O-mannosylation precedes and potentially controls the N-glycosylation of a yeast cell wall glycoprotein

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.

O-mannosyltransferase MaPmt2 contributes to stress tolerance, cell wall integrity and virulence in Metarhizium acridum

As a conserved post-translational modification, O-mannosyltransferase families play important roles in many cellular processes. Three subfamilies (MaPmt1, MaPmt2 and MaPmt4) are grouped in Metarhizium acridum according to sequence homology. The functions of MaPmt1 and MaPmt4 have been characterized in M. acridum previously. In this study, the functions of another member belonging to the Pmt2 subfamily, MaPmt2, were identified through RNAi strategy. The three MaPmt2 knockdown mutants showed dramatically decreased expression of MaPmt2. Phenotypic analyses showed that the mutants exhibited decreased tolerances to wet-heat, UV-B irradiation and cell wall perturbing chemicals. Further studies revealed that the mutants presented thinner cell walls observed by transmission electron microscope combined with changed cell wall components. Besides, knockdown of MaPmt2 decelerated conidial germination and decreased conidial yield. Compared with the wild-type strain, the MaPmt2 knockdown mutants caused impaired virulence only by topical inoculation. Results illustrated that the decreased virulence by inoculation could result from the delayed conidial germination on locust wings, reduced appressorium formation, as well as reduced turgor pressure in MaPmt2 knockdown mutants.

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.

Recognition of Fungal Components by the Host Immune System

By being the first point of contact of the fungus with the host, the cell wall plays an important role in the pathogenesis, having many molecules that participate as antigens that are recognized by immune cells, and also that help the fungus to establish infection. The main molecules reported to trigger an immune response are chitin, glucans, oligosaccharides, proteins, melanin, phospholipids, and others, being present in the principal pathogenic fungi with clinical importance worldwide, such as Histoplasma capsulatum, Paracoccidioides brasiliensis, Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans, Blastomyces dermatitidis, and Sporothrix schenckii. Knowledge and understanding of how the immune system recognizes and responds to fungal antigens are relevant for the future research and development of new diagnostic tools and treatments for the control of mycosis caused by these fungi.

The Degree and Length of O-Glycosylation of Recombinant Proteins Produced in Pichia pastoris Depends on the Nature of the Protein and the Process Type

The methylotrophic yeast Pichia pastoris is known as an efficient host for the production of heterologous proteins. While N-linked protein glycosylation is well characterized in P. pastoris there is less knowledge of the patterns of O-glycosylation. O-glycans produced by P. pastoris consist of short linear mannose chains, which in the case of recombinant biopharmaceuticals can trigger an immune response in humans. This study aims to reveal the influence of different cultivation strategies on O-mannosylation profiles in P. pastoris. Sixteen different model proteins, produced by different P. pastoris strains, are analyzed for their O-glycosylation profile. Based on the obtained data, human serum albumin (HSA) is chosen to be produced in fast and slow growth fed batch fermentations by using common promoters, P_GAP and P_AOX1 . After purification and protein digestion, glycopeptides are analyzed by LC/ESI-MS. In the samples expressed with P_GAP it is found that the degree of glycosylation is slightly higher when a slow growth rate is used, regardless of the efficiency of the producing strain. The highest glycosylation intensity is observed in HSA produced with P_AOX1 . The results indicate that the O-glycosylation level is markedly higher when the protein is produced in a methanol-based expression system.

Structure of the eukaryotic protein O-mannosyltransferase Pmt1-Pmt2 complex

In eukaryotes, a nascent peptide entering the endoplasmic reticulum (ER) is scanned by two Sec61-translocon-associated large membrane machines for protein N-glycosylation and protein O-mannosylation, respectively. While the structure of the eight-protein oligosaccharyltransferase complex has been determined recently, the structures of mannosyltransferases of the PMT family, which are an integral part of ER protein homeostasis, are still unknown. Here we report cryo-EM structures of the S. cerevisiae Pmt1–Pmt2 complex bound to a donor and an acceptor peptide at 3.2-Å resolution, showing that each subunit contains 11 transmembrane helices and a lumenal β-trefoil fold termed the MIR domain. The structures reveal the substrate recognition model and confirm an inverting mannosyl-transferring reaction mechanism by the enzyme complex. Furthermore, we found that the transmembrane domains of Pmt1 and Pmt2 share a structural fold with the catalytic subunits of oligosaccharyltransferases, confirming a previously proposed evolutionary relationship between protein O-mannosylation and protein N-glycosylation.

TPR-containing proteins control protein organization and homeostasis for the endoplasmic reticulum

The endoplasmic reticulum (ER) is a complex, multifunctional organelle comprised of a continuous membrane and lumen that is organized into a number of functional regions. It plays various roles including protein translocation, folding, quality control, secretion, calcium signaling, and lipid biogenesis. Cellular protein homeostasis is maintained by a complicated chaperone network, and the largest functional family within this network consists of proteins containing tetratricopeptide repeats (TPRs). TPRs are well-studied structural motifs that mediate intermolecular protein-protein interactions, supporting interactions with a wide range of ligands or substrates. Seven TPR-containing proteins have thus far been shown to localize to the ER and control protein organization and homeostasis within this multifunctional organelle. Here, we discuss the roles of these proteins in controlling ER processes and organization. The crucial roles that TPR-containing proteins play in the ER are highlighted by diseases or defects associated with their mutation or disruption.

