Protein O-glycosylation in fungi: diverse structures and multiple functions

Protein secretion and associated stress in industrially employed filamentous fungi

Application of filamentous fungi for the production of commercial enzymes such as amylase, cellulase, or xylanase is on the rise due to the increasing demand to degrade several complex carbohydrates as raw material for biotechnological processes. Also, protein production by fungi for food and feed gains importance. In any case, the protein production involves both cellular synthesis and secretion outside of the cell. Unfortunately, the secretion of proteins or enzymes can be hampered due to accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) as a result of too high synthesis of enzymes or (heterologous) protein expression. To cope with this ER stress, the cell generates a response known as unfolded protein response (UPR). Even though this mechanism should re-establish the protein homeostasis equivalent to a cell under non-stress conditions, the enzyme expression might still suffer from repression under secretory stress (RESS). Among eukaryotes, Saccharomyces cerevisiae is the only fungus, which is studied quite extensively to unravel the UPR pathway. Several homologs of the proteins involved in this signal transduction cascade are also found in filamentous fungi. Since RESS seems to be absent in S. cerevisiae and was only reported in Trichoderma reesei in the presence of folding and glycosylation inhibitors such as dithiothreitol and tunicamycin, more in-depth study about this mechanism, specifically in filamentous fungi, is the need of the hour. Hence, this review article gives an overview on both, protein secretion and associated stress responses in fungi. KEY POINTS: • Enzymes produced by filamentous fungi are crucial in industrial processes • UPR mechanism is conserved among many fungi, but mediated by different proteins • RESS is not fully understood or studied in industrially relevant filamentous fungi.

Glycans in the oral bacteria and fungi: Shaping host-microbe interactions and human health

The human oral cavity serves as the natural entry port to both the gastrointestinal and respiratory tracts, and hosts a diverse microbial community essential for maintaining health. Dysbiosis of this microbiome can lead to various diseases. Glycans, as vital carriers of biological information, are indispensable structural components of living organisms and play key roles in numerous biological processes. In the oral microbiome, glycans influence microbial binding to host receptors, promote colonization, and mediate communication among microbial communities, as well as between microbes and the host immune system. Targeting glycans may provide innovative strategies for modulating the composition of the oral microbiome, with broader implications for human health. Additionally, exogenous glycans regulate the oral microbiome by serving as carbon and energy sources for microbes, while certain specific glycans can inhibit microbial growth and activity. This review summarizes glycosylation pathways in oral bacteria and fungi, explores the regulation of host-microbiota interactions by glycans, and discusses the effects of exogenous glycans on oral microbiome. The review aims to highlight the multifaceted role of glycans in shaping the oral microbiome and its impact on the host, while also indicates potential future applications.

Glycosylation of cellulase: a novel strategy for improving cellulase

Protein glycosylation is the most complex posttranslational modification process. Most cellulases from filamentous fungi contain N-glycosylation and O-glycosylation. Here, we discuss the potential roles of glycosylation on the characteristics and function of cellulases. The use of certain cultivation, inducer, and alteration of engineering glycosylation pathway can enable the rational control of cellulase glycosylation. Glycosylation does not occur arbitrarily and may tend to modify the 3D structure of cellulases by using specially distributed glycans. Therefore, glycoengineering should be considered comprehensively along with the spatial structure of cellulases. Cellulase glycosylation may be an evolution phenomenon, which has been considered as an economical way for providing different functions from identical proteins. In addition to gene and transcription regulations, glycosylation may be another regulation on the protein expression level. Enhanced understanding of the potential regulatory role of cellulase glycosylation will enable synthetic biology approaches for the development of commercial cellulase.

Fungal chitin-binding glycoprotein induces Dectin-2-mediated allergic airway inflammation synergistically with chitin

Although chitin in fungal cell walls is associated with allergic airway inflammation, the precise mechanism underlying this association has yet to be elucidated. Here, we investigated the involvement of fungal chitin-binding protein and chitin in allergic airway inflammation. Recombinant Aspergillus fumigatus LdpA (rLdpA) expressed in Pichia pastoris was shown to be an O-linked glycoprotein containing terminal α-mannose residues recognized by the host C-type lectin receptor, Dectin-2. Chitin particles were shown to induce acute neutrophilic airway inflammation mediated release of interleukin-1α (IL-1α) associated with cell death. Furthermore, rLdpA-Dectin-2 interaction was shown to promote phagocytosis of rLdpA-chitin complex and activation of mouse bone marrow-derived dendritic cells (BMDCs). Moreover, we showed that rLdpA potently induced T helper 2 (Th2)-driven allergic airway inflammation synergistically with chitin, and Dectin-2 deficiency attenuated the rLdpA-chitin complex-induced immune response in vivo. In addition, we showed that serum LdpA-specific immunoglobulin levels were elevated in patients with pulmonary aspergillosis.

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.

Recent Progress in 1,2-cis glycosylation for Glucan Synthesis

Controlling the stereoselectivity of 1,2-cis glycosylation is one of the most challenging tasks in the chemical synthesis of glycans. There are various 1,2-cis glycosides in nature, such as α-glucoside and β-mannoside in glycoproteins, glycolipids, proteoglycans, microbial polysaccharides, and bioactive natural products. In the structure of polysaccharides such as α-glucan, 1,2-cis α-glucosides were found to be the major linkage between the glucopyranosides. Various regioisomeric linkages, 1→3, 1→4, and 1→6 for the backbone structure, and 1→2/3/4/6 for branching in the polysaccharide as well as in the oligosaccharides were identified. To achieve highly stereoselective 1,2-cis glycosylation, including α-glucosylation, a number of strategies using inter- and intra-molecular methodologies have been explored. Recently, Zn salt-mediated cis glycosylation has been developed and applied to the synthesis of various 1,2-cis linkages, such as α-glucoside and β-mannoside, via the 1,2-cis glycosylation pathway and β-galactoside 1,4/6-cis induction. Furthermore, the synthesis of various structures of α-glucans has been achieved using the recent progressive stereoselective 1,2-cis glycosylation reactions. In this review, recent advances in stereoselective 1,2-cis glycosylation, particularly focused on α-glucosylation, and their applications in the construction of linear and branched α-glucans are summarized.

