Aberrant processing of the WSC family and Mid2p cell surface sensors results in cell death of Saccharomyces cerevisiae O-mannosylation mutants

Expression and characterization of scFv-6009FV in Pichia pastoris with improved ability to neutralize the neurotoxin Cn2 from Centruroides noxius

Small single-chain variable fragments (scFv) are promising biomolecules to inhibit and neutralize toxins and to act as antivenoms. In this work, we aimed to produce a functional scFv-6009FV in the yeast Pichia pastoris, which inhibits the pure Cn2 neurotoxin and the whole venom of Centruroides noxius. We were able to achieve yields of up to 31.6 ± 2 mg/L in flasks. Furthermore, the protein showed a structure of 6.1 % α-helix, 49.1 % β-sheet, and 44.8 % of random coil by CD. Mass spectrometry confirmed the amino acid sequence and showed no glycosylation profile for this molecule. Purified scFv-6009FV allowed us to develop anti-scFvs in rabbits, which were then used in affinity columns to purify other scFvs. Determination of its half-maximal inhibitory concentration value (IC50) was 40 % better than the scFvs produced by E. coli as a control. Finally, we found that scFv-6009FV was able to inhibit ex vivo the pure Cn2 toxin and the whole venom from C. noxius in murine rescue experiments. These results demonstrated that under the conditions assayed here, P. pastoris is suited to produce scFv-6009FV that, compared to scFvs produced by E. coli, maintains the characteristics of an antibody and neutralizes the Cn2 toxin more effectively.

Involvement of the Cell Wall-Integrity Pathway in Signal Recognition, Cell-Wall Biosynthesis, and Virulence in Magnaporthe oryzae

The fungal cell wall is the first layer exposed to the external environment. The cell wall has key roles in regulating cell functions, such as cellular stability, permeability, and protection against stress. Understanding the structure of the cell wall and the mechanism of its biogenesis is important for the study of fungi. Highly conserved in fungi, including Magnaporthe oryzae, the cell wall-integrity (CWI) pathway is the primary signaling cascade regulating cell-wall structure and function. The CWI pathway has been demonstrated to correlate with pathogenicity in many phytopathogenic fungi. In the synthesis of the cell wall, the CWI pathway cooperates with multiple signaling pathways to regulate cell morphogenesis and secondary metabolism. Many questions have arisen regarding the cooperation of different signaling pathways with the CWI pathway in regulating cell-wall synthesis and pathogenicity. In this review, we summarized the latest advances in the M. oryzae CWI pathway and cell-wall structure. We discussed the CWI pathway components and their involvement in different aspects, such as virulence factors, the possibility of the pathway as a target for antifungal therapies, and crosstalk with other signaling pathways. This information will aid in better understanding the universal functions of the CWI pathway in regulating cell-wall synthesis and pathogenicity in M. oryzae. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Sec61 channel subunit Sbh1/Sec61β promotes ER translocation of proteins with suboptimal targeting sequences and is fine-tuned by phosphorylation

The highly conserved endoplasmic reticulum (ER) protein translocation channel contains one nonessential subunit, Sec61β/Sbh1, whose function is poorly understood so far. Its intrinsically unstructured cytosolic domain makes transient contact with ER-targeting sequences in the cytosolic channel vestibule and contains multiple phosphorylation sites suggesting a potential for regulating ER protein import. In a microscopic screen, we show that 12% of a GFP-tagged secretory protein library depends on Sbh1 for translocation into the ER. Sbh1-dependent proteins had targeting sequences with less pronounced hydrophobicity and often no charge bias or an inverse charge bias which reduces their insertion efficiency into the Sec61 channel. We determined that mutating two N-terminal, proline-flanked phosphorylation sites in the Sbh1 cytosolic domain to alanine phenocopied the temperature-sensitivity of a yeast strain lacking SBH1 and its ortholog SBH2. The phosphorylation site mutations reduced translocation into the ER of a subset of Sbh1-dependent proteins, including enzymes whose concentration in the ER lumen is critical for ER proteostasis. In addition, we found that ER import of these proteins depended on the activity of the phospho-S/T-specific proline isomerase Ess1 (PIN1 in mammals). We conclude that Sbh1 promotes ER translocation of substrates with suboptimal targeting sequences and that its activity can be regulated by a conformational change induced by N-terminal phosphorylation.

Protein O-mannosylation across kingdoms and related diseases: From glycobiology to glycopathology

The post-translational glycosylation of proteins by O-linked α-mannose is conserved from bacteria to humans. Due to advances in high-throughput mass spectrometry-based approaches, a variety of glycoproteins are identified to be O-mannosylated. Various proteins with O-mannosylation are involved in biological processes, providing essential necessity for proper growth and development. In this review, we summarize the process and regulation of O-mannosylation. The multi-step O-mannosylation procedures are quite dynamic and complex, especially when considering the structural and functional inspection of the involved enzymes. The widely studied O-mannosylated proteins in human include α-Dystroglycan (α-DG), cadherins, protocadherins, and plexin, and their aberrant O-mannosylation are associated with many diseases. In addition, O-mannosylation also contributes to diverse functions in lower eukaryotes and prokaryotes. Finally, we present the relationship between O-mannosylation and gut microbiota (GM), and elucidate that O-mannosylation in microbiome is of great importance in the dynamic balance of GM. Our study provides an overview of the processes of O-mannosylation in mammalian cells and other organisms, and also associated regulated enzymes and biological functions, which could contribute to the understanding of newly discovered O-mannosylated glycoproteins.