Mycobacteria and their sweet proteins: An overview of protein glycosylation and lipoglycosylation in M. tuberculosis

Post-translational modifications represent a key aspect of enzyme and protein regulation and function. Post-translational modifications are involved in signaling and response to stress, adaptation to changing environments, regulation of toxic and damaged proteins, proteins localization and host-pathogen interactions. Glycosylation in Mycobacterium tuberculosis (Mtb), is a post-translational modification often found in conjunction with acylation in mycobacterial proteins. Since the discovery of glycosylated proteins in the early 1980's, important advances in our understanding of the mechanisms of protein glycosylation have been made. The number of known glycosylated substrates in Mtb has grown through the years, yet many questions remain. This review will explore the current knowledge on protein glycosylation in Mtb, causative agent of Tuberculosis and number one infectious killer in the world. The mechanism and significance of this post-translational modification, as well as maturation, export and acylation of glycosylated proteins will be reviewed. We expect to provide the reader with an overall view of protein glycosylation in Mtb, as well as the significance of this post-translational modification to the physiology and host-pathogen interactions of this important pathogen. The mass spectrometry proteomics data have been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the dataset identifier PXD011081 and 10.6019/PXD011081.

Monitoring Protein Dynamics in Protein O-Mannosyltransferase Mutants In Vivo by Tandem Fluorescent Protein Timers

For proteins entering the secretory pathway, a major factor contributing to maturation and homeostasis is glycosylation. One relevant type of protein glycosylation is O-mannosylation, which is essential and evolutionarily-conserved in fungi, animals, and humans. Our recent proteome-wide study in the eukaryotic model organism Saccharomyces cerevisiae revealed that more than 26% of all proteins entering the secretory pathway receive O-mannosyl glycans. In a first attempt to understand the impact of O-mannosylation on these proteins, we took advantage of a tandem fluorescent timer (tFT) reporter to monitor different aspects of protein dynamics. We analyzed tFT-reporter fusions of 137 unique O-mannosylated proteins, mainly of the secretory pathway and the plasma membrane, in mutants lacking the major protein O-mannosyltransferases Pmt1, Pmt2, or Pmt4. In these three pmtΔ mutants, a total of 39 individual proteins were clearly affected, and Pmt-specific substrate proteins could be identified. We observed that O-mannosylation may cause both enhanced and diminished protein abundance and/or stability when compromised, and verified our findings on the examples of Axl2-tFT and Kre6-tFT fusion proteins. The identified target proteins are a valuable resource towards unraveling the multiple functions of O-mannosylation at the molecular level.

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.

Cellular Consequences of Diminished Protein O-Mannosyltransferase Activity in Baker's Yeast

O-Mannosylation is a type of protein glycosylation initiated in the endoplasmic reticulum (ER) by the protein O-mannosyltransferase (PMT) family. Despite the vital role of O-mannosylation, its molecular functions and regulation are not fully characterized. To further explore the cellular impact of protein O-mannosylation, we performed a genome-wide screen to identify Saccharomyces cerevisiae mutants with increased sensitivity towards the PMT-specific inhibitor compound R3A-5a. We identified the cell wall and the ER as the cell compartments affected most upon PMT inhibition. Especially mutants with defects in N-glycosylation, biosynthesis of glycosylphosphatidylinositol-anchored proteins and cell wall β-1,6-glucan showed impaired growth when O-mannosylation became limiting. Signaling pathways that counteract cell wall defects and unbalanced ER homeostasis, namely the cell wall integrity pathway and the unfolded protein response, were highly crucial for the cell growth. Moreover, among the most affected mutants, we identified Ost3, one of two homologous subunits of the oligosaccharyltransferase complexes involved in N-glycosylation, suggesting a functional link between the two pathways. Indeed, we identified Pmt2 as a substrate for Ost3 suggesting that the reduced function of Pmt2 in the absence of N-glycosylation promoted sensitivity to the drug. Interestingly, even though S. cerevisiae Pmt1 and Pmt2 proteins are highly similar on the sequence, as well as the structural level and act as a complex, we identified only Pmt2, but not Pmt1, as an Ost3-specific substrate protein.

RNA interference-mediated silencing of ppGalNAc-T1 and ppGalNAc-T2 inhibits invasion and increases chemosensitivity potentially by reducing terminal α2,3 sialylation and MMP14 expression in triple‑negative breast cancer cells

Glycopeptide-preferring polypeptide N-acetylgalactosamine transferase (ppGalNAc‑T) is a key enzyme that initiates the formation of the first GalNAc monosaccharide to polypeptides at Thr/Ser residues by O‑linked glycosylation. In order to investigate the effects of ppGalNAc‑T1 and ppGalNAc‑T2 on the initiation of O‑glycosylation, siRNA‑ppGalNAc‑T1 (si‑T1) and siRNA‑ppGalNAc‑T2 (si‑T2) were transfected into highly‑invasive estrogen receptor‑negative MDA‑MB‑231 cells to inhibit O‑glycosylation. Downregulation of ppGalNAc‑T1 demonstrated a significant reduction in the number of terminal α2,3 sialic acids, when compared to cells transfected with si‑T2 or si‑T1/T2. This downregulation led to a decrease in the invasion capabilities of the breast carcinoma cells, as well as enhanced chemosensitivity, which was the result antineoplastic drug effects. In addition, immunoprecipitation assays demonstrated that downregulation of ppGalNAc‑T1 led to a reduction in the number of terminal α2,3 sialic acids on O‑linked glycans of the matrix metalloproteinase‑14 (MMP14) glycoprotein. Furthermore, MMP14 and vascular endothelial growth factor were downregulated in the si‑T1 groups when compared with the si‑T2 and si‑T1/T2 groups. In conclusion, the results of the present study suggest that ppGalNAc‑T1 may serve a pivotal role in the initiation of O‑glycosylation, which may lead to a low density of α2,3 sialic acids on O‑linked glycans of MMP14 when downregulated. Glycosylation serves a significant role in regulating the sensitivity of MMP14 to self‑proteolysis, which ultimately decreases the invasion capabilities of breast cancer cells. The results of the present study may be useful in establishing the function of ppGalNAc‑T1 during breast cancer invasion and metastasis.

