Fluorescence resonance energy transfer (FRET) and proximity ligation assays reveal functionally relevant homo- and heteromeric complexes among hyaluronan synthases HAS1, HAS2, and HAS3

In vertebrates, hyaluronan is produced in the plasma membrane from cytosolic UDP-sugar substrates by hyaluronan synthase 1-3 (HAS1-3) isoenzymes that transfer N-acetylglucosamine (GlcNAc) and glucuronic acid (GlcUA) in alternative positions in the growing polysaccharide chain during its simultaneous extrusion into the extracellular space. It has been shown that HAS2 immunoprecipitates contain functional HAS2 homomers and also heteromers with HAS3 (Karousou, E., Kamiryo, M., Skandalis, S. S., Ruusala, A., Asteriou, T., Passi, A., Yamashita, H., Hellman, U., Heldin, C. H., and Heldin, P. (2010) The activity of hyaluronan synthase 2 is regulated by dimerization and ubiquitination. J. Biol. Chem. 285, 23647-23654). Here we have systematically screened in live cells, potential interactions among the HAS isoenzymes using fluorescence resonance energy transfer (FRET) and flow cytometric quantification. We show that all HAS isoenzymes form homomeric and also heteromeric complexes with each other. The same complexes were detected both in Golgi apparatus and plasma membrane by using FRET microscopy and the acceptor photobleaching method. Proximity ligation assays with HAS antibodies confirmed the presence of HAS1-HAS2, HAS2-HAS2, and HAS2-HAS3 complexes between endogenously expressed HASs. C-terminal deletions revealed that the enzymes interact mainly via uncharacterized N-terminal 86-amino acid domain(s), but additional binding site(s) probably exist in their C-terminal parts. Of all the homomeric complexes HAS1 had the lowest and HAS3 the highest synthetic activity. Interestingly, HAS1 transfection reduced the synthesis of hyaluronan obtained by HAS2 and HAS3, suggesting functional cooperation between the isoenzymes. These data indicate a general tendency of HAS isoenzymes to form both homomeric and heteromeric complexes with potentially important functional consequences on hyaluronan synthesis.

Streptococcus agalactiae capsule polymer length and attachment is determined by the proteins CpsABCD

The production of capsular polysaccharides (CPS) or secreted exopolysaccharides is ubiquitous in bacteria, and the Wzy pathway constitutes a prototypical mechanism to produce these structures. Despite the differences in polysaccharide composition among species, a group of proteins involved in this pathway is well conserved. Streptococcus agalactiae (group B Streptococcus; GBS) produces a CPS that represents the main virulence factor of the bacterium and is a prime target in current vaccine development. We used this human pathogen to investigate the roles and potential interdependencies of the conserved proteins CpsABCD encoded in the cps operon, by developing knock-out and functional mutant strains. The mutant strains were examined for CPS quantity, size, and attachment to the cell surface as well as CpsD phosphorylation. We observed that CpsB, -C, and -D compose a phosphoregulatory system where the CpsD autokinase phosphorylates its C-terminal tyrosines in a CpsC-dependent manner. These Tyr residues are also the target of the cognate CpsB phosphatase. An interaction between CpsD and CpsC was observed, and the phosphorylation state of CpsD influenced the subsequent action of CpsC. The CpsC extracellular domain appeared necessary for the production of high molecular weight polysaccharides by influencing CpsA-mediated attachment of the CPS to the bacterial cell surface. In conclusion, although having no impact on cps transcription or the synthesis of the basal repeating unit, we suggest that these proteins are fine-tuning the last steps of CPS biosynthesis (i.e. the balance between polymerization and attachment to the cell wall).

Domain interactions control complex formation and polymerase specificity in the biosynthesis of the Escherichia coli O9a antigen

The Escherichia coli O9a O-polysaccharide (O-PS) is a prototype for bacterial glycan synthesis and export by an ATP-binding cassette transporter-dependent pathway. The O9a O-PS possesses a tetrasaccharide repeat unit comprising two α-(1→2)- and two α-(1→3)-linked mannose residues and is extended on a polyisoprenoid lipid carrier by the action of a polymerase (WbdA) containing two glycosyltransferase active sites. The N-terminal domain of WbdA possesses α-(1→2)-mannosyltransferase activity, and we demonstrate in this study that the C-terminal domain is an α-(1→3)-mannosyltransferase. Previous studies established that the size of the O9a polysaccharide is determined by the chain-terminating dual kinase/methyltransferase (WbdD) that is tethered to the membrane and recruits WbdA into an active enzyme complex by protein-protein interactions. Here, we used bacterial two-hybrid analysis to identify a surface-exposed α-helix in the C-terminal mannosyltransferase domain of WbdA as the site of interaction with WbdD. However, the C-terminal domain was unable to interact with WbdD in the absence of its N-terminal partner. Through deletion analysis, we demonstrated that the α-(1→2)-mannosyltransferase activity of the N-terminal domain is regulated by the activity of the C-terminal α-(1→3)-mannosyltransferase. In mutants where the C-terminal catalytic site was deleted but the WbdD-interaction site remained, the N-terminal mannosyltransferase became an unrestricted polymerase, creating a novel polymer comprising only α-(1→2)-linked mannose residues. The WbdD protein therefore orchestrates critical localization and coordination of activities involved in chain extension and termination. Complex domain interactions are needed to position the polymerase components appropriately for assembly into a functional complex located at the cytoplasmic membrane.