Cells under pressure: how yeast cells respond to mechanical forces

In their natural habitats, unicellular fungal microbes are exposed to a myriad of mechanical cues such as shear forces from fluid flow, osmotic changes, and contact forces arising from microbial expansion in confined niches. While the rigidity of the cell wall is critical to withstand such external forces and balance high internal turgor pressure, it poses mechanical challenges during physiological processes such as cell growth, division, and mating that require cell wall remodeling. Thus, even organisms as simple as yeast have evolved complex signaling networks to sense and respond to intrinsic and extrinsic mechanical forces. In this review, we summarize the type and origin of mechanical forces experienced by unicellular yeast and discuss how these forces reorganize cell polarity and how pathogenic fungi exploit polarized assemblies to track weak spots in host tissues for successful penetration. We then describe mechanisms of force-sensing by conserved sets of mechanosensors. Finally, we elaborate downstream mechanotransduction mechanisms that orchestrate appropriate cellular responses, leading to improved mechanical fitness.

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.

Letting go of O-glycans

The generation of free N-glycans, or unconjugated oligosaccharides derived from N-linked glycoproteins, is well understood, but whether a similar fate awaits O-linked glycoprotein carbohydrates was unknown. Hirayama et al. now reveal, by using only mannose as an energy source, the generation of free O-glycans in Saccharomyces cerevisiae, in the lumen of a secretory compartment, possibly the vacuole. These findings uncover the presence of a possible regulated degradation pathway for O-mannosylated glycoproteins.

Free glycans derived from O-mannosylated glycoproteins suggest the presence of an O-glycoprotein degradation pathway in yeast

In eukaryotic cells, unconjugated oligosaccharides that are structurally related to N-glycans (i.e. free N-glycans) are generated either from misfolded N-glycoproteins destined for the endoplasmic reticulum-associated degradation or from lipid-linked oligosaccharides, donor substrates for N-glycosylation of proteins. The mechanism responsible for the generation of free N-glycans is now well-understood, but the issue of whether other types of free glycans are present remains unclear. Here, we report on the accumulation of free, O-mannosylated glycans in budding yeast that were cultured in medium containing mannose as the carbon source. A structural analysis of these glycans revealed that their structures are identical to those of O-mannosyl glycans that are attached to glycoproteins. Deletion of the cyc8 gene, which encodes for a general transcription repressor, resulted in the accumulation of excessive amounts of free O-glycans, concomitant with a severe growth defect, a reduction in the level of an O-mannosylated protein, and compromised cell wall integrity. Our findings provide evidence in support of a regulated pathway for the degradation of O-glycoproteins in yeast and offer critical insights into the catabolic mechanisms that control the fate of O-glycosylated proteins.

FGB1 and WSC3 are in planta-induced β-glucan-binding fungal lectins with different functions

In the root endophyte Serendipita indica, several lectin-like members of the expanded multigene family of WSC proteins are transcriptionally induced in planta and are potentially involved in β-glucan remodeling at the fungal cell wall. Using biochemical and cytological approaches we show that one of these lectins, SiWSC3 with three WSC domains, is an integral fungal cell wall component that binds to long-chain β1-3-glucan but has no affinity for shorter β1-3- or β1-6-linked glucose oligomers. Comparative analysis with the previously identified β-glucan-binding lectin SiFGB1 demonstrated that whereas SiWSC3 does not require β1-6-linked glucose for efficient binding to branched β1-3-glucan, SiFGB1 does. In contrast to SiFGB1, the multivalent SiWSC3 lectin can efficiently agglutinate fungal cells and is additionally induced during fungus-fungus confrontation, suggesting different functions for these two β-glucan-binding lectins. Our results highlight the importance of the β-glucan cell wall component in plant-fungus interactions and the potential of β-glucan-binding lectins as specific detection tools for fungi in vivo.

Identification and Functional Testing of Novel Interacting Protein Partners for the Stress Sensors Wsc1p and Mid2p of Saccharomyces cerevisiae

Wsc1p and Mid2p are transmembrane signaling proteins of cell wall stress in the budding yeast Saccharomyces cerevisiae. When an environmental stress compromises cell wall integrity, they activate a cell response through the Cell Wall Integrity (CWI) pathway. Studies have shown that the cytoplasmic domain of Wsc1p initiates the CWI signaling cascade by interacting with Rom2p, a Rho1-GDP-GTP exchange factor. Binding of Rom2p to the cytoplasmic tail of Wsc1p requires dephosphorylation of specific serine residues but the mechanism by which the sensor is dephosphorylated and how it subsequently interacts with Rom2p remains unclear. We hypothesize that Wsc1p and Mid2p must be physically associated with interacting proteins other than Rom2p that facilitate its interaction and regulate the activation of CWI pathway. To address this, a cDNA plasmid library of yeast proteins was expressed in bait strains bearing membrane yeast two-hybrid (MYTH) reporter modules of Wsc1p and Mid2p, and their interacting preys were recovered and sequenced. 14 previously unreported interactors were confirmed for Wsc1p and 29 for Mid2p. The interactors’ functionality were assessed by cell growth assays and CWI pathway activation by western blot analysis of Slt2p/Mpk1p phosphorylation in null mutants of each interactor under defined stress conditions. The susceptibility of these strains to different stresses were tested against antifungal agents and chemicals. This study reports important novel protein interactions of Wsc1p and Mid2p that are associated with the cellular response to oxidative stress induced by Hydrogen Peroxide and cell wall stress induced by Caspofungin.