Functional Similarities between the Protein O-Mannosyltransferases Pmt4 from Bakers' Yeast and Human POMT1

Protein O-mannosylation is an essential post-translational modification. It is initiated in the endoplasmic reticulum by a family of protein O-mannosyltransferases that are conserved from yeast (PMTs) to human (POMTs). The degree of functional conservation between yeast and human protein O-mannosyltransferases is uncharacterized. In bakers' yeast, the main in vivo activities are due to heteromeric Pmt1-Pmt2 and homomeric Pmt4 complexes. Here we describe an enzymatic assay that allowed us to monitor Pmt4 activity in vitro. We demonstrate that detergent requirements and acceptor substrates of yeast Pmt4 are different from Pmt1-Pmt2, but resemble that of human POMTs. Furthermore, we mimicked two POMT1 amino acid exchanges (G76R and V428D) that result in severe congenital muscular dystrophies in humans, in yeast Pmt4 (I112R and I435D). In vivo and in vitro analyses showed that general features such as protein stability of the Pmt4 variants were not significantly affected, however, the mutants proved largely enzymatically inactive. Our results demonstrate functional and biochemical similarities between POMT1 and its orthologue from bakers' yeast Pmt4.

Mapping the O-Mannose Glycoproteome in Saccharomyces cerevisiae

O-Mannosylation is a vital protein modification conserved from fungi to humans. Yeast is a perfect model to study this post-translational modification, because in contrast to mammals O-mannosylation is the only type of O-glycosylation. In an essential step toward the full understanding of protein O-mannosylation we mapped the O-mannose glycoproteome in baker's yeast. Taking advantage of an O-glycan elongation deficient yeast strain to simplify sample complexity, we identified over 500 O-glycoproteins from all subcellular compartments for which over 2300 O-mannosylation sites were mapped by electron-transfer dissociation (ETD)-based MS/MS. In this study, we focus on the 293 O-glycoproteins (over 1900 glycosylation sites identified by ETD-MS/MS) that enter the secretory pathway and are targets of ER-localized protein O-mannosyltransferases. We find that O-mannosylation is not only a prominent modification of cell wall and plasma membrane proteins, but also of a large number of proteins from the secretory pathway with crucial functions in protein glycosylation, folding, quality control, and trafficking. The analysis of glycosylation sites revealed that O-mannosylation is favored in unstructured regions and β-strands. Furthermore, O-mannosylation is impeded in the proximity of N-glycosylation sites suggesting the interplay of these types of post-translational modifications. The detailed knowledge of the target proteins and their O-mannosylation sites opens for discovery of new roles of this essential modification in eukaryotes, and for a first glance on the evolution of different types of O-glycosylation from yeast to mammals.

Hansenula polymorpha Pmt4p Plays Critical Roles in O-Mannosylation of Surface Membrane Proteins and Participates in Heteromeric Complex Formation

O-mannosylation, the addition of mannose to serine and threonine residues of secretory proteins, is a highly conserved post-translational modification found in organisms ranging from bacteria to humans. Here, we report the functional and molecular characterization of the HpPMT4 gene encoding a protein O-mannosyltransferase in the thermotolerant methylotrophic yeast Hansenula polymorpha, an emerging host for the production of therapeutic recombinant proteins. Compared to the deletion of HpPMT1, deletion of another major PMT gene, HpPMT4, resulted in more increased sensitivity to the antibiotic hygromycin B, caffeine, and osmotic stresses, but did not affect the thermotolerance of H. polymorpha. Notably, the deletion of HpPMT4 generated severe defects in glycosylation of the surface sensor proteins HpWsc1p and HpMid2p, with marginal effects on secreted glycoproteins such as chitinase and HpYps1p lacking a GPI anchor. However, despite the severely impaired mannosylation of surface sensor proteins in the Hppmt4∆ mutant, the phosphorylation of HpMpk1p and HpHog1p still showed a high increase upon treatment with cell wall disturbing agents or high concentrations of salts. The conditional Hppmt1pmt4∆ double mutant strains displayed severely impaired growth, enlarged cell size, and aberrant cell separation, implying that the loss of HpPMT4 function might be lethal to cells in the absence of HpPmt1p. Moreover, the HpPmt4 protein was found to form not only a homomeric complex but also a heteromeric complex with either HpPmt1p or HpPmt2p. Altogether, our results support the function of HpPmt4p as a key player in O-mannosylation of cell surface proteins and its participation in the formation of heterodimers with other PMT members, besides homodimer formation, in H. polymorpha.

