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State Key Laboratory of Microbial Resources, Institute of Microbiology, Chinese Academy of Sciences, Beijing 100101, PR China
Correspondence
Cheng Jin
jinc{at}sun.im.ac.cn
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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-Mannosidases play an important role in the processing of mannose-containing glycans in eukaryotes. A deficiency in -mannosidase is lethal in humans and cattle. In contrast to mammals, Saccharomyces cerevisiae does not require the endoplasmic reticulum -mannosidase gene for growth. However, little is known of the consequence of loss of function of class I -mannosidases in filamentous fungi. In this study, the msdS/AfmsdC gene was identified to encode 1,2--mannosidase MsdS in Aspergillus fumigatus. Soluble MsdS expressed in Escherichia coli was characterized as a typical class I -mannosidase. The msdS gene was deleted by replacement of the msdS gene with a pyrG gene. Although the mutant showed a defect in N-glycan processing, as well as a reduction of cell wall components and a reduced ability of conidiation, it appeared that the rate of hyphal growth was not affected. Morphology analysis revealed abnormal polarity and septation at the stages of germination, hyphal growth and conidiation. Although the mechanism by which the N-glycan processing affects polarity and septation is unclear, our results show that msdS is involved in polarity and septation in A. fumigatus.
The Genbank/EMBL/DDBJ accession number for the nucleotide sequence of msdS/AfmsdC is AY573554.
A table of LC-MS/MS data is available with the online version of this paper.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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-glucosidases, which remove the glucose residues, and it is followed by the action of various 1,2--mannosidases, which can remove one or more of the four 1,2--linked mannose residues. In mammalian cells, Man9GlcNAc2 is converted to Man5GlcNAc2 by the action of ER and Golgi -mannosidases, and Man5GlcNAc2 is the precursor for complex hybrid and N-glycans with a high mannose content (Kornfeld & Kornfeld, 1985
). In Saccharomyces cerevisiae, a specific ER 1,2--mannosidase converts Man9GlcNAc2 into Man8GlcNAc2, which is elongated in the Golgi to form an outer chain containing up to 200 mannose residues (Kukuruzinska et al., 1987; Tanner & Lehle, 1987). Aspergillus saitoi and Trichoderma reesei have been found to produce N-glycan structures containing five mannose units (Man5GlcNAc2), suggesting further processing of the Man9GlcNAc2 precursor (Chiba et al., 1993; Maras et al., 1997). Based on these observations, N-glycan synthesis in filamentous fungi seems to differ from that in yeast, and may be more similar to the processing in higher eukaryotes (Eades & Hintz, 2000).
The -mannosidases have been classified into two groups: class I and class II (Daniel et al., 1994; Eades et al., 1998; Moremen et al., 1994). Class I -mannosidases include ER Man9-mannosidase, endomannosidase and Golgi mannosidase I. Class II -mannosidases contain the lysosomal mannosidases, Golgi mannosidase II, yeast vacular mannosidase (Yoshihisa & Anraku, 1989, 1990), and ER -mannosidase II (Weng & Spiro, 1993, 1996). Several Golgi -mannosidases have been cloned and characterized from Pencillium citrinum (Yoshida & Ichishima, 1995; Yoshida et al., 1993), A. saitoi (Ichishima et al., 1999), Aspergillus oryzae (Akao et al., 2006), T. reesei (Maras et al., 2000), and Aspergillus nidulans (Eades & Hintz, 2000). These enzymes are monomeric, with a molecular mass of 50–60 kDa, and they show maximal activity in the semi-acidic condition (pH 4–6).
Aspergillus fumigatus is known to cause fatal invasive aspergillosis in immunocompromised patients (Latgé, 1999). The crude mortality rate for invasive aspergillosis is over 90 %, and falls to around 50–70 % if treatment is given (Steinbach et al., 2003). The main reason for patient death is the low efficiency of the drug therapies available to treat invasive aspergillosis, and the lack of an assay that can detect the fungus early during the infection. The fungal cell wall is essential for fungal life, and therefore it is a unique specific target for antifungal drug development. Many glycoproteins are directly or indirectly involved in the synthesis and organization of the fungal cell wall, and thus it is of importance to assess the role of N-glycan processing in trafficking, localization and function of proteins in A. fumigatus.
