Interaction among the subunits of Golgi membrane mannosyltransferase complexes of the yeast Saccharomyces cerevisiae

Discovery of α-(1→6)-linked mannan structures resembling yeast N-glycan outer chains in Aspergillus fumigatus mycelium

The cellular surface of the pathogenic filamentous fungus Aspergillus fumigatus is enveloped in a mannose layer, featuring well-established fungal-type galactomannan and O-mannose-type galactomannan. This study reports the discovery of cell wall component in A. fumigatus mycelium, which resembles N-glycan outer chains found in yeast. The glycosyltransferases involved in its biosynthesis in A. fumigatus were identified, with a focus on two key α-(1→2)-mannosyltransferases, Mnn2 and Mnn5, and two α-(1→6)-mannosyltransferases, Mnn9 and Van1. In vitro examination revealed the roles of recombinant Mnn2 and Mnn5 in transferring α-(1→2)-mannosyl residues. Proton nuclear magnetic resonance (1H-NMR) analysis of cell wall extracts from the ∆mnn2∆mnn5 strain indicated the existence of an α-(1→6)-linked mannan backbone in the A. fumigatus mycelium, with Mnn2 and Mnn5 adding α-(1→2)-mannosyl residues to this backbone. The α-(1→6)-linked mannan backbone was absent in strains where mnn9 or van1 was disrupted in the parental ∆mnn2∆mnn5 strain in A. fumigatus. Mnn9 and Van1 functioned as α-(1→6)-linked mannan polymerases in heterodimers when co-expressed in Escherichia coli, indicating their crucial role in biosynthesizing the α-(1→6)-linked mannan backbone. Disruptions of these mannosyltransferases did not affect fungal-type galactomannan biosynthesis. This study provides insights into the complexity of fungal cell wall architecture and a better understanding of mannan biosynthesis in A. fumigatus.

IMPORTANCE

This study unravels the complexities of mannan biosynthesis in A. fumigatus, a key area for antifungal drug discovery. It reveals the presence of α-(1→6)-linked mannan structures resembling yeast N-glycan outer chains in A. fumigatus mycelium, offering fresh insights into the fungal cell wall's design. Key enzymes, Mnn2, Mnn5, Mnn9, and Van1, are instrumental in this process, with Mnn2 and Mnn5 adding specific mannose residues and Mnn9 and Van1 assembling the α-(1→6)-linked mannan structures. Although fungal-type galactomannan's presence in the cell wall is known, the existence of an α-(1→6)-linked mannan adds a new dimension to our understanding. This intricate web of mannan biosynthesis opens avenues for further exploration and enhances our understanding of fungal cell wall dynamics, paving the way for targeted drug development.

Pleiotropic roles of N-glycans for enzyme activities and stabilities of MIPC synthases, Csh1 and Sur1/Csg1, in Saccharomyces cerevisiae

Mannosyl phosphorylceramide (MIPC) is a membrane lipid classified as a complex sphingolipid in Saccharomyces cerevisiae. MIPC is synthesized by two redundant enzymes, Sur1/Csg1 and Csh1, in the Golgi lumen. MIPC consists of five subtypes (A, B', B, C, and D-type) according to the position and number of hydroxyl groups on the ceramide moiety. Sur1 exerts higher impact on synthesis of MIPC-B and MIPC-C than Csh1. In this study, we elucidated the roles played by N-glycans attached to Sur1 and Csh1, and dissected the mechanisms underlying substrate recognition by these two enzymes. Sur1 carries an N-glycan on Asn-224, while Csh1 has N-glycans on Asn-51 and Asn-247. Although intracellular proteins usually harbor core-type N-glycans, the N-glycan on Asn-51 of Csh1 exhibited a unique mannan-like structure containing a long backbone of mannose. Sur1 N224Q and Csh1 N51Q mutants exhibited a decrease in the activity to synthesize specific MIPC subtypes for each enzyme, suggesting that these N-glycans play a role in substrate recognition through their catalytic domains. Moreover, ectopic insertion of an N-glycosylation consensus sequence (NST) at codon 51 of Sur1 (Sur1-NST51) resulted in an artificial modification with mannan, which markedly decreased protein stability. Our results suggest that the diminished stability of the Sur1-NST51 mutant protein could be attributable to potential structural alterations by the mannan. Collectively, the present study reveals essential luminal domains of Sur1 and Csh1 that dictate substrate specificity and/or the protein stabilities via mannan modification.

