Overexpression, purification, biochemical characterization, and molecular modeling of recombinant GDP-mannosyltransferase (GumH) from Xylella fastidiosa

Biochemical characterization of a novel bifunctional glycosyl-1-phosphate transferase involved in the exopolysaccharide biosynthesis

Ebosin produced by Streptomyces sp. 139 is a novel exopolysaccharide with anti-rheumatic arthritis activity in vivo and its biosynthesis gene cluster (ste) has been previously identified. In our previous research, ste5 gene has been identified as priming glycosyltransferase involved in Ebosin biosynthesis. However, it remains unclear how ste5 initiated Ebosin biosynthesis in molecular level. Here we show that Ebosin derivative produced by ste5 mutant lost the antagonist activities for IL-1R and Overexpression of ste5 in mutant dramatically enhanced the antagonist activities for IL-1R. For biochemical characterization of Ste5, the ste5 gene was cloned and expressed in Escherichia coli BL21. We identified that the recombinant Ste5 can transfer galactose-1-Phosphate (Gal-1-P) or glucose-1-Phosphate (Glc-1-P) from UDP-galactose and UDP-glucose to the lipid carrier located in the cytoplasmic membrane of Streptomyces sp. 139 (ste5(-)) with a continuous coupled spectrophotometric assay. 12.6 μM of Km was for UDP-galactose and 23.9 μM for UDP-glucose respectively. Our results indicate that Ste5 is bifunctional Gal-1-P and Glc-1-P transferase to initiate Ebosin biosynthesis and may be further applied in remoulding carbohydrate compounds.

The exopolysaccharide of Xylella fastidiosa is essential for biofilm formation, plant virulence, and vector transmission

Exopolysaccharides (EPS) synthesized by plant-pathogenic bacteria are generally essential for virulence. The role of EPS produced by the vector-transmitted bacterium Xylella fastidiosa was investigated by knocking out two genes implicated in the EPS biosynthesis, gumD and gumH. Mutant strains were affected in growth characteristics in vitro, including adhesion to surfaces and biofilm formation. In addition, different assays were used to demonstrate that the mutant strains produced significantly less EPS compared with the wild type. Furthermore, gas chromatography-mass spectrometry showed that both mutant strains did not produce oligosaccharides. Biologically, the mutants were deficient in movement within plants, resulting in an avirulent phenotype. Additionally, mutant strains were affected in transmission by insects: they were very poorly transmitted by and retained within vectors. The gene expression profile indicated upregulation of genes implicated in cell-to-cell signaling and adhesins while downregulation in genes was required for within-plant movement in EPS-deficient strains. These results suggest an essential role for EPS in X. fastidiosa interactions with both plants and insects.

Role of PelF in pel polysaccharide biosynthesis in Pseudomonas aeruginosa

Pseudomonas aeruginosa produces three exopolysaccharides, Psl, Pel, and alginate, that play vital roles in biofilm formation. Pel is a glucose-rich, cellulose-like exopolysaccharide. The essential Pel biosynthesis proteins are encoded by seven genes, pelA to pelG. Bioinformatics analysis suggests that PelF is a cytosolic glycosyltransferase. Here, experimental evidence was provided to support this PelF function. A UDP-glucose dehydrogenase-based assay was developed to quantify UDP-glucose. UDP-glucose was proposed as the substrate for PelF. The isogenic pelF deletion mutant accumulated 1.8 times more UDP-glucose in its cytosol than the wild type. This suggested that PelF, which was found localized in the cystosol, uses UDP-glucose as substrate. Additionally, in vitro experiments confirmed that PelF uses UDP-glucose as substrate. To analyze the functional roles of conserved residues in PelF, site-directed mutagenesis was performed. The presence of the EX7E motif is characteristic for various glycosyltransferase families, and in PelF, E405/E413 are the conserved residues in this motif. Replacement of E405 with A resulted in a reduction of PelF activity to 30.35% ± 3.15% (mean ± standard deviation) of the wild-type level, whereas replacement of the second E, E413, with A did not produce a significant change in the activity of PelF. Moreover, replacement of both E residues did not result in a loss of PelF function, but replacement of the conserved R325 or K330 with A resulted in a complete loss of PelF activity. Overall, our data show that PelF is a soluble glycosyltransferase that uses UDP-glucose as the substrate for Pel synthesis and that conserved residues R325 and K330 are important for the activity of PelF.