To investigate N-glycan processing in A. fumigatus, a gene, which was previously designated AfmsdC and is now annotated as msdS, was identified to encode a class I 1,2--mannosidase. In this report, MsdS/AfMsdC was expressed and characterized. Also, the phenotypes associated with the deletion of the msdS/AfmsdC gene were analysed.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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). A. fumigatus strain CEA17 (Weidner et al., 1998), a kind gift from C. d'Enfert, Institut Pasteur, Paris, France, was propagated at 37 °C on YGA (0.5 % yeast extract, 2 % glucose, and 1.5 % Bacto-agar), complete medium (Cove, 1966), or minimal medium with 0.5 mM sodium glutamate as a nitrogen source (Cove, 1966). Uridine and uracil were added to the media at a concentration of 5 mM when CEA17 or a complemented strain was grown. Mycelium was harvested from strains grown in complete liquid medium at 37 °C, with shaking at 250 r.p.m. At the specified culture time point, mycelium was harvested, washed with distilled water, frozen in liquid N2, and then ground by hand. The powder was stored at –70 °C for DNA, RNA and protein extraction. Conidia were prepared by growing A. fumigatus strains on solid complete medium with uridine and uracil (CMU) for 48 h at 37 °C. The spores were collected, washed twice with 0.01 % Tween-20 in physiological saline, and resuspended in 0.01 % Tween-20 in saline. The concentration of spores was confirmed by haemocytometer counting and viable counting. Vectors and plasmids were propagated in Escherichia coli DH5 (BRL).
Computer analysis.
Sequence analysis of cDNA clones and multiple sequence alignments were performed using OMIGA v2.0, and a BLAST search was performed.
Molecular cloning of A. fumigatus msdS.
The AfmsdC/msdS genomic sequence was identified in a search of the A. fumigatus genome database (http://www.tigr.org/tdb/e2k1/afu1/), using a TBLASTN program to search for sequences corresponding to the conserved amino acid sequence of Penicillium citrinum msdC that were homologous between A. nidulans, S. cerevisiae and human. A 1.6 kb genomic DNA fragment was found to contain the entire ORF. Based on the nucleotide sequence, the forward primer (5'-ATGCATTTACCCTCTTTGTCC-3') and the reverse primer (5'-TCACGTATGATGAATTCGGAC-3') were designed for cloning the cDNA of AfmsdC by PCR. The PCR products were subcloned into pGEM-T easy Vector (Promega), sequenced (T-msdC), and the position of the intron was determined by comparing the cDNA with the genomic sequence.
Expression and purification of MsdS.
The sequence encoding soluble MsdS (sMsdS) was amplified from T-msdC by truncation of the coding region for the transmembrane domain. Expression of sMsdS fused with a His-tag in E. coli was done by following Novagen protocols. The plasmid construct pET-msdC was introduced into E. coli BL21(DE3). The recombinant strain was induced by the addition of 0.4 mM IPTG at 27 °C. After induction, cell extracts were prepared, and separated by metal chelation affinity chromatography (Novagen). Proteins were analysed by SDS-PAGE electrophoresis, and recombinant protein was identified by Western blotting with a His-tag-specific antibody.
Construction of the msdS null mutant and the complemented strain.
To delete msdS, a deletion construct was designed to replace the entire coding region of msdS with a pyrG cassette by homologous recombination (d'Enfert, 1996). PCR primers were designed to amplify a 1.8 kb upstream non-coding region of the msdS before the ATG start codon (5' primer pair: 5'-ACGCGTCGAC GCGGCCGCTCAGCTTGACTGAGAGAGGAG-3' and 5'-GGTGGTGATATCGATACCGACCAACGAAAGAAT-3'; the SalI, NotI and EcoRV restriction sites are underlined, respectively), and a 1.8 kb downstream non-coding region of the msdS after the stop codon (3' primer pair: 5'-GGTGGTGATATCTGTACATACCCTAGCTGGCT-3'and 5'-GCTCTAGAGAGCTGCACCACAAAAAGCAC-3'; the EcoRV and XbaI restriction sites are underlined, respectively). These PCR fragments were cloned into the relevant sites of pBlueScript II SK. The pyrG-blaster cassette (8.6 kb), released by the digestion of pCDA14 (d'Enfert, 1996) with HpaI, was cloned into the site between the up- and downstream non-coding regions of the msdS, to yield the deletion construct pBK70. At a unique NotI site, the linearized pBK70 was transformed into strain CEA17 by protoplast transformation (Yelton et al., 1984), and screened for mutants with uridine and uracil autotrophy. The deletions in the mutants were confirmed by PCR and Southern blotting.