Aspergillus fumigatus Mnn9 is responsible for mannan synthesis and required for covalent linkage of mannoprotein to the cell wall

Owing to the essential role in protection of the Aspergillus fumigatus cell against human defense reactions, its cell wall has long been taken as a promising antifungal target. The cell wall of A. fumigatus composed of chitin, glucan and galactomannan and mannoproteins. Although galactomannan has been used as a diagnostic target for a long time, its biosynthesis remains unknown in A. fumigatus. In this study, a putative α1,6-mannosyltransferase gene mnn9 was identified in A. fumigatus. Deletion of the mnn9 gene resulted in an increased sensitivity to calcofluor white, Congo red, or hygromycin B as well as in reduced cell wall components and abnormal polarity. Although there was no major effect on N-glycan synthesis, covalently-linked cell wall mannoprotein Mp1 was significantly reduced in the mutant. Based on our results, we propose that Mnn9p is a mannosyltransferase responsible for the formation of the α-mannan in cell wall mannoproteins, potentially via elongation of O-linked mannose chains.

Prohibitin: A Novel Molecular Player in KDEL Receptor Signalling

The KDEL receptor (KDELR) is a seven-transmembrane-domain protein involved in retrograde transport of protein chaperones from the Golgi complex to the endoplasmic reticulum. Our recent findings have shown that the Golgi-localised KDELR acts as a functional G-protein-coupled receptor by binding to and activating Gs and Gq. These G proteins induce activation of PKA and Src and regulate retrograde and anterograde Golgi trafficking. Here we used an integrated coimmunoprecipitation and mass spectrometry approach to identify prohibitin-1 (PHB) as a KDELR interactor. PHB is a multifunctional protein that is involved in signal transduction, cell-cycle control, and stabilisation of mitochondrial proteins. We provide evidence that depletion of PHB induces intense membrane-trafficking activity at the ER–Golgi interface, as revealed by formation of GM130-positive Golgi tubules, and recruitment of p115, β-COP, and GBF1 to the Golgi complex. There is also massive recruitment of SEC31 to endoplasmic-reticulum exit sites. Furthermore, absence of PHB decreases the levels of the Golgi-localised KDELR, thus preventing KDELR-dependent activation of Golgi-Src and inhibiting Golgi-to-plasma-membrane transport of VSVG. We propose a model whereby in analogy to previous findings (e.g., the RAS-RAF signalling pathway), PHB can act as a signalling scaffold protein to assist in KDELR-dependent Src activation.

Yeast Mnn9 is both a priming glycosyltransferase and an allosteric activator of mannan biosynthesis

The fungal cell possesses an essential carbohydrate cell wall. The outer layer, mannan, is formed by mannoproteins carrying highly mannosylated O- and N-linked glycans. Yeast mannan biosynthesis is initiated by a Golgi-located complex (M-Pol I) of two GT-62 mannosyltransferases, Mnn9p and Van1p, that are conserved in fungal pathogens. Saccharomyces cerevisiae and Candida albicans mnn9 knockouts show an aberrant cell wall and increased antibiotic sensitivity, suggesting the enzyme is a potential drug target. Here, we present the structure of ScMnn9 in complex with GDP and Mn²⁺, defining the fold and catalytic machinery of the GT-62 family. Compared with distantly related GT-78/GT-15 enzymes, ScMnn9 carries an unusual extension. Using a novel enzyme assay and site-directed mutagenesis, we identify conserved amino acids essential for ScMnn9 ‘priming’ α-1,6-mannosyltransferase activity. Strikingly, both the presence of the ScMnn9 protein and its product, but not ScMnn9 catalytic activity, are required to activate subsequent ScVan1 processive α-1,6-mannosyltransferase activity in the M-Pol I complex. These results reveal the molecular basis of mannan synthesis and will aid development of inhibitors targeting this process.