Structural and functional characterization of a multi-domain GH92 α-1,2-mannosidase from Neobacillus novalis

Many secreted eukaryotic proteins are N-glycosylated with oligosaccharides composed of a high-mannose N-glycan core and, in the specific case of yeast cell-wall proteins, an extended α-1,6-mannan backbone carrying a number of α-1,2- and α-1,3-mannose substituents of varying lengths. α-Mannosidases from CAZy family GH92 release terminal mannose residues from these N-glycans, providing access for the α-endomannanases, which then degrade the α-mannan backbone. Most characterized GH92 α-mannosidases consist of a single catalytic domain, while a few have extra domains including putative carbohydrate-binding modules (CBMs). To date, neither the function nor the structure of a multi-domain GH92 α-mannosidase CBM has been characterized. Here, the biochemical investigation and crystal structure of the full-length five-domain GH92 α-1,2-mannosidase from Neobacillus novalis (NnGH92) with mannoimidazole bound in the active site and an additional mannoimidazole bound to the N-terminal CBM32 are reported. The structure of the catalytic domain is very similar to that reported for the GH92 α-mannosidase Bt3990 from Bacteroides thetaiotaomicron, with the substrate-binding site being highly conserved. The function of the CBM32s and other NnGH92 domains was investigated by their sequential deletion and suggested that whilst their binding to the catalytic domain was crucial for the overall structural integrity of the enzyme, they appear to have little impact on the binding affinity to the yeast α-mannan substrate. These new findings provide a better understanding of how to select and optimize other multi-domain bacterial GH92 α-mannosidases for the degradation of yeast α-mannan or mannose-rich glycans.

Biochemical characterization of a glycoside hydrolase family 43 β-D-galactofuranosidase from the fungus Aspergillus niger

β-D-Galactofuranose (Galf) and its polysaccharides are found in bacteria, fungi and protozoa but do not occur in mammalian tissues, and thus represent a specific target for anti-pathogenic drugs. Understanding the enzymatic degradation of these polysaccharides is therefore of great interest, but the identity of fungal enzymes with exclusively galactofuranosidase activity has so far remained elusive. Here we describe the identification and characterization of a galactofuranosidase from the industrially important fungus Aspergillus niger. Analysis of glycoside hydrolase family 43 subfamily 34 (GH43_34) members via conserved unique peptide patterns and phylogeny, revealed the occurrence of distinct clusters and, by comparison with specificities of characterized bacterial members, suggested a basis for prediction of enzyme specificity. Using this rationale, in tandem with molecular docking, we identified a putative β-D-galactofuranosidase from A. niger which was recombinantly produced in Escherichia coli. The Galf-specific hydrolase, encoded by xynD demonstrates maximum activity at pH 5, 25 °C towards 4-nitrophenyl-β-galactofuranoside (pNP-β-Galf), with a K_m of 17.9 ± 1.9 mM and V_max of 70.6 ± 5.3 µM min^-1. The characterization of this first fungal GH43 galactofuranosidase offers further molecular insight into the degradation of Galf-containing structures.

Extension of O-Linked Mannosylation in the Golgi Apparatus Is Critical for Cell Wall Integrity Signaling and Interaction with Host Cells in Cryptococcus neoformans Pathogenesis

The human-pathogenic yeast Cryptococcus neoformans assembles two types of O-linked glycans on its proteins. In this study, we identified and functionally characterized the C. neoformans CAP6 gene, encoding an α1,3-mannosyltransferase responsible for the second mannose addition to minor O-glycans containing xylose in the Golgi apparatus. Two cell surface sensor proteins, Wml1 (WSC/Mid2-like) and Wml2, were found to be independent substrates of Cap6-mediated minor or Ktr3-mediated major O-mannosylation, respectively. The double deletion of KTR3 and CAP6 (ktr3Δ cap6Δ) completely blocked the mannose addition at the second position of O-glycans, resulting in the accumulation of proteins with O-glycans carrying only a single mannose. Tunicamycin (TM)-induced phosphorylation of the Mpk1 mitogen-activated protein kinase (MAPK) was greatly decreased in both ktr3Δ cap6Δ and wml1Δ wml2Δ strains. Transcriptome profiling of the ktr3Δ cap6Δ strain upon TM treatment revealed decreased expression of genes involved in the Mpk1-dependent cell wall integrity (CWI) pathway. Consistent with its defective growth under several stress conditions, the ktr3Δ cap6Δ strain was avirulent in a mouse model of cryptococcosis. Associated with this virulence defect, the ktr3Δ cap6Δ strain showed decreased adhesion to lung epithelial cells, decreased proliferation within macrophages, and reduced transcytosis of the blood-brain barrier (BBB). Notably, the ktr3Δ cap6Δ strain showed reduced induction of the host immune response and defective trafficking of ergosterol, an immunoreactive fungal molecule. In conclusion, O-glycan extension in the Golgi apparatus plays critical roles in various pathobiological processes, such as CWI signaling and stress resistance and interaction with host cells in C. neoformans. IMPORTANCE Cryptococcus neoformans assembles two types of O-linked glycans on its surface proteins, the more abundant major O-glycans that do not contain xylose residues and minor O-glycans containing xylose. Here, we demonstrate the role of the Cap6 α1,3-mannosyltransferase in the synthesis of minor O-glycans. Previously proposed to be involved in capsule biosynthesis, Cap6 works with the related Ktr3 α1,2-mannosyltransferase to synthesize O-glycans on their target proteins. We also identified two novel C. neoformans stress sensors that require Ktr3- and Cap6-mediated posttranslational modification for full function. Accordingly, the ktr3Δ cap6Δ double O-glycan mutant strain displays defects in stress signaling pathways, CWI, and ergosterol trafficking. Furthermore, the ktr3Δ cap6Δ strain is completely avirulent in a mouse infection model. Together, these results demonstrate critical roles for O-glycosylation in fungal pathogenesis. As there are no human homologs for Cap6 or Ktr3, these fungus-specific mannosyltransferases are novel targets for antifungal therapy.

On the impact of carbohydrate-binding modules (CBMs) in lytic polysaccharide monooxygenases (LPMOs)

Lytic polysaccharide monooxygenases (LPMOs) have revolutionized our understanding of how enzymes degrade insoluble polysaccharides. Compared with the substantial knowledge developed on the structure and mode of action of the catalytic LPMO domains, the (multi)modularity of LPMOs has received less attention. The presence of other domains, in particular carbohydrate-binding modules (CBMs), tethered to LPMOs has profound implications for the catalytic performance of the full-length enzymes. In the last few years, studies on LPMO modularity have led to advancements in elucidating how CBMs, other domains, and linker regions influence LPMO structure and function. This mini review summarizes recent literature, with particular focus on comparative truncation studies, to provide an overview of the diversity in LPMO modularity and the functional implications of this diversity.

Comparison of the stability of Mucor-derived flavin adenine dinucleotide-dependent glucose dehydrogenase and glucose oxidase

Long-term stability at near-body temperature is important for continuous glucose monitoring (CGM) sensors. However, the stability of enzymes used in CGM sensors has often been evaluated by measuring their melting temperature (T_m) values and by short heat treatment but not at around 37 °C. Glucose oxidase (GOD) is used in current CGM sensors. In this study, we evaluated the stability of modified Mucor-derived flavin adenine dinucleotide-dependent glucose dehydrogenase (designated Mr144-297) with improved thermal stability at medium to high temperatures and compared it with that of GOD. The T_m value of Mr144-297 was 61.6 ± 0.3 °C and was similar to that of GOD (61.4 ± 1.2 °C). However, Mr144-297 was clearly more stable than GOD at 40 °C and 55 °C. At 37 °C, the stability of a carbon electrode with immobilized Mr144-297 was higher than that of an electrode with GOD. Our data indicate that Mr144-297 is a more suitable enzyme for CGM sensors than is GOD.