The complemented strain was constructed by the replacement of pyrG with a wild-type copy of msdS in the 
msdS mutant. msdS, with its 1.8 kb upstream and 1.8 kb downstream non-coding regions, was amplified by PCR (primer pair: 5'-TCAGCTTGACTGAGAGAGGAG-3' and 5'-GAGCTGCACCACAAAAAGCAC-3'), and the product was cloned into pGEM-T Easy Vector, and sequenced. The resulting plasmid T-long70 was then linearized with XhoI, before transformation into the msdS null mutant. The transformants were chosen by PCR, and then the transformation was confirmed by Southern blot analysis, using the upstream non-coding region as a probe. The probe was labelled by following the protocol of the DIG-labelled hybridization kit (catalogue no. 1093657; Roche Applied Science).
Assay for 1,2--mannosidase activity.
The standard assay mixture containing 50 µg Man(1,2)Man-OMe and the enzyme, in a total volume of 100 µl with 10 mM sodium acetate (pH 5.5), was incubated at 37 °C. The reaction was terminated by heating, and the amount of reducing sugars was determined (Matta & Bahl, 1972). One unit of 1,2--mannosidase is defined as the amount of enzyme that releases 1 µmol mannose per hour at 37 °C. A reaction mixture without enzyme was used as a control in each instance.
For the substrate specificity assay, 10 µg purified sMsdS was incubated with 100 pmol Man9GlcNAc2 (Sigma) or Man8GlcNAc2 (Calbiochem) at 37 °C for 10–360 min. The reaction was terminated by heating at 100 °C for 5 min. Upon addition of 2 vols ethanol, proteins in the reaction mixture were precipitated, and removed by centrifugation. The supernatant was analysed with a CarboPac PA-100 column and a high-performance anion-exchange chromatography pulsed ampere detector (HPAEC-PAD; Dionex). The products were eluted with 250 mM NaOH at a flow rate of 1.0 ml min–1. Man7GlcNAc2, Man6GlcNAc2 and Man5GlcNAc2 (Calbiochem) were used as standards.
The -mannosidase activity of the mutant was assayed as follows: the mutant was grown on a shaker (250 r.p.m.) in 100 ml liquid CMU medium at 37 °C for 48 h. The mycelium was filtered, washed extensively with water, and ground in liquid nitrogen. The powder was dissolved in 10 mM sodium acetate (pH 5.5). After removal of cell debris by centrifugation at 12 000 g at 4 °C for 10 min, the supernatant was used as a crude enzyme for the activity assay, under standard conditions.
Western blotting of ChiB.
Chitinase ChiB, secreted by A. fumigatus, was induced and purified as previously described (Xia et al., 2001). Anti-ChiB antibody developed in mouse was used in Western blotting. Proteins in the cell lysate or culture supernatant were run on a 12 % SDS-PAGE gel, and transferred to PVDF (Bio-Rad) at 300 mA for 1.5 h. The anti-ChiB mouse serum was diluted at 1 : 5000. Protein was detected with the enhanced chemoluminescence substrate (Pierce) and autoradiography on film.
Phenotypic analysis of the mutant.
Growth kinetics of A. fumigatus strains were assayed as follows. A 100 µl slurry of spores (1x109 ml–1) was inoculated into 100 ml liquid CMU medium. After incubation at 37 °C with shaking (200 r.p.m.), three 1 ml aliquots of liquid were taken for each strain at set time intervals, and dried and weighed. The mean weight was used to plot the growth kinetics. The experiment was repeated three times.
To test the sensitivities of the mutant to antifungal reagents, conidiospores were collected from the wild-type, the mutant and the complemented strain, and, for each strain, similar numbers of conidiospores were spotted on CMU plates in the presence of 100 µg Calcofluor white ml–1, 250 µg Congo red ml–1 or 60 µg SDS ml–1. After incubation at 37 or 50 °C for 24–48 h, the plates were photographed.
For examination of conidial germination, 20 ml complete liquid medium was inoculated with 107 freshly harvested conidia, poured into a Petri dish containing a glass coverslip, and incubated at 37 °C for the time indicated in each experiment. At the specified times, the coverslips with adhering germlings were removed, and spore germination was observed and counted under differential interference contrast microscopy.
For examination of nuclei, septa and cell wall staining at the germination stage, the coverslips with adherent germlings were removed, and fixed in fixative solution (4 % formaldehyde, 50 mM phosphate buffer, pH 7.0, and 0.2 % Triton X-100) for 30 min. Coverslips were then washed with PBS, incubated for 15 min with 1 µg 4',6-diamidino-2-phenylindole (DAPI) ml–1 (Sigma), washed with PBS, and then incubated for 5 min with a 10 mg ml–1 solution of fluorescent brightener 28 (Sigma), washed again, and germlings were photographed using a microscope.