Lipotoxicty in yeast: a focus on plasma membrane signalling and membrane contact sites

Lipotoxicity is a pathophysiological process triggered by lipid overload. In metazoans, lipotoxicity is characterised by the ectopic deposition of lipids on organs other than adipose tissue. This leads to organ dysfunction, cell death, and is intimately linked to lipid-associated diseases such as cardiac dysfunction, atherosclerosis, stroke, hepatosteatosis, cancer and the metabolic syndrome. The molecules involved in eliciting lipotoxicity include FAs and their acyl-CoA derivatives, triacylglycerol (TG), diacylglycerol (DG), ceramides, acyl-carnitines and phospholipids. However, the cellular transport of toxic lipids through membrane contact sites (MCS) and vesicular mechanisms as well as lipid metabolism that progress lipotoxicity to the onset of disease are not entirely understood. Yeast has proven a useful model organism to study the molecular mechanisms of lipotoxicity. Recently, the Rim101 pathway, which senses alkaline pH and the lipid status at the plasmamembrane, has been connected to lipotoxicity. In this review article, we summarise recent research advances on the Rim101 pathway and MCS in the context of lipotoxicity in yeast and present a perspective for future research directions.

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.

Vps74 Connects the Golgi Apparatus and Telomeres in Saccharomyces cerevisiae

In mammalian cell culture, the Golgi apparatus fragment upon DNA damage. GOLPH3, a Golgi component, is a phosphorylation target of DNA-PK after DNA damage and contributes to Golgi fragmentation. The function of the yeast (Saccharomyces cerevisiae) ortholog of GOLPH3, Vps74, in the DNA damage response has been little studied, although genome-wide screens suggested a role at telomeres. In this study we investigated the role of Vps74 at telomeres and in the DNA damage response. We show that Vps74 decreases the fitness of telomere defective cdc13-1 cells and contributes to the fitness of yku70Δ cells. Importantly, loss of Vps74 in yku70Δ cells exacerbates the temperature dependent growth defects of these cells in a Chk1 and Mec1-dependent manner. Furthermore, Exo1 reduces the fitness of vps74Δ yku70Δ cells suggesting that ssDNA contributes to the fitness defects of vps74Δ yku70Δ cells. Systematic genetic interaction analysis of vps74Δ, yku70Δ and yku70Δ vps74Δ cells suggests that vps74Δ causes a milder but similar defect to that seen in yku70Δ cells. vps74Δ cells have slightly shorter telomeres and loss of VPS74 in yku70Δ or mre11Δ cells further shortens the telomeres of these cells. Interestingly, loss of Vps74 leads to increased levels of Stn1, a partner of Cdc13 in the CST telomere capping complex. Overexpression of Stn1 was previously shown to cause telomere shortening, suppression of cdc13-1 and enhancement of yku70Δ growth defects, suggesting that increased levels of Stn1 may be the route by which Vps74 affects telomere function. These results establish Vps74 as a novel regulator of telomere biology.

Fungal Hybrid B heme peroxidases - unique fusions of a heme peroxidase domain with a carbohydrate-binding domain

Heme peroxidases, essential peroxide converting oxidoreductases are divided into four independently evolved superfamilies. Within the largest one – the peroxidase-catalase superfamily - two hybrid lineages were described recently. Whereas Hybrid A heme peroxidases represent intermediate enzymes between ascorbate peroxidases and cytochrome c peroxidases, Hybrid B heme peroxidases are unique fusion proteins comprised of a conserved N-terminal heme peroxidase domain and a C-terminal domain of various sugar binding motifs. So far these peculiar peroxidases are only found in the kingdom of Fungi. Here we present a phylogenetic reconstruction of the whole superfamily with focus on Hybrid B peroxidases. We analyse the domain assembly and putative structure and function of the newly discovered oligosaccharide binding domains. Two distinct carbohydrate binding modules (CBM21 and CBM34) are shown to occur in phytopathogenic ascomycetous orthologs of Hybrid B heme peroxidases only. Based on multiple sequence alignment and homology modeling the structure-function relationships are discussed with respect to physiological function. A concerted action of peroxide cleavage with specific cell-wall carbohydrate binding can support phytopathogens survival within the plant host.

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.

The Metacaspase (Mca1p) Restricts O-glycosylation During Farnesol-induced Apoptosis in Candida albicans

Protein glycolysation is an essential posttranslational modification in eukaryotic cells. In pathogenic yeasts, it is involved in a large number of biological processes, such as protein folding quality control, cell viability and host/pathogen relationships. A link between protein glycosylation and apoptosis was established by the analysis of the phenotypes of oligosaccharyltransferase mutants in budding yeast. However, little is known about the contribution of glycosylation modifications to the adaptive response to apoptosis inducers. The cysteine protease metacaspase Mca1p plays a key role in the apoptotic response in Candida albicans triggered by the quorum sensing molecule farnesol. We subjected wild-type and mca1-deletion strains to farnesol stress and then studied the early phase of apoptosis release in quantitative glycoproteomics and glycomics experiments on cell-free extracts essentially devoid of cell walls. We identified and characterized 62 new glycosylated peptides with their glycan composition: 17 N-glycosylated, 45 O-glycosylated, and 81 additional sites of N-glycosylation. They were found to be involved in the control of protein folding, cell wall integrity and cell cycle regulation. We showed a general increase in the O-glycosylation of proteins in the mca1 deletion strain after farnesol challenge. We identified 44 new putative protein substrates of the metacaspase in the glycoprotein fraction enriched on concanavalin A. Most of these substrates are involved in protein folding or protein resolubilization and in mitochondrial functions. We show here that key Mca1p substrates, such as Cdc48p or Ssb1p, involved in degrading misfolded glycoproteins and in the protein quality control system, are themselves differentially glycosylated. We found putative substrates, such as Bgl2p (validated by immunoblot), Srb1p or Ugp1p, that are involved in the biogenesis of glycans. Our findings highlight a new role of the metacaspase in amplifying cell death processes by affecting several critical protein quality control systems through the alteration of the protein glycosylation machinery.Data are available via ProteomeXchange with identifier PXD003677.