Analysis of fungal high-mannose structures using CAZymes

Glycoengineering ultimately allows control over glycosylation patterns to generate new glycoprotein variants with desired properties. A common challenge is glycan heterogeneity, which may affect protein function and limit the use of key techniques such as mass spectrometry. Moreover, heterologous protein expression can introduce nonnative glycan chains that may not fulfill the requirement for therapeutic proteins. One strategy to address these challenges is partial trimming or complete removal of glycan chains, which can be obtained through selective application of exoglycosidases. Here, we demonstrate an enzymatic O-deglycosylation toolbox of a GH92 α-1,2-mannosidase from Neobacillus novalis, a GH2 β-galactofuranosidase from Amesia atrobrunnea and the jack bean α-mannosidase. The extent of enzymatic O-deglycosylation was mapped against a full glycosyl linkage analysis of the O-glycosylated linker of cellobiohydrolase I from Trichoderma reesei (TrCel7A). Furthermore, the influence of deglycosylation on TrCel7A functionality was evaluated by kinetic characterization of native and O-deglycosylated forms of TrCel7A. This study expands structural knowledge on fungal O-glycosylation and presents a ready-to-use enzymatic approach for controlled O-glycan engineering in glycoproteins expressed in filamentous fungi.

Demystifying the O-GlcNAc Code: A Systems View

Post-translational modification with O-linked β-N-acetylglucosamine (O-GlcNAc), a process referred to as O-GlcNAcylation, occurs on a vast variety of proteins. Mounting evidence in the past several decades has clearly demonstrated that O-GlcNAcylation is a unique and ubiquitous modification. Reminiscent of a code, protein O-GlcNAcylation functions as a crucial regulator of nearly all cellular processes studied. The primary aim of this review is to summarize the developments in our understanding of myriad protein substrates modified by O-GlcNAcylation from a systems perspective. Specifically, we provide a comprehensive survey of O-GlcNAcylation in multiple species studied, including eukaryotes (e.g., protists, fungi, plants, Caenorhabditis elegans, Drosophila melanogaster, murine, and human), prokaryotes, and some viruses. We evaluate features (e.g., structural properties and sequence motifs) of O-GlcNAc modification on proteins across species. Given that O-GlcNAcylation functions in a species-, tissue-/cell-, protein-, and site-specific manner, we discuss the functional roles of O-GlcNAcylation on human proteins. We focus particularly on several classes of relatively well-characterized human proteins (including transcription factors, protein kinases, protein phosphatases, and E3 ubiquitin-ligases), with representative O-GlcNAc site-specific functions presented. We hope the systems view of the great endeavor in the past 35 years will help demystify the O-GlcNAc code and lead to more fascinating studies in the years to come.

Bioinformatic characterization of a triacylglycerol lipase produced by Aspergillus flavus isolated from the decaying seed of Cucumeropsis mannii

Lipases are enzymes of industrial importance responsible for the hydrolysis of ester bonds of triglycerides. A lipolytic fungus was isolated and subsequently identified based on the ITS sequence analysis as putative Aspergillus flavus with accession number LC424503. The gene coding for extracellular triacylglycerol lipase was isolated from Aspergillus flavus species, sequenced, and characterised using bioinformatics tools. An open reading frame of 420 amino acid sequence was obtained and designated as Aspergillus flavus lipase (AFL) sequence. Alignment of the amino acid sequence with other lipases revealed the presence GHSLG sequence which is the lipase consensus sequence Gly-X1-Ser-X2-Gly indicating that it a classical lipase. A catalytic active site lid domain composed of TYITDTIIDLS amino acids sequence was also revealed. This lid protects the active site, control the catalytic activity and substrate selectivity in lipases. The 3-Dimensional structural model shared 34.08% sequence identity with a lipase from Yarrowia lipolytica covering 272 amino acid residues of the template model. A search of the lipase engineering database using AFL sequence revealed that it belongs to the class GX-lipase, superfamily abH23 and homologous family abH23.02, molecular weight and isoelectric point values of 46.95 KDa and 5.7, respectively. N-glycosylation sites were predicted at residues 164, 236 and 333, with potentials of 0.7250, 0.7037 and 0.7048, respectively. O-glycosylation sites were predicted at residues 355, 358, 360 and 366. A signal sequence of 37 amino acids was revealed at the N-terminal of the polypeptide. This is a short peptide sequence that marks a protein for transport across the cell membrane and indicates that AFL is an extracellular lipase. The findings on the structural and molecular properties of Aspergillus flavus lipase in this work will be crucial in future studies aiming at engineering the enzyme for biotechnology applications.Communicated by Ramaswamy H. Sarma.

Secretome-based screening of fusion partners and their application in recombinant protein secretion in Saccharomyces cerevisiae

For the efficient production of heterologous proteins in the yeast Saccharomyces cerevisiae, we screened for a novel fusion partner from the yeast secretome. From twenty major proteins identified from the yeast secretome, we selected Scw4p, a cell wall protein with similarity to glucanase, and modified to develop a general fusion partner for the secretory expression of heterologous proteins in yeast. The optimal size of the SCW4 gene to act as an efficient fusion partner was determined by C-terminal truncation analysis; two of the variants, S1 (truncated at codon 115Q) and S2 (truncated at codon 142E), were further used for the secretion of heterologous proteins. When fused with S2, the secretion of three target proteins (hGH, exendin-4, and hPTH) significantly increased. Conserved O-glycosylation sites (Ser/Thr-rich domain) and hydrophilic sequences of S2 were deemed important for the function of S2 as a secretion fusion partner. Approximately 5 g/L of the S2-exendin-4 fusion protein was obtained from fed-batch fermentation. Intact target proteins were easily purified by affinity chromatography after in vitro processing of the fusion partner. This system may be of general application for the secretory production of heterologous proteins in S. cerevisiae. KEY POINTS : • Target proteins were efficiently secreted with their N-terminus fused to Scw4p. • O-glycosylation and hydrophilic stretches in Scw4p were important for protein secretion. • A variant of Scw4p (S2) was successfully applied for the secretory expression of heterologous proteins.

Fungal cellulases: protein engineering and post-translational modifications

Enzymatic degradation of lignocelluloses into fermentable sugars to produce biofuels and other biomaterials is critical for environmentally sustainable development and energy resource supply. However, there are problems in enzymatic cellulose hydrolysis, such as the complex cellulase composition, low degradation efficiency, high production cost, and post-translational modifications (PTMs), all of which are closely related to specific characteristics of cellulases that remain unclear. These problems hinder the practical application of cellulases. Due to the rapid development of computer technology in recent years, computer-aided protein engineering is being widely used, which also brings new opportunities for the development of cellulases. Especially in recent years, a large number of studies have reported on the application of computer-aided protein engineering in the development of cellulases; however, these articles have not been systematically reviewed. This article focused on the aspect of protein engineering and PTMs of fungal cellulases. In this manuscript, the latest literatures and the distribution of potential sites of cellulases for engineering have been systematically summarized, which provide reference for further improvement of cellulase properties. KEY POINTS: •Rational design based on virtual mutagenesis can improve cellulase properties. •Modifying protein side chains and glycans helps obtain superior cellulases. •N-terminal glutamine-pyroglutamate conversion stabilizes fungal cellulases.