Real-time investigation of mannosyltransferase function of a Xylella fastidiosa recombinant GumH protein using QCM-D

Xylella fastidiosa is a gram-negative bacterium that causes serious diseases in economically important crops, including grapevine, coffee, and citrus fruits. X. fastidiosa colonizes the xylem vessels of the infected plants, thereby blocking water and nutrient transport. The genome sequence of X. fastidiosa has revealed an operon containing nine genes possibly involved in the synthesis of an exopolisaccharide (EPS) named fastidian gum that can be related with the pathogenicity of this bacterium. The α-1,3-mannosyltransferase (GumH) enzyme from X. fastidiosa is involved in fastidian gum production. GumH is responsible for the transfer of mannose from guanosine diphosphate mannose (GDP-man) to the cellobiose-pyrophosphate-polyprenol carrier lipid (CPP-Lip) during the assembly and biosynthesis of EPS. In this work, a method for real-time detection of recombinant GumH enzymatic activity was successfully developed using a Quartz Crystal Microbalance with dissipation monitoring (QCM-D). The QCM-D transducer was strategically modified with CPP-Lip by using a solid-supported lipid bilayer that makes use of a self-assembled monolayer of 1-undecanethiol. Monitoring the real-time CPP-Lip QCM-D transducer in the presence of GDP-man and GumH enzyme shows a mass increase, indicating the transfer of mannose. The real-time QCM-D determination of mannosyltransferase function was validated by a High Performance Liquid Chromatography (LC) method developed for determination of GDP produced by enzymatic reaction. LC results confirmed the activity of recombinant GumH protein, which is the first enzyme involved in the biosynthesis of the EPS from X. fastidiosa enzymatically characterized.

Biochemical characterization, membrane association and identification of amino acids essential for the function of Alg11 from Saccharomyces cerevisiae, an alpha1,2-mannosyltransferase catalysing two sequential glycosylation steps in the formation of the lipid-linked core oligosaccharide

The biosynthesis of asparagine-linked glycans occurs in an evolutionarily conserved manner with the assembly of the unique lipid-linked oligosaccharide precursor Glc3Man9GlcNAc2-PP-Dol at the ER (endoplasmic reticulum). In the present study we characterize Alg11 from yeast as a mannosyltransferase catalysing the sequential transfer of two alpha1,2-linked mannose residues from GDP-mannose to Man3GlcNAc2-PP-Dol and subsequently to Man4GlcNAc2-PP-Dol forming the Man5GlcNAc2-PP-Dol intermediate at the cytosolic side of the ER before flipping to the luminal side. Alg11 is predicted to contain three hydrophobic transmembrane-spanning helices. Using Alg11 topology reporter fusion constructs, we show that only the N-terminal domain fulfils this criterion. Surprisingly, this domain can be deleted without disturbing glycosyltransferase function and membrane association, indicating also that the other two hydrophobic domains contribute to ER localization, but in a non-transmembrane manner. By site-directed mutagenesis we investigated amino acids important for transferase activity. We demonstrate that the first glutamate residue in the EX7E motif, conserved in a variety of glycosyltransferases, is more critical than the second, and loss of Alg11 function occurs only when both glutamate residues are exchanged, or when the mutation of the first glutamate residue is combined with replacement of another amino acid in the motif. This indicates that perturbations in EX7E are not restricted to the second glutamate residue. Moreover, Gly85 and Gly87, within a glycine-rich domain as part of a potential flexible loop, were found to be required for Alg11 function. Similarly, a conserved lysine residue, Lys319, was identified as being important for activity, which could be involved in the binding of the phosphate of the glycosyl donor.