Subcellular localization of five singular WSC domain-containing proteins and their roles in Beauveria bassiana responses to stress cues and metal ions

Some model fungi have three or four proteins with each vectoring a single cell Wall Stress-responsive Component (WSC) domain at N-terminus. In this study, five proteins, each vectoring only a single WSC domain in N-terminal, central or even C-terminal region, were found in Beauveria bassiana, a filamentous fungal entomopathogen, and named Wsc1A-1E due to the domain singularity. Four of them lack either transmembrane domain or C-terminal conserved signature sequence (DXXD) compared with the homologues in the model fungi. Intriguingly, all the eGFP-tagged fusion proteins of Wsc1A-1E were evidently localized to the cell wall and membrane of transgenic hyphae. Single deletions of the five wsc genes resulted in significant, but differential, increases in cellular sensitivity to cell wall perturbation, oxidation, high osmolarity, and four to six metal ions (Zn(2+) , Mg(2+) , Fe(2+) , K(+) , Ca(2+) and Mn(2+) ). Each deletion mutant also showed a delay of germination and a decrease of conidial UV-B resistance, thermotolerance or both. However, none of the deletions affected substantially the fungal growth, conidiation and virulence. Our results indicate a significance of each WSC protein for the B. bassiana adaptation to diverse habitats of host insects.

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.

The plant cell wall integrity maintenance mechanism--a case study of a cell wall plasma membrane signaling network

Some of the most important functions of plant cell walls are protection against biotic/abiotic stress and structural support during growth and development. A prerequisite for plant cell walls to perform these functions is the ability to perceive different types of stimuli in both qualitative and quantitative manners and initiate appropriate responses. The responses in turn involve adaptive changes in cellular and cell wall metabolism leading to modifications in the structures originally required for perception. While our knowledge about the underlying plant mechanisms is limited, results from Saccharomyces cerevisiae suggest the cell wall integrity maintenance mechanism represents an excellent example to illustrate how the molecular mechanisms responsible for stimulus perception, signal transduction and integration can function. Here I will review the available knowledge about the yeast cell wall integrity maintenance system for illustration purposes, summarize the limited knowledge available about the corresponding plant mechanism and discuss the relevance of the plant cell wall integrity maintenance mechanism in biotic stress responses.

N-terminal entrance loop of yeast Yps1 and O-glycosylation of substrates are determinant factors controlling the shedding activity of this GPI-anchored endopeptidase

Background

S. cerevisiae Yps1 is the prototypical aspartic endopeptidase of the fungal yapsin family. This glycosylphosphatidylinositol (GPI) anchored enzyme was recently shown to be involved in the shedding of the GPI proteins Utr2, Gas1 and itself. It was also proposed to be part of a novel quality control mechanism that eliminates excess and/or misfolded GPI proteins. What regulates its shedding activity at the cell surface is however poorly understood. Yps1 is initially synthesized as a zymogen requiring proteolytic activation to remove a pro-peptide and further processing within a large insertion loop (N-entrance loop) generates a two-subunit endopeptidase. To investigate the role of this loop on its shedding activity, which typically takes place within Ser/Thr-rich domains, it was replaced with the short peptide found at the analogous position in Yps3. We also tested whether O-glycosylation might protect against proteolytic processing by Yps1.

Results

We show here that replacement of the N-entrance loop (N-ent loop) of Yps1 generates a single chain endopeptidase that undergoes partial (pH 6.0) or complete (pH 3.0) pro-peptide removal. At both pH, the shedding activity of the chimeric endopeptidase (Yps1-DL) toward Gas1 and itself is strongly and drastically increased, respectively. A direct correlation between endoproteolytic cleavage of this loop in native Yps1 and its shedding is observed. The Yps1-dependent shedding of two model GPI proteins (Gas1 and Yps1) is also stimulated by the absence of the O-mannosyltransferases, Pmt4 and Pmt2 respectively, involved in O-glycosylation of their Ser/Thr-rich domains. Under these conditions, some Yps1-independent shedding is also observed.

Conclusions

Partial pro-peptide removal is essential to produce a functional Yps1 endopeptidase. The Yps1 N-ent loop plays a major role in regulating the shedding activity of the endopeptidase, most likely by limiting access to the active site, and its cleavage in native Yps1 is associated with its shedding. O-glycosylation protects against Yps1-dependent and -independent shedding of GPI proteins. It is postulated that hypoglycosylation of cell surface proteins, which may occur for misfolded proteins that escaped the ER-associated degradation, might target their elimination through shedding by Yps1 and possibly other yapsin members.

Unraveling the novel structure and biosynthetic pathway of O-linked glycans in the Golgi apparatus of the human pathogenic yeast Cryptococcus neoformans

Cryptococcus neoformans is an encapsulated basidiomycete causing cryptococcosis in immunocompromised humans. The cell surface mannoproteins of C. neoformans were reported to stimulate the host T-cell response and to be involved in fungal pathogenicity; however, their O-glycan structure is uncharacterized. In this study, we performed a detailed structural analysis of the O-glycans attached to cryptococcal mannoproteins using HPLC combined with exoglycosidase treatment and showed that the major C. neoformans O-glycans were short manno-oligosaccharides that were connected mostly by α1,2-linkages but connected by an α1,6-linkage at the third mannose residue. Comparison of the O-glycan profiles from wild-type and uxs1Δ mutant strains strongly supports the presence of minor O-glycans carrying a xylose residue. Further analyses of C. neoformans mutant strains identified three mannosyltransferase genes involved in O-glycan extensions in the Golgi. C. neoformans KTR3, the only homolog of the Saccharomyces cerevisiae KRE2/MNT1 family genes, was shown to encode an α1,2-mannosyltransferase responsible for the addition of the second mannose residue via an α1,2-linkage to the major O-glycans. C. neoformans HOC1 and HOC3, homologs of the Saccharomyces cerevisiae OCH1 family genes, were shown to encode α1,6-mannosyltransferases that can transfer the third mannose residue, via an α1,6-linkage, to minor O-glycans containing xylose and to major O-glycans without xylose, respectively. Moreover, the C. neoformans ktr3Δ mutant strain, which displayed increased sensitivity to SDS, high salt, and high temperature, showed attenuated virulence in a mouse model of cryptococcosis, suggesting that the extended structure of O-glycans is required for cell integrity and full pathogenicity of C. neoformans.

