Phosphorylation-dependent regulation of amidotransferase during the development of Blastocladiella emersonii

Cell compensatory responses of fungi to damage of the cell wall induced by Calcofluor White and Congo Red with emphasis on Sporothrix schenckii and Sporothrix globosa. A review

The cell wall (CW) of fungi exhibits a complex structure and a characteristic chemical composition consisting almost entirely of interacting crystalline and amorphous polysaccharides. These are synthesized by a number of sugar polymerases and depolymerases encoded by a high proportion of the fungal genome (for instance, 20% in Saccharomyces cerevisiae). These enzymes act in an exquisitely coordinated process to assemble the tridimensional and the functional structure of the wall. Apart from playing a critical role in morphogenesis, cell protection, viability and pathogenesis, the CW represents a potential target for antifungals as most of its constituents do not exist in humans. Chitin, β-glucans and cellulose are the most frequent crystalline polymers found in the fungal CW. The hexosamine biosynthesis pathway (HBP) is critical for CW elaboration. Also known as the Leloir pathway, this pathway ends with the formation of UDP-N-GlcNAc after four enzymatic steps that start with fructose-6-phosphate and L-glutamine in a short deviation of glycolysis. This activated aminosugar is used for the synthesis of a large variety of biomacromolecules in a vast number of organisms including bacteria, fungi, insects, crustaceans and mammalian cells. The first reaction of the HBP is catalyzed by GlcN-6-P synthase (L-glutamine:D-fructose-6-phosphate amidotransferase; EC 2.6.1.16), a critical enzyme that has been considered as a potential target for antifungals. The enzyme regulates the amount of cell UDP-N-GlcNAc and in eukaryotes is feedback inhibited by the activated aminosugar and other factors. The native and recombinant forms of GlcN-6-P synthase has been purified and characterized from both prokaryotic and eukaryotic organisms and demonstrated its critical role in CW remodeling and morphogenesis after exposure of some fungi to agents that stress the cell surface by interacting with wall polymers. This review deals with some of the cell compensatory responses of fungi to wall damage induced by Congo Red and Calcofluor White.

Genetic defects in the hexosamine and sialic acid biosynthesis pathway

BACKGROUND

Congenital disorders of glycosylation are caused by defects in the glycosylation of proteins and lipids. Classically, gene defects with multisystem disease have been identified in the ubiquitously expressed glycosyltransferases required for protein N-glycosylation. An increasing number of defects are being described in sugar supply pathways for protein glycosylation with tissue-restricted clinical symptoms.

SCOPE OF REVIEW

In this review, we address the hexosamine and sialic acid biosynthesis pathways in sugar metabolism. GFPT1, PGM3 and GNE are essential for synthesis of nucleotide sugars uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) and cytidine-5'-monophospho-N-acetylneuraminic acid (CMP-sialic acid) as precursors for various glycosylation pathways. Defects in these enzymes result in contrasting clinical phenotypes of congenital myasthenia, immunodeficiency or adult-onset myopathy, respectively. We therefore discuss the biochemical mechanisms of known genetic defects in the hexosamine and CMP-sialic acid synthesis pathway in relation to the clinical phenotypes.

MAJOR CONCLUSIONS

Both UDP-GlcNAc and CMP-sialic acid are important precursors for diverse protein glycosylation reactions and for conversion into other nucleotide-sugars. Defects in the synthesis of these nucleotide sugars might affect a wide range of protein glycosylation reactions. Involvement of multiple glycosylation pathways might contribute to disease phenotype, but the currently available biochemical information on sugar metabolism is insufficient to understand why defects in these pathways present with tissue-specific phenotypes.

GENERAL SIGNIFICANCE

Future research on the interplay between sugar metabolism and different glycosylation pathways in a tissue- and cell-specific manner will contribute to elucidation of disease mechanisms and will create new opportunities for therapeutic intervention. This article is part of a Special Issue entitled "Glycans in personalised medicine" Guest Editor: Professor Gordan Lauc.

Sporothrix schenckii complex and sporotrichosis, an emerging health problem

Sporothrix schenckii, now named the S. schenckii species complex, has largely been known as the etiological agent of sporotrichosis, which is an acute or chronic subcutaneous mycosis of humans and other mammals. Gene sequencing has revealed the following species in the S. schenckii complex: Sporothrix albicans, Sporothrix brasiliensis, Sporothrix globosa, Sporothrix luriei, Sporothrix mexicana and S. schenckii. The increasing number of reports of Sporothrix infection in immunocompromised patients, mainly the HIV-infected population, suggests sporotrichosis as an emerging global health problem concomitant with the AIDS pandemic. Molecular studies have demonstrated a high level of intraspecific variability. Components of the S. schenckii cell wall that act as adhesins and immunogenic inducers, such as a 70-kDa glycoprotein, are apparently specific to this fungus. The main glycan peptidorhamnomannan cell wall component is the only O-linked glycan structure known in S. schenckii. It contains an α-mannobiose core followed by one α-glucuronic acid unit, which may be mono- or di-rhamnosylated. The oligomeric structure of glucosamine-6-P synthase has led to a significant advance in the development of antifungals targeted to the enzyme’s catalytic domain in S. schenckii.

