A novel screening method for cell wall mutants in Aspergillus niger identifies UDP-galactopyranose mutase as an important protein in fungal cell wall biosynthesis

GfsA encodes a novel galactofuranosyltransferase involved in biosynthesis of galactofuranose antigen of O-glycan in Aspergillus nidulans and Aspergillus fumigatus

The cells walls of filamentous fungi in the genus Aspergillus have galactofuranose (Galf)-containing polysaccharides and glycoconjugates, including O-glycans, N-glycans, fungal-type galactomannan and glycosylinositolphosphoceramide, which are important for cell wall integrity. Here, we attempted to identify galactofuranosyltransferases that couple Galf monomers onto other wall components in Aspergillus nidulans. Using reverse-genetic and biochemical approaches, we identified that the AN8677 gene encoded a galactofuranosyltransferase, which we called GfsA, involved in Galf antigen biosynthesis. Disruption of gfsA reduced binding of β-Galf-specific antibody EB-A2 to O-glycosylated WscA protein and galactomannoproteins. The results of an in-vitro Galf antigen synthase assay revealed that GfsA has β1,5- or β1,6-galactofuranosyltransferase activity for O-glycans in glycoproteins, uses UDP-d-Galf as a sugar donor, and requires a divalent manganese cation for activity. GfsA was found to be localized at the Golgi apparatus based on cellular fractionation experiments. ΔgfsA cells exhibited an abnormal morphology characterized by poor hyphal extension, hyphal curvature and limited formation of conidia. Several gfsA orthologues were identified in members of the Pezizomycotina subphylum of Ascomycota, including the human pathogen Aspergillus fumigatus. To our knowledge, this is the first characterization of a fungal β-galactofuranosyltransferase, which was shown to be involved in Galf antigen biosynthesis of O-glycans in the Golgi.

The transcriptional repressor TupA in Aspergillus niger is involved in controlling gene expression related to cell wall biosynthesis, development, and nitrogen source availability

The Tup1-Cyc8 (Ssn6) complex is a well characterized and conserved general transcriptional repressor complex in eukaryotic cells. Here, we report the identification of the Tup1 (TupA) homolog in the filamentous fungus Aspergillus niger in a genetic screen for mutants with a constitutive expression of the agsA gene. The agsA gene encodes a putative alpha-glucan synthase, which is induced in response to cell wall stress in A. niger. Apart from the constitutive expression of agsA, the selected mutant was also found to produce an unknown pigment at high temperatures. Complementation analysis with a genomic library showed that the tupA gene could complement the phenotypes of the mutant. Screening of a collection of 240 mutants with constitutive expression of agsA identified sixteen additional pigment-secreting mutants, which were all mutated in the tupA gene. The phenotypes of the tupA mutants were very similar to the phenotypes of a tupA deletion strain. Further analysis of the tupA-17 mutant and the ΔtupA mutant revealed that TupA is also required for normal growth and morphogenesis. The production of the pigment at 37°C is nitrogen source-dependent and repressed by ammonium. Genome-wide expression analysis of the tupA mutant during exponential growth revealed derepression of a large group of diverse genes, including genes related to development and cell wall biosynthesis, and also protease-encoding genes that are normally repressed by ammonium. Comparison of the transcriptome of up-regulated genes in the tupA mutant showed limited overlap with the transcriptome of caspofungin-induced cell wall stress-related genes, suggesting that TupA is not a general suppressor of cell wall stress-induced genes. We propose that TupA is an important repressor of genes related to development and nitrogen metabolism.

Elucidation of substrate specificity in Aspergillus nidulans UDP-galactose-4-epimerase

The frequency of invasive fungal infections has rapidly increased in recent years. Current clinical treatments are experiencing decreased potency due to severe host toxicity and the emergence of fungal drug resistance. As such, new targets and their corresponding synthetic pathways need to be explored for drug development purposes. In this context, galactofuranose residues, which are employed in fungal cell wall construction, but are notably absent in animals, represent an appealing target. Herein we present the structural and biochemical characterization of UDP-galactose-4-epimerase from Aspergillus nidulans which produces the precursor UDP-galactopyranose required for galactofuranose synthesis. Examination of the structural model revealed both NAD⁺ and UDP-glucopyranose were bound within the active site cleft in a near identical fashion to that found in the Human epimerase. Mutational studies on the conserved catalytic motif support a similar mechanism to that established for the Human counterpart is likely operational within the A. nidulans epimerase. While the K_m and k_cat for the enzyme were determined to be 0.11 mM and 12.8 s^-1, respectively, a single point mutation, namely L320C, activated the enzyme towards larger N-acetylated substrates. Docking studies designed to probe active site affinity corroborate the experimentally determined activity profiles and support the kinetic inhibition results.