Pen and Pal are nucleotide-sugar dehydratases that convert UDP-GlcNAc to UDP-6-deoxy-D-GlcNAc-5,6-ene and then to UDP-4-keto-6-deoxy-L-AltNAc for CMP-pseudaminic acid synthesis in Bacillus thuringiensis

CMP-pseudaminic acid is a precursor required for the O-glycosylation of flagellin in some pathogenic Gram-negative bacteria, a process known to be critical in bacterial motility and infection. However, little is known about flagellin glycosylation in Gram-positive bacteria. Here, we identified and functionally characterized an operon, named Bti_pse, in Bacillus thuringiensis israelensis ATCC 35646, which encodes seven different enzymes that together convert UDP-GlcNAc to CMP-pseudaminic acid. In contrast, Gram-negative bacteria complete this reaction with six enzymes. The first enzyme, which we named Pen, converts UDP-d-GlcNAc to an uncommon UDP-sugar, UDP-6-deoxy-D-GlcNAc-5,6-ene. Pen contains strongly bound NADP(+) and has distinct UDP-GlcNAc 4-oxidase, 5,6-dehydratase, and 4-reductase activities. The second enzyme, which we named Pal, converts UDP-6-deoxy-D-GlcNAc-5,6-ene to UDP-4-keto-6-deoxy-L-AltNAc. Pal is NAD(+)-dependent and has distinct UDP-6-deoxy-d-GlcNAc-5,6-ene 4-oxidase, 5,6-reductase, and 5-epimerase activities. We also show here using NMR spectroscopy and mass spectrometry that in B. thuringiensis, the enzymatic product of Pen and Pal, UDP-4-keto-6-deoxy-L-AltNAc, is converted to CMP-pseudaminic acid by the sequential activities of a C4″-transaminase (Pam), a 4-N-acetyltransferase (Pdi), a UDP-hydrolase (Phy), an enzyme (Ppa) that adds phosphoenolpyruvate to form pseudaminic acid, and finally a cytidylyltransferase that condenses CTP to generate CMP-pseudaminic acid. Knowledge of the distinct dehydratase-like enzymes Pen and Pal and their role in CMP-pseudaminic acid biosynthesis in Gram-positive bacteria provides a foundation to investigate the role of pseudaminic acid and flagellin glycosylation in Bacillus and their involvement in bacterial motility and pathogenicity.

The biosynthesis of UDP-d-FucNAc-4N-(2)-oxoglutarate (UDP-Yelosamine) in Bacillus cereus ATCC 14579: Pat and Pyl, an aminotransferase and an ATP-dependent Grasp protein that ligates 2-oxoglutarate to UDP-4-amino-sugars

Surface glycan switching is often observed when micro-organisms transition between different biotic and abiotic niches, including biofilms, although the advantages of this switching to the organism are not well understood. Bacillus cereus grown in a biofilm-inducing medium has been shown to synthesize an unusual cell wall polysaccharide composed of the repeating subunit →6)Gal(α1-2)(2-R-hydroxyglutar-5-ylamido)Fuc2NAc4N(α1-6)GlcNAc(β1→, where galactose is linked to the hydroxyglutarate moiety of FucNAc-4-amido-(2)-hydroxyglutarate. The molecular mechanism involved in attaching 2-hydroxyglutarate to 4-amino-FucNAc has not been determined. Here, we show two genes in B. cereus ATCC 14579 encoding enzymes involved in the synthesis of UDP-FucNAc-4-amido-(2)-oxoglutarate (UDP-Yelosamine), a modified UDP-sugar not previously reported to exist. Using mass spectrometry and real time NMR spectroscopy, we show that Bc5273 encodes a C4″-aminotransferase (herein referred to as Pat) that, in the presence of pyridoxal phosphate, transfers the primary amino group of l-Glu to C-4″ of UDP-4-keto-6-deoxy-d-GlcNAc to form UDP-4-amino-FucNAc and 2-oxoglutarate. Pat also converts 4-keto-xylose, 4-keto-glucose, and 4-keto-2-acetamido-altrose to their corresponding UDP-4-amino-sugars. Bc5272 encodes a carboxylate-amine ligase (herein referred as Pyl) that, in the presence of ATP and Mg(II), adds 2-oxoglutarate to the 4-amino moiety of UDP-4-amino-FucNAc to form UDP-Yelosamine and ADP. Pyl is also able to ligate 2-oxoglutarate to other 4-amino-sugar derivatives to form UDP-Yelose, UDP-Solosamine, and UDP-Aravonose. Characterizing the metabolic pathways involved in the formation of modified nucleotide sugars provides a basis for understanding some of the mechanisms used by bacteria to modify or alter their cell surface polysaccharides in response to changing growth and environmental challenges.