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.

Hyperosmotic medium partially restores the growth defect and the impaired production of sterigmatocystin of an Aspergillus nidulans ΔpmtC mutant in a HogA-independent manner

The protein O-mannosyltransferase catalyzes O-mannosylation in the endoplasmic reticulum by transferring mannose to the seryl or threonyl residues of substrate proteins. We previously reported a deletion mutant of O-mannosyltransferase C (ΔpmtC) in Aspergillus nidulans with impaired vegetative growth and sterigmatocystin (ST) production. In this study, we investigated whether osmotic conditions contribute to the developmental processes and ST biosynthesis of the ΔpmtC deletion mutant. We found that hyphal growth and ST production partially improved in the presence of NaCl, KCl or sorbitol as osmotic stabilizers. Conidiation of the ΔpmtC deletion mutant was not restored under osmotic stress conditions when the hogA gene was deleted. The hogA gene encodes a protein required for the cellular response to osmotic pressure. However, the yield of ST and the vegetative growth of the ΔhogA ΔpmtC double deletant was restored by high osmolarity in a HogA-independent manner.

Ero1-Pdi1 module-catalysed dimerization of a nucleotide sugar transporter, FonNst2, regulates virulence of Fusarium oxysporum on watermelon

Fusarium oxysporum f. sp. niveum (Fon) is a soil-borne fungus causing vascular Fusarium wilt on watermelon; however, the molecular network regulating Fon virulence remains to be elucidated. Here, we report the function and mechanism of nucleotide sugar transporters (Nsts) in Fon. Fon genome harbours nine FonNst genes with distinct functions in vegetative growth, asexual production, cell wall stress response and virulence. FonNst2 and FonNst3 are required for full virulence of Fon on watermelon and FonNst2 is mainly involved in fungal colonization of the plant tissues. FonNst2 and FonNst3 form homo- or hetero-dimers but function independently in Fon virulence. FonNst2, which has UDP-galactose transporter activity in yeast, interacts with FonEro1 and FonPdi1, both of which are required for full virulence of Fon. FonNst2, FonPdi1 and FonEro1 target to endoplasmic reticulum (ER) and are essential for ER homeostasis and function. FonEro1-FonPdi1 module catalyses the dimerization of FonNst2, which is critical for Fon virulence. Undimerized FonNst2 is unstable and degraded via ER-associated protein degradation in vivo. These data demonstrate that FonEro1-FonPdi1 module-catalysed dimerization of FonNst2 is critical for Fon virulence on watermelon and provide new insights into the regulation of virulence in plant fungal pathogens via disulfide bond formation of key pathogenicity factors.

Trends and Applications of Omics Technologies to Functional Characterisation of Enzymes and Protein Metabolites Produced by Fungi

Identifying and adopting industrial applications for proteins and enzymes derived from fungi strains have been at the focal point of several studies in recent times. To facilitate such studies, it is necessary that advancements and innovation in mycological and molecular characterisation are concomitant. This review aims to provide a detailed overview of the necessary steps employed in both qualitative and quantitative research using the omics technologies that are pertinent to fungi characterisation. This stems from the understanding that data provided from the functional characterisation of fungi and their metabolites is important towards the techno-economic feasibility of large-scale production of biological products. The review further describes how the functional gaps left by genomics, internal transcribe spacer (ITS) regions are addressed by transcriptomics and the various techniques and platforms utilised, including quantitive reverse transcription polymerase chain reaction (RT-qPCR), hybridisation techniques, and RNA-seq, and the insights such data provide on the effect of environmental changes on fungal enzyme production from an expressional standpoint. The review also offers information on the many available bioinformatics tools of analysis necessary for the analysis of the overwhelming data synonymous with the omics approach to fungal characterisation.

Membrane traffic related to endosome dynamics and protein secretion in filamentous fungi

In eukaryotic cells, membrane-surrounded organelles are orchestrally organized spatiotemporally under environmental situations. Among such organelles, vesicular transports and membrane contacts occur to communicate each other, so-called membrane traffic. Filamentous fungal cells are highly polarized and thus membrane traffic is developed to have versatile functions. Early endosome (EE) is an endocytic organelle that dynamically exhibits constant long-range motility through the hyphal cell, which is proven to have physiological roles, such as other organelle distribution and signal transduction. Since filamentous fungal cells are also considered as cell factories, to produce valuable proteins extracellularly, molecular mechanisms of secretory pathway including protein glycosylation have been well investigated. In this review, molecular and physiological aspects of membrane traffic especially related to EE dynamics and protein secretion in filamentous fungi are summarized, and perspectives for application are also described.

Galactofuranose (Galf)-containing sugar chain contributes to the hyphal growth, conidiation and virulence of F. oxysporum f.sp. cucumerinum

The ascomycete fungus Fusarium oxysporum f.sp. cucumerinum causes vascular wilt diseases in cucumber. However, few genes related to morphogenesis and pathogenicity of this fungal pathogen have been functionally characterized. BLASTp searches of the Aspergillus fumigatus UgmA and galatofuranosyltransferases (Galf-transferases) sequences in the F. oxysporum genome identified two genes encoding putative UDP-galactopyranose mutase (UGM), ugmA and ugmB, and six genes encoding putative Galf-transferase homologs. In this study, the single and double mutants of the ugmA, ugmB and gfsB were obtained. The roles of UGMs and GfsB were investigated by analyzing the phenotypes of the mutants. Our results showed that deletion of the ugmA gene led to a reduced production of galactofuranose-containing sugar chains, reduced growth and impaired conidiation of F. oxysporum f.sp. cucumerinum. Most importantly, the ugmA deletion mutant lost the pathogenicity in cucumber plantlets. Although deletion of the ugmB gene did not cause any visible phenotype, deletion of both ugmA and ugmB genes caused more severe phenotypes as compared with the ΔugmA, suggesting that UgmA and UgmB are redundant and they can both contribute to synthesis of UDP-Galf. Furthermore, the ΔgfsB exhibited an attenuated virulence although no other phenotype was observed. Our results demonstrate that the galactofuranose (Galf) synthesis contributes to the cell wall integrity, germination, hyphal growth, conidiation and virulence in Fusarium oxysporum f.sp. cucumerinum and an ideal target for the development of new anti-Fusarium agents.