Cloning, expression, purification and characterization of recombinant glutathione-S-transferase from Xylella fastidiosa

Xylella fastidiosa is an important pathogen bacterium transmitted by xylem-feedings leafhoppers that colonizes the xylem of plants and causes diseases on several important crops including citrus variegated chlorosis (CVC) in orange and lime trees. Glutathione-S-transferases (GST) form a group of multifunctional isoenzymes that catalyzes both glutathione (GSH)-dependent conjugation and reduction reactions involved in the cellular detoxification of xenobiotic and endobiotic compounds. GSTs are the major detoxification enzymes found in the intracellular space and mainly in the cytosol from prokaryotes to mammals, and may be involved in the regulation of stress-activated signals by suppressing apoptosis signal-regulating kinase 1. In this study, we describe the cloning of the glutathione-S-transferase from X. fastidiosa into pET-28a(+) vector, its expression in Escherichia coli, purification and initial structural characterization. The purification of recombinant xfGST (rxfGST) to near homogeneity was achieved using affinity chromatography and size-exclusion chromatography (SEC). SEC demonstrated that rxfGST is a homodimer in solution. The secondary and tertiary structures of recombinant protein were analyzed by circular dichroism and fluorescence spectroscopy, respectively. The enzyme was assayed for activity and the results taken together indicated that rxfGST is a stable molecule, correctly folded, and highly active. Several members of the GST family have been extensively studied. However, xfGST is part of a less-studied subfamily which yet has not been structurally and biochemically characterized. In addition, these studies should provide a useful basis for future studies and biotechnological approaches of rxfGST.

Living in two worlds: the plant and insect lifestyles of Xylella fastidiosa

Diseases caused by Xylella fastidiosa have attained great importance worldwide as the pathogen and its insect vectors have been disseminated. Since this is the first plant pathogenic bacterium for which a complete genome sequence was determined, much progress has been made in understanding the process by which it spreads within the xylem vessels of susceptible plants as well as the traits that contribute to its acquisition and transmission by sharpshooter vectors. Although this pathogen shares many similarities with Xanthomonas species, such as its use of a small fatty acid signal molecule to coordinate virulence gene expression, the traits that it utilizes to cause disease and the manner in which they are regulated differ substantially from those of related plant pathogens. Its complex lifestyle as both a plant and insect colonist involves traits that are in conflict with these stages, thus apparently necessitating the use of a gene regulatory scheme that allows cells expressing different traits to co-occur in the plant.

Expression and purification of a functionally active recombinant GDP-mannosyltransferase (PimA) from Mycobacterium tuberculosis H37Rv

Lipoarabinomannans (LAM), especially mannose-capped LAM, abundant in the cell wall of Mycobacterium tuberculosis (Mtb) exhibit a broad spectrum of immunomodulatory functions and emerge as key virulence factors that may be relevant drug targets. The pimA gene of mycobacteria encodes a alpha-mannosyltransferase involved in the transfer reaction of the very first mannose from GDP-mannose to the carrier lipid phosphatidyl-myo-inositol, a precursor in the synthesis of LAM. PimA has been proposed to play an essential role in the growth of mycobacteria. In this study, the pimA gene from M. tuberculosis H37Rv was cloned into the pET28a vector and the recombinant plasmid was transformed into Escherichia coli BL21 (DE3) strain, allowing the expression of the Mtb PimA in fusion with a histidine-rich peptide on the N-terminal. The Mtb PimA was purified from the supernatant of the lysed cells under native conditions by immobilized metal affinity chromatography. The purity and molecular weight of Mtb PimA were determined by high performance liquid chromatography and matrix-assisted laser desorption ionization time-of-flight. Circular dichroism spectroscopy study on Mtb PimA showed that the protein was folded. The enzyme assays revealed that Mtb PimA showed a requirement for Mg(2+) for the activity and the K(m) and V(max) values of Mtb PimA were estimated at 18 +/- 2 microM and 0.1 +/- 0.05 nmol/min/microg, respectively. This is the first report describing cloning and expression of GDP-mannosyltransferase gene of M. tuberculosis in E. coli.