Trefoil factor family domains represent highly efficient conformational determinants for N-linked N,N'-di-N-acetyllactosediamine (LacdiNAc) synthesis

The disaccharide N,N'-di-N-acetyllactose diamine (LacdiNAc, GalNAcβ1-4GlcNAcβ) is found in a limited number of extracellular matrix glycoproteins and neuropeptide hormones indicating a protein-specific transfer of GalNAc by the glycosyltransferases β4GalNAc-T3/T4. Whereas previous studies have revealed evidence for peptide determinants as controlling elements in LacdiNAc biosynthesis, we report here on an entirely independent conformational control of GalNAc transfer by single TFF (Trefoil factor) domains as high stringency determinants. Human TFF2 was recombinantly expressed in HEK-293 cells as a wild type full-length probe (TFF2-Fl, containing TFF domains P1 and P2), as single P1 or P2 domain probes, as a series of Cys/Gly mutant forms with aberrant domain structures, and as a double point-mutated probe (T68Q/F59Q) lacking aromatic residues within a hydrophobic patch. The N-glycosylation probes were analyzed by mass spectrometry for their glycoprofiles. In agreement with natural gastric TFF2, the recombinant full-length and single domain probes expressed nearly exclusively fucosylated LacdiNAc on bi-antennary complex-type chains indicating that a single TFF domain was sufficient to induce transfer of this modification. Contrasting to this, the Cys/Gly mutants showed strongly reduced LacdiNAc levels and instead preponderant LacNAc expression. The probe with point mutations of two highly conserved aromatic residues in loop 3 (T68Q/F59Q) revealed that these are essential determinant components, as the probe lacked LacdiNAc expression. The structural features of the LacdiNAc-inducing determinant on human TFF2 are discussed on the basis of crystal structures of porcine TFF2, and a series of extracellular matrix-related LacdiNAc-positive glycoproteins detected as novel candidate proteins in the secretome of HEK-293 cells.

Crystal structure of the Chlamydomonas starch debranching enzyme isoamylase ISA1 reveals insights into the mechanism of branch trimming and complex assembly

The starch debranching enzymes isoamylase 1 and 2 (ISA1 and ISA2) are known to exist in a large complex and are involved in the biosynthesis and crystallization of starch. It is suggested that the function of the complex is to remove misplaced branches of growing amylopectin molecules, which would otherwise prevent the association and crystallization of adjacent linear chains. Here, we investigate the function of ISA1 and ISA2 from starch producing alga Chlamydomonas. Through complementation studies, we confirm that the STA8 locus encodes for ISA2 and sta8 mutants lack the ISA1·ISA2 heteromeric complex. However, mutants retain a functional dimeric ISA1 that is able to partly sustain starch synthesis in vivo. To better characterize ISA1, we have overexpressed and purified ISA1 from Chlamydomonas reinhardtii (CrISA1) and solved the crystal structure to 2.3 Å and in complex with maltoheptaose to 2.4 Å. Analysis of the homodimeric CrISA1 structure reveals a unique elongated structure with monomers connected end-to-end. The crystal complex reveals details about the mechanism of branch binding that explains the low activity of CrISA1 toward tightly spaced branches and reveals the presence of additional secondary surface carbohydrate binding sites.

A structural and biochemical model of processive chitin synthesis

Chitin synthases (CHS) produce chitin, an essential component of the fungal cell wall. The molecular mechanism of processive chitin synthesis is not understood, limiting the discovery of new inhibitors of this enzyme class. We identified the bacterial glycosyltransferase NodC as an appropriate model system to study the general structure and reaction mechanism of CHS. A high throughput screening-compatible novel assay demonstrates that a known inhibitor of fungal CHS also inhibit NodC. A structural model of NodC, on the basis of the recently published BcsA cellulose synthase structure, enabled probing of the catalytic mechanism by mutagenesis, demonstrating the essential roles of the DD and QXXRW catalytic motifs. The NodC membrane topology was mapped, validating the structural model. Together, these approaches give insight into the CHS structure and mechanism and provide a platform for the discovery of inhibitors for this antifungal target.