Membrane Traffic in Aspergillus oryzae and Related Filamentous Fungi

The industrially important filamentous fungus Aspergillus oryzae, known as the yellow Koji mold and also designated the Japanese National fungus, has been investigated for understanding the intracellular membrane trafficking machinery due to the great ability of valuable enzyme production. The underlying molecular mechanisms of the secretory pathway delineate the main secretion route from the hyphal tip via the vesicle cluster Spitzenkörper, but also there is a growing body of evidence that septum-directed and unconventional secretion occurs in A. oryzae hyphal cells. Moreover, not only the secretory pathway but also the endocytic pathway is crucial for protein secretion, especially having a role in apical endocytic recycling. As a hallmark of multicellular filamentous fungal cells, endocytic organelles early endosome and vacuole are quite dynamic: the former exhibits constant long-range motility through the hyphal cells and the latter displays pleiomorphic structures in each hyphal region. These characteristics are thought to have physiological roles, such as supporting protein secretion and transporting nutrients. This review summarizes molecular and physiological mechanisms of membrane traffic, i.e., secretory and endocytic pathways, in A. oryzae and related filamentous fungi and describes the further potential for industrial applications.

The Saccharomyces cerevisiae Ncw2 protein works on the chitin/β-glucan organisation of the cell wall

The NCW2 gene was recently described as encoding a GPI-bounded protein that assists in the re-modelling of the Saccharomyces cerevisiae cell wall (CW) and in the repair of damage caused by the polyhexamethylene biguanide (PHMB) polymer to the cell wall. Its absence produces a re-organization of the CW structure that result in resistance to lysis by glucanase. Hence, the present study aimed to extend the analysis of the Ncw2 protein (Ncw2p) to determine its physiological role in the yeast cell surface. The results showed that Ncw2p is transported to the cell surface upon O-mannosylation mediated by the Pmt1p-Pmt2p enzyme complex. It co-localises with the yeast bud scars, a region in cell surface formed by chitin deposition. Once there, Ncw2p enables correct chitin/β-glucan structuring during the exponential growth. The increase in molecular mass by hyper-mannosylation coincides with the increasing in chitin deposition, and leads to glucanase resistance. Treatment of the yeast cells with PHMB produced the same biological effects observed for the passage from exponential to stationary growth phase. This might be a possible mechanism of yeast protection against cationic biocides. In conclusion, we propose that Ncw2p takes part in the mechanism involved in the control of cell surface rigidity by aiding on the linkage between chitin and glucan layers in the modelling of the cell wall during cell growth.

Structural, Evolutionary, and Functional Analysis of the Protein O-Mannosyltransferase Family in Pathogenic Fungi

Protein O-mannosyltransferases (Pmts) comprise a group of proteins that add mannoses to substrate proteins at the endoplasmic reticulum. This post-translational modification is important for the faithful transfer of nascent glycoproteins throughout the secretory pathway. Most fungi genomes encode three O-mannosyltransferases, usually named Pmt1, Pmt2, and Pmt4. In pathogenic fungi, Pmts, especially Pmt4, are key factors for virulence. Although the importance of Pmts for fungal pathogenesis is well established in a wide range of pathogens, questions remain regarding certain features of Pmts. For example, why does the single deletion of each pmt gene have an asymmetrical impact on host colonization? Here, we analyse the origin of Pmts in fungi and review the most important phenotypes associated with Pmt mutants in pathogenic fungi. Hence, we highlight the enormous relevance of these glycotransferases for fungal pathogenic development.

Compromised chitin synthesis in lager yeast affects its Congo red resistance and release of mannoproteins from the cells

A mutant lager strain resistant to the cell wall-perturbing agent Congo red (CR) was isolated and the genetic alterations underlying CR resistance were investigated by whole genome sequencing. The parental lager strain was found to contain three distinct Saccharomyces cerevisiae (Sc)-type CHS6 (CHitin Synthase-related 6) alleles, two of which have one or two nonsense mutations in the open reading frame, leaving only one functional allele, whereas the functional allele was missing in the isolated CR-resistant strain. On the other hand, the Saccharomyces eubayanus-type CHS6 alleles shared by both the parental and mutant strains appeared to contribute poorly to chitin synthase-activating function. Therefore, the CR resistance of the mutant strain was attributable to the overall compromised activity of CHS6 gene products. The CR-resistant mutant cells exhibited less chitin production on the cell surface and smaller amounts of mannoprotein release into the medium. All these traits, in addition to the CR resistance, were complemented by the functional ScCHS6 gene. It is of great interest whether the frequent nonsense mutations found in ScCHS6 open reading frame in lager yeast strains are a consequence of the domestication process of lager yeast.

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.

Effect of O-glycosylation on amyloid fibril formation of the variable domain in the Vλ6 light chain mutant Wil

Glycosylation is one of the major post-translational modifications in eukaryotic cells and has been reported to affect the amyloid fibril formation in several amyloidogenic proteins and peptides. In this study, we expressed a Vλ6 light chain mutant, Wil, which is an amyloidogenic mutant in AL amyloidosis, by the yeast Pichia pastoris. After separation by cation exchange chromatography, we obtained the O-glycosylated and non-glycosylated Wil mutants in high yield. The structures of these Wil mutants were identical except with respect to glycosylation, and the stabilities were also identical. On the other hand, the O-glycosylation retarded the amyloid fibril formation in a sugar size-dependent manner. From these results, we discussed the role of covalently attached glycan in the retardation of amyloid fibril formation.

Protein O-mannosylation affects protein secretion, cell wall integrity and morphogenesis in Trichoderma reesei

Protein O-mannosyltransferases (PMTs) initiate O-mannosylation of proteins in the ER. Trichoderma reesei strains displayed a single representative of each PMT subfamily, Trpmt1, Trpmt2 and Trpmt4. In this work, two knockout strains ΔTrpmt1and ΔTrpmt4were obtained. Both mutants showed retarded growth, defective cell walls, reduced conidiation and decreased protein secretion. Additionally, the ΔTrpmt1strain displayed a thermosensitive growth phenotype, while the ΔTrpmt4 strain showed abnormal polarity. Meanwhile, OETrpmt2 strain, in which the Trpmt2 was over-expressed, exhibited increased conidiation, enhanced protein secretion and abnormal polarity. Using a lectin enrichment method and MS/MS analysis, 173 O-glycoproteins, 295 O-glycopeptides and 649 O-mannosylation sites were identified as the targets of PMTs in T. reesei. These identified O-mannoproteins are involved in various physiological processes such as protein folding, sorting, transport, quality control and secretion, as well as cell wall integrity and polarity. By comparing proteins identified in the mutants and its parent strain, the potential specific protein substrates of PMTs were identified. Based on our results, TrPMT1 is specifically involved inO-mannosylation of intracellular soluble proteins and secreted proteins, specially glycosidases. TrPMT2 is involved inO-mannosylation of secreted proteins and GPI-anchor proteins, and TrPMT4 mainly modifies multiple transmembrane proteins. The TrPMT1-TrPMT4 complex is responsible for O-mannosylation of proteins involved in cell wall integrity. Overexpression of TrPMT2 enhances protein secretion, which might be a new strategy to improve expression efficiency in T. reesei.

Biomedical applications of yeasts - a patent view, part two: era of humanized yeasts and expanded applications

INTRODUCTION

Yeast humanization, ranging from a simple point mutation to substitution of yeast gene(s) or even a complete pathway by human counterparts has enormously expanded yeast biomedical applications.

