Glucosamine-phosphate N-acetyltransferase 1 and its DNA methylation can be biomarkers for the diagnosis and prognosis of lung cancer.

OBJECTIVE: Lung cancer ranking high in the cancer-related list has long perplexed patients, in which glucosamine-phosphate N-acetyltransferase 1 (GNPNAT1) is found to be highly expressed. Besides, DNA methylation is perceived as a biomarker to assess the prognosis of patients with various cancers. However, the correlation between GNPNAT1 and DNA methylation and the role of GNPNAT1 in lung cancer remain vague. METHODS: Principal component analysis (PCA), heatmap, volcano map, Venn diagram, gene ontology (GO), and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis were used to screen out the candidate genes. The viability, migration, and invasion of lung cancer cells were detected by CCK-8 and Transwell assays. An xenograft tumor mouse model was established. The relative expressions of GNPNAT1, E-cadherin, vimentin, Matrix metalloproteinase-2 (MMP-2), tissue inhibitor of metalloproteinase-2 (TIMP-2), E2F1, and cyclin D1 in cells or xenograft tumor tissues were quantified by Western blot, RT-qPCR, or immunohistochemistry assay. RESULTS: GNPNAT1 was screened as the research object. GNPNAT1 methylation was downregulated, while GNPNAT1 expression was upregulated in lung cancer tissues. The methylation and mRNA levels of GNPNAT1 were correlated with the patient prognosis. GNPNAT1 increased cell viability, migration and invasion, and promoted the xenograft tumor volume and weight, whereas shGNPNAT1 acted oppositely. Moreover, expressions of Vimentin, MMP-2, E2F1, and cyclin D1 were increased, but E-cadherin and TIMP-2 expressions were decreased by overexpressed GNPNAT1, whilst GNPNAT1 knockdown ran conversely. CONCLUSION: GNPNAT1 and methylated GNPNAT1 coverage are biomarkers for the diagnosis and prognosis of lung cancer.

Potential role of glucosamine-phosphate N-acetyltransferase 1 in the development of lung adenocarcinoma.

Glucosamine-phosphate N-acetyltransferase 1 (GNPNAT1) is a key enzyme associated with glucose metabolism and uridine diphosphate-N-acetylglucosamine biosynthesis. Abnormal GNPNAT1 expression might be associated with carcinogenesis. We analyzed multiple lung adenocarcinoma (LUAD) gene expression databases and verified GNPNAT1 higher expression in LUAD tumor tissues than in normal tissues. Moreover, we analyzed the survival relationship between LUAD patients' clinical status and GNPNAT1 expression, and found higher GNPNAT1 expression in LUAD patients with unfavorable prognosis. We built GNPNAT1 gene co-expression networks and further annotated the co-expressed genes' Gene Ontology (GO) terms, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, and various associated regulatory factors. These co-expression genes' functional networks mainly participate in chromosome segregation, RNA metabolic process, and RNA transport. We analyzed GNPNAT1 genetic alterations and co-occurrence networks, and the functional networks of these genes showed that GNPNAT1 participates in multiple steps of cell cycle transition and in the development of some cancers. We assessed the correlation between GNPNAT1 expression and cancer immune infiltrates and showed that GNPNAT1 expression is correlated with several immune cells, chemokines, and immunomodulators in LUAD. We found that GNPNAT1 correlates with LUAD development and prognosis, laying a foundation for further research, especially in immunotherapy.

Independent Prognostic Potential of GNPNAT1 in Lung Adenocarcinoma.