For examination of nuclei, septa and cell wall staining at the conidiation stage, 100 ml complete liquid medium was inoculated with 106 conidia, and incubated with shaking (200 r.p.m.) at 37 °C for 17 h. The mycelia were taken, and placed on a glass coverslip. The glass coverslip was then put in a Petri dish containing two layers of filter paper saturated with complete liquid medium. After incubation at 37 °C for 2–8 h, the coverslip was removed, and stained with DAPI and fluorescent brightener.
A 100 ml volume of complete liquid medium was inoculated with 108 freshly harvested conidia, and incubated at 37 or 50 °C with shaking (200 r.p.m.) for 12 h. Mycelia were harvested by filtering the culture through two layers of Miracloth (Calbiochem), washed twice with distilled water, and streaked on minimal medium agar. After incubation at 37 or 50 °C for 2, 4, 6 and 8 h, mycelia were suspended in 5 ml distilled water, and conidial production was expressed as the mean number of conidia per volume (millilitre).
Conidia or mycelia produced on solid complete medium were fixed with 2.5 % glutaraldehyde and 1 % osmium tetroxide, and then examined with a Quanta 200 scanning electron microscope (FEI). Conidia were fixed in 2.5 % glutaraldehyde in 0.1 M phosphate buffer, pH 7.0, for 4 h, or overnight at 4 °C. Hyphal cells were fixed in 2.5 % glutaraldehyde in 0.1 M phosphate, washed three times with 0.1 M phosphate, post-fixed in 1 % osmium tetroxide, incubated for 2–4 h in 0.1 M phosphate, and the cells were then treated for 15–20 min in each of 30, 50, 70, 85, 95 and 100 % methanol, and post-fixed in 2 % uranyl acetate/30 % methanol. Cells were rinsed, dehydrated, and embedded in Epon 812 by the floating sheet method. The sections were examined with an H-600 electron microscope (Hitachi).
Chemical analysis of the cell wall.
Conidia were inoculated into 100 ml complete medium at a concentration of 106 conidia ml–1 and incubated at 37 °C with shaking (200 r.p.m.) for 48 h. The mycelium was harvested by filtering the culture through two layers of Miracloth, washed twice with distilled water, and lyophilized. Three aliquots of 10 mg dry mycelium were used as independent samples for cell wall analysis, and the experiment was repeated twice. To remove unbound cell wall proteins and water-soluble sugar, each sample was boiled for 5 min in 2 ml 2 % SDS in 50 mM Tris/HCl buffer supplemented with 100 mM Na-EDTA, 40 mM β-mercaptoethanol, and 1 mM PMSF (Elorza et al., 1985; Hearn & Sietsma, 1994; Schoffelmeer et al., 1999). Mannoprotein was extracted with 3 % NaOH at 75 °C for 1 h, and quantitatively determined by using the Lowry protein assay (Lowry et al., 1951). Glucan and chitin were digested in 96 % formic acid at 100 °C for 4 h. Formic acid was evaporated by lyophilization, and the residues were dissolved in 1 ml distilled water. Glucan and chitin were estimated by measuring the released glucose and N-acetylglucosamine after digestion. Glucose was measured by the phenol–sulfuric acid method (Dubois et al., 1956). N-Acetylglucosamine was measured by the method described by Lee et al. (2005).
Glycan analysis by HPAEC-PAD.
The chitinase ChiB used for N-glycan analysis was induced from the wild-type, the mutant and the complemented strain of A. fumigatus as previously described (Xia et al., 2001). The ChiB in the culture supernatant was precipitated with 30–60 % ammonium sulfate. For comparison of N-glycans from different strains, the partially purified ChiB of the wild-type, the mutant and the complemented strain was run on SDS-PAGE, and then transferred to PVDF. The ChiB on PVDF was recovered, and treated with PNGase F (NEB) at 37 °C for 48 h. Upon the addition of 2 vols ethanol, the proteins were precipitated, and they were then removed by centrifugation. The supernatant was analysed with a CarboPac PA-100 column and HPAEC-PAD. The glycans were eluted with 250 mM NaOH at a flow rate of 1.0 ml min–1. Man8GlcNAc2 and Man6GlcNAc2 were used as standards.