Targeting UDP-galactopyranose mutases from eukaryotic human pathogens

UDP-Galactopyranose mutase (UGM) is a unique flavin-dependent enzyme that catalyzes the conversion of UDP-galactopyranose(UDP-Galp) to UDP-galactofuranose (UDP-Galf). The product of this reaction is the precursor to Galf, a major component of the cell wall and of cell surface glycoproteins and glycolipids in many eukaryotic and prokaryotic human pathogens. The function of UGM is important in the virulence of fungi, parasites, and bacteria. Its role in virulence and its absence in humans suggest that UGM is an ideal drug target. Significant structural and mechanistic information has been accumulated on the prokaryotic UGMs; however, in the past few years the research interest has shifted to UGMs from eukaryotic human pathogens such as fungi and protozoan parasites. It has become clear that UGMs from prokaryotic and eukaryotic organisms have different structural and mechanistic features. The amino acid sequence identity between these two classes of enzymes is low, resulting in differences in oligomeric states, substrate binding, active site flexibility, and interaction with redox partners. However, the unique role of the flavin cofactor in catalysis is conserved among this enzyme family. In this review, recent findings on eukaryotic UGMs are discussed and presented in comparison with prokaryotic UGMs.

Aspergillus nidulans galactofuranose biosynthesis affects antifungal drug sensitivity

The cell wall is essential for fungal survival in natural environments. Many fungal wall carbohydrates are absent from humans, so they are a promising source of antifungal drug targets. Galactofuranose (Galf) is a sugar that decorates certain carbohydrates and lipids. It comprises about 5% of the Aspergillus fumigatus cell wall, and may play a role in systemic aspergillosis. We are studying Aspergillus wall formation in the tractable model system, A. nidulans. Previously we showed single-gene deletions of three sequential A. nidulans Galf biosynthesis proteins each caused similar hyphal morphogenesis defects and 500-fold reduced colony growth and sporulation. Here, we generated ugeA, ugmA and ugtA strains controlled by the alcA(p) or niiA(p) regulatable promoters. For repression and expression, alcA(p)-regulated strains were grown on complete medium with glucose or threonine, whereas niiA(p)-regulated strains were grown on minimal medium with ammonium or nitrate. Expression was assessed by qPCR and colony phenotype. The alcA(p) and niiA(p) strains produced similar effects: colonies resembling wild type for gene expression, and resembling deletion strains for gene repression. Galf immunolocalization using the L10 monoclonal antibody showed that ugmA deletion and repression phenotypes correlated with loss of hyphal wall Galf. None of the gene manipulations affected itraconazole sensitivity, as expected. Deletion of any of ugmA, ugeA, ugtA, their repression by alcA(p) or niiA(p), OR, ugmA overexpression by alcA(p), increased sensitivity to Caspofungin. Strains with alcA(p)-mediated overexpression of ugeA and ugtA had lower caspofungin sensitivity. Galf appears to play an important role in A. nidulans growth and vigor.

Structural insight into the unique substrate binding mechanism and flavin redox state of UDP-galactopyranose mutase from Aspergillus fumigatus