BACKGROUND: Glucosamine-Phosphate N-Acetyltransferase 1 (GNPNAT1) is a critical enzyme in the biosynthesis of uridine diphosphate-N-acetylglucosamine. It has many important functions, such as protein binding, monosaccharide binding, and embryonic development and growth. However, the role of GNPNAT1 in lung adenocarcinoma (LUAD) remains unclear. METHODS: In this study, we explored the expression pattern and prognostic value of GNPNAT1 in LUAD across TCGA and GEO databases and assessed its independent prognostic value via Cox analysis. LinkedOmics and GEPIA2 were applied to investigate coexpression and functional networks associated with GNPNAT1. The TIMER web tool was deployed to assess the correlation between GNPNAT1 and the main six types of tumor-infiltrating immune cells. Besides, the correlations between GNPNAT1 and the LUAD common genetic mutations, TMB, and immune signatures were examined. RESULTS: GNPNAT1 was validated upregulated in tumor tissues in TCGA-LUAD and GEO cohorts. Moreover, in both TCGA and GEO cohorts, high GNPNAT1 expression was found to be associated with poor overall survival. Cox analysis showed that high GNPNAT1 expression was an independent risk factor for LUAD. Functional network analysis suggested that GNPNAT1 regulates cell cycle, ribosome, proteasome, RNA transport, and spliceosome signaling through pathways involving multiple cancer-related kinases and E2F family. In addition, GNPNAT1 correlated with infiltrating levels of B cells, CD4+ T cells, and dendritic cells. B cells and dendritic cells could predict the outcome of LUAD, and B cells and CD4+ T cells were significant independent risk factors. The TMB and mutations of KRAS, EGFR, STK11, and TP53 were correlated with GNPNAT1. At last, the correlation analysis showed GNPNAT1 correlated with most of the immune signatures we performed. CONCLUSION: Our findings showed that GNPNAT1 was correlated to the prognosis and immune infiltration of LUAD. In particular, the tight relationship between GNPNAT1 and B cell marker genes may be the epicenter of the immune response and one of the key factors affecting the prognosis. Our findings laid the foundation for further research on the immunomodulatory role of GNPNAT1 in LUAD.

Plasmodium falciparum Apicomplexan-Specific Glucosamine-6-Phosphate N-Acetyltransferase Is Key for Amino Sugar Metabolism and Asexual Blood Stage Development.

UDP-N-acetylglucosamine (UDP-GlcNAc), the main product of the hexosamine biosynthetic pathway, is an important metabolite in protozoan parasites since its sugar moiety is incorporated into glycosylphosphatidylinositol (GPI) glycolipids and N- and O-linked glycans. Apicomplexan parasites have a hexosamine pathway comparable to other eukaryotic organisms, with the exception of the glucosamine-phosphate N-acetyltransferase (GNA1) enzymatic step that has an independent evolutionary origin and significant differences from nonapicomplexan GNA1s. By using conditional genetic engineering, we demonstrate the requirement of GNA1 for the generation of a pool of UDP-GlcNAc and for the development of intraerythrocytic asexual Plasmodium falciparum parasites. Furthermore, we present the 1.95 Å resolution structure of the GNA1 ortholog from Cryptosporidium parvum, an apicomplexan parasite which is a leading cause of diarrhea in developing countries, as a surrogate for P. falciparum GNA1. The in-depth analysis of the crystal shows the presence of specific residues relevant for GNA1 enzymatic activity that are further investigated by the creation of site-specific mutants. The experiments reveal distinct features in apicomplexan GNA1 enzymes that could be exploitable for the generation of selective inhibitors against these parasites, by targeting the hexosamine pathway. This work underscores the potential of apicomplexan GNA1 as a drug target against malaria.IMPORTANCE Apicomplexan parasites cause a major burden on global health and economy. The absence of treatments, the emergence of resistances against available therapies, and the parasite's ability to manipulate host cells and evade immune systems highlight the urgent need to characterize new drug targets to treat infections caused by these parasites. We demonstrate that glucosamine-6-phosphate N-acetyltransferase (GNA1), required for the biosynthesis of UDP-N-acetylglucosamine (UDP-GlcNAc), is essential for P. falciparum asexual blood stage development and that the disruption of the gene encoding this enzyme quickly causes the death of the parasite within a life cycle. The high-resolution crystal structure of the GNA1 ortholog from the apicomplexan parasite C. parvum, used here as a surrogate, highlights significant differences from human GNA1. These divergences can be exploited for the design of specific inhibitors against the malaria parasite.