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.

The fission yeast cell wall stress sensor-like proteins Mtl2 and Wsc1 act by turning on the GTPase Rho1p but act independently of the cell wall integrity pathway

Sensing stressful conditions that affect the cell wall reorganization is important for yeast survival. Here, we studied two proteins SpWsc1p and SpMtl2p with structural features indicative of plasma membrane-associated cell wall sensors. We found that Mtl2p and Wsc1p act by turning on the Rho1p GTPase. Each gene could be deleted individually without affecting viability, but the deletion of both was lethal and this phenotype was rescued by overexpression of the genes encoding either Rho1p or its GDP/GTP exchange factors (GEFs). In addition, wsc1Δ and mtl2Δ cells showed a low level of Rho1p-GTP under cell wall stress. Mtl2p-GFP (green fluorescent protein) localized to the cell periphery and was necessary for survival under different types of cell wall stress. Wsc1p-GFP was concentrated in patches at the cell tips, it interacted with the Rho-GEF Rgf2p, and its overexpression activated cell wall biosynthesis. Our results are consistent with the notion that cell wall assembly is regulated by two different networks involving Rho1p. One includes signaling from Mtl2p through Rho1p to Pck1p, while the second one implicates signaling from Wsc1p and Rgf2p through Rho1p to activate glucan synthase (GS). Finally, signaling through the mitogen-activated protein kinase (MAPK) Pmk1p remained active in mtl2Δ and wsc1Δ disruptants exposed to cell wall stress, suggesting that the cell wall stress-sensing spectrum of Schizosaccharomyces pombe sensor-like proteins differs from that of Saccharomyces cerevisiae.

Botrytis cinerea protein O-mannosyltransferases play critical roles in morphogenesis, growth, and virulence

Protein O-glycosylation is crucial in determining the structure and function of numerous secreted and membrane-bound proteins. In fungi, this process begins with the addition of a mannose residue by protein O-mannosyltransferases (PMTs) in the lumen side of the ER membrane. We have generated mutants of the three Botrytis cinerea pmt genes to study their role in the virulence of this wide-range plant pathogen. B. cinerea PMTs, especially PMT2, are critical for the stability of the cell wall and are necessary for sporulation and for the generation of the extracellular matrix. PMTs are also individually required for full virulence in a variety of hosts, with a special role in the penetration of intact plant leaves. The most significant case is that of grapevine leaves, whose penetration requires the three functional PMTs. Furthermore, PMT2 also contributes significantly to fungal adherence on grapevine and tobacco leaves. Analysis of extracellular and membrane proteins showed significant changes in the pattern of protein secretion and glycosylation by the pmt mutants, and allowed the identification of new protein substrates putatively glycosylated by specific PMTs. Since plants do no possess these enzymes, PMTs constitute a promising target in the development of novel control strategies against B. cinerea.

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.

Mtl1 O-mannosylation mediated by both Pmt1 and Pmt2 is important for cell survival under oxidative conditions and TOR blockade

Mtl1 is a cell surface sensor and member of the Pkc1-MAPK pathway that senses oxidative stress and nutrient starvation. Here we demonstrate that the Mtl1 cytoplasmic domain physically interacts with the GEF (GTPase Exchange Factor) protein Rom2 of the CWI (Cell wall Integrity) pathway. Mtl1 is N-glycosylated protein, highly O-mannosylated by Pmt1, Pmt4 and mostly by Pmt2. Mtl1 localises to the bud, septum, the tip of the shmoo and the cell periphery. The O-mannosylation deficiency that occurs in both the pmt1 and pmt2 mutants adversely affects the distribution of Mtl1 on the septum and also hinders Mtl1 localisation in the tip of the shmoo. Here we present results demonstrating that: (i) O-mannosylation and, more specifically that affecting Mtl1 protein is required for cell survival in response to both oxidative stress and TOR blockade; (ii) Slt2 activity is impaired upon rapamycin treatment in both pmt2 and mtl1 mutants; (iii) Mtl1 is transcriptionally upregulated in quiescent conditions, (iv) O-mannosylation mediated by Pmt1 and Pmt2 favours Mtl1 protein stability. We propose a relevant role for Mtl1 O-mannosylation mediated by both Pmt1 and Pmt2 in the response to oxidative stress and in rapamycin treatment.

A systematic approach to pair secretory cargo receptors with their cargo suggests a mechanism for cargo selection by Erv14

A systematic approach to visualize proteins exiting the endoplasmic reticulum paired with their cargo receptors identifies novel cargo for known receptors and reveals the mechanism of one conserved receptor, Erv14.