In vitro biosynthesis and chemical identification of UDP-N-acetyl-d-quinovosamine (UDP-d-QuiNAc)

N-acetyl-d-quinovosamine (2-acetamido-2,6-dideoxy-d-glucose, QuiNAc) occurs in the polysaccharide structures of many Gram-negative bacteria. In the biosynthesis of QuiNAc-containing polysaccharides, UDP-QuiNAc is the hypothetical donor of the QuiNAc residue. Biosynthesis of UDP-QuiNAc has been proposed to occur by 4,6-dehydration of UDP-N-acetyl-d-glucosamine (UDP-GlcNAc) to UDP-2-acetamido-2,6-dideoxy-d-xylo-4-hexulose followed by reduction of this 4-keto intermediate to UDP-QuiNAc. Several specific dehydratases are known to catalyze the first proposed step. A specific reductase for the last step has not been demonstrated in vitro, but previous mutant analysis suggested that Rhizobium etli gene wreQ might encode this reductase. Therefore, this gene was cloned and expressed in Escherichia coli, and the resulting His6-tagged WreQ protein was purified. It was tested for 4-reductase activity by adding it and NAD(P)H to reaction mixtures in which 4,6-dehydratase WbpM had acted on the precursor substrate UDP-GlcNAc. Thin layer chromatography of the nucleotide sugars in the mixture at various stages of the reaction showed that WbpM converted UDP-GlcNAc completely to what was shown to be its 4-keto-6-deoxy derivative by NMR and that addition of WreQ and NADH led to formation of a third compound. Combined gas chromatography-mass spectrometry analysis of acid hydrolysates of the final reaction mixture showed that a quinovosamine moiety had been synthesized after WreQ addition. The two-step reaction progress also was monitored in real time by NMR. The final UDP-sugar product after WreQ addition was purified and determined to be UDP-d-QuiNAc by one-dimensional and two-dimensional NMR experiments. These results confirmed that WreQ has UDP-2-acetamido-2,6-dideoxy-d-xylo-4-hexulose 4-reductase activity, completing a pathway for UDP-d-QuiNAc synthesis in vitro.

Substrate specificity and kinetic characterization of peptidoglycan O-acetyltransferase B from Neisseria gonorrhoeae

The O-acetylation of the essential cell wall polymer peptidoglycan is a major virulence factor identified in many bacteria, both Gram-positive and Gram-negative, including Staphylococcus aureus, Bacillus anthracis, Neisseria gonorrhoeae, and Neisseria meningitidis. With Gram-negative bacteria, the translocation of acetyl groups from the cytoplasm is performed by an integral membrane protein, PatA, for its transfer to peptidoglycan by O-acetyltransferase PatB, whereas a single bimodal membrane protein, OatA, appears to catalyze both reactions of the process in Gram-positive bacteria. Only phenotypic evidence existed in support of these pathways because no in vitro biochemical assay was available for their analysis, which reflected the complexities of investigating integral membrane proteins that act on a totally insoluble and heterogeneous substrate, such as peptidoglycan. In this study, we present the first biochemical and kinetic analysis of a peptidoglycan O-acetyltransferase using PatB from N. gonorrhoeae as the model system. The enzyme has specificity for muropeptides that possess tri- and tetrapeptide stems on muramyl residues. With chitooligosaccharides as substrates, rates of reaction increase with increasing degrees of polymerization to 5/6. This information will be valuable for the identification and development of peptidoglycan O-acetyltransferase inhibitors that could represent potential leads to novel classes of antibiotics.

The capsular polysaccharide of Staphylococcus aureus is attached to peptidoglycan by the LytR-CpsA-Psr (LCP) family of enzymes

Envelope biogenesis in bacteria involves synthesis of intermediates that are tethered to the lipid carrier undecaprenol-phosphate. LytR-CpsA-Psr (LCP) enzymes have been proposed to catalyze the transfer of undecaprenol-linked intermediates onto the C6-hydroxyl of MurNAc in peptidoglycan, thereby promoting attachment of wall teichoic acid (WTA) in bacilli and staphylococci and capsular polysaccharides (CPS) in streptococci. S. aureus encodes three lcp enzymes, and a variant lacking all three genes (Δlcp) releases WTA from the bacterial envelope and displays a growth defect. Here, we report that the type 5 capsular polysaccharide (CP5) of Staphylococcus aureus Newman is covalently attached to the glycan strands of peptidoglycan. Cell wall attachment of CP5 is abrogated in the Δlcp variant, a defect that is best complemented via expression of lcpC in trans. CP5 synthesis and peptidoglycan attachment are not impaired in the tagO mutant, suggesting that CP5 synthesis does not involve the GlcNAc-ManNAc linkage unit of WTA and may instead utilize another Wzy-type ligase to assemble undecaprenyl-phosphate intermediates. Thus, LCP enzymes of S. aureus are promiscuous enzymes that attach secondary cell wall polymers with discrete linkage units to peptidoglycan.