Mechanisms of protein retention in the Golgi

The protein composition of the Golgi is intimately linked to its structure and function. As the Golgi serves as the major protein-sorting hub for the secretory pathway, it faces the unique challenge of maintaining its protein composition in the face of constant influx and efflux of transient cargo proteins. Much of our understanding of how proteins are retained in the Golgi has come from studies on glycosylation enzymes, largely because of the compartment-specific distributions these proteins display. From these and other studies of Golgi membrane proteins, we now understand that a variety of retention mechanisms are employed, the majority of which involve the dynamic process of iterative rounds of retrograde and anterograde transport. Such mechanisms rely on protein conformation and amino acid-based sorting signals as well as on properties of transmembrane domains and their relationship with the unique lipid composition of the Golgi.

The genetic interaction network of CCW12, a Saccharomyces cerevisiae gene required for cell wall integrity during budding and formation of mating projections

Background

Mannoproteins construct the outer cover of the fungal cell wall. The covalently linked cell wall protein Ccw12p is an abundant mannoprotein. It is considered as crucial structural cell wall component since in baker's yeast the lack of CCW12 results in severe cell wall damage and reduced mating efficiency.

Results

In order to explore the function of CCW12, we performed a Synthetic Genetic Analysis (SGA) and identified genes that are essential in the absence of CCW12. The resulting interaction network identified 21 genes involved in cell wall integrity, chitin synthesis, cell polarity, vesicular transport and endocytosis. Among those are PFD1, WHI3, SRN2, PAC10, FEN1 and YDR417C, which have not been related to cell wall integrity before. We correlated our results with genetic interaction networks of genes involved in glucan and chitin synthesis. A core of genes essential to maintain cell integrity in response to cell wall stress was identified. In addition, we performed a large-scale transcriptional analysis and compared the transcriptional changes observed in mutant ccw12Δ with transcriptomes from studies investigating responses to constitutive or acute cell wall damage. We identified a set of genes that are highly induced in the majority of the mutants/conditions and are directly related to the cell wall integrity pathway and cell wall compensatory responses. Among those are BCK1, CHS3, EDE1, PFD1, SLT2 and SLA1 that were also identified in the SGA. In contrast, a specific feature of mutant ccw12Δ is the transcriptional repression of genes involved in mating. Physiological experiments substantiate this finding. Further, we demonstrate that Ccw12p is present at the cell periphery and highly concentrated at the presumptive budding site, around the bud, at the septum and at the tip of the mating projection.

Conclusions

The combination of high throughput screenings, phenotypic analyses and localization studies provides new insight into the function of Ccw12p. A compensatory response, culminating in cell wall remodelling and transport/recycling pathways is required to buffer the loss of CCW12. Moreover, the enrichment of Ccw12p in bud, septum and mating projection is consistent with a role of Ccw12p in preserving cell wall integrity at sites of active growth.

The microarray data produced in this analysis have been submitted to NCBI GEO database and GSE22649 record was assigned.

The cytoplasmic region of alpha-1,6-mannosyltransferase Mnn9p is crucial for retrograde transport from the Golgi apparatus to the endoplasmic reticulum in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, Och1p and Mnn9p mannosyltransferases are localized in the cis-Golgi. Attempts to live image Och1p and Mnn9p tagged with green fluorescent protein or red fluorescent protein, respectively, using a high-performance confocal laser scanning microscope system resulted in simultaneous visualization of the native proteins in a living cell. Our observations revealed that Och1p and Mnn9p are not always colocalized to the same cisternae. The difference in the dynamics of these mannosyltransferases may reflect differences in the mechanisms for their retention in the cis-Golgi, since it has been reported that Mnn9p cycles between the endoplasmic reticulum and the cis-Golgi whereas Och1p does not (Z. Todorow, A. Spang, E. Carmack, J. Yates, and R. Schekman, Proc. Natl. Acad. Sci. USA 97:13643-13648, 2000). We investigated the localization of chimeric proteins of Mnn9p and Och1p in sec12 and erd1 mutant cells. A chimeric protein, M16/O16, which consists of the N-terminal cytoplasmic region of Mnn9p and the transmembrane and luminal region of Och1p, behaved like Mnn9p, suggesting that the N-terminal cytoplasmic region is important for the intracellular dynamics of Mnn9p. This observation is supported by results from subcellular-fractionation experiments. Mutational analysis revealed that two arginine residues in the N-terminal region of Mnn9p are important for the chimeric protein to cycle between the endoplasmic reticulum and the Golgi apparatus.