The endoplasmic reticulum (ER) is the site of synthesis of secreted and membrane proteins. To exit the ER, proteins are packaged into COPII vesicles through direct interaction with the COPII coat or aided by specific cargo receptors. Despite the fundamental role of such cargo receptors in protein traffic, only a few have been identified; their cargo spectrum is unknown and the signals they recognize remain poorly understood. We present here an approach we term “PAIRS” (pairing analysis of cargo receptors), which combines systematic genetic manipulations of yeast with automated microscopy screening, to map the spectrum of cargo for a known receptor or to uncover a novel receptor for a particular cargo. Using PAIRS we followed the fate of ∼150 cargos on the background of mutations in nine putative cargo receptors and identified novel cargo for most of these receptors. Deletion of the Erv14 cargo receptor affected the widest range of cargo. Erv14 substrates have a wide array of functions and structures; however, they are all membrane-spanning proteins of the late secretory pathway or plasma membrane. Proteins residing in these organelles have longer transmembrane domains (TMDs). Detailed examination of one cargo supported the hypothesis that Erv14 dependency reflects the length rather than the sequence of the TMD. The PAIRS approach allowed us to uncover new cargo for known cargo receptors and to obtain an unbiased look at specificity in cargo selection. Obtaining the spectrum of cargo for a cargo receptor allows a novel perspective on its mode of action. The rules that appear to guide Erv14 substrate recognition suggest that sorting of membrane proteins at multiple points in the secretory pathway could depend on the physical properties of TMDs. Such a mechanism would allow diverse proteins to utilize a few receptors without the constraints of evolving location-specific sorting motifs.

All cells sense their environment, respond to it, and communicate with neighboring cells. To perform these functions, cells use an impressive array of proteins that they display on their surface membranes and secrete into their external environment. Newly synthesized proteins destined for the surface of nucleated cells, or to be secreted into the environment must enter the secretory pathway through the endoplasmic reticulum. Those that reside there remain behind, but most leave for their next destination as cargo proteins in lipid vesicles. To be packaged into vesicles, many of them require a “cargo receptor,” which recognizes and tethers specific cargo proteins in the vesicles. Our study takes a systematic approach to identify the range of cargo proteins that bind to each of the known receptors in yeast. By using this approach, we both discover new cargo for known cargo receptors and delineate the rule that governs cargo selection for one cargo receptor, Erv14. Thus, our study demonstrates a novel approach to identify the cargo for any receptor or to discover new cargo receptors.

Regulation of cell wall biogenesis in Saccharomyces cerevisiae: the cell wall integrity signaling pathway

The yeast cell wall is a strong, but elastic, structure that is essential not only for the maintenance of cell shape and integrity, but also for progression through the cell cycle. During growth and morphogenesis, and in response to environmental challenges, the cell wall is remodeled in a highly regulated and polarized manner, a process that is principally under the control of the cell wall integrity (CWI) signaling pathway. This pathway transmits wall stress signals from the cell surface to the Rho1 GTPase, which mobilizes a physiologic response through a variety of effectors. Activation of CWI signaling regulates the production of various carbohydrate polymers of the cell wall, as well as their polarized delivery to the site of cell wall remodeling. This review article centers on CWI signaling in Saccharomyces cerevisiae through the cell cycle and in response to cell wall stress. The interface of this signaling pathway with other pathways that contribute to the maintenance of cell wall integrity is also discussed.

Cell wall integrity maintenance in plants: lessons to be learned from yeast?

The plant cell wall is involved in different biological processes like cell morphogenesis and response to biotic/abiotic stress. Functional integrity of the wall is apparently being maintained during these processes by changing structure/composition and coordinating cell wall with cellular metabolism. In S.cerevisiae a well-characterized mechanism exists that is maintaining functional integrity of yeast the cell wall during similar processes. During the last years it has become obvious that plants have evolved a mechanism to monitor and maintain functional integrity of their cell walls. However, our understanding of the mechanism is rather limited. The available evidence suggests that similar signaling cascades may be involved and particular protein activities may be conserved between plants and yeast. Here we review the available evidence briefly and highlight similarities between yeast and plants that could help us to understand the mode of action of the signaling cascades maintaining plant cell wall integrity.

Like-acetylglucosaminyltransferase (LARGE)-dependent modification of dystroglycan at Thr-317/319 is required for laminin binding and arenavirus infection

α-dystroglycan is a highly O-glycosylated extracellular matrix receptor that is required for anchoring of the basement membrane to the cell surface and for the entry of Old World arenaviruses into cells. Like-acetylglucosaminyltransferase (LARGE) is a key molecule that binds to the N-terminal domain of α-dystroglycan and attaches ligand-binding moieties to phosphorylated O-mannose on α-dystroglycan. Here we show that the LARGE modification required for laminin- and virus-binding occurs on specific Thr residues located at the extreme N terminus of the mucin-like domain of α-dystroglycan. Deletion and mutation analyses demonstrate that the ligand-binding activity of α-dystroglycan is conferred primarily by LARGE modification at Thr-317 and -319, within the highly conserved first 18 amino acids of the mucin-like domain. The importance of these paired residues in laminin-binding and clustering activity on myoblasts and in arenavirus cell entry is confirmed by mutational analysis with full-length dystroglycan. We further demonstrate that a sequence of five amino acids, Thr(317)ProThr(319)ProVal, contains phosphorylated O-glycosylation and, when modified by LARGE is sufficient for laminin-binding. Because the N-terminal region adjacent to the paired Thr residues is removed during posttranslational maturation of dystroglycan, our results demonstrate that the ligand-binding activity resides at the extreme N terminus of mature α-dystroglycan and is crucial for α-dystroglycan to coordinate the assembly of extracellular matrix proteins and to bind arenaviruses on the cell surface.

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.