Analysis of virulence of the mutant.
The wild-type and mutant strains were used for experimental infections in white male BALB/c mice (18–20 g). Conidia were suspended in 0.01 % Tween-20 in saline to give a challenge inoculum of 3x105 c.f.u. (g body wt)–1 in a 30 µl volume. Mice were immunosuppressed by intraperitoneal injection of cyclophosphamide [150 mg (kg body wt)–1] on days –3 and –1, and one subcutaneous injection of hydrocortisone acetate [40 mg (kg body wt)–1] on day –1. On day 0, mice were anaesthetized by the inhalation of diethyl ether, and infected intranasally with 30 µl spore suspension containing 6x106 conidia. A concurrent control group consisted of mice that had been immunosuppressed, and then inoculated with 30 µl 0.01 % Tween 20 in saline. Immunosuppression was prolonged by cyclophosphamide injections [150 mg (kg body wt)–1] on days 3, 6 and 9. Mice were kept in sterile cages with filter tops, and they received sterile food and bedding. Tetracycline (1 mg ml–1) was added to the drinking water, which was changed twice daily. Four groups, each containing 20 mice, were inoculated, monitored twice daily for 30 days after inoculation, and mortality was recorded. Mice surviving the course of the experiment were killed humanely on day 30. The survival rate was analysed statistically by using the methods of Kaplan–Meier, with SPSS13 software. P values of <0.05 were considered significant in this analysis.
Proteins secreted by the msdS mutant.
The A. fumigatus strains were incubated in the complete liquid medium at 37 °C at 250 r.p.m. for 60 h. The secreted proteins in the culture medium were precipitated from the culture filtrates by the addition of 4 vols ice-cold acetone, and the culture filtrates were kept at 4 °C overnight. The pellet was then recovered by centrifugation, resuspended in water, and separated by PAGE. After staining with Coomassie brilliant blue R-250, the protein bands of interest were cut, and analysed by liquid chromatography/tandem MS (LC-MS/MS).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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) of the A. fumigatus genome database was performed with the protein sequence of the MsdC (P31723) of P. citrinum, and one gene, AfmsdC/msdS, was found. The msdS gene consists of a total of 1677 nt, and contains three introns. Its cDNA encodes a polypeptide of 504 aa with an estimated molecular mass of 55 kDa. The predicted sequence of MsdS exhibits a high score of amino acid identity with other class I -mannosidases: 78.1 % with A. oryzae -mannosidase, 73.5 % with A. saitoi -mannosidase, 72.3 % with A. nidulans -mannosidase IB, and 70.7 % with P. citrinum MsdC.
sMsdS was expressed and purified, as described in Methods. As shown in Fig. 1, a 55 kDa protein was induced. After purification on a nickel column, sMsdS was purified to homogeneity, and exhibited a specific activity of 6.3 U mg–1 toward Man(1,2)Man-OMe.
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-mannoside. Using Man(1,2)Man-OMe as a substrate, the maximal activity (mean±SD, 9.15±0.26 units; assigned a relative activity of 100 %) of sMsdS occurred at 37–40 °C in 10 mM sodium acetate buffer (pH 5.5), and more than 80 % of the maximal activity was detected at 30–50 °C. Addition of 1 mM 1-deoxymannojirimycin to the standard assay mixture caused a complete loss of the -mannosidase activity of sMsdS, while swainsonine, a class II -mannosidase inhibitor, had no effect on its activity (9.02±0.15; 98.6 % relative activity); these results indicate that MsdS is a class I mannosidase. Addition of 1 mM EDTA resulted in a small decrease in activity (8.08±0.29 units; 88.3 % relative activity). sMsdS was able to degrade Man9GlcNAc2 and Man8GlcNAc2 to produce a final mixture of major Man5GlcNAc2 and minor Man6GlcNAc2. It appeared that Man8GlcNAc2 was the better substrate, since it was completely degraded within 30 min, while minor Man9GlcNAc2 was detected in the reaction mixture after incubation for 60 min (Fig. 2).
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). Southern analysis of the EcoRV-digested genomic DNA of the mutant demonstrated that the 5.8 kb EcoRV fragment in the wild-type had been converted into a 6.5 kb EcoRV fragment (Fig. 3b). These results clearly demonstrated that the msdS gene was replaced by a pyrG gene in the msdS mutant. To ensure that all phenotypes noted for the msdS strain were the result of the specific deletion of msdS, the complemented strain was constructed by reintroduction of a wild-type copy of msdS directly into the mutated locus, under the control of its own promoter. The transformation of the complemented strain was also confirmed by PCR and Southern blot (Fig. 3).