UDP-galactopyranose mutase (UGM) is a flavin-containing enzyme that catalyzes the reversible conversion of UDP-galactopyranose (UDP-Galp) to UDP-galactofuranose (UDP-Galf). As in prokaryotic UGMs, the flavin needs to be reduced for the enzyme to be active. Here we present the first eukaryotic UGM structures from Aspergillus fumigatus (AfUGM). The structures are of UGM alone, with the substrate UDP-Galp and with the inhibitor UDP. Additionally, we report the structures of AfUGM bound to substrate with oxidized and reduced flavin. These structures provide insight into substrate recognition and structural changes observed upon substrate binding involving the mobile loops and the critical arginine residues Arg-182 and Arg-327. Comparison with prokaryotic UGM reveals that despite low sequence identity with known prokaryotic UGMs the overall fold is largely conserved. Structural differences between prokaryotic UGM and AfUGM result from inserts in AfUGM. A notable difference from prokaryotic UGMs is that AfUGM contains a third flexible loop (loop III) above the si-face of the isoalloxazine ring that changes position depending on the redox state of the flavin cofactor. This loop flipping has not been observed in prokaryotic UGMs. In addition we have determined the crystals structures and steady-state kinetic constants of the reaction catalyzed by mutants R182K, R327K, R182A, and R327A. These results support our hypothesis that Arg-182 and Arg-327 play important roles in stabilizing the position of the diphosphates of the nucleotide sugar and help to facilitate the positioning of the galactose moiety for catalysis.

Submerged conidiation and product formation by Aspergillus niger at low specific growth rates are affected in aerial developmental mutants

Exposure to an aerial environment or severe nutrient limitation induces asexual differentiation in filamentous fungi. Submerged cultivation of Aspergillus niger in carbon- and energy-limited retentostat cultures both induces and fuels conidiation. Physiological and transcriptomic analyses have revealed that this differentiation strongly affects product formation. Since conidiation is inherent in the aerial environment, we hypothesized that product formation near zero growth can be influenced by affecting differentiation or development of aerial hyphae in general. To investigate this idea, three developmental mutants (ΔfwnA, scl-1, and scl-2 mutants) that have no apparent vegetative growth defects were cultured in maltose-limited retentostat cultures. The secondary-metabolite profile of the wild-type strain defined flavasperone, aurasperone B, tensidol B, and two so far uncharacterized compounds as associated with conidium formation, while fumonisins B(2), B(4), and B(6) were characteristic of early response to nutrient limitation by the vegetative mycelium. The developmental mutants responded differently to the severe substrate limitation, which resulted in distinct profiles of growth and product formation. fwnA encodes the polyketide synthase responsible for melanin biosynthesis during aerial differentiation, and we show that conidial melanin synthesis in submerged retentostat cultures and aurasperone B production are fwnA dependent. The scl-1 and scl-2 strains are two UV mutants generated in the ΔfwnA background that displayed reduced asexual conidiation and formed sclerotium-like structures on agar plates. The reduced conidiation phenotypes of the scl-1 and scl-2 strains are reflected in the retentostat cultivation and are accompanied by elimination or severely reduced accumulation of secondary metabolites and distinctly enhanced accumulation of extracellular protein. This investigation shows that submerged conidiation and product formation of a mitosporic fungus cultured at low specific growth rates can be fundamentally affected by interfering with the genetic program for differentiation of aerial hyphae, opening new perspectives for tailoring industrial performance.

Quantifying the importance of galactofuranose in Aspergillus nidulans hyphal wall surface organization by atomic force microscopy

The fungal wall mediates cell-environment interactions. Galactofuranose (Galf), the five-member ring form of galactose, has a relatively low abundance in Aspergillus walls yet is important for fungal growth and fitness. Aspergillus nidulans strains deleted for Galf biosynthesis enzymes UgeA (UDP-glucose-4-epimerase) and UgmA (UDP-galactopyranose mutase) lacked immunolocalizable Galf, had growth and sporulation defects, and had abnormal wall architecture. We used atomic force microscopy and force spectroscopy to image and quantify cell wall viscoelasticity and surface adhesion of ugeAΔ and ugmAΔ strains. We compared the results for ugeAΔ and ugmAΔ strains with the results for a wild-type strain (AAE1) and the ugeB deletion strain, which has wild-type growth and sporulation. Our results suggest that UgeA and UgmA are important for cell wall surface subunit organization and wall viscoelasticity. The ugeAΔ and ugmAΔ strains had significantly larger surface subunits and lower cell wall viscoelastic moduli than those of AAE1 or ugeBΔ hyphae. Double deletion strains (ugeAΔ ugeBΔ and ugeAΔ ugmAΔ) had more-disorganized surface subunits than single deletion strains. Changes in wall surface structure correlated with changes in its viscoelastic modulus for both fixed and living hyphae. Wild-type walls had the largest viscoelastic modulus, while the walls of the double deletion strains had the smallest. The ugmAΔ strain and particularly the ugeAΔ ugmAΔ double deletion strain were more adhesive to hydrophilic surfaces than the wild type, consistent with changes in wall viscoelasticity and surface organization. We propose that Galf is necessary for full maturation of A. nidulans walls during hyphal extension.