Sporothrix schenckii: purification and partial biochemical characterization of glucosamine-6-phosphate synthase, a potential antifungal target

The first committed step of the biosynthetic pathway leading to uridine-5'-diphospho-N-acetyl-D-glucosamine (UDP-GlcNAc) is catalyzed by glucosamine-6-phosphate synthase (GlcN-6-P synthase), an enzyme proposed as a potential antifungal chemotherapy target. Here, we describe the purification and biochemical characterization of the native enzyme from the dimorphic pathogenic fungus Sporothrix schenckii. The availability of the pure protein facilitated its biochemical characterization. The enzyme exhibited subunit and native molecular masses of 79 and 350+/-5 kDa, respectively, suggesting a homotetrameric structure. Isoelectric point was 6.26 and K(m) values for fructose-6-phosphate and L-glutamine were 1.12+/-0.3 and 2.2+/-0.7 mM, respectively. Inhibition of activity by UDP-GlcNAc was enhanced by Glc-6-P and phosphorylation stimulated GlcN-6-P synthase activity without affecting the enzyme sensitivity to the aminosugar. A glutamine analogue, FMDP [N(3)-(4-methoxyfumaroyl)-L-2,3-diaminopropanoic acid] was a more potent inhibitor of activity than ADMP (2-Amino-2-deoxy-D-mannitol-6-phosphate) but the latter was a stronger inhibitor of growth in two culture media. To our knowledge, this is the first report on the purification and biochemical characterization of a non-recombinant GlcN-6-P synthase from a true dimorphic fungus. Inhibition of enzyme activity and fungal growth by specific inhibitors of GlcN-6-P synthase strongly reinforces the role of this enzyme as a potential target for antifungal chemotherapy.

Highlights of glucosamine-6P synthase catalysis

L-Glutamine:d-fructose-6-phosphate amidotransferase, also known as glucosamine-6-phosphate synthase (GlcN6P synthase), which catalyzes the first step in a pathway leading to the formation of uridine 5'-diphospho-N-acetyl-d-glucosamine (UDP-GlcNAc), is a key point in the metabolic control of the biosynthesis of amino sugar-containing macromolecules. The molecular mechanism of the reaction catalyzed by GlcN6P synthase is complex and involves amide bond cleavage followed by ammonia channeling and sugar isomerization. This article provides a comprehensive overview of the present knowledge on this multi-faceted enzyme emphasizing the progress made during the last five years.

Functional domains and interdomain communication in Candida albicans glucosamine-6-phosphate synthase

Functional and structural properties of several truncated or mutated variants of Candida albicans Gfa1p (glucosamine-6-phosphate synthase) were compared with those of the wild-type enzyme. Fragments encompassing residues 1-345 and 346-712 of Gfa1p, expressed heterogeneously in bacterial host as His6 fusions, were identified as the functional GAH (glutamine amidehydrolysing) and ISOM (hexose phosphate-isomerizing) domains respectively. It was found that the native GAH domain is monomeric, whereas the native ISOM domain forms tetramers, as does the whole enzyme. Spectrofluorimetric and kinetic studies of the isolated domains, the Delta218-283Gfa1p mutein and the wild-type enzyme revealed that the binding site for the feedback inhibitor, uridine 5'-diphospho-N-acetyl-D-glucosamine, is located in the ISOM domain. Inhibitor binding affects amidohydrolysing activity of the GAH domain and, as a consequence, the GlcN-6-P (D-glucosamine-6-phosphate)-synthetic activity of the whole enzyme. The fragment containing residues 218-283 is neither involved in ligand binding nor in protein oligomerization. Comparison of the catalytic activities of Gfa1p(V711F), Delta709-712Gfa1p, Gfa1p(W97F) and Gfa1p(W97G) with those of the native Gfa1p and the isolated domains provided evidence for an intramolecular channel connecting the GAH and ISOM domains of Gfa1p. The channel becomes leaky upon deletion of amino acids 709-712 and in the W97F and W97G mutants. The Trp97 residue was found to function as a molecular gate, opening and closing the channel. The W97G and V711F mutations resulted in an almost complete elimination of the GlcN-6-P-synthetic activity, with the retention of the amidohydrolase and sugar phosphate-isomerizing activities.