Putative stress sensors WscA and WscB are involved in hypo-osmotic and acidic pH stress tolerance in Aspergillus nidulans

Wsc proteins have been identified in fungi and are believed to be stress sensors in the cell wall integrity (CWI) signaling pathway. In this study, we characterized the sensor orthologs WscA and WscB in Aspergillus nidulans. Using hemagglutinin-tagged WscA and WscB, we showed both Wsc proteins to be N- and O-glycosylated and localized in the cell wall and membrane, implying that they are potential cell surface sensors. The wscA disruptant (ΔwscA) strain was characterized by reduced colony and conidia formation and a high frequency of swollen hyphae under hypo-osmotic conditions. The deficient phenotype of the ΔwscA strain was facilitated by acidification, but not by alkalization or antifungal agents. In contrast, osmotic stabilization restored the normal phenotype in the ΔwscA strain. A similar inhibition was observed in the wscB disruptant strain, but to a lesser extent. In addition, a double wscA and wscB disruptant (ΔwscA ΔwscB) strain was viable, but its growth was inhibited to a greater degree, indicating that the functions of the products of these genes are redundant. Transcription of α-1,3-glucan synthase genes (agsA and agsB) was significantly altered in the wscA disruptant strain, resulting in an increase in the amount of alkali-soluble cell wall glucan compared to that in the wild-type (wt) strain. An increase in mitogen-activated protein kinase (MpkA) phosphorylation was observed as a result of wsc disruption. Moreover, the transient transcriptional upregulation of the agsB gene via MpkA signaling was observed in the ΔwscA ΔwscB strain to the same degree as in the wt strain. These results indicate that A. nidulans Wsc proteins have a different sensing spectrum and downstream signaling pathway than those in the yeast Saccharomyces cerevisiae and that they play an important role in CWI under hypo-osmotic and acidic pH conditions.

How do I begin? Sensing extracellular stress to maintain yeast cell wall integrity

The cell wall integrity (CWI) signalling pathway is necessary to remodel the yeast cell wall during normal morphogenesis and in response to cell surface stress. In the Baker's yeast Saccharomyces cerevisiae, a set of five membrane-spanning sensors, namely Wsc1, Wsc2, Wsc3, Mid2 and Mtl1, detect perturbations in the cell wall and/or the plasma membrane and activate a downstream signal transduction pathway with a central MAP kinase module. As a consequence, the expression of genes whose products are involved in cell wall structure and remodelling is induced. This review summarises our recent results on sensor structure and function, as well as the advances made regarding sensor mechanics.

Cloning and functional analysis of the dpm2 and dpm3 genes from Trichoderma reesei expressed in a Saccharomyces cerevisiae dpm1Δ mutant strain

InTrichoderma reesei, dolichyl phosphate mannose (dpm) synthase, a key enzyme in the O-glycosylation process, requires three proteins for full activity. In this study, thedpm2anddpm3genes coding for the DPMII and DPMIII subunits ofT. reeseiDPM synthase were cloned and functionally analyzed after expression in theSaccharomyces cerevisiae dpm1Δ[genotype (BY4743;his3Δ1; /leu2Δ0; lys2Δ0; /ura3Δ0; YPR183w::kanMX4] mutant. It was found that apart from the catalytic subunit DPMI, the DPMIII subunit is also essential to form an active DPM synthase in yeast. Additional expression of the DPMII protein, considered to be a regulatory subunit of DPM synthase, decreased the enzymatic activity. We also characterizedS. cerevisiaestrains expressing thedpm1,2,3ordpm1, 3genes and analyzed the consequences ofdpmexpression on protein O-glycosylationin vivoand on the cell wall composition.

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

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

Defect in dolichol-dependent glycosylation increases sensitivity of Saccharomyces cerevisiae towards anti-fungal drugs

Two temperature-sensitive Saccharomyces cerevisiae mutants, sec59-1 and dpm1-6, impaired, respectively, in dolichol kinase (Sec59p) and dolichyl phosphate mannose (DolPMan) synthase (Dpm1p), have an aberrant cell wall structure and composition. We tested their sensitivity to four classes of antifungal drugs used in clinical practice: 5-fluorocytosine, amphotericin B, caspofungin and itraconasole. The strains were resistant to itraconazole and sensitive to the other drugs used. The minimal inhibitory concentration (MIC) of caspofungin and amphotericin B was two-fold lower for sec59-1 and dpm1-6 than for the respective wild-type strains. The sensitivity of both mutants could be brought back to the wild-type level by a multicopy suppressor of the thermosensitive phenotype, the RER2 gene, encoding cis-prenyltransferase involved in dolichol biosynthesis. Biochemical analysis revealed slight changes of the cell wall composition, different in the mutants as compared to the wild-type in response to the drugs. Our data strongly support a relationship between dolichol phosphate level, protein glycosylation and antifungal sensitivity.

Stability control of MTL1 mRNA by the RNA-binding protein Khd1p in yeast

Khd1p (KH-domain protein 1) is a yeast RNA-binding protein highly homologous to mammalian hnRNP K. Khd1p associates with hundreds of potential mRNA targets including a bud-localized ASH1 mRNA and mRNAs encoding membrane-associated proteins such as Mid2p and Mtl1p. While Khd1p negatively regulates gene expression of Ash1p by translational repression, Khd1p positively regulates gene expression of Mtl1p by mRNA stabilization. To investigate how Khd1p regulates the stability of MTL1 mRNA, we searched for cis-acting elements and trans-acting factors controlling MTL1 mRNA stability. Regional analysis revealed that partial deletion of the coding sequences of MTL1 mRNA restored the decreased MTL1 mRNA and protein levels in khd1Δ mutants. This region, encompassing nucleotides 532 to 1032 of the Mtl1p coding sequence, contains CNN repeats that direct Khd1p-binding. Insertion of this sequence into other mRNAs conferred mRNA instability in khd1Δ mutants. We further searched for factors involved in the destabilization of MTL1 mRNA. Mutations in CCR4 and CAF1/POP2, encoding major cytoplasmic deadenylases, or of SKI genes, which code for components of a complex involved in 3' to 5' degradation, did not restore the decreased MTL1 mRNA levels caused by khd1Δ mutation. However, mutations in DCP1 and DCP2, encoding a decapping enzyme complex, and XRN1, encoding a 5'-3' exonuclease, restored the decreased MTL1 mRNA levels. Furthermore, Khd1p colocalized with Dcp1p in processing bodies, cytoplasmic sites for mRNA degradation. Our results suggest that MTL1 mRNA bears a cis-acting element involved in destabilization by the decapping enzyme and the 5'-3' exonuclease, and Khd1p stabilizes MTL1 mRNA through binding to this element.