Characterization of recombinant UDP-galactopyranose mutase from Aspergillus fumigatus

UDP-galactopyranose mutase (UGM) is a flavin-containing enzyme that catalyzes the conversion of UDP-galactopyranose to UDP-galactofuranose, the precursor of galactofuranose, which is an important cell wall component in Aspergillus fumigatus and other pathogenic microbes. A. fumigatus UGM (AfUGM) was expressed in Escherichia coli and purified to homogeneity. The enzyme was shown to function as a homotetramer by size-exclusion chromatography and to contain approximately 50% of the flavin in the active reduced form. A k(cat) value of 72 +/- 4 s(-1) and a K(M) value of 110 +/- 15 microM were determined with UDP-galactofuranose as substrate. In the oxidized state, AfUGM does not bind UDP-galactopyranose, while UDP and UDP-glucose bind with K(d) values of 33 +/- 9 microM and 90 +/- 30 microM, respectively. Functional and structural differences between the bacterial and eukaryotic UGMs are discussed.

Spatial and developmental differentiation of mannitol dehydrogenase and mannitol-1-phosphate dehydrogenase in Aspergillus niger

The presence of a mannitol cycle in fungi has been subject to discussion for many years. Recent studies have found no evidence for the presence of this cycle and its putative role in regenerating NADPH. However, all enzymes of the cycle could be measured in cultures of Aspergillus niger. In this study we have analyzed the localization of two enzymes from the pathway, mannitol dehydrogenase and mannitol-1-phosphate dehydrogenase, and the expression of their encoding genes in nonsporulating and sporulating cultures of A. niger. Northern analysis demonstrated that mpdA was expressed in both sporulating and nonsporulating mycelia, while expression of mtdA was expressed only in sporulating mycelium. More detailed studies using green fluorescent protein and dTomato fused to the promoters of mtdA and mpdA, respectively, demonstrated that expression of mpdA occurs in vegetative hyphae while mtdA expression occurs in conidiospores. Activity assays for MtdA and MpdA confirmed the expression data, indicating that streaming of these proteins is not likely to occur. These results confirm the absence of the putative mannitol cycle in A. niger as two of the enzymes of the cycle are not present in the same part of A. niger colonies. The results also demonstrate the existence of spore-specific genes and enzymes in A. niger.

Aspergillus nidulans UDP-glucose-4-epimerase UgeA has multiple roles in wall architecture, hyphal morphogenesis, and asexual development

Aspergillus nidulans UDP-glucose-4-epimerase UgeA interconverts UDP-glucose and UDP-galactose and participates in galactose metabolism. The sugar moiety of UDP-galactose is predominantly found as galactopyranose (Galp, the six-membered ring form), which is the substrate for UDP-galactopyranose mutase (encoded by ugmA) to generate UDP-galactofuranose (Galf, the five-membered ring form) that is found in fungal walls. In A. fumigatus, Galf residues appear to be important for virulence. The A. nidulans ugeA Delta strain is viable, and has defects including wide, slow growing, highly branched hyphae and reduced conidiation that resemble the ugmA Delta strain. As for the ugmA Delta strain, ugeA Delta colonies had substantially reduced sporulation but normal spore viability. Conidia of the ugeA Delta strain could not form colonies on galactose as a sole carbon source, however they produced short, multinucleate germlings suggesting they ceased to grow from starvation. UgeA purified from an expression plasmid had a relative molecular weight of 40.6 kDa, and showed in vitro UDP-glucose-4-epimerase activity. Transmission electron microscope cross-sections of wildtype, ugeA Delta, and ugmA Delta hyphae showed they had similar cytoplasmic contents but the walls of each strain were different in appearance and thickness. Both deletion strains showed increased substrate adhesion. Localization of UgeA-GFP and UgmA-GFP was cytoplasmic, and was similar on glucose and galactose. Neither gene product had a longitudinal polarized distribution. Localization of a UgmA-mRFP in a strain that resembled the ugmA Delta strain was cytoplasmic and lacked a longitudinal polarized distribution. The roles of UgeA in A. nidulans growth and morphogenesis are consistent with the importance of Galf, and are related but not identical to the roles of UgmA.