Glucosamine-6-phosphate synthase--the multi-facets enzyme

L-Glutamine: D-fructose-6-phosphate amidotransferase, known under trivial name of glucosamine-6-phosphate synthase, as the only member of the amidotransferase subfamily of enzymes, does not display any ammonia-dependent activity. This enzyme, catalysing the first committed step in a pathway leading to the eventual formation of uridine 5'-diphospho-N-acetyl-D-glucosamine (UDP-GlcNAc), is an important point of metabolic control in biosynthesis of amino sugar-containing macromolecules. The molecular mechanism of reaction catalysed by GlcN-6-P synthase is complex and involves both amino transfer and sugar isomerisation. Substantial alterations to the enzyme structure and properties have been detected in different neoplastic tissues. GlcN-6-P synthase is inflicted in phenomenon of hexosamine-induced insulin resistance in diabetes. Finally, this enzyme has been proposed as a promising target in antifungal chemotherapy. Most of these issues, especially their molecular aspects, have been extensively studied in recent years. This article provides a comprehensive overview of the present knowledge on this multi-facets enzyme.

Involvement of GFA1, which encodes glutamine-fructose-6-phosphate amidotransferase, in the activation of the chitin synthesis pathway in response to cell-wall defects in Saccharomyces cerevisiae

Cell-wall damage caused by mutations of cell-wall-related genes triggers a compensatory mechanism which eventually results in hyperaccumulation of chitin reaching 20% of the cell-wall dry mass. We show that activation of chitin synthesis is accompanied by a rise, from 1.3-fold to 3.5-fold according to the gene mutation, in the expression of most of the genes encoding enzymes of the chitin metabolic pathways. Evidence that GFA1, which encodes glutamine-fructose-6-Phosphate amidotransferase (Gfa1p), the first committed enzyme of this pathway, plays a major role in this process was as follows. Activation of chitin synthesis in the cell-wall mutants correlated with activation of GFA1 and with a proportional increase in Gfa1p activity. Overexpression of GFA1 caused an approximately threefold increase in chitin in the transformed cells, whereas chitin content was barely affected by the joint overexpression of CHS3 and CHS7. Introduction of a gfa1-97 allele mutation in the cell-wall-defective gas1Delta mutant or cultivation of this mutant in a hyperosmotic medium resulted in reduction in chitin synthesis that was proportional to the decrease in Gfa1p activity. Finally, the stimulation of chitin production was also accompanied by an increase in pools of fructose 6-Phosphate, a substrate of Gfa1p. In quantitative terms, we estimated the flux-coefficient control of Gfa1p to be in the range of 0.90, and found that regulation of the chitin metabolic pathway was mainly hierarchical, i.e. dominated by regulation of the amount of newly synthesized GFA1 protein. In the search for the mechanism by which GFA1 is activated in response to cell-wall perturbations, we could only show that neither MCM1 nor RLM1, which encode two transcriptional factors of the MADS box family that are required for expression of cell-cycle and cell-wall-related genes, was involved in this process.

Functional regulation of glutamine:fructose-6-phosphate aminotransferase 1 (GFAT1) of Drosophila melanogaster in a UDP-N-acetylglucosamine and cAMP-dependent manner

Glutamine:fructose-6-phosphate aminotransferase (GFAT; EC 2.6.1.16) expression is tightly regulated in the context of amino sugar synthesis in many organisms from yeast to humans by transcriptional and post-translational processes. We have cloned the cDNA of the GFAT1 of Drosophila melanogaster (Dmel/Gfat1). One of the two putative protein kinase A (PKA) phosphorylation sites proposed for the regulation of human GFAT1 [Zhou, Huynh, Hoffmann, Crook, Daniels, Gulve and McClain (1998) Diabetes 47, 1836-1840] is conserved in Dmel/GFAT1. In the other one the reactive serine has been converted to a cysteine, making further access by PKA unlikely. The Dmel/Gfat1 gene is localized at position 81F on the right arm of chromosome 3. By whole-mount in situ hybridization specific expression of Dmel/GFAT1 was detected in embryonic chitin-synthesizing tissues and in the corpus cells of salivary glands from late third larval instar. Expressing Dmel/GFAT1 in yeast we showed that Dmel/GFAT1 activity is controlled by UDP-N-acetylglucosamine and PKA in the yeast total protein extract system. We propose a model for the independent regulation of the Dmel/GFAT1 enzyme by feedback inhibition and PKA.