AREAS COVERED

General and patent-oriented insights into the application of native and humanized yeasts for production of human glycoproteins (gps) and antibodies (Abs), toxicity/mutagenicity assays, treatments of gastrointestinal (GI) disorders and potential drug delivery as a probiotic (with emphasis on Saccharomyces bulardii) and studies on human diseases/cancers and screening effective drugs.

EXPERT OPINION

Humanized yeasts cover the classical advantageous features of a 'microbial eukaryote' together with advanced human cellular processes. These unique characteristics would permit their use in the production of functional and stable therapeutic gps and Abs in lower prices compared to mammalian (CHO) production-based systems. Availability of yeasts humanized for cytochrome P450 s will expand their application in metabolism-related chemical toxicity assays. Engineered S. bulardii for expression of human proteins might expand its application by synergistically combining the probiotic activity with the treatment of metabolic diseases such as phenylketonuria via GI-delivery. Yeast models of human diseases will facilitate rapid functional/phenotypic characterization of the disease-producing mutant genes and screening of the therapeutic compounds using yeast-based high-throughput research techniques (Yeast one/two hybrid systems) and viability assays.

In-depth characterization of Trichoderma reesei cellobiohydrolase TrCel7A produced in Nicotiana benthamiana reveals limitations of cellulase production in plants by host-specific post-translational modifications

Sustainable production of biofuels from lignocellulose feedstocks depends on cheap enzymes for degradation of such biomass. Plants offer a safe and cost-effective production platform for biopharmaceuticals, vaccines and industrial enzymes boosting biomass conversion to biofuels. Production of intact and functional protein is a prerequisite for large-scale protein production, and extensive host-specific post-translational modifications (PTMs) often affect the catalytic properties and stability of recombinant enzymes. Here we investigated the impact of plant PTMs on enzyme performance and stability of the major cellobiohydrolase TrCel7A from Trichoderma reesei, an industrially relevant enzyme. TrCel7A was produced in Nicotiana benthamiana using a vacuum-based transient expression technology, and this recombinant enzyme (TrCel7Arec ) was compared with the native fungal enzyme (TrCel7Anat ) in terms of PTMs and catalytic activity on commercial and industrial substrates. We show that the N-terminal glutamate of TrCel7Arec was correctly processed by N. benthamiana to a pyroglutamate, critical for protein structure, while the linker region of TrCel7Arec was vulnerable to proteolytic digestion during protein production due to the absence of O-mannosylation in the plant host as compared with the native protein. In general, the purified full-length TrCel7Arec had 25% lower catalytic activity than TrCel7Anat and impaired substrate-binding properties, which can be attributed to larger N-glycans and lack of O-glycans in TrCel7Arec . All in all, our study reveals that the glycosylation machinery of N. benthamiana needs tailoring to optimize the production of efficient cellulases.

A comprehensive genome-scale model for Rhodosporidium toruloides IFO0880 accounting for functional genomics and phenotypic data

Rhodosporidium toruloides is a red, basidiomycetes yeast that can accumulate a large amount of lipids and produce carotenoids. To better assess this non-model yeast's metabolic capabilities, we reconstructed a genome-scale model of R. toruloides IFO0880's metabolic network (iRhto1108) accounting for 2204 reactions, 1985 metabolites and 1108 genes. In this work, we integrated and supplemented the current knowledge with in-house generated biomass composition and experimental measurements pertaining to the organism's metabolic capabilities. Predictions of genotype-phenotype relations were improved through manual curation of gene-protein-reaction rules for 543 reactions leading to correct recapitulations of 84.5% of gene essentiality data (sensitivity of 94.3% and specificity of 53.8%). Organism-specific macromolecular composition and ATP maintenance requirements were experimentally measured for two separate growth conditions: (i) carbon and (ii) nitrogen limitations. Overall, iRhto1108 reproduced R. toruloides's utilization capabilities for 18 alternate substrates, matched measured wild-type growth yield, and recapitulated the viability of 772 out of 819 deletion mutants. As a demonstration to the model's fidelity in guiding engineering interventions, the OptForce procedure was applied on iRhto1108 for triacylglycerol overproduction. Suggested interventions recapitulated many of the previous successful implementations of genetic modifications and put forth a few new ones.

Endo-β-1,3-glucanase (GH16 Family) from Trichoderma harzianum Participates in Cell Wall Biogenesis but Is Not Essential for Antagonism Against Plant Pathogens

Trichoderma species are known for their ability to produce lytic enzymes, such as exoglucanases, endoglucanases, chitinases, and proteases, which play important roles in cell wall degradation of phytopathogens. β-glucanases play crucial roles in the morphogenetic-morphological process during the development and differentiation processes in Trichoderma species, which have β-glucans as the primary components of their cell walls. Despite the importance of glucanases in the mycoparasitism of Trichoderma spp., only a few functional analysis studies have been conducted on glucanases. In the present study, we used a functional genomics approach to investigate the functional role of the gluc31 gene, which encodes an endo-β-1,3-glucanase belonging to the GH16 family in Trichoderma harzianum ALL42. We demonstrated that the absence of the gluc31 gene did not affect the in vivo mycoparasitism ability of mutant T. harzianum ALL42; however, gluc31 evidently influenced cell wall organization. Polymer measurements and fluorescence microscopy analyses indicated that the lack of the gluc31 gene induced a compensatory response by increasing the production of chitin and glucan polymers on the cell walls of the mutant hyphae. The mutant strain became more resistant to the fungicide benomyl compared to the parental strain. Furthermore, qRT-PCR analysis showed that the absence of gluc31 in T. harzianum resulted in the differential expression of other glycosyl hydrolases belonging to the GH16 family, because of functional redundancy among the glucanases.

Ultrastructural and SINS analysis of the cell wall integrity response of Aspergillus nidulans to the absence of galactofuranose

With lethal opportunistic fungal infections on the rise, it is imperative to explore new methods to examine virulence mechanisms. The fungal cell wall is crucial for both the virulence and viability of Aspergillus nidulans. One wall component, Galf, has been shown to contribute to important fungal processes, integrity of the cell wall and pathogenesis. Here, we explore gene deletion strains lacking the penultimate enzyme in Galf biosynthesis (ugmAΔ) and the protein that transports Galf for incorporation into the cell wall (ugtAΔ). In applying gene deletion technology to the problem of cell wall integrity, we have employed multiple micro- and nano-scale imaging tools, including confocal fluorescence microscopy, electron microscopy, X-Ray fluorescence and atomic force microscopy. Atomic force microscopy allows quantification of ultrastructural cell wall architecture while near-field infrared spectroscopy provides spatially resolved chemical signatures, both at the nanoscale. Here, for the first time, we demonstrate correlative data collection with these two emerging modalities for the multiplexed in situ study of the nanoscale architecture and chemical composition of fungal cell walls.