A single UDP-galactofuranose transporter is required for galactofuranosylation in Aspergillus fumigatus

Galactofuranose (Galf) containing molecules have been described at the cell surface of several eukaryotes and shown to contribute to the virulence of the parasite Leishmania major and the fungus Aspergillus fumigatus. It is anticipated that a number of the surface glycoconjugates such as N-glycans or glycolipids are galactofuranosylated in the Golgi apparatus. This raises the question of how the substrate for galactofuranosylation reactions, UDP-Galf, which is synthesized in the cytosol, translocates into the organelles of the secretory pathway. Here we report the first identification of a Golgi-localized nucleotide sugar transporter, named GlfB, with specificity for a UDP-Galf. In vitro transport assays established binding of UDP-Galf to GlfB and excluded transport of several other nucleotide sugars. Furthermore, the implication of glfB in the galactofuranosylation of A. fumigatus glycoconjugates and galactomannan was demonstrated by a targeted gene deletion approach. Our data reveal a direct connection between galactomannan and the organelles of the secretory pathway that strongly suggests that the cell wall-bound polysaccharide originates from its glycosylphosphatidylinositol-anchored form.

X-ray crystallography reveals a reduced substrate complex of UDP-galactopyranose mutase poised for covalent catalysis by flavin

The flavoenzyme uridine 5'-diphosphate galactopyranose mutase (UGM or Glf) catalyzes the interconversion of UDP-galactopyranose and UDP-galactofuranose. The latter is a key building block for cell wall construction in numerous pathogens, including Mycobacterium tuberculosis. Mechanistic studies of UGM suggested a novel role for the flavin, and we previously provided evidence that the catalytic mechanism proceeds through a covalent flavin-galactose iminium. Here, we describe 2.3 and 2.5 A resolution X-ray crystal structures of the substrate-bound enzyme in oxidized and reduced forms, respectively. In the latter, C1 of the substrate is 3.6 A from the nucleophilic flavin N5 position. This orientation is consistent with covalent catalysis by flavin.

Characterization of the Caenorhabditis elegans UDP-galactopyranose mutase homolog glf-1 reveals an essential role for galactofuranose metabolism in nematode surface coat synthesis

Galactofuranose (Gal(f)), the furanoic form of d-galactose produced by UDP-galactopyranose mutases (UGMs), is present in surface glycans of some prokaryotes and lower eukaryotes. Absence of the Gal(f) biosynthetic pathway in vertebrates and its importance in several pathogens make UGMs attractive drug targets. Since the existence of Gal(f) in nematodes has not been established, we investigated the role of the Caenorhabditis elegans UGM homolog glf-1 in worm development. glf-1 mutants display significant late embryonic and larval lethality, and other phenotypes indicative of defective surface coat synthesis, the glycan-rich outermost layer of the nematode cuticle. The glf homolog from the protozoan Leishmania major partially complements C. elegans glf-1. glf-1 mutants rescued by L. major glf, which behave as glf-1 hypomorphs, display resistance to infection by Microbacterium nematophilum, a pathogen of rhabditid nematodes thought to bind to surface coat glycans. To confirm the presence of Gal(f) in C. elegans, we analyzed C. elegans nucleotide sugar pools using online electrospray ionization-mass spectrometry (ESI-MS). UDP-Gal(f) was detected in wild-type animals while absent in glf-1 deletion mutants. Our data indicate that Gal(f) likely has a pivotal role in maintenance of surface integrity in nematodes, supporting investigation of UGM as a drug target in parasitic species.