Targeting a critical step in fungal hexosamine biosynthesis.

Aspergillus fumigatus is a human opportunistic fungal pathogen whose cell wall protects it from the extracellular environment including host defenses. Chitin, an essential component of the fungal cell wall, is synthesized from UDP-GlcNAc produced in the hexosamine biosynthetic pathway. As this pathway is critical for fungal cell wall integrity, the hexosamine biosynthesis enzymes represent potential targets of antifungal drugs. Here, we provide genetic and chemical evidence that glucosamine 6-phosphate N-acetyltransferase (Gna1), a key enzyme in this pathway, is an exploitable antifungal drug target. GNA1 deletion resulted in loss of fungal viability and disruption of the cell wall, phenotypes that could be rescued by exogenous GlcNAc, the product of the Gna1 enzyme. In a murine model of aspergillosis, the Δgna1 mutant strain exhibited attenuated virulence. Using a fragment-based approach, we discovered a small heterocyclic scaffold that binds proximal to the Gna1 active site and can be optimized to a selective submicromolar binder. Taken together, we have provided genetic, structural, and chemical evidence that Gna1 is an antifungal target in A. fumigatus.

Combinatorial pathway enzyme engineering and host engineering overcomes pyruvate overflow and enhances overproduction of N-acetylglucosamine in Bacillus subtilis.

BACKGROUND: Glucosamine-6-phosphate N-acetyltransferase (GNA1) is the key enzyme that causes overproduction of N-acetylglucosamine in Bacillus subtilis. Previously, we increased GlcNAc production by promoting the expression of GNA1 from Caenorhabditis elegans (CeGNA1) in an engineered B. subtilis strain BSGN12. In this strain overflow metabolism to by-products acetoin and acetate had been blocked by mutations, however pyruvate accumulated as an overflow metabolite. Although overexpression of CeGNA1 drove carbon flux from pyruvate to the GlcNAc synthesis pathway and decreased pyruvate accumulation, the residual pyruvate reduced the intracellular pH, resulting in inhibited CeGNA1 activity and limited GlcNAc production. RESULTS: In this study, we attempted to further overcome pyruvate overflow by enzyme engineering and host engineering for enhanced GlcNAc production. To this end, the key enzyme CeGNA1 was evolved through error-prone PCR under pyruvate stress to enhance its catalytic activity. Then, the urease from Bacillus paralicheniformis was expressed intracellularly to neutralize the intracellular pH, making it more robust in growth and more efficient in GlcNAc production. It was found that the activity of mutant CeGNA1 increased by 11.5% at pH 6.5-7.5, with the catalytic efficiency increasing by 27.5% to 1.25 s-1 µM-1. Modulated expression of urease increased the intracellular pH from 6.0 to 6.8. The final engineered strain BSGN13 overcame pyruvate overflow, produced 25.6 g/L GlcNAc with a yield of 0.43 g GlcNAc/g glucose in a shake flask fermentation and produced 82.5 g/L GlcNAc with a yield of 0.39 g GlcNAc/g glucose by fed-batch fermentation, which was 1.7- and 1.2-times, respectively, of the yield achieved previously. CONCLUSIONS: This study highlights a strategy that combines pathway enzyme engineering and host engineering to resolve overflow metabolism in B. subtilis for the overproduction of GlcNAc. By means of modulated expression of urease reduced pyruvate burden, conferred bacterial survival fitness, and enhanced GlcNAc production, all of which improved our understanding of co-regulation of cell growth and metabolism to construct more efficient B. subtilis cell factories.

The Apicomplexa-specific glucosamine-6-phosphate N-acetyltransferase gene family encodes a key enzyme for glycoconjugate synthesis with potential as therapeutic target.