Determination of carbohydrate structure recognized by prostate-specific F77 monoclonal antibody through expression analysis of glycosyltransferase genes

This study reports the determination of the carbohydrate epitope of monoclonal antibody F77 previously raised against human prostate cancer PC-3 cells (Zhang, G., Zhang, H., Wang, Q., Lal, P., Carroll, A. M., de la Llera-Moya, M., Xu, X., and Greene, M. I. (2010) Proc. Natl. Acad. Sci. U. S. A. 107, 732–737). We performed a series of co-transfections using mammalian expression vectors encoding specific glycosyltransferases. We thereby identified branching enzymes and FUT1 (required for Fucα1→2Gal linkage) as being essential for F77 antigen formation. When immortalized normal prostate 267B1 cells were transfected with FUT1 alone, cells showed weak expression of F77 antigen. By contrast, cells co-transfected with FUT1 plus either GCNT1, GCNT2, or GCNT3 (an enzyme required to form GlcNAcβ1→6Gal/GalNAc) showed robust F77 antigen expression, suggesting that F77 specifically binds to Fucα1→2Galβ1→4GlcNAcβ1→6Gal/GalNAc. RT-PCR for FUT1, GCNT1, GCNT2, and GCNT3 showed that F77-positive cell lines indeed express transcripts encoding FUT1 plus one GCNT. F77-positive prostate cancer cells transfected with siRNAs targeting FUT1, GCNT2, and GCNT3 showed significantly reduced F77 antigen, confirming the requirement of these enzymes for epitope synthesis. We also found that hypoxia induces F77 epitope expression in immortalized prostate RWPE1 cells, which express F77 antigen moderately under normoxia but at an elevated level under hypoxia. Quantitative RT-PCR demonstrated up-regulation of FUT1, GCNT2, and GCNT3 transcripts in RWPE1 cells under hypoxia, suggesting that hypoxia up-regulates glycosyltransferase expression required for F77 antigen synthesis. These results define the F77 epitope and provide a potential mechanism for F77 antigen synthesis in malignant prostate cancer.

Lipopolysaccharide engineering in Neisseria meningitidis: structural analysis of different pentaacyl lipid A mutants and comparison of their modified agonist properties

Engineering the lipopolysaccharide (LPS) biosynthetic pathway offers the potential to obtain modified derivatives with optimized adjuvant properties. Neisseria meningitidis strain H44/76 was modified by expression of the pagL gene encoding lipid A 3-O-deacylase from Bordetella bronchiseptica and by inactivation of the lgtB gene encoding the terminal oligosaccharide galactosyltransferase. Mass spectrometry analysis of purified mutant LPS was used for detailed compositional analysis of all present molecular species. This determined that the modified LPS was mainly pentaacylated, demonstrating high efficiency of conversion from the hexaacyl to the 3-O-deacylated form by heterologous lipid A 3-O-deacylase (PagL) expression. MS analyses also provided evidence for expression of only one major oligosaccharide glycoform, which lacked the terminal galactose residue as expected from inactivation of the lgtB gene. The immunomodulatory properties of PagL-deacylated LPS were compared with another pentaacyl form obtained from an lpxL1(-) mutant, which lacks the 2' secondary acyl chain. Although both LPS mutants displayed impaired capacity to induce production of the pro-inflammatory cytokine IL-6 in the monocytic cell line Mono Mac 6, induction of the Toll-interleukin-1 receptor domain-containing adaptor-inducing interferon-β-dependent chemokine interferon-γ-induced protein 10 was largely retained only for the lgtB(-)/pagL(+) mutant. Removal of remaining hexaacyl species exclusively present in lgtB(-)/pagL(+) LPS demonstrated that these minor species potentiate but do not determine the activity of this LPS. These results are the first to indicate a qualitatively different response of human innate cells to pentaacyl lpxL1(-) and pagL(+) LPS and show the importance of detailed structure-function analysis when working with modified lipid A structures. The pagL(+) LPS has significant potential as immune modulator in humans.