Ligand binding and substrate discrimination by UDP-galactopyranose mutase

Galactofuranose (Galf) residues are present in cell wall glycoconjugates of numerous pathogenic microbes. Uridine 5'-diphosphate (UDP) Galf, the biosynthetic precursor of Galf-containing glycoconjugates, is produced from UDP-galactopyranose (UDP-Galp) by the flavoenzyme UDP-galactopyranose mutase (UGM). The gene encoding UGM (glf) is essential for the viability of pathogens, including Mycobacterium tuberculosis, and this finding underscores the need to understand how UGM functions. Considerable effort has been devoted to elucidating the catalytic mechanism of UGM, but progress has been hindered by a lack of structural data for an enzyme-substrate complex. Such data could reveal not only substrate binding interactions but how UGM can act preferentially on two very different substrates, UDP-Galp and UDP-Galf, yet avoid other structurally related UDP sugars present in the cell. Herein, we describe the first structure of a UGM-ligand complex, which provides insight into the catalytic mechanism and molecular basis for substrate selectivity. The structure of UGM from Klebsiella pneumoniae bound to the substrate analog UDP-glucose (UDP-Glc) was solved by X-ray crystallographic methods and refined to 2.5 A resolution. The ligand is proximal to the cofactor, a finding that is consistent with a proposed mechanism in which the reduced flavin engages in covalent catalysis. Despite this proximity, the glucose ring of the substrate analog is positioned such that it disfavors covalent catalysis. This orientation is consistent with data indicating that UDP-Glc is not a substrate for UGM. The relative binding orientations of UDP-Galp and UDP-Glc were compared using saturation transfer difference NMR. The results indicate that the uridine moiety occupies a similar location in both ligand complexes, and this relevant binding mode is defined by our structural data. In contrast, the orientations of the glucose and galactose sugar moieties differ. To understand the consequences of these differences, we derived a model for the productive UGM-substrate complex that highlights interactions that can contribute to catalysis and substrate discrimination.

Galactofuranose attenuates cellular adhesion of Aspergillus fumigatus

Galactofuranose (Galf) is a major molecule found in cell wall polysaccharides, secreted glycoproteins, membrane lipophosphoglycans and sphingolipids of Aspergillus fumigatus. The initial step in the Galf synthetic pathway is the re-arrangement of UDP-galactopyranose to UDP-Galf through the action of UDP-galactopyranose mutase. A mutant lacking the AfUGM1 gene encoding the UDP-galactopyranose mutase has been constructed. In the mutant, though there is a moderate reduction in the mycelial growth associated with an increased branching, it remains as pathogenic and as resistant to cell wall inhibitors and phagocytes as the wild-type parental strain. The major phenotype seen is a modification of the cell wall surface that results in an increase in adhesion of the mutants to different inert surfaces (glass and plastic) and epithelial respiratory cells. The adhesive phenotype is due to the unmasking of the mannan consecutive to the removal of galactofuran by the ugm1 mutation. Removal of the mannan layer from the mutant surface by a mannosidase treatment abolishes mycelial adhesion to surfaces.

Contribution of galactofuranose to the virulence of the opportunistic pathogen Aspergillus fumigatus

The filamentous fungus Aspergillus fumigatus is responsible for a lethal disease called invasive aspergillosis that affects immunocompromised patients. This disease, like other human fungal diseases, is generally treated by compounds targeting the primary fungal cell membrane sterol. Recently, glucan synthesis inhibitors were added to the limited antifungal arsenal and encouraged the search for novel targets in cell wall biosynthesis. Although galactomannan is a major component of the A. fumigatus cell wall and extracellular matrix, the biosynthesis and role of galactomannan are currently unknown. By a targeted gene deletion approach, we demonstrate that UDP-galactopyranose mutase, a key enzyme of galactofuranose metabolism, controls the biosynthesis of galactomannan and galactofuranose containing glycoconjugates. The glfA deletion mutant generated in this study is devoid of galactofuranose and displays attenuated virulence in a low-dose mouse model of invasive aspergillosis that likely reflects the impaired growth of the mutant at mammalian body temperature. Furthermore, the absence of galactofuranose results in a thinner cell wall that correlates with an increased susceptibility to several antifungal agents. The UDP-galactopyranose mutase thus appears to be an appealing adjunct therapeutic target in combination with other drugs against A. fumigatus. Its absence from mammalian cells indeed offers a considerable advantage to achieve therapeutic selectivity.