Apicomplexa form a phylum of obligate parasitic protozoa of great clinical and veterinary importance. These parasites synthesize glycoconjugates for their survival and infectivity, but the enzymatic steps required to generate the glycosylation precursors are not completely characterized. In particular, glucosamine-phosphate N-acetyltransferase (GNA1) activity, needed to produce the essential UDP-N-acetylglucosamine (UDP-GlcNAc) donor, has not been identified in any Apicomplexa. We scanned the genomes of Plasmodium falciparum and representatives from six additional main lineages of the phylum for proteins containing the Gcn5-related N-acetyltransferase (GNAT) domain. One family of GNAT-domain containing proteins, composed by a P. falciparum sequence and its six apicomplexan orthologs, rescued the growth of a yeast temperature-sensitive GNA1 mutant. Heterologous expression and in vitro assays confirmed the GNA1 enzymatic activity in all lineages. Sequence, phylogenetic and synteny analyses suggest an independent origin of the Apicomplexa-specific GNA1 family, parallel to the evolution of a different GNA1 family in other eukaryotes. The inability to disrupt an otherwise modifiable gene target suggests that the enzyme is essential for P. falciparum growth. The relevance of UDP-GlcNAc for parasite viability, together with the independent evolution and unique sequence features of Apicomplexa GNA1, highlights the potential of this enzyme as a selective therapeutic target against apicomplexans.

Nanoparticle abraxane possesses impaired proliferation in A549 cells due to the underexpression of glucosamine 6-phosphate N-acetyltransferase 1 (GNPNAT1/GNA1).

Abraxane (Abr), a US Food and Drug Administration-approved albumin-bound nanoparticle applied for the treatment of non-small-cell lung cancer, has been reported to be more effective than paclitaxel (PTX). To further understand the molecular mechanisms that produce this superior drug efficacy of Abr, a quantitative proteomic approach has been applied to investigate the global protein expression profiles of lung cancer cell A549 treated with Abr and PTX. Only one protein, namely, glucosamine 6-phosphate N-acetyltransferase 1 (GNA1), showed significant differential expression (P<0.05) in the cutoff of 2.0 fold, suggesting that Abr can be used safely as a substitute for PTX. GNA1 is a key enzyme in the biosynthesis of uridine diphosphate-N-acetylglucosamine, which is an important donor substrate for N-linked glycosylation and has several important functions such as embryonic development and growth. Albumin plays a major role in the regulation of this protein. In summary, this study first shows that the superior drug effect of Abr is mainly due to the downregulation of GNA1, which causes proliferative delay and cell adhesion defect. It is also noteworthy that the deficiency of GNA1 might reduce insulin secretion which correlates with type 2 diabetes.

Protective role of endogenous plasmalogens against hepatic steatosis and steatohepatitis in mice.

Free cholesterol (FC) accumulation in the liver is an important pathogenic mechanism of nonalcoholic steatohepatitis (NASH). Plasmalogens, key structural components of the cell membrane, act as endogenous antioxidants and are primarily synthesized in the liver. However, the role of hepatic plasmalogens in metabolic liver disease is unclear. In this study, we found that hepatic levels of docosahexaenoic acid (DHA)-containing plasmalogens, expression of glyceronephosphate O-acyltransferase (Gnpat; the rate-limiting enzyme in plasmalogen biosynthesis), and expression of Pparα were lower in mice with NASH caused by accumulation of FC in the liver. Cyclodextrin-induced depletion of FC transactivated Δ-6 desaturase by increasing sterol regulatory element-binding protein 2 expression in cultured hepatocytes. DHA, the major product of Δ-6 desaturase activation, activated GNPAT, thereby explaining the association between high hepatic FC and decreased Gnpat expression. Gnpat small interfering RNA treatment significantly decreased peroxisome proliferator-activated receptor α (Pparα) expression in cultured hepatocytes. In addition to GNPAT, DHA activated PPARα and increased expression of Pparα and its target genes, suggesting that DHA in the DHA-containing plasmalogens contributed to activation of PPARα. Accordingly, administration of the plasmalogen precursor, alkyl glycerol (AG), prevented hepatic steatosis and NASH through a PPARα-dependent increase in fatty acid oxidation. Gnpat+/- mice were more susceptible to hepatic lipid accumulation and less responsive to the preventive effect of fluvastatin on NASH development, suggesting that endogenous plasmalogens prevent hepatic steatosis and NASH. CONCLUSION: Increased hepatic FC in animals with NASH decreased plasmalogens, thereby sensitizing animals to hepatocyte injury and NASH. Our findings uncover a novel link between hepatic FC and plasmalogen homeostasis through GNPAT regulation. Further study of AG or other agents that increase hepatic plasmalogen levels may identify novel therapeutic strategies against NASH. (Hepatology 2017;66:416-431).