Saccharomyces cerevisiae alpha1,6-mannosyltransferase has a catalytic potential to transfer a second mannose molecule

In yeast, the N-linked oligosaccharide modification in the Golgi apparatus is initiated by alpha1,6-mannosyltransferase (encoded by the OCH1 gene) with the addition of mannose to the Man(8)GlcNAc(2) or Man(9)GlcNAc(2) endoplasmic reticulum intermediates. In order to characterize its enzymatic properties, the soluble form of the recombinant Och1p was expressed in the methylotrophic yeast Pichia pastoris as a secreted protein, after truncation of its transmembrane region and fusion with myc and histidine tags at the C-terminus, and purified using a metal chelating column. The enzymatic reaction was performed using various kinds of pyridylaminated (PA) sugar chains as acceptor, and the products were separated by high performance liquid chromatography. The recombinant Och1p efficiently transferred a mannose to Man(8)GlcNAc(2)-PA and Man(9)GlcNAc(2)-PA acceptors, while Man(5)GlcNAc(2)-PA, which completely lacks alpha1,2-linked mannose residues, was not used as an acceptor. At high enzyme concentrations, a novel product was detected by HPLC. Analysis of the product revealed that a second mannose was attached at the 6-O-position of alpha1,3-linked mannose branching from the alpha1,6-linked mannose that is attached to beta1,4-linked mannose of Man(10)GlcNAc(2)-PA produced by the original activity of Och1p. Our results indicate that Och1p has the potential to transfer two mannoses from GDP-mannose, and strictly recognizes the overall structure of high mannose type oligosaccharide.

Vesicular transport and the Golgi apparatus in yeast

Eukaryotic cells have developed a complex intracellular membrane system to divide the cell into various compartments where specific biochemical reactions are efficiently conducted locally. They also have developed systems to deliver appropriate materials to each specific compartment. Vesicular transport is a delivery system that also links most of the main organelles in the cell. The Golgi apparatus occupies the central position of the traffic between the endoplasmic reticulum and the endosome/vacuole/plasma membrane by maturating and sorting delivery of materials. Every important feature of vesicular transport has been identified by studying the Golgi apparatus, and the unicellular microorganism Saccharomyces cerevisiae has been an extremely excellent material for this study. Cycles of production and consumption of the transport vesicles by sorting the cargo, budding from the donor, tethering, docking and fusion to the target can now be explained to a large extent at the molecular level. The functional and structural aspects of the Golgi have also been well studied in the last decade.

Localization of GDP-mannose transporter in the Golgi requires retrieval to the endoplasmic reticulum depending on its cytoplasmic tail and coatomer

The Saccharomyces cerevisiae GDP-mannose transporter (GMT) encoded by the essential gene VRG4/VIG4 is a member of the nucleotide-sugar transporter family in the Golgi apparatus. We examined GMT in the secretory mutant cells to investigate the mechanism of its localization in the Golgi. At the nonpermissive temperature, most GMT was found in the endoplasmic reticulum of sec23ts cells, which have defective COPII, and in the vacuole of sec21ts cells, which have defective COPI. The C-terminal hydrophilic peptide of GMT that is exposed to the cytosol binds to Ret2p, a subunit of the COPI coat. Mutant peptide derivatives that have lost a cluster of lysine in the vicinity of the transmembrane domain had reduced binding activity to Ret2p and the GMT with this sequence was delivered to the vacuole. Our results indicate that GMT escapes from delivery to the vacuole by recycling to the endoplasmic reticulum and retrieval requires the lysine-rich C-terminal tail that can bind to the COPI coat.

Disruption of the gene enhances protein secretion in and

Screening for genes affecting super-secreting phenotype of the over-secreting mutant of Kluyveromyces lactis resulted in isolation of the gene named KlMNN10, sharing high homology with Saccharomyces cerevisiae MNN10. The disruption of the KlMNN10 in Kluyveromyces lactis, as well as of MNN10 and MNN11 in Saccharomyces cerevisiae, conferred the super-secreting phenotype. MNN10 isolated from Saccharomyces cerevisiae suppressed the super-secretion phenotype in Kluyveromyces lactis klmnn10, as did the homologous KlMNN10. The genes MNN10 and MNN11 of Saccharomyces cerevisiae encode mannosyltransferases responsible for the majority of the alpha-1,6-polymerizing activity of the mannosyltransferase complex. These data agree with the view that the structure of glycoproteins in a yeast cell wall strongly influences the release of homologous and heterologous proteins in the medium. The set of genes namely the suppressors of the over-secreting phenotype, could be attractive for further analysis of gene functions, over-secreting mechanisms and for construction of new strains optimized for heterologous protein secretion. KlMNN10 has EMBL accession no. AJ575132.