2-Acylamido analogues of N-acetylglucosamine prime formation of chitin oligosaccharides by yeast chitin synthase 2

Chitin, a homopolymer of β1,4-linked N-acetylglucosamine (GlcNAc) residues, is a key component of the cell walls of fungi and the exoskeletons of arthropods. Chitin synthases transfer GlcNAc from UDP-GlcNAc to preexisting chitin chains in reactions that are typically stimulated by free GlcNAc. The effect of GlcNAc was probed by using a yeast strain expressing a single chitin synthase, Chs2, by examining formation of chitin oligosaccharides (COs) and insoluble chitin, and by replacing GlcNAc with 2-acylamido analogues of GlcNAc. Synthesis of COs was strongly dependent on inclusion of GlcNAc in chitin synthase incubations, and N,N'-diacetylchitobiose (GlcNAc2) was the major reaction product. Formation of both COs and insoluble chitin was also stimulated by GlcNAc2 and by N-propanoyl-, N-butanoyl-, and N-glycolylglucosamine. MALDI analyses of the COs made in the presence of 2-acylamido analogues of GlcNAc showed they that contained a single GlcNAc analogue and one or more additional GlcNAc residues. These results indicate that Chs2 can use certain 2-acylamido analogues of GlcNAc, and likely free GlcNAc and GlcNAc2 as well, as GlcNAc acceptors in a UDP-GlcNAc-dependent glycosyltransfer reaction. Further, formation of modified disaccharides indicates that CSs can transfer single GlcNAc residues.

Identification of multiple phosphorylation sites on maize endosperm starch branching enzyme IIb, a key enzyme in amylopectin biosynthesis

Starch branching enzyme IIb (SBEIIb) plays a crucial role in amylopectin biosynthesis in maize endosperm by defining the structural and functional properties of storage starch and is regulated by protein phosphorylation. Native and recombinant maize SBEIIb were used as substrates for amyloplast protein kinases to identify phosphorylation sites on the protein. A multidisciplinary approach involving bioinformatics, site-directed mutagenesis, and mass spectrometry identified three phosphorylation sites at Ser residues: Ser(649), Ser(286), and Ser(297). Two Ca(2+)-dependent protein kinase activities were partially purified from amyloplasts, termed K1, responsible for Ser(649) and Ser(286) phosphorylation, and K2, responsible for Ser(649) and Ser(297) phosphorylation. The Ser(286) and Ser(297) phosphorylation sites are conserved in all plant branching enzymes and are located at opposite openings of the 8-stranded parallel β-barrel of the active site, which is involved with substrate binding and catalysis. Molecular dynamics simulation analysis indicates that phospho-Ser(297) forms a stable salt bridge with Arg(665), part of a conserved Cys-containing domain in plant branching enzymes. Ser(649) conservation appears confined to the enzyme in cereals and is not universal, and is presumably associated with functions specific to seed storage. The implications of SBEIIb phosphorylation are considered in terms of the role of the enzyme and the importance of starch biosynthesis for yield and biotechnological application.

Promotion of enzyme flexibility by dephosphorylation and coupling to the catalytic mechanism of a phosphohexomutase

The enzyme phosphomannomutase/phosphoglucomutase (PMM/PGM) from Pseudomonas aeruginosa catalyzes an intramolecular phosphoryl transfer across its phosphosugar substrates, which are precursors in the synthesis of exoproducts involved in bacterial virulence. Previous structural studies of PMM/PGM have established a key role for conformational change in its multistep reaction, which requires a dramatic 180° reorientation of the intermediate within the active site. Here hydrogen-deuterium exchange by mass spectrometry and small angle x-ray scattering were used to probe the conformational flexibility of different forms of PMM/PGM in solution, including its active, phosphorylated state and the unphosphorylated state that occurs transiently during the catalytic cycle. In addition, the effects of ligand binding were assessed through use of a substrate analog. We found that both phosphorylation and binding of ligand produce significant effects on deuterium incorporation. Phosphorylation of the conserved catalytic serine has broad effects on residues in multiple domains and is supported by small angle x-ray scattering data showing that the unphosphorylated enzyme is less compact in solution. The effects of ligand binding are generally manifested near the active site cleft and at a domain interface that is a site of conformational change. These results suggest that dephosphorylation of the enzyme may play two critical functional roles: a direct role in the chemical step of phosphoryl transfer and secondly through propagation of structural flexibility. We propose a model whereby increased enzyme flexibility facilitates the reorientation of the reaction intermediate, coupling changes in structural dynamics with the unique catalytic mechanism of this enzyme.