Human acetyl-CoA:glucosamine-6-phosphate N-acetyltransferase 1 has a relaxed donor specificity and transfers acyl groups up to four carbons in length.

Glucosamine-6-phosphate N-acetyltransferase1 (GNA1) catalyses the transfer of an acetyl group from acetyl coenzyme A (AcCoA) to glucosamine-6-phosphate (GlcN6P) to form N-acetylglucosamine-6-phosphate (GlcNAc6P), which is an essential intermediate in UDP-GlcNAc biosynthesis. An analog of GlcNAc, N-butyrylglucosamine (GlcNBu) has shown healing properties for bone and articular cartilage in animal models of arthritis. The goal of this work was to examine whether GNA1 has the ability to transfer a butyryl group from butyryl-CoA to GlcN6P to form GlcNBu6P, which can then be converted to GlcNBu. We developed fluorescent and radioactive assays and examined the donor specificity of human GNA1. Acetyl, propionyl, n-butyryl, and isobutyryl groups were all transferred to GlcN6P, but isovaleryl-CoA and decanoyl-CoA did not serve as donor substrates. Site-specific mutants were produced to examine the role of amino acids potentially affecting the size and properties of the AcCoA binding pocket. All of the wild type and mutant enzymes showed activities of both acetyl and butyryl transfer and can therefore be used for the enzymatic synthesis of GlcNBu for biomedical applications.

Characterization of the amino acid residues mediating the unique amino-sugar-1-phosphate acetyltransferase activity of the archaeal ST0452 protein.

The ST0452 protein from the thermophilic archaean Sulfolobus tokodaii has been identified as an enzyme with multiple sugar-1-phosphate nucleotidylyltransferase and amino-sugar-1-phosphate acetyltransferase (amino-sugar-1-P AcTase) activities. Analysis of the protein showed that in addition to glucosamine-1-phosphate (GlcN-1-P) AcTase activity, it possesses unique galactosamine-1-phosphate (GalN-1-P) AcTase activity not detected in any other proteins. Comparison of the crystal structures of the ST0452 protein and GlmU from Escherichia coli (EcGlmU), which possesses only GlcN-1-P AcTase activity, showed that the overall sequence identity between these two proteins is less than 25 %, but the amino acid residues predicted to comprise the catalytic center of EcGlmU are conserved in the ST0452 protein. To understand the molecular mechanism by which the ST0452 amino-sugar-1-P AcTase activity recognizes two independent substrates, several ST0452 substitution and truncation mutant proteins were constructed and analyzed. We found that His308 is essential for both GalN-1-P and GlcN-1-P AcTase activities, whereas Tyr311 and Asn331 are important only for the GalN-1-P AcTase activity. In addition, deletion of the C-terminal 5 or 11 residues showed that the 11-residue C-terminal region exerts a modest stimulatory effect on GalN-1-P AcTase activity but dramatically suppresses GlcN-1-P AcTase activity. This region also appears to make an important contribution to the thermostability of the entire ST0452 protein. Systematic deletions from the C-terminus also demonstrated that the C-terminal region with the ß-helix structure has an important role mediating the trimerization of the ST0452 protein. This is the first report of an analysis of a thermostable archaeal enzyme exhibiting multiple amino-sugar-1-P AcTase activities.