Role of the unfolded protein response in regulating the mucin-dependent filamentous-growth mitogen-activated protein kinase pathway

Signaling mucins are evolutionarily conserved regulators of signal transduction pathways. The signaling mucin Msb2p regulates the Cdc42p-dependent mitogen-activated protein kinase (MAPK) pathway that controls filamentous growth in yeast. The cleavage and release of the glycosylated inhibitory domain of Msb2p is required for MAPK activation. We show here that proteolytic processing of Msb2p was induced by underglycosylation of its extracellular domain. Cleavage of underglycosylated Msb2p required the unfolded protein response (UPR), a quality control (QC) pathway that operates in the endoplasmic reticulum (ER). The UPR regulator Ire1p, which detects misfolded/underglycosylated proteins in the ER, controlled Msb2p cleavage by regulating transcriptional induction of Yps1p, the major protease that processes Msb2p. Accordingly, the UPR was required for differentiation to the filamentous cell type. Cleavage of Msb2p occurred in conditional trafficking mutants that trap secretory cargo in the endomembrane system. Processed Msb2p was delivered to the plasma membrane, and its turnover by the ubiquitin ligase Rsp5p and ESCRT attenuated the filamentous-growth pathway. We speculate that the QC pathways broadly regulate signaling glycoproteins and their cognate pathways by recognizing altered glycosylation patterns that can occur in response to extrinsic cues.

Glycosylation variants of a β-glucosidase secreted by a Taiwanese fungus, Chaetomella raphigera, exhibit variant-specific catalytic and biochemical properties

Cellulosic biomass is an abundant and promising energy source. To make cellulosic biofuels competitive against conventional fuels, conversion of rigid plant materials into sugars must become efficient and cost-effective. During cellulose degradation, cellulolytic enzymes generate cellobiose (β-(1→4)-glucose dimer) molecules, which in turn inhibit such enzymes by negative feedback. β-Glucosidases (BGLs) cleave cellobiose into glucose monomers, assisting overall cellulolytic activities. Therefore, BGLs are essential for efficient conversion of cellulosic biomass into biofuels, and it is important to characterize newly isolated BGLs for useful traits. Here, we report our discovery that the indigenous Taiwanese fungus Chaetomella raphigera strain D2 produces two molecular weight variants of a single BGL, D2-BGL (shortened to “D2”), which differ in O-glycosylation. The more extensively O-glycosylated form of native D2 (nD2L) has increased activity toward the natural substrate, cellobiose, compared to the less O-glycosylated form (nD2S). nD2L is more stable at 60°C, in acidic pH, and in the presence of the ionic detergent sodium dodecyl sulfate than nD2S. Furthermore, unlike nD2S, nD2L does not display substrate inhibition by an artificial substrate p-nitrophenyl glucopyranoside (pNPG), and the glucose feedback inhibition kinetics of nD2L is competitive (while it is non-competitive for nD2S), suggesting that these two glycovariants of D2 bind substrates differently. Interestingly, D2 produced in a heterologous system, Pichia pastoris, closely mimics properties of nD2S. Our studies suggest that O-glycosylation of D2 is important in determining its catalytic and biochemical properties.

ERAD and how viruses exploit it

Endoplasmic reticulum (ER)-associated degradation (ERAD) is a universally important process among eukaryotic cells. ERAD is necessary to preserve cell integrity since the accumulation of defective proteins results in diseases associated with neurological dysfunction, cancer, and infections. This process involves recognition of misfolded or misassembled proteins that have been translated in association with ER membranes. Recognition of ERAD substrates leads to their extraction through the ER membrane (retrotranslocation or dislocation), ubiquitination, and destruction by cytosolic proteasomes. This review focuses on ERAD and its components as well as how viruses use this process to promote their replication and to avoid the immune response.

Aspergillus nidulans Pmts form heterodimers in all pairwise combinations

Eukaryotic protein O-mannosyltransferases (Pmts) are divided into three subfamilies (Pmt1, Pmt2, and Pmt4) and activity of Pmts in yeasts and animals requires assembly into complexes. In Saccharomyces cerevisiae, Pmt1 and Pmt2 form a heteromeric complex and Pmt 4 forms a homomeric complex. The filamentous fungus Aspergillus nidulans has three Pmts: PmtA (subfamily 2), PmtB (subfamily 1), and PmtC (subfamily 4). In this study we show that A. nidulans Pmts form heteromeric complexes in all possible pairwise combinations and that PmtC forms homomeric complexes. We also show that MsbA, an ortholog of a Pmt4-modified protein, is not modified by PmtC.

Protein O-mannosyltransferases associate with the translocon to modify translocating polypeptide chains

O-Mannosylation and N-glycosylation are essential protein modifications that are initiated in the endoplasmic reticulum (ER). Protein translocation across the ER membrane and N-glycosylation are highly coordinated processes that take place at the translocon-oligosaccharyltransferase (OST) complex. In analogy, it was assumed that protein O-mannosyltransferases (PMTs) also act at the translocon, however, in recent years it turned out that prolonged ER residence allows O-mannosylation of un-/misfolded proteins or slow folding intermediates by Pmt1-Pmt2 complexes. Here, we reinvestigate protein O-mannosylation in the context of protein translocation. We demonstrate the association of Pmt1-Pmt2 with the OST, the trimeric Sec61, and the tetrameric Sec63 complex in vivo by co-immunoprecipitation. The coordinated interplay between PMTs and OST in vivo is further shown by a comprehensive mass spectrometry-based analysis of N-glycosylation site occupancy in pmtΔ mutants. In addition, we established a microsomal translation/translocation/O-mannosylation system. Using the serine/threonine-rich cell wall protein Ccw5 as a model, we show that PMTs efficiently mannosylate proteins during their translocation into microsomes. This in vitro system will help to unravel mechanistic differences between co- and post-translocational O-mannosylation.