Two KTR Mannosyltransferases Are Responsible for the Biosynthesis of Cell Wall Mannans and Control Polarized Growth in Aspergillus fumigatus

The fungal cell wall is a complex and dynamic entity essential for the development of fungi. It allows fungal pathogens to survive environmental challenge posed by nutrient stress and host defenses, and it also is central to polarized growth. The cell wall is mainly composed of polysaccharides organized in a three-dimensional network. Aspergillus fumigatus produces a cell wall galactomannan whose biosynthetic pathway and biological functions remain poorly defined. Here, we described two new mannosyltransferases essential to the synthesis of the cell wall galactomannan. Their absence leads to a growth defect with misregulation of polarization and altered conidiation, with conidia which are bigger and more permeable than the conidia of the parental strain. This study showed that in spite of its low concentration in the cell wall, this polysaccharide is absolutely required for cell wall stability, for apical growth, and for the full virulence of A. fumigatus.

Fungal cell wall mannans are complex carbohydrate polysaccharides with different structures in yeasts and molds. In contrast to yeasts, their biosynthetic pathway has been poorly investigated in filamentous fungi. In Aspergillus fumigatus, the major mannan structure is a galactomannan that is cross-linked to the β-1,3-glucan-chitin cell wall core. This polymer is composed of a linear mannan with a repeating unit composed of four α1,6-linked and α1,2-linked mannoses with side chains of galactofuran. Despite its use as a biomarker to diagnose invasive aspergillosis, its biosynthesis and biological function were unknown. Here, we have investigated the function of three members of the Ktr (also named Kre2/Mnt1) family (Ktr1, Ktr4, and Ktr7) in A. fumigatus and show that two of them are required for the biosynthesis of galactomannan. In particular, we describe a newly discovered form of α-1,2-mannosyltransferase activity encoded by the KTR4 gene. Biochemical analyses showed that deletion of the KTR4 gene or the KTR7 gene leads to the absence of cell wall galactomannan. In comparison to parental strains, the Δktr4 and Δktr7 mutants showed a severe growth phenotype with defects in polarized growth and in conidiation, marked alteration of the conidial viability, and reduced virulence in a mouse model of invasive aspergillosis. In yeast, the KTR proteins are involved in protein 0- and N-glycosylation. This study provided another confirmation that orthologous genes can code for proteins that have very different biological functions in yeasts and filamentous fungi. Moreover, in A. fumigatus, cell wall mannans are as important structurally as β-glucans and chitin.

Stereodivergent Mannosylation Using 2- O-( ortho-Tosylamido)benzyl Group

We report a novel strategy for obtaining both anomers from a single mannosyl donor equipped with a C2- o-TsNHbenzyl ether (2- O-TAB) by switching reaction conditions. In particular, the formation of various β-mannosides was achieved with high selectivity by using a mannosyl phosphite in the presence of ZnI₂.

Mass spectrometric analysis of the N-glycoproteome in statin-treated liver cells with two lectin-independent chemical enrichment methods

Protein N-glycosylation is essential for mammalian cell survival and is well-known to be involved in many biological processes. Aberrant glycosylation is directly related to human disease including cancer and infectious diseases. Global analysis of protein N-glycosylation will allow a better understanding of protein functions and cellular activities. Mass spectrometry (MS)-based proteomics provides a unique opportunity to site-specifically characterize protein glycosylation on a large scale. Due to the complexity of biological samples, effective enrichment methods are critical prior to MS analysis. Here, we compared two lectin-independent methods to enrich glycopeptides for the global analysis of protein N-glycosylation by MS. The first boronic acid-based enrichment (BA) method benefits from the universal and reversible interactions between boronic acid and sugars; the other method utilizes metabolic labeling and click chemistry (MC) to incorporate a chemical handle into glycoproteins for future affinity enrichment. We comprehensively compared the performance of the two methods in the identification and quantification of glycoproteins in statin-treated liver cells. Based on the current results, the BA method is more universal in enriching glycopeptides, while with the MC method, cell surface glycoproteins were highly enriched, and the quantification results appear to be more dynamic because only the newly-synthesized glycoproteins were analyzed. In addition, we normalized the glycosylation site ratios by the corresponding parent protein ratios to reflect the real modification changes. In combination with MS-based proteomics, effective enrichment methods will vertically advance protein glycosylation research.

The impact of O-glycan chemistry on the stability of intrinsically disordered proteins

Protein glycosylation is a diverse post-translational modification that serves myriad biological functions. O-linked glycans in particular vary widely in extent and chemistry in eukaryotes, with secreted proteins from fungi and yeast commonly exhibiting O-mannosylation in intrinsically disordered regions of proteins, likely for proteolysis protection, among other functions. However, it is not well understood why mannose is often the preferred glycan, and more generally, if the neighboring protein sequence and glycan have coevolved to protect against proteolysis in glycosylated intrinsically disordered proteins (IDPs). Here, we synthesized variants of a model IDP, specifically a natively O-mannosylated linker from a fungal enzyme, with α-O-linked mannose, glucose, and galactose moieties, along with a non-glycosylated linker. Upon exposure to thermolysin, O-mannosylation, by far, provides the highest extent of proteolysis protection. To explain this observation, extensive molecular dynamics simulations were conducted, revealing that the axial configuration of the C2-hydroxyl group (2-OH) of α-mannose adjacent to the glycan-peptide bond strongly influences the conformational features of the linker. Specifically, α-mannose restricts the torsions of the IDP main chain more than other glycans whose equatorial 2-OH groups exhibit interactions that favor perpendicular glycan-protein backbone orientation. We suggest that IDP stiffening due to O-mannosylation impairs protease action, with contributions from protein-glycan interactions, protein flexibility, and protein stability. Our results further imply that resistance to proteolysis is an important driving force for evolutionary selection of α-mannose in eukaryotic IDPs, and more broadly, that glycan motifs for proteolysis protection likely coevolve with the protein sequence to which they attach.

Oxygen Activation by Cu LPMOs in Recalcitrant Carbohydrate Polysaccharide Conversion to Monomer Sugars

Natural carbohydrate polymers such as starch, cellulose, and chitin provide renewable alternatives to fossil fuels as a source for fuels and materials. As such, there is considerable interest in their conversion for industrial purposes, which is evidenced by the established and emerging markets for products derived from these natural polymers. In many cases, this is achieved via industrial processes that use enzymes to break down carbohydrates to monomer sugars. One of the major challenges facing large-scale industrial applications utilizing natural carbohydrate polymers is rooted in the fact that naturally occurring forms of starch, cellulose, and chitin can have tightly packed organizations of polymer chains with low hydration levels, giving rise to crystalline structures that are highly recalcitrant to enzymatic degradation. The topic of this review is oxidative cleavage of carbohydrate polymers by lytic polysaccharide mono-oxygenases (LPMOs). LPMOs are copper-dependent enzymes (EC 1.14.99.53-56) that, with glycoside hydrolases, participate in the degradation of recalcitrant carbohydrate polymers. Their activity and structural underpinnings provide insights into biological mechanisms of polysaccharide degradation.