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-Mannosidase activity of the msdS mutant was determined by measuring the mannose released from Man(1,2)Man-OMe (see Methods). The mean (SD) -mannosidase activity of the msdS mutant was 1.31±0.07 units mg–1, which was 44 % of the activity of the wild-type (2.98±0.23 units mg–1). The complemented strain showed an activity of 2.16±0.07 units mg–1. The MsdS that was secreted by the wild-type and the complemented strain was not found in the culture supernatant of the msdS mutant (Fig. 4a). Therefore, the activity detected in the mutant could be due to other -mannosidases able to cleave Man(1,2)Man-OMe. In addition, the proteins, such as ChiB and Alg1, secreted by the msdS mutant were identified to be larger than their counterparts secreted by the wild-type or the complemented strain (see supplementary Table S1, available with the online version of this paper). Western blotting revealed that only one ChiB band was detected in the intracellular and secreted preparations of the mutant, and this band was slightly larger than the those from the wild-type and the complemented strain (Fig. 4b). Moreover, the N-glycan released from the ChiB secreted by the mutant was determined to be Man8GlcNAc2, whereas the N-glycan from the wild-type and the complemented strain was Man6GlcNAc2 (Fig. 5). These results indicate that deletion of msdS in A. fumigatus leads to a complete loss of MsdS activity, and a defect in N-glycan processing. However, it appeared that msdS was not required for protein secretion, at least for secretion of ChiB.
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msdS mutant, the hyphal growth of the msdS mutant was similar to that of the wild-type or the complemented strain. When the mutant was grown in the presence of Calcofluor white or Congo red, hyphal growth was not affected at 37 °C. However, the mutant showed a slightly increased sensitivity to Calcofluor white and Congo red at 50 °C. Also, the mutant was not sensitive to SDS, as compared with the wild-type strain. These observations demonstrated that the cell wall integrity of the mutant was slightly affected at a higher temperature.
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-glucan, mannoprotein, β-glucan and chitin in the mycelial cell wall of the mutant grown at 37 or 50 °C was reduced to some extent (10–27 %) compared with the wild-type and the complemented strain (Table 1). Interestingly, when the temperature was elevated to 50 °C, the wild-type showed increases of 15, 47 and 34 % for -glucan, β-glucan and chitin, respectively, and mannoprotein was decreased by 17 %, as compared with cultivation at 37 °C. Also at 50 °C, the mutant exhibited increases of 25, 33 and 55 % in -glucan, β-glucan and chitin, respectively, and a decrease of 13 % in mannoprotein, as compared with growth at 37 °C. These data suggest that an elevated temperature could induce an increase of -glucan and chitin in the cell wall of the mutant. This is probably the reason that the mutant showed a minor temperature-sensitive defect in cell wall integrity. Electron microscopic analysis revealed that the mycelial cell wall of the mutant grown at 37 °C was normal, while the cell wall of the mutant at 50 °C was less dense, as judged by electron density, but there was no change in thickness (Fig. 7a).
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), a severe defect in metulae formation of the mutant was observed at 50 °C (Fig. 7c). Conidia counting confirmed that the mean ± SD (x107) conidia count of the mutant at 37 °C was 105±13, and this was one-third of that of the wild-type strain (326±19). The number of conidia produced by the mutant at 50 °C was 0.4±0.1, which was only 3 % of that produced by the wild-type (12±1). These results suggest a severe defect in conidiation of the mutant, especially at a higher temperature. Conidia counts for the complemented strain were 247±22 and 11±0.9, and for CEA17 they were 273±13 and 10.6±0.7, at 37 and 50 °C, respectively.
Moreover, in the immunocompromised mouse model, the difference in virulence between the wild-type and the msdS mutant was not statistically significant (P>0.05). Taking these reults together, we concluded that the msdS gene was not essential for the growth and virulence of A. fumigatus, and that deletion of this gene led to a defect in conidia formation, and a minor defect in cell wall integrity at 50 °C.
Morphogenesis of the msdS mutant
In general, a filamentous fungus initiates its life cycle from conidial germination, and terminates it with conidiation. When the conidia break dormancy, nuclear division is accompanied by a series of ordered morphological events, including the switch from isotropic to polar growth, the emergence of second germ tubes from the conidia, and septation. In A. fumigatus, it has been shown that the switch from isotropic to polar growth precedes the first mitosis during the early stage of germination. The earliest emergence of second germ tubes from the conidia occurs after the third mitotic division, and the first septation usually occurs in germlings that have undergone four rounds of mitosis (Momany & Taylor, 2000).