Endoplasmic reticulum glucosidases and protein quality control factors cooperate to establish biotrophy in Ustilago maydis

Secreted fungal effectors mediate plant-fungus pathogenic interactions. These proteins are typically N-glycosylated, a common posttranslational modification affecting their location and function. N-glycosylation consists of the addition, and subsequent maturation, of an oligosaccharide core in the endoplasmic reticulum (ER) and Golgi apparatus. In this article, we show that two enzymes catalyzing specific stages of this pathway in maize smut (Ustilago maydis), glucosidase I (Gls1) and glucosidase II β-subunit (Gas2), are essential for its pathogenic interaction with maize (Zea mays). Gls1 is required for the initial stages of infection following appressorium penetration, and Gas2 is required for efficient fungal spreading inside infected tissues. While U. maydis Δgls1 cells induce strong plant defense responses, Δgas2 hyphae are able to repress them, showing that slight differences in the N-glycoprotein processing can determine the extent of plant-fungus interactions. Interestingly, the calnexin protein, a central element of the ER quality control system for N-glycoproteins in eukaryotic cells, is essential for avoiding plant defense responses in cells with defective N-glycoproteins processing. Thus, N-glycoprotein maturation and this conserved checkpoint appear to play an important role in the establishment of an initial biotrophic state with the plant, which allows subsequent colonization.

Mining the O-mannose glycoproteome reveals cadherins as major O-mannosylated glycoproteins

The metazoan O-mannose (O-Man) glycoproteome is largely unknown. It has been shown that up to 30% of brain O-glycans are of the O-Man type, but essentially only alpha-dystroglycan (α-DG) of the dystrophin-glycoprotein complex is well characterized as an O-Man glycoprotein. Defects in O-Man glycosylation underlie congenital muscular dystrophies and considerable efforts have been devoted to explore this O-glycoproteome without much success. Here, we used our SimpleCell strategy using nuclease-mediated gene editing of a human cell line (MDA-MB-231) to reduce the structural heterogeneity of O-Man glycans and to probe the O-Man glycoproteome. In this breast cancer cell line we found that O-Man glycosylation is primarily found on cadherins and plexins on β-strands in extracellular cadherin and Ig-like, plexin and transcription factor domains. The positions and evolutionary conservation of O-Man glycans in cadherins suggest that they play important functional roles for this large group of cell adhesion glycoproteins, which can now be addressed. The developed O-Man SimpleCell strategy is applicable to most types of cell lines and enables proteome-wide discovery of O-Man protein glycosylation.

The secretory pathway: exploring yeast diversity

Protein secretion is an essential process for living organisms. In eukaryotes, this encompasses numerous steps mediated by several hundred cellular proteins. The core functions of translocation through the endoplasmic reticulum membrane, primary glycosylation, folding and quality control, and vesicle-mediated secretion are similar from yeasts to higher eukaryotes. However, recent research has revealed significant functional differences between yeasts and mammalian cells, and even among diverse yeast species. This review provides a current overview of the canonical protein secretion pathway in the model yeast Saccharomyces cerevisiae, highlighting differences to mammalian cells as well as currently unresolved questions, and provides a genomic comparison of the S. cerevisiae pathway to seven other yeast species where secretion has been investigated due to their attraction as protein production platforms, or for their relevance as pathogens. The analysis of Candida albicans, Candida glabrata, Kluyveromyces lactis, Pichia pastoris, Hansenula polymorpha, Yarrowia lipolytica, and Schizosaccharomyces pombe reveals that many - but not all - secretion steps are more redundant in S. cerevisiae due to duplicated genes, while some processes are even absent in this model yeast. Recent research obviates that even where homologous genes are present, small differences in protein sequence and/or differences in the regulation of gene expression may lead to quite different protein secretion phenotypes.

Detection and characterization of megasatellites in orthologous and nonorthologous genes of 21 fungal genomes

Megasatellites are large DNA tandem repeats, originally described in Candida glabrata, in protein-coding genes. Most of the genes in which megasatellites are found are of unknown function. In this work, we extended the search for megasatellites to 20 additional completely sequenced fungal genomes and extracted 216 megasatellites in 203 out of 142,121 genes, corresponding to the most exhaustive description of such genetic elements available today. We show that half of the megasatellites detected encode threonine-rich peptides predicted to be intrinsically disordered, suggesting that they may interact with several partners or serve as flexible linkers. Megasatellite motifs were clustered into several families. Their distribution in fungal genes shows that different motifs are found in orthologous genes and similar motifs are found in unrelated genes, suggesting that megasatellite formation or spreading does not necessarily track the evolution of their host genes. Altogether, these results suggest that megasatellites are created and lost during evolution of fungal genomes, probably sharing similar functions, although their primary sequences are not necessarily conserved.

Protein O-mannosylation: what we have learned from baker's yeast

BACKGROUND

Protein O-mannosylation is a vital type of glycosylation that is conserved among fungi, animals, and humans. It is initiated in the endoplasmic reticulum (ER) where the synthesis of the mannosyl donor substrate and the mannosyltransfer to proteins take place. O-mannosylation defects interfere with cell wall integrity and ER homeostasis in yeast, and define a pathomechanism of severe neuromuscular diseases in humans.