Five Genes Involved in Biosynthesis of the Pyruvylated Galβ1,3-Epitope in Schizosaccharomyces pombe N-Linked Glycans

The N-linked galactomannans of Schizosaccharomyces pombe have pyruvylated Galbeta1,3-(PvGal) caps on a portion of the Galalpha1,2-residues in their outer chains (Gemmill, T. R., and Trimble, R. B. (1998) Glycobiology 8, 1087-1095). PvGal biosynthesis was investigated by ethyl methanesulfonate mutagenesis of S. pombe, followed by the isolation of cells devoid of negatively charged N-glycans by Q-Sepharose exclusion and failure to bind human serum amyloid P component, which acts as a lectin for terminal PvGal residues. Mutant glycans were characterized by lectin binding, saccharide composition, exoglycosidase sensitivity, and NMR spectroscopy. Restoration of the cell surface negative charge by complementation with an S. pombe genomic library led to the identification of five genes involved in PvGal biosynthesis, which we designated pvg1-pvg5. Pvg1p may be a pyruvyltransferase, since NMR of pvg1(-) mutant N-glycans revealed the absence of only the pyruvyl moiety. Pvg2p-Pvg5p are crucial for attachment of the Galbeta1,3-residue that becomes pyruvylated. Pvg3p is predicted to be a member of the beta1,3-galactosyltransferase family, and Pvg3p-green fluorescent protein labeling was consistent with Golgi localization. Predicted Pvg1p and Pvg3p functions imply that Galbeta1,3-is added to the galactomannans and is then pyruvylated in situ, rather than by an en bloc addition of PvGalbeta1,3-caps to the outer chain. Pvg4p-green fluorescent protein targeted to the nucleus, and its sequence contains a MADS-box DNA-binding and dimerization domain; however, it does not appear to solely control transcription of the other identified genes. Pvg2p and/or Pvg5p may contribute to an enzyme complex. Whereas a functional role for the PvGal epitope in S. pombe remains unclear, it is nonessential for either cell growth or mating under laboratory conditions.

A kinetic proof-reading mechanism for protein sorting

Resident proteins of the exocytic pathway are maintained at various levels through coatomer protein I (COPI)-mediated recycling. Sorting of cargo by COPI requires GTP hydrolysis by ADP-ribosylation factor 1 (ARF-1). This small GTPase recruits coatomer onto Golgi membranes and upon hydrolysis, is thought to release coatomer back into the cytosol. This step requires the activating protein, ARFGAP1. By coupling sorting to a cargo-induced sequestering of ARFGAP1, we have formulated a kinetic proof-reading model that explains how a GTP hydrolysis-driven coat release can yield an active sorting event. The sorting scheme predicts a dependency on the amount of ARFGAP1 and explains the recent experimental findings that ARF-1 and COPI detach with different time constants from the Golgi membrane in vivo.

The components of the Saccharomyces cerevisiae mannosyltransferase complex M-Pol I have distinct functions in mannan synthesis

The yeast Saccharomyces cerevisiae processes N-linked glycans in the Golgi apparatus in two different ways. Whereas most of the proteins of internal membranes receive a simple core-type structure, a long branched polymer termed mannan is attached to the glycans of many of the proteins destined for the cell wall. The first step in mannan synthesis is the initiation and extension of an alpha-1,6-linked polymannose backbone. This requires the sequential action of two enzyme complexes, mannan polymerases (M-Pol) I and II. M-Pol I contains the proteins Mnn9p and Van1p, although the stoichiometry and individual contributions to enzyme action are unclear. We report here that the two proteins are each present as a single copy in the complex. Both proteins contain a DXD motif found in the active site of many glycosyltransferases, and mutations in this motif in Mnn9p or Van1p reveal that both proteins contribute to mannose polymerization. However, the effects of these mutations on both the in vivo and in vitro activity are distinct, suggesting that the two proteins may have different roles in the complex. Finally, we show that a simple glycoprotein based on hen egg lysozyme can be used as a substrate for modification by purified M-Pol I in vitro.