As shown in Fig. 8, when incubated at 37 °C in rich medium containing glucose as the carbon source, the wild-type conidium germinated in a typical bi-polar pattern at an angle of 18 °, and the second germ tube and the first septation occurred after four rounds of mitosis (7–8 h). The septum formed at the ‘neck’ site near the region of what was once the conidium. In comparison with the wild-type, the earliest emergence of the second germ tube occurred in the msdS mutant after the second mitotic division (5 h) at an angle of 12 °, and the third germ tube, or branching of the germling, was found after the third or fourth nuclear division (6–7 h). After four rounds of mitotic division, some germinated conidia of the mutant were not able to form a septum, while for those conidia that could form a septum, it was usually formed at the neck site of the newly emerged germ tube or germling, instead of the neck site of the first germling, as in the wild-type strain. Moreover, the germling of the mutant was more swollen than that of the wild-type. As summarized in Table 2, after two rounds of mitotic division (5 h), over 15 % of the conidia of the mutant formed a second germ tube, while about 30 % were found with a third germ tube after four rounds of mitosis (6 h). About 40 % of conidia of the mutant formed a fourth germ tube or branching after four rounds of mitosis. These results clearly demonstrated that deletion of the msdS gene led to random budding and septation at an early stage of germination.
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), suggesting reduced polar growth and septation during hyphal growth.
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, the mutant was able to form a conidiophore vesicle; however, this vesicle contained more nuclei and formed less metulae than the vesicle of the wild-type, and this again suggested aberrant polarity and septation at an early stage of conidiation. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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-Mannosidase I is known to play an important role in the processing of mannose-containing glycans. In humans (Rose, 1967) and cattle (Burditt et al., 1978; Norden et al., 1973; Phillips et al., 1974), a deficiency in -mannosidase can result in the lethal disease mannosidosis. In Drosophila melanogaster, deletion of the Golgi mannosidase I (MAS-1) is viable, and the null organisms synthesize the same range of oligosaccharides as the wild-type, albeit with different ratios (Roberts et al., 1998). In S. cerevisiae, disruption of the ER -mannosidase gene does not prevent outer chain synthesis (Puccia et al., 1993). These observations suggest that the consequences of -mannosidase I activity vary in different species.
In filamentous fungi, investigations into -mannosidases have mostly concentrated on characterization, and their role in processing N-glycans. Little is known about their physiological importance in filamentous fungi. Our investigation of -mannosidase in A. fumigatus was initiated before the completion of the genome sequencing of A. fumigatus, and three genes were identified (including AY573554, AY852252 and AY852253). Since we originally identified the putative class I -mannosidase gene (AY573554) by a TBLASTN search with the MsdC (P31723) of P. citrinum, we named the gene AfmsdC. In the latest release of the TIGR database (www.tigr.org/tdb/e2k1/afu1/) (Galagan et al., 2005), nine genes are annotated to encode -mannosidases, and these include: XP_749038.1, XP_754794.1, XP_751252.1, XP_751819.1, XP_752444.1, XP_752825.1, XP_753592.1, XP_751114.1 and XP_750572.1. Among these mannosidases, MsdS (XP_752825.1) has an overall protein sequence identity of 98 % with AfMsdC. Therefore, the gene reported here is referred to as AfmsdC/msdS.
In this study, we showed that 1-deoxymannojirimycin was able to completely inhibit MsdS, while swainsonine had no effect. We found that sMsdS could act on all 1,2 linkages in Man9GlcNAc2 and Man8GlcNAc2, to produce a final product of Man5GlcNAc2, and that deletion of the msdS gene resulted in conversion of the N-glycan on mature ChiB from Man8GlcNAc2 to Man6GlcNAc2, suggesting that MsdS hydrolyses Man8GlcNAc2 to yield Man6GlcNAc2.
Although 44 % of the -mannosidase activity of the wild-type was detected in the mutant, there were several lines of evidence to show that the mutant was completely devoid of the MsdS: (i) PCR and Southern blotting analysis confirmed that the msdS gene was deleted in the mutant; (ii) the substrate we used for activity assay was Man(1,2)Man-OMe, which is not a specific substrate for class I -mannosidase, and it can be cleaved by other -mannosidases found in A. fumigatus; (iii) the MsdS detected in the culture supernatant of the wild-type was missing in the supernatant of the mutant; and (iv) the proteins secreted by the mutant were larger than their counterparts secreted by the wild-type or the complemented strain. Indeed, the N-glycan on mature ChiB secreted by the mutant was Man8GlcNAc2, instead of Man6GlcNAc2, as in the wild-type and the complemented strain.