SCOPE OF REVIEW

On the molecular level, the O-mannosylation pathway and the function of O-mannosyl glycans have been characterized best in the eukaryotic model yeast Saccharomyces cerevisiae. In this review we summarize general features of protein O-mannosylation, including biosynthesis of the mannosyl donor, characteristics of acceptor substrates, and the protein O-mannosyltransferase machinery in the yeast ER. Further, we discuss the role of O-mannosyl glycans and address the question why protein O-mannosylation is essential for viability of yeast cells.

GENERAL SIGNIFICANCE

Understanding of the molecular mechanisms of protein O-mannosylation in yeast could lead to the development of novel antifungal drugs. In addition, transfer of the knowledge from yeast to mammals could help to develop diagnostic and therapeutic approaches in the frame of neuromuscular diseases. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Architecture and biosynthesis of the Saccharomyces cerevisiae cell wall

The wall gives a Saccharomyces cerevisiae cell its osmotic integrity; defines cell shape during budding growth, mating, sporulation, and pseudohypha formation; and presents adhesive glycoproteins to other yeast cells. The wall consists of β1,3- and β1,6-glucans, a small amount of chitin, and many different proteins that may bear N- and O-linked glycans and a glycolipid anchor. These components become cross-linked in various ways to form higher-order complexes. Wall composition and degree of cross-linking vary during growth and development and change in response to cell wall stress. This article reviews wall biogenesis in vegetative cells, covering the structure of wall components and how they are cross-linked; the biosynthesis of N- and O-linked glycans, glycosylphosphatidylinositol membrane anchors, β1,3- and β1,6-linked glucans, and chitin; the reactions that cross-link wall components; and the possible functions of enzymatic and nonenzymatic cell wall proteins.

Transcription of N- and O-linked mannosyltransferase genes is modulated by the pacC gene in the human dermatophyte Trichophyton rubrum

In fungi, ambient pH sensing involves the activation of the Pal/PacC signalling pathway. In the dermatophyte Trichophyton rubrum, pH-dependent secretion of keratinases, which are major virulence determinants, is affected by disruption of the pacC gene. Here, the transcription profiling of the genes coding for N- and O-linked mannosyltransferases, enzymes involved in protein glycosylation, was evaluated in T. rubrum in response to disruption of the pacC gene and growth in keratin, glucose, and glucose plus glycine. We show that transcription of these mannosyltransferase genes is affected by nutrients at acidic pH and by PacC.

Carbohydrate synthesis and biosynthesis technologies for cracking of the glycan code: recent advances

The glycan code of glycoproteins can be conceptually defined at molecular level by the sequence of well characterized glycans attached to evolutionarily predetermined amino acids along the polypeptide chain. Functional consequences of protein glycosylation are numerous, and include a hierarchy of properties from general physicochemical characteristics such as solubility, stability and protection of the polypeptide from the environment up to specific glycan interactions. Definition of the glycan code for glycoproteins has been so far hampered by the lack of chemically defined glycoprotein glycoforms that proved to be extremely difficult to purify from natural sources, and the total chemical synthesis of which has been hitherto possible only for very small molecular species. This review summarizes the recent progress in chemical and chemoenzymatic synthesis of complex glycans and their protein conjugates. Progress in our understanding of the ways in which a particular glycoprotein glycoform gives rise to a unique set of functional properties is now having far reaching implications for the biotechnology of important glycodrugs such as therapeutical monoclonal antibodies, glycoprotein hormones, carbohydrate conjugates used for vaccination and other practically important protein-carbohydrate conjugates.

Protein Glycosylation in Aspergillus fumigatus Is Essential for Cell Wall Synthesis and Serves as a Promising Model of Multicellular Eukaryotic Development

Glycosylation is a conserved posttranslational modification that is found in all eukaryotes, which helps generate proteins with multiple functions. Our knowledge of glycosylation mainly comes from the investigation of the yeast Saccharomyces cerevisiae and mammalian cells. However, during the last decade, glycosylation in the human pathogenic mold Aspergillus fumigatus has drawn significant attention. It has been revealed that glycosylation in A. fumigatus is crucial for its growth, cell wall synthesis, and development and that the process is more complicated than that found in the budding yeast S. cerevisiae. The present paper implies that the investigation of glycosylation in A. fumigatus is not only vital for elucidating the mechanism of fungal cell wall synthesis, which will benefit the design of new antifungal therapies, but also helps to understand the role of protein glycosylation in the development of multicellular eukaryotes. This paper describes the advances in functional analysis of protein glycosylation in A. fumigatus.

Members of protein O-mannosyltransferase family in Aspergillus fumigatus differentially affect growth, morphogenesis and viability

O-mannosylation is an essential protein modification in eukaryotes. It is initiated at the endoplasmic reticulum by O-mannosyltransferases (PMT) that are evolutionary conserved from yeast to humans. The PMT family is phylogenetically classified into PMT1, PMT2 and PMT4 subfamilies, which differ in protein substrate specificity and number of genes per subfamily. In this study, we characterized for the first time the whole PMT family of a pathogenic filamentous fungus, Aspergillus fumigatus. Genome analysis showed that only one member of each subfamily is present in A. fumigatus, PMT1, PMT2 and PMT4. Despite the fact that all PMTs are transmembrane proteins with conserved peptide motifs, the phenotype of each PMT deletion mutant was very different in A. fumigatus. If disruption of PMT1 did not reveal any phenotype, deletion of PMT2 was lethal. Disruption of PMT4 resulted in abnormal mycelial growth and highly reduced conidiation associated to significant proteomic changes. The double pmt1pmt4 mutant was lethal. The single pmt4 mutant exhibited an exquisite sensitivity to echinocandins that is associated to major changes in the expression of signal transduction cascade genes. These results indicate that the PMT family members play a major role in growth, morphogenesis and viability of A. fumigatus.