Genomic screen for vacuolar protein sorting genes in Saccharomyces cerevisiae

The biosynthetic sorting of hydrolases to the yeast vacuole involves transport along two distinct routes referred to as the carboxypeptidase Y and alkaline phosphatase pathways. To identify genes involved in sorting to the vacuole, we conducted a genome-wide screen of 4653 homozygous diploid gene deletion strains of Saccharomyces cerevisiae for missorting of carboxypeptidase Y. We identified 146 mutant strains that secreted strong-to-moderate levels of carboxypeptidase Y. Of these, only 53 of the corresponding genes had been previously implicated in vacuolar protein sorting, whereas the remaining 93 had either been identified in screens for other cellular processes or were only known as hypothetical open reading frames. Among these 93 were genes encoding: 1) the Ras-like GTP-binding proteins Arl1p and Arl3p, 2) actin-related proteins such as Arp5p and Arp6p, 3) the monensin and brefeldin A hypersensitivity proteins Mon1p and Mon2p, and 4) 15 novel proteins designated Vps61p-Vps75p. Most of the novel gene products were involved only in the carboxypeptidase Y pathway, whereas a few, including Mon1p, Mon2p, Vps61p, and Vps67p, appeared to be involved in both the carboxypeptidase Y and alkaline phosphatase pathways. Mutants lacking some of the novel gene products, including Arp5p, Arp6p, Vps64p, and Vps67p, were severely defective in secretion of mature alpha-factor. Others, such as Vps61p, Vps64p, and Vps67p, displayed defects in the actin cytoskeleton at 30 degrees C. The identification and phenotypic characterization of these novel mutants provide new insights into the mechanisms of vacuolar protein sorting, most notably the probable involvement of the actin cytoskeleton in this process.

Functional expression of Pseudomonas aeruginosa GDP-4-keto-6-deoxy-D-mannose reductase which synthesizes GDP-rhamnose

Pseudomonas aeruginosa is an opportunistic Gram-negative bacterium that causes severe infections in a number of hosts from plants to mammals. A-band lipopolysaccharide of P. aeruginosa contains d-rhamnosylated O-antigen. The synthesis of GDP-D-rhamnose, the d-rhamnose donor in d-rhamnosylation, starts from GDP-D-mannose. It is first converted by the GDP-mannose-4,6-dehydratase (GMD) into GDP-4-keto-6-deoxy-D-mannose, and then reduced to GDP-D-rhamnose by GDP-4-keto-6-deoxy-D-mannose reductase (RMD). Here, we describe the enzymatic characterization of P. aeruginosa RMD expressed in Saccharomyces cerevisiae. Previous success in functional expression of bacterial gmd genes in S. cerevisiae allowed us to convert GDP-D-mannose into GDP-4-keto-6-deoxy-D-mannose. Thus, coexpression of the Helicobacter pylori gmd and P. aeruginosa rmd genes resulted in conversion of the 4-keto-6-deoxy intermediate into GDP-deoxyhexose. This synthesized GDP-deoxyhexose was confirmed to be GDP-rhamnose by HPLC, matrix-assisted laser desorption/ionization time-of-flight MS, and finally NMR spectroscopy. The functional expression of P. aeruginosa RMD in S. cerevisiae will provide a tool for generating GDP-rhamnose for in vitro rhamnosylation of glycoprotein and glycopeptides.

What can yeast tell us about N-linked glycosylation in the Golgi apparatus?

The N-glycans found on eukaryotic glycoproteins occur in a vast range of different structures. A universal N-glycan core is attached to proteins during synthesis in the endoplasmic reticulum, and then diversity is generated as the proteins pass through the Golgi apparatus. Many of the Golgi-localised glycosyltransferases have now been identified in both yeast and mammalian cells, but it is still unclear how these enzymes are integrated into the Golgi and the rest of the cell so as to ensure efficient and specific processing of passing substrates. This review discusses the potential of the yeast system to address these issues.