Biosynthesis of galactomannans found in filamentous fungi belonging to Pezizomycotina

The galactomannans (GMs) that are produced by filamentous fungi belonging to Pezizomycotina, many of which are pathogenic for animals and plants, are polysaccharides consisting of α-(1→2)-/α-(1→6)-mannosyl and β-(1→5)-/β-(1→6)-galactofuranosyl residues. GMs are located at the outermost layer of the cell wall. When a pathogenic fungus infects a host, its cell surface must be in contact with the host. The GMs on the cell surface may be involved in the infection mechanism of a pathogenic fungus or the defense mechanism of a host. There are two types of GMs in filamentous fungi, fungal-type galactomannans and O-mannose type galactomannans. Recent biochemical and genetic advances have facilitated a better understanding of the biosynthesis of both types. This review summarizes our current information on their biosynthesis.

Distinct roles of N- and O-glycans in cellulase activity and stability

In nature, many microbes secrete mixtures of glycoside hydrolases, oxidoreductases, and accessory enzymes to deconstruct polysaccharides and lignin in plants. These enzymes are often decorated with N- and O-glycosylation, the roles of which have been broadly attributed to protection from proteolysis, as the extracellular milieu is an aggressive environment. Glycosylation has been shown to sometimes affect activity, but these effects are not fully understood. Here, we examine N- and O-glycosylation on a model, multimodular glycoside hydrolase family 7 cellobiohydrolase (Cel7A), which exhibits an O-glycosylated carbohydrate-binding module (CBM) and an O-glycosylated linker connected to an N- and O-glycosylated catalytic domain (CD)-a domain architecture common to many biomass-degrading enzymes. We report consensus maps for Cel7A glycosylation that include glycan sites and motifs. Additionally, we examine the roles of glycans on activity, substrate binding, and thermal and proteolytic stability. N-glycan knockouts on the CD demonstrate that N-glycosylation has little impact on cellulose conversion or binding, but does have major stability impacts. O-glycans on the CBM have little impact on binding, proteolysis, or activity in the whole-enzyme context. However, linker O-glycans greatly impact cellulose conversion via their contribution to proteolysis resistance. Molecular simulations predict an additional role for linker O-glycans, namely that they are responsible for maintaining separation between ordered domains when Cel7A is engaged on cellulose, as models predict α-helix formation and decreased cellulose interaction for the nonglycosylated linker. Overall, this study reveals key roles for N- and O-glycosylation that are likely broadly applicable to other plant cell-wall-degrading enzymes.

Identification of membrane proteome of Paracoccidioides lutzii and its regulation by zinc

Aim:

During infection development in the host, Paracoccidioides spp. faces the deprivation of micronutrients, a mechanism called nutritional immunity. This condition induces the remodeling of proteins present in different metabolic pathways. Therefore, we attempted to identify membrane proteins and their regulation by zinc in Paracoccidioides lutzii.

Materials & methods:

Membranes enriched fraction of yeast cells of P. lutzii were isolated, purified and identified by 2D LC–MS/MS detection and database search.

Results & conclusion:

Zinc deprivation suppressed the expression of membrane proteins such as glycoproteins, those involved in cell wall synthesis and those related to oxidative phosphorylation. This is the first study describing membrane proteins and the effect of zinc deficiency in their regulation in one member of the genus Paracoccidioides.

The methodology of protein identification allows the characterization of biological processes performed by those molecules. Therefore, we performed a membrane proteomic analysis of Paracoccidioides lutzii and further evaluated the responses of the fungus to zinc deprivation. The results obtained in the work allowed the characterization of membrane proteins present in organelles that are related to different cellular functions. Zinc deprivation changes processes related to cellular physiology and metabolism. These results help us to understand the process of pathogen–host interaction, since zinc deprivation is a condition present during infection.

The Botrytis cinerea elicitor protein BcIEB1 interacts with the tobacco PR5-family protein osmotin and protects the fungus against its antifungal activity

The broad-range phytopathogenic fungus Botrytis cinerea secretes hundreds of proteins during infection of its plant hosts. One of these proteins, BcIEB1, is abundantly secreted and is able to elicit plant defenses, probably as a pathogen-associated molecular pattern, although its native function in B. cinerea biology remains unknown. Pull-down experiments designed to isolate the molecular target of BcIEB1 in tobacco resulted in the identification of osmotin, a pathogenesis-related protein of family 5 that shows antifungal activity. The expression of osmotin in Escherichia coli allowed the verification of the BcIEB1-osmotin interaction with pure proteins by pull-down and far Western blot experiments, as well as the confirmation of the activity of osmotin against B. cinerea. Interestingly, B. cinerea Δbcieb1 mutants are more susceptible than the wild-type to osmotin, and the external addition of pure BcIEB1 protects the Δbcieb1 mutants, as well as Saccharomyces cerevisiae, from the antifungal action of osmotin, thus pointing at PR5 inhibition as the primary native function of BcIEB1. The question of whether osmotin is also involved in the activation of plant defenses by BcIEB1 is also addressed, and the data suggest that osmotin does not participate in the elicitation process.

Enhancing structural characterisation of glucuronidated O-linked glycans using negative mode ion trap higher energy collision-induced dissociation mass spectrometry

RATIONALE

High protein production and secretion with eukaryotic glycosylation machinery make T. reesei RUT-C30 a suitable expression host for recombinant proteins. The N-glycosylation of secreted proteins of RUT-C30 is known to vary depending on culture nutrients but O-glycosylation has been less extensively studied.

METHODS

O-Glycans and glycopeptides from secreted proteins were separated by porous graphitised carbon and C-18 liquid chromatography, respectively. O-Glycans were analysed in negative ion mode by electrospray ionisation linear ion trap mass spectrometry and glycopeptides in positive ion mode by electrospray ionisation hybrid quadrupole-orbitrap mass spectrometry. Tandem mass spectrometry was used on O-glycans and glycopeptides including ion trap higher energy collision-induced dissociation (tHCD) to detect glycan fragments not detectable with standard ion trap fragmentation. tHCD allowed targeted MS³ experiments to be performed on structures containing hexuronic acid, which was not possible with ion trap CID, validating this novel O-glycan composition. Positive mode C18-LC/ESI-MS/MS was used to identify and characterise glycopeptides found to be modified with this class of O-glycans, identifying cellobiohydrolase I as a carrier of these novel O-glycans.

RESULTS

Negative mode ion trap higher energy collision-induced dissociation allowed detection and targeted MS³ experiments to be performed on the hexuronic acid substituent of O-glycan structures, which was not possible with ion trap CID, validating the novel O-glycan composition to include hexuronic acid. Using glycopeptide analysis, this novel O-glycan composition was found to be present on the catalytic domain of cellobiohydrolase I, the most abundant secreted protein by T. reesei.

CONCLUSIONS

These are the first reported O-glycans to contain acidic sugars in fungi and they could have significant implications for cellobiohydrolase I structure and activity as well as the activity of recombinant proteins expressed in this host system. Copyright © 2017 John Wiley & Sons, Ltd.