polymicrobial infections. The fungus Candida albicans and the bacterium Staphylococcus aureus are important microbial pathogens responsible for the majority of infections in hospitalized patients and are often coisolated from a host. In this study, we demonstrated that when grown together, the fungus provides the bacterium with enhanced tolerance to antimicrobial drugs. This process was mediated by polysaccharides secreted by the fungal cell into the environment. The biofilm matrix formed by these polysaccharides prevented penetration by the drugs and provided the bacteria with protection. Importantly, we show that by inhibiting the production of the fungal polysaccharides, a specific antifungal agent indirectly sensitized the bacteria to antimicrobials. Understanding the therapeutic implications of the interactions between these two diverse microbial species will aid in overcoming the limitations of current therapies and in defining new targets for treating complex polymicrobial infections.

Yeast is an integral part of mammalian microbiome, and like commensal bacteria, has the potential of being harnessed to influence immunity in clinical settings. However, functional specificities of yeast-derived immunoregulatory molecules remain elusive. Here we find that while under steady state, β-1,3-glucan-containing polysaccharides potentiate pro-inflammatory properties, a relatively less abundant class of cell surface polysaccharides, dubbed mannan/β-1,6-glucan-containing polysaccharides (MGCP), is capable of exerting potent anti-inflammatory effects to the immune system. MGCP, in contrast to previously identified microbial cell surface polysaccharides, through a Dectin1-Cox2 signaling axis in dendritic cells, facilitates regulatory T (Treg) cell induction from naïve T cells. Furthermore, through a TLR2-dependent mechanism, it restrains Th1 differentiation of effector T cells by suppressing IFN-γ expression. As a result, administration of MGCP display robust suppressive capacity towards experimental inflammatory disease models of colitis and experimental autoimmune encephalomyelitis (EAE) in mice, thereby highlighting its potential therapeutic utility against clinically relevant autoimmune diseases. Yeast form part of the host microbiome with known impact on host immunity. Here the authors identify and investigate the impact of commensal yeast-derived polysaccharides in modulating host inflammation, and show its potential for inhibiting inflammation in a number of models of inflammatory diseases.

Fungal insect pathogens have evolved diverse mechanisms to evade host immune recognition and defense responses. However, identification of fungal factors involved in host immune evasion during cuticular penetration and subsequent hemocoel colonization remains limited. Here, we report that the entomopathogenic fungus Beauveria bassiana expresses an endo-β-1,3-glucanase (BbEng1) that functions in helping cells evade insect immune recognition/ responses. BbEng1 was specifically expressed during infection, in response to host cuticle and hemolymph, and in the presence of osmotic or oxidative stress. BbEng1 was localized to the fungal cell surface/ cell wall, where it acts to remodel the cell wall pathogen associated molecular patterns (PAMPs) that can trigger host defenses, thus facilitating fungal cell evasion of host immune defenses. BbEng1 was secreted where it could bind to fungal cells. Cell wall β-1,3-glucan levels were unchanged in ΔBbEng1 cells derived from in vitro growth media, but was elevated in hyphal bodies, whereas glucan levels were reduced in most cell types derived from the BbEng1 overexpressing strain ( BbEng1 OE ). The BbEng1 OE strain proliferated more rapidly in the host hemocoel and displayed higher virulence as compared to the wild type parent. Overexpression of their respective Eng1 homologs or of BbEng1 in the insect fungal pathogens, Metarhizium robertsii and M . acridum also resulted in increased virulence. Our data support a mechanism by which BbEng1 helps the fungal pathogen to evade host immune surveillance by decreasing cell wall glucan PAMPs, promoting successful fungal mycosis.

Motor responses of plants related to tropisms are usually carried out using elongation growth mechanisms. However, this study examined gravitropism at the level of cells (primary phloem fibers) that have completed their growth and are forming a thickened tertiary cell wall. A stocktaking and analysis of the gene expression of enzymes for callose metabolism was carried out at different stages of development of phloem fibers of flax ( Linum usitatissimum L.) and during the gravitational response. Genes of putative β-1,3-glucan synthases (GSLs) and β-1,3-glucanases (BGs) were identified, which have a differential expression pattern in the studied cells, among which genes with the maximum level of expression at a certain stage of development were noted. Generally, the expression of β-1,3-glucan synthase genes was decreased during gravitropism, while β-1,3-glucanase genes were characterized by different expression profiles, among which genes with an increased expression level ( LusBG1 and LusBG3 ) were identified only during the graviresponse. The data obtained allowed the authors to assume the presence of active callose metabolism in the cell wall of the studied fibers at different stages of development and that callose degradation dominates during the gravitational response in such fibers. The results of the work lay the foundation for further studies of the function of callose in the development of fibers and the implementation of the motor response of plants.