Overlapping and distinct roles of Aspergillus fumigatus UDP-glucose 4-epimerases in galactose metabolism and the synthesis of galactose-containing cell wall polysaccharides

The cell wall of Aspergillus fumigatus contains two galactose-containing polysaccharides, galactomannan and galactosaminogalactan, whose biosynthetic pathways are not well understood. The A. fumigatus genome contains three genes encoding putative UDP-glucose 4-epimerases, uge3, uge4, and uge5. We undertook this study to elucidate the function of these epimerases. We found that uge4 is minimally expressed and is not required for the synthesis of galactose-containing exopolysaccharides or galactose metabolism. Uge5 is the dominant UDP-glucose 4-epimerase in A. fumigatus and is essential for normal growth in galactose-based medium. Uge5 is required for synthesis of the galactofuranose (Galf) component of galactomannan and contributes galactose to the synthesis of galactosaminogalactan. Uge3 can mediate production of both UDP-galactose and UDP-N-acetylgalactosamine (GalNAc) and is required for the production of galactosaminogalactan but not galactomannan. In the absence of Uge5, Uge3 activity is sufficient for growth on galactose and the synthesis of galactosaminogalactan containing lower levels of galactose but not the synthesis of Galf. A double deletion of uge5 and uge3 blocked growth on galactose and synthesis of both Galf and galactosaminogalactan. This study is the first survey of glucose epimerases in A. fumigatus and contributes to our understanding of the role of these enzymes in metabolism and cell wall synthesis.

Increased phagocytosis of Mycobacterium marinum mutants defective in lipooligosaccharide production: a structure-activity relationship study

Mycobacterium marinum is a waterborne pathogen responsible for tuberculosis-like infections in ectotherms and is an occasional opportunistic human pathogen. In the environment, M. marinum also interacts with amoebae, which may serve as a natural reservoir for this microorganism. However, the description of mycobacterial determinants in the early interaction with macrophages or amoebae remains elusive. Lipooligosaccharides (LOSs) are cell surface-exposed glycolipids capable of modulating the host immune system, suggesting that they may be involved in the early interactions of M. marinum with macrophages. Herein, we addressed whether LOS composition affects the uptake of M. marinum by professional phagocytes. Mutants with various truncated LOS variants were generated, leading to the identification of several previously uncharacterized biosynthetic genes (wbbL2, MMAR_2321, and MMAR_2331). Biochemical and structural approaches allowed resolving the structures of LOS precursors accumulating in this set of mutants. These strains with structurally defined LOS profiles were then used to infect both macrophages and Acanthamoebae. An inverse correlation between LOS completeness and uptake of mycobacteria by phagocytes was found, allowing the proposal of three mutant classes: class I (papA4), devoid of LOS and highly efficiently phagocytosed; class II, accumulating only early LOS intermediates (wbbL2 and MMAR_2331) and efficiently phagocytosed but less than class I mutants; class III, lacking LOS-IV (losA, MMAR_2319, and MMAR_2321) and phagocytosed similarly to the control strain. These results indicate that phagocytosis is conditioned by the LOS pattern and that the LOS pathway used by M. marinum in macrophages is conserved during infection of amoebae.

Enzymatic basis for N-glycan sialylation: structure of rat α2,6-sialyltransferase (ST6GAL1) reveals conserved and unique features for glycan sialylation

Glycan structures on glycoproteins and glycolipids play critical roles in biological recognition, targeting, and modulation of functions in animal systems. Many classes of glycan structures are capped with terminal sialic acid residues, which contribute to biological functions by either forming or masking glycan recognition sites on the cell surface or secreted glycoconjugates. Sialylated glycans are synthesized in mammals by a single conserved family of sialyltransferases that have diverse linkage and acceptor specificities. We examined the enzymatic basis for glycan sialylation in animal systems by determining the crystal structures of rat ST6GAL1, an enzyme that creates terminal α2,6-sialic acid linkages on complex-type N-glycans, at 2.4 Å resolution. Crystals were obtained from enzyme preparations generated in mammalian cells. The resulting structure revealed an overall protein fold broadly resembling the previously determined structure of pig ST3GAL1, including a CMP-sialic acid-binding site assembled from conserved sialylmotif sequence elements. Significant differences in structure and disulfide bonding patterns were found outside the sialylmotif sequences, including differences in residues predicted to interact with the glycan acceptor. Computational substrate docking and molecular dynamics simulations were performed to predict and evaluate the CMP-sialic acid donor and glycan acceptor interactions, and the results were compared with kinetic analysis of active site mutants. Comparisons of the structure with pig ST3GAL1 and a bacterial sialyltransferase revealed a similar positioning of donor, acceptor, and catalytic residues that provide a common structural framework for catalysis by the mammalian and bacterial sialyltransferases.