Oligomeric structure and regulation of Candida albicans glucosamine-6-phosphate synthase

Candida albicans glucosamine-6-phosphate (GlcN-6-P) synthase was purified to apparent homogeneity with 52% yield from recombinant yeast YRSC-65 cells efficiently overexpressing the GFA1 gene. The pure enzyme exhibited Km(Gln) = 1.56 mM and Km(Fru-6-P) = 1.41 mM and catalyzed GlcN-6-P formation with kcat = 1150 min-1. The isoelectric point of 4.6 +/- 0.05 was estimated from isoelectric chromatofocusing. Gel filtration, native polyacrylamide gel electrophoresis, subunit cross-linking, and SDS-polyacrylamide gel electrophoresis showed that the native enzyme was a homotetramer of 79.5-kDa subunits, with an apparent molecular mass of 330-340 kDa. Results of chemical modification of the enzyme by group-specific reagents established an essential role of a cysteinyl residue at the glutamine-binding site and histidyl, lysyl, arginyl, and tyrosyl moieties at the Fru-6-P-binding site. GlcN-6-P synthase in crude extract was effectively inhibited by UDP-GlcNAc (IC50 = 0.67 mM). Purification of the enzyme markedly decreased the sensitivity to the inhibitor, but this could be restored by addition of another effector, glucose 6-phosphate. Binding of UDP-GlcNAc to the pure enzyme in the presence of Glc-6-P showed strong negative cooperativity, with nH = 0.54, whereas in the absence of this sugar phosphate no cooperative effect was observed. Pure enzyme was a substrate for cAMP-dependent protein kinase, the action of which led to the substantial increase of GlcN-6-P synthase activity, correlated with an extent of protein phosphorylation. The maximal level of activity was observed for the enzyme molecules containing 1. 21 +/- 0.08 mol of phosphate/mol of GlcN-6-P synthase. Monitoring of GlcN-6-P synthase activity and its sensitivity to UDP-GlcNAc during yeast --> mycelia transformation of C. albicans cells, under in situ conditions, revealed a marked increase of the former and a substantial fall of the latter.

Isolation and characterization of the GFA1 gene encoding the glutamine:fructose-6-phosphate amidotransferase of Candida albicans

Glutamine:fructose-6-phosphate amidotransferase (glucosamine-6-phosphate synthase) catalyzes the first step of the hexosamine pathway required for the biosynthesis of cell wall precursors. The Candida albicans GFA1 gene was cloned by complementing a gfa1 mutation of Saccharomyces cerevisiae (previously known as gcn1-1; W. L. Whelan and C. E. Ballou, J. Bacteriol. 124:1545-1557, 1975). GFA1 encodes a predicted protein of 713 amino acids and is homologous to the corresponding gene from S. cerevisiae (72% identity at the nucleotide sequence level) as well as to the genes encoding glucosamine-6-phosphate synthases in bacteria and vertebrates. In cell extracts, the C. albicans enzyme was 4-fold more sensitive than the S. cerevisiae enzyme to UDP-N-acetylglucosamine (an inhibitor of the mammalian enzyme) and 2.5-fold more sensitive to N3-(4-methoxyfumaroyl)-L-2,3-diaminopropanoic acid (a glutamine analog and specific inhibitor of glucosamine-6-phosphate synthase). Cell extracts from the S. cerevisiae gfa1 strain transformed with the C. albicans GFA1 gene exhibited sensitivities to glucosamine-6-phosphate synthase inhibitors that were similar to those shown by the C. albicans enzyme. Southern hybridization indicated that a single GFA1 locus exists in the C. albicans genome. Quantitative Northern (RNA) analysis showed that the expression of GFA1 in C. albicans is regulated during growth: maximum mRNA levels were detected during early log phase. GFA1 mRNA levels increased following induction of the yeast-to-hyphal-form transition, but this was a response to fresh medium rather than to the morphological change.

Developmental regulation of hexosamine biosynthesis by protein phosphatases 2A and 2C in Blastocladiella emersonii

Extracts of the aquatic fungus Blastocladiella emersonii were found to contain protein phosphatases type 1, type 2A, and type 2C with properties analogous to those found in mammalian tissues. The activities of all three protein phosphatases are developmentally regulated, increasing during sporulation, with maximum level in zoospores. Protein phosphatases 2A and 2C, present in zoospore extracts, catalyze the dephosphorylation of L-glutamine:fructose-6-phosphate amidotransferase (EC 2.6.1.16, amidotransferase), a key regulatory enzyme in hexosamine biosynthesis. The protein phosphatase inhibitor okadaic acid induces encystment and inhibits germ tube formation but does not affect the synthesis of the chitinous cell wall. These results strongly suggest that phosphatase 2C is responsible for the dephosphorylation of amidotransferase in vivo. This dephosphorylation is inhibited by uridine-5'-diphospho-N-acetylglucosamine, the end product of hexosamine synthesis and the substrate for chitin synthesis. This result demonstrates a dual role of uridine-5'-diphospho-N-acetylglucosamine by inhibiting the activity of the phosphorylated form of amidotransferase and by preventing its dephosphorylation by protein phosphatases.