Reduced expression of the O-mannosyltransferase 2 (AfPmt2) leads to deficient cell wall and abnormal polarity in Aspergillus fumigatus

Protein O-mannosyltransferases (PMTs) initiate O-mannosylation of secretory proteins, which are of fundamental importance in eukaryotes. The human fungal pathogen Aspergillus fumigatus possesses three genes encoding for PMTs, namely, Afpmt1, Afpmt2 and Afpmt4. We have previously shown that lack of AfPmt1 leads to a temperature-sensitive phenotype featured with severe defects in hyphal growth, conidiation, cell wall integrity and morphology at elevated temperatures. In this study, a conditional mutant P2 was constructed by replacing the native promoter of the Afpmt2 with the Aspergillus nidulans alcA promoter. Reduced expression of the Afpmt2 gene led to a lagged germination, retarded hyphal growth, reduced conidiation and defect in cell wall integrity; however, no temperature-sensitive growth was observed. Further analysis revealed that reduced expression of the Afpmt2 caused a failure of the actin re-arrangement. Our results suggest that Afpmt2 gene was required for growth and played a role distinct from that of the Afpmt1 in A. fumigatus.

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.

Protein O-mannosylation: conserved from bacteria to humans

Protein O-mannosylation is an essential modification in fungi and animals. Different from most other types of O-glycosylation, protein O-mannosylation is initiated in the endoplasmic reticulum by the transfer of mannose from dolichol monophosphate-activated mannose to serine and threonine residues of secretory proteins. In recent years, it has emerged that even bacteria are capable of O-mannosylation and that the biosynthetic pathway of O-mannosyl glycans is conserved between pro- and eukaryotes. In this review, we summarize the observations that have opened up the field and highlight characteristics of O-mannosylation in the different domains/kingdoms of life.

Glycosylation defects activate filamentous growth Kss1 MAPK and inhibit osmoregulatory Hog1 MAPK

The yeast filamentous growth (FG) MAP kinase (MAPK) pathway is activated under poor nutritional conditions. We found that the FG-specific Kss1 MAPK is activated by a combination of an O-glycosylation defect caused by disruption of the gene encoding the protein O-mannosyltransferase Pmt4, and an N-glycosylation defect induced by tunicamycin. The O-glycosylated membrane proteins Msb2 and Opy2 are both essential for activating the FG MAPK pathway, but only defective glycosylation of Msb2 activates the FG MAPK pathway. Although the osmoregulatory HOG (high osmolarity glycerol) MAPK pathway and the FG MAPK pathway share almost the entire upstream signalling machinery, osmostress activates only the HOG-specific Hog1 MAPK. Conversely, we now show that glycosylation defects activate only Kss1, while activated Kss1 and the Ptp2 tyrosine phosphatase inhibit Hog1. In the absence of Kss1 or Ptp2, however, glycosylation defects activate Hog1. When Hog1 is activated by glycosylation defects in ptp2 mutant, Kss1 activation is suppressed by Hog1. Thus, the reciprocal inhibitory loop between Kss1 and Hog1 allows only one or the other of these MAPKs to be stably activated under various stress conditions.

Comparative genomics and molecular dynamics of DNA repeats in eukaryotes

Repeated elements can be widely abundant in eukaryotic genomes, composing more than 50% of the human genome, for example. It is possible to classify repeated sequences into two large families, "tandem repeats" and "dispersed repeats." Each of these two families can be itself divided into subfamilies. Dispersed repeats contain transposons, tRNA genes, and gene paralogues, whereas tandem repeats contain gene tandems, ribosomal DNA repeat arrays, and satellite DNA, itself subdivided into satellites, minisatellites, and microsatellites. Remarkably, the molecular mechanisms that create and propagate dispersed and tandem repeats are specific to each class and usually do not overlap. In the present review, we have chosen in the first section to describe the nature and distribution of dispersed and tandem repeats in eukaryotic genomes in the light of complete (or nearly complete) available genome sequences. In the second part, we focus on the molecular mechanisms responsible for the fast evolution of two specific classes of tandem repeats: minisatellites and microsatellites. Given that a growing number of human neurological disorders involve the expansion of a particular class of microsatellites, called trinucleotide repeats, a large part of the recent experimental work on microsatellites has focused on these particular repeats, and thus we also review the current knowledge in this area. Finally, we propose a unified definition for mini- and microsatellites that takes into account their biological properties and try to point out new directions that should be explored in a near future on our road to understanding the genetics of repeated sequences.

Membrane topology and post-translational modification of the Saccharomyces cerevisiae essential protein Rot1

ROT1 is an essential gene that has been related to cell wall biosynthesis, the actin cytoskeleton and protein folding. In order to help to understand its molecular function, we carried out a characterization of the Rot1 protein. It is primarily located at the endoplasmic reticulum-nuclear membrane facing the lumen. Rot1 migrates more slowly than expected, which might suggest post-translational modification. Our results indicate that Rot1 is a protein that is neither GPI-anchored nor O-glycosylated. In contrast, it is N-glycosylated. By a directed mutagenesis of several Asn residues, we identified that the protein is simultaneously glycosylated at N103, N107 and N139. Although the mutation of these three N sites is not lethal, cellular growth is impaired. Sequence analysis predicts a transmembrane domain at the C-terminus. This fragment affects neither the targeting of the Rot1 protein to the ER nor its N-glycosylation, although it is important for the anchoring of the protein to the membrane and for its functionality. The existence of a signal sequence at the N-terminus has been suggested. However, deletion of this fragment impedes neither translocation to the ER nor N-glycosylation, but it is required for cell viability. Finally, we found that Rot1 is translocated to the ER by an SRP-independent post-translational mechanism which depends on Sec62.