A novel bacterial enzyme with D-glucuronyl C5-epimerase activity

Glycosaminoglycans are biologically active polysaccharides that are found ubiquitously in the animal kingdom. The biosynthesis of these complex polysaccharides involves complicated reactions that turn the simple glycosaminoglycan backbone into highly heterogeneous structures. One of the modification reactions is the epimerization of D-glucuronic acid to its C5-epimer L-iduronic acid, which is essential for the function of heparan sulfate. Although L-iduronic acid residues have been shown to exist in polysaccharides of some prokaryotes, there has been no experimental evidence for the existence of a prokaryotic D-glucuronyl C5-epimerase. This work for the first time reports on the identification of a bacterial enzyme with D-glucuronyl C5-epimerase activity. A gene of the marine bacterium Bermanella marisrubri sp. RED65 encodes a protein (RED65_08024) of 448 amino acids that has an overall 37% homology to the human D-glucuronic acid C5-epimerase. Alignment of this peptide with the human and mouse sequences revealed a 60% similarity at the carboxyl terminus. The recombinant protein expressed in Escherichia coli showed epimerization activity toward substrates generated from heparin and the E. coli K5 capsular polysaccharide, thereby providing the first evidence for bacterial D-glucuronyl C5-epimerase activity. These findings may eventually be used for modification of mammalian glycosaminoglycans.

The UDP-glucose dehydrogenase of Escherichia coli K-12 displays substrate inhibition by NAD that is relieved by nucleotide triphosphates

UDP-glucose dehydrogenase (Ugd) generates UDP-glucuronic acid, an important precursor for the production of many hexuronic acid-containing bacterial surface glycostructures. In Escherichia coli K-12, Ugd is important for biosynthesis of the environmentally regulated exopolysaccharide known as colanic acid, whereas in other E. coli isolates, the same enzyme is required for production of the constitutive group 1 capsular polysaccharides, which act as virulence determinants. Recent studies have implicated tyrosine phosphorylation in the activation of Ugd from E. coli K-12, although it is not known if this is a feature shared by bacterial Ugd proteins. The activities of Ugd from E. coli K-12 and from the group 1 capsule prototype (serotype K30) were compared. Surprisingly, for both enzymes, site-directed Tyr → Phe mutants affecting the previously proposed phosphorylation site retained similar kinetic properties to the wild-type protein. Purified Ugd from E. coli K-12 had significant levels of NAD substrate inhibition, which could be alleviated by the addition of ATP and several other nucleotide triphosphates. Mutations in a previously identified UDP-glucuronic acid allosteric binding site decreased the binding affinity of the nucleotide triphosphate. Ugd from E. coli serotype K30 was not inhibited by NAD, but its activity still increased in the presence of ATP.

Comparison of predicted epimerases and reductases of the Campylobacter jejuni D-altro- and L-gluco-heptose synthesis pathways

Uniquely modified heptoses found in surface carbohydrates of bacterial pathogens are potential therapeutic targets against such pathogens. Our recent biochemical characterization of the GDP-6-deoxy-D-manno- and GDP-6-deoxy-D-altro-heptose biosynthesis pathways has provided the foundation for elucidation of the more complex L-gluco-heptose synthesis pathway of Campylobacter jejuni strain NCTC 11168. In this work we use GDP-4-keto,6-deoxy-D-lyxo-heptose as a surrogate substrate to characterize three enzymes predicted to be involved in this pathway: WcaGNCTC (also known as Cj1427), MlghB (Cj1430), and MlghC (Cj1428). We compare them with homologues involved in d-altro-heptose production: WcaG81176 (formerly WcaG), DdahB (Cjj1430), and DdahC (Cjj1427). We show that despite high levels of similarity, the enzymes have pathway-specific catalytic activities and substrate specificities. MlghB forms three products via C3 and C5 epimerization activities, whereas its DdahB homologue only had C3 epimerase activity along its cognate pathway. MlghC is specific for the double C3/C5 epimer generated by MlghB and produces L-gluco-heptose via stereospecific C4 reductase activity. In contrast, its homologue DdahC only uses the C3 epimer to yield d-altro-heptose via C4 reduction. Finally, we show that WcaGNCTC is not necessary for L-gluco-heptose synthesis and does not affect its production by MlghB and MlghC, in contrast to its homologue WcaG81176, that has regulatory activity on d-altro-heptose synthesis. These studies expand our fundamental understanding of heptose modification, provide new glycobiology tools to synthesize novel heptose derivatives with biomedical applications, and provide a foundation for the structure function analysis of these enzymes.