Artificial environments for the co-translational stabilization of cell-free expressed proteins.

An approach for designing individual expression environments that reduce or prevent protein aggregation and precipitation is described. Inefficient folding of difficult proteins in unfavorable translation environments can cause significant losses of overexpressed proteins as precipitates or inclusion bodies. A number of chemical chaperones including alcohols, polyols, polyions or polymers are known to have positive effects on protein stability. However, conventional expression approaches can use such stabilizing agents only post-translationally during protein extraction and purification. Proteins that already precipitate inside of the producer cells cannot be addressed. The open nature of cell-free protein expression systems offers the option to include single chemicals or cocktails of stabilizing compounds already into the expression environment. We report an approach for systematic screening of stabilizers in order to improve the solubility and quality of overexpressed proteins co-translationally. A comprehensive list of representative protein stabilizers from the major groups of naturally occurring chemical chaperones has been analyzed and their concentration ranges tolerated by cell-free expression systems have been determined. As a proof of concept, we have applied the method to improve the yield of proteins showing instability and partial precipitation during cell-free synthesis. Stabilizers that co-translationally improve the solubility and functional folding of human glucosamine 6-phosphate N-acetyltransferase have been identified and cumulative effects of stabilizers have been studied.

Engineering of an N-acetylneuraminic acid synthetic pathway in Escherichia coli.

N-acetylneuraminic acid (NeuAc) has recently drawn much attention owing to its wide applications in many aspects. Besides extraction from natural materials, production of NeuAc was recently focused on enzymatic synthesis and whole-cell biocatalysis. In this study, we designed an artificial NeuAc biosynthetic pathway through intermediate N-acetylglucosamine 6-phosphate in Escherichia coli. In this pathway, N-acetylglucosamine 2-epimerase (slr1975) and glucosamine-6-phosphate acetyltransferase (GNA1) were heterologously introduced into E. coli from Synechocystis sp. PCC6803 and Saccharomyces cerevisiae EBY100, respectively. By derepressing the feedback inhibition of glucosamine-6-phosphate synthase, increasing the accumulation of N-acetylglucosamine and pyruvate, and blocking the catabolism of NeuAc, we were able to produce 1.62 g l⁻¹ NeuAc in recombinant E. coli directly from glucose. The NeuAc yield reached 7.85g l⁻¹ in fed-batch fermentation. This process offered an efficient fermentative method to produce NeuAc in microorganisms using glucose as carbon source and can be optimized for further improvement.

A missense mutation in the glucosamine-6-phosphate N-acetyltransferase-encoding gene causes temperature-dependent growth defects and ectopic lignin deposition in Arabidopsis.

To study the regulatory mechanisms underlying lignin biosynthesis, we isolated and characterized lignescens (lig), a previously undescribed temperature-sensitive mutant of Arabidopsis thaliana that exhibits ectopic lignin deposition and growth defects under high-temperature conditions. The lig mutation was identified as a single base transition in GNA1 encoding glucosamine-6-phosphate N-acetyltransferase (GNA), a critical enzyme of UDP-N-acetylglucosamine (UDP-GlcNAc) biosynthesis. lig harbors a glycine-to-serine substitution at residue 68 (G68S) of GNA1. Enzyme activity assays of the mutant protein (GNA1(G68S)) showed its thermolability relative to the wild-type protein. The lig mutant exposed to the restrictive temperature contained a significantly smaller amount of UDP-GlcNAc than did the wild type. The growth defects and ectopic lignification of lig were suppressed by the addition of UDP-GlcNAc. Since UDP-GlcNAc is an initial sugar donor of N-glycan synthesis and impaired N-glycan synthesis is known to induce the unfolded protein response (UPR), we examined possible relationships between N-glycan synthesis, UPR, and the lig phenotype. N-glycans were reduced and LUMINAL BINDING PROTEIN3, a typical UPR gene, was expressed in lig at the restrictive temperature. Furthermore, treatment with UPR-inducing reagents phenocopied the lig mutant. Our data collectively suggest that impairment of N-glycan synthesis due to a shortage of UDP-GlcNAc leads to ectopic lignin accumulation, mostly through the UPR.