A block of endocytosis of the yeast cell wall integrity sensors Wsc1 and Wsc2 results in reduced fitness in vivo

The response to cell surface stress in yeast is mediated by a set of five plasma membrane sensors. We here address the relation of intracellular localization of the sensors Wsc1, Wsc2, and Mid2 to their turnover and signaling function. Growth competition experiments indicate that Wsc2 plays an important role in addition to Wsc1 and Mid2. The two Wsc sensors appear at the bud neck during cytokinesis and employ different routes of endocytosis, which govern their turnover. Whereas Wsc1 uses a clathrin-dependent NPFDD signal, Wsc2 relies on a specific lysine residue (K495). In end3 and doa4 endocytosis mutants, both sensors accumulate at the plasma membrane, and a hypersensitivity to cell wall-specific drugs and to treatment with zymolyase is observed. A haploid strain in which endocytosis of the two sensors is specifically blocked displays a reduced fitness in growth competition experiments. If the Mid2 sensor is mobilized by the addition of an endocytosis signal, it mimics the dynamic distribution of the Wsc sensors, but is unable to complement the specific growth defects of a wsc1 deletion. These data suggest that sensor distribution is not the major determinant for its specificity.

Is there anyone out there?--Single-molecule atomic force microscopy meets yeast genetics to study sensor functions

The ability to react to environmental stress is a key feature of microbial cells, which frequently involves the fortification of their cell wall as a primary step. In the model yeast Saccharomyces cerevisiae the biosynthesis of the cell wall is regulated by the so-called cell wall integrity signal transduction pathway, which starts with the detection of cell surface stress by a small family of five membrane-spanning sensors (Wsc1-Wsc3, Mid2, Mtl1). Although genetic evidence indicated that these proteins act as mechanosensors, direct in vivo evidence for their function remained scarce. Here, we review a new approach integrating the tools and concepts of genetics with those of nanotechnology. We show how atomic force microscopy can be combined with advanced protein design by yeast genetics, to study the function and the mechanical properties of yeast sensors in living cells down to the single molecule level. We anticipate that this novel integrated technology will enable a paradigm shift in cell biology, so that pertinent questions can be addressed, such as the nanomechanics of single sensors and receptors, and how they distribute across the cell surface when they respond to extracellular stress.

Evasion of endoplasmic reticulum surveillance makes Wsc1p an obligate substrate of Golgi quality control

In the endoplasmic reticulum (ER), most newly synthesized proteins are retained by quality control mechanisms until folded. Misfolded molecules are sorted to ER-associated degradation (ERAD) pathways for disposal. Reports of mutant proteins degraded in the vacuole/lysosome suggested an independent Golgi-based mechanism also at work. Although little is understood of the post-ER pathway, the growing number of variants using it suggests a major role in quality control. Why seemingly redundant mechanisms in sequential compartments are needed is unclear. To understand their physiological relationship, the identification of endogenous pathway-specific substrates is a prerequisite. With ERAD substrates already well characterized, the discovery of Wsc1p as an obligate substrate of Golgi quality control enabled detailed cross-pathway analyses for the first time. By analyzing a panel of engineered substrates, the data show that the surveillance mode is determined by each polypeptide's intrinsic design. Although most secretory pathway proteins can display ERAD determinants when misfolded, the lack thereof shields Wsc1p from inspection by ER surveillance. Additionally, a powerful ER export signal mediates transport whether the luminal domain is folded or not. By evading ERAD through these passive and active mechanisms, Wsc1p is fully dependent on the post-ER system for its quality control.

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.

Drosophila Dystroglycan is a target of O-mannosyltransferase activity of two protein O-mannosyltransferases, Rotated Abdomen and Twisted

Recent studies highlighted an emerging possibility of using Drosophila as a model system for investigating the mechanisms of human congenital muscular dystrophies, called dystroglycanopathies, resulting from the abnormal glycosylation of alpha-dystroglycan. Several of these diseases are associated with defects in O-mannosylation, one of the most prominent types of alpha-dystroglycan glycosylation mediated by two protein O-mannosyltransferases. Drosophila appears to possess homologs of all essential components of the mammalian dystroglycan-mediated pathway; however, the glycosylation of Drosophila Dystroglycan (DG) has not yet been explored. In this study, we characterized the glycosylation of Drosophila DG using a combination of glycosidase treatments, lectin blots, trypsin digestion, and mass spectrometry analyses. Our results demonstrated that DG extracellular domain is O-mannosylated in vivo. We found that the concurrent in vivo activity of the two Drosophila protein O-mannosyltransferases, Rotated Abdomen and Twisted, is required for O-mannosylation of DG. While our experiments unambiguously determined some O-mannose sites far outside of the mucin-type domain of DG, they also provided evidence that DG bears a significant amount of O-mannosylation within its central region including the mucin-type domain, and that O-mannose can compete with O-GalNAc glycosylation of DG. We found that Rotated Abdomen and Twisted could potentiate in vivo the dominant-negative effect of DG extracellular domain expression on crossvein development, which suggests that O-mannosylation can modulate the ligand-binding activity of DG. Taken together these results demonstrated that O-mannosylation of Dystroglycan is an evolutionarily ancient mechanism conserved between Drosophila and humans, suggesting that Drosophila can be a suitable model system for studying molecular and genetic mechanisms underlying human dystroglycanopathies.