Although we observed a reduction in a number of cell wall components, and a reduced ability of conidia formation, it was apparent that deletion of msdS had no effect on the growth and virulence.
O-Mannosylation has been shown to be involved in polarized growth of A. nidulans. The swoA mutant has been identified as a single-locus temperature-sensitive mutant that fails to switch from isotropic to polar growth. At 42 °C, each cell is swollen, and contains 64 or more nuclei after a 14 h period of growth. During growth at a restrictive temperature, multiple points of polarity are established, but polar growth can not be maintained (Momany et al., 1999). The swoA mutant can be complemented by pmtA, which is the gene encoding the Pmt2 subfamily O-mannosyltransferase. Disruption of pmtA leads to a phenotype identical to that of the swoA mutant (Shaw & Momany, 2002). In addition, a hypersensitivity to Congo red and a decrease in conidia formation are observed (Oka et al., 2004). Similarly, screening of the polarity-defective mutants in Neurospora crassa has led to the identification of two genes in the mannosylation pathway: alg-1 and sec-53, which encode 1,4-β-mannosyltransferase (ALG1) and phosphomannomutase (SEC53), respectively (Seiler & Plamann, 2003). Recently, a polarity-defective phenotype has also been observed by disruption of the AapmtA gene in Aspergillus awamori (Oka et al., 2005). These observations clearly demonstrate the involvement of the O-mannosylation in cell wall integrity and polarized growth in filamentous fungi. More recently, we have shown that the Afpmt1, a gene encoding the Pmt1 subfamily O-mannosyltransferase 1, is crucial for cell wall integrity and conidia morphology. However, disruption of the Afpmt1 gene does not affect polarized growth of A. fumigatus (Zhou et al., 2007), suggesting that proteins involved in the polarity of filamentous fungi might be O-mannosylated by Pmt2 subfamily O-mannosyltransferase.
For what is believed to be the first time, we showed that the N-glycan processing was involved in the polarity of A. fumigatus. However, deletion of the msdS gene did not give a phenotype identical to A. nidulans mutants with defective O-mannosylation. The msdS mutant showed a slight increase in sensitivity to Congo red and Calcofluor white at a higher temperature. Analysis of morphogenesis revealed that the mutant displayed a random emergence of germ tube and septum formation at an early stage of germination, swollen and multinucleate basal cells and hyphal tips during hyphal growth, and multinucleate conidiospore vesicles and reduced metulae formation, suggesting abnormalities of polarity establishment and septation during development of the mutant. Since N-glycosylation of proteins is known to play a variety of roles, such as protein folding, trafficking, localization and function, the phenotypes associated with the msdS mutant could be explained as: (i) the proteins that are required for polar growth and septation are substrates of MsdS, and require N-glycosylation for their correct localization and function; (ii) for the proteins that are involved in cell wall synthesis and organization, their localization and function are less dependent on N-glycosylation, as compared with O-mannosylation. Obviously, an understanding of the role of the msdS in polar growth and septation will depend on identification of the substrates of MsdS, and their roles in the cell.
It is not surprising that both the complemented strain and CEA17 displayed phenotypes similar to the mutant when they were used as controls. Both the complemented strain and CEA17 are devoid of the pyrG gene that encodes an orotidine-5'-phosphate decarboxylase, which is an enzyme that catalyses the last step of de novo UMP biosynthesis, which is a precursor for synthesis of glycoproteins. It has been shown that uridine/uracil deprivation in A. fumigatus CEA17 results in a low rate of conidium swelling, and in the inability of the conidia of the mutant to produce germ tubes (d'Enfert, 1996). Therefore, we attributed the abnormal polarity of the complemented strain to the depletion of glycoprotein synthesis, which then led to abnormal polarity.
In conclusion, we have shown that the msdS gene is not essential for growth and virulence of A. fumigatus. Although the mechanism remains unclear, our results clearly show that -mannosidase I activity of MsdS is involved in polarity and septation in A. fumigatus.
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ACKNOWLEDGEMENTS |
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Edited by: S. D. Harris
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REFERENCES |
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Received 14 February 2008;
revised 25 March 2008;
accepted 25 March 2008.
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