Crystal structure and functional characterization of a glucosamine-6-phosphate N-acetyltransferase from Arabidopsis thaliana.

GlcNAc (N-acetylglucosamine) is an essential part of the glycan chain in N-linked glycoproteins. It is a building block for polysaccharides such as chitin, and several glucosaminoglycans and proteins can be O-GlcNAcylated. The deacetylated form, glucosamine, is an integral part of GPI (glycosylphosphatidylinositol) anchors. Both are incorporated into polymers by glycosyltransferases that utilize UDP-GlcNAc. This UDP-sugar is synthesized in a short pathway comprising four steps starting from fructose 6-phosphate. GNA (glucosamine-6-phosphate N-acetyltransferase) catalyses the second of these four reactions in the de novo synthesis in eukaryotes. A phylogenetic analysis revealed that only one GNA isoform can be found in most of the species investigated and that the most likely Arabidopsis candidate is encoded by the gene At5g15770 (AtGNA). qPCR (quantitative PCR) revealed the ubiquitous expression of AtGNA in all organs of Arabidopsis plants. Heterologous expression of AtGNA showed that it is highly active between pH 7 and 8 and at temperatures of 30-40°C. It showed Km values of 231 µM for glucosamine 6-phosphate and 33 µM for acetyl-CoA respectively and a catalytic efficiency comparable with that of other GNAs characterized. The solved crystal structure of AtGNA at a resolution of 1.5 Å (1 Å=0.1 nm) revealed a very high structural similarity to crystallized GNA proteins from Homo sapiens and Saccharomyces cerevisiae despite less well conserved protein sequence identity.

Characterization, localization, essentiality, and high-resolution crystal structure of glucosamine 6-phosphate N-acetyltransferase from Trypanosoma brucei.

A gene predicted to encode Trypanosoma brucei glucosamine 6-phosphate N-acetyltransferase (TbGNA1; EC 2.3.1.4) was cloned and expressed in Escherichia coli. The recombinant protein was enzymatically active, and its high-resolution crystal structure was obtained at 1.86 Å. Endogenous TbGNA1 protein was localized to the peroxisome-like microbody, the glycosome. A bloodstream-form T. brucei GNA1 conditional null mutant was constructed and shown to be unable to sustain growth in vitro under nonpermissive conditions, demonstrating that there are no metabolic or nutritional routes to UDP-GlcNAc other than via GlcNAc-6-phosphate. Analysis of the protein glycosylation phenotype of the TbGNA1 mutant under nonpermissive conditions revealed that poly-N-acetyllactosamine structures were greatly reduced in the parasite and that the glycosylation profile of the principal parasite surface coat component, the variant surface glycoprotein (VSG), was modified. The significance of results and the potential of TbGNA1 as a novel drug target for African sleeping sickness are discussed.

Structural and kinetic differences between human and Aspergillus fumigatus D-glucosamine-6-phosphate N-acetyltransferase.

Aspergillus fumigatus is the causative agent of aspergillosis, a frequently invasive colonization of the lungs of immunocompromised patients. GNA1 (D-glucosamine-6-phosphate N-acetyltransferase) catalyses the acetylation of GlcN-6P (glucosamine-6-phosphate) to GlcNAc-6P (N-acetylglucosamine-6-phosphate), a key intermediate in the UDP-GlcNAc biosynthetic pathway. Gene disruption of gna1 in yeast and Candida albicans has provided genetic validation of the enzyme as a potential target. An understanding of potential active site differences between the human and A. fumigatus enzymes is required to enable further work aimed at identifying selective inhibitors for the fungal enzyme. In the present study, we describe crystal structures of both human and A. fumigatus GNA1, as well as their kinetic characterization. The structures show significant differences in the sugar-binding site with, in particular, several non-conservative substitutions near the phosphate-binding pocket. Mutagenesis targeting these differences revealed drastic effects on steady-state kinetics, suggesting that the differences could be exploitable with small-molecule inhibitors.