Megasatellites: a peculiar class of giant minisatellites in genes involved in cell adhesion and pathogenicity in Candida glabrata

Minisatellites are DNA tandem repeats that are found in all sequenced genomes. In the yeast Saccharomyces cerevisiae, they are frequently encountered in genes encoding cell wall proteins. Minisatellites present in the completely sequenced genome of the pathogenic yeast Candida glabrata were similarly analyzed, and two new types of minisatellites were discovered: minisatellites that are composed of two different intermingled repeats (called compound minisatellites), and minisatellites containing unusually long repeated motifs (126–429 bp). These long repeat minisatellites may reach unusual length for such elements (up to 10 kb). Due to these peculiar properties, they have been named ‘megasatellites’. They are found essentially in genes involved in cell–cell adhesion, and could therefore be involved in the ability of this opportunistic pathogen to colonize the human host. In addition to megasatellites, found in large paralogous gene families, there are 93 minisatellites with simple shorter motifs, comparable to those found in S. cerevisiae. Most of the time, these minisatellites are not conserved between C. glabrata and S. cerevisiae, although their host genes are well conserved, raising the question of an active mechanism creating minisatellites de novo in hemiascomycetes.

O-Mannosyltransferase 1 in Aspergillus fumigatus (AfPmt1p) is crucial for cell wall integrity and conidium morphology, especially at an elevated temperature

Protein O-mannosyltransferases initiate O mannosylation of secretory proteins, which are of fundamental importance in eukaryotes. In this study, the PMT gene family of the human fungal pathogen Aspergillus fumigatus was identified and characterized. Unlike the case in Saccharomyces cerevisiae, where the PMT family is highly redundant, only one member of each PMT subfamily, namely, Afpmt1, Afpmt2, and Afpmt4, is present in A. fumigatus. Mutants with a deletion of Afpmt1 are viable. In vitro and in vivo activity assays confirmed that the protein encoded by Afpmt1 acts as an O-mannosyltransferase (AfPmt1p). Characterization of the DeltaAfpmt1 mutant showed that a lack of AfPmt1p results in sensitivity to elevated temperature and defects in growth and cell wall integrity, thereby affecting cell morphology, conidium formation, and germination. In a mouse model, Afpmt1 was not required for the virulence of A. fumigatus under the experimental conditions used.

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

Protein glycosylation is essential for eukaryotic cells from yeasts to humans. When compared to N-glycosylation, O-glycosylation is variable in sugar components and the mode of linkages connecting the sugars. In fungi, secretory proteins are commonly mannosylated by protein O-mannosyltransferase (PMT) in the endoplasmic reticulum, and subsequently glycosylated by several glycosyltransferases in the Golgi apparatus to form glycoproteins with diverse O-glycan structures. Protein O-glycosylation has roles in modulating the function of secretory proteins by enhancing the stability and solubility of the proteins, by affording protection from protease degradation, and by acting as a sorting determinant in yeasts. In filamentous fungi, protein O-glycosylation contributes to proper maintenance of fungal morphology, hyphal development, and differentiation. This review describes recent studies of the structure and function of protein O-glycosylation in industrially and medically important fungi.

Membrane association is a determinant for substrate recognition by PMT4 protein O-mannosyltransferases

Protein O-mannosylation represents an evolutionarily conserved, essential posttranslational modification with immense impact on a variety of cellular processes. In humans, O-mannosylation defects result in Walker-Warburg syndrome, a severe recessive congenital muscular dystrophy associated with defects in neuronal migration that produce complex brain and eye abnormalities. In mouse and yeasts, loss of O-mannosylation causes lethality. Protein O-mannosyltransferases (PMTs) initiate the assembly of O-mannosyl glycans. The evolutionarily conserved PMT family is classified into PMT1, PMT2, and PMT4 subfamilies, which mannosylate distinct target proteins. In contrast to other types of glycosylation, signal sequences for O-mannosylation have not been identified to date. In the present study, we identified signals that determine PMT4-dependent O-mannosylation. Using specific model proteins, we demonstrate that in yeast Pmt4p mediates O-mannosylation of Ser/Thr-rich membrane-attached proteins. The nature of the membrane-anchoring sequence is nonrelevant, as long as it is flanked by a Ser/Thr-rich domain facing the endoplasmic reticulum lumen. Our work shows that, in contrast to several other types of glycosylation, PMT4 O-mannosylation signals are not just linear protein's primary structure sequences but rather are highly complex. Based on these findings, we performed in silico analyses of the Saccharomyces cerevisiae proteome and identified previously undescribed Pmt4p substrates. This tool for proteome-wide identification of O-mannosylated proteins is of general interest because several of these proteins are major players of a wide variety of cellular processes.

Characterization of O-linked saccharides on glycoproteins

Recombinant and native proteins of Pichia pastoris can be O-mannosylated on serine and threonine residues, allowing further elongation reactions to generate short O-linked oligosaccha-rides of mannose. Methods for release from the protein with alkaline beta-elimination with or without reduction of the released saccharides, and for subsequent chromatographic and enzymatic characterization of these saccharides are described.