Identification and characterization of RHOA-interacting proteins in bovine spermatozoa.

In somatic cells, RHOA mediates actin dynamics through a GNA13-mediated signaling cascade involving RHO kinase (ROCK), LIM kinase (LIMK), and cofilin. RHOA can be negatively regulated by protein kinase A (PRKA), and it interacts with members of the A-kinase anchoring (AKAP) family via intermediary proteins. In spermatozoa, actin polymerization precedes the acrosome reaction, which is necessary for normal fertility. The present study was undertaken to determine whether the GNA13-mediated RHOA signaling pathway may be involved in acrosome reaction in bovine caudal sperm, and whether AKAPs may be involved in its targeting and regulation. GNA13, RHOA, ROCK2, LIMK2, and cofilin were all detected by Western blot in bovine caudal sperm. Overlay, immunoprecipitation, and subsequent mass spectrometry analysis identified several RHOA-interacting proteins, including proacrosin, angiotensin-converting enzyme, tubulin, aldolase C, and AKAP4. Using overlay and pulldown techniques, we demonstrate that phosphorylation of AKAP3 increases its interaction with the RHOA-interacting proteins PRKAR2 (the type II regulatory subunit of PRKA, formerly RII) and ropporin (ROPN1, a PRKAR2-like protein, or R2D2). Varying calcium concentrations in pulldown assays did not significantly alter binding to R2D2 proteins. These data suggest that the actin-regulating GNA13-mediated RHOA-ROCK-LIMK-cofilin pathway is present in bovine spermatozoa, that RHOA interacts with proteins involved in capacitation and the acrosome reaction, and that RHOA signaling in sperm may be targeted by AKAPs. Finally, AKAP3 binding to PRKAR2 and ROPN1 is regulated by phosphorylation in vitro.

Glucose-6-phosphate as a probe for the glucosamine-6-phosphate N-acetyltransferase Michaelis complex.

Glucosamine-6-phosphate N-acetyltransferase (GNA1) catalyses the N-acetylation of d-glucosamine-6-phosphate (GlcN-6P), using acetyl-CoA as an acetyl donor. The product GlcNAc-6P is an intermediate in the biosynthesis UDP-GlcNAc. GNA1 is part of the GCN5-related acetyl transferase family (GNATs), which employ a wide range of acceptor substrates. GNA1 has been genetically validated as an antifungal drug target. Detailed knowledge of the Michaelis complex and trajectory towards the transition state would facilitate rational design of inhibitors of GNA1 and other GNAT enzymes. Using the pseudo-substrate glucose-6-phosphate (Glc-6P) as a probe with GNA1 crystals, we have trapped the first GNAT (pseudo-)Michaelis complex, providing direct evidence for the nucleophilic attack of the substrate amine, and giving insight into the protonation of the thiolate leaving group.

Enzymes of UDP-GlcNAc biosynthesis in yeast.

D-Glucosamine is an important building block of major structural components of the fungal cell wall, namely chitin, chitosan and mannoproteins. Other amino sugars, such as D-mannosamine and D-galactosamine, relatively abundant in higher eukaryotes, rarely occur in fungal cells and are actually absent from yeast and yeast-like fungi. The glucosamine-containing sugar nucleotide UDP-GlcNAc is synthesized in yeast cells in a four-step cytoplasmic pathway. This article provides a comprehensive overview of the present knowledge on the enzymes catalysing the particular steps of the pathway in Candida albicans and Saccharomyces cerevisiae, with a special emphasis put on mechanisms of the catalysed reactions, regulation of activity and perspectives for exploitation of enzymes participating in UDP-GlcNAc biosynthesis as potential targets for antifungal chemotherapy.